Steel plate and manufacturing method thereof

Abstract

The present invention relates to a steel plate and a manufacturing method thereof and, more specifically, to an ultra-high strength steel plate having excellent impact resistance in a bend forming part, and to a manufacturing method thereof.

Description

Technical Field

[0001] The present disclosure relates to a steel plate and a method of manufacturing the same, and more specifically, to an ultra-high strength steel plate having excellent impact resistance in a bend forming portion, and a method of manufacturing the same.Background Art

[0002] Ultra-high strength hot-rolled steel plates, manufactured to primarily utilize a conventional martensite-based microstructure to have high strength, durability, and impact resistance, suffer from problems of having poor impact resistance in formed products due to insufficient ductility, a non-uniform microstructure, segregation of alloy elements, or the like, in an initial hot-rolled steel plate, even when relatively simple-shaped components are manufactured through processes such as roll forming, bending, or the like. In particular, to ensure safety during collisions in a vehicle exposed to various external environments, impact resistance should be secured even at temperature ranges below room temperature, but technical solutions to address this issue are insufficient, and merely to overcome lack of a bend forming portion, the following technology have been proposed.

[0003] Patent Document 1 attempts to improve bendability by a method of primarily controlling alloying elements such as C, Si, Mn, or the like in steel and minimizing rolling at temperatures outside of those of a non-recrystallization zone of austenite during hot-rolling to reduce an aspect ratio of prior austenite grains while simultaneously suppressing a texture enhancing anisotropy. However, Patent Document 1 discloses technology requiring high-temperature rolling and requiring control of a texture of the steel, which may be difficult to control. Therefore, there are difficulties in terms of a manufacturing process, and since bainite is included as a main phase in a central portion, in addition to martensite, which is mainly formed in a surface portion in thickness, there is a disadvantage that it is difficult to secure a uniform material and lacks bending processability. In addition, there is no review of impact resistance after bending may be provided.

[0004] In Patent Document 2, a surface portion includes ferrite and pearlite, which are structures that are too soft, to have low strength, as compared to a central portion, and there is a disadvantage in that deformation is concentrated in the surface portion during bending processing. Therefore, bending moldability may be good, but when the molding part collides, cracks are likely to occur in the surface portion where deformation is concentrated, so impact resistance at low temperatures may be inferior.

[0005] Patent Document 3 relates to high-strength steel having at least 5% of ferrite, as a soft phase, and a composite structure including martensite, auto-tempered martensite, bainite, or the like, as a hard phase, to secure high strength and excellent formability at the same time. This is a manufacturing technology that mainly targets cold rolled products, and it is difficult to manufacture steel having high yield strength because the soft and hard phases exist at the same time.

[0006] Alloying elements such as Si, Mn, Mo, Cr, Cu, Ni, or the like, commonly used to manufacture ultra-high strength steels, enhance hardenability and may thus be effective in improving strength. However, excessive addition of these elements may lead to segregation of the alloying elements and heterogeneity of a microstructure, resulting in poor bending formability. In particular, steel having high hardenability is susceptible to a change in microstructure during cooling, leading to uneven formation of a low-temperature transformation structure, making it difficult to achieve even higher bending formability and impact resistance.

[0007] Furthermore, excessive addition of the alloying elements increases hot deformation resistance. Furthermore, when Ti, Nb, and the like are added together, dynamic strain-induced precipitation during hot-rolling causes rapid changes in deformation resistance, resulting in poor shape quality of the rolled plate. This, in turn, leads to an uneven microstructure and ultimately, poor properties of a final component.[Prior Art Documents][Patent Documents]

[0008] (Patent Document 1) Japanese Patent Laid-Open Publication No. 2013-117068 (Patent Document 2) Korean Patent Laid-Open Publication No. 10-2021-0088646 (Patent Document 3) Korean Patent No. 10-1225321 Disclosure of InventionTechnical Problem

[0009] According to an embodiment of the present disclosure, a steel plate and a method for manufacturing the same may be provided.

[0010] According to an embodiment of the present disclosure, an ultra-high strength steel plate having excellent impact resistance in a bend forming portion, and a method for manufacturing the same may be provided.

[0011] The objectives of the present disclosure are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present disclosure from the overall contents of the present specification.Solution to Problem

[0012] According to an embodiment of the present disclosure, a steel plate including, by weight, C: 0.10~0.30%, Si: 0.01~0.50%, Mn: 0.5~1.5%, Al: 0.01~0.10%, P: 0.0100~0.0500%, S: 0.0010~0.0100%, N: 0.0010~0.0100%, at least one of Sb: 0.005~0.100%, Sn: 0.005~0.100%, or As: 0.005~0.100%, at least one of Cu: 0.010~0.300%, Cr: 0.010~0.500%, Ni: 0.010∼0.200%, Mo: 0.010~0.300%, or B: 0.0010~0.0050%, at least one of Nb: 0.005~0.030% or Ti: 0.005~0.050%, with a remainder of Fe and unavoidable impurities, wherein an A value defined in Relationship 1 below is 10.0 or less, wherein a T value defined in Relationship 2 below is 40.0~100.0, wherein a microstructure includes, by area, 3% or less of ferrite, with a remainder of bainite and martensite, and wherein the steel plate has a 90° bend formability (R / t) of 4.0 or less, and has a temperature of -20°C or less at which cracking occurs in an impact test after 90° bend forming: A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N

[0013] (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element.) T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element.)

[0014] The A value defined in the Relationship 1 may be 0 or greater.

[0015] The microstructure of the steel plate may include, by area, 97% or greater of the bainite and the martensite, in total.

[0016] The microstructure may include, by area, 50% or greater of martensite.

[0017] A tensile strength of the steel plate may be 1400 MPa or greater and a yield ratio of the steel plate may be 0.75 or greater.

[0018] An area fraction of defects caused by red scale on a surface of the steel plate may be less than 30%.

[0019] According to an embodiment of the present disclosure, a method of manufacturing a steel plate, including first heating a steel slab containing, by weight, C: 0.10~0.30%, Si: 0.01~0.50%, Mn: 0.5~1.5%, Al: 0.01~0.10%, P: 0.0100~0.0500%, S: 0.0010~0.0100%, N: 0.0010~0.0100%, at least one of Sb: 0.005~0.100%, Sn: 0.005~0.100%, or As: 0.005~0.100%, at least one of Cu: 0.010~0.300%, Cr: 0.010~0.500%, Ni: 0.010~0.200%, Mo: 0.010~0.300%, or B: 0.0010~0.0050%, at least one of Nb: 0.005~0.030% or Ti: 0.005~0.050%, with a remainder of Fe and unavoidable impurities, wherein an A value defined in Relationship 1 below is 10.0 or less, and wherein a T value defined in Relationship 2 below is 40.0~100.0, to a first heating temperature range in which an X value defined in Relationship 3 below satisfies 1.0~10.0; second heating the firstly heated steel slab; hot-rolling the secondarily heated steel slab; first cooling the hot-rolled steel plate to a temperature within a range of 300 to 400°C at an average cooling rate of 50 to 100°C / s; and second cooling the firstly cooled steel plate to a temperature within a range of 50 to 200°C at an average cooling rate of 1 to 30°C / s, and then coiling the second cooled steel plate: A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element.) T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element.) X = 160 × A × Exp − 37 , 000 / 8.314 × 273 + Tp A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, and Tp is a first heating temperature.)

