Steel sheet, member, and manufacturing method therefor

Abstract

The present invention relates to a steel sheet, a member, and a manufacturing method therefor and, more specifically, to a steel sheet having excellent strength and notch impact toughness after quenching and tempering heat treatment, a member using same, and a manufacturing method therefor.

Description

Steel plate, component and method of manufacturing the same

[0001] The present invention relates to a steel plate, a member, and a method for manufacturing the same, and more specifically, to a steel plate having excellent strength and notch impact toughness after quenching and tempering heat treatment, a member using the same, and a method for manufacturing the same.

[0002] Recently, as regulations on carbon dioxide emission reduction or fuel efficiency for the protection of the global environment have been strengthened, interest in lightweighting components of passenger cars and commercial vehicles continues. For example, there is an intensifying effort to apply high-strength steel to components such as side rail frames of commercial vehicles with high curb weight or freight weight in order to achieve lightweighting. In particular, the frame component can be manufactured using a cold forming method on hot-rolled steel sheets after sufficiently securing the desired high strength or high yield ratio by controlling the precipitation strengthening elements to an appropriate content. In this case, there is a disadvantage that dimensional changes including springback or forming cracks may occur during the cold forming process due to the high strength of the hot-rolled steel sheet itself. On the other hand, the frame component can be manufactured by using hot-rolled steel sheets with relatively lower strength by controlling the solid solution strengthening elements to an appropriate content, forming the shape using a hot or cold forming method, and then performing quenching and tempering heat treatments. However, while this method allows for the production of parts with high dimensional precision, it has the disadvantage of causing increased manufacturing costs due to subsequent heat treatment. It is believed that parts manufacturers with established heat treatment facilities prefer the latter method for manufacturing high-strength, lightweight frame parts.

[0003] Meanwhile, it is known that when the above-mentioned frame parts are manufactured using the quenching and tempering heat treatment method, low-temperature tempering heat treatment can be applied in a temperature range of 150 to 250°C to ensure high strength, and conversely, high-temperature tempering heat treatment can be applied in a temperature range of 350 to 650°C to ensure high toughness or resistance to external impact.

[0004] Patent Document 1 proposes forming a martensitic structure by processing a steel composed of 0.10~0.80% C, 0.8% or less Si, 0.8~3.0% Mn, and other alloying elements at a temperature of Ar3 or higher to achieve an area reduction of 30% or more, followed by water cooling, as an example of a representative composition. Additionally, it proposes a method for manufacturing martensitic steel with high toughness by applying only processing and cooling methods, such as heating to a temperature range of 300~500℃ at a heating rate of 100℃ / sec or more, maintaining it for 10~30 sec, and then rapidly cooling it. Specifically, it suggests that the method for performing the area processing can be selected from anvil compression processing or rolling processing, and that the processing can be performed at a temperature lower than the austenite recrystallization temperature (processing of 30% or more at a temperature where austenite recrystallization does not occur) and in a temperature range of 650~1000℃. In particular, it is characterized that only when tempering heat treatment is performed at a rapid heating rate to the target temperature, the former austenite grain boundaries (PAGS) constituting the tempered martensite structure have regions free of carbides, having periods of 5 μm or less and undulations with an amplitude of 200 nm or more, and the final steel has a tensile strength of 1397 to 1423 MPa and a ductile-brittle transition temperature (DBTT) of 0 to 30°C. Meanwhile, the above tempering heat treatment conditions, rapid heating or heating rate, and holding time are difficult to implement for manufacturers heat-treating conventional automotive parts, which means that carbides may remain at the former austenite grain boundaries in most heat-treated parts.

[0005] (Patent Document 1) Korean Registered Patent Publication No. 10-0522409

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

[0007] According to one embodiment of the present invention, the invention aims to provide a steel plate having excellent strength and notch impact toughness after quenching and tempering heat treatment, a member using the same, and a method for manufacturing the same.

[0008] The problems of the present invention are not limited to those described above. A person skilled in the art will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0009] A steel sheet according to one embodiment of the present invention comprises, in weight percent, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, the remainder being Fe and unavoidable impurities, and

[0010] The X value defined in the following Equation 1 is 0.4 to 0.6, and

[0011] The Y value defined in the following Equation 2 is 0.15 to 3.6, and

[0012] The Z value defined in the following Equation 3 is 50 or less, and

[0013] The R value defined in the following Equation 4 is 10 or less, and

[0014] The yield strength may be 350 MPa or higher.

[0015] [Relationship 1]

[0016] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15

[0017] (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.)

[0018] [Relationship 2]

[0019] Y = [Ti] / ([Al]+[Si])

[0020] (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.)

[0021] [Relationship 3]

[0022] Z = [Ni] / ([C]+[Mn]) X 100

[0023] (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.)

[0024] [Relationship 4]

[0025] R = ([Ti]+[B]) / [N]

[0026] (In the formula, [Ti], [B], and [N] are the weight percent of each element.)

[0027] The above steel plate may include one or more of the following (a) to (c).

[0028] (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less

[0029] (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less

[0030] (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less

[0031] The above steel plate may contain 25 to 80% pearlite, 20 to 75% ferrite, and unavoidable structure in terms of area %.

[0032] The surface layer of the above steel plate may include a decarburized layer of 150㎛ or less.

[0033] The above steel plate may have a tensile strength of 500 MPa or more and an elongation of 15% or more.

[0034] The surface layer hardness of the above steel plate may be 160 Hv or higher.

[0035] The above steel plate may have a hardness difference of 35 Hv or less between the surface layer and the 1 / 4 point in the thickness direction.

[0036] A method for manufacturing a steel plate, which is another embodiment of the present invention, is:

[0037] A step of heating a steel slab comprising, in weight%, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, and the remainder being Fe and unavoidable impurities, wherein the X value defined in the following Equation 1 is 0.4~0.6, the Y value defined in the following Equation 2 is 0.15~3.6, the Z value defined in the following Equation 3 is 50 or less, and the R value defined in the following Equation 4 is 10 or less;

[0038] A step of hot rolling the above heated steel slab at a finishing rolling temperature of Ar3 or higher; and

[0039] The above hot-rolled steel plate may include a step of cooling to a temperature range of 550 to 700°C and coiling.

