High elongation and high hole-expandability cold-rolled steel sheet of 1300 MPa or higher grade and method for manufacturing the same

Abstract

The present invention discloses a cold-rolled steel sheet of 1300 MPa or higher grade with high elongation and high hole expansion property. The cold-rolled steel sheet contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: 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 contents of C and Mn satisfy C + Mn / 6 ≥ 0.52; provided that the microstructure of the cold-rolled steel sheet has nano-scale precipitates with an average diameter of less than 30 nm. The cold-rolled steel sheet has ultra-high strength and also has high elongation and high hole expansion property, and is excellent in formability.

Description

Technical Field

[0001] The present invention relates to steel materials and a manufacturing method thereof, and particularly to a cold-rolled steel sheet and a manufacturing method thereof.

Background Art

[0002] In recent years, with the intensification of the global energy crisis and environmental problems, "energy conservation" and "safety" have become the main development directions of the automobile manufacturing industry. As one of the important measures for energy conservation and emission reduction, when manufacturing automobiles, the weight of automobiles is reduced by lightweight design.

[0003] In recent years, the use of ultra-high strength steel in the automobile industry has been very common. Ultra-high strength steel has good mechanical properties and service performance, can be applied to the manufacture of automobile structural parts, realize the lightweight of parts, and effectively reduce the weight of automobiles.

[0004] There are many ultra-high strength steels for automobiles in the current automobile industry, usually including dual-phase steel, quenched and partitioned steel, bainite steel, complex-phase steel, etc. Among them, dual-phase steel and quenched and partitioned steel are excellent in strength and plasticity, but the hole expansion rate (about 20% - 35%) is much lower than that of conventional soft steel for automobiles; on the other hand, bainite steel and composite steel have a high hole expansion rate but too low elongation rate. Therefore, in order to meet the more diversified market requirements, it is necessary to develop an ultra-high strength cold-rolled steel sheet with high elongation rate and high hole expansion property.

[0005] Therefore, in view of the technical problems existing in the existing ultra-high strength steel, the present invention hopes to obtain a cold-rolled steel sheet of 1300 MPa or more grade with high elongation rate and high hole expansion property in order to obtain ultra-high strength while ensuring excellent formability.

[0006] In the current prior art, although some researchers have already developed ultra-high strength steel materials, none of these technical solutions can obtain a steel sheet like the present invention with corresponding high elongation rate and high hole expansion property.

[0007] For example, in a Chinese patent document with publication number CN104451436A, publication date March 25, 2015, and title "Bainite-Martensite-Austenite Multiphase Wear-Resistant Steel Sheet and Method for Manufacturing", a bainite-martensite-austenite multiphase wear-resistant steel sheet and method for manufacturing is disclosed, and its chemical composition, in weight percentage content, 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, the remainder being Fe and unavoidable impurity elements. From rolled material, a bainite-martensite-retaining austenite multiphase structure can be obtained, with a retained austenite volume fraction of 5-15%, a yield strength exceeding 1000 MPa, a tensile strength exceeding 1300 MPa, an elongation exceeding 15%, a hardness HB of 420-500, and machinability and welding performance meeting the requirements of equipment manufacturing; abrasive resistance is more than 1.3 times that of Hardox 450, and more than 1.5 times that of Hardox 450 under weakly acidic working environment conditions. In this technical solution, a sufficient amount of retained austenite is obtained by adding large amounts of Si and Al, and high elongation is obtained due to the TRIP effect of retained austenite, but the hole-expanding properties of the steel material are not considered.

