cold-rolled steel sheet

Abstract

This invention relates to a cold-rolled steel sheet with high bake hardening and excellent bendability after bake hardening. It has a specified composition, containing 20%–70% ferrite and more than 30% tempered martensite by area ratio, with the total ferrite and tempered martensite exceeding 90%. A 30μm × 30μm microstructure image obtained by photographing the microstructure at 2000x magnification is placed in an xy coordinate system with the thickness direction as the x-axis and the rolling direction as the y-axis. The microstructure image is then divided into 1024 segments along the x-axis and 1024 segments along the y-axis to form 1024 × 1024 segmented regions. The values ​​in each segmented region are set to "1" if the microstructure is ferrite and "0" otherwise, and then converted to 2-grayscale to create a two-dimensional image. The heterogeneity α obtained by performing a two-dimensional discrete Fourier transform on the two-dimensional image is 1.20 or less.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201980061030.4, which entered the Chinese national phase of PCT international application PCT / JP2019 / 039241 filed on October 4, 2019. Technical Field

[0002] This invention relates to cold-rolled steel sheets, and more specifically, to cold-rolled steel sheets with excellent paint bake hardening properties and impact resistance, suitable for structural components of automobiles and the like, which are primarily used through pressure processing. This application claims priority based on Japanese Patent Application No. 2018-189164, filed on October 4, 2018, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, the application of high-strength steel sheets has been expanding from the perspective of lightweighting, which helps improve the fuel efficiency of automobiles. However, since most automotive parts are manufactured through pressure forming, excellent formability is required in addition to high strength. Furthermore, from the perspective of ensuring passenger safety, improved crash resistance is also desired, requiring raw materials with high strength and excellent bending deformation capacity relative to the bending stress generated during a collision. Therefore, raw materials that are relatively soft and easy to form during forming, have a large bake hardening amount during painting and baking after forming, and excellent bending properties after bake hardening are required.

[0004] The aforementioned bake hardening is a phenomenon in which interstitial elements (mainly carbon) move into dislocations (line defects that are the basic process of plastic deformation) introduced by pressure forming (hereinafter also referred to as "pre-strain") and become bonded, thus hindering their movement and thereby increasing strength. It is also known as strain aging. The amount of bake hardening in ferritic single-phase structures such as mild steel sheets can be controlled by the amount of dissolved carbon.

[0005] On the other hand, in high-strength steel plates, to ensure workability, they are mostly composite structures containing hard (martensite) and soft (ferrite) structures. The hard structure (martensite) containing a large amount of dissolved carbon is responsible for high bake hardening properties. However, while the hard structure containing a large amount of dissolved carbon can achieve high strength, it is difficult to balance bake hardening and bendability after bake hardening. That is, compared with ferrite, martensite has a higher amount of dissolved carbon and a higher dislocation density, thus exhibiting excellent bake hardening properties but poor bendability.

[0006] For example, Patent Document 1 discloses a cold-rolled steel sheet that ensures high bake hardening properties by primarily comprising a microstructure of bainite and martensite and limiting the ferrite area ratio to below 5%. However, because this steel sheet contains a large amount of hard bainite and martensite, bake hardening occurs in both the hard and soft phases of the composite microstructure when the pre-strain is above 2%. Therefore, the microstructure after bake hardening treatment becomes uneven in strength and does not exhibit excellent bendability after bake hardening.

[0007] Patent Document 2 discloses a steel sheet that improves workability and bake hardening properties by including tempered martensite or tempered bainite. However, in the case of Patent Document 2, no sufficient research has been conducted on improving the bending properties after bake hardening.

[0008] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2008-144233 Patent Document 2: Japanese Patent Application Publication No. 2003-277884 Summary of the Invention

[0009] The problem that the invention aims to solve Therefore, the object of the present invention is to provide a cold-rolled steel sheet with high bake hardening and excellent bending properties after bake hardening.

[0010] Methods for solving problems To achieve the above-mentioned objectives, the inventors investigated the bake-hardening amount and the bending properties after bake-hardening. As a result, the inventors discovered that in the microstructure of cold-rolled steel sheets containing ferrite and tempered martensite, when a cross-linked structure is adopted in which ferrite is finely and homogeneously divided by tempered martensite in both the rolling direction and the thickness direction, the cold-rolled steel sheet exhibits high bake-hardening amount and excellent bending properties after bake-hardening. Furthermore, the inventors also discovered that such a cross-linked structure can be quantified using the frequency spectrum obtained by performing a two-dimensional Fourier transform on a microstructure image of the cold-rolled steel sheet, thus completing the present invention.

[0011] The cold-rolled steel sheet that can achieve the above objectives is described below.

[0012] (1) A cold-rolled steel sheet, wherein, by mass%, it contains C: 0.05~0.30% Si: 0.200~2.000% Mn: 2.00~4.00% P: Below 0.100% S: Below 0.010% Al: 0.001~2.000% N: below 0.010% Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100% Cu: 0~1.000%, Ni: 0~1.000%, Mo: 0~1.000%, Cr: 0~1.000%, W: 0~0.005%, Ca: 0–0.005% Mg: 0–0.005% REM: 0~0.010%, B: 0~0.0030%, The remaining portion contains Fe and impurities; It contains 20%–70% ferrite and more than 30% tempered martensite by area ratio. The total content of ferrite and tempered martensite is over 90%; In the section of the cold-rolled steel sheet perpendicular to the width direction of the sheet, at a position from 1 / 8 to 7 / 8 of the sheet width, a 30μm×30μm microstructure image obtained by taking a picture at a magnification of 2000 times is placed in an xy coordinate system with the sheet thickness direction as the x-axis and the rolling direction as the y-axis. Then, the microstructure image is divided into 1024 segments in the x-axis direction and 1024 segments in the y-axis direction to form 1024×1024 segmented regions. For the value in each of the above segmented regions, it is set to "1" when the structure is ferrite, and set to "0" in other cases, and then 2-grayscale conversion is performed to produce a two-dimensional image. For the above two-dimensional image, the heterogeneity α defined by equation (1) is 1.20 or less. [Mathematical Expression 1] In equation (1), Su is defined by equation (2), and Sv is defined by equation (3). [Mathematical Expression 2] In equations (2) and (3), F(u, v) is defined by equation (4). [Mathematical Expression 3] In equation (4), f(x, y) represents the gray level of the coordinates (x, y) of the two-dimensional image above.

[0013] (2) The cold-rolled steel sheet according to (1) further contains, by mass%, one or more of the following: Ti: 0.003-0.100%, Nb: 0.003-0.100%, V: 0.003-0.100% or less in total.

[0014] (3) The cold-rolled steel sheet according to any one of (1) or (2), wherein the microstructure image is a 30μm×30μm microstructure image obtained by taking a photograph at a magnification of 2000 times on the microstructure at a position from 1 / 4 to 3 / 8 of the thickness of the sheet to the surface in a section of the sheet thickness perpendicular to the width direction of the sheet at the center of the width of the cold-rolled steel sheet.

[0015] The effects of the invention According to the present invention, a cold-rolled steel sheet with a composite microstructure having ferrite finely and homogeneously divided into tempered martensite in both the rolling direction and the thickness direction can be provided, exhibiting high bake hardening and excellent bendability after bake hardening. This cold-rolled steel sheet has excellent pressure formability, and its strength is further enhanced by baking during coating after pressure forming, resulting in excellent subsequent bendability. Therefore, since the steel sheet exhibits high impact absorption even for bending stresses generated when subjected to impact forces and deformed in a bellows shape, it is suitable as a structural component in fields such as automobiles. Attached Figure Description

[0016] Figure 1 It is a two-dimensional image obtained by converting the microstructure of a cold-rolled steel sheet according to an embodiment of the present invention into 2-grayscale.

[0017] Figure 2 It is Figure 1 The spectrum is obtained by performing a two-dimensional discrete Fourier transform on a two-dimensional image.

