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. It is possible, according to the cold-rolled steel sheet and the method for manufacturing same, to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

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] The steel industry is one of the industries with very high carbon emissions, and there is a growing demand for the development of eco-friendly steel materials utilizing electric furnaces instead of conventional blast furnaces to reduce these emissions. Unlike the conventional blast furnace method of manufacturing steel materials, which emits carbon through the reduction and oxidation of iron ore, electric furnaces reduce carbon emissions by utilizing scrap to produce steel materials. However, since scrap contains some tramp elements such as copper (Cu), antimony (Sb), and tin (Sn), there is a problem in that this leads to a degradation of the steel material's quality.

[0003] Patent Document 1 (Korean Published Patent Application No. 10-1998-0009493) discloses a method for manufacturing steel for processing using an electric furnace-vacuum degassing process with iron scrap as the main raw material. However, a problem arises in that the material properties of the steel material deteriorate due to the inclusion of a large amount of nitrogen.

[0004] Therefore, cold-rolled steel sheets and a method for manufacturing the same are required to solve these problems.

[0005] The objective of the present application is to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality even when the nitrogen content increases, and a method for manufacturing the same.

[0006] To solve the above problem, the cold-rolled steel sheet of the present application comprises, in weight%, C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, and Sn: greater than 0% It contains 0.050% or less, and the remainder consists of Fe and other unavoidable impurities, and the value of a represented by Formula 1 below may be 0 or greater and 600 or less.

[0007] [Equation 1]

[0008] a = X Ti - AMU Ti / AMU N × X N

[0009] In the above Equation 1, X Ti and X N are the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

[0010] In addition, the above cold-rolled steel sheet may have a b value of 10 or more, represented by the following Equation 2.

[0011] [Equation 2]

[0012] b = X B - AMU B / AMU N × {(X N - AMU N / AMU Ti × X Ti )}

[0013] In Equation 2 above, X B , XTi , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU N ε₀ and ε₀ are the atomic weights of B, Ti, and N, respectively. However, (X₀ N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ).

[0014] In addition, the above cold-rolled steel sheet can satisfy the following Equation 3.

[0015] [Equation 3]

[0016] 1.00 ≤ a × b × 10 -3 ≤ 13.00

[0017] In the above Equation 3, a is the value of a represented by Equation 1, and b is the value of b represented by Equation 2.

[0018] In addition, the above cold-rolled steel sheet may have an AlN precipitate formation amount of less than 9 ppm.

[0019] In addition, the microstructure of the above cold-rolled steel sheet may consist of 30% or more and 50% or less of ferrite and 45% or more and 60% or less of martensite in terms of area fraction, and the remainder being bainite, austenite, and pearlite.

[0020] In addition, the above cold-rolled steel sheet may have a tensile strength of 980 MPa or more and an elongation of 12.0% or more.

[0021] In addition, the method for manufacturing a cold-rolled steel sheet of the present application comprises, in weight%, C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, and Sn: greater than 0% and 0.050%. The method comprises the steps of: preparing a slab comprising the following, and the remainder being Fe and other unavoidable impurities; reheating the slab; hot-rolling the reheated slab to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and annealing the cold-rolled steel sheet, wherein the value of a represented by the following formula 1 may be 0 or greater and 600 or less.

[0022] [Equation 1]

[0023] a = X Ti - AMU Ti / AMU N × X N

[0024] In the above Equation 1, X Ti and X N are the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

[0025] In addition, the method for manufacturing the above cold-rolled steel sheet may have a value of b represented by the following Equation 2 that is 10 or more.

[0026] [Equation 2]

[0027] b = X B - AMU B / AMU N× {(X N - AMU N / AMU Ti × X Ti )}

[0028] In Equation 2 above, X B , X Ti , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU N ε₀ and ε₀ are the atomic weights of B, Ti, and N, respectively. However, (X₀ N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ).

[0029] In addition, the method for manufacturing the above cold-rolled steel sheet can satisfy the following Equation 3.

[0030] [Equation 3]

[0031] 1.00 ≤ a × b × 10 -3 ≤ 10.00

[0032] In the above Equation 3, a is the value of a represented by Equation 1, and b is the value of b represented by Equation 2.

[0033] In addition, the method for manufacturing the cold-rolled steel sheet may further include a step of coiling the hot-rolled steel sheet at a temperature of 400°C or higher and 700°C or lower prior to the step of manufacturing the cold-rolled steel sheet.

[0034] In addition, the method for manufacturing the above cold-rolled steel sheet may include, in the annealing step, increasing the temperature at an average rate of 1℃ / s or more and 10℃ / s or less, and heat treating at 760℃ or more and 860℃ or less for 10 seconds or more and 500 seconds or less.

[0035] In addition, the method for manufacturing the cold-rolled steel sheet described above may include, in the annealing step, a first cooling step of cooling to 500°C or higher and 800°C or lower at an average cooling rate of 0.5°C / s or higher and 20°C / s or lower after heat treatment, and a second cooling step of cooling to 200°C or higher and 400°C or lower at an average cooling rate of 1°C / s or higher and 100°C / s or lower.

