EP2876179B1

EP2876179B1 - Ni-CONTAINING STEEL PLATE

Info

Publication number: EP2876179B1
Application number: EP13823858.9A
Authority: EP
Inventors: Shinichi Miura; Yukio Shimbo; Nobuyuki Ishikawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2012-07-23
Filing date: 2013-07-18
Publication date: 2017-10-11
Anticipated expiration: 2033-07-18
Also published as: US20150147222A1; JP5594329B2; WO2014017057A1; EP2876179A1; CN104487602B; KR101702480B1; EP2876179A4; IN2014DN10853A; CN104487602A; WO2014017057A8; JP2014019936A; KR20150023724A

Description

TECHNICAL FIELD

The present invention relates to an Ni-containing steel plate with excellent low-temperature toughness, in particular to a steel plate which is suitable for use as members such as storage tanks for liquefied natural gas.

BACKGROUND ART

Conventionally, for members such as overland storage tanks for liquefied natural gas (hereinafter, referred to as LNG), high Ni-containing steel plates which are excellent in mechanical properties at low temperatures have been commonly used. In particular, steel plates composed of high Ni-containing steel which contains Ni by 9 mass% (hereinafter, referred to as 9 % Ni steel) have been commonly used, and they have actually been applied in many cases.
Regarding 9 % Ni steel, considerations on various properties such as mechanical properties and weldability have been made. For example, Steel and Iron by Furukimi Osamu, Suzuki Shigeharu, Nakano Yoshifumi, 69(1982)5, S492 (NPL 1) discloses that low-temperature toughness is improved by reducing the amount of impurity elements such as P and S. Further, Handbook of Metal, 4th revised edition, edited by The Japan Institute of Metals and Materials, Maruzen, p801 (NPL 2) discloses that low-temperature toughness is improved by stabilizing retained austenite. However, since Ni is an expensive metal, it is desired to reduce Ni content.
Techniques for obtaining steel plates which can be made to have an Ni content smaller than that of 9 % Ni steel and has good low temperature toughness are disclosed in for example, WO2007/034576 (PTL 1), WO2007/080645 (PTL 2), JP2011-214099A (PTL 3). PTL 1 discloses that mechanical properties of a steel plate can be improved by predetermining the chemical composition of the steel plate, defining the amount, aspect ratio, and average equivalent circular diameter of austenite contained in the steel plate, and manufacturing the steel plate with a method to satisfy such definitions.
Further, PTL 2 discloses that toughness of the heat-affected zone of a steel plate can be improved when the steel plate has a predetermined chemical composition and the Fe content obtained by an extraction residue method after a heat-cycle simulation test is more than a predetermined value. Further, PTL 3 discloses that a brittle crack-arrest property of steel can be improved when the steel has a predetermined chemical composition, with certain textures developed.
PTL 4 describes a Ni-added steel sheet containing, by mass%, 0.03-0.10% C, 0.02-0.40% Si, 0.3-1.2% Mn, 5.0-7.5% Ni, 0.4-1.5% Cr, 0.02-0.4% Mo, 0.01-0.08% Al, and 0.0001-0.0050% total O and which has P, S, and N contents reduced to 0.0100% or less, 0.0035% or less, and 0.0070% or less, respectively, with the remainder comprising Fe and incidental impurities. In the steel sheet, a portion thereof located at a distance of 1/4 the sheet thickness from a surface of the steel sheet in the width direction has a Ni segregation ratio of 1.3 or less. After a subzero treatment, the steel sheet has an austenite content of 2% or more, and the austenite after the subzero treatment has an index to unevenness of 5.0 or less and an average equivalent-circle diameter of 1 µm or less.

CITATION LIST

Patent Literature

PTL 1: WO2007/034576
PTL 2: WO2007/080645
PTL 3: JP2011-214099A
PTL 4: WO2012/005330

Non-patent Literature

NPL 1: Steel and Iron by Furukimi Osamu, Suzuki Shigeharu, Nakano Yoshifumi, 69(1982)5, 5492
NPL 2: Handbook of Metal, 4th revised edition, edited by The Japan Institute of Metals and Materials, Maruzen, p800-802

SUMMARY OF INVENTION

(Technical Problem)

However, the techniques disclosed in PTL 1, 2 and 3 do not include definitions regarding the amount of austenite at around -165 °C where the LNG tanks are actually used, and consideration regarding low-temperature toughness when the techniques are applied to actual structures were not made. Further, there were no specific disclosures regarding the manufacturing method of the steel plates.
The present invention has been developed in view of such situation, and an object thereof is to provide an Ni-containing steel plate which is low in cost and has excellent low-temperature toughness.