[0040] [Relationship 1]

[0041] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15

[0042] (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.)

[0043] [Relationship 2]

[0044] Y = [Ti] / ([Al]+[Si])

[0045] (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.)

[0046] [Relationship 3]

[0047] Z = [Ni] / ([C]+[Mn]) X 100

[0048] (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.)

[0049] [Relationship 4]

[0050] R = ([Ti]+[B]) / [N]

[0051] (In the formula, [Ti], [B], and [N] are the weight percent of each element.)

[0052] The above steel slab may include one or more of the following (a) to (c).

[0053] (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less

[0054] (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less

[0055] (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less

[0056] The step of heating the above steel slab can be performed in a temperature range of 1150 to 1300°C.

[0057] A member of another embodiment of the present invention comprises, in weight percent, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, the remainder being Fe and unavoidable impurities, and

[0058] The X value defined in the following Equation 1 is 0.4 to 0.6, and

[0059] The Y value defined in the following Equation 2 is 0.15 to 3.6, and

[0060] The Z value defined in the following Equation 3 is 50 or less, and

[0061] The R value defined in the following Equation 4 is 10 or less, and

[0062] The microstructure may include more than 97% tempered martensite, less than 3% retained austenite and cementite, and unavoidable microstructures in terms of area %.

[0063] [Relationship 1]

[0064] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15

[0065] (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.)

[0066] [Relationship 2]

[0067] Y = [Ti] / ([Al]+[Si])

[0068] (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.)

[0069] [Relationship 3]

[0070] Z = [Ni] / ([C]+[Mn]) X 100

[0071] (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.)

[0072] [Relationship 4]

[0073] R = ([Ti]+[B]) / [N]

[0074] (In the formula, [Ti], [B], and [N] are the weight percent of each element.)

[0075] The above member may include one or more of the following (a) to (c).

[0076] (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less

[0077] (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less

[0078] (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less

[0079] The above member may have a prior austenite grain size of 20 μm or less.

[0080] The above member may have a yield strength of 1000 MPa or more, a tensile strength of 1100 MPa or more, and an elongation of 6% or more.

[0081] The above member may have a notch impact toughness of 40J or more at 20℃ and a notch impact toughness of 20J or more at -40℃.

[0082] A method for manufacturing another component of the present invention comprises the step of providing the aforementioned steel plate;

[0083] A step of manufacturing a member by cold forming the above steel plate;

[0084] A step of first heating the above cold-formed member to a temperature range of 850~930℃ and first maintaining it for 15~20 minutes;

[0085] A step of first cooling the above-mentioned first heated and first maintained member to a temperature range below the martensite transformation end temperature (Mf) at an average cooling rate of 30 to 100℃ / s; and

[0086] It may include a step of secondarily heating the cooled member to a temperature range of 350 to 450°C and secondarily maintaining it for 10 to 30 minutes.

[0087] The above-mentioned secondary heating and secondary maintenance may further include a secondary cooling step of furnace cooling or air cooling the member.

[0088] According to one embodiment of the present invention, a steel plate, a member, and a method for manufacturing the same can be provided.

[0089] According to one embodiment of the present invention, a steel plate having excellent strength and notch impact toughness after quenching and tempering heat treatment, a member using the same, and a method for manufacturing the same can be provided.

[0090] According to one embodiment of the present invention, a steel plate that can be used for side rail frame parts of medium or large commercial vehicles, a member using the same, and a method for manufacturing the same can be provided.

[0091] FIG. 1 is a micrograph of the microstructure of a hot-rolled steel sheet of specimen 6 according to one embodiment of the present invention.

[0092] FIG. 2 is an optical micrograph of a member after QT heat treatment of specimen 6 according to one embodiment of the present invention.

[0093] FIG. 3 is an EBSD micrograph of the member after QT heat treatment of specimen 6 according to one embodiment of the present invention.

[0094] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0095] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0096] The attached drawings are for understanding the invention only and are not intended to limit the invention. The shapes and sizes of elements in the drawings may be exaggerated for clearer explanation.

[0097] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0098] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0099] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0100] The present invention will be described in detail below.

[0101] The steel composition of the present invention will be described in detail below.

[0102] Unless otherwise specifically stated in the present invention, the % indicating the content of each element is based on weight.

[0103] A steel plate and a member according to one embodiment of the present invention may comprise, in weight percent, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, and N: 0.008% or less.

[0104] Carbon (C): 0.23~0.35%

[0105] Carbon (C) is an effective element for increasing the strength of steel and can increase strength after quenching and tempering heat treatments. If the carbon (C) content is less than 0.23%, it may be difficult to secure a sufficient yield strength above the desired level during tempering heat treatment. In one embodiment of the present invention, carbon (C) may be included in an amount of 0.28% or more. On the other hand, if the content exceeds 0.35%, while it is possible to secure strength after tempering heat treatment, it may be difficult to secure the target Charpy V-notch impact toughness. If the tempering heating temperature can be raised to 450°C or higher to increase the Charpy V-notch impact toughness, if other alloying elements other than carbon (C) are not sufficiently controlled, tempering softening occurs rapidly, which limits the securing of a yield strength of 1000 MPa or more. According to one embodiment of the present invention, carbon (C) may be included in an amount of 0.33% or less.

[0106] Manganese (Mn): 0.50~1.20%

[0107] Manganese (Mn) is an essential element for increasing the strength of steel and increases strength after quenching heat treatment. If the content is less than 0.50%, it may be difficult to sufficiently secure the desired level of yield strength after tempering heat treatment. In one embodiment of the present invention, it may contain 0.90% or more. On the other hand, if the manganese (Mn) content exceeds 1.20%, while securing strength is advantageous, it may be difficult to secure the desired Charpy notch impact toughness because excessive segregation of manganese (Mn) elements within the steel may result in a large amount of MnS inclusions remaining or increase sensitivity to temper brittleness during tempering heat treatment. In one embodiment of the present invention, manganese (Mn) may be contained in an amount of 1.10% or less.

