A cold-rolled steel sheet and a manufacturing method thereof

Abstract

This invention discloses a cold-rolled steel sheet, which, by mass percentage, comprises the following chemical elements: C: 0.190–0.250%, Si: 0.60–1.30%, Mn: 2.00–2.50%, Al: 0.20–0.80%, with the remainder being Fe and unavoidable impurities. The microstructure of the cold-rolled steel sheet includes ferrite, retained austenite, and at least one of the following: distributed martensite and tempered martensite; wherein the carbon content of the retained austenite satisfies: C RA ≥(833-Tp-7400*W) Mn -W Si ) / 24100*100%, C RA W represents the carbon content (wt%) of the retained austenite. Mn and W Si These represent the mass percentages (wt%) of Mn and Si elements in the cold-rolled steel sheet, respectively, and Tp represents the distribution temperature during continuous annealing, which is 350–430°C. This invention discloses a method for manufacturing cold-rolled steel sheets. The cold-rolled steel sheet of this invention possesses high strength-ductility product, high yield strength, and low alloy cost.

Description

Technical Field

[0001] This invention relates to the field of steel, and in particular to a cold-rolled steel sheet and its manufacturing method. Background Technology

[0002] With the advancement of the "carbon peaking and carbon neutrality" strategy, the automotive industry, a major carbon emitter, is facing increasingly stringent requirements for energy conservation, consumption reduction, and emission reduction. Lightweight vehicle body design, as a crucial measure for reducing vehicle weight and consumption, is receiving increasing attention in today's booming new energy vehicle market.

[0003] Ultra-high strength steel is a crucial material for lightweighting automotive bodies due to its superior strain distribution, high strain hardening characteristics, uniform mechanical properties, minimal springback fluctuations, better absorption performance, and longer fatigue life, making it a preferred material for automotive structural components. The complex design of automotive structural components necessitates excellent formability of ultra-high strength steel. Third-generation advanced high-strength steel, currently the best-performing ultra-high strength steel overall, is characterized by a strength-ductility product (tensile strength * elongation at break) of approximately 20–23 GPa·%, meaning that at a tensile strength of 1000 MPa, the elongation at break is approximately 20%–23%. When the tensile strength is stable within a fixed range, a key method to increase the strength-ductility product is to increase the content of ferrite and retained austenite, utilizing the coordinated deformation capacity of ferrite and the TRIP effect of retained austenite to improve the elongation at break.

[0004] For example, the invention patent CN109414904A, entitled "High-Strength Steel Products and Annealing Process for Manufacturing Them," discloses a high-strength annealed steel plate with the following composition by mass percentage: C: 0.12–0.5%, Mn: 1–3%, Si+Al: 0.8–3%. The microstructure consists of ferrite and essentially equiaxed retained austenite with an average aspect ratio less than 3:1. The product of the tensile strength and total elongation (UTS·TE) of the steel plate is ≥25000 MPa·%. The steel plate undergoes a two-step annealing process: the first step annealing to obtain a microstructure mainly composed of martensite, and the second step annealing involving homogenization in the critical region at 720–850°C, followed by holding at 370–430°C. This patent has a yield strength of 530–685 MPa and employs a two-step annealing process, resulting in a long production process and high manufacturing costs.

[0005] Patent CN106244924A, entitled "A Cold-Rolled Quenched and Ductile Steel and its Preparation Method," discloses a cold-rolled quenched and ductile steel with a strength-ductility product of 25 GPa·% or higher. Its chemical composition by mass percentage is: C: 0.18–0.23%, Mn: 1.5–2.2%, Si: 1.3–1.8%, P≤0.02%, S≤0.008%, Nb: 0–0.05%, Ti: 0–0.1%, with the balance being Fe and unavoidable impurities. The microstructure consists of 20–50% polygonal ferrite, 30–70% carbide-free bainite and martensite, and 8–16% retained austenite. The yield strength of this patent is below 650 MPa, even below 600 MPa.

