Steel plate

Abstract

The present disclosure discloses a steel plate that has excellent gas cutting crack resistance, and that has both toughness and hardness after reheating and quenching, while suppressing the use of Ni and Mo. The steel plate disclosed herein is characterized in that it contains specified elements and has a hardness of 220HV10 or less in an L-direction cross section at a position 1 mm from the surface in the thickness direction.

Description

[Technical field]

[0001] The present application discloses a steel sheet. [Background technology]

[0002] A steel plate with excellent wear resistance is called a wear-resistant steel plate. Since wear-resistant steel plates are required to have hardness near the surface layer, they contain elements that enhance hardenability. Cr, Ni, and Mo are elements that enhance hardenability. Wear-resistant steel plates containing Cr, Ni, Mo, and the like have been proposed (see, for example, Patent Documents 1 to 4). Since wear-resistant steel plates have high strength, cutting cracks due to hydrogen embrittlement may occur during gas cutting of the steel plate in the manufacturing process. Therefore, wear-resistant steel plates that have improved gas cutting crack resistance by reheating the steel plate to 200°C or higher as a dehydrogenation treatment have been proposed (see, for example, Patent Documents 5 to 6). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-204575 [Patent Document 2] Japanese Patent Application Publication No. 10-102185 [Patent Document 3] Japanese Patent Publication No. 59-129724 [Patent Document 4] Japanese Patent Application Publication No. 59-70721 [Patent Document 5] JP 2002-226916 A [Patent Document 6] JP 2002-256339 A Summary of the Invention [Problem to be solved by the invention]

[0004] The wear-resistant steel plates of Patent Documents 1 and 2 contain 1.0 mass% or more of Mo. The wear-resistant steel plates of Patent Documents 3 and 4 contain 2.0 mass% or more of Ni. Ni and Mo are expensive elements, and it is desirable to reduce their content from the viewpoint of alloy costs. The wear-resistant steel plates of Patent Documents 5 and 6 improve their gas cutting crack resistance by performing a dehydrogenation treatment in which the steel plate is reheated to 200°C or higher, but the dehydrogenation treatment tends to increase costs.

[0005] The present application discloses a steel plate in which the use of Ni and Mo is suppressed, which has gas cutting crack resistance before reheating and quenching, and which is capable of achieving both toughness and hardness after reheating and quenching. [Means for solving the problem]

[0006] The present application discloses the following aspects as means for solving the above problems. (1) In mass%, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, less than 7.50%, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of: The hardness in the L-direction cross section at a position 1 mm from the surface in the thickness direction is 220HV10 or less. steel plate. (2) In mass%, V: 0.001% or more, 0.200% or less, Mo: 0.001% or more, 0.200% or less, W: 0.001% or more, 0.200% or less, Ti: 0.001% or more, 0.100% or less, and Nb: 0.001% or more, 0.100% or less, One or more selected from the group consisting of The steel plate according to (1). (3) In mass%, Ni: 0.001% or more, 1.000% or less, and Cu: 0.001% or more, 1.000% or less, One or two selected from the group consisting of The steel plate according to (1) or (2). (4) In mass%, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, and REM: 0.0001% or more, 0.1000% or less, One or more selected from the group consisting of The steel sheet according to any one of (1) to (3). (5) In mass%, Sn: 0.001% or more, 0.100% or less, and Sb: 0.001% or more, 0.100% or less, One or two selected from the group consisting of Steel plate of any of (1) to (4). (6) It has the following properties: Steel plate of any of (1) to (5). Properties: When the above steel plate was subjected to quenching treatment by heating to 950°C, holding for 15 minutes, and then cooling at a rate of 15°C / sec, When an L-direction cross section at a position 1 mm in the thickness direction from the surface of the steel plate after the quenching treatment is observed by EBSD, in a 400 μm × 400 μm area, the EBSD pixel KAM value is 1.5 ° or less, and the average of the top 10 grain sizes of ferrite grains defined as closed regions surrounded by EBSD pixel boundaries having an inclination angle difference of 15 ° or more is 40 μm or less, The area ratio of cementite in the steel plate after the quenching treatment is 0% or more and 0.5% or less, and the average equivalent circle diameter of the cementite is 500 nm or less. (7) Have a thickness of 6 mm or more and 50 mm or less, The steel sheet according to any one of (1) to (6). (8) This is the original plate before heat treatment. The steel sheet according to any one of (1) to (7). (9) Obtaining a slab having a chemical composition according to any one of (1) to (5); The slab is heated to a temperature range of 1100°C to 1300°C, and is rolled at a temperature of 1100°C or higher with a rolling reduction of 10% or more three or more times to obtain a rough rolled material with a total rolling reduction of 50% or more. The rough rolled material is rolled three or more times at a rolling reduction of 5% or more in a temperature range of 900°C or more and less than 1100°C, and the rough rolled material is subjected to finish rolling at a total rolling reduction of 50% or more to obtain a finish rolled material; Cooling the finish rolled material to 200°C or less at a cooling rate of 0.10°C / sec or less; Manufacturing method of steel plate. (10) In mass%, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, less than 7.50%, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of: When observing an L-direction cross section at a position 1 mm from the surface in the thickness direction by EBSD, in a 400 μm × 400 μm area, the average of the top 10 grain sizes of ferrite grains, defined as closed regions surrounded by EBSD pixel boundaries in which the EBSD pixel KAM value is 1.5° or less and the tilt angle difference is 15° or more, is 40 μm or less; The area ratio of cementite is 0% or more and 0.5% or less, and the average circle equivalent diameter of the cementite is 500 nm or less. steel plate. (11) In mass%, V: 0.001% or more, 0.200% or less, Mo: 0.001% or more, 0.200% or less, W: 0.001% or more, 0.200% or less, Ti: 0.001% or more, 0.100% or less, and Nb: 0.001% or more, 0.100% or less, One or more selected from the group consisting of The steel plate according to (10). (12) In mass%, Ni: 0.001% or more, 1.000% or less, and Cu: 0.001% or more, 1.000% or less, One or two selected from the group consisting of The steel plate according to (10) or (11). (13) In mass%, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, and REM: 0.0001% or more, 0.1000% or less, One or more selected from the group consisting of The steel sheet according to any one of (10) to (12). (14) In mass%, Sn: 0.001% or more, 0.100% or less, and Sb: 0.001% or more, 0.100% or less, One or two selected from the group consisting of The steel sheet according to any one of (10) to (13). (15) The hardness in the L-direction cross section at a position 1 mm from the surface in the thickness direction is 360HV10 or more, The average Charpy absorbed energy at -40℃ is 27J or more. The steel sheet according to any one of (10) to (14). (16) Have a thickness of 6 mm or more and 50 mm or less, The steel sheet according to any one of (10) to (15). (17) Obtaining a slab having a chemical composition according to any one of (10) to (14). The slab is heated to a temperature range of 1100°C to 1300°C, and is rolled at a temperature of 1100°C or higher with a rolling reduction of 10% or more three or more times to obtain a rough rolled material with a total rolling reduction of 50% or more. The rough rolled material is rolled three or more times at a rolling reduction of 5% or more in a temperature range of 900°C or more and less than 1100°C, and the rough rolled material is subjected to finish rolling at a total rolling reduction of 50% or more to obtain a finish rolled material. Cooling the finish rolled material to 200°C or less at a cooling rate of 0.10°C / sec or less; and The cooled finish rolled material is heated to 900 ° C. or higher, held for 10 minutes or more, and then cooled at 10 ° C. / sec or higher for quenching treatment. Manufacturing method of steel plate. Effect of the Invention