(Solution to Problem)

The inventors of the present invention, as a result of intense investigation for providing an Ni-containing steel plate with excellent low-temperature toughness, discovered that by containing C, Si, Mn, P, S, Al, and Ni as essential elements of a steel, and setting the amount of retained austenite contained in the steel after performing sub-zero treatment where cooling is performed until reaching liquid nitrogen temperature to be less than 1.7 %, and setting the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more to 5 µm or less by equivalent circle diameter, excellent low-temperature toughness can be achieved even when the Ni content is reduced compared to conventional 9 % Ni steel.
If the Ni content in steel is reduced to be smaller than that of 9 % Ni steel, even if retained austenite is stable at room temperature, it will be unstable at -165 °C where LNG tanks are used. Further, it is considered that toughness decreases when retained austenite exists at -165 °C, because the retained austenite is transformed into martensite phase due to deformation induced transformation, at the tip of a crack formed in the steel material when the LNG tank fractures. Under the situation, by reducing the amount of retained austenite remaining after sub-zero treatment corresponding to -165 °C where LNG tanks are used, and forming a fine microstructure as described above, it is assumed that low-temperature toughness can be improved even if the Ni content in steel is reduced to be smaller than that of conventional 9 % Ni steel.
The present invention is based on the above discoveries and it provides:

An Ni-containing steel plate having a chemical composition consisting of by mass% C: 0.01 % to 0.15 %, Si: 0.02 % to 0.10 %, Mn: 0.45 % to 2.00 %, P: 0.020 % or less, S: 0.005 % or less, Al: 0.005 % to 0.100 %, Ni: 5.0 % to 8.0 %, optionally at least one element selected from Cr: 1.00 % or less and Mo: 1.000 % or less, optionally at least one element selected from Cu: 1.00 % or less, V: 0.100 % or less, Nb: 0.100 % or less, Ti: 0.100 % or less, and B: 0.0030 % or less, optionally at least one element selected from Ca: 0.0050 % or less and REM: 0.0050 % or less, and the balance being Fe and incidental impurities, wherein
the steel plate has a microstructure containing less than 1.7 % by volume fraction of retained austenite when cooled to liquid nitrogen temperature, and in the microstructure the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more is 5 µm or less by equivalent circle diameter, wherein the volume fraction of retained austenite is determined by collecting a sample from a steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction, subjecting it to sub-zero treatment for 10 minutes in liquid nitrogen, and then measuring the austenite volume fraction by X-ray diffraction, and
wherein the average grain size by equivalent circle diameter of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more is determined by collecting a sample from a steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction were polished and mirror finished, subjecting it to EBSP analysis, selecting among the obtained data a high-angle grain boundary where the orientation difference between two crystal grains contacting the grain boundary is 15° or more and obtaining the average grain size by equivalent circle diameter of the region surrounded by the high-angle grain boundary.

(Advantageous Effect of Invention)

According to the present invention, an Ni-containing steel plate containing less Ni content compared to 9 % Ni steel but having low-temperature toughness equivalent to that of 9 % Ni steel can be easily manufactured, and an industrially remarkable effect is provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the Ni-containing steel plate according to the present invention will be explained in detail and separately based on chemical composition, microstructure, and manufacturing method.
Unless otherwise specified, the indication of "%" regarding composition shall stand for "mass%".

(1) Chemical Composition

First, the chemical composition will be described.

C: 0.01 % to 0.15 %

C is an important element for solid solution strengthening of steel. If C content is less than 0.01 %, sufficient strength cannot be obtained. On the other hand, adding C in an amount exceeding 0.15 % would cause deterioration of weldability and workability. Therefore, C content is set to be in the range of 0.01 % to 0.15 %. Preferably, the range is from 0.03 % to 0.10 %.

