An ultra-high strength cold-rolled steel strip having a tensile strength of 1450 MPa or more and a method of manufacturing the same

Abstract

The application discloses an ultrahigh-strength cold-rolled steel strip with tensile strength above 1450MPa, which contains Fe and inevitable impurity elements, and further contains the following chemical elements with mass percentage 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%, and Al: 0.01-0.08%; the microstructure has a matrix and carbide particles uniformly and dispersedly distributed in the matrix, wherein the matrix has volume fraction of 95% or above tempering martensite, and the average diameter of the carbide particles is not above 0.5 microns. In addition, the application further discloses a manufacturing method of the ultrahigh-strength cold-rolled steel strip, which comprises the following steps: (1) smelting and casting; (2) hot rolling; (3) cold rolling after pickling; (4) continuous annealing; (5) tempering; and (6) flattening.

Description

Technical Field

[0001] This invention relates to a type of steel and its manufacturing method, and more particularly to a high-strength cold-rolled steel and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of the automotive industry, the market demand for vehicle weight reduction and safety has also increased. Many automakers, driven by the need for weight reduction and safety, are demanding the use of higher-strength steel plates.

[0003] However, research has found that the higher the strength of the steel plate, the worse its formability and toughness. Furthermore, when it inevitably comes into contact with hydrogen during processing and use, it is prone to brittle fracture, exhibiting hydrogen-induced embrittlement. This severely reduces the safety protection effect of ultra-high strength steel and causes great trouble in its application.

[0004] In the current technology, some researchers have carried out relevant optimization designs for high-strength steel plates and achieved certain results, obtaining some high-strength steels with tensile strengths of over 1450MPa.

[0005] For example, Chinese technical document CN110684932A, published on January 14, 2020, entitled "A 1500MPa Grade Cold-Formed Strip Steel and Its Production Method," discloses a 1500MPa grade cold-formed strip steel and its production method. 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 production method of this technical solution includes hot metal pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing, and leveling processes. The continuous annealing process includes: heating and soaking temperature of 820–860℃, soaking time of 50–100s, rapid cooling start temperature of 660–680℃, cooling rate of 80–100℃ / s, and over-aging temperature of 260–300℃. The resulting strip steel has a microstructure consisting of uniformly distributed Mao islands and a very small amount of ferrite, exhibiting ultra-high strength and excellent weldability and cold formability.

[0006] For example, Chinese patent document CN112981252A, published on June 18, 2021, entitled "A 1500MPa Grade Automotive Steel Sheet and Its Production Method," discloses a 1500MPa grade automotive steel sheet and its production method. Its chemical composition is designed as follows: 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%, N: ≤0.005%. The production method of this technical solution includes hot rolling, cold rolling, annealing, leveling, and hot forming processes. The automotive steel sheet finally obtained using this technical solution has a tensile strength of 1500-1600MPa, a yield strength of 1000-1200MPa, and an elongation ≥5%.

[0007] For example, Chinese patent document CN112522573A, published on March 19, 2021, entitled "A B-containing martensitic steel strip and its manufacturing method," discloses a B-containing martensitic steel strip and its manufacturing method. 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%. It also includes one or both of Sn: 0.005-0.04% or Cu: 0.1-0.6%, Nb: 0.01-0.08% or Mo: 0.1-0.4%, and Mn / S ≥ 250. The martensitic steel strip has a yield strength of 800-1200MPa, a tensile strength of 1100-1900MPa, and an elongation of 3-12%, and can be widely used in the field of high-strength automotive steel.

[0008] The ultra-high strength steels disclosed in the aforementioned patent documents all involve strengths exceeding 1450 MPa, and some involve improvements in delayed cracking or hydrogen-induced cracking. However, the inventors' research revealed that none of these patent documents considered the delayed cracking that occurs after the steel sheet is formed into parts and then painted. It should be understood that when steel is actually used in the manufacture of automotive parts, it must be formed and then painted. If the delayed cracking problem after painting and baking is not considered, quality defects can easily appear after the parts have been painted. Summary of the Invention

[0009] One of the objectives of this invention is to provide an ultra-high strength cold-rolled steel strip with a tensile strength of over 1450 MPa. Through the combination of reasonable composition design and process design, this invention can obtain an ultra-high strength cold-rolled steel strip with a tensile strength of over 1450 MPa. While possessing ultra-high strength, it also has excellent cold bending performance and good resistance to hydrogen-induced cracking. Furthermore, after the simulated forming of the part is heated and held at 170°C for 20 minutes (the baking process of automotive parts), it still has excellent toughness and resistance to hydrogen-induced cracking.

