Ultra-high strength cold-rolled steel strip with tensile strength of 1450 MPa or more 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 a steel material and a manufacturing method thereof, and more particularly to a high strength cold rolled steel material and a manufacturing method thereof. [Background technology]

[0002] Background technology In recent years, with the rapid development of the automobile industry, the market demands for lighter weight and safety of automobiles are increasing. Many automobile manufacturers are seeking to use higher strength steel sheets to meet the demands for lighter weight and safety.

[0003] However, it has been found that the higher the strength of a steel plate, the more its formability and toughness deteriorate, and it is prone to brittle fracture due to the inevitable contact with hydrogen during processing and use, i.e., it has hydrogen embrittlement properties, which severely reduces the safety and protective effect of ultra-high strength steel and causes many problems during use.

[0004] In terms of existing technology, some researchers have carried out optimization design related to high-strength steel plates, and have achieved certain results, resulting in some high-strength steels with tensile strengths of 1450 MPa or more.

[0005] For example, a Chinese technical document with publication number CN110684932A, publication date January 14, 2020, and title "1500 MPa level cold-formed strip steel and its manufacturing method" discloses a 1500 MPa level cold-formed strip steel and its manufacturing method, whose 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 solution includes hot metal pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing and leveling processes. However, the continuous annealing process is: heating and soaking temperature are both 820-860℃, soaking time is 50-100s, quenching start temperature is 660-680℃, cooling rate is 80-100℃ / s, and overaging temperature is 260-300℃. The final strip steel structure is uniformly distributed island martensite and a small amount of ferrite, with ultra-high strength and excellent weldability and cold formability.

[0006] Also, for example, a Chinese patent document with publication number CN112981252A, publication date June 18, 2021, and title "1500MPa level automotive steel plate and its manufacturing method" discloses a 1500MPa level automotive steel plate and its manufacturing method, and 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%. At the same time, the manufacturing method of this technical solution includes hot rolling, cold rolling, annealing, leveling, and hot forming processes. The automotive steel plate finally obtained by adopting this technical solution has a tensile strength of 1500-1600 MPa, a yield strength of 1000-1200 MPa, and an elongation rate of ≧ 5%.

[0007] Furthermore, for example, a Chinese patent document with publication number CN112522573A, publication date March 19, 2021, and title "B-containing martensitic strip steel and its manufacturing method" discloses a B-containing martensitic strip steel and a manufacturing method thereof, whose chemical composition, in 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, and N: 0.004-0.01%. Further, 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, and Mn / S is ≧ 250. The martensitic strip steel has a yield strength of 800-1200 MPa, a tensile strength of 1100-1900 MPa, and an elongation rate of 3-12%, and can be widely applied in the field of high-strength automotive steel.

[0008] The ultra-high strength steels disclosed in the above patent technical documents all relate to strengths of 1450 MPa or more, 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 patent documents take into consideration delayed cracking after forming a part from a steel plate and then baking paint. When steel materials are actually used to manufacture automobile parts, they need to be baked after forming. However, it should be known that if the problem of delayed cracking after baking paint after part forming is not taken into consideration, the parts are likely to have quality defects after baking paint. Summary of the Invention [Means for solving the problem]

[0009] Contents of the invention One object of the present invention is to provide an ultra-high strength cold rolled strip steel having a tensile strength of 1450 MPa or more. By combining rational component design and process design, the present invention makes it possible to obtain an ultra-high strength cold rolled strip steel having a tensile strength of 1450 MPa or more which has ultra-high strength, excellent cold bending properties and good hydrogen-induced cracking resistance, and which has excellent toughness and hydrogen-induced cracking resistance even in parts which have been heated and kept at 170°C for 20 minutes after forming (a baking paint process for automotive parts)

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

[0011] In order to achieve the above object, the present invention provides a steel sheet containing Fe and unavoidable impurity elements, and further containing 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%; The microstructure of the present invention is an ultra-high strength cold rolled strip steel having a tensile strength of 1450 MPa or more, the microstructure of which has a matrix and carbide particles uniformly and dispersively distributed in the matrix, the matrix having a tempered martensite volume fraction of 95% or more, and the carbide particles having 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 percentage of each chemical element is: 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%, and the balance is Fe and unavoidable impurities.

