Ultra-high strength cold-rolled steel sheet and method for producing 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 idea of the present invention relates to cold-rolled steel sheets, and more particularly, to an ultra-high strength cold-rolled steel sheet having a fine structure controlled to have high strength and elongation, and a method for manufacturing the same.

Background Art

[0002] For the purpose of improving the collision safety of automobiles and reducing the weight of vehicle bodies, the materials for the structural parts of automobiles are required to have the characteristics of high strength and high formability. As methods for satisfying high strength and formability, there are dual-phase steel composed of ferrite and martensite structures, and transformation-induced plasticity steel (TRIP, Transformation induced plasticity steel) that uses the effect of phase transformation during the deformation of retained austenite. Transformation-induced plasticity steel with a matrix structure composed of ferrite and bainite is disadvantageous in ensuring strength by the mixture law, so high-strength transformation-induced plasticity steel with a matrix structure of martensite has attracted 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 partitioning (QP) heat treatment.

[0003] Steel materials with a strength of 1.2 GPa or more, particularly 1.5 GPa or more, and high formability require not only high tensile strength but also high yield strength, and at the same time, an appropriate fraction of retained austenite structure for ensuring elongation and the stability of retained austenite. Conventional technologies have limitations in that it is insufficient to simultaneously ensure a tensile strength of 1470 MPa or more and an elongation of 15% or more.

[0004] In addition, when the microstructure is composed only of martensite and retained austenite, the tissue fraction is determined to be overly sensitive to the quenching end point temperature. In particular, even with a fine component deviation such as casting segregation that is difficult to avoid, there is a difference in the fraction of martensite due to 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 has been used as the main microstructure, and elongation has been ensured through retained austenite or ferrite tissue. The characteristics of quenching and reheat heat treatment are such that the tissue fractions of tempered martensite, martensite, and retained austenite change according to the quenching end point temperature. In order to ensure the target physical properties, an optimal quenching end point temperature range is determined according to the alloy components to control the fraction of the microstructure. However, if the quenching end point temperature is too low, the size of the retained austenite becomes fine, but its fraction becomes very small. If the quenching end point temperature is too high, the size of the austenite is large and the carbon enrichment is insufficient, so after the final cooling, it transforms into a martensite structure or is unstable, resulting in a small contribution to ensuring elongation.

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

[0006] As a related prior document, there is Korean Patent Application No. 10-2018-0047388.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The technical problem to be achieved by the technical idea of the present invention is to provide an ultra-high strength cold-rolled steel sheet having high strength and elongation by controlling the microstructure and a method for manufacturing the same.

[0008] However, such problems are exemplary and the technical idea of the present invention is not limited thereto.

Means for Solving the Problems

[0009] According to one aspect of the present invention, there is provided 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 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 an ultra-high strength cold-rolled steel sheet containing iron (Fe) and other inevitable impurities. In the region between the surface portion and the central portion of the cold-rolled steel sheet, when observing an area of 100 μm 2 in the width direction of the steel sheet, the ratio (B / A) of the area (B) of crystal grains having a carbon content of 0.5% or less in austenite to the area (A) of austenite is less than 0.1, and the yield strength (YP): 1180 MPa or more, the tensile strength (TS): 1470 MPa or more, the elongation (El): 15% or more, the yield ratio (YR): 75% or more, and the bendability (R / t): 3.0 or less are satisfied.

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

[0012] According to an embodiment of the present invention, when observing retained austenite crystal grains by electron backscatter diffraction (EBSD) analysis in the width direction of a cold-rolled steel sheet in a region between the surface portion and the central portion of the cold-rolled steel sheet, in the process of associating an average value of crystal orientation differences obtained by averaging the crystal orientation differences between an arbitrary region in the retained austenite crystal grain and a comparison region adjacent to the one region with the one region, when calculating the distribution of the average value of the crystal orientation differences in the retained austenite crystal grain, 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 can satisfy the relationship of (Kmax - Kavg) / (Kmax - Kmin)>0.4.

[0013] According to an embodiment of the present invention, when observing retained austenite crystal grains by electron backscatter diffraction (EBSD) analysis in the width direction of a cold-rolled steel sheet in a region between the surface portion and the central portion of the cold-rolled steel sheet, the comparison region adjacent to the one region includes a first comparison region positioned in contact with the one region, a second comparison region further separated from the first comparison region with respect to the one region and positioned in contact with the first comparison region, and a third comparison region further separated from the second comparison region with respect to the one region and positioned in contact with the second comparison region. The average value of the crystal orientation differences obtained by averaging the crystal orientation differences between an arbitrary region in the retained austenite crystal grain and the comparison region adjacent to the one region may be the average value of the crystal orientation differences obtained by averaging the crystal orientation differences between the arbitrary region and the third comparison region.

