阅读说明：本技术 结构用高强度厚钢板 (High-strength thick steel plate for structure ) 是由 崎本隆洋 半田恒久 伊木聪 于 2016-09-15 设计创作，主要内容包括：本发明提供板厚为70mm以上且脆性裂纹传播停止性能优良的结构用高强度厚钢板。本发明的高强度厚钢板由具备具有高的脆性裂纹传播停止性能的内部区域和在其两外侧脆性裂纹传播停止性能低的区域的三层构成。板厚t为70mm以上,板厚方向的1/6t、1/3t、1/2t各位置处的脆性裂纹传播停止性能指标Y(＝vTrs-12×I<Sub>(100)</Sub>-22×I<Sub>(211)</Sub>)满足Y<Sub>1/3t</Sub>≤0.9Y<Sub>1/2t</Sub>和Y<Sub>1/6t</Sub>≥0.8Y<Sub>1/2t</Sub>。需要说明的是,vTrs为V缺口夏比冲击试验的断口转变温度(℃),I<Sub>(100)</Sub>为与轧制面(板面)平行的(100)面的X射线衍射强度比,I<Sub>(211)</Sub>为与轧制面平行的(211)面的X射线衍射强度比。(The invention provides a structural high-strength thick steel plate with a plate thickness of 70mm or more and excellent brittle crack propagation stopping performance. The high-strength thick steel sheet of the present invention is composed of three layers including an inner region having a high brittle crack propagation stopping property and regions having a low brittle crack propagation stopping property on both outer sides thereof. A sheet thickness t of 70mm or more, and a brittle crack propagation stopping performance index Y (═ vTrs-12 xI) at 1/6t, 1/3t and 1/2t positions in the sheet thickness direction (100) ‑22×I (211) ) Satisfy Y 1/3t ≤0.9Y 1/2t And Y 1/6t ≥0.8Y 1/2t . In addition, vTrs is the fracture transition temperature (. degree. C.) of the V-notch Charpy impact test, I (100) Is the X-ray diffraction intensity ratio of a (100) plane parallel to the rolled plane (plate surface), I (211) The X-ray diffraction intensity ratio of the (211) plane parallel to the rolled surface was obtained.)

1. A high-strength thick steel plate for construction, characterized in that,

has a composition containing C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, N: 0.0050% or less and the balance of Fe and inevitable impurities,

a plate thickness t of 70mm or more, and a brittle crack propagation stopping performance index Y at a position 1/2t of the plate thickness defined by the following expression (a)_1/2t(DEG C) and a brittle crack propagation stopping performance index Y at a sheet thickness of 1/3t defined by the following formula (b)_1/3tA brittle crack propagation stopping performance index Y at a sheet thickness of 1/6t defined by the following formula (c)_1/6tThe (. degree. C.) satisfies the following formulae (1) and (2),

Y_1/3t≤0.9Y_1/2t…(1)

Y_1/6t≥0.8Y_1/2t…(2)

Y_1/2t＝(vTrs)_1/2t-12×{I₍₁₀₀₎}_1/2t-22×{I₍₂₁₁₎}_1/2t‥(a)

Y_1/3t＝(vTrs)_1/3t-12×{I₍₁₀₀₎}_1/3t-22×{I₍₂₁₁₎}_1/3t‥(b)

Y_1/6t＝(vTrs)_1/6t-12×{I₍₁₀₀₎}_1/6t-22×{I₍₂₁₁₎}_1/6t‥(c)

wherein, (vTrs)_1/2t、(vTrs)_1/3t、(vTrs)_1/6tThe fracture transition temperature (. degree. C.) of the V-notch Charpy impact test at each position of the sheet thickness,

{I₍₁₀₀₎}_1/2t、{I₍₁₀₀₎}_1/3t、{I₍₁₀₀₎}_1/6tis X-ray diffraction of (100) plane parallel to the plate surface at each position of the plate thicknessThe ratio of the intensity to the intensity of the light,

{I₍₂₁₁₎}_1/2t、{I₍₂₁₁₎}_1/3t、{I₍₂₁₁₎}_1/6tthe X-ray diffraction intensity ratio of the (211) plane parallel to the plate surface at each position of the plate thickness.

2. A high-strength thick steel plate for construction according to claim 1, wherein the plate thickness is 100mm or less.

3. A thick, high-strength steel plate for structural use according to claim 1 or 2, wherein said brittle crack propagation stopping performance index Y_1/3tThe temperature is below-150 ℃.

4. The structural high-strength thick steel sheet according to claim 1 or 2, wherein the steel sheet has a structure in which a bainite phase is mainly contained at a volume fraction of 80% or more, and the second phase is composed of one or two or more selected from ferrite, pearlite, and martensite at a total volume fraction of 20% or less (including 0%).

5. A structural high-strength thick steel sheet according to claim 3, wherein the steel sheet has a structure mainly composed of a bainite phase at a volume fraction of 80% or more, and the second phase is composed of one or two or more selected from ferrite, pearlite, and martensite at a total volume fraction of 20% or less (including 0%).

6. The structural high-strength thick steel plate as set forth in claim 1 or 2, wherein the composition further comprises at least one selected from the following (a) and (B):

(A) selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% of one or more than two;

(B) selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

7. A high-strength thick steel plate for construction according to claim 3, wherein said composition further comprises at least one selected from the group consisting of the following (a) and (B):

(A) selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% of one or more than two;

(B) selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

8. The high-strength thick steel plate for construction according to claim 4, wherein the composition further comprises at least one selected from the following (A) and (B):

(A) selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% of one or more than two;

(B) selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

9. A high-strength thick steel plate for construction according to claim 5, wherein said composition further comprises at least one selected from the group consisting of the following (A) and (B):

(A) selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% of one or more than two;

(B) selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

Technical Field

The present invention relates to a structural thick steel sheet suitable for use in large steel structures such as ships, marine structures, cryogenic storage tanks, and building/civil engineering structures, and a method for producing the same, and particularly relates to improvement in brittle crack propagation stopping performance of a thick steel sheet having a sheet thickness of 70mm or more.

Background

In large steel structures such as ships, marine structures, cryogenic storage tanks, and building/civil engineering structures, if an accident such as extensive damage or damage associated with brittle fracture occurs, the economic efficiency and the environment are greatly affected. Therefore, in particular, in the case of large steel structures, it is generally required to improve the safety of the structures from the viewpoint of preventing brittle fracture.

In recent years, large-sized containers such as 6000 to 20000 TEUs are being built or planned. With the increase in the size of such ships, steel sheets used as hull plates have been thickened and strengthened, and have begun to be used in thicknesses of 50 to 100mm and yield strengths of 390N/mm²Grade, 470N/mm²High-strength steel plate. The TEU (Twenty-foot Equivalent Unit) represents the number of containers converted into a length of 20 feet, and represents an index of the carrying capacity of the container ship.

