Thick steel plate and method for manufacturing the same

Abstract

The present invention aims to provide a thick steel plate having high strength, excellent elongation and fatigue crack propagation properties through the thickness, and toughness, and a method for producing the same. The thick steel plate has a composition comprising, by mass%, 0.05-0.20% C, 0.01-0.50% Si, 0.50-2.00% Mn, 0.05% or less P, and 0.02% or less S, with the balance consisting of Fe and inevitable impurities, and a microstructure comprising a ferrite phase comprising 80% or more by area from the surface to 100 μm below the surface along the plate thickness direction, and a ferrite phase comprising 80% or less by area from 100 μm below the surface to a position ¼ of the plate thickness along the plate thickness direction, the balance consisting of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, wherein the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

Description

Technical Field

[0001] The present invention relates to thick steel plates and methods for manufacturing the same, and more particularly to thick steel plates having excellent elongation properties, fatigue crack propagation properties, and toughness across the thickness, and methods for manufacturing the same. The thick steel plates of the present invention are suitable for use in welded structures such as ships, marine structures, bridges, buildings, and tanks, where structural safety is highly required. Background Art

[0002] Thick steel plates are widely used in structures such as ships, marine structures, bridges, buildings, and tanks. These thick steel plates are required to have excellent mechanical properties such as strength and toughness, as well as excellent weldability, and also excellent fatigue properties.

[0003] When such structures are used, repeated loads such as vibrations caused by wind and earthquakes are applied to the structures. Therefore, thick steel plates are required to have fatigue properties that can ensure the safety of the structures even when subjected to such repeated loads.

[0004] Fatigue fracture refers to a phenomenon in which a fine crack (fatigue crack) initially develops, followed by a stage of expansion (progression) of this crack. Fatigue fracture usually occurs when fatigue cracks originate from welds and propagate through the steel until fracture. This is because welds, due to their shape, tend to become stress concentration areas, and tensile residual stresses are generated after welding. Therefore, as a means of suppressing cracks from welds, techniques such as introducing compressive residual stresses by hammering are widely known.

[0005] However, it is not realistic to perform such treatment on all welds, which are numerous in a structure, from the perspective of workability and manufacturing cost. Therefore, even if fatigue cracks occur in welds, it is important to delay crack propagation in the subsequent steel material to extend the fatigue life of the welded structure. Therefore, it is desirable to improve the fatigue crack propagation resistance of the steel material itself.

[0006] For example, Patent Document 1 describes a thick steel plate, in which, in a method for manufacturing a thick steel plate with a plate thickness of 20 mm or less, the Ceq (carbon equivalent) is controlled within a specific range by reducing the amount of C added and the cooling stop temperature is lowered to achieve a balance between elongation and fatigue crack propagation resistance.

[0007] Patent Document 2 describes a method for producing a thick steel plate having low anisotropy in crack propagation characteristics by combining heating, rolling, accelerated cooling, and heat treatment according to yield stress.

[0008] Patent Document 3 improves fatigue crack propagation characteristics by producing a dual-phase steel having a microstructure consisting of bainite and ferrite with an area ratio of 38 to 52%, and controlling the Vickers hardness of the ferrite phase and the density of the boundary between the ferrite phase and the bainite phase.

[0009] In Patent Document 4, in order to improve excellent fatigue crack propagation resistance and elongation characteristics through the entire thickness, a thick steel plate is proposed in which the microstructure in the range from the surface to 100 μm below the surface along the plate thickness direction contains a ferrite phase with an area ratio of not less than 80%, the microstructure in the range from 100 μm below the surface to 1 / 2 of the plate thickness contains a ferrite phase with an area ratio of not more than 80%, and the balance is composed of a pearlite phase, a bainite phase, or a mixed phase of a pearlite phase and a bainite phase.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 2010-196109

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 2007-332402

[0014] Patent Document 3: Japanese Patent Application Laid-Open No. 08-225882

[0015] Patent Document 4: Japanese Patent Application Publication No. 2019-026927 Summary of the Invention

[0016] Problems to be solved by the invention

[0017] However, the conventional technologies described in Patent Documents 1 to 4 have the following problems.

[0018] The method described in Patent Document 1 manufactures thick steel plates through an in-line process based on controlled rolling and accelerated cooling. Therefore, especially for thin plates with a thickness of 20 mm or less, temperature deviations are likely to occur at the leading and trailing ends of the steel plate during hot rolling and accelerated cooling, making it impossible to achieve stable mechanical properties over the entire length.

[0019] Furthermore, in the method described in Patent Document 2, if quenching is performed immediately after reheating in the dual-phase region, the thick steel plate deteriorates in shape due to phase transformation and shrinkage. Furthermore, the outermost layer of the thick steel plate becomes finer and hardened due to quenching, resulting in deterioration in elongation characteristics throughout the thickness. These tendencies are particularly pronounced when the plate thickness is thin.

[0020] The method described in Patent Document 3 manufactures thick steel plates through an in-line process based on rolling and accelerated cooling control, similar to Patent Document 1. Therefore, particularly for thin products with a thickness of 20 mm or less, there is a problem: temperature deviations are likely to occur at the leading and trailing ends of the steel plate during hot rolling and accelerated cooling, making it impossible to achieve stable mechanical properties over the entire length.

[0021] 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. Due to the high cooling rate, this method prioritizes the formation of a bainite phase over a pearlite phase, and the presence of islands of martensite within the bainite phase degrades the toughness.

[0022] Therefore, conventional production methods have the problem of being unable to produce thick steel plates that have all the properties of elongation through thickness (also called through-thickness elongation), fatigue crack propagation properties, and toughness.

[0023] 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 plate having high strength, excellent elongation characteristics through thickness, fatigue crack propagation characteristics, and toughness, and a method for producing the same.

[0024] Methods used to solve problems

[0025] The present inventors conducted studies to solve the above-mentioned problems and, as a result, obtained the following findings.

[0026] (1) After hot rolling and cooling, the thick steel plate has uneven structure due to cooling deviation. However, this uneven structure can be eliminated by reheating to a temperature above the dual phase region.

[0027] (2) Even when the plate thickness is thin, by controlling the cooling pattern after the reheating heat treatment, it is possible to achieve both the elongation characteristics in the total thickness and the fatigue crack propagation resistance characteristics over the entire length.

