KR20230041060A - Thick steel plate and its manufacturing method - Google Patents

Abstract

It is an object of the present invention to provide a thick steel plate with high strength and excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at full thickness, and a manufacturing method thereof. In mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.00%, P: 0.05% or less, S: 0.02% or less, the balance being Fe and unavoidable impurities. In addition, the microstructure includes a ferrite phase of 80% or more in area ratio in the range from the surface to the subsurface 100μm in the plate thickness direction, and the plate thickness direction from 100μm below the surface. In the range of 1/4 of the thickness, the area ratio of the ferrite phase is 80% or less, the balance is composed of a pearlite phase or a mixed phase of the pearlite phase and the bainite phase, and the area ratio of the pearlite phase is the bainite phase More thick steel plates than the area ratio.

Description

Thick steel plate and its manufacturing method

The present invention relates to a thick steel plate and a manufacturing method thereof, and more particularly, to a thick steel plate excellent in elongation characteristics, fatigue crack propagation characteristics, and toughness at full thickness and a manufacturing method thereof. . The steel plate of the present invention can be suitably used for welded structures where structural safety is strongly required, such as ships, offshore structures, bridges, buildings, and tanks.

Thick steel plates are widely used in structures such as ships, offshore structures, bridges, buildings, and tanks. Such a thick steel plate is required to have excellent mechanical properties such as strength and toughness and excellent weldability, as well as excellent fatigue properties.

When using a structure as described above, a repeated load such as vibration due to wind or earthquake is applied to the structure. For this reason, the thick steel plate is required to have fatigue characteristics capable of ensuring the safety of the structure even when such repeated loads are applied.

Fatigue failure is a phenomenon in which fine cracks (fatigue cracks) first occur, and then the cracks spread (proliferation). In fatigue fracture, there are many cases in which fatigue cracks generally occur at welds, propagate through steel materials, and lead to fracture. It is believed that this is due to the fact that the welded portion tends to become a stress concentration portion due to its shape, and that tensile residual stress is generated after welding. For this reason, as a means for suppressing the generation of cracks from welded parts, a technique of introducing residual stress in compression by peening or the like is widely known.

However, it is not realistic from the standpoint of workability and manufacturing cost to apply this treatment to all of the many welded parts in the structure. Therefore, even if a fatigue crack occurs at a welded joint or the like, it is important to prolong the fatigue life as a welded structure by delaying the subsequent crack propagation in the steel material, and it is desirable to improve the fatigue crack propagation resistance of the steel material itself. It is becoming.

For example, in Patent Document 1, in a method for manufacturing a thick steel sheet having a thickness of 20 mm or less, while controlling Ceq (carbon equivalent) to a specific range by lowering the amount of C added, and lowering the cooling stop temperature, elongation and A thick steel sheet in which crack propagation characteristics are compatible with endothelial fatigue is described.

Further, Patent Literature 2 describes a method for manufacturing a thick steel sheet having small crack propagation property anisotropy by combining heating, rolling, accelerated cooling, and heat treatment according to the yield stress.

In Patent Literature 3, a dual-phase steel having a microstructure composed of bainite and ferrite with an area ratio of 38 to 52% is used, and the Vickers hardness of the ferrite phase portion and the ferrite phase and the bay The fatigue crack propagation characteristics are improved by controlling the density of the boundary between the night phases.

In Patent Document 4, in order to improve excellent fatigue resistance crack propagation characteristics and elongation characteristics at full thickness, the microstructure in the range from the surface to the subsurface of 100 μm in the sheet thickness direction is 80 in area ratio. % or more of the ferrite phase, and the microstructure in the range from 100 μm below the surface to the position of 1/2 the sheet thickness contains 80% or less of the ferrite phase in area ratio, and the remainder is a pearlite phase or a bainite phase , or a thick steel sheet composed of a mixed phase of a pearlite phase and a bainite phase has been proposed.

Patent Document 1: Japanese Unexamined Patent Publication No. 2010-196109 Patent Document 2: Japanese Unexamined Patent Publication No. 2007-332402 Patent Document 3: Japanese Unexamined Patent Publication No. Hei 08-225882 Patent Document 4: Japanese Unexamined Patent Publication No. 2019-026927

However, the conventional techniques described in Patent Literatures 1 to 4 have the following problems.

In the method described in Patent Literature 1, a thick steel sheet is manufactured by an online process by controlling rolling and accelerated cooling. Therefore, in particular, in the case of a thin steel sheet having a sheet thickness of 20 mm or less, during hot rolling and accelerated cooling, temperature deviations at the stern end of the steel sheet tend to occur, Stable mechanical properties cannot be obtained over the entire length.

Further, in the method described in Patent Literature 2, if quenching is performed immediately after reheating in the two-phase region, the shape of the thick steel sheet deteriorates due to transformation shrinkage, and the outermost layer of the thick steel sheet is quenched. It is miniaturized by this, and by hardening, the elongation property in the entire thickness is deteriorated. These tendencies are remarkable especially when the plate thickness is thin.

In the method described in Patent Literature 3, as in Patent Literature 1, a thick steel sheet is manufactured by an online process by rolling and accelerated cooling control. Therefore, in particular, in the case of thin materials having a sheet thickness of 20 mm or less, during hot rolling and accelerated cooling, temperature variation at the stern end of the steel sheet tends to occur, and stable mechanical properties cannot be obtained over the entire length. There is a problem called

In Patent Document 4, the reheated hot-rolled sheet is cooled and quenched at an average cooling rate of 7.7 to 16.9° C./s. In this method, since the cooling rate is high, the bainite phase is formed preferentially over the pearlite phase, and since island martensite exists in the bainite phase, the toughness value deteriorates.

Thus, in the conventional manufacturing method, there has been a problem that it is not possible to manufacture a thick steel sheet having all of the elongation at full thickness (also referred to as full thickness elongation), fatigue crack propagation characteristics, and toughness.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel sheet having high strength and excellent in elongation characteristics at full thickness, fatigue crack propagation characteristics, and toughness, and a manufacturing method thereof.

The present inventors obtained the following knowledge as a result of examining in order to solve the said subject.

(1) In the thick steel sheet after hot rolling is finished and cooled, there is an imbalance (difference) in the structure due to the cooling deviation, but this imbalance in the structure can be resolved by reheating to a temperature equal to or higher than the temperature of the two-phase region. .

