Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet suitable as a material for a vehicle structural member and the like, and, more specifically, to a high-strength composite structure steel sheet with a tensile strength of 780 MPa or higher, and a method for manufacturing same.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel plate suitable as a material for automobile structural members, and more specifically, to a high-strength composite structure steel plate having a tensile strength of 780 MPa or higher and a method for manufacturing the same.

[0002] As regulations on automobile fuel efficiency are strengthened to protect the global environment, efforts are being made to reduce the weight of automobile bodies. One such measure is to reduce the weight of automotive materials by increasing the strength of steel plates.

[0003] Generally, high-strength automotive materials are categorized into precipitation-hardened steel, bake-hardened steel, solution-hardened steel, and transformation-hardened steel. Among these, transformation-hardened steels include dual-phase steel (DP steel), complex-phase steel (CP steel), and transformation-induced plasticity (TRIP steel). Such transformation-hardened steels are also called advanced high-strength steels (AHSS).

[0004] Among transformation-hardened steels, DP steel achieves high strength through the fine, even dispersion of hard martensite within soft ferrite. CP steel contains two or more phases of ferrite, martensite, and bainite in its microstructure, and contains precipitation-hardening elements such as Ti and Nb to enhance strength. TRIP steel, on the other hand, achieves high strength and ductility by transforming fine, evenly dispersed retained austenite into martensite during processing at room temperature.

[0005] Recently, as automotive materials, steel plates with higher strength are required to improve fuel efficiency and durability of the vehicle body, and the use of high-strength steel plates with a tensile strength of 780 MPa or more as structural or reinforcing materials for the vehicle body is increasing for crash safety and passenger protection.

[0006] Until now, steel plate development has primarily focused on improving stretchability from the perspective of ductility and tensile strength. However, as the strength of automotive steel plates continues to increase, the ductility of the cut-edge surface (i.e., the cut edge) when shearing these steel plates is becoming increasingly low, leading to frequent cracking at the edges during processing. This is particularly true for components requiring stretch-flangeability, such as sill sides and seat components. Even with excellent elongation, poor burring (hole expandability, HER) renders the material unusable.

[0007] Automobile manufacturers, who have mainly used DP steel with excellent formability for these parts, are currently demanding the development of DP steel that satisfies the characteristics of DP steel, such as low yield ratio and high ductility, while also having excellent work hardening rate and burring resistance, in order to solve this problem.

[0008] Meanwhile, automotive steel sheets also require high corrosion resistance, so hot-dip galvanized steel sheets, which boast excellent corrosion resistance, have traditionally been used as automotive materials. This hot-dip galvanized steel sheet can be manufactured using continuous hot-dip galvanizing equipment, where the recrystallization annealing and plating processes are performed on the same line, offering the advantage of low-cost production of steel sheets with superior corrosion resistance. Furthermore, alloyed hot-dip galvanized steel sheets, which undergo an alloying process involving reheating after hot-dip galvanizing, are widely used due to their superior corrosion resistance, weldability, and formability.

[0009] However, there is difficulty in securing the surface quality of the plated steel sheet due to Si and Mn, which are hardenable and oxidizing elements added to secure high strength of the steel.

[0010] Accordingly, in order to reduce the weight of automobiles, it is necessary to develop DP steel that has the characteristics of DP steel, such as low yield ratio and high ductility, while also having excellent burring and stretch flangeability. In addition, there is a demand for the development of hot-dip galvanized steel sheets that have excellent corrosion resistance and weldability.

[0011] As a prior art, Patent Document 1, which improves the workability of a high-strength steel plate, proposes a composite-structure steel plate mainly composed of martensite, and discloses a manufacturing method for dispersing fine copper precipitation particles with a particle size of 1 to 100 nm within the structure to improve the workability of the steel plate. In this document, Cu is excessively added at 2 to 5% to precipitate fine Cu particles, which raises concerns about the occurrence of red-hot embrittlement due to Cu, and there is a problem of excessively increasing the manufacturing cost.

[0012] Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet with excellent hole expandability. Specifically, Patent Document 2 discloses a steel sheet having a matrix structure of ferrite and pearlite at 2-10 area%, and wherein carbonitride-forming elements such as Nb, Ti, and V are added for precipitation strengthening, thereby enhancing strength through precipitation strengthening and grain refinement. However, while the steel sheet disclosed in this document has excellent hole expandability, it has limitations in improving tensile strength, and has concerns about cracking during press forming due to its high yield strength and low ductility.

[0013] Meanwhile, Patent Document 3 discloses a method for manufacturing a composite structure steel sheet with improved workability by utilizing a retained austenite phase. However, the steel sheet according to this document has problems such as difficulty in ensuring plating quality due to the addition of large amounts of Si and Al as alloy composition, and difficulty in ensuring the surface quality of the steel during steelmaking and casting. Furthermore, the high content of retained austenite results in a high initial yield strength (YS) due to transformation-induced plasticity, making it difficult to secure the low yield ratio required for automobiles, and there is a problem of processing cracks occurring during press forming.

[0014] (Patent Document 1) Japanese Patent Publication No. 2005-264176

[0015] (Patent Document 2) Korean Patent Publication No. 10-2015-0073844

[0016] (Patent Document 3) Japanese Patent Publication No. 2015-113504

[0017] One aspect of the present invention is to provide a steel sheet suitable for use as an automobile structural member, which steel sheet has a high tensile strength of 780 MPa or higher, as well as excellent work hardening rate and hole expandability (burring resistance). A method for manufacturing the steel sheet is also provided.

[0018] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0019] According to one aspect of the present invention, a cold-rolled steel sheet can be provided that includes, in wt%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder being Fe and other unavoidable impurities.

[0020] In one embodiment of the present invention, the cold rolled steel sheet can satisfy the following relational expression 1.

[0021] [Relationship 1]

[0022] 2×{(Si+Cr+(3×Mo)+Ti+Nb) / C} ≥ 15.0

[0023] (In equation 1, each element represents the weight content, and if not added, 0 is substituted.)

[0024] In one embodiment of the present invention, the cold-rolled steel sheet may have a microstructure composed of an area fraction of 25 to 60% ferrite, 25 to 60% bainite, 20% or less martensite, and the remainder retained austenite.

[0025] In addition, in one embodiment of the present invention, the martensite has a number of fine martensites having a size of 3 ㎛ or less in the equivalent diameter per unit area (1 mm 2 ) per 2.0×10 5 It could be more than a dog.

[0026] Cold-rolled steel sheets having such alloy composition and microstructure can have a low yield ratio and high ductility, and at the same time, can have excellent work hardening rate and burring resistance.

