Ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or higher and its manufacturing method.

Abstract

The present invention discloses an ultra-high strength cold rolled strip steel having a tensile strength of 1450 MPa or more, which contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.19-0.245%, Si: 0.03-0.45%, Mn: 0.8-1.2%, B: 0.001-0.004%, Cu: 0.05-0.15%, Zr: 0.05-0.15%, Ti: 0.005-0.05%, Al: 0.01-0.08%; its microstructure has a matrix and carbide particles uniformly and dispersively distributed in the matrix, the matrix having a volume fraction of 95% or more of tempered martensite, and the carbide particles have an average diameter of 0.5 microns or less. The present invention also discloses a method for producing the above ultra-high strength cold rolled strip steel, which includes the following steps: (1) smelting and casting; (2) hot rolling; (3) cold rolling after pickling; (4) continuous annealing; (5) tempering; and (6) leveling.

Description

Technical Field

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

Background Art

[0002] Background Art In recent years, with the rapid development of the automobile industry, the market's requirements for the lightweight and safety of automobiles have been increasing. Many automobile manufacturers are demanding the use of higher-strength steel sheets due to the requirements of lightweight and safety.

[0003] However, it has been found that the higher the strength of the steel sheet, the more deteriorated its formability and toughness, and it is easily brittle fractured due to the inevitable contact with hydrogen during the processing and use processes, that is, it has hydrogen embrittlement characteristics. This severely reduces the safety and protection effect of ultra-high-strength steel and causes many problems during use.

[0004] In the existing technology, there are some researchers who have carried out optimization designs related to high-strength steel sheets and achieved certain results, and obtained some high-strength steels with a tensile strength of 1450 MPa or more.

[0005] For example, in the Chinese technical document with publication number CN110684932A, publication date January 14, 2020, and title "1500MPa level cold-formed strip steel and method for manufacturing the same," a 1500MPa level cold-formed strip steel and method for manufacturing the same are disclosed, and its chemical composition is designed as follows: C:0.25~0.4%, Si:0.1~0.3%, Mn:1.1~1.7%, Cr:0.2~0.4%, P:≦0.02%, S:≦0.012%, Al:0.03~0.05%, Ti:0.035~0.05%, B:0.001~0.003%, V:0.15~0.3%, N:≦0.003%. The manufacturing method of this technical proposal includes processes of molten iron pretreatment, steelmaking in a converter, refining in an LF furnace, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing, and leveling. However, the continuous annealing process is as follows: heating and soaking temperatures are both 820-860°C, soaking time is 50-100 s, quenching start temperature is 660-680°C, cooling rate is 80-100°C / s, and overaging temperature is 260-300°C. The resulting strip steel has a uniformly distributed island-like martensite structure with a small amount of ferrite, exhibiting ultra-high strength and excellent weldability and cold formability.

[0006] Furthermore, for example, in Chinese patent document No. CN112981252A, published on June 18, 2021, titled "1500MPa Level Automotive Steel Sheet and Method for Manufacturing the Same," a 1500MPa level automotive steel sheet and a method for manufacturing the same are disclosed. The chemical composition is designed to be C: 0.17~0.21%, Si: 0.12~0.2%, Mn: 1.3~1.7%, P: ≤0.03%, S: ≤0.01%, Al: 0.03~0.05%, and N: ≤0.005%. Simultaneously, the manufacturing method of this technical scheme includes hot rolling, cold rolling, annealing, leveling, and thermoforming processes. The automotive steel sheet finally obtained by adopting this technical scheme has a tensile strength of 1500~1600MPa, a yield strength of 1000~1200MPa, and an elongation of ≥5%.

[0007] Furthermore, for example, in the Chinese patent document with publication number CN112522573A, publication date March 19, 2021, titled "B-containing martensitic strip and method for manufacturing the same," B-containing martensitic strip and a method for manufacturing the same are disclosed, and its chemical composition by mass percentage is C: 0.16~0.26%, Si: 0.1~0.5%, Mn: 0.4~1.7%, P: ≤0.02%, S: ≤0.007%, Al: ≤0.001%, B: 0.001~0.006%, V: 0.15~0.3%, N: 0.004~0.01%. Furthermore, it contains either or both of Sn: 0.005-0.04% and Cu: 0.1-0.6%, and either or both of Nb: 0.01-0.08% and Mo: 0.1-0.4%, with Mn / S ≥ 250. This martensitic strip steel has a yield strength of 800-1200 MPa, a tensile strength of 1100-1900 MPa, and an elongation of 3-12%, making it widely applicable to the high-strength automotive steel sector.

[0008] The ultra-high-strength steels disclosed in the above-mentioned patent technical documents all relate to strengths of 1450 MPa or higher, and some of them also relate to improvements in delayed cracking and hydrogen-induced cracking. However, according to the inventors' research, none of the above-mentioned patent documents consider delayed cracking after the steel sheet has been formed into a part and then baked and painted. When steel materials are actually used in the manufacture of automobile parts, it is necessary to bake and paint them after forming, but it is important to know that if the problem of delayed cracking after baking and painting of the part after forming is not considered, the part is likely to develop quality defects after baking and painting. [Overview of the project] [Means for solving the problem]

[0009] Content of the invention One objective of the present invention is to provide an ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more. Through the cooperation of rational component design and process design, the present invention makes it possible to obtain an ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more that has ultra-high strength, excellent cold bending properties and good resistance to hydrogen-induced cracking, and also has excellent toughness and resistance to hydrogen-induced cracking even in parts that have undergone heating and holding at 170°C for 20 minutes after forming (a baking and coating process for automotive parts).

