Steel sheet and manufacturing method thereof

Abstract

The present invention relates to a steel sheet and a manufacturing method thereof and, more specifically, to a steel sheet having excellent abrasion resistance and a manufacturing method thereof.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to a steel plate having excellent wear resistance and a method for manufacturing the same.

[0002] In the case of construction machinery and industrial machinery used in many industrial fields such as construction, civil engineering, mining, and cement industries, the application of materials with wear-resistant properties is necessary as wear due to friction occurs severely during operation.

[0003] In general, wear resistance and hardness are correlated in thick-walled steel plates. Therefore, increased hardness is necessary for steel plates subject to wear. To ensure more stable wear resistance, a uniform hardness is required from the surface to the inner thickness (near t / 2, where t is the plate thickness). In other words, the surface and inner thickness of the steel plate must be of equal hardness.

[0004] Accordingly, in order to secure not only excellent wear resistance by securing surface hardness but also to secure the life of the material by securing center hardness, a method is required that can secure a certain level of hardness at both the surface and center.

[0005] (Patent Document 1) Korean Patent Publication No. 2018-0073368

[0006] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same are provided.

[0007] According to one embodiment of the present invention, an object is to provide a steel plate having excellent wear resistance and a method for manufacturing the same.

[0008] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0009] According to one embodiment of the present invention, it contains, in wt%, carbon (C): 0.100 to 0.190%, silicon (Si): 0.50% or less, manganese (Mn): 0.50 to 3.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.05% or less, chromium (Cr): 1.50% or less, molybdenum (Mo): 1.00% or less, titanium (Ti): 0.020% or less, boron (B): 50 ppm or less, the remainder iron (Fe) and other unavoidable impurities.

[0010] The R1 value defined in the following relational expression 1 is 2.00 or greater,

[0011] The R2 value defined in the following relational expression 2 is 0.70 or greater,

[0012] The microstructure can provide a steel sheet containing martensite of 15% or more in area% at a point 1 / 2 from the surface toward the center of the thickness.

[0013] [Relationship 1]

[0014] R1 = ([Ti] / [B])×[Cr]×[Mo]

[0015] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight percent of each element.)

[0016] [Relationship 2]

[0017] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5

[0018] (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight percent of each element.)

[0019] The above steel plate may further contain one or more of nickel (Ni): 0.50% or less, copper (Cu): 0.50% or less, niobium (Nb): 0.05% or less, and calcium (Ca): 2 to 100 ppm.

[0020] The microstructure of the above steel plate may include martensite at an area% of 90% or more at a point 2 mm from the surface toward the center of the thickness.

[0021] The microstructure of the above steel plate may include bainite as a residual structure at a point 1 / 2 from the surface toward the center of the thickness, and may include bainite as a residual structure at a point 2 mm from the surface toward the center of the thickness.

[0022] The above steel plate may have a surface hardness of 360 to 440 HBW at a point 2 mm from the surface toward the center of the thickness, and a center hardness of 335 HBW or more at a point 1 / 2 from the surface toward the center of the thickness.

[0023] The above steel plate may have a thickness of 80 mm or more.

[0024] A step of heating a steel slab containing, in wt%, carbon (C): 0.100 to 0.190%, silicon (Si): 0.50% or less, manganese (Mn): 0.50 to 3.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.05% or less, chromium (Cr): 1.50% or less, molybdenum (Mo): 1.00% or less, titanium (Ti): 0.020% or less, boron (B): 50 ppm or less, the remainder iron (Fe) and other unavoidable impurities, and having an R1 value defined in the following equation 1 of 2.00 or more and an R2 value defined in the following equation 2 of 0.70 or more;

[0025] A step of rolling the above heated steel slab;

[0026] A step of finishing hot rolling the above-mentioned rolled steel sheet;

[0027] A step of cooling the above-mentioned hot-rolled steel sheet to room temperature;

[0028] A step of reheating the above-mentioned air-cooled steel plate at a temperature range of 850 to 930°C; and

[0029] A method for manufacturing a steel plate can be provided, including a step of cooling the reheated steel plate to 200°C or lower at a cooling rate of 0.50°C / s or higher.

