High-strength-and-ductility co-deformable ferrite / austenite double-heterostructure steel plate and preparation method thereof

Abstract

The invention relates to a high-strength and ductility co-deformable ferrite / austenite double-heterostructure hot-rolled low-carbon and low-manganese steel plate based on multi-stage carbon and manganese element partition regulation and a preparation method, and belongs to the field of development of advanced high-strength steel for third-generation automobiles. The steel plate comprises the chemical components of Fe, C, Si, Mn, Al, V, P, S and inevitable impurities, and the thickness of the steel plate is 2-4 mm. The preparation method comprises the following steps: 1) alloy smelting, casting and forging; 2) hot rolling and cooling to room temperature at a cooling speed of 2-20 DEG C / s after rolling; and 3) one-stage or two-stage critical zone annealing treatment. The yield strength of the steel plate ranges from 400 MPa to 800 MPa, the tensile strength ranges from 850 MPa to 1050 MPa, the percentage elongation after fracture ranges from 25% to 40%, and the product of strength and elongation ranges from 24 GPa% to 34 GPa%. The steel plate provided by the invention has a micron or submicron ferrite / austenite component / structure double heterostructure, the retained austenite has obvious chemical component heterogeneity, and an obvious multistage discontinuous TRIP plasticizing effect exists in the deformation process. The preparation method provides a novel low-cost and high-efficiency process scheme for remarkably improving the strength and plasticity of the low-carbon and low-manganese steel.

Description

Technical Field

[0001] The present invention belongs to the field of development of advanced steel materials for automotive structures, and particularly relates to a high strength-plasticity product form co-deformable ferrite / austenite dual heterogeneous structure steel plate and a preparation method thereof. Background Art

[0002] In order to meet the collision safety while achieving vehicle body lightweighting, automotive structural steels have been continuously developed towards high strength and high plasticity, thus giving rise to the third generation of advanced high strength steels. Due to its unique TRIP effect, retained austenite can significantly improve the work hardening ability of materials during deformation, thereby synchronously enhancing strength and plasticity. Therefore, retained austenite has been widely introduced into the organizational structure design of advanced high strength steels, forming a series of TRIP steels with different body-centered cubic structure matrices. In recent years, more and more studies have shown that the ideal occurrence form of the TRIP effect is that retained austenite can be orderly and continuously activated at different stages of plastic deformation. To achieve this goal, researchers have been committed to constructing a mechanical stability gradient of retained austenite by regulating the chemical composition, morphology, size, crystallographic orientation and other chemical / structural heterogeneity characteristics of retained austenite. It is expected that the content of retained austenite with different mechanical stabilities will exhibit continuous and effective TRIP effects at different deformation degrees.

[0003] Currently, the mainstream method for constructing the chemical / structural heterogeneity of retained austenite is to introduce body-centered cubic structure matrices (such as ferrite, bainite and martensite) with obvious differences in composition, morphology and substructure density. However, a large number of research practices have shown that such matrix structure components with significant hardness differences will have very serious strain partitioning behaviors during deformation, resulting in strain mainly concentrated in the soft-phase matrix components. The local strain in the hard-phase matrix components is extremely low, which makes it difficult for the retained austenite distributed therein to effectively exert its TRIP plasticizing effect. At the same time, the strong strain contrast between the hard and soft phase matrix components will accelerate the fracture failure of the hard and soft phase interfaces. Therefore, in order to improve the deformation coordination between the hard and soft phase structure components and delay the microscale interface cracking, while fully exerting the effectiveness of the TRIP effect of retained austenite, it is urgent to develop a high strength and high plasticity TRIP steel with a ferrite / austenite dual heterogeneous structure with good co-deformability. Summary of the Invention

[0004] Aiming at the above technical problems, the present invention provides a high strength-plasticity product form co-deformable ferrite / austenite dual heterogeneous structure steel plate and a preparation method thereof.

[0005] A high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention has a chemical composition by mass percentage as follows: C: 0.1 - 0.2, Si: 0.5 - 1.0, Mn: 2.0 - 3.0, Al: 0.5 - 2.0, V: 0.1 - 0.15, P: 0.001 - 0.1, S: ≤0.004, and the balance is Fe and inevitable impurities; the thickness of the steel plate is 2.0 - 4.0 mm.

[0006] A high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention has a yield strength of 400 - 800 MPa, a tensile strength of 850 - 1050 MPa, an elongation after fracture of 25% - 40%, and a product of strength and ductility of 24 - 34 GPa%.