Steel materials such as thick steel plates used for large steel structures such as ships are required to have excellent low-temperature toughness and an excellent brittle crack propagation arrest toughness value at the use temperature from the viewpoint of ensuring the safety of the structures. In particular, even if a brittle crack should occur, it is necessary to stop the propagation of the brittle crack before the brittle crack is broken in a large scale, and therefore, the brittle crack propagation stop toughness value (hereinafter, also referred to as "crack arrest performance") of a steel material such as a thick steel plate to be used is an important characteristic.

Under such circumstances, various steel materials or large welded structures having improved "crack arrest performance" have been developed and manufactured.

For example, patent document 1 describes a structural high-strength thick steel sheet having excellent brittle crack propagation stop characteristics. In the technique described in patent document 1, a composition containing, by mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.0%, Al: 0.005-0.08%, or further contains Ti: 0.005-0.03%, Nb: 0.005-0.05%, Cu: 0.01 to 0.5%, Ni: 0.01-1.0%, Cr: 0.01 to 0.5%, Mo: 0.01-0.5%, V: 0.001-0.1%, B: 0.003% or less, Ca: less than 0.005 percent,REM: heating one or two or more kinds of steel materials of 0.01% or less to 900-1200 deg.C, and Ar is performed at the central part of the plate thickness₃A cumulative reduction ratio of 30% or more at a temperature not lower than the above point, and a temperature Ar at the central part of the sheet thickness₃Below and Ar₃The method comprises rolling at a temperature range of-60 ℃ or higher with a cumulative reduction of 30% or more, and then cooling at a cooling rate of 2 ℃/sec or higher to 600 ℃ or lower, thereby producing a thick steel sheet having a texture in which the (100) plane X-ray intensity ratio in the rolled surface at the central portion of the sheet thickness is 2.0 or more and the (110) plane X-ray intensity ratio in the rolled surface at the portion of 1/4 sheet thickness is 1.5 or more. When such a thick steel plate is applied to the flange portion of the T-joint, the brittle crack that has started to develop from the web portion can be stopped at the flange portion. In the technique described in patent document 1, a specific texture is developed so as to increase the X-ray intensity ratio of a specific plane parallel to the rolling plane at each position of 1/2 parts of the sheet thickness and 1/4 parts of the sheet thickness, and the propagation direction of a brittle crack is changed, thereby improving crack arrest properties.

Patent document 2 discloses a thick steel sheet having a thickness of 50mm or more and excellent in brittle crack propagation stopping characteristics, and a method for manufacturing the same. In the technique described in patent document 2, a composition containing, in mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80-1.80%, S: 0.001 to 0.05%, and contains a metal selected from the group consisting of Ti: 0.005-0.050% or Nb: 0.001 to 0.1% of at least one kind selected from the group consisting of Cu: 2.0% or less, V: 0.2% or less, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, W: 0.5% or less, B: 0.0050% or less, Zr: one or two or more kinds of steel materials of 0.5% or less are heated to 900 to 1300 ℃, rolled at a cumulative reduction of 10% or more in a temperature range of 1000 to 850 ℃ in terms of surface temperature, brought into a state where the surface temperature is 900 to 600 ℃ and the internal temperature is 50 to 150 ℃ higher than the surface temperature, and then hot rolled at a reduction of 7% or more per pass, a cumulative reduction of 50% or more and a rolling completion temperature of 800 to 500 ℃ in terms of surface temperature. After the hot rolling is completed, the steel sheet may be cooled to 400 ℃ at a cooling rate of 5 ℃/sec or more. Thereby, the following thick steel plate can be obtained: the X-ray intensity ratio of the (211) plane or the (100) plane in the rolling plane of the part in the region of at least 20% of the plate thickness at the plate thickness central part is more than 1.5, the X-ray intensity ratio of the (211) plane or the (100) plane in the rolling plane of the region of 1/4-1/10 as the plate thickness or the region of 3/4-9/10 as the plate thickness is more than 1.3, the tip shape of the brittle crack propagation stop portion in the cross section in the plate thickness direction of the fracture at the time of performing the broad ESSO test is formed as a recessed portion in which the stopped crack length in the region of 20% of the width of the plate thickness center portion is recessed at least by shortening the plate thickness length in the traveling direction of the brittle crack by the maximum crack length in the region of 1/4-1/10 of the plate thickness or 3/4-9/10 of the plate thickness from the surface of the steel plate, and the propagation of the broad brittle crack which has been difficult in the past in the thick steel plate can be stopped without stress reflection. In the technique described in patent document 2, the X-ray intensity ratio of a specific surface parallel to the rolling surface is adjusted so as to increase in a region of at least 20% of the plate thickness at 1/2 parts of the plate thickness and in regions of 1/4 to 1/10 or 3/4 to 9/10 of the plate thickness, thereby improving the brittle fracture propagation stopping property.

Patent document 3 describes a method for producing a structural high-strength thick steel sheet having a thickness of 50mm or more and excellent brittle crack propagation stopping properties. In the technique described in patent document 3, a composition containing, by mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.2%, Al: 0.005-0.08%, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005-0.03%, or further comprises at least one element selected from the group consisting of Nb: 0.005-0.05%, Cu: 0.01 to 0.5%, Ni: 0.01-1.0%, Cr: 0.01 to 0.5%, Mo: 0.01-0.5%, V: 0.001-0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: the method comprises heating at least one steel material of 0.010% or less to 900-1150 ℃, rolling the steel material so that the total of the cumulative reduction ratios in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, the cumulative reduction ratio is 20% or more in a state where the central portion of the plate thickness is in the austenite recrystallization temperature range, and the average reduction ratio per pass is 5.0% or less, then rolling the steel material so that the cumulative reduction ratio is 40% or more and the average reduction ratio per pass is 7.0% or more in a state where the central portion of the plate thickness is in the austenite non-recrystallization temperature range, and then cooling the steel material to 600 ℃ or less at an accelerated cooling rate of 4.0 ℃/sec or more. Thereby, a high-strength thick steel sheet for a structure excellent in brittle crack propagation stop characteristics can be obtained as follows: has a texture in which the microstructure is a ferrite main body, the degree of integration of RD// (110) planes in the surface layer portion is 1.3 or more, and the degree of integration of RD// (110) planes in the central portion of the sheet thickness is 1.8 or more, and has a Charpy fracture transformation temperature in the surface layer portion of-60 ℃ or less and a Charpy fracture transformation temperature in the central portion of the sheet thickness of-50 ℃ or less. In the technique described in patent document 3, a specific texture is developed at the surface layer portion and the plate thickness center portion to improve the brittle crack propagation stopping property of the thick steel plate. In patent document 3, "degree of integration of RD// (110) plane" means a value obtained by integrating values of three-dimensional crystal orientation density functions of orientations of the (110) plane parallel to the Rolling Direction (RD) to obtain an integrated value and dividing the integrated value by the number of the integrated orientations.