[0010] The ultra-high-strength cold-rolled steel strip according to the present invention can be efficiently applied to the automotive industry and the manufacture of automotive parts in order to achieve weight reduction in automobiles while ensuring safety, and has good prospects for widespread adoption and application value.

[0011] 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: C:0.19~0.245%, Si:0.03~0.45%, Mn:0.8~1.2%, B:0.001~0.004%, Cu:0.05~0.15%, Zr:0.05~0.15%, Ti:0.005~0.05%, Al:0.01~0.08%; The microstructure provides an ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more, wherein the matrix has tempered martensite at a volume fraction of 95% or more, and the carbide particles have an average diameter of 0.5 microns or less.

[0012] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the mass percentages of each chemical element are: The composition is as follows: C: 0.19-0.245%, Si: 0.03-0.45%, Mn: 0.8-1.2%, B: 0.001-0.004%, Cu: 0.05-0.15%, Zr: 0.05-0.15%, Ti: 0.005-0.05%, Al: 0.01-0.08%, with the remainder being Fe and unavoidable impurities.

[0013] In the above technical solution, the ultra-high-strength cold-rolled steel strip according to the present invention employs C, Si, Mn, and B as basic additive elements in the chemical composition design to ensure strength while keeping the carbon equivalent as low as possible. According to a general empirical formula for calculating the strength of martensitic steel, TS(MPa) = 2880C + 800, and this formula is applicable when C is 0.2 to 0.5%. However, the carbon content of the steel material designed by the present invention to obtain the same strength is always lower than the value calculated by the empirical formula.

[0014] Furthermore, in this invention, by adding strong carbide-forming elements such as Zr and Ti to the steel, a large amount of finely dispersed precipitates such as ZrC and Ti(C,N) are formed on the steel sheet during the processing process, improving the tempering resistance characteristics of martensite and suppressing the precipitation of Fe3C during the tempering process. This suppresses the adsorption of large amounts of hydrogen by reversible hydrogen traps and prevents the problem of excessively high levels of diffused hydrogen within the steel sheet. In addition, an appropriate amount of Cu is added to the ultra-high-strength cold-rolled steel strip to improve the corrosion resistance of the steel material.

[0015] In the ultra-high-strength cold-rolled steel strip according to the present invention, the design principles for each chemical element are as follows.

[0016] C: In the ultra-high-strength cold-rolled steel strip according to the present invention, element C improves the strength of the steel by influencing the hardness of martensite, and adding an appropriate amount of C to the steel is advantageous for the strength of the material. However, it should be noted that the higher the element C content in the steel, the harder the martensite becomes, and the greater the brittleness, which is unfavorable for welding. For this reason, in the present invention, considering the effect of element C content on the performance of the steel, the mass percentage content of element C is actually controlled to 0.19 to 0.245%.

[0017] Of course, in some preferred embodiments, the mass percentage content of element C may be further controlled to 0.195-0.24% to obtain better implementation effects.

[0018] Si: In the ultra-high-strength cold-rolled steel strip according to the present invention, the Si element has a solid solution strengthening effect, but it tends to promote the formation of retained austenite. Therefore, in order to avoid the formation of retained austenite as much as possible, it is necessary to control the Si element content in the steel to a low level, and in the present invention, the mass percentage content of the Si element is controlled to 0.03 to 0.45%.

[0019] Of course, in some preferred embodiments, the mass percentage content of Si element may be further controlled to 0.03-0.4% to obtain better implementation effects.

[0020] Mn: In the ultra-high-strength cold-rolled steel strip according to the present invention, Mn is an important element for improving hardenability, and can improve the hardenability of the steel material, which is also advantageous in terms of strength. However, Mn increases the carbon equivalent of the steel, and it is necessary to design the material with as little Mn as possible depending on the cooling method. Considering the effect of the Mn element content on the performance of the steel material, the mass percentage content of Mn element in the ultra-high-strength cold-rolled steel strip according to the present invention is controlled to 0.8 to 1.2%.

[0021] Of course, in some preferred embodiments, the mass percentage content of Mn element may be further controlled to 0.9-1.1% to obtain better implementation effects.

[0022] B: In the ultra-high-strength cold-rolled steel strip according to the present invention, element B can similarly improve the hardenability of the steel material, and the beneficial effects of element B are utilized to ensure the hardenability of the steel material. In the present invention, the mass percentage content of element B is controlled to 0.001 to 0.004%.

[0023] Of course, in some preferred embodiments, the mass percentage content of element B may be further controlled to 0.0015-0.0035% to obtain better implementation effects.

[0024] Cu: In the ultra-high strength cold-rolled strip steel according to the present invention, the Cu element can improve the corrosion resistance of the steel material and is also beneficial to the improvement of the hydrogen-induced cracking characteristics of the material. However, it is equally unfavorable to add excessive Cu to the steel, and it should be noted that the excessive addition of the Cu element leads to the hot brittleness of the steel material. Therefore, it is necessary to strictly control the content of the Cu element. In the present invention, the mass percentage content of the Cu element is controlled to be 0.05 - 0.15%.

[0025] Zr: In the ultra-high strength cold-rolled strip steel according to the present invention, Zr is a strong carbide-forming element. By adding an appropriate amount of Zr to the steel, not only can the formation of retained austenite be effectively suppressed, but it is also beneficial to the improvement of the strength and toughness of the steel material. Therefore, in the present invention, the mass percentage content of the Zr element is controlled to be 0.05 - 0.15%.

