Cold-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof and, more specifically, to a high-strength cold-rolled steel sheet having excellent ductility and workability, and a manufacturing method thereof.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent ductility and workability and a method for manufacturing the same.

[0002]

[0003] In recent years, the automotive industry has seen increasingly stringent regulations for environmental protection. Consequently, carbon reduction and fuel efficiency regulations are becoming increasingly stringent. To ensure passenger safety in the event of a collision or other accident, the use of high-strength steel is increasing. Furthermore, to enhance the impact resistance of the vehicle body, high-strength steel with superior yield strength is being adopted for structural components such as members, seat rails, and pillars.

[0004] However, high strength of steel plates may result in a decrease in ductility and formability.

[0005] To address this, the development of materials that simultaneously satisfy both high strength and high formability is required. Typically, as the strength of steel plates increases, the elongation decreases, resulting in reduced workability. Therefore, the development of materials that can compensate for this issue is urgently needed. Conventional methods for strengthening steel include solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. However, among the aforementioned methods, the steels utilizing solution strengthening and grain refinement have the problem that it is very difficult to produce high-strength steels with a tensile strength exceeding 490 MPa.

[0006] Meanwhile, precipitation-strengthened high-strength steel is a technology that secures strength by precipitating carbon and nitride by adding carbon and nitride-forming elements such as Nb, Ti, and V, thereby refining the grains through suppression of grain growth by fine precipitates. This technology has the advantage of easily securing high strength at a low manufacturing cost, but has the disadvantage of requiring high-temperature annealing to secure ductility by causing sufficient recrystallization because the recrystallization temperature rises rapidly due to the fine precipitates. In addition, precipitation-strengthened steel, which strengthens by precipitating carbon and nitride in a ferrite matrix, has the problem that it is difficult to obtain high-strength steel of the 600 MPa class or higher.

[0007] Various types of transformation-hardened high-strength steels have been developed, including DP (Dual Phase) steels composed of a soft ferrite matrix and hard martensite, TRIP (Transformation Induced Plasticity) steels that secure high ductility by utilizing the transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steels composed of a composite structure of ferrite and hard bainite or martensite.

[0008] Recently, automotive steel sheets are required to have higher strength to improve fuel efficiency and durability, and the demand for ultra-high strength steel sheets with a tensile strength of 1180 MPa or more as body structures or reinforcing materials is increasing to ensure collision safety and protect passengers.

[0009] Among these, DP steel offers excellent ductility and is the most commonly used automotive steel. However, it suffers from a low yield ratio (YR) and poor formability and workability. Furthermore, as steel sheets increasingly require higher strength, cracks and wrinkles can occur during press forming of automotive parts, making it difficult to manufacture complex parts.

[0010] Among ultra-high strength steels, TRIP and XF steels have higher ductility and better yield ratio than conventional DP steels, so they have good workability. However, they have the disadvantage of poor weldability due to the addition of large amounts of Si and Al to secure high elongation.

[0011] To overcome these shortcomings of existing ultra-high-strength steels, the application of ultra-high-strength steels to more complex components can be expanded by reducing the Si and Al content to create a composition that achieves good weldability while also satisfying a certain yield ratio. This can be achieved by utilizing Quenching and Partitioning (Q&P) heat treatment, a cutting-edge heat treatment technology that secures retained austenite.

[0012] The invention disclosed in Patent Document 1 is a technology for simultaneously securing ductility and workability of the high-strength steel. The technology of Patent Document 1 requires careful selection of the heat treatment temperature to secure high elongation by forming retained austenite through precise control of slow and rapid cooling temperatures.

[0013] The invention disclosed in Patent Document 2 is characterized by manufacturing a steel plate having a high yield ratio through a Q&P process and a painting treatment.

[0014] The invention disclosed in Patent Document 3 provides a method for manufacturing a high-strength cold-rolled steel sheet having a high bainite fraction by cooling to a bainite region, but has a problem in that the carbon partitioning effect is inferior to that of the Q&P process, resulting in inferior elongation.

[0015] (Patent Document 1) Korean Patent Publication No. 10-2020-0076795

[0016] (Patent Document 2) Japanese Patent Publication No. 2010-090432

[0017] (Patent Document 3) Japanese Patent Publication No. 2016-216808

[0018]

[0019] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0020] According to one embodiment of the present invention, it is an object to provide a high-strength cold-rolled steel sheet having excellent ductility and workability and a method for manufacturing the same.

[0021] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0022]

[0023] According to one embodiment of the present invention, it contains, in wt%, carbon (C): 0.08 to 0.16%, silicon (Si): 1.00% or less, manganese (Mn): 1.60 to 3.00%, molybdenum (Mo): 0.40% or less, chromium (Cr): 1.0% or less, phosphorus (P): 0.10% or less, sulfur (S): 0.020% or less, aluminum (sol.Al): 0.60% or less, titanium (Ti): 0.001 to 0.040%, niobium (Nb): 0.001 to 0.040%, nitrogen (N): 0.01% or less, boron (B): 0.010% or less, antimony (Sb): 0.05% or less, the remainder iron (Fe) and other unavoidable impurities.