[0006] As can be seen from the existing technology, while improving the strength-ductility product of materials, it is impossible to guarantee that the materials also possess high yield strength. However, a lower yield strength cannot meet the manufacturing requirements for parts with both high elongation and strength. Therefore, it is desirable to develop an ultra-high-strength steel for cold stamping that possesses both high strength-ductility product and high yield strength. Summary of the Invention

[0007] To address the aforementioned technical problems, embodiments of the present invention provide a cold-rolled steel sheet and a method for manufacturing the same.

[0008] In a first aspect, embodiments of the present invention disclose a cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises the following chemical elements by mass percentage:

[0009] C: 0.190–0.250%, Si: 0.60–1.30%, Mn: 2.00–2.50%, Al: 0.20–0.80%, with the remainder being Fe and unavoidable impurities. The microstructure of the cold-rolled steel sheet includes ferrite, retained austenite, and at least one of the following: distributed martensite and tempered martensite.

[0010] Wherein, the carbon content of the retained austenite satisfies: C RA ≥(833-Tp-7400*W Mn -W Si ) / 24100*100%, C RA W represents the carbon content (wt%) of the retained austenite. Mn and W Si These represent the mass percentages (wt%) of Mn and Si elements in the cold-rolled steel sheet, respectively. Tp represents the distribution temperature during continuous annealing, which is 350–430℃.

[0011] By adopting the above technical solution, the cold-rolled steel sheet of the present invention can have high strength and low alloy cost while having high strength and ductility.

[0012] Optionally, by volume percentage, the microstructure of the cold-rolled steel sheet is 20-50% ferrite, 13-16% retained austenite, and 40-65% of at least two of martensite, partitioned martensite, and tempered martensite.

[0013] Optionally, the Al and Si elements also satisfy the following condition: 1.00% ≤ Al + Si ≤ 1.80%.

[0014] Optionally, the unavoidable impurities, by mass percentage, include: P ≤ 0.020%, S ≤ 0.0120%, N ≤ 0.008%, and B ≤ 0.0005%.

[0015] Optionally, the cold-rolled steel sheet has a tensile strength ≥980MPa, a yield strength of 700~850MPa, an elongation at break ≥24%, and a strength-ductility product ≥25GPa·%.

[0016] Secondly, embodiments of the present invention disclose a method for manufacturing cold-rolled steel sheets, the method comprising the following steps:

[0017] Smelting and casting;

[0018] Hot rolling and coiling;

[0019] Insulation;

[0020] Pickling and cold rolling;

[0021] Continuous annealing: The heating temperature is (Ts-180)℃ to (Ts-100)℃, then rapidly cooled, and then heated to Tp and held for 210 to 630 seconds;

[0022] Where Ts represents the annealing temperature, which is Ac1 to 900℃, and Ac1 represents the temperature at which austenite begins to form; Tp represents the partitioning temperature, which is 350 to 430℃.

[0023] By adopting the above technical solution, through reasonable chemical composition design and optimized process, the cold-rolled steel sheet of the present invention can have high strength-ductility product and high yield strength.

[0024] Optionally, in the hot rolling and coiling steps, the heating temperature of the hot rolling is 1180-1300℃, the holding time is 60-240 min, and the final rolling temperature is 900-960℃.

[0025] Optionally, in the hot rolling and coiling steps, the coiling temperature is controlled to be 400–520°C.

[0026] Optionally, in the heat preservation step, the temperature is controlled at (Tc-50)~550℃ for 60~240min, where Tc represents the winding temperature.

[0027] Optionally, in the pickling and cold rolling steps, the pickling speed is controlled to be ≤200m / min, and / or the cold rolling reduction is controlled to be 30-70%. Attached Figure Description

[0028] Figure 1 The diagram shows the microstructure of Embodiment 4 of the present invention. Detailed Implementation

[0029] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0030] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0032] In this embodiment of the invention, the strength-ductility product is the product of tensile strength and elongation at break.

