Steel plate and its manufacturing method

Abstract

The present invention relates to a high-strength steel sheet used for automobile structural members and the like, and more particularly to a steel sheet having excellent formability and fracture resistance, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet used for automobile structural members and the like, and more particularly to a steel sheet having excellent formability and fracture resistance, and a method for producing the same. [Background technology]

[0002] In recent years, as environmental and safety regulations in the automotive industry have become increasingly strict, carbon dioxide emission and fuel economy regulations have also become increasingly stringent. The Insurance Institute for Highway Safety in the United States has been gradually strengthening crash safety regulations for passenger protection, and since 2013 has required severe crash performance such as a 25% small overlap.

[0003] The only solution to these environmental and safety issues is to reduce the weight of automobiles. To achieve this, steel materials must be made stronger, and they must also have excellent formability. Furthermore, to ensure the crashworthiness of the vehicle body, they must also have excellent fracture resistance.

[0004] Generally, high-strength automotive materials can be classified into precipitation-hardened steel, bake-hardened steel, solid-solution-hardened steel, transformation-hardened steel, and the like.

[0005] Examples of transformation strengthened steels include dual phase steel (DP steel), complex phase steel (CP steel), transformation induced plasticity steel (TRIP steel), etc. These transformation strengthened steels are also called advanced high strength steels (AHSS).

[0006] Among these, DP steel is a steel that ensures high strength by finely and uniformly dispersing hard martensite in soft ferrite, CP steel is a steel that contains two or three phases (ferrite, martensite, and bainite) and is added with precipitation hardening elements such as Ti and Nb to improve strength, and TRIP steel is a steel that contains finely and uniformly dispersed retained austenite, which transforms into martensite during room temperature processing, ensuring high strength and high ductility.

[0007] Meanwhile, there is a growing demand for stronger steel sheets for automobiles in order to improve fuel efficiency and durability, and there has been an increasing use of ultra-high strength steel sheets with a tensile strength of 980 MPa or more for the body structure and as reinforcement materials for the sake of collision safety and passenger protection.

[0008] In particular, to improve the crashworthiness of vehicle bodies, high-strength steel with excellent yield strength is used for structural components such as members, seat rails, pillars, etc. A structural component has the advantage of being more crash-resistant the higher its yield strength (YS) to tensile strength (TS), i.e., the higher its yield ratio (YR = YS / TS).

[0009] However, in general, as the strength of steel plate increases, its ductility decreases, resulting in problems such as reduced formability and workability, and there is currently a need to develop materials that can compensate for this.In other words, to simultaneously ensure collision safety and part formability, it is essential to develop materials that have high yield strength and excellent ductility.

[0010] To increase the yield ratio (YR) of steel, the yield strength relative to the tensile strength must be increased. A typical method for achieving this is to use water cooling during continuous annealing. This involves soaking the steel in water during the annealing process, forming martensite, and then performing a tempering process to produce steel sheets with a tempered martensite microstructure. However, this method has serious drawbacks, including poor shape quality due to temperature variations in the width and length directions during water cooling, which can lead to cracks during forming, poor workability, and uneven material properties across different positions.

[0011] As a prior art related to the above technology, Patent Document 1 discloses a martensitic steel material with a martensite volume fraction of 80 to 97%, which is produced by subjecting a steel material containing 0.18% or more of carbon (C) to continuous annealing, water cooling to room temperature, and then overaging at a temperature of 120 to 300° C. for 1 to 15 minutes. When ultra-high strength steel is produced using this method of tempering after water cooling, the yield ratio is very high, but problems arise in that the shape quality of the coil deteriorates due to temperature unevenness in the width and length directions, and problems arise such as the occurrence of cracks during forming and reduced workability.

[0012] Patent Document 2 discloses a method for producing a high-tensile steel sheet having a composite structure mainly composed of martensite, in which fine precipitated copper particles with a particle size of 1 to 100 nm are dispersed within the structure to improve workability. However, adding an excessive amount of Cu (2 to 5 wt%) to precipitate the fine copper particles may cause red shortness due to Cu, resulting in excessive increases in production costs.

[0013] On the other hand, Patent Document 3 discloses a precipitation-hardened steel sheet having a ferrite matrix and containing 2 to 10% by area of ​​pearlite, and aims to improve strength through precipitation hardening and grain refinement by adding carbonitride-forming elements such as Nb, Ti, and V. While this steel sheet has good hole expandability, there is a limit to how much tensile strength can be increased, and it has a problem of cracking during press forming due to its high yield strength and low ductility.

[0014] Patent Document 4 discloses a method for producing a cold-rolled steel sheet that simultaneously secures high strength and high ductility by utilizing tempered martensite and has an excellent sheet shape after continuous annealing. However, the carbon content is high at 0.2% or more, which results in poor weldability, and the high Si content may cause dent defects in the furnace. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Publication No. 4-289120 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-264176 [Patent Document 3] Korean Patent Publication No. 2015-0073844 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-090432 Summary of the Invention [Problem to be solved by the invention]

[0016] An embodiment of the present invention provides a steel sheet suitable for use in automobile structural members, which has not only high strength but also excellent formability and fracture resistance, and a method for manufacturing the same.

[0017] The object of the present invention is not limited to the above-mentioned content, but can be understood from the entire content of this specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention. [Means for solving the problem]

[0018] One aspect of the present invention is a steel sheet containing, by weight, 0.1 to 0.2% carbon (C), 0.5 to 1.3% silicon (Si), 0.5% or less (excluding 0%) aluminum (Al), 1.9 to 3.0% manganese (Mn), 0.3% or less molybdenum (Mo), 1% or less chromium (Cr), 0.1% or less phosphorus (P), 0.1% or less sulfur (S), and the balance being Fe and other unavoidable impurities, The C, Si, Al, Mn, Cr, and Mo satisfy the following relational formula 1: The microstructure is, in area %, ferrite: 10 to 35%, retained austenite: 3 to 15%, fresh martensite: 20% or less (excluding 0%), and the remainder includes one or more of tempered martensite and bainite. The fresh martensite relates to a steel plate in which the fraction of fresh martensite in which the interphase distance of the fresh martensite is three times or more the grain size of the fresh martensite is 30% or more.

[0019] [Equation 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

[0020] The fraction of fresh martensite may satisfy the following relational expression 2.

