Low-cost high-strength plastic QP steel plate based on flash thermal excitation effect and preparation method

Abstract

The invention relates to a chemical / structural double-heterogeneous high-strength plastic low alloy Qamp based on a flash thermal excitation effect. A P steel plate and a preparation method belong to the field of development of advanced high-strength steel for automobiles. The steel plate comprises the following chemical components: Fe, C, Si, Mn, V, P, S and inevitable impurities, and the thickness of the steel plate is 1.0-1.5 mm. The preparation method comprises the following steps: 1) alloy smelting, casting and forging; 2) hot rolling; 3) cold rolling; and 4) flash thermal excitation annealing treatment. The yield strength of the steel plate ranges from 660 MPa to 920 MPa, the tensile strength ranges from 1100 MPa to 1350 MPa, the yield ratio ranges from 0.5 to 0.7, the uniform elongation ranges from 8.0% to 12.0%, the percentage elongation after fracture ranges from 13.0% to 16.0%, and the product of strength and elongation ranges from 16000 MPa% to 21000 MPa%. The steel plate disclosed by the invention has a body-centered cubic / face-centered cubic double heterostructure consisting of non-recrystallized ferrite, recrystallized ferrite, island-shaped martensite, lath martensite, blocky retained austenite and film-shaped retained austenite, and obvious chemical component heterogeneity exists in the retained austenite. According to the preparation method disclosed by the invention, cold rolling Qamp is remarkably shortened; the annealing time of the P steel is long, and the method is used for developing a low-cost, high-performance and low-alloy heterostructure Qamp; a novel method is provided for P steel.

Description

Technical Field

[0001] The present invention belongs to the field of development of advanced high-strength steels for automobiles, and particularly relates to a preparation method of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect. Background Art

[0002] With the continuous aggravation of environmental problems and energy crises, the design of automobile bodies urgently needs to further promote the lightweighting of structural parts. According to statistics, steel parts account for more than 60% of the steel structural parts in automobile bodies. Therefore, the lightweighting of steel structural parts is particularly important for automobile design. In order to maintain the collision safety of automobiles while reducing the weight by thinning the thickness of parts, it is urgent to develop advanced high-strength steels with both ultra-high strength and plasticity. At present, the advanced high-strength steels for automobiles have developed to the third generation, and the representative steel type among them is quenching and partitioning (Q&P) steel. The basic principle of Q&P steel is to fully austenitize or partially austenitize the steel, and then quench it to a temperature between the martensite start (Ms) temperature and the martensite finish (Mf) temperature to obtain a certain content of martensite and untransformed austenite. Subsequently, isothermal treatment is carried out at the quenching temperature or a certain temperature higher than the quenching temperature to achieve carbon enrichment in the austenite. Finally, it is cooled to room temperature to obtain a high-strength and high-ductility steel with ferrite, martensite and retained austenite.

[0003] Traditional cold-rolled annealed Q&P steels usually adopt a heating rate below 20 °C / s, and the isothermal treatment time in the austenite region or partial austenite region is usually up to several minutes. This low heating rate and long high-temperature isothermal treatment process usually result in a body-centered cubic-face-centered cubic homogeneous duplex structure with relatively uniform composition and structure. In recent years, more and more studies have shown that introducing a matrix structure of soft and hard phases with different recrystallization states and appropriately promoting the chemical composition inhomogeneity in the tissue, and then constructing a typical chemical / structural double heterogeneous structure helps to simultaneously improve the strength and plasticity of the material. Related studies on duplex steels and ordinary low-carbon low-alloy TRIP steels have shown that the instantaneous thermal excitation effect in the flash heating mode can significantly inhibit the homogenization diffusion of chemical components, delay the recrystallization behavior of steel materials and promote austenite phase transformation, which is helpful to enhance the interaction between recrystallization and phase transformation behaviors and has great potential in constructing chemical / structural double heterogeneous structure steel materials. However, at present, the research and development of chemical / structural double heterogeneous low-cost high-strength and high-ductility Q&P steels regulated by the flash thermal excitation effect are still very lacking. Summary of the Invention

[0004] In view of the above technical problems, the present invention provides a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect and a preparation method thereof.

[0005] A low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention, its chemical composition by mass percentage: C: 0.2-0.25, Si: 1.0-2.0, Mn: 1.0-2.0, V: 0.15-0.3, P: 0.001-0.1, S: ≤0.004, the balance is Fe and unavoidable impurities; the thickness of the steel plate is 1.0-1.5mm.

[0006] A low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention, the yield strength is 660-920MPa, the tensile strength is 1100-1350MPa, the yield ratio is 0.5-0.7, the uniform elongation is 8.0%-12.0%, the elongation after fracture is 13.0%-16.0%, and the strength-ductility product is 16000-21000MPa%.