Si: 0.02 % to 0.10 %

Si is an effective element as a deoxidizer in molten steel and an effective element for solid solution strengthening. If Si content is less than 0.02 %, deoxidizing effect cannot be sufficiently obtained. On the other hand, adding Si in an amount exceeding 0.10 % would cause problems such as reduction in ductility and toughness, and an increase of inclusions. Therefore, Si content is set to be in the range of 0.02 % to 0.10 %, and preferably in the range of 0.03 % to 0.10 %.

Mn: 0.45 % to 2.00 %

Mn is an effective element from the viewpoint of ensuring quench hardenability and enhancing strength. If Mn content is less than 0.45 %, the effect thereof cannot be sufficiently obtained. On the other hand, adding Mn in an amount exceeding 2.00 % would cause deterioration of weldability. Therefore, Mn content is set to be in the range of 0.45 % to 2.00 %, and preferably in the range of 0.55 % to 1.00 %.

P: 0.020 % or less

Although high P content in steel leads to deterioration of low temperature toughness, the content thereof of 0.020 % or less would be acceptable. Therefore, the upper limit of P content is set to be 0.020 %.

S: 0.005 % or less

High S content in steel causes precipitation as MnS, and this, as an inclusion, becomes the fracture generation origin of high tensile strength steel and leads to deterioration of toughness. However, if the content thereof is 0.005 % or less, it would cause no problem. Therefore, the upper limit of S content is set to be 0.005 %.

Al: 0.005 % to 0.100 %

Al is an effective element as a deoxidizer in molten steel and an effective element for improving low-temperature toughness. If Al content is less than 0.005 %, these effects cannot be sufficiently obtained. On the other hand, if the content thereof exceeds 0.100 %, weldability will decrease. Therefore, Al content is set to be in the range of 0.005 % to 0.100 %, and preferably in the range of 0.020 % to 0.050 %.

Ni: 5.0 % to 8.0 %

Ni is an important element for the present invention, and it is an element that enhances quench hardenability and improves toughness of ferrite matrix. If Ni content is less than 5.0 %, these effects cannot be sufficiently exhibited. On the other hand, if the content thereof exceeds 8.0 %, costs will increase. Therefore, Ni content is set to be in a range of 5.0 % to 8.0 %. In addition, from the viewpoint of further reducing costs, it is desirable for Ni content to be in the range of 5.0 % to 7.5 %.
In addition to the above basic chemical compositions, it is possible to contain at least one element selected from Cr and Mo, as a first group of selected components, if necessary, in the following ranges.

Cr: 1.00 % or less

Cr enhances quench hardenability and provides an effect of improving low-temperature toughness by refining martensite phase. However, if the content thereof exceeds 1.00 %, it would cause deterioration of weldability and an increase in manufacturing costs. Therefore, when containing Cr, the content thereof is set to be in the range of 1.00 % or less. In order to effectively exhibit the above effect, it is preferable for the Cr content to be 0.05 % or more, and more preferably in the range of 0.10 % to 0.75 %.

Mo: 1.000 % or less

Mo enhances quench hardenability and provides an effect of improving low-temperature toughness by refining martensite phase. However, if the content thereof exceeds 1.000 %, it would cause deterioration of weldability and an increase in manufacturing costs. Therefore, when containing Mo, the content thereof is set to be in the range of 1.000 % or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005 % or more, and more preferably in the range of 0.010 % to 0.500 %.
Further, in the present invention, it is possible to contain at least one element selected from Cu, V, Nb, Ti, and B as a second group of selected components, if necessary, in the following ranges.

Cu: 1.00 % or less

Cu is an element that enhances quench hardenability. However, if the content thereof exceeds 1.00 %, it would cause reduction of hot workability and an increase in costs. Therefore, when containing Cu, the content thereof is set to be in the range of 1.00 % or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.05 % or more.

V: 0.100 % or less

V is an element that precipitates as carbonitride, has an effect of refining microstructures, and is useful for improving toughness. However, if the content thereof exceeds 0.100 %, it would cause deterioration of weldability. Therefore, when containing V, the content thereof is set to be in the range of 0.100 % or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005 % or more.

Nb: 0.100 % or less

Nb is an element that precipitates as carbonitride, has an effect of refining microstructures, and is useful for improving toughness. However, if the content thereof exceeds 0.100 %, it would cause deterioration of weldability. Therefore, when containing Nb, the content thereof is set to be in the range of 0.100 % or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005 % or more.