[0018] Figure 3 This is an illustrative schematic diagram of a two-dimensional image obtained by converting the microstructure of a cold-rolled steel sheet to grayscale.

[0019] Figure 4 It is Figure 3 The spectrum is obtained by performing a two-dimensional discrete Fourier transform on a two-dimensional image.

[0020] Figure 5 This is an illustrative schematic diagram of a two-dimensional image obtained by converting the microstructure of a cold-rolled steel sheet to grayscale.

[0021] Figure 6 It is Figure 5 The spectrum is obtained by performing a two-dimensional discrete Fourier transform on a two-dimensional image.

[0022] Figure 7 This is a graph showing the relationship between heterogeneity α and baking hardening amount BH.

[0023] Figure 8 It is a graph showing the relationship between the heterogeneity α and the ratio of the minimum bending radius and the plate thickness after baking and hardening, i.e., R / t. Detailed Implementation

[0024] Cold-rolled steel sheet The cold-rolled steel sheet according to the embodiments of the present invention is characterized in that it contains, by mass%, a certain percentage of the following: C: 0.05~0.30% Si: 0.200~2.000% Mn: 2.00~4.00% P: Below 0.100% S: Below 0.010% Al: 0.001~2.000% N: below 0.010% Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100% Cu: 0~1.000%, Ni: 0~1.000%, Mo: 0~1.000%, Cr: 0~1.000%, W: 0~0.005%, Ca: 0–0.005% Mg: 0–0.005% REM: 0~0.010%, B: 0~0.0030%, The remaining portion contains Fe and impurities; It contains 20%–70% ferrite and more than 30% tempered martensite by area ratio. The total content of ferrite and tempered martensite is over 90%; In the section of the cold-rolled steel sheet perpendicular to the width direction of the sheet, at a distance of 1 / 8 to 7 / 8 of the sheet width, a 30μm × 30μm microstructure image obtained by taking a photograph at a magnification of 2000 times is placed in an xy coordinate system with the thickness direction as the x-axis and the rolling direction as the y-axis. Then, the microstructure image is divided into 1024 segments in the x-axis direction and 1024 segments in the y-axis direction to form 1024 × 1024 segmented regions. For the values ​​in each of the above segmented regions, the value is set to "1" when the microstructure is ferrite and "0" otherwise, and then 2-grayscale is performed to create a two-dimensional image. For the above two-dimensional image, the inhomogeneity α defined by equation (1) is 1.20 or less. [Mathematical Expression 4] In equation (1), Su is defined by equation (2), and Sv is defined by equation (3). [Mathematical Expression 5] In equations (2) and (3), F(u, v) is defined by equation (4). [Mathematical Expression 6] In equation (4), f(x, y) represents the gray level of the coordinates (x, y) of the two-dimensional image above.

[0025] For example, in order to improve the bake hardening properties of steel sheets containing a composite microstructure of ferrite and martensite, it is necessary to uniformly introduce pre-strain into both the ferrite and martensite in the steel sheet. Furthermore, from the viewpoint of improving the flexural properties after bake hardening, it is important to make the microstructure of the steel sheet homogeneous. Based on the above insights, in the steel sheet of this embodiment, the inventors have specified the heterogeneity α, as defined by the above formula, to be 1.20 or less. The inventors have found that when the heterogeneity α is 1.20 or less, the bake hardening properties and flexural properties after bake hardening of the cold-rolled steel sheet can be significantly improved.

[0026] In cold-rolled steel sheets containing a composite microstructure of ferrite and tempered martensite, when the heterogeneity α is set to 1.20 or less, for example, in the rolling direction and thickness direction of the cold-rolled steel sheet, a cross-linked structure is formed in which ferrite is finely and homogeneously divided by tempered martensite. Here, the phrase "cross-linked structure in the rolling direction and thickness direction of the cold-rolled steel sheet" is intended to describe a structure in which tempered martensite is randomly linked within the steel sheet, extending along both the rolling and thickness directions, and ferrite is finely and homogeneously dispersed within it. When observing the microstructure from a cross-section of the steel sheet containing the thickness direction x and the rolling direction y, multiple tempered martensites extend into the same thickness region, and these same thickness regions are connected by parallel lines extending along the thickness direction x in a random configuration (see reference). Figure 3 As a result, in the aforementioned cross-section, ferrite is finely segmented by tempered martensite. However, it is important to note that this cross-linked structure is merely one example of the microstructure composition in a steel sheet with a heterogeneity α of 1.20.

[0027] To obtain structures with a heterogeneity α of 1.20 or less, it becomes necessary to control the manufacturing conditions described later. The quantification of such cross-linked structures using Fourier transform will be explained in detail below.

[0028] First, using a scanning electron microscope (SEM), microstructure images of 30 μm × 30 μm were observed at 2000x magnification using grayscale (256 gray levels) in a section of the cold-rolled steel sheet perpendicular to its width direction, at a distance of 1 / 8 to 7 / 8 of the sheet width and from the surface to 1 / 4 to 3 / 8 of the sheet thickness. The resulting microstructure images were arranged in an xy coordinate system with the thickness direction as the x-axis and the rolling direction as the y-axis, having 1024 × 1024 pixels (corresponding to the aforementioned segmented region). Next, the values ​​of each of the 1024 × 1024 pixels were set to "1" if the microstructure was ferrite, and "0" otherwise, resulting in a 2D image. Furthermore, in a specific embodiment of the present invention, the aforementioned microstructure image may also be a 30μm×30μm microstructure image obtained by taking a photograph at a magnification of 2000 times on the microstructure at a position from 1 / 4 to 3 / 8 of the thickness of the cold-rolled steel sheet at the center of the sheet width in a section perpendicular to the width direction of the steel sheet.

[0029] Image processing for grayscale conversion can be performed using image analysis software such as ImageJ. For each pixel, binarization is performed so that it is black when the microstructure is ferrite and white otherwise. The binarization threshold is determined using the average of the brightness values ​​described in "Glasbey, CA (1993), 'An analysis of histogram-based thresholding algorithms, CVGIP: Graphical Models and Image Processing 55: 532-537'". This algorithm is installed in ImageJ and automatically binarizes the image by setting the threshold determination method to Method=Mean using the Auto threshold function. That is, the binarization threshold is set in ImageJ to Method=Mean and radius=15, and each pixel value is replaced by the average of the pixel values ​​within a radius of 15 pixels centered on the pixel being examined, and is automatically determined by the smoothed histogram.

[0030] An example of a two-dimensional image obtained through such operations is shown below. Figure 1 middle. Figure 1 It is a two-dimensional image obtained by grayscale conversion of the microstructure 2 of the cold-rolled steel sheet according to an embodiment of the present invention. Figure 1 The x-axis corresponds to the thickness direction of the plate, and the y-axis corresponds to the rolling direction. Figure 1 In the diagram, the black areas represent ferrite, and the white areas represent tempered martensite. (The text abruptly ends here.) Figure 1 As indicated, the black ferrite phase is finely and homogeneously segmented by the white tempered martensite phase in both the rolling direction and thickness direction of the cold-rolled steel sheet, thereby forming a cross-linked structure.

[0031] Next, the two-dimensional image obtained by 2-grayscale conversion yields two-dimensional data f(x,y) for each pixel (x,y) (x=0~1023, y=0~1023). f(x,y) represents the grayscale of the pixel at coordinates (x,y). For the obtained two-dimensional data, the two-dimensional discrete Fourier transform (2D DFT) defined in equation (4) is implemented.

[0032] [Mathematical Expression 7] Here, F(u, v) is the two-dimensional spectrum of the two-dimensional data f(x, y) after its two-dimensional discrete Fourier transform. The spectrum F(u, v) is generally a complex number and contains information about the periodicity and regularity of the two-dimensional data f(x, y). In other words, the spectrum F(u, v) contains information about the periodicity and regularity of the two-dimensional data f(x, y). Figure 1The information relates to the periodicity and regularity of the ferrite and tempered martensite structure within the two-dimensional image shown.

