Steel sheet and method for manufacturing same

Abstract

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

Description

Technical Field

[0001] The present invention relates to a high-strength steel plate for automotive structural components and the like. More specifically, the present invention relates to a steel plate having excellent formability and fracture resistance and a method for manufacturing the same. Background Art

[0002] In recent years, as environmental and safety regulations in the automotive industry have become increasingly strict, carbon dioxide emissions regulations and fuel efficiency regulations have also been gradually strengthened. The Insurance Institute for Highway Safety in the United States has gradually strengthened regulations on collision safety for protecting passengers and has required harsh collision performances such as 25% small overlap collisions since 2013.

[0003] The only solution to address such environmental and safety issues is the lightweighting of automobiles. To achieve lightweighting, it is necessary to increase the strength of steel, and excellent formability is also required while ensuring high strength. Moreover, to ensure the collision performance of the vehicle body, excellent fracture resistance is needed.

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

[0005] The transformation-strengthened steels include dual-phase steel (DP steel), complex-phase steel (CP steel), transformation-induced plasticity steel (TRIP steel), and the like. These transformation-strengthened steels are also referred to as advanced high-strength steels (AHSS).

[0006] Among them, DP steel is a steel in which hard martensite is finely and uniformly dispersed in soft ferrite to ensure high strength. CP steel is a steel containing two or three phases of ferrite, martensite, and bainite, and precipitation hardening elements such as Ti and Nb are added to increase the strength. TRIP steel is a steel containing finely and uniformly dispersed retained austenite, and during room-temperature processing, the retained austenite phase is transformed into martensite, thereby ensuring high strength and high ductility.

[0007] In addition, in recent years, steel plates for automobiles are required to have higher strength to improve fuel efficiency, durability, etc. In terms of collision safety and passenger protection, the use of ultra-high-strength steel plates with a tensile strength of 980 MPa or more as vehicle body structures or reinforcements is increasing.

[0008] In particular, in order to improve the crashworthiness of the vehicle body, high-strength steels with excellent yield strength are used for structural members such as members, seat rails, and pillars. The higher the yield strength (YS) of the structural member relative to the tensile strength (TS), that is, the higher the yield ratio (YR = YS / TS), the more beneficial it is to the crashworthiness characteristics.

[0009] However, generally, as the strength of the steel sheet increases, the ductility decreases, so problems such as a decrease in formability and workability occur. Therefore, it is necessary to develop a material that can improve this problem. That is, in order to ensure both collision stability and component formability, it is necessary to develop a material with high yield strength and excellent ductility.

[0010] In order to increase the yield ratio (YR) of steel, it is necessary to increase the yield strength relative to the tensile strength. A representative method to achieve this is to use water cooling during continuous annealing. This is to perform soaking in the annealing process, then immerse in water to form martensite, and then through the tempering process to manufacture a steel sheet with a tempered martensite phase in the fine structure. However, this method has very serious drawbacks such as poor shape quality due to temperature deviation in the width direction and length direction during water cooling, resulting in deteriorated workability such as cracks during forming and causing material deviation at each position.

[0011] As a prior art related to the above technology, Patent Document 1 discloses a martensitic steel. The martensitic steel is obtained by subjecting a steel containing 0.18% or more of carbon (C) to continuous annealing treatment, then water cooling to room temperature, and then performing overaging treatment at a temperature of 120 - 300°C for 1 - 15 minutes, so that the volume ratio of martensite is 80 - 97%. As described above, when manufacturing ultra-high-strength steel by the method of tempering after water cooling, the yield ratio is very high, but due to temperature deviation in the width direction and length direction, the shape quality of the rolled sheet deteriorates, resulting in problems such as cracks during forming and reduced workability.

[0012] Patent Document 2 discloses a method for manufacturing a high-tensile steel sheet. The high-tensile steel sheet is a steel sheet composed of a composite structure with martensite as the main body, and fine precipitated copper particles with a particle size of 1 - 100 nm are dispersed in the structure to improve workability. However, in order to precipitate fine copper particles, 2 - 5 wt% of excessive Cu is added, which may cause red-hot brittleness caused by Cu, and there is a problem of excessive increase in manufacturing cost.

[0013] In addition, Patent Document 3 is a precipitation-strengthened steel sheet with a ferrite matrix structure containing 2-10 area% of pearlite. By adding carbide and nitride forming elements such as Nb, Ti, V, etc., the improvement of strength is promoted through precipitation strengthening and grain refinement. This steel sheet has good hole expansion properties, but there are limitations in increasing the tensile strength, and due to the high yield strength and low ductility, there is a problem of cracking during stamping forming.

[0014] Patent Document 4 discloses a method for manufacturing a cold-rolled steel sheet. The method uses tempered martensite to ensure both high strength and high ductility, and has excellent sheet shape after continuous annealing. However, the carbon content is as high as more than 0.2%, resulting in poor weldability, and a large amount of Si is contained, so furnace dent defects may occur.

[0015] (Patent Document 1) Japanese Unexamined Patent Publication No. 1992-289120

[0016] (Patent Document 2) Japanese Unexamined Patent Publication No. 2005-264176

[0017] (Patent Document 3) Korean Unexamined Patent Publication No. 2015-0073844

[0018] (Patent Document 4) Japanese Unexamined Patent Publication No. 2010-090432 SUMMARY OF THE INVENTION

[0019] (1) Technical Problems to be Solved

[0020] An object of one aspect of the present invention is to provide a steel sheet and a manufacturing method thereof. The steel sheet is suitable for automotive structural components, etc., and not only has high strength, but also has excellent formability and fracture resistance.

[0021] The technical problems of the present invention are not limited to the above. The technical problems of the present invention can be understood from the overall content of this specification, and those skilled in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention.

[0022] (2) Technical Solutions

[0023] One embodiment of the present invention relates to a steel plate which, by weight %, comprises: carbon (C): 0.1 - 0.2%, silicon (Si): 0.5 - 1.3%, aluminum (Al): less than 0.5% (except 0%), manganese (Mn): 1.9 - 3.0%, molybdenum (Mo): less than 0.3%, chromium (Cr): less than 1% (except 0%), phosphorus (P): less than 0.1%, sulfur (S): less than 0.1%, the balance being Fe and other inevitable impurities, and C, Si, Al, Mn, Cr and Mo satisfy the following relational expression 1. The microstructure, by area %, comprises: ferrite: 10 - 35%, retained austenite: 3 - 15%, fresh martensite: less than 20% (except 0%), and the balance is one or more of tempered martensite and bainite. The fraction of fresh martensite in which the interphase distance of fresh martensite is more than 3 times the particle size of fresh martensite is more than 30%.

[0024] [Relational expression 1]

[0025] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo) ≥ 0.7

[0026] (wherein each element represents the weight content.)

[0027] The fraction of the fresh martensite may satisfy the following relational expression 2.

[0028] [Relational expression 2]

[0029] (FM TM+B / FM T ) × 100 ≥ 80%

[0030] (wherein FM T is the total fraction of fresh martensite, and FM TM+B refers to the fraction of fresh martensite that is in contact with tempered martensite or bainite in the fraction of fresh martensite.)