[0007] The steel plate of the present disclosure has reduced use of Ni and Mo, has gas cutting crack resistance, and is capable of achieving both toughness and hardness after reheating and quenching. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, as a steel plate according to one embodiment of the present invention (hereinafter referred to as "this embodiment"), a steel plate as a base plate before heat treatment and a steel plate (mainly a wear-resistant steel plate) after the heat treatment will be described. In the numerical ranges described in stages in this specification, the upper limit value of a certain numerical range may be replaced with the upper limit value of another numerical range described in stages, and the lower limit value of a certain numerical range may be replaced with the lower limit value of another numerical range described in stages. The term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0009] First, the results of the research conducted by the present inventors that led to the completion of the present invention and the new findings obtained will be described in detail. (1) Cr is a cheaper element than Ni and Mo. If the properties required for a steel sheet can be achieved by suppressing the Ni and Mo contents and increasing the Cr content, it is possible to reduce the alloying cost of the steel sheet. According to the new findings of the present inventors, in order to ensure the properties required for a steel sheet while intentionally not including Ni and Mo or reducing the Ni and Mo contents, the Cr content of the steel sheet needs to exceed 2.00%. (2) According to the new findings of the present inventors, by setting the Cr content to more than 2.00% and the Si content to more than 0.50%, for example, even in a steel plate having a plate thickness of 6 mm or more and 50 mm or less, hardenability is ensured up to the center position of the plate thickness, and the toughness and hardness after reheating and quenching are more optimal. (3) According to the new findings of the present inventors, by slowly cooling a steel sheet from a temperature range of Ac3 point or higher to 200°C or lower at a cooling rate of 0.1°C / sec or less after rough rolling and finish rolling, even if the steel sheet has the chemical compositions described in (1) and (2) above, the hardness of the surface layer of the steel sheet after cooling becomes 220HV10 or less, and gas cutting crack resistance is significantly improved. (4) A steel plate having the above-mentioned (1) to (3) specified chemical components and a surface hardness of 220 HV or less can achieve both toughness and hardness after reheating and quenching. For example, the steel plate after reheating and quenching has reduced use of Ni and Mo, and the grain size of ferrite grains, the area ratio of cementite, and the average equivalent circle diameter of cementite are appropriately controlled, resulting in excellent toughness and hardness.

[0010] 1. Steel plate Based on the above findings, the present application discloses the following steel plates. The steel plate according to the first embodiment described below has reduced use of Ni and Mo, has gas cutting crack resistance, and is capable of achieving both toughness and hardness after reheating and quenching. The steel plate according to the second embodiment described below has reduced use of Ni and Mo, and has excellent toughness and hardness.

[0011] The steel plate according to the first embodiment has, in mass%, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, less than 7.50%, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of: It is characterized in that the hardness in the L-direction cross section at a position 1 mm from the surface in the thickness direction is 220HV10 or less.

[0012] The steel plate according to the second embodiment has, in mass%, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, less than 7.50%, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of: When observing an L-direction cross section at a position 1 mm from the surface in the thickness direction by EBSD, in a 400 μm × 400 μm area, the average of the top 10 grain sizes of ferrite grains, defined as closed regions surrounded by EBSD pixel boundaries in which the EBSD pixel KAM value is 1.5° or less and the tilt angle difference is 15° or more, is 40 μm or less; The area ratio of cementite is 0% or more and 0.5% or less, and the average equivalent circle diameter of the cementite is 500 nm or less.

[0013] <Chemical composition> The chemical composition of the steel sheet according to this embodiment will be described. In the following description of elements, "%" for the content means "mass %".

[0014] [C: 0.10% or more, 0.35% or less] C (carbon) is an element that improves the hardenability of steel and increases its hardness. From the viewpoint of ensuring hardness, the C content is 0.10% or more. The C content is preferably 0.15% or more. On the other hand, C is an element that forms cementite. If cementite is formed in excess, it may impair toughness. From the viewpoint of ensuring toughness, the C content is 0.35% or less. The C content is preferably 0.25% or less, and more preferably 0.22% or less.

[0015] [Si: more than 0.50%, less than 2.00%] Si (silicon) is a deoxidizing element. In addition, when a certain amount of Si is contained together with Cr described later, the hardenability, toughness after reheating and hardening, and hardness are significantly improved. From this viewpoint, the Si content is more than 0.50%. The Si content is preferably 0.55% or more, more preferably 0.60% or more. On the other hand, if Si is contained in excess, there is a possibility that the toughness is impaired. From the viewpoint of ensuring toughness, the Si content is 2.00% or less. The Si content is preferably 1.50% or less, more preferably 1.00% or less.

[0016] [Mn: 0.10% or more, 2.00% or less] Mn (manganese) is an element that enhances the hardenability of steel and improves hardness. From the viewpoint of ensuring hardness, the Mn content is 0.10% or more. The Mn content is preferably 0.50% or more, more preferably 0.80% or more, and further preferably 1.00% or more. On the other hand, Mn is an element that may embrittle the grain boundaries of steel and deteriorate toughness if contained in excess. From the viewpoint of ensuring toughness, the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less, and more preferably 1.60% or less.

[0017] [P:0.020% or less] P (phosphorus) is an element that is mixed into steel during the manufacturing process. If P is contained in excess, toughness may deteriorate. From the viewpoint of ensuring toughness, the P content is 0.020% or less. The P content is preferably 0.012% or less, and more preferably 0.010% or less. It is preferable to reduce the P content, and although there is no lower limit, it may be 0% or more, and from the viewpoint of manufacturing costs, it may be more than 0%. The P content may be 0.001% or more.

[0018] [S:0.010% or less] S (sulfur) is an element that is mixed into steel during the manufacturing process. Excessive S content may cause a deterioration in toughness. From the viewpoint of ensuring toughness, the S content is 0.010% or less. The S content is preferably 0.005% or less, and more preferably 0.003% or less. It is preferable to reduce the S content, and although there is no lower limit, it may be 0% or more, and from the viewpoint of manufacturing costs, it may be more than 0%. The S content may be 0.0001% or more.

[0019] [Cr: more than 2.00%, less than 7.50%] Cr (chromium) is an element that enhances the hardenability of steel. Cr is an important element from the viewpoint of ensuring the hardness and toughness of the wear-resistant steel by suppressing the content of expensive elements such as Ni and Mo. The Cr content is more than 2.00% from the viewpoint of ensuring hardenability. The Cr content is preferably 2.10% or more, and more preferably 2.40% or more. On the other hand, the Cr content is 7.50% or less, and may be 7.00% or less, from the viewpoint of cost. The Cr content is preferably 5.00% or less, and more preferably 3.00% or less.

[0020] [Al: 0.010% or more, 0.100% or less] Al (aluminum) is a deoxidizing element and also an element that forms nitrides. In order to obtain the deoxidizing effect, the Al content is 0.010% or more. From the viewpoint of refining the metal structure by AlN, the Al content is more preferably 0.020% or more. On the other hand, if Al is contained excessively, coarse inclusions may be generated, which may impair the mechanical properties of the steel sheet. From the viewpoint of suppressing the generation of coarse inclusions, the Al content is 0.100% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less.

[0021] [N: 0.0020% or more, 0.0100% or less] N (nitrogen) is an element that is mixed into steel during the manufacturing process. Excessive N content may cause a deterioration in toughness. The N content is 0.0100% or less from the viewpoint of ensuring toughness. The N content is preferably 0.0070% or less, and more preferably 0.0060% or less. When Al or Ti is contained, it combines with N to form fine nitrides such as AlN and TiN. The N content is 0.0020% or more from the viewpoint of refining the metal structure by the nitrides.

[0022] [B: 0.0003% or more, 0.0030% or less] B (boron) is an element that significantly improves the hardenability of steel even in a small amount. In order to obtain the effect of hardenability, the B content is 0.0003% or more, more preferably 0.0005% or more, and further preferably 0.0008% or more. On the other hand, even if B is contained in excess, the effect is saturated, so the B content is 0.0030% or less. The B content is preferably 0.0025% or less, and more preferably 0.0020% or less.

[0023] The basic chemical composition of the steel sheet in this embodiment is as described above. Furthermore, the steel sheet in this embodiment may contain one or more of the following optional elements as necessary. These elements do not necessarily have to be contained, so the lower limit is 0%.

[0024] The steel sheet according to this embodiment may contain one or more elements (group a) of V, Mo, W, Ti and Nb, which form compounds such as carbides and nitrides, as necessary.