[0028] (3) Furthermore, by generating a larger amount of pearlite phase than bainite phase, the toughness value can be improved.

[0029] (4) During the cooling process after hot rolling, by properly controlling the cooling rate, it is possible to eliminate structural unevenness, ensure high strength over the entire length, and take into account both the elongation characteristics and fatigue crack propagation resistance characteristics over the entire thickness.

[0030] The present invention has been accomplished based on the above findings, and its gist is as follows.

[0031] [1] A thick steel plate having a composition comprising, in mass%, 0.05 to 0.20% C, 0.01 to 0.50% Si, 0.50 to 2.00% Mn, 0.05% or less P, 0.02% or less S, with the balance being Fe and unavoidable impurities,

[0032] The microstructure is as follows: the area ratio of the ferrite phase is more than 80% in the range from the surface to 100 μm below the surface along the plate thickness direction, and the area ratio of the ferrite phase is less than 80% in the range from 100 μm below the surface to the 1 / 4 position of the plate thickness along the plate thickness direction, and the balance is composed of pearlite phase, or a mixed phase of pearlite phase and bainite phase, and the area ratio of the above-mentioned pearlite phase is greater than the area ratio of the above-mentioned bainite phase.

[0033] [2] The thick steel plate according to [1], wherein the above-mentioned composition further contains, in terms of mass%, selected from the group consisting of Cr: 0.01-1.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Mo: 0.01-1.00%, Co: 0.01-1.00%, Sn: 0.005-0.500%, Sb: 0.005-0.200%, Nb: One or more of the following: 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: 0.0001-0.020% and REM: 0.0001-0.020%.

[0034] [3] A method for manufacturing a thick steel plate, wherein:

[0035] The steel material having the composition described in [1] or [2] is heated to 900 to 1200°C.

[0036] The heated steel material is hot rolled at a cumulative reduction ratio of 50% or more to produce a hot-rolled sheet.

[0037] The hot rolled plate is cooled,

[0038] Then, it is reheated to a reheating temperature of not less than the Ac1 transformation point and not more than 950°C.

[0039] The steel sheet reheated to a temperature of not less than the Ac1 transformation point and not more than 950°C is cooled at an average cooling rate of 2 to 7°C / s to a cooling stop temperature of 350 to 600°C.

[0040] The steel plate cooled to the cooling stop temperature of 350 to 600° C. is quenched.

[0041] [4] A method for manufacturing a thick steel plate, wherein:

[0042] The steel material having the composition described in [1] or [2] is heated to 900 to 1200°C.

[0043] The heated steel material is hot rolled at a cumulative reduction ratio of 50% or more to produce a hot-rolled sheet.

[0044] Next, the steel sheet cooled to a temperature not less than the Ar1 transformation point and not more than the Ar3 transformation point is cooled at an average cooling rate of 2 to 7°C / s to a cooling stop temperature of 350 to 600°C.

[0045] The steel plate cooled to the cooling stop temperature of 350 to 600° C. is quenched.

[0046] Effects of the Invention

[0047] According to the present invention, thick steel plates with high strength, excellent elongation properties throughout the thickness, fatigue crack propagation characteristics, and toughness can be obtained. Even if fatigue cracks develop over time in stress concentration areas or welds, the thick steel plates of the present invention can suppress subsequent crack propagation, thereby improving the overall safety of steel structures. Furthermore, by appropriately using the thick steel plates of the present invention in structures such as bridges, ships, building structures, and construction machinery, the maintenance costs of such structures can be reduced, thereby reducing lifecycle costs, making them extremely useful industrially. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a schematic diagram of a single-side notched simple tension fatigue test specimen used in fatigue crack propagation testing. DETAILED DESCRIPTION

[0049] Next, a method for implementing the present invention will be specifically described. It should be noted that the following description shows preferred embodiments of the present invention, but the present invention is not limited to the following description.

[0050] [Ingredients]

[0051] The reasons for limiting the chemical composition of the thick steel plate of the present invention are described below. It should be noted that, unless otherwise specified, "%" in the following description means "mass %".

[0052] C: 0.05~0.20%

[0053] C is an element that increases the hardness of the matrix phase (matrix) and improves the strength. In addition, it has the effect of generating a pearlite phase as a collection of cementite phases, so that fatigue resistance is improved. In order to obtain such an effect, it is necessary to set the C content to 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more, and further preferably 0.12% or more. On the other hand, when the C content exceeds 0.20%, the hardness of the matrix phase increases excessively and the elongation over the total thickness deteriorates. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, more preferably 0.16% or less, and further preferably 0.14% or less.

[0054] Si: 0.01~0.50%

[0055] Si is an element that acts as a deoxidizer and increases the hardness of the matrix phase by solid solution strengthening when dissolved in steel. In order to achieve such an effect, the Si content needs to be set to 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.1% or more, further 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 over the total thickness decrease. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less, further preferably 0.35% or less, and most preferably 0.30% or less.

[0056] Mn: 0.50~2.00%

[0057] Mn is an element that increases the hardness of the matrix phase and improves its strength. To achieve this effect, the Mn content must be set to 0.50% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more, even more preferably 0.80% or more, and most preferably 1.00% or more. On the other hand, if the Mn content exceeds 2.00%, weldability decreases, and MnS, as an inclusion, segregates excessively, degrading toughness. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.85% or less, more preferably 1.70% or less, even more preferably 1.55% or less, and most preferably 1.40% or less.

[0058] P: 0.05% or less

[0059] P is an element contained in steel as an unavoidable impurity. P segregates at grain boundaries, causing adverse effects such as reduced toughness of the base material and welds, so it is preferable to minimize its content, but a content of 0.05% or less is permissible. Therefore, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, and more preferably 0.03% or less. On the other hand, there is no lower limit for the P content, and excessive reduction leads to increased refining costs, so the P content is preferably set to 0.001% or more. The P content is preferably 0.002% or more, and more preferably 0.003% or more.