[0030] [Relationship 1]

[0031] R1 = ([Ti] / [B])×[Cr]×[Mo]

[0032] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight percent of each element.)

[0033] [Relationship 2]

[0034] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5

[0035] (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight percent of each element.)

[0036] The above steel slab may further contain one or more of nickel (Ni): 0.50% or less, copper (Cu): 0.50% or less, niobium (Nb): 0.05% or less, and calcium (Ca): 2 to 100 ppm.

[0037] The above heating step is performed at a temperature range of 1050 to 1250°C,

[0038] The above rolling step is performed at a temperature range of 950 to 1050°C.

[0039] The above finishing hot rolling step is performed at a temperature range of 750 to 950°C.

[0040] The above reheating step can be performed with a reheating time of 1.3t+10 minutes to 1.3t+60 minutes (t: plate thickness, mm).

[0041] The above cooled steel plate may have a thickness of 80 mm or more.

[0042] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same can be provided.

[0043] According to one embodiment of the present invention, a steel plate having excellent wear resistance and a method for manufacturing the same can be provided.

[0044] According to one embodiment of the present invention, a steel plate having excellent surface and center hardness and being usable in construction machinery and industrial machinery used in many industrial fields such as construction, civil engineering, mining, and cement industries, and a method for manufacturing the same can be provided.

[0045] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0046] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0047] Hereinafter, the present invention will be described in detail.

[0048] Below, the steel composition of the present invention is described in detail.

[0049] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0050] According to one embodiment of the present invention, a steel sheet may include, in wt%, carbon (C): 0.100 to 0.190%, silicon (Si): 0.50% or less, manganese (Mn): 0.50 to 3.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.05% or less, chromium (Cr): 1.50% or less, molybdenum (Mo): 1.00% or less, titanium (Ti): 0.020% or less, and boron (B): 50 ppm or less.

[0051] Carbon (C): 0.100~0.190%

[0052] Carbon (C) is an element that is effective in increasing strength and hardness in steel with a martensitic structure and is effective in improving hardenability. In order to sufficiently secure the above-described effect, carbon (C) may be added in an amount of 0.100% or more. According to one embodiment of the present invention, it may be 0.110% or more. On the other hand, if the carbon (C) content exceeds 0.190%, not only the upper limit of the target surface hardness may be exceeded, but there may also be problems of deteriorating weldability and toughness in the future. Therefore, according to one embodiment of the present invention, the carbon (C) content may be 0.190% or less. According to one embodiment of the present invention, it may be 0.180% or less.

[0053] Silicon (Si): 0.50% or less

[0054] Silicon (Si) is an effective element for improving strength through deoxidation and solid solution strengthening. To achieve the above effects, silicon (Si) can be added in an amount of 0.50% or less. If the content exceeds 0.50%, excessive scale may be generated and grown on the surface during hot rolling, potentially remaining as surface defects in the final sheet material. Furthermore, weldability may deteriorate. Therefore, in the present invention, the silicon (Si) content may be controlled to 0.50% or less. In one embodiment of the present invention, it may be 0.45% or less. However, 0% is excluded in consideration of increased load and manufacturing costs during the steelmaking process.

[0055] Manganese (Mn): 0.50~3.00%

[0056] Manganese (Mn) is an element that suppresses ferrite formation and effectively increases hardenability by lowering the Ar3 temperature, thereby improving the strength and toughness of steel. According to one embodiment of the present invention, in order to secure the hardness of the ultra-thick material, the manganese (Mn) content may be 0.50% or more. In one embodiment of the present invention, it may be 0.60% or more. On the other hand, if the content exceeds 3.00%, there is a risk of MnS segregation at the center of the rolled plate, which may lead to plate breakage in the worst case after heat treatment. In one embodiment of the present invention, it may be 2.80% or less.

