High-formability and high-surface-quality 1300MPa-grade cold-rolled martensitic steel for automobiles and preparation method thereof

Abstract

The invention belongs to the field of automobile steel, and particularly relates to high-formability and high-surface-quality 1300MPa-grade cold-rolled martensitic steel for an automobile and a preparation method. The steel comprises the following chemical components in percentage by weight: 0.135%-0.148% of C, 0.35%-0.48% of Si, 1.80%-1.95% of Mn, less than or equal to 0.010% of P, less than or equal to 0.0050% of S, less than or equal to 0.0050% of N, 0.020%-0.035% of Als, 0.015%-0.025% of Ti and the balance of Fe and other inevitable impurities. The tensile strength of the steel is 1300 MPa or above, the yield strength is 1030-1200 MPa, the ductility is larger than or equal to 8%, and the performance requirements of relevant standards for the 1300MPa-grade cold-rolled martensitic steel are met.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of automobile steel, and particularly relates to a 1300MPa-grade cold-rolled martensitic steel with high formability and high surface quality for automobiles and a preparation method. BACKGROUND

[0002] With the increasing demand for individualized material performance in the automobile industry and the increasingly fierce competition in the automobile market, the demand for high-formability high-strength individualized performance steel materials is increasing. Cold-rolled martensitic steel can meet the demand for relatively high formability on the basis of its ultra-high strength mechanical performance and relatively simple production process, thereby providing automobile manufacturers with more flexible material selection. The microstructure of the cold-rolled martensitic steel is mainly fine martensitic structure, and the formation of the martensitic structure mainly relies on the martensitic transformation of austenite under the condition of rapid cooling. Compared with the cold speed limitation of the traditional continuous annealing unit air cooling process, the water cooling process can efficiently and stably provide higher cooling speed for the strip steel, reduce the equipment condition requirement, and realize high-forming performance through individualized design of the alloy, thereby gradually becoming a more optimal scheme for producing high-forming cold-rolled martensitic steel.

[0003] The 1300MPa-grade high-forming cold-rolled martensitic steel can provide good forming performance while considering high strength, has higher yield ratio and tensile performance, and has better flanging and bending performance than traditional martensitic steel. Compared with other cold-rolled automobile high-strength steel, the cold-rolled martensitic steel has significant advantages in mechanical performance and production process. Therefore, it is of great significance to explore a 1300MPa-grade cold-rolled martensitic steel with high formability and high surface quality for automobiles and a preparation method around the high-formability alloy design and water cooling process technical route.

[0004] Patent document CN114086071B "Low-cost 1200Mpa grade cold-rolled high-strength martensitic steel and its manufacturing method", the invention discloses a 1200MPa cold-rolled martensitic steel and its preparation method, the 1200MPa cold-rolled martensitic steel includes the following chemical components: C: 0.15%-0.19%; Si: 0.2%-0.4%; Mn: 1.5%-2.0%; Als: 0.02%-0.08%; P: ≤0.015%; S: ≤0.010%; Ti: 0.020%-0.040%; Cr: 0.15%-0.45%; B: 0.0005%-0.0030%; the balance is Fe and inevitable impurities. The invention adjusts the process requirements of the heating process, welding process and continuous annealing process in the existing manufacturing method; at the same time, after the annealing process, a quenching process is added. Compared with the starting point of the invention based on the water cooling process, the invention adopts air cooling process to complete the martensitic phase change, although B element is selected to replace Mo, Nb and V elements to reduce the cost of alloy, but Cr element is still added to adapt to the cooling rate limit of air cooling process, so the invention has significant difference from the invention in the starting point of composition design; due to the difference in composition design, the process parameters are designed based on the composition characteristics in the hot rolling, cold rolling and annealing process; at the same time, the invention relates to a product with a tensile strength of about 1200MPa and an elongation of 8.5%, while the product described in the invention has similar forming performance and a tensile strength of 1300MPa grade.

