Cold-rolled steel sheet of 1300 MPa or higher grade and method for producing the same

Abstract

The present invention discloses a cold-rolled steel sheet of 1300 MPa or higher grade and a method for manufacturing the same. 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: 0.10% - 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 contents of C and Mn satisfy C + Mn / 6 ≥ 0.35%. The cold-rolled steel sheet of 1300 MPa or higher grade adopts a reasonable chemical composition design and manufacturing process, and has excellent delayed fracture resistance and bending performance while having ultra-high strength.

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 the 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. In the current automobile industry, during actual automobile manufacturing, due to the requirements of lightweight and safety, the use of higher strength steel sheets is required. Among them, ultra-high strength steel with a tensile strength of 1000 Mpa or more has considerable potential for weight reduction in terms of lightweight and safety performance, can be used in the manufacture of safety parts, reinforcement parts, and structural parts, and has a good prospect of generalization.

[0004] However, high-strength steel with a tensile strength of 1000 MPa or more naturally has the characteristics of stress corrosion cracking (delayed cracking). Such high-strength steel sheets are very likely to crack very slowly under the action of stress and corrosive media. The delayed cracking generated in this way causes considerable trouble in the practical application of ultra-high strength steel and greatly limits the practical application of ultra-high strength steel.

[0005] Delayed cracking refers to the problem that although a component does not crack during manufacturing, over time, under the dual action of stress and a corrosive medium, stress corrosion cracking occurs, ultimately leading to the destruction of the component and the loss of its safety and protective functions. In this process, hydrogen has the effect of promoting the appearance and expansion of cracks. Generally, among currently available high-strength steels, the higher the strength of the steel, the stronger the tendency for delayed cracking. In the practical application of advanced high-strength steels, delayed cracking becomes the greatest risk.

[0006] In the current prior art, although some researchers have been able to develop ultra-high-strength steel materials, none of these technical solutions can successfully solve the problem of delayed cracking existing in ultra-high-strength steels.

[0007] For example, in the Chinese patent document with the publication number CN102822375A, the publication date of December 12, 2012, and the title "Ultra-high-strength cold-rolled steel sheet and its manufacturing method", an ultra-high-strength cold-rolled steel sheet and its manufacturing method are disclosed. However, the composition is 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 order to obtain a high-strength steel with a tensile strength of 1320 MPa or more and a flatness of the steel sheet of 10 mm or less, in continuous annealing, it is necessary to cool at a cooling rate of 20°C / s or more from Ac3 to the range of Ms point to Ms point + 200°C (gas cooling), hold for 0.1 - 60 s, and then cool to 100°C or less at a cooling rate of 100°C / s or more (water cooling).

[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 unavoidable 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 cost, 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 unavoidable impurity elements. The 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] In the above three patent documents, although the obtained steel materials all have ultra-high strength and good mechanical properties, none of these three technical solutions are involved in improving the delayed fracture resistance of ultra-high strength steel materials.

Summary of the Invention

Problems to be Solved by the Invention

[0011] One object of the present invention is to provide a novel cold-rolled steel sheet of 1300 MPa or higher grade. The cold-rolled steel sheet of 1300 MPa or higher grade adopts a reasonable chemical composition design and manufacturing process, has ultra-high strength, and also has excellent delayed fracture resistance and bending performance. When the prestress is 1.05 times or more of the tensile strength, the cold-rolled steel sheet will not generate delayed fracture even when immersed in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more. It is particularly suitable for the manufacture of safety structural parts for automobiles and has good prospects for generalization.

Means for Solving the Problems

[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: 0.10% - 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 contents of C and Mn satisfy C + Mn / 6 ≥ 0.35%, and provide a cold-rolled steel sheet of 1300 MPa or higher grade.

[0013] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the mass percentage contents of its respective chemical elements are: 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%; the balance is iron and other inevitable impurities; And the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.35%.

[0014] In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the design principle of each chemical element is as follows.

[0015] C: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, by adding C element, not only the strength of the steel material can be improved, but also the hardness of martensite can be improved to 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 0.1%, it affects the strength of the steel sheet and is disadvantageous to the generation amount and stability of 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 0.10% - 0.30%.