[0027] In one embodiment of the present invention, the cold rolled steel sheet may further include at least one of titanium (Ti): 0.040% or less and niobium (Nb): 0.040% or less.

[0028] According to one embodiment of the present invention, a cold-rolled steel sheet may have a content of C, Si, Cr, and Mo in ferrite that satisfies the following relational expression 2, and in this case, the hardness of the ferrite may be increased, thereby obtaining the effect of reducing the hardness difference with the hard phase.

[0029] [Relationship 2]

[0030] 2×{(Si F +Cr F +(3×Mo F )) / C F} ≥ 500

[0031] (In equation 2, each element represents the weight content contained in ferrite.)

[0032] In a cold-rolled steel sheet according to one embodiment of the present invention, the hardness relationship between the microstructures can satisfy the following relational expression 3, and the relationship between the work hardening rate (n), elongation (El), hole expandability (HER), and yield ratio (YR) in a 4 to 6% strain range can satisfy the following relational expression 4.

[0033] [Relationship 3]

[0034] H M / (H F +H B ) ≤ 2.0

[0035] (H in relation 3 M The hardness value of martensite, H F is the hardness value of ferrite, H B refers to the hardness value of bainite.)

[0036] [Relationship 4]

[0037] (n×El×HER) / YR ≥ 150.0

[0038] (In equation 4, the units of each property are not considered.)

[0039] A cold-rolled steel sheet according to one embodiment of the present invention may have a yield strength of 450 MPa or more, a tensile strength of 780 MPa or more, a yield ratio of 0.70 or less, and an elongation of 15% or more.

[0040] According to another aspect of the present invention, in manufacturing a cold rolled steel sheet, the method comprises the steps of: preparing a steel slab; heating the steel slab to a temperature range of 1050 to 1300°C; finishing hot rolling the heated steel slab at a temperature higher than the Ar3 transformation point to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a temperature range of 400 to 700°C; cold rolling the coiled hot rolled steel sheet at a total reduction ratio of 40 to 80% to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at a temperature range of 780 to 835°C; first cooling the continuously annealed cold rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 1 to 14°C / s; second cooling the first-cooled cold rolled steel sheet to a temperature range of 350 to 480°C at a cooling rate of 10°C / s or more; A step of maintaining the secondarily cooled cold-rolled steel sheet for 100 seconds or longer; and a step of finally cooling the maintained cold-rolled steel sheet to a temperature of Ms-100°C or lower at a cooling rate of 3°C / s or higher may be included.

[0041] In one embodiment of the present invention, the steel slab has the above-described alloy composition and can satisfy relational expression 1.

[0042] In one embodiment of the present invention, the step of obtaining a galvanized steel sheet by galvanizing the maintained cold-rolled steel sheet before final cooling may be further included.

[0043] In one embodiment of the present invention, the galvanized steel sheet may further be optionally subjected to alloying heat treatment.

[0044] In one embodiment of the present invention, a step of temper rolling at a reduction ratio of less than 1% after final cooling may be further included.

[0045] According to the present invention, a cold-rolled steel sheet (composite structure steel) having a low yield ratio and high ductility, which are characteristics of DP steel, and excellent work hardening rate and burring resistance can be provided.

[0046] The cold-rolled steel sheet according to the present invention has an effect that makes it suitable for use as an automotive material, particularly as a structural member.

[0047] FIG. 1 is a diagram showing a graph of the relationship between relational expression 1 and mechanical properties (relational expression 3) according to one embodiment of the present invention.

[0048] FIG. 2 is a diagram showing a graph of the relationship between relational expression 1 and mechanical properties (relational expression 4) according to one embodiment of the present invention.

[0049] The inventors of the present invention have conducted in-depth research on a method to improve the work hardening rate as well as the hole expandability (burring property) of DP steel, which has been used as a material for existing automobile structural members, while maintaining the low yield ratio and high elongation.

[0050] Through repeated research, the inventors of the present invention have confirmed that a structural composition advantageous for improving work hardening rate and burring resistance can be established by optimizing the alloy composition, manufacturing conditions, and particularly the annealing process for steel plates with a tensile strength of 780 MPa or higher. As a result, the present invention is provided.

[0051] Hereinafter, the present invention will be described in detail.

[0052] According to one aspect of the present invention, a steel sheet may contain, in wt%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), and nitrogen (N): 0.010% or less (excluding 0%).

[0053] Below, the reasons for limiting the alloy composition of a steel plate according to one embodiment of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0054] Carbon (C): 0.060~0.120%

[0055] Carbon (C) is a very important element added to strengthen the transformation structure, and is effective in promoting the formation of martensite phase in composite structure steel while promoting the high strength of steel.

[0056] As the C content increases, the amount of martensite formed in the steel increases. In one embodiment of the present invention, when the C content exceeds 0.120%, the strength is improved by the martensite formed in large quantities, but the difference in strength with ferrite having a relatively low carbon concentration increases, and this difference in strength easily causes fracture at the interface between phases when stress is applied, so the bending characteristics and extension flangeability of the steel are inferior. In addition, the weldability is inferior, so that welding defects occur when the customer processes the parts. On the other hand, when the C content is less than 0.060%, it is very difficult to secure the desired strength.

[0057] Therefore, in one embodiment of the present invention, C may be included in an amount of 0.060 to 0.120%. In another embodiment of the present invention, C may be 0.070% or more, or 0.080% or more. In yet another embodiment, C may be 0.150% or less, or 0.110% or less.

[0058] Silicon (Si): 1.20% or less (excluding 0%)

[0059] Silicon (Si) is a useful element that can secure strength without reducing the ductility of steel. Furthermore, Si promotes the formation of ferrite and promotes the enrichment of carbon into untransformed austenite, thereby promoting the formation of martensite. Furthermore, Si, due to its high solid-solution strengthening ability, increases the strength of ferrite, making it an effective element for reducing the hardness difference between phases.

[0060] In one embodiment of the present invention, if the content of Si exceeds 1.20%, the surface quality deteriorates during the plating process of steel, making it impossible to secure the surface quality of the plated steel sheet. In one embodiment of the present invention, there is no particular limitation on the lower limit content of Si, and since the effect of Si can be obtained if it is added at a level during the steel manufacturing process, taking this into consideration, it can be limited to exceeding 0%.

[0061] Therefore, in one embodiment of the present invention, Si may be included in an amount of more than 0% to 1.20%. In another embodiment of the present invention, the Si may be 0.05% or more, or 0.11% or more. In yet another embodiment, the Si may be 1.10% or less, or 1.05% or less.