[0031] The yield strength (YS), uniform elongation (Uniform Elongation, U-El) and tensile strength (TS) of the steel plate may satisfy the following relational expression 3.

[0032] [Relational expression 3]

[0033] YS × U-El / TS ≥ 6

[0034] The yield strength (YS), non-uniform elongation (Post Uniform Elongation, P-El) and tensile strength (TS) of the steel plate may satisfy the following relational expression 4.

[0035] [Relational expression 4]

[0036] YS×P-El / TS≥3

[0037] The steel plate may further contain boron (B): 0.01% or less.

[0038] The steel plate may further contain one or more of titanium (Ti): 0.05% or less and niobium (Nb): 0.05% or less.

[0039] The steel plate may further include a zinc-based coating.

[0040] Another embodiment according to the present invention relates to a method for manufacturing a steel plate, which includes the following steps: heating a steel billet in a temperature range of 1100 - 1300 °C, and by weight, the steel billet contains: carbon (C): 0.1 - 0.2%, silicon (Si): 0.5 - 1.3%, aluminum (Al): 0.5% or less (except 0%), manganese (Mn): 1.9 - 3.0%, molybdenum (Mo): 0.3% or less, chromium (Cr): 1% or less (except 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.1% or less, the balance being Fe and other inevitable impurities, and the C, Si, Al, Mn, Cr, and Mo satisfy the following relational expression 1; hot finish rolling the reheated steel billet in a temperature range above Ar3 to manufacture a hot-rolled steel plate; coiling the hot-rolled steel plate in a temperature range of 400 - 700 °C; after the coiling, cooling the hot-rolled steel plate at a cooling rate of 0.1 °C / second to room temperature; after the cooling, cold rolling the hot-rolled steel plate with a total reduction ratio of 30 - 80% to manufacture a cold-rolled steel plate; continuously annealing the cold-rolled steel plate at a temperature of Ac1 + 30 °C to Ac3 + 30 °C; cooling the continuously annealed cold-rolled steel plate at a cooling rate of 10 °C / second or less for the first cooling to 450 - 700 °C; after the first cooling, cooling at a cooling rate of 5 °C / second or more for the second cooling to 250 - 500 °C; and reheating the cold-rolled steel plate after the second cooling to a temperature of 490 °C or less and holding for 20 seconds or more, wherein the cold rolling is performed with a cumulative reduction ratio of 25% or more in the initial No. 1 to No. 3 stands.

[0041] [Relational Expression 1]

[0042] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo)≥0.7

[0043] (wherein each element represents a weight content.)

[0044] The continuous annealing may be performed at a temperature of 800 - 880 °C.

[0045] The cooling rate during the secondary cooling can be faster than that during the primary cooling.

[0046] The secondary cooling can be carried out in a hydrogen rapid cooling device using hydrogen (H2) gas.

[0047] After the secondary cooling, a step of holding for 30 seconds can be further included.

[0048] After the reheating and holding, a step of hot-dip galvanizing in a plating bath at 430 - 490 °C can be further included.

[0049] After the hot-dip galvanizing, a step of performing an alloying heat treatment can be further included.

[0050] After the alloying heat treatment, a step of cooling at a cooling rate of 5 °C / second or more, cooling to a temperature below Ms to 100 °C, and then performing a skin pass rolling of less than 2% can be further included.

[0051] (III) Beneficial Effects

[0052] According to the present invention, by providing a steel sheet having a high strength with a tensile strength of 980 MPa or more and excellent formability, processing defects generated during component processing can be prevented, and thus various components with complex shapes can be manufactured. In addition, the fracture resistance is improved, which can contribute to improving the safety of components and vehicles.

[0053] The various and beneficial advantages and effects of the present invention are not limited to the above, and can be more easily understood during the description of the specific embodiments of the present invention. Description of the Drawings

[0054] Figure 1 It is a graph showing the change of relational expression 3 according to relational expression 1 in the results of the examples of the present invention.

[0055] Figure 2 It is a graph showing the change of relational expression 4 according to relational expression 1 in the results of the examples of the present invention. Best Embodiment

[0056] The terms used in this specification are for explaining the present invention and are not intended to limit the present invention. In addition, unless the relevant definitions indicate the exact opposite meaning, the singular forms used in this specification also include the plural forms.

[0057] The meaning of "comprising" or "including" used in the specification is to specify the constitution and does not exclude the existence or addition of other constitutions.

[0058] Unless otherwise defined, all terms used in this specification, including technical and scientific terms, have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in a dictionary shall be construed to have a meaning consistent with the relevant technical literature and the content disclosed herein.

[0059] The inventors of the present invention have conducted in-depth research to provide a high-strength steel sheet having excellent formability and fracture resistance.

[0060] As a result, by optimizing the alloy composition system and manufacturing conditions of the steel, a structure conducive to ensuring the desired physical properties can be obtained, and it has been confirmed that a steel sheet suitable for use as a structural member for automobiles that need to be processed into complex shapes can be provided, thereby completing the present invention.

[0061] Hereinafter, the present invention will be described in detail.

[0062] First, the alloy composition of the steel sheet according to one aspect of the present invention will be described in detail.

[0063] Unless otherwise specifically stated, in the present invention, the content of each element is based on weight, and the proportion of the structure is based on area.

[0064] The steel sheet may contain: carbon (C): 0.1 - 0.2%, silicon (Si): 0.5 - 1.3%, aluminum (Al): 0.5% or less (except 0%), manganese (Mn): 1.9 - 3.0%, molybdenum (Mo): 0.3% or less, chromium (Cr): 1% or less (except 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.1% or less, and the balance of Fe and other inevitable impurities.

[0065] Carbon (C): 0.1 - 0.2%

[0066] The C is a very important element added to the steel for strengthening the phase transformation structure. This C promotes the high strength of the steel and promotes the formation of martensite in the duplex structure steel. When the content of the C increases, the amount of martensite increases.

[0067] When the content of C exceeds 0.2%, the strength increases due to the formation of martensite, but the strength difference with ferrite having a low carbon concentration increases. This strength difference makes it easy to fracture at the interphase boundary during plastic deformation, so there are problems of reduced ductility and work hardening rate. In addition, due to poor weldability, welding defects are generated during component processing, and liquid metal embrittlement (LME) cracks are generated during welding, thus hindering the performance of the components. Additionally, when the content of C is less than 0.1%, it is difficult to ensure the target level of strength and it is difficult to ensure a specified fraction of retained austenite phase required for ductility. It is more favorable when the content of C is 0.10 - 0.20%, and further favorably, the content of C can be 0.12 - 0.18%.

