Thick steel sheet and manufacturing method therefor

Abstract

Provided is a thick steel sheet having excellent tensile properties in the sheet thickness direction even when a large amount of impurity elements are contained. The thick steel sheet has a component composition containing, in terms of mass%: 0.050-0.200% of C; 0.02-0.60% of Si; 0.50-1.80% of Mn; 0.003-0.030% of P; 0.0005-0.0080% of S; 0.02-0.50% of Cu; 0.02-0.50% of Cr; 0.005-0.050% of Sn; 0.002-0.030% of Ti; 0.010-0.100% of Al; and 0.0010-0.0080% of N, with the balance consisting of Fe and unavoidable impurities. The thick steel sheet has a sheet thickness of 50 mm or less and a reduction of area of 50% or more as measured by a tensile test in the sheet thickness direction.

Description

Thick steel plate and method for manufacturing the same

[0001] The present invention relates to a thick steel plate and a method for manufacturing the same.

[0002] Conventionally, steel plates have mainly been manufactured using high-quality blast furnace pig iron. However, in recent years, due to the trend of decarbonization, it is considered that the usage ratio of scrap in steel plate manufacturing will increase. When the usage ratio of scrap is increased, the concentration of alloying elements (impurity elements) such as Cu and Sn mixed in from the scrap increases compared to the case of using conventional blast furnace pig iron. When a large amount of these impurity elements is contained in the steel plate, it is known that surface defects are likely to occur during manufacturing. For example, Patent Document 1 discloses a manufacturing method for suppressing surface defects by appropriately controlling the components.

[0003] Japanese Patent Application Laid-Open No. 2022-175772

[0004] Regarding techniques for suppressing surface defects, studies have been conducted as in Patent Document 1. On the other hand, among steel plates, thick steel plates used for welded structures such as ships, buildings, and bridges are required to have tensile properties in the plate thickness direction from the perspective of the soundness of the structures. With conventional techniques, it is difficult to ensure tensile properties in the plate thickness direction when a large amount of impurity elements is contained, and there have been few examples of studies on tensile properties in the plate thickness direction.

[0005] The present invention has been made in view of the above problems, and an object thereof is to provide a thick steel plate having excellent tensile properties in the plate thickness direction even when a large amount of impurity elements is contained.

[0006] In order to achieve the above problems and obtain a thick steel plate suitable as a welded structural material, the inventors of the present invention conducted intensive research on the component composition, manufacturing method, etc. of the steel plate, and obtained the following findings.

[0007] (1) When impurity elements such as Sn are contained, the impurity elements concentrate in the center of the plate thickness, and porosity is formed, resulting in a decrease in tensile properties in the plate thickness direction.

[0008] (2) In order to suppress the center segregation of impurity elements, it is necessary to perform a process for reducing center segregation from the casting stage. Furthermore, it is also necessary to apply sufficient processing to the center of the plate thickness during hot rolling.

[0009] This invention was made by further considering the above findings, and its gist is as follows.

[0010] 1. A thick steel sheet having a composition in mass%, containing C: 0.050-0.200%, Si: 0.02-0.60%, Mn: 0.50-1.80%, P: 0.003-0.030%, S: 0.0005-0.0080%, Cu: 0.02-0.50%, Cr: 0.02-0.50%, Sn: 0.005-0.050%, Ti: 0.002-0.030%, Al: 0.010-0.100%, and N: 0.0010-0.0080%, with the remainder being Fe and unavoidable impurities, having a thickness of 50 mm or less, and a reduction in area measured by a tensile test in the thickness direction of the sheet, of 50% or more.

[0011] 2. The thick steel plate according to 1, wherein the component composition further contains one or more selected from the group consisting of, by mass%, Ni: 0.50% or less, Mo: 0.50% or less, Nb: 0.030% or less, V: 0.050% or less, B: 0.0050% or less, Sb: 0.300% or less, W: 2.00% or less, Co: 2.00% or less, Ca: 0.0050% or less, Mg: 0.0100% or less, Zr: 0.0050% or less, Ta: 0.20% or less, Y: 0.010% or less, and REM: 0.0200% or less.

