High-strength and high-toughness bainite medium manganese steel as well as preparation method and application thereof

Abstract

The invention discloses high-strength and high-toughness bainite medium manganese steel as well as a preparation method and application thereof, and relates to the technical field of high-strength steel processing. The method comprises the steps that a casting blank of medium manganese steel is prepared, heat preservation is conducted on the casting blank for 2-4 h at the temperature of 1150-1250 DEG C, and then the casting blank is hot-rolled to form a hot-rolled steel plate with the thickness of 10-12 mm; the hot-rolled steel plate is heated to the austenite phase change interval temperature, and a steel plate subjected to solid solution treatment is obtained; the steel plate obtained after solid solution is heated to the bainite phase change interval temperature, and a bainite steel plate is obtained; the bainite steel plate is placed at the room temperature, and a cold-rolled steel plate is obtained; and the cold-rolled steel plate is subjected to heat preservation for 10-20 min at the temperature of 280-350 DEG C and then subjected to water quenching to the room temperature, and the high-strength and high-toughness bainite medium manganese steel is obtained. Bainite and pearlite structures are introduced into an existing medium manganese steel system, the mechanical property of the medium manganese steel is improved, and the toughness of the material is concerned. And a new window is opened for the performance design of the medium manganese steel.

Description

Technical Field

[0001] The invention relates to the technical field of high-strength steel processing, and in particular to a high-strength and high-toughness bainite medium manganese steel and a preparation method and application thereof. Background Art

[0002] Medium manganese steel, with a Mn content of about 4-12wt.%, and a strength-plasticity product between 20GPa% and 50GPa%, is a typical representative of the third generation of advanced high-strength steel. It uses austenite as a soft phase for coordinated deformation, and ferrite / martensite as a hard phase to provide strength, but such a combination of organizations will bring about problems such as low yield strength and poor plasticity. Therefore, it is considered to introduce other types of organizations or phases in the microstructure design to expand the strength and plasticity control window of medium manganese steel. Generally speaking, compared with ferrite and martensite, bainite has a better match between strength and deformation ability. Therefore, the introduction of bainite helps to broaden the current ideas for the construction of microstructures and the development of mechanical properties of medium manganese steel. Moreover, the bainite phase transformation is one of the most basic solid-state phase transformations in the field of physical metallurgy. Its research has a history of more than 90 years and is widely used in the design of various advanced high-strength steels.

[0003] At present, most of the research on bainite transformation of steel materials focuses on introducing dislocations to accelerate the bainite transformation rate and strengthen the strength of bainite, and its Mn content is generally less than 3wt.%; while the research on bainite transformation of medium manganese steel is in the primary stage. In the prior art, the process of multiple warm rolling after salt bath to the specified temperature is adopted, which not only obtains the similar properties of low-temperature bainite, the tensile strength is 2154MPa, the total elongation is 14%, but also greatly reduces the time of bainite transformation, from 72h at 200℃ to 2h at 300℃. The prior art also discloses a high-strength and high-plasticity bainite steel and a production method, which is mainly to roll at different rolling amounts after cooling twice at different speeds to the specified rolling start temperature above the bainite transformation start temperature, with good surface quality, low energy consumption of deformation process, short production cycle, and the yield strength of the material is 1392-1435MPa, the tensile strength is 1771-1900MPa, and the elongation is 16.6-19.2%. Also disclosed is an ultra-high strength bainitic cold heading steel and a manufacturing method thereof. The yield strength of the prepared steel is between 1224 and 1350 MPa, the tensile strength is between 1510 and 1530 MPa, the elongation is between 15.5 and 16%, the cross-sectional shrinkage is between 57 and 58%, and the yield strength ratio is between 0.80 and 0.88. The bainitic steel is a bainitic steel that is rolled into a wire rod and then spheroidized and annealed before undergoing a bainitic phase transformation. An uncommon cold heading technology is used, and attention is paid to the toughness of the material.

