Preparation method of 1400 MPa-grade medium manganese steel

Abstract

The invention relates to the technical field of steel materials for automobiles, in particular to a preparation method of 1400 MPa-grade medium manganese steel. According to the specific technical scheme, the method comprises the following steps: heating a medium manganese steel ingot to 1150-1250 DEG C, preserving heat for 2 + / -0.5 hours, forging into a plate blank, carrying out air cooling, preserving heat for 20 minutes at 1100-1200 DEG C, carrying out multi-pass hot rolling deformation, carrying out air cooling, heating to 700-750 DEG C, preserving heat for 20 minutes, carrying out warm rolling deformation at 620-720 DEG C, carrying out air cooling, heating to 700-750 DEG C, preserving heat for at least 20 minutes, and carrying out air cooling, and the 1400 MPa grade medium manganese steel with the ultralow yield ratio is obtained. According to the method, rolling is conducted in a warm rolling mode, the normalizing process is combined, and finally the ultrahigh-strength medium manganese steel with the yield ratio ranging from 0.3 to 0.306, the yield strength ranging from 270 MPa to 430 MPa and the tensile strength ranging from 800 MPa to 1400 MPa is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of automobile steel materials, and in particular to a method for preparing 1400MPa-level medium manganese steel. Background Art

[0002] The automobile industry is developing rapidly, automobile production is increasing explosively, environmental and energy problems are becoming increasingly severe, and more and more companies are paying attention to green manufacturing and energy conservation and emission reduction. Steel used in automobile body structural parts often requires high strength, high plasticity and good processing performance. On the one hand, steel will produce cracks and cracks when deformed, while the appropriate yield strength and low yield strength ratio can have lower processing energy consumption and larger forming space, reducing the production cost of steel mills. On the other hand, automobile collision safety is very important. Higher tensile strength can delay the occurrence of fracture and improve the safety of the vehicle. Therefore, the appropriate yield strength and how to reduce the yield strength ratio while increasing the tensile strength are the key to designing steel for automobile structures.

[0003] Medium manganese steel for automobile structure is an advanced high-strength steel widely used in the automobile field. Its structure is austenite and ferrite. Austenite can improve the strength and plasticity of steel through TRIP phase transformation and plastic deformation, and has high-strength mechanical properties. When deformed by external collision, the shear fracture of steel will not break due to its ultra-low yield strength ratio, which effectively improves the safety factor of the car. Medium manganese steel is often used in anti-collision parts such as automobile suspension systems, ABC columns and bumpers. With the development of society, the performance requirements for medium manganese steel for automobile structure are getting higher and higher.

[0004] Chinese invention patent application CN117144261A discloses a room temperature quenching-partitioning medium manganese steel with low yield ratio and high tensile strength and a preparation method thereof, wherein the composition by mass percentage is C: 0.1-0.3%, Mn: 8-12%, Si: 1.0-3.0%, P: ≤ 0.005%, S: ≤ 0.005%, and the balance is Fe and inevitable impurities. The yield ratio can be 0.55-0.60 and the tensile strength is 1250-1350MPa. However, the yield ratio is too high, the tensile strength is also slightly low, and there is an extra step of partitioning heat treatment process.

[0005] Chinese invention patent application CN116288030A discloses a 600MPa grade hot-dip galvanized dual-phase steel for automobile outer panels and a preparation method thereof, wherein the composition by mass percentage is: C: 0.11-0.15%, Si: 0.05-0.10%, Mn: 1.5-1.7%, P≤0.005%, S≤0.005%, A1s: 0.4-0.6%, Mo: 0.01-0.05%, and the rest is Fe and unavoidable impurities, and the yield strength ratio is 0.55, the yield strength is 300-360MPa, and the tensile strength is 620-670M. The yield strength ratio of the steel plate is too high, and the yield strength and tensile strength are both low. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for preparing 1400MPa grade medium manganese steel, which is rolled by warm rolling and combined with a normalizing process to finally obtain an ultra-high strength medium manganese steel with a yield ratio of 0.3 to 0.306, a yield strength of 270MPa to 430MPa, and a tensile strength of 800MPa to 1400MPa.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0008] The invention discloses a preparation method of 1400MPa grade medium manganese steel. The medium manganese steel ingot is heated to 1150-1250°C and kept warm for 2±0.5 hours, then forged into a slab, air-cooled, kept warm at 1100-1200°C for 20 minutes, subjected to multiple hot rolling deformation, air-cooled, heated to 700-750°C and kept warm for 20 minutes, then warm-rolled at 620-720°C, air-cooled, heated to 700-750°C and kept warm for at least 20 minutes, and air-cooled to obtain 1400MPa grade ultra-low yield ratio medium manganese steel.