[0012] 3. The thick steel plate according to 1 or 2, wherein the number of Mn segregation grains having a length of 500 μm or more in the width direction of the plate in the microstructure of the thick steel plate is 0.3 or less per 1 mm in the width direction of the plate.

[0013] 4. A method for manufacturing a thick steel plate according to any one of the above 1 to 3, comprising: continuously casting molten steel to form a slab; heating the slab; and hot rolling the heated slab to form a thick steel plate, wherein in the continuous casting, a reduction gradient of 0.30 mm / m or more is performed upstream of the final solidification position of the slab; in the heating, the heating temperature is 1000 to 1250°C; and in the hot rolling, the number of rolling passes in which the plate thickness on the pass entry side is 80 mm or less and the pass reduction ratio is 7% or more is 3 or more, and the temperature at the center of the plate thickness at the start of the first rolling pass is 900°C or higher.

[0014] 5. The method for manufacturing a thick steel plate according to item 4, wherein the thick steel plate after hot rolling is subjected to accelerated cooling.

[0015] According to the present invention, it is possible to provide a thick steel sheet that has excellent tensile properties in the thickness direction even when it contains a large amount of impurity elements.

[0016] The embodiments of the present invention will be described below. The following description illustrates a preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

[0017] [Component Composition] The component composition of the thick steel plate of the present invention and the reasons for its limitations are described below. Unless otherwise specified, the "%" representing the content of each component means "mass %".

[0018] C: 0.050 to 0.200% C is an element that has the effect of increasing the strength of thick steel plates. If the C content is less than 0.050%, this effect cannot be obtained. Therefore, the C content should be 0.050% or more, preferably 0.060% or more. On the other hand, if the C content exceeds 0.200%, the properties of the welded joint deteriorate. Therefore, the C content should be 0.200% or less, preferably 0.180% or less, and more preferably 0.160% or less.

[0019] Si: 0.02-0.60% Si is an element that acts as a deoxidizing agent in the steelmaking process and also has the effect of increasing the strength of steel sheets through solid solution strengthening. If the Si content is less than 0.02%, these effects cannot be obtained. Therefore, the Si content should be 0.02% or more. On the other hand, if the Si content exceeds 0.60%, the weldability and surface properties deteriorate. Therefore, the Si content should be 0.60% or less, preferably 0.50% or less, and more preferably 0.30% or less.

[0020] Mn: 0.50-1.80% Mn is an element that enhances hardenability and increases the strength of thick steel plates. To obtain these effects, the Mn content should be 0.50% or more, preferably 0.60% or more. On the other hand, if the Mn content exceeds 1.80%, central segregation is promoted, and the tensile properties in the thickness direction of the plate deteriorate. Therefore, the Mn content should be 1.80% or less, preferably 1.60% or less.

[0021] P: 0.003 to 0.030%. P is an element that reduces grain boundary strength by segregating at grain boundaries and contributes to the formation of fracture initiation points, so it is preferable to reduce it as much as possible. If the P content exceeds 0.030%, the desired tensile properties in the thickness direction of the plate cannot be obtained. For this reason, the P content should be 0.030% or less, preferably 0.020% or less. On the other hand, there are industrial limits to reducing the P content. For this reason, the P content should be 0.003% or more.

[0022] S: 0.0005 to 0.0080% S is an element that forms MnS in steel and reduces the tensile properties in the thickness direction, so it is preferable to reduce it as much as possible. If the S content exceeds 0.0080%, the desired tensile properties in the thickness direction cannot be obtained. For this reason, the S content should be 0.0080% or less. On the other hand, there are industrial limits to how much the S content can be reduced. For this reason, the S content should be 0.0005% or more.