[0068] Silicon (Si): 0.5 - 1.3%

[0069] The Si is a ferrite stabilizing element that promotes the phase transformation of ferrite and promotes the enrichment of carbon (C) in the untransformed austenite, thus contributing to the formation of martensite. In addition, the solid solution strengthening ability of the Si is excellent, thus effectively reducing the hardness difference between phases by increasing the strength of ferrite. In addition, by effectively suppressing the precipitation of carbides in bainite during holding in the bainite region, it promotes the enrichment of C in the untransformed austenite, thus extending the martensite phase transformation during rapid cooling at low temperature and forming the retained austenite required for ductility. Therefore, the silicon (Si) is an element useful for improving the ductility of the steel plate. That is, the Si is a useful element that can ensure strength without reducing the ductility of the steel plate.

[0070] When the content of this Si exceeds 1.3%, surface scale defects are caused, which has an adverse effect on the surface quality of the coating, hinders the chemical conversion treatability, and has poor weldability, so there is a problem of generating welding defects during component processing. In particular, LME cracks are generated during welding, thus reducing the component performance. On the other hand, when the content of the Si is less than 0.5%, it is difficult to ensure a specified fraction of retained austenite phase required for ductility, and the solid solution strengthening property is poor, reducing the strength of ferrite, so there are limitations in reducing the hardness difference between phases, which may reduce the formability. It is more favorable when the content of the Si is 0.50 - 1.30%, and further favorably, the content of the Si can be 0.7 - 1.2%.

[0071] Aluminum (Al): 0.5% or less (except 0%)

[0072] The aluminum, more preferably acid-soluble aluminum (Sol.Al), is an element added for grain refinement and deoxidation of the steel. Similar to Si, aluminum is a ferrite stabilizing element. The Al is a useful element for improving the hardenability of martensite by distributing carbon in ferrite into austenite. In addition, when the aluminum remains in the bainite region during the annealing process, it can effectively suppress the precipitation of carbides in bainite, promote the enrichment of C into the untransformed austenite, delay the martensite phase transformation during rapid cooling at low temperature, and generate a retained austenite phase, thereby improving the ductility of the steel plate.

[0073] When the content of this Al exceeds 0.5%, excessive inclusions may be formed during the steelmaking continuous casting operation, increasing the possibility of surface defects on the steel plate surface, and there is also a problem of increasing the manufacturing cost. In addition, the weldability is poor, resulting in the possibility of welding defects during component processing. Therefore, the content of the Al can be 0.5% or less, excluding 0%. It is more advantageous when the content of the Al is 0.50% or less.

[0074] Manganese (Mn): 1.9 - 3.0%

[0075] The Mn is an element effective in strengthening the steel while refining the particles without reducing the ductility and completely precipitating sulfur (S) in the steel as MnS to prevent hot brittleness caused by the formation of FeS. In addition, the Mn makes it easier to form martensite by reducing the critical cooling rate for obtaining the martensite phase in the duplex steel.

[0076] When the content of this Mn is less than 1.9%, it is difficult to ensure the strength desired in the present invention. On the other hand, when the content of the Mn exceeds 3.0%, the possibilities of problems such as weldability and hot rolling property are high. Not only is excessive martensite formed and the material is unstable, but also a manganese oxide band (Mn - Band) is formed in the structure, increasing the risk of defects such as processing cracks and plate fracture. In addition, Mn oxides are dissolved out on the surface during the annealing process, thus posing a problem of significantly hindering the surface quality.

[0077] Therefore, in the present invention, the content of the Mn can be 1.9 - 3.0%, more preferably, it can be 1.90 - 3.00%. Further preferably, the content of the Mn can be 2.0 - 2.8%.

[0078] Molybdenum (Mo): 0.3% or less

[0079] Mo is an element that can be optionally included while delaying the transformation of austenite to pearlite and for the purpose of improving the refinement and strength of ferrite. This Mo increases the hardenability of the steel and has the advantage of being able to control the yield ratio by finely forming martensite at grain boundaries. However, Mo is an expensive element, and as the content of the Mo increases, the manufacturing cost increases, so it is economically disadvantageous.

[0080] To fully obtain the above effects, the Mo can be added up to 0.3% at most. When the content of the Mo exceeds 0.3%, it causes a sharp increase in alloy cost, a decrease in economy, and an excessive generation of grain refinement effect and solid solution strengthening effect, resulting in a problem that the ductility of the steel is reduced instead. It is more advantageous when the content of the Mo is 0.30% or less.

[0081] Chromium (Cr): 1.0% or less (except 0%)

[0082] The Cr is an element added to increase the hardenability of the steel and ensure high strength, and plays an important role in the formation of martensite. In addition, it minimizes the decrease in elongation with respect to the increase in strength, which is beneficial for manufacturing a composite structure steel with high ductility. Especially, Cr 23 Cr-based carbides such as C6, a part of the carbide is dissolved during the annealing process, and a part is not dissolved and remains, so that the amount of solid-solution C in the martensite can be controlled below an appropriate level after cooling, suppressing the occurrence of yield point elongation (YP-El), which is beneficial for manufacturing a composite structure steel with a low yield ratio.

[0083] When the content of the Cr exceeds 1.0%, not only do the above effects saturate, but the hot rolling strength increases excessively, resulting in a problem of poor cold rolling property. In addition, since too much Cr-based carbide is formed and coarsens, the martensite size becomes coarse after annealing, resulting in a problem of reduced elongation. It is more advantageous when the content of the Cr is 1.00% or less.

[0084] Phosphorus (P): 0.1% or less

[0085] The P is a substitutional element with a large solid solution strengthening effect, and is the most favorable element for improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, when the content of the P is excessive, the possibility of brittle fracture increases significantly, resulting in the possibility of slab fracture during hot rolling and problems that hinder the plating surface characteristics. Therefore, the content of the P can be 0.1% or less, and 0% can be excluded considering the level inevitably added during the steel manufacturing process.

[0086] Sulfur (S): 0.1% or less

[0087] The S is an impurity inevitably added to the steel and is an element that hinders ductility and weldability. Therefore, it is preferable to control the content of the S at as low a level as possible. In particular, since the possibility of causing red hot brittleness is high, it is preferable to control the content of the S below 0.01%. However, 0% may be excluded in consideration of the level inevitably added during the steel manufacturing process.

[0088] Boron (B): below 0.01%

[0089] The B is an element that delays the phase transformation of austenite to pearlite during the cooling in annealing, and is an element that may optionally be included to suppress the formation of ferrite and to ensure the hardenability for promoting the formation of martensite. When the content of the B exceeds 0.01%, B is excessively enriched on the steel surface, resulting in problems such as a reduction in plating adhesion. Therefore, the content of the B may be 0.01% or less. It is more advantageous when the content of the B is 0.010% or less.

[0090] In addition, in addition to the above components, one or more of titanium (Ti): below 0.05% and niobium (Nb): below 0.05% may be further included.

[0091] The Ti and Nb are elements effective for increasing the strength of the steel and grain refinement caused by the formation of nano-precipitates. When these elements are added, they combine with carbon to form very fine nano-precipitates, and these nano-precipitates play a role in reducing the interphase hardness difference by strengthening the matrix structure.

