1200MPa-grade hot continuous rolling high-strength medium manganese steel

Abstract

The invention discloses 1200MPa-grade hot continuous rolling high-strength medium manganese steel. The 1200MPa-grade hot continuous rolling high-strength medium manganese steel comprises the following chemical components in percentage by weight: 0.15 to 0.25 percent of C, 5.5 to 6.9 percent of Mn, 1.5 to 2.2 percent of Si, 0.30 to 0.50 percent of Mo, 0.05 to 0.10 percent of V, less than 0.01 percent of S, less than 0.01 percent of P and the balance of Fe and other inevitable impurities. According to the invention, after-rolling waste heat is utilized to regulate and control the microstructure and mechanical properties, and the method has the advantages of simple preparation process, low energy consumption and easy industrial implementation. In the continuous cooling process, austenite stable elements are re-distributed in martensite and surrounding austenite phases, and an obtained matrix structure is a high-strength lath martensite and a small amount of metastable state austenite structure and has high strength and good plasticity.

Description

Technical Field

[0001] The present invention belongs to the production of hot-rolled positive phase transformation high-strength steel plates, and specifically relates to a hot-rolled high-strength medium manganese steel. Background Art

[0002] There is an urgent need for high-strength automotive steel with green and energy-saving production processes, which has significant effects in energy conservation and emission reduction, structural component safety improvement and lightweighting. 980 / 1050 / 1180 high-strength steel has been mass-produced and supplied to the European market, which can achieve a 10% to 15% weight reduction for parts, further reducing the weight of the body-in-white and increasing fuel economy. Domestic commercial vehicle companies such as FAW Jiefang, Dongfeng Commercial Vehicle, and Sinotruk are also developing higher-strength automotive steels based on 800MPa. As the third generation of advanced high-strength automotive steel, high-strength medium-manganese steel has outstanding advantages in lightweighting structural parts and improving product life. At present, Q690 grade high-strength medium-manganese steel has been widely used in automobile manufacturing and engineering machinery and other fields, but in the application of high-strength medium-manganese steel above 1000MPa, it still faces the problem of a good match between preparation process properties, strength and plasticity. Therefore, the research and development of medium-manganese steel with low energy consumption in the preparation process and high strength and plasticity plays an important role in accelerating the low-carbon transformation and development of my country's steel industry and improving my country's energy structure system.

[0003] Currently, conventional high-strength medium-manganese steels are mostly produced using a reverse transformation process involving post-rolling intercritical annealing. This process involves diffusion of the Mn element from the α phase into the γ phase during annealing, resulting in a fine ferrite + moderate retained austenite composite structure. This metastable retained austenite stimulates the transformation-induced plasticity (TRIP) effect during deformation, achieving high strength and plasticity. However, the strength loss caused by the high-strength martensite during annealing limits the application of this type of steel in structural components requiring high crash resistance. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the present invention utilizes post-rolling residual heat to regulate microstructure and mechanical properties. By controlling the redistribution of austenite-stabilizing elements within the martensite and surrounding austenite phases during continuous cooling, this direct transformation process produces a matrix structure composed of high-strength lath martensite and a small amount of metastable austenite, resulting in high strength and good ductility. This invention offers the advantages of a simple production process, low energy consumption, and ease of industrial implementation.

[0005] The technical solutions of the present invention are as follows:

[0006] A 1200MPa grade hot-rolled high-strength medium manganese steel, the chemical composition of which is as follows by weight: C: 0.15-0.25%, Mn: 5.5-6.9%, Si: 1.5-2.2%, Mo: 0.30-0.50%, V: 0.05-0.10%, S: <0.01%, P: <0.01%, and the balance is Fe and other inevitable impurities;

[0007] The preparation method comprises the following steps:

[0008] Step 1, billet heating and hot rolling:

[0009] Smelting is performed according to the above chemical composition and casting is performed into slabs; the slabs are heated in a furnace to 1100-1250°C and kept warm for 2-4 hours; the iron oxide scale is removed and hot rolled, and the steel plates are hot rolled in 6-8 passes to a target thickness of 18-24 mm, and the finishing rolling temperature is controlled at 780-900°C;

[0010] Step 2: Coil and cool:

[0011] The hot-rolled steel plate is coiled at a coiling temperature of 550-650°C and then slowly cooled to room temperature; ultimately, a lath martensite and film-like retained austenite duplex structure is obtained.