[0026] Ti: In the ultra-high strength cold-rolled strip steel according to the present invention, the Ti element can play a role in fixing N and can fully exert the hardening effect improvement function of B. Also, TiC formed by the Ti element in the steel is beneficial to the dispersion of hydrogen aggregation, but TiN is unfavorable to the plasticity of the steel. Therefore, in the present invention, considering the influence of the Ti element on the performance of the steel material, the mass percentage content of the Ti element is controlled to be 0.005 - 0.05%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage content of the Ti element may be further controlled to be 0.005 - 0.04%.

[0028] Al: In the ultra-high strength cold-rolled strip steel according to the present invention, the Al element can play a role in deoxidation and is added as a deoxidizer to ensure the performance of the steel material. Therefore, in the present invention, in order to exert the beneficial effects of the Al element, the mass percentage content of the Al element is controlled to be 0.01 - 0.08%.

[0029] In summary, the present invention rationally designs the chemical composition and, after smelting, casting, and rolling, rapidly quenching during continuous annealing yields at least 95% martensite by volume fraction, with the remainder being bainite. If unavoidable, small amounts of ferrite and retained austenite may be included, but the content (volume fraction) of both ferrite and retained austenite is <0.5%. This compositional and process design minimizes the retained austenite content and avoids the adverse effects of internal stress and brittle phases caused by the transformation of retained austenite into high-carbon martensite during the forming process.

[0030] Furthermore, by adding a combination of strong carbide-forming elements in appropriate types and amounts, and dispersing and precipitating a large amount of fine carbide particles in the matrix, these carbide particles are uniformly and dispersedly distributed within the matrix metal, have an average diameter of 0.5 microns or less, and are resistant to growth during tempering. This design effectively improves the tempering resistance of martensite, and ensures that the precipitate particles remain fine even after firing, preventing deterioration of the material's hydrogen-induced cracking resistance.

[0031] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, among the unavoidable impurities, P ≤ 0.015%, S ≤ 0.002%, and N ≤ 0.005%.

[0032] In the ultra-high-strength cold-rolled steel strip according to the present invention, elements P, S, and N are all impurity elements in the steel. When technical conditions are permissible, in order to obtain ultra-high-strength cold-rolled steel strip with superior performance and quality, the content of impurity elements in the steel should be reduced as much as possible.

[0033] P: In this invention, P is an impurity element in steel, which reduces the toughness of the steel and adversely affects delayed cracking. Therefore, in this invention, it is necessary to strictly control the P element content in the steel, and the mass percentage content of P element is controlled so that P ≤ 0.015%.

[0034] S: In this invention, S is also an impurity element in steel, and S forms MnS in steel, which has a significant effect on the hole expansion rate of the steel material. Therefore, in this invention, it is necessary to strictly control the S element content in the steel, and the mass percentage content of S element is controlled so that S ≤ 0.002%.

[0035] N: In this invention, N is also an impurity element in steel, and N can react with Ti in steel to precipitate large TiN particles. When large TiN particles are located near the surface of the steel plate, they tend to form regions of hydrogen aggregation, which in turn become a source of cracking. Therefore, in order to minimize the amount of TiN, it is necessary to control the mass percentage content of element N so that N ≤ 0.005%.

[0036] Of course, in some preferred embodiments, the content of impurity elements P, S, and N may be further limited to obtain better implementation effects, preferably controlled to P ≤ 0.012%, S ≤ 0.0015%, and N ≤ 0.004%.

[0037] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the chemical elements further include at least one of the following: W: 0.05~0.15%; Mo: 0.05~0.15%; Ni: 0.05~0.15%; Ca: 0.0005~0.0035%; Nb: 0.015~0.045%; V: 0.005~0.015%.

[0038] In this invention, in order to obtain an ultra-high-strength cold-rolled steel strip with superior performance, elements W, Mo, Ni, Ca, Nb, and V may be added.

[0039] W: In the technical solution of the present invention, W is a strong carbide-forming element, and adding an appropriate amount of W to steel is advantageous not only for the formation of dispersedly distributed fine precipitates but also for the dispersion of localized hydrogen condensation. Therefore, in order to exert the beneficial effects of element W, preferably 0.05 to 0.15% of W may be added to the ultra-high-strength cold-rolled steel strip according to the present invention.

[0040] Mo: In the technical solution of the present invention, the element Mo can improve the hardenability of steel materials and is advantageous not only for the formation of dispersed fine TiMoC precipitates but also for the dispersion of localized hydrogen condensation. Therefore, in order to exert the beneficial effects of the element Mo, preferably 0.05 to 0.15% of Mo may be added to the ultra-high-strength cold-rolled steel strip according to the present invention.

[0041] Nb, V: In the technical solution of the present invention, elements Nb and V play a role in refining the crystal grains and, by being dispersed and precipitated, are advantageous for the dispersion of hydrogen aggregates. Therefore, in the present invention, appropriate amounts of Nb and V may be added, and it is preferable to control the amount of Nb to 0.015-0.045% and the amount of V to 0.005-0.015%.

[0042] Furthermore, like the elements Zr and Ti in steel, the elements W, Mo, V, and Nb, which are preferably added, are all strong carbide-forming elements. By adding strong carbide-forming elements such as Zr, W, Mo, V, Nb, and Ti to steel, a large amount of finely dispersed precipitates such as ZrC, WC, TiMoC, Ti(C,N), and Nb(C,N) are formed on the steel sheet during the processing process, improving the tempering resistance characteristics of martensite and suppressing the precipitation of Fe3C during the tempering process.