[0092] When these Ti and Nb are added, when the contents of the Ti and Nb respectively exceed 0.05%, the manufacturing cost increases, and too many precipitates are formed, resulting in a problem of a significant reduction in ductility. Therefore, when one or more of the Ti and Nb are added, the contents of the Ti and Nb may be 0.05% or less respectively. It is more advantageous when the contents of the Ti and Nb are 0.050% or less.

[0093] The remaining component is iron (Fe). However, in the conventional manufacturing process, inevitable impurities may be inadvertently mixed in from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those skilled in the art of ordinary steel manufacturing, so not all of their contents will be specifically described in this specification.

[0094] It is effective that C, Si, Al, Mn, Cr and Mo in the above alloy composition of the steel plate satisfy the following relational expression 1.

[0095] [Relational expression 1]

[0096] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo) ≥ 0.7

[0097] (Wherein, each element represents a weight content.)

[0098] When the relational expression 1 is satisfied, retained austenite that can contribute to ductility can be sufficiently ensured. In addition, through the optimization of the manufacturing process, the spacing between very high-hardness fresh martensite phases with relatively high local stress concentration during stamping or collision can be widened, and it can be distributed finely and uniformly around tempered martensite and bainite structures, alleviating the local stress concentration generated in the fresh martensite, delaying the generation and aggregation of voids generated around the fresh martensite, thereby ensuring excellent formability without crack generation during the stamping process. In addition, fracture can also be delayed during vehicle collision, thereby promoting the improvement of fracture resistance. On the other hand, when the relational expression 1 is not satisfied, the fraction of retained austenite cannot be sufficiently ensured, and the fine fresh martensite cannot be distributed uniformly, making it difficult to ensure formability and fracture resistance. It is more favorable when the relational expression 1 is 0.70 or less.

[0099] Next, the microstructure of the steel sheet of the present invention will be described in detail. The microstructure, in terms of area%, may include: ferrite: 10 - 35%, retained austenite: 3 - 15%, fresh martensite: 20% or less (excluding 0%), and the balance is one or more of tempered martensite and bainite.

[0100] The ferrite is very important for ensuring formability and strength. When the fraction of ferrite increases, carbon accumulates in the untransformed austenite, lowering the martensite transformation temperature below room temperature, thereby contributing to the retention of retained austenite at room temperature. In addition, the ferrite is a soft phase and is easily deformed, so it can contribute to the ensuring of ductility by itself. For this reason, it is more effective when the ferrite is 10% or more. When the fraction of the ferrite exceeds 35%, the strength is reduced, so the required strength cannot be ensured. When the fraction of the ferrite is less than 10%, the ductility contribution effect of the ferrite itself cannot be expected, and it hinders the accumulation of carbon in austenite, making it difficult to obtain the required fraction of retained austenite.

[0101] In addition, in the present invention, by generating bainite, or the generated martensite is tempered, the carbon introduced into the bainite or martensite is transferred to the surrounding untransformed austenite and accumulates, lowering the martensite transformation temperature below room temperature, thereby ensuring retained austenite at room temperature. When 3% or more of the retained austenite is ensured, transformation-induced plasticity is induced during forming, which is beneficial to ensuring the ductility of the steel sheet. However, when there is too much retained austenite, in the case of a plated steel sheet, during the assembly of automotive parts, it tends to be easily affected by liquid metal embrittlement during spot welding, so it is more effective when the retained austenite does not exceed 15%.

[0102] The newly formed martensite is very effective in ensuring strength. However, when its fraction exceeds 20%, it is impossible to disperse the newly formed martensite finely and uniformly around the tempered martensite or bainite structure. When the newly formed martensite is concentrated, due to its high hardness, large local stress concentration may occur during forming or collision, which may cause brittleness or fracture. That is, the formability may be reduced.

[0103] It is more effective when the balance in the fine structure is one or more of the tempered martensite and bainite phases. When the content of at least one of the phases is 40% or more, carbon can be enriched in the untransformed austenite, thereby generating retained austenite with a fraction of 3 - 15% that contributes to ductility, and the fraction of the newly formed martensite can be controlled below 20%. Therefore, the newly formed martensite can be dispersed finely and uniformly around the tempered martensite or bainite structure. However, when its fraction is less than 40%, the required fraction of retained austenite may not be ensured, and the finally generated fraction of the newly formed martensite will also exceed 20%, making it difficult to disperse the fine newly formed martensite evenly and finely around the tempered martensite or bainite.

[0104] Specifically, the newly formed martensite is the fine structure with the highest strength in the above fine structure phases. When the newly formed martensite is dispersed finely and uniformly, during the stamping forming process of the component, when deformation is applied, the local stress concentration applied to the newly formed martensite will be dispersed to the adjacent fine structures and relieved. Therefore, the generation and aggregation of pores are delayed, so that no processing cracks occur during component forming and excellent formability is achieved. On the other hand, when the newly formed martensite is coarse and unevenly distributed, stress will be locally concentrated on the newly formed martensite, thus easily leading to the generation and aggregation of pores. Therefore, processing cracks occur during component forming, reducing the formability. In addition, during vehicle collision, when the newly formed martensite is dispersed finely and uniformly, local stress concentration is relieved. Therefore, the generation and aggregation of pores are delayed, so that the anti-collision fracture resistance can be improved. On the other hand, when the newly formed martensite is coarse and unevenly distributed, stress will be locally concentrated on the newly formed martensite, easily leading to the generation and aggregation of pores, so the fracture resistance during vehicle collision becomes poor.

[0105] In addition, in the steel plate, the fraction of fresh martensite with the interphase distance of fresh martensite being more than 3 times the particle size of fresh martensite can be 30% or more. When the fraction of fresh martensite with the interphase distance (L) of fresh martensite being more than 3 times the particle size (d) of fresh martensite (L>3d) is 30% or more, local stress concentration in the fresh martensite described above can be prevented, and the effect of delaying the generation and aggregation of pores can be maximized, thereby improving formability and anti-collision fracture resistance. However, when the fraction of fresh martensite with the interphase distance (L) of fresh martensite being more than 3 times the particle size (d) of fresh martensite (L>3d) is less than 30%, local stress concentration in the fresh martensite increases, promoting the generation and aggregation of pores, thereby reducing formability and anti-collision fracture resistance.

[0106] In addition, it can be more effective when the fraction of the fresh martensite satisfies the following relational expression 2. This indicates that when a microstructure in which fine fresh martensite is uniformly dispersed around tempered martensite or bainite phase is achieved, it is more effective in terms of formability and fracture resistance. When the fraction of fresh martensite in contact with tempered martensite or bainite mentioned in relational expression 2 exceeds 80% of the total fraction of fresh martensite, through the uniform dispersion effect of fine fresh martensite, the effect of greatly alleviating local stress concentration in fresh martensite and delaying the formation and aggregation of pores can be maximized, thereby improving formability and anti-collision fracture resistance. However, when the fraction of fresh martensite in contact with tempered martensite or bainite mentioned in the relational expression 2 is less than 80%, the uniform dispersion effect of fine fresh martensite disappears, increasing local stress concentration in fresh martensite and making the generation and aggregation of pores easier, thereby reducing formability and anti-collision fracture resistance.