[0033] Figure 2 It is Figure 1 The spectrum is obtained by performing a two-dimensional discrete Fourier transform on a two-dimensional image. Figure 2 The horizontal axis is the v-axis, with a range of v = -1023 to 1023, and the vertical axis is the u-axis, with a range of u = -1023 to 1023. Figure 2 The spectrum is a black and white grayscale image (grayscale image), with the maximum value of the spectral intensity represented by white and the minimum value represented by black. Figure 2 In the middle, the part with high spectral intensity ( Figure 2 The white part in the middle has a shape that extends from the center along the v-axis and u-axis, and the boundaries are not clear.

[0034] In the spectrum F(u, v), the sum of the absolute values ​​(i.e., spectral intensities) of the spectrum along the u-axis, Su, is defined by equation (2). Similarly, in the spectrum F(u, v), the sum of the absolute values ​​of the spectrum along the v-axis, Sv, is defined by equation (3). Furthermore, the ratio of Su to Sv is defined by equation (1), which is referred to as the inhomogeneity α in this invention. In the sum of equations (2) and (3) defining Su and Sv, the absolute values ​​of the spectrum at coordinate (0, 0) are not included in the (u, v) space.

[0035] [Mathematical Expression 8] The following will Figure 1 The microstructure shown is referred to as structure 1. Structure 1, as described above, has a cross-linked structure of ferrite separated by tempered martensite. Additionally, the spectrogram of structure 1 (…) Figure 2 Similarly, as described above, it has a shape in which the white part extends from the center of the image along the u-axis and v-axis.

[0036] For ease of understanding, Figure 1 and 2 The relationship between the cross-linked structure shown and the spectrum diagram is illustrated using a schematic diagram. Figures 3-6 The following is a detailed explanation. Figure 3 and 5 This is an illustrative schematic diagram of a two-dimensional image obtained by grayscale conversion of the microstructure of cold-rolled steel sheet. Figure 3 and 5 In the diagram, the black part represents ferrite, and the white part represents tempered martensite. Figure 4 and 6 They are respectively to Figure 3 and 5 The spectrum is obtained by performing a two-dimensional discrete Fourier transform on a two-dimensional image. If referenced... Figure 3 and Figure 5 Then it can be known Figure 5 Two-dimensional images and Figure 3 Compared to the two-dimensional image, it has a cross-linked structure with finer and more homogeneous division of ferrite (black parts) into tempered martensite (white parts). Furthermore, if referring to the spectrum... Figure 4 and Figure 6 ,but Figure 4 Spectrum diagram and Figure 6 Compared to the spectrum diagram, the expansion of the white region along the u-axis is more significant than that along the v-axis. As a result, regarding the heterogeneity α, compared to... Figure 3 compared to Figure 5 A lower value is adopted. In summary, it can be seen that the lower the heterogeneity α, the less difference there is between the expansion of the white portion along the u-axis and the expansion along the v-axis; that is, the microstructure of the cold-rolled steel sheet has a finer and more homogeneously segmented cross-linked structure. In fact, if for... Figure 1 In one embodiment of the present invention, the heterogeneity α of tissue 1 is calculated to be 1.14, which is controlled to be below 1.20.

[0037] Furthermore, the bake-hardening amount of structure 1 was 105 MPa, and similarly, the minimum bending radius / thickness ratio of structure 1 after bake-hardening was 0.4. A smaller minimum bending radius / thickness ratio indicates better bendability after bake-hardening. Moreover, these values ​​were measured under the same conditions as in the examples described later.

[0038] Figure 7 This is a graph showing the relationship between heterogeneity α and baking hardening amount BH. Figure 8 It is a graph showing the relationship between the heterogeneity α and the ratio of the minimum bending radius and the plate thickness after baking and hardening, i.e., R / t. Figure 7 and Figure 8 This is a graph plotted from data obtained by manufacturing multiple cold-rolled steel sheets with different chemical compositions and microstructures within the scope of the embodiments of the present invention described above, and then subjecting these cold-rolled steel sheets to the same bake-hardening treatment and bending test as in the examples. (Refer to...) Figure 7 and 8 It can be seen that if α decreases, especially if α becomes below 1.20, there is a tendency for a significant increase in bake hardening amount BH and a significant decrease in the ratio of minimum bending radius to plate thickness after bake hardening, i.e., R / t. The results indicate that in cold-rolled steel sheets containing a composite microstructure of ferrite and tempered martensite, forming a cross-linked structure in the rolling direction and thickness direction of the cold-rolled steel sheet, where tempered martensite finely and homogeneously divides the ferrite, i.e., a cross-linked structure where α is below 1.20, can significantly improve the bake hardening properties and the bending properties after bake hardening.

[0039] Hereinafter, an example of one embodiment of the present invention will be described.

[0040] (I) Chemical composition First, the chemical composition of the steel plate according to embodiments of the present invention and the slab used in its manufacture will be described. In the following description, the unit of the content of each element contained in the steel plate and slab, i.e., "%", refers to "mass %" unless otherwise specified.

[0041] (C: 0.05%~0.30%) Carbon (C) improves hardenability and increases strength by being incorporated into martensitic structures. It also improves bake hardening properties. To effectively exert these effects, the C content is set to 0.05% or more, preferably 0.07% or more, and more preferably 0.09% or more. On the other hand, weldability deteriorates when the C content exceeds 0.30%. Therefore, the C content is set to 0.30% or less, preferably 0.20% or less, and more preferably 0.14% or less.

[0042] (Si: 0.200%~2.000%) Si is an essential element for suppressing carbide formation and ensuring the solid solution C required for bake hardening. When the Si content is below 0.200%, its effect is sometimes insufficient. Therefore, the Si content is set to 0.200% or more. Furthermore, Si is also useful for increasing the strength of steel sheets with excellent bake hardening properties. To effectively exert this effect, the Si content is preferably set to 0.500% or more, more preferably 0.800% or more. On the other hand, when the Si content exceeds 2.000%, surface properties deteriorate, or the additive effect becomes saturated, leading to a futile increase in cost. Therefore, the Si content is set to 2.000% or less, preferably 1.500% or less, more preferably 1.100% or less.

[0043] (Mn: 2.00%~4.00%) Mn is a hardenability-enhancing element, which is useful for increasing the strength of steel plates. To effectively exert this effect, the Mn content is set to 2.00% or more, preferably 2.30% or more, and more preferably 2.60% or more. However, since excessive Mn addition can lead to a decrease in low-temperature toughness through MnS precipitation, the Mn content is set to 4.00% or less, preferably 3.50% or less, and more preferably 3.00% or less.

[0044] (Al: 0.001%~2.000%) Al is effective in improving the yield of deoxidizing and carbide-forming elements. To effectively exert this effect, the Al content is set to 0.001% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Al content exceeds 2.000%, weldability decreases, or oxide inclusions increase, leading to deterioration of surface properties. Therefore, the Al content is set to 2.000% or less, preferably 1.000% or less, and more preferably 0.030% or less.

[0045] (P: below 0.100%) Phosphorus (P) is not an essential element; for example, it is present in steel as an impurity. From a weldability perspective, a lower P content is better. In particular, weldability decreases significantly when the P content exceeds 0.100%. Therefore, the P content is set to 0.100% or less, preferably 0.030% or less, and more preferably 0.020% or less. Reducing the P content incurs costs, and the cost increases significantly if it is to be reduced to below 0.0001%. Therefore, the P content can also be set to 0.0001% or more, or even 0.010% or more. Furthermore, since P contributes to increased strength, from this perspective, the P content can also be set to 0.0001% or more, or even 0.010% or more.