[0005] Patent document CN115044831B "1100MPa grade cold rolled martensitic steel and its manufacturing method", the invention discloses a kind of 1100MPa grade cold rolled martensitic steel and its manufacturing method, wherein the chemical composition of 1100MPa grade cold rolled martensitic steel is C:0.13-0.17%, Si:1.05-1.15%, Mn:1.70-1.80%, P:≤0.020%, S:≤0.010%, Cr:0.25-0.35%, Alt:0.030-0.050%, Ti:0.020-0.030%, Ce:10-20ppm, the rest is Fe and inevitable impurities.Compared with the invention based on high cooling rate water cooling process as the design starting point, combined with its cooling rate design can be judged that the invention adopts gas cooling process, at the same time, the invention adopts higher Si component design and additionally adds Cr and Ce elements, while the component design of the present invention is fully combined with the high cooling rate technical characteristics of water cooling process to effectively control the surface quality through lower Si content design;Due to the difference in component design, new process parameter design is carried out based on the component characteristics in hot rolling, cold rolling and annealing process;At the same time, the invention relates to a product with a tensile strength of about 1100MPa and an elongation of 6.5%, while the product described in the present invention has a tensile strength of 1300MPa grade under the premise of better forming performance.

[0006] Patent document CN109898018A "A cold-rolled 1300MPa grade martensitic steel and its production method", the invention discloses a cold-rolled 1300MPa grade martensitic steel and its production method. C: 0.19%-0.25%, Si: 1.0%-1.5%, Mn: 1.8%-2.5%, Al: 0.02%-0.07%, P: ≤0.02%, S: ≤0.005%, N ≤0.005%, the balance is Fe and inevitable impurities. The slab heating temperature is 1230-1280℃, the holding time is more than 180min; the hot rolling opening rolling temperature is greater than 1100℃, the finish rolling temperature is 900-1000℃; the coiling temperature is 650-700℃; the cold rolling reduction is controlled between 40-50%; the heating temperature is 839-880℃, the holding time is 90-120s, water quenching, the water entry temperature is 700-750℃, the cooling rate is 300-500℃ / s, tempering, the tempering temperature is 300-350℃, the aging time is 370-400s, and then air cooling to room temperature. Compared with the technical solution of the invention, although the same water cooling process path is adopted, there is a significant difference between the alloy design idea and the invention. The invention mainly increases the hardenability by adding high C and Mn to form a large number of high internal stress martensitic structure, and then uses higher temperature tempering treatment to reduce the internal stress to ensure the deformation ability of the material, and uses high Si design to avoid the precipitation of carbide; and the invention provides the basis conditions for alloy reduction and matching of suitable annealing temperature by combining with the higher compression ratio of cold rolling to provide more deformation energy for subsequent recrystallization, on the other hand, part of the strengthening and fine grain strengthening are realized by adding trace Ti, and the effective improvement of formability is realized by using lower tempering temperature, and on this basis, the lower Si content design is realized which is beneficial to the surface quality. SUMMARY

[0007] The purpose of the present application is to provide a high formability and high surface quality 1300MPa grade cold rolled martensitic steel for automobiles and a preparation method. The present application is different from the previous process path using air cooling and the design scheme of high alloy cumulative hardenability. The technical advantages of high cooling rate of water cooling process path are fully utilized. On the basis of ensuring that the material realizes martensite base, the production purpose of high formability and high surface quality is realized through reasonable composition and whole process process design. The tensile strength of the steel is more than 1300MPa, the yield strength is 1030-1200MPa, the elongation is ≥8%, and the surface quality of the steel plate after flash nickel plating treatment can reach FB level.

[0008] In order to achieve the above purpose, the following technical scheme is adopted in the present application:

[0009] The application discloses a high-formability high-surface-quality 1300MPa-grade cold-rolled martensite steel for automobiles, and chemical components of the steel include the following in percentage by weight: C: 0.135% to 0.148%, Si: 0.35% to 0.48%, Mn: 1.80% to 1.95%, P: less than or equal to 0.010%, S: less than or equal to 0.0050%, N: less than or equal to 0.0050%, Al: 0.020% to 0.035%, Ti: 0.015% to 0.025%, and the balance of Fe and other inevitable impurities; the tensile strength of the steel is greater than or equal to 1300MPa, the yield strength is 1030MPa to 1200MPa, and the elongation is greater than or equal to 8%, thereby meeting the performance requirements of the 1300MPa-grade cold-rolled martensite steel in the relevant standards.