[0107] [Relational expression 2]

[0108] (FM TM+B / FM T )×100≥80%

[0109] (where FM T is the total fraction of fresh martensite, and FM TM+B refers to the fraction of fresh martensite in contact with tempered martensite or bainite in the fraction of fresh martensite.)

[0110] The steel plate of the present invention has a high strength with a tensile strength (TS) of 980 MPa or more, a yield strength (YS) of 700 MPa or more, and a total elongation rate (T-El) of 13% or more. Moreover, the relationship among the tensile strength (TS), yield strength (YS), and uniform elongation rate (U-El) of the steel plate of the present invention satisfies the following relational expression 3, and the relationship among the tensile strength (TS), yield strength (YS), and non-uniform elongation rate (P-El) satisfies the following relational expression 4. Therefore, a steel plate with excellent formability and fracture resistance can be provided. 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 %.

[0111] [Relational expression 3]

[0112] YS × U-El / TS ≥ 6

[0113] [Relational expression 4]

[0114] YS × P-El / TS ≥ 3

[0115] During the tensile test of the material, the deformation rate until fracture in the load-displacement curve is defined as the total elongation rate (Total Elongation, T-El), the deformation rate until the maximum load point is defined as the uniform elongation rate (U-El), and the deformation rate from the maximum load point to fracture is defined as the non-uniform elongation rate (P-El). When deforming the material, the deformation of the entire material will proceed uniformly until the position of the uniform elongation rate. Beyond this position, necking will occur in a certain part of the material. Therefore, when the uniform elongation rate is excellent, during component forming, even at a high deformation rate, it has excellent formability without necking. Relational expression 3 means that when the fine microstructure is controlled to uniformly distribute fine newly formed martensite as described above, even if the yield strength is high, it has excellent formability. In addition, when the non-uniform elongation rate is excellent, even if necking occurs, more deformation should accompany until the final fracture of the material. It is more advantageous when the said relational expression 3 is 6.0 or more. Expression 4 means that when the fine microstructure is controlled to uniformly distribute fine newly formed martensite as described above, the yield strength is high and the non-uniform elongation rate is also excellent. Therefore, during vehicle collision, the generation and aggregation of pores in the tissue are delayed, and thus the anti-collision fracture resistance is excellent. It is more advantageous when the said relational expression 4 is 3.0 or more. Generally speaking, when relational expressions 3 and 4 are satisfied, it means that the steel has excellent component formability during vehicle collision and fracture resistance even with a high yield strength.

[0116] In addition, the steel plate of the present invention can be a cold-rolled steel plate, and can be a hot-dip galvanized steel plate including a zinc-based coating on at least one surface of the cold-rolled steel plate, or an alloyed hot-dip galvanized steel plate obtained by alloying the hot-dip galvanized steel plate.

[0117] Although not particularly limited, the zinc-based coating may be a galvanized coating mainly containing zinc, or a zinc alloy coating containing aluminum and / or magnesium in addition to zinc.

[0118] Hereinafter, a method for manufacturing a steel sheet according to another specific embodiment of the present invention will be described in detail.

[0119] Briefly, the present invention can be manufactured through a process of [reheating of the steel billet - hot rolling - coiling - cold rolling - continuous annealing - cooling - reheating and holding] to manufacture the desired steel sheet, and then processes such as [hot dip galvanizing - alloying heat treatment] can be further performed.

[0120] The conditions for each step will be described in detail below.

[0121] Heating of the steel billet

[0122] First, prepare a steel billet that satisfies the above alloy composition and relational expression 1, and then the steel billet can be heated. This process is carried out to smoothly carry out the subsequent hot rolling process and fully obtain the physical properties of the desired steel sheet.

[0123] The heating process is not particularly limited as long as it is under normal heating conditions. As a preferred example, it can be carried out in the temperature range of 1100 - 1300 °C. When the heating temperature is lower than 1100 °C, the friction between the steel sheet and the rolling mill increases, resulting in a problem that the load on the roll during hot rolling increases sharply. On the other hand, when the heating temperature exceeds 1300 °C, not only does the energy cost required to increase the temperature increase, but also the amount of surface scale increases, which may lead to material loss.

[0124] Hot rolling

[0125] The heated steel billet as described above can be hot finish rolled above the Ar3 transformation point to manufacture a hot rolled steel sheet. At this time, the hot rolling conditions are not particularly limited and can be carried out at a normal hot rolling temperature. As a preferred example, the hot finish rolling can be carried out in the temperature range of 800 - 1000 °C.

[0126] Coiling

[0127] The hot rolled steel sheet manufactured as described above can be coiled, and at this time, it can be carried out in the temperature range of 400 - 700 °C.

[0128] When the coiling temperature is lower than 400°C, the strength of the hot-rolled steel sheet increases excessively, which may cause rolling load during subsequent cold rolling. In addition, excessive costs and time are required to cool the hot-rolled steel sheet to the coiling temperature, which becomes a reason for increasing the process cost. On the other hand, when the coiling temperature exceeds 700°C, excessive scale is generated on the surface of the hot-rolled steel sheet, and the possibility of surface defects is high, which becomes a reason for weakening the plating property.

[0129] Cooling

[0130] Preferably, the coiled hot-rolled steel sheet is cooled at a cooling rate of 0.1°C / second or less (except 0°C / second) to room temperature. Here, cooling represents the average cooling rate.

[0131] As described above, by cooling the coiled hot-rolled steel sheet at a specified speed, a hot-rolled steel sheet in which carbides that will become nucleation sites of austenite are finely dispersed can be obtained. That is, fine carbides are uniformly dispersed in the steel during hot rolling, and during subsequent annealing, while the carbides dissolve, they can be finely dispersed and form an austenite phase in the steel. Therefore, a uniformly dispersed fine martensite phase can be obtained after annealing is completed.

[0132] Cold rolling

[0133] The coiled hot-rolled steel sheet as described above can be cold-rolled to manufacture a cold-rolled steel sheet, and at this time, the cold rolling reduction ratio (total reduction ratio) can be 30 - 80%.

[0134] In particular, in the present invention, by making the cumulative reduction ratio of the initial stands during cold rolling, preferably the 1st stand to the 3rd stand, 25% or more, the stored energy inside the steel is increased, and thus the effect of acting as a driving force for promoting the recrystallization of ferrite during subsequent annealing can be obtained. Thereby, the effect of reducing the fraction of unrecrystallized ferrite in the steel can be imparted.

[0135] When unrecrystallized ferrite exists in the steel, due to local concentration of deformation and stress, the ductility of the steel becomes poor. On the other hand, recrystallized ferrite relieves the concentration of deformation and stress, which helps to improve the ductility.

