A 1900MPa grade cold-rolled medium manganese steel and its preparation method

Abstract

The present invention relates to the technical field of high-strength steel, and specifically to a 1900MPa grade cold-rolled medium manganese steel and a preparation method thereof. The specific technical scheme is as follows: a preparation method of 1900MPa grade cold-rolled medium manganese steel, wherein the components of the medium manganese steel are melted and cast into ingots and then forged into slabs, the slabs are kept at 1120℃ to 1180℃ for 2 to 3 hours, the slabs are subjected to multiple hot rolling deformations, and after hot rolling, the slabs are air-cooled to room temperature to obtain hot-rolled plates, the hot-rolled plates are heated to 700℃ to 750℃ and kept at this temperature for 2 to 3 hours, and then warm-rolled at 650℃ to 700℃ and air-cooled to room temperature to obtain warm-rolled plates; after cold rolling the warm-rolled plates, the plates are kept at 200℃ to 250℃ for at least 120 minutes to obtain medium manganese steel. The present invention performs rolling in a cold rolling manner and combines air cooling and low-temperature tempering processes to finally obtain high-strength medium manganese steel with a yield strength of 1200MPa to 1900MPa and a tensile strength of 1600MPa to 1900MPa.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel, and in particular to a 1900 MPa grade cold-rolled medium manganese steel and a preparation method thereof. Background Art

[0002] Since the 1920s, steel has been used in various fields due to its low cost, ease of forming and welding, and has played an important role in the progress of human society and industrial development. Steel has high strength and excellent plasticity, and occupies a dominant position in metal structural materials. Traditional high-strength steel has the advantages of abundant reserves and diverse performance, and is widely used in industries such as construction, railways, energy and machinery. In the automotive manufacturing field, it is often used to make the main structural components of automobiles due to its excellent mechanical properties. Traditional high-strength steel generally has common key problems such as low toughness and plasticity, complex production processes, and high costs. Compared with traditional high-strength steel, advanced high-strength steel has a higher strength and plasticity combination. Advanced high-strength steel also has better formability and higher work hardening.

[0003] Advanced high-strength steel has undergone three generations of upgrades. As one of the most promising candidates for the third generation of ultra-high-strength steel, medium-manganese steel (MMS) is a key research focus in terms of composition, process design, and mechanical property optimization. MMS is categorized into three types based on its Mn content and the range and amount of other alloying elements. The first type has a low overall alloying element content. The second type has a higher Mn content (5% < Mn < 10%) and incorporates a certain amount of other alloying elements. The third type has a Mn content between 9% and 12% and incorporates a high content of other alloying elements, such as Al. MMS boasts a high strength-ductility product. This excellent strength-ductility match stems primarily from the transformation-induced plasticity of metastable retained austenite. MMS offers significant advantages in processability, weldability, and casting properties, finding applications in the automotive, shipbuilding, and military sectors. The rolling of MMS is primarily accomplished through hot and cold rolling, but this presents certain challenges. Cold-rolled MMS requires one or more annealing steps during processing, making the process cumbersome and costly. Some medium-manganese steels are produced through multiple forging and annealing processes, which is relatively complex. Others incorporate precious metals such as rare earth elements, resulting in higher costs. These issues have hindered the production and application of medium-manganese steel. It is essential to utilize low-cost, short-process technologies to produce medium-manganese steel with excellent mechanical properties.

[0004] The steel contains no or minimal precious metal elements, resulting in low cost and excellent overall performance, a trend that is trending in the steel industry. The present invention, which is free of precious metal elements such as Cr, Mo, and Ti, offers low cost and excellent mechanical properties, generating significant social demand and is expected to capture a significant share of the steel market.