[0010] The application discloses a high-formability high-surface-quality 1300MPa-grade cold-rolled martensite steel for automobiles, and chemical components of the steel include the following in percentage by weight: C: 0.135% to 0.148%, Si: 0.35% to 0.48%, Mn: 1.80% to 1.95%, P: less than or equal to 0.010%, S: less than or equal to 0.0050%, N: less than or equal to 0.0050%, Al: 0.020% to 0.035%, Ti: 0.015% to 0.025%, and the balance of Fe and other inevitable impurities; the tensile strength of the steel is greater than or equal to 1300MPa, the yield strength is 1030MPa to 1200MPa, and the elongation is greater than or equal to 8%, thereby meeting the performance requirements of the 1300MPa-grade cold-rolled martensite steel in the relevant standards.

[0011] Carbon (C): C elements are dissolved in the iron matrix to cause lattice distortion, play a role of solid solution strengthening as interstitial atoms, and can effectively improve the hardenability and promote the martensitic transformation of austenite. However, too high C content is prone to cause the precipitation of carbides in the tempering stage and affect the welding performance of subsequent processes. In order to ensure the realization of the 1300MPa mechanical properties, the C element content in the application is designed to be 0.135% to 0.148%.

[0012] Silicon (Si): since the solubility of Si elements in Fe3C is low, the precipitation of Fe3C in the aging process is inhibited, and the formation of carbides is avoided to degrade the final performance of the product, but too much Si element generates Fe2SiO4 in the hot rolling stage to affect the surface quality of the steel plate. Therefore, the Si element content in the application is designed to be 0.35% to 0.48%.

[0013] Manganese (Mn): Mn elements provide part of the solid solution strengthening effect, and can effectively reduce the transformation temperature of austenite to ferrite, which is beneficial to the realization of the martensitic transformation in cooperation with the process window of the continuous annealing furnace. However, too high Mn element will affect the welding performance of subsequent processes and generate composition segregation in the hot rolling process. Therefore, the Mn element content in the application is designed to be 1.80% to 1.95%.

[0014] Aluminum (Al): Al elements are a kind of elements that are prone to oxidation, and can effectively deoxidize molten steel in the steelmaking process, but too much Al will affect the continuous casting process and cause problems such as water gap blockage. Therefore, the Al element content in the application is designed to be 0.020% to 0.035%.

[0015] Titanium (Ti): Ti forms Ti(C, N) with C and N, which plays a role in refining the original austenite grain; at the same time, Ti combines with N in the steel to form TiN, which plays a role in fixing N, avoiding free N atoms from deteriorating the toughness of the product. However, if the content of Ti is too high, it will not only increase the cost of the alloy, but also easily lead to the size of TiN being too large, which will deteriorate the performance of the steel plate. Therefore, in the present application, the content of Ti is controlled at 0.015% to 0.025%.

[0016] Phosphorus (P), sulfur (S) and nitrogen (N): phosphorus, sulfur and nitrogen are harmful elements in the product described in the present application, and their content should be reduced as much as possible. However, too low a content requirement will significantly increase the cost of alloy raw materials and steelmaking process, which is not conducive to realizing the characteristics of low-cost products. Therefore, the design of the present application is P≤0.010%, S≤0.0050%, and N≤0.0050%.

[0017] A preparation method of a high-formability high-surface-quality 1300MPa-grade cold-rolled martensitic steel for automobiles, comprising smelting, hot rolling, cold rolling and continuous annealing, specifically comprising:

[0018] Smelting and continuous casting: smelting is carried out by a converter to obtain molten steel with designed composition, and after smelting, the molten steel is continuously cast into a slab.