[0108] Silicon (Si): 0.02~0.15%

[0109] Silicon (Si) is used as a deoxidizer for steel and is an effective element for increasing the strength of steel. To secure the aforementioned effects, silicon (Si) may be included in an amount of 0.02% or more. In one embodiment of the present invention, it may be included in an amount of 0.05% or more. On the other hand, if the content exceeds 0.15%, it may cause scale delamination not only on the surface scale of the hot-rolled steel sheet itself but also when cold-forming parts using the steel or heating cold parts in a furnace. Furthermore, silicon (Si) elements may react with oxygen in the steel to form silicate inclusions such as SiO2, and these inclusions may reduce the low-temperature impact toughness of the final part or act as a starting point for fatigue failure. In one embodiment of the present invention, the upper limit of the content may be limited to 0.10%.

[0110] Phosphorus (P): 0.02% or less

[0111] Phosphorus (P) is useful as it is effective in increasing the strength of steel, but it can segregate at austenite grain boundaries, causing processing defects in cold-formed parts such as laminations or causing temper brittleness during the tempering heat treatment process. Meanwhile, to control the phosphorus (P) content to a low level, quicklime (CaO) treatment can be performed for dephosphorization, which may lead to an increase in steelmaking process costs. Therefore, in the present invention, the phosphorus (P) content is maintained as low as possible, but the upper limit can be limited to 0.02%. In one embodiment of the present invention, the upper limit of the content can be limited to 0.01%.

[0112] Sulfur (S): 0.005% or less

[0113] Sulfur (S) can cause high-temperature cracks by segregating into MnS non-metallic inclusions within the steel or during continuous casting solidification. Therefore, in the present invention, the sulfur (S) content can be limited to 0.005% or less. In one embodiment of the present invention, it can be limited to 0.003% or less. However, 0% is excluded considering the extent to which it is inevitably contained during steel manufacturing.

[0114] Aluminum (Al): 0.05% or less

[0115] Aluminum (Al) is an element added as a deoxidizer. Aluminum (Al) can react with nitrogen in steel to form AlN deposits, which can cause cracks in the cast billet during the continuous casting process. Additionally, it can form aluminum (Al)-containing inclusions within the steel, which can reduce the low-temperature impact toughness of the final part or act as a starting point for fatigue failure. Therefore, the content of aluminum (Al) can be limited to 0.05% or less. In one embodiment of the present invention, the content can be limited to 0.03% or less. However, 0% is excluded to account for the extent that is inevitably contained during steel manufacturing.

[0116] Titanium (Ti): 0.015~0.07%

[0117] Titanium (Ti) is an element that forms crystals or precipitates within steel and can increase the strength of steel by inhibiting the growth of austenite grains. When the titanium (Ti) content is less than 0.015%, sufficient nitrogen and TiN precipitates cannot be formed within the steel, which can reduce the effective boron content. In one embodiment of the present invention, titanium (Ti) may be included in an amount of 0.02% or more. On the other hand, if the content exceeds 0.07%, it may cause nozzle clogging during the continuous casting process or allow Ti-containing oxides to remain in the cast billet or steel, thereby reducing the toughness of the steel. In one embodiment of the present invention, the upper limit of the content may be limited to 0.06%.

[0118] Boron (B): 0.0005~0.003%

[0119] Boron (B) is a beneficial element that increases the hardenability of steel. When added in an appropriate amount, it is effective in increasing hardenability by suppressing ferrite formation, but when added in an excessive amount, it can impair the toughness of the steel. When the boron (B) content is less than 0.0005%, there is a limit to increasing the hardenability of the steel, and even if tempering heat treatment is applied, the elongation of the steel after heat treatment is less than 8%, which may result in temper brittleness. In one embodiment of the present invention, the lower limit may be 0.001%. On the other hand, if the content exceeds 0.003%, it may impair room temperature and low temperature impact toughness. In one embodiment of the present invention, the upper limit may be limited to 0.0025%.

[0120] Nitrogen (N): 0.008% or less

[0121] Nitrogen (N) is an austenite-stabilizing and nitride-forming element. If its content exceeds 0.008%, it forms AlN nitrides, which can cause cracks at slab corners during continuous casting or act as a starting point for fatigue cracks in the final part, thereby degrading durability. Therefore, in the present invention, the content can be limited to 0.008% or less. In one embodiment of the present invention, the upper limit can be limited to 0.006%.

[0122] The steel of the present invention may contain the remainder of iron (Fe) and unavoidable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally incorporated during the conventional manufacturing process, they cannot be excluded. As such impurities are known to any person skilled in the field of ordinary steel manufacturing, all details thereof are not specifically mentioned in this specification.

[0123] A steel plate and a member according to one embodiment of the present invention may include one or more of the following (a) to (c).

[0124] (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less

[0125] (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less

[0126] (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less

[0127] According to one embodiment of the present invention, the above-described components are basically included, and the remainder includes Fe and unavoidable impurities, but additional elements may be added to the extent that the present invention is not impaired.

[0128] (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less

[0129] Chromium (Cr) and molybdenum (Mo) elements can increase the hardenability of steel and suppress tempering softening during high-temperature tempering, thereby contributing to increasing the strength of steel or parts after heat treatment. However, if the upper limits of each of the above are exceeded, it may be difficult to simultaneously secure the desired level of strength and notch impact toughness.

[0130] (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less

[0131] Niobium (Nb) is an element that increases strength through solid solution strengthening or precipitation strengthening. It can be added in a certain amount to refine the size of austenite grains in order to improve the impact toughness of steel with a martensitic structure. On the other hand, if its content exceeds 0.01%, it increases the hot-rolled strength of the steel, which may make it difficult to control the yield strength of the hot-rolled steel sheet to a level of 550 MPa or less. Therefore, in the present invention, its content can be added at 0.01% or less. In one embodiment of the present invention, it may be included at 0.008% or less.