[0025] [V: 0% or more, 0.200% or less] V (vanadium) is an element that forms precipitates such as carbides and nitrides, and has the effect of enhancing hardenability. However, in the steel sheet according to the present embodiment, the effect of V is not essential. The V content is 0% or more, and may be 0.001% or more, or 0.005% or more. On the other hand, the V content is 0.200% or less from the viewpoint of cost. It is preferably 0.150% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[0026] [Mo: 0% or more, 0.200% or less] Mo (molybdenum) is an element that enhances the hardenability of steel. However, in the steel sheet according to the present embodiment, the effect of Mo is not essential. The Mo content is 0% or more, and may be 0.001% or more, or 0.030% or more. On the other hand, the Mo content is 0.200% or less from the viewpoint of cost. It is preferably 0.150% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[0027] [W: 0% or more, 0.200% or less] W (tungsten) is an element that enhances the hardenability of steel. However, in the steel sheet according to the present embodiment, the effect of W is not essential. The W content is 0% or more, and may be 0.001% or more, or 0.030% or more. On the other hand, the W content is 0.200% or less from the viewpoint of cost. It is preferably 0.150% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[0028] [Ti: 0% or more, 0.100% or less] Ti (titanium) is a deoxidizing element and also an element that forms nitrides. In addition, when B is contained in the steel, Ti can be used to suppress the formation of BN and improve hardenability. However, in the steel plate according to the present embodiment, the effect of Ti is not essential. The Ti content is 0% or more, and may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of suppressing the generation of coarse inclusions that may adversely affect toughness, the Ti content is 0.100% or less. The Ti content is more preferably 0.050% or less, and further preferably 0.030% or less. The Ti content may be less than 0.010%.

[0029] [Nb: 0% or more, 0.100% or less] Nb (niobium) is an element that forms precipitates such as carbides and nitrides, and can contribute to the refinement of the metal structure by the precipitates. However, in the steel sheet according to the present embodiment, the effect of Nb is not essential. The Nb content is 0% or more, and may be 0.001% or more, or 0.010% or more. On the other hand, even if Nb is excessively contained, the effect is saturated. In this respect, the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less, and more preferably 0.030% or less.

[0030] The steel plate according to this embodiment may contain one or both of Ni and Cu (group b), which contribute to the toughness of the base material through solid solution, as necessary.

[0031] [Ni: 0% or more, 1.000% or less] Ni (nickel) is an element that enhances the hardenability of steel and improves toughness. However, in the steel plate according to this embodiment, the effect of Ni is not essential. The Ni content is 0% or more, and may be 0.001% or more, or 0.030% or more. On the other hand, the Ni content is 1.000% or less from the viewpoint of cost. The Ni content is preferably 0.300% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[0032] [Cu: 0% or more, 1.000% or less] Cu (copper) is an element that enhances the hardenability of steel and improves toughness. However, in the steel plate according to the present embodiment, the effect of Cu is not essential. The Cu content is 0% or more, and may be 0.001% or more, or 0.030% or more. On the other hand, the Cu content is 1.000% or less from the viewpoint of cost. The Cu content is preferably 0.300% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[0033] The steel plate according to this embodiment may contain, as necessary, one or more of Ca, Mg and REM (group c) which form oxides and contribute to the toughness of the weld heat affected zone structure through grain refinement.

[0034] [Ca: 0% or more, 0.0100% or less] Ca (calcium) is an element that forms oxides and controls the morphology of inclusions. However, in the steel sheet according to the present embodiment, the effect of Ca is not essential. The Ca content is 0% or more, and may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of suppressing the generation of coarse inclusions, the Ca content is 0.0100% or less. The Ca content is preferably 0.0080% or less, and more preferably 0.0060% or less.

[0035] [Mg: 0% or more, 0.0100% or less] Mg (magnesium) is an element that forms oxides and controls the morphology of inclusions. However, in the steel sheet according to the present embodiment, the effect of Mg is not essential. The Mg content is 0% or more, and may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of cost, the Mg content is 0.0100% or less. The Mg content is preferably 0.0080% or less, and more preferably 0.0060% or less.

[0036] [REM: 0% or more, 0.1000% or less] REM (rare earth elements) refers to a collective term for 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. The REM content refers to the total content of the 17 elements. REM is an element that forms oxides or sulfides and controls the form of inclusions. However, the effect of REM is not essential for the steel sheet according to the present embodiment. The REM content is 0% or more, and may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of cost, the REM content is 0.1000% or less. The REM content is preferably 0.0100% or less, and more preferably 0.0060% or less.

[0037] The steel plate according to this embodiment may contain one or both of Sn and Sb (group d), which contribute to the toughness of the base metal through solid solution, segregation, and precipitation, as necessary.

[0038] [Sn: 0% or more, 0.100% or less] Sn (tin) is an element that is mixed into steel during the manufacturing process. The Sn content is 0.100% or less from the viewpoint of ensuring toughness. The Sn content is preferably 0.070% or less, more preferably 0.050% or less, further preferably less than 0.030%, and particularly preferably 0.010% or less. The Sn content may be 0% or more, but may be 0.001% or more from the viewpoint of manufacturing costs.

[0039] [Sb: 0% or more, 0.100% or less] Sb (antimony) is an element that is mixed into steel during the manufacturing process. The Sb content is 0.100% or less from the viewpoint of ensuring toughness. The Sb content is preferably 0.007% or less, and more preferably 0.006% or less. The Sb content may be 0% or more, but may be 0.001% or more from the viewpoint of manufacturing costs.

[0040] The balance of the above chemical components of the steel sheet according to the present embodiment is Fe and impurities. Here, the impurities are components that are mixed in due to raw materials such as ores and scraps, or various factors in the manufacturing process, when the steel sheet according to the present embodiment is industrially manufactured. The inclusion of impurities is permitted within a range that does not adversely affect the properties of the steel sheet according to the present embodiment.

[0041] <Metal structure> Next, the metal structure of the steel plate according to this embodiment will be described. In the following, "%" in the metal structure means "area %". In the following, the steel plate used as the original plate before the heat treatment for making it into the wear-resistant steel plate is referred to as "original plate before heat treatment". For example, the steel plate according to the first embodiment may be the original plate before heat treatment. In addition, among the steel plates according to this embodiment, the steel plate obtained by subjecting the original plate before heat treatment to heat treatment and used mainly as the wear-resistant steel plate is referred to as "wear-resistant steel plate". For example, the steel plate according to the second embodiment may be the wear-resistant steel plate. However, the uses of the steel plate according to this embodiment are not limited to these.

[0042] [Metal structure of steel sheet according to the first embodiment] The metal structure of the steel plate according to the first embodiment (e.g., the original plate before heat treatment) may contain pearlite, ferrite, upper bainite, and lower bainite, and is desirably composed of pearlite, ferrite, upper bainite, and lower bainite.

[0043] In the steel sheet according to the first embodiment, the area ratio of pearlite contained in the metal structure is preferably 0% or more and 10% or less. Also, in the steel sheet according to the first embodiment, in a 400 μm×400 μm region, the KAM value of the EBSD pixel is 1.5° or less, and the average of the top 10 grain sizes of ferrite grains defined as a closed region surrounded by EBSD pixel boundaries with an inclination difference of 15° or more is preferably 40 μm or less.

[0044] In the steel plate according to the first embodiment, the area ratio of pearlite and the average of the top 10 grain sizes of ferrite grains are in the above-mentioned ranges, so that the following effects can be achieved. For example, if the generation rate of pearlite increases excessively, the strength increases excessively, which may cause a decrease in hydrogen embrittlement resistance. In addition, if the crystal grain size of the remaining structure excluding pearlite (for example, the remaining structure composed of one or more of ferrite, upper bainite, and lower bainite) becomes excessively coarse, the hydrogen embrittlement resistance may be decreased. In the steel plate according to the first embodiment, the area ratio of pearlite and the average of the top 10 grain sizes of ferrite grains are in the above-mentioned ranges, so that such problems related to hydrogen embrittlement resistance are unlikely to occur. In the steel plate according to the first embodiment, in order to reduce the strength and obtain hydrogen embrittlement resistance, it is preferable that the metal structure is composed of only both ferrite and pearlite. The total area ratio of ferrite is preferably 70% or more.

