Ultra-high strength self-tempering steel plate and preparation method thereof

Abstract

The present invention discloses an ultra-high strength self-tempered automobile steel plate and a preparation method thereof. The chemical composition of the steel plate includes, by mass percentage, C: 0.15-0.23%, Mn: 1.8-2.5%, Si: 0.8-1.5%, Cr: 1.0-1.5%, Al: 0.2-0.8%, Ti+Nb≤0.03%, P<0.02%, S<0.01%, and the balance is Fe. A 1.8-2.0 mm hot-rolled plate is heated to 900° C. and kept warm for at least 3 minutes, and then air-cooled to obtain a test steel. The present invention solves the problem of how to obtain a test steel with appropriate composition under hot rolling process conditions, so that the test steel can achieve mechanical properties of a tensile strength of not less than 1400 MPa and an elongation of not less than 10% due to the self-tempering phenomenon of martensite during quenching and air cooling. The present invention helps to realize the short-process preparation of ultra-high strength steel plates, and has significant application value for the development of lightweight automobiles.

Description

Technical Field

[0001] The invention relates to an ultra-high strength self-tempering automobile steel plate and a preparation method thereof, belonging to a special steel preparation process technology. Background Art

[0002] In the face of energy crisis, environmental pollution and the growth of automobile demand, how to achieve energy conservation and emission reduction has always been a realistic problem that has plagued the development of my country's automobile industry. It is reported that 75% of automobile fuel consumption is related to the weight of the vehicle. Compared with other lightweight materials such as magnesium alloy, aluminum alloy and other composite materials, steel materials, especially steel plates, are still the main materials of my country's automobiles due to their obvious advantages in cost and performance. They can account for more than 70% of the weight of the vehicle body. Theoretically, when the thickness of the steel plate is reduced by 0.05mm, 0.10mm, and 0.15mm, the body weight can be reduced by about 6%, 12%, and 18%. It can be seen that the development of ultra-high reinforcement of automobile steel plates can not only achieve lightweight, but also ensure safety in the event of a collision.

[0003] At present, based on the global industry trend of green, efficient and intelligent development of steel, the near-net-shape short-process production of high-performance steel grades has gradually become the mainstream of the steel industry. The production process of self-tempering steel is relatively simple, and it is an ideal steel grade for promoting near-net-shape production. However, the matching of the alloy composition of ultra-high-strength automotive steel and the self-tempering process requires comprehensive research, and its strength and plasticity also need to be further improved to meet the needs of industrial applications. Summary of the invention

[0004] In order to solve the problems of the prior art, the purpose of the present invention is to overcome the shortcomings of the existing technology and provide an ultra-high strength self-tempering steel plate and a preparation method thereof. The steel is based on low-carbon and low-alloy components, and is regulated by special components. A hot rolling + quenching and air cooling process is adopted to cause the martensite in the steel to undergo self-tempering transformation, which can not only effectively improve its strength and plasticity, but also enable it to have the characteristics of a short process preparation, and has significant application value in the field of ultra-high strength automotive steel research and development.

[0005] In order to achieve the above invention purpose, the present invention adopts the following technical solution:

[0006] An ultra-high strength self-tempered steel plate, calculated by mass percentage, has the following main chemical compositions and proportions:

[0007] C: 0.15-0.23%, Mn: 1.8-2.5%, Si: 0.8-1.5%, Cr: 1.0-1.5%, Al: 0.2-0.8%, Ti+Nb≤0.03%, P<0.02%, S<0.01%, the balance is Fe;

[0008] The ultra-high strength self-tempering steel plate is prepared by the following method:

[0009] (1) Melting and casting process:

[0010] The steel billet having the above-mentioned composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, it is cut into ingot blocks with a thickness of no more than 35 mm;

[0011] (2) Rolled steel plate:

[0012] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be not less than 1200°C, and the final rolling temperature is controlled to be not less than 950°C, and a hot-rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8 to 2.0 mm; the hot-rolled steel plate is heated to 900°C and kept warm for at least 3 minutes for heat treatment, and then air-cooled to obtain the ultra-high strength self-tempering steel plate.