[0012] The Mn element significantly expands the austenite phase region and improves the hardenability of the material, so that only martensite transformation still occurs during slow cooling in the furnace, thereby obtaining a high-strength martensite matrix and an appropriate amount of retained austenite structure.

[0013] The addition of 1.5-2.2% Si inhibits the precipitation of cementite, ultimately obtaining a lath martensite matrix and an appropriate amount of retained austenite structure; during the plastic deformation process, martensite provides high strength and retained austenite effectively relaxes stress concentration, thereby obtaining mechanical properties with high strength, plasticity and toughness matching.

[0014] The single-pass reduction rate is 10% to 25%, and the total reduction rate is 76% to 82%.

[0015] The steel plate has a yield strength of 820-980 MPa, a tensile strength of 1220-1380 MPa, a Vickers hardness of 330-380 HV, and an elongation of 12%-18%.

[0016] The beneficial effects of the present invention are:

[0017] 1. Low preparation cost. This process uses residual heat after rolling for heat treatment, and does not require additional heat treatment processes. It has the advantages of low energy consumption and easy industrial implementation.

[0018] 2. The steel plate has a simple preparation process and a wide process window. Combined with the aforementioned phase transition characteristics, the material's high-temperature coiling process at 550-650°C significantly reduces the difficulty of coiling medium and thick plates, improves internal stress within the structure, and effectively reduces the difficulty of straightening during product use.

[0019] 3. The steel plate has excellent comprehensive performance.

[0020] 4. The deformed original austenite of the material of the present invention has high thermal stability. During the slow cooling process after coiling, the supercooled austenite only undergoes martensite transformation, which is a phase transformation feature that traditional "quenching-partitioning" steel does not have. After slowly cooling to the martensite transformation temperature range, the supersaturated carbon element in the martensite is first transformed and diffuses into the surrounding untransformed austenite, so that part of the austenite is stabilized to room temperature. Due to the addition of higher Si elements, the precipitation of cementite is suppressed, and finally a lath martensite matrix and an appropriate amount of residual austenite structure are obtained. The matrix structure of the steel of this composition system is high-strength martensite. With the change of the content of austenite stabilizing elements, the content of retained austenite changes slightly, but there is no high-temperature phase transformation products such as pearlite, ferrite, and bainite. During the plastic deformation process, martensite provides high strength, and the retained austenite effectively relaxes stress concentration, thereby obtaining excellent mechanical properties of high-strength and plastic matching. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the metallographic structure diagram of the experimental steel in Example 1;

[0022] Figure 2 This is the metallographic diagram of the original austenite grain boundary of the experimental steel in Example 3

[0023] Figure 3 is the SEM micrograph of the experimental steel in Example 3;

[0024] Figure 4 TEM micrograph of the experimental steel in Example 3;

[0025] Figure 5 is the metallographic structure diagram of the experimental steel in Comparative Example 1;

[0026] Figure 6 is the metallographic structure diagram of the experimental steel in Comparative Example 2;

[0027] Figure 7 This is the TEM micrograph of the experimental steel in Comparative Example 2. DETAILED DESCRIPTION

[0028] The device for observing the metallographic structure in the embodiment of the present invention is a Leica DMIRM-2500M metallographic microscope;

[0029] The device for observing SEM tissue in the embodiment of the present invention is a Zeiss Ultra55 scanning electron microscope;

[0030] The device for observing TEM tissue in the embodiment of the present invention is Tecnai G 2 F20 field emission transmission electron microscope.

[0031] Example 1

[0032] The process steps for preparing hot-rolled high-strength medium manganese steel with a thickness of 24 mm are as follows:

[0033] Step 1: Billet heating and hot rolling:

[0034] The chemical composition of the steel slab, by weight, is: C: 0.15%, Mn: 6.9%, Si: 1.5%, Mo: 0.30%, V: 0.05%, S: 0.005%, P: 0.01%, with the balance being Fe and other unavoidable impurities. The slab is furnace-heated to 1250°C and held for 3 hours. It is then hot-rolled in six passes to a target thickness of 24mm, with a total reduction of 76%. The final rolling temperature is 950°C.