[0016] Si: In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, by adding Si element, the hardenability of the steel material can be improved. Moreover, Si dissolved in the steel affects the interaction of dislocations, increases the work hardening rate, and can appropriately improve the elongation rate, contributing to obtaining good formability of the steel material. Therefore, in order to exert the beneficial effect of Si element, 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 0.1% - 0.5%.

[0017] 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 0.8% - 2.5% is that when the mass percentage content of Mn in the steel is less than 0.8%, the hardenability of the produced steel material becomes insufficient, and 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 significantly increases, which has an adverse effect on both the welding performance and the resistance to delayed cracking 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 0.8% - 2.5%.

[0018] 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 the present invention, the mass percentage content of Al element is controlled to 0.01% - 0.03% in order to exert the beneficial effects of the Al element.

[0019] 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 B element, the formation of martensite can be promoted, and the strength of the martensitic steel can be ensured. However, when the defects at the grain boundaries are filled, adding more B will, due to the precipitation of "boron phase" at the grain boundaries, instead increase the grain boundary energy level, and the "boron phase" will also become the nucleus of a new phase, promoting the increase in the nucleation rate and leading to the decrease in the hardenability of the steel. Therefore, it is necessary to pay attention that the content of B element in the steel should not be too high. 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 B element is controlled to 0.001 - 0.003%.

[0020] 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 crystal grains is strongly manifested at high temperatures, and in addition, the addition of Ti element also contributes to the refinement of crystal grains. Therefore, in order to exert the beneficial effect of Ti element, in the present invention, the mass percentage content of Ti element is controlled to 0 to 0.05%. Preferably, the mass percentage content of Ti element is controlled to 0.01 to 0.05%.

[0021] 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 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 in the steel so as to satisfy C + Mn / 6 ≧ 0.35%. Preferably, 0.35% ≦ C + Mn / 6 ≦ 0.60%; more preferably, 0.37% ≦ C + Mn / 6 ≦ 0.56%.

[0022] 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%.

[0023] In the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, 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 P element in the steel should be reduced as much as possible. Specifically, the mass percentage content of P element is controlled to satisfy P ≦ 0.015%.

[0024] In addition, MnS formed by incorporating impurity element S has a severe impact on the formability of steel materials. Therefore, in the present invention, the mass percentage content of S element in the steel is strictly controlled to satisfy S ≦ 0.003%. In addition, since impurity element N is likely to lead to cracks and bubble generation on the slab surface, in the present invention, the mass percentage content of N element is controlled to satisfy N ≦ 0.006%.

[0025] 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 tempered martensite + bainite.

[0026] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the volume fraction ratio of tempered martensite is ≧ 55%, and the volume fraction ratio of bainite is more than 0 and < 15%. Preferably, the volume fraction ratio of tempered martensite is 55 - 90% or 65 - 90%. Preferably, the volume fraction ratio of bainite is 5 - 15%.

[0027] Furthermore, in the cold-rolled steel sheet of 1300 MPa or higher grade according to the present invention, the diameter of the tempered martensite is 10 micrometers or less. Preferably, the diameter of the tempered martensite is 4 - 9 micrometers.

[0028] In the present invention, the composition designed for the steel according to the present invention is a composition system in which C + Mn + B is dominant. By combining and designing the C, Mn, and B elements, it is possible to ensure that the volume fraction content 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, it is ensured that a certain volume fraction of bainite exists in the finally obtained microstructure and that the volume fraction ratio of bainite is less than 15%.

[0029] It is necessary to explain that in the present invention, according to past experience and research results, through the rational design of alloying elements and manufacturing processes, a microstructure of retained austenite + fine lath-shaped tempered martensite (the diameter of the lath-shaped martensite is less than 10 micrometers) + bainite can be reliably obtained in the cold-rolled steel sheet. However, after tempering, martensite reduces stress, decreases hardness, and can generate fine dispersed precipitates that can function as hydrogen traps inside, all of which are factors contributing to the improvement of the delayed cracking performance. On the other hand, the acquisition of retained austenite not only contributes to delayed cracking but also to the improvement of the formability of the cold-rolled steel sheet.

