Steel sheet and method for producing steel sheet

Abstract

To disclose a steel sheet that suppresses the use of costly elements such as Ni and Mo, achieves reduction in alloy and production costs, and enables compatibility between hardness and bendability.SOLUTION: A steel sheet of the present disclosure contains, in mass%, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, with the balance being Fe and impurities. The steel sheet has a hardness of 360 HV10 or more at a position 2 mm from the surface in an L-direction cross-section in the thickness direction. The surface oxide scale has a thickness of 5 μm or less.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] A steel plate excellent in wear resistance is called a wear-resistant steel plate. Since the hardness near the surface layer is required for the wear-resistant steel plate, it contains elements that enhance hardenability. Cr, Ni, and Mo are elements that enhance hardenability. Conventionally, wear-resistant steel plates containing Cr, Ni, Mo, etc. have been proposed (see, for example, Patent Documents 1 to 5). Further, the wear-resistant steel plate preferably has excellent bend formability. For example, it has been proposed to improve the bend formability of the wear-resistant steel plate by controlling the finish rolling conditions when manufacturing the wear-resistant steel plate (see, for example, Patent Document 6).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventional steel plates have room for improvement in achieving both hardness and bend formability.

Means for Solving the Problems

[0005] The gist of the present invention is as follows.

[0006] (1) By mass, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: More than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, and the balance consists of Fe and impurities, the hardness in the L-direction cross-section at a position 2 mm in the thickness direction from the surface is 360 HV10 or more, the thickness of the surface oxide scale is 5 μm or less, a steel sheet. (2) By mass, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: More than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, and contains one or more selected from the group consisting of the following group a, group b, group c, and group d, the balance consists of Fe and impurities, the hardness in the L-direction cross-section at a position 2 mm in the thickness direction from the surface is 360 HV10 or more, the thickness of the surface oxide scale is 5 μm or less, Steel plate [Group a] V: not less than 0.001% and not more than 0.200%, Mo: not less than 0.001% and not more than 0.200%, W: not less than 0.001% and not more than 0.200%, Ti: not less than 0.001% and not more than 0.100%, and Nb: not less than 0.001% and not more than 0.100%, one or more selected from the group consisting of [Group b] Ni: not less than 0.001% and not more than 1.000%, and Cu: not less than 0.001% and not more than 1.000%, one or two selected from the group consisting of [Group c] Ca: not less than 0.0001% and not more than 0.0100%, Mg: not less than 0.0001% and not more than 0.0100%, and REM: not less than 0.0001% and not more than 0.1000%, one or more selected from the group consisting of [Group d] Sn: not less than 0.001% and not more than 0.100%, and Sb: not less than 0.001% and not more than 0.100%, one or two selected from the group consisting of (3) containing the said Group a, the steel plate according to (2). (4) containing the said Group b, the steel plate according to (2) or (3). (5) containing the said Group c, the steel plate according to any one of (2) to (4). (6) containing the said Group d, the steel plate according to any one of (2) to (5). (7) having a thickness of not less than 6 mm and not more than 50 mm, the steel plate according to any one of (1) to (6). (8) Obtaining a slab having a chemical composition according to any one of (1) to (6). Performing rough rolling on the slab to obtain a rough rolled material. For the rough rolled material, at a temperature of Ar3 - 30°C or higher and Ar3 or lower, performing finish rolling with a reduction ratio of each rolling pass being 5% or less and the number of rolling passes being 3 or more to obtain a finish rolled material, and Cooling the finish rolled material to 200°C or lower at a cooling rate of 10°C / sec or higher, including A method for manufacturing a steel plate.

Effect of the Invention

[0007] According to the present invention, a steel plate having excellent hardness and bend formability is provided.

Mode for Carrying Out the Invention

[0008] Hereinafter, a steel plate according to an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. In the numerical ranges described step by step in this specification, the upper limit value of a certain stepwise numerical range may be replaced with the upper limit value of the numerical range of other stepwise descriptions, and the lower limit value of a certain stepwise numerical range may be replaced with the lower limit value of the numerical range of other stepwise descriptions. The term "step" includes not only an independent step but also this term even when it cannot be clearly distinguished from other steps as long as the intended purpose of that step is achieved.

[0009] First, the research results and new findings obtained by the inventors who have completed the present invention will be described in detail.

[0010] Cr is an element that is cheaper than Ni and Mo. If the required properties of the steel plate can be achieved by suppressing the contents of Ni and Mo and increasing the Cr content, it is possible to reduce the alloy cost of the steel plate. As a result of the research by the inventors, it was found that the Cr content required for a non-alloy type steel plate when Ni and Mo are not intentionally contained or the contents of Ni and Mo are each suppressed to 0.5% or less is more than 2.00%.

