A 1300 mpa or more grade cold-rolled steel sheet and a method for manufacturing the same

Abstract

The application discloses a 1300MPa or more grade cold-rolled steel plate, which contains Fe and inevitable impurity elements, and also contains the following chemical elements with mass percentage as follows: C: 0.10%-0.30%, Si: 0.1%-0.5%, Mn: 0.8%-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>=0.35%. In addition, the application also discloses a manufacturing method of the above-mentioned 1300MPa or more grade cold-rolled steel plate, which comprises the following steps: (1) smelting and casting; (2) hot rolling; (3) cold rolling; (4) annealing; (5) continuous tempering temperature: the tempering temperature is 400-550 DEG C, the tempering time is 10-300s, and then the temperature is cooled to room temperature at a speed of 30 DEG C / s or more; (6) flattening; (7) discontinuous tempering: the tempering temperature is 180-260 DEG C, and the tempering time is 0.5-6h.

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 possesses excellent mechanical and performance properties, enabling the manufacture of automotive structural components and achieving lightweighting of parts, thereby effectively reducing vehicle weight. In the current automotive industry, the need for weight reduction and safety in vehicle manufacturing necessitates the use of higher strength steel plates. Among these, ultra-high strength steel with a tensile strength of 1000 MPa and above shows significant potential for weight reduction and safety performance improvements. It can be used in the manufacture of safety components, reinforcements, and structural parts, and has promising prospects for widespread application.

[0004] However, high-strength steel with a tensile strength of over 1000 MPa naturally exhibits stress corrosion cracking (delayed cracking). This type of high-strength steel plate is prone to slow cracking under stress and corrosive media. The resulting delayed cracking has caused considerable trouble for the application of high-strength steel and greatly limited the application of ultra-high-strength steel.

[0005] Delayed cracking refers to the phenomenon where a part does not crack during manufacturing, but over time, under the combined effects of stress and corrosive media, stress corrosion cracking occurs, ultimately leading to part failure and loss of its protective function. In this process, hydrogen plays a role in promoting crack initiation and propagation. Generally speaking, among currently available high-strength steels, the higher the strength of the steel, the more severe the tendency for delayed cracking. Delayed cracking is the greatest risk in the application of advanced high-strength steels.

[0006] While some researchers have developed ultra-high strength steel in the current technology, these technical solutions have not effectively solved the problem of delayed cracking in ultra-high strength steel.

[0007] For example, Chinese patent document CN102822375A, published on December 12, 2012, entitled "Ultra-high strength cold-rolled steel sheet and its manufacturing method," discloses an ultra-high strength cold-rolled steel sheet and its manufacturing method, with the following composition: C: 0.05-0.4%, Si≤2.0%, Mn: 1.0-3.0%, P≤0.05%, S≤0.02%, Al: 0.01-0.05%, N≤0.05%. In continuous annealing, this cold-rolled steel sheet needs to be cooled from Ac3 at a cooling rate of 20℃ / s or higher (gas cooling) to the range of Ms point - Ms point + 200℃, held for 0.1-60s, and then cooled to below 100℃ at a cooling rate of 100℃ / s or higher (water cooling) to obtain high-strength steel with a tensile strength of 1320MPa or higher, and a flatness of less than 10mm.

[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 features low production cost and can be industrially produced in thicknesses of 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] While the steel obtained in these three patent documents all possesses extremely high strength and good mechanical properties, none of these three technical solutions involve improving the delayed cracking resistance of ultra-high strength steel. Summary of the Invention

[0011] One objective of this invention is to provide a new type of cold-rolled steel sheet with a tensile strength of 1300 MPa or higher. This cold-rolled steel sheet employs a rational chemical composition design and manufacturing process, exhibiting not only ultra-high strength but also excellent resistance to delayed cracking and bending performance. Under a pre-stress greater than or equal to 1.05 times the tensile strength, this cold-rolled steel sheet can be immersed in 1 mol / L hydrochloric acid for over 300 hours without delayed cracking. It is particularly suitable for the manufacture of automotive safety structural components and has promising prospects for widespread application.