[0057] Phosphorus (P): 0.050% or less

[0058] Phosphorus (P) is an element that is inevitably contained in steel, but it also degrades the steel's toughness. Therefore, the phosphorus (P) content can be controlled to 0.050% or less, as much as possible. However, considering the level of inevitability contained in steel, 0% is excluded.

[0059] Sulfur (S): 0.020% or less

[0060] Sulfur (S) is an element that forms MnS inclusions in steel, thereby reducing the toughness of the steel. Therefore, the sulfur (S) content can be controlled to 0.020% or less, as much as possible. However, considering the level that is unavoidably contained in steel, 0% is excluded.

[0061] Aluminum (Al): 0.05% or less

[0062] Aluminum (Al) is an effective deoxidizer for steel, reducing the oxygen content in molten steel. However, if the aluminum (Al) content exceeds 0.05%, the cleanliness of the steel may be compromised. According to one embodiment of the present invention, the aluminum (Al) content may be 0.04% or less. However, 0% is excluded due to factors such as increased load and manufacturing costs during the steelmaking process.

[0063] Chromium (Cr): 1.50% or less

[0064] Chromium (Cr) increases the hardenability, thereby increasing the strength of steel, and is also an element that is advantageous in securing hardness. For the above-mentioned effect, chromium (Cr) can be added in an amount of 1.50% or less. On the other hand, if the content exceeds 1.50%, there may be a problem that fracture due to thermal shock may occur during the process of reheating the slab cooled to room temperature after steelmaking and then charging it into a heating furnace for hot rolling. In addition, the carbon equivalent may be excessively increased, which may ultimately cause poor weldability. According to one embodiment of the present invention, the chromium (Cr) content may be 1.45% or less. However, 0% is excluded in consideration of the load and increased manufacturing cost during the steelmaking process.

[0065] Molybdenum (Mo): 1.00% or less

[0066] Molybdenum (Mo) increases the hardenability of steel, and is particularly effective in improving the hardness of thick-walled steel. To fully achieve the above-described effects, molybdenum (Mo) can be added. However, molybdenum (Mo) is an expensive element, and if its content exceeds 1.00%, not only does the manufacturing cost increase, but also weldability deteriorates. Therefore, in the present invention, the content of molybdenum (Mo) can be controlled to 1.00% or less. In one embodiment of the present invention, it can be 0.95% or less. Meanwhile, in one embodiment of the present invention, the lower limit of the content of molybdenum (Mo) can be 0.20% to achieve the above-described effects. In one embodiment of the present invention, the lower limit can be 0.25%. In one embodiment of the present invention, the lower limit can be 0.30%.

[0067] Titanium (Ti): 0.020% or less

[0068] Titanium (Ti) is an element that maximizes the effect of B, which is an effective element in improving the hardenability of steel. Specifically, the titanium (Ti) can combine with nitrogen to form TiN precipitates, thereby inhibiting the formation of BN, thereby increasing the solid solution B and maximizing the improvement in hardenability. However, if the content of the titanium (Ti) exceeds 0.020%, coarse TiN precipitates are formed, which causes a problem in that the toughness of the steel is inferior. Therefore, according to one embodiment of the present invention, when adding the titanium (Ti), it can be limited to 0.020% or less. According to one embodiment of the present invention, it can be 0.018% or less.

[0069] Boron (B): 50 ppm or less

[0070] Boron (B) is a highly effective element that can enhance the hardenability and strength of steel, even with small additions. However, excessive boron content can actually impair the steel's toughness and weldability, so its content should be controlled to 50 ppm or less.

[0071] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0072] A steel sheet according to one embodiment of the present invention may further include at least one of nickel (Ni): 0.50% or less, copper (Cu): 0.50% or less, niobium (Nb): 0.05% or less, and calcium (Ca): 2 to 100 ppm.

[0073] According to one embodiment of the present invention, in addition to the above-described alloy composition, elements advantageous for securing the target properties may be further included.