[0019] Hot rolling: the heating temperature of the slab is between 1200 and 1250℃, the heating time is 120 to 150min, the rough rolling start temperature is between 1040 and 1100℃, the finish rolling temperature is between 850 and 900℃, the middle temperature during coiling is between 500 and 530℃, and the head and tail within 100 meters are 600 to 630℃.

[0020] The design of the heating temperature and the heating time of hot rolling can ensure the full homogenization of alloy elements, ensure the Ti atom precipitation behavior and play a role in fixing N, and at the same time ensure the precipitation of Ti(C, N), pin the original austenite grain boundary and promote the refinement of the original austenite grain. Therefore, the heating temperature of the present application is 1200 to 1250℃, and the heating time is 120 to 150min; the rough rolling start temperature and the finish rolling temperature should also not be too low, in order to ensure the dynamic recrystallization behavior of the original austenite grain in the hot rolling stage; the middle temperature during coiling is controlled at 500 to 530℃, and the coiling method of heating the head and tail is adopted, which can effectively avoid the problem of flat coiling after coiling and improve the stability of cold rolling.

[0021] Cold rolling: the hot-rolled plate needs to be laser welded before cold rolling production to ensure continuous production of cold rolling, the laser power is 11.2-11.8 KW, the welding speed is 4.0-5.0 m / min, the front induction heating power is 5-12 KW, and the rear induction heating power is 10-15 KW; the pickling process adopts a shallow tank turbulent pickling process, the stretch straightening elongation is 0.5-0.8%, the acid liquid temperature is 80-83℃, and the pickling time is 70-90 s. After the pickling process removes the iron oxide scale, cold rolling is carried out, and the cold rolling reduction is between 55% and 60%.

[0022] The welding process scheme can ensure that the weld does not break during cold rolling, and the reasonable design of laser power ensures the weld penetration. Through the reasonable matching of welding speed and front and rear induction heating power, the weld microstructure performance of the product is controlled, the microstructure performance transition with the base material is improved on the basis of ensuring that the high cold speed area in the center of the weld does not appear micro-cracks, and combined with the design of the cold rolling compression ratio, the welding rolling breakage during rolling is avoided. Combined with the design of the composition and hot rolling process of the present application, and considering the proportion and structure characteristics of the silicon-rich layer, the higher stretch straightening elongation and acid liquid temperature can effectively improve the pickling efficiency and ensure the surface quality on the basis of considering the pickling difficulty of the silicon-rich layer. The design of too low cold rolling compression ratio is not conducive to providing reasonable deformation energy storage for the subsequent annealing process to provide a more reasonable process window for the subsequent continuous annealing and recrystallization stage.

[0023] The continuous annealing includes: the furnace atmosphere during the continuous annealing process is 15%-25% H2, and the rest is N2, and the dew point temperature is controlled between-15 and-10℃. The annealing process is designed to heat to 850-880℃ at 2-6℃ / s, and the isothermal time is 100-150 s; the cooling speed during the slow cooling stage is 3-6℃ / s, and the temperature is cooled to 680-700℃; then enter the water quenching and fast cooling stage, and the cooling rate is higher than 200℃ / s to cool to room temperature, then heat to the aging temperature, the aging temperature is 300-350℃, the aging time is 200-280 s, then cool to room temperature for pickling and flash nickel plating, and the specific process includes degreasing-pickling-water washing-flash nickel plating-finishing, on the basis of considering the interaction between the product characteristics (composition, microstructure and performance) and the flash nickel plating process, the flash nickel plating current density is 4-6 A / dm 2 , the plating liquid temperature is 45-50℃, the PH value is 3.8-4.2, the time is 15-50 s, and the finishing rolling force is 6000-7000 KN.

[0024] The heating temperature is selected as 850-880 DEG C to ensure full austenitization, and the subsequent quenching process is more favorable to martensite transformation; and the low slow cooling temperature can cause ferrite phase change, which is not conducive to the formation of the martensite matrix of the final product and the realization of the performance of 1300 MPa; the water quenching process with super-high cooling speed can obtain high-strength martensite structure, and the performance requirement of the product is realized; under the grain refinement effect of Ti, the deformation capacity of the product can be improved and the high formability can be ensured through low-temperature tempering.