[0012] To achieve the above objectives, the present invention provides a cold-rolled steel sheet with a strength of 1300 MPa or higher, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0013] C: 0.10% ~ 0.30%, Si: 0.1% ~ 0.5%, Mn: 0.8% ~ 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 ≥ 0.35%.

[0015] 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:

[0016] C: 0.10%–0.30%, Si: 0.1%–0.5%, Mn: 0.8%–2.5%, Al: 0.01%–0.03%, B: 0.001–0.003%; Ti: 0–0.05%, balance being Fe and other unavoidable impurities;

[0017] Furthermore, the mass percentage content of C and Mn satisfies: C + Mn / 6 ≥ 0.35%.

[0018] The design principles of each chemical element in the 1300MPa and above grade cold-rolled steel sheet described in this invention are as follows:

[0019] 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 increases the hardness of martensite, ensuring the occurrence of martensitic phase transformation. The inventors have found that when the mass percentage of C in the steel is below 0.1%, the strength of the steel sheet is affected, and it is detrimental to the formation and stability of 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 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 0.10% and 0.30%.

[0020] Si: In the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, the addition of Si can improve the hardenability of the steel. Furthermore, Si dissolved in the steel can affect dislocation interactions, increase the work hardening rate, and appropriately increase the elongation, which is beneficial for obtaining better formability of the steel. Therefore, to maximize the beneficial effects of Si, the mass percentage of Si in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention is controlled between 0.1% and 0.5%.

[0021] 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 mass percentage of Mn in the steel is selected to be between 0.8% and 2.5% because: when the mass percentage of Mn in the steel is below 0.8%, the hardenability of the prepared steel is insufficient, and it cannot generate enough martensite during annealing, resulting in insufficient strength of the steel sheet; while when the mass percentage of Mn in the steel is above 2.5%, the carbon equivalent increases significantly, which negatively impacts the weldability and resistance to delayed cracking of the steel. Therefore, considering the influence of Mn content on the properties of the steel, the mass percentage of Mn in the cold-rolled steel sheet of 1300MPa and above described in this invention is controlled between 0.8% and 2.5%.

[0022] 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.

[0023] 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 increase the grain boundary energy potentials. At the same time, the "boron phase" will also act as the nucleus of the new phase, promoting the nucleation rate and causing the hardenability of the steel to decrease. Therefore, considering the influence of B content on the steel properties, 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%.

[0024] Ti: In the cold-rolled steel sheet with a strength of 1300 MPa or higher 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 in this invention is controlled between 0% and 0.05%.

[0025] 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≥0.35%.

[0026] 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%.

[0027] 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%.

[0028] Furthermore, MnS formed by the combination of impurity element S can severely affect the formability of steel. Therefore, in this invention, the mass percentage of S in the steel is strictly controlled to ensure that S ≤ 0.003%. Additionally, since impurity element N easily causes cracks or bubbles on the slab surface, the mass percentage of N in this invention is controlled to ensure that N ≤ 0.006%.

[0029] 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 tempered martensite, and bainite.

[0030] Furthermore, in the cold-rolled steel sheet of 1300MPa and above described in this invention, the volume proportion of tempered martensite is ≥55%, and the volume proportion of bainite is greater than 0 and less than 15%.

[0031] Furthermore, in the cold-rolled steel sheet of 1300MPa and above described in this invention, the diameter of the tempered martensite is not greater than 10 micrometers.

[0032] In this invention, the steel is designed with a composition system primarily composed of C, Mn, and B. Through the coordinated design of C, Mn, and B elements, the martensite volume fraction is guaranteed to be greater than 55%. Simultaneously, the C-curve for bainite is shifted to the left, while the C-curves for ferrite and pearlite are shifted to the right, ensuring that the final microstructure contains a certain volume fraction of bainite, with the bainite volume ratio being less than 15%.