(2) Even when the plate thickness is thin, by controlling the cooling pattern after the reheating heat treatment, both the elongation characteristics at the full thickness and the crack propagation characteristics in the endothelial layer can be compatible over the entire length.

(3) Further, the toughness value can be improved by forming more pearlite phase than bainite phase.

(4) In the process of cooling after hot rolling is finished, by appropriately controlling the cooling rate, the imbalance of the structure is resolved and high strength is ensured over the entire length, elongation properties in the entire thickness and crack propagation in the inner skin characteristics are compatible.

The present invention has been made based on the above knowledge, and the gist configuration thereof is as follows.

[1] In mass %,

C: 0.05 to 0.20%;

Si: 0.01 to 0.50%;

Mn: 0.50 to 2.00%;

P: 0.05% or less;

S: contains 0.02% or less, and has a component composition consisting of the balance Fe and unavoidable impurities,

The microstructure includes a ferrite phase of 80% or more in area ratio in a range from the surface to the bottom of the surface of 100 μm in the plate thickness direction,

In the plate thickness direction, in the range from 100 μm below the surface to 1/4 of the plate thickness,

Contains a ferrite phase of 80% or less in area ratio,

A thick steel sheet in which the remainder is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

[2] The above component composition, in mass%,

Cr: 0.01 to 1.00%;

Cu: 0.01 to 2.00%;

Ni: 0.01 to 2.00%;

Mo: 0.01~1.00%,

Co: 0.01~1.00%,

Sn: 0.005~0.500%,

Sb: 0.005 to 0.200%;

Nb: 0.005 to 0.200%;

V: 0.005 to 0.200%;

Ti: 0.005 to 0.050%;

B: 0.0001 to 0.0050%;

Zr: 0.005~0.100%,

Ca: 0.0001 to 0.020%;

Mg: 0.0001 to 0.020%, and

REM: The thick steel sheet according to [1], containing one or two or more selected from 0.0001 to 0.020%.

[3] Heat the steel material having the component composition described in [1] or [2] above at 900 to 1200 ° C,

The heated steel material is subjected to hot rolling with a cumulative reduction of 50% or more to obtain a hot-rolled sheet,

Cooling the hot-rolled sheet,

Subsequently, reheating to a reheating temperature equal to or higher than the Ac1 transformation point and equal to or lower than 950 ° C,

The steel sheet reheated to a temperature above the Ac1 transformation point and below 950 ° C is cooled to a cooling stop temperature of 350 to 600 ° C at an average cooling rate of 2 to 7 ° C / s,

A method for producing a thick steel plate, in which quenching is performed on a steel plate cooled to the cooling stop temperature of 350 to 600 ° C.

[4] Heat the steel material having the component composition described in [1] or [2] above at 900 to 1200 ° C,

The heated steel material is subjected to hot rolling with a cumulative reduction ratio of 50% or more to obtain a hot-rolled sheet,

continue,

The steel sheet cooled to a temperature equal to or higher than the Ar1 transformation point and equal to or lower than the Ar3 transformation point is cooled to a cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C. / s,

A method for producing a thick steel plate, in which quenching is performed on a steel plate cooled to the cooling stop temperature of 350 to 600 ° C.

According to the present invention, a thick steel sheet having high strength and excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at full thickness can be obtained. In the thick steel plate of the present invention, even if fatigue cracks occur over time at stress concentration zones, weld zones, etc., subsequent propagation of cracks is suppressed, so that the safety of the entire steel structure can be improved. In addition, by using the steel plate of the present invention suitably for structures such as bridges, ships, building structures, construction industrial machinery, etc., it becomes possible to reduce the maintenance cost and consequently the life cycle cost of these structures, which is extremely industrial. useful.

[Fig. 1] Fig. 1 is a schematic diagram of a one-sided notch simple tensile type fatigue test piece used in a fatigue crack propagation test.

Next, a method of implementing the present invention will be described in detail. In addition, the following description shows preferred embodiment of this invention, and this invention is not limited at all by the following description.

[Ingredient Composition]

Regarding the component composition of the thick steel sheet of the present invention, the reason for the limitation will be explained below. In addition, "%" in the following description shall represent "mass %" unless otherwise indicated.

C: 0.05 to 0.20%

C is an element having an effect of increasing matrix (matrix) hardness and improving strength. In addition, since there is an effect of generating a pearlite phase, which is an aggregate of cementite phases, fatigue resistance is increased. In order to obtain such an effect, it is necessary to make the C content 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more, still more preferably 0.12% or more. On the other hand, when the C content exceeds 0.20%, the hardness of the matrix excessively increases and elongation at full thickness deteriorates. For this reason, C content is made into 0.20 % or less. The C content is preferably 0.18% or less, more preferably 0.16% or less, still more preferably 0.14% or less.

Si: 0.01 to 0.50%

Si is an element that acts as a deoxidizer and increases the hardness of the base phase by solid solution in steel through solid solution strengthening. In order to obtain such an effect, it is necessary to make the Si content 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.1% or more, still more preferably 0.15% or more, and most preferably 0.20% or more. On the other hand, when the Si content exceeds 0.50%, the elongation and toughness in the entire thickness decrease. For this reason, Si content is made into 0.50 % or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less, still more preferably 0.35% or less, and most preferably 0.30% or less.

Mn: 0.50 to 2.00%

Mn is an element having an effect of increasing the hardness of the base phase and improving the strength. In order to obtain such an effect, it is necessary to make the Mn content 0.50% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more, still more preferably 0.80% or more, and most preferably 1.00% or more. On the other hand, when the Mn content exceeds 2.00%, in addition to the decrease in weldability, MnS as an inclusion is excessively segregated and the toughness deteriorates. For this reason, the Mn content is made 2.00% or less. The Mn content is preferably 1.85% or less, more preferably 1.70% or less, still more preferably 1.55% or less, and most preferably 1.40% or less.

