High elongation rate and high hole expansion property cold rolled steel sheet of 1300 MPa or more grade and method for producing 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 method for manufacturing the same, and particularly to a cold-rolled steel sheet and a method for manufacturing the same.

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 exhaust gas reduction, when manufacturing automobiles, the weight of automobiles is reduced by means of 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 the elongation rate is too low. 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 and high hole expandability.

[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 and high hole expandability 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 having corresponding high elongation and high hole expandability.

[0007] For example, in a Chinese patent document with publication number CN104451436A, publication date of March 25, 2015, and title "Bainite-Martensite-Austenite Duplex Wear-Resistant Steel Plate and Manufacturing Method", a bainite-martensite-austenite duplex wear-resistant steel plate and manufacturing method are disclosed. 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, and the balance is Fe and inevitable impurity elements. From the rolled material, a bainite-martensite-retained austenite duplex structure can be obtained, with the volume fraction of retained austenite being 5 - 15%, the yield strength of the material exceeding 1000 MPa, the tensile strength exceeding 1300 MPa, the elongation rate exceeding 15%, and the hardness HB being 420 - 500, and the machining performance and welding performance meeting the requirements of equipment manufacturing; the wear resistance against abrasive wear is more than 1.3 times that of Hardox450, and more than 1.5 times that of Hardox450 under weak acidic working environment conditions. In this technical solution, by adding a large amount of Si and Al, a sufficient amount of retained austenite is obtained, and a high elongation rate is obtained by the TRIP effect of the retained austenite, but the hole expansion property of the steel is not considered.

[0008] Also, for example, in a Chinese patent document with publication number CN102776438A, publication date November 14, 2012, and title "Niobium-lanthanum microalloyed Mn-B series ultra-high strength steel plate and its heat treatment process", a niobium-lanthanum microalloyed Mn-B series ultra-high strength steel plate and its heat treatment process are disclosed. 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%, and the balance is Fe and inevitable impurities. In this technical solution, the adopted heat treatment process has an austenitizing temperature of 880 - 940°C, is water quenched after holding for 0.5 - 5 hours, has a tempering temperature of 190 - 250°C, and a holding time of 1 - 15 hours. In the technical solution of this patent, the designed steel plate has excellent mechanical properties, with a tensile strength reaching 1200 - 1400 MPa, a yield strength of 1000 - 1300 MPa, an elongation of 6 - 15%, low production costs, and the characteristic of enabling industrial production of steel plates with a thickness specification 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 its manufacturing method", a steel for truck shoes with a tensile strength of 1300 MPa and its manufacturing method 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%, and the balance is Fe and inevitable impurity elements. This steel material designed according to this technical solution has a tensile strength of 1340 MPa or more, an elongation at break of less than 12%, a "U" notch impact absorption energy exceeding 72 J, high strength, few burning cracks and internal cracks, and a long service life.

[0010] The ultra-high strength steels disclosed in the above two patent documents CN102776438A and CN102321841A respectively obtained good mechanical properties by adding micro-alloying elements such as niobium, lanthanum, nickel, cadmium, and copper. However, the performance of the finally prepared steel plates cannot achieve the indicators covering high elongation rate and high hole expansion property as in the present invention.

Summary of the Invention

Means for Solving the Problems

[0011] One object of the present invention is to provide a cold-rolled steel plate of 1300 MPa or higher grade with high elongation rate and high hole expansion property. The cold-rolled steel plate of 1300 MPa or higher grade adopts a reasonable chemical composition design and manufacturing process, has ultra-high strength, and also has high elongation rate and high hole expansion property, excellent formability, can be effectively used in the automotive field, and has good application prospects.

[0012] In order to achieve the above object, the present invention 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%; However, its microstructure is a cold-rolled steel plate of 1300 MPa or higher grade with high elongation rate and high hole expansion property, having nano-scale precipitates with an average diameter of less than 30 nm.

[0013] Furthermore, in the cold-rolled steel plate of 1300 MPa or higher grade according to the present invention, the mass percentage contents of its respective chemical elements are: 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 balance is Fe and other inevitable impurities; Moreover, the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.52%; However, the microstructure of the cold-rolled steel sheet has nano-scale 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 grade according to the present invention has nano-scale precipitates with an average diameter of 15 to 28 nm.

