AU2023255622B2 - Steel plate and method of producing same - Google Patents

Description

WO 2023/203815 A1 CG, CI, CM, GA, GN, GQ, GW, KM, ML, MR, NE, SN, TD, TG).

:

- (21(3)) (321)

18 Aug 2025

TITLE STEEL PLATE AND METHOD OF PRODUCING SAME 5 TECHNICAL FIELD

[0001] The present disclosure relates to a steel plate, particularly a steel plate 2023255622

that can stably ensure excellent strength, low-temperature toughness, and low yield ratio and is suitable for use at low temperatures, and a method of 10 producing the same. The steel plate according to the present disclosure can be suitably used in large low-temperature liquefied gas storage tanks because, for example for liquefied gas storage tanks for ships, stress relief can be performed mechanically instead of by post weld heat treatment.

15 BACKGROUND

[0002] When carbon steel and/or carbon manganese steel is used for tanks that are classified as independent Type-C tanks and have a design temperature below –10 °C among liquefied gas storage tanks for ships, stress relief after welding is required. Stress relief is usually performed by post weld heat 20 treatment (PWHT), but mechanical stress relief is also possible if the yield ratio of the steel material is 0.8 or less. For large tanks, PWHT is difficult, and accordingly a low yield ratio material that can be subjected to mechanical stress relief is desired. For example, in the case where carbon steel is used for large storage tanks for liquefied CO 2 , high-strength steel that ensures excellent 25 toughness at a low temperature of –50 °C and has a tensile strength (hereafter also referred to as TS) of 690 MPa or more is needed. Thus, a steel material having low yield ratio, high strength, and excellent low-temperature toughness is required. Since weld defects are checked by ultrasonic testing during welding process in some cases, the material may also be required to have low 30 acoustic anisotropy.

[0003] For steel plates with low-temperature toughness and low yield ratio, for example, JP 2016-507649 A (PTL 1) discloses a steel plate having an impact toughness of 150 J or more at –75 °C, a yield ratio of 0.8 or less, and a tensile strength of 530 MPa or more. 35 CITATION LIST

Ref. No. PO225838-PCT-ZZ (1/24)

18 Aug 2025

Patent Literature

[0004] PTL 1: JP 2016-507649 A

5 [0005] However, the TS of the steel plate described in PTL 1 is 620 MPa at a maximum, and a steel plate with a TS of 690 MPa or more is not yet provided. Conventionally, since carbon steel with low yield ratio, excellent low- 2023255622

temperature toughness, and high strength of 690 MPa or more is unavailable, expensive nickel steel such as 9 % Ni steel needs to be used, causing an 10 increase in material costs.

[0006] It could therefore be helpful to provide a steel plate having high strength of TS ≥ 690 MPa, excellent low-temperature toughness, low yield ratio, and low acoustic anisotropy, and a method of producing the same. SUMMARY OF THE INVENTION 15 [0007] Upon careful examination on the chemical composition and microstructure of high-strength steel to achieve low-temperature toughness and low yield ratio, we discovered the following, especially regarding the microstructure: an effective way of achieving desired property improvement may be that, in the microstructure at a depth position of 1/4 of the plate 20 thickness from the surface of the steel plate in the plate thickness direction, the ferrite fraction is 5 % to 95 % and the martensite austenite constituent fraction is 1 % to 30 % with the residual microstructure consisting of tempered martensite and/or bainite, the average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation 25 difference of 15 degrees or more is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm 2 or less.

[0008] Herein, the term “ferrite” refers to BCC phase that remains without reverse transformation even when martensite and bainite are heat-treated to a 30 temperature of Ac 1 point or more and inherits the original lath-like microstructure. By setting this relatively soft ferrite phase to 5 % to 95 % and finely distributing martensite austenite constituent, low yield ratio may be achieved. Moreover, when regions surrounded by large-angle grain boundaries 35 with an orientation difference of 15 degrees or more are defined as crystal grains, by suppressing the formation of coarse grains so that the number density

Ref. No. PO225838-PCT-ZZ (2/24)

18 Aug 2025

of crystal grains having an equivalent circular diameter of more than 30 µm will be 250/mm 2 or less and limiting the residual microstructure other than ferrite and martensite austenite constituent to be tempered martensite and/or bainite phase, low-temperature toughness may be achieved. 5 Furthermore, by setting the average aspect ratio of crystal grains to less than 2.0, reduction in acoustic anisotropy may also be achieved.

[0009] The present invention provides the following. 2023255622

1. A steel plate comprising a chemical composition containing (consisting of), in mass%, C: 0.02 % or more and 0.15 % or less, Si: 0.01 % or 10 more and 0.50 % or less, Mn: 0.05 % or more and 2.50 % or less, Ni: 0.5 % or more and less than 5.0 %, P: 0.03 % or less, S: 0.005 % or less, and N: 0.0010 % or more and 0.0080 % or less, with a balance consisting of Fe and inevitable impurities, wherein in a microstructure at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction, 15 a ferrite fraction is 5 % to 95 % and a martensite austenite constituent fraction is 1 % to 30 % with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains 20 having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm 2 or less.

[0010] 2. The steel plate according to 1., wherein the chemical composition further contains, in mass%, one or more selected from Al: 0.100 % or less, Nb: 0.1 % or less, Cr: 2.00 % or less, Mo: 1.0 % or less, Cu: 2.0 % or less, V: 0.05 25 % or less, Ti: 0.03 % or less, B: 0.0030 % or less, Ca: 0.007 % or less, REM: 0.010 % or less, and Mg: 0.007 % or less.

[0011] 3. A method of producing a steel plate, the method comprising subjecting a steel material having the chemical composition according to claim 1 or 2 to hot rolling, thereafter to first heating retention, thereafter to 30 quenching, thereafter to second heating retention, and thereafter to cooling treatment, wherein in the hot rolling, a finish temperature is 900 °C or more, in the first heating retention, a heating temperature is in a temperature range of Ac 3 point or more and 1000 °C or less, in the quenching, an average cooling rate from 600 °C to 300 °C at a depth position of 1/4 of a plate thickness from 35 a surface of the steel plate in a plate thickness direction is 3 °C/s or more, and a cooling end temperature is 300 °C or less, in the second heating retention, a

Ref. No. PO225838-PCT-ZZ (3/24)

18 Aug 2025

heating temperature is in a temperature range of Ac 1 point or more and less than the Ac 3 point, and in the cooling treatment, an average cooling rate from 700 °C to 500 °C at the depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3 °C/s or more, and 5 a cooling end temperature is 500 °C or less and 200 °C or more.

[0012] The steel plate according to the present disclosure, despite its material 2023255622

being carbon steel or carbon manganese steel, can be used for steel structures that are used in low-temperature environments, for example, large low- 10 temperature storage tanks for ships such as liquefied CO 2 tanks and LPG tanks. The steel plate according to the present disclosure also contributes to lower construction costs than nickel steel. This yields industrially great advantageous effects.

15 DETAILED DESCRIPTION

[0013] Embodiments of the present disclosure will be described in detail below. The following description shows preferred embodiments of the present disclosure, and the present disclosure is not limited to such.

[0014] [Chemical composition] 20 A steel plate according to the present disclosure has a certain chemical composition. Preferably, the steel material used for producing the steel plate according to the present disclosure also has the certain chemical composition. Each element contained in the chemical composition will be described below. In this specification, “%” as a unit of content of each element denotes “mass%” 25 unless otherwise specified.

[0015] C: 0.02 % or more and 0.15 % or less C is an element that has the effect of improving the strength of the steel plate. In order to achieve this effect, the C content is 0.02 % or more. The C content is preferably 0.03 % or more. If the C content is more than 0.15 %, the 30 amount of martensite austenite constituent in the steel plate increases and low- temperature toughness decreases. The C content is therefore 0.15 % or less. The C content is preferably 0.12 % or less.

[0016] Si: 0.01 % or more and 0.50 % or less Si is an element that acts as a deoxidizer. In order to achieve this effect,

Ref. No. PO225838-PCT-ZZ (4/24) the Si the Si content content is 0.01  is 0.01 % or or more. more. The Si content The Si content is is preferably 0.03  preferably 0.03 or more. % or more. If the If the Si Si content content is is excessively excessively high, toughness decreases. high, toughness decreases.The TheSiSicontent contentis is therefore 0.50%  therefore0.50 or or less. less. TheThe Si content Si content is preferably is preferably 0.30 % 0.30  or or less. less.

[0017]

[0017] Mn: 0.05%  Mn:0.05 or or more more andand 2.50 2.50  less % or or less

Mnisisananelement Mn elementthat thatenhances enhances thethe hardenability hardenability of the of the steel steel andand is is effective in effective in increasing increasingthethe strength strength of the of the steel steel plate. plate. In order In order to achieve to achieve this this effect, the effect, theMn Mn content content is 0.05  is 0.05 % or or more. more. The Mncontent The Mn contentisispreferably 0.10% preferably0.10 or more. or If the more. If the Mn contentisis more Mn content morethan 2.50%,,toughness than2.50 toughness degrades. degrades. TheThe Mn Mn contentisis therefore content 2.50% or therefore2.50 2.50% or less. or less. less. TheThe The Mn content Mn content Mn content is preferably is preferably is preferably or2.00 2.00 % 2.00% or  or less. less. less.

[0018]

[0018] Ni: 0.5 %orormore Ni: 0.5 moreand andless lessthan 5.0%  than5.0 Ni is Ni is an an element effective in element effective in improving the low-temperature improving the toughness low-temperature toughness

of the of the steel steelplate. plate.InInorder order to to achieve achieve this this effect, effect, the the Ni Ni content content is 0.5 %isor0.5  or more.Since more. SinceNi Ni is is an an expensive expensive element, element, the plate the steel steel costs plate increase costs increase as as the Ni the Ni contentincreases. content increases.Accordingly, Accordingly, in present in the the present disclosure, disclosure, the Nithe Ni content content is less is less

than 5.0 than .The 5.0 %. TheNiNi content content is is preferably preferably 0.80.8  more. % or or more. The The Ni Ni content content is is preferably preferably 3.5%  preferably3.5 3.5% oror or less. less. less.

[0019] 0.03 %ororless P: 0.03

[0019] P: less P is P is an an inevitable inevitable impurity, impurity, and and isis aa harmful harmfulelement element that that adversely adversely

affects the affects the low-temperature low-temperature toughness toughness of theof the plate. steel steel plate. For example, For example, in order in order

to obtain to a sound obtain a sound base basemetal metaland and weld weld joint joint when when welding welding the steel the steel plate plate to to producea awelded produce welded structure, structure, it isit preferable is preferable to reduce to reduce the P the P content content as much as as much as possible. The possible. The PP content content is is therefore 0.03 %ororless. therefore 0.03 less. Since Sincelower lowerP Pcontent contentisis better from better from the the viewpoint of low-temperature viewpoint of low-temperaturetoughness, toughness,the thelower lowerlimit limitofofthe the P content P content is is not not set set and andmay may be be . Excessively 0 %.0 Excessively reducing reducing the P content, the P content,

however,causes however, causesananincrease increase in in cost. cost. Accordingly, Accordingly, the the lower lower limitlimit of Pthe of the P content is content is preferably 0.001  preferably 0.001 fromthe % from the viewpoint viewpointofofcost. cost.