[0043] Ni: In the technical solution of the present invention, Ni is advantageous for improving the corrosion resistance of steel and is also advantageous for mitigating the brittleness caused by Cu. Therefore, in the ultra-high-strength cold-rolled steel strip according to the present invention, preferably 0.05 to 0.15% Ni may be added.

[0044] Ca: In the technical solution of the present invention, the aspect ratio of the inclusions can be improved by adding an appropriate amount of Ca, and in the present invention, preferably 0.0005 to 0.0035% of Ca may be added.

[0045] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the mass percentage content of each chemical element further satisfies at least one of the following conditions: C: 0.195~0.24%, Si: 0.03~0.4%, Mn: 0.9~1.1%, B: 0.0015~0.0035%, Ti: 0.005~0.04%.

[0046] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the matrix of its microstructure further contains bainite, preferably the volume fraction of bainite is 1.2 to 3.3%.

[0047] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the volume fractions of ferrite and retained austenite in the matrix of its microstructure are both <0.5%; preferably, the volume fractions of ferrite and retained austenite are 0.3 to 0.4%, respectively.

[0048] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, the carbide particles include Fe3C and at least one of Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

[0049] Furthermore, in the ultra-high-strength cold-rolled steel strip according to the present invention, its tensile strength is ≥1450 MPa; and its room-temperature impact toughness (Charpy V notch) is ≥38 J / cm 2The hydrogen-induced cracking resistance is such that a U-shaped bending test specimen with a prestress equal to 1 times its tensile strength does not crack even when immersed in 1 mol / L hydrochloric acid for 300 hours or more; and after heating and holding at 170°C for 20 minutes, the hydrogen-induced cracking resistance is such that a U-shaped bending test specimen with a prestress of 1.2 times its tensile strength or more does not crack even when immersed in 1 mol / L hydrochloric acid for 300 hours or more. Preferably, the yield strength is in the range of 1190 to 1370 MPa, the tensile strength is in the range of 1470 to 1650 MPa, the elongation is in the range of 5 to 7%, and the room temperature impact toughness is 44 to 47 J / cm 2 It is within the range.

[0050] Accordingly, another object of the present invention is to provide a method for manufacturing the above-mentioned ultra-high-strength cold-rolled steel strip, and the inventors have further optimized the design of the preparation process of the manufacturing method to suit the design of the chemical composition of the steel strip. By adopting this manufacturing method, the ultra-high-strength cold-rolled steel strip according to the present invention can be efficiently prepared and has good potential for application.

[0051] To achieve the above objective, the method for producing the ultra-high-strength cold-rolled steel strip provided by the present invention includes the following steps: (1) Smelting and casting; (2) Hot rolling: Heat to 1150~1200°C, maintain temperature for 40~50 min, finish rolling at 870~920°C, rapidly cool to winding temperature after rolling at a cooling rate of 20~50°C / s, winding temperature at 500~600°C, and controlled cooling after winding; (3) Cold rolling after pickling; (4) Continuous annealing; (5) Tempering: Heat to a tempering temperature of 200-300°C using induction heating, and maintain the temperature for 150 seconds or more.

[0052] In the method for manufacturing ultra-high-strength cold-rolled steel strip according to the present invention, the inventors have optimized the design of the hot rolling process, employing a process of low-temperature heating, low-temperature rolling termination, and low-temperature winding. Of course, by further rapid cooling of the steel coil after winding by blowing it with a blower, the precipitation and growth of precipitates (WC / TiMoC / Ti(C,N) / Nb(C,N)) can be suppressed, thereby inducing secondary precipitation during the continuous annealing process of the cold-rolled sheet and obtaining finer strong carbide precipitation.

[0053] Furthermore, in step (5) of the present invention, the inventors set the tempering temperature to 200-300°C in order to reduce the hardness of the martensite by low-temperature tempering and control the size of the Fe3C precipitates.

[0054] Furthermore, in the manufacturing method according to the present invention, in step (2), after winding, the steel coil is cooled by air cooling so that the surface temperature reaches 400-500°C, and then cooled by blowing with a blower until the surface temperature of the steel coil is less than 200°C.

[0055] In the above-described technical solution of the present invention, the purpose of controlled cooling after winding is to accelerate cooling, thereby suppressing the precipitation of some strong carbides (WC, TiMoC, Ti(C,N), Nb(C,N), etc.) during the cooling process of the hot-rolled coil, and to induce secondary precipitation during the continuous annealing process of the cold-rolled sheet, thereby obtaining finer strong carbide precipitation.

[0056] Furthermore, in the manufacturing method according to the present invention, in step (3), the cold rolling reduction ratio is controlled to 30-65%.

[0057] Furthermore, in the manufacturing method according to the present invention, in step (4), the material is heated to the austenite single-phase region at a heating rate of 5°C / s or more, kept warm for 30 to 120 seconds, then cooled to 700 to 780°C at a rate of 3 to 10°C / s, then water-cooled to 100°C or below at a rate of 700°C / s or more, and then pickled.

[0058] Accordingly, in some preferred embodiments, after the completion of step (4) above, the pickled steel sheet may be further subjected to alkaline washing and rinsing to remove residual acid from the surface of the steel sheet, and after drying the steel sheet, it may be heated to 200-300°C by induction heating to perform tempering, preferably by tempering for 200 s or more to obtain tempered martensite with fine secondary precipitate particles.