[0007] A high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention has a room-temperature microstructure of micron-sized polygonal blocky ferrite with extremely low dislocation density, nano-sized lath-shaped ferrite, sub-micron-sized blocky retained austenite, nano-sized film-shaped retained austenite, and a small amount of sub-micron-sized lath-shaped martensite islands. Among them, the blocky retained austenite is distributed near the polygonal ferrite, while the film-shaped retained austenite is distributed between the ferrite laths, and the retained austenite has obvious chemical composition heterogeneity. The area fraction of the blocky ferrite is 20 - 30%, the area fraction of the lath-shaped ferrite is 20 - 40%, the content of the retained austenite is 20 - 35 vol%, and the rest is lath-shaped martensite islands. The ferrite contains nano-sized granular VC precipitates distributed dispersedly.

[0008] A preparation method of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention includes the following steps:

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

[0010] According to the composition ratio of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate, smelt and cast into an ingot by a vacuum induction electric furnace, and forge the ingot into a forging billet with a thickness of 60 - 90 mm by free forging.

[0011] Step 2, hot rolling:

[0012] (1) Keep the forging billet with a thickness of 60 - 90 mm obtained in Step 1 at 1200 °C - 1300 °C for 2 - 3 h to achieve element homogenization;

[0013] (2) Rolling: Start rolling at 1050 - 1150 °C, roll to a thickness of 3.0 - 4.0 mm through 7 - 8 passes, control the finishing rolling temperature at 800 - 900 °C, control the pass deformation amount at 20% - 40%, and control the total hot rolling deformation amount at 90% - 95%;

[0014] (3) The hot-rolled sheet is cooled to room temperature by using an air-cooling or slow-cooling method with asbestos covering, and the cooling rate is controlled below 15 °C / s to obtain a hot-rolled sheet with a thickness of 2.0 - 4.0 mm.

[0015] Step 3, one-step (or two-step) short-time intercritical annealing treatment:

[0016] (1) One-step intercritical annealing treatment: The hot-rolled sheet obtained in step 2 is heated to 800 - 900 °C at a heating rate of 10 - 20 °C / s and held for 20 - 40 min, and then cooled to room temperature at a cooling rate of 40 - 60 °C / s to obtain a high strength-ductility product co-deformable dual heterogeneous structure steel plate;

[0017] (2) Two-step intercritical annealing treatment: The hot-rolled sheet obtained in step 2 is heated to 880 - 900 °C at a heating rate of 10 - 20 °C / s and held for 20 - 40 min, and then cooled to room temperature at a cooling rate of 40 - 60 °C / s. Then it is heated to 800 - 860 °C at a heating rate of 10 - 20 °C / s and held for 20 - 40 min, and finally cooled to room temperature at a cooling rate of 40 - 60 °C / s to obtain a high strength-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate.

[0018] The technical idea of the present invention is as follows:

[0019] A low-carbon silicon-manganese-aluminum-vanadium-based composition with a manganese content of 2.0 - 3.0 wt% is adopted, which is between conventional low-carbon low-manganese TRIP steel and low-carbon medium-manganese TRIP steel. Relatively low carbon (0.1 - 0.2 wt%) and manganese contents are used to ensure that micron-scale polygonal proeutectoid ferrite can be formed at a cooling rate below 15 °C / s after hot rolling. A small amount of Si (0.5 - 1.0 wt%) is added to inhibit the formation of cementite during annealing. A small amount of Al (0.5 - 2.0 wt%) is added to inhibit the formation of cementite and can expand the ferrite phase region to a certain extent, promoting the formation of polygonal ferrite after hot rolling. A small amount of V (0.1 - 0.15 wt%) is added to strengthen the ferrite, improve the strength of the matrix, and reduce the hardness difference between the soft and hard phases of the matrix.

[0020] Based on the basic principle of constructing a co-deformable ferrite / austenite dual heterogeneous structure by hot rolling and one-step (two-step) intercritical annealing treatment: The steel plate is cooled to room temperature at a cooling rate below 15 °C / s after hot rolling to obtain a mixed structure of polygonal ferrite and lath martensite. Then, through one-step or two-step intercritical (800 - 900 °C) short-time (20 - 40 min) annealing, a ferrite / austenite dual heterogeneous structure material composed of polygonal ferrite, lath-shaped ferrite, massive retained austenite, and thin-film retained austenite is obtained. Among them, the retained austenite has not only significant morphological size heterogeneity but also obvious chemical composition heterogeneity.

[0021] A high-strength and high-ductility co-deformable ferrite / austenite dual heterogeneous structure steel plate and a preparation method thereof according to the present invention have the following beneficial effects compared with the prior art:

[0022] (1) Significantly different from conventional low-carbon low-alloy TRIP steels and medium-manganese TRIP steels, the high-strength and high-ductility steel plate of the present invention has a typical ferrite / austenite dual heterogeneous structure, and there is a low hardness difference between the body-centered cubic matrix components, and it has very excellent co-deformability. The specific manifestation of the dual heterogeneous structure is that the ferrite has micron-scale polygonal blocky and nano-scale lath-like morphologies, and the dislocation density in the ferrite is extremely low. The retained austenite has two morphologies of blocky and film-like, and there is an obvious chemical composition gradient inside the retained austenite, and there is a significant mechanical stability gradient between the retained austenites.