[0004] It is worth noting that although a lot of research has been carried out on bainitic steel, it is still concentrated on low-alloy steel, and less research is involved in steels with higher manganese content. Now, medium manganese steel, as a representative of the third generation of advanced high-strength steel, is a hot topic in industry research. Therefore, the introduction of bainite, a strong and tough structure, is of great reference significance for the development of new medium manganese steel. Summary of the invention

[0005] In view of the deficiencies in the above-mentioned background technology, the present invention mainly solves the following technical problems: The present invention solves the problem of generally low yield strength in the existing medium manganese steel system. Provides a high-strength and high-toughness bainite medium manganese steel and its preparation method and application. The present invention introduces bainite and pearlite structures into the existing medium manganese steel system, improves the mechanical properties of the medium manganese steel and pays attention to the toughness of the material. It opens a new window for the performance design of medium manganese steel.

[0006] The first object of the present invention is to provide a method for preparing high-strength and high-toughness bainite medium-manganese steel, comprising the following steps:

[0007] The ingot of medium manganese steel is prepared, the ingot is kept at 1150-1250°C for 2-4 hours, and then hot-rolled to a hot-rolled steel plate with a thickness of 10-12 mm;

[0008] The hot-rolled steel plate is heated to the austenite phase transformation range temperature, and kept at this temperature for 30 to 60 minutes, and then water-quenched to room temperature to obtain a solution-treated steel plate;

[0009] The solution-treated steel plate is heated to a bainite phase transformation temperature range and kept at that temperature for 48 to 72 hours, and then water-cooled to room temperature to obtain a bainite steel plate;

[0010] The bainite steel sheet is cold-rolled at room temperature to reduce the thickness by 30 to 60% to obtain a cold-rolled steel sheet;

[0011] The cold rolled steel plate is kept at 280-350°C for 10-20 minutes and then quenched with water to room temperature to obtain high strength and high toughness bainitic medium manganese steel.

[0012] Preferably, the temperature of the austenite transformation range is 800-900°C.

[0013] Preferably, the temperature of the bainite phase transformation range is 400-500°C.

[0014] Preferably, the heating rate for heating to the temperature in the austenite transformation range is 3 to 8 K / s.

[0015] Preferably, the heating rate for heating to the temperature in the bainite transformation range is 3 to 8 K / s.

[0016] Preferably, during hot rolling, the initial rolling temperature is 1050-1100°C, and the rolling termination temperature is not less than 850-900°C.

[0017] Preferably, the following components in mass percentage are selected when preparing the ingot of medium manganese steel: C: 0.4-0.5%, Mn: 11-13%, Al: 1-3%, V: 0.6-0.8%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe.

[0018] Preferably, the ingot of medium manganese steel is produced by weighing raw materials according to the mass percentage of chemical composition and smelting them.

[0019] The second object of the present invention is to provide a high-strength and high-toughness bainitic medium manganese steel.

[0020] The third object of the present invention is to provide a high-strength and high-toughness bainitic medium manganese steel for use in automobile manufacturing.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention provides a high-strength and high-toughness bainitic medium manganese steel and a preparation method and application thereof. The high-strength and high-toughness bainitic medium manganese steel provided by the present invention, the existing medium manganese steel microstructure type is generally martensite / ferrite as a strengthening phase matched with austenite as a matrix, and its development has reached a bottleneck due to the inherent properties of the microstructure itself, but the present invention introduces a bainitic microstructure into the medium manganese steel system and proves that it has good mechanical properties, that is, the medium manganese steel prepared by the present invention still retains a certain toughness while reaching a yield of 2GPa, opening a new window for the regulation of the microstructure and properties of the medium manganese steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a heat treatment process diagram for high-strength and high-toughness medium manganese steel designed based on bainite phase transformation in the present invention.

[0024] Figure 2 This is the microstructure diagram of the medium manganese steel sample obtained in Example 1 of the present invention.

[0025] Figure 3 This is the microstructure diagram of the medium manganese steel sample obtained in Example 2 of the present invention.

[0026] Figure 4 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 1 of the present invention.

[0027] Figure 5 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 2 of the present invention.

[0028] Figure 6 This is a comparison chart of the toughness of the medium manganese steel sample obtained in Example 2 of the present invention and other samples. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention is further described below in conjunction with specific embodiments and drawings, but the embodiments are not intended to limit the present invention.