[0059] Of course, after removing residual acid from the steel sheet surface by alkaline cleaning and rinsing and drying the steel sheet, it is even more preferable to heat it to a tempering temperature of 200-250°C by induction heating and perform tempering for 400 seconds or more, or to level it after the tempering treatment.

[0060] Furthermore, in the manufacturing method according to the present invention, in step (6), the leveling rate is set to ≤0.3%.

[0061] The ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more according to the present invention and its manufacturing method have the following advantages and beneficial effects compared to the prior art: Compared to conventional steel materials, the present invention provides ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more at a lower carbon equivalent. By employing a rational chemical composition, the inventors effectively suppress the opportunity for retained austenite formation, keeping the retained austenite content below 0.5%, and avoiding additional stress caused by the transformation of retained austenite into high-carbon martensite during the forming process.

[0062] Furthermore, the inventors have found that by adding strong carbide-forming elements when designing the chemical composition, they can effectively suppress the enrichment of locally dissolved carbon, further inhibit the formation of retained austenite, and improve the toughness of the steel sheet by dispersing and precipitating a large amount of fine precipitates in the steel. This effectively avoids the reduction in toughness due to the aggregation and growth of carbides during the subsequent baking and coating process of the parts. In addition, thanks to the internal stress release effect during the baking process, the delayed crack resistance of the parts is also improved.

[0063] This ultra-high-strength cold-rolled steel strip not only possesses ultra-high strength of 1450 MPa or more, but also has excellent cold bending properties and good resistance to hydrogen-induced cracking. Furthermore, even after heating and holding at 170°C for 20 minutes, the steel retains excellent toughness and resistance to hydrogen-induced cracking. The microstructure of this ultra-high-strength cold-rolled steel strip is predominantly tempered martensite, containing at least 95% tempered martensite, with the remainder being bainite. Where unavoidable, it may also contain small amounts of ferrite and retained austenite, but their content is <0.5% by volume fraction. In terms of substructure, the outstanding feature of this ultra-high-strength cold-rolled steel strip is the large amount of fine carbide particles dispersed and precipitated in the matrix. These carbide particles include Fe3C and, depending on cost, Ti(C,N), Nb(C,N), ZrC, WC, VC, TiMoC, etc. These carbide particles are uniformly and dispersedly distributed in the matrix metal, and their average diameter is 0.5 microns or less.

[0064] This ultra-high-strength cold-rolled steel strip exhibits excellent resistance to hydrogen-induced cracking. A U-shaped bending test specimen prepared from it, with a prestress equal to one times its tensile strength, will not crack even after being immersed in 1 mol / L hydrochloric acid for more than 300 hours. After simulating the forming and baking / painting processes of automotive parts by pre-deforming the steel sheet by 2% and heating and holding it at 170°C for 20 minutes, its resistance to hydrogen-induced cracking is further improved. In other words, a U-shaped bending test specimen with a prestress 1.2 times its tensile strength will not crack even after being immersed in 1 mol / L hydrochloric acid for more than 300 hours. Due to these properties, the above-mentioned ultra-high-strength cold-rolled steel strip has relatively good performance when used in the manufacture of automotive safety structural components. [Brief explanation of the drawing]

[0065] [Figure 1] Figure 1 schematically shows a comparison of the impact toughness of the ultra-high-strength cold-rolled steel strip according to Example 3 and the comparative steel strip according to Comparative Example 3, both in their annealed state and in the annealed + 2% pre-deformed + baked state. [Modes for carrying out the invention]

[0066] Specific Embodiments The ultra-high-strength cold-rolled steel strip and its manufacturing method according to the present invention will be further interpreted and explained below based on specific examples, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0067] Examples 1-8 and Comparative Examples 1-3 The ultra-high-strength cold-rolled steel strips in Examples 1 to 8 of the present invention and the comparative steel strips in Comparative Examples 1 to 3 were all prepared by the following process: (1) Smelting and casting were carried out according to the chemical composition shown in Table 1.

[0068] (2) Hot rolling: The obtained billet was heated to 1150-1200°C and held at that temperature for 40-50 minutes to perform hot rolling, controlling the rolling completion temperature to 870-920°C. After rolling, it was rapidly cooled to the winding temperature, controlling the cooling rate to 20-50°C / s, controlling the winding temperature to 500-600°C. After winding, the steel coil was cooled by air cooling until the surface temperature was between 400-500°C, and then cooled by blowing with a blower until the surface temperature of the steel coil was below 200°C.

[0069] (3) Cold rolling after pickling: After pickling using a conventional pickling process, cold rolling was performed, and the cold rolling reduction ratio was controlled to 30-65%.

[0070] (4) Continuous annealing: Cold-rolled steel sheets were continuously annealed, heated to the austenite single-phase region at a heating rate of 5°C / s or more, held at a temperature of 3 to 120 seconds, then cooled to 700 to 780°C at a rate of 3 to 10°C / s, then water-cooled to below 100°C at a rate of 700°C / s or more, and finally pickled.

[0071] (5) Tempering: The steel material was heated to a tempering temperature of 200-300°C by induction heating and held at that temperature for 150 seconds or more; preferably, the holding time was controlled to 200 seconds or more; of course, in some preferred embodiments, the tempering temperature may be controlled to 200-250°C and tempering may be performed for 400 seconds or more.

