A processing method for improving the product of strength and plasticity of medium manganese steel plates

Abstract

The present invention discloses a processing method for improving the strength-ductility product of medium manganese steel plates, which relates to the technical field of medium manganese steel plate processing. The present invention adjusts the composition of manganese steel and then smelts and casts it into an ingot, and successively performs forging, hot rolling, soft annealing, cold rolling, tempering, and critical annealing treatments to obtain a medium manganese steel plate with high strength and toughness. Without much increase in production cost, the present invention effectively improves the strength-ductility product of medium manganese steel plates as a whole, and promotes the industrial production and application of medium manganese steel.

Description

Technical Field

[0001] The invention belongs to the technical field of medium manganese steel plate processing, and specifically relates to a processing method for improving the strength and plasticity of medium manganese steel plate. Background Art

[0002] As the environment deteriorates and the number of cars increases, energy conservation and emission reduction have received widespread attention in the world, and lightweighting of cars has become one of the key research and development directions of the automotive manufacturing industry. Lightweighting can be achieved through the application of advanced high-strength steel, high-strength steel hot / cold forming technology, etc.

[0003] Medium manganese steel (manganese mass fraction between 3 and 12%) is the third generation of advanced automotive high-strength steel. Due to its excellent strength-plasticity product (20 to 70 GPa%), it is possible to achieve a streamlined cold forming process and contribute to lightweighting. After the cold-rolled medium manganese steel undergoes austenite reversal by conventional annealing, its microstructure is usually a two-phase structure of austenite and ferrite. During the deformation process, the residual austenite partially or completely undergoes transformation-induced plasticity (TRIP) effect. The TRIP effect can delay necking and significantly increase its elongation. The generated new phase martensite increases the material strength, thereby optimizing the mechanical properties.

[0004] At present, the stability of austenite is the main factor to ensure the occurrence of TRIP effect, among which the manganese content in austenite, austenite grain size, austenite volume fraction and other multiple factors are coupled to determine its stability. In addition, on the one hand, it is necessary to prevent the austenite stability from being too poor, so that a large amount of martensitic phase transformation occurs in the early stage of plastic deformation, and the remaining austenite content cannot meet the needs of subsequent deformation, resulting in premature fracture. On the other hand, it is also necessary to prevent the austenite from being too stable and unable to fully exert the role of TRIP effect.

[0005] In recent years, domestic and foreign scholars have introduced high manganese cementite particles by isothermal tempering before the reverse transformation of austenite in medium manganese steel. High manganese cementite particles can serve as additional austenite nucleation sites. In the reverse transformation process of austenite nucleation and growth, compared with austenite nucleated and grown at the martensite boundary, austenite that inherits the nucleation of high manganese cementite has excellent properties such as high manganese content, fine grain size, and resistance to plastic deformation. However, the additional isothermal tempering process prolongs the heat treatment time, resulting in excessive formation and grain growth of austenite, which in turn leads to poor stability of austenite and further deteriorates its mechanical properties.

[0006] In addition, for the current research status of medium manganese steel plates, the strength-plasticity product of medium manganese steel plates is relatively low, so how to optimize the processing technology of medium manganese steel plates to further improve the strength-plasticity product of medium manganese steel plates is a hot topic in current research. Summary of the invention

[0007] Based on this, the objective of the present invention is to provide a processing method for improving the strength-ductility product of medium manganese steel sheets. This method, based on the design concept of medium manganese steel, adjusts the processing technology by means of the influence of a large number of copper-rich particles and cementite double nano-particles precipitated during tempering on the reverse transformation of austenite in medium manganese steel, thereby manufacturing a medium manganese steel material with high strength and toughness, significantly improving the strength-ductility product of medium manganese steel sheets to meet the use indexes of different automotive parts.