[0020] The first heating may be performed at a temperature within a range of 800 to 1000°C.

[0021] The first heating may have a holding time of 30 to 90 minutes.

[0022] The second heating may be performed at a temperature within a range of 1100 to 1350°C, and the hot-rolling may be performed at a temperature within a range of 800 to 1150°C.Advantageous Effects of Invention

[0023] According to an embodiment of the present disclosure, a steel plate and a method for manufacturing the same may be provided.

[0024] According to an embodiment of the present disclosure, an ultra-high strength steel plate having excellent impact resistance in a bend forming portion, and a method for manufacturing the same may be provided.

[0025] According to an embodiment of the present disclosure, a hot-rolled steel plate that may be used for chassis components of passenger cars and commercial vehicles, particularly components manufactured by roll forming or bending forming, such as a bumper beam, a rear safety plate (under run protection device (URPD)), a frame, a cross member, or the like, which require impact resistance, may be provided.Brief Description of Drawings

[0026] FIG. 1 illustrates distribution diagrams of Relationships 1 and 2 of Inventive Examples and Comparative Examples according to an embodiment of the present disclosure. FIGS. 2 (a) and 2 (b) respectively illustrate surface photographs of Invention Example 6 and Comparative Example 2 according to an embodiment of the present disclosure. Best Mode for the Invention

[0027] Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various manners, and the scope of the present disclosure should not be construed as limited to the embodiments described below. These embodiments may be provided to further explain the present disclosure to those skilled in the art.

[0028] To address the problems of the prior art, with regard to ultra-high strength hot-rolled steels with various components and different microstructures, the inventors investigated bending formability and impact resistance of bending-formed samples according to characteristics of the components and the microstructures. In particular, the inventors investigated dependence of bending formability, impact resistance, and surface red scale defects for alloying elements prone to segregation and alloying elements with high hardenability, in steel. Based on these results, the present disclosure was completed.

[0029] Hereinafter, the present disclosure will be described in detail.

[0030] Below, a steel composition of the present disclosure may be described in detail.

[0031] Unless otherwise specified, percentages indicating amounts of respective elements in the present disclosure are based on weight.

[0032] According to an embodiment of the present disclosure, a steel plate may include, by weight, C: 0.10~0.30%, Si: 0.01~0.50%, Mn: 0.5~1.5%, Al: 0.01~0.10%, P: 0.0010~0.0500%, S: 0.0010~0.0100%, N: 0.0010~0.0100%, at least one of Sb: 0.005~0.100%, Sn: 0.005~0.100%, or As: 0.005~0.100%, at least one of Cu: 0.010~0.300%, Cr: 0.010~0.500%, Ni: 0.010∼0.200%, Mo: 0.010~0.300%, or B: 0.0010~0.0050%, at least one of Nb: 0.005~0.030% or Ti: 0.005~0.050%, with a remainder of Fe and unavoidable impurities.Carbon (C): 0.10~0.30%

[0033] Carbon (C) may be the most economical and effective element for strengthening steel and may significantly affect the strength and ductility of each component. As an addition amount increases, hardenability may increase to increase a fraction of a hard phase such as bainite, martensite, or the like in a microstructure, thereby increasing tensile strength. When the carbon (C) amount is less than 0.10%, it may be difficult to fully achieve the aforementioned effects. According to an embodiment of the present disclosure, the carbon (C) amount is 0.15% or more. On the other hand, when the amount exceeds 0.30%, a fraction of each phase, including bainite, martensite, or the like increases, and hardness values also increase, resulting in excessive strength increases and potential problems such as reduced elongation and formability. According to an embodiment of the present disclosure, an upper limit may be limited to 0.25% to stably secure target levels of strength and formability.Silicon (Si): 0.01~0.50%

[0034] Silicon (Si) deoxidizes molten steel, has a solid solution strengthening effect, delays formation of coarse carbides, and promotes ferrite formation, to effectively improve ductility of steel. When the silicon (Si) amount is less than 0.01%, it may be difficult to fully achieve the aforementioned effects. According to an embodiment of the present disclosure, a lower limit may be 0.03%. However, when the silicon (Si) amount exceeds 0.50%, severe red scales due to silicon (Si) form on the steel plate surface during hot-rolling, significantly degrading a quality of the steel plate surface. Furthermore, localized surface roughness variations caused by the severe red scale defects may also result in reduced bending formability. According to an embodiment of the present disclosure, an upper limit may be 0.40%. According to another embodiment of the present disclosure, the upper limit may be 0.30%. According to another embodiment of the present disclosure, the upper limit may be 0.20%.Manganese (Mn): 0.5~1.5%

[0035] Manganese (Mn), like silicon (Si), may be an element effective in solid solution strengthening steel, and may increase hardenability of the steel and may facilitate formation of hard phases, which are bainite and martensite, during cooling after hot-rolling. However, when the manganese (Mn) amount is less than 0.5%, the aforementioned effects of addition may be difficult to achieve. According to an embodiment of the present disclosure, a lower limit may be 0.6%. According to an embodiment of the present disclosure, the lower limit may be 0.8%. On the other hand, when the amount exceeds 1.5%, hardenability significantly increases, increasing a fraction and hardness of each phase, including bainite and martensite, which may lead to excessive strength increases and reduced formability. Furthermore, during a casting process, segregation zones develop significantly in a central portion in thickness during slab casting, and during cooling after hot-rolling, a microstructure may become uneven in the thickness direction, resulting in poor formability. According to an embodiment of the present disclosure, an upper limit may be 1.4%.Aluminum (Al): 0.01~0.10%

[0036] Aluminum (Al) may be an element to be primarily added for deoxidation, and may have a ferrite transformation promoting effect. When an amount thereof is less than 0.01%, its effect may be insufficient. According to an embodiment of the present disclosure, it may be added in an amount of 0.015% or more. Conversely, when an amount thereof exceeds 0.10%, Aluminum combines with nitrogen to form AlN, which may easily cause corner cracks in slab during continuous casting and defects due to inclusion formation. According to an embodiment of the present disclosure, an upper limit may be 0.08%.Phosphorus (P): 0.0100~0.0500%

