A 1300mpa or more grade cold rolled steel sheet having high elongation and hole expandability and a method for manufacturing the same

Abstract

The application discloses a 1300MPa or above grade cold-rolled steel plate with high elongation and high hole expanding performance, which contains Fe and inevitable impurity elements, and also contains the following chemical elements with mass percentage as follows: C: 0.15%-0.30%, Si: 0.3%-0.5%, Mn: 1.8%-2.5%, Al: 0.01%-0.03%, B: 0.001-0.003%, Ti: 0-0.05%, and the mass percentage of C and Mn satisfies C+Mn / 6>=0.52; wherein the microstructure of the cold-rolled steel plate has nano precipitates with an average diameter less than 30nm. In addition, the application also discloses a manufacturing method of the above-mentioned 1300MPa or above grade cold-rolled steel plate, which comprises the following steps: (1) smelting and casting; (2) hot rolling; (3) hot coil heat preservation cover annealing; (4) cold rolling; (5) annealing; (6) isothermal heat preservation treatment; (7) cooling; (8) flattening.

Description

Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a cold-rolled steel sheet and a method for manufacturing the same. Background Technology

[0002] In recent years, with the escalation of the global energy crisis and environmental problems, "energy conservation" and "safety" have become the main development directions for the automotive manufacturing industry. Among these, adopting lightweight design to reduce vehicle weight during automobile manufacturing is one of the important measures for energy conservation and emission reduction.

[0003] In recent years, ultra-high strength steel has been widely used in the automotive industry. Ultra-high strength steel has good mechanical and performance properties, and it can be used to manufacture automotive structural parts and achieve lightweighting of parts, thereby effectively reducing vehicle weight.

[0004] In the current automotive industry, there are many varieties of ultra-high-strength steel used in automobiles, which typically include: dual-phase steel, quenched ductile steel, bainitic steel, and multiphase steel. Dual-phase steel and quenched ductile steel possess good strength and plasticity, but their porosity (approximately 20%-35%) is far lower than that of traditional mild steel used in automobiles. While bainitic steel and multiphase steel have higher porosity, their elongation is too low. Therefore, to meet increasingly diverse market demands, it is necessary to develop an ultra-high-strength cold-rolled steel sheet that combines high elongation and high porosity performance.

[0005] Therefore, in view of the technical problems existing in the current ultra-high strength steel, the present invention aims to obtain a cold-rolled steel sheet with high elongation and high hole expansion performance of 1300MPa and above, so as to obtain ultra-high strength while ensuring excellent formability.

[0006] In the current technology, although some researchers have developed ultra-high strength steel, none of these technical solutions can achieve the high elongation and high hole expansion performance of the steel plate of the present invention.

[0007] For example, Chinese patent document CN104451436A, published on March 25, 2015, entitled "Bainitic-Martensitic-Austenitic Dual-Phase Wear-Resistant Steel Plate and Manufacturing Method Thereof", discloses a bainitic-martensitic-austenitic dual-phase wear-resistant steel plate and its manufacturing method. Its chemical composition by weight percentage is: C: 0.20-0.40; Mn: 0.30-1.50; Si: 0.80-1.20; Cr: 0.60-1.00; Ni: 0.20-0.60; Mo: 0.20-0.40; Cu: 0.20-0.50; B: 0.0005-0.003; S≤0.010, P≤0.015, with the balance being Fe and unavoidable impurity elements. The rolled material yields a bainitic-martensite-retained austenite multiphase microstructure, with a retained austenite volume fraction of 5-15%. The material exhibits a yield strength greater than 1000 MPa, a tensile strength greater than 1300 MPa, an elongation greater than 15%, and a hardness of HB420-500. Its machinability and weldability meet equipment manufacturing requirements. Its abrasive wear resistance is more than 1.3 times that of Hardox 450, and more than 1.5 times that of Hardox 450 under weakly acidic conditions. This technical solution obtains sufficient retained austenite through the addition of high Si and Al content, and achieves high elongation through the TRIP effect of the retained austenite. However, it does not consider the hole-expanding properties of the steel, which differs from the design concept of this patent.

