A high-strength and high-toughness medium manganese steel plate and its preparation method

Abstract

The present invention relates to the technical field of medium manganese steel production, and specifically to a high-strength and tough medium manganese steel plate. Its chemical components and weight percentages are as follows: 0.01% ≤ C ≤ 0.1%, 10% ≤ Mn ≤ 12%, and the rest are Fe and inremovable impurities. Its preparation method includes: Step 1, smelting: Take materials according to the described chemical components and their mass percentages, and then use a vacuum induction furnace for smelting. After smelting, it is successively cast and forged to obtain a medium manganese steel ingot; Step 2, solution treatment: Solution-treat the obtained ingot in a heating furnace at 900 ± 50 °C for 2 h; Step 3, warm rolling: After the solution treatment is completed, keep the ingot in the austenite single-phase region for a period of time, perform warm rolling, and then air-cool to room temperature; Step 4, annealing: Heat the plate obtained after warm rolling to the critical region for annealing to improve the mechanical properties of the material. The present invention uses the warm rolling process, which requires less rolling force compared to room temperature rolling, and the obtained material has better toughness and is easier to process.

Description

Technical Field

[0001] The present invention relates to the technical field of medium manganese steel production, and particularly relates to a high-strength and high-toughness medium manganese steel plate and a preparation method thereof. Background Art

[0002] For many years, the automotive industry has been an important market for steel manufacturing. The main reason why steel materials will continue to be the key materials in automotive manufacturing is that while meeting increasingly stringent engineering requirements, they also form a system in terms of cost, lifespan, and strength level. In addition, steel has excellent formability, weldability, recyclability, and good collision energy absorption ability. In recent decades, the increasing global demand for lighter and more economical vehicles has led to the continuous development of advanced high-strength steels with high strength and good ductility. As the third generation of advanced high-strength steel, medium manganese steel has a better combination of strength and toughness than the first generation of advanced high-strength steel, and its production cost is lower than that of the second generation of advanced high-strength steel due to lower alloying elements.

[0003] Medium manganese steel is characterized by low-carbon medium alloy, usually containing 3-12wt.% of Mn and small amounts of Si, Al, and micro-alloying additives. The microstructure of medium manganese steel usually contains a certain volume of metastable retained austenite. The austenite undergoes martensitic transformation during deformation, and this transformation greatly increases the work-hardening rate, which can delay necking and achieve the unity of high strength and high toughness. At present, although there are industrial prototypes of medium manganese steel, the composition design and process technology have not been finalized, and large-scale industrial production cannot be carried out. In addition, during the cold rolling process of medium manganese steel at room temperature, the retained austenite generates martensite structure due to deformation, the deformation resistance increases, the rolling mill will bear a higher load, increasing the wear of the rolls and raising the production cost. The microstructure of medium manganese steel obtained by cold rolling and reverse transformation annealing process is uneven, reducing the mechanical properties of medium manganese steel. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-strength and high-toughness medium manganese steel plate and a preparation method thereof. The present invention uses warm rolling process, which requires less rolling force than room temperature rolling, and the obtained material has better toughness and is easier to process, so as to solve the problems raised in the above background art.

[0005] To achieve the above purpose, one aspect of the present invention provides a high-strength and high-toughness medium manganese steel plate. The chemical composition of the high-strength and high-toughness medium manganese steel plate is calculated by mass percentage as follows: 0.01% ≤ C ≤ 0.1%, 10% ≤ Mn ≤ 12%, and the rest is Fe and unavoidable impurities.

[0006] Preferably, the mass percentage of Mn is: 11.8-12.0%.

[0007] Preferably, the yield strength of the high-strength and high-toughness medium manganese steel sheet is ≥380 MPa, the tensile strength is ≥950 MPa, and the uniform elongation is ≥7%.

[0008] On the other hand, the present invention provides a method for preparing a high-strength and high-toughness medium manganese steel sheet, which is used to prepare the high-strength and high-toughness medium manganese steel sheet described in Claims 1 and 2, and the preparation method includes the following steps:

[0009] Step 1, smelting: Taking materials according to the alloy components by the mass percentage of the chemical components, and then using a vacuum induction furnace for smelting. After smelting, casting and forging are carried out in sequence to obtain a medium manganese steel ingot;

[0010] Step 2, solution treatment: Subjecting the obtained ingot to solution treatment in a heating furnace at 900±50 °C for 2 h;

[0011] Step 3, warm rolling: After the solution treatment is completed, keeping the ingot in the two-phase region for a period of time, performing warm rolling, and then air-cooling to room temperature;

[0012] Step 4, annealing: Heating the sheet obtained after warm rolling to the critical region for annealing to improve the uniformity of the material.