[0037] Phosphorus (P), like silicon (Si), has both solid solution strengthening and ferrite transformation promoting effects. The present disclosure does not specifically limit a lower limit of phosphorus (P). However, manufacturing at an amount below 0.0100% may be economically disadvantageous due to the high manufacturing costs, and may also result in insufficient strength. On the other hand, when the phosphorus amount exceeds 0.0500%, embrittlement occurs due to grain boundary segregation, cracks may easily form during forming, and ductility and formability may be significantly reduced.Sulfur (S): 0.0010~0.0100%

[0038] Sulfur (S) may be an impurity present in steel. When an amount thereof exceeds 0.0100%, it combines with manganese and other elements to form excessive non-metallic inclusions. This may lead to micro-cracks during cutting and significantly reduced bending formability. While the present disclosure does not specifically limit a lower limit of sulfur (S) amount, producing steel with an amount less than 0.0010% may require significant time during steelmaking, reducing productivity. According to an embodiment of the present disclosure, an upper limit of sulfur (S) amount is 0.0050%.Nitrogen (N): 0.0010~0.0100%

[0039] Nitrogen (N), along with carbon (C), may be a representative solid-solution strengthening element and a hardenable element. It may form coarse nitrides with elements such as titanium (Ti), aluminum (Al), or the like. While the solid-solution strengthening effect of nitrogen (N) may be generally superior to that of carbon (C), increasing the amount of nitrogen (N) in steel significantly reduces toughness. Therefore, in the present disclosure, an upper limit of the nitrogen amount is limited to 0.0100%. However, producing steel with a nitrogen (N) amount less than 0.0010% requires significant time during steelmaking, potentially reducing productivity.

[0040] At least one of Antimony (Sb): 0.005~0.100%, Tin (Sn): 0.005~0.100%, or Arsenic (As): 0.005~0.100%.

[0041] Antimony (Sb), tin (Sn), and arsenic (As) may inevitably be contained in steel. When contained at appropriate levels and present in a solid solution state, they enhance strength of the steel through solid solution strengthening. Furthermore, they tend to concentrate primarily on a surface, which has an effect of suppressing the formation of red scale (Fayalite, Fe 2 SiO 4 ). To achieve the effect, an amount of each of antimony (Sb), tin (Sn), and arsenic (As) may be 0.005% or greater. In an embodiment of the present disclosure, a lower limit of each amount is 0.010%. Furthermore, in an embodiment of the present disclosure, the lower limit of each amount is 0.030%. However, the elements may be elements that may be prone to segregation at grain boundaries and surface portions, and when they may be contained excessively, ductility of the steel may be poor, and defects due to uneven scales on the surface may occur. That is, when the elements are added excessively, the elements may be detrimental to securing bending formability and impact resistance of the ultra-high strength steel to be solved in the present disclosure. Therefore, in the present disclosure, an upper limit of the amount of each of the antimony (Sb), tin (Sn), and arsenic (As) may be limited to 0.100%. Accordingly, the present disclosure may contain one or more of the elements. In the present disclosure, other than the antimony (Sb), tin (Sn), and arsenic (As), rare-earth metal (REM) elements such as W, Ta, Zr, Y, La, and Ce may not be specifically added and used, but when these alloy elements may be added excessively, inclusions may be formed, and therefore the sum of amounts thereof may be limited to 0.020% or less.

[0042] At least one of Copper (Cu): 0.010~0.300%, Chromium (Cr): 0.010~0.500%, Nickel (Ni): 0.010~0.200%, Molybdenum (Mo): 0.010~0.300%, or Boron (B): 0.0010~0.0050%

[0043] Copper (Cu), Chromium (Cr), Nickel (Ni), Molybdenum (Mo), and Boron (B) may all enhance hardenability of steel. Furthermore, when nickel (Ni) is present with copper (Cu), it may effectively suppress intergranular cracking in slabs caused by copper (Cu). Therefore, the present disclosure may include at least one of the components.

[0044] As described above, copper (Cu) enhances the hardenability of steel. Therefore, to ensure its effectiveness, the amount is increased to 0.010% or more. According to an embodiment of the present disclosure, an amount of 0.050% or more may be more effective. However, as described above, excessive copper (Cu) amount may cause intergranular cracking in the slab. Therefore, the amount is limited to 0.300% or less.

[0045] Chromium (Cr) strengthens the steel and, upon cooling, delays ferrite phase transformation, thereby aiding formation of bainite and martensite. However, when the amount is less than 0.010%, the aforementioned effects may be difficult to achieve. Conversely, when the chromium (Cr) amount exceeds 0.500%, excessive formation of high-hardness martensite may occur, resulting in poor elongation. Furthermore, similar to manganese (Mn), segregation in the center of the steel thickness may be significantly developed, leading to a non-uniform thickness-wise microstructure, which may result in poor formability. According to an embodiment of the present disclosure, an upper limit may be limited to 0.300%.

[0046] Nickel (Ni) may be effective in improving hardenability of steel, and when containing copper, it has an effect of suppressing slab intergranular cracking of copper. To achieve the above-described effects, nickel (Ni) may be contained in an amount of 0.010% or more. However, nickel (Ni) may be an expensive element, so excessive inclusion may be economically undesirable. Therefore, an upper limit of the amount may be limited to 0.200%.

[0047] The molybdenum (Mo) may increase hardenability of steel and facilitate formation of bainite and martensite. However, when the amount is less than 0.010%, the above-mentioned effects cannot be achieved. Conversely, when the molybdenum (Mo) amount exceeds 0.300%, excessive hardenability increases, leading to excessive increases in the martensite phase fraction and phase hardness, which may drastically deteriorate formability. Furthermore, this may be economically disadvantageous and may also be detrimental to weldability. According to an embodiment of the present disclosure, an upper limit of molybdenum (Mo) may be 0.200%.

[0048] When boron (B) is present in a solid solution state in steel, it stabilizes grain boundaries, thereby improving low-temperature toughness of the steel at low temperatures. By forming BN with solid solution nitrogen, it may suppress the formation of coarse nitrides such as AlN, TiN, or the like. When the boron (B) amount is less than 0.0010%, the above-mentioned effects may be difficult to achieve. On the other hand, when the amount exceeds 0.0050%, the effect no longer increases, ductility decreases, recrystallization behavior may be delayed during hot-rolling, and rolling loads may significantly increase during rolling.

[0049] At least one of Niobium (Nb): 0.005~0.030% or Titanium (Ti): 0.005~0.050%

[0050] Niobium (Nb) and titanium (Ti) may be representative precipitation-strengthening elements. Together with carbon and nitrogen, they form precipitates in steel, contributing to increased strength. Furthermore, they inhibit grain boundary movement, inhibiting grain growth during slab reheating and delaying recrystallization during hot-rolling, effectively forming a fine, uniform microstructure. The present disclosure may contain one or more of the elements to achieve the effects.