[0062] Manganese (Mn): 1.80~2.50%

[0063] Manganese (Mn) is an element that strengthens steel by minimizing grain size without compromising ductility and by completely precipitating sulfur (S) in the steel as MnS, thereby preventing hot embrittlement caused by the formation of FeS. Furthermore, in composite phase steels, it lowers the critical cooling rate for martensite formation, contributing to easier martensite formation.

[0064] In one embodiment of the present invention, if the content of Mn is less than 1.80%, the strengthening effect of the steel is insufficient, making it difficult to secure the target level of strength. On the other hand, if the content exceeds 2.50%, there is a high possibility that problems will occur in the weldability and hot-rollability of the steel, and there is a problem that the martensite phase is formed excessively, making the material unstable, and Mn-Band (band of Mn oxide) is formed in the structure, increasing the risk of processing cracks and plate breakage. In addition, there is a problem that Mn oxide is dissolved on the surface of the steel during the annealing process, which significantly inhibits the plating property during subsequent plating.

[0065] Therefore, in one embodiment of the present invention, Mn may be included in an amount of 1.80 to 2.50%. In another embodiment of the present invention, the Mn may be included in an amount of 1.85% or more, or 1.90% or more. In yet another embodiment, the Mn may be included in an amount of 2.45% or less.

[0066] Molybdenum (Mo): 0.200% or less (excluding 0%)

[0067] Molybdenum (Mo) delays the transformation of austenite to pearlite, while also contributing to the refinement and strength of ferrite. Mo enhances the hardenability of steel, forming fine martensite at grain boundaries, thereby facilitating yield ratio control. However, as it is an expensive element, higher Mo contents pose manufacturing challenges. Therefore, appropriate control of its content is crucial.

[0068] In one embodiment of the present invention, if the content of Mo exceeds 0.200%, there is a problem that the alloy cost increases rapidly, which reduces economic feasibility, the crystal grains become excessively fine, and the solid solution strengthening becomes excessive, which rather reduces the ductility of the steel. In one embodiment of the present invention, the lower limit content of Mo is not particularly limited, and since the effect of Mo can be obtained if it is added at a level during the steel manufacturing process, taking this into consideration, it can be limited to exceeding 0%.

[0069] Therefore, in one embodiment of the present invention, Mo may be included in an amount of more than 0% to 0.200%. In another embodiment of the present invention, Mo may be 0.001% or more. In yet another embodiment, Mo may be 0.180% or less, or 0.170% or less.

[0070] Chromium (Cr): 0.800% or less (excluding 0%)

[0071] Chromium (Cr) is an element with similar properties to the aforementioned Mn, and can be added to improve the hardenability of steel and secure high strength. Cr is effective in the formation of martensite, and is an advantageous element for manufacturing composite phase steel with high ductility by minimizing the decrease in elongation compared to the increase in strength. In particular, during the hot rolling process, Cr 23 It forms Cr-based carbides such as C6, some of which dissolve during the annealing process, while others remain undissolved, allowing the amount of dissolved C in martensite to be controlled below an appropriate level after subsequent cooling. As a result, it suppresses the occurrence of yield point elongation (YP-El), which is advantageous for the production of composite phase steels with low yield ratios.

[0072] In one embodiment of the present invention, when the Cr content exceeds 0.800%, the hardenability of the steel is improved, which is advantageous for the formation of martensite. However, not only is the amount excessive, but the fraction of Cr-based carbides also increases and coarsens, so that martensite coarsens after annealing, which causes a problem of lowering the elongation of the steel. In one embodiment of the present invention, the lower limit content of Cr is not particularly limited, and since the effect of Cr can be obtained if it is added at a level during the steel manufacturing process, it can be limited to more than 0% in consideration of this.

[0073] Therefore, in one embodiment of the present invention, Cr may be included in an amount of greater than 0% to 0.800%. In another embodiment of the present invention, Cr may be 0.010% or more, or 0.011% or more. In yet another embodiment, Cr may be 0.750% or less, or 0.700% or less.

[0074] Phosphorus (P): 0.150% or less (excluding 0%)

[0075] Phosphorus (P) is the most advantageous element for securing strength without significantly impairing steel formability. However, excessive addition significantly increases the risk of brittle fracture, increasing the likelihood of slab fracture during hot rolling. Furthermore, it also has the problem of acting as an element that impairs the surface properties of galvanized steel sheets.

[0076] In one embodiment of the present invention, since the aforementioned problem may occur when the P content exceeds 0.150%, the P may be included at 0.150% or less. Meanwhile, since the P may be inevitably added during the steel manufacturing process, a content of 0% may be excluded.

[0077] Sulfur (S): 0.010% or less (excluding 0%)

[0078] Sulfur (S) is an unavoidable impurity added to steel, and it is effective to keep its content as low as possible. In particular, S in steel is highly likely to cause red-hot embrittlement.

[0079] In one embodiment of the present invention, since the aforementioned problem may occur when the content of S exceeds 0.010%, the S may be included at 0.010% or less. Meanwhile, considering the level at which the S is inevitably added during the steel manufacturing process, 0% may be excluded from the content.

[0080] Aluminum (sol.Al): 0.100% or less (excluding 0%)

[0081] Aluminum (sol.Al) is an element added to refine the grain size and deoxidize steel.

[0082] In one embodiment of the present invention, if the sol.Al content is 0%, aluminum killed steel cannot be manufactured in a stable state, so 0% can be excluded for that content. On the other hand, if the sol.Al content exceeds 0.100%, while it is advantageous for increasing the strength of the steel due to the grain refinement effect, it increases the possibility of excessive formation of inclusions during the steelmaking continuous casting operation, which causes surface defects in the plated steel sheet. In addition, there is a concern that the manufacturing cost will increase, thereby reducing economic feasibility.

[0083] Therefore, in one embodiment of the present invention, sol.Al may be included in an amount of more than 0% to 0.100%. According to another embodiment of the present invention, the sol.Al may be 0.005% or more, or 0.010% or more. According to yet another embodiment, the sol.Al may be 0.095% or less, or 0.090% or less.

[0084] Nitrogen (N): 0.010% or less (excluding 0%)

[0085] Nitrogen (N) is an impurity that is inevitably added to steel, and it is effective to manage its content as low as possible.

[0086] In one embodiment of the present invention, since there is a concern that the cost of steel refining will rapidly increase when the N content is extremely low, the content may be limited to a range that allows for operation. As one example, the N content may be limited to 0.010% or less; however, considering the level that is inevitably added during the steel manufacturing process, 0% may be excluded from the content.

[0087] A steel plate according to one embodiment of the present invention may further include at least one of Ti and Nb in addition to the above-described alloy composition for the purpose of further improving the mechanical properties of the steel plate.

