Cold-rolled steel sheet and method for manufacturing same

Abstract

The present application relates to a cold-rolled steel sheet and a method for manufacturing same. According to the present application, it is possible to provide a high strength cold-rolled steel sheet having excellent hydrogen embrittlement properties, and a method for manufacturing same.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present application relates to a cold-rolled steel sheet and a method for manufacturing the same.

[0002] In the automotive industry, the demand for crash safety in vehicle bodies has been continuously increasing. Recently, as the adoption of electric vehicles has expanded, the number of vehicle parts has decreased; however, the increase in vehicle weight due to the introduction of batteries has further amplified the requirements for crash safety. Accordingly, the ultra-high strength of crash components that contribute to crash safety, such as front bumper beams, side sills, and door impact beams, is also continuously being pursued. In particular, the application of martensitic steel, which possesses the highest strength among cold-rolled steel sheets, is expanding as the use of roll forming techniques increases.

[0003] Patent Document 1 (Japanese Published Patent No. 1992-289120) proposes a method for manufacturing general high-yield martensitic steel by immersing a hot plate (microstructure: austenite) in cold water and rapidly cooling it. At this time, the rapidly cooled steel plate forms martensite with a very high dislocation density inside, and when reheated, transition carbides or fine cementite are formed on these dislocations, increasing the yield strength and improving hydrogen embrittlement.

[0004] However, this method has two major problems. First, the plate formation becomes distorted due to the rapid cooling speed of the water cooling method. Second, since the types of steel that use the water cooling immersion method are limited, it is difficult to produce them in combination with the general gas cooling method. In particular, steel companies that generally do not have water cooling equipment cannot produce these types of steel.

[0005] Therefore, to solve these problems, a cold-rolled steel sheet having high yield capacity and high hydration embrittlement, which is produced by increasing dislocation density in general martensitic steel rather than by increasing dislocation density through a water-cooled immersion method, and a method for manufacturing the same are required.

[0006] The objective of the present application is to provide a cold-rolled steel sheet having high yield and high odor embrittlement and a method for manufacturing the same.

[0007] To solve the above problem, the cold-rolled steel sheet of the present application comprises, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, and the remainder is composed of Fe and other unavoidable impurities, and in a region corresponding to 25% of the thickness from the surface, it comprises a tempered martensite structure with an area fraction greater than 80% and less than 100%, and the remainder is composed of one or more selected from the martensite structure, ferrite structure, bainite structure and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more.

[0008] In addition, the cold-rolled steel sheet may further include, in weight%, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V and Ti: greater than 0% and less than or equal to 0.1%.

[0009] In addition, the above cold-rolled steel sheet may have a tensile strength (TS) of 1470 MPa or more.

[0010] In addition, the method for manufacturing a cold-rolled steel sheet according to the present application comprises the steps of: reheating and hot-rolling a slab comprising, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, and the remainder being Fe and other unavoidable impurities; coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet while unwinding it; annealing the cold-rolled steel sheet; cooling the annealed cold-rolled steel sheet; over-aging the cooled cold-rolled steel sheet; and temper-rolling the over-aged cold-rolled steel sheet. A method for manufacturing a cold-rolled steel sheet comprising the step of tempering a cold-rolled steel sheet that has been tempered, wherein the tempered cold-rolled steel sheet comprises a tempered martensite structure with an area fraction of more than 80% and less than 100% in an area corresponding to 25% of the thickness from the surface, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more.

[0011] In addition, the above temper rolling step can be performed by temper rolling the over-aged cold-rolled steel sheet with a reduction rate of 0.05% or more and 1.0% or less.

[0012] Additionally, the cooling step may include a first cooling step of cooling to a cooling temperature of 500°C or higher and 650°C or lower; and a second cooling step of cooling to a cooling temperature of 100°C or higher and less than 300°C.

[0013] In addition, the method for manufacturing the above cold-rolled steel sheet can satisfy the following general formula 1.

[0014] [General Formula 1]

[0015] 1470 ≤ T 1C +200×(10C+Mn+Cr+Mo)

[0016] In the above general formula 1, T 1Cis the cooling temperature (°C) in the first cooling stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0017] In addition, the method for manufacturing the above cold-rolled steel sheet can satisfy the following general formula 2.

[0018] [General Formula 2]

[0019] 200 ≤ ((T T +273)(20+Log(t T ))) / (T 2C ×R SPM )

[0020] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage, and T 2C is the cooling temperature (°C) in the second cooling stage, and R SPM is the reduction rate (%) at the temper rolling stage.

[0021] In addition, the tempering step may be performed at a tempering temperature of 150°C or higher and 300°C or lower for a tempering time of 1 hour or more and 30 hours or less.

[0022] In addition, the overaging treatment step described above can be performed at an overaging treatment temperature of 150°C or higher and 350°C or lower.

[0023] According to the present application, a cold-rolled steel sheet having high yield and high odor-relief properties and a method for manufacturing the same can be provided.

[0024] In the description of numerical ranges in this specification, the notation “X~Y” indicates X or greater and Y or less, unless otherwise specifically stated. Additionally, “greater than or equal to” may be replaced with “greater than,” and “less than or equal to” may be replaced with “less than.”