[0051] Niobium (Nb) has a significant grain refinement effect, effectively improving strength and impact toughness of steel. When the niobium (Nb) amount is less than 0.005%, the effect may be difficult to achieve. Conversely, when the amount exceeds 0.030%, excessive recrystallization delays during hot-rolling, leading to formation of elongated grains and coarse composite precipitates, which may degrade formability.

[0052] Titanium (Ti) has a strong affinity for nitrogen (N), forming coarse TiN in the steel. Therefore, containing boron (B) may effectively inhibit the formation of BN, maintaining B in a solid solution state. Furthermore, the remaining titanium (Ti) reacts with nitrogen and combines with carbon (C) in the steel, forming fine TiC precipitates, a useful element for improving strength of the steel. When the titanium (Ti) amount is less than 0.005%, the effect may be difficult to achieve. Conversely, when the amount exceeds 0.050%, formation of coarse TiN may degrade formability.

[0053] In addition to the composition described above, steel of the present disclosure may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities may be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities may be readily apparent to anyone skilled in the art of steel manufacturing, their full amounts may not be specifically discussed herein.

[0054] A steel plate according to an embodiment of the present disclosure may have an A value of 10.0 or less, as defined in the following Relationship 1. A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element.)

[0055] The present disclosure provides a steel material with excellent strength, and more particularly, a steel material with excellent resistance to external impact after being manufactured into a component. Therefore, to ensure these properties, uniformity of components and microstructure of the steel material may be important, and it has been confirmed that these properties are interrelated.

[0056] Relationship 1 of the present disclosure relates to uniformity of the components, and ensuring the uniformity of the components also locally affects formation of the microstructure.

[0057] When the A value defined in Relationship 1 exceeds 10.0, segregation of the alloying elements Cu, Sb, As, and Sn in the grain boundaries and the surface portion of the microstructure of the steel plate may be excessive, forming surface defects and weakening the grain boundaries, which may deteriorate ductility and formability of the steel. In addition, Cu and Ni may be elements that significantly increase hardenability of steel, and Sb, As, and Sn may solid-solution strengthen the steel, so that the fractions of martensite and bainite, which may be hard phases, locally increase, which may deteriorate formability and impact resistance. According to an embodiment of the present disclosure, the A value may be 5.0 or less. In the present disclosure, a lower limit of the A value may not be particularly limited, but when it is less than 0, red scale due to Si may be excessively formed on the surface of the steel plate, which not only significantly deteriorates surface quality of the steel plate, but also causes a problem of reduced bending formability due to local surface roughness differences formed by the severe red scale defects. Therefore, according to an embodiment of the present disclosure, the A value may be 0 or greater. According to an embodiment of the present disclosure, the A value may be 1.0 or greater. According to an embodiment of the present disclosure, when an element is not added among the alloying elements mentioned in Relationship 1, 0 may be substituted.

[0058] The steel plate according to an embodiment of the present disclosure may have a T value of 40.0 to 100.0, as defined in Relationship 2 below. T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N

[0059] (In the Relationship, [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element.)

[0060] Among the microstructures proposed in the present disclosure, formation of bainite and martensite, which are hard phases, should be maintained at an appropriate level to secure target tensile strength. Accordingly, in the present disclosure, a combination of alloying elements that facilitate the formation of the hard phases was used to derive Relationship 2. As the T value defined in Relationship 2 increases, the formation of the hard phases, which are bainite and martensite, increases, and the hardness value of each hard phase may also increase. Therefore, a larger T value may be advantageous in securing strength. On the other hand, when the T value is less than 40.0, it may be difficult to secure the strength of the steel proposed in the present disclosure. In the present disclosure, a lower limit of the T value may be limited to 40.0. According to an embodiment of the present disclosure, the lower limit may be 50. On the other hand, when the value may be excessively large, the ductility of the steel may be reduced, and when an external force such as a collision is applied from the outside, cracks may be likely to occur due to non-uniform microstructures, inclusions, and coarse carbonitrides that may be included in the steel, and there may be a problem in that the material deviation increases in overall length and width of the hot-rolled steel plate. Therefore, in the present disclosure, an upper limit of the T value may be limited to 100.0. According to an embodiment of the present disclosure, the upper limit may be 80.0. According to an embodiment of the present disclosure, when any of the alloying elements mentioned in Relationship 2 above may not be added, 0 may be substituted.

[0061] Hereinafter, a microstructure of steel of the present disclosure will be described in detail.

[0062] Unless otherwise specified, percentages indicating fractions of a microstructure in the present disclosure may be based on area.

[0063] A microstructure of a steel plate according to an embodiment of the present disclosure may include, by area, at least 97% of bainite and martensite, and may include 3% or less of ferrite as other structures. According to an embodiment of the present disclosure, the other structures may include less than 3% of ferrite.

[0064] In the present disclosure, respective area fractions of bainite and martensite in the microstructure may not be specifically limited. However, since strength increases as the area fraction of martensite increases, a lower limit may be limited to 50%. In an embodiment, the fraction of martensite may be 70% or greater.

[0065] Furthermore, when the microstructure contains more than 3% of ferrite, which is a relatively soft phase, the yield strength may decrease or uneven deformation may occur in the bending portion during bending. In an embodiment, the ferrite amount is limited to less than 3%. According to an embodiment of the present disclosure, the ferrite may be included to have 0%.

[0066] In the present disclosure, tempered martensite containing fine carbides may be considered martensite. Other unavoidable structures, such as martensite and austenite (MA) and retained austenite, may also form, but an upper limit may be limited to 2%.

[0067] According to an embodiment of the present disclosure, a steel plate has a tensile strength of 1400 MPa or greater, a yield ratio of 0.75 or greater, a 90° bend formability (R / t) of 4.0 or less, and a temperature at which cracking occurs at -20°C or less during an impact test after 90° bending. In this case, r may be a bending radius and t may be a thickness of the steel plate (mm). Furthermore, an area fraction of defects caused by red scales on a steel plate surface may be less than 30%. An area fraction of surface defects caused by red scales may be calculated by dividing a defect area by an observed area, based on images identified by an optical surface defect detector (SDD) system. Considering that surface defects caused by red scales typically occur in stripes in a rolling direction, lengths of the defects may be directly measured by drawing a straight line, perpendicular to the rolling direction, on the rolled plate, and a resulting ratio may be calculated as a percentage of a width of the rolled plate.

[0068] Hereinafter, the steel manufacturing method of the present disclosure will be described in detail.

[0069] A steel plate according to an embodiment of the present disclosure may be manufactured by first heating, second heating, hot-rolling, first cooling, second cooling, and coiling a steel slab satisfying the above-described alloy composition.First Heating

[0070] A steel slab satisfying the alloy composition of the present disclosure may be first heated to a first heating temperature range in which an X value defined in Relationship 3 below satisfies 1.0~10.0. X = 160 × A × Exp − 37 , 000 / 8.314 × 273 + Tp A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, and Tp is a first heating temperature.)