[0008] Furthermore, for example, in a Chinese patent document with publication number CN102776438A, publication date November 14, 2012, and title "Niobium-Lanthanum Microalloid Mn-B System Ultra-High Strength Steel Sheet and its Heat Treatment Process," a niobium-lanthanum microalloid Mn-B system ultra-high strength steel sheet and its heat treatment process are disclosed. The chemical composition and content (by weight percentage) of the steel sheet 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 remainder being Fe and unavoidable impurities. In this technical solution, the heat treatment process employed involves an austenitization temperature of 880-940°C, followed by water granulation after a holding time of 0.5-5 hours, and a tempering temperature of 190-250°C with a holding time of 1-15 hours. The designed steel sheet in this patented technical solution has excellent mechanical properties, with a tensile strength of 1200-1400 MPa, a yield strength of 1000-1300 MPa, and an elongation of 6-15%, and is characterized by low production costs and the ability to industrially produce steel sheets of standard thicknesses of 5-25 mm.

[0009] Furthermore, for example, in a Chinese patent document with publication number CN102321841A, publication date January 18, 2012, and title "Steel for truck shoes with a tensile strength of 1300 MPa and a method for manufacturing the same," a steel for truck shoes with a tensile strength of 1300 MPa and a method for manufacturing the same are disclosed. 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. Designed according to this technical plan, this steel material has a tensile strength of 1340 MPa or more, a fracture elongation of less than 12%, and a "U" notch impact absorption energy exceeding 72 J, resulting in high strength, fewer quench cracks and internal cracks, and a long service life.

[0010] The ultra-high-strength steels disclosed in the two aforementioned patent documents, CN102776438A and CN102321841A, respectively, achieved good mechanical properties through the addition of microalloy elements such as niobium, lanthanum, nickel, cadmium, and copper. However, the performance of the final prepared steel sheets could not achieve the high elongation and high hole-expanding properties required by the present invention. [Overview of the project] [Means for solving the problem]

[0011] One objective of the present invention is to provide a cold-rolled steel sheet of 1300 MPa or higher with high elongation and high hole-expandability. This cold-rolled steel sheet of 1300 MPa or higher employs a rational chemical composition design and manufacturing process, possessing ultra-high strength while also having high elongation and high hole-expandability, excellent formability, and can be effectively used in the automotive sector, offering promising application prospects.

[0012] To achieve the above objective, the present invention contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages: Contains: 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%; Furthermore, the mass percentage content of C and Mn satisfies the condition C + Mn / 6 ≥ 0.52%; However, the microstructure provides a cold-rolled steel sheet with high elongation and high hole-expanding properties of 1300 MPa or higher, having nanoscale precipitates with an average diameter of less than 30 nm.

[0013] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the mass percentage content of each chemical element is: The composition is 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%; the remainder is Fe and other unavoidable impurities; Furthermore, the mass percentage content of C and Mn satisfies the condition C + Mn / 6 ≥ 0.52%; However, the microstructure of the cold-rolled steel sheet has nanoscale precipitates with an average diameter of less than 30 nm.

[0014] Furthermore, the microstructure of the cold-rolled steel sheet of 1300 MPa or higher according to the present invention has nanoscale precipitates with an average diameter of 15 to 28 nm.

[0015] In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the design principles for each chemical element are as follows.

[0016] C: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the addition of element C not only improves the strength of the steel material but also ensures the occurrence of martensitic transformation. According to the inventors' research, if the mass percentage content of element C in the steel is less than 0.15%, it affects the strength of the steel sheet and is unfavorable for the amount and stability of retained austenite formation. However, if the mass percentage content of element C in the steel exceeds 0.30%, the martensite hardness becomes too high, making it easy for the grain size to coarse, which is unfavorable for the formability of the steel sheet. Therefore, considering the effect of element C content on the performance of the steel material, the mass percentage content of element C in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention is controlled to 0.15% to 0.30%.

[0017] Si: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the Si element can exert a solid solution strengthening effect; therefore, in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the mass percentage content of the Si element is controlled to 0.3% to 0.5%.