[0030] Furthermore, in the cold-rolled steel sheet of grade 1300 MPa or higher according to the present invention, its performance satisfies the following: When the tensile strength is 1300 - 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 2.5; when the tensile strength is more than 1400 MPa and ≤ 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3; when the tensile strength is more than 1500 MPa and ≤ 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3.5; when the tensile strength is 1650 MPa or higher, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 4, where R represents the bending radius and t represents the plate thickness. When the prestress is 1.05 times or more of the tensile strength, no delayed cracking occurs even after immersion in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more.

[0031] Preferably, when the tensile strength is 1300 - 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 2.0 - 2.5; when the tensile strength is more than 1400 MPa and ≤ 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 2.5 - 3; when the tensile strength is more than 1500 MPa and ≤ 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 3.0 - 3.5; when the tensile strength is 1650 MPa or higher, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 3.5 - 4.

[0032] Preferably, when the prestress is 1.1 times or more of the tensile strength, no delayed cracking occurs even when immersed in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more. Preferably, when the prestress is 1.15 times or more of the tensile strength, no delayed cracking occurs even when immersed in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more. Preferably, when the prestress is 1.2 times or more of the tensile strength, no delayed cracking occurs even when immersed in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more.

[0033] It should be noted that in the present invention, the acquisition of retained austenite + fine lath-shaped tempered martensite + bainite together determines the good formability of this cold-rolled steel sheet of 1300 MPa or more grade designed according to the present invention. In this cold-rolled steel sheet of 1300 MPa or more grade designed according to the present invention, when the tensile strength is 1300 - 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 2.5; when the tensile strength is 1401 - 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3; when the tensile strength is 1501 - 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3.5; when the tensile strength is 1650 MPa or more, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 4.

[0034] Correspondingly, another object of the present invention is to provide a manufacturing method of the above-mentioned cold-rolled steel sheet of 1300 MPa or more grade. By optimizing the design of the manufacturing process, the cold-rolled steel sheet obtained by this manufacturing method has excellent delayed fracture resistance and bending performance while having ultra-high strength.

[0035] To achieve the above object, the manufacturing method of the above-mentioned cold-rolled steel sheet of 1300 MPa or more grade provided by the present invention includes 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°C, the tempering time is 10 - 300 s, and then it is cooled to room temperature at a rate of 30°C / s or more; (6) Levelling; (7) Discontinuous tempering: The tempering temperature is 180 - 260°C, and the tempering time is 0.5 - 6 h.

[0036] In this technical solution designed according to the present invention, the acquisition of bainite is one of the features of the present invention. In the cooling process of continuous annealing in step (4), the steel can first acquire a part of bainite, so that the martensite formed later can be ensured not to grow violently around the fine dispersed nuclei of bainite, and finally fine lumpy martensite is formed, and fine lumpy tempered martensite with a diameter of 10 micrometers or less can be obtained.

[0037] Also, as another feature of the present invention, in the manufacturing method, two tempering processes are designed. After the first continuous tempering process is completed, after levelling, discontinuous secondary tempering is further carried out. The purpose of such a design is to temper the martensite structure, localize carbon in the undeformed austenite, and enable the acquisition of the final retained austenite + fine lumpy tempered martensite + bainite structure after cooling.

[0038] In the present invention, in the continuous tempering process of step (5), the final form and size of martensite are determined by this process. Specifically, the tempering temperature is controlled at 400 - 550°C, and the tempering time is controlled at 10 - 300 s. In this manufacturing method designed according to the present invention, the present invention can finally obtain fine lumpy tempered martensite with a diameter of 10 micrometers or less. The tempering temperature and tempering time of various specific components therein need to be specifically set based on the dynamic CCT curve in order not to have a great influence on the strength of the steel and ensure a bainite ratio of 15% or less.

