WO2012133872A1 - Thick steel sheet having superior fatigue resistance properties in sheet thickness direction, method for producing same, and fillet welded joint using said thick steel sheet - Google Patents

Abstract

Provided are: a thick steel sheet that has superior fatigue resistance properties in the direction of sheet thickness and that is favorable for a welded steel structure of a pressure vessel or the like; a method for producing the thick steel sheet; and a fillet welded joint that uses the thick steel sheet. Specifically, the thick steel sheet has an aggregate structure having an x-ray intensity ratio of at least 2.0 in the (110) plane parallel to the sheet surface in at least the range from the position 2 mm in the direction of sheet thickness from one or both of the rolled surfaces of the steel sheet to the 3/10 position of the sheet thickness, and has a structure having an average value for the compressive remaining stress in the direction of sheet thickness of at least 160 MPa, having an x-ray intensity ratio in the (100) plane parallel to the sheet surface of no greater than 1.1, and containing: C, Si, and Mn; Ti and/or Nb; and, as needed, one or at least two of Cu, Ni, Cr, Mo, V, W, Zr, B, and Al, the remainder comprising Fe and unavoidable impurities.

Description

Thick steel plate with excellent fatigue resistance in the thickness direction, manufacturing method thereof, fillet welded joint using the thick steel plate

The present invention is a plate suitable for a welded steel structure such as a ship, a marine structure, a bridge, a structure, a pressure vessel, or the like. The present invention relates to a steel plate excellent in fatigue resistance in the thickness direction, a manufacturing method thereof, and a fillet welded joint using the steel plate.

Steel plates used for welded steel structures such as ships, offshore structures, bridges, buildings, pressure vessels, etc. are mechanical properties such as strength and toughness, and weldability. Of course, it is of course excellent in steady cyclic load during operation (steady cyclic load) and unsteady cyclic load caused by vibration such as wind and earthquake. However, it is required to have a characteristic that can ensure the structural safety of the structure. Particularly in recent years, steel sheets are strongly required to have excellent fatigue resistance.

In welded steel structures, there are many stress concentration parts at the weld toe, etc., but stress tends to concentrate at the weld toe part, and tensile residual stress also acts, so repeated loads were applied. In some cases, fatigue cracks are likely to occur from the weld toe, and the weld toe is often the source of fatigue cracks.

Measures such as improvement of the shape of the toe and introduction of compressive residual stress are known to prevent the occurrence of such fatigue cracks. However, since there are a large number of weld toes in a welded steel structure, it takes a lot of labor and time to implement the above-described measures for preventing the occurrence of fatigue cracks for each weld toe. , Increase in the number of construction steps and increase in construction cost.

Therefore, it is considered to improve the fatigue resistance characteristics of the welded steel structure by improving the fatigue resistance characteristics of the steel sheet to be used instead of measures for preventing the occurrence of such fatigue cracks. By improving the fatigue resistance of the steel sheet itself, the growth of fatigue cracks is suppressed, and the fatigue life of the welded steel structure can be extended.

In response to such a request, for example, in Patent Document 1, a striped second phase extending in the steel plate rolling direction has a microstructure in which an area ratio of 5 to 50% is scattered in the matrix phase. , hardness of the second phase (hardness) _{H V} is less than 30% higher than the hardness _{H V} of the mother phase, excellent steel fatigue crack growth properties (fatigue crack propagation properties) have been proposed.

The technique described in Patent Document 1 disperses a high-hardness second phase in the matrix phase, and when a fatigue crack reaches the vicinity of the hard second phase, the propagation of cracks is significantly delayed, and the steel plate It is intended to improve fatigue crack propagation resistance, and the aspect ratio of the second phase is preferably 4 or more. It is described that if such a steel plate is used for a large structure in which fatigue cracks are generated from the surface and propagated, no special consideration is required and a high fatigue crack propagation preventing property can be imparted to the large structure. ing.

Also, among the welded joints, fillet welded joints such as corner welding, cross welding, cover plate welding, stud welding, stud welding, etc. ) Fatigue strength is known to be the lowest, and especially in the fillet welded joints of heavy steel plates (heavy gauge steel) applied to recent large container vessels, etc. It is regarded as an urgent issue. In the case of fillet welded joints, fatigue cracks generated from the weld toes propagate in the plate thickness direction, so using a steel plate with excellent fatigue resistance in the plate thickness direction improves the fatigue resistance of the joint. It is effective for.

Further, in Patent Document 2, by mass, C: 0.015 to 0.20%, Si: 0.05 to 2.0%, Mn: 0.1 to 2.0%, P: 0.05 %, S: 0.02% or less, consisting of the balance Fe and inevitable impurities, and a (200) diffraction intensity ratio (diffracted intensity ratio) measured by X-ray of 2.0 to 15. Fatigue crack growth rate in the plate thickness direction with a recovery ferrite grain (recovery ferritic grain) or recrystallized ferrite grain (recrystallized ferritic grain) area ratio (area ratio) of 15 to 40% A thick steel plate with a low rate is described.

