Ultra-high-strength cold-rolled steel sheet and method for manufacturing the same

Abstract

The present invention provides an ultra-high strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure, and a method for manufacturing the same. According to an embodiment of the present invention, the ultra-high strength cold-rolled steel sheet contains, by weight %, carbon (C): 0.28% to 0.45%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): more than 0% to 1.0%, molybdenum (Mo): more than 0% to 0.5%, the total of niobium (Nb), titanium (Ti) and vanadium (V): more than 0% to 0.1%, phosphorus (P): more than 0% to 0.03%, sulfur (S): more than 0% to 0.03%, nitrogen (N): more than 0% to 0.01%, and the balance is iron (Fe) and other inevitable impurities. The ultra-high strength cold-rolled steel sheet can satisfy a yield strength (YP) of 1180 MPa or more, a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 15% or more, a yield ratio (YR) of 75% or more, and a bendability (R / t) of 3.0 or less.

Description

[Technical Field]

[0001] The technical concept of the present invention relates to cold-rolled steel sheets, and more particularly to ultra-high-strength cold-rolled steel sheets having high strength and elongation by controlling the microstructure, and to a method for manufacturing the same. [Background technology]

[0002] To improve collision safety and reduce vehicle weight, the materials used in the structural components of automobiles are required to possess high strength and high formability. Methods to satisfy these high strength and formability requirements include dual-phase steel, composed of ferrite and martensite structures, and transformation-induced plasticity steel (TRIP), which utilizes the phase transformation effect of retained austenite during deformation. Transformation-induced plasticity steel with a matrix composed of ferrite and bainite is disadvantageous in ensuring strength due to the mixing law; therefore, high-strength transformation-induced plasticity steel with a martensite matrix is ​​attracting attention. As a method for producing martensite-based high-strength transformation-induced plasticity steel, martensite or tempered martensite and retained austenite structures can be realized through quenching and parting (QP) heat treatment.

[0003] High-strength and highly formable steel materials of 1.2 GPa or higher, especially 1.5 GPa or higher, require not only high tensile strength but also high yield strength, and simultaneously, an appropriate fraction of retained austenite structure and stability of retained austenite are necessary to ensure elongation. Conventional technology has limitations in that it is insufficient to simultaneously ensure a tensile strength of 1470 MPa or higher and an elongation of 15% or higher.

[0004] Furthermore, when the microstructure consists only of martensite and retained austenite, the microstructure fraction is excessively sensitive to the quenching endpoint temperature. In particular, even with unavoidable microstructure deviations such as casting segregation, a difference in the martensite fraction occurs depending on the Ms temperature and the quenching temperature, making it difficult to achieve a uniform microstructure and uniform retained austenite. Conventionally, in order to ensure high strength and formability, martensite or tempered martensite was used as the main microstructure, and elongation was ensured through retained austenite or ferrite structures. A characteristic of quenching and reheating heat treatment is that the microstructure fraction of tempered martensite, martensite, and retained austenite changes depending on the quenching endpoint temperature. To ensure the target physical properties, the optimal rapid cooling endpoint temperature range is determined according to the alloy composition to control the microstructure fraction. However, if the rapid cooling endpoint temperature is too low, the retained austenite will be fine in size, but its fraction will be very small. If the rapid cooling endpoint temperature is too high, the austenite will be large, and carbon concentration will be insufficient. As a result, after final cooling, it may transform into a martensitic structure or become unstable, thus contributing little to ensuring elongation.

[0005] Therefore, in order to ensure formability, an appropriate fraction and fine shape of retained austenite, as well as stability through carbon concentration, must be ensured. On the other hand, in high-strength steels of 1470 MPa or higher, ensuring elongation via ferrite can lead to a decrease in yield strength or tensile strength, so ferrite must be limited.

[0006] A relevant prior art document is Korean Patent Application No. 10-2018-0047388. [Overview of the project] [Problems that the invention aims to solve]

[0007] The technical problem that the technical concept of this invention aims to solve is to provide an ultra-high-strength cold-rolled steel sheet with high strength and elongation by controlling the microstructure, and a method for manufacturing the same.

[0008] However, these challenges are illustrative, and the technical concept of the present invention is not limited to them. [Means for solving the problem]

[0009] According to one aspect of the present invention, the present invention provides an ultra-high-strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure, and a method for manufacturing the same.

[0010] According to one embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet is an ultra-high-strength cold-rolled steel sheet containing, by weight %, carbon (C): 0.28%~0.45%, silicon (Si): 1.0%~2.5%, manganese (Mn): 1.5%~3.0%, aluminum (Al): 0.01%~0.05%, chromium (Cr): greater than 0%~1.0%, molybdenum (Mo): greater than 0%~0.5%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0%~0.1%, phosphorus (P): greater than 0%~0.03%, sulfur (S): greater than 0%~0.03%, nitrogen (N): greater than 0%~0.01%, and the remainder being iron (Fe) and other unavoidable impurities, wherein the region between the surface and the center of the cold-rolled steel sheet has a thickness of 100 μm in the width direction of the steel sheet. 2 When observing the above areas, the ratio (B / A) of the area of ​​crystal grains with a carbon content of 0.5% or less in austenite to the area of ​​austenite (A) is less than 0.1, and the following conditions are met: yield strength (YP): 1180 MPa or higher, tensile strength (TS): 1470 MPa or higher, elongation (El): 15% or higher, yield ratio (YR): 75% or higher, and bendability (R / t): 3.0 or lower.

[0011] According to one embodiment of the present invention, the ratio (C / A) of the area of ​​martensite-austenite (MA) grains to the area of ​​austenite (A) may be less than 0.5.

[0012] According to one embodiment of the present invention, when observing retained austenite crystal grains in the width direction of the steel sheet by backscattered electron diffraction (EBSD) analysis in the region between the surface and the center of the cold-rolled steel sheet, when calculating the distribution of the average value of the crystal orientation difference within the retained austenite crystal grain by relating the average value of the crystal orientation difference obtained by averaging the difference in crystal orientation between any one region within the retained austenite crystal grain and a comparison region adjacent to the one region, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) shown in the distribution of the region where the average value of the crystal orientation difference is 0° or more and 3° or less can satisfy the relationship (Kmax-Kavg) / (Kmax-Kmin)>0.4.

[0013] According to one embodiment of the present invention, when observing retained austenite crystal grains in the width direction of the steel sheet by backscattered electron diffraction (EBSD) analysis in the region between the surface and the center of the cold-rolled steel sheet, the comparison region adjacent to the region includes a first comparison region located in contact with the region, a second comparison region located further away from the region than the first comparison region and in contact with the first comparison region, and a third comparison region located further away from the region than the second comparison region and in contact with the second comparison region. The average value of the crystal orientation difference obtained by averaging the crystal orientation difference between any region within the retained austenite crystal grains and the comparison regions adjacent to the region may be the average value of the crystal orientation difference obtained by averaging the crystal orientation difference between any region and the third comparison region.