P: 0.05% or less

P is an element contained in steel as an unavoidable impurity. P segregates at grain boundaries and exerts adverse effects, such as reducing the toughness of base materials and welded parts. Therefore, it is desirable to reduce P as much as possible, but a content of 0.05% or less is permissible. For this reason, P content is made into 0.05 % or less. The P content is preferably 0.04% or less, and more preferably 0.03% or less. On the other hand, although the lower limit of the P content is not limited, it is preferable to set the P content to 0.001% or more, since an excessive reduction causes a sharp increase in the refining cost. The P content is preferably 0.002% or more, more preferably 0.003% or more.

S: 0.02% or less

S is an element contained in steel as an unavoidable impurity. S is present in steel as sulfide-based inclusions such as MnS, and becomes a starting point for brittle fracture and deteriorates toughness. Therefore, it is desirable to reduce it as much as possible, but a content of 0.02% or less is acceptable. For this reason, S content is made into 0.02 % or less. It is preferable to make S content into 0.01 % or less. On the other hand, although the lower limit of the S content is not limited, it is preferable to set the S content to 0.0005% or more, since an excessive reduction causes a sharp increase in the refining cost.

The balance consists of Fe and unavoidable impurities. In addition, when the content of oxygen (O) contained as an unavoidable impurity exceeds 0.0050%, the presence ratio of inclusions on the surface of the steel sheet increases, so that cracks originating from the inclusions tend to occur. Therefore, it is preferable to make O content into 0.0050% or less. Similarly, when the content of N contained as an unavoidable impurity exceeds 0.0050%, the presence ratio of inclusions on the steel sheet surface increases, so that cracks originating from the inclusions tend to occur. Therefore, it is preferable to make N content into 0.0050 % or less. As for N content, it is more preferable to make it 0.0040 % or less. Similarly, when the content of sol.Al contained as an unavoidable impurity exceeds 0.060%, Al is mixed into the weld metal portion during welding, and the toughness of the weld portion deteriorates. Therefore, the sol.Al content is preferably 0.060% or less. The sol.Al content is more preferably 0.050% or less, and still more preferably 0.040% or less.

Also, in the present invention, Cr: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Co: 0.01 to 1.00%, Sn: 0.005 to 0.500%, Sb : 0.005~0.200%, Nb: 0.005~0.200%, V: 0.005~0.200%, Ti: 0.005~0.050%, B: 0.0001~0.0050%, Zr: 0.005~0.100%, Ca: 0.0001~0.020%, Mg: One or two or more selected from 0.0001 to 0.020% and REM: 0.0001 to 0.020% may be optionally contained.

Cr: 0.01~1.00%

Cr is an element having an effect of further improving the strength. In addition, Cr is an element that promotes the formation of cementite and promotes the formation of a pearlite phase that is advantageous for fatigue resistance. In the case of containing Cr, the Cr content is made 0.01% or more in order to obtain the above effects. Preferably it is 0.10% or more. On the other hand, when the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, when it contains Cr, it is made into 1.00 % or less. The Cr content is preferably 0.80% or less, more preferably 0.50% or less.

Cu: 0.01~2.00%

Cu is an element that further increases strength by solid solution. When containing Cu, in order to acquire the said effect, Cu content is made into 0.01 % or more. It is preferable to make Cu content into 0.05% or more, and it is more preferable to make it 0.10% or more. On the other hand, when the Cu content exceeds 1.00%, weldability is impaired, and also defects tend to occur during production of a thick steel sheet. Therefore, when containing Cu, it is set as 2.00% or less. The Cu content is preferably 0.70% or less, more preferably 0.60% or less, still more preferably 0.50% or less.

Ni: 0.01 to 2.00%

Ni is an element having an effect of improving low-temperature toughness, and Ni improves hot brittleness when it contains Cu. When containing Ni, in order to acquire the said effect, Ni content is made into 0.01% or more. It is preferable to make Ni content into 0.05 % or more. On the other hand, when the Ni content exceeds 1.00%, weldability is impaired and steel material cost increases. Therefore, when containing Ni, it is set as 1.00% or less. The Ni content is preferably 0.70% or less, more preferably 0.40% or less.

Mo: 0.01~1.00%

Mo is an element having an effect of increasing the hardness of the base phase, and may be optionally contained depending on desired properties. When containing Mo, in order to acquire this effect, Mo content is made into 0.01 % or more. It is preferable to make Mo content into 0.05 % or more. However, since weldability and toughness are impaired when Mo content exceeds 1.00%, when it contains, Mo content is made into 1.00% or less. It is preferable to make Mo content into 0.80 % or less, and it is more preferable to make it into 0.70 % or less.

Co: 0.01~1.00%

Co is an element having an effect of increasing the hardness of the base phase, and may be optionally contained depending on desired properties. In order to obtain this effect, in the case of containing Co, the Co content is made 0.01% or more. Preferably it is 0.10% or more, More preferably, it is 0.20% or more, More preferably, it is 0.35% or more. On the other hand, even if the Co content exceeds 1.00%, the effect is saturated, and the alloy cost increases. For this reason, when it contains, Co content is made into 1.00% or less. The Co content is preferably 0.50% or less.

Sn: 0.005~0.500%

Sn is an element having an effect of increasing the hardness of the base phase, and may be optionally contained depending on desired properties. In order to fully acquire these effects, when containing Sn, it is set as 0.005% or more. It is preferably 0.010% or more, more preferably 0.020% or more, and still more preferably 0.030% or more. On the other hand, when Sn content exceeds 0.500%, deterioration of ductility and toughness of steel will be caused. For this reason, when it contains, it is set as 0.500% or less. Preferably, it is 0.300% or less, More preferably, it is 0.200% or less, More preferably, it is 0.100% or less.

Sb: 0.005 to 0.200%

Sb is an element having an effect of increasing the hardness of the base phase, and may be optionally contained depending on desired properties. In order to fully acquire these effects, when containing Sb, it is set as 0.005% or more. The Sb content is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, when the Sb content exceeds 0.200%, deterioration of ductility and toughness of steel is caused. For this reason, when it contains, Sb content is made into 0.200 % or less. It is preferably 0.150% or less, more preferably 0.100% or less, still more preferably 0.080% or less, and most preferably 0.050% or less.

Nb: 0.005 to 0.200%

Nb is an element having an effect of suppressing recrystallization of austenite during hot rolling and refining the finally obtained crystal grains. Further, Nb precipitates during air cooling after accelerated cooling to further improve strength. In the case of containing Nb, the Nb content is made 0.005% or more in order to obtain the above effects. The Nb content is preferably 0.007% or more, more preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.200%, hardenability becomes excessive and bainite is excessively formed, making it impossible to obtain a desired structure and lowering toughness. Therefore, when Nb is contained, Nb content is made into 0.200% or less. The Nb content is preferably 0.070% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less.