[0025] In addition, regarding the numerical ranges described stepwise in this specification, any upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of another numerical range described stepwise, or may be substituted with a value shown in the examples.

[0026] The present application relates to a cold-rolled steel sheet. The cold-rolled steel sheet comprises, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, with the remainder being Fe and other unavoidable impurities; comprises a tempered martensite structure with an area fraction greater than 80% and less than 100% in a region corresponding to 25% of the thickness from the surface, and comprises one or more selected from the remainder being a martensite structure, a ferrite structure, a bainite structure, and a retained austenite structure, and has a yield strength (YP) of 1180 MPa or more. The cold-rolled steel sheet of the present application can have high yield capacity and high decomposition embrittlement by satisfying the aforementioned composition ratios and being composed of the aforementioned structure satisfying the aforementioned area fraction range in the aforementioned region.

[0027] Specifically, the area fraction of the tempered martensite structure formed in the region corresponding to 25% of the thickness from the surface may be 82% or more and 95% or less, or 84% or more and 90% or less.

[0028] In addition, the above cold-rolled steel sheet may have a yield strength of 1210 MPa or more or 1220 MPa or more. At this time, the upper limit of the yield strength of the above cold-rolled steel sheet may be 1500 MPa or less or 1300 MPa or less. By satisfying the aforementioned range of yield strength, the above cold-rolled steel sheet may have a high yield strength, and as a result, when applied to automobile body parts, it contributes to improving passenger safety and enables thinning of the thickness, thereby contributing to vehicle body weight reduction and fuel efficiency improvement. At this time, the above yield strength was measured through a tensile test described later.

[0029] For example, the area fraction of the ferrite structure formed in an area corresponding to 25% of the thickness from the surface of the cold-rolled steel sheet may be greater than 0% and less than 10%. Additionally, the area fraction of the bainite structure formed in an area corresponding to 25% of the thickness from the surface of the cold-rolled steel sheet may be greater than 0% and less than 20%. Furthermore, the area fraction of the retained austenite structure formed in an area corresponding to 25% of the thickness from the surface of the cold-rolled steel sheet may be greater than 0% and less than or equal to 1%.

[0030] In one example, the cold-rolled steel sheet may contain carbides including cementite, transition carbides, and fine precipitates in an area corresponding to 25% of the thickness from the surface.

[0031] The above cementite (Fe3C) is a carbide with an atomic ratio of iron (Fe) to carbon (C) of 3:1. It is formed when the tempering temperature is high or the tempering time is long, and because it is formed coarsely in a needle-like shape, it can reduce bendability and hydrogen embrittlement. Therefore, the area fraction of the above cementite may be 0% or more and 5% or less. At this time, the above cementite may contain some nitrogen.

[0032] In addition, the above-mentioned transition carbide is a carbide comprising an ε-carbide having an atomic ratio of carbon to a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), or molybdenum (Mo), of 2.5:1, or an η-carbide having an atomic ratio of 2:1, which can increase the Yield Ratio (YR), which is the ratio of yield strength to tensile strength, while simultaneously improving bendability and hydrogen embrittlement. Accordingly, the area fraction of the above-mentioned transition carbide may be greater than 0% and less than or equal to 5%. At this time, the above-mentioned transition carbide may contain some nitrogen.

[0033] In addition, the above-mentioned fine precipitates are carbides in which the atomic ratio of carbon (C) or nitrogen (N) to an alloying element, which is one of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo), is 1:1, and are formed during hot rolling or during coiling after hot rolling, and contain fine alloying elements (Ti, Nb, V, Mo, etc.), and unlike the above-mentioned cementite and the above-mentioned transition carbides, iron (Fe) is not present as a constituent element. The area fraction of the above-mentioned fine precipitates may be greater than 0% and less than or equal to 5%. By having the aforementioned area fraction, the above-mentioned fine precipitates may have a precipitation strengthening effect and a grain refinement effect. Conversely, if the area fraction of the above-mentioned fine precipitates exceeds the aforementioned range, the size of the precipitates (Ti, Nb, V and / or N) becomes coarse during the continuous casting process, and the precipitation strengthening effect and grain refinement effect may be reduced. At this time, the above-mentioned fine precipitates may contain some nitrogen.

[0034] The area fraction of the above cementite, transition carbides, and microprecipitates can be measured through transmission electron microscopy (TEM) analysis using FIB sampling.

[0035] The alloy composition of the above cold-rolled steel sheet is described below.

[0036] C: 0.23 wt% or more, 0.30 wt% or less

[0037] Carbon (C) is an element that is dissolved in the martensite structure to secure strength through improved hardness. If the carbon is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it may reduce strength. Additionally, if the carbon is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, weldability and workability may be reduced. Accordingly, the carbon may be included in the cold-rolled steel sheet in an amount of 0.23 wt% or more and 0.30 wt% or less, and specifically, in an amount of 0.24 wt% or more and 0.26 wt% or less.