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

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

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

Advantages 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 performs heat treatment between hot rolling coiling and cold rolling to appropriately adjust the carbon distribution. 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 bend 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] In particular, by inducing a plurality of stages of phase transformation such as martensite transformation (primary), lower bainite transformation (secondary), and upper bainite transformation (tertiary) during 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 inevitably present 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 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

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Figure 11

Best Mode for Carrying Out the Invention

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more fully explain the technical idea of the present invention to those having ordinary knowledge in the relevant technical field. The following embodiments can be modified into various other forms, and the scope of the technical idea of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to further enrich and complete the present disclosure and to fully convey the technical idea of the present invention to those skilled in the art. Throughout this specification, the same reference numerals mean the same elements. Furthermore, various elements and regions in the drawings are schematically shown. Therefore, the technical idea of the present invention is not limited by the relative sizes and intervals shown in the accompanying drawings.

[0031] The technical idea of the present invention provides 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, and an elongation of 15% or more, a yield ratio (yield strength / tensile strength) of 75% or more, and a 90° bending workability of 3.0R / t or less, and a method for manufacturing the same.

[0032] Hereinafter, the ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention will be described in detail.

[0033] The ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention 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 contains iron (Fe) and other inevitable impurities.

[0034] Hereinafter, the roles and contents of each component contained in the ultra-high strength cold-rolled steel sheet according to the present invention will be described as follows. At this time, the content of the component element all means weight% with respect to the entire steel sheet.

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

[0036] Carbon is added to ensure the strength of the steel, and in particular, to increase the strength of the martensite structure. In addition, since it can be classified to stabilize austenite and ensure elongation through the transformation-induced plasticity (TRIP) effect, a sufficient carbon content is required. When the content of the carbon is less than 0.28%, it may be difficult to obtain the target strength and elongation at the same time. When the content of the carbon exceeds 0.45%, the weldability may decrease and hydrogen embrittlement may occur. Therefore, it is preferable to add the carbon content at 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, which delays the formation of carbides in ferrite and martensite and has the effect of solid solution strengthening. In particular, it is essential to delay the formation of carbides in martensite and partition carbon into austenite. When the content of silicon is less than 1.0%, the effect of suppressing carbide formation is small, so it may be difficult to sufficiently ensure the stability of retained austenite. When the content of silicon exceeds 2.5%, oxides such as Mn2SiO4 are formed during the manufacturing process, which inhibits the plating property and may increase the carbon equivalent and reduce the 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 the effect of solid solution strengthening, increases hardenability, and delays the formation of ferrite and bainite during cooling. When the content of manganese is less than 1.5%, the effect of adding manganese is not sufficient, so it may be difficult to ensure hardenability. When the content of manganese exceeds 3.0%, the transformation of bainite may be excessively delayed, the formation of inclusions such as MnS and segregation may cause a decrease in workability, and the carbon equivalent may be increased to 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 is used as a deoxidizer and can help suppress the formation of carbides similar to silicon. When the content of aluminum is less than 0.01%, the deoxidation effect may be insufficient. When the content of aluminum exceeds 0.05%, AlN is formed during the production of slabs, which may induce cracks 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): More than 0% - 1.0%

[0044] Chromium has the effect of solid solution strengthening, increases hardenability and contributes to the improvement of strength. Acting together with C and Mn, it refines the martensite and bainite structures and contributes to the stabilization of retained austenite. When the content of the chromium exceeds 1.0%, the transformation of bainite may be excessively delayed, and the manufacturing cost of the steel may increase. Therefore, it is preferable to add chromium in an amount exceeding 0% to 1.0% of the total weight of the steel plate.

[0045] Molybdenum (Mo): More than 0% - 0.5%

[0046] Molybdenum has the effect of solid solution strengthening, increases hardenability and contributes to the improvement of strength. Acting together with C and Mn, it refines the martensite and bainite structures and contributes to the stabilization of retained austenite. When the content of the molybdenum exceeds 0.5%, the transformation of bainite may be excessively delayed, and the manufacturing cost of the steel may increase. Therefore, it is preferable to add molybdenum in an amount exceeding 0% to 0.5% of the total weight of the steel plate.

[0047] Sum of Niobium (Nb), Titanium (Ti) and Vanadium (V): More than 0% - 0.1%

[0048] In the present invention, at least one or more of niobium, titanium, and vanadium can be contained. Niobium, titanium, and vanadium are precipitate-forming elements, and can increase strength by the precipitate strengthening effect, and can also obtain the effect of refining crystal grains. When the sum of the niobium, titanium, and vanadium is added in excess of 0.1% respectively, the manufacturing cost of the steel may increase significantly, the rolling load may increase greatly due to a large number of precipitates during rolling, and the elongation may decrease. Therefore, it is preferable to add the sum of niobium, titanium, and vanadium in an amount exceeding 0% to 0.1% of the total weight of the steel plate respectively. Also, each of niobium, titanium, and vanadium is preferably added in an amount of 0.1% or less of the total weight of the steel plate. For example, each of niobium, titanium, and vanadium can be added in an amount exceeding 0% to 0.05%.