[0033] In this embodiment of the invention, "martensite" refers to a supersaturated solid solution of carbon in α-Fe.

[0034] In this embodiment of the invention, "tempered martensite" refers to a mixed microstructure of extremely fine transitional carbides and supersaturated (low carbon content) α-phase obtained from the decomposition of martensite.

[0035] In this embodiment of the invention, "partitioned martensite" refers to the low-carbon structure formed by the decomposition of martensite to obtain carbides, while some carbon atoms are "partitioned" into the retained austenite.

[0036] In this embodiment of the invention, "rapid cooling" refers to the process of rapidly cooling a steel plate from a high temperature to a low temperature. "Slow cooling" refers to the process of cooling a steel plate from a high temperature to a low temperature at a relatively slow rate. It should be noted that the descriptions of "high temperature" and "low temperature" are only relative to each other, meaning that there is a temperature difference between the two temperatures. For those skilled in the art, the temperature ranges for "high temperature" and "low temperature" can be defined according to specific circumstances.

[0037] In a first aspect, embodiments of the present invention disclose a cold-rolled steel sheet, which, by mass percentage, comprises the following chemical elements:

[0038] C: 0.190–0.250%, Si: 0.60–1.30%, Mn: 2.00–2.50%, Al: 0.20–0.80%, with the remainder being Fe and unavoidable impurities. The microstructure of the cold-rolled steel sheet includes ferrite, retained austenite, and at least one selected from partitioned martensite and tempered martensite. That is, the microstructure of the cold-rolled steel sheet of the present invention includes ferrite, retained austenite, and partitioned martensite; or the microstructure of the cold-rolled steel sheet of the present invention includes ferrite, retained austenite, and tempered martensite; or the microstructure of the cold-rolled steel sheet of the present invention includes ferrite, retained austenite, partitioned martensite, and tempered martensite.

[0039] Among them, the carbon content of the retained austenite satisfies: C RA ≥(833-Tp-7400*W Mn -W Si ) / 24100*100%, C RA W represents the carbon content (wt%) of the retained austenite. Mn and W Si These represent the mass percentages (wt%) of Mn and Si elements in the cold-rolled steel sheet, respectively. Tp represents the distribution temperature during continuous annealing, which is 350–430℃.

[0040] In the above technical solution, the present invention is an improvement specifically for cold-rolled steel sheets based on a reasonable chemical composition design. It employs a relatively simple alloy element design, primarily using C, Si, Mn, and Al, without adding expensive alloying elements such as Cr, Mo, Nb, and Ti, resulting in lower alloy costs.

[0041] Furthermore, the present invention also achieves effective control over the microstructure of cold-rolled steel sheets by regulating the above-mentioned chemical composition, that is, the microstructure of the cold-rolled steel sheets of the present invention includes ferrite, retained austenite, and at least one of the following: distributed martensite and tempered martensite.

[0042] This invention utilizes carbides in partitioned or tempered martensite to promote the decomposition of retained austenite while simultaneously increasing the yield strength of cold-rolled steel sheets. Furthermore, to leverage the TRIP (Transformation-Induced Plasticity) effect of retained austenite, it is necessary to ensure the presence of stable retained austenite in the steel sheet, thereby controlling the carbon content of the retained austenite, i.e., the carbon content must satisfy C... RA ≥(833-Tp-7400*W Mn -W Si The carbon content of the retained austenite is 100% (24100*100%), ensuring sufficient carbon content and thus guaranteeing stable retained austenite in the steel sheet. Therefore, the "TRIP" effect of retained austenite can be effectively utilized, improving the strength and plasticity of the cold-rolled steel sheet of this invention. Furthermore, the fracture elongation of the cold-rolled steel sheet is improved by utilizing the coordinated deformation capacity of ferrite and the TRIP effect of retained austenite.