[0046] (S: below 0.010%) Sulfur (S) is not an essential element; for example, it is present in steel as an impurity. From a weldability perspective, the lower the S content, the better. Higher S content leads to increased MnS precipitation and decreased low-temperature toughness. In particular, when the S content exceeds 0.010%, the decrease in weldability and low-temperature toughness is significant. Therefore, the S content is set to 0.010% or less, preferably 0.007% or less, and more preferably 0.003% or less. Reducing the S content incurs costs, and if a reduction to below 0.0001% is desired, the cost increases significantly. Therefore, the S content can also be set to 0.0001% or more, or 0.003% or more.

[0047] (N: below 0.010%) Nitrogen (N) is not an essential element; for example, it is present in steel as an impurity. From a weldability perspective, the lower the N content, the better. In particular, weldability decreases significantly when the N content exceeds 0.010%. Therefore, the N content is set to 0.010% or less, preferably 0.006% or less, and more preferably 0.003% or less. Reducing the N content incurs costs, and if a reduction to below 0.0001% is desired, the cost increases significantly. Therefore, the N content can also be set to 0.0001% or more.

[0048] The basic composition of the steel plate and the slab used in the manufacture of the present invention is as described above. Furthermore, the steel plate and slab may also contain any of the following elements as needed.

[0049] (Ti: less than 0.100%, Nb: less than 0.100%, V: less than 0.100%) Ti, Nb, and V contribute to increased strength. Therefore, Ti, Nb, or V, or any combination thereof, may be included. To fully achieve this effect, the total content of Ti, Nb, or V, or any combination thereof, is preferably set to 0.003% or more, more preferably 0.010% or more. On the other hand, when the total content of Ti, Nb, or V, or any combination thereof, exceeds 0.100%, hot rolling and cold rolling become difficult. Therefore, the total content of Ti, Nb, or V, or any combination thereof, is set to 0.100% or less, more preferably 0.030% or less. That is, it is preferable to set the limits for each component individually as follows: Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100%, and the total content when they are combined in any way is also set to 0.003% to 0.100%.

[0050] (Cu: less than 1.000%, Ni: less than 1.000%, Mo: less than 1.000%, Cr: less than 1.000%) Cu, Ni, Mo, and Cr contribute to increased strength. Therefore, it may contain Cu, Ni, Mo, or Cr, or any combination thereof. To fully achieve this effect, the content of Cu, Ni, Mo, and Cr, individually, is preferably in the range of 0.005% to 1.000%, more preferably 0.010% to 1.000%. Furthermore, the total content of any combination of two or more of the selected Cu, Ni, Mo, and Cr is preferably 0.005% to 1.000%, more preferably 0.010% to 1.000%. On the other hand, when the content of Cu, Ni, Mo, and Cr, or the total content of any combination of two or more of them, exceeds 1.000%, the effect of the above-mentioned action saturates, resulting in unnecessarily high costs. Therefore, the upper limit of the content of Cu, Ni, Mo, and Cr, or the total content of any combination of two or more of them, is set to 1.000%. That is, the preferred values ​​are Cu: 0.005% to 1.000%, Ni: 0.005% to 1.000%, Mo: 0.005% to 1.000%, and Cr: 0.005% to 1.000%, and the total content when they are combined in any way is also preferably 0.005% to 1.000%.

[0051] (W: less than 0.005%, Ca: less than 0.005%, Mg: less than 0.005%, REM: less than 0.010%) W, Ca, Mg, and REM contribute to the fine dispersion of inclusions, thus improving toughness. Therefore, W, Ca, Mg, or REM, or any combination thereof, may be included. To fully achieve this effect, the total content of W, Ca, Mg, and REM, or any combination of two or more thereof, is preferably set to 0.0003% or more, more preferably 0.003% or more. On the other hand, when the total content of W, Ca, Mg, and REM exceeds 0.010%, the surface properties deteriorate. Therefore, the total content of W, Ca, Mg, and REM is set to 0.010% or less, more preferably 0.009% or less. That is, preferably W: 0.005% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.01% or less, and the total content of any two or more thereof is 0.0003% to 0.010%. The upper limit of the total content of any two or more of them is more preferably 0.009%, and the lower limit of the total content of any two or more of them is more preferably 0.003%.

[0052] REM (Rare Earth Metals) refers to a total of 17 elements, including Sc, Y, and the lanthanides. "REM content" refers to the total content of these 17 elements. In industry, lanthanides are added, for example, as misch metals.

[0053] (B: below 0.0030%) Boron (B) is a hardenability-enhancing element and is useful for increasing the strength of steel plates. A B content of 0.0001% (1 ppm) or higher is preferred. However, since adding B at levels exceeding 0.0030% (30 ppm) saturates the aforementioned effects and is economically wasteful, the B content is set to 0.0030% (30 ppm) or less, preferably 0.0025% (25 ppm) or less, and more preferably 0.0019% (19 ppm) or less.

[0054] In the steel sheet of the embodiments of the present invention, the remaining portion other than the above-mentioned components includes Fe and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of steel sheets through various manufacturing processes involving raw materials such as ores or waste, and are not components that are intentionally added for the steel sheet of the embodiments of the present invention.

[0055] (II) The microstructure of steel The cold-rolled steel sheet according to embodiments of the present invention is characterized by comprising a composite microstructure containing at least two or more microstructures. By controlling this composite microstructure, the distribution of pre-strain is altered, thereby improving bake hardening properties. The rationale for specifying the area fraction of each microstructure will be explained. In the following description, unless otherwise specified, the unit of percentage for each microstructure contained in the steel sheet, "%", refers to "area %".

[0056] (Ferrite: 20%–70%) Ferrite is a microstructure characterized by low yield stress, excellent ductility, and work hardening properties. Therefore, excessively increasing the ferrite area ratio increases the strength before bake hardening and decreases the yield stress after bake hardening. Since bake hardening properties are significantly deteriorated in this case, the ferrite area ratio in the steel sheet is set to 70% or less. To further improve bake hardening properties, the ferrite area ratio is preferably set to 50% or less, more preferably 45% or less. On the other hand, when the ferrite area ratio is less than 20%, excessive pre-strain is introduced into the hard microstructure, which conversely deteriorates bake hardening properties and results in poor ductility. Therefore, the ferrite area ratio is set to 20% or more, preferably 25% or more, more preferably 30% or more.

[0057] (Tempered martensite: 30% or more) In embodiments of the present invention, the structure is configured to contain at least 30% tempered martensite in addition to the aforementioned ferrite. Tempered martensite is a microstructure that improves the strength, bake hardening properties, and bending properties of the steel sheet after bake hardening. Generally, due to its higher carbon concentration in hard microstructures compared to ferrite, it exhibits excellent bake hardening properties. In embodiments of the present invention, to increase the amount of bake hardening, such hard microstructure must be tempered martensite. Furthermore, to improve the bending properties and ultimate deformation capacity after bake hardening, it is also necessary to temper the quenched martensite in the composite microstructure. However, when the composite microstructure contains both soft ferrite and tempered martensite, the pre-strain is primarily borne by the ferrite, thus the bake hardening properties of the tempered martensite cannot be fully and effectively utilized in the past. To increase bake hardening properties, it is important that the tempered martensite bears the deformation. However, since if there is too little tempered martensite, only the ferrite phase bears the deformation, a content of at least 30% is required. Therefore, the area ratio of tempered martensite is set to 30% or more, preferably 40% or more, and more preferably 50% or more. On the other hand, the area ratio of tempered martensite is preferably set to 80% or less, and more preferably 70% or less.

[0058] (The total of ferrite and tempered martensite is over 90%) In embodiments of the present invention, the total area ratio of ferrite and tempered martensite is set to 90% or more. If the total area ratio of ferrite and tempered martensite falls below 90%, sufficient bake hardening and ferrite and bake hardened flexibility cannot be obtained. Therefore, the total area ratio of ferrite and tempered martensite is set to 90% or more, preferably 95% or more, more preferably 97% or more, and may also be 100%.