[0045] Upper bainite and lower bainite are plate-like or lath-like hard phases, and are generally distinguished by the presence or absence of cementite. When the metal structure of the steel plate according to the first embodiment is composed of ferrite, pearlite, upper bainite, and lower bainite, the metal structure containing lamellar cementite formed by eutectoid reaction is regarded as pearlite, and the metal structure containing other cementite is regarded as upper bainite or lower bainite. In this embodiment, it is not necessary to distinguish between upper bainite and lower bainite. When upper bainite and lower bainite are not observed by a scanning electron microscope (SEM), it is determined that the metal structure is composed of one or both of ferrite and pearlite. The steel plate according to the first embodiment may have the area ratio of pearlite and the average grain size of the crystals of the remaining structure excluding pearlite within the above-mentioned range, and may contain a metal structure other than ferrite and pearlite.

[0046] The metal structure is observed in an L-direction cross-section region that does not contain surface scale, located 1 mm from the surface in the thickness direction. The observation surface of the sample used for metal structure observation is a cross-section cut in the thickness direction along the L (longitudinal) direction (rolling direction) of the sample (hereinafter sometimes referred to as the "L-direction cross-section of the sample"), which is wet polished and etched with nital. The metal structure is observed at a magnification of 200 times, and the area ratios of ferrite, pearlite, upper bainite, and lower bainite are determined by observing five fields of view.

[0047] [Area ratio of pearlite: 0% or more, 10% or less] In the metal structure of the steel plate according to the first embodiment, the area ratio of pearlite is desirably 0% or more and 10% or less, preferably 5% or less, and more preferably 3% or less.

[0048] The pearlite area ratio is measured by SEM. The pearlite area ratio is measured in an L-direction cross-section region that does not contain surface scale, located 1 mm from the surface in the thickness direction. The observation surface of the sample used to measure the pearlite area ratio is the L-direction cross-section of the sample, which is wet polished and etched with nital. The pearlite area ratio is measured by image analysis of a photograph taken at 200x magnification.

[0049] [Average of top 10 ferrite grain sizes: 40μm or less] In the steel sheet according to the first embodiment, it is desirable that the average of the top 10 grain sizes of ferrite grains in a 400 μm×400 μm region is 40 μm or less. As described above, the ferrite grains are defined as closed regions surrounded by EBSD pixel boundaries in which the KAM value of the EBSD pixel is 1.5° or less and the inclination difference is 15° or more. Hereinafter, the grain size of the ferrite grains defined as closed regions surrounded by EBSD pixel boundaries in which the KAM value of the EBSD pixel is 1.5° or less and the inclination difference is 15° or more may be referred to as the "high-angle grain size". When the high-angle grain size is large, fracture is more likely to occur, and when the high-angle grain size is small, fracture is suppressed and toughness is improved. As described above, the high-angle grain size is evaluated as the average value of the top 10 (the largest 10) crystal grains in a 400 μm×400 μm region. The high-angle grain size is more preferably 30 μm or less.

[0050] In this embodiment, the high-angle grain size is measured by electron backscattered diffraction pattern (hereinafter also referred to as "EBSD"). The EBSD measurement is performed in a field of view of 400 μm × 400 μm with a pitch of 0.4 μm. The average of the top 10 circle-equivalent diameters of ferrite grains, which are defined as closed regions surrounded by EBSD pixel boundaries with an EBSD pixel KAM value of 1.5° or less and an inclination difference of 15° or more, and do not include the end of the observation region, is calculated using commercially available analysis software (OIM-Analysis manufactured by TSL). The EBSD measurement is performed in a region of the L-direction cross section not including the surface scale, located 1 mm from the surface in the thickness direction. The sample used for the EBSD measurement has an observation surface of 10 mm square and is cut out from the end of the steel plate in the width direction. The observation surface is the L-direction cross section of the sample, and is electrolytically polished.

[0051] [Hardness and hydrogen content] The hardness in the L-direction cross section at a position 1 mm from the surface of the steel plate according to the first embodiment and the hydrogen content at a position 1 mm from the surface will be described.

[0052] [Hardness at L-section 1mm from surface in thickness direction: 220HV10 or less] In the steel plate according to the first embodiment, the hardness of the region not including the surface scale in the L-direction cross section at a position 1 mm from the surface in the thickness direction is 220 HV10 or less in Vickers hardness in order to ensure gas cutting crack resistance. The hardness of the L-direction cross section at a position 1 mm from the surface in the thickness direction of the steel plate according to the first embodiment is preferably 200 HV10 or less, more preferably 150 HV10 or less, the lower the better, and there is no lower limit. The Vickers hardness test is performed in accordance with JIS Z 2244:2009, and the load is 10 kgf. The Vickers hardness is the average value of three points measured on the L-direction cross section of the sample. The sample used for the Vickers hardness test is cut out from the end in the width direction of the steel plate. The cut out test surface is wet-polished to a thickness of 50 μm or more with water-resistant abrasive paper of #200 grain size, and then finish-polished with wet abrasive paper of #800 grain size. When the surface Vickers hardness is 120HV10 or less, it can be determined that the metal structure of the wear-resistant steel substrate before heat treatment is formed of ferrite and pearlite, and the total area ratio of pearlite is 5% or less.

[0053] [Hydrogen content at a position 1 mm from the surface in the thickness direction: 0.1 mass ppm or less] In the steel plate according to the first embodiment, the hydrogen content at a position 1 mm from the surface in the thickness direction is desirably 0.1 mass ppm or less in terms of the amount of detected hydrogen measured by gas chromatography mass spectrometry. The hydrogen content at a position 1 mm from the surface in the thickness direction of the steel plate is measured by gas chromatography mass spectrometry when a metal piece cut out from a position 1 mm from the surface of the steel plate in the thickness direction, not including surface scale, with t (thickness direction) 1 mm, W (width direction) 10 mm, and L (longitudinal direction) 40 mm is heated in the temperature range of -50°C to 250°C at a heating rate of 100°C / hour.

[0054] [Metal structure of steel sheet according to the second embodiment] Next, the metal structure of the steel plate (for example, abrasion-resistant steel plate) according to the second embodiment will be described. In the following, "%" in the metal structure means "area %".

[0055] The area ratio of cementite contained in the metal structure of the steel sheet according to the second embodiment is more than 0% and 0.5% or less. The average circle equivalent diameter of the cementite is 500 nm or less. In addition, in a 400 μm × 400 μm region of the steel sheet according to the second embodiment, it is preferable that the KAM value of the EBSD pixel is 1.5° or less, and the average of the top 10 grain sizes of ferrite grains defined as a closed region surrounded by EBSD pixel boundaries with an inclination difference of 15° or more is 40 μm or less.

[0056] The metal structure of the steel plate according to the second embodiment preferably has the area ratio and average equivalent circle diameter of cementite and the average grain size in the above-mentioned ranges. For example, when upper bainite or pearlite is generated, the area ratio of cementite increases excessively, and it is likely to become coarse. When ferrite is generated, it is likely to become insufficient in hardness, and toughness may be reduced. Retained austenite becomes hard martensite by deformation (deformation-induced martensite), and toughness may be reduced. Therefore, the metal structure of the steel plate according to the second embodiment preferably consists of only one or both of martensite and lower bainite. The total area ratio of martensite and lower bainite is preferably 95% or more. The remainder excluding martensite and lower bainite consists of one or more of ferrite, pearlite, retained austenite, and upper bainite.