[0060] S: 0.02% or less

[0061] S is an element contained in steel as an unavoidable impurity. S, present in steel as sulfide inclusions such as MnS, acts as a starting point for brittle fracture and degrades toughness. Therefore, it is best to minimize its content, but a content of 0.02% or less is acceptable. Therefore, the S content is set to 0.02% or less. The S content is preferably set to 0.01% or less. While there is no lower limit for the S content, excessive reduction leads to increased refining costs. Therefore, the S content is preferably set to 0.0005% or more.

[0062] The balance is made up of Fe and inevitable impurities. It should be noted that when the content of oxygen (O) contained as an inevitable impurity exceeds 0.0050%, the proportion of inclusions on the surface of the steel plate increases, and cracks starting from inclusions are easily generated. Therefore, the O content is preferably set to 0.0050% or less. Similarly, when the content of N contained as an inevitable impurity exceeds 0.0050%, the proportion of inclusions on the surface of the steel plate increases, and cracks starting from inclusions are easily generated. Therefore, the N content is preferably set to 0.0050% or less. The N content is more preferably set to 0.0040% or less. Similarly, when the content of sol.Al contained as an inevitable impurity exceeds 0.060%, Al is mixed into the weld metal during welding, and the toughness of the weld is deteriorated. Therefore, the sol.Al content is preferably set to 0.060% or less. The sol.Al content is more preferably set to 0.050% or less, and further preferably set to 0.040% or less.

[0063] Furthermore, in the present invention, one or more selected from the group consisting of Cr: 0.01-1.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Mo: 0.01-1.00%, Co: 0.01-1.00%, Sn: 0.005-0.500%, 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: 0.0001-0.020% and REM: 0.0001-0.020% may be optionally contained.

[0064] Cr: 0.01~1.00%

[0065] Cr is an element that further improves strength. It also promotes cementite formation, thereby accelerating the formation of pearlite, which is beneficial for fatigue resistance. When Cr is included, to achieve the aforementioned effects, the Cr content is set to 0.01% or higher. It is preferably set to 0.10% or higher. On the other hand, a Cr content exceeding 1.00% impairs weldability and toughness. Therefore, when Cr is included, it is set to 1.00% or lower. The Cr content is preferably set to 0.80% or lower, and more preferably to 0.50% or lower.

[0066] Cu: 0.01~2.00%

[0067] Cu is an element that further increases strength by solid solution. In the case of containing Cu, in order to obtain the above-mentioned effect, the Cu content is set to 0.01% or more. It is preferred to set the Cu content to 0.05% or more, and more preferably to set it to 0.10% or more. On the other hand, when the Cu content exceeds 1.00%, weldability is impaired, and defects are easily generated when manufacturing thick steel plates. Therefore, in the case of containing Cu, it is set to 2.00% or less. The Cu content is preferably set to 0.70% or less, more preferably to 0.60% or less, and even more preferably to 0.50% or less.

[0068] Ni: 0.01~2.00%

[0069] Nickel is an element that improves low-temperature toughness and also improves hot brittleness when Cu is included. When nickel is included, the nickel content is set to 0.01% or higher to achieve the above-mentioned effects. It is preferably set to 0.05% or higher. On the other hand, a nickel content exceeding 2.00% impairs weldability and increases steel material costs. Therefore, when nickel is included, it is set to 2.00% or lower. The nickel content is preferably set to 0.70% or lower, and more preferably to 0.40% or lower.

[0070] Mo: 0.01~1.00%

[0071] Mo is an element that increases the hardness of the matrix phase and can be optionally included depending on the desired properties. When Mo is included, to achieve this effect, the Mo content is set to 0.01% or higher. Preferably, the Mo content is set to 0.05% or higher. However, if the Mo content exceeds 1.00%, weldability and toughness are impaired. Therefore, when Mo is included, the Mo content is set to 1.00% or lower. Preferably, the Mo content is set to 0.80% or lower, and more preferably, to 0.70% or lower.

[0072] Co: 0.01~1.00%

[0073] Co is an element that increases the hardness of the matrix phase and can be optionally contained depending on the desired properties. To achieve this effect, when Co is contained, the Co content is set to 0.01% or more. It is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 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. Therefore, when Co is contained, the Co content is set to 1.00% or less. The Co content is preferably set to 0.50% or less.

[0074] Sn: 0.005~0.500%

[0075] Sn is an element that increases the hardness of the matrix phase and can be optionally contained according to the desired properties. In order to fully achieve this effect, when Sn is contained, the content is set to 0.005% or more. It is preferably set to 0.010% or more, more preferably set to 0.020% or more, and further preferably set to 0.030% or more. On the other hand, when the Sn content exceeds 0.500%, the ductility and toughness of the steel deteriorate. Therefore, when Sn is contained, it is set to 0.500% or less. It is preferably 0.300% or less, more preferably 0.200% or less, and further preferably 0.100% or less.

[0076] Sb: 0.005~0.200%

[0077] Sb is an element that increases the hardness of the matrix phase and can be optionally contained depending on the desired properties. In order to fully achieve this effect, when Sb is contained, the content is set to 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%, the ductility and toughness of the steel deteriorate. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. It is preferably 0.150% or less, more preferably 0.100% or less, further preferably 0.080% or less, and most preferably 0.050% or less.

[0078] Nb: 0.005~0.200%

[0079] Nb is an element that has the effect of suppressing the recrystallization of austenite during hot rolling and making the final grains finer. In addition, Nb precipitates during air cooling after accelerated cooling, further improving the strength. When Nb is contained, in order to obtain the above-mentioned effect, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.007% or more, and more preferably set to 0.010% or more. On the other hand, when the Nb content exceeds 0.200%, the hardenability becomes excessive and bainite is excessively generated, so the desired structure cannot be obtained and the toughness is reduced. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.070% or less, more preferably set to 0.050% or less, further preferably set to 0.040% or less, and most preferably set to 0.030% or less.

[0080] V: 0.005~0.200%

[0081] V, like Nb, is an element that suppresses the recrystallization of austenite during hot rolling and refines it, and precipitates during the air cooling process after hot rolling to increase the strength. It can be optionally contained according to the desired properties. In order to achieve the above-mentioned effects, when V is contained, the V content is set to 0.005% or more. The V content is preferably set to 0.010% or more, more preferably 0.020% or more, and further 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 set to 0.200% or less. The V content is preferably set to 0.150% or less, more preferably 0.100% or less, and further preferably 0.070% or less.