[0074] Nickel (Ni): 0.50% or less

[0075] Nickel (Ni) is generally an effective element for improving both strength and toughness. However, if its content exceeds 0.50%, it can increase manufacturing costs. Therefore, according to one embodiment of the present invention, nickel (Ni) may be added at a level of 0.50% or less.

[0076] Copper (Cu): 0.50% or less

[0077] Copper (Cu) is an element that improves the hardenability of steel and enhances the strength and hardness of steel through solid solution strengthening. However, if the copper (Cu) content exceeds 0.50%, it causes surface defects and hinders hot workability. Therefore, according to one embodiment of the present invention, the content may be limited to 0.50% or less.

[0078] Niobium (Nb): 0.05% or less

[0079] Niobium (Nb) can be dissolved in austenite to increase the hardenability of the austenite. In addition, it is an effective element for increasing the strength of steel by forming carbonitrides such as Nb (C,N) and inhibiting the growth of austenite grains. However, if the content of niobium (Nb) exceeds 0.05%, not only is an ultra-high temperature operation of 1200℃ or higher required in the reheating step for hot rolling, but coarse grains and precipitates are formed as a result, which may have a problem of lowering the toughness of the steel. Therefore, according to one embodiment of the present invention, when adding niobium (Nb), the content may be limited to 0.05% or less.

[0080] Calcium (Ca): 2~100ppm

[0081] Calcium (Ca) has excellent bonding strength with S, and thus has the effect of suppressing the formation of MnS, which is segregated in the center of the thickness of steel, by generating CaS. In addition, the CaS generated by the addition of calcium (Ca) has the effect of increasing corrosion resistance in a humid external environment. According to one embodiment of the present invention, calcium (Ca) may be added in an amount of 2 ppm or more for the above-described effect. On the other hand, if the content exceeds 100 ppm, problems such as nozzle clogging may occur during steelmaking.

[0082] A steel plate according to one embodiment of the present invention may have an R1 value defined in the following relational expression 1 of 2.00 or more.

[0083] [Relationship 1]

[0084] R1 = ([Ti] / [B])×[Cr]×[Mo]

[0085] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight percent of each element.)

[0086] According to one embodiment of the present invention, the target center hardness can be secured by satisfying the condition that the R1 value defined in relational expression 1 is 2.00 or higher. According to one embodiment of the present invention, by controlling the relationship between the contents of Ti, B, Cr, and Mo through relational expression 1, the hardenability of the steel can be increased, thereby securing the hardness proposed in the present invention.

[0087] According to one embodiment of the present invention, the center hardness may mean the hardness at a point halfway from the surface of the steel plate in the direction of the center of the thickness.

[0088] According to one embodiment of the present invention, the R1 value defined in the above relational expression 1 may be 2.10 or greater. Meanwhile, the upper limit of the R1 value of relational expression 1 is not particularly limited, but may be 7.00 or less. According to one embodiment of the present invention, it may be 6.00 or less.

[0089] According to one embodiment of the present invention, the R2 value defined in the following relational expression 2 may be 0.70 or more.

[0090] [Relationship 2]

[0091] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5

[0092] (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight percent of each element.)

[0093] According to one embodiment of the present invention, by satisfying the condition that the R2 value defined in relational expression 2 is 0.70 or higher, the target center hardness and surface hardness can be secured simultaneously. According to one embodiment of the present invention, by controlling the content relationship of C, Mn, Ni, Cu, Cr, and Mo through relational expression 2, the strength and hardness of the steel can be increased, thereby securing the hardness proposed in the present invention.

[0094] According to one embodiment of the present invention, the center hardness may mean the hardness at a point 1 / 2 from the surface of the steel plate in the direction of the center of the thickness, and the surface hardness may mean the hardness at a point 2 mm from the surface of the steel plate.

[0095] According to one embodiment of the present invention, the R2 value defined in the above relational expression 2 may be 0.71 or greater. Meanwhile, the upper limit of the R1 value of relational expression 1 is not particularly limited, but may be 0.90 or less. According to one embodiment of the present invention, it may be 0.85 or less.