Patent document 4 describes a method for producing a structural high-strength thick steel sheet having a thickness of 50mm or more and excellent brittle crack propagation stopping properties. In the technique described in patent document 4, a composition containing, by mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.5%, Al: 0.005-0.08%, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005 to 0.03%, or further contains an Nb: 0.005-0.05%, Cu: 0.01 to 0.5%, Ni: 0.01-1.0%, Cr: 0.01 to 0.5%, Mo: 0.01-0.5%, V: 0.001-0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: the method comprises heating at least one steel material of 0.010% or less to a temperature of 1000-1200 ℃, carrying out rolling in which the total of the cumulative reduction ratios in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, the cumulative reduction ratio in the state where the plate thickness central portion is in the austenite recrystallization temperature range is 20% or more, the cumulative reduction ratio in the state where the plate thickness central portion is in the austenite non-recrystallization temperature range is 40% or more, and the difference between the rolling temperatures of the first pass and the final pass in the rolling in the state where the plate thickness central portion is in the austenite non-recrystallization temperature range is 40 ℃ or less, and then cooling at a cooling rate of 4 ℃/sec or more to 450 ℃. Thus, a high-strength steel plate for a structural object having a texture in which the microstructure is mainly bainite and the degree of integration of RD// (110) planes in the surface layer portion is 1.5 or more, and having excellent brittle crack propagation stopping properties in which the Charpy fracture transition temperature in the surface layer portion and the plate thickness center portion is-40 ℃ or less can be obtained. In the technique described in patent document 4, a specific texture is developed in the surface layer portion and the plate thickness central portion, and the brittle crack propagation stop property of the thick steel plate is improved. In patent document 4, "degree of integration of RD// (110) plane" means a value obtained by integrating values of three-dimensional crystal orientation density functions of orientations of the (110) plane parallel to the Rolling Direction (RD) to obtain an integrated value and dividing the integrated value by the number of the integrated orientations.

Patent document 5 describes "a high-strength thick steel plate having excellent crack arrestability". The steel plate described in patent document 5 is a high-strength steel plate as follows: has a composition containing C: 0.04-0.16%, Si: 0.01-0.5%, Mn: 0.75 to 2.5%, Al: 0.001 to 0.1%, Nb: 0.003 to 0.05%, Ti: 0.005-0.05%, N: 0.001 to 0.008% of a composition in which P, S, Cu, Ni, Mo, V, B, Ca, Mg, REM are limited to specific values or less, the balance is Fe and unavoidable impurities, the carbon equivalent Ceq is 0.30 to 0.50%, the composition has a microstructure containing 70% or less of ferrite and 30% or more of bainite in terms of area ratio, and the total length per unit area of grain boundaries having a crystal misorientation of 15 DEG or more, that is, the grain boundary density is 400 to 1000mm/mm at 1/4 parts of the sheet thickness²The area ratio of a (100) plane having an angle of 15 DEG or less with respect to a plane perpendicular to the main rolling direction is 10 to 40%, and the grain boundary density is 300 to 900mm/mm at 1/2 parts of the plate thickness²The area ratio of a (110) plane having an angle of 15 DEG or less with respect to a plane perpendicular to the main rolling direction is 40-70%. In the technique described in patent document 5, it is preferable that the above-mentioned method be used as a production methodThe steel sheet is charged into a heating furnace having an atmospheric temperature of 1000 to 1250 ℃, rough rolling is performed at a plate thickness center temperature of 850 ℃ and at 1150 ℃ or lower for 4 to 15 passes with a reduction rate per pass of 3 to 30%, a reduction rate per pass within 3 passes of less than 3%, and a cumulative reduction rate of 15 to 70%, finish rolling is performed at 750 to 850 ℃ for 4 to 15 passes with a plate thickness center temperature, an average value of shape ratios of 0.5 to 1, and a cumulative reduction rate of 40 to 80%, and then, the steel sheet is cooled to 550 ℃ or lower at a plate thickness center cooling rate of 2 to 10 ℃/sec from 700 ℃ or higher. In the technique described in patent document 5, propagation of a brittle crack is suppressed by changing a developed texture at 1/2 portions of the plate thickness and 1/4 portions of the plate thickness.

Disclosure of Invention

Problems to be solved by the invention

In the techniques described in patent documents 1 to 5, the propagation of brittle cracks is suppressed by developing the texture at a specific position in the thickness direction, and the brittle crack propagation stopping characteristics in the thick steel plate are improved. However, the techniques described in patent documents 1 to 5 still have the following problems: in the case of a high-strength thick steel sheet having a thickness of 70mm or more, it has not been achieved to maintain sufficiently excellent brittle crack propagation stopping characteristics such that a broad brittle crack can be stopped even under the most severe conditions in the actual structure, such as stress-free reflection in which a crack enters through a complete penetration weld.

In view of the problems of the prior art, the present invention has an object to provide a brittle crack propagation material having a plate thickness of 70mm or moreA high-strength thick steel sheet excellent in stopping performance and a method for producing the same. The term "high strength" as used herein means a yield strength of 400N/mm²The above is the case. In addition, the "brittle crack propagation stopping property" referred to herein means a hull design temperature: brittle crack propagation arrest toughness value Kca at-10 deg.C_-10℃Is 9500N/mm^3/2The above is the case.

Means for solving the problems

In order to achieve the above object, the present inventors have made intensive studies on the relationship between the distribution state of the brittle crack propagation stopping performance at each position in the plate thickness direction and the brittle crack propagation stopping performance of the entire thick steel plate, focusing on the difference in the brittle crack propagation stopping performance at each position in the plate thickness direction of the thick steel plate. As a result, it was found that a thick steel plate having a plate thickness of 70mm or more had an appropriate distribution of brittle crack propagation stopping performance at each position in the plate thickness direction, which significantly improved the brittle crack propagation stopping performance of the entire thick steel plate.

Namely, it was found that: in order to improve the brittle crack propagation stopping performance of the entire thick steel plate, it is first necessary to produce a thick steel plate having three layers including an inner region (plate thickness center region) having a high brittle crack propagation stopping performance and regions relatively lower in brittle crack propagation stopping performance than the inner region on both outer sides. For such a thick steel plate, it was found that: when a brittle crack develops (propagates), a level difference occurs in the vicinity of the boundary between the center of the thickness of the steel sheet and the portions on both outer sides thereof, and the crack tip develops in the form of a brittle crack branching into three layers.