[0024] The R value defined in the following relational expression 1 is 0.270 or greater,

[0025] The microstructure may be a cold rolled steel sheet containing, in area %, 30 to 70% of bainite and tempered martensite combined, 50% or less of ferrite, 5% or less of retained austenite, and the remainder fresh martensite.

[0026] [Relationship 1]

[0027] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5

[0028] (In the formula, [C], [Si], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0029] The above microstructure may include, in area %, 30 to 50% of bainite and tempered martensite combined, 20 to 50% of ferrite, 2 to 5% of retained austenite, and the remainder of fresh martensite.

[0030] The above cold-rolled steel sheet may have a yield ratio of 0.60 or more, a hole expansion ratio (HER) of 30% or more, and a product of the hole expansion ratio (HER) and the elongation (EL) (HER*EL) of 800 or more.

[0031]

[0032] According to one embodiment of the present invention, a steel sheet is provided, which contains, in wt%, carbon (C): 0.08 to 0.16%, silicon (Si): 1.00% or less, manganese (Mn): 1.60 to 3.00%, molybdenum (Mo): 0.40% or less, chromium (Cr): 1.0% or less, phosphorus (P): 0.10% or less, sulfur (S): 0.020% or less, aluminum (sol.Al): 0.60% or less, titanium (Ti): 0.001 to 0.040%, niobium (Nb): 0.001 to 0.040%, nitrogen (N): 0.01% or less, boron (B): 0.010% or less, antimony (Sb): 0.05% or less, the remainder iron (Fe) and other unavoidable impurities, and has an R value of 0.270 or more defined in the following relational expression 1. Steps to reheat the slab;

[0033] A step of hot rolling the above reheated steel slab;

[0034] A step of coiling the hot-rolled steel sheet;

[0035] A step of cold rolling the above-mentioned rolled steel plate;

[0036] A step of continuously annealing the above cold-rolled steel sheet at a temperature range of 790 to 830°C;

[0037] A step of first cooling the continuously annealed steel plate to a temperature range of 630 to 680°C;

[0038] A step of secondarily cooling the above-mentioned first-cooled steel plate to a temperature range of 320 to 360°C;

[0039] A step of reheating and maintaining the secondarily cooled steel plate to a temperature range of Ms or higher; and

[0040] It may be a method for manufacturing a cold-rolled steel sheet, including a step of thirdly cooling the reheated and maintained steel sheet to a temperature range of 150°C or lower.

[0041] [Relationship 1]

[0042] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5

[0043] (In the formula, [C], [Si], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0044] The above reheating step is performed at a temperature range of 1100 to 1300°C,

[0045] The above hot rolling step is performed at a finishing rolling temperature of Ar3 to Ar3+50℃.

[0046] The above coiling step is performed by coiling at a temperature range of 400 to 650°C and then cooling to room temperature at an average cooling rate of 0.1°C / s or less.

[0047] The above cold rolling step is performed at a reduction ratio of 40 to 70%.

[0048] The above first cooling step cools at an average cooling rate of 10.0℃ / s or less,

[0049] The above secondary cooling step cools at an average cooling rate of 5℃ / s or more,

[0050] The above reheating and maintaining step is maintained for more than 60 seconds after reheating,

[0051] The above third cooling step can cool at an average cooling rate of 5°C / s or more.

[0052] The step of subjecting the thirdly cooled steel sheet to a temper rolling process of less than 1% may be further included.

[0053]

[0054] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0055] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet having excellent ductility and processability and a method for manufacturing the same can be provided.

[0056] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet and a method for manufacturing the same can be provided, which can be used for automotive structural members having complex shapes requiring high formability, while improving processability by increasing the yield ratio and having excellent ductility and hole expandability.

[0057] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0058]

[0059] FIG. 1 is a graph showing the change in the product of hole expandability (HER) and elongation (EL) according to relational expression 1 according to one embodiment of the present invention.

[0060] FIG. 2 is a graph showing changes in hole expandability (HER) according to relational expression 1 according to one embodiment of the present invention.

[0061] Figure 3 is a graph showing the change in the yield ratio according to relational expression 1 according to one embodiment of the present invention.

[0062]

[0063] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0064]

[0065] Hereinafter, the present invention will be described in detail.

[0066]

[0067] Below, the steel composition of the present invention is described in detail.

[0068] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0069]

[0070] According to an embodiment of the present invention, a cold-rolled steel sheet may include, in wt%, carbon (C): 0.08 to 0.16%, silicon (Si): 1.00% or less, manganese (Mn): 1.60 to 3.00%, molybdenum (Mo): 0.40% or less, chromium (Cr): 1.0% or less, phosphorus (P): 0.10% or less, sulfur (S): 0.020% or less, aluminum (sol.Al): 0.60% or less, titanium (Ti): 0.001 to 0.040%, niobium (Nb): 0.001 to 0.040%, nitrogen (N): 0.01% or less, boron (B): 0.010% or less, and antimony (Sb): 0.05% or less.