[0071] When the X value defined in the Relationship 3 exceeds 10, an amount of segregation increases in a surface portion during reheating of the slab, to form surface defects or segregate at grain boundaries, thereby lowering ductility and formability of the steel. Therefore, in the present disclosure, an upper limit of the X value may be limited to 10.0. According to an embodiment of the present disclosure, the upper limit may be limited to 7.0. Meanwhile, when the X value is less than 1.0, there may be a risk of increased surface defects due to red scale on the surface of the steel plate. According to an embodiment of the present disclosure, when the alloying elements mentioned in Relationship 2 may not be added, 0 may be substituted.

[0072] Meanwhile, as the first heating temperature (Tp) of Relationship 3 increases, the X value increases, which may cause the problem. Thus, a degree of segregation caused by excessive inclusion in the steel may be changed, depending on a steel slab heating method. This may be because solid solubility and diffusion rate of the alloying elements change rapidly above a certain temperature. Therefore, in the present disclosure, the reheating process may be divided into first and second heating stages and controlled based on Relationship 3.

[0073] In particular, during the reheating process of the slab loaded into a furnace and heated, heat transfer proceeds from a surface portion to a core portion, and the steel undergoes a phase transformation from ferrite to austenite. Therefore, a temperature of the slab may be typically highest at the surface portion, which may facilitate segregation of the alloying elements there. However, by dividing the heating process into the first and second heating stages, and briefly maintaining the first heating temperature within the temperature range where the steel undergoes the phase transformation, followed by second heating, rapid dissolution and diffusion of the alloying elements in the steel may be suppressed. Consequently, excessive segregation of the alloying elements in the surface portion may also be suppressed.

[0074] In the present disclosure, a holding time for the first heating may not be particularly limited, but may be 30 minutes or more. In an embodiment, the holding time may be 90 minutes or less. When the holding time is less than 30 minutes, the above-described effect may be insufficient. On the other hand, when the holding time exceeds 90 minutes, the effect no longer increases and may be economically disadvantageous.

[0075] During the first heating, the heating temperature (Tp) may be limited by Relationship 3, which also reflects influence of alloying elements. Therefore, there may be no need to specifically define lower and upper limits thereof. However, since solubility of alloying elements in steel changes significantly before and after ferrite-to-austenite phase transformation, it is desirable to set A1 and A3 temperatures, which represent phase transformation onset and end temperatures of the steel during heating, as the lower and upper limits of the first heating temperature. When the temperature falls below the A1 temperature during the first heating, the steel will undergo phase transformation during the second heating, making it difficult to suppress the segregation of alloying elements at corner and surface portions of the slab. Conversely, when the temperature exceeds the A3 temperature during the first heating, the segregation of alloying elements already formed at the slab corners and surface may be exacerbated during the second heating. According to an embodiment of the present disclosure, a lower limit of the heating temperature (Tp) during the first heating may be 800°C. According to an embodiment of the present disclosure, an upper limit of the heating temperature (Tp) during the first heating may be 1000°C.Second Heating

[0076] The first heated steel slab may be reheated at a temperature ranging from 1100°C to 1350°C.

[0077] When the reheating temperature is less than 1100°C during the second heating, precipitates will not be sufficiently re-dissolved, reducing precipitate formation in subsequent processes after hot-rolling. This will result in retention of coarse TiN. Furthermore, the slab will not be sufficiently heated, making it difficult to consistently control a temperature of the steel plate during hot-rolling. Conversely, when the temperature exceeds 1350°C, austenite grain growth may lead to a decrease in strength.Hot-Rolling

[0078] The second heated steel slab may be hot-rolled at a temperature ranging from 800 to 1150°C.

[0079] During hot-rolling, when the temperature exceeds 1150°C, a temperature of the hot-rolled steel plate increases, resulting in coarser grains and poor surface quality. Conversely, when the temperature is less than 800°C, excessive recrystallization delays may lead to the development of elongated grains, exacerbating anisotropy and deteriorating formability. According to an embodiment of the present disclosure, conditions for the heating and hot-rolling processes may not be particularly limited, but may be performed in a process that directly combines continuous casting and hot-rolling.First Cooling

[0080] The hot-rolled steel plate may be first cooled to a temperature within a range of 300 to 400°C at an average cooling rate of 50 to 100°C / s.

[0081] During the first cooling, when the average cooling rate is less than 50°C / s, a ferrite fraction may become excessively high, making it difficult to secure the properties targeted by the present disclosure. On the other hand, when the cooling rate exceeds 100°C / s, it may be difficult to control the cooling rate uniformly across the entire steel plate, and variations in the hardness value of the formed hard phase may be likely to occur, which may locally result in poor formability.

[0082] During the first cooling, when the cooling end temperature exceeds 400°C, excessive bainite formation occurs, particularly an increase in upper bainite, making it difficult to achieve target strength. Conversely, when the temperature is less than 300°C, martensite with high hardness increases significantly, resulting in poor formability and a poor shape of the steel plate.Second Cooling

[0083] The first cooled steel plate may be coiled after second cooling at an average cooling rate of 1 to 30°C / s to a temperature within a range of 50 to 200°C.

[0084] During the second cooling, when the cooling end temperature exceeds 200°C, a martensite fraction in the hard phase decreases, making it difficult to secure strength. According to an embodiment of the present disclosure, an upper limit of a cooling end temperature may be 150°C. On the other hand, when the temperature is less than 50°C, excessive martensite formation occurs, resulting in reduced elongation of the steel. Furthermore, residual cooling water may cause corrosion of the steel plate. According to an embodiment of the present disclosure, a lower limit of the cooling end temperature during second cooling may be 70°C.

[0085] When the average cooling rate exceeds 30°C / s during second cooling, the hardness of the hard phase may increase excessively, resulting in reduced elongation. While there is no specific limitation on a lower limit of the average cooling rate, controlling the cooling rate to less than 1°C / s requires a long cooling zone, making it difficult to manufacture at target cooling end temperature.

[0086] A steel plate according to an embodiment of the present disclosure may be pickled and oiled, after coiling. Furthermore, according to an embodiment of the present disclosure, after the pickling process, the steel plate may be heated to a temperature within a range of 450 to 740°C, followed by hot-dip galvanizing. In this case, the plating bath may contain, by wt, Mg: 0.01 to 30%, Al: 0.01 to 50%, and the remainder Zn and unavoidable impurities.Mode for the Invention

[0087] Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples may be intended only to illustrate and further explain the present disclosure and may not be intended to limit the scope of the present disclosure.(Examples)Table 1 below shows the component compositions for each steel grade.