[0023] Cu: 0.02-0.50% Cu is an element that is mixed in from raw materials such as scrap, and is difficult to remove. Therefore, the Cu content should be 0.02% or more. On the other hand, if the Cu content exceeds 0.50%, the surface properties of the steel material deteriorate and the manufacturability decreases. Therefore, the Cu content should be 0.50% or less, preferably 0.30% or less.

[0024] Cr: 0.02-0.50% Cr is an element that is mixed in from raw materials such as scrap. Therefore, the Cr content should be 0.02% or more. On the other hand, if the Cr content exceeds 0.50%, it will lead to increased costs and a decrease in toughness due to carbide formation. Therefore, the Cr content should be 0.50% or less, preferably 0.30% or less.

[0025] Sn: 0.005-0.050% Sn is an element that is mixed in from raw materials such as scrap, and is difficult to remove. Therefore, the Sn content should be 0.005% or more, preferably 0.010% or more. On the other hand, if the Sn content exceeds 0.050%, the tensile properties in the thickness direction of the plate will decrease. Therefore, the Sn content should be 0.050% or less, preferably 0.030% or less.

[0026] Ti: 0.002-0.030% Ti is an element that precipitates finely as nitrides or carbonitrides, improving the tensile properties in the thickness direction of the sheet. To obtain this effect, the Ti content should be 0.002% or more, preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, if the Ti content exceeds 0.030%, coarse carbonitrides will precipitate, and the tensile properties in the thickness direction of the sheet will decrease. For this reason, the Ti content should be 0.030% or less, preferably 0.020% or less, and more preferably 0.018% or less.

[0027] Al: 0.010-0.100% Al acts as a deoxidizing agent and is therefore the most commonly used in molten steel deoxidation processes. It also fixes dissolved nitrogen in the steel to form AlN, and has the effect of suppressing toughness deterioration by reducing dissolved nitrogen. To obtain such effects, the Al content should be 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 0.100%, the number of inclusions in the steel increases and the tensile properties in the thickness direction of the plate deteriorate. For this reason, the Al content should be 0.100% or less, preferably 0.070% or less, and more preferably 0.060% or less.

[0028] N: 0.0010 to 0.0080% N is an element that degrades surface properties by forming coarse nitrides and carbonitrides, and it is preferable to reduce it as much as possible. Therefore, the N content should be 0.0080% or less, preferably 0.0060% or less, and more preferably 0.0040% or less. On the other hand, there are industrial limits to reducing the N content. Therefore, the N content should be 0.0010% or more.

[0029] The component composition in one embodiment of the present invention has a component composition that contains the above-mentioned elements, with the remainder being Fe and unavoidable impurities.

[0030] Furthermore, in other embodiments of the present invention, the above component composition may optionally further contain at least one selected from the group consisting of Ni, Mo, Nb, V, B, Sb, W, Co, Ca, Mg, Zr, Ta, Y, and REM.

[0031] Ni: 0.50% or less. Ni is an element that further improves the properties of thick steel plates and can be included in any amount depending on the desired properties. However, since Ni is an expensive element, the cost of thick steel plates increases significantly if the Ni content is high. Therefore, when Ni is included, the Ni content should be 0.50% or less. Preferably, the Ni content is 0.30% or less. On the other hand, there is no particular lower limit to the Ni content, but in order to fully obtain the above effect, it is preferable that the Ni content be 0.02% or more. More preferably, the Ni content is 0.10% or more.

[0032] Mo: 0.50% or less. Mo is an element that further improves the strength of thick steel plates and can be included in any amount depending on the required strength. However, if the Mo content exceeds 0.50%, it will lead to an increase in costs. Therefore, when Mo is included, the Mo content should be 0.50% or less. Preferably, the Mo content is 0.30% or less. On the other hand, there is no particular lower limit to the Mo content, but in order to fully obtain the above effect, it is preferable that the Mo content be 0.005% or more. More preferably, the Mo content is 0.05% or more.