[0018] Mn: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the addition of Mn not only improves the hardenability of the steel material but also effectively improves the strength of the steel sheet. The reason for setting the mass percentage content of Mn in the steel to 1.8% to 2.5% is that, in the present invention, a large amount of carbides are generated during hot rolling, resulting in insufficient carbon equivalent in the matrix structure. If the mass percentage content of Mn in the steel falls below 1.8%, the insufficient carbon equivalent results in insufficient hardenability of the manufactured steel material, preventing the formation of a sufficient amount of martensite during the annealing process, and resulting in insufficient strength of the steel sheet. However, if the mass percentage content of Mn in the steel exceeds 2.5%, the carbon equivalent increases significantly, negatively affecting both the weldability and delayed crack resistance of the steel material. Therefore, considering the influence of Mn element content on steel performance, the mass percentage content of Mn element in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention is controlled to 1.8% to 2.5%.

[0019] Al: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the addition of an appropriate amount of Al element can produce deoxygenation and grain refinement effects. Therefore, in order to exert the beneficial effects of Al element, the present invention controls the mass percentage content of Al element to 0.01% to 0.03%.

[0020] B: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, B is an element that significantly improves the hardenability of the steel. By adding element B, the formation of martensite is promoted, and the strength of martensitic steel can be ensured. However, if defects at the grain boundaries are filled, adding more B will cause the precipitation of the "boron phase" at the grain boundaries, which will conversely reduce the plasticity of the steel. Therefore, it is important to note that the content of element B in the steel should not be too high. According to the inventors' research, if the content of element B in the steel is less than 0.001%, the effect of element B cannot be effectively exerted, but if the content of element B in the steel is more than 0.003%, it adversely affects the plasticity of the steel. Therefore, considering the effect of element B content on steel performance, in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the mass percentage content of element B is controlled to 0.001 to 0.003%.

[0021] Ti: In the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the addition of Ti, a strong carbide-forming element, strongly suppresses the growth of austenite crystal grains at high temperatures, and the addition of Ti also contributes to the refinement of crystal grains. Therefore, in order to exert the beneficial effects of Ti, the present invention controls the mass percentage content of Ti to 0-0.05%. In some embodiments, the mass percentage content of Ti is controlled to 0.01-0.05%.

[0022] Furthermore, it is important to note that in order to increase the strength of the steel to over 1300 MPa, in this cold-rolled steel sheet of the 1300 MPa or higher class according to the present invention, the inventors control the mass percentage content of a single chemical element, and at the same time, further control the mass percentage content of C and Mn elements in the steel to satisfy C+Mn / 6 ≥ 0.52%. In some embodiments, the mass percentage content of C and Mn elements in the steel is controlled to satisfy 0.52% ≤ C+Mn / 6 ≤ 0.61%. In some embodiments, the mass percentage content of C and Mn elements in the steel is controlled to satisfy 0.55% ≤ C+Mn / 6 ≤ 0.605%.

[0023] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, among inevitable impurities, P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.006%.

[0024] In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the P element, S element, and N element are all impurity elements in the steel. When technically acceptable, in order to obtain steel materials with better performance and quality, the content of impurity elements in the steel should be reduced as much as possible. Therefore, unless specifically required, the content of the P element in the steel should be reduced as much as possible. Specifically, the mass percentage content of the P element is controlled to satisfy P ≤ 0.015%.

[0025] In addition, MnS formed by incorporating the impurity element S severely affects the formability of the steel material. Therefore, in the present invention, the mass percentage content of the S element in the steel is strictly controlled to satisfy S ≤ 0.003%. Also, since the impurity element N is likely to lead to cracking and bubble generation on the slab surface, in the present invention, the mass percentage content of the N element is controlled to satisfy N ≤ 0.006%.

[0026] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, its microstructure is retained austenite + fine blocky martensite + bainite + the above-mentioned nanoscale precipitates.

[0027] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the volume ratio of martensite is ≥ 55%, and the volume ratio of bainite is greater than 0 and < 15%.