[0011] The bendability of the steel sheet deteriorates due to the appearance of grain boundary oxide scale and the increase in the thickness of the surface oxide scale. To improve the bendability, it is necessary to suppress the generation of grain boundary oxide scale and surface oxide scale by suppressing the Si content, suppress the generation of surface oxide scale by suppressing the Mn content, and further precisely control the thickness of the surface oxide scale by making full use of rolling technology. As a result of the study by the present inventors, by suppressing the Si content to 0.30% or less, the generation of grain boundary oxide scale and surface oxide scale caused by Si is suppressed, and by suppressing the Mn content to 1.00% or less, the generation of surface oxide scale caused by Mn is reduced. Furthermore, during the production of the steel sheet, at a temperature immediately below the Ar3 transformation point (specifically, Ar3 - 30 °C or more and Ar3 or less), the reduction ratio of each rolling pass is 5% or less, and finish rolling with 3 or more passes of the rolling pass is performed, whereby the thickness of the surface oxide scale can be significantly reduced. As a result of these, it was found that a steel sheet excellent in bendability can be obtained while ensuring sufficient hardness.

[0012] 1. Steel sheet Based on the above findings, the present application discloses the following steel sheet.

[0013] The steel sheet according to the first embodiment contains, by mass%, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: More than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, the balance consists of Fe and impurities, the hardness in the L-direction cross-section at a position 2 mm from the surface in the thickness direction is 360 HV10 or more, It is characterized in that the thickness of the surface oxidation scale is 5 μm or less.

[0014] The steel sheet according to the second embodiment contains, by mass%, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: More than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, contains one or more selected from the group consisting of the following group a, group b, group c, and group d, the balance consists of Fe and impurities, the hardness in the L-direction cross-section at a position 2 mm from the surface in the thickness direction is 360 HV10 or more, It is characterized in that the thickness of the surface oxidation scale is 5 μm or less. [Group a] V: 0.001% or more and 0.200% or less, Mo: 0.001% or more and 0.200% or less, W: 0.001% or more and 0.200% or less, Ti: 0.001% or more and 0.100% or less, and Nb: 0.001% or more and 0.100% or less, one or more selected from the group consisting of [Group b] Ni: 0.001% or more and 1.000% or less, and Cu: 0.001% or more and 1.000% or less, one or two selected from the group consisting of [Group c] Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, and REM: Above 0.0001% and below 0.1000%, One or more selected from the group consisting of [Group d] Sn: Above 0.001% and below 0.100%, and Sb: Above 0.001% and below 0.100%, One or two selected from the group consisting of

[0015] [Chemical composition] The elements constituting the chemical composition of the steel sheet according to this embodiment will be described. In the following description of the elements, "%" of the content means "% by mass".

[0016] [C: Above 0.10% and below 0.35%] C (carbon) is an element that increases the hardenability of steel and raises the 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. Excessive formation of cementite may reduce toughness. Also, when the C content becomes excessive, cracking of the steel sheet is likely to occur, and the bend formability may deteriorate. From the viewpoints of ensuring toughness and bend formability, the C content is 0.35% or less. The C content is preferably 0.25% or less, and more preferably 0.22% or less.

[0017] [Si: Above 0.01% and below 0.30%] Si (silicon) is a deoxidizing element. Also, Si can contribute to increasing the hardenability of steel and raising the hardness. From the viewpoints of the deoxidizing effect and ensuring hardness, the Si content is 0.01% or more. The Si content is preferably 0.06% or more, and more preferably 0.10% or more. On the other hand, when the Si content becomes excessive, excessive formation of grain boundary oxide scale is likely to occur, impairing the bend formability of the steel sheet. From the viewpoint of ensuring the bend formability of the steel sheet, the Si content is 0.30% or less. The Si content is preferably 0.25% or less, and more preferably 0.20% or less.

[0018] [Mn: 0.10% or more, 1.00% or less] Mn (manganese) is an element that enhances the hardenability of steel and improves its hardness. From the viewpoint of ensuring hardness, the Mn content is 0.10% or more. The Mn content is preferably 0.20% or more, and more preferably 0.30% or more. On the other hand, if the Mn content is excessive, surface oxide scale tends to be excessively generated, impairing the bending workability of the steel sheet. From the viewpoint of ensuring the bending workability of the steel sheet, the Mn content is 1.00% or less. The Mn content is preferably 0.90% or less, and more preferably 0.80% or less.

[0019] [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 is deteriorated and there is a possibility that cracks are induced and bending workability is deteriorated. The P content is 0.020% or less from the viewpoint of ensuring toughness and bending workability. 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.

[0020] [S:0.010% or less] S (sulfur) is an element that is mixed into steel during the manufacturing process. Excessive S content may deteriorate toughness and induce cracking, thereby deteriorating bending workability. The S content is 0.010% or less from the viewpoint of ensuring toughness and bending workability. 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.

[0021] [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 hardenability and toughness of the steel sheet 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 the hardenability. The Cr content is preferably 2.10% or more, and more preferably 2.40% or more. On the other hand, if the Cr content is excessive, the cost increases and there is a possibility that cracks are induced and bending workability is deteriorated. From the viewpoint of cost and bending workability, the Cr content is 7.50% or less, and may be 7.00% or less. The Cr content is preferably 5.00% or less, and more preferably 3.00% or less.

[0022] [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 effect of deoxidization, 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 induce cracking of the steel sheet and deteriorate bending workability. From the viewpoint of suppressing the generation of coarse inclusions and ensuring bending workability, the Al content is 0.100% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less.