[0010] The ultra-high strength cold-rolled steel strip described in this invention can be effectively applied in the automotive industry and used to manufacture automotive parts, so as to achieve vehicle weight reduction while ensuring safety. It has good prospects for promotion and application value.

[0011] To achieve the above objectives, the present invention provides an ultra-high strength cold-rolled steel strip with a tensile strength of 1450 MPa or higher, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0012] 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%;

[0013] Its microstructure has a matrix and carbide particles uniformly dispersed in the matrix, wherein the matrix has tempered martensite with a volume fraction of more than 95%, and the average diameter of the carbide particles does not exceed 0.5 micrometers.

[0014] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, the mass percentage of each chemical element is as follows:

[0015] 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 balance being Fe and unavoidable impurities.

[0016] In the above technical solution, the ultra-high strength cold-rolled steel strip of this invention uses C, Si, Mn, and B as basic additive elements in its chemical composition design, which, while ensuring strength, also controls the carbon equivalent to be as low as possible. According to the commonly used empirical formula for calculating the strength of martensitic steel, TS (MPa) = 2880C + 800, this formula is applicable when C is between 0.2% and 0.5%. The steel designed in this invention achieves the same strength with a carbon content lower than the value calculated by the empirical formula.

[0017] Furthermore, in this invention, strong carbide-forming elements such as Zr and Ti are added to the steel to enable the steel plate to form more precipitates during processing, such as fine and dispersed precipitates like ZrC and Ti(C,N), thereby improving the tempering resistance of martensite, inhibiting the precipitation of Fe3C during tempering, and thus suppressing the adsorption of excessive hydrogen by reversible hydrogen traps, preventing the problem of excessive diffusible hydrogen within the steel plate. Additionally, an appropriate amount of Cu is added to this ultra-high strength cold-rolled steel strip to improve the corrosion resistance of the steel.

[0018] The design principles of each chemical element in the ultra-high strength cold-rolled steel strip of this invention are as follows:

[0019] C: In the ultra-high strength cold-rolled steel strip described in this invention, carbon (C) element improves the strength of the steel by affecting the hardness of martensite. Adding an appropriate amount of C to the steel is beneficial to the strength of the material. However, it should be noted that the higher the C content in the steel, the harder the martensite and the greater the tendency to brittleness, which is not conducive to welding. Therefore, considering the influence of C content on the properties of the steel, in this invention, the mass percentage of C element is actually controlled between 0.19% and 0.245%.

[0020] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be further controlled between 0.195-0.24%.

[0021] Si: In the ultra-high strength cold-rolled steel strip described in this invention, Si has a solid solution strengthening effect, but it also easily promotes the formation of retained austenite. Therefore, the Si content in the steel must be controlled at a low level. In this invention, the mass percentage of Si is controlled between 0.03% and 0.45% to minimize the formation of retained austenite.

[0022] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Si element can be further controlled between 0.03-0.4%.

[0023] Mn: In the ultra-high strength cold-rolled steel strip described in this invention, Mn is an important element for improving hardenability, which can enhance the hardenability of the steel and improve its strength. However, Mn increases the carbon equivalent of the steel. Therefore, a design with as little Mn as possible should be adopted according to the cooling method. Considering the impact of Mn content on the properties of the steel, the mass percentage of Mn in the ultra-high strength cold-rolled steel strip described in this invention is controlled between 0.8% and 1.2%.

[0024] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be further controlled between 0.9% and 1.1%.

[0025] B: In the ultra-high strength cold-rolled steel strip described in this invention, element B can also improve the hardenability of the steel. To maximize the beneficial effects of element B and ensure the hardenability of the steel, the mass percentage content of element B is controlled between 0.001% and 0.004% in this invention.

[0026] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element B can be further controlled between 0.0015-0.0035%.

[0027] Cu: In the ultra-high strength cold-rolled steel strip described in this invention, Cu can improve the corrosion resistance of the steel and is beneficial for improving the hydrogen-induced cracking characteristics of the material. However, it should be noted that excessive Cu should not be added to the steel, as too much Cu can easily lead to high-temperature brittleness. Therefore, the Cu content must be strictly controlled. In this invention, the mass percentage of Cu is controlled between 0.05% and 0.15%.

[0028] Zr: In the ultra-high strength cold-rolled steel strip described in this invention, Zr is a strong carbide-forming element. Adding an appropriate amount of Zr to the steel can not only effectively inhibit the formation of retained austenite, but also improve the strength and toughness of the steel. Therefore, in this invention, the mass percentage content of Zr is controlled between 0.05% and 0.15%.

