High-toughness low-carbon medium-manganese steel plate and preparation method thereof

Abstract

The application belongs to the field of advanced steel material, and particularly relates to a high-strength and high-toughness low-carbon medium-manganese steel plate and a preparation method thereof. The element composition and mass percentage of the high-strength and high-toughness low-carbon medium-manganese steel plate are as follows: C: 0.005-0.10%, Si: ≤0.027%, Mn: 9-12%, and the rest is Fe and inevitable impurities. The preparation method comprises the following steps: step 1, ingredients are prepared according to the alloy composition, and then a medium-manganese steel forging is obtained through vacuum induction furnace smelting and subsequent forging; step 2, the forging is placed in a heating furnace at 900±50 DEG C for 2h for solid solution treatment, and then water quenching is performed; step 3, after the solid solution treatment, the forging is subjected to cold rolling; step 4, the plate obtained after the cold rolling is subjected to aging treatment, and then slow cooling is performed in air; and step 5, the plate after annealing is subjected to ultrafast heating, and then rapid cooling is performed. Compared with other types of medium-manganese steel, the high-strength and high-toughness low-carbon medium-manganese steel plate provided by the application requires smaller rolling force for room temperature rolling, has lower requirements for rolling mills, and has a simple thermal mechanical treatment process, and the obtained plate has high strength and high toughness.

Description

Technical Field

[0001] This invention belongs to the field of advanced steel materials, specifically relating to a high-strength, high-toughness, low-carbon, medium-manganese steel plate and its preparation method. Background Technology

[0002] Currently, the automotive industry is moving towards higher safety standards and lighter weight. The space for weight reduction through simplifying the vehicle body structure is becoming increasingly limited. The most effective way to achieve lightweighting while maintaining vehicle safety performance is to use advanced high-strength steel with higher strength and toughness. Among these, medium-manganese steel, as a typical third-generation advanced high-strength steel, typically contains 4-12 wt.% Mn and small amounts of Si, Al, and other microalloying elements. Compared to high-manganese steel (second-generation advanced high-strength steel), it has a lower alloy element content, resulting in lower difficulty and cost in large-scale manufacturing, and thus holds significant application potential in the automotive field.

[0003] Numerous research institutions and steel companies both domestically and internationally are strengthening their research and development on the composition, microstructure, properties, and production processes of third-generation automotive steels, represented by medium-manganese steel, and have achieved certain research results. However, medium-manganese steel has not yet achieved large-scale application in automotive body-in-white due to issues such as strength-ductility-toughness matching, formability, and weldability. Among these issues, cold-rolled medium-manganese steel using bell-type annealing exhibits a long yield plateau during stretching and produces Lüders bands during stamping, causing significant wrinkling on the surface of parts; these are critical problems that must be addressed. Summary of the Invention

[0004] The purpose of this invention is to provide a high-strength and tough low-carbon medium-manganese steel plate and its preparation method. Specifically, it is a method for preparing high-strength and tough low-carbon medium-manganese steel plate by cold rolling + aging + rapid heat treatment process. The plate obtained by this method has a yield strength ≥820MPa, tensile strength ≥1100MPa, and uniform elongation ≥0.06.

[0005] The technical solution for achieving the objective of this invention is as follows:

[0006] The present invention discloses a high-strength, high-toughness, low-carbon, medium-manganese steel plate, the elemental composition and mass percentage of which are: C: 0.005-0.10%, Si≤0.027%, Mn: 9-12%, with the remainder being Fe and unavoidable impurities.

[0007] Preferably, the mass percentage of Mn is 11.0% to 12.0%.

[0008] This invention also provides a process for preparing high-strength, high-ductility, medium-manganese steel plates, the specific steps of which are as follows:

[0009] Step 1: Prepare the materials according to the alloy composition and mass percentage of the high-strength and tough low-carbon manganese steel plate, then smelt them in a vacuum induction furnace, and then forge them to obtain medium manganese steel forgings.

[0010] Step 2: Place the forgings obtained from smelting into a heating furnace at 900±50℃ for 2 hours for solution treatment, and then perform water quenching to rapidly reduce the temperature to room temperature.

[0011] Step 3: After the solution treatment is completed, the above forgings are cold rolled.

[0012] Step 4: Aging treatment is performed on the cold-rolled sheet material, followed by slow cooling in air.

[0013] Step 5: Rapidly heat the annealed sheet to austenitization, then rapidly cool it.

[0014] Preferably, the cold rolling process in step 3 includes: the initial pressing amount is 1 mm per pass, the pressing amount is changed to 0.5 mm after the deformation amount reaches 40%, and the final rolling amount is 90% ± 2%.

[0015] Preferably, the aging temperature in step 4 is 550°C and the annealing time is 1 hour.