V: 0.005~0.200%

V, like Nb, is an element having an effect of suppressing recrystallization of austenite during hot rolling to refine it and increasing strength by precipitating in the air cooling process after hot rolling, and is optionally contained according to desired characteristics. can do. In order to obtain the above effect, when V is contained, the V content is made 0.005% or more. The V content is preferably 0.010%, more preferably 0.020% or more, and still more preferably 0.030% or more. However, when the V content exceeds 0.200%, a large amount of VC precipitates and the toughness is impaired. Therefore, when V is contained, the V content is made 0.200% or less. The V content is preferably 0.150% or less, more preferably 0.100% or less, and still more preferably 0.070% or less.

Ti: 0.005 to 0.050%

Ti has a strong nitride formation tendency, and since it fixes N and reduces solute N, it has an effect of improving the toughness of a base material and a welded part. Moreover, when B is contained, Ti fixes N and it can suppress that B precipitates as BN by including Ti together. As a result, the hardenability improvement effect of B can be promoted, and the strength can be further improved. Therefore, it can be contained arbitrarily according to a desired characteristic. In order to obtain the said effect, when it contains Ti, it is set as 0.005% or more. The Ti content is preferably 0.007% or more, more preferably 0.010% or more. However, when the Ti content exceeds 0.050%, a large amount of TiC precipitates and the toughness is impaired. Therefore, when it contains Ti, Ti content is made into 0.050 % or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, and still more preferably 0.020% or less.

B: 0.0001 to 0.0050%

B is an element having an effect of remarkably improving hardenability and increasing strength even when contained in a small amount, and may be contained depending on desired characteristics. In order to obtain the said effect, when B is contained, it is set as 0.0001% or more. The B content is preferably 0.0005% or more, more preferably 0.001% or more. However, when the B content exceeds 0.0050%, the effect not only saturates, but also reduces weldability, so when B is contained, the B content is made 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0030% or less, and still more preferably 0.0020% or less.

Zr: 0.005~0.100%

Zr is an element having an effect of further increasing the strength. In order to sufficiently obtain the above effect, when Zr is contained, the Zr content is made 0.005% or more. The Zr content is preferably 0.010% or more, more preferably 0.030% or more, and still more preferably 0.050% or more. On the other hand, when the Zr content exceeds 0.100%, the effect of improving the strength is saturated. Therefore, when Zr is contained, Zr content is made into 0.100% or less.

Ca: 0.0001 to 0.020%

Ca bonds with S, suppresses the formation of MnS or the like that elongates in the rolling direction, controls the shape of sulfide-based inclusions so that they appear spherical, and contributes to improving the toughness of welds and the like, so desired characteristics may be included depending on When containing Ca, in order to acquire this effect, Ca content is made into 0.0001% or more. The Ca content is preferably 0.0005% or more, more preferably 0.0010% or more. However, when the Ca content exceeds 0.020%, not only the effect is saturated, but also the cleanliness of the steel is lowered, surface defects are frequent, and the surface properties are lowered. For this reason, when it contains Ca, Ca content is made into 0.020 % or less. The Ca content is preferably 0.010% or less, more preferably 0.006% or less, and still more preferably 0.002% or less.

Mg: 0.0001 to 0.020%

Mg is an element having an effect of improving toughness through refinement of crystal grains. In the case of containing Mg, the Mg content is made 0.0001% or more in order to obtain the above effect. The Mg content is preferably 0.0003% or more, more preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.020%, the effect is saturated. Therefore, when Mg is contained, Mg content is made into 0.020% or less. The Mg content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.005% or less.

REM: 0.0001 to 0.020%

REM (rare earth metal) is an element having an effect of improving toughness. When adding REM, the REM content is made 0.0001% or more in order to obtain the above effect. The REM content is preferably 0.0003% or more. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, when adding REM, REM content is made into 0.020 % or less. The REM content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.001% or less.

[micro organization]

Next, the reason for limiting the microstructure of the thick steel plate will be explained. In addition, "%" in the description of the microstructure shall indicate an area ratio unless otherwise specified. In the following description, the "front end" of the thick steel sheet is defined as a position 100 mm inward from the front end of the steel sheet in the rolling direction to the tail end side. Similarly, the "tail end" of a thick steel sheet is defined as a position 100 mm inward from the tail end of the steel sheet in the rolling direction toward the front end side. In addition, the "center" of a thick steel plate is defined as the position of the center of a steel plate in the rolling direction (longitudinal direction).

Tissue ranging from surface to subsurface 100 μm (superficial tissue)

In the thick steel sheet of the present invention, the microstructure in the range from the surface to the subsurface 100 μm in the sheet thickness direction (hereinafter sometimes referred to simply as “surface layer portion”) is defined as a ferrite phase having an area ratio of 80% or more. to include In the two-phase region above the Ac1 transformation point and below the Ac3 transformation point, a surface layer decarburization reaction occurs, and 80% or more of ferrite is generated in the surface layer portion to soften the surface layer of the thick steel sheet, thereby significantly improving the elongation characteristics in the entire thickness. can

This surface layer decarburization reaction occurs by passing through the two-phase region or holding it in the two-phase region in the reheating process.

When the area ratio of the ferrite phase in the surface layer portion is less than 80%, a large amount of hard residual structures composed of bainite phase, pearlite phase, martensite phase, or mixed phases thereof are present. As a result, the hardness of the surface layer portion increases, and elongation characteristics at the desired full thickness cannot be obtained. Moreover, there are cases where the tensile strength becomes excessive.