JP-A-7-90478 JP-A-8-199286

However, in the technique described in Patent Document 1, in order to lower the fatigue crack propagation rate and significantly delay the propagation of fatigue cracks, the hardness of the second phase can be increased compared to the parent phase and dispersed in a large amount. Necessary, there arises a problem that the ductility and toughness of the steel sheet are significantly reduced. Although reduction of the ductility and toughness of the steel sheet may be prevented by the inclusion of a large amount of alloy elements, the problem of incurring an increase in material costs is inevitable.

In the technique described in Patent Document 2, the (200) diffraction intensity ratio in the plate thickness direction is set to 2.0 or more, that is, a texture in which the (100) plane is aligned parallel to the plate surface is developed. The fatigue crack propagation in the plate thickness direction is suppressed by causing various slip systems to act at the fatigue crack tip, thereby causing interference between dislocations and suppressing crack propagation. The speed is low. However, the (100) plane is a cleavage plane, and the thick steel plate with the (100) plane aligned in parallel to the plate surface has a problem that the toughness in the plate thickness direction deteriorates.

Furthermore, the techniques described in Patent Documents 1 and 2 reduce the fatigue crack propagation rate, but do not significantly increase the total fatigue life.

As described above, the thick steel plates with excellent fatigue resistance described in Patent Documents 1 and 2 have room to be improved in terms of cost and performance for use in welded structures, while manufacturing fillet welded joints. However, a welding method for improving fatigue resistance as a joint is not disclosed.

An object of the present invention is to advantageously solve such problems of the prior art, and to provide a thick steel plate excellent in fatigue resistance in the thickness direction and suitable for a welded steel structure and a method for producing the same.

Another object of the present invention is to provide a fillet welded joint which is a fillet joint using a thick steel plate having excellent fatigue resistance in the thickness direction and which has excellent fatigue resistance.

In order to improve the fatigue characteristics without accompanying a decrease in toughness in the thickness direction, the present inventors have made extensive studies focusing on the texture and obtained the following knowledge.
(1) In order to improve the fatigue characteristics, the (110) plane is parallel to the plate surface in the range from the position of 2 mm in the plate thickness direction to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. It is effective to use an organization that has been developed (sometimes referred to as (110) texture).
(2) In order to suppress a reduction in toughness in the plate thickness direction, it is effective to have a structure in which the development of the (100) plane is suppressed in parallel with the plate surface in the above range.
(3) In order to improve fatigue properties without accompanying toughness reduction in the thickness direction, it is effective to introduce residual stress in the thickness direction and make the average value as small as possible (to the compression side). is there.
(4) The texture having the characteristics (1) and (2) described above is for hot rolling from a position 2 mm in the thickness direction from both sides or one side of the rolling surface of the steel plate to a 3/10 position of the thickness. In the temperature range where the range is a two-phase temperature range, rolling is performed so that the average reduction rate of one pass is less than 3.5%, so that the cumulative reduction rate is 50% or more. Residual stress is introduced by adjusting the cooling rate of accelerated cooling after two-phase rolling or hot rolling with a cumulative rolling reduction of 50% or more.

Also, (5) restricting the welding heat input and the number of laminations when producing fillet welded joints is effective in improving the fatigue strength of fillet welds.

The present invention is intended for a steel sheet having a thickness of 50 mm or more, and “excellent in fatigue resistance” means that a three-point bending fatigue specimen having a dimension and shape shown in FIG. 1 is used. The fatigue test is performed under the condition that the stress ratio (= minimum load / maximum load) is 0.1, the fatigue life in the plate thickness direction is obtained, and the stress range (stress range) is obtained. ) The fatigue life at 340 MPa is 2 million times or more.