[0039] Also, in the discontinuous tempering process of step (7), specifically, the tempering temperature is controlled to be 180 - 260°C, and the tempering time is controlled to be 0.5 - 6 h. Since this process is independent of the above-mentioned steps (4) and (5) in terms of process, it is called a discontinuous tempering process, which is substantially a discontinuous low-temperature overaging tempering process and can be carried out in a bell-type furnace. According to this discontinuous tempering process, the martensite structure is tempered, carbon is localized in the untransformed austenite, and after cooling, it is possible to obtain a final structure of retained austenite + fine lumpy tempered martensite + bainite. After tempering, the martensite has reduced stress, decreased hardness, and fine dispersed precipitates are generated inside it, which can function as hydrogen traps, and all of them are factors contributing to the improvement of the delayed cracking performance. On the other hand, the acquisition of retained austenite not only contributes to delayed cracking but also to the improvement of the formability of the steel material.

[0040] It should be noted that if the tempering temperature is too high and / or the tempering time is too long, it will lead to a decrease in the strength of the steel material, or a severe yield platform will appear in the material, which may affect the press formability. However, if the tempering temperature is too low and / or the tempering time is too short, the martensite cannot be significantly tempered, a sufficient amount of retained austenite cannot be obtained, and the formability cannot be improved. Therefore, for this discontinuous tempering process of step (7), it is also necessary to rationally design the process based on the specific composition. Therefore, in order to ensure the performance of the steel material, in the present invention, specifically, the tempering temperature in the discontinuous tempering process is controlled to be 180 - 260°C, and the tempering time is controlled to be 0.5 - 6 h.

[0041] Furthermore, in the manufacturing method according to the present invention, in step (2), first, it is heated to 1100 - 1250°C and held for 0.3 hours or more (for example, 0.3 - 2 h), then hot-rolled at a temperature above Ar3 (austenite transformation temperature), and after rolling, rapidly cooled at a rate of 30 - 80°C / s, and the coiling temperature is controlled to be 530 - 600°C.

[0042] Furthermore, in the manufacturing method according to the present invention, in step (2), the hot rolling temperature is 920°C or lower.

[0043] Furthermore, in the manufacturing method according to the present invention, in step (3), the cold rolling reduction rate is controlled to be 45 - 65%.

[0044] Furthermore, in the manufacturing method according to the present invention, in step (4), the annealing soaking temperature is controlled to be 830 - 870°C, the holding time is controlled to be 30 - 150 s, then it is cooled to 730 - 780°C at a cooling rate of 5 - 15°C / s, and then cooled to the continuous annealing and tempering temperature at a rate of 50 - 700°C / s.

[0045] In the above technical solution of the present invention, in the annealing step of step (4), in order to achieve soaking annealing at the full austenitization temperature, the annealing soaking temperature is limited to 830 - 870°C and the holding time is limited to 30 - 150 s. If the annealing soaking temperature adopted in step (4) is less than 830°C and less than 30 s, sufficient tensile strength cannot be obtained. However, if the annealing soaking temperature adopted exceeds 870°C and exceeds 150 s, it will lead to a significant decrease in the formability of the steel material.

[0046] 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 good formability can be obtained. Therefore, the annealing soaking temperature is preferably controlled to be 850 - 860°C.

[0047] Furthermore, in the manufacturing method according to the present invention, in step (4), the annealing soaking temperature is controlled to be 850 - 860°C.

[0048] Furthermore, in the manufacturing method according to the present invention, in step (5), the tempering temperature is set to 430 - 550°C.

[0049] Furthermore, in the manufacturing method according to the present invention, in step (5), the tempering time is set to 50 - 300 s.

[0050] Furthermore, in the manufacturing method according to the present invention, in step (5), the cooling rate is set to 30 to 50 °C / s.

[0051] Furthermore, in the manufacturing method according to the present invention, in step (6), the leveling rate is controlled to 0 to 0.3%.

[0052] 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 as 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 low delayed cracking susceptibility and high bending performance can be obtained.

[0053] The cold-rolled steel sheet of 1300 MPa or higher grade has sufficiently excellent delayed cracking resistance. When the prestress is 1.05 times or more of the tensile strength, no delayed cracking occurs even after immersion in 1 mol / L hydrochloric acid for 300 hours or more. At the same time, the microstructure of retained austenite + fine blocky tempered martensite + bainite in this cold-rolled steel sheet directly determines the good formability of the cold-rolled steel sheet designed according to the present invention. For this cold-rolled steel sheet designed, when the tensile strength is 1300 to 1400 MPa, the limit value of the 90° cold bending performance evaluation parameter R / t is ≤ 2.5; when the tensile strength is more than 1400 MPa and ≤ 1500 MPa, the limit value of the 90° cold bending performance evaluation parameter R / t is ≤ 3; when the tensile strength is more than 1500 MPa and ≤ 1650 MPa, the limit value of the 90° cold bending performance evaluation parameter R / t is ≤ 3.5; when the tensile strength is 1650 MPa or higher, the limit value of the 90° cold bending performance evaluation parameter R / t is ≤ 4, where R represents the bending radius and t represents the plate thickness.

