High-strength austenite TWIP steel and preparation method thereof

Abstract

The high-strength austenite TWIP steel is characterized by comprising the following components: C, Mn, Al, Si, Mo, V, Nb, Ce and the balance of Fe. The invention further discloses a preparation method of the high-strength austenite TWIP steel. The preparation method comprises the following steps: (1) selecting cast ingots smelted according to the components and the weight ratio; (2) forging the cast ingot to obtain a plate blank; (3) firstly carrying out hot rolling on the plate blank to obtain a hot-rolled plate, and then carrying out warm rolling on the hot-rolled plate to obtain a warm-rolled plate; and (4) carrying out heat treatment on the warm-rolled plate. The preparation method is low in cost, simple in process and good in finished product performance.

Description

Technical Field

[0001] The present invention relates to a steel, in particular to a twinning-induced plasticity (TWIP) steel, and also relates to a preparation method of the steel. Background Art

[0002] With the rapid development of new energy vehicles, China has achieved good results in the field of new energy vehicle industry. Automobile lightweighting has always been a key point in the research and development of the automotive field, such as the research on lightweight replacement of material types (aluminum alloy replacing steel), the topological optimization design of part structures, etc. The twinning-induced plasticity (TWIP) steel, as a key steel for automotive parts, has been the focus of the industry research field in recent years. Due to its high manganese content in its composition, its microstructure presents austenitization, and the TWIP effect will occur during stress deformation, that is, twin crystals will form at the grain boundaries of austenite grains. The nucleation and growth of twin crystals will greatly consume energy, thereby hindering the initiation of cracks in TWIP steel to improve plasticity and enhancing the resistance of materials to increase strength. At present, the conventional preparation process of TWIP steel mainly involves air cooling after hot rolling, then cold rolling, and finally annealing treatment.

[0003] At the present stage, the main difficulty in the popularization and application of TWIP steel is that its complicated preparation process greatly increases its use cost, making it difficult to gain technical and economic advantages when competing with other similar steel types. Therefore, the research on simplifying the preparation process of TWIP steel is the key to its further popularization and application. At the same time, in response to the demand for automobile lightweighting, improving the comprehensive mechanical properties of TWIP steel through composition and process optimization, and reducing the use thickness of TWIP steel under the same service conditions to achieve the purpose of part weight reduction are also research difficulties at the present stage. Summary of the Invention

[0004] The first technical problem to be solved by the present invention is to provide a high-strength austenitic TWIP steel with simple process and excellent comprehensive mechanical properties in view of the above technical status.

[0005] The second technical problem to be solved by the present invention is to provide a preparation method of a high-strength austenitic TWIP steel with simple process and excellent comprehensive mechanical properties in view of the above technical status.

[0006] The technical solution adopted by the present invention to solve the above first technical problem is as follows: a high-strength austenitic TWIP steel, characterized by including the following components and their weight ratios:

[0007] C 0.55% - 0.65%;

[0008] Mn 21.0% - 23.0%;

[0009] Al 1.45% - 1.55%;

[0010] Si 1.45% - 1.55%;

[0011] Mo 0.95% - 1.10%;

[0012] V 0.19% - 0.20%;

[0013] Nb 0.02% - 0.03%;

[0014] Ce 0.02% - 0.03%;

[0015] The balance is Fe.

[0016] The technical solution adopted by the present invention to solve the above - mentioned second technical problem is: a preparation method of a high - strength austenitic TWIP steel, which is characterized by including the following steps:

[0017] ① Select an ingot melted according to the component and weight ratio;

[0018] ② Forge the ingot to obtain a slab;

[0019] ③ First, hot - roll the slab to obtain a hot - rolled sheet, and then warm - roll the hot - rolled sheet to obtain a warm - rolled sheet;

[0020] ④ Heat - treat the warm - rolled sheet.

[0021] Preferably, in step ②, the ingot is held at 1150°C - 1250°C for 1.5 h - 2 h and then forged into a slab.

[0022] Preferably, in step ③, the starting rolling temperature of the hot - rolling is 1000°C - 1100°C, and the final rolling temperature is 850°C - 950°C.

[0023] Preferably, the number of passes of the hot - rolling in step ③ is 3 - 5 times.

[0024] Preferably, the starting rolling temperature of the warm - rolling in step ③ is 600°C - 700°C, and the final rolling temperature is 400°C - 500°C.

[0025] Preferably, the number of passes of the warm - rolling in step ③ is 2 - 4 times.

[0026] Preferably, the hot - rolled sheet in step ③ is air - cooled to the starting rolling temperature of the warm - rolling for warm - rolling.

[0027] Preferably, the warm - rolled sheet after final rolling in step ③ is air - cooled.

[0028] Preferably, the heat - treatment conditions in step ④ are as follows: the warm - rolled sheet is held at 800°C - 900°C for 20 min - 30 min and then water - cooled.