[0013] In the above technical solution, the ultra-high strength cold rolled steel strip of the present invention adopts C, Si, Mn, and B as basic additive elements in the design of the chemical composition, ensuring 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, which is applicable when C is 0.2-0.5%, but the carbon content of the steel material designed according to the present invention to obtain the same strength is always lower than the calculated value of the empirical formula.

[0014] In addition, in the present 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 in the steel sheet during processing, improving the tempering resistance of martensite and suppressing the precipitation of Fe3C during the tempering process, thereby suppressing the large amount of hydrogen adsorption by reversible hydrogen traps and preventing the problem of excessively high diffusive hydrogen in the steel sheet. Furthermore, an appropriate amount of Cu element is also added to the ultra-high strength cold rolled strip steel in order 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 of each chemical element are as follows:

[0016] C: In the ultra-high strength cold rolled steel strip according to the present invention, the C element affects the hardness of martensite to improve the strength of the steel material, and adding an appropriate amount of C to the steel is advantageous to the strength of the material. However, it should be noted that the higher the C element content in the steel, the harder the martensite becomes and the greater the brittleness, which is disadvantageous for welding, tends to be. Therefore, in the present invention, taking into consideration the effect of the C element content on the performance of the steel material, the mass percentage content of the C element is actually controlled to 0.19 to 0.245%.

[0017] Of course, in some preferred embodiments, the mass percentage content of C element may be further controlled to 0.195-0.24% in order 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 is likely to promote the formation of retained austenite. Therefore, in order to prevent 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% in order 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 the hardenability, and can improve the hardenability of the steel material, which is also advantageous for strength. However, Mn increases the carbon equivalent of the steel, and therefore it is necessary to design the Mn content as low 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 the Mn element is controlled to 0.8 to 1.2% in the ultra-high strength cold rolled steel strip according to the present invention.

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

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

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

[0024] Cu: In the ultra-high strength cold rolled steel strip according to the present invention, the Cu element can improve the corrosion resistance of the steel material and is also advantageous in improving the hydrogen induced cracking properties of the material. However, it is also not preferable to add excessive Cu to the steel, and it should be noted that excessive addition of Cu element leads to high temperature brittleness of the steel material. Therefore, it is necessary to strictly control the Cu element content, and in the present invention, the mass percentage content of Cu element is controlled to 0.05 to 0.15%.

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

[0026] Ti: In the ultra-high strength cold rolled steel strip according to the present invention, the Ti element can fix N and can fully exert the hardenability improving effect of B. Also, TiC formed by the Ti element in the steel is advantageous for dispersing hydrogen coagulation, but TiN is disadvantageous for the plasticity of the steel. Therefore, in the present invention, taking into consideration the effect of the Ti element on the performance of the steel material, the mass percentage content of the Ti element is controlled to 0.005 to 0.05%.

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

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

[0029] In summary, the present invention rationally designs the chemical composition, and after smelting, casting and rolling, rapid quenching during continuous annealing can obtain martensite with a volume fraction of at least 95%, the remainder being bainite, and in unavoidable cases, it may contain a small amount of ferrite and retained austenite, but the contents (volume fractions) of ferrite and retained austenite are both <0.5%. Through such component design and process design, the content of retained austenite can be minimized, and the adverse effects of internal stress and brittle phase caused by the transformation of retained austenite into high carbon martensite during the forming process can be avoided.

[0030] In addition, by adding a combination of strong carbide forming elements with appropriate types and contents, a large amount of fine carbide particles are dispersed and precipitated in the matrix, so that these carbide particles are uniformly and dispersively distributed in the matrix metal, have an average diameter of 0.5 microns or less, and are not likely to grow during tempering. This design can effectively improve the temper resistance of martensite, ensure that the precipitate particles are fine after tempering, and ensure that the hydrogen induced cracking resistance of the material is not deteriorated.