[0035] Step 2: Coil and cool:

[0036] The hot rolled steel sheet is coiled at a temperature of 650°C and then slowly cooled to room temperature. Figure 1 As shown, it can be seen that the original austenite presents a typical equiaxed recrystallization morphology, and a fine lath martensite matrix is ​​obtained.

[0037] Mechanical property tests were conducted on the experimental steel, and the yield strength of the experimental steel was 820 MPa, the tensile strength was 1220 MPa, the Vickers hardness was 340 HV, and the elongation was 18%.

[0038] Example 2

[0039] The process steps for preparing hot-rolled high-strength medium manganese steel with a thickness of 20 mm are as follows:

[0040] Step 1: Billet heating and hot rolling:

[0041] The chemical composition of the steel billet, by weight, is: C: 0.20%, Mn: 6.0%, Si: 1.7%, Mo: 0.35%, V: 0.09%, S: 0.01%, P: 0.008%, with the balance being Fe and other unavoidable impurities. The billet is furnace-heated to 1200°C and held for 2 hours. It is then hot-rolled in seven passes to a target thickness of 20mm, with a total reduction of 80%. The final rolling temperature is 820°C.

[0042] Step 2: Coil and cool:

[0043] After hot rolling, the steel sheet is coiled at 580°C and then slowly cooled to room temperature. This results in a dual-phase structure of lath martensite and film-like retained austenite.

[0044] Mechanical property tests were conducted on the experimental steel, and the yield strength of the experimental steel was 870 MPa, the tensile strength was 1280 MPa, the Vickers hardness was 370 HV, and the elongation was 15%.

[0045] Example 3

[0046] The process steps for preparing hot-rolled high-strength medium manganese steel with a thickness of 18 mm are as follows:

[0047] Step 1: Billet heating and hot rolling:

[0048] The chemical composition of the steel slab, by weight, is: C: 0.25%, Mn: 5.5%, Si: 2.2%, Mo: 0.50%, V: 0.10%, S: 0.004%, P: 0.007%, with the balance being Fe and other unavoidable impurities. The slab is furnace-heated to 1100°C and held for 4 hours. It is then hot-rolled in eight passes to a target thickness of 18mm, with a total reduction of 82%. The final rolling temperature is 780°C.

[0049] Step 2: Coil and cool:

[0050] The hot rolled steel sheet is coiled at a temperature of 550°C and then slowly cooled to room temperature. Figure 2 、 3 As shown in Figure 4, during low temperature rolling, the original austenite presents a flat shape, and fine lath martensite and an appropriate amount of retained austenite structure are obtained.

[0051] Mechanical property tests were conducted on the experimental steel, and the yield strength of the experimental steel was 980 MPa, the tensile strength was 1380 MPa, the Vickers hardness was 380 HV, and the elongation was 12%.

[0052] Comparative Example 1

[0053] The process steps for preparing hot-rolled high-strength medium manganese steel with a thickness of 20 mm are as follows:

[0054] Step 1: Billet heating and hot rolling:

[0055] The chemical composition of the steel billet is as follows by weight: C: 0.15%, Mn: 3.5%, Si: 0.20%, Mo: 0.40%, V: 0.05%, S: 0.005%, P: 0.008%, and the balance is Fe and other inevitable impurities.

[0056] The steel billet is heated to 1150℃ in the furnace and kept warm for 2h. It is then hot rolled in 7 passes to a target thickness of 20mm with a total reduction of 80%. The final rolling temperature is 780℃.

[0057] Step 2: Coil and cool:

[0058] The hot rolled steel sheet is coiled at a temperature of 600°C and then slowly cooled to room temperature. Figure 5 As shown, lath martensite + bainite and a small amount of retained austenite structure are obtained.

[0059] Mechanical property tests of the experimental steel showed that the yield strength was 680 MPa, the tensile strength was 980 MPa, the Vickers hardness was 310 HV, and the elongation was 16%.

[0060] Comparative Example 2

[0061] The process steps for preparing hot-rolled high-strength medium manganese steel with a thickness of 20 mm are as follows:

[0062] Step 1: Billet heating and hot rolling:

[0063] The chemical composition of the steel slab, by weight, is: C: 0.21%, Mn: 4.77%, Si: 0.22%, Mo: 0.30%, V: 0.08%, S: 0.004%, P: 0.006%, with the balance being Fe and other unavoidable impurities. The slab is furnace-heated to 1200°C and held for 3 hours. It is then hot-rolled in seven passes to a target thickness of 20mm, with a total reduction of 80%. The final rolling temperature is 780°C.