[0008] For example, Chinese patent document CN102776438A, published on November 14, 2012, entitled "A Niobium-Lantern Microalloyed Mn-B Series Ultra-High Strength Steel Plate and Its Heat Treatment Process", discloses a niobium-lanthanum microalloyed Mn-B series ultra-high strength steel plate and its heat treatment process. The chemical composition and content (weight percentage) of the steel plate are: C 0.14%-0.35%, Mn 1.5%-2.0%, Si 0.6%-1.0%, P≤0.015%, S≤0.002%, Nb 0.01%-0.06%, B 0.0005%-0.0040%, La 0.001%-0.5%, with the balance being Fe and unavoidable impurities. In this technical solution, the heat treatment process is as follows: austenitizing temperature of 880-940℃, holding time of 0.5-5 hours followed by water quenching; tempering temperature of 190-250℃, holding time of 1-15 hours. The steel plate designed using this patented technology possesses excellent mechanical properties, with a tensile strength of 1200-1400MPa, a yield strength of 1000-1300MPa, and an elongation of 6-15%. It also features low production cost and the ability to industrially produce steel plates with thicknesses ranging from 5-25mm.

[0009] For example, Chinese patent document CN102321841A, published on January 18, 2012, entitled "Steel for Track Plates with Tensile Strength Reaching 1300MPa and its Manufacturing Method Thereof," discloses a steel for track plates with a tensile strength of 1300MPa and its manufacturing method. Its chemical composition by weight percentage is C: 0.20-0.30%, Mn: 0.80-1.40%, Si: 0.15-0.35%, P: 0-0.015%, S: 0-0.016%, Cr: 0-0.30%, Ni: 0-0.25%, Cu: 0-0.30%, Ti: 0.01-0.02%, Al: 0.02-0.06%, B: 0.0005-0.0035%, with the remainder being Fe and unavoidable impurity elements. The steel designed by this technical solution has a tensile strength of over 1340MPa, an elongation at break of less than 12%, and an impact absorption energy of over 72J with a U-shaped notch. It has high strength, few quenching cracks and internal cracks, and a long service life.

[0010] The ultra-high strength steels disclosed in the aforementioned patent documents CN102776438A and CN102321841A achieve good mechanical properties by adding microalloying elements such as niobium, lanthanum, nickel, cadmium, and copper. However, the performance of the steel plates ultimately produced by these patents cannot meet the high elongation and high hole expansion performance indicators covered by this invention. Summary of the Invention

[0011] One of the objectives of this invention is to provide a cold-rolled steel sheet with a strength of 1300 MPa or higher that has high elongation and high hole expansion performance. This cold-rolled steel sheet with a strength of 1300 MPa or higher adopts a reasonable chemical composition design and manufacturing process. While having ultra-high strength, it also has high elongation and high hole expansion performance. Its formability is excellent, and it can be effectively applied in the automotive industry with a very broad application prospect.

[0012] To achieve the above objectives, the present invention provides a cold-rolled steel sheet with high elongation and high hole expansion performance, having a strength of 1300 MPa or higher, containing Fe and unavoidable impurity elements, and further containing the following chemical elements in the following mass percentages:

[0013] C: 0.15% ~ 0.30%, Si: 0.3% ~ 0.5%, Mn: 1.8% ~ 2.5%, Al: 0.01% ~ 0.03%, B: 0.001-0.003%; Ti: 0 ~ 0.05%;

[0014] Furthermore, the mass percentage content of C and Mn satisfies: C + Mn / 6 ≥ 0.52%;

[0015] The microstructure of the cold-rolled steel sheet has nanoprecipitates with an average diameter of less than 30 nm.

[0016] Furthermore, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, the mass percentage content of each chemical element is as follows:

[0017] C: 0.15%–0.30%, Si: 0.3%–0.5%, Mn: 1.8%–2.5%, Al: 0.01%–0.03%, B: 0.001–0.003%; Ti: 0–0.05%, balance being Fe and other unavoidable impurities;

[0018] Furthermore, the mass percentage content of C and Mn satisfies: C + Mn / 6 ≥ 0.52%;

[0019] The microstructure of the cold-rolled steel sheet has nanoprecipitates with an average diameter of less than 30 nm.

[0020] The design principles of each chemical element in the 1300MPa and above grade cold-rolled steel sheet described in this invention are as follows:

[0021] C: In the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, the addition of carbon (C) not only improves the strength of the steel but also ensures the occurrence of martensitic transformation. The inventors have found that when the mass percentage of C in the steel is below 0.15%, the strength of the steel sheet is affected, and it is detrimental to the formation and stability of retained austenite; while when the mass percentage of C in the steel is above 0.30%, it easily leads to excessively high martensite hardness and coarse grain size, which is detrimental to the formability of the steel sheet. Therefore, considering the influence of C content on the properties of the steel, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, the mass percentage of C is controlled between 0.15% and 0.30%.