[0033] Nb: 0.030% or less. Nb is an element that has the effect of further improving the strength of thick steel plates, and can be included as desired depending on the required strength. However, if the Nb content exceeds 0.030%, carbonitrides precipitate and coarseen, and the tensile properties in the thickness direction of the plate deteriorate. Therefore, when Nb is included, the Nb content should be 0.030% or less. The Nb content is preferably 0.025% or less, and more preferably 0.022% or less. On the other hand, there is no particular lower limit to the Nb content, but in order to fully obtain the above effect, an Nb content of 0.002% or more is preferred. The Nb content is more preferably 0.005% or more, and even more preferably 0.007% or more.

[0034] V: 0.050% or less. V is an element that has the effect of further improving the strength of thick steel plates, and can be included in any amount depending on the required strength. However, if the V content exceeds 0.050%, coarse carbonitrides will precipitate, and the tensile properties in the thickness direction of the plate will decrease. Therefore, when V is included, the V content should be 0.050% or less. Preferably, the V content is 0.040% or less. On the other hand, there is no particular lower limit to the V content, but in order to fully obtain the above effect, a V content of 0.002% or more is preferable.

[0035] B: 0.0050% or less. B is an element that has the effect of further improving the strength of the base material and can be included as desired depending on the required strength. However, if the B content exceeds 0.0050%, coarse B precipitates are formed, and the tensile properties in the thickness direction of the plate decrease. Therefore, when B is included, the B content should be 0.0050% or less. Preferably, the B content is 0.0020% or less. On the other hand, there is no particular lower limit to the B content, but in order to fully obtain the above effect, a B content of 0.0003% or more is preferable.

[0036] Sb: 0.300% or less. Sb is an element that promotes central segregation and reduces tensile properties in the thickness direction of the plate. Therefore, if Sb is included, the Sb content should be 0.300% or less. Preferably, the Sb content is 0.050% or less, and more preferably 0.030% or less. On the other hand, Sb may be introduced from raw materials such as scrap, and its removal is often difficult. Therefore, the Sb content may be 0.002% or more.

[0037] W: 2.00% or less. W is an element that further improves corrosion resistance and can be included in any amount depending on the required corrosion resistance. However, if the W content exceeds 2.00%, the tensile properties in the thickness direction of the plate will decrease due to the formation of precipitates. Therefore, when W is included, the W content should be 2.00% or less. Preferably, the W content is 0.50% or less. On the other hand, there is no particular lower limit to the W content. Since W is effective even in small amounts, the W content may be greater than 0%.

[0038] Co: 2.00% or less. Co is an element that further improves corrosion resistance and can be included as desired depending on the required corrosion resistance. However, if the Co content exceeds 2.00%, it becomes disadvantageous from a cost standpoint. Therefore, when Co is included, the Co content should be 2.00% or less. Preferably, the Co content is 1.50% or less. On the other hand, there is no particular lower limit to the Co content. Since Co is effective even in small amounts, the Co content may be greater than 0%. Preferably, the Co content is 0.10% or more.

[0039] Ca: 0.0050% or less. Ca is an effective element for controlling the morphology of inclusions such as MnS, and can be included as needed. Morphological control of inclusions refers to converting elongated sulfide-based inclusions into granular inclusions, which can further improve the tensile properties in the thickness direction of the sheet. However, if the Ca content exceeds 0.0050%, the amount of nonmetallic inclusions increases, and the tensile properties in the thickness direction of the sheet decrease. Therefore, when Ca is included, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0040% or less. On the other hand, there is no particular lower limit to the Ca content, but in order to fully obtain the above-mentioned effects, a Ca content of 0.0005% or more is preferable. More preferably, the Ca content is 0.0010% or more.