In addition, the area ratio of the ferrite phase in the surface layer portion here refers to the average value of the area ratio of the ferrite phase in the range from the surface to the subsurface of 100 μm of the thick steel sheet. In addition, the microstructure in the surface layer portion refers to the microstructure in the surface layer portion at the front end, center, and tail end in the rolling direction of the thick steel sheet. Therefore, in the thick steel sheet of the present invention, the average value of the area ratio of the ferrite phase in the range from the surface to the bottom of the surface of 100 μm is 80% or more at the leading edge, center, and tail end in the rolling direction of the thick steel sheet. In addition, usually, when the microstructure of the surface layer portion at the front end, center, and tail end satisfies the above condition, the above condition is satisfied over the entire length of the thick steel sheet in the rolling direction. Therefore, it can be said that the thick steel sheet of the present invention has an area ratio of the ferrite phase in the surface layer portion of 80% or more over the entire length in the rolling direction. That is, in the present invention, the area ratio of the ferrite phase in the surface layer portion is 80% or more means that the area rate of the ferrite phase in the surface layer portion is 80% or more at any of the front end, center, and tail end over the entire length in the rolling direction. means you can

The remainder other than the ferrite phase in the microstructure of the surface layer portion preferably consists of a pearlite phase or a mixed phase of bainite and pearlite phases, but since the bainite phase contains island-like martensite and deteriorates toughness, It is so preferable that it is small, and it is more preferable to use only the pearlite phase.

Tissue ranging from 100 μm below the surface to 1/4 of the plate thickness (tissue inside the plate thickness)

The area ratio of the microstructure in the range from 100 μm below the surface to the position of 1/4 of the sheet thickness in the sheet thickness direction in the thick steel sheet of the present invention (hereinafter sometimes referred to simply as “inside the sheet thickness”) It is assumed that 80% or less of the ferrite phase is included. When the microstructure within the plate thickness satisfies the above conditions, desired strength and crack propagation characteristics to the inner skin can be obtained.

Note that the area ratio of the ferrite phase within the plate thickness here refers to the average value of the area ratio of the ferrite phase in the range from 100 μm below the surface of the thick steel plate to the position of 1/4 of the plate thickness. In addition, here, the microstructure inside the sheet thickness refers to the microstructure inside the sheet thickness at the front end, center, and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the microstructure in the range from 100 μm below the surface to 1/4 of the sheet thickness at the leading edge, center, and tail end in the rolling direction of the thick steel sheet satisfies the above conditions. In addition, similarly to the structure of the surface layer portion, usually, if the microstructure inside the sheet thickness at the leading end, center, and tail end satisfies the above condition, the above condition is satisfied over the entire length of the thick steel sheet in the rolling direction. Therefore, the thick steel sheet of the present invention can be said to have a ferrite phase in which the microstructure in the thickness of the sheet is 80% or less in area ratio over the entire length in the rolling direction.

In the present invention, the remainder of the microstructure in the inside of the sheet thickness is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase. The bainite phase contains island martensite and deteriorates toughness. For this reason, desired toughness can be obtained by making the area fraction of the pearlite phase larger than the area fraction of the bainite phase. The area fraction of the bainite phase is preferably 15% or less. More preferably, it is 13% or less, and even more preferably, it is 11% or less.

In addition, the remainder of the thick steel plate of the present invention refers to the surface layer portion at the front end, the center, and the tail end, and the remainder within the thickness of the plate. That is, over the entire length of the thick steel sheet in the rolling direction, the rest of the microstructure is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

In addition, the microstructure in the surface layer portion and inside the plate thickness can be evaluated by the method described in Examples.

[full thickness elongation]

The total thickness elongation of the thick steel sheet is not particularly limited, but is preferably 19% or more when the sheet thickness exceeds 16 mm, and 15% or more when the sheet thickness is 16 mm or less. In the present invention, it is preferable that the above conditions for full thickness elongation are satisfied at the front end, center and tail end of the thick steel sheet in the rolling direction. In addition, usually, when the front end, the center, and the tail end satisfy the above condition, the above condition is satisfied over the entire length of the thick steel sheet in the rolling direction. In addition, full-thickness elongation can be measured by the method described in the Example.

[tensile strength]

The tensile strength (TS) of the thick steel sheet is not particularly limited, but is preferably 490 MPa or more. In addition, the upper limit of TS is not particularly limited either, but, for example, when setting it to 490 MPa (50 kgf/mm ² ) class in JIS, TS may be 610 MPa or less. In addition, in the case of 570 MPa (60 kgf/mm ² ) class in JIS, the upper and lower limits of TS may be set to 570 MPa and 720 MPa, respectively. In the present invention, it is preferable that the conditions of TS are satisfied at the front end, center, and tail end of the thick steel sheet in the rolling direction. In addition, usually, when the front end, the center, and the tail end satisfy the above condition, the above condition is satisfied over the entire length of the thick steel sheet in the rolling direction. In addition, TS can be measured by the method described in the Example.

[tenacity]

The thick steel sheet of the present invention has excellent toughness as a result of having the above component composition and microstructure. Although the toughness of the thick steel sheet of the present invention is not particularly limited, in the case of a test piece thickness of 10 mm, the Charpy absorbed energy vE ₀ at 0°C, which is one of the toughness indicators, is preferably set to 100 J or more, and preferably set to 130 J or more. More preferably, it is more preferably 150J or more, and most preferably 200J or more. On the other hand, although it is not limited also about the upper limit of _vE0 , it may be 400J or less, 300J or less, or 270J or less, for example. In the case of a test piece thickness of 5 mm, it is preferable to set the Charpy absorbed energy vE ₀ at 0°C to 50 J or more. On the other hand, the upper limit of vE ₀ is also not limited, but may be, for example, 200 J or less, 150 J or less, or 135 J or less. In addition, vE ₀ can be measured by the method described in the Example.

[Fatigue crack propagation characteristics]

m에서의 피로 균열 전파 속도 4.25×10^-8m/cycle 이하가 바람직하다.The thick steel sheet of the present invention can have excellent fatigue crack propagation characteristics as a result of having the above component composition and microstructure. As an index of fatigue crack propagation characteristics, fatigue crack propagation speed (da/dN) can be used. The value of the fatigue crack propagation speed is not particularly limited, but in the present invention, ΔK = 25 MPa

A fatigue crack propagation velocity of 4.25×10 ^-8 m/cycle or less in m is preferred.

[plate thickness]

The "thick steel plate" in the present invention refers to a steel plate having a thickness of 6 mm or more according to the usual definition in this technical field. On the other hand, the upper limit of the plate thickness of the thick steel plate in the present invention is not particularly limited and can be set to any value. However, as described above, the effect of the present invention is particularly remarkable for thin materials in which the temperature deviation at the tip end of the steel sheet tends to increase and excellent elongation properties in the entire thickness are required. Therefore, the plate thickness of the thick steel plate is preferably 25 mm or less, and more preferably 20 mm or less.