[0013] Preferably, the warm rolling in Step 2 includes: Keeping the sheet in a muffle furnace at 650~700 °C for 10 minutes before rolling, and then keeping it in a furnace at 680 °C for 3 minutes before each pass. The final rolling reduction is 90%±3%.

[0014] Preferably, the annealing temperature in Step 4 is 550 °C, and the annealing time is 0~4 h.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] The preparation method of the high-strength and high-toughness medium manganese steel sheet provided by the embodiment of the present invention adopts a warm rolling process. Preheating the material can reduce the deformation resistance during rolling, and the rolling force required is smaller than that of room temperature rolling. Moreover, the obtained material has better toughness and is easier to process; the equipment used in the present invention is simple and easy to operate, which is conducive to large-scale industrial production; the present invention provides medium manganese steel sheets with different strength and toughness combinations, which can be applied to different material requirements. Description of the Drawings

[0017] Figure 1 It is a process flow diagram of the preparation method of the high-strength and high-toughness medium manganese steel sheet provided by the embodiment of the present invention;

[0018] Figure 2 It is the EBSD characterization of Example 1 provided by the embodiment of the present invention;

[0019] Figure 3 Engineering stress-strain curves of the tensile test of the sheets of Example 1 and Comparative Examples 1, 2, and 3 provided for the embodiments of the present invention;

[0020] Figure 4 Engineering stress-strain curves of Example 1 and Example 2 provided for the embodiments of the present invention;

[0021] Figure 5 XRD results of Example 1 pre-stretched with different strains provided for the embodiments of the present invention. Detailed implementation manners

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] The present invention provides a high-strength and high-toughness medium manganese steel sheet, and the chemical composition of the high-strength and high-toughness medium manganese steel sheet is calculated by mass percentage as follows: 0.01% ≤ C ≤ 0.1%, 10% ≤ Mn ≤ 12%, and the balance is Fe and impurities that cannot be removed.

[0024] In a preferred embodiment of the present invention, the mass percentage of Mn in the high-strength and high-toughness medium manganese steel sheet is: 11.8 - 12.0%.

[0025] Furthermore, the yield strength of the high-strength and high-toughness medium manganese steel sheet ≥ 380 MPa, the tensile strength ≥ 950 MPa, and the uniform elongation ≥ 7%.

[0026] The present invention provides a preparation method for a high-strength and high-toughness medium manganese steel sheet, and the preparation method is carried out according to the treatment process of smelting - solution treatment - warm rolling - annealing, as Figure 1 shown, and the preparation method specifically includes:

[0027] Step 1, smelting: Weigh the corresponding high-purity metal powders according to the alloy composition, and use a pressurized vacuum induction furnace to smelt to obtain an ingot;

[0028] Step 2, solution treatment: Subject the obtained ingot to solution treatment in a heating furnace at 900 ± 50 °C for 2 h;

[0029] Step 3, warm rolling: After the solution treatment is completed, keep the ingot in the two-phase region for a period of time, carry out warm rolling, and then air-cool to room temperature;

[0030] Step 4, annealing: Heat the sheet obtained after warm rolling to the critical region for annealing to improve the uniformity of the material.

[0031] The warm rolling process is described in detail below: Before rolling, the sample is placed in a muffle furnace at 650 - 700 °C and kept warm for 10 minutes. After taking it out, it is rolled along the rolling direction. Then, before each pass, it is kept warm in the furnace at 680 °C for 3 minutes. The final rolling reduction is 90% ± 3%. The reduction per pass in the early stage is 1 mm. After reaching a deformation of 60%, the reduction per pass is changed to 0.5 mm until a high-strength and high-toughness medium-manganese steel sheet with a rolling reduction of 90% is obtained.

[0032] The equipment for the room temperature tensile test in the following examples and comparative examples is carried out on an Instron 5982 universal testing machine with a displacement rate of 3 mm / min.

[0033] The equipment for the EBSD microstructural characterization technology in Example 1 below is based on the NORDLYS 2S probe produced by Oxford Company equipped on a Gemini 500 field emission scanning electron microscope to collect Kikuchi pattern signals, and the data analysis is completed by the HKL Channel 5 system software.