[0038] Mn: 1.0 wt% or more and 1.5 wt% or less

[0039] Manganese (Mn) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability. If the manganese is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, it may reduce strength. Additionally, if the manganese is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, the resistance to hydrogen embrittlement may be reduced due to the formation of manganese bands, MnS, etc., which may reduce bendability. Therefore, the manganese may be included in the cold-rolled steel sheet in an amount of 1.0 wt% or more and 1.5 wt% or less, specifically, in an amount of 1.2 wt% or more and 1.5 wt% or less.

[0040] Si: 0.03 wt% or more, 0.3 wt% or less

[0041] Silicon (Si) is an element used to ensure hydrogen embrittlement resistance by suppressing the formation of cementite. If the silicon is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, ductility may be reduced. Additionally, if the silicon is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, strength may be reduced by forming a ferrite structure. Accordingly, the silicon may be included in the cold-rolled steel sheet in an amount of 0.03 wt% or more and 0.3 wt% or less, and specifically, in an amount of 0.05 wt% or more and 0.2 wt% or less, or in an amount of 0.08 wt% or more and 0.1 wt% or less.

[0042] Cr: Greater than 0 wt% and less than or equal to 0.8 wt%

[0043] Chromium (Cr) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability. If the chromium is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may reduce laser weldability and ductility. Therefore, the chromium may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.8 weight%.

[0044] Mo: Greater than 0 wt% and less than or equal to 0.3 wt%

[0045] Molybdenum (Mo) is an element that contributes to strength improvement through solid solution strengthening and increased hardenability, and can refine Ti-based precipitates. If the above molybdenum is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, the cost may increase. Therefore, the above molybdenum may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.3 weight%.

[0046] Remaining Fe and other unavoidable impurities

[0047] The aforementioned unavoidable impurities are impurities introduced during the steelmaking and manufacturing processes of cold-rolled steel sheets. Since this is widely known in the industry, a detailed description is omitted. In one embodiment of this application, the addition of elements other than the components of the cold-rolled steel sheet described above is not excluded, and various elements may be included within a scope that does not impair the technical concept of this application. If additional elements are included, they may be included to replace the remainder, which is iron (Fe).

[0048] In one example, the cold-rolled steel sheet may further include, in weight%, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V and Ti: greater than 0% and less than or equal to 0.1%.

[0049] Sol. Al: Greater than 0 wt% and less than or equal to 0.1 wt%

[0050] Soluble aluminum (Sol. Al) is aluminum that exists in a solid solution state within the iron of a cold-rolled steel sheet and is an element used as a deoxidizer by combining with oxygen within the cold-rolled steel sheet. If the soluble aluminum is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the deoxidation effect may be reduced. Furthermore, if the soluble aluminum is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may form AlN and cause cracking of the slab. Accordingly, the soluble aluminum may be included in the cold-rolled steel sheet in an amount greater than 0 wt% and less than or equal to 0.1 wt%, and specifically, in an amount greater than or equal to 0.01 wt% and less than or equal to 0.08 wt% or greater than or equal to 0.02 wt% and less than or equal to 0.05 wt%.

[0051] P: Greater than 0 wt% and less than or equal to 0.05 wt%

[0052] Phosphorus (P) is an element that improves the strength of cold-rolled steel sheets but increases brittleness through grain boundary segregation and reduces spot weldability. Therefore, the above phosphorus may be included in the cold-rolled steel sheets in an amount greater than 0 weight% and less than or equal to 0.05 weight%.

[0053] S: Greater than 0 wt% and less than or equal to 0.01 wt%

[0054] Sulfur (S) is an element that reduces hydrogen embrittlement resistance by forming non-metallic inclusions of MnS. Therefore, the sulfur may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.01 weight%.

[0055] N: Greater than 0 wt% and less than or equal to 0.01 wt%

[0056] Nitrogen (N) is an element that is inevitably added. Therefore, the nitrogen may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.01 weight%.

[0057] B: Greater than 0 wt% and less than or equal to 0.004 wt%

[0058] Boron (B) is a strong hardenable element and contributes significantly to improving the hardenability of cold-rolled steel sheets. If the boron is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, grain boundary brittleness may increase due to BN formation. Therefore, the boron may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.004 weight%.

[0059] Total of Nb, V, and Ti: Greater than 0 wt% and less than or equal to 0.1 wt%

[0060] The above niobium (Nb), the above vanadium (V), and the above titanium (Ti) are elements that help refine the grain size by forming fine precipitates (NbC, VC, TiC, etc.) during the hot rolling and cold rolling stages. If the total of the above niobium, the above vanadium, and the above titanium included in the cold-rolled steel sheet exceeds the upper limit of the aforementioned range, a problem may occur in which coarse nitrides (TiN, NbN, VN, etc.) are formed during the steelmaking and continuous casting stages, resulting in uneven solid solution. Therefore, the above niobium, the above vanadium, and the above titanium may be included in the cold-rolled steel sheet in a total amount of more than 0 weight% and less than or equal to 0.1 weight%.