[0043] Therefore, the cold-rolled steel sheet of the present invention, through the above-mentioned chemical composition design and control of the microstructure composition and the carbon content of retained austenite, enables the cold-rolled steel sheet of the present invention to have high strength-ductility product, high yield strength, and low alloy cost, thus meeting the requirements of more complex automotive safety and structural components and achieving better lightweighting effects. Specifically, in this embodiment, the yield strength of the cold-rolled steel sheet of the present invention is 700-850 MPa, and the strength-ductility product is ≥25 GPa·%.

[0044] The design principles and functions of each chemical element in the cold-rolled steel sheet of this invention are explained in detail below.

[0045] Carbon (C) has a much higher solubility in austenite than in ferrite, which can prolong the incubation period before austenite transformation and lower the Ms temperature. The higher the mass percentage of C in the steel, the greater the fraction of retained austenite. During quenching and partitioning, the enrichment of C in the retained austenite is higher, which is beneficial for enhancing the stability of retained austenite, producing the TRIP effect, and improving the material's plasticity. However, excessively high carbon equivalents can reduce the weldability of the material. Therefore, in this invention, the mass percentage of C is controlled between 0.190% and 0.250% (including the endpoints of 0.190% and 0.250%).

[0046] Si: Si strongly inhibits cementite precipitation and promotes carbon enrichment into retained austenite during quenching-partitioning processes, increasing the content of stable retained austenite in steel and improving material plasticity. However, excessively high Si content reduces the high-temperature plasticity of steel and easily leads to surface defects such as red iron scale and internal oxidation. Therefore, in this invention, the mass percentage of Si is controlled between 0.60% and 1.30% (including the endpoints of 0.60% and 1.30%).

[0047] Mn: Mn can improve the stability of austenite and the hardenability of steel, expand the austenite phase region, and reduce the critical transformation rate of martensite, thereby improving the stability of retained austenite in steel and increasing its plasticity; the solid solution strengthening effect of Mn in steel can also improve the strength of steel. However, excessively high Mn content will reduce the weldability and corrosion resistance of steel. Therefore, in this invention, the mass percentage of Mn is controlled at 2.00% to 2.50% (including the endpoint values ​​of 2.00% and 2.50%).

[0048] Al: Al has a similar effect to Si in inhibiting cementite precipitation, promoting the enrichment of carbon into retained austenite during quenching and partitioning, inhibiting martensite transformation, and also playing a partial solid solution strengthening role. However, excessively high Al content will deteriorate the continuous casting performance of steel, resulting in a decrease in the number of consecutive castings, and a large amount of AlN inclusions can easily cause slab cracking defects. Therefore, in this invention, the mass percentage of Al is controlled at 0.20% to 0.80% (including the endpoint values ​​of 0.20% and 0.80%).

[0049] In some possible embodiments provided by the present invention, the microstructure of the cold-rolled steel sheet, by volume percentage, comprises 20-50% ferrite, 13-16% retained austenite, and 40-65% at least two types selected from martensite, partitioned martensite, and tempered martensite. That is, the microstructure of the cold-rolled steel sheet of the present invention comprises ferrite, retained austenite, martensite, and partitioned martensite; by volume percentage, ferrite comprises 20-50%, retained austenite comprises 13-16%, and martensite and partitioned martensite together comprise 40-65%. Alternatively, the microstructure of the cold-rolled steel sheet of the present invention comprises ferrite, retained austenite, martensite, and tempered martensite; by volume percentage, ferrite comprises 20-50%, retained austenite comprises 13-16%, and martensite and tempered martensite together comprise 40-65%. Alternatively, the microstructure of the cold-rolled steel sheet of the present invention comprises ferrite, retained austenite, partitioned martensite, and tempered martensite; by volume percentage, ferrite comprises 20-50%, retained austenite comprises 13-16%, and partitioned martensite and tempered martensite together comprise 40-65%. Alternatively, the microstructure of the cold-rolled steel sheet of the present invention comprises ferrite, retained austenite, martensite, partitioned martensite, and tempered martensite; by volume percentage, ferrite comprises 20-50%, retained austenite comprises 13-16%, and martensite, partitioned martensite, and tempered martensite together comprise 40-65%. Thus, by reasonably controlling the volume percentage of the microstructure composition in the cold-rolled steel sheet of the present invention, the tensile strength of the cold-rolled steel sheet of the present invention is ≥980MPa, and the elongation at break is ≥24%.