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

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

[0033] P: In the present invention, P is an impurity element in steel, which reduces the toughness of the steel material and has an adverse effect on delayed cracking. Therefore, in the present invention, it is necessary to strictly control the P element content in the steel, and the mass percentage content of P is controlled to be P≦0.015%.

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

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

[0036] Of course, in some preferred embodiments, in order to obtain better performance, the contents of the impurity elements P, S, and N may be further limited, and preferably controlled to be 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 the present invention, W, Mo, Ni, Ca, Nb and V elements may be further added in order to obtain an ultra-high strength cold rolled steel strip having better performance.

[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 not only advantageous for forming fine precipitates distributed in a dispersed manner, but also advantageous for dispersing local hydrogen cohesion. Therefore, in order to exert the beneficial effects of W element, it is preferable to add 0.05-0.15% of W to the ultra-high strength cold rolled strip steel of the present invention.

[0040] Mo: In the technical solution of the present invention, Mo element can improve the hardenability of the steel material, and is not only favorable for forming fine TiMoC precipitates distributed in a dispersed manner, but also favorable for dispersing local hydrogen aggregation. Therefore, in order to exert the beneficial effects of Mo element, Mo may be added in an amount of preferably 0.05 to 0.15% in the ultra-high strength cold rolled strip steel of the present invention.

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

[0042] Like the Zr and Ti elements in steel, the above-mentioned W, Mo, V and Nb which are preferably added are all strong carbide-forming elements, and 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 in the steel sheet during processing, improving the tempering resistance properties 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 in improving the corrosion resistance of steel and in alleviating the brittleness caused by Cu. Therefore, in the ultra-high strength cold rolled steel strip of the present invention, Ni may be added in an amount of preferably 0.05 to 0.15%.

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

[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: C: 0.195-0.24%, Si: 0.03 to 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 the microstructure further contains bainite, and preferably the volume fraction of the bainite is 1.2 to 3.3%.

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

[0048] Furthermore, in the ultra-high strength cold rolled steel strip according to the present invention, the carbide particles contain 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 of the present invention, its tensile strength is ≧1450MPa; its room temperature impact toughness (Charpy V-notch) is ≧38J / cm 2and its hydrogen-induced cracking resistance satisfies the requirement that a U-shaped bend test piece having a prestress equal to 1 times the tensile strength will not crack even if immersed in hydrochloric acid with a concentration of 1 mol / L for 300 hours or more; and after heating and holding at 170°C for 20 minutes, its hydrogen-induced cracking resistance satisfies the requirement that a U-shaped bend test piece having a prestress equal to 1.2 times the tensile strength will not crack even if immersed in hydrochloric acid with a concentration of 1 mol / L 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 is in the range.

[0050] Accordingly, another object of the present invention is to provide a method for producing the above ultra-high strength cold rolled steel strip, and the inventors further optimize the design of the preparation process of the production method to meet the design of the chemical composition of the steel strip, which can efficiently prepare the ultra-high strength cold rolled steel strip of the present invention, and has good application prospects.

[0051] In order to achieve the above object, the manufacturing method of the above ultra-high strength cold rolled steel strip provided by the present invention includes the following steps: (1) Smelting and casting; (2) Hot rolling: heating to 1150-1200°C, keeping for 40-50min, finishing temperature is 870-920°C, quenching to coiling temperature after rolling, cooling rate is 20-50°C / s, coiling temperature is 500-600°C, and controlled cooling after coiling; (3) Cold rolling after pickling; (4) Continuous annealing; (5) Tempering: Heat to a tempering temperature of 200 to 300°C using induction heating, and keep at that temperature for 150 seconds or more.

[0052] In the manufacturing method of the ultra-high strength cold rolled steel strip according to the present invention, the inventors have optimized the design of the hot rolling process, and the hot rolling adopts the processes of low temperature heating, low temperature rolling end, and low temperature coiling. Of course, the steel coil after coiling can be further quenched by blowing with a blower to suppress the precipitation and growth of precipitates (WC / TiMoC / Ti(C,N) / Nb(C,N)), which can induce secondary precipitation during the continuous annealing process of the cold rolled sheet, resulting in finer and stronger carbide precipitation.