[0132] Vanadium (V) is an element that increases strength by forming precipitates such as VC within steel. On the other hand, if its content exceeds 0.1%, it may be difficult to excessively increase the hot-rolled strength of the steel or to secure an elongation of 8% or more after high-temperature tempering heat treatment. Therefore, in the present invention, its content can be limited to 0.1% or less.

[0133] (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less

[0134] Nickel (Ni) and copper (Cu) are elements that simultaneously increase the hardenability and toughness of steel. Meanwhile, in the present invention, nickel (Ni) and copper (Cu) can be effective in reducing sensitivity to temper brittleness or securing Charpy notch impact toughness during tempering heat treatment. However, if the content of nickel (Ni) and copper (Cu) is excessive, tempering strength increases, but there is a limit to the improvement of impact toughness, and there may be a problem of increasing the manufacturing cost of the steel sheet. Therefore, the upper limit of the content can be limited to 0.50%. In one embodiment of the present invention, the upper limit can be limited to 0.03%.

[0135] According to one embodiment of the present invention, the steel plate may have an X value defined in the following relationship 1 of 0.4 to 0.6.

[0136] [Relationship 1]

[0137] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15

[0138] (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.)

[0139] Equation 1 of the present invention is a relationship representing the carbon equivalent of steel and can indicate the level of alloying elements required to secure an equivalent level of hardenability. If the value of X defined in Equation 1 is less than 0.4, there may be a problem in securing the target yield strength after quenching and tempering. On the other hand, if the value exceeds 0.6, there may be a problem in simultaneously securing the target strength and impact toughness. Preferably, it may be 0.40 to 0.60.

[0140] A steel plate according to one embodiment of the present invention may have a Y value defined in the following equation 2 of 0.15 to 3.6.

[0141] [Relationship 2]

[0142] Y = [Ti] / ([Al]+[Si])

[0143] (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.)

[0144] In the present invention, to secure the desired Charpy notch impact toughness, the ratio of hard oxides within the steel can be limited through Equation 2. When the titanium (Ti) content is high and the Y value defined in Equation 2 is less than 0.15, titanium (Ti)-containing oxides may remain within the steel, thereby reducing the impact toughness of the heat-treated steel and parts. However, since the titanium (Ti) content has an advantageous aspect in securing the yield strength of the steel by forming fine precipitates such as TiC or the hardenability of the steel, it can be added at the level suggested in the present invention. On the other hand, when the aluminum (Al) or silicon (Si) content is high and the Y value exceeds 3.6, it is difficult to suppress the reaction with oxygen in the molten steel during the casting process. Consequently, oxides such as Al2O3 and SiO2 may be formed in single or composite forms with a high fraction of coarse size or clustering, and if these oxides remain in the heat-treated steel or parts, the impact toughness may be reduced. Accordingly, as a method to improve the strength and impact toughness of heat-treated steel and parts, the value of Y in Equation 2 of the present invention may be limited. Preferably, it may be 0.15 to 3.60.

[0145] A steel plate according to one embodiment of the present invention may have a Z value of 50 or less as defined in the following relationship 3.

[0146] [Relationship 3]

[0147] Z = [Ni] / ([C]+[Mn]) X 100

[0148] (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.)

[0149] In the present invention, by controlling the above-mentioned Equation 3, it is possible to secure a yield strength of 1000 MPa or more or an elongation of 8% or more after quenching and tempering heat treatment, while also securing a Charpy notch impact toughness of 20 Joule or more at a temperature of -40℃. This is achieved by delaying the tempering softening of the quenched steel within a certain range, as the nickel (Ni) element suppresses the rapid growth of cementite (Fe3C) particles within the martensite during the relatively high-temperature tempering heat treatment process. Furthermore, it is believed that the relatively high Charpy impact toughness value is achieved because the movement of dislocations that remain and are not extinguished even at the tempering temperature becomes possible, allowing for the ductility or toughness of the tempered martensite to be possessed, thereby hindering the propagation of notch cracks. Meanwhile, when nickel (Ni) is included in minute amounts (impurity levels), it is necessary to control the carbon (C) and manganese (Mn) content as low as possible within the ranges presented in the present invention to ensure high strength and high toughness. Accordingly, in the present invention, the Z value defined in Equation 3 may be 50 or less. Preferably, it may be 50.0 or less. In one embodiment of the present invention, the Z value may be 48 or less. Meanwhile, in the present invention, the lower limit of the Z value may be 0. In one embodiment of the present invention, the lower limit may be 0.09.

[0150] A steel plate according to one embodiment of the present invention may have an R value defined in the following equation 4 that is 10 or less.

[0151] [Relationship 4]

[0152] R = ([Ti]+[B]) / [N]

[0153] (In the formula, [Ti], [B], and [N] are the weight percent of each element.)

[0154] If the R value defined in Equation 4 exceeds 10, it is necessary to control the nitrogen (N) content in the steel to less than 0.003%, which may lead to an increase in the manufacturing cost of the molten steel. Furthermore, even when the nitrogen (N) element is excessively present in the steel at a level of 0.008%, it is effective to secure an effective boron (B) content through an appropriate combination of titanium (Ti) elements. Meanwhile, satisfying the above Equation 4 of the present invention is effective in securing the desired strength and Charpy notch impact toughness. In addition, it is possible to have an elongation of 8% or more despite high-temperature tempering heat treatment, or to suppress the occurrence of temper brittleness. Preferably, it may be 10.0 or less. In one embodiment of the present invention, the R value may be 9 or less. Meanwhile, to secure more effective physical properties, the lower limit may be 6.3 or more.

[0155] The steel microstructure of the present invention will be described in detail below.

[0156] Unless otherwise specifically stated in the present invention, the % representing the fraction of the microstructure is based on area.

[0157] A steel sheet according to one embodiment of the present invention may contain, in area %, 25 to 80 percent pearlite, 20 to 75 percent ferrite, and unavoidable structure.

[0158] In the present invention, the microstructure fraction can be measured using an optical microscope, and the microstructure fraction can be measured at a point 1 / 4 of the way from the surface of the steel plate in the thickness direction.