[0022] Si: In the cold-rolled steel sheet with a strength of 1300MPa or above described in this invention, the element Si can play a solid solution strengthening role. In the cold-rolled steel sheet with a strength of 1300MPa or above described in this invention, the mass percentage content of the element Si is controlled between 0.3% and 0.5%.

[0023] Mn: In the cold-rolled steel sheet of 1300MPa and above described in this invention, the addition of Mn not only improves the hardenability of the steel but also effectively enhances its strength. The reason for selecting a Mn mass percentage of 1.8% to 2.5% in the steel is that: during hot rolling, a large amount of carbides are generated, resulting in insufficient carbon equivalent in the matrix structure. When the Mn mass percentage in the steel is below 1.8%, the insufficient carbon equivalent leads to insufficient hardenability of the prepared steel, preventing the formation of sufficient martensite during annealing, resulting in insufficient strength of the steel sheet. Conversely, when the Mn mass percentage in the steel is above 2.5%, the carbon equivalent increases significantly, negatively impacting the weldability and resistance to delayed cracking of the steel. Therefore, considering the influence of Mn content on steel properties, the Mn mass percentage in the cold-rolled steel sheet of 1300MPa and above described in this invention is controlled between 1.8% and 2.5%.

[0024] Al: In the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, adding an appropriate amount of Al can deoxidize and refine the grains. Therefore, to maximize the beneficial effects of Al, the mass percentage of Al is controlled between 0.01% and 0.03% in this invention.

[0025] B: In the 1300MPa and above grade cold-rolled steel sheet described in this invention, B is an element that can significantly improve the hardenability of steel. Adding B can promote martensite formation and ensure the strength of martensitic steel. However, it should be noted that the B content in the steel should not be too high. After the grain boundary defects are filled, if more B is added, the precipitation of the "boron phase" at the grain boundaries will actually reduce the plasticity of the steel. The inventors have found that when the B content in the steel is below 0.001%, the role of B cannot be effectively exerted, while when the B content in the steel is above 0.003%, it will have an adverse effect on the plasticity of the steel. Therefore, considering the influence of B content on the performance of steel, in the 1300MPa and above grade cold-rolled steel sheet described in this invention, the mass percentage of B is controlled between 0.001% and 0.003%.

[0026] Ti: In the cold-rolled steel sheet of 1300MPa and above described in this invention, the added strong carbide-forming element Ti exhibits a strong effect of inhibiting austenite grain growth at high temperatures. Simultaneously, the addition of Ti to the steel also helps to refine the grains. Therefore, to maximize the beneficial effects of Ti, the mass percentage of Ti is controlled between 0% and 0.05% in this invention.

[0027] In addition, it should be noted that, in order to ensure that the strength of the steel is greater than 1300MPa, in the cold-rolled steel sheet with a strength of 1300MPa or higher designed in this invention, the inventors, while controlling the mass percentage content of a single chemical element, further controlled the mass percentage content of C and Mn elements in the steel to meet the following condition: C+Mn / 6≥0.52%.

[0028] Furthermore, in the cold-rolled steel sheet of 1300MPa and above described in this invention, among the unavoidable impurities, P≤0.015%, S≤0.003%, and N≤0.006%.

[0029] In the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, phosphorus (P), sulfur (S), and nitrogen (N) are all impurity elements in the steel. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of these impurity elements should be reduced as much as possible. Therefore, unless otherwise specified, the content of phosphorus (P) in the steel should be reduced as much as possible, specifically controlling the mass percentage of P to ≤ 0.015%.

[0030] Furthermore, the MnS formed by the impurity element S can severely affect the formability of steel. Therefore, this invention requires strict control of the mass percentage of S in the steel to ensure that S ≤ 0.003%. Additionally, since the impurity element N easily causes cracks or bubbles on the slab surface, this invention controls the mass percentage of N to ensure that N ≤ 0.006%.

[0031] Furthermore, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, its microstructure consists of retained austenite + fine blocky martensite + bainite + the aforementioned nanoprecipitates.

[0032] Furthermore, in the cold-rolled steel sheet of 1300MPa and above described in this invention, the volume proportion of martensite is ≥55%, and the volume proportion of bainite is greater than 0 and less than 15%.