[0050] In some possible embodiments provided by the present invention, the Al and Si elements, by mass percentage, also satisfy the following: 1.00% ≤ Al + Si ≤ 1.80%. Thus, appropriate amounts of Al and Si can ensure a certain strengthening effect and promote the stabilization of residual austenite during the partitioning process, without causing deterioration of continuous casting performance or an increase in surface defects such as red iron scale and internal oxidation on the hot-rolled surface.

[0051] Furthermore, in the cold-rolled steel sheet of the present invention, unavoidable impurities, by mass percentage, include: P≤0.020%, S≤0.0120%, N≤0.008%, and B≤0.0005%.

[0052] In some embodiments of the present invention, P, S, and N are all unavoidable impurity elements in cold-rolled steel sheets. Excessive P content weakens grain boundaries, increases material brittleness, and deteriorates weldability. An excessively high mass percentage of N in the steel sheet makes steelmaking and continuous casting difficult and hinders inclusion control. Excessively high contents of P, S, and N impurity elements in the steel sheet significantly affect its performance. The main function of B is to improve the hardenability of the steel sheet. B tends to segregate at austenite grain boundaries, delaying the transformation of austenite to ferrite. Excessively high B content increases the strength of the steel sheet but deteriorates its plasticity.

[0053] In some possible embodiments provided by the present invention, the tensile strength of the cold-rolled steel sheet is ≥980MPa, the yield strength is 700~850MPa, the elongation at break is ≥24%, and the strength-ductility product is ≥25GPa·%.

[0054] Secondly, embodiments of the present invention disclose a method for manufacturing cold-rolled steel sheet, the method comprising the following steps:

[0055] Smelting and casting;

[0056] Hot rolling and coiling;

[0057] Insulation;

[0058] Pickling and cold rolling;

[0059] Continuous annealing: The heating temperature is (Ts-180)℃ to (Ts-100)℃, then rapidly cooled to 180~250℃, and then heated to Tp and held for 210~630s;

[0060] Where Ts represents the annealing temperature, which is Ac1 to 900℃, and Ac1 represents the temperature at which austenite begins to form; Tp represents the partitioning temperature, which is 350 to 430℃.

[0061] By employing the above technical solution, the temperature and time during the continuous annealing process are controlled. Specifically, the heating temperature is controlled to be (Ts-180)℃ to (Ts-100)℃, where Ts is Ac1~900℃, thereby ensuring the formation of ferrite and austenite. Further, rapid cooling is performed to 180~250℃, specifically at a rate of 50-500℃ / s. This yields a cold-rolled steel sheet with a microstructure of ferrite, martensite, and retained austenite. The faster cooling rate and lower rapid cooling temperature ensure higher hardness and martensite content, resulting in higher yield strength. Further, the partitioning temperature and time are controlled. The partitioning time refers to a holding time of 210~630s, allowing martensite to precipitate carbides, forming tempered or partitioned martensite with carbides. Simultaneously, carbon atoms in the martensite diffuse into the retained austenite, resulting in tempered or partitioned martensite containing an appropriate amount of carbides and retained austenite with a higher carbon content.