[0013] Preferably, the ultra-high strength self-tempering steel plate of the present invention is composed of the following chemical components in terms of mass percentage:

[0014] C: 0.16~0.22%, Mn: 1.8~2.4%, Si: 0.8~1.4%, Cr: 1.1~1.5%, Al: 0.3~0.6%, Ti≤0.01%, Nb≤0.02%, P≤0.01%, S≤0.008%, and the balance is Fe.

[0015] Preferably, in terms of mass percentage, the ultra-high strength self-tempering steel plate of the present invention has a yield strength of not less than 767 MPa, a tensile strength of not less than 1357 MPa, and an elongation of not less than 9.1% under quenching conditions at 880-920°C.

[0016] Further preferably, in terms of mass percentage, the ultra-high strength self-tempering steel plate of the present invention has a yield strength of not less than 857 MPa, a tensile strength of not less than 1410 MPa, and an elongation of not less than 10.2% under 900° C. quenching conditions.

[0017] A method for preparing the ultra-high strength self-tempering steel plate of the present invention comprises the following steps:

[0018] (1) Melting and casting process:

[0019] The steel billet having the above-mentioned composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, it is cut into ingot blocks with a thickness of no more than 35 mm;

[0020] (2) Rolled steel plate:

[0021] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be not less than 1200°C, and the final rolling temperature is controlled to be not less than 950°C, and a hot-rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8 to 2.0 mm; the hot-rolled steel plate is heated to 900°C and kept warm for at least 3 minutes for heat treatment, and then air-cooled to obtain the ultra-high strength self-tempering steel plate.

[0022] Principle of the invention:

[0023] As we all know, the martensite structure formed after quenching of low-carbon and low-alloy steel in the Fe-Mn-Si-Cr-C system has the performance characteristics of high strength and high hardness. Therefore, in order to achieve the goal of lightweight, martensite is an important organizational factor for the development of automotive steel towards ultra-high strengthening. However, while pursuing ultra-high strength, the loss of plasticity is inevitable. How to balance the strength and plasticity indicators to obtain ideal mechanical properties has always been a bottleneck problem in the research and development of ultra-high strength steels. Current research mostly uses processes such as continuous annealing or quenching and partitioning treatment to introduce residual austenite into the structure, so that it can exert phase transformation-induced plasticity effects to obtain increased strength and plastic growth. However, to achieve the development towards ultra-high strengthening, these steels have complex compositions, high alloy element content, and strict requirements on the corresponding process parameters, making it difficult to achieve short-process preparation.

[0024] Studies have shown that for steels with a low carbon content in the matrix, the martensitic transformation starting temperature (Ms) is higher. During the quenching and cooling process, martensite will be formed at a higher temperature. At this time, since the supersaturated carbon atoms still have a strong diffusion ability, they will escape from the interstitial positions of the martensitic phase crystal lattice to form metastable carbides, causing the martensite to spontaneously undergo a tempering transformation, namely, self-tempering. As a result, the solid solution strengthening effect of martensite is weakened, while the plasticity is improved accordingly.

[0025] However, different cooling rates during quenching have a significant impact on the self-tempering phenomenon. The present invention uses the calculation formula for the cumulative diffusion distance of carbon atoms in martensite in the literature, that is, equations (1) and (2) to compare the diffusion of carbon atoms in the experimental steel under different cooling conditions, and finds that the diffusion distance of carbon atoms in air cooling is 4 times that in water cooling. Obviously, the possibility of martensite self-tempering is greater in air cooling, so that the experimental steel can obtain ultra-high strength and ideal plasticity without additional tempering treatment.

[0026]

[0027] D = 2 × 10 6 exp(-1.092×10 5 / 8.314T) (2)

[0028] Where t is time, D is the diffusion coefficient of carbon, and T is temperature (K).

[0029] In addition, in terms of alloying design, the present invention combines calculation and analysis to find that the increase of alloying elements carbon, manganese, chromium, and silicon will reduce the starting temperature of martensitic transformation, among which carbon has the most significant effect. At the same time, carbon content is the fundamental factor affecting martensitic strength. With the increase of carbon content, the starting temperature of martensitic transformation decreases, while the strength of the formed martensitic structure increases. In other words, although the martensitic strength increases with a high carbon content, it will inhibit the martensitic self-tempering transformation, which makes it difficult to achieve the purpose of balancing strength and plasticity indicators. Therefore, the present invention introduces the alloying element aluminum, and thermodynamic calculations show that the addition of an appropriate amount of aluminum will significantly increase the starting temperature of martensitic transformation and will basically not affect other phase transformation behaviors, which is conducive to the generation of self-tempering effect. In addition, the present invention has scientifically found that, unlike the traditional automotive steel plate preparation process, there is no need to perform a cold rolling process after hot rolling. The test steel using niobium and vanadium composite addition has a better organizational state after being processed by a multi-pass hot rolling process, and the fine grain strengthening and precipitation strengthening effects of the microalloying elements further improve the strength and plasticity of the test steel.