[0021] [Equation 2] (FM TM+B / FM T )×100≧80% (Here, FM T is the total fraction of fresh martensite, and FM TM+Bmeans the fraction of fresh martensite that is in contact with tempered martensite or bainite among the fraction of fresh martensite.)

[0022] The steel sheet may have a yield strength (YS), a uniform elongation (U-El), and a tensile strength (TS) that satisfy the following relational expression 3:

[0023] [Equation 3] YS×U-El / TS≧6

[0024] The steel sheet may have a yield strength (YS), a post uniform elongation (P-El), and a tensile strength (TS) that satisfy the following relational expression 4.

[0025] [Equation 4] YS×P-El / TS≧3

[0026] The steel sheet may further contain boron (B): 0.01% or less.

[0027] The steel plate may further contain one or more of titanium (Ti): 0.05% or less and niobium (Nb): 0.05% or less.

[0028] The steel sheet may further include a zinc-based plating layer.

[0029] Another aspect of the present invention is a method for producing a steel slab containing, by weight, 0.1 to 0.2% carbon (C), 0.5 to 1.3% silicon (Si), 0.5% or less (excluding 0%) aluminum (Al), 1.9 to 3.0% manganese (Mn), 0.3% or less molybdenum (Mo), 1% or less chromium (Cr), 0.1% or less phosphorus (P), 0.1% or less sulfur (S), and the balance being Fe and other unavoidable impurities, wherein the C, Si, Al, Mn, Cr, and Mo satisfy the following relational expression 1: Finish hot rolling the reheated steel slab at a temperature range of Ar3 or higher to produce a hot-rolled steel sheet; Coiling the hot-rolled steel sheet at a temperature in the range of 400 to 700 ° C.; After the coiling, cooling the hot-rolled steel sheet to room temperature at a cooling rate of 0.1 ° C. / s; After the cooling, the hot-rolled steel sheet is cold-rolled at a total reduction rate of 30 to 80% to produce a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of Ac1+30°C to Ac3+30°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to 450 to 700 ° C. at a cooling rate of 10 ° C. / s or less; After the primary cooling, a secondary cooling step is performed to cool the material to 250 to 500 ° C. at a cooling rate of 5 ° C. / s or more. The second-cooled cold-rolled steel sheet is reheated to a temperature of 490°C or less and maintained for 20 seconds or more, The method relates to a steel sheet manufacturing method in which the cold rolling is carried out at a cumulative reduction rate of 25% or more in the first 1st to 3rd stands.

[0030] [Equation 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

[0031] The continuous annealing may be carried out at a temperature of 800 to 880°C.

[0032] The cooling rate during the secondary cooling may be faster than the cooling rate during the primary cooling.

[0033] The secondary cooling may be performed in a hydrogen quenching facility using hydrogen (H2) gas.

[0034] After the secondary cooling, the method may further include a step of holding for 30 seconds.

[0035] After the reheating and holding, the method may further include a step of hot dip galvanizing in a coating bath at 430 to 490°C.

[0036] After the hot dip galvanizing, the method may further include a step of performing an alloying heat treatment.

[0037] After the alloying heat treatment, the method may further include a step of cooling to a temperature of Ms to 100° C. at a cooling rate of 5° C. / s or more, and then performing temper rolling of less than 2%. [Effects of the Invention]

[0038] According to the present invention, a steel sheet having a high strength of 980 MPa or more and excellent formability is provided, which prevents defects that occur during part processing and enables the manufacture of various parts with complex shapes. In addition, the fracture resistance is improved, which contributes to improving the stability of parts and vehicles.

[0039] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0040] [Figure 1] 1 is a graph showing the change in Relational Expression 3 according to Relational Expression 1 in the results of an example of the present invention. [Figure 2] 1 is a graph showing the change in Relational Expression 4 according to Relational Expression 1 in the results of an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0041] The terms used herein are for the purpose of describing the invention and are not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly indicates otherwise.

[0042] As used herein, "comprises" embodies features and does not exclude the presence or addition of other features.

[0043] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0044] The inventors of the present invention have conducted extensive research to provide a high strength steel sheet having excellent formability and fracture resistance.

[0045] As a result, the inventors confirmed that by optimizing the alloy composition system and manufacturing conditions of the steel, it is possible to obtain a structure that is advantageous for ensuring the target physical properties, and that it is possible to provide a steel sheet suitable for use in automobile structural components that require processing into complex shapes, and thus completed the present invention.

[0046] The present invention will be described in detail below.

[0047] First, the alloy composition of the steel sheet according to one embodiment of the present invention will be described in detail.

[0048] Unless otherwise specified in the present invention, the content of each element is based on weight, and the proportion of the structure is based on area.

[0049] The steel plate may contain carbon (C): 0.1 to 0.2%, silicon (Si): 0.5 to 1.3%, aluminum (Al) 0.5% or less (excluding 0%), manganese (Mn): 1.9 to 3.0%, molybdenum (Mo): 0.3% or less, chromium (Cr): 1% or less (excluding 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.1% or less, the balance being Fe and other unavoidable impurities.

[0050] Carbon (C): 0.1~0.2% C is a very important element added to strengthen the transformed structure in steel. C increases the strength of steel and promotes the formation of martensite in dual-phase steel. As the C content increases, the amount of martensite increases.

[0051] If the C content exceeds 0.2%, the strength increases due to the formation of martensite, but the strength difference with ferrite, which has a low carbon concentration, becomes large. This strength difference easily causes fracture at the interphase interface during plastic deformation, resulting in problems of reduced ductility and work hardening rate. Furthermore, poor weldability leads to welding defects during part processing, and LME (Liquid Metal Embrittlement) cracks during welding, impairing part performance. On the other hand, if the C content is less than 0.1%, it is difficult to achieve the target level of strength and to ensure the required fraction of retained austenite required for ductility. The C content is more preferably 0.10 to 0.20%, and even more preferably 0.12 to 0.18%.

[0052] Silicon (Si): 0.5 to 1.3% The Si element is a ferrite stabilizing element that promotes ferrite transformation and contributes to the formation of martensite by promoting the enrichment of carbon (C) in untransformed austenite. Furthermore, Si has excellent solid solution strengthening properties and is effective in increasing the strength of ferrite and reducing the hardness difference between phases. Furthermore, Si effectively suppresses the precipitation of carbides in bainite when retained in the bainite region, thereby promoting the enrichment of C in untransformed austenite and delaying martensitic transformation during low-temperature rapid cooling, thereby forming retained austenite necessary for ductility. Therefore, Si is a useful element for improving the ductility of steel sheets. In other words, Si is a useful element that can ensure the strength of steel sheets without reducing their ductility.