[0036] In addition, the method for manufacturing the above cold-rolled steel sheet allows the cold-rolled steel sheet to have a formation amount of AlN precipitates of less than 9 ppm.

[0037] In addition, the method for manufacturing the above cold-rolled steel sheet may result in the cold-rolled steel sheet having a microstructure comprising, in terms of area fraction, 30% or more and 50% or less of ferrite and 45% or more and 60% or less of martensite, with the remainder consisting of bainite, austenite, and pearlite.

[0038] In addition, the method for manufacturing the above cold-rolled steel sheet allows the cold-rolled steel sheet to have a tensile strength of 980 MPa or more and an elongation of 12.0% or more.

[0039] In addition, in the step of preparing the above slab, the slab may be manufactured by refining molten steel tapped from a converter or electric furnace in a vacuum degassing facility.

[0040] According to the cold-rolled steel sheet and the method for manufacturing the same of the present application, a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality can be manufactured.

[0041] Figure 1 is a graph showing the Continuous Cooling Transformation Curve (CCT Curve) according to boron content.

[0042] Figure 2 is a photograph showing the formation of AlN within the slab using a scanning electron microscope, taken of a cold-rolled steel sheet manufactured in Comparative Example 1.

[0043] Embodiments of the present invention will be described in detail below. Furthermore, the scope of the present invention is not limited to the embodiments described below, and may be implemented with arbitrary modifications within the scope that does not deviate from the essence of the present invention.

[0044] 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.”

[0045] In the description of numerical ranges in this specification, the term “to” is used to mean that the values ​​described before and after it are included as lower and upper limits, unless otherwise specifically stated.

[0046] In the numerical ranges described stepwise in this specification, an 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 also be substituted with a value shown in the examples.

[0047] The present application relates to a cold-rolled steel sheet comprising, in weight percent, C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, and Sn: greater than 0% and 0.050% or less. The remainder may consist of Fe and other unavoidable impurities.

[0048] The composition of the above cold-rolled steel sheet is explained below.

[0049] C: 0.05 wt% or more, 0.20 wt% or less

[0050] Carbon (C) is an element that plays a role in increasing strength by being dissolved in austenite during annealing heat treatment and causing an increase in the strength of martensite after final cooling. If the carbon is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the strength may decrease. In addition, if the carbon is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, weldability may decrease, and there is a risk of reducing hole expansionability by causing an excessive increase in strength and increasing the hardness difference between the ferrite and martensite phases. Therefore, the carbon may be included in the cold-rolled steel sheet in an amount of 0.05 wt% or more and 0.20 wt% or less.

[0051] Si: 0.20 wt% or more, 2.00 wt% or less

[0052] Silicon (Si) is a ferrite-stabilizing element that can be added to increase ferrite strength by being dissolved in the ferrite. Additionally, the silicon is an element effective in improving elongation by causing the purification of the ferrite. If the silicon is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, there is a risk of reducing strength and elongation. Furthermore, if the silicon is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may form Si-based oxides on the surface, thereby degrading surface and / or plating properties. Therefore, the silicon may be included in the cold-rolled steel sheet in an amount of 0.20 wt% or more and 2.00 wt% or less.

[0053] Mn: 1.50 wt% or more, 3.00 wt% or less

[0054] Manganese (Mn) is an element that plays a role in stabilizing the austenite phase. It suppresses the transformation into ferrite, pearlite, and bainite during cooling and increases the martensite fraction, thereby causing an increase in strength. If the manganese is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, ferrite, pearlite, and bainite may be formed, which may lead to a decrease in strength. Additionally, if the manganese is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, there is a risk that hole expansion properties may be reduced due to the formation of a Mn enrichment layer. Therefore, the manganese may be included in the cold-rolled steel sheet in an amount of 1.50 wt% or more and 3.00 wt% or less.

[0055] P: Greater than 0 wt% and less than or equal to 0.1 wt%

[0056] Phosphorus (P) is an element that segregates during the steel manufacturing process and causes a decrease in toughness and delayed fracture. Therefore, the phosphorus 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 preferably, in an amount greater than 0 wt% and less than or equal to 0.0200 wt%.

[0057] S: Greater than 0 wt% and less than or equal to 0.01 wt%

[0058] Since sulfur (S) can reduce the toughness and weldability of steel, 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%, and preferably, in an amount greater than 0 weight% and less than or equal to 0.0030 weight%.

[0059] Al: Greater than 0 wt% and less than or equal to 0.100 wt%

[0060] Aluminum (Al) is an element that plays a role in stabilizing the ferrite phase, increasing the fraction of ferrite, causing ferrite purification, and is effective in improving elongation. If the above aluminum is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may form coarse AlN nitrides, thereby reducing elongation. Furthermore, if the above aluminum combines with nitrogen to form AlN nitrides, it may segregate at grain boundaries during slab formation, thereby degrading the quality of the slab. Therefore, the above aluminum may be included in the cold-rolled steel sheet in an amount of 0% or more and 0.100% or less.