Further, the present invention is directed to a fillet welded joint of a thick steel plate having a thickness of 50 mm or more. When the plate thickness is less than 50 mm, the decrease in fatigue strength due to the plate thickness effect is not so significant, and it conforms to various fatigue design curves based on many past fatigue test databases (databases). Thus, fatigue resistance safety is ensured without using the present invention. “Excellent fatigue resistance” means that a fatigue test piece with a three-point bend fillet welded joint with the dimensions shown in FIG. The fatigue life in the plate thickness direction is obtained, and the fatigue life in the stress range of 340 MPa is assumed to be 250,000 times or more.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2 at least in the range from the position 2 mm in the plate thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. A thick steel plate having an excellent fatigue resistance in the plate thickness direction, having a texture of 0.0 or more and an average value of compressive residual stress in the plate thickness direction being 160 MPa or more.
(2) In the texture, the X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less, and excellent in fatigue resistance in the plate thickness direction according to (1) Thick steel plate.
(3) The thick steel plate contains, by mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, and Ti: 0.005. -0.050%, Nb: containing one or two selected from 0.001-0.1%, and having a composition comprising the balance Fe and inevitable impurities (1) Or the thick steel plate excellent in the fatigue resistance of the thickness direction as described in (2).
(4) In addition to the above composition, Cu: 2.0% or less; Ni: 2.0% or less; Cr: 0.6% or less; Mo: 0.6% or less; 2 or less, W: 0.5% or less, Zr: 0.5% or less, and B: 0.0050% or less, containing one or more selected from (3) A steel plate with excellent fatigue resistance in the thickness direction as described in 1.
(5) In addition to the above composition, the composition further contains Al: 0.1% or less in terms of mass%. Excellent thick steel plate.
(6) When the steel material having the composition according to any one of (3) to (5) is heated and hot-rolled to obtain a thick steel plate, the hot-rolling is performed at an austenite partial recrystallization temperature ( the first rolling at a cumulative reduction ratio of 10% or more in a temperature range of more than or equal to the ultimate partial recrystallization temperature), and 3/10 of the plate thickness from the position of 2 mm from both sides or one side of the rolled surface of the thick steel plate. In the temperature range where the range corresponding to the position is a two-phase structure, the average rolling reduction of each pass is less than 3.5%, and the cumulative rolling reduction is 50% or more. After completion of hot rolling at 600 ° C. or higher, accelerated cooling at a cooling rate of 1 ° C./s or higher is applied to cool to 400 ° C. or lower. A method for producing a thick steel plate having excellent fatigue resistance in the thickness direction.
(7) A fillet portion of a thick steel plate having a thickness of 50 mm or more and excellent in fatigue resistance in the plate thickness direction has a heat input of 30 kJ / cm or less, 3 layers or less (3 layers or less), 6 passes (6 passes or less) A fillet welded joint with excellent fatigue strength, characterized by welding in the following lamination.
(8) The steel plate having a thickness of 50 mm or more is parallel to the plate surface at least in the range from the position of 2 mm in the plate thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate ( 110) The fillet welded joint having excellent fatigue strength according to (7), wherein the X-ray intensity ratio of the surface is 2.0 or more.
(9) The fatigue according to (8), wherein the structure of the thick steel plate having a thickness of 50 mm or more has an X-ray intensity ratio of (100) plane parallel to the plate surface of 1.1 or less. Fillet welded joint with excellent strength.
(10) The fillet weld having excellent fatigue strength according to (8) or (9), wherein an average value of compressive residual stress in the thickness direction of the thick steel plate having a thickness of 50 mm or more is 160 MPa or more. Fittings.

According to the present invention, a thick steel plate having a thickness of 50 mm or more that has excellent fatigue resistance in the thickness direction can be easily and inexpensively manufactured without impairing ductility and toughness, and has a remarkable industrial effect.
In addition, according to the present invention, the fatigue properties of the fillet welded portion of the thick steel plate having a thickness of 50 mm or more in which the fatigue strength is a particular problem can be easily obtained using a steel plate having ductility and toughness as a welded structure. In addition, it can be improved at a low cost and has a remarkable industrial effect.

The schematic diagram explaining the dimension shape of the 3 point | piece bending test piece used for a fatigue test. The schematic diagram explaining the generation | occurrence | production state of the slip in the fatigue crack tip which progresses in a sheet thickness direction cross section. Explanatory drawing which shows typically the dimension shape of the 3 point bending fillet welded joint fatigue test piece with a notch used for a fatigue test. The figure explaining the welding conditions of a fillet welded joint.

Hereinafter, the structure, the thickness direction compressive residual stress, the preferred component composition, and the production conditions defined in the present invention will be described.
[Organization]
The steel plate according to the present invention has an X-ray intensity of (110) plane parallel to the plate surface at least in the range from 2 mm in the plate thickness direction to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. It has a texture with a ratio of 2.0 or more.

In order to suppress the progress (propagation) of fatigue cracks that propagate in the plate thickness direction (the crack surface is the plate thickness surface), a structure in which the (110) surface is inclined by 90 ° from the crack surface (plate thickness surface), that is, a plate A structure ((110) texture) in which the (110) planes are accumulated parallel to the plane is set, and the X-ray intensity ratio is set to 2.0 or more.

FIG. 2 is a schematic diagram for explaining the occurrence of slip at the tip of a developing fatigue crack in the cross section in the thickness direction. Generally, fatigue cracks cause irreversible slip at the crack tip at a crack angle of about 45 ° from the crack surface where the shear stress is maximum due to the action of repeated stress, which accumulates and propagates (crack tip). In the slip system (slip direction in the slip plane) where the shear stress is highest due to the relationship between the stress field and crystal orientation, slip deformation (slip deformation) occurs and the crack progresses.