[0029] Ti: In the ultra-high strength cold-rolled steel strip described in this invention, Ti can fix N, and it can fully utilize the hardenability-enhancing effect of B. Furthermore, TiC formed by Ti in the steel is beneficial for the dispersion of hydrogen aggregation, but TiN is detrimental to the plasticity of the steel. Therefore, considering the influence of Ti on the properties of the steel, in this invention, the mass percentage content of Ti is controlled between 0.005% and 0.05%.

[0030] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Ti element can be further controlled between 0.005-0.04%.

[0031] Al: In the ultra-high strength cold-rolled steel strip described in this invention, Al element can play a deoxidizing role. It is added as a deoxidizer to ensure the performance of the steel. Therefore, in order to give full play to the beneficial effects of Al element, in this invention, the mass percentage content of Al element is controlled between 0.01% and 0.08%.

[0032] In summary, this invention features a rationally designed chemical composition. Through smelting, casting, and rolling, followed by rapid quenching in continuous annealing, it achieves a martensite volume fraction of at least 95%, with the remainder being bainite. When unavoidable, a small amount of ferrite and retained austenite may be present, but the content (volume fraction) of both ferrite and retained austenite is <0.5%. The composition and process design minimize the content of retained austenite, avoiding the adverse effects of internal stress and brittle phases caused by the transformation of retained austenite into high-carbon martensite during the forming process.

[0033] Furthermore, by incorporating appropriate types and amounts of strong carbide-forming elements, a large number of fine carbide particles are dispersed and precipitated in the matrix. These carbide particles are uniformly dispersed in the matrix metal, with an average particle diameter not exceeding 0.5 micrometers, and are not easily grown during tempering. This design effectively improves the tempering resistance of martensite, ensuring that the precipitated particles remain fine after baking, thus maintaining the material's resistance to hydrogen-induced cracking.

[0034] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, among the unavoidable impurities, P≤0.015%, S≤0.002%, and N≤0.005%.

[0035] In the ultra-high strength cold-rolled steel strip described in this invention, P, S, and N are all impurity elements in steel. Under the condition that technical conditions permit, in order to obtain ultra-high strength cold-rolled steel strip with better performance and higher quality, the content of impurity elements in steel should be reduced as much as possible.

[0036] P: In this invention, P is an impurity element in steel, which reduces the toughness of the steel and has an adverse effect on delayed cracking. Therefore, in this invention, the P content in the steel must be strictly controlled, and the mass percentage of P must be controlled to meet the following condition: P ≤ 0.015%.

[0037] S: In this invention, S is also an impurity element in steel. S forms MnS in steel and severely affects the porosity of the steel. Therefore, in this invention, it is necessary to strictly control the content of S in the steel and control the mass percentage of S to meet the following requirement: S≤0.002%.

[0038] N: In this invention, N is also an impurity element in steel. 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 are prone to become areas where hydrogen accumulates, thereby forming crack sources. Therefore, it is required to control the mass percentage content of N element to meet the following requirement: N≤0.005% in order to minimize the amount of TiN.

[0039] Of course, in some preferred embodiments, in order to obtain better implementation results, the content of impurity elements P, S and N can be further limited, and can preferably be controlled as follows: P≤0.012%, S≤0.0015%, N≤0.004%.

[0040] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, its chemical elements also contain at least one of the following:

[0041] W: 0.05-0.15%;

[0042] Mo: 0.05-0.15%;

[0043] Ni: 0.05-0.15%;

[0044] Ca: 0.0005-0.0035%;

[0045] Nb: 0.015-0.045%;

[0046] V: 0.005-0.015%.

[0047] In order to obtain ultra-high strength cold-rolled steel strip with better performance, W, Mo, Ni, Ca, Nb and V elements may be added to this invention.

[0048] W: In the technical solution described in this invention, W is a strong carbide-forming element. Adding an appropriate amount of W to steel is not only beneficial for forming finely dispersed precipitates, but also beneficial for dispersing the accumulation of hydrogen in local areas. Therefore, in order to give full play to the beneficial effects of W, 0.05-0.15% W can preferably be added to the ultra-high strength cold-rolled steel strip described in this invention.

[0049] Mo: In the technical solution described in this invention, Mo can improve the hardenability of steel. It not only facilitates the formation of finely dispersed TiMoC precipitates but also helps disperse the accumulation of hydrogen in localized areas. Therefore, to maximize the beneficial effects of Mo, 0.05-0.15% Mo can preferably be added to the ultra-high strength cold-rolled steel strip described in this invention.