[0014] According to one embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet may contain a mixed structure comprising ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite. The fraction of ferrite is in the range of greater than 0% to 5%, the fraction of martensite is in the range of greater than 0% to 20%, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of greater than 0% to 30%, the fraction of lower bainite is in the range of greater than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction. The minimum value of the sum of the fraction of upper bainite and the fraction of lower bainite may be 10%.

[0015] According to one embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet contains a mixed structure comprising tempered martensite, martensite, retained austenite, upper bainite, and lower bainite, wherein the fraction of martensite is in the range of more than 0% to 20%, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of more than 0% to 30%, the fraction of lower bainite is in the range of more than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction.

[0016] According to one embodiment of the present invention, the average diameter of the retained austenite may be 1.0 μm or less. [Effects of the Invention]

[0017] In the case of the technical idea of the present invention, the ultra-high strength cold-rolled steel sheet is a transformation-induced plasticity steel formed through rapid cooling and reheat heat treatment. The ultra-high strength cold-rolled steel sheet adjusts the carbon distribution appropriately by performing heat treatment between hot rolling coiling and cold rolling. After cold rolling and heat treatment, the carbon content in austenite is appropriately adjusted, and it can have a yield strength of 1180 MPa or more, a high tensile strength of 1470 MPa or more, an elongation of 15% or more, a yield ratio of 75% or more, and a bending formability (R / t) of 3.0 or less based on a 90° bend standard. The ultra-high strength cold-rolled steel sheet uses tempered martensite, upper bainite, and lower bainite transformation structures as the microstructure to refine and stabilize retained austenite, and can stably provide the yield strength and yield ratio.

[0018] Particularly, by inducing multiple stages of phase transformation such as martensite transformation (primary), lower bainite transformation (secondary), and upper bainite transformation (tertiary) in the rapid cooling (secondary cooling), maintenance of rapid cooling, reheat, and partitioning processes, it can also help control the problem of non-uniformity of the structure due to casting segregation and the like that inevitably exist in the steel material, and can refine and stabilize retained austenite. When simply composed of the structure of martensite and retained austenite, the Ms point changes due to the non-uniformity of components such as casting segregation in the structure, and it has different fractions of martensite and retained austenite at the same rapid cooling temperature. On the contrary, in the ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention, such problems can be solved.

[0019] The effects of the present invention described above are described exemplarily, and the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

[0020] [Figure 1] It is a diagram for explaining the concept of a method of calculating the average value (K) of the crystal orientation difference obtained by averaging the crystal orientation differences between a comparison region adjacent to one region in the manufacturing method of the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention. [Figure 2] It is a graph showing the distribution pattern of the average value (K) of the crystal orientation difference.

[0021] [Figure 3] This is a scanning electron microscope image showing the microstructure of the steel material after primary heat treatment in Example 1 of the experimental examples of the present invention.

[0022] [Figure 4] This is a scanning electron microscope image showing the microstructure of the steel material after primary heat treatment in Comparative Example 2 of the experimental examples of the present invention.

[0023] [Figure 5] This is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Example 1 of the experimental examples of the present invention.

[0024] [Figure 6] This is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet produced by Comparative Example 1 of the experimental examples of the present invention.

[0025] [Figure 7] This is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet produced by Comparative Example 2 of the experimental examples of the present invention.

[0026] [Figure 8] This is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet produced by Comparative Example 3 of the experimental examples of the present invention.

[0027] [Figure 9] This figure shows the shape and distribution of retained austenite mediated by EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Example 1 of the experimental examples of the present invention.

[0028] [Figure 10] This figure shows the shape and distribution of retained austenite mediated by EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 1 of the experimental examples of the present invention.

[0029] [Figure 11]This figure shows the shape and distribution of retained austenite mediated by EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 2 of the experimental examples of the present invention. [Best Mode for Carrying Out the Invention]

[0030] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are provided to further fully illustrate the technical idea of ​​the invention to those ordinary skill in the art, and the embodiments described below can be modified into various other forms; the scope of the technical idea of ​​the invention is not limited to these embodiments. Rather, these embodiments are provided to further enrich and complete this disclosure and to fully communicate the technical idea of ​​the invention to those skilled in the art. Throughout this specification, the same reference numerals mean the same elements. Furthermore, various elements and areas in the drawings are shown schematically. Therefore, the technical idea of ​​the invention is not limited by the relative sizes or spacing shown in the accompanying drawings.

[0031] The technical concept of the present invention is to provide an ultra-high-strength cold-rolled steel sheet having a yield strength of 1180 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 15% or more, a yield ratio (yield strength / tensile strength) of 75% or more, and a 90° bendability of 3.0 R / t or less, as well as a method for manufacturing the same.

[0032] The following describes in detail the ultra-high-strength cold-rolled steel sheet based on the technical concept of the present invention.

[0033] An ultra-high-strength cold-rolled steel sheet according to one embodiment of the present invention contains, by weight percent, carbon (C): 0.28% to 0.45%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.03%, nitrogen (N): greater than 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities.

[0034] The roles and content of each component in the ultra-high-strength cold-rolled steel sheet according to the present invention are described below. In this case, the content of each component element is all expressed as a weight percentage of the total steel sheet.

[0035] Carbon (C): 0.28%~0.45%

[0036] Carbon is added to ensure the strength of steel, and in particular, to increase the strength of the martensitic structure. Furthermore, a sufficient carbon content is required because it stabilizes austenite through segmentation and ensures elongation through the transformation-induced plasticity (TRIP) effect. If the carbon content is less than 0.28%, it may be difficult to simultaneously achieve the target strength and elongation. If the carbon content exceeds 0.45%, weldability may decrease, and hydrogen embrittlement may occur. Therefore, it is preferable to add carbon in an amount of 0.28% to 0.45% of the total weight of the steel sheet.

[0037] Silicon (Si): 1.0%~2.5%

[0038] Silicon is a ferrite-stabilizing element that delays the formation of carbides in ferrite and martensite, thereby providing solid solution strengthening. In particular, it is essential for delaying the formation of carbides in martensite and for partitioning carbon in austenite. If the silicon content is less than 1.0%, the carbide formation inhibitory effect is small, making it difficult to ensure sufficient stability of retained austenite. If the silicon content exceeds 2.5%, oxides such as Mn2SiO4 are formed during the manufacturing process, inhibiting plating properties and increasing the carbon equivalent, which can reduce weldability. Therefore, it is preferable to add silicon in an amount of 1.0% to 2.5% of the total weight of the steel sheet.

[0039] Manganese (Mn): 1.5%~3.0%

[0040] Manganese has a solid solution strengthening effect, increasing hardenability and delaying the formation of ferrite and bainite during cooling. If the manganese content is less than 1.5%, the effect of manganese addition is insufficient, making it difficult to ensure hardenability. If the manganese content exceeds 3.0%, the transformation of bainite may be excessively delayed, leading to the formation of inclusions such as MnS and a decrease in workability due to segregation, and an increase in carbon equivalent, which can reduce weldability. Therefore, it is preferable to add manganese in an amount of 1.5% to 3.0% of the total weight of the steel sheet.