[0082] Ti: 0.005~0.050%

[0083] Ti has a strong tendency to form nitrides, fixes N and reduces dissolved N, thus improving the toughness of the base material and the weld. In addition, when B is contained, by simultaneously containing Ti, Ti fixes N and can suppress the precipitation of B in the form of BN. As a result, the hardenability-enhancing effect of B can be promoted, further improving the strength. Therefore, it can be optionally contained according to the desired characteristics. In order to achieve the above-mentioned effect, when Ti is contained, it is set to 0.005% or more. The Ti content is preferably set to 0.007% or more, and more preferably set to 0.010% or more. However, when the Ti content exceeds 0.050%, TiC precipitates in large quantities, impairing the toughness. Therefore, when Ti is contained, the Ti content is set to 0.050% or less. The Ti content is preferably set to 0.040% or less, more preferably set to 0.030% or less, and even more preferably set to 0.020% or less.

[0084] B: 0.0001~0.0050%

[0085] B is an element that significantly improves hardenability and increases strength even in trace amounts, and can be contained depending on the desired properties. To achieve these effects, when B is contained, the content is set to 0.0001% or more. The B content is preferably set to 0.0005% or more, and more preferably to 0.001% or more. However, when the B content exceeds 0.0050%, not only does its effect saturate, but weldability also decreases. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably set to 0.0040% or less, more preferably to 0.0030% or less, and even more preferably to 0.0020% or less.

[0086] Zr: 0.005~0.100%

[0087] Zr is an element that further enhances strength. To fully achieve this effect, when Zr is present, the Zr content is set to 0.005% or higher. The Zr content is preferably set to 0.010% or higher, more preferably 0.030% or higher, and even more preferably 0.050% or higher. On the other hand, the strength-enhancing effect saturates when the Zr content exceeds 0.100%. Therefore, when Zr is present, the Zr content is set to 0.100% or lower.

[0088] Ca: 0.0001~0.020%

[0089] Ca combines with S to suppress the formation of MnS and other inclusions that extend long in the rolling direction, controlling the morphology of sulfide inclusions so that they are spherical, which helps improve the toughness of welds, etc., so it can be contained according to the desired properties. When Ca is contained, in order to achieve this effect, the Ca content is set to 0.0001% or more. The Ca content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more. However, when the Ca content exceeds 0.020%, not only is the effect saturated, but the cleanliness of the steel is reduced, a large number of surface flaws are generated, and the surface properties are reduced. Therefore, when Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably set to 0.010% or less, more preferably set to 0.006% or less, and even more preferably set to 0.002% or less.

[0090] Mg: 0.0001~0.020%

[0091] Mg is an element that improves toughness by reducing grain size. When Mg is included, the Mg content is set to 0.0001% or more to achieve this effect. The Mg content is preferably set to 0.0003% or more, and more preferably 0.0005% or more. On the other hand, the effect saturates when the Mg content exceeds 0.020%. Therefore, when Mg is included, the Mg content is set to 0.020% or less. The Mg content is preferably set to 0.015% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.

[0092] REM: 0.0001~0.020%

[0093] REM (rare earth metals) are elements that have the effect of improving toughness. When adding REM, the REM content is set to 0.0001% or more to achieve the above effect. The REM content is preferably set to 0.0003% or more. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, when adding REM, the REM content is set to 0.020% or less. The REM content is preferably set to 0.010% or less, more preferably set to 0.005% or less, and even more preferably set to 0.001% or less.

[0094] [Microstructure]

[0095] Next, the reasons for limiting the microstructure of thick steel plates will be explained. It should be noted that, unless otherwise specified, the "%" in the description of the microstructure refers to the area ratio. Furthermore, the "front end" of a thick steel plate in the following description is defined as the position 100 mm from the front end toward the rear end in the rolling direction of the steel plate. Similarly, the "rear end" of a thick steel plate is defined as the position 100 mm from the rear end toward the front end in the rolling direction of the steel plate. Furthermore, the "center" of a thick steel plate is defined as the center of the steel plate in the rolling direction (longitudinal direction).

[0096] The tissue from the surface to 100 μm below the surface (surface tissue)

[0097] The microstructure of the thick steel plate of the present invention, from the surface to 100 μm below the surface along the plate thickness direction (hereinafter sometimes referred to as the "surface portion"), contains a ferrite phase comprising 80% or more by area. A surface decarburization reaction occurs in the dual-phase region above the Ac1 transformation point and below the Ac3 transformation point, generating 80% or more of ferrite in the surface portion, thereby softening the surface layer of the thick steel plate. This significantly improves the elongation characteristics through the entire thickness.

[0098] The skin decarburization reaction occurs by passing through or remaining in the two-phase region during reheating.

[0099] If the surface ferrite phase area ratio is less than 80%, a large amount of hard residual structure composed of bainite, pearlite, martensite, or a mixture thereof will be present. As a result, the surface hardness increases, and the desired elongation characteristics through the entire thickness cannot be achieved. In addition, the tensile strength may become too high.

[0100] It should be noted that, here, the area ratio of the ferrite phase in the surface portion refers to the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel plate. In addition, the microstructure of the surface portion refers to the microstructure of the surface portion at the front end, center and tail end in the rolling direction of the thick steel plate. Therefore, the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface at the front end, center and tail end in the rolling direction of the thick steel plate of the present invention is 80% or more. It should be noted that, generally, as long as the microstructure of the surface portion at the front end, center and tail end meets the above conditions, the above conditions are met over the entire length of the thick steel plate in the rolling direction. Therefore, it can be said that the area ratio of the ferrite phase in the surface portion of the thick steel plate of the present invention over the entire length in the rolling direction is 80% or more. That is, in the present invention, the area ratio of the ferrite phase in the surface part is 80% or more, which means that the area ratio of the ferrite phase in the surface part can be obtained at any point of the front end, center, and tail end along the entire length in the rolling direction.

[0101] The balance other than the ferrite phase in the microstructure of the surface layer is preferably composed of a pearlite phase or a mixed phase of a bainite phase and a pearlite phase. However, the bainite phase contains island martensite, which deteriorates the toughness. Therefore, the less bainite phase, the better, and it is more preferable to set it to only the pearlite phase.