[0062] The inventors also considered that when the partitioning time is short, a large number of free dislocations generated during quenching remain inside the martensite. These dislocations can be activated under low loads, resulting in high work hardening rate and low yield strength in the steel plate. When the partitioning time is extended, dispersed carbides precipitate in the martensite laths. These carbides promote the decomposition of retained austenite and also increase the yield strength of the steel plate. When the partitioning time is further extended, the degree of decomposition of retained austenite intensifies, the content of stable retained austenite in the steel plate decreases, and the TRIP effect cannot be effectively exerted, resulting in a decrease in the fracture elongation of the steel plate. Therefore, in this invention, the partitioning time must be controlled within the above range to ensure that the retained austenite has a sufficient carbon content (C). RA The yield strength and fracture elongation are improved by utilizing the interaction of high dislocation density in martensite and the TRIP effect of retained austenite.

[0063] Therefore, this invention, through rational chemical composition design and optimized process, obtains a microstructure containing ferrite, retained austenite, and at least one of the following: distributed martensite and tempered martensite, wherein the carbon content of the retained austenite satisfies: C RA ≥(833-Tp-7400*W Mn -W Si The cold-rolled steel sheet of this invention, with a strength-ductility product of 24100*100%, possesses both high strength and high yield strength. Furthermore, the manufacturing process requires only one annealing step, simplifying the process, shortening the manufacturing flow, and reducing production costs. Among these, C... RA W represents the carbon content (wt%) of the retained austenite. Mn and W Si These represent the mass percentages (wt%) of Mn and Si elements in the cold-rolled steel sheet, respectively. Tp represents the distribution temperature during continuous annealing, which is 350–430℃.

[0064] In the hot rolling and coiling steps of this invention, the heating temperature for hot rolling is controlled at 1180–1300°C, the holding time is 60–240 min, and the final rolling temperature is 900–960°C. Specifically, controlling the heating temperature of the slab hot rolling at 1180–1300°C is to prevent excessively high heating temperatures from causing coarse microstructure and severe surface decarburization. Conversely, if the heating temperature of the slab is too low, the finishing rolling and coiling temperatures after high-pressure water descaling and initial rolling will be too low, resulting in excessive resistance to final rolling deformation. Accordingly, in some preferred embodiments, the heating temperature of the slab hot rolling can be controlled between 1210–1270°C.

[0065] The holding time is controlled to be 60–240 min. This is because if the holding time is too short, the internal temperature and microstructure of the slab cannot be uniform; if the holding time is too long, the grains of the slab will coarsen severely and the surface will be severely decarburized. Therefore, the holding time is controlled to be 60–240 min.

[0066] Furthermore, the inventors considered that excessively low final rolling temperatures result in high deformation resistance, which is detrimental to sheet shape control. Therefore, in this invention, the final rolling temperature is controlled at 900–960°C.

[0067] In some possible embodiments provided by this invention, the inventors also discovered that due to elements such as Si and Mn, internal oxidation and red iron scale can occur on the surface of the steel plate. To solve the above problems, the upper limit of the coiling temperature is controlled at 520°C. However, when the coiling temperature is low, the high strength of the hot-rolled coil leads to increased resistance to pickling uncoiling and cold rolling deformation. Therefore, this invention controls the coiling temperature to be between 400°C and 520°C.

[0068] In the heat preservation step of this invention, in order to further obtain uniform and suitable strength of the hot-rolled coil after pickling and cold rolling, the hot-rolled coil obtained by hot rolling and coiling of the slab is kept at (Tc-50)~550℃ for 60~240min after the coiling step, where Tc represents the coiling temperature, that is, Tc is 400~520℃, so that the hot-rolled coil is uniformly softened and the content of hard phases such as martensite and bainite is reduced.

[0069] Furthermore, in the pickling and cold rolling steps of the present invention, the pickling speed is controlled to be ≤200m / min, and / or the cold rolling reduction is controlled to be 30-70%.

[0070] The present invention will now be described in conjunction with more specific embodiments.

