Superplastic medium manganese steel and its preparation method based on rare earth element grain boundary segregation and TWIP effect regulation

Abstract

The invention discloses a superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect and a preparation method thereof. The alloy composition is C 0.2%, Mn 9%, Nb 0.1%, Mo 0.1%, rare earth 0.3%, and the balance is Fe and inevitable impurities. The invention optimizes the composition of the medium manganese steel and improves the rolling process, firstly utilizes the TWIP effect to prepare the ultrafine-grained medium manganese steel, then utilizes the rare earth segregation to inhibit the growth of grains, and finally realizes the preparation of the superplastic ultrafine-grained medium manganese steel with a stable grain size under high temperature conditions.

Description

Technical Field

[0001] The invention relates to the technical field of medium manganese steel, and in particular to a superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect and a preparation method thereof. Background Art

[0002] Energy saving, safety and environmental protection have become a development direction of the automotive industry, which has promoted the development of lightweight automobiles. As the third generation of advanced lightweight steel materials, medium manganese steel has the advantages of good performance and low cost, and has attracted the attention of academia and industry. At present, the formed parts of medium manganese steel are mainly concentrated in relatively simple A-pillars, B-pillars and bumpers, etc., and are less used in complex integrated forming parts. The superplasticity of medium manganese steel has the characteristics of good forming performance, which provides ideas for the integrated forming preparation of complex medium manganese steel parts. However, the superplasticity of medium manganese steel has not been effectively utilized. The main reason is that the superplasticity of medium manganese steel has the characteristics of high deformation temperature and low strain rate, which is not conducive to industrial production. Therefore, how to make the superplasticity of medium manganese steel take into account both low deformation temperature and high strain rate has become the focus of research.

[0003] Grain refinement is an effective way to achieve low temperature and high strain rate superplasticity of medium manganese steel, which has been proven in the applicant's previous patent (CN 114318161 A medium manganese steel with low temperature and high strain rate superplasticity and its preparation method). However, although grain refinement can make medium manganese steel have good superplasticity, during the superplastic thermal deformation process, the refined grains will grow rapidly, which will reduce the superplasticity of medium manganese steel. Therefore, how to prepare ultrafine and high temperature stable ultrafine grain structure of medium manganese steel is of great significance for further developing the potential of low temperature and high speed superplasticity of medium manganese steel. Summary of the invention

[0004] To achieve the above object, the present invention provides a method for preparing superplastic medium manganese steel based on the regulation of rare earth element grain boundary segregation and TWIP effect.

[0005] The technical solution adopted by the present invention is:

[0006] A superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect, wherein the alloy composition is: C 0.03-0.3%, Mn 3-12%, Nb 0.01-0.15%, Mo 0.01-0.15%, rare earth 0.01-0.3%, and the balance is Fe and unavoidable impurities.

[0007] The method for preparing the superplastic medium manganese steel comprises the following steps:

[0008] Step 1, smelting: according to C 0.03-0.3%, Mn 3-12%, Nb 0.01-0.15%, Mo 0.01-0.15%, rare earth 0.01-0.3%, the balance is Fe and unavoidable impurities, proportioning, smelting, casting to obtain a steel ingot;

[0009] Step 2, forging: heating the steel ingot to 1000-1200°C, keeping the temperature for 3-4 hours, and forging it into a steel billet;

[0010] Step 3, low temperature hot rolling: heating the steel billet to 600-700°C in the two-phase region, keeping the temperature for 1-2 hours, then cooling to 350-500°C, and performing 5-6 passes of low temperature rolling in the temperature range of 350-500°C to obtain a warm rolled plate;

[0011] Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a cold rolled sheet;

[0012] Step 5, critical heat treatment: subjecting the cold rolled sheet to critical heat treatment at a two-phase region temperature of 600-700° C. to obtain ultrafine grained medium manganese steel.

[0013] Furthermore, in step 1, the rare earth is a rare earth ferrosilicon alloy, wherein the mass percentage of rhenium is 15-20%.

[0014] Furthermore, in step 2, the thickness of the steel billet is between 8 mm and 10 mm.

[0015] Furthermore, in step 3, the total reduction ratio of warm rolling is 50% to 75%, which is beneficial to provide mechanical driving force for the TWIP effect.

[0016] Furthermore, in step 4, the total reduction ratio of cold rolling is 50-60%, which is beneficial to accumulate higher strain energy storage in ferrite and provide driving force for recrystallization in the subsequent critical heat treatment process.