[0008] The present invention is implemented by adopting the following technical solutions:

[0009] A processing method for improving the strength-ductility product of medium manganese steel sheets, comprising the following steps:

[0010] Step 1, melting: melting the raw materials and casting them into an ingot; the mass percentages of the main chemical components in the ingot are as follows:

[0011] C: 0.15 - 0.18%, Mn: 10.0 - 10.32%, Al: 1.9 - 2.1%, Cu: 1.8 - 2.2%, and the rest is Fe and inevitable impurities;

[0012] Step 2, forging: heating the ingot to 1200°C, holding for 2 h, then forging it into a slab, and subsequently air-cooling it to room temperature;

[0013] Step 3, hot rolling: heating the slab to 1200°C, holding for 2 h, and then performing rolling, wherein the starting rolling temperature is 1200 ± 50°C, the final rolling temperature is not lower than 900°C, and after 5 - 7 passes of rolling, a thin sheet with a thickness of 4 ± 0.5 mm is obtained, and then it is air-cooled to room temperature;

[0014] Step 4, soft annealing: heating the thin sheet to 600°C, holding for 30 min, and then air-cooling it to room temperature;

[0015] Step 5, cold rolling: performing cold rolling on the thin sheet after soft annealing treatment at room temperature with a large reduction ratio, and after 5 - 7 passes of rolling, a cold-rolled thin sheet with a thickness of 1.2 ± 0.2 mm is obtained;

[0016] Step 6, tempering: heating the cold-rolled thin sheet to 450°C, holding for 30 min, and then water-cooling it to room temperature;

[0017] Step 7, critical annealing: quickly heating the cold-rolled thin sheet after tempering treatment to 600°C, with a heating rate not lower than 50°C / s, holding for 30 min, and then water-cooling it to room temperature to obtain a cold-rolled medium manganese steel sheet with high strength and toughness.

[0018] Preferably, in Step 2, the cross-section of the slab is 100 mm × 30 mm.

[0019] Preferably, in Step 3, the final rolling temperature is 900 - 1000°C.

[0020] Preferably, the reduction in the heavy reduction cold rolling described in step 5 is 70%.

[0021] Preferably, the heating rate in step 7 is 50 - 70 °C / s.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] On the one hand, the present invention combines the fixing effect of copper-rich particles on the phase boundary and recrystallized grains, preventing the excessive formation and growth of austenite; on the other hand, it fully retains the manganese-rich fine-grained austenite of the genetic cementite. Under the combined action of these two kinds of precipitated particles, an appropriate austenite grain size and austenite content are prepared, ensuring its appropriate stability, enabling the TRIP effect to occur sufficiently and coordinately during the deformation process, and achieving excellent mechanical properties. For the medium manganese steel sheet processed by the present invention, the austenite grain size and austenite content are 0.74 μm and 49.3% respectively, and the manganese content in the manganese-rich fine-grained austenite of the inherited cementite reaches ~20 wt.%, which is significantly higher than the manganese content of austenite formed at the martensite boundary (~15 wt%). The tensile strength of the medium manganese steel sheet of the present invention is 1250 ± 15 MPa, the elongation is 45.1 ± 2.9%, and the product of strength and plasticity is 56.4 ± 2.0 Pa%. Without much increase in production cost, the product of strength and plasticity of the medium manganese steel sheet is effectively improved as a whole, promoting the industrial production and application of medium manganese steel. Description of the Drawings

[0024] Figure 1 It is the transmission electron microscope image of the cold-rolled thin sheet after the tempering treatment in step 6 of the embodiment;

[0025] Figure 2 It is the transmission electron microscope image (a) and manganese element composition distribution map (b) of the cold-rolled medium manganese steel sheet of the embodiment;

[0026] Figure 3 It is the scanning electron microscope image of the cold-rolled medium manganese steel sheet of the embodiment and the manganese steel sheet prepared in the comparative example;

[0027] Figure 4 It is the engineering stress-strain curve of the cold-rolled medium manganese steel sheet of the embodiment and the manganese steel sheet prepared in the comparative example;

[0028] Figure 5 It is the change in austenite content corresponding to the tensile process of the cold-rolled medium manganese steel sheet of the embodiment and the manganese steel sheet prepared in the comparative example. Detailed Embodiments

[0029] The present invention will be further described below in conjunction with specific embodiments.

[0030] Embodiment

[0031] A processing method for improving the strength-ductility product of medium manganese steel sheets, the specific steps are as follows:

[0032] Step 1, melting: melting the raw materials and casting them to obtain an ingot; the mass percentages of the main chemical components in the ingot are as follows:

[0033] C: 0.15%, Mn: 10.32%, Al: 2.01%, Cu: 1.98%, and the balance is Fe and inevitable impurities;

[0034] Step 2, forging: heating the ingot to 1200 °C, holding for 2 h, then forging it into a slab with a cross-section of 100 mm × 30 mm, and then air-cooling it to room temperature;