[0071] Examples 1-12 and Comparative Examples 1-3

[0072] The cold-rolled steel sheets in Examples 1-12 were obtained using the following steps:

[0073] (1) Smelting and casting: Smelting and casting are carried out in accordance with the chemical composition shown in Table 1 below to obtain slabs;

[0074] (2) Hot rolling and coiling: In the hot rolling step, the heating temperature of the slab is controlled at 1180~1300℃, the holding time is 60~240min, and the final rolling temperature is 900~960℃; the coiling temperature Tc is controlled at 400~520℃.

[0075] (3) Heat preservation: Keep the temperature at (Tc-50)~550℃ for 60~240min;

[0076] (4) Pickling and cold rolling: Pickling and cold rolling are carried out, and the pickling speed is controlled to be ≤200m / min, and the cold rolling reduction is controlled to be 30% to 70%;

[0077] (5) Continuous annealing: The heating temperature is (Ts-180)℃ to (Ts-100)℃. The temperature is held at Ts for 30 to 180s, and then slowly cooled to 650 to 750℃ at a rate of 1 to 10℃ / s. Then, the temperature is rapidly cooled to 180 to 250℃ at a rate of 50 to 500℃ / s and held for 10 to 90s. The temperature is then raised to Tp and held for 210 to 630s. Finally, the strip is cooled to room temperature.

[0078] Wherein, Ts represents the annealing temperature, which is Ac1 to 900℃, and Ac1 represents the temperature at which austenite begins to form (Ac1 is calculated using JmatPro thermodynamic software in this invention); Tp represents the partitioning temperature, which is 350 to 430℃.

[0079] Table 1 lists the mass percentage of each chemical element in the steel grades corresponding to the cold-rolled steel sheets of Examples 1-12 and the steel sheets of Comparative Examples 1-3.

[0080] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, N, and B)

[0081]

[0082] Tables 2-1 and 2-2 list the specific process parameters for the cold-rolled steel sheets of Examples 1-12 and the steel sheets of Comparative Examples 1-3.

[0083] Table 2-1

[0084]

[0085] Table 2-2

[0086]

[0087] Table 3 lists the performance test results of the cold-rolled steel sheets of Examples 1-12 and the steel sheets of Comparative Examples 1-3.

[0088] Table 3

[0089]

[0090] As shown in Table 3, the tensile strength of Examples 1-12 of the present invention is ≥980MPa, the yield strength is 700-850MPa, the elongation at break is ≥24%, and the strength-ductility product is ≥25GPa·%. Specifically, the tensile strength is 980-1100MPa, the elongation at break is 24-29%, and the strength-ductility product is 25-30GPa·%. Furthermore, the microstructure contains 20-50% ferrite, 40-65% martensite, at least two of the following: distributed martensite and tempered martensite, and 13-16% retained austenite. The carbon content in the retained austenite meets the C... RA ≥(833-Tp-7400*W Mn -W Si ) / 24100*100%.

[0091] also, Figure 1 This is a microstructure diagram of Embodiment 4 of the present invention, combined with... Figure 1 It can be seen that the microstructure of the cold-rolled steel sheet in Example 4 is ferrite + partitioned martensite + tempered martensite + retained austenite, where 1 represents ferrite, 2 represents tempered martensite, 3 represents retained austenite, and 4 represents partitioned martensite.

[0092] In summary, the chemical composition of the cold-rolled steel sheet of this invention is simple, mainly composed of C, Si, Mn, and Al. Through the combination of composition and preparation method, it achieves excellent properties such as tensile strength ≥980MPa, yield strength of 700-850MPa, elongation at break ≥24%, and strength-ductility product ≥24GPa·%. Its microstructure contains 20-50% ferrite, 40-65% martensite, at least two of the following: distributed martensite and tempered martensite, and at least 13% retained austenite, wherein the carbon content in the retained austenite meets the C... RA ≥(833-Tp-7400*W Mn -W Si ) / 24100*100%, where C RA W represents the carbon content (wt%) in the retained austenite. Mn and W Si Tp represents the mass percentage of Mn and Si in the steel, respectively, and Tp represents the partitioning temperature during continuous annealing (unit: °C).