Accordingly, when the (110) plane, which is the principal slip plane of a steel plate having a body-centered cubic (bcc) structure (body-centered cubic structure), is inclined by 90 ° from the crack plane, the shear stress is reduced. Slip on a surface inclined by about 45 ° from the maximum crack surface is suppressed.

Further, if the X-ray intensity ratio of the (110) plane parallel to the plate surface is less than 2.0, the effect of reducing the fatigue crack propagation rate and improving the fatigue characteristics in the plate thickness direction cannot be obtained sufficiently. 0 or more. The X-ray intensity ratio of the (110) plane parallel to the plate surface is based on the X-ray intensity from the (110) plane parallel to the plate surface in a steel plate having a random orientation. The ratio of the X-ray intensity from the (110) plane parallel to the plate surface. The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more means that the (110) plane parallel to the plate surface is 2.0 times or more compared to a steel plate having a random crystal orientation. It means that it is highly accumulated and forms a (110) texture.

The steel plate according to the present invention has a texture in which the X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more, at least 2 mm in the plate thickness direction from both sides or one side of the rolled surface of the steel plate. Prepare for the range from the position to the 3/10 position of the plate thickness.

Fatigue cracks propagating in the plate thickness direction are generated from a stress concentration area near the surface of the steel sheet, for example, a welded part such as a member attached to the surface. In this case, the texture given by the welding heat for attaching the member or the like disappears.

On the other hand, the fatigue crack that has progressed to the center of the plate thickness has a large crack, the stress intensity factor at the crack tip is large, and the fatigue crack growth amount per cycle of the repeated load (fatigue crack growth). It becomes large, and the effect of reducing the fatigue crack propagation rate due to the presence of the (110) texture is hardly obtained.

Therefore, the texture is formed in a range from a position of 2 mm in the thickness direction from at least one side or one side of the rolling surface of the steel plate to a 3/10 position of the thickness. However, the effect of the present invention is not impaired even if the entire steel sheet is made into a (110) texture, and the thick steel sheet according to the present invention does not prevent the entire texture in the thickness direction from being the texture.

In the body-centered cubic (bcc) structure steel plate, the (100) plane is a cleavage plane, and the presence of the (100) plane parallel to the plate surface reduces the toughness in the plate thickness direction, and the (100) plane becomes the plate surface. When it develops in parallel, the formation of (110) texture is inhibited, so at least in the range from 2 mm in the thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the plate. The X-ray intensity ratio of the parallel (100) plane is 1.1 or less, preferably reduced as much as possible. The X-ray intensity ratio of the (100) plane parallel to the plate surface is based on the X-ray intensity from the (100) plane parallel to the plate surface in a steel plate having a random orientation, The ratio of X-ray intensity from (100) planes existing in parallel. The X-ray intensity ratio of the (100) plane parallel to the plate surface is 1.1 or less means that the accumulation of the (100) plane parallel to the plate surface is 1.1 times or less compared to the steel plate having a random orientation. It means that (100) texture is hardly formed.

[Compressive residual stress in the thickness direction]
The compressive residual stress in the plate thickness direction is effective for suppressing toughness reduction in the plate thickness direction and reducing the fatigue crack propagation rate in the plate thickness direction. However, if it is less than 160 MPa, the excellent fatigue resistance characteristics described above cannot be obtained. , 160 MPa or more. The average value of compressive residual stress in the plate thickness direction is obtained by measuring the residual stress in the plate thickness direction (crack propagation direction) at a pitch of 4 mm in the plate thickness direction by X-ray measurement (minus). The absolute value of the average value).

In order to combine the strength and toughness (tensile strength TS: 490 MPa or more, absorbed energy at −40 ° C. (absorbed energy): 200 J or more) as a steel plate according to the present invention for a welded steel structure, The preferred component composition and production conditions are as follows.
[Component Composition] In the description, “%” is “mass%”.

C: 0.03-0.15%
C is an element having an effect of increasing the strength of steel, and in order to ensure a desired high strength, it is preferably contained in an amount of 0.03% or more. The toughness of the welded heat-affected zone is reduced. For this reason, C is preferably limited to a range of 0.03 to 0.15%.

Si: 0.60% or less Si is an element that acts as a deoxidizing agent and has the effect of increasing the strength of the steel by solid solution. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 0.60% lowers the toughness of the weld heat affected zone. For this reason, it is preferable to limit Si to 0.60% or less. In addition, More preferably, it is 0.50% or less.