[0041] Aluminum (Al): 0.01%~0.05%

[0042] Aluminum can be used as a deoxidizing agent and, similar to silicon, can help suppress the formation of carbides. If the aluminum content is less than 0.01%, the deoxidizing effect may be insufficient. If the aluminum content exceeds 0.05%, it may form AlN during slab production, inducing cracking during casting or hot rolling. Therefore, it is preferable to add aluminum in an amount of 0.01% to 0.05% of the total weight of the steel sheet.

[0043] Chromium (Cr): Over 0% to 1.0%

[0044] Chromium has a solid solution strengthening effect, increasing hardenability and contributing to improved strength. It also works with C and Mn to refine the martensite and bainite structures and contribute to the stabilization of retained austenite. If the chromium content exceeds 1.0%, the transformation of bainite may be excessively delayed, which can increase the manufacturing cost of the steel. Therefore, it is preferable to add chromium in an amount of more than 0% to 1.0% of the total weight of the steel sheet.

[0045] Molybdenum (Mo): Over 0% to 0.5%

[0046] Molybdenum has a solid solution strengthening effect, increasing hardenability and contributing to improved strength. It also works with C and Mn to refine the martensite and bainite structures and contribute to the stabilization of retained austenite. If the molybdenum content exceeds 0.5%, the transformation of bainite may be excessively delayed, which can increase the manufacturing cost of the steel. Therefore, it is preferable to add molybdenum in an amount of more than 0% to 0.5% of the total weight of the steel sheet.

[0047] Total amount of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.1%

[0048] In this invention, the material may contain at least one of niobium, titanium, and vanadium. Niobium, titanium, and vanadium are precipitate-forming elements that can increase strength through precipitation strengthening and also provide grain refinement. If the sum of niobium, titanium, and vanadium added exceeds 0.1% each, the manufacturing cost of the steel may increase significantly, the rolling load may increase significantly due to a large amount of precipitation during rolling, and the elongation may decrease. Therefore, it is preferable that the sum of niobium, titanium, and vanadium each be added in an amount greater than 0% to 0.1% of the total weight of the steel sheet. Furthermore, it is preferable that each of niobium, titanium, and vanadium be added in an amount of 0.1% or less of the total weight of the steel sheet. For example, each of niobium, titanium, and vanadium can be added in an amount greater than 0% to 0.05%.

[0049] Phosphorus (P): Over 0% ~ 0.03%

[0050] Phosphorus is an impurity found in the steel manufacturing process. While it can contribute to increased strength through solid solution strengthening, high concentrations can cause low-temperature brittleness. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.03% of the total weight of the steel sheet.

[0051] Sulfur (S): More than 0%~0.03%

[0052] Sulfur is an impurity present in steel during the manufacturing process, and it can form nonmetallic inclusions such as FeS and MnS, which can reduce bendability, toughness, and weldability. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.03% of the total weight of the steel sheet.

[0053] Nitrogen (N): greater than 0% to 0.01%

[0054] Nitrogen is an element that is inevitably present during steel production and can help stabilize austenite, but it can react with Al to form AlN, which can induce cracking during continuous casting. Therefore, it is preferable to limit the nitrogen content to more than 0% to 0.01% of the total weight of the steel sheet.

[0055] On the other hand, the ultra-high-strength cold-rolled steel sheet according to a modified embodiment of the present invention may further contain, in addition to the alloying elements described above, at least one of the following elements having a compositional range.

[0056] Nickel (Ni): Over 0% to 0.5%

[0057] Nickel can also help stabilize austenite and increase the hardenability of steel. However, a nickel content exceeding 0.5% is undesirable because it increases the manufacturing cost of the steel. Therefore, it is preferable to add nickel in an amount between 0% and 0.5% of the total weight of the steel sheet.

[0058] Copper (Cu): More than 0%~0.5%

[0059] Copper can also help stabilize austenite and increase the hardenability of steel. However, a copper content exceeding 0.5% is undesirable because it increases the manufacturing cost of the steel. Therefore, it is preferable to add copper in an amount between 0% and 0.5% of the total weight of the steel sheet.

[0060] Furthermore, it is preferable that the total amount of nickel and copper added is greater than 0% but less than 1.0%.

[0061] Boron (B): Over 0% ~ 0.005%

[0062] Boron, like Mn, Cr, and Mo, can improve hardenability. If the boron content exceeds 0.005%, it may concentrate on the surface, leading to a deterioration in quality such as plating adhesion. Therefore, it is preferable to add boron in an amount of more than 0% to 0.005% of the total weight of the steel sheet.

[0063] The remaining component of the aforementioned ultra-high-strength cold-rolled steel sheet is iron (Fe). However, in the normal steelmaking process, unintended impurities from the raw materials and surrounding environment inevitably become mixed in, and it is not possible to eliminate them. Since these impurities are known to any engineer in a normal manufacturing process, their details will not be specifically mentioned in this specification.

[0064] An ultra-high-strength cold-rolled steel sheet according to one embodiment of the present invention may include a mixed structure comprising ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite. The fraction of ferrite is in the range of 0% to 5% (including 0%), the fraction of martensite is in the range of more than 0% to 20%, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of more than 0% to 30%, the fraction of lower bainite is in the range of more than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction. The minimum sum of the fraction of upper bainite and the fraction of lower bainite may be 10%. The fractions refer to the ratio of areas derived from photographs of the microstructure through an image analyzer. The ferrite may include polygonal ferrite. The average diameter of the retained austenite may be, for example, 1.0 μm or less, and may be in the range of, for example, 0.1 μm to 1.0 μm.

[0065] The retained austenite is finely distributed in the lath and grain boundaries of the tempered martensite and bainite, thereby stabilizing the retained austenite and ensuring stable strength and elongation.

[0066] Furthermore, the ultra-high-strength cold-rolled steel sheet does not have to contain ferrite. In such cases, the ultra-high-strength cold-rolled steel sheet contains a mixed structure in which tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed, the fraction of martensite is in the range of more than 0% to 20%, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of more than 0% to 30%, the fraction of lower bainite is in the range of more than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction. Also, the sum of the fraction of upper bainite and the fraction of lower bainite may be 10% to 60%. The minimum value of the sum of the fraction of upper bainite and the fraction of lower bainite may be 10%.

[0067] The ultra-high-strength cold-rolled steel sheet according to the technical concept of the present invention has a thickness of t / 4 between the surface and the center of the cold-rolled steel sheet, with a width of 100 μm in the width direction (TD) of the steel sheet. 2 When observing the above areas, the ratio (B / A) of the area of ​​a crystal grain with a carbon content of 0.5% or less in the austenite to the area of ​​austenite (A) is less than 0.1. This ratio (B / A) can be understood as a measure of the stability of the composition of retained austenite (RA) formed in the steel sheet. If the ratio (B / A) is 0.1 or greater, the stability of the austenite composition is insufficient, and therefore the effect of improving elongation due to retained austenite cannot be obtained. To measure the carbon content within individual crystal grains, the distance between lattice planes is measured through transmission electron microscopy (TEM) observation, and C γ =( α γ The carbon content was derived through the relationship (-3.592) / 0.033. α γ This is the austenite lattice constant measured by a transmission electron microscope.