[0088] Titanium (Ti): 0.040% or less and niobium (Nb): 0.040% or less

[0089] Titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of steel and for refining grains by forming fine precipitates.

[0090] In one embodiment of the present invention, when the content of Ti and Nb exceeds 0.040%, the manufacturing cost increases and there is a problem that the ductility of the steel is significantly reduced due to excessive formation of precipitates.

[0091] Therefore, in one embodiment of the present invention, when at least one of Ti and Nb is added, each may be included at 0.040% or less. According to another embodiment of the present invention, each of Ti and Nb may be 0.035% or less, or 0.030% or less.

[0092] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0093] A steel plate according to one embodiment of the present invention may be composed of the above-described alloy elements, and the relationship between some of the alloy elements may be limited to specific conditions.

[0094] In one embodiment of the present invention, the content relationship of C, Si, Cr, Mo, Ti and Nb in the alloy composition can be defined by the following relational expression 1.

[0095] [Relationship 1]

[0096] 2×{(Si+Cr+(3×Mo)+Ti+Nb) / C} ≥ 15.0

[0097] (In equation 1, each element represents the weight content, and if not added, 0 is substituted.)

[0098] The inventors of the present invention have repeatedly studied and found that reducing the difference in hardness between phases is effective in obtaining excellent work hardening rate and burring resistance while securing low yield ratio and high ductility of composite structure steel. Accordingly, they have factored out a combination of specific alloying elements capable of increasing the solid solution strengthening of ferrite in composite structure as equation 1.

[0099] According to one embodiment of the present invention, when the value of relational expression 1 is controlled to 15.0 or more, the concentrations of Si, Mo, and Cr within the ferrite increase, which causes a solid solution strengthening effect, thereby effectively reducing the hardness difference between phases with a hard phase (e.g., martensite phase, etc.). In addition, Ti or Nb-based precipitates are formed within the ferrite grains, which relatively increases the hardness of the ferrite, thereby maximizing the effect of reducing the hardness difference between phases.

[0100] Therefore, according to one embodiment of the present invention, the value of relation 1 may be 15.0 or greater, and according to another embodiment, it may be 16.0 or greater, 16.5 or greater.

[0101] Hereinafter, the microstructural characteristics of the steel plate according to one embodiment of the present invention will be described in detail.

[0102] The inventors of the present invention have found that, in order to improve the work hardening rate and hole expandability (burring property) of a steel plate, i.e., a composite structure steel, according to one embodiment of the present invention, control of the fraction range of each phase is effective.

[0103] Accordingly, a steel plate according to one embodiment of the present invention may include ferrite of 25 to 60% in area fraction, bainite of 25 to 60%, martensite of 20% or less, and retained austenite as a remainder structure.

[0104] If the ferrite fraction of the steel sheet according to one embodiment of the present invention is less than 25%, high ductility cannot be achieved and the work hardening rate cannot be improved. Conversely, if the fraction exceeds 60%, the target level of strength cannot be secured.

[0105] In one embodiment of the present invention, the bainite phase is a structure that is advantageous in reducing the hardness difference between phases in a composite structure steel. If the fraction is less than 25%, the effect of reducing the hardness difference between phases is insufficient, and thus burring properties cannot be improved. On the other hand, if the fraction exceeds 60%, the fraction of martensite, which is a relatively hard phase, becomes low, making it impossible to secure the intended strength. In particular, the bainite phase is formed between the ferrite and martensite phases, and thus can effectively reduce the hardness difference between the soft ferrite phase and the hard martensite phase.

[0106] In one embodiment of the present invention, if the fraction of the martensite phase, which is advantageous in securing the strength of the steel plate, exceeds 20%, the strength increases excessively, resulting in a decrease in ductility, and the difference in hardness between the phases with the soft ferrite increases, resulting in poor burring properties. Such a martensite phase can be formed by finely and evenly dispersing an appropriate fraction within the steel. In particular, in one embodiment of the present invention, the martensite phase is formed such that the number of fine martensites having a size of 3 ㎛ or less in terms of a circle-equivalent diameter is 1 mm per unit area. 2 ) per 2.0×10 5 It can be ideal. The number of the above fine martensite is per unit area (1 mm 2 ) per 2.0×10 5 If it is less than that, the work hardening ability of the steel plate cannot be improved due to the presence of a large number of martensite phases of relatively large size.

[0107] A steel plate according to one embodiment of the present invention may include a retained austenite phase as a residual structure, and the fraction thereof is not particularly limited.

[0108] In particular, in a steel sheet according to one embodiment of the present invention, the contents of C, Si, Cr, and Mo in the ferrite can satisfy the following relational expression 2 by controlling the relationship between specific elements in the alloy composition.

[0109] [Relationship 2]

[0110] 2×{(Si F +Cr F +(3×Mo F )) / C F} ≥ 500

[0111] (In equation 2, each element represents the weight content contained in ferrite.)

[0112] That is, according to one embodiment of the present invention, the content of elements advantageous to the solid solution strengthening effect in the ferrite phase increases, thereby generating the solid solution strengthening effect, and also, by forming precipitates in the ferrite, the hardness of the soft ferrite phase increases.

[0113] In one embodiment of the present invention, if the value of relational expression 2 is less than 500, the hardness difference between the ferrite and martensite phases is not effectively reduced, so that the burring property of the steel sheet becomes inferior.

[0114] A steel plate according to one embodiment of the present invention can have a characteristic of a low difference in hardness between phases of a composite structure by controlling the alloy composition and microstructure.

[0115] In particular, the steel plate according to one embodiment of the present invention can satisfy the hardness relationship between the ferrite, bainite, and martensite phases as shown in the following equation 3.

[0116] [Relationship 3]

[0117] H M / (H F +H B ) ≤ 2.0

[0118] (H in relation 3 M The hardness value of martensite, H F is the hardness value of ferrite, H B refers to the hardness value of bainite.)

[0119] In one embodiment of the present invention, if the value of relational expression 3 exceeds 2.0, it means that the difference in hardness between the phases of the composite structure is large, and in this case, the burring property of the steel plate cannot be secured.

[0120] In addition, the steel sheet according to one embodiment of the present invention includes a microstructure of ferrite and bainite phases at a certain percentage or more, and a fine martensite phase is evenly dispersed and formed, so that deformation begins at a low stress in the initial stage of plastic deformation, thereby exhibiting characteristics such as a low yield ratio and a high work hardening rate. This change in microstructure can alleviate local stress and deformation, thereby delaying the creation, growth, and coalescence of pores, and as a result, the ductility of the steel sheet can be improved.