[0023] [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 deteriorate toughness and induce cracking, thereby deteriorating bending workability. The N content is 0.0100% or less from the viewpoint of ensuring toughness and bending workability. 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.

[0024] [B: Above 0.0003% and below 0.0030%] B (boron) is an element that significantly enhances the hardenability of steel even in trace amounts. To obtain the effect of hardenability, the B content is 0.0003% or more, more preferably 0.0005% or more, and still more preferably 0.0008% or more. On the other hand, even if B is contained in excess, the effect saturates, and there is a possibility of inducing cracks and deteriorating bendability. From the viewpoint of ensuring bendability, the B content is 0.0030% or less. The B content is preferably 0.0025% or less, and more preferably 0.0020% or less.

[0025] The steel sheet according to this embodiment may contain one or more of V, Mo, W, Ti, and Nb (group a) that form compounds such as carbides and nitrides, if necessary.

[0026] [V: Above 0.001% and below 0.200%] V (vanadium) is an element that forms precipitates such as carbides and nitrides and has the effect of enhancing hardenability. To obtain the effect of hardenability, the V content is 0.001% or more, more preferably 0.005% or more. On the other hand, from the viewpoint of cost, the V content is 0.200% or less. Preferably it is 0.150% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. In the steel sheet according to this embodiment, the effect of V is not essential, and in this regard, the V content may be 0% or more.

[0027] [Mo: Above 0.001% and below 0.200%] Mo (molybdenum) is an element that enhances the hardenability of steel. To obtain the effect of hardenability, the Mo content is 0.001% or more, more preferably 0.030% or more. On the other hand, from the viewpoint of cost, the Mo content is 0.200% or less. Preferably it is 0.150% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. In the steel sheet according to this embodiment, the effect of Mo is not essential, and in this regard, the Mo content may be 0% or more.

[0028] [W: 0.001% or more and 0.200% or less] W (tungsten) is an element that enhances the hardenability of steel. In order to obtain the effect of hardenability, the W content is 0.001% or more, more preferably 0.030% or more. On the other hand, from the viewpoint of cost, the W content is 0.200% or less. Preferably it is 0.150% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. In addition, in the steel sheet according to the present embodiment, the effect by W is not essential, and in this regard, the W content may be 0% or more.

[0029] [Ti: 0.001% or more and 0.100% or less] Ti (titanium) is a deoxidizing element and also an element that forms nitrides. Further, when B is contained in the steel, Ti is used to suppress the formation of BN and enhance the hardenability. In order to obtain the effect of hardenability and from the viewpoint of refinement of the metal structure by TiN, the Ti content is 0.001% or more, preferably 0.005% or more. On the other hand, from the viewpoint of suppressing the generation of coarse inclusions that may have an adverse effect on toughness, the Ti content is 0.100% or less. The Ti content is more preferably 0.050% or less, and still more preferably 0.030% or less. The Ti content may be less than 0.010%. In addition, in the steel sheet according to the present embodiment, the effect by Ti is not essential, and in this regard, the Ti content may be 0% or more.

[0030] [Nb: 0.001% or more and 0.100% or less] Nb (niobium) is an element that forms precipitates such as carbides and nitrides. From the viewpoint of refinement of the metal structure by the precipitates, the Nb content is 0.001% or more, preferably 0.010% or more. On the other hand, since the effect saturates even if Nb is contained in excess, the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less, and still more preferably 0.030% or less. In addition, in the steel sheet according to the present embodiment, the effect by Nb is not essential, and in this regard, the Nb content may be 0% or more.

[0031] The steel sheet according to this embodiment may contain one or two types (group b) of Ni and Cu that contribute to the toughness of the base metal through solid solution as needed.

[0032] [Ni: 0.001% or more and 1.000% or less] Ni (nickel) is an element that increases the hardenability of steel and improves toughness. To obtain the effect of improving toughness, the Ni content is 0.001% or more, preferably 0.030% or more. On the other hand, from the perspective of cost, the Ni content is 1.000% or less. The Ni content is preferably 0.300% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. In addition, in the steel sheet according to this embodiment, the effect of Ni is not essential, and in this regard, the Ni content may be 0% or more.

[0033] [Cu: 0.001% or more and 1.000% or less] Cu (copper) is an element that increases the hardenability of steel and improves toughness. To surely obtain the effect of improving toughness, the Cu content is 0.001% or more, preferably 0.030% or more. On the other hand, from the perspective of cost, the Cu content is 1.000% or less. The Cu content is preferably 0.300% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. In addition, in the steel sheet according to this embodiment, the effect of Cu is not essential, and in this regard, the Cu content may be 0% or more.

[0034] The steel sheet according to this embodiment may contain one or more types (group c) of Ca, Mg, and REM that form oxides and contribute to the toughness of the heat-affected zone structure of welding through grain refinement as needed.

[0035] [Ca: 0.0001% or more and 0.0100% or less] Ca (Calcium) is an element that forms oxides and controls the morphology of inclusions. To obtain the effect of oxide formation, the Ca content is 0.0001% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, from the viewpoint of suppressing the formation 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. In addition, in the steel sheet according to this embodiment, the effect of Ca is not essential, and in this regard, the Ca content may be 0% or more.