[0049] Phosphorus (P): More than 0% - 0.03%

[0050] Phosphorus is an impurity contained in the steel manufacturing process. It can contribute to the improvement of strength through solid solution strengthening, but when contained in a large amount, it 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 contained in the steel manufacturing process. It can form non-metallic inclusions such as FeS and MnS, which may 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): More than 0% - 0.01%

[0054] Nitrogen is an element inevitably contained during steel manufacturing. It can contribute to the stabilization of austenite, but it can react with Al to form AlN, which may 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 the modified embodiment of the present invention may further contain at least one or more of the elements having the following composition ranges in addition to the alloy elements described above.

[0056] Nickel (Ni): More than 0% - 0.5%

[0057] Nickel can also help stabilize austenite and increase the hardenability of steel. When the nickel content exceeds 0.5%, it is not preferable because it increases the manufacturing cost of steel. Therefore, nickel is preferably added at more than 0% to 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. When the copper content exceeds 0.5%, it is not preferable because it increases the manufacturing cost of steel. Therefore, it is preferable to add copper in an amount of more than 0% to 0.5% of the total weight of the steel sheet.

[0060] Also, the total of nickel and copper is preferably added in an amount of more than 0% to 1.0%.

[0061] Boron (B): More than 0% - 0.005%

[0062] Boron can improve hardenability like Mn, Cr, Mo, etc. When the boron content exceeds 0.005%, it may concentrate on the surface and cause deterioration of 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 components of the ultra-high strength cold-rolled steel sheet are iron (Fe). However, in the normal steelmaking process, unintended impurities may inevitably mix in from raw materials and the surrounding environment, and thus it is impossible to eliminate them. Since these impurities are known to any engineer in the normal manufacturing process, not all of their contents are specifically mentioned in this specification.

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

[0065] Retained austenite is finely distributed at the laths and grain boundaries of the tempered martensite and the bainite, whereby the retained austenite is stabilized and strength and elongation can be stably ensured.

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

[0067] The ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention has a 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 of 100 μm 2 When observing an area of 0.5% or more, the ratio (B / A) of the area (B) of crystal grains in which the carbon content in austenite is 0.5% or less to the area (A) of austenite is less than 0.1. The ratio (B / A) can be understood as a measure of the stability of the composition of retained austenite (RA) generated in the steel sheet. When the ratio (B / A) is 0.1 or more, the stability of the austenite composition is not sufficient, and thus the effect of improving elongation by retained austenite cannot be obtained. For measuring the carbon content in individual crystal grains, the distance between lattice planes was measured through transmission electron microscope (TEM) observation, and C γ =(α γ -3.592) / 0.033, and the carbon content was derived through this relational expression. α γ is the austenite lattice constant measured by a transmission electron microscope.

[0068] The ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention has a 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 of 100 μm 2 When observing an area of 0.5% or more, the ratio (C / A) of the area (C) of martensite-austenite crystal grains to the area (A) of austenite is less than 0.5. The ratio (C / A) can be understood as a measure of the stability of retained austenite (RA) by position in the steel sheet. When the ratio (C / A) is 0.5 or more, the martensite-austenite crystal grains that do not participate in the deformation-induced martensite transformation become excessive, and thus 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 to the one region, when calculating the distribution of the average value of the crystal orientation difference in the retained austenite crystal grains, the maximum value (Kmax), the minimum value (Kmin), and the 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 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 averaged with 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) that are in contact with the one region (A0), the second comparison regions (A7 to A 18 ) that are further separated from the first comparison regions (A1 to A6) with respect to the one region (A0) and are in contact with the first comparison regions (A1 to A6), and the third comparison regions (A 18 ) that are further separated from the second comparison regions (A7 to A 18 ) with respect to the one region (A0) and are in contact with the second comparison regions (A7 to A 19 ~A 36) can be included. In this case, based on any one region (A0) within the retained austenite crystal grains, the average value (K) of the crystal orientation difference obtained by averaging the crystal orientation differences between the comparison region adjacent to the one region (A0) is the average value (K) of the crystal orientation difference obtained by averaging the crystal orientation differences between the third comparison region (A 19 ~A 36 ) and the one region (A0).

[0072] For example, the average value (K) of the crystal orientation difference obtained by averaging the crystal orientation differences between the third comparison region (A 19 ~A 36 ) and the one region (A0) can be represented by the following mathematical formula 1. Here, (MA) i represents the crystal orientation difference between the one region (A0) and any one region of the third comparison region (A 19 ~A 36 ), n is 19, and m can have a value of 36.

[0073]

Equation

[0074] Referring to FIG. 2, the distribution of the average value (K) of the crystal orientation difference is shown from 0° to 5°. Among them, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) in the distribution of the region where the average value of the crystal orientation difference is 0° or more and 3° or less can be calculated.