[0020] 0.005%ororless S: 0.005

[0020] S: less S forms MnS S forms MnSin in thethe steeland steel and significantlydegrades significantly degrades low-temperature low-temperature

toughness, and toughness, andaccordingly accordinglyitit is is desirable desirable to to reduce the SS content reduce the content as as much muchasas

possible with possible the upper with the limit being upper limit being 0.005 . The 0.005 %. TheSScontent contentis is preferably preferably 0.002 0.002 ororless. % less.Since Since lower lower S content S content is better, is better, the lower the lower limit limit of the Sofcontent the S is content is not set not set and and may . be 00 %. may be

[0021]

[0021] N: 0.0010%  N:0.0010 oror more more andand % orless 0.0080 0.0080 or less Nforms N forms precipitates precipitates in the in the steel. steel. If the If the N content N content is than is more more0.0080 than 0.0080

, the %, the toughness toughnessofofthe the base base metal metaldecreases. decreases.NNisisalso also an anelement elementthat thatforms forms AlNand AIN andthus thuscontributes contributestotograin grainrefinement refinementofofthe thebase basemetal. metal.This Thiseffect effectisis

Ref. No. Ref. No. PO225838-PCT-ZZ (5/24) PO225838-PCT-ZZ (5/24) achieved when achieved whenthe theN N content content is is0.0010 0.0010  or % or more. more. The The N content N content is therefore is therefore

0.0010 %%or 0.0010 0.0010 ormore or moreand more and and 0.0080 0.0080 0.0080  less. %% or or or less. less. The TheThe N content NN content content is is is preferably preferably preferably 0.0020 0.0020 0.0020 %%  or more. or The NNcontent more. The contentisis preferably 0.0060% ororless. preferably 0.0060 less.

[0022] In one

[0022] In one embodiment embodimentof of thethe presentdisclosure, present disclosure,the thechemical chemicalcomposition composition

contains the contains the certain certain amounts of the amounts of the elements describedabove elements described abovewith withthe thebalance balance consistingofofFeFeandand consisting inevitable inevitable impurities. impurities.

[0023]

[0023] In another embodiment In another embodimentof ofthethe present present disclosure,thethe disclosure, chemical chemical compositionmay composition mayoptionally optionallyfurther furthercontain containone oneorormore more selectedfrom selected from Cr,Mo, Cr, Mo, Al, Cu, Al, Nb, V, Cu, Nb, V, Ti, Ti, B, B, Ca, Ca, REM, andMgMg REM, and preferably preferably in in thefollowing the following amounts. amounts.

[0024]

[0024] Cr: 2.00 %ororless Cr: 2.00 less Cr is Cr is an an element elementthat thatcan can improve improve the strength the strength ofsteel of the the steel plate plate without without

significantly impairing significantly low-temperaturetoughness. impairing low-temperature toughness. In In order order to achieve to achieve this this

effect, the effect, the Cr Cr content is preferably content is 0.01 %or preferably 0.01 ormore. more.The The Cr Cr content content is more is more

preferably 0.30 %orormore. preferably 0.30 more. If If theCrCr the content content is is more more thanthan 2.002.00 , low- %, the the low-

temperature toughness temperature toughnessof of thethe steel steel plate plate maymay decrease. decrease. The The Cr Cr content content is is therefore preferably therefore preferably 2.00  or 2.00 % or less. less. The Cr content The Cr content is is more 0.80% preferably 0.80 more preferably

or less. or less.

[0025] Mo:1.0

[0025] Mo: 1.0%  1.0% or oror less less less

Moisisananelement Mo element thatthat contributes contributes to improve to improve strength strength of theand of the steel, steel, and

maybebeoptionally may optionallyadded addeddepending depending on on thethe desired desired strength. strength. If Ifthe theMoMo content content

is more is more than than 1.0 1.0 , toughness %, toughness 1.0%, may toughness may may degrade. degrade. degrade. Accordingly, Accordingly, Accordingly, inin in the the the casewhere case case where where MoMo Mo

is added, is the Mo added, the Mocontent contentisispreferably 1.0%  preferably1.0 or or less.From less. From thethe viewpoint viewpoint of of achieving the achieving the strength strength improving improvingeffect effectbybyMo, Mo, thethe Mo Mo content content is preferably is preferably

0.01 %or 0.01 or more. more.

[0026]

[0026] Al: A1: 0.100% Al: 0.100 0.100% oror or less less less

Al is Al is an an element that acts element that acts as as aa deoxidizer, deoxidizer, and is most and is widely used most widely usedinin the molten the moltensteel steeldeoxidation deoxidation process process for tensile for high high tensile strength strength steel. steel. In to In order order to achievethis achieve thiseffect, effect,the theAl Alcontent contentisispreferably preferably 0.001 0.001 0.001% or % or or more. more. more. AlThe The Al The Al content content content

is more is more preferably 0.010%orormore. preferably 0.010 more. If If theAlAlcontent the content is ismore more than than 0.100 0.100 %, ,

the toughness the toughnessofofthe thebase basemetal metal maymay decrease. decrease. Thecontent The Al Al content is therefore is therefore

preferably 0.100 %ororless. preferably 0.100 less. The Al content The Al content is is more 0.07%ororless. preferably 0.07 more preferably less.

[0027] 2.0%ororless Cu:2.0

[0027] Cu: less Cu isis an Cu anelement elementthat thatcancan increase increase strength strength while while maintaining maintaining high high

toughness, and toughness, and may maybebeoptionally optionallyadded added depending depending on the on the desired desired strength. strength. If If

the Cu the content is Cu content is more morethan 2.0%,,hot than2.0 hotbrittleness brittleness may mayoccur occurand and degrade degrade thethe

surfacecharacteristics surface characteristicsof of thethe steel steel plate. plate. Accordingly, Accordingly, incase in the the where case Cu where is Cu is

Ref. No. Ref. No. PO225838-PCT-ZZ (6/24) PO225838-PCT-ZZ (6/24) added, the added, the Cu Cucontent contentisispreferably preferably2.02.0  less. % or or less. TheThe Cu content Cu content is is more more preferably1.0% preferably  less. 1.0 or or less. In In order order to achieve to achieve the foregoing the foregoing effect, effect, the Cuthe Cu content content is preferably is 0.01 %orormore. preferably 0.01 more.The The Cu Cu content content is more is more preferably preferably 0.10 0.10 % or  or more, and more, and further further preferably 0.20 %orormore. preferably 0.20 more.

[0028]

[0028] Nb: 0.1%ororless Nb:0.1 less Nbisisananelement Nb element that that contributes contributes to improve to improve strength strength of theand of the steel, steel, and maybebeoptionally may optionallyadded addeddepending depending on on thethe desired desired strength. strength. If If theNbNb the content content

is more is more than 0.1 , than 0.1 the toughness %, the of the toughness of the base base metal metal may degrade.Accordingly, may degrade. Accordingly, in the in case where the case whereNbNb is added, is added, the the Nb content Nb content is preferably is preferably  orFrom 0.1less. 0.1 % or less. From

the viewpoint the of achieving viewpoint of achieving the the strength strength improving effect by improving effect Nb, the by Nb, the Nb content Nb content

is preferably is 0.005  preferably 0.005 or more. % or more.

[0029] 0.05%ororless V: 0.05

[0029] V: less Visis an V aneffective effectiveelement element thatthat enhances enhances the strength the strength of theplate of the steel steel plate through strengthening through strengtheningbybyprecipitation. precipitation.IfIf the the VVcontent contentisisexcessively excessivelyhigh, high,

the low-temperature the toughnessofofthe low-temperature toughness thesteel steel plate plate may decrease. Accordingly, may decrease. Accordingly,in in the case the case where whereVVisisadded, added,the theV Vcontent contentisispreferably 0.05%  preferably0.05 or or less.The less. The V V content content is more is content is more preferably morepreferably preferably % or 0.040.04 0.04% ororless. less.less. Although Although Although nonolower no lower lower limit limit limit is ison is placed placed on placed on the V the content, the V content, the V content is V content is preferably 0.010  preferably 0.010 or or more more % or in in in more order order to to to order achieve achieve achieve

the foregoing the foregoingeffect. effect.

[0030]

[0030] Ti: 0.03 %ororless Ti: 0.03 less Ti is Ti is an an element that has element that the effect has the effect of of enhancing the toughness enhancing the toughnessofofthe the weld without weld withoutdegrading degrading thethe mechanical mechanical properties properties of base of the the metal base metal when when weldingthethesteel welding steel plate plate to to produce produce a welded a welded structure. structure. Intoorder In order to this achieve achieve this effect, the effect, the Ti Ti content contentisispreferably preferably0.003 % ormore. 0.003 or more. A Ti content A Ti content of more of more than than

0.03 , however, 0.03 %, however, may maycause causea decrease a decreasein intoughness. toughness.The The Ti Ti content content is is thereforepreferably therefore preferably % orless. 0.03 0.03 or less.

[0031]

[0031] B: 0.0030%  B:0.0030 oror less less

B is B is an an element elementthat thatenhances enhances hardenability hardenability when when addedadded in a small in a small

amount.InInorder amount. order to to sufficiently sufficiently achieve achieve this this effect, effect, the Bthe B content content is preferably is preferably

0.0003 %orormore. 0.0003 more. If If thethe B content B content is more is more thanthan 0.0030 0.0030 , toughness %, toughness may may degrade.Accordingly, degrade. Accordingly, in the in the casecase where where B is added, B is added, the B content the B content is preferably is preferably

0.0030 %ororless. 0.0030 less.

[0032] 0.007% ororless Ca:0.007

[0032] Ca: less Ca is Ca is an element that an element that has has the the effect effect of of improving the low-temperature improving the low-temperature

toughnessof of toughness thethe steel steel plate plate by controlling by controlling the of the form form of inclusions inclusions in the in the steel. steel . If the If Ca content the Ca contentisisexcessively excessivelyhigh, high, thethe cleanliness cleanliness of of the the steel steel may may be be

Ref. No. Ref. No. PO225838-PCT-ZZ (7/24) PO225838-PCT-ZZ (7/24) impaired and impaired andthe theCharpy Charpyabsorbed absorbed energy energy at low at low temperatures temperatures (hereafter (hereafter alsoalso referred to referred to as asCharpy Charpy toughness) maydecrease. toughness) may decrease.Accordingly, Accordingly,ininthe thecase casewhere where Ca is Ca is added, added, the the Ca Cacontent contentisis preferably 0.007%  preferably0.007 or or less.The less. The Ca Ca content content is is morepreferably more 0.004%  preferably0.004 oror less.Although less. Althoughno no lower lower limit limit is is placed placed onon thethe Ca Ca content, the content, the Ca content is Ca content is preferably 0.001 %or preferably 0.001 ormore moreininorder ordertotoachieve achievethe the foregoingeffect. foregoing effect.

[0033] REM:

[0033] REM: 0.010 0.010  less % or or less REM(rare REM (rareearth earthmetal) metal)isisananelement elementthat thathas hasthe theeffect effect of of improving improving the low-temperature the low-temperaturetoughness toughness of of thethe steel steel plate plate by by controlling controlling thethe form form of of

inclusionsininthe inclusions thesteel, steel,asaswith withCa.Ca. If If thethe REMREM content content is excessively is excessively high, high, the the cleanliness of cleanliness of the the steel steelmay may be be impaired and Charpy impaired and Charpytoughness toughness maymay decrease . decrease.