[0059] (Other organizations) In the preferred manufacturing method of the cold-rolled steel sheet of the present invention described below, retained austenite may sometimes be generated depending on the manufacturing conditions. The area ratio of this microstructure is a value obtained by subtracting the area ratio of ferrite and tempered martensite as measured above from 100%. In embodiments of the present invention, since the control of pre-strain distribution in ferrite and tempered martensite is important, the influence of other microstructures, such as retained austenite, can be ignored if they are in small amounts. As described above, in embodiments of the present invention, since the microstructure is composed of ferrite and tempered martensite or more (preferably 95% or more), the influence of retained austenite can also be ignored.

[0060] Similarly, in the preferred manufacturing method of the cold-rolled steel sheet of the present invention described below, during the tempering process, carbides such as cementite precipitate from martensite and ferrite. Because these carbides precipitate in fine and abundant quantities, they are difficult to measure as an area ratio. Therefore, when ferrite and tempered martensite contain carbides, the area ratio of these structures is measured as the area ratio of the parent phase containing the carbides.

[0061] In this invention, the area ratio of ferrite and the area ratio of tempered martensite are determined as follows: First, a sample is collected using a section of the plate thickness perpendicular to the rolling direction of the steel plate as the observation surface. The observation surface is ground, and the microstructure at 1 / 4 of the thickness of the steel plate is observed using SEM-EBSD (scanning electron microscope with electron backscatter diffraction device) at a magnification of 5000x. The area ratio of ferrite is determined by image analysis in a field of view of 100μm×100μm. The average of the measured values ​​from any five or more fields of view is determined as the area ratio of ferrite in this invention.

[0062] Next, secondary electron images of the area from a depth of 3t / 8 to t / 2 of the steel plate surface are captured using SEM. For example, the magnification is set to 1500x. Since the white portion of the obtained image data represents hard microstructure and the black portion represents ferrite, the area ratio of the hard microstructure is determined based on this image data. The tempering state of the hard microstructure is determined as follows: When observing the secondary electron images of the SEM, the contrast of the laths and lath blocks contained in the martensite is considered clear. For example, when observing at 5000x or 10000x magnification, if fine carbides precipitate within the microstructure, it can be said that the microstructure has been tempered, and the hard microstructure is determined to be tempered martensite.

[0063] (Heterogeneity α) The inhomogeneity α of the cold-rolled steel sheet in this embodiment is 1.20 or less as defined by formula (1). The inhomogeneity α is obtained by the following method. In a section of the cold-rolled steel sheet with a thickness perpendicular to the width direction of the sheet, at a position from 1 / 8 to 7 / 8 of the sheet width, the microstructure at a position from 1 / 4 to 3 / 8 of the sheet thickness away from the surface is photographed at a magnification of 2000. The obtained 30μm × 30μm microstructure image is arranged in an xy coordinate system with the thickness direction as the x-axis and the rolling direction as the y-axis, and each pixel of 1024 × 1024 is represented as a gray level. Therefore, the microstructure image represented by the gray level (256 gray levels) is obtained from a section of the cold-rolled steel sheet with a surface including the thickness direction and the rolling direction. Next, for each of the 1024×1024 segmented regions, a 2-grayscale image is created by setting the value to "1" when the microstructure is ferrite and "0" otherwise. Finally, a 2-dimensional image is obtained by using a 2-dimensional discrete Fourier transform to calculate the inhomogeneity α defined by equation (1) from the 2-grayscale microstructure image. Furthermore, in a specific embodiment of the present invention, the above-mentioned microstructure image can also be a 30μm×30μm microstructure image obtained by taking a photograph of the microstructure at a distance of 1 / 4 to 3 / 8 of the thickness of the cold-rolled steel sheet from the center of the sheet width in a section perpendicular to the width direction of the steel sheet, at a magnification of 2000x.

[0064] [Mathematical Expression 9] In equation (1), Su is defined by equation (2), and Sv is defined by equation (3). [Mathematical Expression 10] In equations (2) and (3), F(u, v) is defined by equation (4). [Mathematical Expression 11] In equation (4), f(x, y) represents the gray level of the coordinates (x, y) of the two-dimensional image above.

[0065] As mentioned above, α has baking hardening properties Figure 7 The relationship shown is that α has a relationship with the bendability after baking and hardening. Figure 8 The relationship shown. If the α value determined from the microstructure of the cold-rolled steel sheet according to one embodiment of the present invention is 1.20 or less, then... Figure 7 and 8 As shown, the bake hardening amount BH becomes 100 MPa or more, and the ratio of the minimum bending radius to the sheet thickness after bake hardening, i.e., R / t, becomes less than 1.0. Therefore, the cold-rolled steel sheet of one embodiment of the present invention has excellent bake hardening properties and impact resistance. α is preferably 1.10 or less, more preferably 1.05 or less. The lower limit of α is not particularly specified, but is generally 0.90 or more.

[0066] Based on the above, the cold-rolled steel sheet of one embodiment of the present invention exhibits excellent paint baking hardening performance and excellent impact resistance. Therefore, the cold-rolled steel sheet of this embodiment is preferably used for structural components such as automobiles that are used through pressure processing.

[0067] (Mechanical properties) The cold-rolled steel sheet of this embodiment preferably has a tensile strength of 780 MPa or more, more preferably 800 MPa or more, and even more preferably 900 MPa or more.

[0068] The cold-rolled steel sheet of this embodiment preferably has a bake hardening amount of 100 MPa or more, more preferably 120 MPa or more, and even more preferably 150 MPa or more.

[0069] The cold-rolled steel sheet of this embodiment preferably has a tensile strength at break of 10% or more, and more preferably a tensile strength at break of 12% or more. The cold-rolled steel sheet of this embodiment has excellent bendability after bake hardening, preferably has a minimum bending radius to thickness ratio of less than 1.0, and more preferably has a minimum bending radius to thickness ratio of less than 0.5.

[0070] (III) Manufacturing method Next, a preferred manufacturing method for the cold-rolled steel sheet according to an embodiment of the present invention will be described.

[0071] The following description is an example of a characteristic method intended for manufacturing cold-rolled steel sheets according to embodiments of the present invention, and is not intended to limit the manufacture of the cold-rolled steel sheets to the manufacturing method described below.

[0072] The above manufacturing method is characterized by including the following steps: The process of casting molten steel with the chemical composition described above to form a slab. The roughing process involves roughing the slab in a temperature range of 1050°C to 1250°C. The roughing is performed by reversible rolling with a reduction rate of less than 30% per pass. The reversible rolling includes three or more sets of rolling operations, wherein the rolling is performed as follows (i) and (ii): (i) A round trip with a reduction rate of 20%–30% in the first course and less than 15% in the second course, and (ii) A round trip with a reduction rate of less than 15% in the third course and a reduction rate of 20% to 30% in the fourth course. Two round trips are considered as one group, and the difference in reduction rate between the two tracks during one round trip is more than 5%. The finishing rolling process begins less than 5 seconds after the roughing rolling process and the rough-rolled steel plate is finished rolling in a temperature range of 850°C to 1050°C. The finishing rolling is carried out in 4 or more consecutive rolling stands, the reduction rate of the first stand is less than 15%, and the finished steel plate is coiled in a temperature range of 200°C or lower. The cold rolling process involves cold rolling the obtained hot-rolled steel sheet with a reduction rate of less than 30%. The resulting cold-rolled steel sheet is held in a temperature range of Ac1 to 1000°C for 10 to 1000 seconds, followed by an annealing process where it is cooled to below 200°C at an average cooling rate of 10°C / second to 200°C / second; and The resulting steel sheet is then subjected to a tempering process, maintaining it at a temperature range of 200℃ to 350℃ for at least 100 seconds. The details of each step are explained below.

[0073] (The process of forming a slab) The slab can be manufactured by continuously casting, for example, by melting molten steel having the chemical composition of the steel plate described in the embodiments of the present invention above, using a converter or electric furnace. Alternatively, ingot casting, thin slab casting, etc., can also be used instead of continuous casting.