Ti: 0.100 % or less

Ti has an effect of improving toughness by fixing solute N, which is harmful to toughness, as TiN. However, if the content thereof exceeds 0.100 %, it would cause precipitation of a coarse carbonitride, and deteriorate toughness. Therefore, when containing Ti, the content thereof is set to be in the range of 0.100 % or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.005 % or more, and more preferably in the range of 0.010 % to 0.050 %.

B: 0.0030 % or less

B is an element that enhances quench hardenability when added to steel by a small amount. However, if the content thereof exceeds 0.0030 %, it would cause deterioration of toughness. Therefore, when containing B, the content thereof is set to be in the range of 0.0030 % or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0003 % or more.
Further, in the present invention, it is possible to contain at least one element selected from Ca and REM as a third group of selected components, if necessary, in the following ranges.

Ca: 0.0050 % or less

Ca is an element that fixes S and inhibits generation of MnS which becomes the cause of reduction in toughness. However, if the content thereof exceeds 0.0050 %, it would cause an increase in the amount of inclusions existing in steel and lead to deterioration of toughness rather than providing the above effect. Therefore, when containing Ca, the content thereof is set to be in the range of 0.0050 % or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0005 % or more.

REM: 0.0050 % or less

REM (Rare Earth Metal) is an element that fixes S and inhibits generation of MnS which becomes the cause of reduction in toughness. However, if the content thereof exceeds 0.0050 %, it would cause an increase in the amount of inclusions existing in steel and lead to deterioration of toughness rather than providing the above effect. Therefore, when containing REM, the content thereof is set to be in the range of 0.0050 % or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0005 % or more.
The balance other than the components described above includes Fe and incidental impurities.

(2) Microstructure

Next, the microstructure will be described.
The Ni-containing steel plate of the present invention has the above chemical composition, and also has a microstructure containing less than 1.7 % of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 µm or less by equivalent circle diameter.
Since the steel plate of the present invention is used mainly in storage tanks for LNG, the microstructure at -165 °C where LNG tanks are used is important. Therefore, the microstructure after sub-zero treatment where the steel plate is held at liquid nitrogen temperature, is defined. If the amount of retained austenite remaining after sub-zero treatment is 1.7 % or more by volume fraction, sufficient low-temperature toughness cannot be obtained. Some reports have been made that retained austenite improves low temperature toughness. However, for the Ni-containing steel plate of the present invention, retained austenite has a harmful effect on toughness. It is considered that this is due to the fact that, since in Ni-containing steel plate of the present invention, the Ni content is smaller than the Ni content in conventional 9 % Ni steel, even if retained austenite exists at -165 °C, it is unstable, and if the steel structure undergoes plastic deformation at the tip of a crack, the retained austenite transforms into martensite by plasticity-induced martensite phase transformation. Therefore, the amount of retained austenite when the steel plate is cooled to liquid nitrogen temperature is set to be less than 1.7 % by volume fraction. This amount is preferably 1.0 % or less, and more preferably 0.5 % or less.
Further, if the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more exceeds 5 µm by equivalent circle diameter, sufficient low-temperature toughness cannot be obtained. Therefore, the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more is set to be 5 µm or less by equivalent circle diameter, and preferably 3 µm or less by equivalent circle diameter.

(3) Manufacturing Condition

Next, a preferable manufacturing condition for manufacturing the steel plate of the present invention having the above described chemical composition and the above microstructure will be described. The following manufacturing condition is merely an example of a condition for manufacturing the Ni-containing steel plate of the present invention, and as long as the Ni-containing steel plate of the present invention can be obtained, manufacturing condition for the present invention is not limited to the following manufacturing condition.
In the present invention, it is preferable to heat a slab or a steel billet having the above described chemical composition at a temperature range of 900 °C to 1100 °C for 10 hours or less, and then to subject it to hot rolling at a temperature range of 870 °C or lower so that the cumulative rolling reduction ratio is 40 % or more and 70 % or less and the finisher delivery temperature is between 700 °C and 820 °C, and then to subject the obtained hot rolled steel plate to direct quenching treatment where quenching is immediately performed until reaching a temperature of 200 °C or lower at a cooling rate of 5 °C/s or more, and then to heat the steel plate to a temperature range of 500 °C to 650 °C at a heating rate of 0.05 °C/s to 1.0 °C/s, and then to subject the steel plate to tempering by holding the temperature at the same temperature range for 10 minutes or more and 60 minutes or less.