[0050] Nb and V: In the technical solution described in this invention, Nb and V elements can refine the grains and disperse their precipitation, which is beneficial for the aggregation of dispersed hydrogen. Therefore, in this invention, appropriate amounts of Nb and V can be added, with Nb content controlled between 0.015% and 0.045%, and V content controlled between 0.005% and 0.015%.

[0051] It should be noted that, similar to Zr and Ti in steel, the preferred additions W, Mo, V, and Nb are also strong carbide-forming elements. Adding strong carbide-forming elements such as Zr, W, Mo, V, Nb, and Ti to steel can enable the steel plate to form more fine and dispersed precipitates such as ZrC, WC, TiMoC, Ti(C,N), and Nb(C,N) during processing, thereby improving the tempering resistance of martensite and inhibiting the precipitation of Fe3C during tempering.

[0052] Ni: In the technical solution described in this invention, Ni is beneficial for improving the corrosion resistance of steel and for mitigating the brittleness caused by Cu. Therefore, in the ultra-high strength cold-rolled steel strip described in this invention, 0.05-0.15% Ni can preferably be added.

[0053] Ca: In the technical solution described in this invention, adding an appropriate amount of Ca can improve the aspect ratio of the inclusions. In this invention, it is preferable to add 0.0005-0.0035% Ca.

[0054] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, the mass percentage content of each chemical element further satisfies at least one of the following:

[0055] C: 0.195-0.24%,

[0056] Si: 0.03-0.4%,

[0057] Mn: 0.9-1.1%,

[0058] B: 0.0015-0.0035%,

[0059] Ti: 0.005-0.04%.

[0060] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, the matrix of its microstructure also includes bainite.

[0061] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, the volume fractions of ferrite and retained austenite in its microstructure matrix are both <0.5%.

[0062] Furthermore, in the ultra-high strength cold-rolled steel strip of the present invention, the carbide particles include Fe3C; and at least one of Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

[0063] Furthermore, in the ultra-high strength cold-rolled steel strip described in this invention, its tensile strength is ≥1450MPa; its room temperature impact toughness (Charpy V-notch) is ≥38J / cm. 2 The hydrogen-induced cracking resistance meets the following requirements: U-shaped bending specimens with a prestress equal to one times the tensile strength do not crack after being immersed in 1 mol / L hydrochloric acid for more than 300 hours; after being heated and held at 170℃ for 20 minutes, its hydrogen-induced cracking resistance meets the following requirements: U-shaped bending specimens with a prestress greater than or equal to 1.2 times the tensile strength do not crack after being immersed in 1 mol / L hydrochloric acid for more than 300 hours.

[0064] Accordingly, another objective of this invention is to provide a method for manufacturing the aforementioned ultra-high strength cold-rolled steel strip. The inventors have further optimized the preparation process of this manufacturing method to adapt it to the chemical composition design of the steel strip. This manufacturing method can effectively produce the aforementioned ultra-high strength cold-rolled steel strip, which has promising application prospects.

[0065] To achieve the above objectives, the present invention provides a method for manufacturing ultra-high strength cold-rolled steel strip, comprising the following steps:

[0066] (1) Smelting and casting;

[0067] (2) Hot rolling: The temperature is heated to 1150-1200℃ and held for 40-50 minutes. The final rolling temperature is controlled at 870-920℃. After rolling, the temperature is rapidly cooled to the coiling temperature at a rate of 20-50℃ / s. The coiling temperature is controlled at 500-600℃, and the cooling is controlled after coiling.

[0068] (3) Cold rolling after pickling;

[0069] (4) Continuous annealing;

[0070] (5) Tempering: Heat to tempering temperature of 200-300℃ by induction heating and hold for more than 150 seconds.

[0071] In the manufacturing method of ultra-high strength cold-rolled steel strip described in this invention, the inventors optimized the hot rolling process, employing low-temperature heating, low-temperature final rolling, and low-temperature coiling. Furthermore, the coiled steel can be further cooled using a fan to suppress the precipitation and growth of precipitates (WC / TiMoC / Ti(C,N) / Nb(C,N)), thereby causing secondary precipitation during the continuous annealing process of the cold-rolled sheet, resulting in finer strong carbide precipitates.

[0072] Furthermore, in step (5) of the present invention, the inventors controlled the tempering temperature between 200-300°C in order to reduce the hardness of martensite and control the size of Fe3C precipitates through low-temperature tempering.

[0073] Furthermore, in the manufacturing method described in this invention, in step (2), after coiling, the steel coil is first air-cooled to a surface temperature between 400-500°C, and then cooled by a fan until the surface temperature of the steel coil is below 200°C.