Therefore, the present inventors have conceived that the brittle crack propagation stopping performance at each position in the plate thickness direction can be easily evaluated by using an index Y (° c) defined by the following formula.

Y＝vTrs-12×I₍₁₀₀₎-22×I₍₂₁₁₎…(A)

Wherein, vTrs: fracture transition temperature (DEG C) of V-notch Charpy impact test,

I₍₁₀₀₎: x-ray diffraction intensity ratio of (100) plane parallel to rolled plane (plate surface),

I₍₂₁₁₎: x-ray diffraction intensity ratio of the (211) plane parallel to the rolled surface (plate surface).

The index Y is a parameter introduced by a part of the present inventors in order to define the fracture transition temperature (vTrs) required for improving the brittle crack propagation arrest toughness, based on the fracture transition temperature (vTrs) in the charpy impact test, in consideration of the degree of development of texture that affects the improvement in the brittle crack propagation arrest performance. Good texture development for improving brittle crack propagation stopping performance, I₍₁₀₀₎、I₍₂₁₁₎When the temperature increases, the index Y becomes low.

The present inventors have newly found that, using this index Y as an index of brittle crack propagation stopping performance at each position in the sheet thickness direction: in particular, the brittle crack propagation stopping performance index Y at the 1/6 position of the sheet thickness is adjusted_1/6tBrittle crack propagation stopping performance index Y at 1/3 point of sheet thickness_1/3tAnd a brittle crack propagation stopping performance index Y at the 1/2 position of the sheet thickness_1/2tA thick steel plate having a remarkably improved brittle crack propagation arrest toughness value Kca throughout the thickness thereof is formed while satisfying the following expressions (1) and (2).

Y_1/3t≤0.9Y_1/2t…(1)

Y_1/6t≥0.8Y_1/2t…(2)

The expression (1) indicates that the internal region (plate thickness central region) having a high brittle crack propagation stopping performance exists at least in a region between two positions 1/3 of the plate thickness. The expression (2) indicates that regions having relatively low brittle crack propagation stopping performance are present on both outer sides of the inner region. Here, Y is_1/2t、Y_1/3tAnd Y_1/6tIs less than zero.

First, the experimental results that are the basis of the present invention will be explained.

In order to change the characteristics at each position in the thickness direction, various thick steel sheets having a thickness of 70mm or more were produced by variously changing the composition, production conditions, and the like. From each of the obtained thick steel plates, an ESSO test piece (dimension: t X500 mm) was cut out over the entire thickness, and a reduced ESSO test piece (dimension: 20X 500mm) having a plate thickness of 20mm was cut out with each position in the plate thickness direction as the center.

Then, using these test pieces, a temperature gradient-type ESSO test was performed in accordance with appendix a of brittle crack arrest design guidelines (japan maritime association, 2009, 9 months) to determine the relationship between the crack arrest temperature and the brittle crack propagation arrest toughness value Kca across the entire thickness of each thick steel sheet and at each position in the sheet thickness direction.

From the results obtained, an example of a thick steel plate having a plate thickness of 85mm is shown in FIG. 1.

The thick steel plate showing the brittle crack propagation stopping performance in fig. 1 is a thick steel plate which is composed of three layers and shows a distribution of the brittle crack propagation stopping performance at each position in the plate thickness direction, and shows a high brittle crack propagation stopping toughness value Kca at the 1/2 position of the plate thickness and at the 1/3 position of the plate thickness in the inner region of the plate thickness, and shows a lower brittle crack propagation stopping toughness value Kca at the 1/6 position of the plate thickness on both outer sides thereof.

The brittle crack propagation arrest toughness value Kca at each position in the sheet thickness direction was obtained using a test piece having a sheet thickness of 20mm, and in order to eliminate the influence of the sheet thickness, the sheet thickness effect coefficient f (t) defined by the following formula was used in accordance with the Japan welding society Standard WES 3003,

f(t)＝1-0.05(t-30)；t≤35mm

＝54/65-3t/1300；35mm≤t≤100mm，

kca obtained from a test piece having a plate thickness of 20mm (hereinafter, Kca)_t＝20mm) Kca converted into 85mm thick plate by the following formula_t＝85mmShown in fig. 1.

Kca_t＝85mm＝Kca_t＝20mm×f(85)/f(20)。

According to fig. 1, the brittle crack propagation arrest toughness value Kca of the entire thickness of such a thick steel plate showing the distribution of the brittle crack propagation arrest performance at each position in the plate thickness direction shows a value significantly higher than the brittle crack propagation arrest toughness value at each position in the plate thickness direction.

In this thick steel sheet, V-notch charpy impact test pieces were cut from each position in the sheet thickness direction (1/6 position in the sheet thickness, 1/3 position in the sheet thickness, 1/2 position in the sheet thickness) according to the regulations of JIS Z2242 so that the test piece length direction was the rolling direction, and the fracture transition temperature vTrs was obtained at each position in the sheet thickness direction. Further, test pieces were cut out so that the positions in the thickness direction of the thick steel sheet (1/6 position of the thickness, 1/3 position of the thickness, 1/2 position of the thickness) were measured, and the X-ray diffraction intensity ratio of the (100) plane and the (211) plane parallel to the rolled plane (plate surface) was determined by the X-ray diffraction method. From these values, the index Y (Y) at each position in the sheet thickness direction is calculated using the above expression (A)_1/6t、Y_1/3t、Y_1/2t) When, Y_1/6t＝-129℃、Y_1/3t＝-168℃、Y_1/2t-170 ℃ and satisfies the formulae (1) and (2).

Namely, it was found that: in order to satisfy the above expressions (1) and (2), it is important for a thick steel plate having a thickness of 70mm or more to have a brittle crack propagation arrest performance with a high brittle crack propagation arrest performance in the plate thickness direction, in which the distribution state of the brittle crack propagation arrest performance at each position in the plate thickness direction is adjusted so as to form three layers having a high brittle crack propagation arrest performance of 1/3 thickness with the plate thickness center as the center and a total plate thickness of 1/3 thickness with a brittle crack arrest performance relatively lower than that of the inner region, and having different brittle crack propagation arrest performances in the plate thickness direction.

The present invention has been completed based on the above findings and through further studies. That is, the gist of the present invention is as follows.

[1]A high-strength thick steel plate for structural use, characterized in that the plate thickness t is 70mm or more, and the brittle crack propagation stopping performance index Y at the position of 1/2t is defined by the following expression (a)_1/2t(DEG C) and a brittle crack propagation stopping performance index Y at a sheet thickness of 1/3t defined by the following formula (b)_1/3tA brittle crack propagation stopping performance index Y at a sheet thickness of 1/6t defined by the following formula (c)_1/6t(° c) satisfies the following formulae (1) and (2).