[0053] If the Si content exceeds 1.3%, it can cause surface scale defects, adversely affecting the surface quality of the plating, impairing phosphatability, and poor weldability, resulting in welding defects during part processing. In particular, LME cracks can occur during welding, reducing part performance. On the other hand, if the Si content is less than 0.5%, it is difficult to ensure the required fraction of retained austenite necessary for ductility, and the poor solution hardening ability reduces the strength of ferrite, limiting the reduction in the hardness difference between phases and reducing formability. The Si content is more preferably 0.50 to 1.30%, and even more preferably 0.7 to 1.2%.

[0054] Aluminum (Al) 0.5% or less (excluding 0%) The aluminum, more preferably acid-soluble aluminum (Sol. Al), is an element added to refine the grain size and deoxidize steel, and, like Si, is a ferrite stabilizing element. Al is an element useful for distributing carbon in ferrite to austenite to improve the hardenability of martensite. Furthermore, by effectively suppressing the precipitation of carbides in bainite when maintaining the steel in the bainite region during annealing, Al promotes the enrichment of C in untransformed austenite, delaying martensitic transformation during low-temperature rapid cooling, and generating a retained austenite phase, thereby improving the ductility of the steel sheet.

[0055] If the Al content exceeds 0.5%, excessive inclusions may be formed during continuous casting of steel, increasing the likelihood of surface defects on the surface of the steel sheet and increasing manufacturing costs. Furthermore, poor weldability may result in welding defects during part fabrication. Therefore, the Al content should be 0.5% or less, excluding 0%. It is more advantageous for the Al content to be 0.50% or less.

[0056] Manganese (Mn): 1.9-3.0% Mn is an effective element for strengthening steel, as it refines grains without reducing ductility, completely precipitates sulfur (S) in steel as MnS, preventing hot embrittlement due to the formation of FeS, and also facilitates the formation of martensite in dual-phase steels by lowering the critical cooling rate at which the martensite phase is obtained.

[0057] If the Mn content is less than 1.9%, it is difficult to achieve the strength targeted in the present invention. On the other hand, if the Mn content exceeds 3.0%, problems such as weldability and hot rolling are likely to occur, and not only is the excessive formation of martensite making the material unstable, but also the formation of manganese oxide bands (Mn-Bands) in the structure increases the risk of defects such as processing cracks and sheet breakage. In addition, Mn oxides dissolve onto the surface during the annealing process, significantly degrading surface quality.

[0058] Therefore, in the present invention, the Mn content is 1.9 to 3.0%, more advantageously 1.90 to 3.00%, and even more advantageously 2.0 to 2.8%.

[0059] Molybdenum (Mo): 0.3% or less Mo is an element selectively included to delay the transformation of austenite to pearlite and to refine ferrite and improve strength. Mo has the advantages of improving the hardenability of steel, forming fine martensite at grain boundaries, and enabling control of the yield ratio. However, Mo is an expensive element, and the higher its content, the higher the manufacturing cost, which is economically disadvantageous.

[0060] To fully achieve the above-mentioned effects, the Mo content is added in an amount of up to 0.3%. If the Mo content exceeds 0.3%, the alloy cost rises sharply, reducing the economic viability, and the grain refinement effect and solid solution strengthening effect occur excessively, which in turn reduces the ductility of the steel. It is more advantageous for the Mo content to be 0.30% or less.

[0061] Chromium (Cr): 1.0% or less (excluding 0%) Cr is an element added to improve the hardenability of steel and ensure high strength, and plays an important role in the formation of martensite. It also minimizes the decrease in elongation rate relative to the increase in strength, which is advantageous for producing dual-phase steel with high ductility. In particular, Cr is added during the hot rolling process. 23 It forms Cr-based carbides such as C6, some of which dissolve during the annealing process and some of which remain undissolved. This allows the amount of dissolved C in martensite to be controlled below an appropriate level after cooling, which suppresses the occurrence of yield point elongation (YP-El) and is advantageous for producing dual-phase steels with low yield ratios.

[0062] If the Cr content exceeds 1.0%, not only will the above-mentioned effects saturate, but the hot-rolling strength will excessively increase, resulting in a problem of poor cold-rolling properties. Furthermore, excessive formation of Cr-based carbides will cause coarsening, resulting in coarsening of the martensite size after annealing, leading to a problem of reduced elongation. It is more advantageous for the Cr content to be 1.00% or less.

[0063] Phosphorus (P): 0.1% or less P is a substitutional element with a significant solid-solution strengthening effect, improving in-plane anisotropy without significantly impairing formability, and is the most advantageous element for ensuring strength. However, excessive P content significantly increases the likelihood of brittle fracture, potentially causing slab breakage during hot rolling and impairing the surface properties of the coating. Therefore, P should be contained in a range of 0.1% or less, and 0% can be excluded in consideration of the level of P unavoidably added during the steel manufacturing process.

[0064] Sulfur (S): 0.1% or less S is an unavoidably added impurity in steel and an element that impairs ductility and weldability, so it is advantageous to control its content as low as possible. In particular, since it is highly likely to cause red shortness, it is preferable to control its content to 0.01% or less. However, 0% can be excluded in consideration of the level of S that is inevitably added during the steel manufacturing process.

[0065] Boron (B): 0.01% or less B is an element that delays the transformation of austenite to pearlite during the cooling process during annealing, and is selectively included to ensure hardenability by suppressing the formation of ferrite and promoting the formation of martensite. If the B content exceeds 0.01%, there is a problem that B is excessively concentrated on the steel surface, deteriorating plating adhesion, etc. Therefore, B is included at 0.01% or less. It is more advantageous that the B content is 0.010% or less.

[0066] On the other hand, in addition to the above components, one or more of titanium (Ti): 0.05% or less and niobium (Nb): 0.05% or less may be further contained.

[0067] Ti and Nb are elements that are effective in increasing the strength of steel and refining grains by forming nano-precipitates. When these elements are added, they combine with carbon to form very fine nano-precipitates, which strengthen the matrix structure and reduce the hardness difference between phases.

[0068] When adding Ti and Nb, if the content exceeds 0.05% each, there are problems such as an increase in manufacturing costs and a significant decrease in ductility due to the formation of excessive precipitates. Therefore, when adding one or more of Ti and Nb, each is contained in an amount of 0.05% or less. It is more advantageous that the content of Ti and Nb is 0.050% or less.