[0061] In one example, the cold-rolled steel sheet may have a tensile strength (TS) of 1470 MPa or higher. Specifically, the cold-rolled steel sheet may have a tensile strength of 1530 MPa or higher or 1540 MPa or higher. Additionally, the upper limit of the tensile strength of the cold-rolled steel sheet may be 1800 MPa or lower or 1600 MPa or lower. By satisfying the aforementioned ranges for tensile strength, the cold-rolled steel sheet may possess high tensile strength, thereby contributing to improved passenger safety when applied to automobile body parts, and enabling thinning of the thickness, which can contribute to vehicle body weight reduction and improved fuel efficiency. At this time, the tensile strength was measured through a tensile test described below.

[0062] In addition, the above cold-rolled steel sheet may have an elongation (EL) of 5% or more. Specifically, the above cold-rolled steel sheet may have an elongation of 6% or more or 7% or more. In addition, the upper limit of the elongation of the above cold-rolled steel sheet may be less than 10%. By satisfying the aforementioned range of elongation, the above cold-rolled steel sheet may have excellent formability into a part. At this time, the above elongation was measured through a tensile test described later.

[0063] This application also relates to a method for manufacturing a cold-rolled steel sheet. The above method for manufacturing a cold-rolled steel sheet relates to a method for manufacturing the aforementioned cold-rolled steel sheet. Since specific details regarding the cold-rolled steel sheet described below can be applied in the same way as those described for the cold-rolled steel sheet, they will be omitted.

[0064] The method for manufacturing a cold-rolled steel sheet according to the present application comprises the steps of hot rolling, coiling, cold rolling, annealing, cooling, overaging treatment, temper rolling, and tempering. Furthermore, the cold-rolled steel sheet tempered through the tempering step comprises, internally, a tempered martensite structure with an area fraction of more than 80% and less than 100% in an area corresponding to 25% of the thickness from the surface, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure, and retained austenite structure, and has a yield strength (YP) of 1180 MPa or more. Since the specific description of each structure formed in the aforementioned area and the yield strength is the same as that described in the cold-rolled steel sheet, it will be omitted. According to the method for manufacturing a cold-rolled steel sheet according to the present application, a cold-rolled steel sheet having high strength and high desiccation embrittlement can be manufactured.

[0065] The above hot rolling step is a step for manufacturing a slab into a hot-rolled steel sheet, and is performed by reheating and then hot-rolling a slab comprising, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, with the remainder being Fe and other unavoidable impurities. At this time, since the specific description of the composition of the slab is the same as that described in the above cold-rolled steel sheet, it will be omitted.

[0066] In addition, the above slab may further include, in weight percent, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V, and Ti: greater than 0% and less than or equal to 0.1%. Since a specific description of the additional composition of the above slab is the same as that described in the above cold-rolled steel sheet, it will be omitted.

[0067] Reheating the slab is performed before hot rolling the slab to bring the slab to a temperature suitable for rolling. In one example, the reheating temperature of the slab is not specifically limited, but may be, for example, between 1150°C and 1300°C. By satisfying the aforementioned range, the slab can be reheated to a temperature of Ac3 or higher to re-dissolve the components during casting. Conversely, if the reheating temperature of the slab is below the lower limit of the aforementioned range, the added components are not uniformly re-dissolved, and the rolling load may increase during the hot rolling stage; if it exceeds the upper limit of the aforementioned range, the finishing temperature of the rough rolling and finishing rolling may rise, leading to grain coarsening and a deterioration in the material quality of the final product. Therefore, the reheating temperature of the slab may satisfy the aforementioned range.

[0068] As described above, the hot rolling is performed to manufacture the reheated slab into a hot-rolled steel sheet. For example, the finishing temperature during the hot rolling may be 800°C or higher and 1000°C or lower. If the finishing temperature during the hot rolling is below the lower limit of the aforementioned range, the rolling load increases, which may lead to a decrease in productivity. Furthermore, if the finishing temperature during the hot rolling exceeds the upper limit of the aforementioned range, it may cause grain coarsening, which may lead to a decrease in strength. Therefore, the finishing temperature during the hot rolling may satisfy the aforementioned range. At this time, the reduction rate during the hot rolling may be 80% or higher and 95% or lower. Subsequently, the hot-rolled steel sheet may be cooled at a cooling rate of 1°C / s or higher and 100°C / s or lower.

[0069] The above-mentioned winding step is a step of winding the hot-rolled steel sheet obtained by the above-mentioned hot rolling.

[0070] In one example, the coiling step may be performed at a temperature of 400°C or higher and 740°C or lower. By performing the coiling step within the aforementioned temperature range, the shape control and cold rolling performance of the hot-rolled coil after coiling are excellent, and it may be advantageous for realizing the material of the final product. On the other hand, if the coiling step is performed below the lower limit of the aforementioned temperature range, the load during cold rolling may increase. Furthermore, if the coiling step is performed above the upper limit of the aforementioned temperature range, the crystal grains of the final product may coarsen, and the material may decrease.

[0071] In one example, the method for manufacturing the cold-rolled steel sheet may further include a pickling step. The pickling step is a step for removing the surface scale layer of the coiled hot-rolled steel sheet and can be performed by pickling the coiled hot-rolled steel sheet. Since all conditions known in the art can be applied as the pickling conditions, this is omitted.