[0088] [Table 1]SteelAlloying Composition (wt%)CSiMnCrMoNbTiCuNiSbAsSnAlP*S*N*B*A0.220.071.00.1000.0600.0150.0250.3200.0900.0070.0060.0070.03110306020B0.220.161.10.1000.0700.0120.0200.1100.1000.1200.0050.0050.03100307020C0.220.201.10.2000.1000.0300.0200.1000.0800.0060.1300.0050.0390307020D0.220.101.10.3000.1000.0200.0200.1200.0500.0050.0070.1400.0480208020E0.220.201.10.3000.1000.0200.0200.1000.1000.0100.0200.0100.0480208020F0.220.031.10.3000.1000.0200.0200.1000.1000.0100.0200.0100.0480208020G0.220.101.50.5000.2000.0250.0300.1000.0800.0100.0200.0100.0390206030H0.220.081.00.0050.0050.0200.0300.0900.0900.0100.0080.0090.0410040604I0.220.101.40.4000.1000.0200.0300.1000.0800.0100.0100.0100.03100306020J0.220.101.00.1800.0200.0020.0030.1000.1300.0100.0100.0100.04110306020K0.220.101.00.1800.0200.0010.0170.0900.1000.0030.0030.0030.0390306020L0.220.101.00.0010.0020.0100.0200.0050.0050.0100.0100.0100.0310030605M0.220.101.00.2000.0050.0030.0150.0300.0110.0150.0060.0080.0390206020N0.220.101.00.1800.0050.0010.0160.1000.1200.0100.0100.0200.03100406020O0.220.081.10.0200.2000.0030.0250.0300.0130.0170.0060.0050.03110206015P0.220.101.10.0100.1900.0010.0230.1000.1000.0100.0100.0100.03120206018Q0.230.081.10.0100.1300.0030.0200.0300.0120.0130.0060.0050.04100306020R0.230.101.10.0100.1400.0020.0210.0900.0900.0200.0080.0070.03100206019S0.220.081.10.1500.2000.0010.0250.0400.0150.0120.0060.0050.03110206020T0.220.101.10.1500.2100.0100.0250.1000.1000.0100.0060.0220.03110206020* Units are ppm.

[0089] Table 2 below shows calculation results of Relationships 1 and 2 for steel grades listed in Table 1. Furthermore, manufacturing conditions for each steel grade were shown. For second heating not shown in Table 2, a heating temperature (final reheating temperature) was 1200°C and applied equally to each steel grade. Additionally, a holding time for first heating was applied consistently at 30 minutes, and a thickness of all steel plates immediately after hot-rolling was maintained at 3.2 mm. [Table 2]SampleSteelAlloying CompositionFirst HeatingHot-RollingFirst CoolingSecond CoolingRelationship 1Relationsh ip 2Relationship 3Temp. (°C)End Temp. (°C)Average Cooling Rate (°C / s)End Temp. (°C)Average Cooling Rate (°C / s)A Valu eR*Ti*T ValueX ValueTp1A16.30.5380.00451.957.08908793706683142B25.40.546-0.00454.564.38008873826465213C18.60.546-0.00471.556.78508853706881154D36.30.530-0.00777.591.88008623506585185E3.00.530-0.00783.917.810808453426365176F3.00.530-0.00781.20.64008503606560127G2.90.8330.009160.67.48008653537072208H3.50.0330.00937.68.98008603676960229I3.00.5330.009110.47.580085034068701810J2.90.540-0.01850.07.480086534065802011K-1.90.546-0.00450.9-5.382087935070781512L3.40.093-0.00134.28.780088533563831813M2.90.548-0.00649.57.380087336072581614N2.30.547-0.00549.86.483088838560621915O3.30.3880.00458.79.082088735475562216P1.90.4800.00257.95.583088533669701617Q2.00.523-0.00148.59.298089237565682518R2.90.5120.00053.28.183089236782811919S2.20.5380.00473.37.990087438562922520T3.50.5380.00481.78.783089138964831821M2.90.548-0.00649.59.186087345358801522M2.90.548-0.00649.59.186086623075751423M2.90.548-0.00649.59.18608663201081061824M2.90.548-0.00649.59.186087238220951825M2.90.548-0.00649.59.186088434560375526M2.90.548-0.00649.59.1860890380622652527M2.90.548-0.00649.59.1860878385602522 A=85Cu+3Sb+2As+3Sn−Ni1.2−4.5×R*R*=300B−1.93Al−Ti*Ti*=Ti−3.42N

[0090] *(In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element.) T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N

[0091] (In the Relationship, [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element.) X = 160 × A × Exp − 37 , 000 / 8.314 × 273 + Tp A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N (In the Relationship, [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, and Tp is a first heating temperature.)

[0092] Table 3 below shows microstructures and properties of each sample. The microstructures formed in the steel, including differentiation and area fractions of ferrite (F), bainite (B), and martensite (M), was measured using Electron Back Scattered Diffraction (EBSD, JEOL JSM-7001F) and Secondary Electron Microscopy (SEM), at magnifications of 3,000 to 5,000x. The microstructure was observed at points 1 / 4 to 1 / 2 in a thickness direction of a cross-section of the steel plate in a rolling direction, and no MA phase or retained austenite was observed. For some examples requiring detailed analysis, the microstructure was additionally observed at points 1 / 8 in a thickness direction from the surface portion.

[0093] In addition, tensile tests were performed on each sample to evaluate properties thereof, including bendability and impact resistance at the bending zone. The tensile tests were performed on ASTM-compliant samples prepared in a direction, parallel to the rolling direction. In this case, TS, YS, and El represent tensile strength, 0.2% offset yield strength, and fracture elongation, respectively. In addition, yield ratios (YR, YS / TS) were calculated and presented. Bendability (R / t) was tested by a 90° bending test on samples prepared in a direction, perpendicular to the rolling direction, where R is a bending radius of the upper die and t is a thickness of the steel plate (mm). The impact resistance was evaluated by freely dropping and impacting a 5 kg weight from a height of 0.5 m perpendicular to the bending portion of a sample that had undergone a 90° bending test with R / t=6.0. The impact test was conducted after maintaining the samples at room temperature, 0°C, -10°C, - 20°C, -30°C, and -40°C for 30 minutes. Temperatures at which cracks occurred in the sample after impact were recorded, and when cracks occurred at a temperature -20°C or less, it was judged to be good. Additionally, an area fraction of surface defects caused by red scale on a steel plate surface was calculated by dividing a defect area by an observed area, based on images identified by an optical surface defect detector (SDD) system. Considering that surface defects caused by red scales typically occur in stripes in a rolling direction, lengths of the defects may be directly measured by drawing a straight line, perpendicular to the rolling direction, on the rolled plate, and a resulting ratio may be calculated as a percentage of a width of the rolled plate. [Table 3]SampleSteelMicrostructure (area%)PropertiesBendingSurface QualityB+MBMFTS (MPa)YS (MPa)YREl (%)Bendabil ity (R / t)Crack Occurrence Temp. (°C)Red Scale Fraction (area%)1A9910891149012780.86103.5033Comparative Ex. 12B9910891150512650.84103.5-1035Comparative Ex. 23C1009910156012800.8294.0041Comparative Ex. 34D1008920157213160.8494.0-1032Comparative Ex. 45E1008920156813050.83104.0-1021Comparative Ex. 56F10010900153712920.84104.0-1017Comparative Ex. 67G10001000169213400.7954.5015Comparative Ex. 78H953065513759980.73103.0-4012Comparative Ex. 89I10010900156612480.80103.5-1016Comparative Ex. 910J9913861138511230.82123.0-1015Comparative Ex. 1011K9915841141011400.81123.0-1035Comparative Ex. 1112L965046412809980.78133.0-2016Comparative Ex. 1213M9914851147512200.83122.5-4018Inventive Ex. 114N9911881151011950.79113.0-3013Inventive Ex. 215O999901158512210.77113.0-4014Inventive Ex. 316P1008920155211820.76123.0-4019Inventive Ex. 417Q1008920166213100.79103.5-3013Inventive Ex. 518R1009910162012950.80104.0-308Inventive Ex. 619S9911881157012450.79103.0-4012Inventive Ex. 720T9912871154012300.80113.0-4014Inventive Ex. 821M976037311439900.85103.0-4021Comparative Ex. 1322M1005950163813050.8084.5-2018Comparative Ex. 1423M1009910156512800.8294.5-2016Comparative Ex. 1524M9340537117010110.86103.5O17Comparative Ex. 1625M10010900158012750.8184.5-2021Comparative Ex. 1726M9919800115410580.92123.0O13Comparative Ex. 1827M1007930162013050.8184.5-2015Comparative Ex. 19