Accordingly,in Accordingly, in the the case case where whereREM REM is added, is added, the the REMREM content content is preferably is preferably

0.010 0.010% 0.010 %oror orless. less. The less. The REM TheREM content REMcontent content isis is more more more preferably preferably preferably 0.008 0.008 0.008 less. % or or or less. less. Although Although Although no lower no lower limit limit is is placed placed on on the the REM REM content, content, thethe REMREM content content is preferably is preferably

0.001 %orormore 0.001 moreininorder ordertoto achieve achievethe theforegoing foregoingeffect. effect. Herein, REM Herein, REM isisa ageneric genericterm termfor for17 17elements elementsincluding including1515lanthanoid lanthanoid elements and elements andY Y andand Sc,Sc, and and these these elements elements may may be be contained contained singly singly or in or in combination.The combination. TheREM REM content content is the is the totalcontent total contentofofthese theseelements. elements.

[0034]

[0034] Mg:

[0034] Mg: 0.007%  Mg:0.007 0.007 or or or less less less

Mgisis an Mg an element elementthat that has has the the effect effect of of improving the low-temperature improving the low-temperature toughnessof of toughness thethe steel steel plate plate by controlling by controlling the of the form form of inclusions inclusions in the in the steel, steel , as with as with Ca and REM. Ca and REM.If If theMgMg the content content is is excessively excessively high, high, thecleanliness the cleanlinessofof the steel the steel may be impaired may be impairedand andCharpy Charpy toughness toughness may may decrease decrease. . Accordingly, Accordingly,

in the in the case case where Mgisisadded, where Mg added,the theMgMg content content is is preferably preferably 0.007 0.007  less. % or or less.

The Mg The Mgcontent contentisismore morepreferably 0.004%  preferably0.004 or or less.Although less. Althoughno no lower lower limit limit is is

placed on placed on the the Mg Mgcontent, content,thetheMg Mg content content is preferably is preferably % or 0.001 0.001 or more more in in order to order to achieve achievethethe foregoing foregoing effect. effect.

[0035] [Microstructure]

[0035] [Microstructure]

(Ferrite fraction:55% to (Ferrite fraction: 95 , to 95 martensite austenite %, martensite austenite constituent fraction: 111% constituent fraction: %

to 30 , to 30 residual microstructure: %, residual microstructure: tempered martensiteand/or tempered martensite and/orbainite) bainite) In the In the microstructure microstructure atdepth at a a depth position position of 1/4ofof1/4 the of the thickness plate plate thickness from the from the surface surface of of the the steel steel plate plate according according to to the the present present disclosure disclosure in in the the plate thickness plate thicknessdirection, direction, thethe ferrite ferrite fraction fraction is is  95 5 %5 to to %95andthe andmartensite the martensite austenite constituent austenite constituent fraction fraction is is11% to 1% to 30 to 30  with 30 with % with the thethe residual residual residual microstructure microstructure microstructure

consistingofoftempered consisting tempered martensite martensite and/orand/or bainite. bainite. If the ferrite If the ferrite fractionfraction in the in the microstructure microstructure is is lessthan less than 5 , 5 %, a yield a yield ratio ratio of 0.80 of 0.80 or less or less cannot cannot be achieved . be achieved.

Ref. No. Ref. No. PO225838-PCT-ZZ (8/24) PO225838-PCT-ZZ (8/24)

If the If the ferrite ferritefraction fractionisismore more than 95 , than 95 the fraction %, the fraction of of hard hard phase phasesuch suchasas martensiteaustenite martensite austenite constituent constituent is low is low and aand a yield yield ratio ratio ofor0.80 of 0.80 lessor less cannot cannot be achieved. be achieved.IfIfthe themartensite martensite austenite austenite constituent constituent fraction fraction is than is less less 1than %, a 1 , a yield ratio yield ratio of of 0.80 0.80 or or less less cannot cannotbebeachieved. achieved. If the If the martensite martensite austenite austenite

constituent fraction constituent fraction is is more than3030%,, more than toughness toughness degrades. degrades. If the If the residual residual

microstructure is microstructure is not not tempered temperedmartensite martensiteand/or and/orbainite, bainite,for forexample, example,if ifthe the residual microstructure residual microstructureisisas-quenched as-quenched martensite martensite or bainite, or bainite, the desired the desired

toughness cannot toughness cannotbebeachieved. achieved.

[0036] (Averageaspect

[0036] (Average aspectratio ratioofofcrystal crystal grains: grains: less less than than 2.0, 2.0, number numberdensity density

of crystal of crystal grains grains having havingequivalent equivalentcircular circulardiameter diameter of of more more thanthan 30 30 um: µm: m: 2 less) 250/mmor 250/mm² or less) In the In the microstructure microstructure atdepth at a a depth position position of 1/4ofof1/4 the of the thickness plate plate thickness from the from the surface surface of of the the steel steel plate plate according according to to the the present present disclosure disclosure in in the the plate thickness plate thickness direction, direction, when when regions regions surrounded surrounded by large-angle by large-angle grain grain

boundaries with boundaries withan anorientation orientation difference difference of of 15 degrees or 15 degrees or more moreare are defined defined as as crystal grains, crystal grains,the theaverage average aspect aspect ratioratio ofcrystal of the the crystal grains grains is less is less2.0, than than 2.0 , andthe and thenumber number density density of crystal of crystal grains grains having having an equivalent an equivalent circularcircular diameterdiameter

of more of than 30 more than mamong 30 um µm amongthethe crystalgrains crystal grainsisis 250/mm² 2 less. If the average 250/mmor or less. If the average aspect ratio aspect ratio of of the thecrystal crystalgrains grainsis is2.02.0 or or more, more, the orientation the orientation of theof the

microstructure microstructure tends tends to high, to be be high, causing causing an increase an increase in acoustic in acoustic anisotropy. anisotropy. If If the number the densityofofcrystal number density crystal grains grains having having an an equivalent equivalent circular circular diameter diameter of of morethan more µmm than3030um among among the crystal the crystal grains grains is more is more than than 250/mm 250/mm², 2 , toughness toughness

decreases. The decreases. The number numberdensity densityofofcrystal crystalgrains grainshaving havingananequivalent equivalentcircular circular diameter of diameter of more morethan m than3030um µm isispreferably preferably150/mm² 150/mm or2 less. or less.

[0037] Theplate

[0037] The platethickness thicknessofofthe thesteel steelplate plateisis not notlimited limitedand andmay may be be any any

thickness. For thickness. example,the For example, theplate platethickness thicknessofofthe thesteel steel plate plate is is preferably preferably 66 mmorormore mm more and and 50 50 mm mm or less. or less.

[0038] [Mechanicalproperties]

[0038] [Mechanical properties] (Tensilestrength) (Tensile strength)

Thetensile The tensilestrength strength of of thethe steel steel plate plate is not is not limited, limited, but but is is preferably preferably

690 MPa 690 MPaorormore. more.With Witha atensile tensile strength strength of of 690 690 MPa MPaorormore, more,the theplate plate thickness can thickness can be bereduced reducedwhen when thethe steel steel plate plate is is used used forfor tanks. tanks. TheThe tensile tensile

strength of strength of the the steel steel plate plate isismore more preferably preferably 720 MPaorormore. 720 MPa more. Although Although no no upperlimit upper limitisisplaced placedon on the the tensile tensile strength, strength, the tensile the tensile strength strength is preferably is preferably

1000 MPaororless. 1000 MPa less. The tensile The tensile strength strength can can be be measured measuredby by thethe method method described described in in the the

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EXAMPLES EXAMPLES section section below. below.

[0039] (Yield

[0039] (Yield ratio) ratio)

Theyield The yieldratio ratioofofthethe steel steel plate plate is not is not limited, limited, butpreferably but is is preferably 0.80 0.80 or less. or less. With With aa yield yield ratio ratio of of 0.80 0.80ororless, less,mechanical mechanical stress stress reliefcancan relief be be

performedinstead performed insteadof of post post weld weldheat heattreatment. treatment.

[0040] (Low-temperature

[0040] (Low-temperature toughness) toughness)

The toughness The toughnessvalue valueofofthethesteel steelplate plateisis not notlimited, limited, but but the the Charpy Charpy absorbed energy absorbed –50°CC(vE-50°C) energyatat-50 (vE -50C )isis preferably (vE-50°c) preferably 100 100J Jorormore moreinina afull-size full-size Charpyimpact Charpy impact test. test. vE -50C vE-50°C of the of the steel steel plate plate is more is more preferably preferably 150 150 J or J or more. more.

[0041] [Productionmethod]

[0041] [Production method] A method A methodofofproducing producingthethe steelplate steel plateaccording accordingto tothethepresent present disclosure (production disclosure (productionmethod) method) will will be described be described below. below. In theInfollowing the following description, the description, the term “temperature”refers term "temperature" refers to to the the temperature temperatureatat the the center center of of the plate the plate thickness thicknessunless unless otherwise otherwise specified. specified. The temperature The temperature at the of at the center center of

the plate the plate thickness thickness can be obtained, can be obtained, for for example, example, by byheat heattransfer transfer calculation calculation from the from thesurface surfacetemperature temperature of of thethe steel steel plate plate measured measured with with a radiation a radiation

thermometer. thermometer.

The production The productionmethod methodis is a a steelplate steel plateproduction productionmethod method comprising comprising

subjecting aa steel subjecting steel material material having the above-described having the above-describedchemical chemical composition composition

to hot to hotrolling, rolling,thereafter thereafter to to first first heating heating retention, retention, thereafter thereafter to quenching, to quenching,

thereafter totosecond thereafter second heating heating retention, retention, and thereafter and thereafter to cooling to cooling treatment. treatment. In In the hot the hotrolling, rolling,the thefinish finishtemperature temperature is °C is 900 900or C or In more. more. In the the first first heating heating retention, the retention, the heating heating temperature is in temperature is in the the temperature range of temperature range of Ac3 Acpoint Ac pointoror 3 point or moreand more 1000°CCororless. and1000 less.In In the the quenching, quenching,the the average averagecooling coolingrate rate from from600 600

C toto 300 °C 300°CC at at a depth a depth position position of of of 1/4 1/4theofplate the plate thickness thickness from from the the surface surface of the of the steel steel plate plate in in the the plate plate thickness thickness direction is 33 C/s direction is °C/s or or more, and the more, and the coolingend cooling endtemperature temperature is 300 °C orC is 300 or less. less. In theInsecond the second heating heating retention, retention, the the heating temperature heating temperatureis is in in the the temperature rangeof temperature range of Ac1 Acpoint Ac pointororormore 1point more more and and and less less less

than Ac than Ac3 point.In Ac 3point. point. In the In the cooling the cooling treatment, cooling treatment, the treatment, the average the average cooling average cooling rate cooling rate from rate from 700 from 700 C 700 °C °C

to 500°CCatata adepth to 500 depth position position of of of 1/4 1/4the of plate the plate thickness thickness from from the the of surface surface of the steel the steel plate plate in in the the plate plate thickness thicknessdirection direction is is 3 3 C/s °C/s or or more, more, and and the cooling the cooling

endtemperature end temperatureis is 500500 Cless °C or or less and and 200orC 200 °C or more. more. Theplate The steel steelaccording plate according to the to the present presentdisclosure disclosurecancan be suitably be suitably produced produced underconditions. under these these conditions. Eachstep Each stepwill willbebe described described in detail in detail below. below.