[0136] When the cumulative reduction ratio of the initial stands 1 to 3 during cold rolling is less than 25% or the cold rolling reduction ratio (total reduction ratio) up to the final stand is less than 30%, it is not only difficult to ensure the desired thickness, but also there is a problem that it is difficult to correct the shape of the steel sheet. In addition, there is also a problem that the ductility is reduced due to an increase in the fraction of unrecrystallized ferrite. On the other hand, when the cold rolling reduction ratio up to the final stand during cold rolling exceeds 80%, the strength increases, resulting in a problem of roll load during cold rolling, and there is an increased possibility of cracks occurring in the edge portion of the steel sheet.

[0137] In the present invention, the cold rolling can be carried out using a rolling mill composed of 5 or 6 stands, but is not limited thereto.

[0138] Continuous annealing

[0139] Preferably, the cold-rolled steel sheet manufactured as described above is subjected to a continuous annealing treatment. As an example, the continuous annealing treatment can be carried out in a continuous alloying hot-dip galvanizing furnace.

[0140] The continuous annealing step is a process for forming a ferrite phase and an austenite phase and decomposing carbon while recrystallizing.

[0141] The continuous annealing treatment is preferably carried out in a temperature range of Ac1 + 30°C to Ac3 + 30°C, and more preferably, it can be carried out in a temperature range of 800 - 880°C.

[0142] When the temperature during the continuous annealing is lower than Ac1 + 30°C, not only sufficient recrystallization cannot be achieved, but also it is difficult to form sufficient austenite, so that the fractions of tempered martensite, fresh martensite, and bainite at the desired level cannot be ensured after annealing. On the other hand, when the temperature during the continuous annealing exceeds Ac3 + 30°C, since the austenite grain size becomes coarse, a fine residual austenite phase cannot be uniformly formed around the hard phase. In addition, the productivity is reduced, and elements such as Si, Mn, and B that reduce the wettability of hot-dip galvanizing due to high-temperature annealing exacerbate the formation of surface enrichments, thus the plating surface quality cannot be ensured.

[0143] Stepwise cooling

[0144] Preferably, the cold-rolled steel sheet subjected to the continuous annealing treatment as described above is subjected to stepwise cooling.

[0145] Specifically, the cooling is preferably carried out at an average cooling rate of 10°C / second or less (except 0°C / second), cooled to 450 - 670°C (this cooling is called primary cooling), and then cooled at an average cooling rate of 5°C / second or more, cooled to 250 - 500°C (this cooling is called secondary cooling).

[0146] Primary cooling

[0147] In the present invention, in order to form one or more of tempered martensite and bainite with a fraction of 40% or more as the final structure, it is necessary to ensure the fractions of martensite and bainite during the subsequent secondary cooling process. Therefore, the primary cooling can be carried out at an average cooling rate of 10°C / second or less (excluding 0°C / second) and cooled to 450 - 670°C.

[0148] Specifically, when the subsequent secondary cooling is terminated below Ms (martensite transformation start temperature), a relatively large amount of martensite phase can be formed. For this purpose, it is preferable to control the termination temperature of the primary cooling as low as possible. In addition, when the subsequent secondary cooling is terminated in the bainite temperature range, the bainite phase can be formed relatively favorably. For this purpose, it is preferable to control the termination temperature of the primary cooling higher.

[0149] During primary cooling, it is cooled at an average cooling rate of 10°C / second or less (excluding 0°C / second) to 450 - 700°C. Preferably, when the subsequent secondary cooling is terminated below Ms, the primary cooling is preferably carried out to a temperature range of 450 - 600°C, and when the subsequent secondary cooling is terminated in the bainite temperature range, the primary cooling is preferably carried out to a temperature range of 550 - 700°C.

[0150] When the termination temperature during the primary cooling is lower than 450°C, the load on the equipment for the atmosphere gas in the cooling annealing furnace is large, and the cooling rate becomes fast, and it is impossible to sufficiently ensure the ferrite phase generated during cooling. On the other hand, when the termination temperature during the primary cooling exceeds 700°C, there is a disadvantage that an excessively high cooling rate is required during the subsequent cooling (secondary cooling).

[0151] In addition, when the average cooling rate during the primary cooling exceeds 10°C / second, the diffusion of carbon cannot occur sufficiently. In addition, considering productivity, the primary cooling can be carried out at an average cooling rate of 1°C / second or more.

[0152] Secondary cooling

[0153] After terminating the primary cooling under the above conditions, secondary cooling is preferably carried out. At this time, the desired fine structure can be induced by controlling the cooling termination temperature and the cooling rate.

[0154] When cooled below Ms during the secondary cooling, quenching martensite is formed, and the lower the temperature, the higher the fraction of quenching martensite, thereby inducing an increase in the strength of the steel plate. In addition, during subsequent heat treatment (the reheating process of the present invention), it is tempered to tempered martensite, and the carbon supersaturated in the martensite is distributed to the surrounding untransformed austenite, thereby improving the stability of the retained austenite and enhancing ductility.

[0155] When cooled to a temperature exceeding Ms during the secondary cooling, the fraction of bainite can be increased. At this time, due to the effects of Si and Al during the bainite phase transformation, the precipitation of carbides is delayed, and as carbon is distributed from the bainite to the surrounding untransformed austenite, the stability of the retained austenite is improved, and ductility can be enhanced.

[0156] When the termination temperature of the secondary cooling is lower than 250 °C, the fraction of quenching martensite increases excessively, the fraction of the retained austenite phase decreases instead, and there is a problem of poor shape of the steel plate. On the other hand, when the termination temperature of the secondary cooling exceeds 500 °C, bainite cannot be sufficiently formed, the fraction of the retained austenite phase decreases, and during subsequent processes, the fraction of the newly formed martensite phase increases significantly, resulting in a problem of excessive increase in strength.

[0157] In addition, when the average cooling rate during the secondary cooling is less than 5 °C / second, a pearlite phase is formed, so it may not be possible to form a bainite phase at the desired level. On the other hand, the upper limit of the average cooling rate is not particularly limited, and a person of ordinary skill in the art can appropriately select it considering the specifications of the cooling equipment. As an example, it can be carried out at an average cooling rate of 100 °C / second or less.

[0158] In addition, the secondary cooling can use a hydrogen cooling device using hydrogen (H2 gas). As described above, by cooling using a hydrogen cooling device, the effect of suppressing surface oxidation that may occur during the secondary cooling can be obtained. At this time, although the type of gas used in the hydrogen cooling device is not limited, as an example, it can be controlled by 60 - 70% hydrogen (H2) and the balance nitrogen (N2).

[0159] In addition, when performing segmented cooling as described above, the cooling rate during secondary cooling can be faster than that during primary cooling.

[0160] Hold

[0161] After completing the secondary cooling as described above, a process of holding for 30 seconds or more within the cooling temperature range can be further performed.

[0162] Through the above-described holding process, it is possible to obtain the effect of tempering martensite or further increasing the amount of bainite phase transformation. When the holding time is less than 30 seconds, it is difficult to expect the above effects.