[0057] Martensite and lower bainite are lath-shaped hard phases, and are generally distinguished by the presence or absence of cementite. When the metal structure of the steel plate according to the second embodiment is composed of martensite and lower bainite, a metal structure containing two or more cementites in one crystal grain is regarded as lower bainite. In this embodiment, it is not necessary to distinguish between martensite and lower bainite. When ferrite, pearlite, retained austenite, and upper bainite are not observed by SEM, it is determined that the metal structure is composed of one or both of martensite and lower bainite. The metal structure of the steel plate according to the third or fourth embodiment may include a metal structure other than martensite and lower bainite, so long as the area ratio and average equivalent circle diameter of cementite and the average grain size of the crystals are within the above-mentioned ranges.

[0058] The metal structure is observed in an L-direction cross-section region that does not contain surface scale, located 1 mm from the surface in the thickness direction. The observation surface of the sample used for metal structure observation is a cross-section (L-direction cross-section) cut in the thickness direction along the L (longitudinal) direction (rolling direction) of the sample, which is wet polished and etched with nital. The metal structure is observed at a magnification of 400x, and the presence or absence of ferrite, pearlite, retained austenite, and upper bainite is determined by observing five fields of view.

[0059] [Area ratio of cementite: 0% or more, 0.5% or less] When the metal structure of the steel plate according to the present embodiment is made of martensite, the area ratio of cementite is 0%. As described above, when the metal structure of the steel plate according to the present embodiment is made of martensite and lower bainite, a metal structure containing two or more cementite grains is regarded as lower bainite. From the viewpoint of ensuring toughness, it is desirable that the area ratio of cementite is 0.5% or less. The area ratio of cementite is preferably 0.4% or less, and more preferably 0.3% or less. When the quenching stop temperature is lowered, the generation of upper bainite and pearlite is suppressed, and the area ratio of cementite is reduced.

[0060] In the steel sheet according to the second embodiment, the area ratio of cementite is measured by SEM. The area ratio of cementite is measured in an L-direction cross section region not including surface scale, located 1 mm from the surface in the thickness direction. The observation surface of the sample used for measuring the area ratio of cementite is the L-direction cross section of the sample, which is subjected to electrolytic etching. The area ratio of cementite is measured by image analysis of five photographs taken at 30,000 times magnification.

[0061] [Average equivalent circular diameter of cementite: 500 nm or less] In the steel plate according to the third or fourth embodiment, the average equivalent circle diameter of cementite is desirably 500 nm or less from the viewpoint of ensuring toughness. The average equivalent circle diameter of cementite is preferably 300 nm or less. The average equivalent circle diameter of cementite may be 50 nm or more, or may be 100 nm or more.

[0062] The average equivalent circle diameter of cementite is calculated from the area (total) of cementite not including the edge of the observation area, which is obtained by image analysis of the SEM photograph used to measure the area ratio, and the number of cementite particles. In other words, the average equivalent circle diameter is calculated from the average area obtained by dividing the total area of ​​cementite excluding the cementite present at the edge of the observation area by the number of cementite particles excluding the cementite present at the edge of the observation area. When the area ratio of cementite is 0%, the average equivalent circle diameter is 0 nm. When the quenching stop temperature is lowered, the formation of upper bainite and pearlite is suppressed, and the average equivalent circle diameter of cementite is reduced. The cementite contained in upper bainite and pearlite has a large average equivalent circle diameter and area ratio. If the area ratio of cementite is 0.5% or less and the average equivalent circle diameter is 500 nm or less, it may be determined that the total area ratio of upper bainite and pearlite is less than 5%.

[0063] [Average of top 10 ferrite grain sizes: 40μm or less] In the steel sheet according to the second embodiment, it is preferable that the average of the top 10 grain sizes of ferrite grains defined in a closed region surrounded by EBSD pixel boundaries in which the EBSD pixel KAM value is 1.5° or less and the inclination difference is 15° or more in a 400 μm×400 μm region is 40 μm or less. In the steel sheet according to the second embodiment, as in the steel sheet according to the first embodiment, when the high-angle grain size (the grain size of ferrite grains defined in a closed region surrounded by EBSD pixel boundaries in which the EBSD pixel KAM value is 1.5° or less and the inclination difference is 15° or more) becomes large, fracture is likely to occur, and when the high-angle grain size becomes small, toughness is improved. As described above, the high-angle grain size is evaluated as the average value of the top 10 (the largest 10) crystal grains in a 400 μm×400 μm region. The high-angle grain size is more preferably 30 μm or less.

[0064] The method for measuring the high-angle grain size is as described above.

[0065] The steel plate according to the second embodiment has the above-mentioned chemical composition and metal structure, and thus has, for example, desired hardness and toughness. Hereinafter, the hardness in the L-direction cross section at a position 1 mm from the surface of the steel plate according to the second embodiment, the hardness at the 1 / 2t part, and the low-temperature toughness will be described.

[0066] [Hardness at L-section 1mm from surface in thickness direction: 360HV10 or more] In order to ensure wear resistance, the steel plate according to the second embodiment has a Vickers hardness of 360 HV10 or more in the L-direction cross section at a position 1 mm from the surface in the thickness direction. The surface hardness of the wear-resistant steel plate is preferably 400 HV10 or more, and the higher the better, with no upper limit. The surface hardness of the steel plate may be 450 HV10 or less from the viewpoint of ensuring toughness. The surface hardness of the steel plate is measured at a position 1 mm from the surface in the thickness direction of the steel plate, taking into account the influence of decarburization. The Vickers hardness test is performed in accordance with JIS Z 2244:2009, and the load is 10 kgf. The Vickers hardness is the average value of three points measured on the L-direction cross section of the sample. Details of the sample used in the Vickers hardness test are as described above.

[0067] [Hardness at the center of the steel plate (1 / 2t part): 360HV10 or more] The center part in the thickness direction of the steel plate according to the second embodiment is synonymous with the 1 / 2t part, and hereinafter, the hardness at the center part in the thickness direction of the steel plate is referred to as the 1 / 2t part hardness. The 1 / 2t part hardness is preferably 360HV10 or more, more preferably 380HV10 or more in Vickers hardness, from the viewpoint of preventing deterioration of wear resistance after the surface layer part to the 1 / 4t part is worn by use. The 1 / 2t part hardness is more preferably 400HV10 or more, and the higher the better, and there is no upper limit. The 1 / 2t part hardness may be 450HV10 or less from the viewpoint of ensuring toughness. The Vickers hardness test is performed in accordance with JIS Z 2244:2009, and the load is 10kgf. The Vickers hardness is the average value of three points measured on the L-direction cross section of the sample. Details of the sample used in the Vickers hardness test are as described above. The hardness of the 1 / 2t section is ensured by increasing the content of alloys that enhance hardenability and by increasing the cooling rate for quenching.

[0068] [Average Charpy absorbed energy at -40℃: 27J or more] The steel plate according to the second embodiment may be used in cold regions or at high altitudes. In order to suppress breakage due to impact during processing and use, the steel plate is required to ensure toughness at -40°C. From this viewpoint, the steel plate according to the second embodiment preferably has an average Charpy absorbed energy of 27 J or more at -40°C. The average Charpy absorbed energy at -40°C is preferably 50 J or more. The higher the average Charpy absorbed energy at -40°C, the more preferable it is, and although there is no upper limit, it may be 100 J or less.

[0069] Charpy tests are performed using full-size test pieces with V notches in accordance with JIS Z 2242:2018. When the thickness of the steel plate is 12 mm or less, a sub-size test piece of 5 mm is used. The Charpy test piece is taken from the 1 / 4t section for wear-resistant steel plates with a thickness of 16 mm or more, and from the 1 / 2t section for wear-resistant steel plates with a thickness of less than 16 mm. The longitudinal direction of the Charpy test piece is the rolling direction. Considering the variation in the measured values, the average Charpy absorbed energy is the arithmetic average of the measured values ​​of three test pieces.

[0070] <Thickness> The thickness of the steel plate according to the first embodiment (e.g., the original plate before heat treatment) is not particularly limited. The steel plate according to the first embodiment may be, for example, a thick plate. Specifically, the steel plate according to the first embodiment may have a thickness of 6 mm or more and 50 mm or less. The thickness may be 8 mm or more, 10 mm or more, or 16 mm or more, and may be 45 mm or less, or 40 mm or less.