[0033] Furthermore, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, the diameter of the martensite is no greater than 10 micrometers.

[0034] In this invention, the steel is designed with a composition system mainly composed of C+Mn+B. Through the design of the combination of C, Mn, and B elements, the volume proportion of martensite is guaranteed to be greater than 55%. At the same time, the C-curve of bainite is shifted to the left, and the C-curves of ferrite and pearlite are shifted to the right, ensuring that a certain volume fraction of bainite is obtained in the final microstructure, and the volume proportion of bainite is less than 15%.

[0035] It should be noted that, in this invention, through the rational design of alloying elements and manufacturing processes, a microstructure of: retained austenite + fine blocky martensite (the diameter of the blocky martensite is no greater than 10 micrometers) + bainite + nanoprecipitates can be obtained, and the average diameter of the nanoprecipitates is less than 30 nm. This microstructure determines that the cold-rolled steel sheet of this invention has good elongation and porosity.

[0036] Furthermore, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, its properties satisfy the following:

[0037] When the tensile strength is 1300-1400 MPa, the elongation is greater than 10% and the porosity is greater than 40%; when the tensile strength is greater than 1400 and ≤1500 MPa, the elongation is greater than 9% and the porosity is greater than 40%; when the tensile strength is above 1500 MPa, the elongation is greater than 8% and the porosity is greater than 40%.

[0038] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned cold-rolled steel sheet with a strength of 1300 MPa or above, which optimizes the manufacturing process. The cold-rolled steel sheet with a strength of 1300 MPa or above obtained by this manufacturing method has not only ultra-high strength, but also good elongation and high hole expansion performance.

[0039] To achieve the above objectives, the present invention proposes a method for manufacturing cold-rolled steel sheets with a strength of 1300 MPa or higher, comprising the following steps:

[0040] (1) Smelting and casting;

[0041] (2) Hot rolling;

[0042] (3) Hot-rolled annealing with heat insulation cover: After winding, annealing is carried out quickly with heat insulation cover for 0.5-6 hours, with a temperature drop of less than 6℃ per hour;

[0043] (4) Cold rolling;

[0044] (5) Annealing: Control the annealing temperature to 830-860℃, hold for 40-80s, then cool to 730-780℃ at a cooling rate of 5-15℃ / s; then cool to the isothermal holding temperature at a rate of 50-700℃ / s.

[0045] (6) Isothermal insulation treatment: The insulation temperature is 400-550℃ and the insulation time is 100-300s;

[0046] (7) Cooling; Cool to room temperature at a rate of 30℃ / s-100℃ / s;

[0047] (8) Flat.

[0048] In this technical solution designed by the present invention, the inventors have optimized the manufacturing process and improved the process flow.

[0049] In this invention, the rapid annealing under a heat-insulating cover after hot rolling is one of the inventors' unique innovations. Based on a reasonable composition and process design, in step (3), the steel is annealed at a lower temperature for a longer period of time, which generates finely dispersed nano-precipitates ε-carbides. Then, through a reasonable process design, these finely dispersed ε-carbides can be further inherited into the final continuously annealed finished steel plate. These dispersed precipitates not only improve the overall strength, reduce the strength difference between phases, and reduce the strength difference between grain boundaries and within grains, but they can also strengthen grain boundaries during deformation, thereby playing a dual role in improving the strength and porosity of the steel.

[0050] Furthermore, in the above-mentioned technical solution of the present invention, the annealing temperature in step (5) is limited to between 830-860°C because the desired result is a fully austenitized homogenized annealing. When the annealing temperature is below 830°C, sufficient tensile strength cannot be obtained through complete austenitization; while when the annealing temperature is above 860°C, the porosity of the steel will decrease significantly. Accordingly, in some preferred embodiments, the annealing temperature can be preferably controlled between 830-850°C, which ensures both complete austenitization and prevents grain coarsening, thereby retaining fine, dispersed nanoprecipitates with an average size of less than 30 nm in the final microstructure.