[0028] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the volume ratio of martensite is 55 - 90%, preferably 70 - 86%.

[0029] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the volume ratio of bainite is 7 - 14%.

[0030] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the diameter of the martensite is 10 micrometers or less.

[0031] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher according to the present invention, the average diameter of the martensite is 5 to 9 micrometers.

[0032] In this invention, the composition designed for the steel is a C+Mn+B dominant compositional system, and by designing the combination of C, Mn, and B elements, it is possible to ensure that the volume phase ratio of martensite exceeds 55%. At the same time, by ensuring that the C-curve of bainite is shifted to the left and the C-curves of ferrite and pearlite are shifted to the right, a certain volume fraction of bainite is obtained in the final microstructure, and the volume phase ratio of bainite is ensured to be less than 15%.

[0033] It needs to be explained that, in the present invention, through a rational design of alloying elements and manufacturing processes, a microstructure is obtained consisting of retained austenite + fine massive martensite (the diameter of the massive martensite is 10 micrometers or less) + bainite + nanoscale precipitates, and the average diameter of the nanoscale precipitates is less than 30 nm. This structure determines the good elongation and hole expansion ratio of the cold-rolled steel sheet according to the present invention.

[0034] Furthermore, the cold-rolled steel sheet of 1300 MPa or higher according to the present invention satisfies the following performance requirements: When the tensile strength is 1300-1400 MPa, the elongation is greater than 10% and the hole expansion is greater than 40%; when the tensile strength is greater than 1400 MPa and less than or equal to 1500 MPa, the elongation is greater than 9% and the hole expansion is greater than 40%; and when the tensile strength is 1500 MPa or higher, the elongation is greater than 8% and the hole expansion is greater than 40%.

[0035] Accordingly, another objective of the present invention is to provide a method for manufacturing cold-rolled steel sheets of the 1300 MPa or higher class, and by optimizing the design of the manufacturing process, the cold-rolled steel sheets obtained by this manufacturing method have ultra-high strength, as well as good ductility and high hole-expanding properties.

[0036] To achieve the above objective, the method for manufacturing the above-mentioned cold-rolled steel sheet of 1300 MPa or higher grade provided by the present invention includes the following steps: (1) Smelting and casting; (2) Hot rolling; (3) Hot winding and annealing with heat retention bell: After winding, annealing is performed immediately with a heat retention bell, with an annealing time of 0.5 to 6 hours, and the temperature drop per hour is 6°C or less; (4) Cold rolling; (5) Annealing: The annealing soaking temperature is controlled to 830-860°C, the holding time is controlled to 40-80 s, then the material is cooled to 730-780°C at a cooling rate of 5-15°C / s, and then cooled to the isothermal holding temperature at a rate of 50-700°C / s; (6) Isothermal insulation treatment: The insulation temperature shall be 400-550°C and the insulation time shall be 100-300 seconds; (7) Cooling: Cool to room temperature at a rate of 30°C / s to 100°C / s; (8) Leveling.

[0037] In this technical solution designed according to the present invention, the inventors optimized the manufacturing process and improved its flow.

[0038] In this invention, one of the inventors' unique innovations is to anneal the steel immediately after hot winding using a heat-retaining bell. Based on a rational composition and process design, the process in step (3) allows for the generation of ε-carbides, which are fine, dispersed nanoscale precipitates, by heat-retaining annealing the steel at a low temperature for a long period of time. Furthermore, the rational process design allows these finely dispersed ε-carbides to be transferred to the final, continuously annealed steel sheet. These dispersed precipitates not only improve overall strength, reduce strength differences between phases, and reduce strength differences between grain boundaries and within grains, but they can also strengthen grain boundaries during the deformation process, thus playing a dual role in improving the strength of the steel and increasing the hole expansion rate.