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

[0054] Furthermore, in the manufacturing method according to the present invention, in step (2), after coiling, the steel coil is cooled by air until the surface temperature becomes 400 to 500°C, and then blown with a blower to cool the steel coil until the surface temperature becomes less than 200°C.

[0055] In the above technical solution of the present invention, the purpose of the controlled cooling after coiling is to accelerate the cooling so as to suppress 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 precipitation of strong carbides.

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

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

[0058] Accordingly, in some preferred embodiments, after completing the above step (4), the pickled steel sheet may be further subjected to alkaline cleaning and rinsing to remove residual acid on the surface of the steel sheet, and after drying, the steel sheet may be induction heated to 200-300°C to be tempered. Preferably, the tempering time is 200 s or more to obtain tempered martensite with fine secondary precipitate particles.

[0059] Of course, after removing residual acid on the steel sheet surface by alkaline cleaning and rinsing and drying the steel sheet, more preferably, the steel sheet may be heated to a tempering temperature of 200 to 250°C by induction heating and tempered for 400 s or more, or leveling may be performed after the tempering treatment.

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

[0061] The ultra-high strength cold rolled steel strip having a tensile strength of 1450 MPa or more and the manufacturing method thereof according to the present invention have the following advantages and beneficial effects compared to the prior art: Compared with the current prior art steel materials, the present invention obtains ultra-high strength cold rolled strip steel with a tensile strength of more than 1450 MPa with a lower carbon equivalent. By adopting a reasonable chemical composition, the inventors can effectively suppress the chance of the formation of retained austenite, and the retained austenite content can be kept below 0.5%, so as to avoid the additional stress caused by the transformation of retained austenite into high carbon martensite during the forming process.

[0062] Furthermore, the inventors added strong carbide-forming elements when designing the chemical composition, thereby effectively suppressing the enrichment of local solid solution carbon and further suppressing the formation of retained austenite, and at the same time, by dispersing and precipitating a large amount of fine precipitates in the steel, the toughness of the steel plate is improved, and a decrease in toughness due to the aggregation and growth of carbides during the subsequent baking paint process of the parts can be effectively avoided. In addition, the internal stress relief effect during the baking process also improves the delayed crack resistance of the parts.

[0063] The ultra-high strength cold rolled steel strip not only has an ultra-high strength of 1450 MPa or more, but also has excellent cold bending properties and good resistance to hydrogen-induced cracking, and the steel has excellent toughness and resistance to hydrogen-induced cracking even after heating and holding for 20 minutes at 170° C. The microstructure of the ultra-high strength cold rolled steel strip is dominated by tempered martensite, containing at least 95% or more tempered martensite, the balance being bainite, and may contain small amounts of ferrite and retained austenite in unavoidable cases, with the contents of each being <0.5% by volume. In terms of sub-structure, the outstanding feature of the ultra high strength cold rolled strip steel is a large amount of fine carbide particles dispersed and precipitated in the matrix, which include Fe3C and Ti(C,N), Nb(C,N), ZrC, WC, VC, TiMoC, etc. depending on the material, and these carbide particles are uniformly and dispersively distributed in the matrix metal, and the particles have an average diameter of less than 0.5 microns.

[0064] The ultra-high strength cold rolled strip steel has excellent hydrogen-induced cracking resistance, and a U-shaped bent test piece prepared with the steel and having a prestress equal to one time the tensile strength does not crack even after immersion in hydrochloric acid with a concentration of 1 mol / L for more than 300 hours. The steel plate is pre-deformed by 2% and heated and kept at 170°C for 20 minutes to simulate the forming and baking paint process of automobile parts, and the hydrogen-induced cracking resistance is further improved. That is, a U-shaped bent test piece with a prestress of 1.2 times the tensile strength does not crack even after immersion in hydrochloric acid with a concentration of 1 mol / L for more than 300 hours. Due to this characteristic, the above ultra-high strength cold rolled strip steel has relatively good usability when used in the manufacture of automobile safety structural parts. [Brief description of the drawings]

[0065] [Figure 1] FIG. 1 is a schematic diagram showing a comparison of impact toughness between the ultra-high strength cold rolled steel strip according to Example 3 and the comparative steel strip according to Comparative Example 3 in the annealed state and the annealed + 2% pre-deformed + baked state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0066] Specific embodiments The ultra-high strength cold rolled steel strip and the manufacturing method thereof according to the present invention will be further explained with reference to the following specific examples, but the explanations and interpretations are not intended to unduly limit the technical solution of the present invention.