[0069] The remaining component is iron (Fe). However, since unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to exclude them. Since these impurities would be known to anyone skilled in normal manufacturing processes, the contents of all of them will not be specifically mentioned in this specification.

[0070] In the steel sheet, it is effective that, among the alloy compositions described above, C, Si, Al, Mn, Cr, and Mo satisfy the following relational expression 1.

[0071] [Equation 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

[0072] When the above-mentioned relational expression 1 is satisfied, the amount of retained austenite, which contributes to ductility, can be sufficiently secured. Furthermore, by optimizing the manufacturing process, the interphase distance of the extremely hard fresh martensite phase, which is subject to significant local stress concentration during press forming or collision, can be increased, and its distribution can be finely and uniformly dispersed around the tempered martensite and bainite structures. This relieves local stress concentration in the fresh martensite and delays the formation and coalescence of voids around the fresh martensite, thereby ensuring excellent formability without cracking during press forming. Furthermore, fracture resistance can be improved by delaying fracture during a vehicle collision. On the other hand, when the above-mentioned relational expression 1 is not satisfied, the fraction of retained austenite cannot be sufficiently secured, and fine fresh martensite cannot be uniformly distributed, making it difficult to ensure formability and fracture resistance. It is more advantageous for the above-mentioned relational expression 1 to be 0.70 or less.

[0073] Next, the microstructure of the steel sheet of the present invention will be described in detail. The microstructure may contain, in area fraction, 10 to 35% ferrite, 3 to 15% retained austenite, 20% or less (excluding 0%) fresh martensite, and the remainder being one or more of tempered martensite and bainite.

[0074] The ferrite is very important for ensuring formability and strength. As the ferrite fraction increases, carbon accumulates in untransformed austenite, lowering the martensite transformation temperature below room temperature and contributing to ensuring retained austenite at room temperature. Furthermore, ferrite is easily transformed into a soft phase, which contributes to ensuring ductility. Therefore, it is effective for the ferrite content to be 10% or more. If the ferrite fraction exceeds 35%, the strength decreases and the desired strength cannot be achieved. If the ferrite fraction is less than 10%, the ductility-contributing effect of ferrite itself cannot be expected, and the accumulation of carbon in austenite is inhibited, making it difficult to achieve the desired retained austenite fraction.

[0075] Furthermore, in the present invention, when bainite is formed or the formed martensite is tempered, carbon introduced into the bainite or martensite migrates to and accumulates in the surrounding untransformed austenite, lowering the martensite transformation temperature below room temperature and ensuring retained austenite at room temperature. When the retained austenite is 3% or more, transformation-induced plasticity occurs during forming, which is advantageous for ensuring the ductility of the steel sheet. However, if the retained austenite is too high, plated steel sheets tend to be vulnerable to liquid metal embrittlement during spot welding in the assembly of automotive parts, so it is effective not to exceed 15%.

[0076] The fresh martensite is very effective in ensuring strength, but if its fraction exceeds 20%, the fresh martensite cannot be finely and uniformly dispersed around the tempered martensite or bainite structure. When fresh martensite is concentrated, its high hardness causes a large local stress concentration during forming or impact, which can cause brittleness and fracture. In other words, formability is reduced.

[0077] It is effective that the remainder of the microstructure is one or more of tempered martensite and bainite. When at least one of these phases is contained in an amount of 40% or more, retained austenite, which concentrates carbon in untransformed austenite and contributes to ductility, can be generated in a fraction of 3 to 15%, and the fraction of fresh martensite can be controlled to 20% or less. This allows the fresh martensite to be finely and uniformly dispersed around the tempered martensite or bainite structure. However, if the fraction is less than 40%, the desired fraction of retained austenite cannot be ensured, and the fraction of the finally generated fresh martensite exceeds 20%, making it difficult to uniformly and finely disperse the fine fresh martensite around the tempered martensite or bainite.

[0078] Specifically, fresh martensite is the microstructure with the highest strength among the microstructure phases. When fresh martensite is finely and uniformly dispersed, local stress concentrations on fresh martensite during deformation during press forming of a part are dispersed and alleviated by neighboring microstructures. This delays the formation and coalescence of pores, resulting in excellent formability without the generation of processing cracks during part formation. On the other hand, when fresh martensite is coarse, uneven, and uneven, local stress concentrations in the fresh martensite are likely to cause the formation and coalescence of pores. This causes processing cracks during part formation and reduces formability. Furthermore, when fresh martensite is finely and uniformly dispersed, local stress concentrations are alleviated, delaying the formation and coalescence of pores, thereby improving crash resistance. However, when fresh martensite is coarse, uneven, and uneven, local stress concentrations in the fresh martensite are likely to cause the formation and coalescence of pores, resulting in poor crash resistance during vehicle collisions.

[0079] In addition, the fresh martensite in the steel sheet may have a fraction of 30% or more of fresh martensite, where the interphase distance (L) of the fresh martensite is three times or more the grain size (d) of the fresh martensite. When the fraction of fresh martensite, where the interphase distance (L) of the fresh martensite is three times or more the grain size (d) of the fresh martensite (L>3d), is 30% or more, the aforementioned local stress concentration on the fresh martensite can be prevented, and the effect of delaying the formation and coalescence of pores can be maximized, thereby improving formability and impact fracture resistance. However, when the fraction is less than 30%, the local stress concentration on the fresh martensite increases, promoting the formation and coalescence of pores, thereby deteriorating formability and impact fracture resistance.

[0080] Meanwhile, it is effective for the fraction of fresh martensite to satisfy the following relational expression 2. This means that realizing a microstructure in which fine fresh martensite is uniformly dispersed around tempered martensite or bainite phases is effective in terms of formability and fracture resistance. When the fraction of fresh martensite in contact with tempered martensite or bainite, as shown in relational expression 2, exceeds 80% of the total fraction of fresh martensite, the uniform dispersion of fine fresh martensite maximizes the effects of mitigating local stress concentration on the fresh martensite and delaying the formation and coalescence of pores, thereby improving formability and impact fracture resistance. However, when this fraction is less than 80%, the uniform dispersion effect of fine fresh martensite disappears, increasing local stress concentration on the fresh martensite and facilitating the formation and coalescence of pores, thereby deteriorating formability and impact fracture resistance.