[0072] The above cold rolling step is a step for manufacturing the above hot-rolled steel sheet into a cold-rolled steel sheet, and is performed by cold rolling while unwinding the coiled hot-rolled steel sheet.

[0073] In one example, the reduction rate during cold rolling may be 30% or more and less than 70%. If the reduction rate during cold rolling is below the lower limit of the aforementioned range, it may cause grain coarsening of the final structure, thereby causing a decrease in strength. Additionally, if the reduction rate during cold rolling exceeds the upper limit of the aforementioned range, the rolling load may increase, leading to a decrease in productivity. Therefore, the reduction rate during cold rolling may satisfy the aforementioned range.

[0074] The above annealing step is a step of heat-treating a cold-rolled steel sheet.

[0075] In one example, the annealing step may be performed by annealing the cold-rolled steel sheet at an annealing temperature of 800°C or higher and 900°C or lower. Specifically, the annealing step may be performed by annealing the cold-rolled steel sheet at an annealing temperature of 800°C or higher and 900°C or lower for 60 seconds or more and 180 seconds or less. Performing the annealing step within the aforementioned annealing temperature range may be advantageous for securing an austenite single-phase region. On the other hand, if the annealing step is performed below the lower limit of the aforementioned annealing temperature range, it is disadvantageous for securing a single-phase region, which may result in a decrease in the material properties of the final product. Furthermore, if the annealing step is performed above the upper limit of the aforementioned annealing temperature range, the crystal grains of the final product may coarsen, which may result in a decrease in material properties.

[0076] The above cooling step is a step of cooling annealed cold-rolled steel sheet. At this time, the cooling may be performed using a gas cooling method known in the art to ensure the flatness of the cold-rolled steel sheet.

[0077] In one example, the cooling step may include a first cooling step and a second cooling step.

[0078] The above first cooling step is a step of cooling an annealed cold-rolled steel sheet in a first step. For example, the above first cooling step may be performed by cooling to a cooling temperature of 500°C or higher and 650°C or lower. Specifically, the above first cooling step may be performed by cooling to a cooling temperature of 500°C or higher and 650°C or lower at a cooling rate of 1°C / s or higher and 20°C / s or lower for a period of 10 seconds or more and 100 seconds or less. If the above first cooling step is performed below the lower limit of the aforementioned cooling temperature range, a ferrite structure may be formed within the cold-rolled steel sheet, which may reduce tensile strength. In addition, if the above first cooling step is performed above the upper limit of the aforementioned cooling temperature range, rapid cooling may cause defects in the sheet shape. Therefore, the above first cooling step may be performed by cooling to the aforementioned cooling temperature.

[0079] In one example, it is important to avoid the transformation of the austenite structure into ferrite during the first cooling step, and this can be resolved by including hardenable elements such as manganese (Mn), chromium (Cr), and molybdenum (Mo) in the cold-rolled steel sheet. Accordingly, the method for manufacturing the cold-rolled steel sheet can satisfy the following general formula 1.

[0080] [General Formula 1]

[0081] 1470 ≤ T 1C +200×(10C+Mn+Cr+Mo)

[0082] In the above general formula 1, T 1C is the cooling temperature (°C) in the first cooling stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0083] Specifically, the value calculated by the above general formula 1 may be 1490 or higher or 1510 or higher. In addition, the upper limit of the value calculated by the above general formula 1 may be 1700 or lower or 1650 or lower. By satisfying the above general formula 1, the method for manufacturing the above cold-rolled steel sheet can manufacture a cold-rolled steel sheet that satisfies the tensile strength range of the aforementioned range.

[0084] In addition, the above-mentioned second cooling step is a step of cooling the steel plate cooled in the first step a second time. For example, the above-mentioned second cooling step may be performed by cooling to a cooling temperature of 100°C or higher and less than 300°C. Specifically, the above-mentioned second cooling step may be performed by cooling to a cooling temperature of 100°C or higher and less than 300°C at a cooling rate of 10°C / s or higher and 100°C / s or lower for a period of 10 seconds or more and 60 seconds or less. If the above-mentioned second cooling step is performed below the lower limit of the aforementioned cooling temperature range, productivity may be reduced as it is implemented by gas cooling. In addition, if the above-mentioned second cooling step is performed above the upper limit of the aforementioned cooling temperature range, it may be disadvantageous in terms of securing material. Therefore, the above-mentioned second cooling step may be performed by cooling to the aforementioned cooling temperature.

[0085] The above over-aging treatment step is a step of over-aging a cooled cold-rolled steel sheet.