Y_1/3t≤0.9Y_1/2t…(1)

Y_1/6t≥0.8Y_1/2t…(2)

Y_1/2t＝(vTrs)_1/2t-12×{I₍₁₀₀₎}_1/2t-22×{I₍₂₁₁₎}_1/2t‥(a)

Y_1/3t＝(vTrs)_1/3t-12×{I₍₁₀₀₎}_1/3t-22×{I₍₂₁₁₎}_1/3t‥(b)

Y_1/6t＝(vTrs)_1/6t-12×{I₍₁₀₀₎}_1/6t-22×{I₍₂₁₁₎}_1/6t‥(c)

Wherein, (vTrs)_1/2t、(vTrs)_1/3t、(vTrs)_1/6tThe fracture transition temperature (. degree. C.) of the V-notch Charpy impact test at each position of the sheet thickness,

{I₍₁₀₀₎}_1/2t、{I₍₁₀₀₎}_1/3t、{I₍₁₀₀₎}_1/6tthe X-ray diffraction intensity ratio of a (100) plane parallel to the plate surface at each position of the plate thickness,

{I₍₂₁₁₎}_1/2t、{I₍₂₁₁₎}_1/3t、{I₍₂₁₁₎}_1/6tthe X-ray diffraction intensity ratio of the (211) plane parallel to the plate surface at each position of the plate thickness.

[2] The high-strength thick steel sheet for structural use according to the above [1], wherein the sheet thickness is 100mm or less.

[3]As described above [1]Or [2]]The structural high-strength thick steel plate, wherein the brittle crack propagation stopping performance index Y_1/3tThe temperature is below-150 ℃.

[4] The structural high-strength thick steel sheet according to any one of the above [1] to [3], comprising:

contains, in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, N: 0.0050% or less and the balance of Fe and inevitable impurities; and

the bainite phase is mainly composed of a bainite phase at a volume ratio of 80% or more, and the second phase is composed of one or two or more kinds selected from ferrite phase, pearlite, and martensite at a total volume ratio of 20% or less (including 0%).

[5] The thick, high-strength steel sheet for structural use according to [4], wherein the composition further contains, in mass%, a metal selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002 to 0.003% of one or more than two.

[6] The thick steel plate for structural use according to [4] or [5], wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

[7] A method for producing a high-strength thick steel plate for structural use, wherein a steel material is subjected to a heating step and a hot rolling step to produce a thick steel plate having a plate thickness t of 70mm or more,

the steel material is characterized by containing, in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, N: 0.0050% or less, with the balance being Fe and unavoidable impurities,

the heating step is set to a step of heating the steel material to a heating temperature of 900 to 1200 ℃,

the hot rolling step is set as follows: performing primary rolling with a cumulative reduction of 9% or less in a temperature range of 1000 to 850 ℃ in terms of surface temperature, subsequently performing secondary rolling with a reduction of 7% or more per pass, a cumulative reduction of 55% or more and a rolling completion temperature of 800 to 550 ℃ in terms of surface temperature in a temperature range of 900 to 600 ℃ in terms of surface temperature, subsequently cooling to a cooling stop temperature of 450 to 400 ℃ at a cooling rate of 5 ℃/sec or more on average in a temperature range of 790 to 540 ℃ in terms of surface temperature,

a thick steel plate is produced in which the brittle crack propagation stopping performance index Y at each position in the plate thickness direction satisfies the following expressions (1) and (2).

Y_1/3t≤0.9Y_1/2t…(1)

Y_1/6t≥0.8Y_1/2t…(2)

Wherein, Y_1/2t＝(vTrs)_1/2t-12×{I₍₁₀₀₎}_1/2t-22×{I₍₂₁₁₎}_1/2t‥(a)

Y_1/3t＝(vTrs)_1/3t-12×{I₍₁₀₀₎}_1/3t-22×{I₍₂₁₁₎}_1/3t‥(b)

Y_1/6t＝(vTrs)_1/6t-12×{I₍₁₀₀₎}_1/6t-22×{I₍₂₁₁₎}_1/6t‥(c)

Note that (vTrs)_1/2t、(vTrs)_1/3t、(vTrs)_1/6tThe fracture transition temperature (. degree. C.) in the V-notch Charpy test at each position of the sheet thickness,

{I₍₁₀₀₎}_1/2t、{I₍₁₀₀₎}_1/3t、{I₍₁₀₀₎}_1/6tthe X-ray diffraction intensity ratio of a (100) plane parallel to the plate surface at each position of the plate thickness,

{I₍₂₁₁₎}_1/2t、{I₍₂₁₁₎}_1/3t、{I₍₂₁₁₎}_1/6tthe X-ray diffraction intensity ratio of the (211) plane parallel to the plate surface at each position of the plate thickness.

[8] The method for producing a high-strength thick steel sheet for structural use according to item [7], wherein the composition further contains, in mass%, a component selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002 to 0.003% of one or more than two.

[9] The method for producing a high-strength thick steel sheet for structural use according to the above [7] or [8], wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two.

Effects of the invention

According to the present invention, it is possible to easily manufacture a steel sheet having a thickness of 70mm or more and a yield strength of 400N/mm²Above and hull design temperature: brittle crack propagation arrest toughness value Kca at-10 deg.C_-10℃Is 9500N/mm^3/2The high-strength thick steel sheet having excellent brittle crack propagation stopping performance exhibits industrially significant effects. Further, the high-strength thick steel plate according to the present invention is applied to a hatch coaming or a deck member in a strong deck portion structure of a large container ship or a bulk carrier, and has a very great effect of contributing to improvement of safety of a ship.

Drawings

Fig. 1 is a graph showing the results of the ESSO test by the relationship between the brittle crack propagation arrest toughness value Kca and the brittle crack propagation arrest temperature Tk.

Detailed Description

The high-strength thick steel sheet of the present invention has a high brittle crack propagation stopping performance in a central region of a thickness of 1/3 inclusive of a central position of the sheet thickness and a relatively low brittle crack propagation stopping performance in each outer region of 1/3 thickness of both outer sides of the sheet thickness in a cross section in the sheet thickness direction, and shows a distribution of the brittle crack propagation stopping performance in the sheet thickness direction of three layers. By making the regions on both outer sides of the plate thickness center portion exist which exhibit a different development of brittle cracks from the region of the plate thickness center portion, the development of brittle cracks differs in each layer, and as a result, the brittle crack propagation arrest toughness value of the entire thick steel plate is improved as compared with, for example, a uniform thick steel plate having the same brittle crack propagation arrest performance as that of the plate thickness center portion throughout the entire thickness.

As described above, the high-strength thick steel sheet of the present invention exhibits a distribution of brittle crack propagation stopping performance in the thickness direction of the three layers, that is, the brittle crack propagation stopping performance index Y at each position in the thickness direction satisfies the following expression (1) and expression (2).