[0102] The structure from 100 μm below the surface to 1 / 4 of the plate thickness (structure inside the plate thickness)

[0103] The thick steel plate of the present invention has a microstructure within the range from 100 μm below the surface to a position one-quarter of the plate thickness along the plate thickness direction (hereinafter sometimes referred to as "within the plate thickness") containing a ferrite phase at an area ratio of 80% or less. By ensuring that the microstructure within the plate thickness satisfies these conditions, the desired strength and fatigue crack propagation resistance can be achieved.

[0104] It should be noted that the area ratio of the ferrite phase within the plate thickness refers to the average area ratio of the ferrite phase within the range from 100 μm below the surface to 1 / 4 of the plate thickness of the thick steel plate. Furthermore, the microstructure within the plate thickness refers to the microstructure within the plate thickness at the leading, center, and trailing ends of the thick steel plate in the rolling direction. Therefore, the microstructure of the thick steel plate of the present invention within the range from 100 μm below the surface to 1 / 4 of the plate thickness at the leading, center, and trailing ends of the thick steel plate in the rolling direction satisfies the above-mentioned conditions. It should be noted that, similar to the structure of the surface layer, generally, as long as the microstructure within the plate thickness at the leading, center, and trailing ends satisfies the above-mentioned conditions, the above-mentioned conditions are satisfied over the entire length of the thick steel plate in the rolling direction. Therefore, it can be said that the microstructure within the plate thickness of the thick steel plate of the present invention is ferrite phase at an area ratio of 80% or less over the entire length of the thick steel plate in the rolling direction.

[0105] The present invention is characterized by having the remainder of the microstructure within the plate thickness comprised of pearlite or a mixed phase of pearlite and bainite, with the area fraction of the pearlite phase exceeding that of the bainite phase. Bainite contains islands of martensite, which degrades toughness. Therefore, by increasing the area fraction of the pearlite phase to a greater extent than that of the bainite phase, the desired toughness can be achieved. The area fraction of the bainite phase is preferably set to 15% or less, more preferably 13% or less, and even more preferably 11% or less.

[0106] It should be noted that the excess of the thick steel plate of the present invention refers to the excess in the surface layer at the leading, middle, and trailing ends, and within the plate thickness. That is, along the entire length of the thick steel plate in the rolling direction, the excess of the microstructure consists of a pearlite phase or a mixed phase of pearlite and bainite, and the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

[0107] The microstructures of the surface layer and the inner thickness of the plate can be evaluated by the method described in the Examples.

[0108] [Total thickness elongation]

[0109] The total thickness elongation of a thick steel plate is not particularly limited; however, it is preferably 19% or greater for a plate thickness exceeding 16 mm, and 15% or greater for a plate thickness of 16 mm or less. In the present invention, the above-mentioned total thickness elongation conditions are preferably met at the leading, middle, and trailing ends of the thick steel plate in the rolling direction. It should be noted that, generally, as long as the above-mentioned conditions are met at the leading, middle, and trailing ends, they are met over the entire length of the thick steel plate in the rolling direction. Furthermore, the total thickness elongation can be measured using the method described in the Examples.

[0110] [Tensile strength]

[0111] The tensile strength (TS) of the thick steel plate is not particularly limited, but is preferably 490 MPa or more. In addition, the upper limit of TS is also not particularly limited. For example, when the upper limit is set to 490 MPa (50 kgf / mm2) of JIS, the upper limit of TS is preferably 490 MPa (50 kgf / mm2). 2 ) level, TS is set to 610MPa or less. In addition, when JIS 570MPa (60kgf / mm 2 ) level, the upper and lower limits of TS can be set to 570 MPa and 720 MPa, respectively. In the present invention, the above TS conditions are preferably met at the leading, middle, and trailing ends of the thick steel plate in the rolling direction. It should be noted that, generally, as long as the leading, middle, and trailing ends meet the above conditions, they are met along the entire length of the thick steel plate in the rolling direction. TS can also be measured using the method described in the Examples.

[0112] [toughness]

[0113] The thick steel plate of the present invention has the above-mentioned composition and microstructure, and as a result, has excellent toughness. The toughness of the thick steel plate of the present invention is not particularly limited. When the test piece thickness is 10 mm, the Charpy absorbed energy vE0 at 0°C, which is one of the indicators of toughness, is preferably set to 100 J or more, more preferably to 130 J or more, further preferably to 150 J or more, and most preferably to 200 J or more. On the other hand, there is no upper limit on vE0, and for example, it can be 400 J or less, 300 J or less, or 270 J or less. When the test piece thickness is 5 mm, the Charpy absorbed energy vE0 at 0°C is preferably set to 50 J or more. On the other hand, there is no upper limit on vE0, and for example, it can be 200 J or less, 150 J or less, or 135 J or less. It should be noted that vE0 can be measured by the method described in the examples.

[0114] [Fatigue crack propagation characteristics]

[0115] The thick steel plate of the present invention has the above-mentioned composition and microstructure, and as a result, it can have excellent fatigue crack propagation characteristics. As an indicator of fatigue crack propagation characteristics, the fatigue crack propagation velocity (da / dN) can be used. The value of the fatigue crack propagation velocity is not particularly limited. In the present invention, the fatigue crack propagation velocity when ΔK = 25 MPa√m is preferably 4.25×10 -8 m / cycle or less.

[0116] [Plate thickness]

[0117] In the present invention, "thick steel plate" is generally defined in the technical field as steel plate with a thickness of 6 mm or greater. Meanwhile, the upper limit of the thickness of the thick steel plate in the present invention is not particularly limited and can be set to any value. However, as mentioned above, temperature deviations between the leading and trailing ends of the steel plate can easily become large, and the present invention is particularly effective for thin products requiring excellent elongation properties across the entire thickness. Therefore, the thickness of the thick steel plate is preferably set to 25 mm or less, and more preferably 20 mm or less.

[0118] [Manufacturing method]

[0119] The thick steel plate of the present invention can be obtained by sequentially subjecting a steel material having the above-mentioned composition to heating, hot rolling, cooling, reheating, cooling, and quenching, or by sequentially subjecting a steel material to heating, hot rolling, cooling, and quenching. First, the method of sequentially subjecting a steel material to heating, hot rolling, cooling, reheating, cooling, and quenching will be described.