[0029] Compared with the prior art, the advantages of the present invention are as follows: In the high-strength austenitic TWIP steel provided by the present invention, the contents of Mn element and Al element are both increased, and trace Nb and Ce elements are additionally added. The increase in the content of Mn element can significantly strengthen the stability of austenite in TWIP steel, thereby strengthening the TWIP effect and greatly increasing the energy required for the formation of twins. The increase in the content of Al element inhibits the precipitation of cementite to a certain extent, enhancing the stability of austenite from the side. The addition of trace Nb element and Ce element can refine the grains, improve the comprehensive mechanical properties of TWIP steel, and optimize the corrosion resistance and high-temperature properties. Compared with the hot rolling combined with cold rolling composite process of traditional Fe-20Mn-1Al-0.6C TWIP steel, the present invention adopts a synergistic control process of hot rolling and warm rolling, avoiding the pauses in the rolling process of the cold rolling process, saving production costs, greatly accelerating the rolling process rate, and at the same time improving the tissue stability of TWIP steel, providing sufficient conditions for the formation of twins in the TWIP effect. Compared with the tensile strength of about 850 MPa of traditional Fe-20Mn-1Al-0.6C TWIP steel, the strength of the high-strength austenitic TWIP steel of the present invention is increased by about 15%. By optimizing the component ratio and rolling process of TWIP steel on the basis of traditional Fe-20Mn-1Al-0.6C TWIP steel, the rolling preparation process is simplified, the production efficiency is improved, and the comprehensive mechanical properties of TWIP steel are also improved.

[0030] The high-strength austenitic TWIP steel and its preparation method of the present invention have low cost, simple process, low noise and air pollution, and good finished product performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Microstructure diagram of the high-strength austenitic TWIP steel prepared in Example 1;

[0032] Figure 2 Stress-strain curve diagram of the high-strength austenitic TWIP steel prepared in Example 1;

[0033] Figure 3 Microstructure diagram of the high-strength austenitic TWIP steel prepared in Example 2;

[0034] Figure 4 Stress-strain curve diagram of the high-strength austenitic TWIP steel prepared in Example 2;

[0035] Figure 5 Microstructure diagram of the high-strength austenitic TWIP steel prepared in Example 3;

[0036] Figure 6 Stress-strain curve diagram of the high-strength austenitic TWIP steel prepared in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be further described in detail below in conjunction with the embodiments with reference to the accompanying drawings.

[0038] Embodiment 1

[0039] A preparation method of a 1000MPa grade high-strength austenitic TWIP steel in this embodiment includes the following steps:

[0040] S1: According to the mass percentage of alloy components of the high-strength austenitic TWIP steel: C (0.58%), Mn (21.7%), Al (1.52%), Si (1.49%), Mo (1.02%), V (0.195%), Nb (0.022%), Ce (0.021%), and the remaining content is Fe for chemical composition ratio. In addition, for the 1000MPa grade high-strength austenitic TWIP steel, the addition of two rare earth elements Nb and Ce can be controlled within a deviation of 0.02% to reduce costs.

[0041] S2: The prepared TWIP steel raw materials are successively put into a vacuum melting furnace for melting, the melting temperature is 1500°C, then electromagnetic stirring is carried out under a vacuum state, the working frequency is 3Hz, and finally it is cast into an ingot under a vacuum state and naturally cooled.

[0042] S3: The slab obtained in S2 is hot-rolled and warm-rolled. First, the slab is heated to the rolling start temperature of 1000°C and held for 16 minutes, then hot-rolled through 4 passes, controlling the thickness of the hot-rolled plate to be 7mm, and the final hot-rolling temperature is controlled at 850°C. Then the hot-rolled final rolling plate is air-cooled to the warm-rolling start temperature of 600°C, and then warm-rolled through 3 passes, controlling the thickness of the warm-rolled plate to be 2mm, and the final warm-rolling temperature is controlled at 400°C.

[0043] S4: The warm-rolled plate obtained in S3 is heat-treated, and the heat treatment process is to hold at 900°C for 20 minutes and then water-cooled.

[0044] The microstructure of the 1000MPa grade high-strength austenitic TWIP steel is as Figure 1 shown. The stress-strain curve of the 1000MPa grade high-strength austenitic TWIP steel is as Figure 2 shown. The high-strength austenitic TWIP steel prepared in this embodiment has fine and uniform austenite grains, the yield strength can reach 652MPa, the tensile strength can reach 1038MPa, and the elongation after fracture is 25%.

[0045] Embodiment 2

[0046] A preparation method of a 1100MPa grade high-strength austenitic TWIP steel in this embodiment includes the following steps:

[0047] S1: According to the mass percentages of alloying elements of high-strength austenitic TWIP steel: C (0.62%), Mn (22.7%), Al (1.49%), Si (1.50%), Mo (1.06%), V (0.198%), Nb (0.024%), Ce (0.025%), and the remaining content is Fe, conduct chemical composition proportioning.

[0048] S2: Put the prepared TWIP steel raw materials into a vacuum melting furnace for melting in sequence. The melting temperature is 1550 °C, then conduct electromagnetic stirring under vacuum state, the working frequency is 2.5 Hz, and finally cast into an ingot under vacuum state and cool naturally.