[0096] Below, the steel microstructure of the present invention is described in detail.

[0097] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0098] The microstructure of the steel sheet according to one embodiment of the present invention may include martensite and residual bainite in an area% of 15% or more at a point 1 / 2 from the surface toward the center of the thickness, and may include martensite and residual bainite in an area% of 90% or more at a point 2 mm from the surface toward the center of the thickness.

[0099] In the present invention, the martensite is distinguished from tempered martensite developed by performing a subsequent tempering heat treatment after quenching.

[0100] According to one embodiment of the present invention, if the martensite content is less than 90% at a point 2 mm from the surface of the steel plate toward the center of the thickness, it may be difficult to secure the target level of hardness. According to one embodiment of the present invention, the martensite content may be 100%.

[0101] Meanwhile, at a point halfway from the surface of the steel sheet toward the center of the thickness, it may be difficult to include more than 90% of martensite because the cooling rate is physically slow compared to the surface of the steel sheet. In order to secure the hardness level targeted by the present invention, the martensite content may be 15% or more. If the martensite content is less than 15%, it may be difficult to secure the hardness level targeted by the present invention. According to one embodiment of the present invention, the martensite content may be 17% or more. According to one embodiment of the present invention, the upper limit of the martensite fraction at a point halfway from the surface toward the center of the thickness is not particularly limited, but may be 90% or less.

[0102] The surface and core microstructures of steel plates may contain bainite as a residual structure in addition to martensite. By including bainite as a residual structure, both the surface and core hardness can be maintained to a certain level or higher.

[0103] According to one embodiment of the present invention, a steel plate may have a surface hardness of 360 to 440 HBW at a point 2 mm from the surface toward the center of the thickness, a center hardness of 335 HBW or more at a point 1 / 2 from the surface toward the center of the thickness, and a thickness of 80 mm or more.

[0104] According to one embodiment of the present invention, the steel plate may have a thickness of 90 mm or more. According to one embodiment of the present invention, the steel plate may have a thickness of 100 mm or more. According to one embodiment of the present invention, the upper limit of the thickness of the steel plate is not particularly limited, but may be 140 mm or less. According to one embodiment of the present invention, the steel plate may have a thickness of 130 mm or less. According to one embodiment of the present invention, the center hardness may be 440 HBW or less.

[0105] Below, the steel manufacturing method of the present invention is described in detail.

[0106] A steel plate according to one embodiment of the present invention can be manufactured by heating, rough rolling, finishing hot rolling, air cooling, reheating, and cooling the above-described alloy composition.

[0107] [heating]

[0108] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be heated in a temperature range of 1050 to 1250°C.

[0109] If the above slab heating temperature is below 1050℃, not only will microalloys such as Nb not be sufficiently dissolved, but the slab surface temperature will also be relatively low, which may increase the load applied to the rolling mill during hot rolling. On the other hand, if the temperature exceeds 1250℃, there is a risk that the austenite grains will coarsen, forming an uneven structure. In addition, there is a problem that scales may be excessively generated and grown, which may not be smoothly exfoliated during rolling and may ultimately remain as surface defects.

[0110] [Rough rolling]

[0111] The above heated steel slab can be rolled at a temperature range of 950 to 1050°C.

[0112] The above heated steel slab can be subjected to rough rolling to obtain a rough rolled bar.

[0113] During the above rough rolling, if the temperature is below 950℃, the rolling load increases, resulting in a relatively weak compression, which may prevent sufficient deformation from reaching the center of the slab thickness, and thus may lead to defects such as voids not being eliminated. On the other hand, during the rough rolling, if the temperature exceeds 1050℃, recrystallization may occur simultaneously with rolling, followed by grain growth, which may lead to excessive coarseness of the initial austenite grains.

[0114] [Finishing hot rolling]

[0115] The above-mentioned pre-rolled steel plate can be finished by hot rolling at a temperature range of 750 to 950°C.