[0074] (Rough rolling process) The slab can also be heated to a temperature range of 1000°C to 1300°C before performing the following rough rolling process. There is no particular requirement for the holding time after heating, but to ensure that the temperature reaches the center of the slab as well, it is preferable to set it to 30 minutes or more. Furthermore, to suppress excessive oxide scale loss, it is preferable to set it to 10 hours or less, and more preferably 5 hours or less. In the case of direct feed rolling or direct rolling, if the temperature of the cast slab is 1050°C to 1250°C, it can be directly supplied to the following rough rolling process without heating and holding.

[0075] By implementing rough rolling using only reversible rolling, the Mn segregation in the slab is not formed into a plate-like shape extending in one direction, but can be controlled into a complex shape. Therefore, in subsequent processes, a microstructure that suppresses the formation of banded structures and has complex and interwoven ferrite can be obtained. As a result, a cold-rolled steel sheet with a heterogeneity α controlled to 1.20 or less and containing a composite microstructure with fine and homogeneous cross-linked ferrite separated by tempered martensite can be obtained. Furthermore, cold-rolled steel sheets containing conventional composite microstructures cannot have a heterogeneity α set to 1.20 or less because they do not undergo reversible rolling with a reduction rate difference during a round trip, as described below.

[0076] To explain the complex shape of the aforementioned Mn segregation in more detail, firstly, in the slab before rough rolling, the concentrated portions of alloying elements such as Mn grow in a comb-like, roughly perpendicular pattern from both surfaces of the slab inwards. Secondly, during rough rolling, in each pass, the surface of the slab is extended along the rolling direction. The rolling direction is the direction in which the slab moves relative to the rolls. Thus, the slab surface extends along the rolling direction, causing the Mn segregation growing from the slab surface inwards to be inclined along the slab's rolling direction in each pass.

[0077] In unidirectional rolling, where the slab's direction of advance is always the same in each pass of the roughing mill, the Mn segregation remains almost straight, gradually increasing in inclination towards the same direction in each pass. Furthermore, at the end of the roughing mill, the Mn segregation remains almost straight, becoming almost parallel to the slab surface, forming flattened micro-segregations.

[0078] On the other hand, in reversible rolling where the slab's direction of travel alternately becomes opposite in each pass of roughing, the Mn segregation that is inclined in the previous pass becomes inclined in the opposite direction in the next pass, resulting in the Mn segregation becoming a bent shape. Therefore, in reversible rolling, by repeatedly performing passes that alternately become opposite in direction, the Mn segregation becomes a complexly bent shape.

[0079] When the roughing temperature is below 1050°C, it becomes difficult to complete the rolling process at a temperature above 850°C in the subsequent finishing rolling stage, resulting in poor steel sheet shape. Furthermore, since the oxide scale loss during slab preheating increases above 1250°C and becomes a contributing factor to slab cracking, the roughing temperature is set between 1050°C and 1250°C. The lower limit of the roughing temperature is preferably 1100°C, and the upper limit is preferably 1200°C.

[0080] If the reduction rate in a single pass of roughing exceeds 30%, the shear stress during rolling increases, making it easier for Mn segregation to form bands rather than complex shapes. Therefore, the reduction rate in a single pass of roughing is set to 30% or less. Since a smaller reduction rate results in smaller shear strain during rolling and better inhibits the formation of band structures, there is no specific lower limit for the reduction rate. However, from a productivity point of view, it is preferable to be 10% or more, and more preferably 15%.

[0081] To form complex shapes, more specifically, mesh shapes, of Mn segregation, resulting in a cross-linked structure of tempered martensite and ferrite, the reduction rate must be controlled in each pass due to changes in shear stress during rolling. To prevent Mn segregation from forming bands, it is preferable to repeat reversible rolling twice with different reduction rates in each pass. In this case, to achieve a banded distribution in the first pass (high rolling temperature) by applying a large reduction in the same direction as the forward direction, and then to achieve a complex shape distribution of Mn segregation in the fourth pass (low rolling temperature) by applying a large reduction in the opposite direction to the forward direction, it is preferable to have higher reduction rates in the first and fourth passes than in other passes. That is, rolling is performed in three or more sets, with each set consisting of two passes totaling (i) and (ii) described below.

[0082] (i) A round trip with a reduction rate of 20%–30% in the first pass and less than 15% in the second pass; and (ii) A round trip with a reduction rate of less than 15% in the third course and a reduction rate of 20% to 30% in the fourth course.

[0083] However, since it becomes difficult to ensure sufficient finishing temperature if more than 6 sets of the above rolling process are performed, it is preferable to set it to 5 sets or less.

[0084] Furthermore, it is preferable that each pass, where the directions of advance are opposite to each other, be performed the same number of times, that is, the total number of passes is set to an even number. However, in a typical roughing rolling production line, the inlet and outlet sides of the roughing roll are sandwiched between rolls and located on opposite sides. Therefore, the number of passes (rolling) from the inlet side to the outlet side of the roughing roll is increased by one. In this case, the Mn segregation portion becomes plate-like in the final pass (rolling), making it difficult to form a mesh-like distribution of Mn. When roughing is performed in such a hot rolling production line, it is preferable to set the reduction rate when the rough-rolled plate is finally sent from the inlet side to the outlet side to 5% or less, and more preferably to leave the roll gap empty and omit rolling.

[0085] As will be described later, tandem multi-stage rolling in finishing mills is effective for refining the recrystallized microstructure; however, tandem rolling makes it easier to form flattened microsegregations. To utilize tandem multi-stage rolling, the reduction rate difference in one pass of the aforementioned reversible rolling must be increased, and the microsegregations formed in subsequent tandem rolling processes must be controlled. This effect becomes significant if the reduction rate difference in one pass of the reversible rolling is 5% or more. Therefore, the reduction rate difference in one pass of the reversible rolling is preferably set to 5% or more, and more preferably 10% or more.

[0086] In order to maintain the mesh structure of Mn generated by reversible rolling in roughing, it is necessary to suppress the movement of austenite grain boundaries. Therefore, the holding time from roughing to finishing is preferably set to less than 5 seconds, and more preferably to less than 3 seconds.

[0087] (Precision rolling process) Following the reversible rolling in roughing, to narrow the spacing of Mn segregation bands originating from secondary dendrite arms by increasing the reduction rate of tandem rolling in finishing, finishing is preferably performed using four or more consecutive rolling stands. If finishing is completed below 850°C, sufficient recrystallization does not occur, resulting in a structure extending along the rolling direction, which in turn forms a plate-like structure originating from the extended structure in subsequent processes. Therefore, the finishing temperature is set to 850°C or higher, preferably 900°C or higher. On the other hand, if the finishing temperature exceeds 1050°C, it becomes difficult to generate fine recrystallized austenite grains, Mn segregation at grain boundaries becomes difficult, and Mn segregation bands tend to flatten. Therefore, the finishing temperature is set to 1050°C or lower. If necessary, the rough-rolled steel sheet may be reheated after the roughing process and before the finishing process. Furthermore, by setting the reduction rate of the first stand in the finishing mill to less than 15%, the excessive formation of recrystallized grains is suppressed, making it easier to maintain the mesh structure of Mn formed in the roughing process. In this way, by limiting not only the roughing process but also the finishing process, the microsegregation of flattened Mn can be suppressed. Additionally, the reduction rate of the first stand in the finishing mill is preferably 10% or less.