[0074] In the above technical solution of the present invention, the purpose of controlled cooling after coiling is to suppress the precipitation of some strong carbides (WC, TiMoC, Ti(C,N), Nb(C,N) etc.) during the cooling process of hot-rolled coil by accelerating cooling, so that secondary precipitation occurs during the continuous annealing process of cold-rolled sheet, resulting in finer strong carbide precipitation.

[0075] Furthermore, in the manufacturing method described in this invention, in step (3), the cold rolling reduction rate is controlled to be 30-65%.

[0076] Furthermore, in the manufacturing method described in this invention, in step (4), the temperature is raised to the austenitic single-phase region at a heating rate of 5°C / s or higher and held for 30 to 120 seconds; then cooled to between 700 and 780°C at a rate of 3 to 10°C / s, and then water-cooled to below 100°C at a rate of not less than 700°C / s, and then pickled.

[0077] Accordingly, in some preferred embodiments, after completing the above step (4), for the pickled steel plate, the residual acid on the surface of the steel plate can be further removed by alkaline washing and cleaning, and the steel plate is dried. Then, it is heated to 200-300°C by induction heating for tempering. Preferably, the tempering time is controlled to be not less than 200s to obtain tempered martensite, and the secondary precipitates are fine particles.

[0078] Of course, after removing residual acid from the surface of the steel plate by alkaline washing and cleaning and drying the steel plate, it is preferable to further heat it to a tempering temperature of 200-250℃ by induction heating and temper it for no less than 400 seconds. After tempering, it can be leveled.

[0079] Furthermore, in the manufacturing method described in this invention, in step (6), the flatness is controlled to be ≤0.3%.

[0080] Compared with existing technologies, the ultra-high strength cold-rolled steel strip with a tensile strength of over 1450 MPa and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0081] Compared with existing steel technologies, this invention produces an ultra-high strength cold-rolled steel strip with a tensile strength exceeding 1450 MPa at a lower carbon equivalent. The inventors employed a rational chemical composition to effectively suppress the formation of retained austenite, controlling its content to below 0.5%, thus avoiding additional stress caused by the transformation of retained austenite into high-carbon martensite during the forming process.

[0082] In addition, during the chemical composition design, the inventors effectively suppressed the enrichment of local solid solution carbon by adding strong carbide-forming elements, further suppressing the formation of residual austenite. At the same time, a large number of fine precipitates are dispersed in the steel, which can improve the toughness of the steel plate and effectively avoid the decrease in toughness caused by the aggregation and growth of carbides during the subsequent painting process of the parts. Furthermore, thanks to the stress release effect of the baking process, the resistance to delayed cracking of the parts is improved.

[0083] This ultra-high strength cold-rolled steel strip not only possesses an ultra-high strength exceeding 1450 MPa, but also exhibits excellent cold bending performance and good resistance to hydrogen-induced cracking. Furthermore, after being heated to 170℃ and held for 20 minutes, the steel demonstrates 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. When unavoidable, it may contain small amounts of ferrite and retained austenite, but the volume fraction of each is <0.5%. In terms of substructure, a prominent feature of this ultra-high strength cold-rolled steel strip is the dispersed precipitation of numerous fine carbide particles in the matrix. These carbide particles include Fe3C, and depending on cost, also include Ti(C,N), Nb(C,N), ZrC, WC, VC, TiMoC, etc. These carbide particles are uniformly dispersed in the matrix metal, with an average particle diameter not exceeding 0.5 micrometers.

[0084] This ultra-high strength cold-rolled steel strip exhibits good resistance to hydrogen-induced cracking. U-shaped bending specimens prepared with a prestress equal to one times the tensile strength can be immersed in 1 mol / L hydrochloric acid for over 300 hours without cracking. After 2% pre-deformation and heating at 170℃ for 20 minutes to simulate automotive part forming and painting processes, its resistance to hydrogen-induced cracking further improves: U-shaped bending specimens with a prestress of 1.2 times the tensile strength can be immersed in 1 mol / L hydrochloric acid for over 300 hours without cracking. This characteristic makes the aforementioned ultra-high strength cold-rolled steel strip suitable for use in the manufacture of automotive safety structural components, offering good service performance. Attached Figure Description

[0085] Figure 1 The impact toughness of the ultra-high strength cold-rolled steel strip of Example 3 and the comparative steel strip of Comparative Example 3 are schematically shown in the annealed state and the annealed + 2% pre-deformation + baking state, respectively. Detailed Implementation

[0086] The following will provide further explanation and description of the ultra-high strength cold-rolled steel strip and its manufacturing method according to the present invention with reference to specific embodiments. However, such explanation and description do not constitute an improper limitation on the technical solution of the present invention.

