Hydrogen embrittlement resistant heterostructure medium manganese steel and preparation method thereof

Abstract

The invention discloses a hydrogen embrittlement-resistant heterostructure medium manganese steel and a preparation method thereof, and the medium manganese steel comprises the following components in percentage by mass: 0.05-0.3% of C; 3 to 12% of Mn; 0-8% of Al; 0.05% to 0.3% of Nb; 0.05% to 0.1% of V; 0.02-0.4% of Mo, and the balance Fe and inevitable impurities. According to the method, a gradient structure is introduced by adopting high-differential-ratio rolling and passes through Qamp; p, heat treatment to regulate and control the austenite stability and structure. According to the method, rolling parameters and Qamp are adjusted; and P treatment temperature and time are adopted, so that multi-scale and gradient distribution and film-shaped retained austenite are formed, the toughness and crack propagation resistance of the material are enhanced, and the capture and inhibition effects of hydrogen atoms in the material are effectively improved. The mechanical property and the hydrogen embrittlement resistance of the medium manganese steel are synchronously improved.

Description

Technical Field

[0001] The invention relates to the technical field of advanced high-strength steel, and in particular to a hydrogen embrittlement-resistant heterogeneous medium manganese steel and a preparation method thereof. Background Art

[0002] Lightweighting of automobiles has become an important trend in the development of the world's automobile industry. Increasing the proportion of high-strength steel is one of the key technologies to achieve both lightweighting and improved collision safety. With the increasing demand for high-performance automotive steel in the automotive industry, hot stamping technology has developed rapidly. The most widely used hot stamping parts in the automotive industry are chassis components, such as A-pillars, B-pillars, bumpers, roof rails, etc. Currently, more than 80% of the body frames can be produced by hot stamping of high-strength steel. However, as the strength of steel increases, the sensitivity to hydrogen-induced delayed fracture usually increases significantly, which has become an important factor restricting the promotion and application of high-strength grade steels in industrial fields such as automobiles.

[0003] At present, the research on the hydrogen embrittlement resistance of medium manganese steel mainly focuses on direct heat treatment after ordinary rolling to change the content and stability of austenite, so as to improve its hydrogen embrittlement resistance. The microstructure morphology of medium manganese steel prepared by the existing process is equiaxed, which is not conducive to the mechanical stability of the retained austenite in a large range, causing a large amount of retained austenite to undergo martensitic transformation in a small strain range, thereby causing a large number of hydrogen atoms in the austenite to gather at the grain boundaries, increasing the hydrogen embrittlement tendency of the medium manganese steel, and therefore the prepared medium manganese steel has poor hydrogen embrittlement resistance. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a hydrogen embrittlement resistant heterogeneous structure medium manganese steel and a preparation method thereof.

[0005] The technical solution adopted by the present invention is:

[0006] A hydrogen embrittlement resistant heterogeneous structure medium manganese steel, the composition of which is, by mass percentage, 0.05-0.3% C, 3-12% Mn, 0-8% Al, 0.05-0.3% Nb, 0.05-0.1% V, 0.02-0.4% Mo, and the rest is Fe and unavoidable impurities.

[0007] Furthermore, tensile performance tests were carried out according to the STM-E8-E8M standard and a tensile rate of 1.5 mm / min. The tensile strength of the manganese steel in the novel hydrogen embrittlement-resistant heterogeneous structure was 1028-1115 MPa, the yield strength was 813-863 MPa, and the elongation was 33.1-36.2%.

[0008] The method for preparing any one of the above novel hydrogen embrittlement resistant heterogeneous structure manganese steels comprises the following steps:

[0009] (1) Proportioning, smelting and casting according to the composition to obtain a steel ingot;

[0010] (2) heating the obtained steel ingot to 1100° C. to 1200° C., keeping the temperature for 3 to 4 hours, and forging it into a steel billet;

[0011] (3) subjecting the obtained steel billet to multiple passes of high differential speed ratio warm rolling at 500° C. to 700° C. and a rolling speed ratio of the upper and lower rolls of ≥4 to obtain a warm rolled plate;

[0012] (4) subjecting the obtained warm rolled sheet to high differential speed ratio cold rolling under the condition that the rolling speed ratio of the upper and lower rollers is ≥3 to obtain a cold rolled sheet;

[0013] (5) The obtained cold-rolled sheet is heated to the austenite region and kept warm to be completely austenitized, and then quenched and partitioned at a temperature of 100 to 350° C., and then cooled to room temperature to obtain a new type of hydrogen embrittlement-resistant heterogeneous medium manganese steel.