[0071]

[0072] Carbon (C): 0.08~0.16%

[0073] Carbon (C) is a very important element added to strengthen the transformation structure. Carbon (C) promotes the formation of hard martensite in composite phase steel, thereby improving strength. As the carbon (C) content increases, the amount of martensite also increases. However, when the carbon (C) content exceeds 0.16%, the strength of martensite increases, but the difference in strength with ferrite, which has a low carbon concentration, becomes significant. This difference in strength can easily cause fracture at the interphase interface during plastic deformation, which can reduce ductility and work hardening rate. Furthermore, due to poor weldability, welding defects can occur during component processing. Therefore, according to one embodiment of the present invention, the upper limit may be limited to 0.16%. According to one embodiment of the present invention, it may be 0.15% or less. On the other hand, if the carbon (C) content is less than 0.08%, it may be difficult to secure the desired strength. Therefore, according to one embodiment of the present invention, the lower limit may be limited to 0.08%. According to one embodiment of the present invention, it may be 0.09% or more.

[0074]

[0075] Silicon (Si): 1.00% or less

[0076] Silicon (Si) is a ferrite stabilizing element that promotes ferrite transformation and promotes carbon enrichment in untransformed austenite during the Q&P process, thereby contributing to the formation of retained austenite. Furthermore, it is effective in reducing the hardness difference between phases by increasing the strength of ferrite through solid solution strengthening, and is a useful element that can secure strength without reducing the ductility of steel sheets. However, if its content exceeds 1.00%, it may cause surface scale defects, adversely affecting surface quality. In addition, it may deteriorate weldability and chemical treatment properties. Therefore, according to one embodiment of the present invention, the upper limit may be limited to 1.00%. According to one embodiment of the present invention, it may be 0.90% or less.

[0077]

[0078] Manganese (Mn): 1.60~3.00%

[0079] Manganese (Mn) is an element that refines particles without reducing ductility, completely precipitates S in steel as MnS, prevents hot embrittlement caused by the formation of FeS, and strengthens steel. At the same time, in composite phase steel, it plays a role in lowering the critical cooling rate at which martensite is obtained, thereby facilitating the formation of martensite. If the manganese (Mn) content is less than 1.60%, it may be difficult to secure the strength targeted in the present invention. According to one embodiment of the present invention, it may be 2.00% or more. On the other hand, if the content exceeds 3.00%, there is a high possibility that problems such as weldability and hot rollability will occur, martensite will be formed excessively, making the material unstable, and Mn-Bands (bands of Mn oxide) will be formed in the structure, which may increase the risk of processing cracks and plate breakage. In addition, during annealing, Mn oxide may be dissolved on the surface, which may significantly deteriorate the surface quality. According to one embodiment of the present invention, it may be 2.60% or less.

[0080]

[0081] Molybdenum (Mo): 0.40% or less

[0082] Molybdenum (Mo) is an element that delays the transformation of austenite into pearlite while simultaneously improving the refinement and strength of ferrite. Molybdenum (Mo) improves the hardenability of steel and has the advantage of controlling the yield ratio by forming fine martensite at grain boundaries. However, as it is an expensive element, the manufacturing cost increases as the content increases, which is disadvantageous in terms of cost. Therefore, it is desirable to appropriately control the content. To achieve the above-mentioned effect, molybdenum (Mo) may be added at 0.40% or less. If the content of molybdenum (Mo) exceeds 0.40%, it causes a sharp increase in alloy cost, reducing economic feasibility. In addition, excessive grain refinement and solid solution strengthening effects may lower the ductility of the steel. According to one embodiment of the present invention, it may be 0.30% or less. In the present invention, 0% is excluded in consideration of the amount that is inevitably added during manufacturing.

[0083]

[0084] Chromium (Cr): 1.0% or less

[0085] Chromium (Cr) is an element added to improve the hardenability of steel and secure high strength, and is an element that plays a very important role in the formation of martensite. It can also be advantageous in the production of composite phase steel with high ductility by minimizing the decrease in elongation compared to the increase in strength. In particular, chromium (Cr) is added during the hot rolling process. 23It forms Cr-based carbides such as C6, and some of these carbides are dissolved during the annealing process, and some remain undissolved. This allows the amount of solid solution C in martensite to be controlled below an appropriate level after cooling, thereby suppressing the occurrence of yield point elongation, making it an advantageous element for manufacturing composite phase steel with a low yield ratio. However, if the content exceeds 1.0%, not only will the effect be saturated, but there may be a problem of poor cold rolling properties due to excessive increase in hot-rolled strength. In addition, as the fraction of Cr-based carbides increases and coarsens, there may be a problem of coarsening the martensite size after annealing, resulting in a decrease in elongation. According to one embodiment of the present invention, it may be 0.8% or less.

[0086]

[0087] Phosphorus (P): 0.10% or less

[0088] Phosphorus (P) is the substitutional element with the greatest strengthening effect, improving in-plane anisotropy and securing strength without significantly reducing formability. However, if added excessively, the possibility of brittle fracture increases significantly, which may cause problems such as slab fracture during hot rolling and acting as an element that impairs the surface properties of the plating. Therefore, according to one embodiment of the present invention, the upper limit of phosphorus (P) may be limited to 0.10%. However, 0% is excluded in consideration of the level that is unavoidably added.