[0094] As shown in Table 3, Inventive Examples satisfying conditions of the present disclosure satisfied microstructural characteristics proposed in the present disclosure and also achieved the desired properties.

[0095] FIG. 1 illustrates distribution diagrams of Relationships 1 and 2 of Inventive Examples and Comparative Examples according to an embodiment of the present disclosure.

[0096] FIGS. 2 (a) and 2 (b) respectively illustrate surface photographs of Invention Example 6 and Comparative Example 2 according to an embodiment of the present disclosure. FIGS. 2(a) and 2(b) are photographs of a surface of each sample examined using an optical surface defect analysis system, and FIG. 2(b) shows that stripe-shaped surface defects were observed.

[0097] Meanwhile, Comparative Example 1 failed to satisfy Relationship 1 of the present disclosure, and, specifically, was an example where a Cu amount exceeded the proposed range of the present disclosure. Comparative Example 1 satisfied basic material properties, including yield strength and tensile strength, but exhibited poor bending formability of steel and impact resistance of a bending portion. This was attributed to an excessive amount of Cu, which resulted in segregation at grain boundaries and in a surface portion. Additional microstructural analysis of a 1 / 8 point from the surface portion in a thickness direction, confirmed that the microstructure includes 95% martensite and 5% ferrite. A higher martensite fraction was observed in the surface portion, as compared to the deeper layer, with little bainite and only 5% ferrite. This suggests that localized segregation of hardenable elements such as Cu, Ni, or the like is responsible therefor. Additionally, a size of the microstructure was somewhat uneven, and excessive stripe defects caused by red scales were observed on a surface.

[0098] Comparative Examples 2, 3, and 4 failed to satisfy Relationship 1, and showed severe component segregation in a surface portion, grain boundaries, or the like. Comparative Examples 2, 3, and 4 achieved basic tensile properties and bending formability targeted by the present disclosure, but surface defects due to red scale defects occurred in the surface portion, and low-temperature impact resistance in the bending portion was inferior. As shown in FIG. 2 (b), the surface defects due to the red scale defects were characterized by long stripes occurring in a rolling direction, and in a defective portion, surface wrinkles and microcracks occurred after bending due to a local surface roughness difference formed by the red scale defects, which may be judged to be cause of inferior impact resistance. The microstructure in the surface portion was composed of 88~92% martensite, which was at the same level as a central portion, and there was no difference.

[0099] Comparative Examples 5 and 6 may be examples failed to satisfy Relationship 3, and may be examples that fall outside the first heating temperature range proposed by the present disclosure when reheating a slab. Comparative Example 5 failed to satisfy Relationship 3 proposed by the present disclosure because a heating temperature was excessively high during first heating of a slab. As the heating temperature increases, segregation of segregated elements at grain boundaries and in a surface portion becomes more severe, resulting in poor bending resistance. In addition, Comparative Example 6 failed to satisfy Relationship 3 because a heating temperature was excessively low during first heating. When the heating temperature was lower than a phase transformation initiation temperature, it was difficult to suppress the segregation of alloy elements such as Cu, Sb, As, Sn, or the like in a surface portion of a slab. Therefore, it may be confirmed that Comparative Example 6 has somewhat poor impact resistance of a bending portion.

[0100] Comparative Example 7 may be an example failed to satisfy Relationship 2. Comparative Example 7 contained a relatively high amount of Mn, Cr, Mo, and B, alloying elements that affect hardenability of steel, exceeding the range of Relationship 2 proposed in the present disclosure. Consequently, while satisfying all of the manufacturing conditions proposed in the present disclosure, tensile strength was excessively high, and elongation and bendability were poor. This may be believed to be due to the microstructure consisting solely of martensite, which has a high dislocation density. During bending, a dislocation density further increases in a surface portion, resulting in poor bending formability and impact resistance.

[0101] Comparative Example 8 was deficient in alloying elements that affect hardenability. Cr and Mo were below a lower limit of the range of the present disclosure, failing to satisfy Relationship 2. Consequently, a fraction of martensite, which was a hard phase, decreased, and excessive formation of ferrite, which was a soft phase, occurred in the microstructure. As a result, while elongation and bendability were satisfactory, strength and yield ratio were reduced, making it unsuitable for use as a structure member such as a frame or the like.

[0102] Comparative Example 9, similar to Comparative Example 7, exhibited a slightly higher amount of alloying elements that affect hardenability, exceeding the range of Relationship 2. As a result, Comparative Example 9 exhibited poor impact resistance of a bending portion.

[0103] Comparative Example 10, an example in which neither Nb nor Ti was added, exhibited good bendability, but fell short of the target tensile strength proposed by the present disclosure. Furthermore, impact resistance of a bending portion was also poor. This may be believed to be due to lack of Nb and Ti, which may be effective in grain refinement and homogenization, resulting in an uneven initial microstructure and observation of locally coarse microstructures, resulting in differences in properties.

[0104] Comparative Example 11, an example in which Sb, As, and Sn were not added on the levels proposed by the present disclosure, failed to satisfy Relationship 1. Trace amounts of Sb, As, and Sn detected among steel components shown in Table 1 correspond to trace levels. As a result, Comparative Example 11 exhibited numerous stripe defects caused by red scales on a steel plate surface, and exhibited poor impact resistance of a bending portion.