[0067] Examples 1 to 8 and Comparative Examples 1 to 3 The ultra-high strength cold rolled steel strips according to Examples 1 to 8 of the present invention and the comparative steel strips according to 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 kept at that temperature for 40-50 minutes to perform heating and hot rolling. The rolling end temperature was controlled to 870-920°C. After rolling, the billet was rapidly cooled to the coiling temperature, the cooling rate was controlled to 20-50°C / s, the coiling temperature was controlled to 500-600°C, and after coiling, the billet was cooled by air cooling until the surface temperature of the steel coil reached a temperature between 400-500°C, and then the surface temperature of the steel coil was cooled by blowing with a blower until the surface temperature of the steel coil reached less than 200°C.

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

[0070] (4) Continuous annealing: The cold-rolled steel sheet is subjected to continuous annealing, heated to the austenite single phase region at a heating rate of 5°C / s or more, held at that temperature for 30 to 120 s, and then cooled to 700 to 780°C at a rate of 3 to 10°C / s, water-cooled to below 100°C at a rate of 700°C / s or more, and then pickled.

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

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

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

[0074] In the present invention, the chemical element compositions 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-mentioned process steps, but their chemical element compositions and / or related process parameters included parameters that did not meet the design specifications of the present invention.

[0075] Table 1 shows the mass percentages of the chemical elements 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] Specific process parameters in the above-mentioned processes and steps for producing the ultra-high strength cold rolled steel strips according to Examples 1 to 8 and the comparative steel strips according to Comparative Examples 1 to 3 are shown in Tables 2-1 and 2-2.

[0078] [Table 2-1]

[0079] [Table 2-2]

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

[0081] The observation and analysis results of the microstructures of the ultra-high strength cold rolled steel strips according to Examples 1 to 8 and the comparative steel strips according to Comparative Examples 1 to 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 comprises a matrix and carbide particles uniformly and dispersively distributed in the matrix, with the carbide particles having an average diameter of 0.5 microns or less.

[0084] In the present invention, the microstructural matrix of the ultra-high strength cold rolled steel strips according to Examples 1 to 8 may contain tempered martensite, bainite, ferrite and retained austenite, with the volume fraction of tempered martensite being 96 to 98%, each exceeding 95%, the volume fraction of bainite being 1.2 to 3.3%, the volume fraction of ferrite being 0.3 to 0.4%, and the volume fraction of retained austenite being 0.3 to 0.4%.

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

[0086] After completing the above observation and analysis of the microstructure, the inventors further sampled the obtained ultra-high strength cold rolled steel strips according to Examples 1 to 8 and the finished comparative steel strips according to Comparative Examples 1 to 3 and carried out various performance tests in order to test the performance of the steel strips. The obtained test results are shown in Table 4.

[0087] The relevant performance test measures were: (1) Tensile test: According to the tensile test method for metal sheets of GB / T3076-1982, the yield strength, tensile strength, and elongation (δ) of the steel sheets in the initially annealed state in each of the examples and comparative examples at room temperature were measured using a scale distance of 50 mm.