[0163] Reheating and holding

[0164] By subjecting the cold-rolled steel sheet that has completed the stepwise cooling as described above to a reheating and holding process, the fine microstructure desired in the present invention can be formed. Specifically, it is preferably a process of reheating the cold-rolled steel sheet that has undergone the secondary cooling to a temperature below 490°C and holding for 20 seconds or more.

[0165] By reheating to the above temperature and holding, the quenched martensite generated through the previous cooling process can be transformed into tempered martensite, and bainite phase transformation also occurs.

[0166] During the tempering process, the supersaturated carbon in the martensite is redistributed to the surrounding untransformed austenite. In addition, when the secondary cooling terminates above Ms, the fraction of bainite increases significantly during the reheating and holding process. During this process, the carbon discharged from the bainite is redistributed to the untransformed austenite, and the stability of the retained austenite is improved, thereby achieving the effect of improved ductility.

[0167] However, when the temperature during reheating is excessively increased, the carbides in the tempered martensite and bainite become coarse, resulting in a decrease in strength. Due to the formation of coarse carbides, the redistribution effect of carbon in the untransformed austenite is reduced, so the fraction of retained austenite decreases, and ultimately it is difficult to expect an improvement in ductility.

[0168] Therefore, the reheating temperature can be 490°C or lower, and more preferably, it can be 470°C or lower.

[0169] Preferably, the cold-rolled steel sheet reheated to below 490°C as described above is held at this temperature for 20 seconds or more to fully achieve the above effects. When the holding time during the holding is too long and exceeds 5 minutes, the tempering effect of the martensite becomes excessive, resulting in a problem of reduced strength. Therefore, it is preferably not more than 5 minutes.

[0170] In addition, the present invention can provide a plated steel sheet by plating the cold-rolled steel sheet manufactured as described above.

[0171] Hot-dip galvanizing

[0172] After the reheating and holding process as described above, it is preferable to immerse the steel sheet in a hot-dip zinc-based plating bath to manufacture a hot-dip zinc-based steel sheet.

[0173] At this time, hot-dip galvanization can be carried out under conventional conditions. As an example, it can be carried out in the temperature range of 430 - 490°C. In addition, the composition of the hot-dip zinc-based plating bath during the hot-dip galvanization is not particularly limited and can be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0174] Alloying heat treatment

[0175] In addition, if necessary, the hot-dip galvanized steel sheet can be subjected to alloying heat treatment to obtain an alloyed hot-dip galvanized steel sheet. In the present invention, the process conditions of the alloying heat treatment are not particularly limited as long as they are ordinary conditions. As an example, the alloying heat treatment process can be carried out in the temperature range of 480 - 600°C.

[0176] In addition, after the hot-dip galvanization or alloying heat treatment, a final cooling and temper rolling process can be further carried out.

[0177] Final cooling

[0178] As described above, the steel sheet subjected to hot-dip galvanization or alloying heat treatment is finally cooled, thereby further introducing fresh martensite. At this time, it is preferable to carry out the final cooling at a cooling rate of 5°C / second or more and cool to a temperature below Ms.

[0179] When the cooling rate during the cooling is less than 5°C / second, the desired level of fresh martensite phase cannot be ensured during the cooling process. On the other hand, the upper limit of the cooling rate is not particularly limited, but in order to form a specified fraction of the fresh martensite phase, it can be carried out at a cooling rate of 50°C / second or less.

[0180] Temper rolling

[0181] Furthermore, if necessary, the hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet subjected to final cooling is temper rolled, thereby forming a large number of dislocations in the steel, and thus the bake hardenability can be further improved. At this time, the reduction rate is preferably less than 2% (except 0%). If the reduction rate is 2% or more, although it is beneficial for forming dislocations, due to the limitations of equipment capabilities, side effects such as plate fracture may occur. Detailed implementation manners

[0182] Hereinafter, the embodiments of the present invention will be described. It is natural that those of ordinary skill in the art to which the present invention pertains can make various modifications to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of the rights of the present invention is not limited to the following embodiments, and the scope of the rights of the present invention should be defined by the claims and the content equivalent thereto.

[0183] (Example 1)

[0184] The steel billets having the alloy compositions shown in Table 1 below are heated at a temperature of 1200°C, and then each of the heated steel billets is hot finish rolled at 900°C above the Ar3 transformation point temperature to manufacture hot rolled steel sheets. After pickling, each hot rolled steel sheet is coiled at the temperature disclosed in Table 2, and then cooled to room temperature at a cooling rate of 0.1°C / second or less. Thereafter, the cooled hot rolled steel sheets are cold rolled to manufacture cold rolled steel sheets.

[0185] During the cold rolling, the cumulative reduction ratios of the first to third stands are set to 25% and carried out at a total reduction ratio of 60%.

[0186] Thereafter, each cold rolled steel sheet is subjected to continuous annealing treatment under the conditions shown in Table 2 below, and then subjected to primary cooling (slow cooling), secondary cooling (rapid cooling) and holding, and reheating. After completion of the secondary cooling and holding processes, reheating is carried out at a temperature of 490°C or less and the holding process is carried out at this temperature for 30 seconds. The process of holding after the secondary cooling is carried out for 30 seconds.

[0187] Thereafter, galvanizing treatment is carried out in a hot dip galvanizing bath at 460°C, and then finally cooled to room temperature at a cooling rate of 5°C / second, and then skin pass rolling is carried out at less than 2% to manufacture hot dip galvanized steel sheets. At this time, for a part of the steel, after the galvanizing treatment, alloying heat treatment is carried out.

[0188] [Table 1]

[0189]

[0190] Thereafter, the mechanical properties and fine microstructure properties of each test piece are evaluated, and the results are shown in Table 3 below. At this time, the tensile test of each test piece is carried out along the L direction using the DIN standard to evaluate the tensile physical properties. At this time, the deformation rate from the load-displacement curve to fracture during the tensile test of the material is defined as the total elongation (T-El), the deformation rate up to the maximum load point is defined as the uniform elongation (U-El), and the deformation rate from the maximum load point to fracture is defined as the non-uniform elongation (P-El), and the values are recorded. The fraction of the fine microstructure is measured for tempered martensite (TM), bainite (B), ferrite (F), fresh martensite (FM), and retained austenite (A) at the 1 / 4t position of the thickness of the annealed steel sheet by subjecting the matrix microstructure to nitric acid ethanol (Nital) etching and then using FE-SEM, an image analyzer, EBSD, and XRD.

[0191] [Table 2]

[0192]

[0193] [Table 3]

[0194]

[0195] In the above Tables 1 to 3, the relational expressions 1 to 4 are as follows.

[0196] [Relational expression 1]

[0197] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo) ≥ 0.7

[0198] (wherein each element represents a weight content.)

[0199] [Relational expression 2]

[0200] (FM TM+B / FM T ) × 100 ≥ 80%

[0201] (wherein FM T is the total fraction of newly formed martensite, and FM TM+B refers to the fraction of newly formed martensite that is in contact with tempered martensite or bainite in the fraction of newly formed martensite.)