[0005] Chinese invention patent application CN116926406A discloses a method for preparing a two-phase annealed, low-temperature tempered, high-strength medium-manganese steel. The steel comprises 0.26 wt% C, 7.9 wt% Mn, 1.8 wt% Al, 0.02 wt% Si, ≤0.005 wt% P, ≤0.007 wt% S, and the balance Fe and unavoidable impurities. The heat treatment process is complex and subject to uncertainty. The resulting steel exhibits a tensile strength of 1200-1300 MPa, a tensile strength of 1400 MPa, and an elongation of 25-27%. However, the yield strength and tensile strength are lower than those of the present invention.

[0006] Chinese invention patent application CN117821711A discloses a heat treatment method for cold-rolled medium-manganese steel. Prior to the conventional Q&P heat treatment process, a simple annealing heat treatment is added to preliminarily modify the internal microenvironment of the cold-rolled medium-manganese steel. The steel's composition, by mass percentage, is as follows: C: 0.12%, Mn: 7.69%, Si: 1.45%, Al: 2.76%, Cu: 0.51%, Mo: 0.2%, Cr: 0.35%, Nb: 0.11%, B: 0.001%, Ni: 0.027%, with the remainder being iron and unavoidable impurities. The steel has a tensile strength of 1200-1300 MPa and an elongation of 33%-39%. Compared to the present invention, the steel has a lower tensile strength. Summary of the Invention

[0007] In response to the shortcomings of the existing technology, the present invention provides a 1900MPa grade cold-rolled medium manganese steel and a preparation method thereof. The steel is rolled by cold rolling and combined with air cooling and low-temperature tempering processes to finally obtain a high-strength medium manganese steel with a yield strength of 1200MPa to 1900MPa and a tensile strength of 1600MPa to 1900MPa.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0009] The invention discloses a 1900MPa grade cold-rolled medium manganese steel, which comprises the following components by mass percentage: carbon: 0.15%-0.68%, manganese: 10%-12.5%, aluminum: 1.5%-2.5%, and the balance is Fe.

[0010] Correspondingly, a preparation method of 1900MPa grade cold-rolled medium manganese steel comprises the following steps: melting and casting the various components of the medium manganese steel into ingots and then forging them into slabs; keeping the slabs at 1120°C to 1180°C for 2 to 3 hours; subjecting the slabs to multiple hot rolling deformations; and air-cooling them to room temperature after hot rolling to obtain hot-rolled plates; heating the hot-rolled plates to 700°C to 750°C and keeping them at this temperature for 2 to 3 hours; and then warm-rolling and deforming them at 650°C to 700°C; and air-cooling them to room temperature to obtain warm-rolled plates; and cold-rolling the warm-rolled plates and then keeping them at 200°C to 250°C for at least 120 minutes to obtain the medium manganese steel.

[0011] Preferably, the ingot is heated to 1150° C. to 1250° C. and kept at this temperature for 2 to 3 hours, then forged into a slab and air-cooled.

[0012] Preferably, the thickness of the slab is 18 to 22 mm, and the thickness of the cold-rolled plate obtained by cold-rolling the warm-rolled plate is 1 to 2 mm.

[0013] Preferably, the smelting temperature is 1550°C to 1650°C.

[0014] Preferably, the warm-rolled plate is heated to the two-phase region and kept warm for 5 to 6 hours, and then air-cooled and cold-rolled to obtain the cold-rolled plate.

[0015] Preferably, the hot rolling passes are 7 to 8, the starting rolling temperature is 1120°C to 1180°C, and the finishing rolling temperature is 900°C to 950°C.

[0016] Preferably, the warm rolling passes are 3 to 4 passes, and the cold rolling passes are 5 to 6 passes.

[0017] The present invention has the following beneficial effects:

[0018] 1. The cold-rolled medium manganese steel of the present invention has good mechanical properties and has the advantages of high-strength medium manganese steel. The yield strength reaches 1800MPa and the tensile strength reaches 1900Mpa, which is one level higher than similar steels.