[0072] (6) Optional leveling process: The tempered steel material was subjected to a leveling process to control the leveling rate to ≤0.3% (Example 4 does not include a leveling process).

[0073] Furthermore, the leveling process in step (6) described above is not an essential step for the ultra-high-strength cold-rolled steel strip according to Examples 1 to 8 of the present invention. In Example 4, an ultra-high-strength cold-rolled steel strip with excellent performance could be obtained even without employing the leveling process described above.

[0074] In the present invention, the chemical elemental composition and related process designs of the ultra-high-strength cold-rolled steel strips according to Examples 1 to 8 of the present invention all met the requirements of the design specifications of the present invention. On the other hand, the comparative steel strips according to Comparative Examples 1 to 3 were prepared by the above processes and steps, but their chemical elemental composition and / or related process parameters included parameters that did not satisfy the design of the present invention.

[0075] Table 1 shows the mass percentages of each chemical element in the ultra-high-strength cold-rolled steel strips of Examples 1 to 8 and the comparative steel strips of Comparative Examples 1 to 3.

[0076] [Table 1]

[0077] The specific process parameters for the above processes and steps in the manufacturing methods of the ultra-high-strength cold-rolled steel strips in Examples 1 to 8 and the comparative steel strips in Comparative Examples 1 to 3 are shown in Tables 2-1 and 2-2.

[0078] [Table 2-1]

[0079] [Table 2-2]

[0080] The finished products of the ultra-high-strength cold-rolled steel strips obtained in Examples 1 to 8 and the comparative steel strips obtained in Comparative Examples 1 to 3 were sampled, and the microstructure of the steel strip samples for each example and comparative example was observed and analyzed. It was found that the microstructure of the ultra-high-strength cold-rolled steel strips of Examples 1 to 8 consisted of a matrix and carbide particles uniformly and dispersedly distributed within the matrix. The specific analysis results are shown in Table 3 below.

[0081] The observation and analysis results of the microstructure of the ultra-high-strength cold-rolled steel strips used in Examples 1-8 and the comparative steel strips used in Comparative Examples 1-3 are shown in Table 3.

[0082] [Table 3]

[0083] As is clear from Table 3 above, the microstructure of the ultra-high-strength cold-rolled steel strips according to Examples 1 to 8 of the present invention has a matrix and carbide particles uniformly and dispersedly distributed within the matrix. However, these carbide particles have an average diameter of 0.5 microns or less.

[0084] In the present invention, the matrix of the microstructure of the ultra-high-strength cold-rolled steel strip according to Examples 1 to 8 may include tempered martensite, bainite, ferrite, and retained austenite. However, the volume fraction of tempered martensite is 96-98%, all exceeding 95%, the volume fraction of bainite is 1.2-3.3%, the volume fraction of ferrite is 0.3-0.4%, and the volume fraction of retained austenite is 0.3-0.4%.

[0085] Accordingly, according to the inventors' observations and analyses, in the microstructure of the ultra-high-strength cold-rolled steel strips according to Examples 1 to 8, the strong carbide particles uniformly and dispersedly distributed in the matrix may include Fe3C, Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

[0086] After the observation and analysis of the microstructure described above was completed, the inventors further sampled the finished products of the ultra-high-strength cold-rolled steel strips obtained in Examples 1 to 8 and the comparative steel strips obtained in Comparative Examples 1 to 3, respectively, in order to test the performance of the steel strips. Various performance tests were then conducted, and the results are shown in Table 4.

[0087] The relevant performance testing methods were as follows: (1) Tensile test: The yield strength, tensile strength, and elongation (δ) of the steel sheets in the initial annealed state for each example and comparative example were measured at room temperature using the tensile test method for thin metal sheets GB / T3076-1982, with a scale distance of 50 mm.

[0088] (2) Impact test: The impact toughness of the steel sheets in the initial annealed state for each example and comparative example was measured at room temperature of 25°C or other temperature conditions, according to the metal Charpy notch impact test method of GB / T229-1994. After pre-deforming the steel sheets in the initial annealed state by 2%, and simulating baking at 170°C for 20 minutes, the impact toughness test was performed using the method described above to obtain the impact toughness value of the steel sheets in the initial annealed state after simulating part forming and part baking and painting, and the results are shown in Figure 1 below.

[0089] (3) Cold bending: In accordance with the metal sheet and strip bending performance test method GB / T38806-2020, the steel sheets in the initial annealed state according to Examples 1-8 and Comparative Examples 1-3 were cold-bent to 90° and the minimum bending radius was measured. After cold bending, the ultra-high-strength cold-rolled steel strips according to Examples 1-8 had an internal bending radius / thickness of 3, and the comparative steel strips according to Comparative Examples 1-3 had an internal bending radius / thickness of 3.5.

[0090] (4) The hydrogen-induced cracking resistance of the steel strip before baking was tested by the following non-standard method: The thickness of the test steel plate was 1.2 mm, the dimensions of the U-shaped bent steel plate sample were 150 mm × 30 mm × 1.2 mm, one strain gauge was placed at the point of maximum strain on each sample, the tolerance of the prestress value was ±2%, the immersion temperature was room temperature, and the immersion test should be performed within 4 hours of the bending treatment under prestress (without considering the effect of stress relaxation due to natural aging on the test results). The steel plate in the initial annealed state for each example and comparative example was bent into a U shape to obtain a corresponding U-shaped bent steel plate test specimen (with an internal radius of 8 mm in the U-shape, and the stress level in the vault region of the U-shape bend controlled to 1 times the tensile strength by strain gauges), and thereafter it was immersed in hydrochloric acid with a concentration of 1 mol / L, and the presence or absence of cracks was observed after immersion for 300 hours. If the material does not crack, it indicates excellent resistance to hydrogen-induced cracking. If it does crack, the time at which the crack occurred is recorded, indicating that the cracked steel material had poor resistance to hydrogen-induced cracking.