[0030] Through the above process and component design, an ultra-high strength self-tempering steel plate with a tensile strength of not less than 1357MPa and an elongation of not less than 9.1% is obtained which can be prepared by a short process.

[0031] Compared with the prior art, the present invention has the following obvious outstanding substantive features and significant advantages:

[0032] 1. The Fe-Mn-Si-Cr-Al-C system hot-rolled steel plate designed by the present invention can produce a good martensite self-tempering effect under air-cooling quenching conditions, thereby having ultra-high strength and ideal plasticity, and can meet the development needs of lightweight automobiles;

[0033] 2. The present invention can obtain a uniformly organized steel plate by adding niobium and vanadium in combination with a multi-pass hot rolling process, thus realizing a short-process preparation of ultra-high-strength steel;

[0034] 3. The present invention adopts niobium-titanium composite grain refinement and combines multiple hot rolling to make the hot rolling structure of the test steel uniform; the process of air cooling after heat preservation in the austenite single phase region causes the martensite to self-temper. Compared with the water cooling process, air cooling allows carbon atoms to fully diffuse to increase the tempering degree of martensite, obtain tempered martensite structure, thereby effectively improving strength and plasticity, and obtaining ultra-high strength self-tempering steel with a tensile strength of not less than 1357MPa and an elongation of not less than 9.1%;

[0035] 4. The method of the present invention is simple, easy to implement, low in cost, and suitable for popularization and use. DETAILED DESCRIPTION

[0036] The above scheme is further described below in conjunction with specific implementation examples. The preferred embodiments of the present invention are described in detail as follows:

[0037] Embodiment 1

[0038] In this embodiment, an ultra-high strength self-tempered steel plate, calculated by mass percentage, has the following main chemical compositions and proportions:

[0039] C: 0.16%, Mn: 2.4%, Si: 1.4%, Cr: 1.5%, Al: 0.5%, Ti: 0.01%, Nb: 0.01%, P: 0.01%, S: 0.008%, the balance is Fe;

[0040] The ultra-high strength self-tempering steel plate is prepared by the following method:

[0041] (1) Melting and casting process:

[0042] The steel billet having the above composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, the steel billet is cut into ingot blocks with a thickness of 35 mm;

[0043] (2) Rolled steel plate:

[0044] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be 1200°C, the final rolling temperature is controlled to be 950°C, and the hot rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8-2.0 mm. The hot rolled steel plate is then heated to 900°C and kept warm for 3 minutes for heat treatment, and then air-cooled to room temperature to obtain the ultra-high strength self-tempering steel plate.

[0045] Experimental test analysis:

[0046] Through microstructure analysis and room temperature tensile property test, the steel is mainly composed of tempered martensite and martensite after quenching at different temperatures. The mechanical properties of the test steel plate are shown in Table 1:

[0047] Table 1. Comparison of room temperature tensile properties of the steel after quenching at different temperatures in Example 1

[0048]

[0049] It can be seen from Table 1 that under the quenching condition at 900° C., this embodiment obtains an ultra-high strength self-tempering steel with a tensile strength of 1461 MPa and an elongation of 11%.

[0050] Embodiment 2

[0051] This embodiment is basically the same as the first embodiment, except that:

[0052] In this embodiment, an ultra-high strength self-tempered steel plate, calculated by mass percentage, has the following main chemical compositions and proportions of its components:

[0053] C: 0.18%, Mn: 2.0%, Si: 1.1%, Cr: 1.1%, Al: 0.3%, Ti: 0.01%, Nb: 0.01%, P: 0.01%, S: 0.008%, the balance is Fe;

[0054] The ultra-high strength self-tempering steel plate is prepared by the following method:

[0055] (1) Melting and casting process:

[0056] The steel billet having the above composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, the steel billet is cut into ingot blocks with a thickness of 35 mm;

[0057] (2) Rolled steel plate:

[0058] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be 1200°C, the final rolling temperature is controlled to be 950°C, and the hot rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8-2.0 mm. The hot rolled steel plate is then heated to 900°C and kept warm for 3 minutes for heat treatment, and then air-cooled to room temperature to obtain the ultra-high strength self-tempering steel plate.