[0088] The coiling temperature is preferably below 200°C. By setting the coiling temperature to below 200°C, the austenite transforms into hard martensite during cooling. The phase transformation strain introduced at this temperature introduces a large amount of strain into the soft ferrite near the martensite, which helps to refine and homogenize the recrystallized ferrite during subsequent annealing. When the coiling temperature exceeds 200°C, the formation of martensite is suppressed, and the above-mentioned effect is not obtained; the heterogeneity α does not meet the conditions specified in this invention. Therefore, the coiling temperature is below 200°C, preferably below 100°C, and more preferably below 50°C. Cold rolling is performed on the microstructure obtained by setting the coiling temperature to below 200°C, thereby concentrating stress in the ferrite near the hard martensite and introducing a large amount of strain. Annealing in this state generates many recrystallized ferrite nuclei, resulting in a homogeneous and fine microstructure. Furthermore, the reverse phase transformation γ is also finely generated between the martensite laths. In addition to the mesh structure of Mn formed in the rough rolling process described above, the martensite finely divides the ferrite and forms a cross-linked structure, thereby obtaining the microstructure specified in this invention. Excellent bending performance requires both good machinability and excellent ultimate deformation capacity. However, by finely dividing the ferrite into martensite and forming a cross-linked structure, the work hardening capacity of the ferrite is improved, and consequently, due to the homogeneous microstructure, the ultimate deformation capacity is also excellent.

[0089] On the other hand, when coiling at high temperatures exceeding 200°C, no hard martensite is generated. Therefore, compared with low-temperature coiling, the amount of strain introduced into ferrite after cold rolling is less, and the target microstructure and properties cannot be obtained.

[0090] (Cold rolling process) From the viewpoint of maintaining the cross-linked structure of martensite and ferrite formed in the roughing and finishing rolling processes, it is important to reduce the reduction rate of cold rolling. By suppressing the reduction rate of cold rolling to a low level, the cross-linked structure of martensite and ferrite can be maintained even after annealing. To achieve this effect, the upper limit of the reduction rate of cold rolling is 30%, preferably 20%. If the reduction rate of cold rolling exceeds 30%, the cross-linked structure of martensite and ferrite is crushed along the thickness direction, and the inhomogeneity α does not meet the conditions specified in this invention. From the viewpoint of homogenizing and / or refining the microstructure, the lower limit of cold rolling is 5%, preferably 7%, and more preferably 10%. Setting the reduction rate of cold rolling to 30% or less is an important requirement for satisfying the condition of inhomogeneity α specified in this invention.

[0091] (Annealing process) The steel sheet obtained through the above-described cold rolling process is then subjected to annealing. Heating at the annealing temperature is carried out within a temperature range of Ac1 to 1000°C for 10 to 1000 seconds. This temperature range determines the area ratio of ferrite and hard microstructure. The upper limit of the annealing temperature range is preferably 870°C, more preferably 850°C. To ensure sufficient recrystallization of the cold-worked ferrite and to easily control the area ratio of ferrite and hard microstructure, the annealing time is set to 10 seconds or more. Furthermore, if the annealing time exceeds 1000 seconds, productivity deteriorates. Therefore, the annealing time is set to 10 to 1000 seconds. The upper limit of the annealing time is preferably 300 seconds. The lower limit of the annealing time is preferably 200 seconds.

[0092] Point Ac1 is calculated using the following formula.

[0093] Ac1=751-16×C+35×Si-28×Mn-16×Ni+13×Cr-6×Cu+3×Mo In the above formula, C, Si, Mn, Ni, Cr, Cu, and Mo are the contents (mass%) of each element. For elements that are not present, substitute 0 (mass).

[0094] After holding the annealing temperature, cooling is performed at a rate of 10°C / second to 200°C / second. A rapid cooling rate is preferable to freeze the microstructure and effectively induce the martensitic transformation. However, if the cooling rate is below 10°C / second, martensite is not sufficiently generated, and the desired microstructure cannot be controlled. On the other hand, even if the cooling rate exceeds 200°C / second, its effect saturates; therefore, the cooling rate after annealing is set to 10°C / second to 200°C / second. The upper limit of the cooling rate after annealing is preferably 50°C / second. The lower limit of the cooling rate after annealing is preferably 10°C / second. The above cooling rate differs from the average cooling rate; it refers to a rate not lower than 10°C / second in any temperature range during cooling. The cooling stop temperature is set to 200°C or below. This is to ensure martensite formation after holding the annealing temperature. At this time, a step of stopping cooling at 200°C to 500°C and holding for 10 to 1000 seconds may also be added. The cooling stop temperature is preferably 55°C or below, more preferably 45°C or below.

[0095] (Tempering process) The resulting steel sheet is held in a temperature range of 200°C to 350°C during the tempering process. The holding temperature is preferably set to 250°C to 300°C. When the holding temperature is below 200°C, the pre-strain distribution remains unchanged because the martensite is not tempered. When the temperature exceeds 350°C, the overall dissolved carbon content decreases due to the precipitation of coarse carbides, thus reducing bake hardening properties. Furthermore, if the holding temperature becomes higher than the recrystallization temperature of ferrite, the distribution of the ferrite-matrix interface changes due to the recrystallized ferrite formed in the parent phase, resulting in the occasional truncation or disintegration of the cross-linked structure of martensite and ferrite. On the other hand, to temper the hard structure as a whole, the holding time is set to 100 seconds or more. Afterward, from a productivity point of view, it is cooled to below 100°C at an average cooling rate of 2°C / second or more. The cooling stop temperature is preferably below 50°C, more preferably below 45°C.

[0096] (Surface smoothing process) Alternatively, a final skin finishing rolling (quenching and tempering) can be performed on the cold-rolled steel sheet manufactured by the above method. By performing skin finishing rolling, strain is introduced into the steel sheet even without pre-strain, thus improving bake hardening properties. In order to uniformly introduce strain into the steel sheet, the reduction rate is set to 0.1% or more. Since sheet thickness control becomes difficult, it is preferable to set 0.5% as the upper limit.

[0097] By operating in this way, cold-rolled steel sheets according to embodiments of the present invention can be manufactured.

[0098] The above embodiments are merely specific examples illustrating the implementation of the present invention, and the technical scope of the present invention is not to be interpreted in a limiting manner. That is, the present invention can be implemented in various forms without departing from its technical concept or its main features.

[0099] Example Next, embodiments of the present invention will be described. The conditions in the embodiments are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Various conditions can be adopted in the present invention as long as they do not depart from the spirit of the present invention and achieve the purpose of the present invention.

[0100] A slab with the chemical composition shown in Table 1 is manufactured. The slab is heated at 1300°C for 1 hour, then rough-rolled and finish-rolled under the conditions shown in Table 2. The steel sheet is then coiled and held at the coiling temperature shown in Table 2 for 1 hour to obtain a hot-rolled steel sheet with a thickness of 2 mm. The hot-rolled steel sheet is then pickled and cold-rolled at the reduction rate shown in Table 2 to obtain a cold-rolled steel sheet with the thickness shown in Table 2. Finally, annealing, tempering, and / or surface finishing are performed under the conditions shown in Table 2.

[0101] The microstructure of the obtained cold-rolled steel sheet was observed. During the microstructure observation, the area ratio of ferrite, the area ratio of tempered martensite, and the heterogeneity α were determined using the methods described above.

[0102] In particular, the area ratio of ferrite and the area ratio of tempered martensite are determined as follows: First, a sample is collected using a section of the plate thickness perpendicular to the rolling direction of the steel plate as the observation surface. The observation surface is ground, and the microstructure at 1 / 4 of the thickness of the steel plate is observed using SEM-EBSD at a magnification of 5000x. The area ratio of ferrite is determined by image analysis in a field of view of 100μm×100μm. The average of the measured values ​​in any 5 fields of view is determined as the area ratio of ferrite.

[0103] In addition, SEM double electron images (1500x magnification) were taken of a region at a depth of 3t / 8 to t / 2 from the surface of the steel plate. Since the white areas of the obtained image data represent hard microstructure and the black areas represent ferrite, the area fraction of the hard microstructure was determined based on this image data. Regarding the hard microstructure, when observing the aforementioned SEM double electron images at 5000x or 10000x magnification, the presence of fine carbides within the hard microstructure indicated tempered martensite. The results are shown in Table 3.