[0016] Preferably, the heating rate in step 5 is 100℃ / s, the rapid heating ends at 680℃, the holding time is 1 min, and the subsequent rapid cooling rate is 40℃ / s.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] The high-strength, high-ductility medium-manganese steel sheet provided by the embodiments of the present invention has a very low carbon content in its alloy composition. Compared with other types of medium-manganese steel, it requires less rolling force for room temperature rolling and has lower requirements for rolling mills. The resulting high-strength, high-ductility medium-manganese steel sheet has better toughness and is easier to process. The present invention uses conventional rolling equipment and has a simple operation process. The present invention provides medium-manganese steel sheets with different strength and toughness combinations, which can be applied to different material requirements. The yield strength of the sheet obtained by this method is ≥820MPa, tensile strength is ≥1100MPa, and uniform elongation is ≥0.06. Attached Figure Description

[0019] Figure 1 A schematic diagram illustrating the preparation method and processing flow of high-strength, high-plasticity, medium-manganese steel plates provided for embodiments of the present invention.

[0020] Figure 2 The EBSD characterization results of the sample in Example 1 provided for the implementation of the present invention.

[0021] Figure 3The EBSD characterization results of the sample in Example 2 provided for the implementation of the present invention.

[0022] Figure 4 Engineering stress-strain curves of plate tensile tests provided for embodiments of the present invention in Examples 1 and 2 and Comparative Examples 1 and 2. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] The high-strength, low-carbon, medium-manganese steel plate of the present invention has the following elemental composition and mass percentage: C: 0.005-0.10%, Si≤0.027%, Mn: 9-12%, with the remainder being Fe and unavoidable impurities.

[0025] Preferably, the mass percentage of Mn is 11.0% to 12.0%.

[0026] Furthermore, this method yields a sheet with a yield strength ≥820MPa, a tensile strength ≥1100MPa, and a uniform elongation ≥0.06.

[0027] Another aspect of the present invention provides a process for preparing high-strength, high-toughness, low-carbon, medium-manganese steel plates, the specific steps of which are as follows:

[0028] Step 1: Prepare the materials according to the alloy composition and mass percentage of the high-strength and tough low-carbon manganese steel plate, then smelt them in a vacuum induction furnace, and then forge them to obtain medium manganese steel forgings.

[0029] Step 2: Place the forgings obtained from smelting into a heating furnace at 900±50℃ for 2 hours for solution treatment, and then perform water quenching to rapidly reduce the temperature to room temperature.

[0030] Step 3: After the solution treatment is completed, the above forgings are cold rolled.

[0031] Step 4: Aging treatment is performed on the cold-rolled sheet material, followed by slow cooling in air.

[0032] Step 5: Rapidly heat the annealed sheet to austenitization, then rapidly cool it.

[0033] The cold rolling process is described in detail below: Rolling is carried out along the rolling direction. In the early stage, the amount of reduction per pass is 1 mm. After reaching 40% deformation, the amount of reduction per pass is changed to 0.5 mm. The final rolling amount is 90% ± 2%.

[0034] The room temperature uniaxial tensile tests in the following examples and comparative examples were all performed on a universal testing machine at a displacement rate of 3 mm / min.

[0035] The EBSD microscopic characterization technique described in the following examples uses an Oxford NORDLYS2S probe mounted on a Gemini 500 field emission scanning electron microscope to collect Kikuchi pattern signals, and performs data analysis using Channel 5 software.

[0036] Example 1

[0037] This embodiment provides a high-strength, high-toughness, low-carbon, medium-manganese steel plate, the preparation method of which includes:

[0038] Step 1: Prepare the materials according to the alloy composition and mass percentage of the high-strength and tough low-carbon manganese steel plate, then smelt them in a vacuum induction furnace, and then forge them to obtain medium manganese steel forgings;

[0039] Step 2: Place the forgings obtained from smelting into a heating furnace at 900±50℃ for 2 hours for solution treatment, and then perform water quenching to rapidly reduce the temperature to room temperature.

[0040] Step 3: After the solution treatment is completed, the above forgings are cold rolled.

[0041] Step 4: The cold-rolled sheet is subjected to aging treatment at a temperature of 550℃ for 1 hour, followed by slow cooling in air.

[0042] The medium-manganese steel plate obtained after the above treatment in this embodiment has a yield strength of 844 MPa, a tensile strength of 1040 MPa, and a uniform elongation of 21.4%. The EBSD characterization results of the sample treated in Example 1 are as follows: Figure 2 As shown, the engineering stress-strain curve is as follows: Figure 4 As shown.

[0043] Example 2

[0044] This embodiment provides a high-strength, high-toughness, low-carbon, medium-manganese steel plate, the preparation method of which is basically the same as that in Example 1, except that:

[0045] Step 5: Rapidly heat the annealed sheet to austenitization, then rapidly cool it. The heating rate is 100℃ / s, the rapid heating ends at 680℃, the holding time is 1 min, and then the rapid cooling rate is 40℃ / s.

[0046] The medium-manganese steel plate obtained after the above treatment in this embodiment has a yield strength of 820 MPa, a tensile strength of 1114 MPa, and a uniform elongation of 6.0%. The EBSD characterization results of the sample treated in Example 2 are as follows: Figure 3 As shown, the engineering stress-strain curve is as follows: Figure 4 As shown.