[0081] [Equation 2] (FM TM+B / FM T )×100≧80% (Here, FM T is the total fraction of fresh martensite, and FM TM+B means the fraction of fresh martensite that is in contact with tempered martensite or bainite among the fraction of fresh martensite.)

[0082] The steel sheet of the present invention not only has a high strength of tensile strength (TS) of 980 MPa or more, a yield strength (YS) of 700 MPa or more, and a total elongation (T-El) of 13% or more, but also has excellent formability and fracture resistance, as the relationship between tensile strength (TS), yield strength (YS), and uniform elongation (U-El) satisfies the following relational expression 3, and the tensile strength (TS), yield strength (YS), and non-uniform elongation (P-El) satisfy the following relational expression 4. In the following relational expressions 3 and 4, the units of YS and TS are MPa, and the units of U-El ​​and P-El are %.

[0083] [Equation 3] YS×U-El / TS≧6

[0084] [Equation 4] YS×P-El / TS≧3

[0085] In the load-displacement diagram of a material during tensile testing, the deformation rate up to fracture is defined as total elongation (T-El), the deformation rate up to the maximum load point is defined as uniform elongation (U-El), and the deformation rate from the maximum load point to fracture is defined as post-uniform elongation (P-El). When a material is deformed, deformation proceeds uniformly throughout the material up to the uniform elongation point. Beyond that point, necking occurs in a certain portion of the material. Therefore, excellent uniform elongation results in excellent formability without necking even at high deformation rates during part forming. As mentioned above, the meaning of Relation 3 is that when the microstructure is controlled to uniformly distribute fine fresh martensite, excellent formability is achieved despite high yield strength. Furthermore, excellent non-uniform elongation requires even more deformation before necking occurs. It is advantageous for Relation 3 to be 6.0 or greater. As mentioned above, the meaning of Equation 4 is that when the microstructure is controlled to uniformly distribute fine fresh martensite, the yield strength is high and non-uniform elongation is excellent, and therefore, the generation and coalescence of pores in the structure is delayed during a vehicle collision, resulting in excellent crash fracture resistance. It is more advantageous for the above Equation 4 to be 3.0 or greater. In summary, when Equations 3 and 4 are satisfied, the steel has high yield strength, yet is excellent in part formability and fracture resistance during a vehicle collision.

[0086] On the other hand, the steel sheet of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.

[0087] Although not particularly limited, the zinc-based plating layer may be a zinc plating layer containing mainly zinc, or may be a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0088] Hereinafter, a method for producing a steel sheet according to another embodiment of the present invention will be described in detail.

[0089] In brief, the present invention can produce a target steel sheet through the steps of [steel slab reheating - hot rolling - coiling - cold rolling - continuous annealing - cooling - reheating and holding], and then further steps such as [hot-dip galvanizing - alloying heat treatment] can be carried out.

[0090] The conditions for each stage are explained in detail below.

[0091] Steel slab heating First, a steel slab that satisfies the above-mentioned alloy composition and Relational Formula 1 can be prepared, and then heated. This process is performed to smoothly carry out the subsequent hot rolling process and to sufficiently obtain the desired physical properties of the steel sheet.

[0092] The heating step is not particularly recommended, and normal heating conditions are sufficient. As a preferred example, the heating step can be performed at a temperature range of 1100 to 1300°C. If the heating temperature is less than 1100°C, friction between the steel sheet and the rolling mill increases, resulting in a sudden increase in the load on the rollers during hot rolling. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for the temperature increase increase, but the amount of surface scale also increases, leading to material loss.

[0093] hot rolling The steel slab heated as described above can be finish hot-rolled at a temperature equal to or higher than the Ar3 transformation point to produce a hot-rolled steel sheet. The hot-rolling conditions are not particularly limited, and the hot-rolling can be performed at a normal hot-rolling temperature. As a preferred example, the finish hot-rolling can be performed in the temperature range of 800 to 1000°C.

[0094] Winding The hot-rolled steel sheet produced as described above can be coiled, and this can be done at a temperature range of 400 to 700°C.

[0095] If the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet becomes excessively high, resulting in a rolling load during subsequent cold rolling. Furthermore, excessive costs and time are required to cool the hot-rolled steel sheet to the coiling temperature, resulting in increased process costs. On the other hand, if the temperature exceeds 700°C, excessive scale is likely to form on the surface of the hot-rolled steel sheet, causing surface defects and deteriorating galvanizability.

[0096] cooling The coiled hot-rolled steel sheet is preferably cooled to room temperature at a cooling rate of 0.1°C / s or less (excluding 0°C / s), where cooling refers to an average cooling rate.

[0097] In this way, by cooling the coiled hot-rolled steel sheet at a constant rate, it is possible to obtain a hot-rolled steel sheet in which carbides that serve as austenite nucleation sites are finely dispersed. That is, fine carbides are uniformly dispersed in the steel during the hot-rolling process, and the carbides dissolve during the subsequent annealing, allowing austenite phase to be finely dispersed and formed in the steel, thereby obtaining a uniformly dispersed fine martensite phase after annealing is completed.

[0098] cold rolling The hot-rolled steel sheet thus coiled can be cold-rolled to produce a cold-rolled steel sheet, and the cold rolling can be performed at a cold reduction rate (total reduction rate) of 30 to 80%.

[0099] In particular, in the present invention, the cumulative reduction rate of the initial stands, preferably stands 1 to 3, during cold rolling is set to 25% or more, thereby increasing the stored energy inside the steel, which acts as a driving force for promoting ferrite recrystallization in the subsequent annealing process, thereby reducing the fraction of unrecrystallized ferrite in the steel.

[0100] When unrecrystallized ferrite is present in steel, deformation and stress are concentrated locally, resulting in poor ductility of the steel, whereas recrystallized ferrite can contribute to improving ductility by alleviating deformation and stress concentration.

[0101] During cold rolling, if the cumulative reduction rate of the first to third stands is less than 25% or if the cold reduction rate (total reduction rate) up to the final stand is less than 30%, it is difficult to achieve the target thickness and also difficult to correct the shape of the steel sheet. Furthermore, the proportion of unrecrystallized ferrite increases, resulting in reduced ductility. On the other hand, if the cold reduction rate up to the final stand exceeds 80%, the strength increases, resulting in increased roll load during cold rolling, and there is a high possibility of cracks occurring at the edges of the steel sheet.

[0102] In the present invention, the cold rolling can be carried out using a rolling mill configured with five or six stands, but it is clear that the present invention is not limited thereto.