Mn: 0.80 to 1.80%
Mn is an element that has the effect of increasing the strength of steel. In order to ensure the desired high strength, Mn is preferably contained in an amount of 0.80% or more, but if contained in excess of 1.80%, There is concern about a reduction in material toughness. For this reason, Mn is preferably limited to a range of 0.80 to 1.80%. More preferably, it is 0.9 to 1.60%.

One or two selected from Ti: 0.005 to 0.050% and Nb: 0.001 to 0.1% Ti and Nb increase the strength through precipitation strengthening. At the same time, it is an element that suppresses the growth of austenite grains during heating and contributes to the refinement of the steel sheet structure. In the present invention, it contains one or two elements.

Ti forms carbides and nitrides, contributes to the refinement of austenite grains during steel plate production, suppresses the coarsening of crystal grains in the weld heat affected zone, Improve toughness. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, the content exceeding 0.050% reduces toughness. For this reason, Ti is preferably limited to a range of 0.005 to 0.050%. More preferably, it is 0.005 to 0.02%.

Nb, like Ti, increases the strength through precipitation strengthening, further refines the structure, suppresses recrystallization of austenite, and promotes the effect of rolling to form the desired structure. Have. In order to obtain such an effect, the content is preferably 0.001% or more. However, if the content exceeds 0.1%, the structure becomes needle-like and the toughness tends to decrease. For this reason, Nb is preferably limited to a range of 0.001 to 0.1%. More preferably, it is 0.02 to 0.05%.

When further improving the characteristics, in addition to the above basic components, one or more of Cu, Ni, Cr, Mo, V, W, Zr, B, and Al can be contained.

Cu: 2.0% or less, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, V: 0.2% or less, W: 0.5% or less, Zr: 0.5% or less, B: One or more of 0.0050% or less Cu, Ni, Cr, Mo, V, W, Zr, B are elements that improve the strength and toughness of steel, and are desired It contains 1 type or 2 types or more depending on the characteristics.

Cu contributes to steel strength increase mainly through precipitation strengthening. In order to obtain such an effect, it is desirable to contain 0.05% or more. However, if it exceeds 2.0%, precipitation strengthening is excessive and toughness decreases. For this reason, when it contains, it is preferable to limit Cu to 2.0% or less. In addition, More preferably, it is 0.35% or less.

Ni increases the strength of steel and contributes to improved toughness. Ni acts effectively to prevent cracking during hot rolling with Cu. In order to acquire such an effect, it is desirable to contain 0.05% or more. However, even if it is contained in a large amount exceeding 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected and it is economically disadvantageous, and Ni is an expensive element. Invite the soaring. For this reason, when it contains, it is preferable to limit Ni to 2.0% or less. In addition, More preferably, it is 0.1% or more.

Cr increases the amount of pearlite and contributes to an increase in steel strength. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.6% reduces the toughness of a welded part. For this reason, when it contains, it is preferable to limit Cr to 0.6% or less. More preferably, the content is 0.01 to 0.2%.

Mo contributes to increasing the strength of steel. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.6% reduces the toughness of a welded part. For this reason, when it contains, it is preferable to limit Mo to 0.6% or less. More preferably, the content is 0.01 to 0.08%.

V contributes to increasing the strength of steel through solid solution strength and precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.05% or more, but inclusion exceeding 0.2% significantly reduces the base metal toughness and weldability. For this reason, it is preferable to limit V to 0.2% or less. More preferably, it is 0.05 to 0.1%.

W contributes to an increase in steel strength, especially at high temperatures. In order to acquire such an effect, it is desirable to contain 0.1% or more, but if it contains more than 0.5%, the toughness of the welded portion is lowered. In addition, a large amount of expensive W causes a material cost to rise. For this reason, when contained, W is preferably limited to 0.5% or less. More preferably, it is 0.2 to 0.4%.

Zr contributes to increasing the strength of the steel and improves the resistance to plating cracking in the galvanized material. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.5% lowers the toughness of the weld. For this reason, when it contains, it is preferable to limit to 0.5% or less. More preferably, the content is 0.01 to 0.1%.

B contributes to increasing the strength of the steel through improving hardenability, and precipitates as BN during rolling, contributing to refinement of ferrite grains after rolling. In order to acquire such an effect, it is desirable to contain 0.0010% or more, but inclusion exceeding 0.0050% deteriorates toughness. For this reason, when contained, B is preferably limited to 0.0050% or less. More preferably, the content is 0.0010 to 0.0035%.

Al: 0.1% or less Al acts as a deoxidizer and contributes to the refinement of crystal grains, and in order to obtain such an effect, it is desirable to contain 0.015% or more, An excessive content exceeding 0.1% leads to a decrease in toughness. For this reason, when it contained, Al was limited to 0.1% or less. In addition, Preferably it is 0.08% or less.