[0047] Comparative Example 1

[0048] The quenched manganese steel plate with uniform microstructure obtained after solution treatment according to steps 1 and 2 as described in Example 1 of this experiment.

[0049] In this example, the medium-manganese steel plate obtained using the above method has a yield strength of 440 MPa, a tensile strength of 1002 MPa, and a uniform elongation of 7.5%. The engineering stress-strain curve of the plate after treatment in this example is shown below. Figure 4 As shown.

[0050] Comparative Example 2

[0051] The 1mm thick cold-rolled medium manganese steel sheet obtained after cold rolling according to steps 1, 2 and 3 as described in Example 1 of this experiment;

[0052] Cold rolling: The steel plate is cold rolled in multiple passes, with a final rolling yield of 90%. Initially, the reduction in each pass is 1 mm, and after reaching 40% deformation, the reduction is changed to 0.5 mm to produce a 1 mm thick cold-rolled sheet.

[0053] In this example, the medium-manganese steel plate obtained by the above method has a yield strength of 1190 MPa, a tensile strength of 1240 MPa, and a uniform elongation of 1.8%. The engineering stress-strain curve of the plate after treatment in this example is shown below. Figure 4 As shown.

[0054] from Figure 2 and Figure 3 It can be seen that the cold-rolled manganese steel plates provided in Examples 1 and 2 of this invention have an equiaxed ultrafine grain structure with a grain size of about 100 nm. The black solid lines represent large-angle grain boundaries. The high density of grain boundaries hinders dislocation slip and improves the strength of the material. At the same time, the retained austenite in Examples 1 and 2 undergoes a martensitic phase transformation during the stretching process, leading to transformation-induced plasticity (TRIP effect). The TRIP effect can delay necking and significantly increase its elongation.

[0055] Figure 4The stress-strain curves of Examples 1 and 2 were compared with those of Comparative Examples 1 and 2. Examples 1 and 2 are cold-rolled sheets obtained by the present invention. Compared with Comparative Example 1 (quenched state), it can be seen that the yield strength and tensile strength of the cold-rolled medium-manganese steel sheets obtained by the methods of Examples 1 and 2 are greatly improved. The strength and toughness of Example 1 are significantly higher than those of Comparative Example 1, with a strength-ductility product of 22 GPa%. The strength of Example 2 is higher than that of Comparative Example 1, while the ductility is similar to that of Comparative Example 1, with a strength-ductility product of 7 GPa%. However, the medium-manganese steel sheet (Comparative Example 2) obtained by only cold rolling 90% at room temperature has a significant increase in strength but extremely poor ductility, with a strength-ductility product of only 2 GPa%, which is not suitable for application in the engineering industry. In addition, Example 1 did not undergo a rapid heat treatment stage, and the material had the highest uniform elongation. The yield strength and tensile strength were close to those of Example 2, but the yielding stage showed discontinuous yielding. After rapid heat treatment, the toughness of the material decreased, accompanied by a slight increase in strength, and the yielding was continuous. Meanwhile, the cold rolling process can increase the dislocation density and grain boundary density, thereby improving the strength of the material. The increase in dislocation density during the deformation process of medium manganese steel mainly comes from martensite, which in turn comes from the phase transformation of austenite during the deformation process. Furthermore, the new interface after the phase transformation also increases the strength. Therefore, the preparation method provided by this invention can increase the content of residual austenite and refine the grains, thereby improving the mechanical properties of medium manganese steel and achieving a good balance between strength and toughness in medium manganese steel.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-strength, high-toughness, low-carbon, medium-manganese steel plates, characterized in that, The preparation method includes the following steps: Step 1: Prepare the materials according to the alloy composition and mass percentage of the high-strength and tough low-carbon manganese steel plate, then smelt them in a vacuum induction furnace, and then forge them to obtain medium manganese steel forgings; Step 2: Place the forgings obtained from smelting into a heating furnace at 900±50 ℃ for 2 h for solution treatment, and then perform water quenching to rapidly reduce the temperature to room temperature; Step 3: After the solution treatment is completed, the above forgings are cold rolled; Step 4: The cold-rolled sheet is subjected to aging treatment, followed by slow cooling in air; the aging temperature in Step 4 is 550 ℃, and the aging time is 1 h. Step 5: Rapidly heat the cooled sheet material to austenitization, then rapidly cool it; the rapid heating rate in Step 5 is 100 ℃ / s, the rapid heating end temperature is 680 ℃, the holding time is 1 minute, and then the rapid cooling rate is 40 ℃ / s. The chemical composition of the high-strength, low-carbon, medium-manganese steel plate is as follows (by weight): C: 0.005~0.10%, Si≤0.027%, Mn: 12%, with the remainder being Fe and unavoidable impurities.

2. The preparation method according to claim 1, characterized in that, The cold rolling process in step 3 includes: the initial pressing amount is 1 mm per pass, and after the deformation reaches 40%, the pressing amount is changed to 0.5 mm, and the final rolling amount is 90% ± 2%.

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