[0019] 2. The C and Mn elements in the manganese steel of the present invention promote the stability of austenite and enhance the TRIP effect of retained austenite. Because Si is detrimental to the production of cold-rolled medium-manganese steel sheets, the present invention replaces Si with Al to achieve excellent tensile properties. Furthermore, the chemical composition of the present invention does not contain precious alloying elements such as Cr, Mo, and Ti, reducing production costs.

[0020] 3. The present invention optimizes the processing technology and performs low-temperature tempering treatment after cold rolling, thereby improving the stability of the steel structure and effectively enhancing the TRIP effect during the phase transformation of retained austenite.

[0021] 4. The present invention adopts a vacuum induction furnace for smelting. Under vacuum conditions, the molten 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 cold-rolled medium manganese steel plate of the present invention, and has high heating efficiency, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 1 is a stress-strain curve diagram of the cold-rolled medium manganese steel of Example 1, Example 2 and Comparative Example 1, Comparative Example 2;

[0023] Figure 2 This is the SEM image of the cold-rolled medium manganese steel of Comparative Example 2;

[0024] Figure 3 This is the SEM image of the cold-rolled medium manganese steel of Example 2;

[0025] Figure 4 This is the EBSD image of the cold-rolled medium manganese steel of Comparative Example 2 before the tensile test;

[0026] Figure 5 This is the EBSD image of the cold-rolled medium manganese steel of Example 2 before the tensile test;

[0027] Figure 6 is the XRD pattern of the cold-rolled medium manganese steel of Comparative Example 2;

[0028] Figure 7 This is the XRD pattern of the cold-rolled medium manganese steel of Example 2. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0031] 1. The present invention discloses a 1900MPa grade cold-rolled medium manganese steel, the composition and mass content of the steel are (in terms of mass percentage): carbon C: 0.15% to 0.68%, manganese Mn: 10% to 12.5%, aluminum Al:

[0032] 1.5% to 2.5% of Mn, the balance being Fe and unavoidable impurities. The yield strength of the cold-rolled medium manganese steel reaches 1900 MPa, and the elongation is 6.2% to 16.8%.

[0033] Among them, C: carbon is more conducive to the formation of austenite. Relying on carbon partitioning, a large amount of austenite can be obtained. Carbon can also promote the stability of austenite. It also has solid solution strengthening and precipitation strengthening effects to improve strength and plasticity. However, excessive carbon content will lead to reduced plasticity and toughness. Therefore, the carbon content of the present invention is preferably within 0.15% to 0.28%.

[0034] Mn: Manganese dissolves in austenite, providing solid solution strengthening, promoting austenite stability, lowering the Ms point, and reducing the rate of austenite transformation to martensite. However, excessive manganese content can easily form manganese segregation bands, which can lead to anisotropy in mechanical properties, deteriorating strength and uniform elongation, and increasing production costs. Therefore, the manganese content in the present invention is preferably 11% to 12%.

[0035] Al: Aluminum effectively inhibits cementite precipitation and promotes carbon enrichment in austenite, effectively promoting the stability of retained austenite, enhancing the TRIP effect during the retained austenite transformation process, delaying crack expansion, and improving the toughness of the steel. The aluminum content of the present invention is between 1.5% and 2.5%.

[0036] P, S: Sulfur and phosphorus are harmful elements in steel, causing hot brittleness, reducing ductility and toughness, and causing cracks during forging and rolling. Phosphorus increases cold brittleness, impairs weldability, reduces plasticity, and degrades cold bending properties. Therefore, removing sulfur and phosphorus improves the mechanical properties of this high-strength medium-manganese steel.

[0037] 2. The preparation method of cold-rolled medium manganese steel is carried out according to the following steps:

[0038] (1) Smelting

[0039] TRIP steel is smelted according to its chemical composition. The smelting equipment is a vacuum induction furnace with a temperature of 1550℃~1650℃. After smelting, it is cast into ingots.

[0040] (2) Forging

[0041] First, keep the temperature within the range of 1150℃ to 1250℃ for 2 to 3 hours to homogenize the structure, then forge the ingot into a slab and air-cool it; the thickness of the slab after forging is 18 to 22 mm.