[0036] [Mg: 0.0001% or more, 0.0100% or less] Mg (Magnesium) is an element that forms oxides and controls the morphology of inclusions. To obtain the effect of oxide formation, the Mg content is 0.0001% or more. The Mg content is preferably 0.0005% or more, and more preferably 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. In addition, in the steel sheet according to this embodiment, the effect of Mg is not essential, and in this regard, the Mg content may be 0% or more.

[0037] [REM: 0.0001% or more, 0.1000% or less] REM (rare earth element) means the general term for a total of 17 elements including two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce, and Nd. The REM content means the total content of the above 17 elements. REM is an element that forms oxides and sulfides and controls the morphology of inclusions. To obtain the effect of inclusion formation, the REM content is 0.0001% or more, preferably 0.0005% or more, and more preferably 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. In addition, in the steel sheet according to this embodiment, the effect of REM is not essential, and in this regard, the REM content may be 0% or more.

[0038] The steel sheet according to this embodiment may contain one or both of Sn and Sb (group d) that contribute to the toughness of the base metal through solid solution, segregation, and precipitation, if necessary.

[0039] [Sn: 0.001% or more and 0.100% or less] Sn (tin) is an element that is mixed into steel during the manufacturing process. From the viewpoint of ensuring toughness, the Sn content is 0.100% or less. The Sn content is preferably 0.070% or less, more preferably 0.050% or less, still more preferably less than 0.030%, and particularly preferably 0.010% or less. The Sn content may be 0% or more, but from the viewpoint of manufacturing cost, it may also be 0.001% or more.

[0040] [Sb: 0.001% or more and 0.100% or less] Sb (antimony) is an element that is mixed into steel during the manufacturing process. From the viewpoint of ensuring toughness, the Sb content is 0.100% or less. The Sb content is preferably 0.007% or less, more preferably 0.006% or less. The Sb content may be 0% or more, but from the viewpoint of manufacturing cost, it may also be 0.001% or more.

[0041] The balance of the above chemical components of the steel sheet according to this embodiment is Fe and impurities. Here, the impurities are components that are mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet according to this embodiment is industrially manufactured. The inclusion of impurities is allowed within a range that does not adversely affect the properties of the steel sheet according to this embodiment.

[0042] <Metallographic structure> Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the following, "%" of the metallographic structure is "area %". However, the metallographic structure of the steel sheet according to this embodiment is not limited to that described below.

[0043] The metallographic structure of the steel sheet according to this embodiment may or may not contain cementite. The area ratio of cementite contained in the metallographic structure of the steel sheet according to this embodiment may be, for example, 0% or more and 0.5% or less. Further, the average circle equivalent diameter of the cementite may be 500 nm or less. Further, in the region of 400 μm × 400 μm of the steel sheet according to this embodiment, among the crystal grains surrounded by high-angle grain boundaries of 15° or more, the average grain size of the 10 crystal grains in descending order of grain size may be 40 μm or less.

[0044] It is preferable that the area ratio, average circle equivalent diameter of cementite, and average grain size of crystals of the metallographic structure of the steel sheet according to this embodiment are within the ranges described above. For example, when upper bainite or pearlite is formed, the area ratio of cementite excessively increases and it tends to coarsen. When ferrite is formed, the hardness tends to be insufficient and the toughness may be reduced. Retained austenite may become hard martensite by deformation (work-induced martensite) and the toughness may be reduced. Therefore, it is preferable that the metallographic structure of the steel sheet according to the first embodiment or the second embodiment 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. In this case, the remainder excluding martensite and lower bainite consists of one or more of ferrite, pearlite, retained austenite, and upper bainite.

[0045] Martensite and lower bainite are lath-shaped hard phases and are generally distinguished by the presence or absence of cementite. When the metallographic structure of the steel plate according to the first embodiment or the second embodiment consists of martensite and lower bainite, the metallographic structure containing cementite 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 an optical microscope, it is determined that the metallographic structure consists of one or both of martensite and lower bainite. As described above, the steel plate according to the first embodiment or the second embodiment may contain a metallographic structure other than martensite and lower bainite.

[0046] For the observation of the metallographic structure, when the thickness of the steel plate is less than 16 mm, it is performed at a position in the thickness direction from the surface corresponding to half of the thickness (hereinafter also referred to as the "1 / 2t part"). When the thickness of the steel plate is 16 mm or more, the observation of the metallographic structure is performed at a position in the thickness direction from the surface corresponding to one-fourth of the thickness (hereinafter also referred to as the "1 / 4t part"). The observation surface of the sample used for the observation of the metallographic structure is a cross-section (L-direction cross-section) cut in the thickness direction along the L (longitudinal) direction (rolling direction) of the sample, and is subjected to wet polishing and etching with nital. The observation of the metallographic structure is performed at a magnification of 400 times, and the presence or absence of ferrite, pearlite, retained austenite, and upper bainite is determined by observing 5 fields of view.

[0047] When the metallographic structure of the steel plate according to this embodiment consists of martensite, the area ratio of cementite is 0%. When the metallographic structure of the steel plate according to this embodiment consists of martensite and lower bainite, the metallographic structure containing cementite is regarded as lower bainite. From the viewpoint of ensuring toughness, the area ratio of cementite is desirably 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 formation of upper bainite and pearlite is suppressed, and the area ratio of cementite decreases.