[0061] Ti: 0.020 wt% or more, 0.080 wt% or less

[0062] Titanium (Ti) is an element that improves the quality of the slab by suppressing the formation of AlN through the formation of TiN nitrides by combining with nitrogen in the steel, and maximizes the quenching effect of solid solution B by suppressing the formation of BN. In addition, the titanium forms fine precipitates such as TiC, thereby refining and homogenizing the grain size and improving strength and hole expansion. However, if the titanium combines with nitrogen to form an excessive amount of TiN nitrides, there is a concern that the effects of strength improvement and grain refinement may be reduced. Furthermore, if the titanium is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may reduce the elongation due to excessive precipitation hardening. Therefore, the titanium may be included in the cold-rolled steel sheet in an amount of 0.020 wt% or more and 0.080 wt% or less.

[0063] B: 0.0010 wt% or more, 0.0050 wt% or less

[0064] Boron (B) is a hardenable element that inhibits the phase transformation into ferrite, pearlite, and bainite during cooling. Additionally, the boron segregates at grain boundaries to increase the hardenability of the steel material; however, if the boron combines with nitrogen to form BN nitride, there is a concern that the effect of improving hardenability may be reduced. Furthermore, if the boron is included in the cold-rolled steel sheet in an amount less than the lower limit of the aforementioned range, the strength may decrease due to the reduction in hardenability. Additionally, if the boron is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, the impact properties may be reduced. Therefore, the boron may be included in the cold-rolled steel sheet in an amount of 0.0010 wt% or more and 0.0050 wt% or less.

[0065] N: Greater than 0 wt% and less than or equal to 0.0150 wt%

[0066] Nitrogen (N) is an element that can be excessively incorporated as a tramp element during the electric furnace steelmaking process. If the nitrogen is contained in the cold-rolled steel sheet in excess of the upper limit, hardenability may be reduced due to the formation of BN nitrides, and the quality of the slab may be reduced due to the formation of AlN nitrides. Therefore, the nitrogen may be contained in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.0150 weight%.

[0067] Sb: 0.003 wt% or more, 0.050 wt% or less

[0068] Antimony (Sb) is an element that can be excessively incorporated as a tramp element during the electric furnace steelmaking process. If the antimony is included in the cold-rolled steel sheet at a level below the lower limit of the aforementioned range, the internal oxidation inhibition ability may be weakened. Additionally, if the antimony is included in the cold-rolled steel sheet at a level exceeding the upper limit of the aforementioned range, it may accelerate the effect of red-hot brittleness and degrade the surface quality of the material. Therefore, the antimony may be included in the cold-rolled steel sheet in an amount of 0.003 weight% or more and 0.050 weight% or less.

[0069] Cu: Exceeding 0 wt% and up to 0.500 wt%

[0070] Copper (Cu) is an element that can be excessively incorporated as a tramp element during the electric furnace steelmaking process. If the copper is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, red-hot brittleness may occur, which may degrade the surface quality of the material. Therefore, the copper may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.500 weight%.

[0071] Cr: Greater than 0 wt% and less than or equal to 1,000 wt%

[0072] Chromium (Cr) is an element that increases the hardenability of steel, suppresses ferrite, pearlite, and bainite transformations during cooling, and facilitates the formation of a martensite structure, thereby being effective in increasing strength. Additionally, the chromium may be excessively incorporated as a tramp element during the electric furnace steelmaking process. 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 the elongation. 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 1.000 weight%.

[0073] Ni: Exceeding 0 wt% and up to 0.500 wt%

[0074] Nickel (Ni) is an austenite-stabilizing element that suppresses ferrite, pearlite, and bainite transformations during cooling and is effective in increasing strength. Additionally, the nickel may be excessively incorporated as a tramp element during the electric furnace steelmaking process. Furthermore, if the nickel is included in the cold-rolled steel sheet in an amount exceeding the upper limit of the aforementioned range, it may reduce the elongation. Therefore, the nickel may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.500 weight%.

[0075] Sn: Greater than 0 wt% and less than or equal to 0.050 wt%

[0076] Tin (Sn) is an element that can be excessively incorporated as a tramp element during the electric furnace steelmaking process. If the tin is included in the steel sheet in an amount exceeding the upper limit of the aforementioned range, it can accelerate the effect of red-hot brittleness and degrade the surface quality of the material. Therefore, the tin may be included in the cold-rolled steel sheet in an amount greater than 0 weight% and less than or equal to 0.050 weight%.

[0077] Remaining Fe and other unavoidable impurities

[0078] 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).

[0079] Generally, when cold-rolled steel sheets contain titanium, nitrogen, and boron, titanium and nitrogen combine first to form TiN nitride. If there is residual nitrogen after the formation of TiN nitride, the residual nitrogen combines with boron to form BN nitride. If there is residual nitrogen even after the formation of BN nitride, the residual nitrogen combines with aluminum to form AlN nitride.