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

[0016] C: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the addition of C element can not only improve the strength of the steel material, but also ensure the occurrence of martensite transformation. According to the research of the present inventors, when the mass percentage content of C element in the steel is less than 0.15%, it affects the strength of the steel sheet and is disadvantageous to the generation amount and stability of retained austenite. However, when the mass percentage content of C element in the steel exceeds 0.30%, the martensite hardness becomes too high and the crystal grain size is likely to coarsen, which is disadvantageous to the formability of the steel sheet. Therefore, considering the influence of the C element content on the steel material performance, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the mass percentage content of C element is controlled to be 0.15% to 0.30%.

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

[0018] Mn: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the addition of Mn element can not only improve the hardenability of the steel material, but also effectively improve the strength of the steel sheet. Also, the reason for setting the mass percentage content of Mn in the steel to 1.8% - 2.5% is that in the present invention, a large amount of carbides are generated during hot rolling, resulting in a shortage of the carbon equivalent in the matrix structure. When the mass percentage content of Mn in the steel is less than 1.8%, due to the shortage of the carbon equivalent, the hardenability of the produced steel material becomes insufficient, and a sufficient amount of martensite cannot be generated during the annealing process, resulting in insufficient strength of the steel sheet. However, when the mass percentage content of Mn element in the steel exceeds 2.5%, the carbon equivalent increases significantly, which has an adverse effect on both the welding performance and the delayed cracking resistance of the steel material. Therefore, considering the influence of the Mn element content on the steel material performance, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the mass percentage content of Mn element is controlled to 1.8% - 2.5%.

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

[0020] B: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, B is an element that significantly improves the hardenability of the steel material. By adding the B element, the formation of martensite can be promoted, and the strength of the martensite steel can be ensured. However, when the defects at the grain boundaries are filled, if more B is added, due to the precipitation of the "boron phase" at the grain boundaries, the plasticity of the steel material will conversely decrease. Therefore, it is necessary to pay attention that the content of the B element in the steel should not be too high. According to the research of the present inventors, when the content of the B element in the steel is less than 0.001%, the effect of the B element cannot be effectively exerted. However, when the content of the B element in the steel exceeds 0.003%, it will have an adverse effect on the plasticity of the steel material. Therefore, considering the influence of the B element content on the steel material performance, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the mass percentage content of the B element is controlled to be 0.001 - 0.003%.

[0021] Ti: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, by adding Ti, which is a strong carbide-forming element, the effect of suppressing the growth of austenite grain crystals is strongly manifested at high temperatures. In addition, the addition of the Ti element also contributes to the refinement of the grain crystals. Therefore, in order to exert the beneficial effect of the Ti element, in the present invention, the mass percentage content of the Ti element is controlled to be 0 - 0.05%. In some embodiments, the mass percentage content of the Ti element is controlled to be 0.01 - 0.05%.

[0022] In addition, it should be noted that in order to make the strength of the steel exceed 1300 MPa, in this cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the present 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 so as 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 has a severe impact on 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 aforementioned nanoscale precipitates.

[0027] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the volume fraction ratio of martensite is ≥ 55%, and the volume fraction 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 fraction 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 fraction ratio of bainite is 7 - 14%.

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

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

[0032] In the present invention, the designed composition for the steel according to the present invention is a composition system in which C + Mn + B is dominant. By combining the C, Mn, and B elements for design, it is possible to ensure that the volume fraction 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 can be obtained in the final microstructure, and it is ensured that the volume fraction ratio of bainite is less than 15%.

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

[0034] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, its performance satisfies the following: When the tensile strength is 1300 - 1400 MPa, the elongation rate exceeds 10% and the hole expansion rate exceeds 40%; when the tensile strength exceeds 1400 and ≤ 1500 MPa, the elongation rate exceeds 9% and the hole expansion rate exceeds 40%; when the tensile strength is 1500 MPa or higher, the elongation rate exceeds 8% and the hole expansion rate exceeds 40%.

[0035] Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned cold-rolled steel sheet of 1300 MPa or higher grade. By optimizing the design of the manufacturing process, the cold-rolled steel sheet obtained by this manufacturing method has excellent elongation and high hole expansion property while having ultra-high strength.

[0036] To achieve the above object, 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) Annealing by hot coiling and heat preservation bell: After coiling, immediately anneal with a heat preservation bell, set the annealing time to 0.5 - 6 hours, and the temperature drop per hour to 6°C or less; (4) Cold rolling; (5) Annealing: Control the annealing soaking temperature at 830 - 860°C, control the soaking time at 40 - 80 s, then cool to 730 - 780°C at a cooling rate of 5 - 15°C / s, and then cool to the isothermal holding temperature at a rate of 50 - 700°C / s; (6) Isothermal holding treatment: Set the holding temperature at 400 - 550°C and the holding time at 100 - 300 s; (7) Cooling: Cool to room temperature at a rate of 30°C / s - 100°C / s; (8) Leveling.