[0071] The thickness of the steel plate according to the second embodiment (e.g., abrasion-resistant steel plate) is not particularly limited. The steel plate according to the second embodiment may be a thick plate, like the steel plate according to the first embodiment. The steel plate according to the second embodiment may be used as a steel plate, a steel strip, a steel pipe, etc. The steel plate according to the second embodiment may have a thickness of, for example, 6 mm or more and 50 mm or less. The thickness may be 8 mm or more, 10 mm or more, or 16 mm or more. Moreover, the thickness may be 45 mm or less, or 40 mm or less.

[0072] <Other matters> As described above, the steel plate according to the first embodiment can be used as a pre-heat-treatment original plate. In other words, the steel plate according to the first embodiment can become a wear-resistant steel plate after a predetermined heat treatment. For example, the steel plate according to the first embodiment may have the following properties. Properties: When the above steel plate was subjected to quenching treatment by heating to 950°C, holding for 15 minutes, and then cooling at a rate of 15°C / sec, When an L-direction cross section at a position 1 mm in the thickness direction from the surface of the steel plate after the quenching treatment is observed by EBSD, in a 400 μm × 400 μm area, the EBSD pixel KAM value is 1.5 ° or less, and the average of the top 10 grain sizes of ferrite grains defined as closed regions surrounded by EBSD pixel boundaries having an inclination angle difference of 15 ° or more is 40 μm or less, The area ratio of cementite in the steel plate after the quenching treatment is 0% or more and 0.5% or less, and the average equivalent circle diameter of the cementite is 500 nm or less.

[0073] 2. Steel sheet manufacturing method Next, a method for manufacturing a steel sheet according to this embodiment will be described.

[0074] The steel plate according to the first embodiment (e.g., unheat-treated original plate) is produced by melting steel, adjusting the components, and casting the resulting steel slab, which is then used as a steel material. The steel material is hot-rolled, and then slowly cooled by holding in a furnace, using a heat-insulating material, or stacking hot-rolled sheets. After slow cooling, the steel may be reheated and quenched. The steel plate according to the second embodiment can be produced by reheating the steel plate according to the first embodiment under predetermined conditions and quenching it.

[0075] The manufacturing method of the steel sheet according to the first embodiment is, for example, Obtaining slabs having the above chemical composition, The slab is heated to a temperature range of 1100°C to 1300°C, and is rolled at a temperature of 1100°C or higher with a rolling reduction of 10% or more three or more times to obtain a rough rolled material with a total rolling reduction of 50% or more. The rough rolled material is rolled three or more times at a rolling reduction of 5% or more in a temperature range of 900°C or more and less than 1100°C, and the rough rolled material is subjected to finish rolling at a total rolling reduction of 50% or more to obtain a finish rolled material; and cooling the finish rolled material to 200°C or less at a cooling rate of 0.10°C / sec or less.

[0076] The manufacturing method of the steel sheet according to the second embodiment is, for example, Obtaining slabs having the above chemical composition, A slab heated to a temperature range of 1100°C to 1300°C is rolled at a temperature of 1100°C or higher with a rolling reduction of 10% or more three times or more, and rough rolling is performed with a total rolling reduction of 50% or more to obtain a rough rolled material. The rough rolled material is rolled three or more times at a rolling reduction of 5% or more in a temperature range of 900°C or more and less than 1100°C, and the rough rolled material is subjected to finish rolling at a total rolling reduction of 50% or more to obtain a finish rolled material. Cooling the finish rolled material to 200°C or less at a cooling rate of 0.10°C / sec or less; and and applying a quenching treatment to the cooled finish rolled material by heating it to 900°C or higher, holding it for 10 minutes or more, and then cooling it at a rate of 10°C / sec or more. In other words, the manufacturing method of the steel plate according to the second embodiment is characterized in that the quenching treatment is applied to the steel plate according to the first embodiment.

[0077] [To obtain slabs with the desired chemical composition] The manufacturing method of the slab used for manufacturing the steel plate according to the present embodiment is not limited, and the slab is manufactured by a known method such as a continuous casting method, an ingot casting-blooming method, etc., using molten steel melted to have the above-mentioned chemical components by a normal refining process such as a converter or an electric furnace. The slab is preferably cooled after casting, reheated to a temperature equal to or higher than the Ac3 transformation point as described below, and hot rolled under predetermined conditions. If the slab after continuous casting is charged into a heating furnace by hot charging without being cooled to 400°C or less, coarse austenite generated during casting may remain in the slab after heating. In order to promote the refinement of the structure of the steel plate, the slab after continuous casting is preferably cooled to 400°C or less once.

[0078] [Slab heating temperature before rough rolling: 1100℃ or higher, 1300℃ or lower] The heating of the slab before rough rolling is performed in a temperature range where the surface temperature of the slab is equal to or higher than the Ac3 transformation point. The Ac3 transformation point is the temperature at which transformation from ferrite to austenite begins during heating. The heating temperature of the slab is 1100°C or higher in order to destroy the columnar structure and easily homogenize the metal structure in hot rolling including rough rolling in the subsequent process. The heating temperature is more preferably 1150°C or higher, and even more preferably 1200°C or higher. On the other hand, the heating temperature is 1300°C or lower in order to suppress coarsening of crystal grains. Hot rolling may be divided into "rough rolling" in which hot rolling is performed at 1100°C or higher and "finish rolling" in which hot rolling is performed at less than 1100°C.

[0079] [Rough rolling: rolling at a temperature of 1100℃ or higher with a reduction of 10% or more three times or more, for a total reduction of 50% or more] Rough rolling is performed in a temperature range where the surface temperature of the rolled material is equal to or higher than the Ar3 transformation point. The Ar3 transformation point is the temperature at which transformation from austenite to ferrite begins during cooling. The rolling reduction is 10% or more per rolling pass from the viewpoint of destroying the columnar structure of the steel material, homogenizing the metal structure, and promoting recrystallization of the metal structure. The rolling reduction per pass is preferably 15% or more, more preferably 20% or more. The number of times rough rolling is performed is three or more, and the total rolling reduction is 50% or more, more preferably 60% or more. The rolling reduction per pass in rough rolling is calculated from the thickness of the rolled material before rolling and the thickness of the rolled material after rolling, using the following formula (1). The total rolling reduction in rough rolling is calculated using the following formula (2). Reduction rate per time (%) = 100 × {(thickness of material before rolling) - (thickness of material after rolling)} / thickness of material before rolling (1) Total rolling reduction (%) = 100 × {(thickness of slab before rough rolling) - (thickness of rough-rolled material after rough rolling)} / thickness of slab before rough rolling (2)

[0080] [Finish rolling: rolling at a temperature of 900℃ or higher and lower than 1100℃ with a reduction of 5% or more three times or more, with a total reduction of 50% or more from the above rough rolled material] Finish rolling is performed in a temperature range where the surface temperature of the rolled material is equal to or higher than the Ar3 transformation point. The temperature of the roughly rolled material before the finish rolling is equal to or higher than 900°C. The finish rolling temperature is less than 1100°C in order to suppress further recrystallization of the crystal grains recrystallized by the rough rolling. The rolling reduction is 5% or more in one rolling pass from the viewpoint of introducing dislocations into the crystal grains recrystallized by the rough rolling and obtaining a fine structure by the subsequent ferrite transformation, pearlite transformation, or bainite transformation. The rolling reduction per pass is preferably 10% or more, more preferably 20% or more. The number of finish rolling passes is three or more, and the rolling reduction from the rough rolled material is 50% or more in total, more preferably 60% or more. The rolling reduction per pass in the finish rolling is calculated from the thickness of the rolled material after rolling and the thickness of the rolled material before rolling by the above formula (1). The total rolling reduction in the finish rolling is calculated by the following formula (3). Total rolling reduction (%) = 100 × {(thickness of rough rolled material before finish rolling) - (thickness of finish rolled material after finish rolling)} / thickness of rough rolled material before finish rolling (3)

[0081] [Cooling rate after finish rolling: 0.10℃ / sec or less] The cooling rate of the finish rolled material after the finish rolling is 0.10°C / sec or less in the temperature range up to 200°C or less from the viewpoint of suppressing martensitic transformation by slow cooling and further removing hydrogen contained in the steel sheet. The cooling rate is preferably 0.08°C / sec or less, more preferably 0.05°C / sec or less. It is preferable to reduce the cooling rate, and the lower limit is not limited, but it may be 0.01°C / sec or more from the viewpoint of manufacturing costs. The cooling rate is obtained by dividing the temperature difference between the surface temperature of the finish rolled material immediately after the finish rolling and 200°C by the time until the surface temperature of the finish rolled material falls below 200°C after the finish rolling. The cooling rate of the finish rolled material after the finish rolling is not to be 3°C / sec or more at any time after the finish rolling until the surface temperature falls below 200°C. Most preferably, the cooling rate of the finish rolled material after finish rolling does not exceed 0.1°C / sec at any time after finish rolling until the surface temperature falls below 200°C.