[0054] In summary, the cold-rolled steel sheet designed according to the present invention has ultra-high strength, excellent delayed cracking resistance and bending performance, is useful for the manufacture of automobile parts, can be applied to the automobile industry, and has good prospects for generalization and practical value.

Embodiments for Carrying Out the Invention

[0055] Specific Embodiments Hereinafter, based on specific examples, the cold-rolled steel sheet of 1300 MPa or higher grade and its manufacturing method according to the present invention will be further interpreted and described. However, such interpretation and description do not unduly limit the technical solution of the present invention.

[0056] Examples 1 to 18 The mass percentage ratios of the respective chemical elements designed in the cold-rolled steel sheet of 1300 MPa or higher grade according to Examples 1 to 18 of the present invention are shown in Table 1.

[0057]

Table 1

[0058] 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 billets.

[0059] (2) Hot rolling: The obtained billets were first heated to 1100 - 1250 °C and held for 0.3 hours or more, then hot-rolled at a temperature above Ar3, and 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 530 - 600 °C.

[0060] (3) Cold rolling: The cold rolling reduction rate was controlled to 45 - 65%. (4) Annealing: The annealing soaking temperature was controlled to 830 - 870 °C, preferably 850 - 860 °C, the holding time was controlled to 30 - 150 s, then cooled to 730 - 780 °C at a cooling rate of 5 - 15 °C / s, and then cooled to the continuous annealing temperature at a rate of 50 - 700 °C / s.

[0061] (5) Continuous tempering temperature: The tempering temperature was controlled at 400 - 550 °C, the tempering time was controlled at 10 - 300 s, and then it was cooled to room temperature at a rate of 30 °C / s or more.

[0062] (6) Levelling: The levelling rate was controlled at 0 - 0.3%. (7) Discontinuous tempering: The levelled steel plate was subjected to discontinuous tempering, the tempering temperature was controlled at 180 - 260 °C, and the tempering time was controlled at 0.5 - 6 h.

[0063] For the chemical element compositions of the cold-rolled steel plates of 1300 MPa and above grades according to Examples 1 - 18 of the present invention and the design of the related processes, all met the requirements of the design specifications of the present invention.

[0064] The specific process parameters in the above processes and steps of the cold-rolled steel plates of 1300 MPa and above grades according to Examples 1 - 18 are shown in Table 2-1 and Table 2-2.

[0065]

Table 2-1

[0066]

Table 2-2

[0067] In the present invention, samples were respectively taken from the cold-rolled steel plates of 1300 MPa and above grades according to Examples 1 - 18 of the finished products obtained through the above processes and steps (1) - (7), and the microstructures of the steel plates according to each example were observed and analyzed. As a result, it was found that the microstructures of the cold-rolled steel plates according to each example were all retained austenite + fine blocky tempered martensite + bainite. In this text, the microstructures were observed using a ZEISS Axio Imager M2m type optical microscope. In addition, the details of the nanoscale precipitates and microstructures 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] Furthermore, the inventors further analyzed the volume fraction ratio of each component in the microstructure of the cold-rolled steel sheets of 1300 MPa or higher grade related to Examples 1 to 18 of the finished products, and measured the diameter of the tempered martensite. The results of the analysis and measurement are shown in Table 3 below.

[0069]

Table 3

[0070] As can be seen from the analysis and measurement, in the present invention, the volume fraction ratio of the tempered martensite in the cold-rolled steel sheets of 1300 MPa or higher grade related to Examples 1 to 18 is 68 to 91%, the volume fraction ratio of the bainite is 5 to 14%, and the diameter of the tempered martensite is 4.3 to 8.7 micrometers.