[0051] Furthermore, the austenitic isothermal heat treatment process in step (6) of this invention is another unique innovation of this patent. After annealing, the isothermal treatment is controlled above the bainitic phase transformation end temperature. This process determines the final morphology and size of the martensite. The patented process mainly includes: homogenization at the full austenitization temperature (i.e., homogenization temperature of continuous annealing 830-860℃) - rapid cooling (i.e., cooling at a rate of 50-700℃ / s to the isothermal heat treatment temperature) - heat treatment in the bainitic phase transformation zone (i.e., isothermal heat treatment, heat treatment temperature of 400-550℃) - controlled cooling rate. Obtaining a portion of bainite during the austenitic isothermal quenching process ensures that the subsequently generated martensite does not grow violently around the fine, dispersed nucleation of bainite, thus ultimately forming fine, blocky martensite.

[0052] In the cold-rolled steel sheet designed in this invention, the martensite in the final microstructure is fine blocky martensite with a diameter of no more than 10 micrometers. Through reasonable process design, the bainite content in the steel can be controlled below 15%, thereby avoiding a significant impact on the strength of the steel. Correspondingly, through reasonable control of the cooling rate process design, it is necessary to ensure that a martensite microstructure with a volumetric proportion of ≥55% is generated, while also ensuring that some untransformed austenite is retained as retained austenite after the martensitic transformation. The fine martensite microstructure is beneficial to strength and elongation, while the retained austenite significantly improves elongation through the TRIP effect.

[0053] In the manufacturing method described in this invention, the isothermal holding temperature and isothermal holding time for each specific component need to be set specifically according to the dynamic CCT curve.

[0054] In step (6) of this invention, the isothermal heat treatment is controlled at a temperature of 400-550℃ and a holding time of 100-300s. When the heat treatment temperature is below 400℃ or the holding time is below 100s, it is not conducive to the formation of bainite, nor is it conducive to enriching untransformed austenite with carbon to form retained austenite. When the heat treatment temperature is above 550℃ or the holding time is above 300s, it cannot be guaranteed that the nanoprecipitates generated by hot rolling will not coarsen.

[0055] Furthermore, in the manufacturing method described in this invention, in step (2), the temperature is first heated to 1100-1250°C and held for more than 0.5 hours, and then hot rolled at a temperature of Ar3 or higher. After rolling, the temperature is rapidly cooled at a rate of 30-80°C / s, and the coiling temperature is controlled to be 150-250°C.

[0056] Furthermore, in the manufacturing method described in this invention, in step (4), the cold rolling reduction rate is controlled to be 50-70%.

[0057] Furthermore, in the manufacturing method described in this invention, in step (5), the annealing temperature is controlled to be 830-850°C.

[0058] Furthermore, in the manufacturing method described in this invention, in step (8), the flatness ratio is controlled to be 0-0.3%.

[0059] Compared with the prior art, the cold-rolled steel sheet with a strength of 1300MPa or higher and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0060] This invention develops a new cold-rolled steel sheet with a strength of 1300MPa and above and its manufacturing method. Through reasonable component matching and process design, a cold-rolled steel sheet with a strength of 1300MPa and above can be obtained, which has both high elongation and high hole expansion performance.

[0061] This cold-rolled steel sheet with a tensile strength of 1300 MPa or higher possesses excellent mechanical properties. Its microstructure, consisting of retained austenite, fine blocky martensite, bainite, and nanoprecipitates, ensures excellent elongation, hole expansion performance, and good formability. The cold-rolled steel sheet designed in this invention meets the following performance requirements: when the tensile strength is 1300-1400 MPa, the elongation is greater than 10% and the hole expansion rate is greater than 40%; when the tensile strength is greater than 1400 MPa and ≤1500 MPa, the elongation is greater than 9% and the hole expansion rate is greater than 40%; when the tensile strength is above 1500 MPa, the elongation is greater than 8% and the hole expansion rate is greater than 40%. It can be effectively applied in the automotive industry and has good prospects for promotion and application value. Detailed Implementation

[0062] The following will further explain and illustrate the high elongation and high hole expansion performance of the cold-rolled steel sheet with a strength of 1300MPa and above, and its manufacturing method, with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0063] Examples 1-18

[0064] Table 1 lists the mass percentage of each chemical element designed for the 1300MPa and above cold-rolled steel sheets of Examples 1-18.

[0065] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, and N)

[0066]

[0067]

[0068] The cold-rolled steel sheets with a strength of 1300 MPa or higher described in Examples 1-18 of this invention are all prepared using the following steps:

[0069] (1) Smelting and casting according to the chemical composition shown in Table 1 to obtain a billet.