[0075] In the method for manufacturing an ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention, the maximum value (Kmax), minimum value (Kmin), and average value (Kavg) 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 of (Kmax - Kavg) / (Kmax - Kmin)>0.4.

[0076] In the transformation-induced martensite phase transformation reaction of austenite having an FCC structure, defects inside the crystal grains such as dislocations and stacking faults act as nucleation sites for martensite. Therefore, when the average value of the crystal orientation difference, which is a value indicating the defect distribution within the crystal grains, is too small, TRIP (Transformation Induced Plasticity) nucleation is insufficient, and thus the effect of increasing ductility × tensile strength due to the transformation-induced martensite phase transformation cannot be obtained. On the contrary, when the average value of the crystal orientation difference is too high, the transformation-induced martensite phase transformation occurs concentratedly at the initial stage of tensile deformation, and thus, similarly, the desired effect of increasing ductility cannot be obtained.

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

[0078] Hereinafter, with reference to the accompanying drawings, a method for manufacturing an ultra-high strength cold-rolled steel sheet according to the present invention will be described.

[0079] Method for manufacturing ultra-high strength cold-rolled steel sheet

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

[0081] The method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention includes a step of manufacturing a hot-rolled steel sheet using the steel material of the above composition, a step of cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, a step of annealing and heat-treating the cold-rolled steel sheet, a step of multi-stage cooling the cold-rolled steel sheet, and a step of partitioning and heat-treating the cold-rolled steel sheet.

[0082] In particular, the cold-rolled steel sheet is heated to a temperature above the Ac3 temperature for annealing heat treatment and maintained for a certain period of time, and multi-stage cooling in two stages of slow cooling and rapid cooling is performed until the rapid cooling end temperature. Then, after maintaining at the rapid cooling end temperature for a certain period of time, it is heated to a temperature above the Ms temperature for partitioning heat treatment, and after being maintained constantly during the partitioning heat treatment time, it is finally cooled to a temperature below the Mf temperature.

[0083] Manufacturing steps of hot-rolled steel sheet

[0084] A steel slab having the above 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, re-solution of the components segregated during casting and re-solution of the precipitates occur and homogenization is achieved, enabling hot rolling. If the reheating temperature is less than 1,150°C, re-solution 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 crystal grains may increase, and the process cost due to temperature rise may increase. The reheating time may be, for example, 1 hour to 4 hours. If the reheating time is less than 1 hour, homogenization of segregation may be insufficient. If the reheating time exceeds 4 hours, the size of the austenite crystal grains may increase, and the process cost due to temperature rise may increase.

[0085] After the reheating, hot rolling is performed by a normal method, and for example, hot finish rolling is performed 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 rolling end temperature is less than 850°C, ferrite or pearlite may be generated. If the finish rolling end temperature exceeds 970°C, scale formation increases, the crystal grain size becomes coarser, and it may be difficult to achieve fine homogenization of the structure.

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

[0087] Next, the hot-rolled steel sheet is coiled at a coiling temperature (CT) in the range of, for example, 400°C to 700°C. The range of the coiling temperature can be selected from the viewpoints of cold rolling property and surface quality. When the coiling temperature is less than 400°C, hard phases such as martensite are excessively generated, the material of the hot-rolled steel sheet excessively increases, and the rolling load can significantly increase during cold rolling. When the coiling temperature exceeds 700°C, it can cause non-uniformity of the microstructure of the final product.

[0088] On the other hand, in the method for manufacturing an ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention, after the hot rolling and coiling, 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 select a batch annealing or a continuous heat treatment process, etc. In the microstructure of the steel sheet after the primary heat treatment, within an area of 100 μm 2 there are 2 or less carbides with a crystal grain size of 500 nm or more distributed, and it has a pearlite ratio of 5% or less. If the primary heat treatment process is not performed or the temperature is lower than 500°C, there is a drawback that the cold rolling load increases and the difficulty of the process becomes high. When the temperature of the heat treatment process exceeds 680°C or the time exceeds 12 hours, coarse carbides such as spherical cementite having a diameter of 500 nm or more are formed. This can deepen the inhomogeneity of carbon atoms and create retained austenite with excessive carbon solid solution in the final microstructure after cold rolling. In the case of austenite having a carbon content of about 1.1% or more, as the ratio increases, the TRIP effect decreases and the strength×elongation characteristics are expected to deteriorate. Therefore, after performing the primary heat treatment, an attempt was made to appropriately adjust the microstructure to adjust the stability of the retained austenite.

[0089] Manufacturing steps of cold-rolled steel sheet

[0090] The hot-rolled steel sheet is subjected to pickling treatment with an acid to remove the surface scale layer. Next, the hot-rolled steel sheet is cold-rolled, for example, at an average reduction rate of 40% to 70% to form a cold-rolled steel sheet. The higher the average reduction rate, the more effective the formability due to the grain refinement effect. When the average reduction rate is less than 40%, it is difficult to obtain a uniform fine structure. When the average reduction rate exceeds 70%, the roll force increases and the process load increases. The cold rolling can result in a steel sheet having the thickness of the finally produced steel sheet. The structure of the cold-rolled steel sheet can have a structure in which the structure of the hot-rolled steel sheet is elongated.