[0089]

[0090] Sulfur (S): 0.020% or less

[0091] Sulfur (S) is an unavoidable impurity element added to steel, and it reduces ductility and weldability. Therefore, it is crucial to keep its content as low as possible. In particular, it increases the risk of red-hot embrittlement, so its content can be controlled to 0.020% or less. However, 0% is excluded, considering the level of unavoidable addition during the manufacturing process.

[0092]

[0093] Aluminum (sol.Al): 0.60% or less

[0094] Aluminum (sol.Al) is an element added to refine grain size and deoxidize steel. Similar to Si, it is a ferrite stabilizing element, and it is an effective component for distributing carbon within ferrite to austenite, improving martensite hardenability, and forming retained austenite. In addition, when maintained in the bainite region during annealing, it is a useful element that can effectively suppress the precipitation of carbides within bainite, thereby improving the ductility of the steel sheet. However, if its content exceeds 0.60%, while it is advantageous for increasing strength due to the grain refinement effect, it may increase the possibility of surface defects in the plated steel sheet due to excessive formation of inclusions during steelmaking and continuous casting, and there may be a problem of causing an increase in manufacturing cost. According to one embodiment of the present invention, it may be 0.30% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0095]

[0096] Titanium (Ti): 0.001~0.040%

[0097] Titanium (Ti) is an effective element for increasing the strength of steel sheets and refining grains by forming nano-precipitates. When this element is added, it combines with carbon to form very fine nano-precipitates. These nano-precipitates strengthen the matrix structure and reduce the hardness difference between phases. If the titanium (Ti) content is less than 0.001%, it may be difficult to secure this effect. On the other hand, if the content exceeds 0.040%, the manufacturing cost increases and the excessive precipitates may significantly reduce the ductility. According to one embodiment of the present invention, it may be 0.030% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0098]

[0099] Niobium (Nb): 0.001–0.040%

[0100] Niobium (Nb) is an element that is effective in increasing the strength of steel sheets and refining grains by forming nano-precipitates. When this element is added, it combines with carbon to form very fine nano-precipitates. These nano-precipitates strengthen the matrix structure and reduce the hardness difference between phases. If the niobium (Nb) content is less than 0.001%, it may be difficult to secure such an effect. On the other hand, if the content exceeds 0.040%, the manufacturing cost may increase and the ductility may be significantly reduced due to excessive precipitates. According to one embodiment of the present invention, it may be 0.030% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0101]

[0102] Nitrogen (N): 0.01% or less

[0103] Nitrogen (N) is an element that effectively stabilizes austenite. However, if it exceeds 0.01%, the cost of refining steel increases sharply, and the risk of cracks occurring during casting due to AlN formation, etc., can significantly increase. However, considering the level of unavoidable addition, 0% is excluded.

[0104]

[0105] Boron (B): 0.010% or less

[0106] Boron (B) is a hardenable element that delays the transformation of austenite into pearlite during the cooling process during annealing, inhibits ferrite formation, and promotes martensite formation. However, if its content exceeds 0.010%, excessive boron (B) may be concentrated on the surface, resulting in deterioration of plating adhesion. According to one embodiment of the present invention, the content may be 0.008% or less.

[0107]

[0108] Antimony (Sb): 0.05% or less

[0109] Antimony (Sb) is distributed at grain boundaries, thereby inhibiting the surface enrichment of oxides by delaying the diffusion of oxidizing elements such as manganese, silicon, and aluminum through the grain boundaries. Furthermore, it is excellent in inhibiting the coarsening of surface enrichments due to temperature increases and changes in the hot rolling process. However, if its content exceeds 0.05%, not only is its effect saturated, but manufacturing costs and processability may also be poor. According to one embodiment of the present invention, the content may be 0.03% or less.

[0110]

[0111] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0112]

[0113] A cold rolled steel sheet according to one embodiment of the present invention may have an R value defined in the following relational expression 1 of 0.270 or more.

[0114] [Relationship 1]

[0115] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5

[0116] (In the formula, [C], [Si], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0117]

[0118] According to one embodiment of the present invention, the alloy element content at a point 1 / 4 from the surface of the steel plate toward the center of the thickness can be controlled.

[0119] According to one embodiment of the present invention, the content of C, Si, Al, Cr, and Mo, which are elements related to hardenability, can be controlled. Si and Al are ferrite stabilizing elements that promote ferrite transformation and contribute to the formation of retained austenite and martensite by promoting carbon enrichment in untransformed austenite. C is also an element that contributes to the formation and fraction control of martensite by promoting carbon enrichment in untransformed austenite. In addition, C, Si, and Al have a negative effect on weldability, causing cracks on the surface and inside of the weld. On the other hand, although Mn, Cr, and Mo are elements that contribute to improving hardenability, their effect of contributing to C enrichment in austenite is relatively low compared to C, Si, and Al. Therefore, it is very important to properly adjust the ratio of C, Si, and Al and other hardenable elements Mn, Cr, and Mo.