[0086] In one example, the overaging treatment step may be performed by reheating the cooled cold-rolled steel sheet to an overaging treatment temperature of 150°C or higher and 350°C or lower, and then maintaining it for more than 100 seconds and less than or equal to 600 seconds. Specifically, in the overaging treatment step, the overaging treatment temperature for reheating may be 180°C or higher and 300°C or lower, or 200°C or higher and 250°C or lower. By performing the overaging treatment step by reheating the cooled cold-rolled steel sheet to the aforementioned overaging treatment temperature and then maintaining it for the aforementioned time, dislocations excessively formed in the fresh martensite structure formed in the aforementioned region of the cold-rolled steel sheet cooled through the cooling step can be rearranged to impart toughness. On the other hand, in the overaging treatment step, if the overaging treatment temperature is below the lower limit of the aforementioned range, it is difficult for dislocations within the fresh martensite structure formed in the aforementioned region of the cold-rolled steel sheet cooled through the cooling step to be sufficiently rearranged, so the material is brittle and fracture may occur during forming. In addition, in the overaging treatment step, if the overaging treatment temperature exceeds the upper limit of the aforementioned range, needle-shaped cementite structures may grow within the martensite structure formed in the aforementioned region of the cold-rolled steel sheet cooled through the cooling step, thereby reducing hydrogen embrittlement.

[0087] The above temper rolling step is a step of temper rolling a cold-rolled steel sheet that has undergone over-aging treatment.

[0088] In one example, the temper rolling step may be performed by temper rolling an over-aged cold-rolled steel sheet with a reduction rate of 0.05% or more and 1.0% or less. By performing the temper rolling step by temper rolling the over-aged cold-rolled steel sheet with the aforementioned reduction rate, the plate shape of the over-aged cold-rolled steel sheet can be properly corrected through light rolling, and a deformation exceeding the elastic range is applied to the material, thereby forming dislocations. The formed dislocations act as nucleation sites for transition carbides during the tempering process described later, which can increase the mobile dislocations of the material to a desired value. On the other hand, if the reduction rate in the temper rolling step is below the lower limit of the aforementioned range, the amount of mobile dislocations does not increase significantly, and the precipitation of carbides during tempering decreases, making it difficult to secure the target yield strength. In addition, if the reduction rate exceeds the upper limit of the aforementioned range during the temper rolling step, the material properties increase, but the equipment load becomes severe, making it difficult to apply to a general production process.

[0089] The above tempering step is a step of tempering a tempered cold-rolled steel sheet.

[0090] In one example, the tempering step may be performed at a tempering temperature of 150°C or higher and 300°C or lower for a tempering time of 1 hour or more and 30 hours or less. In the tempering step, if the tempering temperature is below the lower limit of the aforementioned range, the temperature may be too low to facilitate the precipitation of transition carbides. Additionally, in the tempering step, if the tempering temperature exceeds the upper limit of the aforementioned range, the martensite structure within the cold-rolled steel sheet may be over-tempered, resulting in a decrease in tensile strength, and acicular cementite may grow, thereby reducing hydrogen embrittlement. Therefore, the tempering step may be performed within the aforementioned range of tempering temperature and tempering time.

[0091] The cold-rolled steel sheet manufactured according to the above method of manufacturing the cold-rolled steel sheet is a cold-rolled steel sheet using a martensitic steel grade, and its material properties vary significantly depending on the degree of tempering. In this case, if tempering is insufficient, the yield strength does not increase, and if tempering is excessive, both the yield strength and tensile strength may decrease simultaneously. Furthermore, the tempering effect of the cold-rolled steel sheet manufactured according to the above method of manufacturing the cold-rolled steel sheet may vary depending on the reduction ratio applied during temper rolling before tempering is applied. To resolve this, the above method of manufacturing the cold-rolled steel sheet may satisfy the following general formula 2.

[0092] [General Formula 2]

[0093] 200 ≤ ((T T +273)(20+Log(t T ))) / (T 2C ×R SPM )

[0094] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage, and T 2C is the cooling temperature (°C) in the second cooling stage, and R SPM is the reduction rate (%) at the temper rolling stage.

[0095] Specifically, the value calculated by the above general formula 2 may be 240 or higher or 280 or higher. In addition, the upper limit of the value calculated by the above general formula 2 may be 1000 or lower or 900 or lower. By satisfying the above general formula 2, the method for manufacturing the above cold-rolled steel sheet can manufacture a cold-rolled steel sheet that satisfies the aforementioned range of yield strength.

[0096]

[0097] The present application will be described in more detail below through embodiments according to the present application and comparative examples not according to the present application, but the scope of the present application is not limited by the embodiments presented below.

[0098]

[0099] Example 1

[0100] Manufacturing of cold-rolled steel sheets

[0101] The components shown in Table 1 below, the remainder of Fe and other unavoidable impurities were steeled and continuously cast to produce a slab, and then the slab was reheated at a temperature of 1200°C and hot-rolled at a finishing temperature of 900°C and a reduction rate of 90% to produce a hot-rolled steel sheet.

[0102] Afterwards, the hot-rolled steel sheet was cooled at a cooling rate of 50 ℃ / s and then coiled at a coiling temperature of 600℃.

[0103] Afterwards, the above hot-rolled steel sheet was pickled to remove the surface scale layer, and then cold-rolled at a reduction rate of 50% to produce a cold-rolled steel sheet.

[0104] Afterwards, the above cold-rolled steel sheet was annealed at an annealing temperature of 840°C for 108 seconds, as shown in Table 2 below.