[0014] Furthermore, in step (3), the number of high differential speed ratio warm rolling passes is 6 to 7 times, and the total reduction rate is 60% to 70%, mainly to ensure that the grains are gradually refined, the stress distribution is more uniform, and excessive work hardening and crack formation are prevented.

[0015] Furthermore, in step (4), the reduction rate of the high differential speed ratio cold rolling is 85% to 95%, mainly to refine the grains and reduce cracks caused by internal stress accumulation.

[0016] Furthermore, in step (5), the holding time of quenching and partitioning is 10 to 60 minutes. Within this temperature and time range, a large amount of retained austenite can be generated and a film-like retained austenite can be generated, thereby improving its hydrogen embrittlement resistance.

[0017] Beneficial effects of the present invention: The present invention introduces a gradient structure by performing high differential speed ratio rolling, and obtains a film-like residual austenite in combination with the QP treatment process, so as to achieve the purpose of simultaneously improving the mechanical properties and hydrogen embrittlement resistance of medium manganese steel. The high differential speed ratio rolling technology can produce a "rubbing zone" on the workpiece. Due to the effect of external friction, the upper and lower surfaces of the rubbing zone are in opposite directions, resulting in severe shear deformation, which breaks the grains in the material and refines the grain size of the material. Grain refinement increases the density of grain boundaries. As a weak trap for hydrogen, the grain boundary can effectively capture diffused hydrogen atoms and reduce the aggregation of hydrogen into the interior of the grain, thereby reducing the sensitivity to hydrogen embrittlement. In terms of heat treatment process, medium manganese steel is easy to form a film-like residual austenite after Q&P treatment. The film-like residual austenite has high mechanical stability, is difficult to undergo martensitic phase transformation under large strains, and has strong resistance to hydrogen embrittlement.

[0018] The new type of hydrogen embrittlement resistant heterogeneous structure medium manganese steel prepared by the above method is subjected to tensile performance test according to STM-E8-E8M standard and 1.5mm / min tensile rate, and its tensile strength is 1028-1115MPa, yield strength is 813-863MPa, and elongation is 35.8-37.4%. After hydrogen charging, the tensile strength is 1033-1098MPa, the yield strength is 811-856MPa, and the elongation is 27.3-31.3%, which has strong hydrogen embrittlement resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1(a) shows the microstructure morphology of the surface layer of the medium manganese steel after high differential speed ratio rolling in Example 1.

[0020] Figure 1(b) shows the microstructure of the core of the medium manganese steel after high differential speed ratio rolling in Example 1.

[0021] Figure 2 This is the microstructure morphology of the manganese steel in the hydrogen embrittlement resistant heterogeneous structure obtained after Q&P treatment in Example 1. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solution and advantages of the present invention more clear, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and a preferred implementation manner.

[0023] Example 1

[0024] A hydrogen embrittlement resistant heterogeneous structure medium manganese steel, the components of which are as follows by mass percentage: 0.3% C, 7.0% Mn, 0.05% Nb, 0.05% V, 0.02wt.% Mo, and the remainder Fe and unavoidable impurities.

[0025] The preparation steps of the above-mentioned medium manganese steel include:

[0026] Step 1, smelting: in a vacuum induction furnace, smelting and casting are performed to obtain steel ingots according to the chemical composition ratio of medium manganese steel.

[0027] Step 2, forging: heat the steel ingot to 1100°C, keep it warm for 3 hours, and forge it into a steel billet with a cross-sectional area of ​​100mm×40mm.