[0039] Furthermore, in the above technical solution of the present invention, in the annealing step (5), the aim is to achieve somatic annealing at the total austenitization temperature, so the somatic annealing temperature is limited to 830-860°C. If the somatic annealing temperature used is lower than 830°C, sufficient tensile strength cannot be obtained through total austenitization, but if the somatic annealing temperature used is higher than 860°C, it leads to a significant decrease in the hole expansion rate of the steel material. Accordingly, in some preferred embodiments, while ensuring total austenitization, it is also ensured that the size of the obtained crystal grains does not become coarse, and finely dispersed nanoscale precipitates with an average size of less than 30 nm can be retained in the final microstructure, so the somatic annealing temperature is preferably controlled to 830-850°C.

[0040] Furthermore, the austenite isothermal heat retention process in step (6) designed according to the present invention is another unique innovation of this patent, in which the final morphology and size of martensite are determined by isothermal treatment controlled above the bainite transformation completion temperature after annealing. The process of this patent mainly includes soaking at the total austenitization temperature (i.e., soaking temperature of continuous annealing is 830-860°C) - rapid cooling (i.e., cooling to the isothermal heat retention temperature at a rate of 50-700°C / s) - heat retention in the bainite transformation region (i.e., isothermal heat retention treatment with a heat retention temperature of 400-550°C) - cooling at a controlled cooling rate. In the isothermal quenching process of austenite, by first acquiring a portion of bainite, it is possible to ensure that the martensite that is subsequently formed does not grow violently around the fine dispersed nuclei of bainite, ultimately forming fine, massive martensite.

[0041] In this cold-rolled steel sheet designed according to the present invention, the martensite in its final microstructure is fine, massive martensite with a diameter of 10 micrometers or less. Through a rational process design, the amount of bainite in the steel can be controlled to 15% or less, avoiding a severe impact on the strength of the steel. Accordingly, by rationally controlling the cooling rate in the subsequent stages of the process design, the formation of a martensitic structure with a volume phase ratio of ≥55% is ensured, while also ensuring that some of the austenite that did not fully transform after the martensitic transformation is retained as retained austenite. The fine martensite structure contributes to strength and elongation, and the retained austenite significantly improves elongation through the TRIP effect.

[0042] In the manufacturing method according to the present invention, the isothermal insulation temperature and isothermal insulation time for various specific components must be specifically set according to the dynamic CCT curve.

[0043] In step (6) designed according to the present invention, the isothermal heat retention treatment is controlled to maintain a temperature of 400-550°C and a heat retention time of 100-300 s. If the heat retention temperature is below 400°C or the heat retention time is below 100 s, it is unfavorable for the formation of bainite, and also unfavorable for localizing carbon in untransformed austenite to produce retained austenite. If the heat retention temperature is above 550°C or the heat retention time is above 300 s, it becomes impossible to avoid the coarsening of nanoscale precipitates formed by hot rolling.

[0044] Furthermore, in the manufacturing method according to the present invention, in step (2), first the material is heated to 1100-1250°C and kept warm for 0.5 hours or more (for example, 0.5-2 hours), then hot-rolled at a temperature of Ar3 or higher, and after rolling, it is rapidly cooled at a speed of 30-80°C / s to control the winding temperature to 150-250°C.

[0045] Furthermore, in the manufacturing method according to the present invention, the hot rolling temperature in step (2) is 920°C or lower.

[0046] Furthermore, in the manufacturing method according to the present invention, the winding temperature in step (2) is preferably 150 to 230°C.

[0047] Furthermore, in the manufacturing method according to the present invention, in step (3), the starting temperature for annealing with the heat-retaining bell is the same as the winding temperature, and the rate of temperature decrease is 2 to 6°C per hour.

[0048] Furthermore, in the manufacturing method according to the present invention, in step (4), the cold rolling reduction ratio is controlled to 50-70%.

[0049] Furthermore, in the manufacturing method according to the present invention, in step (5), the annealing soaking temperature is controlled to 830-850°C.