[0030] The first aspect of the present invention provides a method for preparing high-strength and high-toughness bainite medium-manganese steel, comprising the following steps:

[0031] The ingot of medium manganese steel is prepared, the ingot is kept at 1150-1250°C for 2-4 hours, and then hot-rolled to a hot-rolled steel plate with a thickness of 10-12 mm;

[0032] The hot-rolled steel plate is heated to the austenite phase transformation range temperature, and kept at this temperature for 30 to 60 minutes, and then water-quenched to room temperature to obtain a solution-treated steel plate;

[0033] The solution-treated steel plate is heated to a bainite phase transformation temperature range and kept at that temperature for 48 to 72 hours, and then water-cooled to room temperature to obtain a bainite steel plate;

[0034] The bainite steel sheet is cold-rolled at room temperature to reduce the thickness by 30 to 60% to obtain a cold-rolled steel sheet;

[0035] The cold rolled steel plate is kept at 280-350°C for 10-20 minutes and then quenched with water to room temperature to obtain high strength and high toughness bainitic medium manganese steel.

[0036] Wherein, the temperature of the austenite phase transformation range is 800-900°C.

[0037] The temperature of the bainite phase transformation range is 400-500°C.

[0038] The heating rate to the austenite transformation temperature is 3-8 K / s, preferably 5 K / s.

[0039] The heating rate for heating to the temperature in the bainite transformation range is 3 to 8 K / s, preferably 5 K / s.

[0040] During hot rolling, the initial rolling temperature is 1050-1100°C, and the rolling termination temperature is not lower than 850-900°C.

[0041] When preparing the ingot of medium manganese steel, the following components are selected in mass percentage: C: 0.4-0.5%, Mn: 11-13%, Al: 1-3%, V: 0.6-0.8%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe.

[0042] The ingot of medium manganese steel is produced by weighing raw materials according to the mass percentage of chemical composition and smelting them.

[0043] In one embodiment, a method for preparing high-strength and high-toughness medium manganese steel, wherein the composition by mass percentage is: C: 0.4-0.5%, Mn: 11-13%, Al: 1-3%, V: 0.6-0.8%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps:

[0044] (1) According to the mass percentage of the chemical components, the raw materials are mixed and smelted in a smelting furnace to obtain a cast ingot, which is then kept at 1200°C for 2 hours for homogenization treatment to dissolve VC, and then kept at 1100°C for 2 hours as the starting temperature for hot rolling for preheating, and then hot rolled to a thin plate, with a final rolling temperature of not less than 850-900°C, and finally machined (cut into small pieces) to a hot-rolled plate specification (the length and width can be appropriately adjusted based on the actual on-site processing situation): length 500 mm*width 100 mm*thickness 10 mm, and finally cooled to room temperature to form a hot-rolled martensitic steel plate.

[0045] (2) The hot-rolled steel sheet is heated to the austenite phase and kept at this temperature for a period of time to obtain a complete austenite structure and completely dissolve the precipitates. It is then water quenched to room temperature, wherein the austenitizing temperature is 850°C and the holding time is 30 minutes.

[0046] (3) The solution-treated steel plate is heated to a bainite phase transformation range (400-500°C) and kept at this temperature for a long time (48-72 hours), and then water-cooled to room temperature.

[0047] (4) The steel sheet is subjected to cold rolling and thinning treatment at room temperature.

[0048] (5) The cold rolled steel sheet is subjected to a short-term heat treatment at a relatively low temperature.

[0049] See also Figure 1 As shown, the present invention provides a method for preparing high-strength and high-toughness medium manganese steel based on bainite phase transformation. The solid solution is used to homogenize the material, the isothermal insulation is used to generate bainite and pearlite structures, the rolling is used to introduce high-density dislocations, and the short-time heat treatment is used to strengthen and eliminate some stresses at the same time by baking. Bainite and pearlite structures are introduced into the existing medium manganese steel system, the mechanical properties of the medium manganese steel are improved, and the toughness of the material is paid attention to. A new window is opened for the performance design of medium manganese steel.