[Manufacturing method]

In the thick steel sheet of the present invention, a method in which heating, hot rolling, cooling, reheating, cooling, and quenching are sequentially performed on a steel material having the above-described component composition, or heating, hot rolling, cooling, and quenching are sequentially performed. It can be obtained according to the implementation method. First, a method of sequentially performing the treatment of heating, hot rolling, cooling, reheating, cooling, and quenching will be described.

river material

As the raw material for the steel of the present invention, any material can be used as long as it has the above component composition and can be hot rolled, but usually a steel slab may be used. For example, molten steel having the above chemical composition can be smelted by a means such as a converter, and used as a steel material such as a slab by a casting method such as a continuous casting method. Further, it can be made into a steel material such as a slab by an ingot-decomposition rolling method.

heating

A steel material having the above component composition is heated at 900 to 1200°C. If the heating temperature is less than 900°C, the deformation resistance of the steel material in the next hot rolling process increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900°C or higher. It is preferable to make heating temperature into 950 degreeC or more. On the other hand, when the heating temperature exceeds 1200°C, the toughness decreases. Therefore, the heating temperature is 1200°C or less. It is preferable to make heating temperature into 1150 degreeC or less.

In addition, when a steel raw material (slab) is manufactured by a method such as continuous casting, the slab may be directly subjected to the heating step without being cooled, or may be subjected to the heating step after being cooled. In addition, although the heating method is not specifically limited, It can heat with a heating furnace according to the usual method, for example.

hot rolled

Subsequently, the heated steel material is hot-rolled to obtain a hot-rolled sheet. At that time, in order to secure toughness, which is the basic performance of the product steel sheet, the cumulative reduction ratio is set to 50% or more. When the cumulative reduction ratio is less than 50%, the ferrite grains in the thickness of the plate become coarse, and a region with low brittleness is locally generated, brittle cracks are likely to occur, and toughness deteriorates. Other conditions regarding the hot rolling process are not particularly limited.

Cooling

Next, the steel sheet after completion of hot rolling is cooled (first cooling step). In the cooling process, when reheating is performed, it is preferable to cool to room temperature. In addition, cooling can be performed by any method, for example, air cooling or accelerated cooling. Also, the cooling conditions are not particularly limited.

reheat

Subsequently, the steel sheet after cooling is reheated to a temperature equal to or higher than the Ac1 transformation point and equal to or lower than 950°C. The reheating temperature is preferably less than the Ac3 transformation point. In this way, by heating to a temperature range that includes the austenite phase above the Ac1 transformation point and below 950° C., the imbalance in the microstructure caused by the cooling deviation can be resolved, and as a result, the imbalance in mechanical properties can be resolved. When the reheating temperature is less than the Ac3 transformation point, the tissue before reheating is not damaged, which is preferable.

When the reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, a decarburization reaction peculiar to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer portion can be set to 80% or more. On the other hand, when the reheating temperature is equal to or higher than the Ac3 transformation point and equal to or lower than 950°C, by making the holding time at the reheating temperature short, the ferrite phase in the surface layer portion generated by the surface layer decarburization reaction at the time of passing through the two-phase region reversely transforms into an austenite phase ( Reaction to reverse is suppressed, and the area ratio of the ferrite phase in the surface layer portion can be made 80% or more.

When the reheating temperature is higher than 950°C, a reaction in which the ferrite phase in the surface layer portion generated by the surface layer decarburization reaction at the time of passing through the two-phase region is reversely transformed into an austenite phase is promoted, and the area ratio of the ferrite phase in the surface layer portion is less than 80%. becomes As a result, the hardness of the surface layer portion increases, and elongation characteristics at the desired full thickness cannot be obtained.

Further, when the reheating temperature is equal to or higher than the Ac3 transformation point and equal to or lower than 950° C., the crystal grain size of the austenite phase inside the plate thickness becomes coarser than when the reheating temperature is lower than the Ac3 transformation point, but it was found that the toughness was not excessively deteriorated. Further, in this temperature range, the rate at which the reverse transformation into the austenite phase within the sheet thickness proceeds increases. For this reason, since the desired parent structure is obtained in a short heating time, the number of thick steel sheets that can be manufactured in a predetermined time increases, so productivity is improved.

On the other hand, when the temperature exceeds 950°C, an austenite phase undergoing reverse transformation within the sheet thickness grows and coarsens, and as a result, a region with low toughness is locally generated and the toughness decreases.

On the other hand, if the reheating temperature is less than the Ac1 transformation point, the reaction of reverse transformation to the austenite phase does not occur, and the desired area ratio of the ferrite phase, pearlite phase, and bainite phase inside the sheet thickness after cooling does not occur. As a result, fatigue characteristics (crack propagation characteristics) deteriorate. In addition, the imbalance in mechanical properties due to the cooling variation in the cooling process after hot rolling cannot be eliminated.

In addition, Ac1 transformation point can be calculated|required by the following formula (1), for example.

Ac1(°C)=723+29.1×Si-10.7×Mn-16.9×Ni+16.9×Cr… (One)

In addition, Ac3 transformation point can be calculated|required by the following formula (2), for example.

Ac3(℃)=961.6-311.9×C+49.5×Si-36.4×Mn+438.1×P-2818×S+12.7×Al-51×Cu-29×Ni-8.7×Cr+13.5×Mo+308.1×Nb -140×V+318.9×Ti+611.2×B-969×N… (2)

Here, the element symbol in the formulas (1) to (2) above means the content (% by mass) of each element, and is set to zero when the element is not contained.

Further, in the reheating treatment, it is preferable to heat to the reheating temperature and then hold it at that temperature. When the reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, and the holding time is less than 10 minutes, the reverse transformation to the austenite phase does not start over the entire length of the steel sheet, and the hardenability is remarkably reduced in some regions. there is. Therefore, it is preferable to make holding time into 10 minutes or more. On the other hand, when the reheating temperature is equal to or higher than the Ac3 transformation point and equal to or lower than 950°C, and the holding time exceeds 30 minutes, the austenite phase grows and coarsens. Therefore, it is preferable to make holding time into 30 minutes or less.