[0068] The ultra-high-strength cold-rolled steel sheet according to the technical concept of the present invention has a thickness of t / 4 between the surface and the center of the cold-rolled steel sheet, with a width of 100 μm in the width direction (TD) of the steel sheet. 2 When observing the above areas, the ratio (C / A) of the area of ​​martensite-austenite crystal grains (C) to the area of ​​austenite (A) is less than 0.5. This ratio (C / A) can be understood as a measure of the positional stability of retained austenite (RA) generated within the steel sheet. If the ratio (C / A) is 0.5 or greater, there is an excess of martensite-austenite crystal grains that do not participate in deformation-induced martensitic transformation, and therefore sufficient elongation and work hardening ability cannot be obtained.

[0069] The ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention, when observing retained austenite crystal grains by the electron backscatter diffraction (EBSD) analysis method in the width direction (TD) of the steel sheet in the region (thickness of t / 4) between the surface portion and the central portion of the cold-rolled steel sheet, through the process of corresponding the average value (K) of the crystal orientation difference with a comparison region adjacent to the one region, by averaging the crystal orientation differences between the one region and the comparison region, when calculating the distribution of the average value of the crystal orientation differences in the retained austenite crystal grains, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) shown in the distribution of the region where the average value of the crystal orientation differences is 0° or more and 3° or less satisfy the relationship of (Kmax - Kavg) / (Kmax - Kmin)>0.4. On the other hand, the maximum value of (Kmax - Kavg) / (Kmax - Kmin) is 1.

[0070] FIG. 1 is a diagram for explaining the concept of a method of calculating the average value (K) of the crystal orientation difference by averaging the crystal orientation differences between a comparison region adjacent to one region in the manufacturing method of the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, and FIG. 2 is a graph showing the distribution aspect of the average value (K) of the crystal orientation difference.

[0071] Referring to FIG. 1, when observing retained austenite crystal grains by the electron backscatter diffraction (EBSD) analysis method in the width direction of the steel sheet in the region between the surface portion and the central portion of the cold-rolled steel sheet, the comparison regions adjacent to the one region (A0) are the first comparison regions (A1 to A6) located in contact with the one region (A0), the first comparison regions (A1 to A6) are further separated from the one region (A0) than the first comparison regions (A1 to A6), and the second comparison regions (A7 to A 18 ) located in contact with the first comparison regions (A1 to A6), and the second comparison regions (A7 to A 18 ) are further separated from the one region (A0) than the second comparison regions (A7 to A 18 ), and the third comparison regions (A 19 to A 36) may include. In this case, the average value (K) of the crystal orientation difference obtained by averaging the crystal orientation difference between any one region (A0) within the retained austenite crystal grain and a comparison region adjacent to the one region (A0) is the third comparison region (A 19 ~A 36 It may also be the average value (K) of the crystal orientation difference obtained by averaging the difference in crystal orientation with ).

[0072] For example, the third comparison region (A) is defined as the arbitrary region (A0) as the reference. 19 ~A 36 The average value (K) of the crystal orientation difference obtained by averaging the crystal orientation differences with ) can be expressed by the following equation 1. Here, (MA) i This is based on one region (A0) as the reference for the third comparison region (A 19 ~A 36 This indicates the difference in crystal orientation with any one of the regions, where n can be 19 and m can have a value of 36.

[0073]

number

[0074] Referring to Figure 2, the distribution of the average value (K) of the crystal orientation difference is shown from 0° to 5°. Within this distribution, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) can be calculated in the region where the average value of the crystal orientation difference is between 0° and 3°.

[0075] In the method for manufacturing ultra-high-strength cold-rolled steel sheets according to the technical concept of the present invention, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) shown in the distribution of the region where the average value (K) of the crystal orientation difference is 0° or more and 3° or less satisfy the relationship (Kmax-Kavg) / (Kmax-Kmin)>0.4.

[0076] In the deformation-induced martensite phase transformation reaction of austenite having an FCC structure, defects within the crystal grains, such as dislocations and stacking faults, act as nucleation sites for martensite. Therefore, if the average value of the crystal orientation difference, which indicates the defect distribution within the crystal grains, is too small, TRIP (Transformation-induced plasticity) nucleation is insufficient, and the effect of increasing ductility × tensile strength due to deformation-induced martensite phase transformation cannot be obtained. On the other hand, if the average value of the crystal orientation difference is too high, the deformation-induced martensite phase transformation occurs concentrated in the early stages of tensile deformation, and similarly, the desired increase in ductility cannot be obtained.

[0077] By controlling the specific components of the alloy composition and their content ranges as described above, ultra-high-strength cold-rolled steel sheets that satisfy the above-mentioned conditions can, for example, meet the following requirements: yield strength (YP): 1180 MPa to 1330 MPa, tensile strength (TS): 1470 MPa to 1770 MPa, elongation (El): 15% or more, yield ratio (YR): 75% or more, and bendability (R / t): 3.0 or less.

[0078] The method for manufacturing ultra-high-strength cold-rolled steel sheets according to the present invention will be described below with reference to the attached drawings.

[0079] Manufacturing method for ultra-high-strength cold-rolled steel sheets

[0080] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling process may, for example, be a slab. A slab in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0081] A method for manufacturing an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention includes the steps of: manufacturing a hot-rolled steel sheet using a steel material of the above composition; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet; multi-stage cooling the cold-rolled steel sheet; and partitioning the cold-rolled steel sheet.

[0082] In particular, the cold-rolled steel sheet is heated to above Ac3 temperature for annealing heat treatment and maintained for a certain period of time, followed by a two-stage multi-stage cooling process of slow cooling and rapid cooling until the rapid cooling completion temperature is reached. Then, after maintaining the rapid cooling completion temperature for a certain period of time, the sheet is heated to above Ms temperature for partitioning heat treatment, maintained at a constant temperature for the duration of the partitioning heat treatment, and finally cooled to below Mf temperature.

[0083] Manufacturing steps for hot-rolled steel sheets

[0084] A steel slab having the aforementioned alloy composition is prepared, and the steel slab is reheated at a reheating temperature (Slab Reheating Temperature, SRT) in the range of, for example, 1,150°C to 1,250°C. Through such reheating, the segregated components and precipitates from casting are redissolved, homogenizing the slab and making it suitable for hot rolling. If the reheating temperature is less than 1,150°C, the redissolution of segregation may be insufficient, and the hot rolling load may increase. If the reheating temperature exceeds 1,250°C, the size of the austenite grains may increase, and the process cost may increase due to the rise in temperature. The reheating time may be, for example, 1 to 4 hours. If the reheating time is less than 1 hour, the homogenization of segregation may be insufficient. If the reheating time exceeds 4 hours, the size of the austenite grains may increase, and the process cost may increase due to the rise in temperature.

[0085] After the aforementioned reheating, hot rolling can be performed by a conventional method, for example, by hot finish rolling at a finish delivery temperature (FDT) in the range of 850°C to 970°C to produce a hot-rolled steel sheet. If the finish delivery temperature is less than 850°C, ferrite or pearlite may be formed. If the finish delivery temperature exceeds 970°C, scale formation increases, the grain size becomes coarser, and it may become difficult to achieve fine and uniform microstructure.