[0059] Experimental test analysis:

[0060] Through microstructure analysis and room temperature tensile property test, the steel is mainly composed of tempered martensite and martensite structure after quenching at different temperatures. The mechanical property values ​​of the test steel plate are shown in Table 2:

[0061] Table 2. Comparison of room temperature tensile properties of the steel after quenching at different temperatures in Example 2

[0062]

[0063] It can be seen from Table 2 that under the quenching condition at 900° C., this embodiment obtains an ultra-high strength self-tempering steel with a tensile strength of 1410 MPa and an elongation of 12.0%.

[0064] Embodiment 3

[0065] This embodiment is basically the same as the above embodiment, with the following features:

[0066] In this embodiment, an ultra-high strength self-tempered steel plate, calculated by mass percentage, has the following main chemical compositions and proportions of its components:

[0067] C: 0.20%, Mn: 1.9%, Si: 1.0%, Cr: 1.3%, Al: 0.4%, Ti: 0.01%, Nb: 0.02%, P: 0.01%, S: 0.008%, the balance is Fe;

[0068] The ultra-high strength self-tempering steel plate is prepared by the following method:

[0069] (1) Melting and casting process:

[0070] The steel billet having the above composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, the steel billet is cut into ingot blocks with a thickness of 35 mm;

[0071] (2) Rolled steel plate:

[0072] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be 1200°C, the final rolling temperature is controlled to be 950°C, and multiple hot rollings are performed to a thickness of 1.8-2.0 mm to obtain a hot-rolled steel plate, and then the hot-rolled steel plate is heated to 900°C and kept warm for 3 minutes for heat treatment, and then air-cooled to room temperature to obtain the ultra-high strength self-tempering steel plate.

[0073] Experimental test analysis:

[0074] Through microstructure analysis and room temperature tensile property test, the steel is mainly composed of tempered martensite and martensite structure after quenching at different temperatures. The mechanical properties of the test steel plate are shown in Table 3:

[0075] Table 3. Comparison of room temperature tensile properties of the steel after quenching at different temperatures in Example 3

[0076]

[0077] It can be seen from Table 3 that under the quenching condition at 900° C., this embodiment obtains an ultra-high strength self-tempering steel with a tensile strength of 1478 MPa and an elongation of 10.2%.

[0078] Embodiment 4

[0079] This embodiment is basically the same as the above embodiment, with the following features:

[0080] In this embodiment, an ultra-high strength self-tempered steel plate, calculated by mass percentage, has the following main chemical compositions and proportions of its components:

[0081] C: 0.22%, Mn: 1.8%, Si: 0.8%, Cr: 1.2%, Al: 0.6%, Ti: 0.01%, Nb: 0.02%, P: 0.01%, S: 0.008%, the balance is Fe;

[0082] The ultra-high strength self-tempering steel plate is prepared by the following method:

[0083] (1) Melting and casting process:

[0084] The steel billet having the above composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, the steel billet is cut into ingot blocks with a thickness of 35 mm;

[0085] (2) Rolled steel plate:

[0086] The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be 1200°C, the final rolling temperature is controlled to be 950°C, and multiple hot rollings are performed to a thickness of 1.8-2.0 mm to obtain a hot-rolled steel plate, and then the hot-rolled steel plate is heated to 900°C and kept warm for 3 minutes for heat treatment, and then air-cooled to room temperature to obtain the ultra-high strength self-tempering steel plate.

[0087] Experimental test analysis:

[0088] Through microstructure analysis and room temperature tensile property test, the steel is mainly composed of tempered martensite and martensite structure after quenching at different temperatures. The mechanical property values ​​of the test steel plate are shown in Table 4:

[0089] Table 4. Comparison of room temperature tensile properties of the steel after quenching at different temperatures in Example 4

[0090]

[0091] It can be seen from Table 4 that under the quenching condition at 900° C., this embodiment obtains an ultra-high strength self-tempering steel with a tensile strength of 1434 MPa and an elongation of 11.2%.