[0105] Afterwards, the annealed cold-rolled steel sheet was first cooled to 550℃ by cooling at a cooling rate of 3.3 ℃ / s for 80 seconds as shown in Table 2 below, and secondly cooled to 200℃ by cooling at a cooling rate of 24 ℃ / s for 14 seconds.

[0106] Afterwards, the cooled cold-rolled steel sheet was reheated to an over-aging treatment temperature of 220°C as shown in Table 2 below, and then over-aged by maintaining it for 360 seconds.

[0107] Afterwards, the over-aged cold-rolled steel sheet was temper-rolled with a reduction rate of 0.1% as shown in Table 2 below.

[0108] Subsequently, a cold-rolled steel sheet was manufactured by tempering the tempered cold-rolled steel sheet at a tempering temperature of 180°C for a tempering time of 6 hours, as shown in Table 2 below.

[0109] Alloy composition of the slab (wg%) CM nSi Sol. Al P S 1.5 0.1 0.0 3 0.0 1 2 0.00 1 0.00 6 0.6 0.0 2 0.0 3 0.2 0.00 3

[0110]

[0111] Examples 2 to 5 and Comparative Examples 1 to 5

[0112] Manufacturing of cold-rolled steel sheets

[0113] A cold-rolled steel sheet was manufactured in the same manner as in Example 1, except that the annealing temperature, first cooling temperature, second cooling temperature, over-aging treatment temperature, tempering rolling reduction ratio, and tempering temperature were changed to the conditions shown in Table 2 below.

[0114] Annealing Temperature (°C) 1st Cooling Temperature (°C) 2nd Cooling Temperature (°C) Over-aging Treatment Temperature (°C) Tempering Rolling Reduction Rate (%) Tempering Temperature (°C) Example 18 40 550 200 2200.05 150 Example 28 40 550 200 2200.1 150 Example 38 40 550 200 2200.1 5 150 Example 48 40 600 200 2200.05 150 Example 58 40 650 200 2200.05 150 Comparative Example 18 40 550 200 2200.2 100 Comparative Example 28 40 550 200 2200.3 150 Comparative Example 38 40 500 200 2200.1 150 Comparative Example 48404502002200.1150Comparative Example 58405505005000.1150

[0115]

[0116] Evaluation Example 1. Evaluation of tissue area fraction

[0117] For a region corresponding to 25% of the surface of the cold-rolled steel sheet produced in each of the examples and comparative examples, the area fraction was evaluated by measuring in a direction perpendicular to the rolling direction of the cold-rolled steel sheet using a scanning electron microscope (SEM), and the results are shown in Table 3 below.

[0118]

[0119] Evaluation Example 2. Evaluation of satisfaction of Equations 1 and 2

[0120] In each of the examples and comparative examples, when manufacturing cold-rolled steel sheets, it was evaluated whether the following general formulas 1 and 2 were satisfied, and the results are shown in Table 3 below.

[0121] [General Formula 1]

[0122] 1470 ≤ T 1C +200×(10C+Mn+Cr+Mo)

[0123] In the above general formula 1, T 1C is the cooling temperature (°C) in the first cooling stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

[0124] [General Formula 2]

[0125] 200 ≤ ((T T +273)(20+Log(t T ))) / (T 2C ×R SPM )

[0126] In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage, and T 2C is the cooling temperature (°C) in the second cooling stage, and R SPM is the reduction rate (%) at the temper rolling stage.

[0127]

[0128] Evaluation Example 3. Tensile Test Evaluation

[0129] Cold-rolled steel sheets produced in each of the examples and comparative examples were taken according to JIS No. 5 standards and prepared as test specimens. Tensile tests were performed on the test specimens in a direction perpendicular to the rolling direction of the cold-rolled steel sheets to measure yield strength (YP), tensile strength (TS), and elongation (EL), and the results are shown in Table 3 below.

[0130]

[0131] Evaluation Example 4. Evaluation of Hydrogen Embrittlement

[0132] In each of the examples and comparative examples, cold-rolled steel sheets were immersed in a 0.1 N hydrochloric acid (HCl) solution for 300 hours under a load of 100% of their yield point (YP), and then it was evaluated whether cracks occurred or fractures occurred due to hydrogen embrittlement. The results are shown in Table 3 below. At this time, if no cracks occurred in the cold-rolled steel sheets, it was evaluated as OK, and if any cracks occurred in the cold-rolled steel sheets, it was evaluated as NG.