[0120] When the R value defined in the above relational expression 1 is 0.270 or more, as mentioned above, the properties proposed in the present invention can be secured. On the other hand, when the value is less than 0.270, it is difficult to secure the target bainite and retained austenite fractions, and thus the target hole expandability, elongation, workability, etc. cannot be secured. According to one embodiment of the present invention, it may be 0.280 or more. The upper limit of the R value in relational expression 1 is not particularly limited, but since weldability may be deteriorated due to excessive C, Si, and Mn, it may be 0.350 or less according to one embodiment of the present invention.

[0121]

[0122] Below, the steel microstructure of the present invention is described in detail.

[0123] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0124]

[0125] The microstructure of a cold-rolled steel sheet according to one embodiment of the present invention may include, in area %, a sum of 30 to 70% of bainite and tempered martensite, 50% or less of ferrite, 5% or less of retained austenite, and the remainder fresh martensite.

[0126] In the present invention, the microstructure can be observed at a point 1 / 4 from the surface of the steel plate toward the center of the thickness.

[0127] In the present invention, in order to simultaneously secure high ductility and workability, the fraction between tempered martensite, bainite, and ferrite is strictly controlled.

[0128] In the present invention, in order to appropriately secure the target strength and ductility, the sum of the bainite and tempered martensite fractions may be included at 30% or more. If the sum of the bainite and tempered martensite fractions is less than 30%, the target strength and workability cannot be sufficiently secured. Meanwhile, according to one embodiment of the present invention, the upper limit may be limited to 70% in order to secure ductility. According to one embodiment of the present invention, it may be limited to 50% or less. According to one embodiment of the present invention, it may be 45% or less. According to one embodiment of the present invention, it may be 40% or less.

[0129] In the present invention, the upper limit of the ferrite fraction can be limited to 50% to ensure ductility. Meanwhile, the ferrite fraction can be secured through ideal annealing, and ductility can be improved by introducing some ferrite during the annealing period. According to one embodiment of the present invention, the lower limit of the ferrite fraction can be 20%.

[0130] Because retained austenite can cause a decrease in workability, the upper limit of the content may be limited to 5% in the present invention. Meanwhile, according to one embodiment of the present invention, retained austenite can contribute to improving the ductility of the steel sheet by inducing transformation-induced plasticity. Therefore, according to one embodiment of the present invention, retained austenite may be included in an amount of 2% or more.

[0131] According to one embodiment of the present invention, the residual structure may include fresh martensite. By introducing a small amount of fresh martensite during final cooling, the desired strength can be secured.

[0132]

[0133] According to one embodiment of the present invention, a cold-rolled steel sheet may have a yield ratio of 0.60 or more, a hole expansion ratio (HER) of 30% or more, and a product of the hole expansion ratio (HER) and the elongation (EL) (HER*EL) of 800 or more.

[0134]

[0135] Below, the steel manufacturing method of the present invention is described in detail.

[0136] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cooling, cold rolling, continuous annealing, primary cooling, secondary cooling, reheating and maintaining, cooling, and temper rolling a steel slab satisfying the above-described alloy composition.

[0137]

[0138] Reheating

[0139] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be reheated to a temperature range of 1100 to 1300°C.

[0140] The slab reheating process can be performed to facilitate the subsequent rolling process and to sufficiently obtain the desired properties of the steel sheet. The reheating temperature of the present invention is not particularly limited and may be conventional reheating conditions. According to one embodiment of the present invention, reheating can be performed at a temperature range of 1100 to 1300°C.

[0141]

[0142] hot rolling

[0143] The above reheated steel slab can be hot rolled at a finishing rolling temperature of Ar3 to Ar3+50℃.

[0144] In the present invention, hot rolling conditions are not particularly limited, and hot rolling can be performed at a typical hot rolling temperature. According to one embodiment of the present invention, hot rolling can be performed at a finishing rolling temperature of 800 to 1000°C.

[0145] [ceremony]

[0146] Ar3 = 910-310[C]-80[Mn]-55[Ni]-15[Cr]-80[Mo]-20[Cu]+0.35(t-8)

[0147] (In the formula, [C], [Mn], [Ni], [Cr], [Mo] and [Cu] are the weight% of each element, and t is the thickness of the steel plate (mm).)

[0148]

[0149] Winding

[0150] The above hot-rolled steel plate can be coiled at a temperature range of 400 to 650°C.

[0151] By performing the above coiling process, a steel sheet can be manufactured with finely dispersed carbides, which serve as austenite nucleation sites. By evenly dispersing fine carbides during the hot rolling process, the carbides dissolve during subsequent annealing, forming finely dispersed austenite. Consequently, fine martensite can be uniformly distributed after annealing.

[0152] If the above coiling temperature is less than 400℃, there may be a problem of material deviation in the width direction of the steel sheet. On the other hand, if the temperature exceeds 650℃, excessive oxide formation on the surface of the steel sheet may occur, resulting in poor plating properties.