Heating Temperature: 900 °C to 1100 °C, Heating duration: 10 hours or less

In a case where the heating temperature is lower than 900 °C, coarse AlN which precipitates during the stage of casting of the steel slab does not dissolve, and toughness decreases. Further, the following rolling conditions cannot be substantially satisfied. If the heating temperature exceeds 1100 °C, austenite becomes coarse grains and toughness will decrease. If the heating duration exceeds 10 hours, austenite grains become coarse and toughness decreases. Therefore, the heating temperature is set to be between 900 °C and 1100 °C, and the heating duration is 10 hours or less.

Rolling Reduction Ratio: Cumulative Rolling Reduction Ratio of 40 % or more and 70 % or less at 870 °C or lower

If the cumulative rolling reduction ratio in the non-recrystallized region of austenite at 870 °C or lower is less than 40 %, refinement of martensite phase will not be sufficient, and toughness decreases. On the other hand, in a case where the cumulative rolling reduction ratio exceeds 70 %, it is difficult to substantially perform rolling at the following finisher delivery temperature. Therefore, the rolling reduction ratio is set to be 40 % or more and 70 % or less at 870 °C or lower.

Finisher delivery temperature: 700 °C to 820 °C

If the finisher delivery temperature is lower than 700 °C, it results in α-γ dual phase rolling so that bainite phase forms, and therefore a desired strength cannot be satisfied. On the other hand, if the finisher delivery temperature exceeds 820 °C, it becomes substantially difficult to perform sufficient rolling reduction in the non-recrystallized region of austenite, a fine microstructure cannot be obtained, and toughness decreases. Therefore, the finisher delivery temperature is set to be in the range of 700 °C to 820 °C.

Cooling (Direct Quenching): Start immediately after rolling

Cooling (direct quenching) is started immediately after rolling is finished. If cooling is not immediately started, bainite phase will generate, and therefore a desired strength cannot be satisfied. Therefore, cooling is started immediately after rolling is finished. Here, "immediately" refers to a point in time within 120 seconds after the completion of rolling.

Cooling Rate: 5 °C/s or more

In a case where the cooling rate is less than 5 °C/s, transformation to martensite phase will not occur, and a desirable strength and toughness cannot be obtained. Therefore, the cooling rate is set to be 5 °C/s or more. Preferably, the cooling rate is 10 °C/s or more.

Cooling Stop Temperature: 200 °C or lower

In a case where the cooling stop temperature exceeds 200 °C, transformation to martensite phase will not occur uniformly in the steel plate, and a desirable strength and toughness cannot be obtained. Therefore, the cooling stop temperature is set to be 200 °C or lower.

Tempering Heating Rate: 0.05 °C/s to 1.0 °C/s

In a case where the tempering heating rate is less than 0.05 °C/s, the precipitated carbide would become coarse, and toughness will decrease. On the other hand, in order to perform rapid short time heating where the tempering heating rate exceeds 1.0 °C/s, induction heating facilities and the like will be required, and costs will increase. Therefore, the tempering heating rate is set to be in the range of 0.05 °C/s to 1.0 °C/s.

Tempering temperature: 500 °C to 650 °C

In a case where the tempering temperature is lower than 500 °C, toughness improving effect caused by precipitation of fine carbides such as cementite cannot be sufficiently obtained. On the other hand, in a case where the tempering temperature exceeds 650 °C, coarse carbide precipitates, and toughness decreases. Therefore, the tempering temperature is set to be in the range of 500 °C to 650 °C.