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

[0038] In the present invention, promptly annealing with a heat preservation bell after hot coiling is one of the unique innovations of the inventors. Based on a reasonable composition and process design, in the process of step (3), by holding and annealing the steel material at a low temperature for a long time, ε-carbide, which is a nano-scale precipitate that is fine and dispersedly distributed, can be generated. Moreover, through a reasonable process design, the finely dispersed ε-carbide can also be inherited to the final continuously annealed finished product, i.e., the steel plate. The carbides precipitated in such a dispersed manner can not only improve the overall strength, reduce the strength difference between each phase, and reduce the strength difference between the grain boundary and the grain interior, but also strengthen the grain boundary during the deformation process, playing a dual role in improving the strength of the steel material and the hole expansion rate.

[0039] In addition, in the annealing process of step (5) in the above technical solution of the present invention, in order to achieve soaking annealing at the full austenitization temperature, the annealing soaking temperature is limited to 830 - 860°C. When the adopted annealing soaking temperature is lower than 830°C, sufficient tensile strength cannot be obtained by full austenitization. However, when the adopted annealing soaking temperature exceeds 860°C, it will lead to a significant and substantial decrease in the hole expansion rate of the steel material. Correspondingly, in some preferred embodiments, while ensuring full austenitization, it is also ensured that the size of the obtained crystal grains does not coarsen, and the final microstructure can retain finely dispersed nano-scale precipitates with an average size of less than 30 nm. Therefore, the annealing soaking temperature is preferably controlled at 830 - 850°C.

[0040] Furthermore, the austenite isothermal holding process in step (6) designed according to the present invention is another unique innovation of this patent. In this process, after annealing, isothermal treatment is carried out by controlling the temperature above the bainite transformation finishing temperature, so as to determine the final morphology and size of martensite. The process of this patent mainly includes soaking at the full austenitizing temperature (i.e., the soaking temperature of continuous annealing is 830 - 860°C) - rapid cooling (i.e., cooling to the isothermal holding temperature at a rate of 50 - 700°C / s) - holding in the bainite transformation region (i.e., isothermal holding treatment with the holding temperature of 400 - 550°C) - cooling at a controlled cooling rate. During the isothermal quenching process of austenite, by first obtaining a part of bainite, it can be ensured that the martensite generated subsequently does not grow violently around the fine dispersed nuclei of bainite, and finally fine blocky martensite is formed.

[0041] In this cold-rolled steel sheet designed according to the present invention, the martensite in its final microstructure is fine blocky martensite with a diameter of 10 micrometers or less. Through reasonable process design, the bainite in the steel can be controlled to be 15% or less, avoiding a severe impact on the strength of the steel. Correspondingly, through the process design of reasonably controlling the cooling rate in the subsequent stage, while ensuring the formation of a martensite structure with a volume ratio ≥ 55%, it is also ensured that a part of the austenite that could not be completely transformed after the martensite transformation is retained as retained austenite. The fine martensite structure contributes to strength and elongation, and the retained austenite significantly improves the elongation rate due to the TRIP effect.

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

[0043] In the process (6) designed according to the present invention, in the isothermal heat preservation treatment, the heat preservation temperature is controlled to be 400 to 550 °C, and the heat preservation time is controlled to be 100 to 300 s. If the heat preservation temperature is lower than 400 °C, or if the heat preservation time is shorter than 100 s, it is disadvantageous for the formation of bainite and also disadvantageous for localizing carbon in the untransformed austenite to generate retained austenite. If the heat preservation temperature exceeds 550 °C, or if the heat preservation time exceeds 300 s, coarsening of the nanoscale precipitates generated by hot rolling cannot be avoided.

[0044] Furthermore, in the manufacturing method according to the present invention, in step (2), first, it is heated to 1100 to 1250 °C and held for 0.5 hours or more (for example, 0.5 to 2 h), then hot rolled at a temperature above Ar3, and after rolling, rapidly cooled at a rate of 30 to 80 °C / s, and the coiling temperature is controlled to be 150 to 250 °C.

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

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

[0047] Furthermore, in the manufacturing method according to the present invention, in step (3), the starting temperature of annealing by the holding bell is the same as the coiling temperature, and the temperature drop rate 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 rate is controlled to be 50 to 70%.

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

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

[0051] The cold-rolled steel sheet of 1300 MPa or higher grade and its manufacturing method according to the present invention have the following advantages and beneficial effects compared with the prior art: The present invention develops a novel cold-rolled steel sheet of 1300 MPa or higher grade and its manufacturing method, and by adopting reasonable component matching and process design, a cold-rolled steel sheet of 1300 MPa or higher grade with both high elongation and high hole expansion property can be obtained.