[0093] Furthermore, the cold-rolled steel sheet of this invention can be completed on existing high-strength steel continuous annealing production lines, further improving the yield strength of ultra-high-strength steel. While possessing high strength-ductility product, it also has high tensile strength and elongation at break, enabling it to meet the requirements of more complex automotive safety and structural components, and achieve better lightweighting results.

[0094] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A cold-rolled steel sheet, characterized in that, The cold-rolled steel sheet comprises the following chemical elements by weight percentage: C: 0.190–0.250%, Si: 0.60–1.30%, Mn: 2.00–2.50%, Al: 0.20–0.80%, with the remainder being Fe and unavoidable impurities. The microstructure of the cold-rolled steel sheet includes ferrite, retained austenite, and at least one of the following: distributed martensite and tempered martensite. Wherein, the carbon content of the retained austenite satisfies: C RA ≥(833-Tp-7400*W Mn -W Si ) / 24100*100%, C RA W represents the carbon content (wt%) of the retained austenite. Mn and W Si These represent the mass percentages (wt%) of Mn and Si elements in the cold-rolled steel sheet, respectively. Tp represents the distribution temperature during continuous annealing, which is 350–430℃.

2. The cold-rolled steel sheet as described in claim 1, characterized in that, The microstructure of the cold-rolled steel sheet, by volume percentage, is 20-50% ferrite, 13-16% retained austenite, and 40-65% at least two of the following: martensite, partitioned martensite, and tempered martensite.

3. The cold-rolled steel sheet as described in claim 2, characterized in that, The elements Al and Si also satisfy the following condition: 1.00% ≤ Al + Si ≤ 1.80%.

4. The cold-rolled steel sheet as described in claim 2, characterized in that, The unavoidable impurities, by mass percentage, include: P ≤ 0.020%, S ≤ 0.0120%, N ≤ 0.008%, and B ≤ 0.0005%.

5. A cold-rolled steel sheet as described in any one of claims 2 to 4, characterized in that, The cold-rolled steel sheet has a tensile strength ≥980MPa, a yield strength of 700~850MPa, an elongation at break ≥24%, and a strength-ductility product ≥25GPa·%.

6. A method for manufacturing a cold-rolled steel sheet as described in any one of claims 1 to 5, characterized in that, The manufacturing method includes the following steps: Smelting and casting; Hot rolling and coiling; Insulation; Pickling and cold rolling; Continuous annealing: The heating temperature is (Ts-180)℃ to (Ts-100)℃, then rapidly cooled to 180~250℃, and then heated to Tp and held for 210~630s; Where Ts represents the annealing temperature, which is Ac1 to 900℃, and Ac1 represents the temperature at which austenite begins to form; Tp represents the partitioning temperature, which is 350 to 430℃.

7. The method for manufacturing a cold-rolled steel sheet as described in claim 6, characterized in that, In the hot rolling and coiling steps, the heating temperature of the hot rolling is 1180-1300℃, the holding time is 60-240 min, and the final rolling temperature is 900-960℃.

8. A method for manufacturing a cold-rolled steel sheet as described in claim 6, characterized in that, In the hot rolling and coiling steps, the coiling temperature is controlled to be 400–520°C.

9. A method for manufacturing a cold-rolled steel sheet as described in claim 8, characterized in that, In the heat preservation step, the temperature is controlled at (Tc-50)~550℃ for 60~240min, where Tc represents the winding temperature.

10. A method for manufacturing a cold-rolled steel sheet as described in claim 6, characterized in that, In the pickling and cold rolling steps, the pickling speed is controlled to be ≤200m / min, and / or the cold rolling reduction is controlled to be 30-70%.

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