[0091] Annealing heat treatment step

[0092] The cold-rolled steel sheet is subjected to annealing heat treatment in a continuous annealing furnace having a normal slow cooling section. The 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 thus can have a great influence on the strength of the cold-rolled steel sheet.

[0093] The annealing heat treatment is heated, for example, at a heating rate of 2°C / second or more, for example, in the range of 2°C / second to 10°C / second. When the heating rate is less than 2°C / second, it takes a long time to reach the target annealing heat treatment temperature, resulting in a decrease in production efficiency and a possible increase in the size of the crystal grains.

[0094] The annealing heat treatment may be performed, for example, at a temperature above Ac3, for example, in the temperature range of 830°C to 930°C, for example, in the temperature 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 reverse transformation to austenite. When the annealing heat treatment temperature is less than 830°C, a single-phase austenite cannot be formed to produce tempered martensite as the final structure. For reference, in order to form a single-phase austenite, the annealing heat treatment must be performed at a temperature above the A3 temperature. When the annealing heat treatment temperature exceeds 900°C, the austenite grain size becomes coarse, and the strength may decrease.

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

[0096] Multi-stage cooling step

[0097] The annealed cold-rolled steel sheet is cooled in multiple stages. The cooling step may be performed in the following two stages.

[0098] First, the annealed cold-rolled steel sheet is slowly cooled in the first cooling at a cooling rate in the range of, for example, 1°C / second to 15°C / second, for example, in the range of 3°C / second to 10°C / second, to a temperature range that suppresses ferrite transformation, for example, to a primary cooling end temperature in the range of 650°C to 800°C. When the primary cooling end temperature of the slow cooling is less than 650°C, ferrite transformation may occur in an undesirable amount, which may cause a decrease in strength. The fraction of ferrite generated by the ferrite transformation is preferably limited to less than 0% to 5%.

[0099] Next, the cold-rolled steel sheet that has been subjected to the first cooling (slow cooling) is rapidly cooled by secondary cooling (quenching) at a cooling rate of, for example, 20°C / second or more, for example, in the range of 20°C / second to 100°C / second, for example, to a temperature below the Ms temperature, for example, in the range of Ms - 140°C to Ms - 30°C, for example, to a final secondary cooling temperature in the range of 180°C to 300°C. The secondary cooling is a quenching step and may be carried out in order as a first secondary quenching step and a second secondary quenching step. The cooling rate of the first secondary quenching step may be, for example, 20°C / second or more, and can quench to a temperature of Ms - 30°C or lower. During the cooling, a part of the austenite transforms into martensite, and the amount is about 20% to 80%. The cooling rate of the second secondary quenching step may be, for example, 30°C / second or more, and cools to the quenching end temperature (Ms - 140°C to Ms - 30°C) to cause martensite transformation.

[0100] By the secondary cooling (quenching), a part of the austenite can be transformed into martensite. The fraction of the generated martensite may be 20% to 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 cooling may be carried out without distinguishing between the first secondary quenching step and the second secondary quenching step.

[0102] Next, the cold-rolled steel sheet that has been subjected to the secondary cooling (quenching) is maintained at the final secondary cooling temperature for a time in the range of, for example, 5 seconds to 90 seconds. In such a holding time after quenching, initially, the temperature homogenization of the steel can be carried out. Next, while maintaining isothermally at the final secondary cooling temperature, a part of the retained austenite can transform into lower bainite or the like.

[0103] Partitioning heat treatment step

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

[0105] When the temperature of the partitioning heat treatment is less than 360°C, the partitioning effect may be insufficient. When the temperature of the partitioning heat treatment exceeds 500°C, the size of the carbide may coarsen and a decrease in strength may occur.

[0106] The holding time of the partitioning heat treatment has a smaller influence compared to the partitioning temperature. When the holding time of the partitioning heat treatment is less than 30 seconds, it may be difficult to obtain a stable partitioning effect. When the holding time of the partitioning heat treatment exceeds 500 seconds, the heat treatment efficiency decreases and the size of the carbide increases, resulting in a decrease in strength.

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

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

[0109] Analysis of changes in fine structure

[0110] Hereinafter, in the process of performing the method for manufacturing an ultra-high strength cold-rolled steel sheet according to the technical idea of the present invention, the change in the microstructure of the ultra-high strength cold-rolled steel sheet will be described in detail.

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

[0112] In the primary cooling of the multi-stage cooling step, the fraction of ferrite generated by ferrite transformation is limited to less than 5%, and ferrite may not be generated. If ferrite with a fraction of 5% or more is generated, there is a risk that the strength will decrease and the target strength cannot be achieved.