[0104] Furthermore, the tensile strength TS, elongation at break EL, bake hardening amount BH, and minimum bending radius R of the obtained cold-rolled steel sheet were measured. In the determination of tensile strength TS, elongation at break EL, and bake hardening amount BH, JIS No. 5 tensile test pieces were collected with the length direction perpendicular to the rolling direction as the longitudinal direction, and tensile tests were performed according to JIS Z 2241. BH is the value obtained by subtracting the stress at which the additional 2% prestrain was applied from the stress at which the test piece was re-stretched after heat treatment at 170°C for 20 minutes with an additional 2% prestrain. To meet the requirements for lightweight automotive bodies, the tensile strength is 780 MPa or higher. Additionally, for ease of forming, the elongation at break is preferably 10% or higher. Regarding BH, since it is difficult to form below 100 MPa and the strength after forming decreases, it is preferably 100 MPa or higher to have excellent bake hardening properties.

[0105] As an indicator for evaluating the bendability after paint baking and hardening treatment, the ratio of minimum bending radius to sheet thickness, R / t, is used. The minimum bending radius R is determined using the V-block method (V-block method: pressing part tip angle: 90°, tip radius R: varying from 0.5 mm in 0.5 mm increments) specified in JIS Z 2248, with the test piece width set to 30 mm. If the ratio of minimum bending radius to sheet thickness, R / t, is 1.0 or higher, the test piece after paint baking and hardening treatment may break immediately due to bending stress generated during wrinkling deformation upon impact. That is, the impact performance of the component is poor. Therefore, the ratio of minimum bending radius to sheet thickness, R / t, after BH measurement, is preferably lower than 1.0.

[0106] [Evaluation Results] As shown in Table 3, excellent TS, BH, and R / t were obtained in Examples 1, 3, 6, 7, 10, 15, 17, 20, 22, 23, 25, 27, 33, 34, and 35. TS was above 780 MPa, BH was above 100 MPa, and R / t became below 1.0, exhibiting high strength and excellent bake-hardening properties, as well as excellent flexural properties after bake-hardening.

[0107] On the other hand, in Comparative Example 2, due to the short tempering time, the tempered martensite did not achieve the desired area ratio, resulting in a low BH and a high R / t in the steel. In Comparative Example 4, due to the high reduction rate during cold rolling, the cross-linked structure of martensite and ferrite could not be maintained, resulting in a larger inhomogeneity α, a low BH, and a high R / t.

[0108] In Comparative Example 5, due to the low tempering holding temperature, the tempered martensite did not achieve the desired area ratio, resulting in a low BH and a high R / t for the steel. In Comparative Example 8, due to the low annealing temperature, the ferrite area ratio became too high and the tempered martensite area ratio became too low, resulting in low TS and BH for the steel.

[0109] In Comparative Example 9, due to the short annealing time, the tempered martensite did not achieve the desired area ratio, resulting in low TS and BH, and high R / t in the steel. In Comparative Example 11, due to the slow cooling rate after annealing, martensite was not sufficiently formed. Therefore, the ferrite area ratio became too high and the tempered martensite area ratio became too low, resulting in low TS and BH. In Comparative Example 12, due to the high tempering holding temperature, coarse carbides precipitated. Furthermore, the formation of recrystallized ferrite prevented the maintenance of the cross-linked structure of martensite and ferrite, resulting in a high inhomogeneity α, low BH, and high R / t.

[0110] In Comparative Example 13, due to the low C content, ferrite and tempered martensite did not achieve the desired area ratio, resulting in low TS and BH of the steel. In Comparative Example 14, due to the low Si content, coarse carbides precipitated, resulting in low BH and high R / t. In Comparative Example 16, due to the low finishing rolling temperature, the inhomogeneity α increased, resulting in low BH and high R / t. In Comparative Example 18, due to the low Mn content, tempered martensite did not achieve the desired area ratio, resulting in low TS and BH and high R / t.

[0111] In Comparative Example 19, due to the low reduction difference between the two passes within one round of rough rolling, the inhomogeneity α is large, BH is low, and R / t is high. In Comparative Example 21, due to the high reduction rate in rough rolling, the inhomogeneity α is large, BH is low, and R / t is high. In Comparative Example 24, due to the high coiling temperature, martensite formation is suppressed, resulting in a large inhomogeneity α, low BH, and high R / t. In Comparative Example 26, due to the few rough rolling passes, a cross-linked structure of tempered martensite and ferrite cannot be obtained, resulting in a large inhomogeneity α, low BH, and high R / t. In Comparative Example 28, the long holding time from rough rolling to finish rolling prevents the formation of a cross-linked structure of tempered martensite and ferrite, resulting in a large inhomogeneity α, low BH, and high R / t.

[0112] In Comparative Example 29, due to the low reduction rate in the first pass of rough rolling and the high reduction rate in the second pass, a cross-linked structure of tempered martensite and ferrite could not be obtained, resulting in a larger inhomogeneity α, lower BH, and higher R / t. In Comparative Example 30, due to the high reduction rate in the third pass of rough rolling and the low reduction rate in the fourth pass, a cross-linked structure of tempered martensite and ferrite could not be obtained, resulting in a larger inhomogeneity α, lower BH, and higher R / t. In Comparative Example 31, due to the high coiling temperature, the formation of martensite in the hot-rolled steel sheet was suppressed, resulting in less strain introduced into the ferrite, leading to a larger inhomogeneity α, lower BH, and higher R / t. In Comparative Example 32, due to the high reduction rate in cold rolling, the cross-linked structure of martensite and ferrite could not be maintained, resulting in a larger inhomogeneity α, lower BH, and higher R / t.

[0113] Industrial availability The cold-rolled steel sheet of the present invention can be used as a structural component of automobiles, especially in the automotive industry.

Claims

1. A cold-rolled steel sheet, wherein, Contains, by mass% C：0.05～0.30%、 Si: 0.200~2.000% Mn: 2.00~4.00% P: Below 0.100% S: Below 0.010% Al：0.001～2.000%、 N: below 0.010% Ti: 0~0.100%, Nb: 0~0.100%, V：0～0.100%、 Cu: 0~1.000%, Ni: 0~1.000%, Mo: 0~1.000%, Cr：0～1.000%、 W：0～0.005%、 Ca: 0–0.005% Mg: 0–0.005% REM: 0~0.010%, B：0～0.0030%， The remaining portion contains Fe and impurities; It contains 20%–70% ferrite and more than 30% tempered martensite by area ratio. The total content of ferrite and tempered martensite is over 90%; In a section of the cold-rolled steel sheet perpendicular to the width direction, at a distance of 1 / 8 to 7 / 8 of the sheet width, a 30μm × 30μm microstructure image obtained by photographing the microstructure at a distance of 1 / 4 to 3 / 8 of the sheet thickness from the surface at a magnification of 2000x is placed in an xy coordinate system with the sheet thickness direction as the x-axis and the rolling direction as the y-axis. The microstructure image is then divided into 1024 segments along the x-axis and 1024 segments along the y-axis to form 1024 × 1024 segmented regions. For each segmented region, the value is set to "1" if the microstructure is ferrite, and "0" otherwise, and then converted to 2-grayscale to create a two-dimensional image. For this two-dimensional image, the inhomogeneity α defined by equation (1) is 1.20 or less. The tensile strength of the cold-rolled steel sheet is above 780 MPa and below 1178 MPa. In equation (1), Su is defined by equation (2), and Sv is defined by equation (3). In equations (2) and (3), F(u, v) is defined by equation (4). In equation (4), f(x, y) represents the gray level of the coordinates (x, y) of the two-dimensional image.

2. The cold-rolled steel sheet according to claim 1, further comprising, by mass%, one or more of the following: Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100% or less in total.

3. The cold-rolled steel sheet according to claim 1 or claim 2, wherein, The microstructure image is a 30μm×30μm microstructure image obtained by taking a picture at a magnification of 2000x in a section of the cold-rolled steel sheet at the center of the sheet width, perpendicular to the sheet width direction, focusing on the microstructure at a distance of 1 / 4 to 3 / 8 of the sheet thickness from the surface.

Images