[0050] A second aspect of the present invention provides a high-strength and high-toughness bainitic medium manganese steel.

[0051] A third aspect of the present invention provides a high-strength and high-toughness bainitic medium manganese steel for use in automobile manufacturing.

[0052] It should be noted that the experimental methods used in the present invention are all conventional methods unless otherwise specified; the reagents and materials used are all commercially available unless otherwise specified.

[0053] Example 1

[0054] Alloy composition (mass percentage): 0.45% C, 12% Mn, 2% Al, 0.7% V, and the rest is Fe.

[0055] Preparation process: (1) Smelting is performed according to the mass percentage of the selected chemical composition to obtain a casting, and the casting is homogenized at 1200°C for 2 hours to dissolve VC, and the casting is preheated at 1100°C for 2 hours as the starting temperature of hot rolling, and then hot rolled to a thin plate. The final rolling temperature is not less than 850-900°C. Finally, the large pieces are cut into small pieces through machining. The specifications of the hot-rolled plate are 500 mm in length, 100 mm in width, and 10 mm in thickness. The length and width can be adjusted appropriately according to the actual situation of on-site processing. Finally, the hot-rolled martensitic steel plate is cooled to room temperature to form a hot-rolled martensitic steel plate.

[0056] (2) The steel plate was heated to 850°C in a muffle furnace and kept at this temperature for 30 min, and then quenched in water to room temperature;

[0057] (3) The steel plate prepared in (2) was sealed in a vacuum glass tube, heated to 450°C in a muffle furnace, and kept at this temperature for 48 h, and then quenched with water to room temperature;

[0058] (4) cold rolling the steel plate in (3) at room temperature to reduce the thickness by 50%;

[0059] (5) The steel plate in (4) is heated to 300° C. using a muffle furnace, and kept at this temperature for 10 min, and then quenched with water to room temperature; thus, a high-strength and high-toughness medium manganese steel containing bainite structure is obtained.

[0060] Tissue: The final room temperature tissue after treatment in Example 1 is as follows Figure 2 As shown, it contains martensite (~26%), retained austenite (~56%), bainite (~15%), and pearlite (~3%).

[0061] Mechanical properties test results, such as Figure 4 As shown, after the introduction of bainite, the yield strength of the steel plate of Example 1 is increased to 1950 MPa, and the total elongation is 5.9%.

[0062] Example 2

[0063] Alloy composition (mass percentage): 0.45% C, 12% Mn, 2% Al, 0.7% V, and the rest is Fe.

[0064] Preparation process: (1) Smelting is performed according to the mass percentage of the selected chemical composition to obtain a casting, and the casting is homogenized at 1200°C for 2 hours to dissolve VC, and the casting is preheated at 1100°C for 2 hours as the starting temperature of hot rolling, and then hot rolled to a thin plate. The final rolling temperature is not less than 850-900°C. Finally, the large pieces are machined and cut into small pieces. The specifications of the hot-rolled plate are 500 mm in length, 100 mm in width, and 10 mm in thickness. The length and width can be adjusted appropriately according to the actual situation of on-site processing. Finally, the hot-rolled martensitic steel plate is cooled to room temperature to form a hot-rolled martensitic steel plate.

[0065] (2) The steel plate was heated to 850°C in a muffle furnace and kept at this temperature for 30 min, and then quenched in water to room temperature;

[0066] (3) The steel plate in (2) was sealed in a vacuum glass tube, heated to 450°C in a muffle furnace, and kept at this temperature for 72 h, and then quenched with water to room temperature;

[0067] (4) cold rolling the steel plate in (3) at room temperature to reduce the thickness by 50%;

[0068] (5) The steel plate in (4) is heated to 300° C. using a muffle furnace, and kept at this temperature for 10 min, and then quenched with water to room temperature; thus, a high-strength and high-toughness medium manganese steel containing bainite structure is obtained.

[0069] Tissue: The final room temperature tissue after treatment in Example 2 is as follows Figure 3 As shown, it contains martensite (~21%), retained austenite (~29%), bainite (~45%), and pearlite (~5%).