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

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

[0090] (4) The hydrogen-induced crack resistance of the strip steel before baking was tested by the following non-standard method: the thickness of the test steel plate is 1.2 mm, the dimensions of the U-shaped bent steel plate sample are 150 mm × 30 mm × 1.2 mm, one strain gauge is installed at the point of maximum strain for each sample, the allowable error of the prestress value is ± 2%, the immersion temperature is room temperature, and the immersion test should be carried out within 4 hours after the bending treatment under prestress (without considering the effect of stress relaxation due to natural aging on the test results), and the steel plate in the initial annealed state according to each embodiment and comparative example is bent into a U-shape to obtain the corresponding U-shaped bent steel plate test piece (the U-shaped bend inner corner radius is 8 mm, and the stress level of the vault area of ​​the U-shaped bend is controlled to 1 times the tensile strength by the strain gauge), which is then immersed in hydrochloric acid with a concentration of 1 mol / L, and the presence or absence of cracks is observed after immersion for 300 hours. If no cracks occurred, it was shown that the steel had excellent resistance to hydrogen-induced cracking. If cracks occurred, the time at which the cracks occurred was recorded, and the steel was shown to have poor resistance to hydrogen-induced cracking.

[0091] (5) The hydrogen-induced cracking resistance of the strip steel after pre-deformation by 2% and heating and holding at 170℃ for 20 minutes (baking paint 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 samples were 150 mm × 30 mm × 1.2 mm, one strain gauge was installed at the point of maximum strain for each sample, the tolerance of the prestress value was ±2%, the immersion temperature was room temperature, and the immersion test should be carried out within 4 hours after the bending process under prestress (stress relaxation due to natural aging on the test results). (not considering the influence of the sum of the two), first, the steel plate was pre-deformed by 2% to simulate the part forming, and then the steel plate was heated and kept at 170 ° C for 20 minutes to simulate the part baking paint process. After the simulation of the pre-forming and baking paint processes was completed, the corresponding U-shaped bent steel material (U-shaped bend inner corner radius is 8 mm, and the stress level of the vault area of ​​the U-shaped bend is controlled to 1.2 times the tensile strength by a strain gauge) according to each embodiment and comparative example 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 there is no crack, it indicates that the hydrogen-induced cracking resistance is excellent, and if there is crack, the time when the crack occurred was recorded, and the cracked steel material indicated poor hydrogen-induced cracking resistance.

[0092] Table 4 shows the test results of the performance of the ultra-high strength cold rolled steel strips according to Examples 1 to 8 and the comparative steel strips according to Comparative Examples 1 to 3.

[0093] [Table 4]

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

[0095] As can be seen from Table 4, the ultra-high strength cold rolled steel strips according to Examples 1 to 8 all have excellent mechanical properties, with their yield strengths being in the range of 1190 to 1370 MPa, tensile strengths being in the range of 1470 to 1650 MPa, elongations being in the range of 5 to 7%, and impact toughness being in the range of 44 to 47 J / cm 2 In addition, the ultra-high strength cold rolled steel strips according to Examples 1 to 8 all have an internal bending radius / sheet thickness of 3 after 90° cold bending, and these ratios of internal bending radius / sheet thickness are smaller than those of Comparative Examples 1 to 3, and have better cold bending properties.

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

[0097] In contrast, the comparative steel strips according to Comparative Examples 1 to 3 did not have good resistance to hydrogen-induced cracking after being heated and held at 170°C for 20 minutes (baking paint process) because the design of their chemical element compositions and the manufacturing process did not satisfy the design requirements of the present invention. When immersed in hydrochloric acid with a concentration of 1 mol / L, they cracked early and were therefore less safe.

[0098] FIG. 1 is a schematic diagram showing a comparison of impact toughness between the ultra-high strength cold rolled steel strip according to Example 3 and the comparative steel strip according to Comparative Example 3 in the annealed state and the annealed + 2% pre-deformed + baked state.

[0099] In Fig. 1, all the toughness tests were performed directly on the steel sheets in the initial annealed state or after simulating part forming and part baking paint. However, "A3 annealed state" corresponds to "the initial annealed state of the steel sheet according to Example 3", "B3 annealed state" corresponds to "the initial annealed state of the steel sheet according to Comparative Example 3", "A3 annealed + 2% pre-deformation + baking state" corresponds to "the state after simulating 2% pre-deformation of the steel sheet in the initial annealed state according to Example 3 and baking paint at 170°C for 20 minutes", and "B3 annealed + 2% pre-deformation + baking state" corresponds to "the state after 2% pre-deformation of the steel sheet in the initial annealed state according to Comparative Example 3 and baking paint at 170°C for 20 minutes".