[0009] Preferably, the raw materials of the medium manganese steel include, by mass percentage, carbon: 0.15% to 0.30%, manganese: 11% to 13.5%, aluminum: 1.5% to 2.5%, sulfur: 0 to 0.005%, phosphorus: 0 to 0.008%, and the balance is iron.

[0010] Preferably, the raw materials of the medium manganese steel are smelted at 1550° C. to 1650° C. to obtain an ingot.

[0011] Preferably, the hot rolling passes are 5 to 7, the start rolling temperature is 1000°C to 1100°C, and the final rolling temperature is 850°C to 950°C.

[0012] Preferably, the warm rolling passes are 10 to 12 passes.

[0013] The present invention has the following beneficial effects:

[0014] 1. The medium manganese steel prepared by the present invention does not contain precious metal elements such as Cr and Mo, has low cost and good mechanical properties, has huge social demand, and is expected to occupy a place in the vast steel market.

[0015] 2. The ultra-high strength medium manganese steel of the present invention has a yield strength ratio of 0.3 to 0.306, a yield strength of 270 MPa to 430 MPa, an elongation of 23% to 25%, and a tensile strength of 800 MPa to 1400 MPa, and has a good work hardening rate, as well as good mechanical properties such as strength and elongation. In addition, no precious alloy elements such as Cr and Mo are added, which contributes to the lightweight and safety of automobiles and has significant economic and social benefits.

[0016] 3. The present invention optimizes the chemical composition. C and Mn elements improve the stability of austenite and reduce the transformation rate of austenite to martensite. Since Si is not good for the surface quality of steel plates, the present invention replaces Si element with Al to obtain excellent tensile properties. At the same time, the chemical composition of the present invention does not contain precious alloy elements such as Cr, Mo, and Ti, which reduces production costs.

[0017] 4. The present invention performs heat treatment after warm rolling, which refines the grains, improves the stability of austenite, and can effectively enhance the TRIP effect during austenite phase transformation.

[0018] 5. A vacuum induction furnace is used for smelting. Under vacuum conditions, the smelting material obtains an induced current through the principle of electromagnetic induction to achieve the purpose of heating. There is no need to use a covering agent to prevent N and O elements in the air from reacting with iron and manganese at high temperatures, which saves the use of covering agents, can effectively reduce the production cost of the ultra-high strength medium manganese steel plate of the present invention, and has high heating efficiency, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 1 is a stress-strain curve diagram of the ultra-high strength medium manganese steel of Example 1 and Comparative Example 1;

[0020] Figure 2 This is a SEM image of the ultra-high strength medium manganese steel of Example 1;

[0021] Figure 3 This is the SEM image of the ultra-high strength medium manganese steel of Comparative Example 1;

[0022] Figure 4 EBSD image of the ultra-high strength medium manganese steel of Example 1 before deformation;

[0023] Figure 5 EBSD image of the ultra-high strength medium manganese steel of Comparative Example 1 before deformation;

[0024] Figure 6The XRD diagrams of the ultra-high strength medium manganese steels of Example 1 and Comparative Example 1 before deformation;

[0025] Figure 7 The XRD diagrams of the ultra-high strength medium manganese steels of Example 1 and Comparative Example 1 after deformation. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0028] Calculated by mass percentage, the raw materials of medium manganese steel include: carbon: 0.15%~0.30%, manganese: 11%~13.5%, aluminum: 1.5%~2.5%, sulfur: 0~0.005%, phosphorus: 0~0.008%, and the balance is iron and unavoidable impurities, and does not contain precious metal elements such as Cr and Mo.