[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 related to Examples 1 to 18 of the obtained finished products, and correlation mechanical property tests were performed on the cold-rolled steel sheet samples of each example to obtain their mechanical strength, elongation rate, and bending performance. The measurement results of the obtained mechanical properties are shown in Table 4.

[0072] The measurement methods of 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 obtain the yield strength, tensile strength, and elongation rate of the cold-rolled steel sheets of 1300 MPa or higher grade related to Examples 1 to 18.

[0073] In addition, the bending performance of the cold-rolled steel sheet according to each example is evaluated by the limit value of the 90-degree cold bending performance evaluation parameter R / t. However, the sheet thickness t is constant, and the bending radius R that ensures no cracking even when bent changes. When the bending radius R that ensures no cracking even when bent is minimized, the limit value of R / t is obtained. The larger the obtained limit value of the 90-degree cold bending performance evaluation parameter R / t, the poorer the bending performance; the smaller the obtained limit value of the 90-degree cold bending performance evaluation parameter R / t, the better the bending performance.

[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, the cold-rolled steel sheets of 1300 MPa or higher grade according to Examples 1 to 18 of the present invention have both ultra-high strength and good cold bending deformation performance. Their yield strength is in the range of 1128 to 1503 MPa, their tensile strength is in the range of 1321 to 1738 MPa, and their elongation is in the range of 6.1 to 10.1%. At the same time, as can be seen from these Examples 1 to 18, when the tensile strength is 1300 to 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 2.5; when the tensile strength is 1401 to 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3; when the tensile strength is 1501 to 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3.5; when the tensile strength is 1650 MPa or higher, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 4. Therefore, the cold-rolled steel sheets according to these Examples 1 to 18 have both ultra-high strength and excellent bending deformation performance.

[0077] Correspondingly, the cold-rolled steel sheets according to these Examples 1 to 18 manufactured in the present invention have excellent mechanical properties as described above and also have excellent resistance to delayed fracture.

[0078] To verify the resistance to delayed fracture of the cold-rolled steel sheets according to Examples 1 to 18, the inventors further sampled from the steel sheets according to each example, and performed an acid immersion test on the cold-rolled steel sheets according to each example, that is, evaluation was carried out by a hydrochloric acid solution immersion test. A load of 1.05, 1.1, 1.15, and 1.2 times the tensile strength was applied to the wire-cut test pieces by bending respectively, and they were immersed in a 0.1 mol / L HCl solution for 300 h without solution replacement. Each time the solution was replaced, the surface corrosion products were removed by brushing, and the test time was controlled to 300 h.

[0079] In the present invention, after the above acid immersion test is completed, the sample plate is observed. If there is no crack in the sample plate, it means that the resistance to delayed fracture under the stress condition is excellent, and it is marked as "OK"; if a crack occurs in the sample plate, it means that the resistance to delayed fracture under the stress condition is poor, and it is marked as "NG".

[0080] The test results after the acid immersion test of the cold-rolled steel sheets according to Examples 1 to 18 are shown in Table 5.

[0081]

Table 5

[0082] As shown in Table 5 above, the cold-rolled steel sheets according to Examples 1 to 18 produced have excellent resistance to delayed fracture. For all the steel sheets according to the examples, when the prestress is 1.05 times or more of the tensile strength, no delayed fracture occurred even after immersion in 1 mol / L hydrochloric acid for 300 h or more.

[0083] In addition, the combination of each technical feature in the present application is not limited to the combination described in the claims of the present application or the combination 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.

[0084] Furthermore, it should also be noted that the above-mentioned embodiments are merely specific embodiments of the present invention. The present invention is not limited to the above embodiments, and it is obvious that any similar changes or modifications that can be directly derived from the disclosure of the present invention by those skilled in the art or can be easily conceived 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: 0.10% to 0.30%, Si: 0.1% to 0.5%, Mn: 0.8% to 2.5%, Al: 0.01% to 0.03%, B: 0.001 - 0.003%, Ti: 0 to 0.05%; And the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.35%, a cold-rolled steel sheet of 1300 MPa or higher grade.