[0025] The contribution of the flash nickel plating process to the surface quality is as follows: on the one hand, the dense nickel layer formed by the flash nickel plating completely isolates the contact of the corrosion medium such as humid air and oxygen with the steel matrix, and prevents the oxidation of the matrix iron; on the other hand, the flash nickel plating can completely eliminate the 'black ash' defects of the strip steel in the annealing process. In the annealing process of the cold-rolled strip steel, a very thin and loose oxide layer is formed on the surface, and the main components are FeO, Fe3O4 and the like. In the subsequent leveling, straightening or simple friction, these oxides will fall off in powder form and adhere to the surface of the strip steel, that is, the 'black ash' defects. Before the annealing and the flash nickel plating, the strip steel will be subjected to pickling activation to remove the original oxide film, and then the flash nickel plating is carried out immediately, that is, a new and dense metal layer is deposited on a clean and activated surface. The nickel layer itself will not produce oxide powder, thereby fundamentally eliminating the 'black ash' defects.

[0026] The application adopts the water quenching process to realize the high-formability cold-rolled martensitic high-strength steel, and the product full-process production process is designed individually, so that the production stability is improved and the production cost is further reduced.

[0027] Compared with the prior art, the application has the following beneficial effects:

[0028] 1) The application realizes the individualized component product design by using the water quenching process with high cooling speed, fully utilizes the high cooling speed advantage of the water quenching process to produce the cold-rolled high-strength martensitic steel product, and realizes the high elongation forming performance by reasonably matching the components such as C, Si, Mn and Ti with the heat treatment process on the premise of meeting the performance requirements of the product.

[0029] 2) The application fully considers the process stability of each process in the whole process, avoids many quality defects in the production process, improves the yield, and meets the development needs of the steel enterprises in the green and low-carbon field.

[0030] 3) The application newly designs the process technical parameters of the full-process multi-process, realizes the stable production of the high-formability high-surface-quality 1300 MPa-grade cold-rolled martensitic steel for automobiles, and can be applied to industrialization without other auxiliary equipment and special process equipment, so that the production process is simplified and the production cost is reduced.

[0031] 4) The component design of the present application realizes effective control of surface quality by lower Si content design on the premise of fully combining the high cooling rate technical characteristics of water cooling process; on the one hand, the higher compression ratio of cold rolling provides more deformation energy storage for subsequent recrystallization, and provides basic conditions for alloy reduction and matching of appropriate annealing temperature, on the other hand, partial precipitation strengthening and fine grain strengthening are realized by adding trace Ti, and the formability is effectively improved by using lower tempering temperature, and on this basis, the lower Si content design beneficial to surface quality is realized. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The microstructure diagram of the 1300MPa grade cold rolled martensitic steel product with high formability and high surface quality for automobile manufactured by the manufacturing method of the present application. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the specific embodiments of the present application will be further described below in combination with examples, the following examples are used to specifically describe the content of the present application, and these examples are only a general description of the content of the present application, and do not limit the content of the present application.

[0034] The chemical composition of the example steel is listed in Table 1, the continuous casting and hot rolling process parameters of the example steel are listed in Table 2, the welding process parameters of the example steel are listed in Table 3, the pickling and rolling process parameters of the example steel are listed in Table 4, the continuous annealing process parameters are listed in Table 5, the flash nickel plating process of the example steel is given in Table 6, and the mechanical properties of the example steel are given in Table 7.