[0033] It should be noted that, based on previous experience and research results, this invention, through the rational design of alloying elements and manufacturing processes, can ensure that cold-rolled steel sheets obtain a microstructure of retained austenite + fine blocky tempered martensite (the diameter of the blocky martensite is no greater than 10 micrometers) + bainite. The martensite, after tempering, exhibits reduced stress and hardness, and simultaneously generates fine, dispersed precipitates that can act as hydrogen traps; all of these factors contribute to improved delayed cracking performance. Furthermore, the acquisition of retained austenite not only delays cracking but also enhances the formability of the cold-rolled steel sheet.

[0034] Furthermore, in the cold-rolled steel sheet with a strength of 1300 MPa or higher described in this invention, its properties satisfy the following:

[0035] When the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤2.5; when the tensile strength is greater than 1400MPa and ≤1500MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤3; when the tensile strength is greater than 1500MPa and ≤1650MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤3.5; when the tensile strength is above 1650MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤4, where R represents the bending radius and t represents the plate thickness.

[0036] When the prestress is greater than or equal to 1.05 times the tensile strength, no delayed cracking occurs after immersion in 1 mol / L hydrochloric acid for more than 300 hours.

[0037] It should be noted that in this invention, the combination of retained austenite, fine blocky tempered martensite, and bainite determines the excellent formability of the cold-rolled steel sheet with a tensile strength of 1300 MPa or higher designed in this invention. In this cold-rolled steel sheet with a tensile strength of 1300-1400 MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤2.5 when the tensile strength is 1401-1500 MPa, ≤3 when the tensile strength is 1501-1650 MPa, ≤3.5 when the tensile strength is 1501-1650 MPa, and ≤4 when the tensile strength is above 1650 MPa.

[0038] Accordingly, another objective of the present invention is to provide a method for manufacturing cold-rolled steel sheets with a strength of 1300 MPa or higher, which optimizes the manufacturing process. The cold-rolled steel sheets produced by this method have ultra-high strength as well as excellent resistance to delayed cracking and bending 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) Cold rolling;

[0043] (4) Annealing;

[0044] (5) Continuous tempering temperature: tempering temperature is 400-550℃, tempering time is 10-300s, and then cooling to room temperature at a rate of more than 30℃ / s.

[0045] (6) Smooth;

[0046] (7) Discontinuous tempering: tempering temperature is 180-260℃, tempering time is 0.5-6h.

[0047] In the technical solution designed in this invention, the acquisition of bainite is one of the features of this invention. In the cooling process of continuous annealing in step (4), the steel can first obtain some bainite to ensure that the martensite generated later does not grow violently around the fine and dispersed nucleation of bainite, so as to finally form fine blocky martensite, which can eventually obtain fine blocky tempered martensite with a diameter of no more than 10 micrometers.

[0048] Furthermore, another feature of this invention is the optimized design of a double tempering process in the manufacturing method. After the first continuous tempering process is completed and leveling is finished, a discontinuous second tempering is further employed. The purpose of this design is to temper the martensitic structure while enriching the untransformed austenite with carbon, so that after cooling, a final structure of retained austenite + fine blocky tempered martensite + bainite is obtained.

[0049] In this invention, the tempering temperature is specifically controlled at 400-550℃ and the tempering time is controlled at 10-300s in the continuous tempering process of step (5) because this process determines the final morphology and size of the martensite. In this manufacturing method designed in this invention, the invention can ultimately obtain fine blocky tempered martensite with a diameter of no more than 10 micrometers. The tempering temperature and tempering time of each specific component need to be specifically set according to the dynamic CCT curve to ensure that a bainite ratio of less than 15% is obtained, which will not have a significant impact on the strength of the steel.