The balance other than the above components is Fe and inevitable impurities, and P: 0.035% or less, S: 0.035% or less, and N: 0.012% or less are acceptable.

[Production conditions]
A method for producing a steel material such as a slab is not particularly limited. Molten steel having the above composition is melted using a conventional melting furnace such as a converter furnace, and used as a steel material such as a slab by a conventional method such as continuous casting. , Heated to a temperature of 900 to 1350 ° C.

When the heating temperature is less than 900 ° C., desired hot rolling becomes difficult. On the other hand, at a heating temperature exceeding 1350 ° C., surface oxidation becomes significant, and coarsening of crystal grains becomes remarkable. For this reason, it is preferable to limit the heating temperature of the steel material to a temperature in the range of 900 to 1350 ° C. More preferably, it is 1150 ° C. or less from the viewpoint of improving toughness.

¡Hot rolling on the heated steel material. The hot rolling includes the first rolling and the second rolling, and the first rolling is a temperature range above the austenite partial recrystallization temperature (in the case of the above component composition, the temperature range above the austenite partial recrystallization temperature is The cumulative rolling reduction is 10% or more at a surface temperature of 1000 to 850 ° C. Since the austenite grains are at least partially recrystallized, the steel sheet structure can be made fine and uniform. In order to at least partially recrystallize the austenite grains, it is preferable that the cumulative rolling reduction is 10% or more. When the rolling temperature range is the austenite non-recrystallization temperature range, it becomes impossible to expect uniform crystal grains. The upper limit of the cumulative rolling reduction is preferably 30% from the viewpoint of securing the rolling reduction of the second rolling.

After the first rolling described above, the range from the position of 2 mm in the plate thickness direction to the 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate is the temperature range where the two-phase structure is formed, and the average of each pass Second rolling is performed at a rolling reduction of less than 3.5%, a cumulative rolling reduction of 50% or more, and a rolling end temperature of 600 ° C. or more.

In the second rolling, the average reduction ratio of each pass is determined by introducing shear strain in the range from the position of 2 mm in the sheet thickness direction to the position of 3/10 of the sheet thickness from both sides or one side of the rolled surface of the sheet. 2. When the rate is 50% or more and the rolling end temperature is 600 ° C. or more, an (110) texture whose X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more is formed. Less than 5%.
If the cumulative rolling reduction is less than 50%, the X-ray intensity ratio of the (110) plane parallel to the plate surface cannot be made 2.0 or more.

In the case of the above composition range, in the temperature range of 900 to 600 ° C., the range from 2 mm position to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate to the plate thickness direction is substantially two-phase. Become an organization. The rolling end temperature is a surface temperature of 600 ° C. or higher.

When the rolling end temperature is less than 600 ° C. at the surface temperature, excessive work strain is introduced into the ferrite and the toughness is lowered, so that it is 600 ° C. or higher, preferably 850 to 600 ° C.

The thick steel plate produced by the above manufacturing method has at least (100) plane X-rays parallel to the plate surface in a range from a position 2 mm in the plate thickness direction to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. The strength ratio is 1.1 or less, and toughness deterioration in the thickness direction is suppressed.

In hot rolling, a steel plate with a thickness of 50 mm or more is used. When the plate thickness is less than 50 mm, at the time of hot rolling, the development of (110) texture occurs at least in the range from 2 mm position to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate in the plate thickness direction. It is difficult to introduce an effective shear strain in the case. Furthermore, if the plate thickness is less than 50 mm, there is a concern that the steel plate buckling performance may be lowered due to the introduction of the plate thickness direction compressive residual stress. From the above, a thick steel plate having a thickness of 50 mm or more is obtained. In addition to the first rolling and the second rolling, the hot rolling may be performed as long as the effects of the rolling are not impaired.

After the second rolling, accelerated cooling is performed at a cooling rate of 1 ° C./s or more, and cooling to 400 ° C. or less. If the cooling stop temperature exceeds 400 ° C. at a cooling rate of less than 1 ° C./s, it is difficult to set the average value of compressive residual stress in the plate thickness direction to 160 MPa or more. Is 400 ° C. or lower. More preferably, it is cooled to 350 ° C. or lower at a cooling rate of 5 ° C./s or higher.

In the present invention, welding heat input (kJ / cm) and a lamination method are defined as welding conditions for a fillet joint of a thick steel plate having excellent fatigue resistance in the thickness direction. Welding heat input (sometimes simply referred to as heat input) is 30 kJ / cm or less. When fillet welding is performed at a heat input exceeding 30 kJ / cm, the structure of the steel sheet or the form of internal residual stress changes due to the thermal effect of the welding, which adversely affects the fatigue characteristics of the steel sheet with excellent fatigue resistance in the thickness direction. Therefore, it is set to 30 kJ / cm or less.