[0017] Furthermore, in step 5, the critical heat treatment time is 45 to 60 minutes, which is conducive to dynamic recrystallization of medium manganese steel, further refinement of grains, and formation of austenite and ferrite dual-phase structure; it is conducive to the segregation of rare earth element rhenium at grain boundaries and inhibiting grain growth.

[0018] Beneficial effects of the present invention:

[0019] The present invention optimizes the composition design and rolling process of medium manganese steel so that the austenite stacking fault can meet the conditions for the occurrence of TWIP effect in the temperature range of 350-500°C, and then in the subsequent warm rolling process, the austenite produces a large number of twins due to the TWIP effect, so that the austenite size is greatly refined, and then cold rolling is performed, which will cause ferrite and austenite to produce a large number of dislocation densities, and recrystallization occurs in the subsequent critical heat treatment process, and the grain size of the material is refined again. Through these two refinements, the grain size of the medium manganese steel can reach about 100nm. Usually, finer grain sizes will grow during the heating process, such as in the critical heat treatment insulation process of the present invention and the superplastic thermal deformation process of the medium manganese steel. In order to solve this problem, the present invention uses rare earth elements to cause segregation at the interface of austenite and ferrite during the rolling process. In the subsequent critical heat treatment and superplastic high-temperature deformation process, this segregation behavior will have a good inhibitory effect on grain boundary migration, so that the ultrafine grain structure of the medium manganese steel has high-temperature stability. Through the combined effect of the above-mentioned inventive designs, the present invention effectively guarantees the superplasticity of medium manganese steel under low temperature and high speed conditions, thereby providing strong support for the industrial application of superplasticity of medium manganese steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 These are the results of tensile properties tests of the ultrafine-grained medium manganese steel obtained in Example 1 at different temperatures.

[0021] Figure 2 These are the results of tensile property tests at different temperatures of the superplastic ultrafine-grained medium manganese steel obtained in Example 1 of the present invention.

[0022] Figure 3 These are the results of tensile property tests at different temperatures of the superplastic ultrafine-grained medium manganese steel obtained in Example 2 of the present invention.

[0023] Figure 4 These are the results of tensile property tests at different temperatures of the superplastic ultrafine-grained medium manganese steel obtained in Example 3 of the present invention.

[0024] Figure 5 This is the metallographic image of the ultrafine-grained medium manganese steel obtained in Example 1 before and after the high-temperature mechanical properties test.

[0025] Figure 6 This is the metallographic image of the ultrafine-grained medium manganese steel obtained in Example 1 after high-temperature mechanical property testing.

[0026] Figure 7 This is a metallographic diagram of the superplastic ultrafine-grained medium manganese steel obtained in Example 1 of the present invention before high-temperature mechanical properties testing.

[0027] Figure 8 This is a metallographic diagram of the superplastic ultrafine-grained medium manganese steel obtained in Example 1 of the present invention after high-temperature mechanical property testing. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with specific examples to facilitate understanding of the present invention, but the present invention is not limited thereto.

[0029] Comparative Example 1

[0030] Ultrafine-grained medium manganese steel is prepared according to the following steps:

[0031] Step 1, smelting: according to C 0.03%, Mn 4%, Nb 0.01%, Mo 0.05%, the balance is Fe and inevitable impurities, proportioning, vacuum melting, casting to obtain a steel ingot;

[0032] Step 2, forging: heat the steel ingot to 1000℃, keep it warm for 3h, and forge it into a steel billet with a thickness of 100mm*40mm;

[0033] Step 3, multi-pass warm rolling: heat the steel billet to 600°C and keep it warm for 1 hour; perform 5-6 passes of low-temperature hot rolling to obtain a 30mm thick low-temperature hot-rolled plate; perform 5-6 passes of rolling the low-temperature hot-rolled plate at 350°C to obtain a 4mm thick warm-rolled plate;

[0034] Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a 1.5 mm thick cold rolled sheet;

[0035] Step 5, critical heat treatment: subjecting the cold-rolled sheet to critical heat treatment at a two-phase region temperature of 600° C. to obtain ultrafine-grained medium manganese steel A with superplasticity;

[0036] The ultrafine-grained medium manganese steel A was prepared into a 10 mm × 10 mm heat-treated sample by wire cutting. The sample was ground step by step to a bright finish using different types of sandpaper, and then electrolytically polished and etched. The microstructure was observed. Figure 5 .