[0050] In addition, in the discontinuous tempering process of step (7), the specific tempering temperature is controlled at 180-260℃, and the tempering time is 0.5-6h. This process is a discontinuous low-temperature over-aging tempering process, which can be implemented using a bell-type furnace. Using this discontinuous tempering process, the martensitic structure can be tempered, while the untransformed austenite is enriched with carbon, so that after cooling, the final residual austenite + fine tempered martensite + bainite structure can be obtained. After tempering, the stress and hardness of the martensite decrease, and fine dispersed precipitates that can act as hydrogen traps are generated inside. All of these are factors that are beneficial to improving the delayed cracking performance. The acquisition of residual austenite is not only beneficial to delayed cracking, but also beneficial to improving the formability of the steel.

[0051] It should be noted that the discontinuous tempering process in step (7) also requires reasonable process design based on the specific composition. When the tempering temperature is too high and / or the tempering time is too long, it may cause the steel to sag or cause the material to have a severe yield plateau, affecting the stamping performance. When the tempering temperature is too low and / or the tempering time is too short, it is not possible to significantly temper the martensite, and sufficient residual austenite cannot be obtained, thus failing to improve the forming performance. Therefore, in order to ensure the performance of the steel, in this invention, the tempering temperature in the discontinuous tempering process is specifically controlled at 180-260℃ and the tempering time is 0.5-6h.

[0052] 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 0.3 hours, and then hot rolled at a temperature above Ar3 (austenite transformation temperature). After rolling, the temperature is rapidly cooled at a rate of 30-80°C / s, and the coiling temperature is controlled at 530-600°C.

[0053] Furthermore, in the manufacturing method described in this invention, in step (3), the cold rolling reduction rate is controlled to be 45-65%.

[0054] Furthermore, in the manufacturing method described in this invention, in step (4), the annealing temperature is controlled to be 830-870℃, the holding time is 30-150s, and then the temperature is cooled to between 730-780℃ at a cooling rate of 5-15℃ / s; and then cooled to the continuous tempering temperature at a rate of 50-700℃ / s.

[0055] In the above technical solution of the present invention, in the annealing step (4), the annealing temperature is limited to between 830-870℃ and the holding time is 30-150s because the desired result is a fully austenitizing temperature homogenization annealing. When the annealing temperature used in step (4) is lower than 830℃ and less than 30s, sufficient tensile strength cannot be obtained; while when the annealing temperature used is higher than 870℃ and greater than 150s, the forming performance of the steel will decrease significantly.

[0056] Accordingly, in some preferred embodiments, the annealing homogenization temperature can be preferably controlled between 850-860°C, which can ensure complete austenitization and prevent the obtained grain size from coarsening, thereby obtaining better forming performance.

[0057] Furthermore, in the manufacturing method described in this invention, in step (4), the annealing temperature is controlled to be 850-860°C.

[0058] Furthermore, in the manufacturing method described in this invention, in step (6), 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, it is possible to obtain cold-rolled steel sheets with a strength of 1300MPa and above that have low delayed cracking sensitivity and high bending performance.

[0061] This cold-rolled steel sheet with a strength of 1300MPa or higher has excellent resistance to delayed cracking. When the prestress is greater than or equal to 1.05 times the tensile strength, it can be immersed in hydrochloric acid with a concentration of 1mol / L for more than 300 hours without delayed cracking. Meanwhile, the microstructure of this cold-rolled steel sheet, consisting of retained austenite, fine blocky tempered martensite, and bainite, directly determines that the cold-rolled steel sheet designed in this invention has excellent formability. When the tensile strength of this cold-rolled steel sheet is 1300-1400 MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤2.5; when the tensile strength is greater than 1400 MPa and ≤1500 MPa, the limit value of R / t is ≤3; when the tensile strength is greater than 1500 MPa and ≤1650 MPa, the limit value of R / t is ≤3.5; and when the tensile strength is above 1650 MPa, the limit value of R / t is ≤, where R represents the bending radius and t represents the plate thickness.