[0153] According to one embodiment of the present invention, after the coiling, cooling to room temperature can be performed. When cooling to room temperature after the coiling, the average cooling rate is not particularly limited and can be any condition applicable in the same technical field. According to one embodiment of the present invention, air cooling can be performed. According to an embodiment of the present invention, when cooling to room temperature, the average cooling rate can be limited to 0.1°C / s or less.

[0154]

[0155] cold rolling

[0156] The above-mentioned rolled steel plate can be cold rolled at a reduction ratio of 40 to 70%.

[0157] If the cold rolling reduction rate is below 40%, it will be difficult to achieve the target thickness and shape correction of the steel sheet may be difficult. Conversely, if it exceeds 70%, cracks are more likely to occur at the steel sheet edges and can lead to problems with cold rolling loads.

[0158]

[0159] continuous annealing

[0160] The above cold-rolled steel sheet can be continuously annealed in a temperature range of 790 to 830°C.

[0161] The above continuous annealing process can secure the desired ferrite fraction through ideal annealing to improve strength and ductility.

[0162] If the continuous annealing temperature is lower than 790°C, it is difficult to secure a sufficient austenite fraction, and thus the ferrite fraction may increase excessively after annealing, making it difficult to secure sufficient strength. According to one embodiment of the present invention, continuous annealing can be performed in a temperature range of 795°C or higher. On the other hand, if the temperature exceeds 830°C, excessive austenite is formed, the ferrite fraction decreases, and it may be difficult to secure the desired ductility. In addition, surface enrichment due to elements that deteriorate the surface quality, such as Si, Mn, and B, may become severe, thereby deteriorating the surface quality. According to one embodiment of the present invention, continuous annealing can be performed in a temperature range of 820°C or lower.

[0163]

[0164] Primary cooling

[0165] The above continuously annealed steel plate can be first cooled at an average cooling rate of 10.0°C / s or less to a temperature range of 630 to 680°C.

[0166] According to one embodiment of the present invention, by controlling the end temperature during primary cooling, the cooling capacity of the equipment can be taken into consideration and a small amount of ferrite can be additionally secured.

[0167] If the final temperature is below 630°C during the first cooling, there may be a problem with insufficient ferrite formation. On the other hand, if the final temperature exceeds 680°C, there may be a problem with the ferrite fraction becoming excessively high.

[0168] During the first cooling, if the average cooling rate exceeds 10.0°C / s, there may be a problem of the bainite fraction becoming excessively high. According to one embodiment of the present invention, it may be 5.0°C / s or less. The lower limit of the average cooling rate is not particularly limited, but according to one embodiment of the present invention, it may be 2.0°C / s or more.

[0169]

[0170] Secondary cooling

[0171] The first-cooled steel plate can be secondarily cooled at an average cooling rate of 5°C / s or more to a temperature range of 320 to 360°C.

[0172] According to one embodiment of the present invention, in a Q&P process in which rapid cooling to a temperature below Ms is followed by reheating to a temperature above Ms, the end temperature is strictly controlled during the first cooling and second cooling, thereby forming a desired microstructure.

[0173] When the cooling end temperature exceeds 360℃ during the secondary cooling, the fraction of initially formed martensite is very small or martensite formation is difficult, so that it is difficult to form tempered martensite and bainite of the target fraction during the final cooling, and thus the target hole expandability cannot be obtained. On the other hand, when the temperature is less than 320℃, the fractions of tempered martensite and bainite are excessive, so that it is difficult to form fresh martensite of the target fraction during the final cooling, and thus the desired strength cannot be obtained.

[0174] When the average cooling rate is less than 5°C / s during secondary cooling, the initial martensite fraction may be very low or martensite formation may be difficult, and thus, there may be a problem in that the fraction of tempered martensite cannot be sufficiently secured during the final cooling. According to one embodiment of the present invention, it may be 10°C / s or more. Meanwhile, the upper limit is not particularly limited, but according to one embodiment of the present invention, it may be 50°C / s or less.

[0175] When cooling, there is no particular limitation on the conditions, but according to one embodiment of the present invention, a hydrogen rapid cooling facility using H2 gas can be used when cooling.

[0176]

[0177] Reheat and maintain

[0178] The above secondarily cooled steel plate can be reheated to a temperature range of Ms or higher and maintained for 60 seconds or longer.

[0179] According to one embodiment of the present invention, martensite formed in a secondary cooling process is reheated to a temperature range of Ms or higher and maintained for 60 seconds or longer to form tempered martensite and bainite through a Q&P process, and carbon is concentrated in the surrounding untransformed austenite. Furthermore, fine fresh martensite can be introduced through the process.

[0180] According to one embodiment of the present invention, the Ms temperature can be determined by the martensite transformation start temperature and the Ms temperature formula applied in the same technical field. According to one embodiment of the present invention, the Ms temperature can be obtained through the following formula.

[0181] [ceremony]

[0182] Ms = 539-423[C]-30.4[Mn]-12.1[Cr]-17.7[Ni]-7.5[Mo]

[0183] (In the formula, [C], [Mn], [Cr], [Ni] and [Mo] are the weight percent of each element.)

[0184]

[0185] tertiary cooling

[0186] The above reheated and maintained steel plate can be cooled a third time at an average cooling rate of 5°C / s or more to a temperature range of 150°C or less.