[0120] Steel raw materials

[0121] The steel material of the present invention can be any material as long as it has the above-mentioned composition and can be hot-rolled, and generally, a steel billet will suffice. For example, molten steel having the above-mentioned composition can be melted in a converter or other means, and then cast into a steel billet or other steel material using a casting method such as continuous casting. Alternatively, a steel billet or other steel material can be produced by an ingot casting and split rolling method.

[0122] heating

[0123] The steel material having the above composition is heated to 900-1200°C. If the heating temperature is lower than 900°C, the deformation resistance of the steel material in the subsequent hot rolling process increases, increasing the load on the hot rolling mill and making hot rolling difficult. Therefore, the heating temperature is set to 900°C or higher. It is preferably set to 950°C or higher. On the other hand, if the heating temperature exceeds 1200°C, the toughness decreases. Therefore, the heating temperature is set to 1200°C or lower. It is preferably set to 1150°C or lower.

[0124] It should be noted that, when the steel raw material (steel slab) is produced by a method such as continuous casting, the steel slab may be directly supplied to the above-mentioned heating step without cooling, or may be supplied to the above-mentioned heating step after cooling. In addition, the heating method is not particularly limited, and for example, heating may be performed using a heating furnace according to a conventional method.

[0125] Hot Rolling

[0126] Next, the heated steel material is hot-rolled to produce hot-rolled plate. To ensure toughness, a fundamental property of the finished steel plate, the cumulative reduction is set to 50% or higher. If the cumulative reduction is less than 50%, the ferrite grains within the plate thickness coarsen, creating localized areas of low brittleness, which can easily cause brittle cracking and degrade toughness. Other conditions for the hot rolling process are not particularly limited.

[0127] cool down

[0128] Next, the steel plate after hot rolling is cooled (first cooling process). In the cooling process, when reheating is performed, it is preferably cooled to room temperature. It should be noted that cooling can be performed by any method, such as air cooling or accelerated cooling. In addition, there is no particular limitation on the cooling conditions.

[0129] Reheating

[0130] Next, the cooled steel sheet is reheated to a temperature between the Ac1 transformation point and 950°C. The reheating temperature is preferably set below the Ac3 transformation point. By heating the steel sheet to a temperature range encompassing the austenite phase, between the Ac1 transformation point and 950°C, microstructural variations caused by cooling variations can be eliminated, thereby eliminating variations in mechanical properties. A reheating temperature below the Ac3 transformation point is preferred because it does not damage the pre-reheating microstructure.

[0131] When the reheating temperature is above the Ac1 transformation point and below the Ac3 transformation point, a unique decarburization reaction occurs in the dual-phase region, enabling the surface ferrite phase area ratio to be increased to 80% or more. On the other hand, when the reheating temperature is above the Ac3 transformation point and below 950°C, by shortening the holding time at the reheating temperature, the austenitic transformation of the surface ferrite phase generated by the surface decarburization reaction during passage through the dual-phase region can be suppressed, enabling the surface ferrite phase area ratio to be increased to 80% or more.

[0132] When the reheating temperature exceeds 950°C, the reverse transformation of the ferrite phase in the surface layer, generated by the surface decarburization reaction during passage through the dual-phase region, into the austenite phase is accelerated, resulting in an area ratio of the ferrite phase in the surface layer falling below 80%. As a result, the hardness of the surface layer increases, and the desired elongation characteristics through the thickness cannot be achieved.

[0133] Furthermore, it can be seen that when the reheating temperature is above the Ac3 transformation point and below 950°C, while the grain size of the austenite phase within the plate thickness increases compared to when the reheating temperature is below the Ac3 transformation point, the toughness does not deteriorate excessively. Furthermore, within this temperature range, the rate of reverse transformation to the austenite phase within the plate thickness increases. Therefore, since the desired parent phase structure is formed within a short heating time, the number of thick steel plates that can be produced within a specified time increases, thereby improving productivity.

[0134] On the other hand, when the temperature exceeds 950°C, the reversely transformed austenite phase grows and coarsens within the plate thickness, resulting in the formation of a localized region with low toughness, thereby reducing the toughness.

[0135] On the other hand, if the reheating temperature is lower than the Ac1 transformation point, reverse transformation to austenite does not occur, and the desired area ratios of ferrite, pearlite, and bainite within the plate thickness after cooling are not achieved. As a result, fatigue properties (crack propagation characteristics) deteriorate. Furthermore, variations in mechanical properties caused by cooling variations during the cooling process after hot rolling cannot be eliminated.

[0136] It should be noted that the Ac1 phase transition point can be obtained, for example, by the following formula (1).

[0137] Ac1(℃)=723+29.1×Si-10.7×Mn-16.9×Ni+16.9×Cr…(1)

[0138] In addition, the Ac3 phase transition point can be obtained by, for example, the following formula (2).

[0139] Ac3(℃)=961.6-311.9×C+49.5×Si-36.4×Mn+438.1×P-2818×S+12.7×Al-51×C u-29×Ni-8.7×Cr+13.5×Mo+308.1×Nb-140×V+318.9×Ti+611.2×B-969×N…(2)

[0140] Here, the element symbols in the above formulas (1) and (2) refer to the content (mass %) of each element, and are set to zero when the element is not contained.

[0141] It should be noted that in the above-mentioned reheating treatment, it is preferred to heat to the reheating temperature and then maintain it at that temperature. When the reheating temperature is above the Ac1 transformation point and below the Ac3 transformation point, if the holding time is less than 10 minutes, the reverse phase transformation to the austenite phase does not start along the entire length of the steel plate, and sometimes the hardenability is significantly reduced in a part of the area. Therefore, the holding time is preferably set to more than 10 minutes. On the other hand, when the reheating temperature is above the Ac3 transformation point and below 950°C, if the holding time exceeds 30 minutes, the austenite phase grows and coarsens. Therefore, the holding time is preferably set to less than 30 minutes.