[0062] In summary, the cold-rolled steel sheet designed in this invention possesses ultra-high strength, as well as excellent resistance to delayed cracking and bending forming performance. It can be effectively used to manufacture automotive parts and applied in the automotive industry, demonstrating promising prospects and application value. Detailed Implementation

[0063] The following will provide further explanation and description of the cold-rolled steel sheet with a strength of 1300MPa or higher and its manufacturing method according to the present invention, with reference to specific embodiments. However, such explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0064] Examples 1-18

[0065] Table 1 lists the mass percentage of each chemical element designed for the 1300MPa and above cold-rolled steel sheets of Examples 1-18.

[0066] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, and N)

[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 0.3 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 530-600℃.

[0071] (3) Cold rolling: Control the cold rolling reduction rate to 45-65%.

[0072] (4) Annealing: Control the annealing temperature to 830-870℃, preferably 850-860℃, and hold for 30-150s. Then cool to 730-780℃ at a cooling rate of 5-15℃ / s. Then cool to the continuous tempering temperature at a rate of 50-700℃ / s.

[0073] (5) Continuous tempering temperature: control the tempering temperature at 400-550℃, control the tempering time at 10-300s, and then cool to room temperature at a rate of 30℃ / s or higher.

[0074] (6) Leveling: Control the leveling rate to 0-0.3%.

[0075] (7) Discontinuous tempering: The flattened steel plate is subjected to discontinuous tempering, and the tempering temperature is controlled at 180-260℃, and the tempering time is 0.5-6h.

[0076] 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.

[0077] 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.

[0078] Table 2-1.

[0079]

[0080] 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 730-850℃ within the process range required by the present invention.

[0081] Table 2-2.

[0082]

[0083] In this invention, samples of the finished cold-rolled steel plates of 1300MPa and above obtained by the above process steps (1)-(7) were taken respectively, and the microstructure of the steel plates of each embodiment was observed and analyzed. It was found that the microstructure of the cold-rolled steel plates of each embodiment was retained austenite + fine blocky tempered martensite + bainite.

[0084] 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 the finished product examples 1-18, and tested the diameter of the tempered martensite. The relevant analysis and test results are listed in Table 3 below.

[0085] Table 3.

[0086]

[0087]

[0088] Analysis and testing show that, in this invention, the volume ratio of tempered martensite in the cold-rolled steel sheets of 1300MPa and above in Examples 1-18 is between 68% and 91%, the volume ratio of bainite is between 5% and 14%, and the diameter of the tempered martensite is between 4.3 and 8.7 micrometers.

[0089] Accordingly, after completing the above observations and analyses, samples of the finished cold-rolled steel sheets with a strength of 1300 MPa or higher 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 bending properties. The results of the mechanical property tests are listed in Table 4.

[0090] The relevant mechanical property testing methods are as follows:

[0091] 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.

[0092] Furthermore, the bending performance of the cold-rolled steel sheets in each embodiment is characterized by the limit value of the 90-degree cold bending performance parameter R / t, where the sheet thickness t is fixed, and the bending radius R that ensures no cracking during bending is variable. The limit value of R / t can be obtained when the bending radius R that ensures no cracking during bending is minimized. The larger the obtained limit value of the 90-degree cold bending performance parameter R / t, the worse the bending ability; the smaller the obtained 90-degree cold bending performance parameter R / t, the better the bending ability.

[0093] Table 4 lists the mechanical property test results of cold-rolled steel with a strength of 1300 MPa or higher in Examples 1-18.

[0094] Table 4.

[0095]

[0096] As shown in Table 4, the cold-rolled steel sheets of 1300MPa and above described in Examples 1-18 of the present invention possess not only ultra-high strength but also good cold bending deformation capability. Their yield strength is between 1128-1503MPa, their tensile strength is between 1321-1738MPa, and their elongation is between 6.1-10.1%. Furthermore, these Examples 1-18 show that when the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤2.5; when the tensile strength is 1401-1500MPa, the limit value of R / t is ≤3; when the tensile strength is 1501-1650MPa, the limit value of R / t is ≤3.5; and when the tensile strength is above 1650MPa, the limit value of R / t is ≤4. It can be seen that the cold-rolled steel sheets of these Examples 1-18 have not only ultra-high strength, but also excellent bending deformation performance.