[0087] Examples 1-8 and Comparative Examples 1-3

[0088] The ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3 of this invention were all prepared using the following steps:

[0089] (1) Smelting and casting according to the chemical composition shown in Table 1.

[0090] (2) Hot rolling: The obtained billet is heated and hot rolled to 1150-1200℃ and held for 40-50 minutes; the final rolling temperature is controlled at 870-920℃, and after rolling, it is rapidly cooled to the coiling temperature, and the cooling rate is controlled at 20-50℃ / s, and the coiling temperature is controlled at 500-600℃; after coiling, controlled cooling is adopted: first, air cooling is used until the surface temperature of the steel coil reaches a certain temperature between 400-500℃, and then the cooling is carried out by a fan until the surface temperature of the steel coil is lower than 200℃.

[0091] (3) Cold rolling after pickling: Pickling is carried out using conventional pickling process, followed by cold rolling, and the cold rolling reduction rate is controlled to be 30-65%.

[0092] (4) Continuous annealing: The cold-rolled steel plate is continuously annealed, heated to the austenitic single-phase region at a heating rate of 5℃ / s or higher and held for 30 to 120s; then cooled to 700 to 780℃ at a rate of 3 to 10℃ / s, and then water-cooled to below 100℃ at a rate of not less than 700℃ / s, and then pickled.

[0093] (5) Tempering: The steel is heated to a tempering temperature of 200-300°C by induction heating and held for more than 150 seconds. Preferably, the holding time can be controlled to be no less than 200 seconds. Of course, in some more preferred embodiments, the tempering temperature can be controlled between 200-250°C and tempering can be carried out for no less than 400 seconds.

[0094] (6) Optional leveling step: Leveling the tempered steel and controlling the leveling rate to be ≤0.3% (Example 4 does not have a leveling step).

[0095] It should be noted that the leveling process in step (6) above is not a necessary step for the ultra-high strength cold-rolled steel strips of Examples 1-8 of the present invention. In Example 4, even without the above leveling step, ultra-high strength cold-rolled steel strips with excellent performance can still be obtained.

[0096] In this invention, the chemical element composition and related process design of the ultra-high strength cold-rolled steel strips in Examples 1-8 all meet the design specifications of this invention. However, although the comparative steel strips in Comparative Examples 1-3 were also prepared using the above process steps, their chemical element composition and / or related process parameters contain parameters that do not conform to the design of this invention.

[0097] Table 1 lists the mass percentage of each chemical element in the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3.

[0098] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, and N)

[0099]

[0100] Tables 2-1 and 2-2 list the specific process parameters in the above process steps for manufacturing the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3.

[0101] Table 2-1.

[0102]

[0103]

[0104] Table 2-2.

[0105]

[0106] Samples of the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3 were taken respectively, and the microstructure of each example and comparative example steel strip sample was observed and analyzed. It was found that the microstructure of the ultra-high strength cold-rolled steel strips of Examples 1-8 has a matrix and carbide particles uniformly dispersed in the matrix. The specific analysis results are listed in Table 3 below.

[0107] Table 3 lists the microstructure observation and analysis results of the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3.

[0108] Table 3.

[0109]

[0110]

[0111] It is evident from Table 3 above that the microstructure of the ultra-high strength cold-rolled steel strips in Examples 1-8 of this invention contains a matrix and carbide particles uniformly dispersed within the matrix. The average diameter of these carbide particles does not exceed 0.5 micrometers.

[0112] In this invention, the matrix of the microstructure of the ultra-high strength cold-rolled steel strip in Examples 1-8 may include: tempered martensite, bainite, ferrite, and retained austenite. The volume fraction of tempered martensite is between 96-98%, all exceeding 95%; the volume fraction of bainite is between 1.2-3.3%; the volume fraction of ferrite is between 0.3-0.4%; and the volume fraction of retained austenite is between 0.3-0.4%.

[0113] Accordingly, through the inventor's observation and analysis, it was found that in the microstructure of the ultra-high strength cold-rolled steel strips of Examples 1-8, the strong carbide particles uniformly dispersed in the matrix may include: Fe3C, Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

[0114] After completing the above observation and analysis of the microstructure, in order to test the performance of the steel strip, the inventors further sampled the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3, and conducted various performance tests. The test results are listed in Table 4.

[0115] The relevant performance testing methods are as follows:

[0116] (1) Tensile test: GB / T3076-1982 Metallic Sheet Tensile Test Method was adopted, with a gauge length of 50 mm. The yield strength, tensile strength and elongation (δ) of the original annealed steel sheet at room temperature of each embodiment and comparative example were tested.