[0064] Step 2: Coil and cool:

[0065] The hot rolled steel sheet is coiled at a temperature of 630°C and then slowly cooled to room temperature. Figure 6 、 Figure 7 As shown in the figure, fine lath martensite, cementite and appropriate amount of retained austenite were obtained.

[0066] Mechanical property tests were conducted on the experimental steel, and the yield strength of the experimental steel was 790 MPa, the tensile strength was 1200 MPa, the Vickers hardness was 350 HV, and the elongation was 9.8%.

[0067] In the present invention, the Mn content is 5.5-6.9%, ensuring that the Mn element significantly expands the austenite phase region, improving the material's hardenability. This allows the material to undergo only martensitic transformation during slow furnace cooling, resulting in a high-strength martensitic matrix and an appropriate amount of retained austenite. The material achieves excellent mechanical properties with a yield strength of 820-980 MPa and a tensile strength of 1220-1380 MPa.

[0068] Furthermore, the mechanical properties and silicon addition ratios of Examples 1-3 and Comparative Examples 1-2 demonstrate that the addition of 1.5-2.2% Si suppresses cementite precipitation, ultimately resulting in a lath martensite matrix with an appropriate amount of retained austenite. During plastic deformation, the martensite provides high strength, while the retained austenite effectively relaxes stress concentration, resulting in excellent mechanical properties with a perfect balance of strength, plasticity, and toughness.

[0069] The content of Mn in Comparative Example 1 is less than 5.5%, which is 3.5%. In addition to martensite transformation, other structures are also present during slow cooling. For example, the lath martensite + bainite and a small amount of retained austenite are finally obtained in Comparative Example 1. Figure 5 shown.

[0070] In Comparative Example 2, the ratio of Si element was set to 0.22%, which was less than 1.5%, but the precipitation of cementite could not be suppressed. Finally, fine lath martensite, cementite and a proper amount of retained austenite were obtained, such as Figure 6 and Figure 7 shown.

Claims

1. A 1200MPa grade hot-rolled high-strength medium manganese steel, characterized by: The chemical composition by weight percentage is: C: 0.15-0.25%, Mn: 5.5-6.9%, Si: 1.5-2.2%, Mo: 0.30-0.50%, V: 0.05-0.10%, S: <0.01%, P: <0.01%, and the balance is Fe and other inevitable impurities; The preparation method comprises the following steps: Step 1, billet heating and hot rolling: Smelting is performed according to the above chemical composition and casting is performed into slabs; the slabs are heated in a furnace to 1100-1250°C and kept warm for 2-4 hours; the iron oxide scale is removed and hot rolled, and the steel plates are hot rolled in 6-8 passes to a target thickness of 18-24 mm, and the finishing rolling temperature is controlled at 780-900°C; Step 2: Coil and cool: The hot-rolled steel plate is coiled at a coiling temperature of 550-650°C and then slowly cooled to room temperature; ultimately, a lath martensite and film-like retained austenite duplex structure is obtained.

2. The 1200 MPa grade hot-rolled high-strength medium manganese steel according to claim 1, characterized in that: The Mn element significantly expands the austenite phase region and improves the hardenability of the material, so that only martensite transformation still occurs during slow cooling in the furnace, thereby obtaining a high-strength martensite matrix and an appropriate amount of retained austenite structure.

3. The 1200 MPa grade hot-rolled high-strength medium manganese steel according to claim 1, characterized in that: The addition of 1.5-2.2% Si inhibits the precipitation of cementite, ultimately obtaining a lath martensite matrix and an appropriate amount of retained austenite structure; during the plastic deformation process, the martensite provides high strength and the retained austenite effectively relaxes stress concentration, thereby obtaining mechanical properties with high strength, plasticity and toughness matching.

4. The 1200 MPa grade hot-rolled high-strength medium manganese steel according to claim 1, characterized in that: The single-pass reduction rate is 10% to 25%, and the total reduction rate is 76% to 82%.

5. The 1200 MPa grade hot-rolled high-strength medium manganese steel according to claim 1, characterized in that: The steel plate has a yield strength of 820-980 MPa, a tensile strength of 1220-1380 MPa, a Vickers hardness of 330-380 HV, and an elongation of 12%-18%.

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