Tempering Holding Time: 10 minutes or more and 60 minutes or less

In a case where the tempering holding time is less than 10 minutes, toughness improving effect caused by precipitation of fine carbides such as cementite cannot be sufficiently obtained. On the other hand, in a case where the tempering holding time exceeds 60 minutes, toughness will decrease due to reasons such as precipitation of a coarse carbide. Further, manufacturing costs will increase. Therefore, the tempering holding time is set to be 10 minutes or more and 60 minutes or less. Cooling after tempering may be performed by either water cooling or air cooling. However, if the cooling rate is too fast, the temperature difference between the surface and the inside of the steel plate becomes large and causes formation of strains inside the steel plate and low temperature toughness decreases. Therefore, the cooling rate is preferably 5 °C/s or less.
In the aforementioned manufacturing condition, after direct quenching, dual phase heat treatment where the steel plate is heated to a temperature range from 650 °C to 800 °C at a heating rate of 0.1 °C/s to 1.5 °C/s, held at the same temperature range for 10 minutes or more and 60 minutes or less, and then subjected to quenching until reaching a temperature of 200 °C or lower at a cooling rate of 5 °C/s or more, may be performed.

Dual Phase Heat Treatment Heating Rate: 0.1 °C/s to 1.5 °C/s

By performing dual phase heat treatment, part of the microstructure transforms into austenite, and as crystal grains become fine, tempering proceeds and thereby improves toughness. However, in a case where the dual phase heat treatment heating rate is less than 0.1 °C/s, austenite grains become coarse and toughness decreases. Further, since the microstructure generated after cooling also becomes coarse, toughness decreases. On the other hand, in a case where the heating rate exceeds 1.5 °C/s, induction heating facilities and the like are required, and costs increase. Therefore, the dual phase heat treatment heating rate is set to be in the range of 0.1 °C/s to 1.5 °C/s.

Dual Phase Heat Treatment Temperature: 650 °C to 800 °C

In a case where the dual phase heat treatment temperature is lower than 650 °C, sufficient austenite reverse transformation does not occur, and refining effect of the microstructure cannot be obtained, and therefore a toughness improving effect cannot be obtained. Further, since the amount of austenite reverse transformation is small, C easily concentrates in austenite, and retained austenite increases. On the other hand, if the dual phase heat treatment temperature exceeds 800 °C, reverse transformation austenite becomes coarse and toughness decreases. Further, since the microstructure after cooling becomes coarse, toughness decreases. Further, manufacturing costs increase. Therefore, the dual phase heat treatment temperature is set to be in the range of 650 °C to 800 °C. In a case where the dual phase heat treatment temperature is high, the amount of reverse transformation austenite increases and the amount of concentration of C in reverse transformation austenite decreases compared to a case where the dual phase heat treatment temperature is low, and therefore the amount of martensite transformation caused by cooling after dual phase heat treatment increases, and the amount of retained austenite decreases. Therefore, the dual phase heat treatment temperature is preferably in the range of 720 °C to 780 °C.

Dual Phase Heat Treatment Holding Time: 10 minutes or more and 60 minutes or less

If the dual phase heat treatment holding time is less than 10 minutes, sufficient austenite reverse transformation does not occur and toughness improving effect caused by refinement of the microstructure cannot be sufficiently obtained. On the other hand, in a case where the dual phase heat treatment holding time exceeds 60 minutes, austenite grains become coarse and toughness decreases. Further, since the microstructure generated after cooling also becomes coarse, toughness decreases. Since C concentrates in austenite, retained austenite increases. Manufacturing costs increase as well. Therefore, the dual phase heat treatment holding time is set to be 10 minutes or more and 60 minutes or less.

Cooling Rate after Dual Phase Heat Treatment: 5 °C/s or more

In a case where the cooling rate is less than 5 °C/s, transformation from austenite to martensite phase will not occur, and a desirable strength and toughness cannot be obtained. Further, if the cooling rate is slow, the amount of solute C in ferrite decreases as the temperature is lowered, and therefore C moves to austenite from the ferrite surrounding the reverse transformed austenite, and C concentrates in the austenite and the austenite tends to remain as retained austenite. Therefore, the cooling rate is set to be 5 °C/s or more. Preferably, the cooling rate is 10 °C/s or more.

Cooling Stop Temperature after Dual Phase Heat Treatment: 200 °C or lower

In a case where the cooling stop temperature exceeds 200 °C, transformation to martensite phase will not occur uniformly in the steel plate, and a desirable strength and toughness cannot be obtained. Further, C concentrates in austenite and tends to remain as retained austenite. Therefore, the cooling stop temperature is set to be 200 °C or lower.
After performing the dual phase heat treatment and cooling until reaching 200 °C or lower, tempering is conducted in the manner previously described. That is, the steel plate is heated to a temperature range of 500 °C to 650 °C at a heating rate of 0.05 °C/s to 1.0 °C/s, and then subjected to tempering by holding the temperature at the same temperature range for 10 minutes or more and 60 minutes or less.