[0086] Next, the hot-rolled steel sheet is cooled to a coiling temperature, for example, in the range of 400°C to 700°C. This cooling can be done by air or water, and can be performed at a cooling rate of, for example, 10°C / second to 30°C / second. A faster cooling rate is advantageous for reducing the average grain size. It is preferable to cool the sheet down to the coiling temperature.

[0087] Next, the hot-rolled steel sheet is coiled at a coiling temperature (CT) in the range of 400°C to 700°C, for example. The coiling temperature range can be selected from the viewpoint of cold-rollability and surface properties. If the coiling temperature is less than 400°C, a hard phase such as martensite may be excessively generated, increasing the material properties of the hot-rolled steel sheet excessively, which may significantly increase the rolling load during cold rolling. If the coiling temperature exceeds 700°C, it may lead to non-uniformity of the microstructure of the final product.

[0088] On the other hand, in the method for manufacturing ultra-high-strength cold-rolled steel sheets according to the technical concept of the present invention, after hot rolling and coiling, and before cold rolling, a primary heat treatment can be performed at a temperature of 500°C to 680°C for 10 seconds to 12 hours. At this time, the primary heat treatment can be selected from batch annealing or a continuous heat treatment process. In the microstructure of the steel sheet after the primary heat treatment, 100 μm 2 Within the area, carbides with a grain size of 500 nm or larger are distributed at a rate of two or fewer particles, and the pearlite ratio is 5% or less. If the primary heat treatment process is not performed, or if the temperature is lower than 500°C, the cold rolling load increases, which has the disadvantage of making the process more difficult. If the temperature of the heat treatment process exceeds 680°C, or if the time exceeds 12 hours, coarse carbides such as spherical cementite with a diameter of 500 nm or larger are formed. This deepens the heterogeneity of carbon atoms, and after cold rolling, it may create retained austenite with an excess of carbon dissolved in the final microstructure. In the case of austenite with a carbon content of approximately 1.1% or more, it is expected that the TRIP effect will decrease and the strength × elongation properties will decrease as the ratio increases, so after the primary heat treatment, we attempted to adjust the stability of retained austenite by appropriately adjusting the microstructure.

[0089] Cold-rolled steel sheet manufacturing steps

[0090] The hot-rolled steel sheet is subjected to an acid pickling treatment to remove the surface scale layer. Next, the hot-rolled steel sheet is cold-rolled to form a cold-rolled steel sheet, for example, with an average reduction ratio of 40% to 70%. The higher the average reduction ratio, the greater the formability due to the refinement effect of the microstructure. If the average reduction ratio is less than 40%, it is difficult to obtain a uniform microstructure. If the average reduction ratio exceeds 70%, the roll force increases, increasing the process load. The cold rolling allows the final steel sheet to have a certain thickness. The microstructure of the cold-rolled steel sheet may have a structure that is an elongated version of the microstructure of the hot-rolled steel sheet.

[0091] Annealing heat treatment step

[0092] The cold-rolled steel sheet is subjected to annealing heat treatment in a continuous annealing furnace with a normal slow cooling section. This annealing heat treatment is performed to form an austenite single-phase structure. The annealing heat treatment temperature and time affect the size of the austenite crystal grains and, therefore, can have a significant impact on the strength of the cold-rolled steel sheet.

[0093] The annealing heat treatment is performed by heating at a heating rate of, for example, 2°C / second or more, and for example, at a heating rate in the range of 2°C / second to 10°C / second. If the heating rate is less than 2°C / second, it will take a long time to reach the target annealing heat treatment temperature, which may reduce production efficiency and increase the size of the crystal grains.

[0094] The annealing heat treatment may be performed, for example, at a temperature of Ac3 or higher, for example, in the range of 830°C to 930°C, for example, in the range of 830°C to 900°C, and maintained for a time in the range of 30 seconds to 120 seconds. In such heating and annealing heat treatment steps, the cold-rolled structure undergoes a reverse transformation to austenite. If the annealing heat treatment temperature is below 830°C, it is not possible to form a single austenite phase in order to create the final structure, tempered martensite. For reference, in order to form a single austenite phase, the annealing heat treatment must be performed at a temperature of A3 or higher. If the annealing heat treatment temperature exceeds 900°C, the austenite grains may become coarse, and the strength may decrease.

[0095] As the annealing heat treatment time increases, it affects the coarsening due to the growth of austenite grains, similar to the annealing heat treatment temperature, but the effect of the annealing heat treatment time is smaller than that of the annealing heat treatment temperature. If the annealing heat treatment time exceeds 120 seconds, the heat treatment efficiency may decrease. If the annealing heat treatment time is less than 30 seconds, the annealing heat treatment effect may be insufficient.

[0096] Multi-stage cooling steps

[0097] The cold-rolled steel sheet that has undergone the annealing heat treatment is cooled in multiple stages. The cooling step may be carried out in the following two stages.

[0098] First, the annealed cold-rolled steel sheet is subjected to primary cooling by slow cooling at a cooling rate of, for example, 1°C / sec to 15°C / sec, or at a cooling rate of, for example, 3°C / sec to 10°C / sec, to a temperature range that suppresses ferrite transformation, for example, to a primary cooling completion temperature in the range of 650°C to 800°C. If the primary cooling completion temperature by slow cooling is less than 650°C, ferrite transformation may occur in an undesirable amount, which may reduce the strength. Preferably, the fraction of ferrite generated by the ferrite transformation is limited to 0% to less than 5%.

[0099] Next, the cold-rolled steel sheet that has undergone primary cooling (slow cooling) is subjected to secondary cooling (rapid cooling) at a cooling rate of, for example, 20°C / second or more, at a cooling rate in the range of 20°C / second to 100°C / second, to a temperature below the Ms temperature, for example, in the range of Ms-140°C to Ms-30°C, for example, a secondary cooling completion temperature in the range of 180°C to 300°C. The secondary cooling is a rapid cooling step, and may be performed sequentially as a 2-1 rapid cooling step and a 2-2 rapid cooling step. The cooling rate of the 2-1 rapid cooling step may be, for example, 20°C / second or more, and it may be rapidly cooled to a temperature of Ms-30°C or lower. During cooling, a portion of the austenite transforms into martensite, and the amount is about 20-80%. The cooling rate in the 2-2 rapid cooling step may be, for example, 30°C / second or more, and the mixture is cooled to the rapid cooling endpoint temperature (Ms-140°C to Ms-30°C) to induce martensitic transformation.

[0100] The aforementioned secondary cooling (rapid cooling) can cause a portion of the austenite to transform into martensite. The fraction of martensite generated may be between 20% and 80%.

[0101] In such heat treatment, if the average cooling rate in the slow cooling to rapid cooling section can be ensured to be faster than 70°C / second, the 2-1 rapid cooling step and the 2-2 rapid cooling step may be performed without distinction.