[0080] In addition, Figure 1 is a graph showing the Continuous Cooling Transformation Curve (CCT Curve) according to boron content. In the graph, "B-less" refers to the case where there is no residual boron, as residual nitrogen remains after the formation of TiN nitride, and the residual nitrogen combines with boron to form BN nitride. "B-retained" refers to the case where there is no residual nitrogen after the formation of TiN nitride, and the boron present in the steel sheet remains in a solid solution state. As can be seen from Figure 1, in the case of "B-retained," the graph is shifted to the right compared to the case of "B-less." In the case of "B-less," more ferrite, pearlite, and bainite phase transformations occur during cooling, and hardenability may decrease due to the formation of BN nitride. Furthermore, if residual nitrogen remains even after the formation of BN nitride, AlN nitride is formed, which may result in inferior high-temperature ductility. Accordingly, in the case of B residue, the strength of the final product may decrease and the elongation may decrease. Therefore, it is important to effectively control the amount of nitrogen to secure the desired strength and satisfy the target material. According to the cold-rolled steel sheet of the present application, by satisfying the aforementioned ranges for the composition of the cold-rolled steel sheet, and in particular by controlling titanium, nitrogen, and boron to satisfy the aforementioned ranges, it is possible to have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0081] In addition, the above cold-rolled steel sheet may have an a value of 0 or more and 600 or less, represented by the following formula 1.

[0082] [Equation 1]

[0083] a = X Ti - AMU Ti / AMU N × X N

[0084] In the above Equation 1, X Ti and X Nare the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

[0085] Specifically, the value of a represented by Equation 1 above may be 0 or greater and 600 or less, 30 or greater and 500 or less, 30 or greater and 400 or less, or 30 or greater and 350 or less. If the value of a represented by Equation 1 above exceeds the upper limit of the aforementioned range, there is a risk that precipitates will be excessively formed, and consequently, the elongation rate may decrease. Furthermore, if the value of a represented by Equation 1 above is below the lower limit of the aforementioned range, BN nitrides may be formed, and the hardenability may decrease. Additionally, if the value of a represented by Equation 1 above is 0 or greater, it means that residual nitrogen after the formation of TiN nitrides can be minimized. Accordingly, BN nitrides are not formed, thereby exhibiting the effect of improving hardenability. Therefore, the cold-rolled steel sheet can have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality by satisfying the aforementioned range for the value of a represented by Equation 1 above.

[0086] In addition, the above cold-rolled steel sheet may have a b value of 10 or more, represented by the following Equation 2.

[0087] [Equation 2]

[0088] b = X B - AMU B / AMU N x {(X N - AMU N / AMU Ti × X Ti )}

[0089] In Equation 2 above, X B , X Ti , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU Nε₀ and ε₀ are the atomic weights of B, Ti, and N, respectively. However, (X₀ N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ).

[0090] Specifically, the value of b represented by the above Equation 2 may be 10 or more, or 10 or more and 50 or less, or 20 or more and 50 or less.

[0091] If the value of b represented by Equation 2 above is below the lower limit of the aforementioned range, the hardenability may be reduced due to the formation of BN nitrides, and it may be difficult to secure the desired strength. Accordingly, by satisfying the aforementioned range for the value of b represented by Equation 2 above, BN nitrides are not formed, and the effect of improving hardenability can be achieved due to the quenching effect of the dissolved B. The cold-rolled steel sheet can have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality by satisfying the aforementioned range for the value of b represented by Equation 2 above.

[0092] In addition, the above cold-rolled steel sheet can satisfy the following Equation 3.

[0093] [Equation 3]

[0094] 1.00 ≤ a × b × 10 -3 ≤13.50

[0095] In the above Equation 3, a is the value of a represented by Equation 1, and b is the value of b represented by Equation 2.

[0096] In addition, the value calculated by the above Equation 3 may be 1.00 or greater and 13.50 or less, specifically, 1.00, 1.10 or 1.20 or greater, and 13.50, 13.00, 12.00, 11.00, 10.00 or 9.00 or less.

[0097] When calculating Equation 3 above, for the value of b represented by Equation 2 above, (X N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Calculate by substituting 0 into ). In addition, if the value calculated by Equation 3 above exceeds the upper limit of the aforementioned range, it may be difficult to secure the desired strength, and there is a risk that the elongation rate may decrease. In addition, if the value calculated by Equation 3 above is below the lower limit of the aforementioned range, it may be difficult to secure the strength. Therefore, the cold-rolled steel sheet can have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality by satisfying the aforementioned range with the value calculated by Equation 3 above.

[0098] The cold-rolled steel sheet of the present application can have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality by controlling the composition of the cold-rolled steel sheet to satisfy the aforementioned range, and by controlling the Ti, N, and B included in the cold-rolled steel sheet such that the value of a represented by Equation 1, the value of b represented by Equation 2, and the value calculated by Equation 3 each satisfy the aforementioned range simultaneously.