[0121] In particular, the steel plate according to one embodiment of the present invention can satisfy the following relational expression 4 in the relationship between the work hardening rate (n), elongation (El), hole expandability (HER), and yield ratio (YR) in the 4 to 6% strain range.

[0122] [Relationship 4]

[0123] (n×El×HER) / YR ≥ 150.0

[0124] (In equation 4, the units of each property are not considered.)

[0125] A steel plate according to one embodiment of the present invention can have high strength, a low yield ratio and high ductility, and at the same time, excellent work hardening rate and hole expandability.

[0126] As an example, the steel plate may have a yield strength of 450 MPa or more, a tensile strength of 780 MPa or more, a yield ratio of 0.70 or less, and an elongation of 15% or more, and may satisfy the aforementioned relationship 4.

[0127] Meanwhile, the steel sheet according to one embodiment 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.

[0128] Although not particularly limited, the zinc-based plating layer may be, for example, a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0129] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a steel plate according to one embodiment of the present invention.

[0130] According to one embodiment of the present invention, a steel plate can be manufactured by going through the process of [heating - hot rolling - cooling - coiling - cold rolling - annealing - cooling] for a prepared steel slab, and each process step is specifically described below.

[0131] [Heating of steel slabs]

[0132] After preparing a steel slab according to one embodiment of the present invention, the steel slab can be heated. The heating process of the steel slab is a process for smoothly performing the hot rolling process described below and sufficiently obtaining the target physical properties of the steel plate. As one example, the steel slab can have the same alloy composition and alloy composition relationship (relationship 1) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the composition relationship are replaced with the above-mentioned matters.

[0133] In one embodiment of the present invention, the heating of the steel slab may be performed at a temperature range of 1050 to 1300°C. If the heating temperature is lower than 1050°C, friction between the steel plate and the rolling mill increases, which causes a problem in that the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required for the temperature increase, but the amount of surface scale increases, which may lead to material loss.

[0134] Therefore, in one embodiment of the present invention, the heating process of the steel slab can be performed in a temperature range of 1050 to 1300°C. According to another embodiment of the present invention, the heating process of the steel slab can be performed at 1100°C or higher, and in yet another embodiment, it can be performed at 1250°C or lower.

[0135] [Hot rolling]

[0136] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0137] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing finishing hot rolling at a temperature higher than the Ar3 transformation point during the hot rolling. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature lower than the Ar3 transformation point, there is a concern that ferrite and austenite dual-phase rolling may occur, resulting in material non-uniformity.

[0138] As an example, the above finishing hot rolling can be performed in a temperature range of 800 to 1000°C, and if the temperature exceeds 1000°C, there is a concern that material unevenness may occur due to the formation of abnormal coarse grains caused by high-temperature rolling, and this causes a problem of coil distortion occurring during subsequent cooling.

[0139] [Winding]

[0140] The hot-rolled steel sheet manufactured above can be coiled.

[0141] In one embodiment of the present invention, the coiling process may be performed at a temperature range of 400 to 700°C. If the coiling temperature is lower than 400°C, the strength of the hot-rolled steel sheet may become excessively high, which may cause rolling load during subsequent cold rolling. In addition, the cost and time required to cool the hot-rolled steel sheet to the coiling temperature are excessive, which causes an increase in process costs. On the other hand, if the temperature exceeds 700°C, scale may excessively form on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and deteriorate the plating properties.

[0142] [Cold rolling]

[0143] The above-mentioned hot-rolled steel sheet can be manufactured into a cold-rolled steel sheet by cold rolling.

[0144] In one embodiment of the present invention, cold rolling can be performed at a cold reduction ratio (total reduction ratio) of 40 to 80%. If the cold reduction ratio during cold rolling is less than 40%, it becomes difficult to secure the target thickness and also difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction ratio exceeds 80%, there is a high possibility of cracks occurring at the edge of the steel sheet, and there is a problem of generating a load during cold pressing.

[0145] [Continuous annealing]

[0146] The cold-rolled steel sheet manufactured above can be subjected to continuous annealing treatment. According to one embodiment of the present invention, the continuous annealing treatment can be performed in a continuous alloying galvanizing furnace.

[0147] In one embodiment of the present invention, the continuous annealing treatment can be performed in a temperature range of 780 to 835°C. That is, by performing the continuous annealing treatment of a cold-rolled steel sheet in a two-phase temperature range where ferrite and austenite coexist, not only austenite but also ferrite can be formed simultaneously with the recrystallization of the structure, while carbon can be distributed.

[0148] In the continuous annealing treatment according to one embodiment of the present invention, if the temperature is below 780℃, not only will recrystallization not be sufficiently achieved, but the austenite phase will not be sufficiently formed, making it impossible to form the target microstructure phase composition after the continuous annealing treatment. On the other hand, if the temperature exceeds 835℃, productivity decreases, and there is a problem that the fraction of the martensite phase becomes excessive after the subsequent cooling process due to the excessive formation of the austenite phase. In this case, while the yield strength increases, the ductility decreases, making it impossible to secure the intended characteristics of low yield ratio and high ductility. In addition, the surface enrichment of elements such as Si, Mn, and B, which inhibit the wettability of the hot-dip galvanizing among the alloy compositions, may be excessive, which may deteriorate the plating surface quality.

[0149] Accordingly, in one embodiment of the present invention, the continuous annealing treatment can be performed at a temperature range of 780 to 835°C. According to another embodiment of the present invention, the continuous annealing treatment can be performed at 785°C or higher, and in another embodiment, at 830°C or lower, or at 820°C or lower.

[0150] In the continuous annealing in the two-phase temperature range according to one embodiment of the present invention, the austenite phase can be formed with an area fraction of 70 to 95%, and the ferrite phase can be formed with an area fraction of 5 to 30%.

[0151] [Cooling and Maintenance]

[0152] The cold rolled steel sheet that has been continuously annealed in the above two-phase temperature range can be cooled.

[0153] In one embodiment of the present invention, the cooling may be performed in stages, and as an example, the continuous annealing process may be a process of first cooling the cold-rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 1 to 14°C / s, and then secondarily cooling the first-cooled cold-rolled steel sheet to a temperature range of 350 to 480°C at a cooling rate of 10°C / s or more. At this time, the cooling rate during the second cooling may be performed faster than the cooling rate during the first cooling.

[0154] In this way, in the continuous annealing process according to one embodiment of the present invention, the type and fraction of the microstructure formed can be controlled by performing stepwise cooling to a specific temperature range according to the cooling rate when cooling a cold-rolled steel sheet in which a certain fraction of a ferrite phase is formed together with austenite.