[0023] (2) The present invention adopts a chemical composition between that of traditional low-carbon low-manganese TRIP steels and conventional medium-manganese TRIP steels. The main feature is that the Mn content is 2.0-3.0 wt%, which is beneficial to the stabilization of retained austenite, and at the same time, it can avoid serious application problems faced by conventional medium-manganese steels such as continuous casting Mn segregation, high rolling difficulty and poor welding performance caused by too high Mn content.

[0024] (3) The present invention adopts a heat treatment process of hot rolling + one-step / two-step short-time intercritical annealing. The single-step annealing time is only 20-40 min, which is much lower than the annealing time of several hours for conventional medium-manganese steels, and greatly reduces energy consumption and production costs. Compared with the isothermal quenching process of conventional low-carbon low-alloy TRIP steels, the process of the present invention has significantly lower process sensitivity and more excellent tissue performance stability. The unique short-time intercritical annealing process also helps to construct the chemical heterogeneity of retained austenite and enhance the formation of the stability gradient of retained austenite.

[0025] (4) During the deformation process of the high-strength and high-ductility co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention, the retained austenite will exhibit an obvious intermittent discontinuous high-frequency multi-stage TRIP effect. This unique TRIP effect is beneficial to triggering strong dislocation multiplication behavior of the co-deformable heterogeneous structure ferrite with extremely low dislocation density at each stage of deformation, thereby significantly improving the work hardening rate of the material and realizing a large synchronous increase in the strength and plasticity of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the hot rolling and heat treatment process in the preparation method of a high-strength and high-ductility co-deformable ferrite / austenite dual heterogeneous structure steel plate according to an example of the present invention, wherein (a) corresponds to Example 1 and (b) corresponds to Example 2.

[0027] Figure 2Secondary electron phase diagram (a) and EBSD map (b) of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 1 of the present invention.

[0028] Figure 3 XRD results of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 1 of the present invention.

[0029] Figure 4 Engineering stress-strain curve of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 1 of the present invention.

[0030] Figure 5 Secondary electron phase diagram (a) and EBSD map (b) of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 2 of the present invention.

[0031] Figure 6 XRD results of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 2 of the present invention.

[0032] Figure 7 Engineering stress-strain curve of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of Example 2 of the present invention. Detailed implementation manners

[0033] Example 1

[0034] For a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention, its chemical composition by mass percentage is: C: 0.18, Si: 0.4, Mn: 2.8, Al: 1.4, V: 0.1, S: ≤0.004, and the balance is Fe and unavoidable impurities; the steel plate thickness is 4.0 mm.

[0035] The preparation method of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention includes the following steps:

[0036] Step 1, alloy smelting, casting and forging:

[0037] According to the chemical composition ratio of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention, smelt at 1630 °C, then cast into an ingot, and use free forging to make a forging blank with a cross-sectional size of 60×60 mm.

[0038] Step 2, hot rolling:

[0039] (1) Keep the forging blank at 1220 °C for 2 h;

[0040] (2) Rolling starts at 1150°C, rolled to a thickness of 4.0 mm in seven passes, and the finish rolling temperature is controlled at about 880°C, then air-cooled to room temperature with a cooling rate of about 15°C / s. The pass deformation amount during hot rolling is controlled at 20%-40%, and the total hot rolling deformation amount is controlled at about 95%.

[0041] Step 3, one-step critical region annealing treatment:

[0042] The hot-rolled plate is heated to 860°C at a heating rate of 10°C / s and held for 30 min, then water-quenched to room temperature.

[0043] A high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate obtained in this example has a yield strength of 585 MPa, a tensile strength of 972 MPa, an elongation after fracture of 34.0%, and a product of strength and ductility of 33048 MPa%.

[0044] Example 2

[0045] For a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention, its chemical composition by mass percentage is: C: 0.18, Si: 0.4, Mn: 2.8, Al: 1.4, V: 0.1, S: ≤0.004, the balance being Fe and unavoidable impurities; the steel plate thickness is 4.0 mm.

[0046] A preparation method of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention includes the following steps:

[0047] Step 1, alloy smelting, casting and forging:

[0048] According to the chemical composition ratio of a high-strength and high-ductility product co-deformable ferrite / austenite dual heterogeneous structure steel plate of the present invention, smelting is carried out at 1620°C, then cast into an ingot, and a forging blank with a cross-sectional size of 60×60 mm is made by free forging.