[0116] A hot-rolled steel sheet can be obtained by final hot rolling the above-mentioned rough-rolled steel sheet.

[0117] During the above finishing hot rolling, if the temperature is below 750℃, dual phase rolling may occur, which may lead to the formation of ferrite in the microstructure. On the other hand, if the temperature exceeds 950℃, the grain size of the final structure may become coarse, which may lead to problems such as poor toughness.

[0118] [Air cooling]

[0119] The above hot-rolled steel sheet can be cooled to room temperature in the air.

[0120] In the present invention, there is no particular limitation on the air cooling conditions after finishing hot rolling, but they may be typical conditions applied in the same technical field.

[0121] [Reheat]

[0122] The above air-cooled steel plate can be reheated in a temperature range of 850 to 930°C for a reheating time of 1.3t+10 minutes to 1.3t+60 minutes (t: plate thickness, mm).

[0123] According to one embodiment of the present invention, the reheating can reversely transform a hot-rolled steel sheet composed of ferrite and pearlite into an austenite single phase.

[0124] If the reheating temperature is below 850°C, austenitization may not be sufficient, resulting in the presence of coarse soft ferrite, which may lower the hardness of the final product. On the other hand, if the reheating temperature exceeds 930°C, the austenite grains become coarser, which has the effect of improving hardenability, but there is a concern that the low-temperature toughness of the steel sheet may be deteriorated.

[0125] During the above reheating, if the reheating time is less than 1.3t+10 minutes (t: plate thickness, mm), austenitization may not occur sufficiently, which may cause difficulties in obtaining a sufficient martensite structure due to the subsequent cooling. On the other hand, during the above reheating, if the reheating time exceeds 1.3t+60 minutes (t: plate thickness, mm), the austenite grains become coarser, which has the effect of improving hardenability, but may cause a problem of inferior toughness.

[0126] [cooling]

[0127] The above reheated steel plate can be cooled to 200°C or less at a cooling rate of 0.50°C / s or more.

[0128] According to one embodiment of the present invention, during the cooling, the cooling end temperature may be based on the temperature at a point halfway from the surface of the steel plate toward the center of the thickness.

[0129] During the above cooling, if the cooling rate is less than 0.50°C / s or the cooling end temperature exceeds 200°C, there is a concern that the bainite phase may be excessively formed after the final cooling. In the present invention, the upper limit of the cooling rate is not particularly limited, but a person skilled in the art can appropriately set it in consideration of the equipment limitations. Meanwhile, according to one embodiment of the present invention, during the above cooling, the cooling rate may be 0.70°C / s or more. According to one embodiment of the present invention, it may be 1.00°C / s or more. According to one embodiment of the present invention, the cooling may be performed through water cooling.

[0130] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0131] (Example)

[0132] After preparing a steel slab having the alloy composition shown in Table 1 below, a hot-rolled steel sheet was manufactured by performing steel slab heating-rough rolling-finish hot rolling-air cooling-reheating-cooling on the steel slab under the conditions shown in Table 2 below.

[0133] Steel alloy composition (weight %) Relationship 1 Relationship 2 CSiMnPSAlCrMoTiBNiCuNbCaR1R2A0.1520.271.510.0090.0020.031.020.650.0160.00210.290.010.020.00125.050.76B0.1560.311.850.0050.0020. 030.830.620.0170.00180.330.030.010.00154.860.78C0.1540.242.100.0 070.0010.030.810.530.0150.00200.150.020.010.00093.220.78D0.0580.2 41.120.0050.0010.050.780.47-0.00151.530.02-0.00100.000.60E0.2810 .251.430.0060.0020.021.500.410.0060.0019-0.01-0.00051.940.90F0.15 00.261.210.0080.0030.030.440.30-0.0021-0.140.01-0.000.51G0.1480. 301.420.0070.0020.040.820.210.0200.00100.210.030.020.00073.440.61

[0134] [Relationship 1]

[0135] R1 = ([Ti] / [B])×[Cr]×[Mo]

[0136] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight percent of each element.)