Moreover, even if the welding heat input is 30 kJ / cm or less, if a fillet welded joint is produced with a laminate exceeding three layers and six passes, the compressive residual stress at the weld toe portion becomes high, and the effect of improving fatigue characteristics cannot be obtained. The lamination is 3 layers or less and 6 passes or less. The welding method is not specified. Hand welding, MIG welding (metal inert gas welding), CO ₂ welding (carbon dioxide welding), etc. can be applied.

The steel material having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a thick steel plate having a thickness of 50 to 80 mm. These thick steel plates were subjected to a structure observation, a tensile test, a toughness test, and a fatigue crack propagation test.

(1) Microstructure observation
From the 1/4 position of the thickness of the obtained thick steel plate (representative of the range of 2 mm to 3/10 position of the plate thickness from the surface to the thickness direction), the specimen for structure observation (size: parallel to the plate surface) (Thickness 1.5 mm × width 25 mm × length 30 mm) was collected, and the X-ray diffraction intensities of the (110) plane and the (100) plane parallel to the plate surface were determined by the X-ray diffraction method. The ratio between the obtained X-ray intensity and the X-ray diffraction intensity of the (110) plane and (100) plane of the random tissue standard sample is determined by the (110) plane X-rays parallel to the plate surface. The intensity ratio was the X-ray intensity ratio of the (100) plane parallel to the plate surface.
(2) Residual stress measurement From the obtained thick steel plate, a test piece for measuring the residual stress by X-ray (size: plate thickness (the original thickness of the steel plate) x 12.5 mm x 300 mm [plate thickness direction dimension x rolling perpendicular direction] Dimension × rolling direction dimension]), and after electropolishing the measurement surface [surface of 12.5 mm × 300 mm] [perpendicular direction dimension × rolling direction dimension], X-rays at 4 mm pitch in the plate thickness direction The thickness direction residual stress was measured. Among the measured residual stresses, the values on the compression side (minus side) were averaged, and the absolute value was taken as the average value of the compressive residual stress in the plate thickness direction.

(3) Tensile test JIS No. 4 tensile specimen (diameter of parallel part) was obtained from the obtained thick steel plate in accordance with the provisions of JIS Z 2201 (1998) so that the tensile direction was perpendicular to the rolling direction of the steel plate. : 14 mm). The sampling position of the test piece was set to 1/4 position of the plate thickness (representing the range of 2 mm to 3/10 position of the plate thickness from the surface to the plate thickness direction). The tensile test is performed in accordance with JIS Z 2241 (1998). YS: Yield strength σ _YS or 0.2% yield strength σ _0.2 , TS: Tensile strength σ _TS , elongation El, static tensile The tensile properties at the time were evaluated.

(4) Toughness test V-notch test specimens were collected from the obtained thick steel sheets in accordance with the provisions of JIS Z 2242 (2005) so that the longitudinal direction was parallel to the rolling direction, and absorption at −40 ° C. Energy was determined and toughness was evaluated. The V-notch test piece was taken from a 1/4 position of the plate thickness (representing a range from 2 mm to 3/10 position of the plate thickness in the plate thickness direction from the surface).

(5) Fatigue test From the obtained thick steel plate, a specimen for fatigue test (size: plate thickness (the original thickness of the steel plate) x 12.5 mm x 300 ~) so that the propagation direction of fatigue cracks is the plate thickness direction. 350 mm [size in the plate thickness direction × size in the direction perpendicular to the rolling × size in the rolling direction]) was collected. The test piece is a notched three-point bending fatigue test piece having the dimensions shown in FIG. 1, and the thickness of the plate is 50 to 65 mm in order to make the bending span 4 times the plate thickness during the fatigue test. In this case, when the dimension in the rolling direction was 300 mm and the plate thickness was 80 mm, the dimension in the rolling direction was 350 mm. In the fatigue test, a fatigue test was performed under the conditions that the stress range was 340 MPa and the stress ratio R (= minimum load / maximum load) was 0.1, and the fatigue characteristics (fatigue life) in the thickness direction were obtained.

The case where the obtained fatigue life was 2 million times or more was evaluated as “Excellent in fatigue resistance in the thickness direction”, and “X” in other cases. The notch of the test piece is a machined notch having a width of 0.1 mm.

Examples of the present invention (Nos. 4, 7, 9, 11, 14, 17) are all 1/4 positions of the plate thickness (representative of a range of 2 mm to 3/10 positions of the plate thickness from the surface to the plate thickness direction) The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more, the average value of the compressive residual stress in the plate thickness direction is 160 MPa or more, and the X-ray intensity ratio of the (100) plane parallel to the plate surface Is 1.1 or less, and there is no decrease in toughness in the plate thickness direction, and the steel plate is excellent in fatigue resistance in the plate thickness direction.