[0082] By going through the above steps, the steel sheet according to the first embodiment can be manufactured.

[0083] [Quenching treatment: After cooling, the finish-rolled material is heated to 900°C or higher, held for 10 minutes or more, and then cooled at a rate of 10°C / sec or more] After the above steps, a predetermined quenching treatment is performed to manufacture the steel plate according to the second embodiment. That is, the finish-rolled material after slow cooling may be gas-cut to any size, reheated, and then quenched. From the viewpoint of preventing the formation of ferrite, the reheating temperature may be a temperature equal to or higher than the Ac3 transformation point. The reheating temperature is preferably 900°C or higher. From the viewpoint of refining the metal structure, the reheating temperature is preferably 1150°C or lower. The reheating temperature is more preferably 1050°C or lower, and even more preferably 1000°C or lower.

[0084] The starting temperature for quenching after reheating is 770°C or higher from the viewpoint of preventing the formation of ferrite. The cooling rate for quenching is 10°C / sec or higher at the surface temperature of the rolled material from the viewpoint of preventing the formation of ferrite and suppressing the precipitation and growth of cementite. The faster the cooling rate, the better, but there is a limit depending on the capacity of the cooling equipment, the thickness of the rolled material, etc., and it may be 50°C / sec or lower.

[0085] The quenching stop temperature after reheating is 200° C. or less in terms of the surface temperature of the rolled material from the viewpoint of suppressing the precipitation and growth of cementite. The quenching stop temperature is preferably 100° C. or less in terms of the surface temperature of the rolled material from the viewpoint of suppressing the growth of cementite in particular.

[0086] In the present application, "Ac3" is the transformation start temperature during heating calculated as follows, and "Ar3" is the transformation start temperature during cooling calculated as follows, and is calculated using the chemical components of the steel.

[0087] Ac3(℃)=902-255×C+19×Si-11×Mn-5×Cr+13×Mo-20×Ni+55×V Ar3(℃)=868-396×C+24.6×Si-68.1×Mn-24.8×Cr In the above formula, C, Si, Mn, Cr, Mo, Ni, and V mean the contents expressed in mass%. EXAMPLES

[0088] An embodiment of the present invention will be described below. However, the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as it does not deviate from the gist of the invention and achieves its object.

[0089] 1.Under heat treatment Slabs were produced by melting steels having various chemical compositions shown in Table 1 below. The balance of the chemical compositions shown in Table 1 is Fe and impurities. These slabs were inserted into a furnace heated to the temperature shown in Table 2 below, and after homogenization treatment, they were taken out into the atmosphere and subjected to rough rolling and finish rolling under the conditions shown in Table 2 below to obtain steel plates (original plates before heat treatment) with a thickness of 50 mm.

[0090] For each of the pre-heat-treatment original sheets, the "hardness in the L-direction cross section at a position 1 mm from the surface in the thickness direction" was measured. The method for measuring the hardness was as described above. The results are shown in Table 2 below.

[0091] For each of the unheated base plates, gas cutting was performed using a mixed gas containing 30% or more hydrogen and other flammable gases. The presence or absence of cracks near the cut part by gas cutting was then confirmed, and the pass / fail of gas cutting crack resistance was judged according to the following criteria. Pass: No cracks were found near the cut area caused by gas cutting. Failed: Cracks were found near the cut area caused by gas cutting.

[0092] 2.Wear-resistant steel plate Each of the above pre-heat-treatment original plates was subjected to quenching at the quenching temperature and post-quenching cooling rate shown in Table 3 below, to obtain abrasion-resistant steel plates.

[0093] For each wear-resistant steel plate, the following were measured: "the average of the top 10 ferrite grain sizes defined as a closed region surrounded by EBSD pixel boundaries with an EBSD pixel KAM value of 1.5° or less and an inclination difference of 15° or more in a 400 μm x 400 μm region," "cementite area ratio," "average equivalent circle diameter of cementite," and "hardness (quenched hardness) in the L-direction cross section at a position 1 mm from the surface in the thickness direction." In addition, for each wear-resistant steel plate, the "average Charpy absorbed energy at -40°C (quenched toughness)" was measured. The measurement methods for each were as described above. The results are shown in Table 3 below.

[0094] 3. Evaluation of Examples and Comparative Examples The steel plates having excellent gas cutting crack resistance before heat treatment and excellent toughness and hardness after heat treatment were judged as "Examples", and the steel plates having poor gas cutting resistance before heat treatment and / or poor toughness and hardness after heat treatment were judged as "Comparative Examples". The judgment results of the Examples and Comparative Examples are shown in Table 3.

[0095] [Table 1]

[0096] [Table 2]

[0097] [Table 3]

[0098] The results shown in Tables 1 to 3 reveal the following:

[0099] In the case of No. 5, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the hardness of the wear-resistant steel plate after heat treatment was insufficient due to insufficient C content, and the toughness was deteriorated due to the coarsening of grain size.

[0100] In the case of No. 6, although the gas cutting crack resistance was ensured in the base plate before the heat treatment, the toughness of the wear-resistant steel plate after the heat treatment was deteriorated due to the excessive C content.

[0101] In the case of No. 7, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the Si content was insufficient, so the hardness of the wear-resistant steel plate after heat treatment was insufficient, and the toughness was deteriorated due to the coarsening of the grain size.

[0102] In the case of No. 8, although the gas cutting crack resistance was ensured in the base plate before the heat treatment, the toughness of the wear-resistant steel plate after the heat treatment was deteriorated due to the excessive Si content.

[0103] In the case of No. 9, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the Mn content was insufficient, so the hardness of the wear-resistant steel plate after heat treatment was insufficient, and the toughness was deteriorated due to the coarsening of grain size.

[0104] As for No. 10, the gas cutting crack resistance was ensured in the base plate before heat treatment, but the toughness of the wear-resistant steel plate after heat treatment was deteriorated due to the excessive Mn content.

[0105] As for No. 11, although the gas cutting crack resistance was ensured in the base plate before the heat treatment, the toughness of the wear-resistant steel plate after the heat treatment was deteriorated due to the excessive P content.

[0106] As for No. 12, although the gas cutting crack resistance was ensured in the base plate before the heat treatment, the toughness of the wear-resistant steel plate after the heat treatment was deteriorated due to the excessive S content.

[0107] In the case of No. 13, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the hardness of the wear-resistant steel plate after heat treatment was insufficient due to an insufficient Cr content, and the toughness was deteriorated due to the coarsening of grain size.

[0108] As for No. 14, although the gas cutting crack resistance was ensured in the base plate before the heat treatment, the toughness of the wear-resistant steel plate after the heat treatment was deteriorated due to the excessive Cr content.

[0109] As for No. 15, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the Al content was insufficient, and therefore the wear-resistant steel plate after heat treatment had insufficient hardness and deteriorated toughness due to poor structure.

[0110] In the case of No. 16, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the Al content was excessive, and therefore the toughness of the wear-resistant steel plate after heat treatment deteriorated due to poor structure.