[0113] In the secondary cooling of the multi-stage cooling step, when the cold-rolled steel sheet is cooled at a high cooling rate, ferrite transformation, pearlite transformation, and bainite transformation are suppressed, and a part of the austenite transforms into martensite. At this time, the fraction of martensite generated by martensite transformation can be limited to 20% - 80%. If the fraction of martensite generated during the secondary cooling exceeds 80%, it may be difficult to ensure an appropriate fraction of retained austenite. If it is less than 20%, since the fraction of retained austenite after cooling is too high, it is difficult to ensure the stability of the retained austenite, and even if the bainite transformation structure is increased, the fraction of martensite decreases, and a decrease in strength may occur. In addition, some martensite structures can increase the internal stress and increase the nucleation rate of bainite, and the bainite transformation can proceed rapidly even at a low temperature below Ms.

[0114] In the secondary cooling of the multi-stage cooling step, during the period of maintaining at the secondary cooling end temperature after rapid cooling, a part of the austenite transforms into bainite, which may mainly be lower bainite. Also, fine precipitates may be formed in the martensite generated in the previous step. At this time, the time for maintaining at the secondary cooling end temperature can be in the range of 5 seconds to 90 seconds. If the maintenance time is less than 5 seconds, there is a risk that the lower bainite transformation will not occur sufficiently. If the maintenance time exceeds 90 seconds, the process cost may increase due to an excessively long heat treatment time.

[0115] In the partitioning heat treatment step, carbon diffuses and concentrates inside the retained austenite, and the retained austenite can be stabilized. Also, bainite transformation can proceed in a part of the retained austenite. The bainite transformation can refine the shape of the retained austenite after rapid cooling, and thereby can contribute to the stabilization of the retained austenite. Due to such an effect, the sum of the fraction of the upper bainite and the fraction of the lower bainite can be 10% or more.

[0116] After performing the partitioning heat treatment step, in the final cooling to room temperature, some unstable austenite can transform into martensite. At this time, when a large amount of martensite is generated, the fraction of the final retained austenite may decrease and have an adverse effect on formability. Therefore, it is preferable to control the generated martensite to less than 20%.

[0117] In order to suppress the martensite transformation in the final cooling, it is preferable to make the secondary cooling end temperature and the holding time at the secondary cooling end temperature such that heat treatment is performed without problems in the partitioning heat treatment step, refine the retained austenite, and allow stabilization to proceed.

[0118] Although bainite transformation may not be considered below the Ms point, there is research indicating that bainite transformation is possible even below the Ms point, and there is research indicating that the nucleation of bainite increases below the Ms point compared to just above the Ms point.

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

[0120] Experimental examples

[0121] Hereinafter, preferred experimental examples are presented to assist in the understanding of the present invention. However, the following experimental examples are merely for assisting in the understanding of the present invention, and the present invention is not limited by the following experimental examples.

[0122] Steel having the composition (unit: wt%) shown in Table 1 below was prepared, and cold-rolled steel sheets according to Examples and Comparative Examples were prepared through a predetermined hot rolling process, cold rolling process, and heat treatment process. The balance 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, 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). In contrast, steel grade E is outside the composition range of the present invention. Specifically, it is below the range of carbon (C): 0.28% to 0.45% and is not satisfactory. Table 2 shows the Ac3 temperature, Ms temperature, temperature of Ms - 140°C, and temperature of Ms - 30°C for the said 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 % of carbon. Ms (°C) = 539 - 423[C] - 30.4[Mn] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo]

[0127] The slabs of the above-described steel grades were reheated at 1200 °C and maintained for 3 hours, hot-rolled to a thickness of 2.4 mm at a finish rolling temperature of 950 °C, and then coiled at 600 °C. Next, the coiled hot-rolled steel sheet was pickled to remove the scale on the surface and cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.2 mm.

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

[0129] Table 3 shows the conditional values of the heat treatment processes for manufacturing the cold-rolled steel sheets of the comparative examples and the examples. In Table 3, 'primary heat treatment' means the heat treatment performed after hot-rolling coiling and 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 adopted Steel Grade E outside the composition range of the present invention. Comparative Example 2 exceeded the temperature range of 500 °C to 680 °C, which is the primary heat treatment temperature range, and was not satisfactory. It was below the annealing temperature range of 830 °C to 930 °C and was not satisfactory. Comparative Example 3 satisfied the annealing temperature range but exceeded the annealing holding time of 30 seconds to 120 seconds and was not satisfactory. After secondary cooling (quenching), it was below the range of 5 seconds to 90 seconds, which is the secondary cooling holding time at the secondary cooling end temperature (180 °C to 300 °C), and was not satisfactory.

[0132] Table 4 shows the item values indicating the microstructures of the cold-rolled steel sheets of the comparative examples and the examples.