[0049] Step 2, hot rolling:

[0050] (1) Keep the forging blank at 1250°C for 2 h;

[0051] (2) Rolling starts at 1100°C, rolled to a thickness of 4.0 mm in seven passes, and the finish rolling temperature is controlled at about 850°C, then covered with asbestos to room temperature with a cooling rate of about 5°C / s. The pass deformation amount during hot rolling is controlled at 20%-40%, and the total hot rolling deformation amount is controlled at about 95%.

[0052] Step 3, two-step critical region annealing treatment:

[0053] (1) The hot-rolled plate is heated to 880°C at a heating rate of 10°C / s and held for 30 min, then water-quenched to room temperature.

[0054] (2) Heat the steel plate to 860 °C at a heating rate of 10 °C / s and hold for 30 min, then water quench to room temperature.

[0055] A high-strength and high-ductility product formable ferrite / austenite dual heterogeneous structure steel plate obtained in this example has a yield strength of 523 MPa, a tensile strength of 988 MPa, an elongation after fracture of 31.0%, and a product of strength and ductility of 30628 MPa%.

Claims

1. A high-strength and high-plasticity product form co-deformable ferrite / austenite dual heterogeneous structure steel plate, characterized in that The components are by mass percentage: C: 0.1 - 0.2, Si: 0.5 - 1.0, Mn: 2.0 - 3.0, Al: 0.5 - 2.0, V: 0.1 - 0.15, P: 0.001 - 0.1, S: ≤0.004, and the balance is Fe and inevitable impurities; The thickness of the steel plate is 2.0 - 5.0 mm.

2. A high-strength plastic product deformable co-ferrite / austenite dual heterogeneous structure steel plate according to claim 1, characterized in that, The yield strength of the steel plate is 400 - 800 MPa, the tensile strength is 850 - 1050 MPa, the elongation after fracture is 25% - 40%, and the product of strength and plasticity is 24 - 34 GPa%.

3. A high-strength plastic product accumulative co-deformable ferrite / austenite dual heterogeneous structure steel plate according to claim 1, characterized in that, The room-temperature microstructure consists of micron-sized polygonal blocky ferrite with extremely low dislocation density, nano-sized lath-shaped ferrite, sub-micron-sized blocky retained austenite, nano-sized thin-film retained austenite, and a small amount of sub-micron-sized lath-shaped martensite islands. Among them, the blocky retained austenite is distributed near the polygonal ferrite, while the thin-film retained austenite is distributed between the ferrite laths, and the retained austenite has obvious chemical composition heterogeneity. The area fraction of the blocky ferrite is 20 - 30%, the area fraction of the lath-shaped ferrite is 20 - 40%, the content of the retained austenite is 20 - 35 vol%, and the rest is lath-shaped martensite islands. The ferrite contains nano-sized granular VC precipitates distributed dispersedly.

4. The preparation method of a high-strength plastic product co-deformable ferrite / austenite dual heterogeneous structure steel plate 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 high-strength and high-plasticity co-deformable ferrite / austenite dual-heterogeneous structure steel plate as described in claim 1, smelt it using a vacuum induction electric furnace and cast it into an ingot, and forge the ingot into a forging blank with a thickness of 60 - 90 mm using free forging. Step 2, hot rolling: (1) Keep the forging blank with a thickness of 60 - 90 mm obtained in Step 1 at 1200°C - 1300°C for 2 - 3 h to achieve element homogenization; (2) Rolling: Start rolling at 1050 - 1150°C, roll it to a thickness of 3.0 - 4.0 mm through 7 - 8 passes, control the final rolling temperature at 800 - 900°C, control the pass deformation amount at 20% - 40%, and control the total hot rolling deformation amount at 90% - 95%; (3) Use air cooling or slow cooling covered with asbestos to cool the hot-rolled plate to room temperature, control the cooling rate below 15°C / s, and obtain a hot-rolled plate with a thickness of 2.0 - 4.0 mm. Step 3, one-step (or two-step) short-time intercritical annealing treatment: (1) One-step intercritical annealing treatment: Heat the hot-rolled plate obtained in Step 2 to 800 - 900°C at a heating rate of 10 - 20°C / s and hold for 20 - 40 min, and then cool it to room temperature at a cooling rate of 40 - 60°C / s to obtain a high-strength and high-plasticity co-deformable dual-heterogeneous structure steel plate; (2) Two-step intercritical annealing treatment: Heat the hot-rolled plate obtained in Step 2 to 880 - 900°C at a heating rate of 10 - 20°C / s and hold for 20 - 40 min, cool it to room temperature at a cooling rate of 40 - 60°C / s, and then heat it to 800 - 860°C at a heating rate of 10 - 20°C / s and hold for 20 - 40 min, and cool it to room temperature at a cooling rate of 40 - 60°C / s to obtain a high-strength and high-plasticity co-deformable ferrite / austenite dual-heterogeneous structure steel plate.

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