[0070] (2) Hot rolling: For the obtained billet, first heat it to 1100-1250℃ and hold it for more than 0.5 hours. Then hot roll it at a temperature of Ar3 or higher. After rolling, cool it rapidly at a rate of 30-80℃ / s. After cooling to the coiling temperature, coil it and control the coiling temperature to 150-250℃.

[0071] (3) Hot-rolled insulation hood annealing: After winding, the insulation hood is used for annealing quickly, and the annealing time is controlled to be 0.5-6 hours. The insulation hood utilizes the internal heat of the steel coil, and the temperature drop is less than 6℃ per hour.

[0072] (4) Cold rolling: Control the cold rolling reduction rate to 50-70%.

[0073] (5) Annealing: control the annealing temperature to be 830-860℃, preferably 830-850℃, and control the holding time to be 40-80s. Then cool to 730-780℃ at a cooling rate of 5-15℃ / s; then cool to the isothermal holding temperature at a rate of 50-700℃ / s.

[0074] (6) Isothermal heat preservation treatment: The annealed steel plate is subjected to isothermal heat preservation treatment, and the heat preservation temperature is controlled at 400-550℃ and the heat preservation time is controlled at 100-300s.

[0075] (7) Cooling: Cool the steel plate after isothermal insulation treatment to room temperature at a rate of 30℃ / s-100℃ / s.

[0076] (8) Leveling: Control the leveling rate to 0-0.3%.

[0077] The chemical element composition and related process design of the cold-rolled steel sheets with a strength of 1300MPa or higher described in Examples 1-18 of this invention all meet the design specifications of this invention.

[0078] Tables 2-1 and 2-2 list the specific process parameters for the 1300MPa and above grade cold-rolled steel sheets of Examples 1-18 in the above process steps.

[0079] Table 2-1.

[0080]

[0081] Note: In Table 2-1 above, the hot rolling temperature used in Examples 1-18 is all greater than Ar3, and the Ar3 in each example is between 740-860℃ within the process range required by the present invention.

[0082] Table 2-2.

[0083]

[0084]

[0085] In this invention, samples of the finished cold-rolled steel plates of 1300MPa and above obtained by the above process steps (1)-(8) were taken and the microstructure of the steel plates of each example was observed and analyzed. It was found that the microstructure of the cold-rolled steel plates of Examples 1-18 all had: retained austenite + fine blocky martensite + bainite + nano precipitates.

[0086] In addition, the inventors further analyzed the volume ratio of each component in the microstructure of the cold-rolled steel sheets of 1300MPa and above in Examples 1-18, and detected the diameter of martensite and nanoprecipitates. The relevant analysis and detection results are listed in Table 3 below.

[0087] Table 3 lists the analysis and test results of the microstructure of cold-rolled steel sheets with a strength of 1300 MPa or higher for Examples 1-18.

[0088] Table 3.

[0089]

[0090]

[0091] Analysis and testing show that, in this invention, the volume ratio of martensite in the cold-rolled steel sheets of 1300MPa and above in Examples 1-18 is between 70-86%, the volume ratio of bainite is between 7-14%, the diameter of the martensite is between 5.1-8.9 micrometers, and the average diameter of the nanoprecipitates is between 15-28nm.

[0092] Accordingly, after completing the above observations and analyses, samples of the finished cold-rolled steel sheets of 1300MPa and above from Examples 1-18 can be taken, and relevant mechanical property tests are performed on the cold-rolled steel sheet samples of each example to obtain their mechanical strength, elongation and expansion rate. The results of the mechanical property tests are listed in Table 4.

[0093] The relevant mechanical property testing methods are as follows:

[0094] Tensile test: The test was conducted according to GB / T 228 (Metallic materials, tensile testing, Part 1: Room temperature test method) to test the yield strength, tensile strength and elongation of the cold-rolled steel sheets with a strength of 1300 MPa or above in Examples 1-18.

[0095] Hole expansion rate test: The hole expansion rate of the cold-rolled steel sheets with a strength of 1300MPa or higher in Examples 1-18 was tested according to GB / T 24524-2021 Test Method for Hole Expansion of Thin Plates and Strips of Metallic Materials.

[0096] Table 4 lists the mechanical property test results of cold-rolled steel with a strength of 1300 MPa or higher in Examples 1-18.

[0097] Table 4.

[0098]

[0099]

[0100] As shown in Table 4 above, the cold-rolled steel sheets of 1300MPa and above designed in Examples 1-18 of this invention have not only ultra-high strength, but also good elongation and hole expansion performance.