[0099] In addition, the above cold-rolled steel sheet may have an AlN precipitate formation amount of less than 9 ppm. If the AlN precipitate formation amount exceeds the upper limit of the aforementioned range, it may be difficult to secure the desired strength. Therefore, the above cold-rolled steel sheet can have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality by controlling the composition of the cold-rolled steel sheet and, accordingly, satisfying the AlN precipitate formation amount within the aforementioned range.

[0100] In addition, the above cold-rolled steel sheet may have a microstructure comprising, in terms of area fraction, 30% or more and 50% or less of ferrite and 45% or more and 60% or less of martensite, with the remainder consisting of bainite, austenite, and pearlite. Furthermore, if the area fraction of the ferrite structure of the above cold-rolled steel sheet exceeds the upper limit of the aforementioned range, the strength of the material may be insufficient, making it difficult to secure the desired strength. Additionally, if the area fraction of the ferrite structure of the above cold-rolled steel sheet is below the lower limit of the aforementioned range, the ductility of the material may be insufficient, making it difficult to achieve the desired level of elongation. Furthermore, the martensite may include one or more of fresh martensite and tempered martensite. Additionally, if the area fraction of the martensite structure of the above cold-rolled steel sheet exceeds the upper limit of the aforementioned range, the ductility of the material may be insufficient, making it difficult to achieve the desired level of elongation. In addition, if the area fraction of the martensite structure of the above cold-rolled steel sheet is below the lower limit of the aforementioned range, the strength of the material becomes insufficient, making it difficult to secure the desired strength. Therefore, the above cold-rolled steel sheet can secure the desired strength and exhibit the effect of improving elongation by satisfying the aforementioned range of microstructure.

[0101] In addition, the above cold-rolled steel sheet may have a tensile strength of 980 MPa or more, specifically 1000 MPa or more or 1030 MPa or more. In addition, the above cold-rolled steel sheet may have an elongation of 12.0% or more, specifically 13.0% or more, 14.0% or more, or 14.4% or more. The above tensile strength and elongation may be measured according to the measurement method of the example described below. In addition, by satisfying the above ranges for tensile strength and elongation, the above cold-rolled steel sheet may have excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0102] 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.

[0103] The method for manufacturing a cold-rolled steel sheet of the present application comprises, in weight%, C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, and Sn: greater than 0% and 0.050% or less. The method includes the steps of preparing a slab comprising the remainder being Fe and other unavoidable impurities, reheating the slab, hot-rolling the reheated slab to produce a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet, and annealing the cold-rolled steel sheet.

[0104] A detailed description of the composition of the above slab is omitted because it is identical to the description of the composition of the above cold-rolled steel sheet.

[0105] In addition, the step of preparing the above-mentioned slab is a step of obtaining molten steel satisfying the aforementioned composition through a steelmaking process, and then manufacturing a slab in the form of a semi-finished product through a continuous casting process. In addition, in the step of preparing the above-mentioned slab, the method of the flux for manufacturing the slab is not limited, and known methods such as a converter or an electric furnace may be adopted. For example, the above-mentioned slab may be manufactured by refining molten steel tapped from a converter or an electric furnace in a vacuum degassing facility.

[0106] In addition, the step of reheating the slab is performed by reheating the slab. The reheating temperature of the slab may be 1150°C or higher and 1300°C or lower.

[0107] In addition, the step of manufacturing the hot-rolled steel sheet is a step for manufacturing a slab into a hot-rolled steel sheet, and is performed by reheating the slab and then hot-rolling it. At this time, the finishing rolling temperature during the hot rolling may be 800°C or higher and 950°C or lower.

[0108] In addition, the step of manufacturing the hot-rolled steel sheet may have a reduction rate of 95% or more during hot rolling, specifically, 97% or more, or 99% or more.

[0109] In addition, the thickness of the hot-rolled steel sheet may be 2 mm or more and 5 mm or less.

[0110] In addition, the above hot-rolled steel sheet may have a microstructure that includes martensite, bainite, pearlite, and ferrite.

[0111] In addition, prior to the step of manufacturing the cold-rolled steel sheet, the step of winding the hot-rolled steel sheet may be further included. The step of winding the hot-rolled steel sheet may be performed by winding the hot-rolled steel sheet obtained by hot rolling at a temperature of 400°C or higher and 700°C or lower.

[0112] In addition, the hot-rolled steel sheet may undergo further pickling. As a result, the oxide layer formed on the surface of the hot-rolled steel sheet can be removed through the pickling solution.

[0113] In addition, the cold rolling step is a step for manufacturing the hot-rolled steel sheet into a cold-rolled steel sheet, and is performed through cold rolling. At this time, the thickness of the cold-rolled steel sheet may be 0.5 mm or more and 3 mm or less.

[0114] In addition, the reduction rate during the above cold rolling may be 30% or more and 80% or less.

[0115] In addition, the annealing step is performed by heat treating the cold-rolled steel sheet. For example, the annealing step may be performed by increasing the temperature at an average rate of 1°C / s or more and 10°C / s or less, and heat treating at 760°C or more and 860°C or less for 10 seconds or more and 500 seconds or less.