[0159] In the present invention, ferrite may be included in an amount of 20% or more to secure a hot-rolled steel sheet having a relatively low yield strength. Meanwhile, if the area fraction exceeds 75%, it is easy to manufacture a steel sheet having a low yield strength that is advantageous for cold forming, but if the pearlite fraction is excessively low, there may be a problem in securing the yield strength or tensile strength after quenching and tempering.

[0160] If the pearlite is less than 25%, the size of the pearlite or cementite (Fe3C) is coarse, so if high-temperature heating of 870°C or higher is not performed during the final heat treatment, undissolved Fe3C may remain, which may limit the ability to secure high strength (corresponding to cases where the solubility of carbon (C) in the austenite matrix is ​​insufficient). In particular, in this case, it may be difficult to secure excellent Charpy notch impact toughness of 20J or more at -40°C. On the other hand, if the pearlite area fraction exceeds 80%, there may be a problem of the yield strength of the hot-rolled steel sheet increasing excessively.

[0161] According to one embodiment of the present invention, the surface layer of the steel plate may include a decarburized layer of 150 μm or less.

[0162] According to one embodiment of the present invention, the surface layer may refer to an area from the surface of the steel plate to a depth of 100 μm in the direction of the thickness center.

[0163] In the present invention, the fatigue life of a final component is increased by controlling the decarburization depth to minimize the decrease in surface strength of the final component, and according to one embodiment of the present invention, this can be limited to 150 μm or less. If this is exceeded, there may be a problem in that the fatigue life is reduced due to a decrease in strength caused by a decrease in surface hardness of the final component.

[0164] A steel plate according to one embodiment of the present invention may have a yield strength of 350 MPa or more, a tensile strength of 500 MPa or more, and an elongation of 15% or more.

[0165] According to one embodiment of the present invention, the surface layer hardness of the steel plate is 160 Hv or more, and the difference between the hardness of the surface layer and the hardness at the 1 / 4 point in the thickness direction may be 35 Hv or less.

[0166] In this invention, tensile strength was measured according to ASTM E8 and ASTM E23 standard test methods, and hardness values ​​were measured using a Vickers hardness tester.

[0167] The microstructure of a member according to one embodiment of the present invention may include, in area %, 97% or more of tempered martensite, 3% or less of retained austenite and cementite, and unavoidable structures.

[0168] In the present invention, the microstructure fraction can be measured using an optical microscope, and the microstructure fraction can be measured at a point 1 / 4 of the way from the surface of the member in the thickness direction.

[0169] If the tempered martensite is less than 97%, there may be a problem in that the target tensile material and impact toughness of the final heat-treated member cannot be secured.

[0170] In addition, the total amount of retained austenite and cementite may be 3% or less. If the total area fraction of retained austenite and cementite exceeds 3%, there is a problem that the yield strength of the final heat-treated member decreases.

[0171] A member according to one embodiment of the present invention may have a prior austenite grain size of 20 μm or less.

[0172] The austenite grain size can be measured at 1 / 4 of the thickness direction from the surface of the member.

[0173] According to one embodiment of the present invention, the austenite grain size can be expressed by etching a steel plate with picric acid, taking about 10 images along the 1 / 4 thickness position using an optical microscope, measuring the grain size in each individual image, and then calculating the average value.

[0174] If the average size of the prior austenite exceeds 20㎛, there may be a problem in that the low-temperature impact toughness of the final heat-treated member drops to less than 20J.

[0175] A member according to one embodiment of the present invention may have a yield strength of 1000 MPa or more, a tensile strength of 1100 MPa or more, and an elongation of 6% or more.

[0176] A member according to one embodiment of the present invention may have a notch impact toughness of 40J or more at 20℃ and a notch impact toughness of 20J or more at -40℃.

[0177] In this invention, tensile strength, yield strength, and impact toughness were measured according to the ASTM E8 and ASTM E23 standard test methods.

[0178] Hereinafter, the steel manufacturing method of the present invention will be described in detail.

[0179] A steel plate according to one embodiment of the present invention can be manufactured by heating, hot rolling, cooling, and coiling a steel slab satisfying the alloy composition described above.

[0180] heating

[0181] A steel slab satisfying the alloy composition of the present invention can be heated to a temperature range of 1150 to 1300°C. The steel slab of the present invention can be manufactured by injecting the steel or molten steel composed as described above into a continuous casting mold through an immersion nozzle, causing the molten steel to solidify initially within the mold, and then cooling the entire structure while passing it through a continuous casting machine. Generally, if titanium (Ti) oxide adheres to the immersion nozzle, it may cause nozzle clogging after a certain period of time, or the titanium (Ti) oxide attached to the nozzle may detach and be mixed into the molten steel inside the mold. In order to minimize this, the present invention [applies] high purity (5~50 ppm → 10 -15By injecting an inert gas (ppm) together, titanium oxide can be prevented from adhering to or detaching around the immersion nozzle, and the reaction between silicon or aluminum elements in the molten steel and oxygen (O) can be suppressed as much as possible, thereby minimizing oxide formation.

[0182] The above heating is intended to ensure a uniform structure and composition distribution within the slab. If the heating temperature is below 1150°C, precipitates formed in the continuous casting slab may not be dissolved, or it may be difficult to ensure compositional uniformity. On the other hand, if the heating temperature exceeds 1300°C, it may be difficult to secure the target surface quality and material properties because an excessively deep decarburized layer may form or austenite grain growth may occur.

[0183] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0184] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0185] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in the present invention may be included in the molten steel within permissible limits.

[0186] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0187] Hot rolling

[0188] The above-mentioned heated steel slab may be hot-rolled at a finishing rolling temperature of Ar3 or higher. The Ar3 temperature in the present invention can be calculated using Equation 1 below. The hot rolling of the present invention may include rough rolling and finishing rolling. If the finishing rolling temperature is less than Ar3, not only does the rolling load increase during hot rolling, but some of the austenite transforms into ferrite during hot rolling, leaving coarse-sized ferrite structures in the surface layer of the final steel, which may increase microstructural non-uniformity. In the present invention, in order to manufacture a hot-rolled steel sheet having a good shape by preventing ferrite transformation before the completion of hot rolling or by suppressing the occurrence of ferrite transformation during cooling on a runout table, the upper limit of the finishing rolling temperature during hot rolling may be limited to Ar3 + 160℃. According to one embodiment of the present invention, the finishing rolling temperature may be 870℃ or higher. According to one embodiment of the present invention, it may be 930℃ or lower.