[0133] In Table 4, the first item value (B / A) means the ratio (B / A) of the area (B) of crystal grains with a carbon content of 0.5% or less in austenite to the area (A) of austenite when observing an area of 100 μm or more in the width direction of the steel sheet in the region between the surface part and the center part of the cold-rolled steel sheet. The second item value (C / A) means the ratio (C / A) of the area (C) of martensite-austenite crystal grains to the area (A) of austenite when observing an area of 100 μm or more in the width direction of the steel sheet in the region between the surface part and the center part of the cold-rolled steel sheet. The third item value ((Kmax - Kavg) / (Kmax - Kmin)) means the relational expression of the maximum value (Kmax), the minimum value (Kmin), and the average value (Kavg) shown in the distribution of the average value of the crystal orientation difference when calculating the distribution of the average value of the crystal orientation difference in the retained austenite crystal grains through the process of corresponding the average value of the crystal orientation difference between an adjacent comparison region and a reference region in the retained austenite crystal grains with an arbitrary one region in the retained austenite crystal grains when observing the 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 part and the center part of the cold-rolled steel sheet, and the average value of the crystal orientation difference is in the range of 0° or more and 3° or less. 2 In Table 4, the first item value (B / A) means the ratio (B / A) of the area (B) of crystal grains with a carbon content of 0.5% or less in austenite to the area (A) of austenite when observing an area of 100 μm or more in the width direction of the steel sheet in the region between the surface part and the center part of the cold-rolled steel sheet. The second item value (C / A) means the ratio (C / A) of the area (C) of martensite-austenite crystal grains to the area (A) of austenite when observing an area of 100 μm or more in the width direction of the steel sheet in the region between the surface part and the center part of the cold-rolled steel sheet. 2 The third item value ((Kmax - Kavg) / (Kmax - Kmin)) means the relational expression of the maximum value (Kmax), the minimum value (Kmin), and the average value (Kavg) shown in the distribution of the average value of the crystal orientation difference when calculating the distribution of the average value of the crystal orientation difference in the retained austenite crystal grains through the process of corresponding the average value of the crystal orientation difference between an adjacent comparison region and a reference region in the retained austenite crystal grains with an arbitrary one region in the retained austenite crystal grains when observing the 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 part and the center part of the cold-rolled steel sheet, and the average value of the crystal orientation difference is in the range of 0° or more and 3° or less.

[0134] [Table 4]

[0135] Referring to Table 4, Examples 1 to 4 all satisfy the range where the first item value (B / A) < 0.1, the second item value (C / A) < 0.5, and the third item value ((Kmax - Kavg) / (Kmax - Kmin)) > 0.4. In contrast, for Comparative Example 1, the first item value (B / A) is greater than 0.1, and the third item value ((Kmax - Kavg) / (Kmax - Kmin)) is less than 0.4. For Comparative Example 2, it can be confirmed that the first item value (B / A) is greater than 0.1 and the second item value (C / A) is not less than 0.5. For Comparative Example 3, it can be confirmed that the second item value (C / A) is not less than 0.5. Table 5 shows the yield strength (YS), tensile strength (TS), elongation (EL), yield ratio (YR), and 90° bendability (R / t) as physical and mechanical properties of the manufactured hot-rolled steel sheets and steel pipes.

[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). Further, 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 more preferably 22,000 or more. In contrast, Comparative Example 1 falls below the range where elongation (EL) is 15% or more and is not satisfactory, and the product of tensile strength and elongation falls below the range of 20,000 or more and is not satisfactory. Comparative Example 2 falls below the range where elongation (EL) is 15% or more and is not satisfactory, falls below the range where yield ratio (YR) is 75% or more and is not satisfactory, exceeds the range where 90° bendability (R / t) is 3.0 or less and is not satisfactory, and the product of tensile strength and elongation falls below the range of 20,000 or more and is not satisfactory. Comparative Example 3 falls below the range where yield strength (YP) is 1180 MPa or more and is not satisfactory, falls below the range where tensile strength (TS) is 1470 MPa or more and is not satisfactory, falls below the range where elongation (EL) is 15% or more and is not satisfactory, falls below the range where yield ratio (YR) is 75% or more and is not satisfactory, exceeds the range where 90° bendability (R / t) is 3.0 or less and is not satisfactory, and the product of tensile strength and elongation falls below the range of 20,000 or more and is not satisfactory.

[0138] To explain the comparative examples that do not satisfy the target physical properties, in the case of Comparative Example 1, it is characterized by a low carbon content, and it was not possible to simultaneously ensure a tensile strength of 1470 MPa and an elongation of 15% or more. In the case of Comparative Example 2, it is characterized by a high primary heat treatment temperature and a low annealing temperature, and sufficient elongation could not be ensured. When the primary heat treatment temperature is high and the annealing temperature is low, a coarse martensite-austenite composite structure is excessively formed compared to retained austenite, and since these do not exhibit the TRIP effect, it is judged that they were not useful for ensuring elongation. In the case of Comparative Example 3, after the secondary cooling, the holding time of rapid cooling was too short, and the yield strength, elongation, and yield ratio were shown to be low. During the holding after the secondary cooling, lower bainite transformation and fine precipitation in martensite occur to increase the yield strength, but in this case, it is judged that the time was insufficient. Also, the austenite phase did not transform into lower bainite, and a part formed a martensite-austenite composite structure, and as a result, an excessive amount of a martensite-austenite phase that does not exhibit the TRIP effect was formed and was judged not to be useful for ensuring elongation.