EXAMPLES

Next, Examples of the present invention will be described.

Molten steels with the chemical compositions shown in table 1 were obtained by steelmaking in a vacuum melting furnace and made into small-sized steel ingots (150 kg). These steels were heated in the conditions shown in table 2, subjected to hot rolling until reaching a plate thickness of 7 mm to 50 mm, and then subjected to quenching just after the rolling. Some of the steel plates were then subjected to tempering treatment. Regarding the rest of the steel plates, after quenching, they were subjected to dual phase heat treatment and then to tempering treatment. The obtained steel plates were each subjected to a tensile test, a Charpy impact test, a measurement of austenite volume fraction, and a measurement of grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more, in the manner described below.
Table 1

Table 1

Steel No.	Chemical Composition (mass%)																Remarks
Steel No.	C	Si	Mn	P	S	Al	Ni	Cr	Mo	Cu	V	Nb	Ti	B	Ca	REM	Remarks
A	0.06	0.06	1.21	0.005	0.0011	0.035	5.7	-	-	-	-	-	-	-	-	-	Inventive Example
B	0.07	0.09	0.95	0.010	0.0009	0.033	7.2	-	-	-	-	-	-	-	-	-	Inventive Example
C	0.05	0.04	0.67	0.003	0.0012	0.029	7.8	-	-	0.12	-	-	-	-	-	-	Inventive Example
D	0.09	0.03	1.06	0.009	0.0010	0.028	6.9	0.12	-	-	0.043	-	-	-	0.0023	-	Inventive Example
E	0.03	0.05	0.88	0.004	0.0012	0.033	7.4	0.72	-	-	-	-	-	-	-	-	Inventive Example
F	0.02	0.06	1.36	0.008	0.0011	0.036	7.6	-	0.03	-	-	-	-	-	-	-	Inventive Example
G	0.05	0.08	0.63	0.006	0.0008	0.024	6.8	-	0.41	-	-	0.014	-	-	-	-	Inventive Example
H	0.04	0.07	0.97	0.011	0.0008	0.031	7.3	-	-	0.23	-	-	0.015	0.0012	-	0.0018	Inventive Example
I	0.06	0.05	1.02	0.005	0.0009	0.030	4.9	-	-	-	-	-	-	-	-	-	Comparative Example

The underlined values are outside the scope of the invention.

[Tensile test]

From each steel plate, at a position of a half the plate thickness, and in the rolling direction, a tensile test specimen having a parallel portion length of 30 mm, GL of 24 mm, a parallel portion diameter of 6 ϕ was collected and subjected to a tensile test at room temperature. From the obtained stress-strain curve, tensile strength (TS) and yield strength (YS) were calculated. TS of 690 MPa or more and YS of 590 MPa or more are each considered as excellent TS and YS.

[Charpy Impact Test]

From each steel plate, at a position of a half the plate thickness, and in a direction orthogonal to the rolling direction, V-notch test specimens were collected in accordance with JIS Z2202 (1998) standard, and subjected to a Charpy impact test with 3 specimens per each temperature for each steel plate in accordance with JIS Z2242 (1998) standard, and absorbed energy at -196 °C was measured to evaluate base material toughness. Steel plates with an average value of absorbed energy (vE_-196) of 3 specimens of 150 J or more are considered as having excellent base material toughness.

[Austenite Volume Fraction]

Samples collected from each steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction were subjected to sub-zero treatment for 10 minutes in liquid nitrogen, and then the austenite volume fraction was measured by X-ray diffraction.

[Measurement of Grain Size of Crystal Grains]

Samples collected from each steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction were polished and mirror finished, and subjected to EBSP analysis. Among the obtained data, a high-angle grain boundary where the orientation difference between two crystal grains contacting the grain boundary is 15° or more was selected and the average grain size by equivalent circle diameter of the region surrounded by the high-angle grain boundary was obtained.

The obtained results are shown in Table 2.

As shown in table 2, it has been confirmed that the inventive examples have excellent low-temperature toughness whereas the comparative examples outside the scope of the present invention have reduced low-temperature toughness.