[0049] S3: Conduct hot rolling and warm rolling on the slab obtained in S2. First, heat the slab to the rolling start temperature of 1050 °C and keep it warm for 12 min, then conduct hot rolling through 5 passes, control the thickness of the hot-rolled plate to be 5 mm, and control the final hot rolling temperature to be 900 °C. Then air-cool the finally hot-rolled plate to the warm rolling start temperature of 650 °C, then conduct warm rolling through 4 passes, control the thickness of the warm-rolled plate to be 1 mm, and control the final warm rolling temperature to be 450 °C, and finally air-cool to room temperature.

[0050] S4: Conduct heat treatment on the warm-rolled plate obtained in S3. The heat treatment process is to keep it warm at 850 °C for 25 min and then water-cool.

[0051] The microstructure of the high-strength austenitic TWIP steel of 1100 MPa grade is as Figure 3 shown. The stress-strain curve of the high-strength austenitic TWIP steel of 1100 MPa grade is as Figure 4 shown. The austenite grains of the high-strength austenitic TWIP steel prepared in this example are fine and have good uniformity. The yield strength can reach 710 MPa, the tensile strength can reach 1136 MPa, and the elongation after fracture is 19%.

[0052] Example 3

[0053] A preparation method of high-strength austenitic TWIP steel of 1200 MPa grade in this example includes the following steps:

[0054] S1: According to the mass percentages of alloying elements of high-strength austenitic TWIP steel: C (0.63%), Mn (22.9%), Al (1.48%), Si (1.53%), Mo (0.97%), V (0.199%), Nb (0.028%), Ce (0.027%), and the remaining content is Fe, conduct chemical composition proportioning.

[0055] S2: The prepared TWIP steel raw materials are sequentially put into a vacuum melting furnace for melting at a melting temperature of 1600 °C, then subjected to electromagnetic stirring under a vacuum state with a working frequency of 2 Hz, and finally cast into an ingot under a vacuum state and naturally cooled.

[0056] S3: The slab obtained in S2 is hot-rolled and warm-rolled. First, the slab is heated to the rolling start temperature of 1100 °C and held for 8 min, then hot-rolled through 3 passes to control the thickness of the hot-rolled plate to 9 mm, and the finish rolling temperature of the hot rolling is controlled at 950 °C. Then the hot-rolled finish-rolled plate is air-cooled to the warm-rolling start temperature of 700 °C, and then warm-rolled through 2 passes to control the thickness of the warm-rolled plate to 3 mm, and the finish rolling temperature of the warm rolling is controlled at 500 °C, and finally air-cooled to room temperature.

[0057] S4: The warm-rolled plate obtained in S3 is heat-treated, and the heat treatment process is to hold at 800 °C for 30 min and then water-cooled.

[0058] The microstructure of the high-strength austenitic TWIP steel of 1200 MPa grade is as Figure 5 shown. The stress-strain curve of the high-strength austenitic TWIP steel of 1200 MPa grade is as Figure 6 shown. The austenite grains of the high-strength austenitic TWIP steel prepared in this example are fine and have good uniformity. The yield strength can reach 763 MPa, the tensile strength can reach 1239 MPa, and the elongation after fracture is 14%.

Claims

1. A high-strength austenitic TWIP steel, characterized in that It includes the following components and their weight ratios: C 0.55％～0.65％； Mn 21.0%~23.0%; Al 1.45％～1.55％； Si 1.45%~1.55%; Mo 0.95%~1.10%; V 0.19％～0.20％； Nb 0.02%~0.03%; Ce 0.02%~0.03%; The rest is Fe.

2. A method for preparing the high-strength austenitic TWIP steel according to claim 1, characterized in that The steps include: ① Select the ingots to be melted according to the components and weight ratio; ② Forging the ingot to obtain a slab; ③ The slab is first hot-rolled to obtain a hot-rolled plate, and then the hot-rolled plate is warm-rolled to obtain a warm-rolled plate; ④ Heat treat the warm rolled plate.

3. The preparation method according to claim 2, characterized in that In step ②, the ingot is kept at 1150°C to 1250°C for 1.5h to 2h and then forged into a slab.

4. The preparation method according to claim 2, characterized in that In step ③, the starting rolling temperature of hot rolling is 1000°C to 1100°C, and the final rolling temperature is 850°C to 950°C.

5. The preparation method according to claim 4, characterized in that The number of hot rolling passes in step ③ is 3 to 5 times.

6. The preparation method according to claim 2, characterized in that The starting rolling temperature of the warm rolling in step ③ is 600°C to 700°C, and the final rolling temperature is 400°C to 500°C.

7. The preparation method according to claim 6, characterized in that The number of warm rolling passes in step ③ is 2 to 4 times.

8. The preparation method according to claim 6, characterized in that In step ③, the hot rolled plate is air-cooled to the warm rolling start temperature for warm rolling.

9. The preparation method according to claim 6, characterized in that In step ③, the warm rolled plate after final rolling is air-cooled.

10. The preparation method according to claim 2, characterized in that The heat treatment conditions in step ④ are as follows: the warm rolled plate is kept at 800°C to 900°C for 20min to 30min and then water-cooled.

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