[0155] In one embodiment of the present invention, a ferrite phase can be additionally introduced into the cold rolled steel sheet by performing primary cooling at a cooling rate of 1 to 14°C / s to a temperature range of 630 to 690°C on the continuously annealed cold rolled steel sheet.

[0156] If the cooling rate during the first cooling is less than 1°C / s, the austenite phase formed during the continuous annealing process will be transformed into an excessive fraction of ferrite, making it impossible to properly secure the bainite and martensite phases in the final microstructure. On the other hand, if the cooling rate exceeds 14°C / s, the additionally introduced ferrite phase will be insufficient, which may reduce the ductility of the steel sheet.

[0157] In addition, if the cooling end temperature during the first cooling is less than 630°C, the ferrite phase cannot be sufficiently formed. On the other hand, if the temperature exceeds 690°C, there is a problem in that the cooling rate must be excessively increased during the subsequent second cooling process, and there is a concern that the fractions of the bainite phase and martensite phase in the final microstructure may not be sufficient.

[0158] According to one embodiment of the present invention, during the first cooling process of a cold-rolled steel sheet subjected to continuous annealing, a ferrite phase having an area fraction of 20 to 30% can be additionally formed, and at the same time, the remaining austenite phase has the effect of being dispersed.

[0159] In one embodiment of the present invention, after the first cooling of the continuously annealed cold-rolled steel sheet, secondary cooling can be performed at a cooling rate of 10°C / s or more to a temperature range of 350 to 480°C, and by maintaining the cold-rolled steel sheet in this cooled state, a bainite phase can be introduced.

[0160] If the cooling rate during the secondary cooling is less than 10°C / s, pearlite may be generated during the cooling process, preventing the bainite phase from being sufficiently formed. Meanwhile, the upper limit of the cooling rate during the secondary cooling is not particularly limited, and a skilled artisan may select it appropriately, taking into account the specifications of the cooling equipment. As an example, the cooling rate may be 100°C / s or less.

[0161] In addition, if the cooling end temperature during the secondary cooling is less than 350°C, the martensite phase is excessively formed, and the martensite phase is tempered during the subsequent holding process, making it impossible to secure the intended yield strength and high ductility. On the other hand, if the temperature exceeds 480°C, the bainite phase is not sufficiently formed, making it impossible to obtain the effect of reducing the hardness difference between phases due to the bainite phase.

[0162] In one embodiment of the present invention, the secondary cooled cold-rolled steel sheet may be maintained for 100 seconds or longer. If the maintenance time is less than 100 seconds, bainite cannot be obtained in a sufficient fraction. The upper limit of the maintenance time is not particularly limited, and can be determined by a person skilled in the art as the time required for the bainite phase to form in the desired fraction.

[0163] In one embodiment of the present invention, the maintenance process performed after secondary cooling can be performed within the temperature range at which the secondary cooling is completed, and therefore, there are no particular limitations on the temperature range. However, as one example, it can be performed at 400°C or lower.

[0164] According to one embodiment of the present invention, a bainite phase having an area fraction of 25 to 60% can be formed during the maintenance process after the secondary cooling of the first-cooled cold-rolled steel sheet, and the formation of the bainite phase in this way enhances the dispersion effect of the remaining austenite phase (approximately 10 to 20% of the fraction). Accordingly, when the remaining austenite phase transforms into martensite during the subsequent cooling process, it can be formed in a fine and uniform distribution.

[0165] [Final cooling]

[0166] The cold rolled steel sheet, which has undergone the above-mentioned stepwise (primary and secondary) cooling and maintenance processes, can be finally cooled.

[0167] In one embodiment of the present invention, the final cooling of the cold rolled steel sheet that has undergone the stepwise cooling and holding process can be performed at a cooling rate of 3°C / s or more to a temperature of Ms-100°C or lower, and a martensite phase can be introduced during this process.

[0168] If the cooling rate during the final cooling is less than 3°C / s or the cooling end temperature exceeds Ms-100°C, the martensite phase cannot be secured at the intended level. In one embodiment of the present invention, there is no particular limitation on the upper limit of the cooling rate and the lower limit of the cooling end temperature during the final cooling, but in terms of forming a martensite phase of a certain fraction, the cooling may be performed at a cooling rate of, for example, 50°C / s or less, and as another example, the final cooling may be performed to room temperature.

[0169] According to one embodiment of the present invention, the austenite that is transformed into ferrite and bainite phases during the stepwise cooling and holding process and remains can be transformed into a martensite phase during the final cooling process. At this time, the austenite phase that is transformed into a martensite phase during the final cooling process is uniformly and finely distributed within the steel sheet due to the dispersion effect during the previous stepwise cooling and holding process, and thus the martensite phase formed during the final cooling process can also be formed in a fine and uniform distribution. The martensite phase formed during the final cooling process can be formed at an area fraction of 20% or less, and among these, the fine martensite phase can be formed at a ratio of 90% or more. As one example of the present invention, the fine martensite phase can refer to a martensite phase having an average crystal grain size of 3 ㎛ or less.

[0170] Meanwhile, the austenite phase remaining after forming the martensite phase during the final cooling process remains as it is, becoming the residual austenite phase of the final microstructure. As an example, the fraction thereof may be 2% or less (including 0%).

[0171] According to one embodiment of the present invention, a steel sheet (cold rolled steel sheet) having a composite structure can be obtained by controlling the alloy composition and manufacturing conditions, and the composite structure of this steel sheet has a low difference in hardness between a soft phase and a hard phase, and further, by introducing an appropriate fraction of ferrite and bainite phases and forming a martensite phase that is finely and uniformly dispersed, it can have the effect of improving the work hardening rate due to alleviation of stress concentration and improving burring property (hole expandability).

[0172] In one embodiment of the present invention, a cold-rolled steel sheet may be plated to obtain a plated steel sheet. As an example, hot-dip galvanizing may be performed on a cold-rolled steel sheet that has undergone a stepwise cooling and maintenance process before final cooling.

[0173] [Hot-dip galvanizing]

[0174] According to one embodiment of the present invention, a cold-rolled steel sheet, that is, a cold-rolled steel sheet that has undergone a stepwise cooling and maintenance process, can be immersed in a molten zinc-based plating bath to produce a molten zinc-based galvanized steel sheet.

[0175] In one embodiment of the present invention, the hot-dip galvanizing process may be performed under normal conditions, but as an example, may be performed at a temperature range of 430 to 490°C. In addition, the composition of the hot-dip galvanizing bath is not particularly limited, and may be a pure zinc plating bath or a zinc alloy plating bath containing Si, Al, Mg, etc.