Cooling

The steel sheet reheated in the reheating step or the hot rolled steel sheet is cooled to a cooling stop temperature of 350 to 600°C (second cooling step). At that time, the average cooling rate is set to 2 to 7°C/s. A lower average cooling rate is preferable from the viewpoint of improving toughness because pearlite transformation is further promoted. However, if the average cooling rate is less than 2° C./s, ferrite grain growth becomes excessive and granulation occurs, so toughness deteriorates. Therefore, the average cooling rate is 2°C/s or more. On the other hand, when the average cooling rate exceeds 7°C/s, pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and bainite transformation and martensite transformation tend to proceed. In this case, since the fraction of the bainite phase or martensite phase increases, the elongation characteristics and toughness in the entire thickness deteriorate. Therefore, the average cooling rate is 7°C/s or less. The average cooling rate is preferably 5°C/s or less, more preferably 4°C/s or less, and still more preferably less than 3°C/s.

In addition, when the cooling stop temperature is less than 350°C, ferrite is excessively formed in the inside of the sheet thickness, so the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling stop temperature is set to 350°C or higher. On the other hand, when the cooling stop temperature exceeds 600°C, hard bainite and martensite are excessively formed because quenching is performed with a large amount of untransformed austenite remaining. As a result, the elongation property at full thickness is lowered, and the toughness is also deteriorated. Therefore, the cooling stop temperature is set to 600°C or less.

quenching

Quenching is performed on the steel sheet cooled to the cooling stop temperature. Therefore, the quenching temperature is in the range of 350 to 600°C. Although quenching can be carried out under arbitrary conditions without any particular limitation, it is preferable to perform water cooling to a temperature below the Ms point, preferably to 200°C or lower. In addition, Ms point can be calculated|required by following Formula (3), for example.

Ms (°C) = 517-300 × C-11 × Si-33 × Mn-17 × Ni-22 × Cr-11 × Mo . (3)

Here, the element symbol in the above formula (3) means the content (mass%) of each element, and is set to zero when the element is not contained.

Next, a method of sequentially performing heating, hot rolling, cooling, and quenching will be described.

The steel material used is the same as the steel material described above. Heating and hot rolling can be performed by the same method as the heating and hot rolling described above.

Cooling after hot rolling is first cooled to a temperature equal to or higher than the Ar1 transformation point and equal to or lower than the Ar3 transformation point, and 350 to 600°C at an average cooling rate of 2 to 7°C/s from the temperature (cooling start temperature) equal to or higher than the Ar1 transformation point and equal to or lower than the Ar3 transformation point. Cool to the cooling stop temperature of The reason why the cooling start temperature is set to a temperature equal to or higher than the Ar1 transformation point and equal to or lower than the Ar3 transformation point (two-phase region) is that a decarburization reaction specific to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer portion can be set to 80% or more. am.

In addition, the following reasons are given as a reason for setting the average cooling rate thereafter to 2 to 7°C/s. If the average cooling rate is less than 2°C/s, ferrite grain growth becomes excessive and granulation occurs, so toughness deteriorates. Therefore, the average cooling rate is 2°C/s or more. On the other hand, when the average cooling rate exceeds 7°C/s, pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and bainite transformation and martensite transformation tend to proceed. In this case, since the fraction of the bainite phase or the martensite phase increases, the elongation characteristics and toughness in the entire thickness deteriorate. Therefore, the average cooling rate is 7°C/s or less. The average cooling rate is preferably 5°C/s or less, more preferably 4°C/s or less, and still more preferably less than 3°C/s.

In addition, the reason why the cooling stop temperature is 350 to 600°C is given for the following reasons. When the cooling stop temperature is less than 350°C, ferrite is excessively formed in the inside of the sheet thickness, so the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling stop temperature is set to 350°C or higher. On the other hand, when the cooling stop temperature exceeds 600°C, hard bainite and martensite are excessively formed because quenching is performed with a large amount of untransformed austenite remaining. As a result, the elongation property at full thickness is lowered, and the toughness is also deteriorated. Therefore, the cooling stop temperature is set to 600°C or less.

Subsequent quenching may be performed in the same manner as the quenching described above.

In addition, Ar1 transformation point can be calculated|required by the following formula (4), for example.

Ar1 = 712-17.8 × C-19.1 × Ni + 20.1 × Si + 11.9 × Cr + 9.8 × Mo . (4)

In addition, Ar3 transformation point can be calculated|required by the following formula (5), for example.

Ar3=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo... (5)

Here, the element symbol in the formulas (4) to (5) above means the content (% by mass) of each element, and is set to zero when the element is not contained.

Example

Hereinafter, the effects of the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

Molten steel having the composition shown in Table 1 was smelted to obtain a steel raw material (slab). In addition, the values of the Ac1 point, Ac3 point, Ms point, Ar1 point, and Ar3 point shown in Table 1 are values obtained from the above-described formulas (1), (2), (3), (4), and (5), respectively. am.

Next, the obtained slab was subjected to heating and hot rolling under the conditions shown in Table 2 to obtain a hot-rolled sheet having a total length of 20 m and a sheet thickness shown in Table 2. Thereafter, the hot-rolled sheet was cooled to room temperature by the cooling method shown in Table 2, reheated to the reheating temperature shown in Table 2, and held for 30 minutes or longer. Subsequently, cooling water was sprayed on both sides of the steel sheet, and after cooling to the cooling stop temperature at the average cooling rate shown in Table 2, a quenching treatment was performed. In the quenching process, it was water-cooled to 150 degrees C or less.

For comparison, in some comparative examples (No. 24 in Table 2), quenching was performed immediately after reheating without performing cooling that satisfies the conditions of the present invention. The quenching conditions in this comparative example were an average cooling rate of 44.0°C/s and a cooling stop temperature of 110°C.

The obtained thick steel sheet was evaluated for (1) microstructure, (2) total thickness elongation, (3) tensile strength (TS), (4) fatigue crack propagation characteristics, and (5) toughness. In order to evaluate the characteristics along the entire length of the thick steel plate, test pieces were taken from each of the front end, center, and tail end of the thick steel plate in the rolling direction. The test method is as follows. In addition, the test piece at the front end and the tail end was taken at a position 100 mm deep from the end of the steel sheet in the rolling direction.