[0091] (5) The hydrogen-induced cracking resistance of the steel strip after pre-deforming it by 2% and heating and holding it at 170°C for 20 minutes (baking process) was tested by the following non-standard method: The thickness of the test steel plate was 1.2 mm, the dimensions of the U-shaped bent steel plate sample were 150 mm × 30 mm × 1.2 mm, one strain gauge was placed at the point of maximum strain on each sample, the tolerance for the prestress value was ±2%, the immersion temperature was room temperature, and the immersion test should be performed within 4 hours of the bending treatment under prestress (stress relaxation due to natural aging on the test results). Without considering the effects of summation, the steel sheet was first pre-deformed by 2% to simulate part forming, then heated and held at 170°C for 20 minutes to simulate the part baking and coating process. After the pre-forming and baking and coating simulations were completed, a corresponding U-shaped bent steel material (with an internal radius of 8 mm for the U-shape bend and the stress level in the vault region of the U-shape bend controlled to 1.2 times the tensile strength using strain gauges) for each example and comparative example was immersed in 1 mol / L hydrochloric acid, and the presence or absence of cracks was observed after 300 hours of immersion. If no cracks occurred, it indicated excellent resistance to hydrogen-induced cracking; if cracks occurred, the time at which cracking occurred was recorded, indicating that the cracked steel material had poor resistance to hydrogen-induced cracking.

[0092] The test results for the performance of the ultra-high-strength cold-rolled steel strips used in Examples 1-8 and the comparative steel strips used in Comparative Examples 1-3 are shown in Table 4.

[0093] [Table 4]

[0094] As is clear from Table 4, the ultra-high-strength cold-rolled steel strips according to Examples 1 to 8 of the present invention exhibit far superior overall performance compared to the comparative steel strips according to Comparative Examples 1 to 3.

[0095] As can be seen in Table 4, the ultra-high-strength cold-rolled steel strips of Examples 1 to 8 all possess excellent mechanical properties, with a yield strength of 1190 to 1370 MPa, a tensile strength of 1470 to 1650 MPa, an elongation of 5 to 7%, and an impact toughness of 44 to 47 J / cm². 2 Furthermore, the ultra-high-strength cold-rolled steel strips according to Examples 1 to 8 all have an internal bending radius / thickness ratio of 3 after 90° cold bending, and their internal bending radius / thickness ratio is smaller than that of Comparative Examples 1 to 3, indicating superior cold bending characteristics.

[0096] Accordingly, even after simulating pre-deformation and heating / holding at 170°C for 20 minutes (baking process) before baking, the corresponding U-shaped bent steel materials of ultra-high-strength cold-rolled steel strip according to Examples 1 to 8 of the present invention do not crack even when immersed in hydrochloric acid at a concentration of 1 mol / L for 300 hours, and exhibit sufficiently excellent hydrogen-induced crack resistance both before and after baking.

[0097] In contrast, the comparative steel strips in Comparative Examples 1 to 3 did not meet the design requirements of the present invention because their chemical elemental composition and manufacturing processes did not meet the requirements. As a result, after heating and holding at 170°C for 20 minutes (baking process), none of them exhibited good hydrogen-induced cracking resistance. When immersed in 1 mol / L hydrochloric acid, they cracked prematurely, resulting in poor safety.

[0098] Figure 1 schematically shows a comparison of the impact toughness of the ultra-high-strength cold-rolled steel strip according to Example 3 and the comparative steel strip according to Comparative Example 3, both in their annealed state and in the annealed + 2% pre-deformed + baked state.

[0099] In Figure 1, all toughness tests were performed either directly on the steel sheet in its initial annealed state or after simulating part forming and part baking. However, "A3 Annealed State" corresponds to "Initial Annealed State of Steel Sheet According to Example 3", "B3 Annealed State" corresponds to "Initial Annealed State of Steel Sheet According to Comparative Example 3", "A3 Annealed + 2% Pre-deformation + Baked State" corresponds to "State after 2% pre-deformation of the steel sheet in the initial annealed state according to Example 3, followed by simulation of baking at 170°C for 20 minutes", and "B3 Annealed + 2% Pre-deformation + Baked State" corresponds to "State after 2% pre-deformation of the steel sheet in the initial annealed state according to Comparative Example 3, followed by simulation of baking at 170°C for 20 minutes".

[0100] As shown in Figure 1, the ultra-high-strength cold-rolled steel strip according to Example 3, prepared according to the present invention, underwent a 2% pre-deformation forming process simulation followed by baking at 170°C for 20 minutes. This further improved the material's hydrogen-induced cracking resistance and toughness, indicating that it would have excellent performance when put into practical use in automotive processes. On the other hand, the comparative steel strip according to Comparative Example 3 showed a decrease in toughness after a 2% pre-deformation forming simulation followed by baking at 170°C for 20 minutes.

[0101] 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.