[0035] Step 3, hot rolling: heating the slab to 1200 °C, holding for 2 h, and then performing rolling, where the starting rolling temperature is 1200 ± 50 °C, the finishing rolling temperature is 900 °C, and a thin sheet with a thickness of 4 ± 0.5 mm is obtained after 6 passes of rolling, and then air-cooling it to room temperature;

[0036] Step 4, softening annealing: heating the thin sheet to 600 °C, holding for 30 min, and then air-cooling it to room temperature;

[0037] Step 5, cold rolling: performing cold rolling with a large reduction ratio (the reduction ratio of the cold rolling with a large reduction ratio is 70%) on the thin sheet after softening annealing at room temperature, and obtaining a cold-rolled thin sheet with a thickness of 1.2 ± 0.2 mm after 7 passes of rolling;

[0038] Step 6, tempering: heating the cold-rolled thin sheet to 450 °C, holding for 30 min, and then water-cooling it to room temperature;

[0039] Step 7, critical annealing: quickly heating the cold-rolled thin sheet after tempering to 600 °C, the heating rate is 50 °C / s, holding for 30 min, and then water-cooling it to room temperature to obtain a cold-rolled medium manganese steel sheet with high strength and toughness.

[0040] Performing transmission electron microscopy observation on the cold-rolled thin sheet after the tempering treatment in Step 6 (as Figure 1 shown), analyzing the morphology and density of the precipitated copper-rich particles and cementite, there are mainly two types of precipitated phases in the tempered martensite matrix, relatively coarser cementite (~70 nm) in size and dispersed fine copper-rich particles (~10 nm), and the number density of cementite (1.73×10 13 m -2 ) is significantly lower than that of the dispersed copper-rich particles (1.48×10 14 m -2 ),

[0041] Figure 2Transmission electron microscopy (TEM) morphology analysis and composition analysis were performed on the as-prepared cold-rolled medium manganese steel sheets. It was found that the austenite (γ(θ)) nucleated on the cementite had a higher manganese content (20.1 wt% vs. 15.2 wt%) and smaller size (∼300 nm vs. ∼700 nm) compared to the austenite matrix (γ) nucleated directly at the martensite boundaries.

[0042] Comparative Example 1

[0043] A processing method for medium manganese steel sheets, with reference to the examples, is different only in that the mass percentages of the main chemical components in the ingot in Step 1 are as follows:

[0044] C: 0.18%, Mn: 10.25%, Al: 2.11%, and the balance is Fe and unavoidable impurities.

[0045] Comparative Example 2

[0046] A processing method for medium manganese steel sheets, the specific steps are as follows:

[0047] Step 1, melting: The raw materials were melted and cast into an ingot; the mass percentages of the main chemical components in the ingot are as follows:

[0048] C: 0.15%, Mn: 10.32%, Al: 2.01%, Cu: 1.98%, and the balance is Fe and unavoidable impurities;

[0049] Step 2, forging: The ingot was heated to 1200 °C, held for 2 h, and then forged into a slab with a cross-section of 100 mm × 30 mm, and then air-cooled to room temperature;

[0050] Step 3, hot rolling: The slab was heated to 1200 °C, held for 2 h, and then rolled, where the starting rolling temperature was 1200 ± 50 °C and the final rolling temperature was 900 °C. After 6 passes of rolling, a thin sheet with a thickness of 4 ± 0.5 mm was obtained, and then air-cooled to room temperature;

[0051] Step 4, soft annealing: The thin sheet was heated to 600 °C, held for 30 min, and then air-cooled to room temperature;

[0052] Step 5, cold rolling: The thin sheet after soft annealing was cold-rolled at room temperature with a large reduction (the reduction of large reduction cold rolling was 70%). After 7 passes of rolling, a cold-rolled thin sheet with a thickness of 1.2 ± 0.2 mm was obtained;

[0053] Step 6, critical tempering: The cold-rolled thin sheet was heated to 600 °C, held for 30 min, and then water-cooled to room temperature to obtain a medium manganese steel sheet.

[0054] Comparative Example 3

[0055] A processing method for medium manganese steel plates, referring to Comparative Example 2, the difference is only that in Step 1, the mass percentages of the main chemical components in the ingot are as follows:

[0056] C: 0.18%, Mn: 10.25%, Al: 2.11%, and the balance is Fe and inevitable impurities.