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

[0140] Referring to FIGS. 3 and 4, as a result of observing the microstructures after the primary heat treatment of Example 1 and Comparative Example 2 having the same composition, in the case of Comparative Example 2, a large amount of coarse cementite with a crystal grain size of 500 nm or more was formed. As a result, although a large amount of retained austenite was ensured, an elongation of 15% or more could not be achieved.

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

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

[0143] Referring to FIGS. 5 and 9, the final microstructure of the ultra-high strength cold-rolled steel sheet according to Example 1 can include a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed. Specifically, the main microstructures are composed of tempered martensite and upper / lower bainite, and it can be confirmed that retained austenite is distributed between martensite and bainite laths and at grain boundaries. In the drawings, the phases denoted as LB and T.MS indicate lower bainite and tempered martensite, respectively. Further, the fraction of the ferrite is in the range of 0% to 5%, the fraction of the martensite is in the range of more than 0% to 20%, the fraction of the 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%, and it can be confirmed that the fraction of the tempered martensite can be included as the remaining fraction. Also, it can be confirmed that the average diameter of the retained austenite is 1.0 μm or less.

[0144] In contrast, referring to FIGS. 6 and 10, the final microstructure of the ultra-high strength cold-rolled steel sheet according to Comparative Example 1 mainly consists of tempered martensite, which is different from FIG. 5 in that the fraction of retained austenite is less than 10%. An example of the tempered martensite region in FIG. 6 is denoted as T.MS. Also, referring to FIGS. 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 in which ferrite, tempered martensite, martensite, martensite-austenite composite structure, retained austenite, upper bainite, and lower bainite are mixed. The martensite-austenite composite structure is denoted as MA in FIG. 7. Also, referring to FIG. 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 should be apparent to those of ordinary skill in the art to which the technical idea of the present invention pertains that the technical idea of the present invention described above is not limited to the foregoing embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible without departing from the technical idea of the present invention.

Claims

1. An ultra-high strength cold-rolled steel sheet containing, 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 being iron (Fe) and other inevitable impurities, In the region between the surface part and the central part of the cold-rolled steel sheet, when observing an area of 100 μm or more in the width direction of the steel sheet, the ratio (B / A) of the area (B) of crystal grains in austenite having a carbon content of 0.5% or less in austenite to the area (A) of austenite is less than 0.1, ２ and The ultra-high strength cold-rolled steel sheet satisfying yield strength (YP): 1180 MPa or more, tensile strength (TS): 1470 MPa or more, elongation (El): 15% or more, yield ratio (YR): 75% or more, and bendability (R / t): 3.0 or less.

2. The ultra-high strength cold-rolled steel sheet according to Claim 1, wherein the ratio (C / A) of the area (C) of martensite-austenite crystal grains to the area (A) of austenite is less than 0.

5.

3. When observing retained austenite crystal grains by electron backscatter diffraction (EBSD) analysis in the width direction of the steel sheet in the region between the surface portion and the center portion of the cold-rolled steel sheet, through the process of corresponding the average value of the crystal orientation difference with a comparison region adjacent to the one region to the one region, when calculating the distribution of the average value of the crystal orientation difference 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 difference is 0° or more and 3° or less satisfy the relationship of (Kmax - Kavg) / (Kmax - Kmin) > 0.

4. The ultra-high strength cold-rolled steel sheet according to Claim 1.

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

5. The ultra-high strength cold-rolled steel sheet is The ultra-high strength cold-rolled steel sheet according to claim 1, comprising a mixed structure in which ferrite, tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed.

6. The fraction of the ferrite is in the range of more than 0% to 5%, The fraction of the martensite is in the range of more than 0% to 20%, The fraction of the 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 fraction of the tempered martensite is included as the remaining fraction. The ultra-high strength cold-rolled steel sheet according to claim 5.

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

8. The ultra-high strength cold-rolled steel sheet is Comprising a mixed structure in which tempered martensite, martensite, retained austenite, upper bainite, and lower bainite are mixed, The fraction of the martensite is in the range of more than 0% to 20%, The fraction of the 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 fraction of the tempered martensite is included as the remaining fraction. The ultra-high strength cold-rolled steel sheet according to claim 1.

9. The average diameter of the retained austenite is 1.0 μm or less. The ultra-high strength cold-rolled steel sheet according to claim 5 or 8.

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