[0102] Next, the cold-rolled steel sheet that has undergone secondary cooling (rapid cooling) is maintained at the secondary cooling completion temperature for a period of time, for example, between 5 and 90 seconds. During this maintenance period after rapid cooling, the temperature of the steel may initially become homogenized. Subsequently, while being maintained at an isothermal temperature at the secondary cooling completion temperature, some of the retained austenite may transform into lower bainite or the like.

[0103] Partitioning heat treatment step

[0104] The multi-stage cooled cold-rolled steel sheet is reheated, for example, at a heating rate in the range of 3°C / second to 20°C / second, and subjected to partitioning heat treatment by maintaining the temperature in the range of 360°C to 500°C, for example, at a temperature in the range of 360°C to 460°C, for example, for a time in the range of 30 seconds to 500 seconds, for example, for a time in the range of 30 seconds to 500 seconds.

[0105] If the temperature of the partitioning heat treatment is below 360°C, the partitioning effect may be insufficient. If the temperature of the partitioning heat treatment exceeds 500°C, the size of the carbides may become coarser, which may lead to a decrease in strength.

[0106] The duration of the partitioning heat treatment has less influence than the partitioning temperature. If the duration of the partitioning heat treatment is less than 30 seconds, it may be difficult to obtain a stable partitioning effect. If the duration of the partitioning heat treatment exceeds 500 seconds, the heat treatment efficiency decreases, the size of the carbides increases, and a decrease in strength may occur.

[0107] The partitioning heat treatment step may be performed immediately after the multi-stage cooling, or after maintaining the temperature at room temperature for several minutes or more.

[0108] After the partitioning heat treatment step is complete, the material is cooled to room temperature, for example, to a temperature in the range of 0°C to 40°C.

[0109] Analysis of changes in microstructure

[0110] In the following section, the changes in the microstructure of the ultra-high-strength cold-rolled steel sheet during the process of manufacturing the ultra-high-strength cold-rolled steel sheet according to the technical concept of the present invention will be described in detail.

[0111] During the annealing heat treatment step, the microstructure of the cold-rolled steel sheet undergoes a reverse transformation to austenite.

[0112] In the primary cooling step of the multi-stage cooling process, the fraction of ferrite generated by ferrite transformation is limited to less than 5%, and it is acceptable for no ferrite to be generated at all. If more than 5% of ferrite is generated, the strength may decrease, and the target strength may not be achieved.

[0113] In the secondary cooling step of the multi-stage cooling process, the cold-rolled steel sheet is cooled at a rapid rate, which suppresses ferrite, pearlite, and bainite transformations, and a portion of the austenite transforms into martensite. At this time, the fraction of martensite generated by the martensitic transformation can be limited to 20% to 80%. If the fraction of martensite generated during the secondary cooling exceeds 80%, it may become difficult to secure an appropriate fraction of retained austenite. If it is less than 20%, the fraction of retained austenite is too high after cooling, making it difficult to ensure the stability of the retained austenite. Even if the bainite transformation structure is increased, the fraction of martensite decreases, which may result in a decrease in strength. Furthermore, some martensitic structures can increase internal stress, increasing the rate of bainite nucleation and causing the bainite transformation to proceed rapidly even at low temperatures below Ms.

[0114] In the secondary cooling step of the multi-stage cooling process, during the period of rapid cooling followed by maintenance at the secondary cooling completion temperature, a portion of the austenite transforms into bainite, which may mainly be lower bainite. Furthermore, fine precipitates may form in the martensite generated in the previous step. The maintenance time at the secondary cooling completion temperature can range from 5 to 90 seconds. If the maintenance time is less than 5 seconds, the lower bainite transformation may not occur sufficiently. If the maintenance time exceeds 90 seconds, the process cost may increase due to the excessively long heat treatment time.

[0115] In the partitioning heat treatment step, carbon diffuses and concentrates within the retained austenite, stabilizing it. Furthermore, a portion of the retained austenite can undergo bainite transformation. This bainite transformation, after rapid cooling, can refine the shape of the retained austenite, thereby contributing to its stabilization. Due to this effect, the sum of the upper bainite fraction and the lower bainite fraction can be 10% or more.

[0116] After the partitioning heat treatment step, during the final cooling to room temperature, some unstable austenite may transform into martensite. If a large amount of martensite is generated at this time, the final fraction of retained austenite may decrease, which may adversely affect the formability. Therefore, it is preferable to control the amount of generated martensite to less than 20%.

[0117] In order to suppress the martensitic transformation during the final cooling, it is preferable to ensure that the heat treatment is carried out without problems in the secondary cooling end temperature, the duration at the secondary cooling end temperature, and the partitioning heat treatment step, thereby refining the retained austenite and promoting stabilization.

[0118] While bainite transformation is sometimes not considered below the aforementioned Ms point, there are studies suggesting that bainite transformation is possible below the Ms point, and studies indicating that bainite nucleation increases below the Ms point compared to just above Ms.

[0119] After such a heat treatment process, the final microstructure may include tempered martensite (20%-80%), retained austenite (10%-30%), lower bainite (0%-30%), upper bainite (0%-30%), and some ferrite (0%-5%) or martensite (0%-20%). The sum of the fractions of upper bainite and lower bainite may be 10% or more. The average diameter of the retained austenite may be 1.0 μm or less.

[0120] Experimental example

[0121] The following are preferred experimental examples to aid in understanding the present invention. However, these experimental examples are merely for the purpose of aiding in understanding the present invention, and the present invention is not limited to these experimental examples.

[0122] Steel having the composition (in weight %) shown in Table 1 below was prepared, and cold-rolled steel sheets according to the examples and comparative examples were prepared through predetermined hot-rolling, cold-rolling, and heat-treatment processes. The remainder is iron (Fe).

[0123] [Table 1]

[0124] Referring to Table 1, steel grades A to D satisfy the composition range of the present invention, specifically, in weight percent, carbon (C): 0.28% to 0.45%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.03%, nitrogen (N): greater than 0% to 0.01%, and the remainder being iron (Fe). Conversely, steel grade E falls outside the composition range of the present invention, specifically, its carbon (C) content is below the range of 0.28% to 0.45%, and therefore does not satisfy the requirements. Table 2 shows the Ac3 temperature, Ms temperature, Ms-140°C temperature, and Ms-30°C temperature for the aforementioned steel grades. The unit is °C.

[0125] [Table 2]

[0126] Referring to Table 2, the Ac3 temperature was calculated using Thermo-Calc and the TCFE9 database. The Ms temperature was calculated using the following empirical formula. In the following empirical formula, for example, "[C]" represents the weight percentage of carbon. Ms(°C) = 539 - 423[C] - 30.4[Mn] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo]

[0127] The slabs of the aforementioned steel types were reheated to 1200°C and maintained at that temperature for 3 hours. After hot-rolling to a thickness of 2.4 mm at a finishing rolling temperature of 950°C, they were wound up at 600°C. Next, the wound hot-rolled steel sheets were pickled to remove surface scale, and then cold-rolled to produce cold-rolled steel sheets with a thickness of 1.2 mm.

[0128] Next, heat treatment was performed under the process conditions shown in Table 3.