[0133] Microstructure (%) Calculated value of General Formula 1 Calculated value of General Formula 2 Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Hydrogen Embrittlement TM FB RA Example 1 Bal.5 10 115 10 8 78.9 15 8 122 7 15 5 0 7.8 OK Example 2 Bal.5 10 115 10 43 9.4 5 79 123 5 15 48 7.3 OK Example 3 Bal.5 10 115 10 29 2.9 71 9 124 9 15 41 7.4 OK Example 4 Bal.2 10 115 6 0 8 78.9 15 8 125 2 15 76 8.4 OK Example 5 Bal.0 10 116 10 8 78.9 15 8 125 8 15 98 8.6 OK Comparative Example 1 Bal.5 10115 1019 3.7563115215377.5NG Comparative Example 2 Bal.5 10115 10146.486114015128.2NG Comparative Example 3 Bal.10 1011460439.45791189146210.2NG Comparative Example 4 Bal.20 10114 10439.45791181138011.6NG Comparative Example 5 Bal.5 15 115 10175.7832105214809.8NGTM: Tempered martensitic structure F: Ferrite structure B: Bainite structure RA: Retained austenite structure Bal.: Balance

[0134] As shown in Table 3 above, unlike the cold-rolled steel sheets produced in Comparative Examples 1 to 5, the cold-rolled steel sheets produced in each of Examples 1 to 5 satisfied both General Formulas 1 and 2 during production, thereby confirming that a tempered martensite structure was included in a specific volume fraction in a specific region. Consequently, it was confirmed that the yield strength, tensile strength, and elongation each satisfied their respective specific ranges, and that hydrogen embrittlement was suppressed. Therefore, it was confirmed that the cold-rolled steel sheets produced in each of Examples 1 to 5 possessed high yield and high hydrogen embrittlement.

Claims

1. In wt%, it comprises C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, and the remainder consists of Fe and other unavoidable impurities, and In an area corresponding to 25% of the thickness from the surface, it comprises a tempered martensite structure with an area fraction greater than 80% and less than 100%, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure, and retained austenite structure, Cold-rolled steel sheet with a yield strength (YP) of 1180 MPa or higher.

2. In Paragraph 1, A cold-rolled steel sheet further comprising, in weight%, one or more selected from Sol. Al: greater than 0% and less than or equal to 0.1%, P: greater than 0% and less than or equal to 0.05%, S: greater than 0% and less than or equal to 0.01%, N: greater than 0% and less than or equal to 0.01%, B: greater than 0% and less than or equal to 0.004%, and the sum of Nb, V, and Ti: greater than 0% and less than or equal to 0.1%.

3. In Paragraph 1, Cold-rolled steel sheet with a tensile strength (TS) of 1470 MPa or higher.

4. A step of reheating and then hot-rolling a slab comprising, in weight percent, C: 0.23% or more and 0.30% or less, Mn: 1.0% or more and 1.5% or less, Si: 0.03% or more and 0.3% or less, Cr: greater than 0% and 0.8% or less, and Mo: greater than 0% and 0.3% or less, and the remainder being Fe and other unavoidable impurities; Step of winding a hot-rolled steel sheet; A step of cold rolling while unwinding the coiled hot-rolled steel sheet; Step of annealing cold-rolled steel sheets; Step of cooling annealed cold-rolled steel sheets; Step of over-aging the cooled cold-rolled steel sheet; A step of temper-rolling an over-aged cold-rolled steel sheet; and A method for manufacturing a cold-rolled steel sheet comprising the step of tempering a temper-rolled cold-rolled steel sheet, The tempered cold-rolled steel sheet comprises a tempered martensite structure with an area fraction of more than 80% and less than 100% in an area corresponding to 25% of the thickness from the surface, and is composed of one or more selected from the remaining martensite structure, ferrite structure, bainite structure, and retained austenite structure. A method for manufacturing cold-rolled steel sheets having a yield strength (YP) of 1180 MPa or higher.

5. In Paragraph 4, The above temper rolling step is a method for manufacturing a cold-rolled steel sheet by temper rolling an over-aged cold-rolled steel sheet with a reduction rate of 0.05% or more and 1.0% or less.

6. In Paragraph 5, The cooling step described above comprises: a first cooling step of cooling to a cooling temperature of 500°C or higher and 650°C or lower; and A method for manufacturing a cold-rolled steel sheet comprising a secondary cooling step of cooling to a cooling temperature of 100℃ or higher and less than 300℃.

7. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet satisfying the following general formula 1: [General Formula 1] 1470 ≤ T 1C +200×(10C+Mn+Cr+Mo) In the above general formula 1, T 1C is the cooling temperature (°C) in the first cooling stage, C is the carbon content (weight%) in the slab, Mn is the manganese content (weight%) in the slab, Cr is the chromium content (weight%) in the slab, and Mo is the molybdenum content (weight%) in the slab.

8. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet satisfying the following general formula 2: [General Formula 2] 200 ≤ ((T T +273)(20+Log(t T ))) / (T 2C ×R SPM ) In the above general formula 2, T T is the tempering temperature (°C) at the tempering stage, and t T is the tempering time (minutes) during the tempering stage, and T 2C is the cooling temperature (°C) in the second cooling stage, and R SPM is the reduction rate (%) at the temper rolling stage.

9. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet in which the above tempering step is performed at a tempering temperature of 150°C or higher and 300°C or lower for a tempering time of 1 hour or more and 30 hours or less.

10. In Paragraph 4, A method for manufacturing a cold-rolled steel sheet, wherein the above-mentioned overaging treatment step is performed by reheating to an overaging treatment temperature of 150°C or higher and 350°C or lower, and then maintaining it for 100 seconds or more and 600 seconds or less.