[0040] Mg: 0.0100% or less. Like Ca, Mg is an effective element for controlling the morphology of inclusions such as MnS, and can be included as needed. However, if the Mg content exceeds 0.0100%, the amount of nonmetallic inclusions increases, and the tensile properties in the thickness direction of the plate deteriorate. Therefore, when Mg is included, the Mg content should be 0.0100% or less. Preferably, the Mg content is 0.0050% or less, and more preferably 0.0040% or less. On the other hand, there is no particular lower limit to the Mg content, but in order to fully obtain the effects described above, a Mg content of 0.0005% or more is preferred. More preferably, the Mg content is 0.0010% or more.

[0041] Zr: 0.0050% or less. Zr, like Ca and Mg, is an effective element for controlling the morphology of inclusions such as MnS, and can be included as needed. However, if the Zr content exceeds 0.0050%, the amount of nonmetallic inclusions increases, and the tensile properties in the thickness direction of the plate decrease. Therefore, when Zr is included, the Zr content should be 0.0050% or less. Preferably, the Zr content is 0.0040% or less. On the other hand, there is no particular lower limit to the Zr content, but in order to fully obtain the effects described above, a Zr content of 0.0005% or more is preferred. More preferably, the Zr content is 0.0010% or more.

[0042] Ta: 0.20% or less. Ta is an element that has the effect of further improving the strength of the thick steel plate and can be optionally contained according to the required strength. However, when Ta exceeds 0.20%, the through-thickness tensile properties deteriorate due to precipitate formation. Therefore, when containing Ta, the Ta content should be 0.20% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but in order to sufficiently obtain the above effect, the Ta content is preferably 0.01% or more.

[0043] Y: 0.010% or less. Y is an element that forms oxides stable at high temperatures, effectively suppresses the coarsening of prior γ grains in the heat-affected zone of welding, and has the effect of further improving the toughness of the welded part. Therefore, Y can be optionally contained according to the required toughness. However, when Y exceeds 0.010%, the amount of inclusions increases and the through-thickness tensile properties deteriorate. Therefore, when containing Y, the Y content should be 0.010% or less. On the other hand, the lower limit of the Y content is not particularly limited, but in order to sufficiently obtain the above effect, the Y content is preferably 0.001% or more.

[0044] REM: 0.0200% or less. REM (rare earth metals), similar to Ca, Mg, and Zr, is an element effective for controlling the morphology of inclusions such as MnS and can be optionally contained from the viewpoints of improving toughness and sulfide stress corrosion cracking resistance. However, when the REM content exceeds 0.0200%, the amount of non-metallic inclusions increases and the through-thickness tensile properties deteriorate. Therefore, when containing REM, the REM content should be 0.0200% or less. The REM content is preferably 0.0100% or less. On the other hand, the lower limit of the REM content is not particularly limited, but in order to sufficiently obtain the above effect, the REM content is preferably 0.0010% or more. The REM content is more preferably 0.0020% or more.

[0045] [Plate thickness] Plate thickness: 50 mm or less. When the plate thickness of the above thick steel plate exceeds 50 mm, the through-thickness tensile properties deteriorate. Therefore, the plate thickness should be 50 mm or less. On the other hand, the lower limit of the plate thickness is not particularly limited, but usually, it can be 6 mm or more. The plate thickness of the above thick steel plate is preferably 10 mm or more.

[0046] [Segregation grains of Mn] In the microstructure of the thick steel plate, the number of segregation grains of Mn with a length in the plate width direction of 500 μm or more is preferably 0.3 or less per 1 mm in the plate width direction. When it is 0.3 or less per mm, segregation of impurity elements is less, so that the tensile properties in the plate thickness direction can be ensured more stably. Further, by manufacturing the thick steel plate so as to reduce the number of segregation grains of Mn, porosity can also be reduced. The number of segregation grains of Mn is more preferably 0.2 or less per 1 mm in the plate width direction. On the other hand, the lower limit of the number of segregation grains of Mn is not particularly limited and may be 0. Note that attention is paid to the number per unit length in the plate width direction because the segregation grains of Mn basically exist in the center of the plate thickness. Here, the number of segregation grains of Mn is measured by an electron probe microanalyzer (EPMA). Specifically, in the Mn concentration distribution obtained by the surface analysis of EPMA, a region where the Mn concentration is 1.33 × C0 or more is specified, and the number of regions having a length of 500 μm or more in the plate width direction among the regions is taken as the number of the segregation grains. Here, C0 is the average concentration of Mn. More specifically, it can be measured by the method described in the examples.