On the other hand, the comparative examples (No. 1, 2, 3, 5, 6, 8, 10, 12, 13, 15, 16) outside the scope of the present invention are (110) plane X-ray intensity parallel to the plate surface. The ratio is less than 2.0 or the average value of compressive residual stress in the plate thickness direction is less than 160 MPa, and the fatigue resistance in the plate thickness direction is inferior.

A fillet welded joint was prepared using the thick steel plate 1 with excellent fatigue properties in the thickness direction with a thickness of 50 to 80 mm whose chemical composition is shown in Table 3 and the manufacturing conditions and properties shown in Table 4, and the shape is shown in FIG. A three-point bending fatigue test was conducted using a notched three-point bending fillet welded joint fatigue test piece. The test method for confirming the structure, mechanical properties and thickness direction fatigue properties of the thick steel plate 1 was carried out in the same manner as in Example 1.

Using the thick steel plate 1 whose characteristics were confirmed by the test described above, a fillet welded joint was produced under the conditions shown in FIG. 4, and a fatigue test was performed. As a fatigue test piece, a notched three-point bending fillet welded joint fatigue test piece having the dimensions shown in FIG. 3 is used, and the stress range is 340 MPa and the stress ratio R (= minimum load / maximum load) is 0.1. The fatigue life was obtained. The results obtained with the thick steel plate 1 are shown in Table 5.

In the thick steel plate 1, all of the inventive examples (test Nos. 3, 4, and 6) obtained a fillet welded joint having excellent fatigue resistance with a fatigue life of 250,000 times or more under severe conditions of a stress range of 340 MPa. It was confirmed that On the other hand, the comparative example (test No. 1 and 2) which deviates from the range of the welding conditions (heat input 30 kJ / cm or less, 3 layers and 6 passes or less) specified in the present invention, and the fatigue life in the plate thickness direction are inferior. The comparative example (test No. 5) using a thick steel plate does not ensure fatigue resistance.

Claims (10)

The X-ray intensity ratio of the (110) plane parallel to the plate surface is 2.0 or more at least in the range from 2 mm in the plate thickness direction to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate. A thick steel plate having a texture that becomes and an average value of compressive residual stress in the plate thickness direction is 160 MPa or more. The thick steel plate according to claim 1, wherein an X-ray intensity ratio of a (100) plane parallel to the plate surface in the texture is 1.1 or less. The thick steel plate contains, by mass%, C: 0.03 to 0.15%, Si: 0.60% or less, Mn: 0.80 to 1.80%, and Ti: 0.005 to 0.00. The thick steel plate according to claim 1 or 2, comprising one or two selected from 050% and Nb: 0.001 to 0.1%, and having a composition comprising the balance Fe and inevitable impurities. In addition to the above-mentioned composition, further, by mass, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, V: 0.2% or less W: 0.5% or less, Zr: 0.5% or less, B: Thick steel plate according to claim 3, comprising one or more selected from 0.0050% or less. . The thick steel plate according to claim 3 or 4, wherein, in addition to the composition, the composition further contains, by mass%, Al: 0.1% or less. When the steel material having the composition according to any one of claims 3 to 5 is heated and hot-rolled to obtain a thick steel plate, the hot-rolling is accumulated in a temperature range equal to or higher than the austenite partial recrystallization temperature. Reduction ratio: 10% or more of the first rolling, and a range corresponding to a position from 2 mm to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the thick steel plate to the 3/10 position of the plate thickness In the temperature range, the second rolling in which the average rolling reduction of each pass is less than 3.5% and the cumulative rolling reduction: 50% or more, after completion of hot rolling at a steel sheet surface temperature of 600 ° C. or more, A method for producing a thick steel plate, which is subjected to accelerated cooling at a cooling rate of 1 ° C./s or more and cooled to 400 ° C. or less. A fillet welded joint in which a fillet portion of a thick steel plate with excellent fatigue resistance in the thickness direction with a thickness of 50 mm or more is welded with a heat input of 30 kJ / cm or less, 3 layers or less and 6 passes or less. The steel plate having a thickness of 50 mm or more is at least a (110) plane parallel to the plate surface in a range from a position of 2 mm to 3/10 position of the plate thickness from both sides or one side of the rolled surface of the steel plate in the plate thickness direction. The fillet welded joint according to claim 7, which has a portion where the X-ray intensity ratio is 2.0 or more. The fillet welded joint according to claim 8, wherein the structure of the thick steel plate having a thickness of 50 mm or more has an X-ray intensity ratio of a (100) plane parallel to the plate surface of 1.1 or less. The fillet welded joint according to claim 8 or 9, wherein an average value of compressive residual stress in the thickness direction of the thick steel plate having a thickness of 50 mm or more is 160 MPa or more.

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