[0048] In the steel sheet according to this embodiment, the area ratio of cementite is measured by a scanning electron microscope (hereinafter referred to as "SEM"). When the thickness of the wear-resistant steel sheet is less than 16 mm, the area ratio of cementite is measured at the 1 / 2t part. When the thickness of the wear-resistant steel sheet is 16 mm or more, the area ratio of cementite is measured at the 1 / 4t part. The observation surface of the sample used for measuring the area ratio of cementite is the L-direction cross-section of the sample, and electrolytic etching is performed. The area ratio of cementite is measured by image analysis of five photos taken at 30,000 times magnification.

[0049] In the steel sheet according to this embodiment, from the viewpoint of ensuring toughness, the equivalent circle diameter of cementite is preferably 500 nm or less. The equivalent circle diameter of cementite is more preferably 300 nm or less. The equivalent circle diameter of cementite may be 50 nm or more, or 100 nm or more.

[0050] The equivalent circle diameter of cementite is calculated from the total area of cementite (total) obtained by image analysis of the SEM photos used for measuring the area ratio and the number of cementite. That is, the equivalent circle diameter is calculated from the average area of cementite obtained by dividing the total area of cementite by the number of cementite. When the area ratio of cementite is 0%, the equivalent circle diameter is 0 nm. When the quenching stop temperature is lowered, the formation of upper bainite and pearlite is suppressed, and the equivalent circle diameter of cementite becomes smaller. The cementite contained in upper bainite and pearlite has a large equivalent circle diameter and area ratio. If the area ratio of cementite is 0.5% or less and the 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%.

[0051] In the steel sheet according to this embodiment, among the crystal grains surrounded by high-angle grain boundaries of 15° or more in a region of 400 μm × 400 μm, the average grain size of the top 10 grain sizes (10 grain sizes in descending order of grain size) is preferably 40 μm or less. Hereinafter, the grain size surrounded by high-angle grain boundaries with a crystal orientation difference of 15° or more is referred to as "high-angle grain size". When the high-angle grain size increases, fracture is likely to occur, and when the high-angle grain size decreases, toughness improves. The high-angle grain size is evaluated as the average value of the top 10 (10 from the largest) among the crystal grains within a region of 400 μm × 400 μm. The high-angle grain size is more preferably 30 μm or less.

[0052] In this embodiment, the high-angle grain size is measured by the electron backscattered diffraction method (Electron Back Scattered Diffraction Pattern, hereinafter also referred to as "EBSD"). The measurement by EBSD is performed at a pitch of 0.4 μm in a field of view of 400 μm × 400 μm. The grain size distribution is displayed by commercially available analysis software (OIM-Analysis manufactured by TSL), and the average of the top 10 grain sizes among the crystal grains surrounded by high-angle grain boundaries of 15° or more is calculated. When the thickness of the base steel sheet is less than 16 mm, the measurement by EBSD is performed at the 1 / 2t part. When the thickness is 16 mm or more, the measurement by EBSD is performed at the 1 / 4t part. The sample used for the measurement by EBSD has an observation surface of 10 mm square and is cut out from a position 1 / 4 of the plate width (1 / 4 width) from the end in the width direction of the steel sheet. The observation surface is a cross-section in the L direction of the sample and is subjected to electrolytic polishing.

[0053] <Hardness> The hardness of the L-direction cross-section at a position 2 mm in the thickness direction from the surface of the steel sheet according to the present embodiment is 360 HV10 or more in terms of Vickers hardness in order to ensure wear resistance. The hardness is preferably 400 HV10 or more, and the higher the better, and the upper limit is not limited. The hardness may be 450 HV10 or less from the viewpoint of ensuring toughness. The hardness is measured at a position 2 mm in the thickness direction from the surface of the steel sheet in consideration of the effects of decarburization and surface oxidation scale. The Vickers hardness test is carried out in accordance with JIS Z 2244-1:2020, and the load is 10 kgf. The Vickers hardness is the average value of three points measured in the L-direction cross-section of the sample.

[0054] The central portion in the thickness direction of the steel sheet according to the present embodiment is synonymous with the 1 / 2t portion, and hereinafter, the hardness at the central portion in the thickness direction of the steel sheet is referred to as the 1 / 2t portion hardness. The 1 / 2t portion hardness is preferably 360 HV10 or more, more preferably 380 HV10 or more in terms of Vickers hardness from the viewpoint of preventing deterioration of wear resistance after the surface layer portion to the 1 / 4t portion has been worn by use. The 1 / 2t portion hardness is more preferably 400 HV10 or more, and the higher the better, and the upper limit is not limited. The 1 / 2t portion hardness may be 450 HV10 or less from the viewpoint of ensuring toughness. The Vickers hardness test is carried out in accordance with JIS Z 2244-1:2020, and the load is 10 kgf. The Vickers hardness is the average value of three points measured in the L-direction cross-section of the sample. The 1 / 2t portion hardness is ensured by increasing the content of the alloy that enhances hardenability and increasing the cooling rate of quenching.