[0052] The cold-rolled steel sheet of 1300 MPa or higher grade has sufficiently excellent mechanical properties, and its microstructure of retained austenite + fine blocky martensite + bainite + nanoscale precipitates can ensure excellent elongation, hole expansion property and good formability of the steel sheet. The performance of the cold-rolled steel sheet designed according to the present invention satisfies that when the tensile strength is 1300 - 1400 MPa, the elongation is over 10% and the hole expansion rate is over 40%; when the tensile strength is over 1400 and ≤ 1500 MPa, the elongation is over 9% and the hole expansion rate is over 40%; when the tensile strength is 1500 MPa or higher, the elongation is over 8% and the hole expansion rate is over 40%, and it can be effectively applied to the automobile industry, having good prospects for generalization and practical value.

Mode for Carrying Out the Invention

[0053] Hereinafter, based on specific examples, the high-elongation and high-hole expansion property cold-rolled steel sheet of 1300 MPa or higher grade and its manufacturing method designed according to the present invention will be further interpreted and explained, but the interpretation and explanation do not unduly limit the technical solution of the present invention.

Example

[0054] Examples 1 - 18 The mass percentage ratios of each chemical element designed in the cold-rolled steel sheet of 1300 MPa or higher grade according to Examples 1 - 18 are shown in Table 1.

[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 steps: (1) Smelting and casting were carried out according to the chemical composition shown in Table 1 to obtain a billet.

[0057] (2) Hot rolling: The obtained billet was first heated to 1100 - 1250 °C and held for 0.5 hours or more, then hot rolled at a temperature above Ar3, rapidly cooled at a rate of 30 - 80 °C / s after rolling, cooled to the coiling temperature and then coiled, and the coiling temperature was controlled to 150 - 250 °C.

[0058] (3) Annealing by a hot coiling and holding bell: After coiling, it was immediately annealed by a holding bell, the annealing time was 0.5 - 6 hours, the internal heat of the steel coil was utilized by the holding bell, and the temperature drop per hour was set to 6 °C or less. (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, then 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 holding treatment: After annealing, the steel sheet was subjected to isothermal holding treatment, the holding temperature was controlled to 400 - 550 °C, and the holding time was controlled to 100 - 300 s.

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

[0062] (8) Levelling: The levelling rate was controlled to 0 - 0.3%. The chemical element compositions of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the present invention and the design of the related processes all met the requirements of the design specifications of the present invention.

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

[0064] [Table 2-1]

[0065] [Table 2-2]

[0066] In the present invention, samples were respectively taken from the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the finished products obtained through the above processes and steps (1) to (8), and the microstructures of the steel sheets according to each example were observed and analyzed. As a result, it was found that the microstructures of the cold-rolled steel sheets according to Examples 1 to 18 all had retained austenite + fine blocky martensite + bainite + nanoscale precipitates.

[0067] In addition, the present inventors further analyzed the volume ratio of each component in the microstructure of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the finished products, 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 type optical microscope. In addition, the details of the nanoscale precipitates and the microstructure were further 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 microstructures of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 are shown in Table 3.

[0069] [Table 3]

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

[0071] Correspondingly, after the above observations and analyses were completed, samples were further taken from the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the obtained finished products, and correlation mechanical property tests were conducted on the cold-rolled steel sheet samples of each example to obtain their mechanical strength, elongation rate, and hole expansion rate. The measurement results of the obtained mechanical properties are shown in Table 4.

[0072] The measurement methods for the related mechanical properties were as follows: Measurement of tensile test: The measurement test was carried out in accordance with GB / T 228 "Tensile test of metallic materials" Part 1: Test method at room temperature to measure the yield strength, tensile strength, and elongation rate of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18.

[0073] Hole expansion rate test: The measurement test was carried out in accordance with GB / T 24524-2021 Test method for hole expansion of metallic materials - Thin sheet and strip to measure the hole expansion rate of the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18.

[0074] The measurement results of the mechanical properties of the 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 cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 designed by the present invention have ultra-high strength while also having good elongation and hole expansion properties.

[0077] As can be seen from reading 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 rate of 43 to 54%.

[0078] Moreover, in these Examples 1 to 18 designed according to the present invention, when the tensile strength of the prepared steel sheet 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 more than 1400 and ≤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%; when the tensile strength was 1500 MPa or higher (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] In addition, the combinations of the technical features in the present application are not limited to the combinations described in the claims of the present application or the combinations described in the specific examples. As long as they do not conflict with each other, all the technical features described in the present application can be freely combined or joined in any form.