[0050] Furthermore, in the manufacturing method according to the present invention, in step (8), the leveling rate is controlled to 0-0.3%.

[0051] The cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention and its manufacturing method have the following advantages and beneficial effects compared to the prior art: This invention develops a novel cold-rolled steel sheet of 1300 MPa or higher grade and a method for manufacturing the same. By employing rational component matching and process design, it is possible to obtain a cold-rolled steel sheet of 1300 MPa or higher grade that combines high elongation and high hole-expanding properties.

[0052] The cold-rolled steel sheet with a tensile strength of 1300 MPa or higher possesses excellent mechanical properties. Its microstructure, consisting of retained austenite, fine massive martensite, bainite, and nanoscale precipitates, ensures excellent elongation, hole-expanding properties, and good formability. The performance of this cold-rolled steel sheet designed according to the present invention satisfies the following conditions: when the tensile strength is 1300-1400 MPa, the elongation is over 10% and the hole-expanding property is over 40%; when the tensile strength is over 1400 but ≤1500 MPa, the elongation is over 9% and the hole-expanding property is over 40%; and when the tensile strength is 1500 MPa or higher, the elongation is over 8% and the hole-expanding property is over 40%. This makes it effectively applicable to the automotive industry and gives it good potential for generalization and practical use. [Modes for carrying out the invention]

[0053] The following interpretation and explanation will be based on specific examples and will further describe the high elongation and high hole-expanding properties of cold-rolled steel sheets of 1300 MPa or higher designed according to the present invention, and the method for manufacturing the same. However, this interpretation and explanation will not unduly limit the technical solutions of the present invention. [Examples]

[0054] Examples 1-18 Table 1 shows the mass percentage ratios of each chemical element designed for the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18.

[0055] [Table 1]

[0056] The cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the present invention were all prepared by the following process: (1) Smelting and casting were carried out according to the chemical composition shown in Table 1 to obtain billets.

[0057] (2) Hot rolling: The obtained billets were first heated to 1100-1250°C and kept warm for 0.5 hours or more, then hot-rolled at a temperature of Ar3 or higher, and after rolling, rapidly cooled at a rate of 30-80°C / s until cooled to the winding temperature, and then wound up, with the winding temperature controlled to 150-250°C.

[0058] (3) Hot winding and annealing with heat retention bell: After winding, the steel coil was immediately annealed with a heat retention bell for 0.5 to 6 hours, utilizing the internal heat of the steel coil with the heat retention bell to keep the temperature drop below 6°C per hour. (4) Cold rolling: The cold rolling reduction ratio was controlled to 50-70%.

[0059] (5) Annealing: The annealing soaking temperature was controlled to 830-860°C, preferably 830-850°C, the holding time was controlled to 40-80 s, and then the material was cooled to 730-780°C at a cooling rate of 5-15°C / s, and then cooled to the isothermal holding temperature at a rate of 50-700°C / s.

[0060] (6) Isothermal heat retention treatment: After annealing, the steel plates were subjected to isothermal heat retention treatment, with the heat retention temperature controlled to 400-550°C and the heat retention time controlled to 100-300 s.

[0061] (7) Cooling: The steel plates after isothermal heat treatment were cooled to room temperature at a rate of 30°C / s to 100°C / s.

[0062] (8) Leveling: The leveling rate was controlled to 0-0.3%. The chemical elemental composition and related process designs of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the present invention all met the requirements of the design specifications of the present invention.

[0063] The specific process parameters for the cold-rolled steel sheets of 1300 MPa or higher grade used in Examples 1 to 18 are shown in Tables 2-1 and 2-2.

[0064] [Table 2-1]

[0065] [Table 2-2]

[0066] In this invention, samples were taken from the cold-rolled steel sheets of 1300 MPa or higher grade obtained through the above processes (1) to (8) in Examples 1 to 18. The microstructure of the steel sheets in each example was observed and analyzed, and it was found that the microstructure of the cold-rolled steel sheets in Examples 1 to 18 all contained retained austenite + fine massive martensite + bainite + nanoscale precipitates.