[0047] [Drawing value] The drawing value measured by the tensile test in the plate thickness direction: 50% or more The drawing value measured by the tensile test in the plate thickness direction is one of the indexes of the tensile properties in the plate thickness direction. When the drawing value is less than 50%, the steel plate peels off in the plate thickness direction in a T-joint part or the like, and the soundness and reliability of the structure are impaired. Therefore, the drawing value of the above thick steel plate measured by the tensile test in the plate thickness direction is 50% or more. The upper limit of the drawing value is not particularly limited and may be 100%. Specifically, the drawing value can be measured by the method described in the examples.

[0048] The lower limit of the yield stress of the above thick steel plate is not particularly limited, but is preferably 300 MPa or more. On the other hand, the upper limit of the yield stress is also not particularly limited and may be, for example, 530 MPa or less.

[0049] The lower limit of the tensile strength of the above thick steel plate is not particularly limited, but is preferably 400 MPa or more. On the other hand, the lower limit of the tensile strength is also not particularly limited and may be, for example, 640 MPa or less.

[0050] [Manufacturing Method] Next, a method for manufacturing a thick steel plate according to one embodiment of the present invention will be described. The above method for manufacturing a thick steel plate comprises the steps of (a) to (c). (a) A continuous casting step in which molten steel is continuously cast to form a slab (b) A heating step in which the slab is heated (c) A hot rolling step in which the heated slab is hot-rolled to form a thick steel plate

[0051] The conditions for each process are described in detail below. Unless otherwise specified, temperature refers to the surface temperature of the workpiece (steel material or thick steel plate). The temperature at the center of the plate thickness can be determined from the surface temperature of the workpiece by heat transfer calculations.

[0052] (Continuous Casting Process) Reduction gradient: 0.30 mm / m or more In the continuous casting process, reduction is performed with a reduction gradient of 0.30 mm / m or more upstream of the final solidification position of the slab. In continuous casting of slabs, voids called porosity, which occur due to solidification shrinkage, and central segregation due to concentrated molten steel occur in the center of the slab in the thickness direction. In particular, when a large amount of Sn is contained, porosity increases and central segregation is promoted. For this reason, by performing reduction with a reduction gradient of 0.30 mm / m or more upstream of the final solidification position in the casting stage, porosity can be mechanically compressed and central segregation can be reduced. If reduction is not performed in the continuous casting process, or if the reduction gradient is less than 0.30 mm / m, the reduction of porosity and central segregation will be insufficient, and it will be difficult to eliminate porosity and central segregation in the subsequent hot rolling process. The reduction gradient is preferably 0.40 mm / m or more. On the other hand, the upper limit of the reduction gradient is not particularly limited, but it may be, for example, 0.60 mm / m or less.

[0053] (Heating process) By heating the slab in the heating process, carbides can be dissolved and the grain size and other properties can be made uniform. Furthermore, the tensile properties in the thickness direction can be improved by porosity bonding.

[0054] The method for providing the slab to this process is not limited; either a method of heating after cooling (reheating) or a method of heating without cooling (heating plate insertion) may be used.

[0055] Heating temperature: 1000-1250°C. In the heating process, the heating temperature should be between 1000 and 1250°C. If the heating temperature is below 1000°C, the temperature of the steel material will drop during rolling, making it impossible to perform the desired processing. On the other hand, if the heating temperature exceeds 1250°C, energy consumption will increase.