[0100] As shown in Fig. 1, the ultra-high strength cold rolled steel strip according to Example 3 prepared according to the present invention was subjected to a 2% pre-deformation to simulate the forming process and then baked at 170°C for 20 minutes, after which the hydrogen induced cracking resistance and toughness of the material were further improved, and thus the material would have excellent performance when put into practical use in the automobile process. Meanwhile, the comparative steel strip according to Comparative Example 3 was subjected to a 2% pre-deformation to simulate the forming process and then baked at 170°C for 20 minutes, but the toughness was reduced.

[0101] Furthermore, the combinations of the technical features in this application are not limited to the combinations described in the claims of this application or the combinations described in the specific examples, and all technical features described in this application can be freely combined or combined in any form, as long as they are not mutually contradictory.

[0102] In addition, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention. The present invention is not limited to the above-mentioned embodiments, and it is clear that any similar changes or modifications that can be directly derived or easily conceived by those skilled in the art from the disclosure of the present invention are included in the protection scope of the present invention.

Claims

1. It contains Fe and unavoidable 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%; The microstructure of this ultra-high strength cold rolled strip steel has a tensile strength of 1450 MPa or more, characterized in that the 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.

2. The mass percentages of each chemical element are: 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%, and the balance being Fe and unavoidable impurities; Preferably, the volume fraction of tempered martensite is between 96 and 98% and the average diameter of said carbide particles is between 0.1 and 0.4 microns. The ultra-high strength cold rolled steel strip according to claim 1 .

3. The ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that, among the unavoidable impurities, P≦0.015%, S≦0.002%, and N≦0.005%; preferably, P≦0.012%, S≦0.0015%, and N≦0.004%.

4. 3. Ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that its 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%.

5. 3. The ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that the mass percentage content of each chemical element further satisfies at least one of the following: 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%.

6. The ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that the microstructural matrix further comprises bainite, preferably the volume fraction of said bainite is 1.2-3.3%.

7. The ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that in its microstructural matrix, the volume fractions of ferrite and retained austenite are both <0.5%, and preferably the volume fractions of ferrite and retained austenite are each 0.3-0.4%.

8. The carbide particles are Fe ３ 3. The ultra-high strength cold rolled strip steel according to claim 1 or 2, characterized in that it contains C and at least one of Ti(C,N), Nb(C,N), ZrC, WC, VC, and TiMoC.

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

10. A method for producing ultra-high strength cold rolled steel strip according to any one of claims 1 to 9, comprising the following steps: (1) Smelting and casting; (2) Hot rolling: heating to 1150-1200°C, keeping at that temperature for 40-50min, finishing rolling at 870-920°C, quenching to coiling temperature after rolling, cooling rate of 20-50°C / s, coiling temperature of 500-600°C, and controlled cooling after coiling; (3) Cold rolling after pickling; (4) Continuous annealing; (5) Tempering: Heat to a tempering temperature of 200 to 300°C by induction heating, and keep at that temperature for 150 seconds or more.

11. The manufacturing method according to claim 10, characterized in that in the step (2), after coiling, the steel coil is cooled by air cooling until the surface temperature of the steel coil reaches 400 to 500°C, and then blown with a blower to cool the steel coil until the surface temperature of the steel coil reaches less than 200°C.

12. The method according to claim 10, wherein in the step (3), the cold rolling reduction is controlled to 30 to 65%.

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

14. In the step (5), before the tempering heating, the steel sheet surface is subjected to alkaline cleaning to remove residual acid solution; preferably, the tempering heating time is controlled to at least 200 s; more preferably, the tempering heating temperature is controlled to 200 to 250 ° C., and the time is set to 400 to 500 s. The manufacturing method according to claim 10, characterized in that

15. The method of claim 10, further comprising step (6) leveling, and controlling the leveling rate to ≦0.3%.

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