2. The mass percentage contents of its respective chemical elements are: C: 0.10% to 0.30%, Si: 0.1% to 0.5%, Mn: 0.8% to 2.5%, Al: 0.01% to 0.03%, B: 0.001 to 0.003%, Ti: 0 to 0.05%; the balance is iron and other inevitable impurities; And the mass percentage contents of C and Mn satisfy C + Mn / 6 ≥ 0.35%; Preferably, Ti: 0.01 to 0.05%, a cold-rolled steel sheet of 1300 MPa or higher grade according to Claim 1.

3. Among the inevitable impurities, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.006%, a cold-rolled steel sheet of 1300 MPa or higher grade according to Claim 1 or 2.

4. Its microstructure is retained austenite + fine blocky tempered martensite + bainite, a cold-rolled steel sheet of 1300 MPa or higher grade according to Claim 1 or 2.

5. The volume ratio of tempered martensite is ≥ 55%, and the volume ratio of bainite is greater than 0 and less than 15%; preferably, the volume ratio of tempered martensite is 55 - 90%, preferably 65 - 90%, and the volume ratio of bainite is 5 - 15%, a cold-rolled steel sheet of 1300 MPa or higher grade according to Claim 4.

6. The diameter of the tempered martensite is 10 micrometers or less, preferably, the diameter of the tempered martensite is 4 - 9 micrometers, a cold-rolled steel sheet of 1300 MPa or higher grade according to Claim 4.

7. Its performance satisfies the following: When the tensile strength is 1300 - 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 2.5; when the tensile strength is more than 1400 MPa and ≤ 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3; when the tensile strength is more than 1500 MPa and ≤ 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 3.5; when the tensile strength is 1650 MPa or more, the limit value of the 90-degree cold bending performance evaluation parameter R / t is ≤ 4, where R represents the bending radius and t represents the plate thickness; When the prestress is 1.05 times the tensile strength, no delayed cracking occurs even after immersion in 1 mol / L hydrochloric acid for 300 hours or more; Preferably, when the tensile strength is 1300 - 1400 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 2.0 - 2.5; when the tensile strength is more than 1400 MPa and ≤ 1500 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 2.5 - 3; when the tensile strength is more than 1500 MPa and ≤ 1650 MPa, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 3.0 - 3.5; when the tensile strength is 1650 MPa or more, the limit value of the 90-degree cold bending performance evaluation parameter R / t is 3.5 - 4 The cold-rolled steel sheet of 1300 MPa or higher grade according to claim 1 or 2, characterized in that.

8. The manufacturing method of the cold-rolled steel sheet of 1300 MPa or higher grade according to any one of claims 1 - 7, characterized by including 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 °C, the tempering time is 10 - 300 s, and then it is cooled to room temperature at a speed of 30 °C / s or more; (6) Levelling; (7) Discontinuous tempering: The tempering temperature is 180 - 260 °C, and the tempering time is 0.5 - 6 h.

9. In step (2), first heat to 1100 - 1250 °C and keep warm for 0.3 hours or more, then hot roll at a temperature above Ar3, and after rolling, rapidly cool at a speed of 30 - 80 °C / s, and control the coiling temperature to 530 - 600 °C. The manufacturing method according to claim 8, characterized in that.

10. Preferably, the hot rolling temperature is 920 °C or lower. The manufacturing method according to claim 9, characterized in that.

11. In step (3), the manufacturing method according to claim 8, characterized in that the cold rolling reduction rate is controlled to 45 to 65%.

12. In step (4), the annealing soaking temperature is controlled to 830 to 870°C, the holding time is controlled to 30 to 150 s, and then it is cooled to 730 to 780°C at a cooling rate of 5 to 15°C / s, and then cooled to the continuous annealing temperature at a rate of 50 to 700°C / s. The manufacturing method according to claim 8, characterized in that.

13. In step (4), the manufacturing method according to claim 11, characterized in that the annealing soaking temperature is controlled to 850 to 860°C.

14. In step (5), preferably, the manufacturing method according to claim 11, characterized in that the cooling rate is 30 to 50°C / s.

15. In step (6), the manufacturing method according to claim 8, characterized in that the leveling rate is controlled to 0 to 0.3%.

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