[0056] The heating time in the heating process is not particularly limited and may be, for example, two hours or more. However, since prolonged heating results in excessive energy consumption, it is preferable that the heating time be eight hours or less.

[0057] (Hot Rolling Process) In the hot rolling process, the slab, which has been heated in the heating process, is subjected to hot rolling consisting of multiple rolling passes.

[0058] Number of rolling passes where the thickness of the plate on the entry side is 80 mm or less and the pass reduction ratio is 7% or more: 3 or more Even if the pass reduction ratio is increased in rolling passes where the thickness of the plate on the entry side exceeds 80 mm, sufficient processing cannot be applied to the center of the plate thickness. Similarly, in rolling passes with a pass reduction ratio of less than 7%, sufficient processing cannot be applied to the center of the plate thickness. Therefore, in order to effectively compress the porosity in the center of the plate thickness, it is effective to increase the number of rolling passes where the thickness of the plate on the entry side is 80 mm or less and the pass reduction ratio is 7% or more. If the number of rolling passes where the thickness of the plate on the entry side is 80 mm or less and the pass reduction ratio is 7% or more is less than 3, the porosity compression effect cannot be sufficiently obtained. Also, segregation of impurity elements cannot be suppressed. Therefore, in the hot rolling process, the number of rolling passes where the thickness of the plate on the entry side is 80 mm or less and the pass reduction ratio is 7% or more is set to 3 or more. The number of rolling passes is preferably 4 or more, more preferably 5 or more, and even more preferably 6 or more. The upper limit of the number of rolling passes is not particularly limited, but it may be, for example, 10 or less.

[0059] The temperature at the center of the plate thickness at the start of the first rolling pass, where the plate thickness on the pass entry side is 80 mm or less and the pass reduction ratio is 7% or more, is 900°C or higher. Furthermore, as mentioned above, there are three or more rolling passes where the plate thickness on the pass entry side is 80 mm or less and the pass reduction ratio is 7% or higher, but if the temperature at the center of the plate thickness at the start of the first pass is less than 900°C, the strength of the material increases, processing cannot be performed sufficiently, and porosity remains. As a result, the desired tensile properties in the thickness direction cannot be obtained. Therefore, the temperature at the center of the plate thickness at the start of the first rolling pass is set to 900°C or higher. The upper limit of the temperature is not particularly limited, but for example, it may be 1000°C or lower.

[0060] The rolling completion temperature is not particularly limited, but it is preferable that the surface temperature be less than 900°C. Furthermore, it is preferable that the rolling completion temperature be 750°C or higher.

[0061] (Cooling) Next, the thick steel plate obtained in the hot rolling process may be cooled. The cooling method is not limited and can be carried out under any conditions. Accelerated cooling is preferable because it allows for even better tensile properties in the thickness direction due to microstructure refinement. Cooling may also be carried out by methods other than accelerated cooling. The method of accelerated cooling is not limited, but examples include water cooling, oil cooling, or a method of injecting air (forced cooling). The cooling rate during accelerated cooling is not particularly limited, but may be 1°C / s or more at the center of the plate thickness. On the other hand, the upper limit of the cooling rate during accelerated cooling is also not particularly limited, but if the cooling rate is excessively fast, a martensitic structure may be formed and the desired reduction in area may not be obtained. Therefore, the cooling rate during accelerated cooling is preferably 100°C / s or less at the center of the plate thickness.

[0062] When performing accelerated cooling, the temperature at which accelerated cooling is stopped is not limited, but it is preferable to stop it at 400°C or higher. Furthermore, it is preferable to stop accelerated cooling at 700°C or lower.

[0063] The present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

[0064] Steel with the component composition shown in Table 1 was melted. Then, thick steel plates with the thicknesses shown in Table 2 were manufactured by continuous casting, heating, hot rolling, and cooling according to the manufacturing conditions shown in Table 2. In the table, the cooling rate during accelerated cooling is expressed as the temperature at the center of the plate thickness.