[0055] <Thickness of surface oxidation scale> It is important that the thickness of the surface oxide scale of the steel sheet according to this embodiment is 5 μm or less. According to the new findings of the present inventors, when the thickness of the surface oxide scale of the steel sheet exceeds 5 μm, cracks and the like are likely to occur on the surface of the steel sheet during bending of the steel sheet. The thickness of the surface oxide scale of the steel sheet may be 4 μm or less. The lower limit of the thickness of the surface oxide scale of the steel sheet is not particularly limited, and may be 0 μm or more, more than 0 μm, 1 μm or more, or 2 μm or more. The thickness of the surface oxide scale of the steel sheet can be controlled by the Mn content of the above-described steel sheet and the finish rolling conditions described below.

[0056] In this embodiment, the "thickness of the surface oxide scale" of the steel sheet can be measured using a scanning electron microscope (SEM). More specifically, the "thickness of the surface oxide scale" is obtained by collecting a sample with a cross-section parallel to the thickness direction of the steel sheet as the observation surface, polishing the observation surface, observing the observation surface at a magnification of 1000 times, and obtaining the distance from the interface between the scale and the base metal to the scale surface at 5 or more points. The measurement is performed in any 3 fields of view, and is determined as the average value of the obtained distances.

[0057] <Thickness> The thickness of the steel sheet according to this embodiment is not particularly limited. The steel sheet according to this embodiment may be, for example, a thick plate. Specifically, the steel sheet according to this 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 also be 45 mm or less, or 40 mm or less.

[0058] 2. Manufacturing method of steel sheet Next, the manufacturing method of the steel sheet according to this embodiment will be described.

[0059] The manufacturing method of the steel sheet according to this embodiment is, for example, obtaining a slab having the above chemical components, performing rough rolling on the slab to obtain a rough rolled material, The finish rolling is performed on the rough rolled material at a temperature of Ar3 - 30 °C or higher and Ar3 or lower, with the reduction ratio of each rolling pass being 5% or lower and the number of passes of the rolling pass being 3 or more to obtain a finish rolled material, and cooling the finish rolled material to 200 °C or lower at a cooling rate of 10 °C / sec or higher.

[0060] [Obtaining a slab having a predetermined chemical composition] The method for manufacturing the slab used in the production of the steel sheet according to this embodiment is not limited and is manufactured by a known method. For example, the slab is manufactured by a known method such as a continuous casting method or an ingot - blooming method using molten steel melted to have the above chemical composition by a normal refining process such as a converter or an electric furnace. The slab is preferably cooled after casting, reheated to a temperature above the Ac3 transformation point, and hot - rolled under predetermined conditions. If the slab after continuous casting is charged into a heating furnace by hot - charge without being cooled to 400 °C or lower, the coarse austenite generated during casting may remain in the slab after heating. In order to promote the refinement of the structure of the steel sheet, it is preferable that the slab after continuous casting is once cooled to 400 °C or lower.

[0061] [Heating of the slab before rough rolling] The heating of the slab before rough rolling is preferably performed at a temperature at which the surface of the slab becomes Ac3 transformation point or higher. The Ac3 transformation point is the temperature at which the transformation from ferrite to austenite starts during heating. The heating temperature of the slab is preferably 1100 °C or higher in order to break the columnar structure and homogenize the metal structure in hot rolling including the subsequent rough rolling. The heating temperature is more preferably 1150 °C or higher, and still more preferably 1200 °C or higher. On the other hand, the heating temperature is preferably 1300 °C or lower in order to suppress the coarsening of crystal grains.

[0062] [Rough rolling] The rough rolling is preferably carried out at a temperature at which the surface temperature of the material to be rolled is above the Ar3 transformation point. The Ar3 transformation point is the temperature at which the transformation from austenite to ferrite starts during cooling. From the viewpoint of destroying the columnar structure of the steel material, homogenizing the metal structure, and further promoting the recrystallization of the metal structure, the reduction ratio in one rolling is preferably 10% or more. The reduction ratio per pass is preferably 15% or more, more preferably 20% or more. The number of rough rolling passes is preferably 3 or more, and the total reduction ratio is preferably 50% or more, more preferably 60% or more. The reduction ratio per pass in rough rolling is obtained by the following formula (1) from the thickness of the material to be rolled before rolling and the thickness of the material to be rolled after rolling. Also, the total reduction ratio in rough rolling is obtained by the following formula (2). Reduction ratio per pass (%) = 100 × {(thickness of the material to be rolled before rolling) - (thickness of the material to be rolled after rolling)} / (thickness of the material to be rolled before rolling) ···(1) Total reduction ratio (%) = 100 × {(thickness of the slab before rough rolling) - (thickness of the rough-rolled material after rough rolling)} / (thickness of the slab before rough rolling) ···(2)

[0063] [Finish rolling] The finish rolling is carried out at a temperature (surface temperature) immediately below the Ar3 transformation point on the surface of the material to be rolled. Specifically, the surface temperature of the rough-rolled material in finish rolling is Ar3 - 30°C or more and Ar3 or less. If the surface temperature of the material to be rolled in finish rolling is too low, cracks will occur in the surface layer of the material to be rolled during finish rolling, making it difficult to manufacture the steel plate. On the other hand, if the surface temperature of the material to be rolled in finish rolling is too high, excessive surface oxidation scale will be generated, and a steel plate with excellent bend formability cannot be manufactured. The surface temperature of the rough-rolled material before finish rolling may be Ar3 - 20°C or more and Ar3 - 1°C or less, or may be Ar3 - 10°C or more and Ar3 - 2°C or less. In the present application, "Ar3" is the transformation start temperature during cooling calculated as follows and is calculated using the chemical composition of the steel.