[0103] continuous annealing The cold-rolled steel sheet produced as described above is preferably subjected to continuous annealing treatment. As an example, the continuous annealing treatment can be carried out in a continuous alloying hot-dip furnace.

[0104] The continuous annealing step is a process for forming ferrite and austenite phases and decomposing carbon at the same time as recrystallization.

[0105] The continuous annealing treatment is preferably carried out in a temperature range of Ac1+30°C to Ac3+30°C, and more advantageously in a temperature range of 800 to 880°C.

[0106] If the continuous annealing temperature is less than Ac1 + 30°C, not only will sufficient recrystallization not occur, but sufficient austenite formation will be difficult, making it impossible to ensure the target levels of tempered martensite, fresh martensite, and bainite fractions after annealing. On the other hand, if the temperature exceeds Ac3 + 30°C, the austenite grain size will become coarse, making it impossible to uniformly form a fine retained austenite phase around the hard phase. Furthermore, productivity will decrease, and high-temperature annealing will deepen the formation of surface concentrates due to elements such as Si, Mn, and B that reduce the wettability of hot-dip galvanizing, making it impossible to ensure the surface quality of the galvanized steel.

[0107] Gradual Cooling It is preferable to cool the cold-rolled steel sheet subjected to the above-described continuous annealing treatment in stages.

[0108] Specifically, the cooling is preferably performed by cooling to 450 to 670°C at an average cooling rate of 10°C / s or less (excluding 0°C / s) (this cooling is called primary cooling), and then cooling to 250 to 500°C at an average cooling rate of 5°C / s or more (this cooling is called secondary cooling).

[0109] Primary cooling In the present invention, in order to form at least one of tempered martensite and bainite in a fraction of 40% or more as the final structure, the fraction of martensite and bainite must be ensured in the subsequent secondary cooling process. To this end, the primary cooling can be performed at an average cooling rate of 10°C / s or less (excluding 0°C / s) to 450 to 670°C.

[0110] Specifically, if the subsequent secondary cooling is completed below Ms (martensitic transformation start temperature), a relatively large amount of martensite phase can be formed, and for this purpose, it is advantageous to control the end temperature of the primary cooling as low as possible.Also, if the subsequent secondary cooling is completed in the bainite temperature range, a relatively large amount of bainite phase can be formed, and for this purpose, it is advantageous to control the end temperature of the primary cooling as high as possible.

[0111] During the primary cooling, cooling is performed to 450 to 700°C at an average cooling rate of 10°C / s or less (excluding 0°C / s). Preferably, if the subsequent secondary cooling is completed at Ms or less, the primary cooling is performed to a temperature range of 450 to 600°C, and if the subsequent secondary cooling is completed in the bainite temperature range, the primary cooling is performed to a temperature range of 550 to 700°C.

[0112] If the end temperature of the primary cooling is less than 450°C, it will place a heavy load on the equipment for cooling the atmospheric gas in the annealing furnace, and the cooling rate will be too fast, making it difficult to secure a sufficient amount of ferrite phase formed during cooling.On the other hand, if the end temperature is more than 700°C, an excessively high cooling rate will be required during the subsequent cooling (secondary cooling).

[0113] Furthermore, if the average cooling rate during the primary cooling exceeds 10°C / s, carbon diffusion will not occur sufficiently. On the other hand, in consideration of productivity, the primary cooling can be carried out at an average cooling rate of 1°C / s or more.

[0114] Secondary cooling It is preferable to carry out secondary cooling after completing the primary cooling under the above conditions. In this case, the target microstructure can be induced to be formed by controlling the cooling end temperature and cooling rate.

[0115] If the secondary cooling is performed below Ms, quenching martensite is formed, and the lower the temperature, the higher the proportion of quenched martensite, which can lead to improved strength of the steel sheet. Furthermore, as the martensite is tempered to form tempered martensite during the subsequent heat treatment (the reheating process of the present invention), the carbon supersaturated in the martensite is distributed to the surrounding untransformed austenite, increasing the stability of the retained austenite and improving ductility.

[0116] During the secondary cooling, if cooling is performed at a temperature above Ms, the fraction of bainite can be increased. During the bainite transformation process, the effects of Si and Al delay the precipitation of carbides, and carbon is distributed from the bainite to the surrounding untransformed austenite, increasing the stability of the retained austenite and improving ductility.

[0117] If the secondary cooling end temperature is less than 250°C, the fraction of quenched martensite increases excessively, which in turn reduces the fraction of retained austenite, resulting in poor shape of the steel sheet. On the other hand, if the temperature exceeds 500°C, bainite is not sufficiently formed, which reduces the fraction of retained austenite, and the fraction of fresh martensite increases significantly in the subsequent process, resulting in excessively high strength.

[0118] Furthermore, if the average cooling rate during the secondary cooling is less than 5°C / s, the pearlite phase may be formed, and the bainite phase may not be formed to the target level. On the other hand, there is no particular upper limit to the average cooling rate, and an ordinary engineer can select an appropriate rate taking into account the specifications of the cooling equipment. As an example, the cooling rate can be 100°C / s or less.

[0119] Furthermore, the secondary cooling can be performed using a hydrogen cooling facility that uses hydrogen gas (H2 gas). By performing cooling using a hydrogen cooling facility in this way, it is possible to obtain the effect of suppressing surface oxidation that may occur during the secondary cooling. Here, the type of gas used in the hydrogen cooling facility is not limited, but as an example, it can be controlled to 60 to 70% hydrogen (H2) and the remainder nitrogen (N2).

[0120] On the other hand, when cooling stepwise as described above, the cooling rate during the secondary cooling can be set faster than the cooling rate during the primary cooling.

[0121] retention After the secondary cooling is completed, a step of maintaining the cooled temperature range for 30 seconds or more may be further carried out.

[0122] The holding step can provide the effect of tempering martensite or further increasing the amount of bainite transformation. If the holding time is less than 30 seconds, it is difficult to expect the above-mentioned effects.

[0123] Reheating and Holding The microstructure intended in the present invention can be formed by reheating and holding the cold-rolled steel sheet after the stepwise cooling process described above. Specifically, it is preferable to reheat the cold-rolled steel sheet after the second cooling process to a temperature of 490°C or less and hold the temperature for 20 seconds or more.

[0124] By reheating to the above-mentioned temperature and holding it there, the quenched martensite formed in the previous cooling process is transformed into tempered martensite, and bainite transformation also occurs.