[0042] (3) Hot rolling

[0043] The slab is subjected to a microstructure homogenization treatment at 1120°C to 1180°C for 2 to 3 hours, and then hot rolled to obtain a hot-rolled plate, which is then cooled to room temperature by air cooling. The hot-rolled plate has a thickness of 5 mm after hot rolling, and the number of hot rolling passes is 7 to 8. The starting rolling temperature during the hot rolling process is 1120°C to 1180°C, and the finishing rolling temperature is 900°C to 950°C.

[0044] (4) Warm rolling

[0045] The second microstructure homogenization treatment is carried out, the hot-rolled plate is heated to 700℃~750℃ and kept warm for 2~3 hours, and warm rolled at 650℃~700℃, and air-cooled to room temperature; the thickness of the warm-rolled plate obtained after warm rolling is 2.5mm, and the number of warm rolling passes is 3~4.

[0046] (5) Cold rolling

[0047] The warm rolled plate is cold rolled to obtain the cold rolled plate. The thickness of the steel plate obtained after the cold rolling is 1 to 2 mm, the reduction rate is 10 to 20%, and the number of cold rolling passes is 5 to 6.

[0048] (6) Tempering

[0049] The resulting cold-rolled sheet was held at 200°C to 250°C for 120-130 minutes and then air-cooled to produce a high-strength medium-manganese steel with a strength of 1900 MPa. The cold-rolled sample was then subjected to low-temperature tempering for two hours. This low-temperature tempering process allows for interfacial migration, increases the martensite volume fraction, improves austenite stability, and enhances the TRIP effect during austenite transformation.

[0050] The present invention will be further described below with reference to specific embodiments.

[0051] Example 1

[0052] The chemical composition and mass percentage of the steel are as follows: C: 0.20%, Mn: 11.40%, Al: 1.97%, and the balance is Fe and unavoidable impurities.

[0053] The preparation method of the 1900 MPa grade cold-rolled medium manganese steel of this embodiment is carried out according to the following steps:

[0054] 1) According to the chemical composition of steel, it is added into the vacuum induction furnace, the melting furnace is heated to 1600℃, and then the smelted molten steel is cast into ingots.

[0055] 2) The ingot was kept at 1200°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 20 mm and then air-cooled.

[0056] 3) The air-cooled slab was subjected to a microstructure homogenization treatment at 1150°C for 2 hours, followed by hot rolling through seven passes to obtain a 5 mm thick hot-rolled plate. The plate was then air-cooled to room temperature. The hot rolling start temperature was 1150°C, and the final rolling temperature was 950°C.

[0057] 4) The hot-rolled plate was heated to 700° C. and kept at this temperature for 2 hours, and then warm-rolled at 650° C. for 4 passes. The thickness of the warm-rolled plate obtained after warm rolling was 2.5 mm, and then air-cooled to room temperature.

[0058] 5) The warm-rolled sheet was cold-rolled to obtain a 2 mm cold-rolled sheet with a reduction of 10% and 5 cold-rolling passes.

[0059] 6) Tempering treatment: heat the cold-rolled sheet to 200°C at a heating rate of 4-7°C / s (preferably 5°C / s) and keep it at 200°C for 120 minutes to improve the stability of the austenite structure.

[0060] The obtained cold-rolled 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. The yield strength of the cold-rolled medium manganese steel prepared in this embodiment was 1654.8 MPa, the tensile strength was 1808.7 MPa, and the elongation after fracture was 14.5%.

[0061] Comparative Example 1

[0062] The chemical composition and mass percentage of the steel are as follows: C: 0.20%, Mn: 11.40%, Al: 1.97%, and the balance is Fe and unavoidable impurities.

[0063] The preparation method of the cold-rolled medium manganese steel of this comparative example is carried out according to the following steps:

[0064] 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.