[0064] Ar3 (°C) = 910 - 203 × C 1 / 2 + 44.7 × Si - 68.1 × Mn - 12.8 × Cr In the above formula, C, Si, Mn, and Cr represent the contents expressed in mass%.

[0065] The reduction ratio of each rolling pass in finish rolling (the reduction ratio in one rolling pass) is 5% or less. The reduction ratio per pass is preferably 4% or less, more preferably 3% or less. The lower limit of the reduction ratio per pass is not particularly limited and is more than 0%, and may be 1% or more or 2% or more. If the reduction ratio in one rolling in finish rolling is too high, cracks will occur in the material to be rolled during finish rolling, making it difficult to manufacture the steel sheet. Also, the number of rolling passes in finish rolling is 3 or more. When the number of passes is 2 or less, excessive surface oxidation scale is generated, and a steel sheet excellent in bend formability cannot be manufactured. The total reduction ratio in finish rolling is not particularly limited, but is, for example, 30% or more and 95% or less. The reduction ratio per pass in finish rolling is obtained by the above formula (1) from the thickness of the material to be rolled after rolling and the thickness of the material to be rolled before rolling. Also, the total reduction ratio in finish rolling is obtained by the following formula (3). Total reduction ratio (%) = 100 × {(thickness of the rough rolled material before finish rolling) - (thickness of the finish rolled material after finish rolling)} / (thickness of the rough rolled material before finish rolling) ··· (3)

[0066] [Cooling after finish rolling] The cooling rate of the finish-rolled material after finish rolling is 10°C / sec or more in the temperature range up to 200°C or lower from the viewpoint of promoting martensite transformation by rapid cooling and ensuring hardness. The cooling rate is preferably 15°C / sec or more, more preferably 20°C / sec or more. The upper limit of the cooling rate is not limited, but for example, it may be 100°C / sec or lower. The cooling rate is obtained by dividing the temperature difference between the surface temperature of the finish-rolled material immediately after finish rolling and the cooling stop temperature by the time until the surface temperature of the finish-rolled material becomes the cooling stop temperature of 200°C or lower after finish rolling. Preferably, the cooling rate of the finish-rolled material after finish rolling does not fall below 10°C / sec at any time until the surface temperature becomes 200°C or lower after finish rolling. The cooling rate after reaching the cooling stop temperature is not particularly limited.

[0067] As described above, by performing each step in the order of slab production, slab preheating, rough rolling, finish rolling, and cooling, the steel sheet according to the present embodiment can be manufactured.

Examples

[0068] Examples according to the present invention are shown below. However, the present invention is not limited to this one conditional example. The present invention can adopt various conditions as long as it does not deviate from the gist and can achieve its purpose.

[0069] 1. Manufacture of steel sheet 1.1 Manufacture of slab ~ rough rolling Steel having various chemical components shown in Table 1 below was melted to manufacture slabs. These slabs were inserted into a furnace heated to 1250°C, given a homogenization treatment, and then taken out into the atmosphere. Each slab was subjected to rough rolling at 1000°C, including 5 rolling passes with an individual reduction rate of 21% and a total reduction rate of 70% to obtain rough-rolled materials. In this example, by changing the thickness of the slab before rough rolling, rough-rolled materials with different thicknesses were obtained, and finally, steel sheets with the same plate thickness were obtained by performing the finish rolling described below.

[0070]

Table 1

[0071] 1.2 Finish rolling For each rough rolled material, finish rolling was performed at the temperatures (surface temperature of the material to be rolled) shown in Table 2 below, the reduction rate per pass (reduction rate of each rolling pass), and the number of rolling passes to obtain a steel plate with a thickness of 30 mm. The chemical composition of each steel plate was the same as that shown in Table 1.

[0072] 2. Evaluation of steel plates 2.1 Hardness measurement For each steel plate, the hardness at the L-direction cross-section at a position 2 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.

[0073] 2.2 Thickness of surface oxidation scale For each steel plate, the thickness of the surface oxidation scale was measured. The method for measuring the thickness was as described above. The results are shown in Table 2 below.

[0074] 2.3 Bend formability In accordance with JIS Z 2248:2022, a test piece with a width of 150 mm × a length of 450 mm × the total thickness was taken from the steel plate so that the longitudinal direction was parallel to the rolling direction, bent at a bending radius of 2.0t up to an angle of 90°, and the presence or absence of cracks was examined. Those without crack generation were considered qualified, and those with crack generation were considered unqualified. The results are shown in Table 2 below.

[0075]

Table 2

[0076] From the results shown in Tables 1 and 2, the following can be understood.