[0042]

[0042] First, First, the steel material the steel material having having the theabove-described above-describedchemical chemical compositionmay composition may be heated be heated before before hot rolling. hot rolling. In thisIncase, this the case, the heating heating

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temperature of temperature of the the steel steel material material is is preferably 900 C preferably 900 °C or or more 1250°CCoror and 1250 more and

less. The less. The method ofproducing method of producingthe thesteel steelmaterial materialisisnot notlimited. limited. For For example, example, molten steel molten steel having having the the above-described above-described chemical chemicalcomposition compositionmaymay be be prepared by prepared by steelmaking steelmakingusing usinga aconventional conventional method method and and castcast to produce to produce the the

steel material. steel material.The The steelmaking maybebeperformed steelmaking may performed using using anyany method method suchsuch as aas a converter, an converter, an electric electricfurnace, furnace,or or an induction an induction furnace. furnace. The casting The casting is is preferably performed preferably bycontinuous performed by continuouscasting castingfrom fromthe theviewpoint viewpointofofproductivity, productivity, but may but beperformed may be performedbyby ingot ingot castingand casting and blooming. blooming. An An example example of steel of the the steel materialisis aasteel material steelslab. slab.

The steel The steel material material obtained obtained as as aa result result of of casting casting and the like and the like may be may be

heatedafter heated aftercooling, cooling,or or directly directly heated heated without without cooling. cooling.

[0043]

[0043] IfIfthe theheating heating temperature temperature ofsteel of the the steel material material is than is less less 900 than due C, due °C,900

to high to high deformation deformation resistance resistance of steel of the the steel material, material, the on the load load the on theinmill mill the in the subsequenthot subsequent hotrolling rolling may mayincrease increaseand andhinder hinderthe thehot hotrolling. rolling. Therefore, Therefore, the the

heating temperature heating temperatureofof the the steel steel material material is is preferably preferably 900 C or 900 °C or more. more.IfIf the the heating temperature heating temperature of of the the steel steel material material isismore more than than 1250 C, due 1250 °C, due toto noticeableoxidation noticeable oxidationof of thethe steel, steel, thethe loss loss incurred incurred byremoval by the the removal of the of the oxide oxide film caused film causedbyby the the oxidation oxidation may may increase, increase, resulting resulting in a decrease in a decrease in yieldinrate. yield rate. Therefore, the Therefore, the heating heating temperature temperatureofofthe the steel steel material material is is preferably preferably 1250 C 1250 °C

or less. or less.

[0044] (Hotrolling)

[0044] (Hot rolling) The steel The steel material material having the above-described having the chemicalcomposition above-described chemical compositionis is

subjectedtotohothot subjected rolling rolling to to obtain obtain a hot-rolled a hot-rolled steel steel plate plate having having a final a final plate plate thickness. The thickness. finish temperature The finish temperatureinin the the hot hot rolling rolling is is 900 C or 900 °C or more. more.IfIf the the

finish temperature finish is less temperature is less than than 900 C, acoustic 900 °C, acoustic anisotropy anisotropy may maydegrade. degrade.The The final plate final thicknessofofthe plate thickness thehot-rolled hot-rolled steelplate steel plate is is notlimited, not limited, butbut is is preferably preferably

6 mm 6 mm orormore moreand and5050 mmmm or less or less as as mentioned mentioned above. above. After After the the hothot rolling,the rolling, the hot-rolled steel hot-rolled steel plate plate may beoptionally may be optionallycooled. cooled.TheThe cooling cooling method method is is not not limited, and limited, and may be, for may be, for example, example, air air cooling cooling or or water water cooling. cooling.

[0045] (Firstheating

[0045] (First heating retention) retention)

After the After thehot hotrolling, rolling,thethe hot-rolled hot-rolled steel steel plate plate is subjected is subjected to heating to heating

retention in retention in an an austenite austenite range range(first (first heating heatingretention). retention).Thus, Thus,thethe heating heating

temperature temperature temperature in ininthethe first first the heating heating first retention retention heating is is Ac3 retention Ac ispoint point Ac3 point or If or more. or more. If the the heating the heating more. If heating temperature of temperature of the the hot-rolled hot-rolled steel steel plate plate is is more than 1000 more than C,crystal 1000°C, crystal grains grains

coarsen and coarsen and toughness toughnessdecreases. decreases.Accordingly, Accordingly, the the heating heating temperature temperature in in thethe

first heating first retentionisis1000 heating retention 1000°C C or less. or less.

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[0046]

[0046] (Quenching) (Quenching) Thehot-rolled The hot-rolled steel steel plate plate after after the the first first heating heating retention retention is subjected is subjected

to quenching to quenching (i.e.accelerated (i.e. accelerated cooling). cooling). It isItimportant is important that, that, in thein the quenching, quenching,

the average the coolingrate average cooling rate from 600°CC from600 to to 300 °C C 300 at at a depth a depth position position of of 1/41/4 of of

the plate the plate thickness from the thickness from the surface surfaceofofthe thesteel steel plate plate in in the the plate plate thickness thickness direction(hereafter direction (hereaftersuch such a depth a depth is also is also denoted denoted as 1/4t) as 1/4t) is 3 °C/s C/s is 3 or or more. more.

[0047] If the

[0047] If the average averagecooling coolingrate rateinin the the quenching quenchingis isless lessthan C/s,itit is than3 3°C/s, is is

difficult toto obtain difficult obtain the the desired desired transformed microstructure,SO transformed microstructure, sothat so thatsufficient sufficient strength cannot strength be achieved. cannot be achieved. Although Althoughnonoupper upper limitisisplaced limit placedononthe theaverage average

coolingrate, cooling rate,ifif the the average average cooling cooling raterate is more is more than than 200 C/s, 200 °C/s, it is difficult it is difficult to to control the control thetemperature temperature at each at each position position insteel in the the steel plate,plate, sothe so that thatmaterial the material quality tends quality tendstotovary varyininthethe plate plate transverse transverse direction direction androlling and the the rolling direction. direction.

Thisisis likely This likely toto cause causevariation variation in in material material properties properties such such as tensile as tensile property property

and toughness. and toughness.Therefore, Therefore,the theaverage averagecooling cooling rateisispreferably rate preferably200200 C/s °C/s or or

less. less.

[0048] If the

[0048] If the cooling coolingstop stoptemperature temperature (i.e.cooling (i.e. cooling endend temperature) temperature) at a at a

position of position of 1/4t 1/4t in inthe thequenching quenching is ismore more than than 300 C, the 300 °C, the desired desired transformed transformed

microstructure cannot microstructure cannotbe beobtained. obtained.The Thecooling coolingstop stoptemperature temperature at at a aposition position of 1/4t of 1/4t in in the the quenching is therefore quenching is 300°CCororless. therefore 300 less.ByByaccelerated accelerated cooling cooling

undersuch under suchconditions, conditions, the the hot-rolled hot-rolled steelsteel plateplate is quenched is quenched well. well.

[0049] Thequenching

[0049] The quenching (accelerated (accelerated cooling) cooling) treatment treatment may may be performed be performed by by any method any methodwithout without limitation.For limitation. For example, example, one one or both or both of cooling of air air cooling and and water cooling water cooling may may bebeused. used.For Forwater watercooling, cooling,any anycooling coolingmethod method (for (for example,spray example, spraycooling, cooling,mist mistcooling, cooling, laminar laminar cooling, cooling, etc.using etc.) ) using water water is is

available. available.

[0050] (Secondheating

[0050] (Second heatingretention) retention) The hot-rolled The hot-rolledsteel steel plate plateafter after the thequenching quenchingis is then then subjected subjected to to heatingretention heating retentioninina adual-phase dual-phase temperature temperature range range (second(second heating heating retention). retention).

The heating The heatingtemperature temperatureininthe thesecond secondheating heating retentionisisAc1 retention AcAc 1 point point point oror or more more more

and less and less than than Ac3 Acpoint. Ac point. 3point. IfIf If the the the heating heating heating temperature temperature temperature is less is less is less than than than Ac1 Ac Ac 1 point, point, point,

martensite austenite martensite austenite constituent constituent cannot be obtained cannot be obtained and and low lowyield yieldratio ratio cannot cannot be achieved. be achieved.IfIfthe theheating heating temperature temperature Acispoint is Ac3 Ac 3 or point point orferrite or more, more, more, phase ferrite ferrite phase phase will will will be less be less than 5  than 5 andtempered % and tempered martensite martensite phase phase will will be be more more thanthan , 90 and 90 %, and low yield low yield ratio ratio cannot cannot be be achieved. achieved.

[0051] Forheating

[0051] For heatingretention retention in in the the dual-phase temperaturerange, dual-phase temperature range,any anyheating heating methodmay method maybe be used used as as long long as the as the heating heating temperature temperature cancontrolled can be be controlled in in

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the foregoing the foregoing manner. manner.AnAn example example of the of the heating heating method method is furnace is furnace heating. heating.

Thefurnace The furnace heating heating is not is not limited, limited, and and a typical a typical heat heat treatment treatment furnacefurnace may be may be used. used.

[0052] After

[0052] After thethe heating heating temperature temperature is reached, is reached, the hot-rolled the hot-rolled steelmay steel plate plate may

be held be held in in the the dual-phase temperaturerange dual-phase temperature rangefor forany anyperiod periodofoftime timebefore beforethe the below-describedcooling below-described coolingtreatment treatmentisisstarted. started. The Theholding holdingtime timeisisnot notlimited, limited, but is but is preferably preferably 55 minutes minutes or or more. more.

[0053] Acpoint

[0053] Ac1 Ac pointcan 1point can can be be be calculatedaccording calculated calculated according according toto to thefollowing the the following following formula formula formula (1): (1): (1):

Ac point(°C) Ac 1point Ac1 point (C)===750.8 (°C) 750.8 26.6XXCCC+++17.6 750.8--–26.6 26.6 17.6XXSiSi 17.6 11.6XXMnMn Si--–11.6 11.6 Mn –22.9 22.9 -- 22.9 CuCu XX Cu

- 23 X Ni – 23 24.1 X Cr Ni ++ 24.1 22.5 X Mo Cr ++ 22.5 39.7X V V Mo- –39.7 - – 5.7 X Ti 5.7Ti +Ti232.4 + +232.4 232.4X X Nb -Nb Nb – 169.4 169.4 - 169.4

 Al X Al Al ... (1). (1). Ac 3point Ac3 Ac pointcan point canbebe can becalculated calculatedaccording calculated accordingtoto according tothe thefollowing the followingformula following formula formula (2): (2): (2):

Ac Ac3 point(°C) Ac 3point point (C)===937.2 (°C) 937.2 436.5XXCCC 937.2--–436.5 436.5 +5656 ++ 56 SiSi XX Si –19.7 19.7 -- 19.7 MnMn XX Mn – 16.3 -- 16.3 16.3  Cu XX Cu Cu

26.6 X Ni -– 26.6 4.9 X Cr Ni -– 4.9 38.1X MoMo Cr+ +38.1 X V + V + 124.8 + 124.8 X Ti - Ti + 136.3 136.3 X Nb  – 19.1 19.1 + Nb +

198.4  Al 198.4 X 3315XBB Al ++ 3315 ... (2). (2).

[0054] (Coolingtreatment)

[0054] (Cooling treatment) Cooling treatment Cooling treatmentisisthen thenperformed. performed. In the In the cooling cooling treatment, treatment, the the average cooling average coolingrate rate from 700°CC from700 to to °C C 500 500 at at a position a position of of C/soror 1/4tisis3 3°C/s 1/4t

more, and more, andthe the cooling coolingend endtemperature temperatureis is500 °C C 500 or or lessandand less °C C 200200 or more. or more.