[0116] In addition, the annealing step may include a first cooling step of cooling to 500°C or higher and 800°C or lower at an average cooling rate of 0.5°C / s or higher and 20°C / s or lower after heat treatment, and a second cooling step of cooling to 200°C or higher and 400°C or lower at an average cooling rate of 1°C / s or higher and 100°C / s or lower.

[0117] In addition, according to the method for manufacturing a cold-rolled steel sheet of the present application, the method of melting in the step of preparing a slab, the reheating temperature in the step of reheating the slab, the step of manufacturing a hot-rolled steel sheet, the step of manufacturing a cold-rolled steel sheet, and the step of annealing satisfy the aforementioned conditions, thereby providing a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0118] In addition, Ti and N included in the above slab may have an a value of 0 or more and 600 or less, represented by the following Equation 1.

[0119] [Equation 1]

[0120] a = X Ti - AMU Ti / AMU N × X N

[0121] In the above Equation 1, X Ti and X N are the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

[0122] Specifically, the value of a represented by Equation 1 above may be 0 or greater and 600 or less, 30 or greater and 500 or less, 30 or greater and 400 or less, or 30 or greater and 350 or less. If the value of a represented by Equation 1 above exceeds the upper limit of the aforementioned range, there is a risk that precipitates will be excessively formed, and consequently, the elongation rate may decrease. Furthermore, if the value of a represented by Equation 1 above is below the lower limit of the aforementioned range, BN nitrides may be formed, and the hardenability may decrease. Additionally, if the value of a represented by Equation 1 above is 0 or greater, it means that residual nitrogen after the formation of TiN nitrides can be minimized. Accordingly, since BN nitrides are not formed, the effect of improving hardenability can be achieved. Therefore, by satisfying the aforementioned range for the value of a represented by Equation 1 above, it is possible to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0123] In addition, B, Ti, and N included in the above slab may have a b value of 10 or more, represented by the following Equation 2.

[0124] [Equation 2]

[0125] b = X B - AMU B / AMU N × {(X N - AMU N / AMU Ti × X Ti )}

[0126] In Equation 2 above, X B , X Ti , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU N ε₀ and ε₀ are the atomic weights of B, Ti, and N, respectively. However, (X₀ N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ).

[0127] Specifically, the value of b represented by the above Equation 2 may be 10 or more, or 10 or more and 50 or less, or 20 or more and 50 or less.

[0128] If the value of b represented by Equation 2 above is below the lower limit of the aforementioned range, the hardenability may be reduced due to the formation of BN nitrides, and it may be difficult to secure the desired strength. Accordingly, by satisfying the aforementioned range for the value of b represented by Equation 2 above, BN nitrides are not formed, and the effect of improving hardenability can be achieved due to the quenching effect of the dissolved B. Therefore, by satisfying the aforementioned range for the value of b represented by Equation 2 above, it is possible to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0129] In addition, B, Ti, and N included in the above slab can satisfy the following Equation 3.

[0130] [Equation 3]

[0131] 1.00 ≤ a × b × 10 -3 ≤ 10.00

[0132] In the above Equation 3, a is the value of a represented by Equation 1, and b is the value of b represented by Equation 2.

[0133] In addition, the value calculated by the above Equation 3 may be 1.00 or greater and 13.50 or less, specifically, 1.00, 1.10 or 1.20 or greater, and 13.50, 13.00, 12.00, 11.00, 10.00 or 9.00 or less.

[0134] When calculating Equation 3 above, for the value of b represented by Equation 2 above, (X N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Calculate by substituting 0 into ). In addition, if the value calculated by Equation 3 above exceeds the upper limit of the aforementioned range, it may be difficult to secure the desired strength, and there is a risk that the elongation rate may decrease. In addition, if the value calculated by Equation 3 above is below the lower limit of the aforementioned range, it may be difficult to secure strength. Therefore, by satisfying the aforementioned range with the value calculated by Equation 3 above, it is possible to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0135] According to the method for manufacturing a cold-rolled steel sheet of the present application, by controlling the composition of the slab to satisfy the aforementioned range, and the Ti, N, and B included in the slab such that the value of a represented by Equation 1, the value of b represented by Equation 2, and the value calculated by Equation 3 each satisfy the aforementioned range simultaneously, it is possible to provide a cold-rolled steel sheet having excellent mechanical properties of high strength, excellent hardenability, and improved surface quality.

[0136]

[0137] 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.

[0138]

[0139] Manufacturing of cold-rolled steel sheets

[0140] Example 1

[0141] Steel having a composition consisting of the components shown in Table 1 below, the remainder being Fe and other unavoidable impurities, was melted in an electric furnace, subjected to secondary refining, and continuously cast to produce a slab. Subsequently, the slab was reheated at a temperature of 1150–1250°C and then hot-rolled at a finishing rolling temperature of 850–950°C to produce a hot-rolled steel sheet with a thickness of 2.0–2.5 mm.

[0142] Afterwards, the above hot-rolled steel sheet was coiled at a coiling temperature of 500 to 600°C.