[0077] For No.1~4, 27~39, steel plates with sufficient hardness and bend formability were obtained.

[0078] For No.5, since the cooling rate after finish rolling was low, the metallographic structure of the steel plate was not properly controlled, resulting in insufficient hardness of the steel plate.

[0079] For No.6, since the cooling stop temperature after finish rolling was too high, the metallographic structure of the steel plate was not properly controlled, resulting in insufficient hardness of the steel plate.

[0080] For No.7, since the finish rolling temperature was too high, the surface oxidation scale of the steel plate became thick, deteriorating the bendability of the steel plate.

[0081] For No.8, since the finish rolling temperature was too low, cracks occurred in the material being rolled during finish rolling, and the steel plate could not be properly manufactured.

[0082] For No.9, since the reduction ratio of each rolling pass in finish rolling was too high, cracks occurred in the material being rolled during finish rolling, and the steel plate could not be properly manufactured.

[0083] For No.10, since the C content was insufficient, the hardness of the steel plate was insufficient.

[0084] For No.11, since the C content was excessive, the steel plate was prone to cracking, deteriorating the bendability of the steel plate.

[0085] For No.12, since Si was not contained, there was insufficient hardening, resulting in insufficient hardness of the steel plate.

[0086] For No.13, since the Si content was excessive, scale was likely to occur at grain boundaries and on the surface during hot rolling, etc., the surface oxidation scale of the steel plate became thick, deteriorating the bendability of the steel plate.

[0087] For No.14, since the Mn content was insufficient, there was insufficient hardening, resulting in insufficient hardness of the steel plate.

[0088] No.15 had an excessive Mn content, which made it easy for scale to form on the surface during hot rolling or the like, resulting in a thick surface oxidation scale on the steel sheet and deteriorating the bending workability of the steel sheet.

[0089] No.16 had an excessive P content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0090] No.17 had an excessive S content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0091] No.18 had an insufficient Cr content, resulting in insufficient hardening and insufficient hardness of the steel sheet.

[0092] No.19 had an excessive Cr content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0093] No.20 had an insufficient Al content, resulting in insufficient hardening and insufficient hardness of the steel sheet.

[0094] No.21 had an excessive Al content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0095] No.22 had an excessive N content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0096] No.23 had an insufficient B content, resulting in insufficient hardening and insufficient hardness of the steel sheet.

[0097] No.24 had an excessive B content, which made the steel sheet prone to cracking and deteriorated the bending workability of the steel sheet.

[0098] No.25 only maintained the temperature during finish rolling and omitted rolling, which made the surface oxidation scale of the steel sheet thick and deteriorated the bending workability of the steel sheet.

[0099] No. 26 had insufficient number of rolling passes in finish rolling, resulting in thick surface oxide scale on the steel plate and deteriorated bendability of the steel plate.

Claims

1. by mass%, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, and the balance consists of Fe and impurities, wherein the hardness in the L-direction cross-section at a position 2 mm from the surface in the thickness direction is 360 HV10 or more, and the thickness of the surface oxide scale is 5 μm or less, steel sheet.

2. by mass%, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.10% or more and 1.00% or less, P: 0.020% or less, S: 0.010% or less, Cr: more than 2.00% and 7.50% or less, Al: 0.010% or more and 0.100% or less, N: 0.0020% or more and 0.0100% or less, and B: 0.0003% or more and 0.0030% or less, and contains one or more selected from the group consisting of the following a group, b group, c group and d group, where the balance consists of Fe and impurities, wherein the hardness in the L-direction cross-section at a position 2 mm from the surface in the thickness direction is 360 HV10 or more, and the thickness of the surface oxide scale is 5 μm or less, steel sheet. [a group] V: 0.001% or more and 0.200% or less, Mo: 0.001% or more and 0.200% or less, W: 0.001% or more and 0.200% or less, Ti: 0.001% or more and 0.100% or less, and Nb: 0.001% or more and 0.100% or less, one or more selected from the group consisting of [b group] Ni: 0.001% or more and 1.000% or less, and Cu: 0.001% or more and 1.000% or less, one or two selected from the group consisting of [c group] Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.1000% or less, one or more selected from the group consisting of [d group] Sn: 0.001% or more and 0.100% or less, and Sb: 0.001% or more and 0.100% or less, one or two selected from the group consisting of

3. containing the a group, the steel sheet according to Claim 2.

4. containing the b group, the steel sheet according to Claim 2.

5. containing the c group, the steel sheet according to Claim 2.

6. containing the d group, the steel sheet according to Claim 2.

7. containing the d group, The steel sheet according to claim 2.

7. having a thickness of 6 mm or more and 50 mm or less, The steel sheet according to any one of claims 1 to 6.

8. obtaining a slab having the chemical composition according to any one of claims 1 to 6, performing rough rolling on the slab to obtain a rough-rolled material, performing finish rolling on the rough-rolled material at a temperature of Ar3 - 30°C or higher and Ar3 or lower, with the reduction ratio of each rolling pass being 5% or less and the number of rolling passes being 3 or more to obtain a finish-rolled material, and cooling the finish-rolled material to 200°C or lower at a cooling rate of 10°C / sec or higher, A method for manufacturing a steel sheet.

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