[0029] The invention discloses a preparation method of 1400MPa-level medium manganese steel, which comprises smelting, forging, rolling and heat treatment, wherein the rolling comprises hot rolling and warm rolling processes.

[0030] The specific steps are as follows:

[0031] (1) adding the metal raw materials according to the above chemical composition into a vacuum induction furnace, and introducing an inert gas to melt the raw materials under the condition of isolating the raw materials from the atmosphere, and the melting temperature of the vacuum induction furnace is 1550° C. to 1650° C. to obtain an ingot;

[0032] (2) heating to 1150°C to 1250°C and keeping the temperature for 2±0.5 hours, then forging into a slab with a thickness of 140 to 160 mm, and cooling by air cooling;

[0033] (3) The slab is subjected to microstructure homogenization treatment: the slab is kept at 1100°C to 1200°C for 20 minutes, and then the microstructure homogenized slab is subjected to multiple hot rolling deformations. After hot rolling to 42 mm, the slab is air-cooled to room temperature to obtain a hot-rolled plate; the number of hot rolling passes is 5 to 7, the starting rolling temperature is 1000°C to 1100°C, and the final rolling temperature is 850°C to 950°C.

[0034] (4) Performing a second microstructure homogenization treatment: heating the hot-rolled plate to 700°C-750°C and keeping it warm for 20 minutes, then performing warm rolling deformation at 620°C-720°C to 3 mm, and air cooling to room temperature; the number of warm rolling passes is 10-12.

[0035] (5) The obtained warm rolled plate is heat treated: heated to 700℃~750℃ and kept warm for at least 20 minutes, the heating rate of the heat treatment process is 2~8℃ / s, and then air-cooled to obtain 1400MPa grade ultra-low yield ratio medium manganese steel for automobile structure. This heat treatment process is used to eliminate the overheated coarse grain structure of castings and forgings, recrystallize ferrite, reverse austenite transformation, refine the grain of steel structure, improve the stability of austenite, and enhance the TRIP effect during austenite transformation.

[0036] In the present invention, the role of each element in the medium manganese steel is analyzed as follows:

[0037] C: Carbon is conducive to the formation of austenite, can improve the stability of austenite, play a role in solid solution strengthening and precipitation strengthening, but too much carbon content will cause component segregation during casting and lead to the precipitation of carbides, which seriously affects the mechanical properties of the obtained ultra-high strength manganese steel. Therefore, the carbon content of the present invention is 0.15% to 0.30%.

[0038] Mn: Manganese has a great influence on the stability of austenite. It can improve the stability of austenite, play a role in solid solution strengthening, and reduce the phase transition temperature of austenite, reduce the transformation rate of austenite to martensite. As the manganese content increases, the effect of reducing the critical transformation rate of martensite is better. At the same time, the microstructure of the ultra-high strength medium manganese steel obtained by the present invention is fine, so as to enhance the effect of fine grain strengthening. However, if the manganese content is too high, it will cause segregation and increase the production cost. Therefore, the manganese content of the present invention is 11% to 13.5%.

[0039] Al: Aluminum can effectively inhibit the precipitation of cementite, promote the enrichment of carbon in austenite, effectively enhance the stability of residual austenite, enhance the TRIP effect during the strain process, delay the expansion of cracks, and improve the tensile properties of medium manganese steel. The aluminum content of the present invention is between 1.5% and 2.5%.