[0189] [Equation 1]

[0190] Ar3(℃) = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]-0.35(t-8)

[0191] (In the formula, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the weight percent of each element, and t is the thickness of the steel plate (mm).)

[0192] Cooling and winding

[0193] The above hot-rolled steel plate can be cooled and coiled in a temperature range of 550 to 700°C.

[0194] The above cooling and coiling can be performed on a runout table after hot rolling, which may be advantageous for ensuring uniform material properties of the hot-rolled steel sheet itself and high strength after quenching and tempering heat treatments. If the coiling temperature is low, below 550°C, low-temperature transformation phases such as bainite or martensite are introduced into the edge portions of the hot-rolled steel sheet in the width direction, which may result in a disadvantage where the strength of the steel sheet increases rapidly and the material variation in the width direction increases. On the other hand, if the coiling temperature exceeds 700°C, decarburization may occur in the surface layer and inner coil portions of the coil during the atmospheric cooling process after coiling the steel sheet. To reduce the difference in hardness between the surface layer of the steel sheet and the 1 / 4 point in the thickness direction, it is necessary to coil at a temperature of 700°C or lower. Furthermore, in the present invention, it was confirmed that when the coiling temperature is high, the pearlite fraction is low or coarse cementite is formed, resulting in a relatively lower strength of the steel or member after heat treatment. This is thought to be because even if steel with a structure in which coarse-sized cementite is formed is heated to a high temperature of 870°C or higher, it may not be completely dissolved within a short time considering the productivity of part manufacturing. Furthermore, in the above case, since it is necessary to further increase the heating temperature to secure the target material after heat treatment, there is a disadvantage that it may lead to an increase in the unit heat consumption during part manufacturing. In addition, it is desirable to control the coiling temperature to be as low as possible so that the size of the cementite constituting the pearlite structure can be formed relatively finely. According to one embodiment of the present invention, the average cooling rate may be 5 to 50°C / s. According to one embodiment of the present invention, the average cooling rate may be 15 to 25°C / s.

[0195] A member according to one embodiment of the present invention can be manufactured by cold forming, first heating, first cooling, second heating, and second cooling a steel plate manufactured under the manufacturing conditions described above.

[0196] Cold forming

[0197] A steel sheet manufactured by the above-described manufacturing method can be cold-formed.

[0198] According to one embodiment of the present invention, a slitting coil can be obtained first by slitting the steel sheet coil into 2 to 3 sections in the longitudinal direction of the steel sheet coil, taking into account the dimensions of the cold-formed member using the manufactured steel sheet.

[0199] A slitting coil manufactured using a multi-stage roll forming facility can be cold-formed to obtain a formed member of a desired shape. In one embodiment of the present invention, a member can be obtained in a "C" shape. During the bending process, the bending radius is set to a range where forming cracks do not occur, but is not specifically limited in the present invention. The method of cold forming is not specifically limited, and methods available in the same technical field may be applied.

[0200] Primary heating and primary maintenance

[0201] The above cold-formed member can be heated first to a temperature range of 850 to 930°C and maintained for 15 to 30 minutes. In the present invention, the member can be heated first to a high temperature to form an austenite structure. The heating temperature can be limited to 850°C or higher to form an austenite structure so that the cementite particles constituting the microstructure of the initial hot-rolled steel sheet can be sufficiently melted and dissolved. If the first heating temperature is below 850°C, a sufficient austenite structure cannot be obtained, and there may be problems that cause non-uniformity in the size of the austenite grains and steel composition. On the other hand, if the temperature exceeds 930°C, the size of the austenite grains may increase excessively, and in this case, there may be problems in securing strength or impact toughness after heat treatment.

[0202] If the first holding time after the first heating is less than 15 minutes, sufficient austenite cannot be obtained as described above, and if the time exceeds 30 minutes, the possibility of decarburization occurring on the surface layer of the member increases, and a ferrite phase may be formed on the surface layer of the member even after quenching and heat treatment, which may cause problems in securing a complete martensite structure. However, in the present invention, the first holding time may vary depending on the thickness of the member.

[0203] Primary cooling

[0204] The above-mentioned first heated and first maintained member can be first cooled to a temperature range below the martensite transformation end temperature (Mf) at an average cooling rate of 30 to 100℃ / s.

[0205] The martensite transformation end temperature (Mf) can be calculated as shown in Equation 2 below.

[0206] In the present invention, rapid cooling may be performed during the first cooling to form martensite. More preferably, water cooling or oil cooling may be performed at a cooling rate greater than the critical cooling rate, but cooling may be performed as slowly as possible within a range where no shape defects occur in the member. According to one embodiment of the present invention, the average cooling rate may be limited to 30°C / s or more. However, the upper limit may be limited to 100°C / s considering the cooling capacity of the cooling equipment (refrigerant temperature control and refrigerant injection speed, etc.) or the cooling method (immersion cooling or injection cooling).

[0207] If the cooling end temperature exceeds the martensite transformation end temperature, the desired level of martensite can be secured. In the present invention, during the first cooling, cooling to room temperature is possible.

[0208] [Equation 2]

[0209] Mf(°C) = Ms-215

[0210] Ms(℃) = 512-453*[C]-16.9*[Ni]+15*[Cr]-9.5*[Mo]+217*[C]2-71.5*[C]*[Mn]-67.6*[C]*[Cr]

[0211] (In the formula, [C], [Ni], [Cr], and [Mo] are the weight percent of each element.)

[0212] Secondary heating and secondary maintenance

[0213] The above-mentioned cooled member can be heated a second time to a temperature range of 350 to 450°C and maintained for 10 to 30 minutes.

[0214] In the present invention, secondary heating can be performed in a temperature range of 350 to 450°C to impart toughness to the rapidly cooled member.