[0065] 2) The ingot was kept at 1200°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 20 mm and then air-cooled.

[0066] 3) The air-cooled slab was subjected to a microstructure homogenization treatment at 1150°C for 2 hours, followed by hot rolling through seven passes to obtain a 5 mm thick hot-rolled plate. The plate was then air-cooled to room temperature. The hot rolling start temperature was 1150°C, and the final rolling temperature was 950°C.

[0067] 4) The hot-rolled plate was heated to 700° C. and kept at this temperature for 2 hours, and then warm-rolled at 650° C. for 4 passes. The thickness of the warm-rolled plate obtained after warm rolling was 2.5 mm, and then air-cooled to room temperature.

[0068] 5) The warm rolled sheet was then cold rolled to obtain a 2 mm cold rolled sheet with a reduction ratio of 10% and 5 cold rolling passes.

[0069] The obtained cold-rolled 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. The yield strength of the cold-rolled medium manganese steel prepared in this comparative example was 1201.6 MPa, the tensile strength was 1607.2 MPa, and the elongation after fracture was 16.8%.

[0070] Example 2

[0071] The chemical composition and mass percentages of the steel are as follows: C: 0.20%, Mn: 11.40%, Al: 1.97%, and the balance being Fe and unavoidable impurities.

[0072] The preparation method of the 1900 MPa grade cold-rolled medium manganese steel of this embodiment is carried out according to the following steps:

[0073] 1) According to the chemical composition of steel, it is added into the vacuum induction furnace, the melting furnace is heated to 1600℃, and then the smelted molten steel is cast into ingots.

[0074] 2) The ingot was kept at 1200°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 20 mm and then air-cooled.

[0075] 3) The air-cooled slab was subjected to a microstructure homogenization treatment at 1150°C for 2 hours, followed by hot rolling through seven passes to obtain a 5 mm thick hot-rolled plate. The plate was then air-cooled to room temperature. The hot rolling start temperature was 1150°C, and the final rolling temperature was 950°C.

[0076] 4) The hot-rolled plate was heated to 700° C. and kept at this temperature for 2 hours, and then warm-rolled at 650° C. for 4 passes. The thickness of the warm-rolled plate obtained after warm rolling was 2.5 mm, and then air-cooled to room temperature.

[0077] 5) The warm-rolled sheet was cold-rolled to obtain a 2 mm cold-rolled sheet with a reduction of 20% and 5 cold-rolling passes.

[0078] 6) Tempering treatment: heat the cold-rolled sheet to 200°C at a heating rate of 4-7°C / s (preferably 5°C / s) and keep it at 200°C for 120 minutes to improve the stability of the austenite structure.

[0079] The obtained cold-rolled 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. The yield strength of the cold-rolled medium manganese steel prepared in this embodiment was 1842.5 MPa, the tensile strength was 1900.1 MPa, the elongation after fracture was 13.5%, and the austenite content was 27%.

[0080] Comparative Example 2

[0081] The chemical composition and mass percentages of the steel are as follows: C: 0.20%, Mn: 11.47%, Al: 1.95%, and the balance being Fe and unavoidable impurities.

[0082] The preparation method of the cold-rolled medium manganese steel of this comparative example is carried out according to the following steps:

[0083] 1) According to the chemical composition of steel, it is added into the vacuum induction furnace, the melting furnace is heated to 1600℃, and then the smelted molten steel is cast into ingots.

[0084] 2) The ingot was kept at 1200°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 20 mm and then air-cooled.

[0085] 3) The air-cooled slab was subjected to a microstructure homogenization treatment at 1150°C for 2 hours, followed by hot rolling through seven passes to obtain a 5 mm thick hot-rolled plate. The plate was then air-cooled to room temperature. The hot rolling start temperature was 1150°C, and the final rolling temperature was 950°C.

[0086] 4) The hot-rolled plate was heated to 700° C. and kept at this temperature for 2 hours, and then warm-rolled at 650° C. for 4 passes. The thickness of the warm-rolled plate obtained after warm rolling was 2.5 mm, and then air-cooled to room temperature.