If the If averagecooling the average cooling raterate is less is less than than 3 C/s, 3 °C/s, there there is is a possibility a possibility that thethat the desired transformed desired microstructure cannot transformed microstructure cannot be obtained and be obtained and strength strength and and toughness decrease. toughness decrease. If If the the cooling cooling stop stop temperature is more temperature is than 500 more than C,there 500°C, there is aa possibility is possibility that that bainite bainite formed after the formed after the cooling coolingstop stopbecomes becomes a main a main

componentand component and low-temperature low-temperature toughness toughness is not is not satisfied. satisfied. If Ifthe thecooling coolingstop stop

temperature isisless temperature lessthan °C, C, than200200 the the desired desired tempering tempering effect effect cannot cannot be be achieved in achieved in the the end and toughness end and toughnessdegrades. degrades.

[0055] After the

[0055] After thecooling coolingstop stop(after (afterthetheendend of of thethe cooling cooling treatment), treatment), air air

cooling may cooling maybebeperformed performed in order in order to improve to improve toughness toughness by self-tempering. by self-tempering.

Thecooling The cooling rate rate in in the the airair cooling cooling is not is not limited. limited. For For example, example, the cooling the cooling rate rate

is typically11C/s is typically °C/sor orless lesswhen when the theplate platethickness thicknessisis 6 mm 6 mm to to50 50mm. mm.

EXAMPLES EXAMPLES

[0056] Steel plates

[0056] Steel plates were were each eachproduced producedaccording according to to thefollowing the following procedure, procedure,

andtheir and theirproperties propertieswere were evaluated. evaluated.

First, molten First, molten steel steel having having the the chemical compositionshown chemical composition shown in in Table Table 1 1 was prepared was preparedbybysteelmaking steelmaking using using a converter, a converter, and and subjected subjected to continuous to continuous

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castingtoto obtain casting obtaina asteel steelslab slab(thickness: (thickness: 200200 mm) mm) as a steel as a steel material. material. Ac1 Ac 1 point Ac point point (C) calculated according (°C) calculated accordingtotothe theforegoing foregoing formula formula (1) (1) and and Ac3 Ac 3 (°C) point Ac point point (°C) (C)

calculated according calculated according to to the the foregoing foregoingformula formula(2)(2)are areshown shown in Table in Table 1 (and 1 (and

Table2). Table 2).

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[0057] [Table 1]

[0057] [Table 1]

point point

(C) point

Ac3

830 826 822 831 876 809 824 829 819 863 757 862 847 828 867 856 843 862 824 843 833 850 842 (°C) 830 826 856 850 Ac3 (C) 830 826 822 822 831 831 876 809 809 824 824 829 829 819 819 863 863 757 757 862 862 847 847 828 828 867 867 856 843 843 862 862 824 824 843 843 833 833 850 842 842

point point point

(C) Ac1

(°C)

689 664 700 697 710 671 673 689 693 720 624 716 719 715 696 711 721 720 690 714 709 703 697 (°C) 689 689 664 664 700 700 697 697 710 710 671 673 673 689 689 693 693 720 720 624 624 716 716 719 719 715 715 696 696 720 720 690 690 714 714 709 709 703 703 697 697 Ac1 Ac 671 711 711 721 721

0.001 REM

REM 0.001 0.001 REM - - - - - - - - - - - -

- - - - - - - - - - - - - -- - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- - - --

0.001 0.001 0.001 Ca

Ca - - - - - - - - - - -

- - - - - - - - - - - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

0.0021 0.0021 0.0021 Mg

Mg Mg - - - - - - - - - - - -

- - - - - - - - - - - - - -- -- -- - -- -- -- - - -- -- -- -- -- -- -- -- -- -- -- -- -I

0.01 0.01 0.01 Nb

Nb - - - - - - - - - - - -

- - - - - - - - - - - - - - -- - -- - - - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

0.04 0.04 0.04 V

- - - - - - - - - - - - - - - - - - - - - - - -

-- -- -- - -- - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- V 0.35 0.55 0.35 0.35 0.55 0.55 Mo

Mo - - - - - - - - - - -

- - - - - - - - - - - - - - -I -- - -- -- - -- - - -- -- -- -- -- - - -- -- -- -- -- -- Cu

0.2 0.2 0.2 Cu - - - - - - - - - - - -

- - - - - - - - - - - - -

-- - -- - -- -- - -- -- -- -- -- -- -- -- - - -- -- -- -- -- -- (mass%) composition Chemical Chemical composition (mass) (mass%) composition Chemical 0.0008

0.0008 0.0008 B

- - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - -- -- - - -- -- -- -- -- - - -- -I -- -- -- -- -- --

B 0.35 0.50

0.35 0.35 0.50 0.50 Cr

Cr - - - - - - - - - - -

- - - - - - - - - - - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- - -I 0.01

0.01 0.01 Ti

Ti - - - - - - - - - - -

- - - - - - - - - - - - -- -- -- -- -- - -- -- -- -- -- -- -- -- -- - - -- -- -- -- -- -I 0.0025 0.0022 0.0028 0.0024 0.0026 0.0025 0.0021 0.0022 0.0021 0.0025 0.0017 0.0026 0.0024 0.0035 0.0028 0.0032 0.0033 0.0023 0.0025 0.0019 0.0025 0.0095 0.0005 0.0022 0.0028 0.0024 0.0026 0.0025 0.0022 0.0017 0.0026 0.0024 0.0035 0.0028 0.0032 0.0033 0.0023 0.0095 0.0025 0.0025 0.0022 0.0028 0.0024 0.0026 0.0025 0.0021 0.0022 0.0021 0.0025 0.0025 0.0017 0.0026 0.0024 0.0035 0.0028 0.0032 0.0033 0.0023 0.0025 0.0025 0.0019 0.0019 0.0025 0.0025 0.0095 0.0005 0.0005 0.0021 0.0021 N

N 1.95 3.00 1.85 1.65 1.55 3.50 2.20 2.40 1.95 0.50 4.80 1.60 1.75 0.95 1.65 1.55 0.25 1.30 1.35 0.85 1.10 1.55 0.09 0.15 0.87 0.01 0.0010 0.008 1.85 1.95 3.00 3.00 1.85 1.65 1.55 3.50 2.20 2.20 2.40 2.40 1.95 0.50 0.50 4.80 4.80 1.60 1.60 1.75 0.95 1.65 1.55 0.25 1.30 1.30 1.35 0.85 1.10 1.10 1.55 1.85 1.95 1.85 1.65 1.55 3.50 1.95 1.75 0.95 1.65 1.55 0.25 1.35 0.85 1.55 1.85 Ni

Ni disclosure. present the of scope the outside indicate Underlines Underlines indicate outside the scope of the present disclosure. disclosure. present the of scope the outside indicate Underlines 0.008

0.008 0.008 0.008 0.008 Al

Al - - - - - - - - - - -

- - - - - - - - - -

- -- -- -- - -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 0.0010 0.0008 0.0008 0.0012 0.0007 0.0009 0.0009 0.0008 0.0050 0.0007 0.0009 0.0010 0.0010 0.0011 0.0005 0.0010 0.0010 0.0010 0.0010 0.0005 0.0070 0.0010

0.0010 0.0008 0.0008 0.0012 0.0007 0.0009 0.0009 0.0008 0.0050 0.0007 0.0009 0.0010 0.0010 0.0005 0.0010 0.0010 0.0010 0.0010 0.0005 0.0070 0.0010 0.0010 0.0010 0.0008 0.0008 0.0012 0.0007 0.0009 0.0009 0.0008 0.0050 0.0007 0.0009 0.0010 0.0010 0.0011 0.0005 0.0010 0.0010 0.0010 0.0010 0.0005 0.0070 0.0010 0.0010 0.0011 S

S 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.01

0.03 0.03 0.01 0.01 0.01 0.04 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 P

P 1.45 1.85 0.65 1.20 1.05 0.06 2.40 0.45 0.95 1.65 1.35 0.85 1.23 1.21 1.85 0.95 2.25 0.03 2.70 1.35 1.25 1.10

1.45 1.45 1.85 1.85 0.65 0.65 1.20 1.20 1.05 1.05 0.06 0.06 2.40 2.40 0.45 0.45 0.95 0.95 1.65 1.65 1.35 1.35 0.85 0.85 1.23 1.23 1.85 1.85 0.95 0.95 2.25 2.25 0.03 2.70 1.35 1.35 1.25 1.25 1.10 1.10 0.87 0.87 1.21 1.21 0.03 2.70 Mn

Mn Mn 0.07 0.25 0.15 0.01 0.48 0.25 0.10 0.07 0.08 0.12 0.09 0.07 0.16 0.26 0.25 0.55 0.32 0.15 0.19 0.05 0.04 0.12

0.07 0.07 0.25 0.25 0.15 0.15 0.48 0.48 0.25 0.25 0.10 0.10 0.07 0.08 0.08 0.12 0.12 0.09 0.09 0.07 0.07 0.16 0.16 0.26 0.26 0.25 0.25 0.55 0.55 0.32 0.32 0.15 0.15 0.19 0.05 0.05 0.04 0.04 0.12 0.12 0.15 0.15 0.01 0.01 0.07 0.19 Si

Si 0.07 0.02 0.14 0.09 0.06 0.11 0.03 0.09 0.12 0.08 0.07 0.09 0.10 0.17 0.01 0.12 0.14 0.11 0.08 0.11 0.12 0.07

0.07 0.07 0.02 0.02 0.14 0.09 0.06 0.06 0.03 0.03 0.09 0.12 0.12 0.08 0.08 0.07 0.07 0.09 0.10 0.17 0.12 0.12 0.14 0.14 0.08 0.12 0.12 0.07 0.07 0.09 0.09 0.14 0.09 0.11 0.11 0.09 0.09 0.10 0.17 0.01 0.01 0.11 0.11 0.08 0.11 0.11 C

C ID sample Steel Steel sample ID

Steel sample ID M

W A D G H K N O Q U V E L T B C R F P S I J

A B C D E F G H I J K L M N O P Q R S T U V W Table 1

Table1 1 Table

Ref. Ref. No. No. PO225838-PCT-ZZ (15/24) PO225838-PCT-ZZ (15/24)

- 16 --

[0058] Subsequently,the

[0058] Subsequently, theobtained obtained steelslab steel slabwas was heated heated andand thenthen hot-rolled hot-rolled

underthe under theconditions conditions shown shown in Table in Table 2 to obtain 2 to obtain a hot-rolled a hot-rolled steelhaving steel plate plate having the corresponding the correspondingplate platethickness thickness (final (final plate plate thickness). thickness). After After this, this, eacheach

treatment was treatment wasperformed performed under under the the conditions conditions shown shown in Table in Table 2 to obtain 2 to obtain a a

steel plate. steel plate.