[0070] Mechanical properties test results, such as Figure 5 As shown, Example 2 having the same nominal composition as Example 1 has a yield strength of 2030 MPa and a total elongation of 10.8% after the bainite isothermal time is changed. Compared with Example 1, the volume fraction of bainite is increased, which promotes further improvement of mechanical properties. Figure 6 This shows that the steel of Example 2 has excellent toughness while having extremely high yield strength.

[0071] in, Figure 6 Other samples mentioned in the article are from the following references:

[0072] [1]A.Kumar,A.Singh,Toughness dependence of nano-bainite on phasefraction and morphology,Mater.Sci.Eng.A 729(2018)439–443.

[0073] [2]H.K.D.H.Bhadeshia,C.Garcia-Mateo,P.Brown,Bainite steel and methodsof man-ufacture thereof,Us 20110126946A1.2(2011).

[0074] [3]A.F.Santacruz O.Rios-Diez,J.A.Jiménez,C.Garcia-Mateo,R.Aristizábal-Sierra,Microstructural and mechanical characterizationof ananostructured bainitic cast steel,Metals(Basel)10(2020)612.

[0075] [4]J.Yang,T.S.Wang,B.Zhang,F.C.Zhang,Microstructure and mechanicalproperties of high-carbon Si-Al-rich steel by low-temperature austempering,Mater.Des.35(2012)170–174.

[0076] [5]G.Gao,H.Zhang,X.Gui,P.Luo,Z.Tan,B.Bai,Enhanced ductility andtoughness in an ultrahigh-strength Mn-Si-Cr-C steel:the great potential ofultrafine filmy retained austenite,Acta Mater.76(2014)425–433.

[0077] [6]M.J.Peet,L.C.D.Fielding,A.A.Hamedany,M.Rawson,P.Hill,H.K.D.H.Bhadeshia,Strength and toughness of clean nanostructured bainite,Mater.Sci.Technol.(United Kingdom)(2017)0836.

[0078] The present invention describes preferred embodiments and their effects. However, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0079] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-strength and high-toughness bainitic medium-manganese steel, characterized in that: The following steps are involved: The ingot of medium manganese steel is prepared, the ingot is kept at 1150-1250°C for 2-4 hours, and then hot-rolled to a hot-rolled steel plate with a thickness of 10-12 mm; The hot-rolled steel plate is heated to the austenite phase transformation range temperature, and kept at this temperature for 30 to 60 minutes, and then water-quenched to room temperature to obtain a solution-treated steel plate; The solution-treated steel plate is heated to a bainite phase transformation temperature range and kept at that temperature for 48 to 72 hours, and then water-cooled to room temperature to obtain a bainite steel plate; The bainite steel sheet is cold-rolled at room temperature to reduce the thickness by 30 to 60% to obtain a cold-rolled steel sheet; The cold rolled steel plate is kept at 280-350°C for 10-20 minutes and then quenched with water to room temperature to obtain high strength and high toughness bainitic medium manganese steel.

2. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: The temperature of the austenite phase transformation range is 800-900°C.

3. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: The temperature of the bainite phase transformation range is 400-500°C.

4. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: The heating rate to the austenite transformation range is 3 to 8 K / s.

5. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: The heating rate to the temperature in the bainite transformation range is 3 to 8 K / s.

6. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: During hot rolling, the initial rolling temperature is 1050-1100°C, and the rolling termination temperature is not lower than 850-900°C.

7. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 1, characterized in that: When preparing the ingot of medium manganese steel, the following components are selected in mass percentage: C: 0.4-0.5%, Mn: 11-13%, Al: 1-3%, V: 0.6-0.8%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe.

8. The method for preparing high-strength and high-toughness bainite medium-manganese steel according to claim 7, characterized in that: The ingot of medium manganese steel is produced by weighing raw materials according to the mass percentage of chemical composition and smelting them.

9. A high-strength and high-toughness bainitic medium-manganese steel produced by the method according to any one of claims 1 to 8.

10. Use of the high-strength and high-toughness bainitic medium-manganese steel according to claim 9 in automobile manufacturing.

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