[0065] For each of the obtained thick steel plates, the number of Mn segregated grains in the microstructure, the reduction of area, the yield strength (YS), and the tensile strength (TS) were measured using the method described below.

[0066] (Number of Mn segregated grains) For the thickness direction, the Mn concentration distribution was measured by surface analysis of EPMA in the area from the center of the plate thickness ± 15 mm (however, if the plate thickness is 30 mm or less, the original plate thickness is used). For the width direction, the Mn concentration distribution was measured in the area from the center of the plate width to the edge of the plate width. The average concentration C0 of Mn was calculated by averaging the obtained data. Then, in the Mn concentration distribution, the areas where the Mn concentration was 1.33 × C0 or higher were identified, and the number of areas with a length of 500 μm or more in the width direction of the plate was counted. The number of grains per 1 mm in the width direction of the plate was evaluated by dividing the obtained count by the length of the measurement area in the width direction of the plate.

[0067] (Aperture value) The aperture value was measured by a tensile test in the thickness direction in accordance with JIS G 3199. In this test, the thickness direction of the thick steel plate was aligned with the tensile direction.

[0068] (Yield Strength and Tensile Strength) The yield strength and tensile strength were measured by tensile testing in accordance with JIS Z 2204. In this test, the width direction of the thick steel plate was oriented as the tensile direction, and measurements were taken using a No. 5 test specimen as defined in JIS Z 2201.

[0069] The results obtained are shown in Table 2. The inventive examples that met the conditions of the present invention (samples No. 1 to 11) exhibited excellent tensile properties in the thickness direction. On the other hand, the comparative examples that did not meet the conditions of the present invention (samples No. 12 to 22) exhibited inferior tensile properties in the thickness direction.

[0070]

[0071]

Claims

1. A thick steel sheet having a composition in mass%, containing C: 0.050-0.200%, Si: 0.02-0.60%, Mn: 0.50-1.80%, P: 0.003-0.030%, S: 0.0005-0.0080%, Cu: 0.02-0.50%, Cr: 0.02-0.50%, Sn: 0.005-0.050%, Ti: 0.002-0.030%, Al: 0.010-0.100%, and N: 0.0010-0.0080%, with the remainder being Fe and unavoidable impurities, having a thickness of 50 mm or less, and a reduction in area measured by a tensile test in the thickness direction of the sheet, of 50% or more.

2. The thick steel plate according to claim 1, wherein the component composition further contains one or more elements selected from the group consisting of, by mass%, Ni: 0.50% or less, Mo: 0.50% or less, Nb: 0.030% or less, V: 0.050% or less, B: 0.0050% or less, Sb: 0.300% or less, W: 2.00% or less, Co: 2.00% or less, Ca: 0.0050% or less, Mg: 0.0100% or less, Zr: 0.0050% or less, Ta: 0.20% or less, Y: 0.010% or less, and REM: 0.0200% or less.

3. The thick steel plate according to claim 1 or 2, wherein the number of Mn segregation grains having a length of 500 μm or more in the width direction of the plate in the microstructure of the thick steel plate is 0.3 or less per 1 mm in the width direction of the plate.

4. A method for manufacturing a thick steel plate according to any one of claims 1 to 3, comprising: continuously casting molten steel to form a slab; heating the slab; and hot rolling the heated slab to form a thick steel plate, wherein in the continuous casting, a reduction gradient of 0.30 mm / m or more is performed upstream of the final solidification position of the slab; in the heating, the heating temperature is 1000 to 1250°C; and in the hot rolling, the number of rolling passes in which the plate thickness on the pass entry side is 80 mm or less and the pass reduction ratio is 7% or more is 3 or more, and the temperature at the center of the plate thickness at the start of the first rolling pass is 900°C or higher.

5. The method for manufacturing a thick steel plate according to claim 4, wherein the thick steel plate after hot rolling is subjected to accelerated cooling.

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