Ref. Ref. No. Ref. No. PO225838-PCT-ZZ No. PO225838-PCT-ZZ (16/24) PO225838-PCT-ZZ (16/24) (16/24)

Table Table2 2 Table 2 material Steel conditions Production material Steel conditions Production Steel material Production conditions heating First heating Second heating Second heating First Heating rolling Hot treatment Cooling treatment Cooling rolling Hot Heatingof Heating of First heating Second heating

of material steel retention retention retention retention Quenching material steel Quenching Hot rolling Quenching Cooling treatment steel material retention retention

[0059]

Steel Steel cooling Average cooling Average Steel

Ac Ac1 Ac3 cooling Average cooling Average Ac3

No. Ac1 Ac3 Remarks

No. Remarks

end Cooling No. Average cooling end Cooling Average cooling end Cooling Remarks

sample Plate

sample Finish

Plate

point

point Heating

Heating

Heating Finish Heating

Heating

Heating sample point point Heating Plate Finish Heating Cooling end Heating Cooling end

Cooling end

rate

rate rate rate (*1) temperature (*1) temperature thickness temperature temperature temperature (*1) temperature temperature (*1) temperature thickness temperature temperature temperature temperature rate rate

ID ID ID

point (°C)

(C) point (°C) (C) (C) (C) temperature thickness temperature temperature temperature (*1) temperature temperature (*1)

(*1)(*3)

(*1)(*2) (*1)(*3)

(*1)(*2) (*1)(*2) (*1)(*3)

(°C)

(mm) (°C)

(°C)

(°C) (°C) (°C)

(C) (C) (C) (C)

(°C) (C)

(mm) (C) (mm) (C) (C) (C) (C) (C)

(C/s) (C/s) (°C/s)

(°C/s) (C/s) (C/s)

[0059] [Table

1200 900

830 200 780

950 250

689 830 950 250

689 900 200 780 Example Example

1200 40 15 1 A 689 830 1200 40 950 900 15 200 780 15 250 Example

1100

830 850 250 350

760

900

689 830 350

760

900 850

689 250 Example Example

40 40 15 15

1100 15 15 2 A 689 830 1100 40 900 850 15 250 760 15 350 Example

[Table 2] 2]

1000 950 790

830 350

150

900

689 830 900 150 350

950 790

689 Example Example

15 30

40 25

1000 15 15 3 A 689 830 1000 25 900 950 30 150 790 15 350 Example

1000

1200 900 100 300

830 710

689 830 710 300

900 100

689 Example Example

25 30 30 25 15 25

1200 1000 4 A 689 830 1200 30 1000 900 25 100 710 25 300 Example

1100 200 300

830 810

950

689 830 950 810 300

900

689 200 Example Example

1100 25 15

30 40 25 25 5 A 689 830 1100 40 950 900 15 200 810 25 300 Example

1150 100

830 780 450

900

689 Example

830 900 450

900 780

100

689 Example

40 40 15 15

1150 25 15

900 900 6 A 689 830 1150 40 900 900 15 100 780 15 450 Example Example Comparative Example Comparative 1200 900 200 300

780

830 750

689 830 780 300

900 200

750

689 40 40

1200 15 15

15 15 7 A 689 830 1200 40 750 900 15 200 780 15 300 Comparative Example Example Comparative Example Comparative 1200 1000 1050 350

200

830 760

689 15

830 760 350

689 200

40 40

1200 15 15 15 15

1000 1050 8 A 689 830 1200 40 1000 1050 15 200 760 15 350 Comparative Example Example Comparative Example Comparative 1150 300

830 100

850

689 680

830 950 850 300

100

689 680

950

40 40

1150 15 15 15

1 2 3 4 5 6 7 8 9 9 A 689 830 1150 40 950 850 15 100 680 15 300 Comparative Example Example Comparative Example Comparative 1100 900 350

900 200

830 860

689 830 350

900 900 200 860

689 15

10 1100 40 40 15 15

10 15 15

A A A A A A A A A A 10 A 689 830 1100 40 900 900 15 200 860 15 350 Comparative Example cooling) (Air 0.5 Example Comparative Example Comparative cooling) (Air 0.5 1200 900 300

830 200 790

689 300

950

830 200 790 15

689 900

950

1200 40 40

11 15 15

11 A 689 830 1200 40 950 900 0.5 (Air cooling) 200 790 15 300 Comparative Example cooling) (Air 0.5 Example Comparative Example Comparative cooling) (Air 0.5 1150

830 900 250 770

689 --

950 250

830 900

689 770

15

40 40

1150 950

11 12 A A A 12 A 689 830 1150 40 950 900 15 250 770 0.5 (Air cooling) - Comparative Example Example Comparative Example Comparative 1090

830 900

900 200 760

689 100

830 760

900 100

900 200

689 1090 40 40

12 13 15 15 13 A 689 830 1090 40 900 900 15 200 760 15 100 Comparative Example Example Comparative Example Comparative 1120

830 950 980 820

200

689 550

820

830 950 550

200

980

689 1120 40 40 15 15

13 14 A A A 14 A 689 830 1120 40 950 980 15 200 820 15 550 Comparative Example Example Comparative Example Comparative 1200

830 900

950 250

780

450

689 830 250

950 900 450 780

689 1200 40 40 15 15

14 15 15 15 15

15 A 689 830 1200 40 950 900 15 450 780 15 250 Comparative Example

1120

826 900

950 250 450

750

664 450

826 250

950 750

900 Example

664 Example

1120 15 50

40 12 15 50

15 16 16 B 664 826 1120 12 950 900 50 250 750 50 450 Example

1200 870

822 300

950 100 780

700 300

950

822 870 100

700 780 Example Example

1200 12 40 50 15

16 17 50 15

17 C 700 822 1200 40 950 870 15 100 780 15 300 Example - 17

1070

831 900 200

900 350

770

697 350

831 900

900 200 Example

770

697 Example

40 40

1070 15 15

17 18 15 15

18 D 697 831 1070 40 900 900 15 200 770 15 350 Example

1120

876 200

880

710 900 780 400 400

900

710 880

876 200 Example

780 17 -

Example

40 30 15 25

1120

18 19 15 25

A A B C D E 19 E 710 876 1120 30 900 880 25 200 780 25 400 Example

1170

809 900 200

950 360

740

671 360

950 740

200

900

671

20 Example

809 Example

25 35

30 20

1170 25 35

19 20 20 F 671 809 1170 20 950 900 35 200 740 35 360 Example

1100

824 250

900 910 300

673 760

250 300

760

824 900 Example

673 910 Example

E F G 20 40 35 15

1100

21 21 G 673 824 1100 40 900 910 15 250 760 15 300 Example

1200 950

829 100

870 320

780

689 950 320

829 870 Example

100 780

689 Example

40 40 15 15

21 22 35 15 15

1200 22 H 689 829 1200 40 950 870 15 100 780 15 320 Example

1150 950 100

880 760

693 350

819 950 760

880 350 Example

100

693 819 Example

22 23 40 40 15 15

1150 15 15

23 I 693 819 1150 40 950 880 15 100 760 15 350 Example

1150

863 200

950

720 900 810 350

950 810

900 350

863 Example

200

720 Example

40 40 15 15

23 24 15 15

1150 24 J 720 863 1150 40 950 900 15 200 810 15 350 Example

1150

757 950 200

890 230

700

624 230

950 700

890 Example

200

757

624 Example

40 40 15 15

24 25 15 15

1150 25 K 624 757 1150 40 950 890 15 200 700 15 230 Example

1200 950 250

716 820

880 320

250 820

950 880 Example

320

862

716 Example

40 40 15 15 15 15

25 26 G H I J K L 1200

862 26 L 716 862 1200 40 950 880 15 250 820 15 320 Example

Ref. 1150 200

847 900 900

719 800 300 Example

847 900 300

800

900 200

719 Example

40 40

26 27 15 15 15 15

1150 27 M 719 847 1150 40 900 900 15 200 800 15 300 Example Example Comparative Example Comparative 1100

828 950 250

900

715 780 300 300

950 250

900

828

715 780

27 28 40 40 15 15 15 15

1100 28 N 715 828 1100 40 950 900 15 250 780 15 300 Comparative Example Example Comparative Example Comparative 1200

Ref. No. 867 950 900 100 800 350

696 350

950 800

867 900 100

696 40 40

28 29 15 15 15 15

1200 29 O 696 867 1200 40 950 900 15 100 800 15 350 Comparative Example Example Comparative Example Comparative 1120 910 810

856 900 250

711 350

900 810 350

250

30 856 910

711

29 30 40 40 15 15 15 15

1120 30 P 711 856 1120 40 900 910 15 250 810 15 350 Comparative Example Example Comparative Example Comparative 1070

843 890

900 200 770 300

721 843 300

900 890 200 770

721 1070

31 40 40 15 15 15 15

31 Q 721 843 1070 40 900 890 15 200 770 15 300 Comparative Example Example Comparative Example Comparative 1150

862 950 250 790

880 400

720 950 250 400

880 790

862

720 1150

31 32 40 40 15 15 15 15

32 R 720 862 1150 40 950 880 15 250 790 15 400 Comparative Example Example Comparative Example Comparative 1100

824 790

900

900 100 320

690 824 900 900 790 320

100

690 1100

32 33 40 40 15 15 15 15

33 S 690 824 1100 40 900 900 15 100 790 15 320 Comparative Example Example Comparative Example Comparative 1160

843 950 890 200 780 350

714 843 950 350

714 780

890 200

33 34 1160 40 40 15 15 15 15

34 T 714 843 1160 40 950 890 15 200 780 15 350 Comparative Example Example Comparative Example Comparative 1200

833 950 880 760

200 350

709 833 950 760 350

709 880 200

1200

34 35 40 40 15 15 15 15

35 U 709 833 1200 40 950 880 15 200 760 15 350 Comparative Example Example Comparative Example Comparative 1200

850 900 800 300

980 200

703 850 800

900 300

703 980 200

1200

35 36 40 40 15 15 15 15

36 V 703 850 1200 40 980 900 15 200 800 15 300 Comparative Example Example Comparative Example Comparative 1200

842 980 800

200 300

920

697 842 40

37 15 300

15

800

980

697 920 200

1200

36 37 40 40

L M N O P Q R S T U V W 15 15 15 15

37 W 697 842 1200 40 disclosure. present the of scope the outside indicate Underlines 980 920 15 200 800 15 300 Comparative Example disclosure. present the of scope the outside indicate Underlines Underlines indicate outside the scope of the present disclosure.