[0125] During the tempering process, carbon that was supersaturated in martensite is redistributed to the surrounding untransformed austenite. In addition, if the secondary cooling ends above Ms, the fraction of bainite increases significantly during the reheating and holding process, and the carbon released from bainite during this process is redistributed to the untransformed austenite, improving the stability of the retained austenite and increasing ductility.

[0126] However, if the reheating temperature is excessively high, the carbides in the tempered martensite and bainite become coarse, reducing the strength, and the formation of coarse carbides reduces the effect of carbon redistribution to untransformed austenite, reducing the fraction of retained austenite. As a result, it is difficult to expect an improvement in ductility.

[0127] Therefore, the reheating temperature can be set to 490°C or less, and it is more advantageous to set the reheating temperature to 470°C or less.

[0128] As described above, it is preferable to hold the cold-rolled steel sheet reheated to 490°C or less at that temperature for 20 seconds or more to fully realize the above-mentioned effects. However, if the holding time is excessive, that is, more than 5 minutes, there is a problem that the tempering effect of martensite becomes excessive, resulting in a decrease in strength, and therefore it is preferable that the holding time does not exceed 5 minutes.

[0129] On the other hand, the present invention can provide a plated steel sheet by plating the cold-rolled steel sheet produced as described above.

[0130] Hot-dip galvanizing After the reheating and holding steps as described above, the steel sheet is preferably immersed in a hot-dip galvanized bath to produce a hot-dip galvanized steel sheet.

[0131] At this time, the hot-dip galvanizing can be carried out under normal conditions, for example, in a temperature range of 430 to 490° C. Furthermore, the composition of the hot-dip galvanizing bath is not particularly limited, and it may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0132] alloying heat treatment Furthermore, if necessary, the hot-dip galvannealed steel sheet can be subjected to an alloying heat treatment to obtain a hot-dip galvannealed steel sheet. In the present invention, the conditions for the alloying heat treatment step are not particularly limited, and ordinary conditions may be used. As an example, the alloying heat treatment step can be performed at a temperature range of 480 to 600°C.

[0133] After the hot dip galvanizing or alloying heat treatment, final cooling and temper rolling steps may be further carried out.

[0134] Final cooling Fresh martensite can be further introduced by final cooling the hot-dip galvanized or alloyed heat-treated steel sheet, which is preferably final cooled to a temperature of Ms or lower at a cooling rate of 5°C / s or higher.

[0135] If the cooling rate is less than 5°C / s, the desired level of fresh martensite cannot be achieved during the cooling process. On the other hand, the upper limit of the cooling rate is not particularly limited, but the cooling rate can be set to 50°C / s or less in order to form a predetermined fraction of pressed martensite.

[0136] temper rolling Furthermore, if necessary, the final-cooled hot-dip galvanized steel sheet or galvannealed hot-dip galvanized steel sheet may be temper-rolled to form a large number of dislocations in the steel, thereby further improving bake hardenability. In this case, the rolling reduction is preferably less than 2% (excluding 0%). If the rolling reduction is 2% or more, this is advantageous in terms of dislocation formation, but side effects such as sheet breakage may occur due to limitations in the equipment capacity. [Example]

[0137] The following examples of the present invention will be described. It goes without saying that those skilled in the art can make various modifications to the following examples without departing from the scope of the present invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the appended claims and their equivalents.

[0138] Example 1 Steel slabs having the alloy compositions shown in Table 1 below were heated at a temperature of 1200°C, and then each heated slab was finish hot-rolled at 900°C, which is above the Ar3 transformation point, to produce hot-rolled steel sheets. After pickling, each hot-rolled steel sheet was coiled at the temperature shown in Table 2 and then cooled to room temperature at a cooling rate of 0.1°C / s or less. The cooled hot-rolled steel sheets were then cold-rolled to produce cold-rolled steel sheets.

[0139] During the cold rolling, the cumulative reduction of stands 1 to 3 was set to 25%, and the total reduction was set to 60%.

[0140] Next, each cold-rolled steel sheet was subjected to continuous annealing under the conditions shown in Table 2 below, followed by primary cooling (slow cooling), secondary cooling (rapid cooling), holding, and reheating. After the secondary cooling and holding steps were completed, the steel sheets were reheated to a temperature of 490°C or less and held at that temperature for 30 seconds. The holding step after the secondary cooling was carried out for 30 seconds.

[0141] Next, the steel sheets were subjected to galvanizing treatment in a hot-dip galvanizing bath at 460°C, followed by final cooling to room temperature at a cooling rate of 5°C / s, and then temper rolling at a rolling rate of less than 2% to produce hot-dip galvanized steel sheets. Here, some of the steel sheets were subjected to alloying heat treatment after the galvanizing treatment.

[0142] [Table 1]

[0143] The mechanical and microstructural properties of each sample were then evaluated, and the results are shown in Table 3 below. Tensile tests were performed on each specimen in the L direction according to the DIN standard to evaluate tensile properties. In the load-displacement diagram during the tensile test, the deformation ratio up to fracture was defined as total elongation (T-El), the deformation ratio up to the maximum load point was defined as uniform elongation (U-El), and the deformation ratio from the maximum load point to fracture was defined as post-uniform elongation (P-El). The microstructural fractions were determined by Nital etching the matrix at the 1 / 4th of the thickness of the annealed steel sheets, followed by measurement of the fractions of tempered martensite (TM), bainite (B), ferrite (F), fresh martensite (FM), and retained austenite (A) using FE-SEM, an image analyzer, EBSD, and XRD.

[0144] [Table 2]

[0145] [Table 3]

[0146] In the above Tables 1 to 3, relational expressions 1 to 4 are as follows.

[0147] [Equation 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

[0148] [Equation 2] (FM TM+B / FM T )×100≧80% (Here, FM T is the total fraction of fresh martensite, and FMTM+B means the fraction of fresh martensite that is in contact with tempered martensite or bainite among the fraction of fresh martensite.)