[0087] 5) The warm-rolled sheet was cold-rolled to obtain a 2 mm cold-rolled sheet with a reduction of 20% and 5 cold-rolling passes.

[0088] The obtained cold-rolled 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. The yield strength of the cold-rolled medium manganese steel prepared in this comparative example was 1287.5 MPa, the tensile strength was 1617.8 MPa, the elongation after fracture was 12.1%, and the austenite content was 45%.

[0089] The cold-rolled medium manganese steel prepared in this comparative example and the embodiment was subjected to relevant performance tests. According to GB / T228-2002 "Metallic Materials Room Temperature Tensile Test Method", the heat-treated steel plate was processed into a standard tensile specimen, and the tensile speed was fixed at 2 mm / min. The stress-strain curve was as follows: Figure 1As shown, the results show that the cold-rolled medium manganese steel of the present invention has extremely high strength, with a yield strength of 1200MPa to 1900MPa and a tensile strength of 1600MPa to 1900MPa.

[0090] In order to analyze the structure and percentage ratio of the 1900MPa grade cold-rolled manganese steel of the present invention, after the steel was prepared, SEM tests were made on Comparative Example 2 and Example 2. Figure 2 and Figure 3 ; and prepared the EBSD maps of Comparative Example 2 and Example 2 before the tensile test, namely Figure 4 and Figure 5 ; At the same time, XRD tests were performed on Comparative Example 2 and Example 2, see Figure 6 and Figure 7 .

[0091] Depend on Figure 2-Figure 7 Analysis shows that the cold-rolled medium-manganese steel of the present invention has a structure composed of austenite and martensite, with the austenite content ranging from 27% to 45% and the remainder being martensite. Furthermore, because the heat treatment temperature of the present invention is 200°C, it can retain the high dislocation density of cold rolling while obtaining more martensite structure through interface movement. Because martensite structure has the characteristics of high strength and high hardness, the obtained cold-rolled medium-manganese steel has extremely high strength. Therefore, the strength of the cold-rolled medium-manganese steel prepared by the present invention can reach the level of 1900MPa, which is characteristic of high-strength medium-manganese steel.

[0092] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing 1900 MPa grade cold-rolled medium manganese steel, characterized by: The components of the medium manganese steel are melted and cast into ingots, and then forged into slabs, wherein the slabs have a thickness of 18 to 22 mm, the slabs are kept at 1120° C. to 1180° C. for 2 to 3 hours, and then subjected to multiple hot rolling deformations on the slabs. After hot rolling, the slabs are air-cooled to room temperature to obtain hot-rolled plates, the hot-rolled plates are heated to 700° C. to 750° C. and kept at this temperature for 2 to 3 hours, and then warm-rolled at 650° C. to 700° C. and air-cooled to room temperature to obtain warm-rolled plates; the warm-rolled plates are cold-rolled to obtain cold-rolled plates with a thickness of 1 to 2 mm, and kept at 200° C. to 250° C. for at least 120 minutes, thereby obtaining medium manganese steel; The hot rolling passes are 7 to 8, the starting rolling temperature is 1120°C to 1180°C, and the finishing rolling temperature is 900°C to 950°C; The medium manganese steel comprises the following components in percentage by mass: Carbon: 0.15%~0.68%, manganese: 10%~12.5%, aluminum: 1.5%~2.5%, and the balance is Fe.

2. The preparation method according to claim 1, wherein: The ingot is heated to 1150° C. to 1250° C. and kept at this temperature for 2 to 3 hours, then forged into a slab and air-cooled.

3. The preparation method according to any one of claims 1 to 2, characterized in that: The smelting temperature is 1550°C to 1650°C.

4. The preparation method according to claim 1, wherein: The warm rolling passes are 3 to 4 times, and the cold rolling passes are 5 to 6 times.

Images