[0007] A low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention, the room temperature structure has a body-centered cubic / face-centered cubic double heterogeneous structure composed of unrecrystallized ferrite, recrystallized ferrite, island martensite, lath martensite, massive retained austenite and thin-film retained austenite, and there is obvious chemical composition heterogeneity in the retained austenite. Among them, the ferrite area fraction is 50-80%, the unrecrystallized ferrite area fraction is 20-40%, the retained austenite content is 14-20vol%, and the ferrite interior contains nano-scale spherical VC precipitates distributed dispersedly.

[0008] A preparation method of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention, comprising the following steps:

[0009] Step 1, alloy melting, casting and forging:

[0010] According to the chemical composition of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect described in claim 1, smelt by a vacuum induction electric furnace and cast into an ingot, and forge the ingot into a forging blank with a thickness of 60-70mm by free forging.

[0011] Step 2, hot rolling:

[0012] (1) Keep the forging blank with a thickness of 60-70mm obtained in step 1 at 1200℃-1250℃ for 2-3h to promote the homogenization of chemical composition;

[0013] (2) Rolling: Start rolling at 1150-1200℃, roll to a thickness of 4.0mm through 7-9 passes, control the finishing rolling temperature at 850-920℃, control the deformation amount of each pass at 30%-40%, and control the total hot rolling deformation amount at 90%-95%;

[0014] (3) After rolling, it is cooled to room temperature by air cooling to obtain a hot-rolled sheet with a thickness of 4.0 mm.

[0015] Step 3, pickling and cold rolling:

[0016] (1) The 4.0-mm-thick hot-rolled sheet obtained in Step 2 is pickled with a dilute hydrochloric acid aqueous solution to remove the surface scale.

[0017] (2) It is cold-rolled to a thickness of 1.0 - 1.5 mm at room temperature, and the cumulative cold-rolling deformation is 60% - 75% to obtain a cold-rolled sheet with a thickness of 1.0 - 1.5 mm.

[0018] Step 4, flash thermal excitation annealing treatment:

[0019] The cold-rolled sheet with a thickness of 1.0 - 1.5 mm obtained in Step 3 is induction-heated, heated to 700 - 900 °C at a heating rate of 100 - 300 °C / s and held for 1 - 60 s, then cooled to 250 - 400 °C at a cooling rate of 30 - 40 °C / s and held for 20 - 100 s, and finally cooled to room temperature at a cooling rate of 20 - 30 °C / s to obtain a body-centered cubic / face-centered cubic chemical / structural double heterogeneous high-strength and high-ductility Q&P steel.

[0020] The technical idea of the present invention is as follows:

[0021] (1) On the basis of the composition of conventional low-carbon silicon-manganese Q&P steel, 0.15 - 0.3 wt% of V is added. The solid solution strengthening and precipitation strengthening effects of V are used to improve the strength of the soft-phase ferrite matrix, reduce the hardness difference between the soft-phase ferrite and the hard-phase martensite, and improve the deformation coordination of the body-centered cubic structure matrix itself.

[0022] (2) A flash thermal excitation effect annealing process with a heating rate of 100 - 300 °C / s and an austenitizing holding time of only 1 - 60 s is adopted. The ultra-fast heating is used to delay recrystallization and promote austenite phase transformation, enhance the interaction between recrystallization and phase transformation, and then construct a body-centered cubic heterogeneous structure matrix with a typical recrystallized zone and an unrecrystallized zone. The short-time austenitizing annealing process is used to inhibit the homogenization of austenite composition, obtain the heterogeneity of the chemical composition of retained austenite, and then construct a mechanical stability gradient of retained austenite. Finally, a body-centered cubic / face-centered cubic chemical / structural double heterogeneous Q&P steel plate with excellent strength and ductility is obtained.

[0023] A low-cost high-strength and high-ductility Q&P steel plate and its preparation method based on the flash thermal excitation effect of the present invention, compared with the prior art, have the beneficial effects as follows:

[0024] (1) Compared with the traditional mainstream low-carbon Si-Mn Q&P steel, a relatively large content (0.15 - 0.3 wt%) of V is added in the composition design of the present invention. This unique composition design can significantly improve the strength of the soft-phase ferrite through solid solution strengthening and precipitation strengthening of V, thereby reducing the hardness difference between the soft-phase ferrite and the hard-phase martensite, effectively improving the deformation coordination between the soft and hard matrix phases, and delaying the cracking at the soft-hard phase interface.