[0202] [Relational expression 3]

[0203] YS × U - El / TS ≥ 6

[0204] [Relational expression 4]

[0205] YS × P - El / TS ≥ 3

[0206] In addition, Figure 1 is a graph showing the relationship between relational expressions 1 and 3 of inventive steels 1 to 6 and comparative steels 1 to 5 in the above-described embodiments, Figure 2 is a graph showing the relationship between relational expressions 1 and 4 of inventive steels 1 to 6 and comparative steels 1 to 5. When the steel composition relational expression 1 of relational expression 1 is 0.7 or more, relational expressions 3 and 4 can be satisfied. As described above, in the present invention, in order to improve formability and anti-collision fracture resistance, relational expressions 3 and 4 should be satisfied. For this purpose, it is effective to ensure the fine microstructure proposed in the present invention. For this purpose, it is important to enrich C in the untransformed austenite. In order to enrich C in the untransformed austenite, a C content of a specified amount or more needs to be included in the steel composition, and further, the formation of carbides in the bainite transformation region is delayed by adding Si and Al. In addition, appropriate ferrite needs to be ensured in order to sufficiently enrich C in austenite. However, when excessive amounts of Mn, Cr, Mo, etc., which are hardenability elements, are added, the formation of ferrite is inhibited, and thus C cannot be sufficiently enriched in austenite.

[0207] When the relational expression 1 is 0.7 or more, the desired ferrite and retained austenite fractions can be ensured. Thus, finally, the newly formed martensite can also be finely dispersed, thereby increasing the uniform elongation and non-uniform elongation. As a result, the relational expressions 3 and 4 can also be satisfied.

[0208] On the other hand, when the relational expression 1 is less than 0.7, the desired retained austenite and ferrite fractions cannot be ensured. Finally, the newly formed martensite cannot be finely dispersed either, thereby reducing the uniform elongation and non-uniform elongation and failing to satisfy the relational expressions 3 and 4.

[0209] In addition, it can be confirmed from Tables 1 to 3 that the inventive steels 1 to 6 that satisfy the alloy composition system and manufacturing conditions proposed in the present invention have a tensile strength of 980 MPa or more and satisfy the relational expressions 3 and 4 at the same time. Therefore, excellent formability and fracture resistance can be ensured.

[0210] In contrast, it can be confirmed that the comparative steels 1 to 8 that deviate from the composition range or the relational expression 1 proposed in the present invention or do not satisfy the manufacturing conditions fail to form the desired fine microstructure and also fail to ensure the physical properties desired in the present invention.

Claims

1. A steel plate, by weight %, the steel plate comprises: carbon (C): 0.1 - 0.2%, silicon (Si): 0.5 - 1.3%, aluminum (Al): less than 0.5% and excluding 0%, manganese (Mn): 1.9 - 3.0%, molybdenum (Mo): less than 0.3%, chromium (Cr): less than 1% and excluding 0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.1%, the balance being Fe and other inevitable impurities, the C, Si, Al, Mn, Cr and Mo satisfy the following relational expression 1, The microstructure, by area %, comprises: ferrite: 10 - 35%, retained austenite: 3 - 15%, fresh martensite: less than 20% and excluding 0%, the balance being one or more of tempered martensite and bainite, In the fresh martensite, the fraction of fresh martensite with an interphase distance of more than 3 times the fresh martensite grain size is 30% or more, [Relational expression 1] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo) ≥ 0.7 Among them, Each element represents the weight content.

2. The steel plate according to claim 1, wherein, The fraction of the fresh martensite satisfies the following relational expression 2, [Relational expression 2] (FM TM+B / FM T )×100≥80% Among them, FM T is the total fraction of newly formed martensite, and FM TM+B refers to the fraction of newly formed martensite in the fraction of newly formed martensite that is in contact with tempered martensite or bainite.

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

4. The steel plate according to claim 1, wherein, The yield strength (YS), non-uniform elongation (P-El) and tensile strength (TS) of the steel plate satisfy the following relational expression 4, [Relational expression 4] YS × P-El / TS ≥ 3.

5. The steel plate according to claim 1, wherein, The steel plate further comprises boron (B): less than 0.01%.

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

7. The steel plate according to claim 1, wherein, The steel plate further includes a zinc-based coating.

8. A method for manufacturing a steel plate, which comprises the following steps: Heating the steel billet in the temperature range of 1100 - 1300 °C, by weight %, the steel billet comprises: carbon (C): 0.1 - 0.2%, silicon (Si): 0.5 - 1.3%, aluminum (Al): less than 0.5% and excluding 0%, manganese (Mn): 1.9 - 3.0%, molybdenum (Mo): less than 0.3%, chromium (Cr): less than 1% and excluding 0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.1%, the balance being Fe and other inevitable impurities, and the C, Si, Al, Mn, Cr and Mo satisfy the following relational expression 1; Hot-rolling the reheated steel billet in the temperature range above Ar3 to manufacture a hot-rolled steel plate; Coiling the hot-rolled steel plate in the temperature range of 400 - 700 °C; After the coiling, cooling the hot-rolled steel plate at a cooling rate of 0.1 °C / second to room temperature; After the cooling, cold-rolling the hot-rolled steel plate with a total reduction ratio of 30 - 80% to manufacture a cold-rolled steel plate; Performing a continuous annealing treatment on the cold-rolled steel plate at a temperature of Ac1 + 30 °C to Ac3 + 30 °C; The cold-rolled steel sheet subjected to the continuous annealing treatment is subjected to a first cooling at a cooling rate of 10°C / second or less, and cooled to 450 - 700°C; After the first cooling, a second cooling is performed at a cooling rate of 5°C / second or more, and cooled to 250 - 500°C; and The cold-rolled steel sheet subjected to the second cooling is reheated to a temperature of 490°C or less and held for 20 seconds or more, wherein the cold rolling is performed with a cumulative reduction ratio of 25% or more in the initial No. 1 to No. 3 stands, [Relationship 1] (10C + Si + Al) / (Mn + 1.3Cr + 2.7Mo) ≥ 0.7 wherein each element represents a weight content.

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

10. The manufacturing method of the steel plate according to claim 8, wherein, The cooling rate during the second cooling is faster than the cooling rate during the first cooling.

11. The manufacturing method of the steel plate according to claim 8, wherein, The second cooling is performed in a hydrogen rapid cooling device using hydrogen (H2) gas.

12. The manufacturing method of the steel plate according to claim 8, wherein, After the second cooling, a step of holding for 30 seconds is further included.

13. The manufacturing method of the steel plate according to claim 8, wherein, After the reheating and holding, a step of hot-dip galvanizing in a plating bath at 430 - 490°C is further included.

14. The manufacturing method of the steel plate according to claim 13, wherein, After the hot-dip galvanizing, a step of performing an alloying heat treatment is further included.

15. The manufacturing method of the steel plate according to claim 14, wherein, After the alloying heat treatment, a step of cooling at a cooling rate of 5°C / second or more, cooling to a temperature of Ms to 100°C or less, and then performing a skin pass rolling of less than 2% is further included.

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