[0215] In the present invention, when heating a second time, the heating speed is not specifically limited, but heating can be performed at a speed of 60℃ / s or less.

[0216] If the secondary heating temperature is less than 350°C, the strength of the member having tempered martensite is high, but it may be difficult to impart sufficient toughness. On the other hand, if the temperature exceeds 450°C, impact toughness can be secured, but it may be difficult to secure the desired strength. In one embodiment of the present invention, it is proposed to heat at the highest possible temperature during secondary heating to avoid the temperature range where temper brittleness may occur.

[0217] When holding for a second time after heating, if the holding time is less than 10 minutes, there is a problem in that sufficient tempering effect cannot be achieved, and the desired impact toughness cannot be sufficiently secured. On the other hand, if the holding time exceeds 30 minutes, it may be difficult to secure the desired level of strength.

[0218] Secondary cooling

[0219] The above secondary heating and secondary maintained member can be secondary cooled.

[0220] In the present invention, the cooling conditions during secondary cooling are not specifically limited, but as one embodiment, furnace cooling or air cooling may be performed.

[0221] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0222] (Example)

[0223] Using steel slabs having the compositions of Tables 1 and 2 below (the remainder being Fe and unavoidable impurities), hot rolling was performed under the conditions of Table 3 below to produce hot-rolled steel sheets with a thickness of 5.3 to 6.3 mm, after which pickling treatment was performed to remove surface scale. Prior to hot rolling, the manufactured steel slabs or lap-manufactured ingots were subjected to homogenization treatment by heating in a temperature range of 1180 to 1220°C for 200 minutes, and during hot rolling, a temperature of Ar3 or higher was applied to each steel grade. In addition, during cooling and coiling, they were cooled uniformly at an average cooling rate of 20°C / s.

[0224]

[0225]

[0226] [Relationship 1]

[0227] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15

[0228] (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.)

[0229] [Relationship 2]

[0230] Y = [Ti] / ([Al]+[Si])

[0231] (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.)

[0232] [Relationship 3]

[0233] Z = [Ni] / ([C]+[Mn]) X 100

[0234] (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.)

[0235] [Relationship 4]

[0236] R = ([Ti]+[B]) / [N]

[0237] (In the formula, [Ti], [B], and [N] are the weight percent of each element.)

[0238]

[0239] For the hot-rolled steel sheet manufactured as described above, the microstructure and physical properties were measured and are shown in Table 4 below. The microstructure fractions were observed using an optical microscope, and all fractions except for pearlite in Table 4 were observed to be ferrite.

[0240] In addition, the depth of the decarburization layer was determined by taking an optical microscope image to check for the presence of ferrite or pearlite in the surface layer of the steel plate thickness, or by using a GDS (Glow Discharge Spectrometer) to evaluate the depth at which the carbon content directly below the surface layer of the steel plate differs from the carbon content in the inner layer of the steel plate, and the depth of the decarburization layer is shown in Table 4 below.

[0241] In addition, for hot-rolled steel sheets, room temperature tensile tests were performed according to the ASTM E8 standard test method to measure and present the yield strength, tensile strength, and elongation. Furthermore, Vickers hardness was measured at the surface layer and at the 1 / 4 thickness point of the steel sheet using a Vickers hardness tester, and the difference was calculated.

[0242]

[0243] A component was manufactured by performing a first heating, first cooling, and second heating on the hot-rolled steel sheet produced above under the conditions disclosed in Table 3. During the first heating, the material was maintained for 15 to 20 minutes, and during the first cooling, it was cooled to room temperature. During the second heating, it was maintained for 10 to 30 minutes, and then the second cooling was performed by air cooling. Subsequently, the microstructure and material properties were observed and are shown in Table 5 below. The microstructure was observed using the same method as applied to the hot-rolled steel sheet.

[0244] In addition, the average grain size of the prior austenite was calculated by measuring the grain size within each individual image and calculating the average value after performing picric acid etching on individual steel specimens after tempering heat treatment at a temperature of 200–300°C for each specimen that was not subjected to secondary heating, taking approximately 10 images along the 1 / 4 thickness position using an optical microscope, and then measuring the grain size within each individual image. Meanwhile, when tempering heat treatment is performed at a temperature of 350–450°C, grain boundary corrosion is insufficient, making it difficult to distinguish grain boundaries using an optical microscope. Furthermore, it was recognized that the grain size of the prior austenite in tempered martensitic steel did not change significantly in the temperature range of 200–450°C, so observation was performed after heat treatment at a temperature of 200–300°C.

[0245] In addition, for the heat-treated members, yield strength, tensile strength, elongation, and Charpy notch impact toughness values ​​were evaluated according to the ASTM E8 and ASTM E23 standard test methods, just as with hot-rolled steel sheets. For Charpy notch impact toughness, tests were conducted at room temperature (20°C) and low temperature (-40°C), respectively. The impact energy values ​​were measured by using an 800J impact tester after controlling the temperature of the individual specimens by immersing them in liquid nitrogen. Meanwhile, the occurrence of brittleness was determined by conducting tensile or impact tests on tempered tensile or impact specimens at the same temperature; brittleness was defined as occurring if the elongation was less than 6% or if the impact energy value measured at -40°C was less than 20 Joules.

[0246]

[0247] As shown in Tables 4 and 5, specimens satisfying the alloy composition and manufacturing conditions of the present invention satisfied the microstructural characteristics proposed in the present invention, and the physical properties intended in the present invention were also secured.

[0248] FIG. 1 is a micrograph of the microstructure of specimen 6 of a hot-rolled steel sheet according to one embodiment of the present invention. The photograph in FIG. 1 shows the microstructure at the 1 / 4 thickness position of the hot-rolled steel sheet, and it can be seen that the fraction of pearlite (dark part) is relatively low.

[0249] FIG. 2 is a micrograph of the microstructure of specimen 6 according to one embodiment of the present invention. The microstructure was observed using an optical microscope and shows the crystal grains of the austenite. The dotted line indicates the crystal grain size of the austenite, and it can be confirmed that it has an average size of 10.4 μm.