[0187] During the third cooling, if the final temperature exceeds 150°C, the phase transformation may not be completed, which may result in failure to secure the desired properties. Furthermore, during the third cooling, if the average cooling rate is less than 5°C / s, there may be a problem in which additional phase transformation occurs.

[0188]

[0189] Temper rolling

[0190] The above thirdly cooled steel plate can be temper rolled to less than 1%.

[0191] In the present invention, temper rolling may be additionally performed to control the plate shape. In the present invention, temper rolling conditions are not particularly limited, and can be performed under normal conditions.

[0192]

[0193] 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 explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0194]

[0195] After preparing a steel slab having the composition shown in Table 1, it was reheated to a temperature range of 1050-1250℃, and finish hot-rolled at a temperature range of Ar3+50℃~950℃, which is higher than the Ar3 transformation point temperature. Thereafter, the hot-rolled steel sheet was coiled at a temperature range of 400-650℃, and cooled at a cooling rate of 0.1℃ / s or less to produce a hot-rolled steel sheet. After pickling the hot-rolled steel sheet, cold rolling was performed at a reduction ratio of 40-70%, and then continuous annealing, primary and secondary cooling were performed under the conditions disclosed in Table 2. In addition, after the reheating and holding process, a third cooling was performed, and temper rolling of less than 1% was performed to produce a cold-rolled steel sheet. During the reheating and holding, all specimens were reheated to a temperature range higher than the Ms temperature and held for 60 seconds or more.

[0196]

[0197] Steel alloy composition (weight %) relationship 1CSiMnPSS.AlNbTiBCrMoR Value A0.140.702.000.010.0010.030.0200.015-0.20.010.305 B0.140.701.900.010.0010.050.0200.006-0.40.020.342 C0.150.902.000.010.0010.040.0200.003-0.10.040.308 D0.140.802.100.010.0010.040.0250.0200.0010-0.050.28 2E0.070.452.300.010.0010.030.0100.020-0.10.020.224F0.090.502.350.010.0010.030.0070.006-0.20.010.266G 0.120.651.800.010.0010.020.0050.0300.0015-0.020.236H0.100.502.000.010.0010.040.0200.001-0.10.070.251

[0198] [Relationship 1] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5

[0199] (In the formula, [C], [Si], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0200]

[0201] Specimen number Steel grade Continuous annealing 1st cooling 2nd coolant heating and maintenance Temperature (℃) Temperature (℃) Speed ​​(℃ / s) Temperature (℃) Speed ​​(℃ / s) Temperature (℃) 1A8046702.9340244502A8506703.9340244503A7706702.2343244504A8106703.0442164505A8106703.0300274506B 8006503.3345224467C8136593.3339234518D8206523.6346224499E8066503.44 001844010F8206603.53902045011G8106523.44041844412H8036553.240518470

[0202] The microstructural characteristics and mechanical properties of each manufactured steel plate were evaluated, and the results are shown in Table 3 below.

[0203] The microstructural fractions were analyzed by analyzing the matrix at a point 1 / 4 of the thickness from the surface of the annealed steel plate toward the center of the thickness. Specifically, the fractions of bainite, tempered martensite, ferrite, fresh martensite, and retained austenite were measured using FE-SEM and an image analyzer after nickel corrosion.

[0204] In addition, a tensile test was conducted on each specimen in the C direction using the JIS standard, and the tensile properties, yield strength (YS), tensile strength (TS), yield ratio (YS / TS), and elongation (El) were evaluated and presented. Hole expandability (HER) was measured using a hole expandability tester.

[0205]

[0206] Specimen number Steel grade Microstructure Property Classification Fraction (area %) YS (MPa) TS (MPa) Yield ratio HER (%) El (%) HER*El B+TMFFMRA1A30452325088170.623823874 Invention example 12A76101136709210.735613728 Comparative example 13A14592524208030.522527675 Comparative example 24A24522404378110.542622572 Comparative example 35A3552945727700.744221882 Comparative example 4 6B32432325228260.633623828Invention Example 27C33392535118180.623425850Invention Example 38D38382225318190.654021840Invention Example 49E24532304228240.512421504Comparative Example 510F17552804108320.492222484Comparative Example 611G13602704108390.492321483Comparative Example 712H20602004148210.502624624Comparative Example 8

[0207]

[0208] * B: Bainite, TM: Tempered Martensite, F: Ferrite, FM: Fresh Martensite, RA: Retained Austenite

[0209] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0210] Figure 1 is a graph showing the change in the product of hole expandability (HER) and elongation (EL) according to relational expression 1 according to one embodiment of the present invention. When the value of relational expression 1 is 0.270 or higher, it can be confirmed that the product of hole expandability and elongation targeted by the present invention can be secured.

[0211] Figure 2 is a graph showing changes in hole expandability (HER) according to relational expression 1 according to one embodiment of the present invention. When the value of relational expression 1 is 0.27 or higher, it can be confirmed that the hole expandability value targeted by the present invention can be secured.