[0129] Table 3 shows the conditions for the heat treatment process used to manufacture cold-rolled steel sheets in the comparative example and the example. In Table 3, 'primary heat treatment' refers to the heat treatment performed after hot rolling and coiling, but before cold rolling.

[0130] [Table 3]

[0131] Referring to Table 3, Examples 1 to 4 satisfy the process range of the present invention. Comparative Example 1 uses steel grade E, which is outside the composition range of the present invention. Comparative Example 2 is not satisfied because it exceeds the primary heat treatment temperature range of 500°C to 680°C and falls below the annealing temperature range of 830°C to 930°C. Comparative Example 3 satisfies the annealing temperature range but exceeds the annealing holding time of 30 seconds to 120 seconds and, after secondary cooling (rapid cooling), falls below the rapid cooling holding time of 5 seconds to 90 seconds at the secondary cooling end temperature (180°C to 300°C) and is not satisfied.

[0132] Table 4 shows the parameter values ​​indicating the microstructure of the cold-rolled steel sheets of the comparative example and the example.

[0133] In Table 4, the first item value (B / A) is 100 μm in the width direction of the cold-rolled steel sheet in the region between the surface and the center of the sheet. 2 When observing the above area, it represents the ratio (B / A) of the area of ​​crystal grains with a carbon content of 0.5% or less in austenite to the area of ​​austenite (A), and the second item value (C / A) is 100 μm in the width direction of the steel sheet in the region between the surface and the center of the cold-rolled steel sheet. 2 When observing the above area, it refers to the ratio (C / A) of the area of ​​martensite-austenite crystal grains (C) to the area of ​​austenite (A). The third item value ((Kmax-Kavg) / (Kmax-Kmin)) refers to the relationship between the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) shown in the distribution of the region where the average crystal orientation difference within the retained austenite crystal grain is between 0° and 3°. This is calculated by using any one region within the retained austenite crystal grain as a reference and associating the average crystal orientation difference obtained by averaging the crystal orientation difference between that region and a comparison region adjacent to that region.

[0134] [Table 4]

[0135] Referring to Table 4, Examples 1 to 4 satisfy all of the following conditions: 1st item value (B / A) < 0.1, 2nd item value (C / A) < 0.5, and 3rd item value ((Kmax-Kavg) / (Kmax-Kmin)) > 0.4. In contrast, Comparative Example 1 has a 1st item value (B / A) greater than 0.1 and a 3rd item value ((Kmax-Kavg) / (Kmax-Kmin)) less than 0.4. Comparative Example 2 confirms that the 1st item value (B / A) is greater than 0.1 and the 2nd item value (C / A) is not less than 0.5. Comparative Example 3 confirms that the 2nd item value (C / A) is not less than 0.5. Table 5 shows the physical and mechanical properties of the manufactured hot-rolled steel sheets and steel pipes, including yield strength (YS), tensile strength (TS), elongation (EL), yield ratio (YR), and 90° bendability (R / t).

[0136] [Table 5]

[0137] Referring to Table 5, the examples satisfy the target ranges for yield strength (YS), tensile strength (TS), elongation (EL), yield ratio (YR), and 90° bendability (R / t). Furthermore, the TS × T.El value, which is the product of tensile strength and elongation, may be 20,000 or more, preferably 21,000 or more, and even more preferably 22,000 or more. In contrast, Comparative Example 1 does not satisfy the target range, as its elongation (EL) is below the range of 15% or more, and its product of tensile strength and elongation is below the range of 20,000 or more. Comparative Example 2 does not satisfy the target range, as its elongation (EL) is below the range of 15% or more, its yield ratio (YR) is below the range of 75% or more, its 90° bendability (R / t) is above the range of 3.0 or less, and its product of tensile strength and elongation is below the range of 20,000 or more. In the comparison, the yield strength (YP) of Comparative Example 3 fell below the range of 1180 MPa or higher and did not meet the requirements. The tensile strength (TS) fell below the range of 1470 MPa or higher and did not meet the requirements. The elongation (EL) fell below the range of 15% or higher and did not meet the requirements. The yield ratio (YR) fell below the range of 75% or higher and did not meet the requirements. The 90° bendability (R / t) fell above the range of 3.0 or lower and did not meet the requirements. The product of tensile strength and elongation fell below the range of 20000 or higher and did not meet the requirements.

[0138] To explain the comparative examples that did not satisfy the target physical properties, Comparative Example 1 was characterized by a low carbon content, and it was not possible to simultaneously secure a tensile strength of 1470 MPa and an elongation of 15% or more. Comparative Example 2 was characterized by a high primary heat treatment temperature and a low annealing temperature, and it was not possible to secure sufficient elongation. When the primary heat treatment temperature is high and the annealing temperature is low, an excessive amount of coarse martensite-austenite composite structure is formed compared to retained austenite, and since these do not exhibit the TRIP effect, it is judged that they did not help to secure elongation. In the case of Comparative Example 3, the rapid cooling maintenance time after secondary cooling was too short, and the yield strength, elongation, and yield ratio were low. Lower bainite transformation and fine precipitation in martensite occur during the maintenance period after secondary cooling, which increases the yield strength, but in this case, it is judged that the maintenance time was insufficient. Furthermore, it is determined that the austenite phase did not transform into lower bainite, and a portion formed a martensite-austenite composite structure. As a result, an excessive amount of martensite-austenite phase, which did not exhibit the TRIP effect, was formed and did not contribute to ensuring elongation.

[0139] Figure 3 is a scanning electron microscope image showing the microstructure of the steel material after primary heat treatment in Example 1 of the experimental examples of the present invention, and Figure 4 is a scanning electron microscope image showing the microstructure of the steel material after primary heat treatment in Comparative Example 2 of the experimental examples of the present invention.

[0140] Referring to Figures 3 and 4, observation of the microstructure after primary heat treatment of Example 1 and Comparative Example 2, which had the same composition, revealed that in the case of Comparative Example 2, a large amount of coarse cementite with a grain size of 500 nm or larger was formed. As a result, despite securing a large amount of retained austenite, an elongation of 15% or more could not be achieved.

[0141] Specifically, according to Example 1, the microstructure of the steel sheet after the primary heat treatment is 100 μm 2Within the area, carbides with a grain size of 500 nm or larger are distributed at a rate of two or fewer, and the pearlite ratio is 5% or less. According to Comparative Example 2, when the temperature of the heat treatment process exceeds 680°C, coarse carbides such as spherical cementite with a diameter of 500 nm or larger are formed. This deepens the heterogeneity of carbon atoms, and after cold rolling, it is possible to create retained austenite in which carbon is excessively dissolved in the final microstructure. In the case of austenite with a carbon content of approximately 1.1% or more, the TRIP effect decreases as the ratio increases, and the strength × elongation properties decrease. Therefore, after the primary heat treatment, it was necessary to adjust the microstructure appropriately to adjust the stability of the retained austenite.