[0080] Furthermore, it should also be noted that the above-mentioned examples are only specific examples of the present invention. The present invention is not limited to the above examples, and it is obvious that any similar changes or modifications that can be directly derived from or easily conceived by those skilled in the art from the disclosure content of the present invention are included in the protection scope of the present invention.

Claims

1. It contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.15% to 0.30%, Si: 0.3% to 0.5%, Mn: 1.8% to 2.5%, Al: 0.01% to 0.03%, B: 0.001 to 0.003%, Ti: 0 to 0.05%; And the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.52%; However, its microstructure is a cold-rolled steel sheet of 1300 MPa or more grade with high elongation and high hole expansion property, having nanoscale precipitates with an average diameter of less than 30 nm.

2. The mass percentage contents of its respective chemical elements are: C: 0.15% to 0.30%, Si: 0.3% to 0.5%, Mn: 1.8% to 2.5%, Al: 0.01% to 0.03%, B: 0.001 to 0.003%, Ti: 0 to 0.05%; the balance is Fe and other inevitable impurities; And the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.52%; However, its microstructure is a cold-rolled steel sheet of 1300 MPa or more grade with high elongation and high hole expansion property as described in Claim 1, having nanoscale precipitates with an average diameter of less than 30 nm.

3. Among the inevitable impurities, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.006%, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 1 or 2.

4. Its microstructure is retained austenite + fine blocky martensite + bainite + the above-mentioned nanoscale precipitates, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 1 or 2.

5. The volume ratio of martensite is ≥ 55%, and the volume ratio of bainite is more than 0 and < 15%, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 4.

6. The volume ratio of martensite is 70 to 86%, and the volume ratio of bainite is 7 to 14%, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 5.

7. The diameter of martensite is 10 micrometers or less, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 4.

8. The diameter of martensite is 5 to 9 micrometers, and it is characterized by the cold-rolled steel sheet of 1300 MPa or more grade according to Claim 7.

9. Its performance satisfies the following: When the tensile strength is 1300 - 1400 MPa, the elongation rate is over 10% and the hole expansion rate is over 40%; When the tensile strength is over 1400 and ≤ 1500 MPa, the elongation rate is over 9% and the hole expansion rate is over 40%; When the tensile strength is 1500 MPa or more, the elongation rate is over 8% and the hole expansion rate is over 40% The cold-rolled steel sheet of 1300 MPa or higher grade according to claim 1 or 2, characterized by the above.

10. The method for manufacturing a cold-rolled steel sheet of 1300 MPa or higher grade according to any one of claims 1 - 9, characterized by including the following steps. (1) Smelting and casting; (2) Hot rolling; (3) Annealing by hot coiling and heat preservation bell: After coiling, quickly anneal by the heat preservation bell, set the annealing time to 0.5 - 6 hours, and set the temperature drop per hour to 6°C or less; (4) Cold rolling; (5) Annealing: Control the annealing soaking temperature at 830 - 860°C, control the heat preservation time at 40 - 80 s, then cool to 730 - 780°C at a cooling rate of 5 - 15°C / s, and then cool to the isothermal heat preservation temperature at a rate of 50 - 700°C / s; (6) Isothermal heat preservation treatment: Set the heat preservation temperature at 400 - 550°C and the heat preservation time at 100 - 300 s; (7) Cooling: Cool to room temperature at a rate of 30°C / s - 100°C / s; (8) Levelling.

11. In step (2), first heat to 1100 - 1250°C and hold for 0.5 hours or more, then hot roll at a temperature above Ar3, and after rolling, rapidly cool at a rate of 30 - 80°C / s, and control the coiling temperature at 150 - 250°C. The manufacturing method according to claim 10, characterized by the above.

12. In step (2), the hot rolling temperature is 920°C or less, and the coiling temperature is preferably 150 - 230°C; preferably, in step (3), the start temperature of annealing by the heat preservation bell is the same as the coiling temperature, and the temperature drop rate is 2 - 6°C per hour. The manufacturing method according to claim 11, characterized by the above.

13. In step (4), control the cold rolling reduction rate at 50 - 70%. The manufacturing method according to claim 10, characterized by the above.

14. In step (5), control the annealing soaking temperature at 830 - 850°C. The manufacturing method according to claim 10, characterized by the above.

15. In step (8), control the levelling rate at 0 - 0.3%. The manufacturing method according to claim 10, characterized by the above.

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