[0117] (2) Impact Test: The impact toughness of the original annealed steel sheets in each embodiment and comparative example was tested according to GB / T229-1994, Charpy Impact Test Method, at room temperature (25℃) or other temperature conditions. The original annealed steel sheets underwent a 2% pre-deformation, followed by simulated baking at 170℃ for 20 minutes. The impact toughness was then tested as described above to obtain the impact toughness values ​​of the original annealed steel sheets after simulated part forming and painting. The results are shown below. Figure 1 middle.

[0118] (3) Cold bending: According to GB / T38806-2020 Test Method for Bending Performance of Sheet Metals and Strips, the original annealed steel sheets of Examples 1-8 and Comparative Examples 1-3 were subjected to 90° cold bending, and the minimum bending radius was tested. After cold bending, the inner bending radius / plate thickness of the ultra-high strength cold-rolled steel strip of Examples 1-8 was 3, and the inner bending radius / plate thickness of the comparative steel strip of Comparative Examples 1-3 was 3.5.

[0119] (4) The hydrogen-induced cracking resistance test of the steel strip before baking was conducted using the following non-standard method: the original annealed steel plates of each embodiment and comparative example were subjected to U-bending. The resulting U-shaped bent steel plate samples (with an inner corner radius of 8 mm and strain gauges controlling the stress level in the U-bending arch area to be 1 times the tensile strength) were then immersed in 1 mol / L hydrochloric acid and observed for cracking after 300 hours. If no cracking occurred, it indicated excellent hydrogen-induced cracking resistance; if cracking occurred, the cracking time was recorded, indicating that the cracked steel had poor hydrogen-induced cracking resistance.

[0120] (5) The hydrogen-induced cracking resistance test of the steel strip after 2% pre-deformation and heating and holding at 170℃ for 20 minutes (painting baking process) was conducted using the following non-standard method: First, the steel plate was pre-deformed by 2% to simulate part forming. Then, the steel plate was heated to 170℃ and held for 20 minutes to simulate the paint baking process of the part. The U-shaped bent steel (U-bend inner corner radius of 8mm, strain gauge controlled stress level of 1.2 times tensile strength in the U-bend arch area) corresponding to each embodiment and comparative example after the pre-forming and simulated paint baking process was immersed in 1mol / L hydrochloric acid and observed whether cracking occurred after 300 hours of immersion. If no cracking occurred, it indicates excellent hydrogen-induced cracking resistance; if cracking occurred, the cracking time was recorded, indicating that the hydrogen-induced cracking resistance of the cracked steel was poor.

[0121] Table 4 lists the performance test results of the ultra-high strength cold-rolled steel strips of Examples 1-8 and the comparative steel strips of Comparative Examples 1-3.

[0122] Table 4.

[0123]

[0124]

[0125] As can be seen from Table 4, the ultra-high strength cold-rolled steel strips of Examples 1-3 of the present invention have significantly better overall performance than the comparative steel strips of Comparative Examples 1-3.

[0126] As shown in Table 4, the ultra-high strength cold-rolled steel strips of Examples 1-8 all possess excellent mechanical properties, with yield strengths between 1190-1370 MPa, tensile strengths between 1470-1650 MPa, elongation between 5-7%, and impact toughness between 44-47 J / cm. 2 Furthermore, after being cold-bent at 90°, the ultra-high strength cold-rolled steel strips of Examples 1-8 all have an inner bending radius / plate thickness of 3, and their ratio of inner bending radius / plate thickness is smaller than that of Comparative Examples 1-3, indicating superior cold bending performance.

[0127] Correspondingly, whether before the paint baking process or after simulated pre-deformation and heating at 170°C for 20 minutes (paint baking process), the U-shaped bent steel corresponding to the ultra-high strength cold-rolled steel strip of Examples 1-8 of the present invention does not crack after being immersed in 1 mol / L hydrochloric acid for 300 hours. It has excellent resistance to hydrogen-induced cracking before and after the paint baking process.

[0128] In contrast, the steel strips in Comparative Examples 1-3 did not meet the design requirements of this invention in terms of chemical element composition and preparation process. After being heated and held at 170°C for 20 minutes (baking process), they did not have good resistance to hydrogen-induced cracking. When immersed in 1 mol / L hydrochloric acid, they cracked early and had poor safety performance.

[0129] Figure 1 The impact toughness of the ultra-high strength cold-rolled steel strip of Example 3 and the comparative steel strip of Comparative Example 3 are schematically shown in the annealed state and the annealed + 2% pre-deformation + baking state, respectively.