(1) Microstructure observation

The microstructure was observed in the following procedure.

Area ratio of the ferrite phase in the surface layer portion, area ratio of the ferrite phase within the plate thickness, area ratio of the pearlite phase and bainite phase within the plate thickness

First, from the obtained thick steel sheet, a test piece for structure observation is taken so that the observation surface is a cross section perpendicular to the rolling direction (cross section in the plate thickness direction), polished until it becomes a mirror surface, and then corroded ( methanol nitrate solution), and using an optical microscope (magnification: 400 times), observations were made from the surface of the steel sheet to the position of 1/4 of the sheet thickness in the sheet thickness direction, and images were taken so that the screen was continuous. Phases were identified by image analysis using the resulting tissue photograph, (a) the average value of the area ratio of the ferrite phase in the range from the surface to the subsurface 100μm of the thick steel plate, (b) from the subsurface 100μm The average value of the area ratio of the ferrite phase in the range of 1/4 of the plate thickness and (c) the area ratio of the pearlite phase and the bainite phase in the range from 100 μm below the surface to the 1/4 position of the plate thickness were obtained.

Table 3 shows the measurement results of the microstructure.

(2) Tensile test

A tensile test was performed using a full-thickness test piece of JIS Z 2201 No. 1A sampled from the center of the width of the thick steel sheet so that the sheet width direction coincided with the tensile direction, and the tensile strength (TS) and full thickness elongation were determined. Tensile strength set 490 MPa or more as pass. As for the elongation characteristics, 15% or more in the case of a plate thickness of 16 mm or less, and 19% or more in the case of a plate thickness of 16 mm or more were considered acceptable.

(3) Fatigue crack propagation test

m의 범위에서 시험을 실시했다. ΔK=25MPa

m에서의 피로 균열 전파 속도 4.25×10^-8m/cycle 이하를 합격으로 했다.A fatigue crack propagation test was conducted using the one-sided notch simple tension type fatigue test piece shown in FIG. 1, and the fatigue crack propagation behavior when a crack propagates in the sheet thickness direction was evaluated. The test conditions were carried out in room temperature air at a stress ratio of 0.1 and a frequency of 10 Hz in accordance with ASTM E647. In the present invention, since the purpose of the present invention is to reduce the propagation speed when a crack generated at a welded part or the like in a welded structure propagates through the steel material, assuming such a situation, the stress magnification factor range (ΔK) is 10 to 30 MPa.

The test was conducted in the range of m. ΔK=25 MPa

A fatigue crack propagation speed of 4.25 × 10 ^-8 m/cycle or less at m was considered acceptable.

(4) toughness

A Charpy impact test piece was sampled in parallel to the rolling direction (L direction) from the center of the thickness of the thick steel sheet. The test piece thickness was set to 10 mm in the case of a plate thickness of 10 mm or more, and 5 mm in the case of a plate thickness of less than 10 mm. In the test, a Charpy impact test was conducted at 0°C in accordance with JIS Z 2202, and absorbed energy vE ₀ was measured. A test piece having a thickness of 10 mm had an absorbed energy of 100 J or more as a pass. The test piece having a test piece thickness of 5 mm had an absorbed energy of 50 J or more as passing.

Table 4 shows the measurement results. As can be seen from these results, in the examples satisfying the conditions of the present invention, thick steel sheets having elongation characteristics in full thickness, crack propagation characteristics in fatigue resistance, and toughness were obtained. On the other hand, in the comparative examples that do not satisfy the conditions of the present invention, at least one of elongation characteristics, fatigue crack propagation speed, and toughness in the full thickness is inferior at least one of the leading end and tail end of the steel sheet.

Claims (4)

in mass %,
C: 0.05 to 0.20%;
Si: 0.01 to 0.50%;
Mn: 0.50 to 2.00%;
P: 0.05% or less;
S: contains 0.02% or less, and has a component composition consisting of the balance Fe and unavoidable impurities,
The microstructure includes a ferrite phase of 80% or more in area ratio in a range from the surface to the subsurface of 100 μm in the plate thickness direction,
In the plate thickness direction, in the range from 100 μm below the surface to 1/4 of the plate thickness,
Contains a ferrite phase of 80% or less in area ratio,
A thick steel plate in which the remainder is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is greater than the area ratio of the bainite phase. The method of claim 1,
The component composition is also in mass%,
Cr: 0.01 to 1.00%;
Cu: 0.01 to 2.00%;
Ni: 0.01 to 2.00%;
Mo: 0.01~1.00%,
Co: 0.01~1.00%,
Sn: 0.005~0.500%,
Sb: 0.005 to 0.200%;
Nb: 0.005 to 0.200%;
V: 0.005 to 0.200%;
Ti: 0.005 to 0.050%;
B: 0.0001 to 0.0050%;
Zr: 0.005~0.100%,
Ca: 0.0001 to 0.020%;
Mg: 0.0001 to 0.020%, and
REM: A thick steel sheet containing one or two or more selected from 0.0001 to 0.020%. Heating the steel material having the component composition according to claim 1 or claim 2 at 900 to 1200 ° C,
The heated steel material is subjected to hot rolling with a cumulative reduction of 50% or more to obtain a hot-rolled sheet,
Cooling the hot-rolled sheet,
Subsequently, reheating to a reheating temperature equal to or higher than the Ac1 transformation point and equal to or lower than 950 ° C,
The steel sheet reheated to a temperature above the Ac1 transformation point and below 950 ° C is cooled to a cooling stop temperature of 350 to 600 ° C at an average cooling rate of 2 to 7 ° C / s,
A method for manufacturing a thick steel plate, wherein quenching is performed on a steel plate cooled to the cooling stop temperature of 350 to 600 ° C. Heating the steel material having the component composition according to claim 1 or claim 2 at 900 to 1200 ° C,
The heated steel material is subjected to hot rolling with a cumulative reduction ratio of 50% or more to obtain a hot-rolled sheet,
continue,
The steel sheet cooled to a temperature equal to or higher than the Ar1 transformation point and equal to or lower than the Ar3 transformation point is cooled to a cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C. / s,
A method for producing a thick steel plate, in which quenching is performed on a steel plate cooled to the cooling stop temperature of 350 to 600 ° C.

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