[0040] P, S: Sulfur exists in steel in the form of sulfide inclusions such as FeS and MnS. Sulfide is usually released in the grain boundary and melts when the temperature reaches its melting point. It will cause cracking during hot deformation of steel during rolling and forging. Phosphorus will seriously affect the cold deformation ability of steel. As the phosphorus content increases, its influence will be aggravated and cold brittleness will occur. Therefore, the present invention needs to remove sulfur and phosphorus elements to improve the mechanical properties of this ultra-high strength medium manganese steel.

[0041] The present invention will be further described below in conjunction with specific embodiments.

[0042] Example 1

[0043] The chemical composition and mass percentage of the steel are as follows: carbon C: 0.21%, manganese Mn: 11.98%, aluminum Al: 1.95%, and the balance is Fe and inevitable impurities.

[0044] The preparation method of the 1400MPa grade ultra-low yield ratio medium manganese steel for automobile structure of this embodiment is carried out according to the following steps:

[0045] 1) According to the chemical composition of steel, it is added into the vacuum induction furnace, the melting furnace is heated to 1550℃~1650℃, and then the smelted molten steel is cast into ingots.

[0046] 2) The ingot is kept at 1150°C for 2 hours to homogenize the structure, and then the ingot is forged into a slab with a thickness of 140 to 160 mm and then air-cooled.

[0047] 3) The air-cooled slab is subjected to a homogenization treatment at 1150°C for 20 minutes, and then subjected to hot rolling to obtain a hot-rolled plate. The hot rolling passes are 5 to 7 times, and the plate thickness after rolling is 42 mm. The plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1000°C, and the final rolling temperature is 850°C.

[0048] 4) heating the hot-rolled plate to 700° C. to 750° C. and keeping the temperature for 20 minutes, and warm-rolling the plate at 620° C. to 720° C. for 10 to 12 passes, obtaining a plate with a thickness of 3 mm after warm rolling, and air cooling the plate to room temperature;

[0049] 5) heating the warm rolled plate to 750°C at a heating rate of 4-7°C / s and keeping the temperature for 20 minutes, and air cooling, i.e. normalizing, to refine the grains and improve the stability of the austenite structure;

[0050] The obtained ultra-high strength medium manganese steel was subjected to a tensile test after grinding the oxide scale to obtain the yield strength, tensile strength and elongation after fracture.

[0051] The ultra-high strength medium manganese steel prepared in this embodiment has a yield strength of 421 MPa, a tensile strength of 1387 MPa, a yield strength ratio of 0.303, and an elongation after fracture of 24.4%.

[0052] Comparative Example 1

[0053] The chemical composition and mass percentage of the steel are as follows: C: 0.21%, Mn: 11.98%, Al: 1.98%, and the balance is Fe and inevitable impurities.

[0054] The preparation method of this comparative example is carried out according to the following steps:

[0055] 1) According to the chemical composition of steel, it is added into the vacuum induction furnace, the melting furnace is heated to 1550℃~1650℃, and then the smelted molten steel is cast into ingots.

[0056] 2) The ingot is kept at 1150°C for 2 hours to homogenize the structure, and then the ingot is forged into a slab with a thickness of 140 to 160 mm and then air-cooled.

[0057] 3) The air-cooled slab is subjected to a homogenization treatment at 1150°C for 20 minutes, and then subjected to hot rolling to obtain a hot-rolled plate. The hot rolling passes are 5 to 7 times, and the plate thickness after rolling is 42 mm. The plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1000°C, and the final rolling temperature is 850°C.

[0058] 4) The hot-rolled plate is heated to 700° C. to 750° C. and kept at this temperature for 20 minutes, and warm-rolled at 620° C. to 720° C., with 10 to 12 warm-rolling passes. The thickness of the warm-rolled plate obtained after warm rolling is 3 mm, and air-cooled to room temperature.

[0059] 5) The warm rolled plate is heated to 750°C at a heating rate of 4-7°C / s, kept at this temperature for 20 minutes, and water-cooled, i.e., quenched.

[0060] The obtained ultra-high strength medium manganese steel was subjected to a tensile test after grinding the oxide scale to obtain the tensile strength, yield strength and elongation after fracture.