[0057] The morphologies of the medium manganese steel plates prepared in the examples and Comparative Examples 1-3 were observed by scanning electron microscopy. As Figure 3 shown, it was found that the microstructures in the plates were all composed of equiaxed austenite grains + ferrite grains. The average size of the austenite grains in the examples (~0.74 μm) was significantly smaller than the average size of the austenite in Comparative Example 1 (~1.12 μm), as Figure 3 shown.

[0058] The relevant mechanical properties of the medium manganese steel plates in the examples and Comparative Examples 1-3 were tested. According to GB / T228-2002 "Metallic materials - Tensile testing at ambient temperature", the heat-treated steel plates were processed into tensile specimens, and the stress-strain curves are as Figure 4 shown. The tensile strength of the medium manganese steel plates in the examples was 1250 MPa. At such a high tensile strength, an elongation rate of 45.1% was still maintained. Although the tensile strength of Comparative Example 1 reached 1382 MPa, the plasticity decreased significantly to 19.2%.

[0059] It can also be seen from Table 1 that compared with Comparative Example 1, the examples had the most excellent combination of strength and plasticity, and the strength-plasticity product reached 56.4 GPa%. XRD phase detection was carried out on the examples and Comparative Example 1 under various stages of deformation to analyze the austenite content and stability. The austenite content in Comparative Example 1 was too high, reaching ~60%. After tensile fracture, the austenite content was less than 5%, and the austenite transformation rate was as high as ~92%. While the austenite content in the examples was ~50%, after tensile fracture, the austenite content was ~15%, and the austenite transformation rate was ~70%. Therefore, the austenite stability in Comparative Example 1 was significantly lower than that in the examples, as Figure 5 shown.

[0060] Thus, compared with Comparative Example 1 without copper, the copper-rich nanoparticles and cementite duplex precipitation in the examples optimized the austenite characteristics, successfully prepared austenite nucleated by cementite, and the introduction of copper-rich particles refined the austenite grain size, preventing excessive formation of austenite, with both austenite content and stability, thus significantly optimizing the strength-plasticity product to meet the usage and future development requirements of modern automotive steels.

[0061] Table 1 Mechanical property curves of the examples and comparative examples of the present invention

[0062]

[0063] It should be noted that the above are only several specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and there may be other variations. All variations directly derived or indirectly extended by those skilled in the art from the disclosed content of the present invention shall be considered as within the protection scope of the present invention.

Claims

1. A processing method for improving the strength-ductility product of medium manganese steel plates, characterized in that, it includes the following steps: Step 1, melting: The raw materials are melted and cast to obtain an ingot; the mass percentages of the main chemical components in the ingot are as follows: C: 0.15 - 0.18%, Mn: 10.0 - 10.32%, Al: 1.9 - 2.1%, Cu: 1.8 - 2.2%, and the rest are Fe and unavoidable impurities; Step 2, forging: Heat the ingot to 1200°C, hold for 2 h, then forge it into a slab, and then air-cool it to room temperature; Step 3, hot rolling: Heat the slab to 1200°C, hold for 2 h, and then perform rolling, where the starting rolling temperature is 1200 ± 50°C, and the final rolling temperature is not lower than 900°C. After 5 - 7 passes of rolling, a thin plate with a thickness of 4 ± 0.5 mm is obtained, and then air-cooled to room temperature; Step 4, soft annealing: Heat the thin plate to 600°C, hold for 30 min, and then air-cool it to room temperature; Step 5, cold rolling: Perform cold rolling with a large reduction ratio on the thin plate after soft annealing treatment at room temperature. After 5 - 7 passes of rolling, a cold-rolled thin plate with a thickness of 1.2 ± 0.2 mm is obtained; Step 6, tempering: Heat the cold-rolled thin plate to 450°C, hold for 30 min, and then water-cool it to room temperature; Step 7, critical annealing: Rapidly heat the cold-rolled thin plate after tempering treatment to 600°C, the heating rate is 50°C / s - 70°C / s, hold for 30 min, and then water-cool it to room temperature; The reduction ratio of the large reduction ratio cold rolling in Step 5 is 70%.

2. The processing method for improving the strength-ductility product of medium manganese steel plates according to claim 1, characterized in that, the cross-section of the slab in Step 2 is 100 mm × 30 mm.

3. The processing method for improving the strength-ductility product of medium manganese steel plates according to claim 1, characterized in that, the final rolling temperature in Step 3 is 900 - 1000°C.

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