[0137] [Relationship 2]

[0138] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5

[0139] (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight percent of each element.)

[0140] Specimen numberSteel gradeHeating and rough rollingFinishingHot rolled materialHeating and coolingTemperature (℃)Temperature (℃)Temperature (℃)Temperature (℃)MaterialTime (min)Speed ​​(℃ / s)Temperature (℃)1A11629958809151571.92192A11269949318681601.53213A11439829249251940.77354B112310409359161531.722 65B11629979268881860.86236B11379789429002090.772197C115210129458901960.6 4188C11429769319121800.47209C11449979179111571.662910D113510219049001702 .012511D115310079259091761.242212D11469909418871970.791913E1160104392192 32000.8815114E113310209539111870.753315E11469979409151631.812216F1129101 78869141781.122617F114310069329261920.7621818F11529889289042000.682019G1 13910178789211482.851920G11419799469181661.6423121G11509869189201900.4320

[0141] Table 3 below shows the microstructure and physical properties of the manufactured steel plates after measurement.

[0142] At this time, the above microstructure was produced by cutting a specimen into an arbitrary size, producing a mirror surface, etching it using a nital etching solution, and then observing the surface point at 2 mm from the surface and the center point at 1 / 2 of the thickness center using an optical microscope and a scanning electron microscope.

[0143] And, the surface and center hardness were measured with a Brinell hardness tester (load 3000kgf, 10mm tungsten indenter). At this time, the surface hardness was recorded as the average value of the values ​​measured three times after milling the surface of the steel plate by 2mm, and the center hardness was recorded as the average value of the values ​​measured three times in the same way as the surface after precisely machine-cutting the 1 / 2 point in the direction of the steel plate thickness and making a mirror-finished surface.

[0144] Specimen number Steel grade Thickness (mm) Microstructure (area %) Physical property Classification Surface center Hardness (HBW) MBMB Surface center 1A9010004951417357 Invention example 12A1009914060421344 Invention example 23A1209821585409338 Invention example 34B959914555411352 Invention example 45B 11010002476416343 Invention Example 56B 12088121288347309 Comparative Example 17C 13010001981419352 Invention Example 68C 12010001485408308 Comparative Example 29C 1009914357400348 Invention Example 710D 9010005149349244 Comparison Example 311D1009913367355231Comparative Example 412D12010002377341219Comparative Example 513E12010004555537400Comparative Example 614E12010004258512397Comparative Example 715E10010004753525379Comparative Example 816F110100010 90406228Comparative Example 917F12089111288349220Comparative Example 1018F1309821486412211Comparative Example 1119G859911288424250Comparative Example 1220G10086141387341245Comparative Example 1321G12010001486418226Comparative Example 14

[0145] * M: Martensite, B: Bainite

[0146] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention could also be secured.

[0147] On the other hand, Comparative Example 1 is an example in which the cooling end temperature deviates from the conditions of the present invention. The cooling end temperature was excessively high, resulting in insufficient martensite formation on the steel plate surface, and consequently, the desired hardness was not achieved.

[0148] Comparative Example 2 is an example where the cooling speed was excessively low. This resulted in an insufficient martensite fraction at the center of the steel sheet. As a result, the desired core hardness was insufficient.

[0149] Comparative Examples 3 to 8 are examples that do not satisfy the alloy composition conditions of the present invention. Comparative Examples 3 to 5 are examples that had insufficient carbon content and did not satisfy Relationship 1 or 2. As a result, the surface hardness and center hardness were inferior. Comparative Examples 6 to 8 are examples that had carbon content exceeding the range of the present invention and did not satisfy Relationship 1. As a result, the center hardness was sufficient, but the surface hardness was not properly secured.

[0150] Comparative Examples 9 to 14 are examples that satisfy the alloy composition range proposed in the present invention, but do not satisfy Relationship 1 or 2. As a result, the surface and center hardness targeted by the present invention were not secured.