Y_1/3t≤0.9Y_1/2t…(1)

Y_1/6t≥0.8Y_1/2t…(2)

Here, Y is_1/2tThe brittle crack propagation stopping performance index at the sheet thickness 1/2t is defined by the following equation (a).

Y_1/2t＝(vTrs)_1/2t-12×{I₍₁₀₀₎}_1/2t-22×{I₍₂₁₁₎}_1/2t‥(a)

(Here, (vTrs)_1/2t: fracture transition temperature (. degree. C.) in V-notch Charpy test at 1/2t thickness, { I₍₁₀₀₎}_1/2t: x-ray diffraction intensity ratio of (100) plane parallel to plate surface at position 1/2t of plate thickness, { I₍₂₁₁₎}_1/2t: x-ray diffraction intensity ratio of (211) plane parallel to the plate surface at position 1/2 t). In the high-strength thick steel sheet of the present invention, Y is preferable in order to ensure a high brittle crack propagation toughness value throughout the entire thickness_1/2tThe temperature is below-150 ℃.

In addition, Y_1/3tThe brittle crack propagation stopping performance index at the 1/3t plate thickness is defined by the following equation (b).

Y_1/3t＝(vTrs)_1/3t-12×{I₍₁₀₀₎}_1/3t-22×{I₍₂₁₁₎}_1/3t‥(b)

(Here, (vTrs)_1/3t: fracture transition temperature (. degree. C.) in V-notch Charpy test at 1/3t thickness, { I₍₁₀₀₎}_1/3t: x-ray diffraction intensity ratio of (100) plane parallel to plate surface at position 1/3t of plate thickness, { I₍₂₁₁₎}_1/3t: x-ray diffraction intensity ratio of (211) plane parallel to the plate surface at position 1/3 t).

In addition, Y_1/6tThe brittle crack propagation stopping performance index at the 1/6t plate thickness is defined by the following equation (c).

Y_1/6t＝(vTrs)_1/6t-12×{I₍₁₀₀₎}_1/6t-22×{I₍₂₁₁₎}_1/6t‥(c)

(Here, (vTrs)_1/6t: fracture transition of V notch Charpy test at 1/6t plate thickness positionTemperature (. degree. C.), { I₍₁₀₀₎}_1/6t: x-ray diffraction intensity ratio of (100) plane parallel to plate surface at position 1/6t of plate thickness, { I₍₂₁₁₎}_1/6t: x-ray diffraction intensity ratio of (211) plane parallel to the plate surface at position 1/6 t).

If the above expression (1) is not satisfied, the brittle crack propagation arrest performance at the 1/3-thick position is degraded, and the thickness of the central region in the sheet thickness having a high brittle crack propagation arrest performance is reduced, so that the brittle crack propagation arrest toughness value Kca cannot be secured at a desired overall thickness. The brittle crack propagation stopping performance Y at the sheet thickness 1/3t position_1/3tPreferably-150 ℃ or lower.

On the other hand, if the above expression (2) is not satisfied, the brittle crack propagation arrest performance at the 1/6 th site of the sheet thickness is excessively improved, the brittle crack progresses in the same manner as in the central portion region of the sheet thickness, and a brittle crack fracture that branches into three layers cannot be formed, so that the brittle crack propagation arrest toughness value Kca of the desired entire thickness cannot be secured.

In view of the above, the present invention is limited to the brittle crack propagation stopping performance Y at the sheet thickness 1/3t position_1/3tBrittle crack propagation stopping performance Y at 1/2t sheet thickness_1/2tAnd brittle crack propagation stopping performance Y at the sheet thickness 1/6t position_1/6tA thick steel plate satisfying the above expressions (1) and (2). Such a thick steel plate is a thick steel plate having a desired high brittle crack propagation arrest toughness value Kca throughout the thickness. In addition, in order to stably produce a thick steel plate exhibiting a brittle crack fracture in which the crack front branches into three layers and having a high value of brittle crack propagation toughness over the entire thickness, it is preferable to further satisfy Y_1/6t≤0.7Y_1/2t。

In addition, if the steel plate satisfies the above conditions, the composition and structure thereof are not particularly limited, and it is preferable to prepare a steel plate having: contains, in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005-0.05%, Ti: 0.005-0.03%, N: 0.0050% or less, or optionally containing Ni selected from: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003%, and/or one or more selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005 to 0.010% of one or two of the above components, with the balance being Fe and unavoidable impurities; and a high-strength thick steel sheet having a structure comprising a bainite phase of 80% or more by volume as a main phase and a ferrite phase of 20% or less (including 0%) by volume as a secondary phase.

Hereinafter, the reason for the limitation of the preferred composition will be described first. Hereinafter, the composition-related mass% is abbreviated as%.

C：0.03～0.20％

C is an element contributing to increase in strength, and is required to be contained by 0.03% or more in order to secure the desired strength of the steel plate of the present invention. On the other hand, if the content exceeds 0.20%, the toughness of the welding heat-affected zone decreases. Therefore, the C content is limited to the range of 0.03 to 0.20%. From the viewpoint of texture control, the amount of the surfactant is preferably 0.05 to 0.09%. More preferably 0.05 to 0.07%.

Si：0.03～0.5％

Si is an element that acts as a deoxidizer and contributes to an increase in strength by forming a solid solution. In order to obtain such an effect, it is necessary to contain 0.03% or more. On the other hand, a large content exceeding 0.5% may lower the toughness of the weld heat-affected zone. Therefore, the Si content is limited to the range of 0.03 to 0.5%. Preferably, the concentration is 0.14 to 0.28%. More preferably 0.14 to 0.17%.

Mn：0.5～2.5％

Mn is an element that contributes to an increase in strength by solid solution strengthening and an increase in hardenability, and also contributes to an increase in toughness and development of a phase change texture. In order to obtain such an effect, the content of the compound is required to be 0.5% or more. On the other hand, if the content exceeds 2.5%, the toughness of the base material may be lowered. Therefore, the Mn content is limited to the range of 0.5 to 2.5%. The content is preferably 1.5 to 2.4%. More preferably 1.8 to 2.0%.

P: less than 0.03%

P is an element which is present in steel as an impurity, segregates at grain boundaries or the like, and adversely affects the toughness of the base material, and is preferably reduced as much as possible. However, up to 0.03% may be allowed. Therefore, the P content is limited to 0.03% or less. It is preferably 0.006% or less. On the other hand, from the viewpoint of dephosphorization cost, the content is 0.0001% or more industrially.