[0037] The tensile properties of the ultrafine-grained medium manganese steel heat-treated sample A were tested at a strain rate of 650-750°C. The test results are as follows: Figure 1 As shown. Figure 1 It can be seen that the experimental steel is in the temperature range of 650-750℃, and in 0.5s -1 The elongation of 608-796% was obtained at the strain rate.

[0038] The microstructure of ultrafine-grained medium manganese steel A after tensile test is observed. Figure 6 .from Figure 5 and Figure 6It can be seen that the average grain size of the superplastic ultrafine-grained manganese steel obtained in Comparative Example 1 changes from the initial 0.4 μm to 0.8 μm, and it can be clearly seen that the grains grow after hot stretching.

[0039] Example 1

[0040] A superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect, wherein the preparation method comprises the following steps:

[0041] Step 1, smelting: Smelting: according to C 0.2%, Mn 9%, Nb 0.1%, Mo 0.1%, rare earth 0.3%, the balance is Fe and inevitable impurities, proportioning, vacuum melting, casting to obtain a steel ingot;

[0042] Step 2, forging: heat the steel ingot to 1200℃, keep it warm for 4h, and forge it into a steel billet with a thickness of 100mm*30mm;

[0043] Step 3, multi-pass warm rolling: heat the steel billet to 700°C and keep it warm for 2 hours; perform 5-6 passes of low-temperature hot rolling to obtain a 25mm thick low-temperature hot-rolled plate; perform 5-6 passes of rolling the low-temperature hot-rolled plate at 500°C to obtain a 4mm thick warm-rolled plate;

[0044] Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a 1.5 mm thick cold rolled sheet;

[0045] Step 5, critical heat treatment: subjecting the cold-rolled sheet to critical heat treatment at a two-phase region temperature of 700° C. to obtain ultrafine-grained medium manganese steel B with superplasticity;

[0046] The ultrafine-grained medium manganese steel B was prepared into a 10 mm × 10 mm heat-treated sample by wire cutting. The sample was ground step by step to a bright finish using different types of sandpaper, and then electrolytically polished and corroded. The microstructure was observed. Figure 6 .

[0047] The ultrafine-grained medium manganese steel heat treatment sample B was subjected to 0.5s -1 The tensile properties test experiment was carried out at a strain rate of 650-750℃. The test results are as follows: Figure 2 As shown. Figure 2 It can be seen that the experimental steel is in the temperature range of 650-750℃, and in 0.5s -1 The elongation of 802-1473% was obtained at the strain rate.

[0048] The microstructure of ultrafine-grained medium manganese steel B after tensile test is observed. Figure 7 .from Figure 7 and Figure 8It can be seen that the average grain size of the superplastic ultrafine-grained manganese steel with rare earth added obtained by the present invention changes from the initial 0.3μm to 0.45μm, which only changes by 0.15μm. It can be seen that in addition to using the TWIP effect to refine the grain size, the present invention can also use the rare earth element grain boundary segregation to inhibit grain growth, thereby further improving the superplastic properties of the material.

[0049] Example 2

[0050] A superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect, wherein the preparation method comprises the following steps:

[0051] Step 1, smelting: according to C 0.31%, Mn 12%, Nb 0.15%, Mo 0.1%, rare earth 0.12%, the balance is Fe and inevitable impurities, proportioning, vacuum melting, casting to obtain a steel ingot;

[0052] Step 2, forging: heat the steel ingot to 1200℃, keep it warm for 3h, and forge it into a steel billet with a thickness of 100mm*40mm;

[0053] Step 3, multi-pass warm rolling: heat the steel billet to 700°C and keep it warm for 1 hour; perform 5-6 passes of low-temperature hot rolling to obtain a 30mm thick low-temperature hot-rolled plate; perform 5-6 passes of rolling the low-temperature hot-rolled plate at 500°C to obtain a 4mm thick warm-rolled plate;

[0054] Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a 1.8 mm thick cold rolled sheet;

[0055] Step 5, critical heat treatment: subjecting the cold rolled sheet to critical heat treatment at a two-phase region temperature of 600° C. to obtain ultrafine-grained medium manganese steel C with superplasticity;

[0056] The ultrafine-grained medium manganese steel C was prepared into a 10 mm × 10 mm heat treatment sample by wire cutting. The ultrafine-grained medium manganese steel heat treatment sample C was -1 The tensile properties test experiment was carried out at a strain rate of 650-750℃. The test results are as follows: Figure 3 As shown. Figure 3 It can be seen that the experimental steel is in the temperature range of 650-750℃, and in 0.5s -1 The elongation of 763-1127% was obtained at the strain rate.