[0105] Comparative Example 12, an example in which alloying elements affecting hardenability were insufficient, also failed to satisfy Relationship 2. Consequently, in a microstructure, a martensite fraction decreased and a ferrite fraction increased. This resulted in a decrease in strength and yield ratio.

[0106] Comparative Examples 13 and 14 were examples in which a cooling end temperature during first cooling after hot-rolling did not meet the range of the present disclosure. In Comparative Example 13, cooling was terminated at a temperature, higher than the suggested temperature range, resulting in second cooling from a relatively high temperature at a slow cooling rate. As a result, excessive ferrite and bainite were formed during first cooling, and a final microstructure was relatively martensite-deficient, with ferrite exceeding 3%, resulting in a tensile strength below the level suggested by the present disclosure. In Comparative Example 14, first cooling was completed late, resulting in a large portion of a cooling process being rapid cooling, increasing a fraction of a hard phase. This resulted in a high martensite fraction, resulting in insufficient elongation and poor bendability and impact resistance of a bending portion.

[0107] Comparative Examples 15 and 16 were cases where a cooling rate during first cooling after hot-rolling was outside the scope of the present disclosure. In Comparative Example 15, a cooling rate during first cooling exceeded the scope of the present disclosure, resulting in a high fraction of a hard phase, and steel was composed mostly of high-hardness martensite. As a result, elongation was insufficient, and both bendability and impact resistance of a bending portion were poor. In Comparative Example 16, a cooling rate during first cooling was slow, falling short of the scope of the present disclosure. As a result, ferrite formed beyond the range proposed by the present disclosure, and a fraction of a hard phase was relatively reduced, failing to achieve the strength level targeted by the present disclosure.

[0108] Comparative Example 17 was a case where a cooling rate was excessively fast during second cooling, resulting in undercooling and failing to meet the target coiling temperature range. Consequently, a hard phase had a high hardness in a microstructure, resulting in insufficient elongation, poor bendability, and poor impact resistance of a bending portion.

[0109] Comparative Examples 18 and 19 were cases where a coiling temperature exceeded the range of the present disclosure. In Comparative Example 18, a second cooling end temperature exceeded the suggested temperature range. As a result, formed martensite consisted entirely of tempered martensite, and tensile strength was inadequate. Comparative Example 19 was a case where a coiling temperature was excessively low, resulting in insufficient elongation due to a high hardness of a hard phase, and poor bendability and impact resistance of a bending portion.

[0110] While the present disclosure has been described in detail through examples, other embodiments may be possible. Therefore, the technical spirit and scope of the claims set forth below may not be limited to the examples.

Claims

1. A steel plate comprising: by weight, C: 0.10~0.30%, Si: 0.01~0.50%, Mn: 0.5~1.5%, Al: 0.01~0.10%, P: 0.0100~0.0500%, S: 0.0010~0.0100%, N: 0.0010~0.0100%, at least one of Sb: 0.005~0.100%, Sn: 0.005~0.100%, or As: 0.005~0.100%, at least one of Cu: 0.010~0.300%, Cr: 0.010~0.500%, Ni: 0.010~0.200%, Mo: 0.010~0.300%, or B: 0.0010~0.0050%, at least one of Nb: 0.005~0.030% or Ti: 0.005~0.050%, with a remainder of Fe and unavoidable impurities, wherein an A value defined in Relationship 1 below is 10.0 or less, wherein a T value defined in Relationship 2 below is 40.0~100.0, wherein a microstructure includes, by area, 3% or less of ferrite, with a remainder of bainite and martensite, and wherein the steel plate has a 90° bend formability (R / t) of 4.0 or less, and has a temperature of -20°C or less at which cracking occurs in an impact test after 90° bend forming: A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N where [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N where [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element.

2. The steel plate of claim 1, wherein the A value defined in Relationship 1 is 0 or greater.

3. The steel plate of claim 1, wherein the microstructure of the steel plate includes, by area, 97% or greater of the bainite and the martensite, in total.

4. The steel plate of claim 1, wherein the microstructure of the steel plate includes, by area, 50% or greater of martensite.

5. The steel plate of claim 1, wherein a tensile strength of the steel plate is 1400 MPa or greater and a yield ratio of the steel plate is 0.75 or greater.

6. The steel plate of claim 1, wherein an area fraction of defects caused by red scale on a surface of the steel plate is less than 30%.

7. A method of manufacturing a steel plate, comprising: first heating a steel slab containing, by weight, C: 0.10~0.30%, Si: 0.01~0.50%, Mn: 0.5~1.5%, Al: 0.01~0.10%, P: 0.0100~0.0500%, S: 0.0010~0.0100%, N: 0.0010~0.0100%, at least one of Sb: 0.005~0.100%, Sn: 0.005~0.100%, or As: 0.005~0.100%, at least one of Cu: 0.010~0.300%, Cr: 0.010~0.500%, Ni: 0.010~0.200%, Mo: 0.010~0.300%, or B: 0.0010~0.0050%, at least one of Nb: 0.005~0.030% or Ti: 0.005~0.050%, with a remainder of Fe and unavoidable impurities, wherein an A value defined in Relationship 1 below is 10.0 or less, and wherein a T value defined in Relationship 2 below is 40.0~100.0, to a first heating temperature range in which an X value defined in Relationship 3 below satisfies 1.0~10.0; second heating the firstly heated steel slab; hot-rolling the secondarily heated steel slab; first cooling the hot-rolled steel plate to a temperature within a range of 300 to 400°C at an average cooling rate of 50 to 100°C / s; and second cooling the firstly cooled steel plate to a temperature within a range of 50 to 200°C at an average cooling rate of 1 to 30°C / s, and then coiling the second cooled steel plate: A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N where [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, T = 33.02 × 1.07 × C / 10 ∧ 0.5 × 0.7 Si + 1 × 5 Mn + 1 × 2.16 Cr + 1 × 3 Mo + 1 × 0.36 Ni + 1 × 0.35 Cu + 1 − 10 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N where [C], [Si], [Mn], [Cr], [Mo], [Ni], [Cu], [B], [Al], [Ti], and [N] are weight percentages of each element, X = 160 × A × Exp − 37 , 000 / 8.314 × 273 + Tp A = 85 Cu + 3 Sb + 2 As + 3 Sn − Ni 1.2 − 4.5 × R * R * = 300 B − 1.93 Al − Ti * Ti * = Ti − 3.42 N where [Cu], [Sb], [As], [Sn], [Ni], [B], [Ti], and [N] are weight percentages of each element, and Tp is a first heating temperature.

8. The method of claim 7, wherein the first heating is performed at a temperature within a range of 800 to 1000°C.

9. The method of claim 7, wherein the first heating has a holding time of 30 to 90 minutes.

10. The method of claim 7, wherein the second heating is performed at a temperature within a range of 1100 to 1350°C, and the hot-rolling is performed at a temperature within a range of 800 to 1150°C.

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