[0101] Referring to Table 4, it is easy to see that the yield strength of the cold-rolled steel sheets with a strength of 1300 MPa or above prepared in Examples 1-18 is between 1067-1292 MPa, the tensile strength is between 1328-1552 MPa, the elongation is between 8.5-12.3%, and the hole expansion rate is between 43-54%.

[0102] Furthermore, in the embodiments 1-18 designed in this invention, when the tensile strength of the prepared steel plate is 1300-1400 MPa (i.e., embodiments 4-6, embodiments 10-12), its elongation is specifically between 11.1-12.3%, and its hole expansion rate is specifically between 53-46%; when the tensile strength is greater than 1400 and ≤1500 MPa (i.e., embodiments 1-3, embodiments 16-18), its elongation is specifically between 9.1-9.7%, and its hole expansion rate is specifically between 46-55%; when the tensile strength is above 1500 MPa (i.e., embodiments 7-9, embodiments 13-15), its elongation is specifically between 8.5-9.7%, and its hole expansion rate is specifically between 47-50%.

[0103] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0104] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A cold-rolled steel sheet with high elongation and high hole-expanding performance, rated at 1300 MPa or higher, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.15%–0.30%, Si: 0.3%–0.5%, Mn: 1.8%–2.5%, Al: 0.01%–0.03%, B: 0.001–0.003%; Ti: 0–0.05%; balance Fe and other unavoidable impurities. Furthermore, the mass percentage content of C and Mn satisfies: C + Mn / 6 ≥ 0.52%; The microstructure of the cold-rolled steel sheet has nanoprecipitates with an average diameter of less than 30 nm. The performance of the cold-rolled steel sheet with a tensile strength of 1300MPa or higher meets the following requirements: when the tensile strength is 1300-1400MPa, the elongation is greater than 10% and the expansion rate is greater than 40%; when the tensile strength is greater than 1400MPa and ≤1500MPa, the elongation is greater than 9% and the expansion rate is greater than 40%; when the tensile strength is above 1500MPa, the elongation is greater than 8% and the expansion rate is greater than 40%.

2. The cold-rolled steel sheet with a strength of 1300 MPa or higher as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.006%.

3. The cold-rolled steel sheet with a strength of 1300 MPa or higher as described in claim 1, characterized in that, Its microstructure consists of retained austenite + fine blocky martensite + bainite + the aforementioned nanoprecipitates.

4. The cold-rolled steel sheet with a strength of 1300 MPa or higher as described in claim 3, characterized in that, The volume proportion of martensite is ≥55%, and the volume proportion of bainite is greater than 0 and less than 15%.

5. The cold-rolled steel sheet with a strength of 1300 MPa or higher as described in claim 3, characterized in that, The diameter of the martensite is no greater than 10 micrometers.

6. The method for manufacturing cold-rolled steel sheets with a strength of 1300 MPa or higher as described in any one of claims 1-5, characterized in that, It includes the following steps: (1) Smelting and casting; (2) Hot rolling; (3) Hot-rolled annealing with heat insulation cover: After winding, annealing is carried out quickly with heat insulation cover for 0.5-6 hours, with a temperature drop of less than 6℃ per hour; (4) Cold rolling; (5) Annealing: Control the annealing temperature to 830-860℃, hold for 40-80s, then cool to 730-780℃ at a cooling rate of 5-15℃ / s; then cool to the isothermal holding temperature at a rate of 50-700℃ / s. (6) Isothermal insulation treatment: The insulation temperature is 400-550℃ and the insulation time is 100-300s; (7) Cooling; Cool to room temperature at a rate of 30℃ / s-100℃ / s; (8) Flat.

7. The manufacturing method as described in claim 6, characterized in that, In step (2), the temperature is first heated to 1100-1250℃ and held for more than 0.5 hours. Then, it is hot rolled at a temperature of Ar3 or higher. After rolling, it is rapidly cooled at a rate of 30-80℃ / s, and the coiling temperature is controlled at 150-250℃.

8. The manufacturing method as described in claim 6, characterized in that, In step (4), the cold rolling reduction rate is controlled to be 50-70%.

9. The manufacturing method as described in claim 6, characterized in that, In step (5), the annealing temperature is controlled to be 830-850℃.

10. The manufacturing method as described in claim 6, characterized in that, In step (8), the flatness ratio is controlled to be 0-0.3%.

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