[0143] Afterwards, pickling was performed on the hot-rolled steel sheet, and the pickled hot-rolled steel sheet was cold-rolled to a thickness of 0.8 to 1.2 mm with a reduction rate of 40 to 70%.

[0144] Afterwards, a cold-rolled steel sheet was manufactured by heat treatment at a temperature of 800 to 840°C for 40 to 80 seconds by increasing the temperature at an average rate of 2 to 3°C / s, first cooling to 590 to 630°C at an average cooling rate of 4 to 8°C / s, and second cooling to 320 to 360°C at an average cooling rate of 15 to 30°C / s.

[0145]

[0146] Examples 2 and 3, Comparative Examples 1 to 4

[0147] Each cold-rolled steel sheet was manufactured in the same manner as in Example 1, except that the composition of the slab was changed as shown in Table 1 below.

[0148]

[0149] Evaluation Example 1. Evaluation of whether Equations 1 to 3 are satisfied

[0150] For the cold-rolled steel sheets produced in each of the examples and comparative examples, it was calculated whether [Ti], [N], and [B] satisfied Equations 1 to 3, and the results are shown in Table 1 below.

[0151] At this time, AMU B , AMU Ti and AMU N are the atomic weights of B, Ti, and N, respectively, AMU B is 10.811, AMU Ti is 47.867, AMU N It was calculated using 14.007.

[0152] In addition, in calculating the value of b represented by Equation 2 and Equation 3, (X N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti It was calculated by substituting 0 into ).

[0153]

[0154] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Slab Composition (wt%) C 0.148 0.134 0.113 0.102 0.156 0.121 0.103 Si 1.30 0.76 1.101.24 1.07 1.19 0.72 Mn 1.71 2.22 2.33 2.37 2.12 2.73 2.38 P 0.0152 0.0125 0.0176 0.0167 0.01110 .01350.0154S0.00190.00140.00110.00110.00120.00130.0013Al0.0460.0460 .0350.0330.0420.0350.293Ti0.0450.0430.0410.0150.0140.0950.032B0.002 80.00230.00320.00210.00250.00250.0005N0.00430.00950.01080.00630.01 070.00850.0042Sb0.0310.0420.0210.0360.0300.0300.001Cu0.0350.2570.24 00.0740.2340.0980.031Cr0.0360.0350.0580.0380.1030.0430.042Ni0.0090.1210.0800.0310.1040.0450.011Sn0.0010.0150.0210.0070.0210.0080.0015a Value 30310537-65-227659176b Value 282332-4-61255 Value calculated by Equation 3 8.482.411.180.2613.8516.480.88AlN formation amount (ppm) 913700000

[0155]

[0156] Evaluation Example 2. Microstructure Observation

[0157] For the cold-rolled steel sheets prepared in each of the examples and comparative examples, the microstructure was observed using a scanning electron microscope (SEM). The results are shown in Table 2 below.

[0158] Here, F represents ferrite, FM represents fresh martensite, and the remainder represents bainite, austenite, and pearlite.

[0159]

[0160] Evaluation Example 3. Measurement of Yield Strength (YP), Tensile Strength (TS), and Elongation (EL)

[0161] For the cold-rolled steel sheets produced in each of the examples and comparative examples, tensile tests were performed using specimens taken according to JIS Standard No. 5 at a 90° angle to the rolling direction, and yield strength, tensile strength, and elongation were measured. The results are shown in Table 2 below.

[0162]

[0163] Microstructure (Area Fraction %) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) FFM Remainder Example 1 435436481,03517.8 Example 2 375857611,10016.9 Example 3 365956791,08614.4 Comparative Example 1 6135451490417.4 Comparative Example 2 5044662997116.4 Comparative Example 3 35587719122011.6 Comparative Example 4 5042856196917.9

[0164]

[0165] As shown in Tables 1 and 2 above, it was confirmed that the cold-rolled steel sheets produced in each of Examples 1 to 3 had a composition that satisfied a specific range, and that the value of a represented by Equation 1, the value of b represented by Equation 2, and Equation 3 satisfied specific ranges, and accordingly, that the microstructure, tensile strength, and elongation satisfied specific ranges. In particular, in the case of the cold-rolled steel sheet produced in Example 3, despite containing a large amount of nitrogen, it was confirmed that the tensile strength and elongation satisfied specific ranges as the value of a represented by Equation 1, the value of b represented by Equation 2, and Equation 3 satisfied specific ranges.

[0166] In contrast, it was confirmed that the composition of the cold-rolled steel sheets produced in each of Comparative Examples 1 to 4 falls outside a specific range, and accordingly, at least one of the values ​​of a represented by Equation 1, b represented by Equation 2, and Equation 3 falls outside a specific range. In addition, it was confirmed that at least one of the microstructure, tensile strength, and elongation of the cold-rolled steel sheets produced in each of Comparative Examples 1 to 4 falls outside a specific range.