[0151] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in weight%, carbon (C): 0.100 to 0.190%, silicon (Si): 0.50% or less, manganese (Mn): 0.50 to 3.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.05% or less, chromium (Cr): 1.50% or less, molybdenum (Mo): 1.00% or less, titanium (Ti): 0.020% or less, boron (B): 50 ppm or less, the remainder iron (Fe) and other unavoidable impurities. The R1 value defined in the following relational expression 1 is 2.00 or greater, The R2 value defined in the following relational expression 2 is 0.70 or greater, A steel plate having a microstructure containing martensite of 15% or more in area% at a point halfway from the surface toward the center of the thickness. [Relationship 1] R1 = ([Ti] / [B])×[Cr]×[Mo] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight % of each element.) [Relationship 2] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5 (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight % of each element.) 2. In claim 1, The above steel plate is a steel plate further containing at least one of nickel (Ni): 0.50% or less, copper (Cu): 0.50% or less, niobium (Nb): 0.05% or less, and calcium (Ca): 2 to 100 ppm.

3. In claim 1, The microstructure of the above steel plate is a steel plate containing martensite of 90% or more in area% at a point of 2 mm from the surface toward the center of the thickness.

4. In claim 3, The microstructure of the above steel plate includes bainite as the residual structure at a point 1 / 2 from the surface toward the center of the thickness, and includes bainite as the residual structure at a point 2 mm from the surface toward the center of the thickness.

5. In claim 1, The above steel plate has a surface hardness of 360 to 440 HBW at a point 2 mm from the surface toward the center of the thickness, and a center hardness of 335 HBW or more at a point 1 / 2 from the surface toward the center of the thickness.

6. In claim 1, The above steel plate is a steel plate having a thickness of 80 mm or more.

7. A step of heating a steel slab containing, by weight%, carbon (C): 0.100 to 0.190%, silicon (Si): 0.50% or less, manganese (Mn): 0.50 to 3.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.05% or less, chromium (Cr): 1.50% or less, molybdenum (Mo): 1.00% or less, titanium (Ti): 0.020% or less, boron (B): 50 ppm or less, the remainder iron (Fe) and other unavoidable impurities, and having an R1 value defined in the following relational expression 1 of 2.00 or more and an R2 value defined in the following relational expression 2 of 0.70 or more; A step of rolling the above heated steel slab; A step of finishing hot rolling the above-mentioned rolled steel plate; A step of cooling the above-mentioned hot-rolled steel sheet to room temperature; A step of reheating the above-mentioned air-cooled steel plate at a temperature range of 850 to 930°C; and A method for manufacturing a steel plate, comprising: a step of cooling the reheated steel plate to 200°C or lower at a cooling rate of 0.50°C / s or higher. [Relationship 1] R1 = ([Ti] / [B])×[Cr]×[Mo] (In the formula, [Ti], [B], [Cr] and [Mo] are the weight % of each element.) [Relationship 2] R2 = [C]+[Mn] / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]) / 5 (In the formula, [C], [Mn], [Ni], [Cu], [Cr] and [Mo] are the weight % of each element.) 8. In claim 7, A method for manufacturing a steel plate, wherein the steel slab further contains at least one of nickel (Ni): 0.50% or less, copper (Cu): 0.50% or less, niobium (Nb): 0.05% or less, and calcium (Ca): 2 to 100 ppm.

9. In claim 7, The above heating step is performed at a temperature range of 1050 to 1250℃. The above rolling step is performed at a temperature range of 950 to 1050℃. The above finishing hot rolling step is performed at a temperature range of 750 to 950°C. A method for manufacturing a steel plate, wherein the above reheating step is performed with a reheating time of 1.3t+10 minutes to 1.3t+60 minutes (t: plate thickness, mm).

10. In claim 7, A method for manufacturing a steel plate, wherein the cooled steel plate has a thickness of 80 mm or more.