[0130] exist Figure 1 In the toughness tests, the steel plates in their original annealed state were tested directly, or the tests were conducted after simulating part forming and baking / painting. "A3 annealed state" corresponds to "the original annealed state of the steel plate in Example 3," and "B3 annealed state" corresponds to "the original annealed state of the steel plate in Comparative Example 3." "A3 annealed + 2% pre-deformation + baking state" corresponds to "the state of the original annealed steel plate of Example 3 after 2% pre-deformation, followed by a simulated baking / painting at 170℃ for 20 minutes." "B3 annealed + 2% pre-deformation + baking state" corresponds to "the state of the original annealed steel plate of Comparative Example 3 after 2% pre-deformation, followed by a simulated baking / painting at 170℃ for 20 minutes."

[0131] like Figure 1As shown, the ultra-high strength cold-rolled steel strip of Example 3 prepared by the present invention, after undergoing a 2% pre-deformation simulated forming process and then baking at 170°C for 20 minutes, exhibits further improvements in its resistance to hydrogen-induced cracking and toughness. This indicates that the material possesses excellent service performance when actually applied in automotive manufacturing processes. In contrast, the toughness of the comparative steel in Comparative Example 3 decreased after the 2% pre-deformation simulated forming and baking at 170°C for 20 minutes.

[0132] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0133] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. An ultra-high strength cold-rolled steel strip with a tensile strength of over 1450 MPa, containing Fe and unavoidable impurity elements, characterized in that, It also 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%; Its microstructure has a matrix and carbide particles uniformly dispersed in the matrix, wherein the matrix has tempered martensite with a volume fraction of more than 95%, and the average diameter of the carbide particles does not exceed 0.5 micrometers; the carbide particles include Fe3C; and at least one of Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC; Its resistance to hydrogen-induced cracking meets the following requirements: U-shaped bending specimens with a prestress equal to one times the tensile strength do not crack after being immersed in 1 mol / L hydrochloric acid for more than 300 hours; after being heated and held at 170℃ for 20 minutes, its resistance to hydrogen-induced cracking meets the following requirements: U-shaped bending specimens with a prestress greater than or equal to 1.2 times the tensile strength do not crack after being immersed in 1 mol / L hydrochloric acid for more than 300 hours.

2. The ultra-high strength cold-rolled steel strip as described in claim 1, characterized in that, Its mass percentage of each chemical element 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 balance being Fe and unavoidable impurities.

3. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, In unavoidable impurities, P ≤ 0.015%, S ≤ 0.002%, and N ≤ 0.005%.

4. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, Its chemical elements also 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％。 5. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, Its 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%.

6. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, Its microstructure also includes bainite.

7. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, The volume fractions of ferrite and retained austenite in its microstructure matrix are both <0.5%.

8. The ultra-high strength cold-rolled steel strip as described in claim 1 or 2, characterized in that, Its tensile strength is ≥1450MPa; room temperature impact toughness is ≥38J / cm. 2 .

9. A method for manufacturing ultra-high strength cold-rolled steel strip as described in any one of claims 1-8, characterized in that, Including the following steps: (1) Smelting and casting; (2) Hot rolling: The temperature is heated to 1150-1200℃ and held for 40-50 minutes; the final rolling temperature is controlled at 870-920℃; after rolling, the temperature is rapidly cooled to the coiling temperature at a cooling rate of 20-50℃ / s; the coiling temperature is controlled at 500-600℃; and the cooling is controlled after coiling. (3) Cold rolling after pickling; (4) Continuous annealing; (5) Tempering: Heat to tempering temperature of 200-300℃ by induction heating and hold for more than 150 seconds.

10. The manufacturing method as described in claim 9, characterized in that, In step (2), after coiling, the coil is first cooled by air cooling until the surface temperature of the steel coil is between 400-500℃, and then cooled by a fan until the surface temperature of the steel coil is below 200℃.

11. The manufacturing method as described in claim 9, characterized in that, In step (3), the cold rolling reduction rate is controlled to be 30-65%.

12. The manufacturing method as described in claim 9, characterized in that, In step (4), at 5℃ / s The above heating rate is used to heat to the austenitic single-phase region and hold for 30-120 seconds; then it is cooled to 700-780℃ at a rate of 3-10℃ / s, and then water-cooled to below 100℃ at a rate of not less than 700℃ / s, followed by pickling.

13. The manufacturing method as described in claim 9, characterized in that, It also includes step (6) leveling, controlling the leveling rate to be ≤0.3%.

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