[0034] The equipment for the XRD result determination in the following examples is a Bruker D8 Advance X-ray diffractometer.

[0035] Example 1

[0036] This example provides a high-strength and high-toughness medium-manganese steel sheet, and its preparation method includes:

[0037] Step 1. Smelting: The molten steel is smelted according to the set composition. The mass percentages of the molten steel are C: 0.01% - 0.1%, Mn: 11.8 - 12.0%, and the balance is Fe and inevitable impurities. Then, it is smelted using a vacuum induction furnace, and after smelting, it is successively cast and forged to obtain a medium-manganese steel ingot.

[0038] Step 2. Solution treatment: The ingot is heated to 900 °C, kept warm for 2 hours, and then water-cooled to room temperature; the slab is wire-cut using wire-cutting technology to cut a steel sheet with a thickness of 15 mm.

[0039] Step 3. Warm rolling: The steel sheet is heated to 680 °C and kept warm for 10 min, and then subjected to multiple warm rolling passes. Then, before each pass, it is kept warm in the furnace at 680 °C for 3 minutes. The final rolling reduction is 90%. The reduction per pass in the early stage is 1 mm. After reaching a deformation of 60%, the reduction per pass is changed to 0.5 mm. A warm-rolled plate with a thickness of 1.5 mm is made and air-cooled to room temperature. The EBSD results of the sample obtained in this example are as Figure 2 shown.

[0040] The yield strength of the medium manganese steel plate obtained by the above method in this example is 806 MPa, the tensile strength is 1215 MPa, and the uniform elongation is 14.8%. The engineering stress-strain curve of the treated plate is as shown in Figure 3 shown.

[0041] Example 2

[0042] This example provides a high-strength and high-toughness medium manganese steel plate, and its preparation method is basically the same as that of Example 1, except that:

[0043] Step 4. Annealing: After Step 3, the same warm-rolled plates are kept at 550 °C for 1 h, 2 h, and 4 h respectively. Then they are air-cooled to room temperature to make 3 groups of warm-rolled medium manganese steel plates.

[0044] The high-strength and high-toughness medium manganese steel obtained in this example by the above method is as follows:

[0045] Insulation time of the sheet Yield strength Tensile strength Uniform elongation 1 h 740 MPa 1142 MPa 17.9% 2 h 682 MPa 1120 MPa 18.3% 4 h 642 MPa 1010 MPa 20.3%

[0046] The engineering stress-strain curve of the treated plate is as shown in Figure 4 shown.

[0047] Comparative Example 1

[0048] The quenched medium manganese steel plate with uniform structure obtained according to Steps 1 and 2 described in Example 1 of this experiment;

[0049] The yield strength of the medium manganese steel plate obtained in this example by the above method is 390 MPa, the tensile strength is 985 MPa, and the uniform elongation is 7.5%. The engineering stress-strain curve of the treated plate is as shown in Figure 3 shown.

[0050] Comparative Example 2

[0051] 1. Obtain a 10-mm-thick medium manganese steel plate with uniform structure according to Steps 1 and 2 described in Example 1;

[0052] 2. Warm rolling: First, put the material into a heating furnace at 680 °C and keep it warm for 10 min, and then keep it warm in the heating furnace for 3 min before each pass of rolling. Roll the plate thickness to about 5 mm in 6 passes, and the total rolling reduction is about 50%;

[0053] The yield strength of the medium manganese steel plate obtained in this example by the above method is 506 MPa, the tensile strength is 1108 MPa, and the uniform elongation is 12.4%. The engineering stress-strain curve of the treated plate is as shown in Figure 3 shown.

[0054] Comparative Example 3

[0055] 1. Obtain a quenched medium manganese steel sheet with a thickness of 15 mm and uniform structure according to Steps 1 and 2 of Example 2;

[0056] 2. Warm rolling at room temperature: At room temperature, roll the sheet thickness to about 1.5 mm in 20 passes, and the total rolling reduction is about 90%;

[0057] The yield strength of the medium manganese steel sheet obtained by the above method in this example is 1190 MPa, the tensile strength is 1215 MPa, and the uniform elongation is 1.5%. The engineering stress-strain curve of the sheet after treatment is as Figure 3 shown.