[0025] (2) Different from the conventional cold rolling continuous annealing Q&P process, the present invention adopts an induction heating flash thermal excitation treatment process, whose heating rate is dozens of times higher than that of the conventional process, and the austenitizing time is much shorter than that of the conventional process. This unique annealing process mode can significantly delay recrystallization, promote austenite phase transformation and inhibit the chemical composition homogenization in austenite, promoting the formation of a body-centered cubic structure matrix with significantly different recrystallization degrees and retained austenite with obvious chemical heterogeneity and mechanical stability gradient, that is, a low-carbon low-alloy Q&P steel with unique body-centered cubic / face-centered cubic chemical / structural double heterogeneity is constructed. Description of the Drawings

[0026] Figure 1 Schematic diagram of the hot rolling and heat treatment process in a preparation method of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to an example of the present invention.

[0027] Figure 2 Secondary electron phase diagram of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 1 of the present invention.

[0028] Figure 3 EBSD diagram of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 1 of the present invention.

[0029] Figure 4 XRD result of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 1 of the present invention.

[0030] Figure 5 Engineering stress-strain curve of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 1 of the present invention.

[0031] Figure 6 Secondary electron phase diagram of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 2 of the present invention.

[0032] Figure 7 EBSD diagram of a low-cost high-strength and high-ductility Q&P steel sheet based on flash thermal excitation effect according to Example 2 of the present invention.

[0033] Figure 8XRD results of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect in Example 2 of the present invention.

[0034] Figure 9 Engineering stress-strain curve of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect in Example 2 of the present invention. Detailed implementation manners

[0035] Example 1

[0036] For a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention, its chemical composition by mass percentage is: C: 0.21, Si: 1.67, Mn: 1.65, V: 0.2, P: 0.001 - 0.1, S: ≤0.004, and the balance is Fe and unavoidable impurities; the thickness of the steel plate is 1.0 mm.

[0037] A preparation method of a low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect of the present invention includes the following steps:

[0038] Step 1, alloy melting, casting and forging:

[0039] Smelt and cast into an ingot by a vacuum induction electric furnace, and forge the ingot into a forging blank with a thickness of 60 mm by free forging.

[0040] Step 2, hot rolling:

[0041] (1) Keep the forging blank with a thickness of 60 mm obtained in Step 1 at 1200 °C for 2 h to promote the homogenization of chemical composition;

[0042] (2) Rolling: Start rolling at 1150 °C, roll to a thickness of 4.0 mm through 7 passes, control the finishing rolling temperature at 900 °C, control the deformation amount of each pass at 30% - 40%, and control the total hot rolling deformation amount at 93%;

[0043] (3) After rolling, cool to room temperature by air cooling to obtain a hot-rolled plate with a thickness of 4.0 mm.

[0044] Step 3, pickling and cold rolling:

[0045] (1) Pickle the 4.0-mm-thick hot-rolled plate obtained in Step 2 with dilute hydrochloric acid aqueous solution to remove the surface scale;

[0046] (2) Cold roll to a thickness of 1.0 mm at room temperature, and the cumulative cold rolling deformation amount is 75% to obtain a cold-rolled plate with a thickness of 1.0 mm.

[0047] Step 4, flash thermal excitation annealing treatment:

[0048] The cold-rolled sheet with a thickness of 1.0 mm obtained in Step 3 is heated by induction heating to 750 °C at a heating rate of 150 °C / s and held for 60 s, then cooled to 330 °C at a cooling rate of 30 °C / s and held for 100 s, and finally cooled to room temperature at a cooling rate of 20 °C / s.

[0049] A low-cost high-strength and high-ductility Q&P steel sheet based on the flash thermal excitation effect obtained in this example has a yield strength of 662 MPa, a tensile strength of 1194 MPa, a yield ratio of 0.55, a uniform elongation of 11.4%, an elongation after fracture of 13.6%, and a strength-ductility product of 16238 MPa%.

[0050] Example 2

[0051] For a low-cost high-strength and high-ductility Q&P steel sheet based on the flash thermal excitation effect of the present invention, its chemical composition by mass percentage is: C: 0.21, Si: 1.67, Mn: 1.65, V: 0.2, P: 0.001 - 0.1, S: ≤0.004, and the balance is Fe and unavoidable impurities; the thickness of the steel sheet is 1.0 mm.

[0052] A preparation method of a low-cost high-strength and high-ductility Q&P steel sheet based on the flash thermal excitation effect of the present invention includes the following steps:

[0053] Step 1, alloy melting, casting and forging:

[0054] Smelt and cast into an ingot using a vacuum induction electric furnace, and forge the ingot into a forging billet with a thickness of 60 mm by free forging.

[0055] Step 2, hot rolling:

[0056] (1) Hold the forging billet with a thickness of 60 mm obtained in Step 1 at 1200 °C for 2 h to promote the homogenization of chemical components;

[0057] (2) Rolling: Start rolling at 1150 °C, roll to a thickness of 4.0 mm in 7 passes, control the finishing rolling temperature at 900 °C, control the deformation per pass at 30% - 40%, and control the total hot rolling deformation at 93%;

[0058] (3) After rolling, cool to room temperature by air cooling to obtain a hot-rolled sheet with a thickness of 4.0 mm.