[0035] Table 1 Chemical composition of example steel (wt%)

[0036]

[0037] Table 2 Hot rolling process of example steel

[0038]

[0039] Table 3 Welding process of example steel

[0040]

[0041] Table 4 Pickling and rolling process of example steel

[0042]

[0043] Table 5 Annealing process of example steel

[0044]

[0045] Table 6 Flash nickel plating process of example steel

[0046]

[0047] Table 7 Mechanical properties of the example steels

[0048]

[0049] From the above examples, it can be seen that the automobile steel product prepared by using the component design, rolling and continuous annealing process of the application has a tensile strength of 1300 MPa or more, a yield strength of 1030-1200 MPa and an elongation of ≥8%, which meets the use requirements of the 1300 MPa grade cold-rolled martensitic steel for automobiles.

[0050] The above examples are only for illustrating the technical concept and characteristics of the application, and the purpose is to enable those skilled in the art to understand the content of the application and implement it, and cannot limit the protection scope of the application. Any equivalent changes or modifications made according to the spirit and principle of the application shall be covered within the protection scope of the application.

Claims

1. A high formability high surface quality 1300 MPa grade cold rolled martensitic steel for automotive applications, characterized in that, The chemical composition of the steel includes, by weight percentage: C: 0.135%-0.148%, Si: 0.35%-0.48%, Mn: 1.80%-1.95%, P≤0.010%, S≤0.0050%, N≤0.0050%, Als: 0.020%-0.035%, Ti: 0.015%-0.025%, and the balance of Fe and other inevitable impurities.

2. The high formability high surface quality 1300 MPa grade cold rolled martensitic steel for automotive use according to claim 1, characterized by, The tensile strength of the steel is above 1300 MPa, the yield strength is 1030-1200 MPa, and the elongation is ≥8%.

3. A method of producing a high-formability high-surface-quality 1300 MPa-grade cold-rolled martensitic steel for automobiles as claimed in claim 1 or 2, comprising smelting, hot rolling, cold rolling, and continuous annealing; characterized by, Specifically comprising: The continuous annealing includes: the furnace atmosphere during the continuous annealing process is 15%-25% of H2 by volume percentage, and the rest is N2, the dew point temperature is controlled between-15 and-10 ℃, the annealing process is designed to heat to 850-880 ℃ at a heating speed of 2-6 ℃ / s, the isothermal time is 100-150 s; the slow cooling stage is cooled to 680-700 ℃ at a cooling speed of 3-6 ℃ / s; then enters the water quenching fast cooling stage, and is cooled to room temperature at a fast cooling rate higher than 200 ℃ / s, then heated to the aging temperature, the aging temperature is 300-350 ℃, the aging time is 200-280 s, and then cooled to room temperature for acid pickling and nickel flash plating.

4. The method of producing a high-formability high-surface-quality 1300 MPa-grade cold-rolled martensitic steel for automobiles according to claim 3, characterized by, The hot rolling includes: the heating temperature of the slab is between 1200 and 1250 ℃, the heating time is 120-150 min, the rough rolling opening temperature is between 1040 and 1100 ℃, the finish rolling temperature is between 850 and 900 ℃, and the middle part temperature of coiling is between 500 and 530 ℃, and the head and tail within 100 meters are 600-630 ℃.

5. The method of producing a high-formability high-surface-quality 1300 MPa-grade cold-rolled martensitic steel for automobiles according to claim 3, characterized by, The cold rolling includes: the hot rolled plate is laser welded before cold rolling production to ensure continuous production of cold rolling, the laser power is 11.2-11.8 KW, the welding speed is 4.0-5.0 m / min, the front induction heating power is 5-12 KW, and the rear induction heating power is 10-15 KW; the stretch reduction elongation is 0.5-0.8%, the acid liquid temperature is 80-83 ℃, the pickling time is 70-90 s, and the scheme is rolled after removing the iron oxide scale through the pickling process.

6. The method of producing a high-formability high-surface-quality 1300 MPa-grade cold-rolled martensitic steel for automobiles according to claim 3, characterized by, The acid pickling flash plating nickel process specifically includes degreasing-pickling-water washing-flash plating nickel-finishing, the flash plating nickel current density is 4-6 A / dm 2 , the plating solution temperature is 45-50 DEG C, the PH value is 3.8-4.2, the time is 15-50 s, and the finishing rolling force is 6000-7000 KN.

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