[0176] [Alloying heat treatment]

[0177] In addition, if necessary, an alloyed zinc-plated steel sheet can be obtained by subjecting a hot-dip galvanized steel sheet manufactured by hot-dip galvanizing according to one embodiment of the present invention to an alloying heat treatment.

[0178] In one embodiment of the present invention, the alloying heat treatment process conditions are not particularly limited, and any conventional conditions may be used. As an example, the alloying heat treatment process may be performed at a temperature range of 480 to 600°C.

[0179] [Temperature rolling]

[0180] Furthermore, if necessary, a temper rolling process can be further performed, and the temper rolling process can be performed not only on a cold rolled steel sheet that has undergone final cooling, but also on a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet that has undergone final cooling.

[0181] According to one embodiment of the present invention, a large amount of dislocations are formed in steel through a temper rolling process, thereby further improving the bake hardenability. As an example, this can be performed at a reduction ratio of less than 1% (excluding 0%). When the reduction ratio during the temper rolling is 1% or more, it is advantageous in terms of dislocation formation, but may cause side effects such as plate fracture due to limitations in equipment capacity.

[0182] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0183] (Example)

[0184] Steel slabs having the alloy compositions shown in Table 1 below were heated at a temperature of 1050 to 1250°C, and then each heated slab was final hot-rolled at Ar3+50°C to 950°C to produce hot-rolled steel sheets. Thereafter, each hot-rolled steel sheet was pickled under normal conditions, coiled at a temperature range of 400 to 700°C, and then cold-rolled at a cold reduction ratio of 40 to 80% to produce cold-rolled steel sheets.

[0185] After this, each cold-rolled steel plate was subjected to continuous annealing treatment under the conditions shown in Table 2 below, and then a stepwise cooling (first - second cooling) and maintenance process was performed.

[0186] Afterwards, the steel was cooled to room temperature at a cooling rate of 5 to 10°C / s, and then temper rolled at a reduction ratio of 0.2% to produce the final cold-rolled steel sheet (composite structure steel).

[0187] Steel alloy composition (weight %) relationship 1CSiMnMoCrTiNbPSol.AlN10.0900.642.450.0010.2500.0010.0200.0130.0040.0260.00420.320.1101.052.000.0300.0110.0020.0200.0500.0070.0310.00521.3 30.0650.112.100.0200.7500.0100.0300.0300.0040.0540.00429.540.0800.201.800.1500.4500.0200.0010.0400.0030.0430.00828.050.1100.552.200.0200.2500.0300.0100.01 00.0050.0250.00516.460.1500.302.300.0300.1700.0250.0210.1000.0070.0220.0038.170.0700.102.500.0200.2200.0240.0330.1200.0050.0200.00212.580.1900.041.500.110 0.3200.0040.0130.1100.0070.0640.0037.490.1420.181.800.0200.4000.0100.0170.0600.0030.0520.0069.4100.1000.102.400.1400.1200.0010.0200.0400.0010.0440.00413.2

[0188] Steel grade Coiling temperature (℃) Annealing temperature (℃) 1st cooling 2nd cooling Maintenance classification End temperature (℃) Cooling speed (℃ / s) End temperature (℃) Cooling speed (℃ / s) Temperature (℃) Time (sec) 1410813675742224365260 Inventive steel 12650810685240126367348 Inventive steel 23620790677437040363488 Inventive steel 34690809685340726373350 Inventive steel 456508356301346016371150 Inventive steel 5565077065084901538191 Comparative steel 16630850675339926365350Comparative strength 27650810648846017372400Comparative strength 38630799650450030388320Comparative strength 49650840675239926365370Comparative strength 510620770649447017386400Comparative strength 6

[0189] For each cold-rolled steel sheet manufactured as described above, the microstructure was measured, the mechanical properties were evaluated, and the results are shown in Table 3.

[0190] First, the microstructure of each cold-rolled steel sheet was measured using FE-SEM and an image analyzer after Nital corrosion for specimens collected at a point 1 / 4t (t: steel sheet thickness (unit: mm)) in the thickness direction. The fractions of ferrite, bainite, and martensite phases were measured, and the fraction of retained austenite phase was measured using XRD.

[0191] In addition, the concentration of specific elements (C, Si, Mo, Cr) in the ferrite was measured using TEM (Transmission Electron Microscopy), EDS (Energy Dispersive Spectroscopy), and EELS (Electron Energy Loss Spectroscopy) analysis equipment. In addition, the hardness of each phase was measured 10 times using a Vickers Micro Hardness Tester, and the average value was taken, and the relationship 3 was calculated using the value.

[0192] And, the size of each particle was measured by converting it into the equivalent diameter from the martensite area results measured using the image analyzer. At this time, 1 mm 2 The number of martensite particles with a size of 3㎛ or less within the unit area was calculated.

[0193] Meanwhile, in order to evaluate the tensile properties of each cold-rolled steel sheet, the same specimens as those used for microstructure measurement were tested in the C direction according to the JIS standard.

[0194] In addition, the strain hardening rate (n) was measured in the 4-6% strain range for the same specimen, and the hole expansion ratio (HER) was evaluated according to the JSF T1001-1996 standard.

[0195] ClassificationMicrostructure CharacteristicsMechanical Properties FBMR-ARelationship 2Micro MNumberYS(MPa)TS(MPa)El(%)YRnHER(%)Relationship 3Relationship 4Inventive steel 141461215952.42×10 5 496826210.600.18351.6220.5 Invention Steel 253291627673.83×10 5 502809210.620.20351.4237.1 Invention Steel 328591306132.61×10 5 548813190.670.18361.2183.8 Invention Steel 442431507332.98×10 5 514841190.610.20331.1205.6 Invention Steel 532472015732.01×10 5 508826220.620.15301.3159.7 Comparison River 159103105721.43×10 5 430850170.510.15262.3131.1 Comparison River 22073613731.01×10 5 645850110.760.09642.183.4 Comparison River 351232602531.79×10 5509818180.620.15182.278.4 Comparison River 445203504631.19×10 5 472843150.560.18232.5110.9 Comparison River 52567804261.21×10 5 580856110.680.09592.985.9 Comparison River 66283004271.39×10 5 In the microstructure, F represents ferrite, B represents bainite, M represents martensite, and RA represents retained austenite. Each phase is expressed as an area fraction %. The number of micro martensite (M) phases is per unit area of ​​1 mm. 2 This is the result measured based on .