S: less than 0.01%

S is an element which is present as a sulfide-based inclusion in steel and lowers hot workability, base metal toughness, base metal ductility, and the like, and is preferably reduced as much as possible. However, up to 0.01% may be allowed. Therefore, the S content is limited to 0.01% or less. It is preferably 0.003% or less. On the other hand, from the viewpoint of desulfurization cost, it is industrially 0.0001% or more.

Al：0.005～0.08％

Al functions as a deoxidizer, and also bonds with nitrogen to precipitate as AlN, thereby inhibiting coarsening of crystal grains. In order to obtain such an effect, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.08% and the content is large, the amount of oxide inclusions increases, and the cleanliness of the steel decreases. Therefore, the Al content is limited to the range of 0.005 to 0.08%. The content is preferably 0.02 to 0.04%.

Nb：0.005～0.05％

Nb contributes to an increase in strength by precipitation strengthening, has an action of suppressing recrystallization of austenite, facilitates working (rolling) in a non-recrystallization temperature range of austenite, and contributes to refinement of crystal grains. In order to obtain such an effect, the content of 0.005% or more is required. On the other hand, a large content exceeding 0.05% tends to result in excessive precipitate formation and a decrease in toughness. Therefore, the Nb content is limited to the range of 0.005 to 0.05%. The content is preferably 0.02 to 0.04%.

Ti：0.005～0.03％

Ti forms nitrides to suppress coarsening of austenite grains, contribute to refining of grains of the base material, and improve toughness of the base material, and also contribute to refining of the structure of the welding heat-affected zone, and improve toughness of the welding heat-affected zone. In order to obtain such an effect, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.03%, the toughness is lowered. Therefore, the Ti content is limited to the range of 0.005 to 0.03%. Preferably, the concentration is 0.008 to 0.015%.

N: 0.0050% or less

N is bonded to Ti, Nb, or the like and contributes to refinement of crystal grains in the form of nitrides, and contributes to improvement of toughness of the base material and toughness of the welding heat-affected zone. In order to obtain such an effect, the content of 0.002% or more is necessary, but if the content exceeds 0.0050%, toughness of the weld portion is lowered. Therefore, the N content is limited to 0.0050% or less.

The above-mentioned component is a basic component, but in the present invention, a component selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003%, and/or one or more selected from the group consisting of Ca: 0.0005 to 0.003%, REM: 0.0005-0.010% of one or two of the above-mentioned optional elements.

Selected from the group consisting of Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02 to 1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002 to 0.003% of one or more

Ni, Cu, Cr, Mo, V and B are elements for increasing the strength, and may be optionally contained in one kind or two or more kinds as required.

Ni is an element that increases strength by forming a solid solution, and also has an effect of improving toughness and preventing thermal cracking when Cu is contained. In order to obtain such an effect, it is necessary to contain 0.05% or more. On the other hand, a large content of more than 3% leads to a rise in material cost. Therefore, when Ni is contained, the Ni content is preferably limited to a range of 0.05 to 3%. More preferably, the concentration is 0.2 to 1%.

Cu is an element that contributes to increase in strength by forming a solid solution, and in order to obtain such an effect, it is necessary to contain 0.05% or more. On the other hand, if the content exceeds 1.5%, the strength excessively increases and the toughness decreases. Therefore, when Cu is contained, the Cu content is preferably limited to the range of 0.05 to 1.5%. More preferably, the concentration is 0.2 to 0.5%.

Cr is an element that contributes to an increase in strength by forming a solid solution, and in order to obtain such an effect, it is necessary to contain 0.02% or more. On the other hand, if the content exceeds 1.0% and the content is large, the toughness of the welding heat-affected zone may be lowered. Therefore, when Cr is contained, the Cr content is preferably limited to a range of 0.02 to 1.0%. More preferably, the concentration is 0.1 to 0.6%.

Mo forms a solid solution or further forms carbide, and has an effect of suppressing a decrease in strength. In order to obtain such an effect, the content of 0.005% or more is required. On the other hand, a large content exceeding 1.0% may decrease the toughness of the weld heat-affected zone. Therefore, when Mo is contained, the Mo content is preferably limited to the range of 0.005 to 1.0%. More preferably, the concentration is 0.005 to 0.01%.

V is an element that contributes to an increase in strength by making solid solution or further forming precipitates (carbides). In order to obtain such an effect, it is necessary to contain 0.002% or more. On the other hand, even if the content exceeds 0.10%, the effect is saturated, and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when V is contained, the content of V is preferably limited to the range of 0.002 to 0.10%. More preferably, the concentration is 0.002 to 0.02%.

B is an element which segregates at grain boundaries, and which is contained in a trace amount to improve hardenability and contribute to increase strength. In order to obtain such an effect, the content of 0.0002% or more is required. On the other hand, if the content exceeds 0.003% and the content is large, the toughness may be rather lowered. Therefore, when B is contained, the content of B is preferably limited to the range of 0.0002 to 0.003%. More preferably, the concentration is 0.0002 to 0.001%.

Is selected from Ca: 0.0005 to 0.003%, REM: 0.0005-0.010 wt% of one or two

Ca. REM is an element contributing to improvement of ductility and toughness by a morphology control action of sulfide-based inclusions. In order to obtain such an effect, Ca: more than 0.0005%, REM: more than 0.0005 percent. On the other hand, even if Ca: more than 0.003%, REM: if the content exceeds 0.010%, the effect is saturated, and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when one or both of Ca and REM are contained, it is preferable to limit Ca: 0.0005 to 0.003%, REM: 0.0005 to 0.010%.

The balance other than the above components is made up of Fe and unavoidable impurities. As inevitable impurities, As: 0.03% or less, Sb: 0.01% or less, Sn: 0.02% or less, Pb: 0.01% or less, Bi: less than 0.01 percent.

Next, the reason why the structure of the high-strength thick steel sheet of the present invention is limited will be described.

The high-strength thick steel sheet of the present invention has the above composition and a structure in which a bainite phase is 80% or more by volume percentage as a main phase and a second phase is 20% or less (including 0%) in total by volume percentage and is composed of one or more types selected from ferrite, pearlite, and martensite throughout the entire region in the thickness direction except for the surface layer of the steel sheet.

In order to maintain the yield strength of 400N/mm under the condition that the plate thickness is more than 70mm²The high-strength thick steel sheet of the present invention has the bainite phase as a main phase. When the main phase is a phase other than the bainite phase, it is difficult to achieve both of the above-described high strength and high toughness in a thick steel sheet having a sheet thickness of 70mm or more. The "main phase" as used herein means a phase that accounts for 80% or more by volume.

In the high-strength thick steel sheet of the present invention, the second phase other than the main phase is set to be one or two or more selected from ferrite phase, pearlite, and martensite in a total volume fraction of 20% or less (including 0%). When the volume fraction of the second phase exceeds 20% and becomes large, the desired high strength cannot be secured. Therefore, the second phase is defined as one or two or more selected from ferrite phase, pearlite, and martensite in a total volume ratio of 20% or less (including 0%). The second phase includes 0% by volume. That is, it may be 100% bainite phase.