[0097] Accordingly, the cold-rolled steel sheets of Examples 1-18 prepared by the present invention not only have the above-mentioned excellent mechanical properties, but also have excellent resistance to delayed cracking.

[0098] To verify the delayed cracking resistance of the cold-rolled steel sheets prepared in Examples 1-18, the inventors took samples of the steel sheets from each example again and controlled the cold-rolled steel sheets of each example to undergo an acid soaking test, that is, an evaluation was conducted by immersion in hydrochloric acid solution. The wire-cut samples were bent to 1.05, 1.1, 1.15, and 1.2 times the tensile strength, respectively, and then immersed in 0.1 mol / L HCl solution for 300 h without changing the solution. Before each solution change, the surface corrosion products were removed with a brush, and the test time was controlled to be 300 h.

[0099] In this invention, after completing the above-mentioned acid soaking experiment, the sample is observed. If the sample does not crack, it means that the resistance to delayed cracking under the stress condition is better and is marked as "OK"; if the sample cracks, it means that the resistance to delayed cracking under the stress condition is poor and is marked as "NG".

[0100] Table 5 lists the test results of the cold-rolled steel sheets of Examples 1-18 after acid soaking tests.

[0101] Table 5.

[0102]

[0103] As shown in Table 5 above, the cold-rolled steel sheets prepared in Examples 1-18 have excellent resistance to delayed cracking. All the steel sheets in the examples, when immersed in hydrochloric acid with a concentration of 1 mol / L for more than 300 hours under a prestress greater than or equal to 1.05 times the tensile strength, did not experience delayed cracking.

[0104] 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.

[0105] 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 a strength of 1300 MPa or higher, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.10%–0.30%, Si: 0.1%–0.5%, Mn: 0.8%–2.5%, Al: 0.01%–0.03%, B: 0.001–0.003%; Ti: 0–0.05%; balance Fe and unavoidable impurities. Furthermore, the mass percentage content of C and Mn satisfies: C + Mn / 6 ≥ 0.35%; The performance requirements of the cold-rolled steel sheets with a strength of 1300 MPa or higher are as follows: When the tensile strength is 1300-1400MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤2.5; when the tensile strength is greater than 1400MPa and ≤1500MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤3; when the tensile strength is greater than 1500MPa and ≤1650MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤3.5; when the tensile strength is above 1650MPa, the limit value of the 90-degree cold bending performance parameter R / t is ≤4, where R represents the bending radius and t represents the plate thickness. When the prestress is greater than or equal to 1.05 times the tensile strength, no delayed cracking occurs after immersion in 1 mol / L hydrochloric acid for more than 300 hours.

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 tempered martensite, and bainite.

4. The cold-rolled steel sheet with a strength of 1300 MPa or higher as described in claim 3, characterized in that, The volumetric proportion of tempered martensite is ≥55%, and the volumetric 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 tempered 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) Cold rolling; (4) Annealing; (5) Continuous tempering temperature: tempering temperature is 400-550℃, tempering time is 10-300s, and then cooling to room temperature at a rate of more than 30℃ / s. (6) Smooth; (7) Discontinuous tempering: tempering temperature is 180-260℃, tempering time is 0.5-6h.

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 0.3 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 530-600℃.

8. The manufacturing method as described in claim 6, characterized in that, In step (3), the cold rolling reduction rate is controlled to be 45-65%.

9. The manufacturing method as described in claim 6, characterized in that, In step (4), the annealing temperature is controlled at 830-870℃, the holding time is 30-150s, and then the temperature is cooled to 730-780℃ at a cooling rate of 5-15℃ / s; then the temperature is cooled to the continuous tempering temperature at a rate of 50-700℃ / s.

10. The manufacturing method as described in claim 9, characterized in that, In step (4), the annealing temperature is controlled to be 850-860℃.

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

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