[0067] Furthermore, the inventors analyzed the volume phase ratio of each component in the microstructure of the 1300 MPa or higher cold-rolled steel sheets of Examples 1 to 18, and measured the diameters of martensite and nanoscale precipitates. The results of the analysis and measurement are shown in Table 3 below. In this text, the microstructure was observed using a ZEISS Axio Imager M2m optical microscope. In addition, the details of the nanoscale precipitates and microstructure were observed and analyzed using a spherical aberration-corrected field emission transmission electron microscope (TEM; model number JEOL ARM-200F) at a TEM operating acceleration voltage of 200 kV.

[0068] The results of the analysis and measurement of the microstructure of cold-rolled steel sheets of 1300 MPa or higher grade used in Examples 1 to 18 are shown in Table 3.

[0069] [Table 3]

[0070] As can be seen from the analysis and measurements, in the cold-rolled steel sheets of 1300 MPa or higher grade according to the present invention, the volume phase ratio of martensite in Examples 1 to 18 was 70 to 86%, the volume phase ratio of bainite was 7 to 14%, the diameter of martensite was 5.1 to 8.9 micrometers, and the average diameter of nanoscale precipitates was 15 to 28 nm.

[0071] After the observation and analysis described above were completed, samples were taken from the 1300 MPa or higher class cold-rolled steel sheets of Examples 1 to 18 of the finished products. Correlative mechanical performance tests were then performed on each cold-rolled steel sheet sample of each example to obtain its mechanical strength, elongation, and hole expansion ratio. The measured results of the obtained mechanical performance are shown in Table 4.

[0072] The methods used to measure the relevant mechanical properties were as follows: Tensile Test Measurement: Measurement tests were conducted according to GB / T 228 "Tensile Tests of Metallic Materials," Part 1: Room Temperature Test Method, and the yield strength, tensile strength, and elongation of cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 were measured and obtained.

[0073] Hole Expansion Rate Test: Measurement tests were conducted according to GB / T 24524-2021, a test method for hole expansion of thin sheets and strips of metal materials, to measure the hole expansion rate of cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18.

[0074] The measurement results of the mechanical performance of cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 are shown in Table 4.

[0075] [Table 4]

[0076] As shown in Table 4 above, the 1300 MPa or higher cold-rolled steel sheets according to Examples 1 to 18 designed by the present invention possess ultra-high strength while also having good ductility and hole-expanding properties.

[0077] As can be seen from Table 4, the cold-rolled steel sheets of 1300 MPa or higher grade prepared in Examples 1 to 18 had a yield strength of 1067 to 1292 MPa, a tensile strength of 1328 to 1552 MPa, an elongation of 8.5 to 12.3%, and a hole expansion ratio of 43 to 54%.

[0078] Furthermore, in these Examples 1 to 18 designed according to the present invention, when the tensile strength of the prepared steel plate was 1300 to 1400 MPa (i.e., Examples 4 to 6, Examples 10 to 12), the elongation was specifically 11.1 to 12.3% and the hole expansion rate was specifically 53 to 46%; when the tensile strength was greater than 1400 and less than or equal to 1500 MPa (i.e., Examples 1 to 3, Examples 16 to 18), the elongation was specifically 9.1 to 9.7% and the hole expansion rate was specifically 46 to 55%; and when the tensile strength was 1500 MPa or more (i.e., Examples 7 to 9, Examples 13 to 15), the elongation was specifically 8.5 to 9.7% and the hole expansion rate was specifically 47 to 50%.

[0079] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0080] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. The mass percentage content of each chemical element in the steel sheet is: The composition is 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%; the remainder is Fe and unavoidable impurities; Furthermore, the mass percentage content of C and Mn satisfies C + Mn / 6 ≥ 0.52%; However, its microstructure consists of ε-carbides, which are nanoscale precipitates with an average diameter of less than 30 nm, and it is a cold-rolled steel sheet with high elongation and high hole-expanding properties of 1300 MPa or higher.