[0212] Figure 3 is a graph showing the change in yield ratio according to relational expression 1 according to one embodiment of the present invention. When the value of relational expression 1 is 0.27 or higher, it can be confirmed that the yield ratio value targeted by the present invention can be secured.

[0213]

[0214] On the other hand, in the case of Comparative Example 1, the temperature was excessively high during continuous annealing, and the desired elongation was not secured.

[0215] In Comparative Example 2, the annealing temperature fell below the suggested range, resulting in excessive formation of abnormal ferrite. As a result, the target strength could not be achieved.

[0216] In Comparative Example 3, the end temperature was excessively high during the secondary cooling, failing to sufficiently secure tempered martensite. As a result, the desired hole expandability was not achieved.

[0217] In Comparative Example 4, the end temperature was not reached during the secondary cooling, so tempered martensite and bainite were excessively formed, resulting in poor strength.

[0218] Comparative Examples 5 to 8 are examples where the value of Equation 1 fell short of the range proposed by the present invention. As a result, excessive ferrite formation occurred, and bainite and tempered martensite formation was insufficient. Consequently, the desired yield ratio and pore expandability could not be achieved.

[0219]

[0220] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in weight%, carbon (C): 0.08 to 0.16%, silicon (Si): 1.00% or less, manganese (Mn): 1.60 to 3.00%, molybdenum (Mo): 0.40% or less, chromium (Cr): 1.0% or less, phosphorus (P): 0.10% or less, sulfur (S): 0.020% or less, aluminum (sol.Al): 0.60% or less, titanium (Ti): 0.001 to 0.040%, niobium (Nb): 0.001 to 0.040%, nitrogen (N): 0.01% or less, boron (B): 0.010% or less, antimony (Sb): 0.05% or less, the remainder iron (Fe) and other unavoidable impurities. The R value defined in the following relational expression 1 is greater than or equal to 0.270, Cold rolled steel sheet containing a microstructure in area % of 30 to 70% of bainite and tempered martensite, 50% or less of ferrite, 5% or less of retained austenite, and the remainder fresh martensite. [Relationship 1] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5 (In the formula, [C], [Si], [Mn], [Cr], and [Mo] are the weight % of each element.) 2. In claim 1, The above microstructure is a cold rolled steel sheet containing, in area %, 30 to 50% of bainite and tempered martensite combined, 20 to 50% of ferrite, 2 to 5% of retained austenite, and the remainder of fresh martensite.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield ratio of 0.60 or more, a hole expansion ratio (HER) of 30% or more, and a product of hole expansion ratio (HER) and elongation (EL) (HER*EL) of 800 or more.

4. A steel slab containing, by weight%, carbon (C): 0.08 to 0.16%, silicon (Si): 1.00% or less, manganese (Mn): 1.60 to 3.00%, molybdenum (Mo): 0.40% or less, chromium (Cr): 1.0% or less, phosphorus (P): 0.10% or less, sulfur (S): 0.020% or less, aluminum (sol.Al): 0.60% or less, titanium (Ti): 0.001 to 0.040%, niobium (Nb): 0.001 to 0.040%, nitrogen (N): 0.01% or less, boron (B): 0.010% or less, antimony (Sb): 0.05% or less, the remainder iron (Fe) and other unavoidable impurities, and having an R value of 0.270 or more as defined in the following relational expression 1. reheating step; A step of hot rolling the above reheated steel slab; A step of coiling the hot-rolled steel plate; A step of cold rolling the above-mentioned rolled steel plate; A step of continuously annealing the above cold-rolled steel plate at a temperature range of 790 to 830°C; A step of first cooling the continuously annealed steel plate to a temperature range of 630 to 680°C; A step of secondarily cooling the above-mentioned first-cooled steel plate to a temperature range of 320 to 360°C; A step of reheating and maintaining the secondarily cooled steel plate to a temperature range of Ms or higher; and A method for manufacturing a cold rolled steel sheet, comprising: a step of re-cooling the reheated and maintained steel sheet to a temperature range of 150°C or less. [Relationship 1] R = [C]+[Si] / 30+[Mn] / 20+([Cr]+[Mo]) / 5 (In the formula, [C], [Si], [Mn], [Cr], and [Mo] are the weight % of each element.) 5. In claim 4, The above reheating step is performed at a temperature range of 1100 to 1300°C. The above hot rolling step is performed at a finishing rolling temperature of Ar3 to Ar3+50℃. The above coiling step is performed by coiling at a temperature range of 400 to 650°C and then cooling to room temperature at an average cooling rate of 0.1°C / s or less. The above cold rolling step is performed at a reduction ratio of 40 to 70%. The above first cooling step cools at an average cooling rate of 10.0℃ / s or less, The above second cooling step cools at an average cooling rate of 5℃ / s or more. The above reheating and maintenance step is maintained for more than 60 seconds after reheating, A method for manufacturing a cold rolled steel sheet, wherein the third cooling step is performed at an average cooling rate of 5°C / s or more.

6. In claim 4, A method for manufacturing a cold rolled steel sheet further comprising the step of subjecting the thirdly cooled steel sheet to a temper rolling process of less than 1%.

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