[0111] As for No. 17, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the N content was insufficient, and therefore the wear-resistant steel plate after heat treatment had a deteriorated toughness due to poor structure.

[0112] In the case of No. 18, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the N content was excessive, and therefore the toughness of the wear-resistant steel plate after heat treatment deteriorated due to poor structure.

[0113] As for No. 19, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the B content was insufficient, and therefore the wear-resistant steel plate after heat treatment had insufficient hardness and deteriorated toughness due to poor structure.

[0114] As for No. 20, although the gas cutting crack resistance was ensured in the base plate before heat treatment, the B content was excessive, so that the toughness of the wear-resistant steel plate after heat treatment deteriorated.

[0115] For Nos. 21 to 26, the gas cutting crack resistance was ensured in the base plate before heat treatment, but the subsequent quenching temperature was inappropriate, resulting in insufficient hardness in the wear-resistant steel plate after heat treatment.

[0116] For Nos. 27 to 33, the cooling rate during production of the unheat-treated black sheets was inappropriate, so that the hardness of the unheat-treated black sheets was excessive, and gas cutting cracking resistance could not be ensured.

[0117] In contrast, for Nos. 1, 2, 4 and 34 to 38, pre-heat treatment base plates excellent in gas cutting cracking resistance were obtained, and after heat treatment, wear-resistant steel plates excellent in hardness and toughness were obtained.

[0118] 4. Supplementary Information In the above embodiment, the plate thickness is 50 mm, but the plate thickness is not limited to this. The same effect can be obtained even if the plate thickness is less than 50 mm or more than 50 mm. As an example, the plate thickness is preferably 6 mm or more and 50 mm or less.

[0119] In addition, the hydrogen content of some of the pre-heat-treated raw sheets according to the above-mentioned examples was measured at a position 1 mm from the surface in the thickness direction by gas chromatography mass spectrometry, and it was found that the hydrogen content was low. For example, the hydrogen content at a position 1 mm from the surface in the thickness direction of the pre-heat-treated raw sheet is preferably 0.1 mass ppm or less.

Claims

1. In mass percent, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, 7.50% or less, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of, The hardness in the cross-section in the L-direction at a position 1 mm from the surface in the thickness direction is 220HV10 or less. steel plate.

2. In mass percent, V: 0.001% or more, 0.200% or less, Mo: 0.001% or more, 0.200% or less, W: 0.001% or more, 0.200% or less, Ti: 0.001% or more, 0.100% or less, Nb: 0.001% or more, 0.100% or less, Includes one or more species selected from the group, The steel plate according to claim 1.

3. In mass percent, Ni: 0.001% or more, 1.000% or less, Cu: 0.001% or more, 1.000% or less, Includes one or two species selected from the group, The steel plate according to claim 1.

4. In mass percent, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.1000% or less, Includes one or more species selected from the group, The steel plate according to claim 1.

5. In mass percent, Sn: 0.001% or more, 0.100% or less, and Sb: 0.001% or more, 0.100% or less, Includes one or two species selected from the group, The steel plate according to claim 1.

6. It has the following properties: A steel plate according to any one of claims 1 to 5. Properties: When the steel plate is subjected to a quenching treatment in which it is heated to 950°C, held for 15 minutes, and then cooled at 15°C / sec, When the cross-section in the L-direction at a position 1 mm in the thickness direction from the surface of the steel plate after the quenching treatment is observed with EBSD, in a 400 μm × 400 μm region, the average of the top 10 ferrite grain sizes defined as a closed region surrounded by EBSD pixel boundaries where the KAM value of the EBSD pixels is 1.5° or less and the tilt angle difference is 15° or more is 40 μm or less. The area ratio of cementite in the steel sheet after the quenching treatment is 0% or more and 0.5% or less, and the average circular diameter of the cementite is 500 nm or less.

7. Having a thickness of 6 mm or more and 50 mm or less, A steel plate according to any one of claims 1 to 5.

8. This is the raw plate before heat treatment. A steel plate according to any one of claims 1 to 5.

9. To obtain a slab having the chemical composition described in any one of claims 1 to 5, The slab, heated to a temperature of 1100°C or higher and 1300°C or lower, is subjected to rolling at a temperature of 1100°C or higher with a reduction ratio of 10% or more three or more times, thereby obtaining a roughly rolled material by performing rough rolling with a total reduction ratio of 50% or more. The rough-rolled material is subjected to rolling at a temperature range of 900°C to less than 1100°C for three or more times with a reduction ratio of 5% or more, and then the rough-rolled material is subjected to finish rolling with a total reduction ratio of 50% or more to obtain a finished-rolled material, and This includes cooling the finished rolled material to 200°C or below at a cooling rate of 0.10°C / sec or less. A method for manufacturing steel plates.

10. In mass percent, C: 0.10% or more, 0.35% or less, Si: more than 0.50%, less than 2.00%, Mn: 0.10% or more, 2.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00%, 7.50% or less, Al: 0.010% or more, 0.100% or less, N: 0.0020% or more, 0.0100% or less, B: 0.0003% or more, 0.0030% or less, V: 0% or more, 0.200% or less, Mo: 0% or more, 0.200% or less, W: 0% or more, 0.200% or less, Ti: 0% or more, 0.100% or less, Nb: 0% or more, 0.100% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.1000% or less, Sn: 0% or more, 0.100% or less, Sb: 0% or more, 0.100% or less, and Remainder: Fe and impurities It consists of, When observing an L-shaped cross-section at a position 1 mm from the surface in the thickness direction using EBSD, in a 400 μm × 400 μm region, the average of the top 10 ferrite grain sizes defined as a closed region enclosed by EBSD pixel boundaries where the KAM value of the EBSD pixels is 1.5° or less and the tilt angle difference is 15° or more is 40 μm or less. The area ratio of cementite is 0% or more and 0.5% or less, and the average circular equivalent diameter of the cementite is 500 nm or less. steel plate.

11. In mass percent, V: 0.001% or more, 0.200% or less, Mo: 0.001% or more, 0.200% or less, W: 0.001% or more, 0.200% or less, Ti: 0.001% or more, 0.100% or less, Nb: 0.001% or more, 0.100% or less, Includes one or more species selected from the group, The steel plate according to claim 10.

12. In mass percent, Ni: 0.001% or more, 1.000% or less, Cu: 0.001% or more, 1.000% or less, Includes one or two species selected from the group, The steel plate according to claim 10.

13. In mass percent, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.1000% or less, Includes one or more species selected from the group, The steel plate according to claim 10.

14. In mass percent, Sn: 0.001% or more, 0.100% or less, and Sb: 0.001% or more, 0.100% or less, Includes one or two species selected from the group, The steel plate according to claim 10.

15. The hardness in the cross-section in the L-direction at a position 1 mm from the surface in the thickness direction is 360HV10 or higher. The average Charpy absorption energy at -40°C is 27 J or higher. A steel plate according to any one of claims 10 to 14.

16. Having a thickness of 6 mm or more and 50 mm or less, A steel plate according to any one of claims 10 to 14.

17. To obtain a slab having the chemical composition described in any one of claims 10 to 14, The slab, heated to a temperature of 1100°C or higher and 1300°C or lower, is subjected to rolling at a temperature of 1100°C or higher with a reduction ratio of 10% or more three or more times, thereby obtaining a roughly rolled material by performing rough rolling with a total reduction ratio of 50% or more. The rough-rolled material is subjected to rolling at a temperature range of 900°C to less than 1100°C for three or more times with a reduction ratio of 5% or more, and then the rough-rolled material is subjected to finish rolling with a total reduction ratio of 50% or more to obtain a finished-rolled material. The aforementioned finished rolled material is cooled to 200°C or below at a cooling rate of 0.10°C / sec or less, and The process includes subjecting the cooled finished rolled material to a quenching treatment, which involves heating it to 900°C or higher, holding it for 10 minutes or more, and then cooling it at 10°C / sec or higher. A method for manufacturing steel plates.