[0196] As shown in Tables 1 to 3, the inventive steels 1 to 5, which satisfy both the alloy composition and manufacturing conditions according to one embodiment of the present invention, formed microstructures as intended. Accordingly, they have a strength of 780 MPa or more, a low yield ratio of 0.70 or less, and high ductility. In addition, since the hardness difference between the phases of the microstructure is minimized, the value of relational expression 3 is 2.0 or less, and hole expandability and work hardening rate are improved, thereby satisfying relational expression 4.

[0197] In this way, the cold-rolled steel sheet according to one embodiment of the present invention has the characteristics of DP steel, such as high yield strength and high ductility, and, despite being a composite structure steel, has a low hardness difference between phases, so it has excellent burring resistance and work hardening rate.

[0198] On the other hand, comparative steel 1, which satisfies the alloy composition system proposed in the present invention but does not satisfy the manufacturing conditions, and comparative steels 2 to 6, which deviate from the relationship 1 proposed in the present invention and do not satisfy the manufacturing conditions, did not form the intended microstructure, and thus had inferior properties in at least one or more properties.

[0199] In particular, Comparative Steels 1 to 6 did not form the ferrite, bainite or martensite phases in the intended fraction, or the content relationship of specific elements in the ferrite did not satisfy the relational expression 2 according to one embodiment of the present invention, so that not only did they not satisfy the relational expression 4 due to the inferior work hardening rate, but also the hardness difference between the phases was large.

[0200] Figure 1 is a graph showing the change in mechanical properties (Relationship 3) according to the content relationship of specific elements (Relationship 1).

[0201] As shown in Fig. 1, it can be seen that when the value of relational expression 1 satisfies 15 or more, the value of relational expression 3 can be secured to 2 or less. That is, according to one embodiment of the present invention, the effect of reducing the hardness difference between phases of composite structure steel can be obtained by controlling the content relationship of specific elements.

[0202] Figure 2 is a graph showing the change in mechanical properties (Relationship 4) according to the content relationship of specific elements (Relationship 1).

[0203] As shown in Fig. 2, it can be seen that when the value of relational expression 1 satisfies 15 or more, the value of relational expression 4 can be secured as 150 or more. That is, according to one embodiment of the present invention, the work hardening rate and burring property of composite structure steel can be improved by controlling the content relationship of specific elements.

[0204] Figures 1 and 2 illustrate invention steels 1 to 5, which are steel plates according to one embodiment of the present invention, and comparative steels 2 to 6, which do not satisfy relational expression 1 according to one embodiment of the present invention.

Claims

1. Contains, in wt%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1, A steel sheet having a microstructure composed of an area fraction of 25 to 60% ferrite, 25 to 60% bainite, 20% or less martensite, and the remainder retained austenite. [Relationship 1] 2×{(Si+Cr+(3×Mo)+Ti+Nb) / C} ≥ 15.0 (In equation 1, each element represents the weight content, and if not added, 0 is substituted.) 2. In paragraph 1, The above martensite has a number of fine martensites with a size of 3 ㎛ or less in diameter per unit area (1 mm). 2 ) 2.0×10 per 5 A plate that is more than a dog.

3. In paragraph 1, The above steel plate is a steel plate further containing at least one of titanium (Ti): 0.040% or less and niobium (Nb): 0.040% or less.

4. In paragraph 1, A steel plate in which the contents of C, Si, Mo and Cr in the above ferrite satisfy the following relationship 2. [Relationship 2] 2×{(Si F +Cr F +(3×Mo F )) / C F} ≥ 500 (In equation 2, each element represents the weight content contained in ferrite.) 5. In paragraph 1, The above steel plate is a steel plate in which the hardness relationship between ferrite, bainite and martensite satisfies the following relationship 3. [Relationship 3] H M / (H F +H B ) ≤ 2.0 (H in relation 3 M is the hardness value of martensite, H F is the hardness value of ferrite, H B ) refers to the hardness value of bainite.

6. In paragraph 1, The above steel plate is a steel plate in which the relationship between the strain hardening rate (n), elongation (El), hole expandability (HER), and yield ratio (YR) in the 4 to 6% strain range satisfies the following relationship 4. [Relationship 4] (n×El×HER) / YR ≥ 150.0 (In equation 4, the units of each property are not considered.) 7. In paragraph 1, The above steel plate is a steel plate having a yield strength of 450 MPa or more, a tensile strength of 780 MPa or more, a yield ratio of 0.70 or less, and an elongation of 15% or more.

8. A step for preparing a steel slab, which contains, by weight%, carbon (C): 0.060 to 0.120%, silicon (Si): 1.20% or less (excluding 0%), manganese (Mn): 1.80 to 2.50%, molybdenum (Mo): 0.200% or less (excluding 0%), chromium (Cr): 0.800% or less (excluding 0%), phosphorus (P): 0.150% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), aluminum (sol.Al): 0.100% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), the remainder being iron and other unavoidable impurities, and satisfying the following relational expression 1; A step of heating the above steel slab to a temperature range of 1050 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab at a temperature higher than the Ar3 transformation point; A step of coiling the above hot-rolled steel plate at a temperature range of 400 to 700°C; A step of cold rolling the above-mentioned hot-rolled steel sheet at a total reduction ratio of 40 to 80% to obtain a cold-rolled steel sheet; A step of continuously annealing the above cold rolled steel sheet at a temperature range of 780 to 835°C; A step of first cooling the continuously annealed cold rolled steel sheet to a temperature range of 630 to 690°C at a cooling rate of 1 to 14°C / s; A step of secondarily cooling the first-cooled cold-rolled steel sheet at a cooling rate of 10°C / s or more to a temperature range of 350 to 480°C; A step of maintaining the secondarily cooled cold rolled steel sheet for 100 seconds or longer; and A method for manufacturing a steel sheet, comprising a step of finally cooling the maintained cold rolled steel sheet to a temperature of Ms-100°C or lower at a cooling rate of 3°C / s or higher. [Relationship 1] 2×{(Si+Cr+(3×Mo)+Ti+Nb) / C} ≥ 15.0 (In equation 1, each element represents the weight content, and if not added, 0 is substituted.) 9. In paragraph 8, A method for manufacturing a steel sheet further comprising the step of obtaining a galvanized steel sheet by galvanizing the cold rolled steel sheet maintained above before final cooling.

10. In paragraph 9, A method for manufacturing a steel sheet further comprising the step of selectively performing alloying heat treatment on the galvanized steel sheet.

11. In paragraph 8, A method for manufacturing a steel sheet further comprising the step of subjecting the above-described finally cooled cold rolled steel sheet to temper rolling at a reduction ratio of less than 1%.

12. In paragraph 8, A method for manufacturing a steel plate, wherein the steel slab further contains at least one of titanium (Ti): 0.040% or less and niobium (Nb): 0.040% or less.

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