[0058] From Figure 2 it can be seen that the warm-rolled sheet provided by the present invention has an ultrafine grain structure, and the grain boundaries hinder the movement of dislocations, improving the strength of the material.

[0059] Figure 3 The engineering stress and engineering strain of Example 1 were compared with those of Comparative Examples 1, 2, and 3. Example 1 is the warm-rolled sheet obtained by the method of the present invention. Comparing Example 1 with Comparative Example 1, it can be seen that while the yield strength and tensile strength of the material after warm rolling are greatly improved, the uniform elongation also increases, achieving a comprehensive improvement in strength and toughness. The strength and toughness of Example 1 are also comprehensively higher than those of the material (Comparative Example 2) with a rolling reduction of 50% at the same temperature (680 °C). In addition, the sheet (Comparative Example 3) cold-rolled at room temperature by 90% has extremely poor toughness although its strength increases significantly, and it is not suitable for application in the engineering industry.

[0060] Figure 4 are the engineering stress-strain curves of Example 1 and Example 2 obtained according to the method of the present invention. Example 1 did not go through the annealing stage, and the material has the highest strength and the lowest toughness among the 4 materials. After annealing treatment, the toughness of the materials all increased significantly, and at the same time, there was a slight loss of strength. Its yield strength and tensile strength are much higher than those of Comparative Example 1. Therefore, the preparation method provided by the method of the present invention realizes the unity of high strength and high toughness of medium manganese steel.

[0061] In order to further analyze the action mechanism of the method of the present invention, pre-tensile experiments with strains of 0, 0.035, 0.07, and 0.10 were carried out on Example 1 ( Figure 5 a), and XRD analysis was performed on the uniform section of the tensile specimen. The results are as Figure 5 shown. The warm-rolled materials at the 4 strains all have FCC phase austenite, HCP phase ε-martensite, and BCC phase α'-martensite, as shown in Figure 5b. Normalize the four specimens by the {110} peak of α'-martensite, and magnify the {110} peak of α'-martensite, the {111} and {200} peaks of austenite, and the {101} peak of ε-martensite. During the deformation process, the content of retained austenite gradually decreases, while the contents of ε-martensite and α'-martensite gradually increase, indicating that the retained austenite undergoes strain-induced martensitic transformation to induce ductile deformation (TRIP effect). The TRIP effect can delay necking, significantly increase its elongation, and the newly formed α'-martensite and ε-martensite improve the material strength. At the same time, the increase in dislocation density and the increase in phase interfaces during the rolling process can improve the material strength. The increase in dislocation density during the deformation of medium manganese steel mainly comes from martensite, and martensite is derived from the phase transformation of austenite during the deformation process, and the new interface after the phase transformation will also enhance the strength. Therefore, the preparation method provided by the present invention can increase the austenite content and refine the grains, improving the mechanical properties of medium manganese steel.

[0062] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made therein without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method of a high-strength and high-toughness medium manganese steel plate, characterized in that, The chemical composition of the high-strength and high-toughness medium manganese steel plate is as follows by mass percentage: 0.01% ≤ C ≤ 0.1%, 10% ≤ Mn ≤ 12%, and the rest is Fe and unavoidable impurities. The preparation method includes the following steps: Step 1, smelting: Take materials according to the described chemical composition and mass percentage, and then use a vacuum induction furnace for smelting. After smelting, successively carry out casting and forging to obtain a medium manganese steel ingot. Step 2, solution treatment: Solution-treat the obtained ingot in a heating furnace at 900 ± 50 °C for 2 h; Use wire cutting technology to perform wire cutting on the slab, and cut a steel plate with a thickness of 15 mm. Step 3, warm rolling: Heat the steel plate to 680 °C and hold for 10 min, then carry out multiple warm rolling operations. Before each pass, hold in a furnace at 680 °C for 3 minutes. The final rolling reduction is 90%; The reduction per pass in the early stage is 1 mm. After reaching a deformation of 60%, the reduction per pass is changed to 0.5 mm. Fabricate a warm-rolled plate with a thickness of 1.5 mm and air-cool it to room temperature. Step 4, annealing: Heat the plate obtained after warm rolling to the critical region for annealing to improve the material uniformity.

2. The preparation method according to claim 1, characterized in that, The annealing temperature in Step 4 is 550 °C, and the annealing time is 0 - 4 h.

3. The preparation method according to claim 1, characterized in that, The mass percentage of Mn is: 11.8 - 12.0%.

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