[0142] Figure 5 is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Example 1 of the experimental examples of the present invention; Figure 6 is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 1 of the experimental examples of the present invention; Figure 7 is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 2 of the experimental examples of the present invention; Figure 8 is a scanning electron microscope image showing the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 3 of the experimental examples of the present invention. Figure 9 is a diagram showing the shape and distribution of retained austenite via EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Example 1 of the experimental examples of the present invention; Figure 10 is a diagram showing the shape and distribution of retained austenite via EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 1 of the experimental examples of the present invention; and Figure 11 is a diagram showing the shape and distribution of retained austenite via EBSD in the final microstructure of an ultra-high-strength cold-rolled steel sheet according to Comparative Example 2 of the experimental examples of the present invention.

[0143] Referring to Figures 5 and 9, the final microstructure of the ultra-high-strength cold-rolled steel sheet according to Example 1 can include a mixed microstructure of ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite. Specifically, the main microstructure consists of tempered martensite and upper / lower bainite, and retained austenite is distributed between the martensite and bainite laths and at the grain boundaries. In the drawings, the phases labeled LB and T.MS represent lower bainite and tempered martensite, respectively. Furthermore, it can be confirmed that the fraction of ferrite is in the range of 0% to 5%, the fraction of martensite is in the range of greater than 0% to 20%, the fraction of retained austenite is in the range of 10% to 30%, the fraction of upper bainite is in the range of greater than 0% to 30%, the fraction of lower bainite is in the range of greater than 0% to 30%, and the fraction of tempered martensite may be included as the remaining fraction. Furthermore, it can be confirmed that the average diameter of the retained austenite is 1.0 μm or less.

[0144] In contrast, referring to Figures 6 and 10, the final microstructure of the ultra-high-strength cold-rolled steel sheet according to Comparative Example 1 differs from Figure 5 in that it mainly consists of tempered martensite and the fraction of retained austenite is less than 10%. In Figure 6, an example of the tempered martensite region is denoted as T.MS. Also, referring to Figures 7 and 11, the final microstructure of the ultra-high-strength cold-rolled steel sheet according to Comparative Example 2 can include a mixed structure containing ferrite, tempered martensite, martensite, martensite-austenite composite structure, retained austenite, upper bainite, and lower bainite. The martensite-austenite composite structure is denoted as MA in Figure 7. Also, referring to Figure 8, the final microstructure of the ultra-high-strength cold-rolled steel sheet according to Comparative Example 3 mainly consists of tempered martensite and martensite-austenite composite structure.

[0145] It will be obvious to a person with ordinary skill in the art to which the technical concept of the present invention pertains, that the technical concept of the present invention described above is not limited to the embodiments and accompanying drawings described above, and that various substitutions, modifications, and changes are possible without departing from the technical concept of the present invention.

Claims

1. This is an ultra-high-strength cold-rolled steel sheet composed of, by weight percent, carbon (C): 0.28% to 0.45%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.03%, nitrogen (N): greater than 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities. When observing an area of ​​100 μm² or more in the width direction of the steel sheet in the region between the surface and the center of the cold-rolled steel sheet, the ratio (B / A) of the area of ​​crystal grains (B) in which the carbon content in the austenite is 0.5% or less to the area of ​​austenite (A) is less than 0.

1. The material satisfies the following requirements: Yield strength (YP): 1180 MPa or higher, Tensile strength (TS): 1470 MPa or higher, Elongation (El): 15% or higher, Yield ratio (YR): 75% or higher, and Flexibility (R / t): 3.0 or lower. The ratio of the area of ​​martensite-austenite grains (C) to the area of ​​austenite (A) (C / A) is less than 0.

5. Ultra-high-strength cold-rolled steel sheet containing a mixed structure of ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite.

2. This is an ultra-high-strength cold-rolled steel sheet composed of, by weight percent, carbon (C): 0.28% to 0.45%, silicon (Si): 1.0% to 2.5%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05%, chromium (Cr): greater than 0% to 1.0%, molybdenum (Mo): greater than 0% to 0.5%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.1%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.03%, nitrogen (N): greater than 0% to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities. When observing an area of ​​100 μm² or more in the width direction of the steel sheet in the region between the surface and the center of the cold-rolled steel sheet, the ratio (B / A) of the area of ​​crystal grains (B) in which the carbon content in the austenite is 0.5% or less to the area of ​​austenite (A) is less than 0.

1. The material satisfies the following requirements: Yield strength (YP): 1180 MPa or higher, Tensile strength (TS): 1470 MPa or higher, Elongation (El): 15% or higher, Yield ratio (YR): 75% or higher, and Flexibility (R / t): 3.0 or lower. The ratio of the area of ​​martensite-austenite grains (C) to the area of ​​austenite (A) (C / A) is less than 0.

5. It contains a mixed structure consisting of tempered martensite, martensite, retained austenite, upper bainite, and lower bainite. The fraction of martensite is in the range of more than 0% to 20%. The fraction of retained austenite is in the range of 10% to 30%. The fraction of the upper bainite is in the range of more than 0% to 30%. The fraction of the lower bainite is in the range of more than 0% to 30%. The aforementioned tempered martensite fraction is included as the remaining fraction in the ultra-high-strength cold-rolled steel sheet.

3. When observing retained austenite crystal grains in the width direction of the steel sheet by backscattered electron diffraction (EBSD) analysis in the region between the surface and the center of the cold-rolled steel sheet, and calculating the distribution of the average value of the crystal orientation difference within the retained austenite crystal grain by relating the average value of the crystal orientation difference obtained by averaging the difference in crystal orientation between any one region within the retained austenite crystal grain and a comparison region adjacent to the one region, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) shown in the distribution of the region where the average value of the crystal orientation difference is 0° or more and 3° or less satisfy the relationship (Kmax - Kavg) / (Kmax - Kmin) > 0.4, as described in claim 1 or 2, the ultra-high-strength cold-rolled steel sheet according to claim 1 or 2.

4. When observing retained austenite crystal grains in the width direction of the steel sheet by backscattered electron diffraction (EBSD) analysis in the region between the surface and the center of the cold-rolled steel sheet, the comparison region adjacent to the one region includes a first comparison region located in contact with the one region, a second comparison region located further away from the one region than the first comparison region and in contact with the first comparison region, and a third comparison region located further away from the one region than the second comparison region and in contact with the second comparison region. The ultra-high-strength cold-rolled steel sheet according to claim 3, characterized in that the average value of the crystal orientation difference obtained by averaging the crystal orientation differences between any one region within the retained austenite crystal grain and a comparison region adjacent to that region is the average value of the crystal orientation difference obtained by averaging the crystal orientation differences between any one region and a third comparison region.

5. The ferrite fraction is in the range of more than 0% to 5%. The fraction of martensite is in the range of more than 0% to 20%. The fraction of retained austenite is in the range of 10% to 30%. The fraction of the upper bainite is in the range of more than 0% to 30%. The fraction of the lower bainite is in the range of more than 0% to 30%. The ultra-high-strength cold-rolled steel sheet according to claim 1, wherein the aforementioned tempered martensite fraction is included as the remaining fraction.

6. The ultra-high-strength cold-rolled steel sheet according to claim 5, wherein the minimum value of the sum of the fraction of upper bainite and the fraction of lower bainite is 10%.

7. The ultra-high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the average diameter of the retained austenite is 1.0 μm or less.

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