[0149] [Equation 3] YS×U-El / TS≧6

[0150] [Equation 4] YS×P-El / TS≧3

[0151] Meanwhile, FIG. 1 is a graph showing the relationship between Relation 1 and Relation 3 for Invention Steels 1-6 and Comparative Steels 1-5 in the above examples, and FIG. 2 is a graph showing the relationship between Relation 1 and Relation 4 for Invention Steels 1-6 and Comparative Steels 1-5. When Relation 1 of the steel composition in Relation 1 is 0.7 or greater, Relations 3 and 4 are satisfied. As described above, in order to improve formability and impact resistance in the present invention, Relations 3 and 4 must be satisfied. To achieve this, it is effective to ensure the microstructure proposed in the present invention, and for this purpose, it is important to concentrate C in the untransformed austenite. To concentrate C in the untransformed austenite, the steel composition must contain a predetermined amount of C or more, and further, the addition of Si and Al must delay the formation of carbides in the bainite transformation region. Furthermore, while adequate ferrite can be ensured to sufficiently concentrate C in austenite, excessive addition of hardening elements such as Mn, Cr, and Mo inhibits ferrite formation, preventing sufficient C concentration in austenite.

[0152] When the above relational expression 1 is 0.7 or more, the target fractions of ferrite and retained austenite can be ensured, and as a result, the final fresh martensite can also be finely dispersed, thereby increasing the uniform elongation and non-uniform elongation, and as a result, the relational expressions 3 and 4 are also satisfied.

[0153] On the other hand, when the value of the relational expression 1 is less than 0.7, it is not possible to ensure the target fractions of retained austenite and ferrite, and as a result, it is not possible to finely disperse fresh martensite, which reduces the uniform elongation and non-uniform elongation, and therefore the relational expressions 3 and 4 are not satisfied.

[0154] On the other hand, it can be seen from Tables 1 to 3 above that invention steels 1 to 6, which satisfy all of the alloy composition systems and manufacturing conditions proposed in the present invention, have tensile strengths of 980 MPa or more and satisfy relations 3 and 4. Therefore, excellent formability and fracture resistance can be ensured.

[0155] In contrast, it can be seen that comparative steels 1 to 8, which deviate from the composition range or Relational Formula 1 proposed in the present invention or do not satisfy the manufacturing conditions, do not form the intended microstructure and do not ensure the physical properties intended in the present invention.

Claims

1. In weight percent, carbon (C): 0.1 to 0.2%, silicon (Si): 0.5 to 1.3%, aluminum (Al) 0.5% or less (excluding 0%), manganese (Mn): 1.9 to 3.0%, molybdenum (Mo): 0.3% or less, chromium (Cr): 1% or less (excluding 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.1% or less, the balance being Fe and other unavoidable impurities, The C, Si, Al, Mn, Cr, and Mo satisfy the following relational expression 1: The microstructure contains, in area percentage, ferrite: 10 to 35%, retained austenite: 3 to 15%, fresh martensite: 20% or less (excluding 0%), and the remainder includes one or more of tempered martensite and bainite, The fresh martensite has a fraction of 30% or more of fresh martensite, where the interphase distance of the fresh martensite is three times or more the grain size of the fresh martensite. [Relationship 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

2. The steel plate according to claim 1 , wherein the fraction of fresh martensite satisfies the following relational expression 2: [Relationship 2] (FM ＴＭ＋Ｂ / FM Ｔ )×100≧80% (Here, FM Ｔ is the fraction of fresh martensite overall, and FM ＴＭ＋Ｂ means the fraction of fresh martensite that is in contact with tempered martensite or bainite among the fraction of fresh martensite.)

3. The steel sheet according to claim 1, wherein the yield strength (YS), uniform elongation (U-El), and tensile strength (TS) satisfy the following relational expression 3: [Relationship 3] YS×U-El / TS≧6

4. The steel sheet according to claim 1, wherein the yield strength (YS), post uniform elongation (P-El), and tensile strength (TS) satisfy the following relational expression 4: [Relationship 4] YS×P-El / TS≧3

5. The steel plate according to claim 1, further comprising boron (B): 0.01% or less.

6. The steel plate according to claim 1, further comprising one or more of titanium (Ti): 0.05% or less and niobium (Nb): 0.05% or less.

7. The steel sheet according to claim 1 , further comprising a zinc-based plating layer.

8. heating a steel slab containing, by weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.5 to 1.3%, aluminum (Al) 0.5% or less (excluding 0%), manganese (Mn): 1.9 to 3.0%, molybdenum (Mo): 0.3% or less, chromium (Cr): 1% or less (excluding 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.1% or less, the balance being Fe and other unavoidable impurities, wherein the C, Si, Al, Mn, Cr, and Mo satisfy the following relational expression 1, at a temperature range of 1100 to 1300°C; Finish hot rolling the reheated steel slab at a temperature range of Ar3 or higher to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature in the range of 400 to 700°C; After the coiling, cooling the hot-rolled steel sheet to room temperature at a cooling rate of 0.1 ° C. / s; After the cooling, the hot-rolled steel sheet is cold-rolled at a total reduction rate of 30 to 80% to produce a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of Ac1+30°C to Ac3+30°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to 450 to 700°C at a cooling rate of 10°C / s or less; After the primary cooling, a secondary cooling step is performed to cool the material to 250 to 500°C at a cooling rate of 5°C / s or more. reheating the second-cooled cold-rolled steel sheet to a temperature of 490°C or less and holding the temperature for 20 seconds or more; The method for producing a steel sheet, wherein the cold rolling is carried out at a cumulative reduction rate of 25% or more in the first 1st to 3rd stands. [Relationship 1] (10C+Si+Al) / (Mn+1.3Cr+2.7Mo)≧0.7 (Here, each element means its weight content.)

9. The method for producing a steel sheet according to claim 8, wherein the continuous annealing is performed at a temperature of 800 to 880°C.

10. The method for producing a steel sheet according to claim 8 , wherein the cooling rate during the secondary cooling is faster than the cooling rate during the primary cooling.

11. The secondary cooling is performed using hydrogen (H ２ 9. The method for producing a steel sheet according to claim 8, wherein the method is carried out in a hydrogen quenching facility using hydrogen quenching gas.

12. The method for manufacturing a steel sheet according to claim 8, further comprising the step of holding the steel sheet for 30 seconds after the secondary cooling.

13. The method for manufacturing a steel sheet according to claim 8, further comprising the step of hot-dip galvanizing in a coating bath at 430 to 490°C after the reheating and holding.

14. The method for manufacturing a steel sheet according to claim 13, further comprising the step of performing an alloying heat treatment after the hot dip galvanizing.

15. The method for producing a steel plate according to claim 14, further comprising the step of cooling to a temperature of Ms to 100°C at a cooling rate of 5°C / s or more after the alloying heat treatment, and then performing temper rolling of less than 2%.

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