Claims

1. In wt%, comprising C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, Sn: greater than 0% and 0.050% or less, and the remainder being Fe and other unavoidable As a cold-rolled steel sheet consisting of impurities, Cold-rolled steel sheet having a value of a represented by the following formula 1 that is 0 or greater and 600 or less. [Equation 1] a = X Ti - AMU Ti / AMU N × X N (In Equation 1, X Ti and X N are the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

2. In Paragraph 1, Cold-rolled steel sheet having a b value of 10 or more as represented by the following Equation 2. [Equation 2] b = X B - AMU B / AMU N × {(X N - AMU N / AMU Ti × X Ti )} (In Equation 2, X B , X Ti , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU N are the atomic weights of B, Ti, and N, respectively. However, (X N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ) 3. In Paragraph 2, Cold-rolled steel sheet satisfying the following Equation 3. [Equation 3] 1.00 ≤ a × b × 10 -3 ≤ 13.00 (In Equation 3, a is the value of a represented by the above Equation 1, b is the value of b represented by Equation 2 above.) 4. In Paragraph 1, Cold-rolled steel sheet with an AlN precipitate formation amount of less than 9 ppm.

5. In Paragraph 1, A cold-rolled steel sheet having a microstructure comprising, in terms of area fraction, 30% or more and 50% or less of ferrite and 45% or more and 60% or less of martensite, with the remainder consisting of bainite, austenite, and pearlite.

6. In Paragraph 1, Cold-rolled steel sheet having a tensile strength of 980 MPa or more and an elongation of 12.0% or more.

7. In wt%, comprising C: 0.05% or more and 0.20% or less, Si: 0.20% or more and 2.00% or less, Mn: 1.50% or more and 3.00% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.01% or less, Al: greater than 0% and 0.100% or less, Ti: 0.020% or more and 0.080% or less, B: 0.0010% or more and 0.0050% or less, N: greater than 0% and 0.0150% or less, Sb: 0.003% or more and 0.050% or less, Cu: greater than 0% and 0.500% or less, Cr: greater than 0% and 1.000% or less, Ni: greater than 0% and 0.500% or less, Sn: greater than 0% and 0.050% or less, and the remainder being Fe and other unavoidable Step of preparing a slab consisting of impurities; Step of reheating the above slab; A step of manufacturing a hot-rolled steel sheet by hot-rolling the above-mentioned reheated slab; A step of manufacturing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet; and A method for manufacturing a cold-rolled steel sheet comprising the step of annealing the above cold-rolled steel sheet, A method for manufacturing a cold-rolled steel sheet, wherein the value of a represented by the following formula 1 is 0 or more and 600 or less. [Equation 1] a = X Ti - AMU Ti / AMU N × X N (In Equation 1, X Ti and X N are the contents of Ti and N in ppm units, respectively, and AMU Ti and AMU N are the atomic weights of Ti and N, respectively.

8. In Paragraph 7, A method for manufacturing cold-rolled steel sheets in which the value of b represented by the following formula 2 is 10 or greater. [Equation 2] b = X B - AMU B / AMU N × {(X N - AMU N / AMU Ti × X Ti )} (In Equation 2, X B , X Ti , X N are the contents of B, Ti, and N in ppm units, respectively, and AMU B , AMU Ti and AMU N are the atomic weights of B, Ti, and N, respectively. However, (X N - AMU N / AMU Ti × X Ti If the calculated value of ) is less than 0, (X N - AMU N / AMU Ti × X Ti Assign 0 to ) 9. In Paragraph 8, A method for manufacturing cold-rolled steel sheets satisfying the following Equation 3. [Equation 3] 1.00 ≤ a × b × 10 -3 ≤ 10.00 (In Equation 3, a is the value of a represented by the above Equation 1, b is the value of b represented by Equation 2 above.) 10. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, further comprising, prior to the step of manufacturing the cold-rolled steel sheet, a step of coiling the hot-rolled steel sheet at a temperature of 400°C or higher and 700°C or lower.

11. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, wherein the annealing step involves increasing the temperature at an average rate of 1℃ / s or more and 10℃ / s or less, and heat treating at 760℃ or more and 860℃ or less for 10 seconds or more and 500 seconds or less.

12. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, wherein the annealing step comprises a first cooling step of cooling to 500°C or higher and 800°C or lower at an average cooling rate of 0.5°C / s or higher and 20°C / s or lower after heat treatment, and a second cooling step of cooling to 200°C or higher and 400°C or lower at an average cooling rate of 1°C / s or higher and 100°C / s or lower.

13. In Paragraph 7, The above cold-rolled steel sheet is a method for manufacturing a cold-rolled steel sheet in which the amount of AlN precipitates formed is less than 9 ppm.

14. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, wherein the microstructure of the above cold-rolled steel sheet comprises, in terms of area fraction, 30% or more and 50% or less of ferrite and 45% or more and 60% or less of martensite, and the remainder being bainite, austenite, and pearlite.

15. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, wherein the above cold-rolled steel sheet has a tensile strength of 980 MPa or more and an elongation of 12.0% or more.

16. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, wherein, in the step of preparing the above-mentioned slab, the slab is manufactured by refining molten steel tapped from a converter or electric furnace in a vacuum degassing facility.

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