No. PO225838-PCT-ZZ thickness plate of 1/4 of position Depth (*1) thickness plate of 1/4 of position Depth (*1) (*1) Depth position of 1/4 of plate thickness °C 300 to °C 600 from range Temperature (*2) °C 300 to °C 600 from range Temperature (*2) (*2) Temperature range from 600 C to 300 C °C 500 to °C 700 from range Temperature (*3) °C 500 to °C 700 from range Temperature (*3) PO225838-PCT-ZZ (17/24) (*3) Temperature range from 700 C to 500 C

(17/24)

- 18 --

[0060] Foreach

[0060] For eachof of thethe obtained obtained steelsteel plates, plates, the microstructure, the microstructure, tensile tensile

strength (TS), strength (TS), yield yieldratio ratio(YR), (YR),low-temperature low-temperature toughness toughness (vE -50C (vE-50°C), (vE-50°c), and), and acoustic anisotropy acoustic anisotropy (sound (soundvelocity velocityratio) ratio)were were evaluated evaluated in the in the following following

manner. The manner. Theevaluation evaluationresults results are are shown shownininTable Table3.3.

[0061] [Microstructure]

[0061] [Microstructure]

A test A test piece piecefor formicrostructure microstructure observation observation was collected was collected from from the the steel steel plate so plate so that that aa position positionofof1/4t 1/4twould wouldbe be thethe observation observation position position. The .test Thepiece test piece was embedded was embedded in resin in resin SO so that so that a cross a cross section section perpendicular perpendicular to rolling to the the rolling directionwould direction wouldbe be thethe observation observation plane, plane, and mirror-polished. and mirror-polished. After After this, this, nital nital

etching was etching performed, and was performed, and then then observation observation was was made madeusing usinga ascanning scanning electron microscope electron with5000 microscope with 5000 magnification magnification andand an image an image of microstructure of microstructure

was taken. was taken. The The obtained obtainedimage imagewaswas analyzed analyzed to identify to identify microstructure microstructure fractions. Microstructures fractions. Microstructures identified identified are are as follows: as follows:

- Tempered - martensiteand/or Tempered martensite and/orbainite: bainite: hard hard phase phasecontaining containingcementite cementite

-- Martensite austenite Martensite austenite constituent: constituent: hardhard phase phase not containing not containing cementite cementite

and having and having an anequivalent equivalentcircular circular diameter diameter of mororless of 11 um µm less - As-quenched - martensite:hard As-quenched martensite: hard phase phase not not containing containing cementite cementite and and having an having an equivalent equivalent circular circular diameter diameter of of more morethan m than11um µm

- Ferrite: - Ferrite: matrix phaseother matrix phase other than than the the above. above.

[0062] Further, microstructure

[0062] Further, microstructureanalysis analysiswas was conducted conducted by EBSD. by EBSD. Defining Defining

regions surrounded regions surrounded by by large-angle large-angle grain grain boundaries boundaries with withananorientation orientation differenceofof1515degrees difference degrees or more or more as crystal as crystal grains, grains, for a for a total total area area of ofX11 mm 1 mm 1 mm,the mm, thecrystal crystal grain grain size size distribution distribution was calculated and was calculated and the the number numberdensity density of crystal of crystal grains grains having an equivalent having an equivalent circular circular diameter diameterofofmore morethan than 30 30 um µm m

was measured. was measured.In In addition, addition, thethe average average value value (average (average aspect aspect ratio) ratio) of of the the aspect ratios aspect ratios (= (= the the length lengthininthe therolling rollingdirection/the direction /thelength lengthin in thethe plate plate

thickness direction) thickness direction) of of crystal crystalgrains grainsobtained obtainedin inan anarea mm  areaofof1 1mm X 11 mm was mm was

calculated. calculated.

[0063] [Tensile strength

[0063] [Tensile strength (TS) (TS) and andyield yield ratio ratio (YR)] (YR)]

A JIS A JISNo. No.4 4tensile tensiletest testpiece piece perpendicular perpendicular to rolling to the the rolling direction direction was was collectedfrom collected from a position a position of of 1/4t 1/4t of the of the steel steel plate. plate. Using Using the tensile the tensile test piece, test piece,

a tensile a tensile test test was conductedininaccordance was conducted accordance with with JISJIS Z 2241 Z 2241 to evaluate to evaluate the the tensile strength tensile strength(TS) (TS)of of thethe steel steel plate. plate. If If thethe tensile tensile strength strength wasMPa was 690 690or MPa or more,the more, thesteel steelplate platewas was evaluated evaluated as having as having high strength high strength andasrated and rated as "pass". “pass”.

Theyield The yieldratio ratio(YR) (YR) was was also also evaluated evaluated based based on on the test the tensile tensile test If result. result. the If the yield ratio yield ratio was was0.80 0.80 or or less, less, thethe steel steel plate plate waswas rated rated as “pass”. as "pass".

Ref. No. Ref. No. PO225838-PCT-ZZ (18/24) PO225838-PCT-ZZ (18/24)

- 19 --

[0064] [Low-temperature

[0064] [Low-temperature toughness toughness (vE -50C )] (vE-50°C)] (vE-50°c)] V-notchedtest V-notched test pieces pieces parallel parallel to to the the rolling rolling direction direction were were collected collected froma aposition from positionofof 1/4t 1/4t of of thethe steel steel plate plate in accordance in accordance withZ 2202. with JIS JIS ZUsing 2202. Using the V-notched the test pieces, V-notched test pieces,Charpy Charpy impact impact test test was was conducted in accordance conducted in with accordance with

JIS ZZ 2242 JIS 2242toto determine determinethe theCharpy Charpy absorbed absorbed energy energy at –50 at -50 C (vE -50CThe °C (vE-50°C). (vE-50°c). ). The Charpyabsorbed Charpy absorbedenergy energy cancan be be regarded regarded as index as an an index of low-temperature of the the low-temperature toughnessofof toughness the the steel steel plate. plate. In In thethe Charpy Charpy impact impact test, three test, three test pieces test pieces (A, B, (A, B, and C) and C) were werecollected collectedper persteel steel plate plate and and measured. measured.The The measurement measurement results results

are shown are shown in in Table Table 3. 3. In In thethe full-size full-size Charpy Charpy impact impact test, test, if if vEof vE-50°C oftest each -50C each test

piece was piece was100 100J Jorormore, more, thethe steel steel plate plate waswas evaluated evaluated as having as having excellent excellent

Charpytoughness Charpy toughnessand andrated ratedasas"pass" “pass”. "pass".

[0065] [Acousticanisotropy

[0065] [Acoustic anisotropy(sound (soundvelocity velocityratio)] ratio)] In order In order to to evaluate evaluatethe theacoustic acousticanisotropy anisotropy of of thethe steel steel plate, plate, thethe

transverse sound transverse soundvelocity velocityratio ratio specified specifiedininJIS JISZ Z3060 3060 waswas evaluated. evaluated. The The

transverse sound transverse sound velocity velocity ratio ratio herein herein is is aa value value defined defined as as CSL/CSC, thatis, CSL/CSC, that is, the ratio the ratio of of the the sound soundvelocity velocity CSLCSL (m/sec) (m/sec) when when the the vibration vibration directiondirection of the of the transversewave transverse waveis is thethe rolling rolling direction direction (L direction) (L direction) to sound to the the sound velocity velocity CSC CSC (m/sec) whenthe (m/sec) when thevibration vibrationdirection directionofofthe thetransverse transversewave waveisisa adirection direction(C(C direction) orthogonal direction) orthogonal toto the therolling rolling direction. direction. The Themeasurement measurement results results are are

shownininTable shown Table3.3. If If CSL/CSC CSL/CSC waswas 1.021.02 or less, or less, thethe steelplate steel platewas was evaluated evaluated

as having as low acoustic having low acoustic anisotropy anisotropyand andrated ratedas as "pass". “pass”.

Ref. No. Ref. No. PO225838-PCT-ZZ (19/24) PO225838-PCT-ZZ (19/24)

Table Table 33 Table 3 results Evaluation results Evaluation Evaluation results Microstructure Microstructure Microstructure greater grains Crystal greater grains Crystal vE-50°C velocity Sound vE-50°C

vE-50°C austenite Martensite vE_50°C velocity Sound vE-50°C vE_50°C austenite Martensite No. Remarks vE-50C vE-50C vE-50C Remarks

[0066]

Crystal grains greater

TS

No. TS fraction Ferrite fraction Ferrite No. Martensite austenite TS Sound velocity Remarks

YR

than Average Average fraction constituent microstructure* Residual YR fraction constituent microstructure* Residual YR

than 30 Ferrite fraction Average

(J)

(J) (J) ratio

(MPa) ratio (J) (J) (J)

30 um than 30 m

µm ratio aspect (MPa) ratio aspect constituent fraction Residual microstructure* (MPa) ratio E ]]ab[e

(%) (%) 2. () aspect ratio

(J) (B) (J) (C) (J) (A) (B) (C)

(A) 2 (A) (B) (C)

(%) (%) ()

(number/mm²)

(number/mm (number/mm ) martensite/bainite Tempered martensite/bainite Tempered 210 230 235

240

1.5

210 720 Example Example

1.00

13 0.69 1.00

0.69 230

1.5 720 240 235 1 40 13 Tempered martensite/bainite 210 1.5 720 0.69 230 240 235 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 710 240

230

170 240

170 220

1.4 710

12 230 Example Example

40 50 1.00

13 12 0.69 1.00

0.69 220

1.4 2 50 12 Tempered martensite/bainite 170 1.4 710 0.69 230 240 220 1.00 Example

[Table

martensite/bainite Tempered martensite/bainite Tempered 730

240 210

200 200

240 210 220

12 1.5 730 Example Example

1.00

50 30 0.70

12 0.70 1.00

1.5 220 3 30 12 Tempered martensite/bainite 240 1.5 730 0.70 200 210 220 1.00 Example martensite/bainite Tempered martensite/bainite Tempered

[9900] 3]

210 700 230 220

230 220

230

210 230

1.4 700 Example Example

30 80 1.00

10 0.71 0.71 1.00

1.4 4 80 10 Tempered martensite/bainite 210 1.4 700 0.71 230 230 220 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 210 220

740 230

1.5 230 230

230 220

740

1.5

210 Example Example

0.74

10 10 1.00

80 10 0.74 1.00

10 5 10 10 Tempered martensite/bainite 210 1.5 740 0.74 230 220 230 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 210

210 210

200

40 720 210

200

210 1.4 210 Example Example

0.77 1.00

5

10 40 720 0.77 1.00

1.4 6 40 5 Tempered martensite/bainite 210 1.4 720 0.77 200 210 210 1.00 Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 760 250 240 250

250

14 130 240 250

760

40 1.03

0.71

40 14 0.71 1.03

130 2.5 7 40 14 Tempered martensite/bainite 130 2.5 760 0.71 250 240 250 1.03 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 680

280 1.5 90

280 680 1.00

0.71 70

50 90

12 80 1.00

0.71

1.5 8 50 12 Tempered martensite/bainite 280 1.5 680 0.71 90 80 70 1.00 Comparative Example Example Comparative Example Comparative 200 210 200

170 700 200

200

170 700

1.4 210

100 0.95 1.00

1 2 3 4 5 6 7 8 9 0.95 1.00

1.4

100 9 100 0 - 170 1.4 700 0.95 200 210 200 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 180 200