[0092] In summary, the chemical composition of the ultra-high strength self-tempered automobile steel plate in the above embodiment of the present invention includes, by mass percentage, C: 0.15-0.23%, Mn: 1.8-2.5%, Si: 0.8-1.5%, Cr: 1.0-1.5%, Al: 0.2-0.8%, Ti+Nb≤0.03%, P<0.02%, S<0.01%, and the balance is Fe. The 1.8-2.0 mm hot-rolled plate is heated to 900°C and kept warm for at least 3 minutes, and then air-cooled to obtain the test steel. The above embodiment of the present invention solves the problem of how to obtain the test steel with the appropriate composition under the hot rolling process conditions, so that the tensile strength is not less than 1400 MPa and the elongation is not less than 10% due to the self-tempering phenomenon of martensite during quenching and air cooling. In particular, the tensile strength of the ultra-high strength self-tempered automobile steel plate of the present invention is not less than 1410 MPa, and the elongation is not less than 10.2%. The above-mentioned embodiments of the present invention are helpful to realize the short-process preparation of ultra-high strength steel plates, and have significant application value for the development of lightweight automobiles.

[0093] The embodiments of the present invention are described above, but the present invention is not limited to the above embodiments. Various changes can be made according to the purpose of the invention. Any changes, modifications, substitutions, combinations or simplifications made according to the spirit and principle of the technical solution of the present invention should be equivalent replacement methods. As long as they meet the purpose of the invention and do not deviate from the technical principles and inventive concepts of the present invention, they belong to the protection scope of the present invention.

Claims

1. An ultra-high strength self-tempering steel plate, characterized in that: Calculated by mass percentage, the chemical composition and proportion of its main components are as follows: C: 0.15-0.23%, Mn: 1.8-2.5%, Si: 0.8-1.5%, Cr: 1.0-1.5%, Al: 0.2-0.8%, Ti≤0.01%, Nb≤0.02%, Ti+Nb≤0.03%, P<0.02%, S<0.01%, the balance is Fe, and the contents of Ti and Nb are not zero; The ultra-high strength self-tempering steel plate is prepared by the following method, and the steps of the method are as follows: (1) Melting and casting process: The steel billet having the above-mentioned composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, it is cut into ingot blocks with a thickness of no more than 35 mm; (2) Rolled steel plates: The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be not less than 1200°C, and the final rolling temperature is controlled to be not less than 950°C, and a hot-rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8 to 2.0 mm; the hot-rolled steel plate is heated to 900°C and kept warm for at least 3 minutes for heat treatment, and then air-cooled to obtain the ultra-high strength self-tempering steel plate.

2. The ultra-high strength self-tempering steel plate according to claim 1, characterized in that: The steel plate has the following chemical components in terms of mass percentage: composition: C: 0.16~0.22%, Mn: 1.8~2.4%, Si: 0.8~1.4%, Cr: 1.1~1.5%, Al: 0.3~0.6%, Ti≤0.01%, Nb≤0.02%, P≤0.01%, S≤0.008%, the balance is Fe, the contents of Ti and Nb are not zero.

3. The ultra-high strength self-tempering steel plate according to claim 1, characterized in that: Under 900℃ quenching condition, its yield strength is not less than 857MPa, its tensile strength is not less than 1410MPa, and its elongation is not less than 10.2%.

4. A method for preparing the ultra-high strength self-tempering steel plate according to claim 1, characterized in that: The method steps are as follows: (1) Melting and casting process: The steel billet having the above-mentioned composition is prepared by smelting and solidifying in a vacuum induction furnace, argon gas is filled for protection during the smelting process, and then mold casting is performed, and after demolding, it is cut into ingot blocks with a thickness of no more than 35 mm; (2) Rolled steel plates: The ingot block is hot rolled, the initial rolling temperature of the hot rolling is controlled to be not less than 1200°C, and the final rolling temperature is controlled to be not less than 950°C, and a hot-rolled steel plate is obtained by multiple hot rolling to a thickness of 1.8 to 2.0 mm; the hot-rolled steel plate is heated to 900°C and kept warm for at least 3 minutes for heat treatment, and then air-cooled to obtain the ultra-high strength self-tempering steel plate.

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