[0102] 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 following chemical elements are contained in the following mass percentages: The composition is as follows: C: 0.19–0.245%, Si: 0.03–0.45%, Mn: 0.8–1.2%, B: 0.001–0.004%, Cu: 0.05–0.15%, Zr: 0.05–0.15%, Ti: 0.005–0.05%, Al: 0.01–0.08%, with the remainder being Fe and unavoidable impurities. The microstructure comprises a matrix and carbide particles uniformly and dispersedly distributed within the matrix, wherein the matrix has tempered martensite at a volume fraction of 95% or more, and the carbide particles have an average diameter of 0.5 microns or less, resulting in an ultra-high-strength cold-rolled steel strip with a tensile strength of 1450 MPa or more.

2. The mass percentages of each chemical element are: The volume fraction of tempered martensite is 96-98%, and the average diameter of the carbide particles is 0.1-0.4 microns. The ultra-high-strength cold-rolled steel strip according to claim 1, characterized in that...

3. The ultra-high-strength cold-rolled steel strip according to claim 1, characterized in that among the unavoidable impurities, P ≤ 0.015%, S ≤ 0.002%, and N ≤ 0.005%.

4. The ultra-high-strength cold-rolled steel strip according to Claim 3, characterized in that among the unavoidable impurities, P ≤ 0.012%, S ≤ 0.0015%, and N ≤ 0.004%.

5. The ultra-high-strength cold-rolled steel strip according to claim 1, characterized in that the chemical element further comprises at least one of the following: W: 0.05-0.15%; Mo: 0.05-0.15%; Ni: 0.05-0.15%; Ca: 0.0005-0.0035%; Nb: 0.015-0.045%; V: 0.005-0.015%.

6. The ultra-high-strength cold-rolled steel according to claim 1, characterized in that the mass percentage content of each chemical element further satisfies at least one of the following conditions: C: 0.195-0.24%, Si: 0.03-0.4%, Mn: 0.9 to 1.1%, B: 0.0015-0.0035%, Ti: 0.005-0.04%.

7. The ultra-high-strength cold-rolled steel strip according to claim 1, wherein the matrix of the microstructure further comprises bainite.

8. The ultra-high-strength cold-rolled steel strip according to claim 7, characterized in that the volume fraction of bainite is 1.2 to 3.3%.

9. The ultra-high-strength cold-rolled steel strip according to claim 1, characterized in that the volume fractions of ferrite and retained austenite in the matrix of its microstructure are each <0.5%.

10. The ultra-high-strength cold-rolled steel strip according to claim 9, characterized in that the volume fractions of ferrite and retained austenite are each 0.3 to 0.4%.

11. The carbide particles are Fe ３ The ultra-high-strength cold-rolled steel strip according to claim 1, characterized by comprising C and at least one of Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

12. The tensile strength is ≥1450 MPa; the room temperature impact toughness is ≥38 J / cm ２ The ultra-high-strength cold-rolled steel strip according to claim 1, characterized in that; the hydrogen-induced cracking resistance satisfies the requirement that a U-shaped bending test specimen having a prestress equal to 1 times the tensile strength does not crack even when immersed in hydrochloric acid at a concentration of 1 mol / L for 300 hours or more; and after heating and holding at 170°C for 20 minutes, the hydrogen-induced cracking resistance satisfies the requirement that a U-shaped bending test specimen having a prestress of 1.2 times or more the tensile strength does not crack even when immersed in hydrochloric acid at a concentration of 1 mol / L for 300 hours or more.

13. The ultra-high-strength cold-rolled steel strip according to Claim 1, characterized in that the yield strength is in the range of 1190 to 1370 MPa, the tensile strength is in the range of 1470 to 1650 MPa, the elongation is in the range of 5 to 7%, and the room temperature impact toughness is in the range of 44 to 47 J / cm².

14. A method for producing ultra-high-strength cold-rolled steel strip according to any one of claims 1 to 13, characterized by comprising the following steps. (1) Smelting and casting; (2) Hot rolling: Heat to 1150-1200°C, hold for 40-50 min, bring the rolling end temperature to 870-920°C, rapidly cool to the winding temperature after rolling, cool at a rate of 20-50°C / s, bring the winding temperature to 500-600°C, and then control cooling after winding; (3) Cold rolling after pickling; (4) Continuous annealing; (5) Tempering: Heat to a tempering temperature of 200-300°C using induction heating, and maintain the temperature for 150 seconds or more.

15. The manufacturing method according to claim 14, characterized in that in step (2), after winding, the steel coil is cooled by air cooling so that the surface temperature is 400 to 500°C, and then cooled by blowing with a blower until the surface temperature of the steel coil is less than 200°C.

16. The manufacturing method according to claim 14, characterized in that in step (3), the cold rolling reduction ratio is controlled to 30 to 65%.

17. The manufacturing method according to claim 14, characterized in that in step (4), the material is heated to the austenite single-phase region at a heating rate of 5°C / s or more, kept warm for 30 to 120 seconds, then cooled to 700 to 780°C at a rate of 3 to 10°C / s, then water-cooled to 100°C or below at a rate of 700°C / s or more, and then pickled.

18. The manufacturing method according to claim 14, characterized in that step (5) involves alkaline cleaning to remove residual acid solution from the surface of the steel plate before tempering heating.

19. The manufacturing method according to claim 14, characterized in that in step (5), the tempering heating time is controlled to at least 200 s.

20. The manufacturing method according to claim 14, characterized in that in step (5), the tempering heating temperature is controlled to 200 to 250°C and the time is set to 400 to 500 s.

21. The manufacturing method according to claim 14, further comprising step (6) leveling, characterized in that the leveling rate is controlled to ≤0.3%.

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