[0142] cool down

[0143] The steel plate reheated by the above-mentioned reheating process or the hot-rolled steel plate is cooled to a cooling stop temperature of 350 to 600°C (second cooling process). At this time, the average cooling rate is set to 2 to 7°C / second. The lower the average cooling rate, the more pearlite transformation is promoted, which is preferable in terms of toughness improvement. However, when the average cooling rate is less than 2°C / second, the grain growth of ferrite becomes excessive and coarsening occurs, thereby deteriorating toughness. Therefore, the average cooling rate is set to 2°C / second or more. On the other hand, when the average cooling rate exceeds 7°C / second, pearlite transformation does not proceed sufficiently in the microstructure inside the steel plate, and bainite transformation and martensite transformation are easy to proceed. In this case, the fraction of bainite phase and martensite phase increases, thereby deteriorating the elongation characteristics and toughness in the total thickness. Therefore, the average cooling rate is set to 7°C / second or less. The average cooling rate is preferably set to 5°C / second or less, more preferably set to 4°C / second or less, and further preferably set to less than 3°C / second.

[0144] Furthermore, if the cooling stop temperature is below 350°C, excessive ferrite forms within the plate thickness, softening the entire steel plate and failing to achieve the desired tensile strength. Therefore, the cooling stop temperature is set above 350°C. On the other hand, if the cooling stop temperature exceeds 600°C, quenching is performed while a large amount of untransformed austenite remains, leading to excessive formation of hard bainite and martensite. As a result, the elongation characteristics through the entire thickness decrease, and toughness also deteriorates. Therefore, the cooling stop temperature is set below 600°C.

[0145] Quenching

[0146] The steel plate cooled to the cooling stop temperature is quenched. Therefore, the quenching temperature is in the range of 350-600°C. Quenching is not particularly limited and can be performed under any conditions, but water cooling to a temperature below the Ms point, preferably below 200°C, is preferred. The Ms point can be calculated, for example, using the following formula (3).

[0147] Ms(℃)=517-300×C-11×Si-33×Mn-17×Ni-22×Cr-11×Mo…(3)

[0148] Here, the element symbols in the above formula (3) refer to the content (mass %) of each element, and are set to zero when the element is not contained.

[0149] Next, a method of sequentially performing the treatments of heating, hot rolling, cooling, and quenching will be described.

[0150] The steel material used is the same material 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.

[0151] Regarding cooling after hot rolling, the steel is first cooled to a temperature above the Ar1 transformation point and below the Ar3 transformation point, and then cooled from the temperature above the Ar1 transformation point and below the Ar3 transformation point (cooling start temperature) to a cooling stop temperature of 350 to 600°C at an average cooling rate of 2 to 7°C / second. The reason for setting the cooling start temperature to a temperature above the Ar1 transformation point and below the Ar3 transformation point (dual phase region) is that a unique decarburization reaction occurs in the dual phase region, which can increase the area ratio of the ferrite phase in the surface layer to 80% or more.

[0152] In addition, as a reason for setting the subsequent average cooling rate to 2 to 7°C / second, the following reasons can be cited. When the average cooling rate is less than 2°C / second, the grain growth of ferrite becomes excessive and coarsened, so the toughness deteriorates. Therefore, the average cooling rate is set to 2°C / second or more. On the other hand, when the average cooling rate exceeds 7°C / second, in the microstructure inside the steel plate, the pearlite transformation does not proceed sufficiently, and the bainite transformation and martensite transformation are easy to proceed. In this case, the fraction of the bainite phase and the martensite phase increases, so the elongation characteristics and toughness over the total thickness deteriorate. Therefore, the average cooling rate is set to 7°C / second or less. The average cooling rate is preferably set to 5°C / second or less, more preferably set to 4°C / second or less, and further preferably set to less than 3°C / second.

[0153] In addition, the following reasons can be cited for setting the cooling stop temperature to 350-600°C. When the cooling stop temperature is lower than 350°C, ferrite is excessively generated inside the plate thickness, so the entire steel plate softens and the desired tensile strength cannot be obtained. Therefore, the cooling stop temperature is set to 350°C or above. On the other hand, when the cooling stop temperature exceeds 600°C, quenching is performed in a state where a large amount of untransformed austenite remains, so hard bainite and martensite are excessively generated. As a result, the elongation characteristics over the total thickness are reduced and the toughness is also deteriorated. Therefore, the cooling stop temperature is set to 600°C or below.

[0154] The subsequent quenching can be performed by the same method as the quenching described above.

[0155] It should be noted that the Ar1 phase transition point can be obtained, for example, by the following formula (4).

[0156] Ar1=712-17.8×C-19.1×Ni+20.1×Si+11.9×Cr+9.8×Mo…(4)

[0157] In addition, the Ar3 transformation point can be obtained by, for example, the following formula (5).

[0158] Ar3=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo…(5)

[0159] Here, the element symbols in the above formulas (4) and (5) refer to the content (mass %) of each element, and are set to zero when the element is not contained.

[0160] Example

[0161] Hereinafter, the effects of the present invention will be described in detail based on Examples, but the present invention is not limited to these Examples.

[0162] Molten steel having the composition shown in Table 1 was melted to produce steel billets. The values ​​of the Ac1 point, Ac3 point, Ms point, Ar1 point, and Ar3 point shown in Table 1 are the values ​​obtained by the above-mentioned equations (1), (2), (3), (4), and (5), respectively.

[0163] The resulting steel slabs were then heated and hot-rolled under the conditions shown in Table 2 to produce hot-rolled plates with a total length of 20 m and the thicknesses shown in Table 2. The hot-rolled plates were then cooled to room temperature using the cooling method described in Table 2, then reheated to the reheating temperature shown in Table 2 and held for at least 30 minutes. Cooling water was then sprayed on both sides of the steel plates, and the plates were cooled to the cooling stop temperature at the average cooling rate shown in Table 2. The plates were then quenched. During the quenching process, the plates were water-cooled to a temperature below 150°C.

[0164] For comparison purposes, in one comparative example (No. 24 in Table 2), cooling that satisfies the conditions of the present invention was not performed after reheating, but quenching was performed immediately. The quenching conditions in this comparative example were an average cooling rate of 44.0°C / second and a cooling stop temperature of 110°C.

[0165] The obtained thick steel plates were 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 properties along the entire length of the thick steel plates, test pieces were cut from the front end, center, and tail end of the thick steel plates in the rolling direction. The test method is as follows. It should be noted that the test pieces at the front end and tail end were cut from a position 100 mm away from the end of the steel plate in the rolling direction.