[0061] The yield strength of the ultra-high strength medium manganese steel prepared in this comparative example is 270 MPa, the tensile strength is 882 MPa, the yield strength ratio is 0.306, and the elongation after fracture is 23.2%.

[0062] The ultra-high strength medium manganese steel prepared in this example was subjected to relevant performance tests. According to GB / T228-2002 "Room Temperature Tensile Test Method for Metallic Materials", the heat-treated steel plate was processed into a standard tensile specimen. The tensile speed was fixed at 2 mm / min. The engineering stress-strain curve is shown in Figure 1 In order to analyze the structure and percentage ratio of the 1400MPa grade ultra-low yield ratio medium manganese steel for automobile structure of the present invention, after preparing the steel, SEM and EBSD tests were made on Example 1 and Comparative Example 1, see Figure 2 , Figure 3 and Figure 4 , Figure 5 ; and prepared the XRD patterns of Example 1 and Example 2 before the tensile test, namely Figure 6 and Figure 7 .

[0063] The performance parameters of Example 1 and Comparative Example 1 are shown in Table 1, and the austenite content is shown in Table 2.

[0064] Table 1 Performance parameters of Example 1 and Comparative Example 1

[0065] project Yield strength(MPa) Tensile strength(MPa) Total elongation after break (%) Example 1 421 1387 24.4 Comparative Example 1 270 882 23.2

[0066] Table 2 Austenite content

[0067] project Example 1 Comparative Example 1 Austenite content (%) 86.7 77.8

[0068] according to Figure 1 As can be seen from Table 1, the yield strength ratio of the ultra-high strength medium manganese steel of the present invention is extremely low, the yield strength ratio of Example 1 is 0.303, the yield strength ratio of Comparative Example 1 is 0.306, the yield strength is 270-420 MPa, and the tensile strength is 880 MPa-1400 MPa.

[0069] According to Table 2, Figures 2 to 5 It can be seen that the structure of the ultra-high strength medium manganese steel of the present invention is austenite and ferrite. The austenite content of the present invention is 86% to 78%, and the rest is ferrite. Because the heat treatment temperature of the present invention is 750°C, the content of ferrite is very small at this higher temperature, but the content of austenite is high. At the same time, because austenite can undergo TRIP phase transformation and TWIP phase transformation during tensile deformation, the work hardening rate and tensile strength of the ultra-high strength medium manganese steel obtained are extremely high, and at the same time, it has good elongation. Therefore, the strength of the ultra-high strength steel prepared by the present invention can reach 1400MPa level, with the characteristics of ultra-high strength.

[0070] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing 1400MPa grade medium manganese steel, characterized in that: The medium manganese steel ingot is heated to 1150℃~1250℃ and kept warm for 2±0.5 hours, then forged into a slab, air-cooled, kept warm at 1100℃~1200℃ for 20 minutes, and subjected to multiple hot rolling deformations. After air-cooling, it is heated to 700℃~750℃ and kept warm for 20 minutes, then warm-rolled at 620℃~720℃, air-cooled, heated to 700℃~750℃ and kept warm for at least 20 minutes, and air-cooled to obtain 1400MPa grade ultra-low yield ratio medium manganese steel.

2. The preparation method according to claim 1, characterized in that: Calculated by mass percentage, the raw materials of the medium manganese steel include: carbon: 0.15% to 0.30%, manganese: 11% to 13.5%, aluminum: 1.5% to 2.5%, sulfur: 0 to 0.005%, phosphorus: 0 to 0.008%, and the balance is iron.

3. The preparation method according to claim 2, characterized in that: The raw materials of medium manganese steel are smelted at 1550°C to 1650°C to obtain ingots.

4. The preparation method according to claim 1, characterized in that: The hot rolling passes are 5 to 7, the starting rolling temperature is 1000°C to 1100°C, and the final rolling temperature is 850°C to 950°C.

5. The preparation method according to claim 1, characterized in that: The warm rolling passes are 10 to 12 passes.

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