[0059] Step 3, pickling and cold rolling:

[0060] (1) Pickle the 4.0 mm thick hot-rolled sheet obtained in Step 2 with dilute hydrochloric acid aqueous solution to remove the surface scale;

[0061] (2) Cold roll to a thickness of 1.0 mm at room temperature, and the cumulative cold rolling deformation is 75% to obtain a cold-rolled sheet with a thickness of 1.0 mm.

[0062] Step 4, flash thermal excitation annealing treatment:

[0063] The cold-rolled sheet with a thickness of 1.0 mm obtained in Step 3 is heated by induction heating to 950 °C at a heating rate of 150 °C / s and held for 60 s, then cooled to 330 °C at a cooling rate of 30 °C / s and held for 100 s, and finally cooled to room temperature at a cooling rate of 20 °C / s.

[0064] A low-cost high-strength and high-ductility Q&P steel sheet based on the flash thermal excitation effect obtained in this example has a yield strength of 912 MPa, a tensile strength of 1310 MPa, a yield ratio of 0.70, a uniform elongation of 8.9%, an elongation after fracture of 15.3%, and a product of strength and ductility of 20043 MPa%.

Claims

1. A low-cost high-strength and high-ductility Q&P steel plate based on flash thermal excitation effect, characterized in that the chemical composition by mass percentage is: C: 0.2 - 0.25, Si: 1.0 - 2.0, Mn: 1.0 - 2.0, V: 0.15 - 0.3, P: 0.001 - 0.1, S: ≤0.004, and the balance is Fe and inevitable impurities; the thickness of the steel plate is 1.0 - 1.5 mm.

2. The low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect according to claim 1, wherein The yield strength is 660 - 920 MPa, the tensile strength is 1100 - 1350 MPa, the yield ratio is 0.5 - 0.7, the uniform elongation is 8.0% - 12.0%, the elongation after fracture is 13.0% - 16.0%, and the product of strength and ductility is 16000 - 21000 MPa%.

3. The low-cost high-strength and high-ductility Q&P steel plate based on the flash thermal excitation effect according to claim 1, wherein The room-temperature structure has a body-centered cubic / face-centered cubic double heterogeneous structure composed of unrecrystallized ferrite, recrystallized ferrite, island martensite, lath martensite, blocky retained austenite and thin-film retained austenite, and there is obvious chemical composition heterogeneity in the retained austenite. Among them, the ferrite area fraction is 50 - 80%, the unrecrystallized ferrite area fraction is 20 - 40%, the retained austenite content is 14 - 20 vol%, and the ferrite interior contains nano-scale spherical VC precipitates distributed dispersedly.

4. The preparation method of a low-cost high-strength and high-ductility Q&P steel sheet based on the flash thermal excitation effect according to claim 1, characterized in that, It includes the following steps: Step 1, alloy melting, casting and forging: According to the chemical composition of a low-cost high-strength and high-ductility Q&P steel plate based on flash thermal excitation effect described in claim 1, smelt and cast into an ingot by a vacuum induction electric furnace, and forge the ingot into a forging blank with a thickness of 60 - 70 mm by free forging. Step 2, hot rolling: (1) Keep the forging blank with a thickness of 60 - 70 mm obtained in Step 1 at 1200 °C - 1250 °C for 2 - 3 h to promote the homogenization of chemical composition; (2) Rolling: Start rolling at 1150 - 1200 °C, roll to a thickness of 4.0 mm through 7 - 9 passes, control the finishing rolling temperature at 850 - 920 °C, control the deformation amount per pass at 30% - 40%, and control the total hot rolling deformation amount at 90% - 95%; (3) After rolling, cool to room temperature by air cooling to obtain a hot-rolled plate with a thickness of 4.0 mm. Step 3, pickling and cold rolling: (1) Pickle the hot-rolled plate with a thickness of 4.0 mm obtained in Step 2 with dilute hydrochloric acid aqueous solution to remove the surface scale; (2) Cold roll to a thickness of 1.0 - 1.5 mm at room temperature, and the cumulative cold rolling deformation amount is 60% - 75% to obtain a cold-rolled plate with a thickness of 1.0 - 1.5 mm. Step 4, flash thermal excitation annealing treatment: For the cold-rolled plate with a thickness of 1.0 - 1.5 mm obtained in Step 3, use induction heating to heat it to 700 - 900 °C at a heating rate of 100 - 300 °C / s and hold for 1 - 60 s, then cool it to 250 - 400 °C at a cooling rate of 30 - 40 °C / s and hold for 20 - 100 s, and finally cool it to room temperature at a cooling rate of 20 - 30 °C / s to obtain a high-strength and high-ductility Q&P steel with a body-centered cubic / face-centered cubic chemical / structural double heterogeneity.

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