[0166] (1) Microstructure observation

[0167] Follow the steps below to observe the microstructure.

[0168] The area ratio of the ferrite phase in the surface layer, the area ratio of the ferrite phase in the inner thickness of the plate, and the area ratio of the pearlite phase and bainite phase in the inner thickness of the plate

[0169] First, a test piece for microstructure observation was cut from the obtained thick steel plate with the observation surface being a cross section perpendicular to the rolling direction (a cross section in the plate thickness direction). After being polished to a mirror surface, it was etched with an etching solution (nitric acid methanol solution). Using an optical microscope (magnification: 400x), the steel plate surface was observed along the plate thickness direction to the 1 / 4 position, and the images were photographed continuously. Using the obtained microstructure photographs, phases were identified by image analysis, and (a) the average area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel plate, (b) the average area ratio of the ferrite phase in the range from 100 μm below the surface to the 1 / 4 position 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 determined.

[0170] Table 3 shows the measurement results of the microstructure.

[0171] (2) Tensile test

[0172] Tensile testing was conducted using JIS Z2201 1A total thickness test pieces cut from the center of the thick steel plate with the plate width aligned with the tensile direction. The tensile strength (TS) and total thickness elongation were determined. A tensile strength of 490 MPa or greater was considered acceptable. For elongation characteristics, a value of 15% or greater was considered acceptable for plate thicknesses of 16 mm or less, and a value of 19% or greater was considered acceptable for plate thicknesses exceeding 16 mm.

[0173] (3) Fatigue crack propagation test

[0174] use Figure 1The fatigue crack propagation test was conducted on the single-side notched simple tensile fatigue test piece shown in the figure, and the fatigue crack propagation behavior when the crack propagates along the plate thickness direction was evaluated. The test conditions were set to 0.1 for the stress ratio and 10 Hz for the frequency according to ASTM E647, and were carried out in the atmosphere at room temperature. In the present invention, since the purpose is to reduce the propagation speed of cracks generated from the weld part and the like in the welded structure when they propagate in the steel, such a situation was assumed and the test was conducted in the range of stress intensity factor (ΔK) of 10 to 30 MPa√m. The fatigue crack propagation speed when ΔK = 25 MPa√m is 4.25×10 -8 m / cycle or less was considered acceptable.

[0175] (4) Toughness

[0176] A Charpy impact test piece is cut from the center of the thick steel plate in parallel with the rolling direction (L direction). Regarding the test piece thickness, the test piece thickness is set to 10 mm when the plate thickness is 10 mm or more, and the test piece thickness is set to 5 mm when the plate thickness is less than 10 mm. The test is carried out at 0°C in accordance with JIS Z 2202, and the absorbed energy vE0 is measured. For test pieces with a thickness of 10 mm, an absorbed energy of 100 J or more is set as qualified. For test pieces with a thickness of 5 mm, an absorbed energy of 50 J or more is set as qualified.

[0177] The measurement results are shown in Table 4. These results show that the Examples, which met the requirements of the present invention, produced thick steel plates with excellent elongation characteristics through the thickness, fatigue crack propagation resistance, and toughness. On the other hand, the Comparative Examples, which did not meet the requirements of the present invention, exhibited poor at least one of the elongation characteristics through the thickness, fatigue crack propagation rate, and toughness at at least one of the leading and trailing ends of the steel plates.

[0178]

[0179]

[0180]

[0181]

Claims

1. A thick steel plate having a composition comprising, in mass%, 0.05-0.20% C, 0.01-0.50% Si, 0.50-2.00% Mn, 0.05% or less P, 0.02% or less S, with the balance being Fe and unavoidable impurities, The microstructure is as follows: the area ratio of the ferrite phase is more than 80% in the range from the surface to 100 μm below the surface along the plate thickness direction, and the area ratio of the ferrite phase is less than 80% in the range from 100 μm below the surface to the 1 / 4 position of the plate thickness along the plate thickness direction, and the balance is composed of pearlite phase, or a mixed phase of pearlite phase and bainite phase, and the area ratio of the pearlite phase is greater than the area ratio of the bainite phase.

2. The thick steel plate according to claim 1, wherein: The above-mentioned composition further contains, in terms of mass%, one or more selected from the group consisting of Cr: 0.01-1.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Mo: 0.01-1.00%, Co: 0.01-1.00%, Sn: 0.005-0.500%, 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: 0.0001-0.020% and REM: 0.0001-0.020%.

3. A method for manufacturing a thick steel plate, wherein: The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200° C. The heated steel material is hot rolled at a cumulative reduction ratio of 50% or more to produce a hot-rolled sheet. Cooling the hot rolled plate, Then, it is reheated to a reheating temperature of not less than the Ac1 transformation point and not more than 950°C. The steel plate reheated to a temperature of not less than the Ac1 transformation point and not more than 950°C is cooled to a cooling stop temperature of 350 to 600°C at an average cooling rate of not less than 2°C / s and less than 3°C / s. The steel plate cooled to the cooling stop temperature of 350 to 600° C. is quenched. The Ac1 phase transition point is obtained by the following formula (1): Ac1=723+29.1×Si-10.7×Mn-16.9×Ni+16.9×Cr…(1) The element symbols in the above formula (1) refer to the mass % content of each element, and are set to zero when the element is not contained.

4. A method for manufacturing a thick steel plate, wherein: The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200° C. The heated steel material is hot rolled at a cumulative reduction ratio of 50% or more to produce a hot-rolled sheet. Next, the steel sheet cooled to a temperature not less than the Ar1 transformation point and not more than the Ar3 transformation point is cooled to a cooling stop temperature of 350 to 600°C at an average cooling rate not less than 2°C / s and less than 3°C / s. The steel plate cooled to the cooling stop temperature of 350 to 600° C. is quenched. The Ar1 phase transition point is obtained by the following formula (4), and the Ar3 phase transition point is obtained by the following formula (5): Ar1=712-17.8×C-19.1×Ni+20.1×Si+11.9×Cr+9.8×Mo…(4) Ar3=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo…(5) The element symbols in the above formulas (4) and (5) refer to the mass % content of each element, and are set to zero when the element is not contained.

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