220

750 230

1.5 230

200

10 220

750

180 0.85

10 1.00

0.85 1.00

1.5

0 10 0 5 Tempered martensite/bainite 180 1.5 750 0.85 220 200 230 1.00 Comparative Example Example Comparative martensite/bainite Tempered martensite/bainite Tempered Example Comparative 1.4 100

650

260 1.4 90

100

650 90

11 1.00

0.72

30 05 8 90 90

0.72 1.00

260 11 30 8 Tempered martensite/bainite 260 1.4 650 0.72 90 100 90 1.00 Comparative Example Example Comparative Example Comparative 110

650

1.5 650 110

260

Bainite Bainite 0.71

11 12 80 1.00

90

15 0.71 1.00

1.5

260 12 45 15 Bainite 260 1.5 650 0.71 90 110 80 1.00 Comparative Example Example Comparative Martensite/bainite Example Comparative Martensite/bainite 740

1.5

220 740

1.5 80 90 1.00

0.68 90

80 1.00

220 13 50 15 Martensite/bainite 220 1.5 740 0.68 80 90 90 1.00 Comparative Example Example Comparative Example Comparative 10

14 1.4 750 80

260

Bainite Bainite 0.73 90 80 1.00

70

13 14 30 45 50 40 80

15 10 0.73 1.00

1.4 750

260 14 40 10 Bainite 260 1.4 750 0.73 70 80 80 1.00 Comparative Example martensite/bainite Tempered Example Comparative martensite/bainite Tempered Example Comparative 120

260 1.5 670 670 120

260

40 0.69 1.00

15 13 80 90 1.00

0.69 15 40 13 Tempered martensite/bainite 260 1.5 670 0.69 120 80 90 1.00 Comparative Example martensite/bainite Tempered martensite/bainite Tempered 210

220 200

1.5 780 210 200

220 1.5 780 210 Example Example

8 0.71 1.00

0.71 1.00

16 40 8 Tempered martensite/bainite 220 1.5 780 0.71 210 210 200 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 170 220

210

770 200 220

25 200

770

170 210

1.4 Example Example

40 30

16 17 1.00

0.68

25 0.68 1.00

1.4 17 30 25 Tempered martensite/bainite 170 1.4 770 0.68 210 200 220 1.00 Example martensite/bainite Tempered martensite/bainite Tempered - 20

210 220

710 230 240

220 230 240

210 1.4 710

40 Example Example

18 1.00

0.71

40 0.71 1.00

1.4 18 40 10 Tempered martensite/bainite 210 1.4 710 0.71 220 230 240 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 720 180 190

180 170

1.5 170 190

180

180 720 Example Example 20 -

0.72

35 1.00

10 15

19 0.72

1.5 19 35 15 Tempered martensite/bainite 180 1.5 720 0.72 180 170 190 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 220 220

730 210 210

220 220

730 210

1.4

20 10 Example

40 Example

40 1.00

0.71

20 10 1.00

1.4 20 40 10 Tempered martensite/bainite 220 1.4 730 0.71 210 210 220 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 230 200

720 215

1.5

230 215

45 210

720 Example Example

0.72

45 1.00

0.72 1.00

210

1.5 21 45 15 Tempered martensite/bainite 230 1.5 720 0.72 210 200 215 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 1.4 720 150 160

170 160

25 12 720 Example Example

21 22 1.00

25 15 12 0.69 1.00

22 25 12 Tempered martensite/bainite 170 1.4 720 0.69 150 160 160 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 720

180 140 150 140

1.5

180 720 Example Example

1.00

35

23 20 0.68 1.00

1.5 23 35 20 Tempered martensite/bainite 180 1.5 720 0.68 140 150 140 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 220 160 180 200

710

1.4

220 Example

160 Example

0.72 1.00

40

24 10 1.00

1.4 710 24 40 10 Tempered martensite/bainite 220 1.4 710 0.72 160 180 200 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 720

1.4 255 256

260

15 190 720 Example Example

0.71 1.00

30 15

25 1.00

190 25 30 15 Tempered martensite/bainite 190 1.4 720 0.71 255 260 256 1.00 Example martensite/bainite Tempered martensite/bainite Tempered 1.5 220 210

180 200

1.5 810 220 210

200

180 Example Example

0.71 1.00

20 15

26 810 0.71 1.00

26 20 15 Tempered martensite/bainite 180 1.5 810 0.71 200 220 210 1.00 Example

Ref. martensite/bainite Tempered martensite/bainite Tempered 220 820 250

240

235

220 820

1.4 Example Example

0.69

13 1.00

27 30 1.00

1.4 27 30 13 Tempered martensite/bainite 220 1.4 820 0.69 235 240 250 1.00 Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 210 760 80

60

210 1.4 80

28 1.00

45 0.68 70

60

35 1.4

Ref. No. 28 45 35 Tempered martensite/bainite 210 1.4 760 0.68 60 70 80 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 210 240

230

630 220

1.5

30 1.00

0.78

1 29 30 1 Tempered martensite/bainite 210 1.5 630 1.5 0.78 230 240 220 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 220 720 90

220 1.4 80

720

29 30 0.71

20 90

12 1.00

80 90

1.4

30 20 12 Tempered martensite/bainite 220 1.4 720 0.71 80 90 90 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 190 710

1.4 80

10 710

50

31 0.71 1.00

10 80 90 80

1.4

31 50 10 Tempered martensite/bainite 190 1.4 710 0.71 80 90 80 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 180 210

200

1.5 630 200

180 210 1.00

0.71

10

32 40 0.71 1.00

32 40 10 Tempered martensite/bainite 180 1.5 630 0.71 200 200 210 1.00 Comparative Example martensite/bainite Tempered Example Comparative Example Comparative martensite/bainite Tempered 760

210 1.5 60

40 50 50 1.00

0.72

33 50

40 60

15 1.00

1.5 50

33 40 15 Tempered martensite/bainite 210 1.5 760 0.72 50 60 50 1.00 Comparative Example Example Comparative martensite/bainite Tempered martensite/bainite Tempered Example Comparative 190 730

1.4 1.00

0.72

34 35 10 60 50 60

1.4 730

34 35 10 Tempered martensite/bainite 190 1.4 730 0.72 60 50 60 1.00 Comparative Example martensite/bainite Tempered Example Comparative Example Comparative martensite/bainite Tempered 170 1.5 750 40

50 60

170 750 0.71 1.00

15

50 70

35 40 60

35 50 15 Tempered martensite/bainite 170 1.5 750 0.71 70 40 60 1.00 Comparative Example martensite/bainite Tempered Example Comparative Example Comparative martensite/bainite Tempered 730

1.4

25 12 210 90

12

25 0.70 1.00

80

36 90 90

210

36 25 12 Tempered martensite/bainite 210 1.4 730 0.70 80 90 90 1.00 Comparative Example Example Comparative martensite/bainite Tempered Example Comparative martensite/bainite Tempered 730 120

150

1.4

260 1.00

0.70

12

37 25 80 1.00

No. PO225838-PCT-ZZ 1.4

260

37 25 12 Tempered martensite/bainite 260 1.4 730 0.70 80 150 120 1.00 Comparative Example microstructure in formed are carbides which in state martensite/bainite: *Tempered microstructure in formed are carbides which in state martensite/bainite: *Tempered *Tempered martensite/bainite: state in which carbides are formed in microstructure precipitated not are carbides which in state Martensite/bainite: precipitated not are carbides which in state Martensite/bainite: PO225838-PCT-ZZ (20/24) Martensite/bainite: state in which carbides are not precipitated

(20/24)

18 Aug 2025

[0067] As can be seen from these tables, in each Example, a steel plate having a certain chemical composition and microstructure and having high strength, high low-temperature toughness, low yield ratio, and low acoustic anisotropy was obtained. 5

[0068] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and 2023255622

should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known 10 matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0069] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a 15 stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Ref. No. PO225838-PCT-ZZ (21/24)

Claims (3)

18 Aug 2025

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

[Claim 1] A steel plate comprising a chemical composition containing, in mass%, 5 C: 0.02 % or more and 0.15 % or less, Si: 0.01 % or more and 0.50 % or less, Mn: 0.05 % or more and 2.50 % or less, 2023255622

Ni: 0.5 % or more and less than 5.0 %, P: 0.03 % or less, 10 S: 0.005 % or less, and N: 0.0010 % or more and 0.0080 % or less, with a balance consisting of Fe and inevitable impurities, wherein in a microstructure at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction, a 15 ferrite fraction is 5 % to 95 % and a martensite austenite constituent fraction is 1 % to 30 % with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains 20 having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm 2 or less.

[Claim 2] The steel plate according to claim 1, wherein the chemical composition further contains, in mass%, one or more selected from 25 Nb: 0.05 % or less, Al: 0.100 % or less, Cr: 2.00 % or less, Mo: 1.0 % or less, Cu: 2.0 % or less, 30 V: 0.05 % or less, Ti: 0.03 % or less, B: 0.0030 % or less, Ca: 0.007 % or less, REM: 0.010 % or less, and 35 Mg: 0.007 % or less.

Ref. No. PO225838-PCT-ZZ (22/24)

- 23 --

[Claim

[Claim 3]

[Claim 3] 3] A method A methodofofproducing producing a steel a steel plate, plate, thethe method method comprising comprising

subjecting aa steel subjecting steelmaterial materialhaving having the thechemical chemical composition accordingto composition according to claim claim 11 or or 22totohothot rolling,thereafter rolling, thereafter to to first first heating heating retention, retention, thereafter thereafter to to

quenching,thereafter quenching, thereafter toto second secondheating heatingretention, retention,andand thereafter thereafter to to cooling cooling

treatment, treatment, treatment,

whereinininthethehothot wherein rolling, rolling, a finish a finish temperature temperature is 900 or C is °C900 or more, more, in the in the first first heating retention,a aheating heating retention, heating temperature temperature is inisa in a temperature temperature

range of range of Ac3 Ac pointor Ac 3point point ormore or moreand more and1000 and 1000 1000 °CC °C oror or less, less, less,

in the in the quenching, an average quenching, an averagecooling coolingrate ratefrom from600 °C C 600 to to °C C 300300 at aat a

depthposition depth positionofof 1/41/4 of of a plate a plate thickness thickness from from a surface a surface of the of theplate steel steelinplate a in a plate thickness plate thickness direction is 33 C/s direction is °C/s or or more, and aa cooling more, and coolingend endtemperature temperatureis is

300°CCororless, 300 less, in the in the second second heating heating retention, retention, aa heating heating temperature temperatureisisinina a temperaturerange temperature rangeofofAc1 Acpoint Ac pointor 1point ormore or more more and and and less less less than than than theAc the the Ac Ac3 point, 3point, point, and and and

in the in the cooling treatment, an cooling treatment, an average averagecooling coolingrate rate from 700°C C from700 to to 500500

C at °C at the the depth depth position position of of 1/4 1/4 of of the the plate plate thickness from the thickness from the surface surface of of the the steel plate steel plate in in the the plate platethickness thicknessdirection direction is is 3 C/s 3 °C/s or more, or more, and aand a cooling cooling end end temperatureis temperature is 500 Cor 500 °C or less less and and 200 Corormore. 200 °C more.

Ref. Ref. No. Ref. No. PO225838-PCT-ZZ No. PO225838-PCT-ZZ (23/24) PO225838-PCT-ZZ (23/24) (23/24)