2. The cold-rolled steel sheet of 1300 MPa or higher grade according to claim 1, characterized in that among the unavoidable impurities, P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.006%.

3. The cold-rolled steel sheet of 1300 MPa or higher according to claim 1, characterized in that its microstructure consists of retained austenite + fine, massive martensite + bainite + the aforementioned nanoscale precipitates.

4. The cold-rolled steel sheet of 1300 MPa or higher according to claim 3, characterized in that the volume phase ratio of martensite is ≥ 55% and the volume phase ratio of bainite is greater than 0 and < 15%.

5. The cold-rolled steel sheet of 1300 MPa or higher according to claim 4, characterized in that the volume phase ratio of martensite is 70 to 86% and the volume phase ratio of bainite is 7 to 14%.

6. The cold-rolled steel sheet of 1300 MPa or higher according to claim 3, characterized in that the diameter of the martensite is 10 micrometers or less.

7. The cold-rolled steel sheet of 1300 MPa or higher according to claim 6, characterized in that the average diameter of the martensite is 5 to 9 micrometers.

8. Its performance meets the following requirements: When the tensile strength is 1300-1400 MPa, the elongation is over 10% and the hole expansion is over 40%; If the tensile strength is greater than 1400 and less than or equal to 1500 MPa, the elongation is greater than 9% and the hole expansion rate is greater than 40%; When the tensile strength is 1500 MPa or higher, the elongation is greater than 8%, and the hole expansion rate is greater than 40%. A cold-rolled steel sheet of 1300 MPa or higher according to claim 1, characterized in that it is a cold-rolled steel sheet of 1300 MPa or higher.

9. A method for manufacturing a cold-rolled steel sheet of 1300 MPa or higher according to any one of claims 1 to 8, characterized by comprising the following steps. (1) Smelting and casting; (2) Hot rolling; (3) Hot winding and annealing with a heat-insulating bell: After winding, annealing is performed immediately with a heat-insulating bell, with an annealing time of 0.5 to 6 hours, and the temperature drop per hour is 6°C or less; (4) Cold rolling; (5) Annealing: The annealing soaking temperature is controlled to 830-860°C, the holding time is controlled to 40-80 s, then the material is cooled to 730-780°C at a cooling rate of 5-15°C / s, and then cooled to the isothermal holding temperature at a rate of 50-700°C / s; (6) Isothermal heat retention treatment: The heat retention temperature shall be 400 to 550°C, and the heat retention time shall be 100 to 300 seconds; (7) Cooling: Cool to room temperature at a rate of 30°C / s to 100°C / s; (8) Leveling.

10. The manufacturing method according to claim 9, characterized in that in step (2), the material is first heated to 1100 to 1250°C and kept warm for 0.5 hours or more, then hot-rolled at a temperature of Ar3 or higher, and after rolling, rapidly cooled at a speed of 30 to 80°C / s to control the winding temperature to 150 to 250°C.

11. The manufacturing method according to claim 10, characterized in that in step (2), the hot rolling temperature is 920°C or less and the winding temperature is 150 to 230°C.

12. The manufacturing method according to claim 9, characterized in that in step (4), the cold rolling reduction ratio is controlled to 50 to 70%.

13. The manufacturing method according to claim 9, characterized in that in step (5), the annealing soaking temperature is controlled to 830 to 850°C.

14. The manufacturing method according to claim 9, characterized in that in step (8), the leveling rate is controlled to 0 to 0.3%.

15. The manufacturing method according to claim 9, characterized in that in step (3), the starting temperature for annealing with the heat retention bell is the same as the winding temperature, and the rate of temperature decrease is 2 to 6°C per hour.

Images