Next, a preferred method for producing the high-strength thick steel sheet of the present invention will be described.

In the present invention, the steel material having the above composition is subjected to a heating step and a hot rolling step to produce a thick steel sheet having a thickness of 70mm or more. The method for producing the steel material is not particularly limited, and from the viewpoint of productivity, the following method is preferred: the molten steel having the above composition is melted in a conventional melting furnace such as a converter, and is formed into a cast slab by a conventional casting method such as a continuous casting method, thereby forming a raw steel material. Needless to say, the steel sheet may be produced by ingot-cogging rolling to produce a steel material.

The obtained steel material is subjected to a heating step for hot rolling. In the heating step, the steel material is heated to a temperature of 900 to 1200 ℃.

Heating temperature: 900 to 1200 DEG C

When the heating temperature is less than 900 ℃, the thermal deformation resistance becomes too high, the load on the rolling mill increases, and it becomes difficult to form a thick steel plate having a predetermined shape. On the other hand, when the heating temperature exceeds 1200 ℃ and becomes high, oxidation becomes remarkable to lower the yield, and the crystal grains become coarse, and desired high toughness cannot be secured. Therefore, the heating temperature is limited to a temperature in the range of 900 to 1200 ℃. In addition, from the viewpoint of forming a phase change texture having a desired degree of integration, 1050 to 1150 ℃ is preferable. In addition, when the temperature of the steel material is kept at a high temperature to such an extent that hot rolling can be performed, hot rolling may be performed in a state where the steel material is not separately heated or after charging into a furnace for a short time.

The hot rolling step is performed on the heated steel material. The hot rolling step is set to include a primary rolling step, a secondary rolling step, and a cooling step after the rolling step.

The first rolling is performed at a cumulative reduction of 9% or less in a temperature range of 1000 to 850 ℃ in terms of surface temperature.

By performing the primary rolling in this temperature range, the austenite grains are not coarsened and are homogenized, and therefore, variation in phase change texture is reduced. When the rolling temperature is a temperature exceeding 1000 ℃ in terms of surface temperature, austenite grains are excessively coarsened, and a desired structure cannot be formed even by hot rolling after passing. On the other hand, when the rolling temperature is less than 850 ℃ in terms of surface thermometer, the temperature is in the range of austenite non-recrystallization temperature, which adversely affects the homogenization of crystal grains. Therefore, the primary rolling is set to be performed at a temperature ranging from 1000 to 850 ℃ in terms of surface temperature.

If the cumulative reduction ratio in this temperature range is increased to more than 9%, a desired reduction ratio in the secondary rolling cannot be secured, and a desired distribution of brittle crack propagation stopping performance in the sheet thickness direction cannot be achieved. For the above reasons, the primary rolling is limited to rolling having a cumulative reduction of 9% or less in a temperature range of 1000 to 850 ℃ in terms of surface temperature. In the primary rolling, the reduction ratio per pass is preferably set to about 3% to about 5% from the viewpoint of the austenite grains being sized.

The secondary rolling is performed at a reduction ratio of 7% or more per pass, a cumulative reduction ratio of 55% or more per pass, and a rolling completion temperature of 800 to 550 ℃ in a temperature range of 900 to 600 ℃ in terms of surface temperature.

By performing the secondary rolling in this temperature range, development of texture in the sheet thickness internal region for improving the brittle crack propagation stopping performance can be promoted.

When the vicinity of the surface of the steel sheet is in a two-phase temperature range in a temperature range of 900 to 600 ℃ in terms of surface temperature and the inside of the steel sheet is in an austenite region and rolling is performed at a reduction ratio of 7% or more per pass, rolling strain is intensively introduced into the inside of the steel sheet, and development of texture can be promoted. As a result, in the central region of the sheet thickness, the X-ray diffraction intensity ratio of the (100) plane and the (211) plane parallel to the sheet surface (rolled surface) effective for improving the brittle crack propagation stopping performance is improved. When the reduction per pass is less than 7%, the introduction of rolling strain into the steel sheet is weak, and a desired texture cannot be formed. In addition, the reduction ratio per pass is preferably set to 9% or more in order to secure a desired width of the region in which the texture is developed in the central portion of the plate thickness.

When the rolling reduction integrated in this temperature range is less than 55%, the X-ray diffraction intensity ratio of the (100) plane and the (211) plane parallel to the plate surface (rolled surface) after transformation, which is effective for improving the brittle crack propagation stopping performance in the central region of the plate thickness, is lower than a desired value, and the desired brittle crack propagation stopping performance cannot be ensured. Therefore, the secondary rolling is limited to rolling in which the reduction per pass is 7% or more and the cumulative reduction is 55% or more in a temperature range of 900 to 600 ℃. It is preferable that the reduction rate per pass is 9% or more and the cumulative reduction rate is 60% or more. On the other hand, from the viewpoint of an excessive load on the rolling mill, the reduction ratio per pass is preferably 15% or less, and the cumulative reduction ratio is preferably 75% or less.

The rolling finishing temperature of the secondary rolling is set to a temperature of 800 to 550 ℃. When the rolling completion temperature is high above 800 ℃, the development of texture becomes insufficient. On the other hand, when the rolling end temperature is less than 550 ℃, the plastic strain accumulated in the grains becomes excessive, and the toughness is lowered, so that the desired brittle crack propagation stopping performance cannot be secured.

After hot rolling, the steel sheet is cooled to a cooling stop temperature of 450 to 400 ℃ at a cooling rate of 5 ℃/sec or more on average in a temperature range of 790 to 540 ℃ in a surface thermometer.

When the cooling rate after hot rolling is less than 5 ℃/sec, the cooling is too slow, and the microstructure up to the 1/2 t-point of the sheet thickness cannot be organized into a microstructure in which the bainite phase is the main phase even with the composition within the range of the present invention. The upper limit of the cooling rate is not particularly limited, but is preferably set to 30 ℃/sec or less from the viewpoint of suppressing the formation of the martensite phase. The average cooling rate is preferably 5 to 15 ℃/sec.

When the cooling stop temperature exceeds 450 ℃, the amount of the second phase other than the bainite phase exceeds 20% by volume, and the desired steel plate structure cannot be secured. On the other hand, below 400 ℃, a martensite phase appears, and a desired steel plate structure cannot be secured. For the above reasons, the cooling after hot rolling is set to a cooling stop temperature of 450 to 400 ℃ at a cooling rate of 5 ℃/sec or more on average in a temperature range of 790 to 540 ℃ in the surface thermometer.