[0057] Example 3

[0058] A superplastic medium manganese steel regulated by rare earth element grain boundary segregation and TWIP effect, wherein the preparation method comprises the following steps:

[0059] Step 1, smelting: Smelting: according to C0.2%, Mn 9%, Nb0.1%, Mo0.05%, rare earth 0.01%, the balance is Fe and inevitable impurities, proportioning, vacuum melting, casting to obtain a steel ingot;

[0060] Step 2, forging: heat the steel ingot to 1200℃, keep it warm for 4h, and forge it into a steel billet with a thickness of 100mm*30mm;

[0061] Step 3, multi-pass warm rolling: heat the steel billet to 700°C and keep it warm for 2 hours; perform 5-6 passes of low-temperature hot rolling to obtain a 25mm thick low-temperature hot-rolled plate; perform 5-6 passes of rolling the low-temperature hot-rolled plate at 500°C to obtain a 4mm thick warm-rolled plate;

[0062] Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a 1.5 mm thick cold rolled sheet;

[0063] Step 5, critical heat treatment: subjecting the cold rolled sheet to critical heat treatment at a two-phase region temperature of 700° C. to obtain ultrafine-grained medium manganese steel D with superplasticity;

[0064] The ultrafine-grained medium manganese steel D was prepared into a 10 mm × 10 mm heat treatment sample by wire cutting. The ultrafine-grained medium manganese steel heat treatment sample D was -1 The tensile properties test experiment was carried out at a strain rate of 650-750℃. The test results are as follows: Figure 4 As shown. Figure 4 It can be seen that the experimental steel is in the temperature range of 650-750℃, and in 0.5s -1 The elongation of 703-912% was obtained at the strain rate.

[0065] It can be seen from the above embodiments and comparative examples that, compared with the prior art, the superplastic medium manganese steel obtained in the present invention has obvious low-temperature superplasticity advantages.

[0066] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications are also within the protection scope of the present invention.

Claims

1. A method for preparing superplastic medium manganese steel based on rare earth element grain boundary segregation and TWIP effect regulation, characterized in that: The alloy composition of the superplastic medium manganese steel is: C 0.03-0.3%, Mn 3-12%, Nb 0.01-0.15%, Mo 0.01-0.15%, rare earth 0.01-0.3%, and the balance is Fe and unavoidable impurities; The method for preparing the superplastic medium manganese steel comprises the following steps: Step 1, smelting: according to C 0.03-0.3%, Mn 3-12%, Nb 0.01-0.15%, Mo0.01-0.15%, rare earth 0.01-0.3%, the balance is Fe and unavoidable impurities, proportioning, smelting, casting to obtain a steel ingot; Step 2, forging: heating the steel ingot to 1000-1200°C, keeping the temperature for 3-4 hours, and forging it into a steel billet; Step 3, low temperature hot rolling: heating the steel billet to 600-700°C in the two-phase region, keeping the temperature for 1-2 hours, then cooling to 350-500°C, and performing 5-6 passes of low temperature rolling in the temperature range of 350-500°C to obtain a warm rolled plate; Step 4, high reduction cold rolling: pickling the warm rolled sheet, and then cold rolling it with a cold rolling mill to obtain a cold rolled sheet; Step 5, intercritical heat treatment: subjecting the cold rolled sheet to intercritical heat treatment at a two-phase region temperature of 600-700° C. to obtain superplastic medium manganese steel.

2. The method for preparing superplastic medium manganese steel according to claim 1, characterized in that: In step 2, the thickness of the steel billet is between 8 and 10 mm.

3. The method for preparing superplastic medium manganese steel according to claim 1, characterized in that: In step 3, the total reduction rate of warm rolling is 50-75%.

4. The method for preparing superplastic medium manganese steel according to claim 1, characterized in that: In step 4, the total reduction ratio of cold rolling is 50-60%.

5. The method for preparing superplastic medium manganese steel according to claim 1, characterized in that: In step 5, the critical heat treatment time is 45 to 60 minutes.

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