[0250] FIG. 3 shows the microstructure of a specimen 6 according to one embodiment of the present invention observed using an EBSD-OIM analysis instrument, and it can be seen that the underlying structure includes packets or blocks of tempered martensite. Individual packets or blocks are distinguished by high-angle grain boundaries with a mis-orientation of 15° or more.

[0251] On the other hand, specimens that did not satisfy the conditions proposed in the present invention failed to secure the microstructural characteristics and physical properties intended by the present invention.

[0252] Although the present invention has been described in detail through embodiments above, other forms of embodiments are also possible. Therefore, the technical concept and scope of the claims described below are not limited to the embodiments.

Claims

1. In wt%, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, the remainder being Fe and unavoidable impurities, and The X value defined in the following Equation 1 is 0.4 to 0.6, and The Y value defined in the following Equation 2 is 0.15 to 3.6, and The Z value defined in the following Equation 3 is 50 or less, and The R value defined in the following Equation 4 is 10 or less, and Steel plate with a yield strength of 350 MPa or higher. [Relationship 1] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15 (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.) [Relationship 2] Y = [Ti] / ([Al]+[Si]) (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.) [Relationship 3] Z = [Ni] / ([C]+[Mn]) X 100 (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.) [Relationship 4] R = ([Ti]+[B]) / [N] (In the formula, [Ti], [B], and [N] are the weight percent of each element.) 2. In Paragraph 1, The above steel plate is a steel plate comprising one or more of the following (a) to (c). (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less 3. In Paragraph 1, The above steel plate is a steel plate comprising, in area %, 25~80% pearlite, 20~75% ferrite, and unavoidable structure.

4. In Paragraph 1, A steel plate having a surface layer of 150㎛ or less.

5. In Paragraph 1, The above steel plate has a tensile strength of 500 MPa or more and an elongation of 15% or more.

6. In Paragraph 1, A steel plate having a surface layer hardness of 160 Hv or higher.

7. In Paragraph 1, The above steel plate is a steel plate in which the difference in hardness between the surface layer and the 1 / 4 point in the thickness direction is 35Hv or less.

8. A step of heating a steel slab comprising, in wt%, C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, and the remainder being Fe and unavoidable impurities, wherein the X value defined in Equation 1 below is 0.4~0.6, the Y value defined in Equation 2 below is 0.15~3.6, the Z value defined in Equation 3 below is 50 or less, and the R value defined in Equation 4 below is 10 or less; A step of hot rolling the above heated steel slab at a finishing rolling temperature of Ar3 or higher; and A method for manufacturing a steel plate comprising the step of cooling the hot-rolled steel plate to a temperature range of 550 to 700°C and coiling it. [Relationship 1] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15 (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.) [Relationship 2] Y = [Ti] / ([Al]+[Si]) (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.) [Relationship 3] Z = [Ni] / ([C]+[Mn]) X 100 (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.) [Relationship 4] R = ([Ti]+[B]) / [N] (In the formula, [Ti], [B], and [N] are the weight percent of each element.) 9. In Paragraph 8, The above steel slab is a method for manufacturing a steel plate comprising one or more of the following (a) to (c). (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less 10. In Paragraph 8, A method for manufacturing a steel plate in which the step of heating the above steel slab is performed in a temperature range of 1150 to 1300℃.

11. In wt%, it comprises C: 0.23~0.35%, Mn: 0.50~1.20%, Si: 0.02~0.15%, P: 0.02% or less, S: 0.005% or less, Al: 0.05% or less, Ti: 0.015~0.07%, B: 0.0005~0.003%, N: 0.008% or less, the remainder being Fe and unavoidable impurities, and The X value defined in the following Equation 1 is 0.4 to 0.6, and The Y value defined in the following Equation 2 is 0.15 to 3.6, and The Z value defined in the following Equation 3 is 50 or less, and The R value defined in the following Equation 4 is 10 or less, and A member whose microstructure, in area %, comprises 97% or more of tempered martensite, 3% or less of retained austenite and cementite, and unavoidable microstructure. [Relationship 1] X = [C] + [Mn] / 6 + ([Cr]+[Mo]+[V]) / 5 + ([Cu]+[Ni]) / 15 (In the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the weight percent of each element.) [Relationship 2] Y = [Ti] / ([Al]+[Si]) (In the formula, [Ti], [Al], and [Si] are the weight percent of each element.) [Relationship 3] Z = [Ni] / ([C]+[Mn]) X 100 (In the formula, [Ni], [C], and [Mn] are the weight percent of each element.) [Relationship 4] R = ([Ti]+[B]) / [N] (In the formula, [Ti], [B], and [N] are the weight percent of each element.) 12. In Paragraph 11, The above member is a member comprising one or more of the following (a) to (c). (a) One or more selected from Cr: 0.2% or less and Mo: 0.1% or less (b) One or more selected from Nb: 0.01% or less and V: 0.1% or less (c) One or more selected from Ni: 0.50% or less and Cu: 0.50% or less 13. In Paragraph 11, The above-mentioned member is a member having a prior austenite grain size of 20 μm or less.

14. In Paragraph 11, The above member is a member having a yield strength of 1000 MPa or more, a tensile strength of 1100 MPa or more, and an elongation of 6% or more.

15. In Paragraph 11, The above member is a member having a notch impact toughness of 40J or more at 20℃ and a notch impact toughness of 20J or more at -40℃.

16. A step of providing a steel plate according to any one of paragraphs 1 to 7; A step of manufacturing a member by cold forming the above steel plate; A step of first heating the above cold-formed member to a temperature range of 850~930℃ and first maintaining it for 15~20 minutes; A step of first cooling the above-mentioned first heated and first maintained member to a temperature range below the martensite transformation end temperature (Mf) at an average cooling rate of 30 to 100℃ / s; and A method for manufacturing a component comprising the step of secondarily heating the cooled component to a temperature range of 350 to 450°C and secondarily maintaining it for 10 to 30 minutes.

17. In Paragraph 16, A method for manufacturing a component, further comprising a step of secondary cooling the above-mentioned secondary heated and secondary maintained component by furnace cooling or air cooling.

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