[0028] Step 3, high differential speed ratio warm rolling: heat the steel billet to 800°C, keep it warm for 2 hours, and perform 6 passes of high differential speed ratio warm rolling. The rolling speed ratio of the upper and lower rolling mills is 4, the rolling temperature range is 500°C to 700°C, and then air-cool it to room temperature to obtain a 16mm thick warm-rolled plate.

[0029] Step 4, high differential speed ratio cold rolling: the high differential speed ratio warm rolled plate is passed through a cold rolling mill for cold rolling with a reduction rate of 85%, wherein the rolling speed ratio of the upper and lower rolling mill rolls is 3, to obtain a cold rolled plate.

[0030] By means of wire cutting, the experimental steel after high differential ratio rolling is processed into hot tensile specimens according to ASTM-E8-E8M standard and along the rolling direction, and then the tensile performance test is carried out at a tensile rate of 1.5mm / min. In this embodiment, the tensile strength of the medium manganese steel after high differential ratio rolling is 912MPa, the yield strength is 746MPa, and the elongation is 32.6%. The microstructure of the medium manganese steel after high differential ratio rolling is shown in Figure 1 (a) and Figure 1 (b).

[0031] It can be seen from Figure 1(a) and Figure 1(b) that after high differential speed ratio rolling, the grain size of the surface layer of the experimental steel becomes finer and an obvious gradient structure appears.

[0032] Step 5, Q&P treatment: The cold-rolled sheet obtained in step 4 is heated to 1150°C and kept at this temperature for 60 minutes to be completely austenitized, then cooled to 150°C for quenching and partitioning, and kept at this temperature for 30 minutes, and then water-cooled to room temperature to obtain the experimental steel. The microstructure of the high differential ratio rolled medium manganese steel after Q&P treatment is shown in Figure 2. Figure 2 shown.

[0033] pass Figure 2 It can be seen that film-like retained austenite appears in the experimental steel after Q&P treatment.

[0034] The obtained experimental steel was processed into hot tensile specimens along the rolling direction according to ASTM-E8-E8M standard by wire cutting, and then the tensile performance test was carried out at a tensile rate of 1.5 mm / min. The tensile strength of the experimental steel was 1028 MPa, the yield strength was 813 MPa, and the elongation was 36.2%.

[0035] The obtained experimental steel was hydrogenated in 0.1mol / L sodium hydroxide (NaOH) + 0.5g / L thiourea (CH4N2S) aqueous solution at 800℃ for 1h, and then the hydrogenated experimental steel was processed into hot tensile specimens along the rolling direction according to ASTM-E8-E8M standard by wire cutting, and then the tensile performance test was carried out at a tensile rate of 1.5mm / min. After hydrogenation, the tensile strength of the experimental steel was 1033MPa, the yield strength was 821MPa, and the elongation was 30.1%;.

[0036] Example 2

[0037] A hydrogen embrittlement resistant heterogeneous structure medium manganese steel, the components of which are as follows by mass percentage: C is 0.05%, Mn is 11.5%, Al is 7.5%, Nb is 0.25%, V is 0.09%, Mo is 0.3wt.%, and the balance is Fe and unavoidable impurities.

[0038] The preparation steps of the above-mentioned medium manganese steel include:

[0039] Step 1, smelting: in a vacuum induction furnace, smelting and casting are performed according to the chemical composition ratio of medium manganese steel to obtain steel ingots.

[0040] Step 2, forging: heat the steel ingot to 1100°C, keep it warm for 3 hours, and forge it into a steel billet with a cross-sectional area of ​​100mm×40mm.

[0041] Step 3, high differential speed ratio warm rolling: heat the billet to 800°C, keep it warm for 2 hours, and perform 6 passes of high differential speed ratio warm rolling. The rolling speed ratio of the upper and lower rolling mills is 5, the rolling temperature range is 500°C to 700°C, and then air-cool it to room temperature to obtain a 16mm thick warm-rolled plate.

[0042] Step 4, high differential speed ratio cold rolling: the high differential speed ratio warm rolled plate is passed through a cold rolling mill for cold rolling with a reduction rate of 85%, wherein the rolling speed ratio of the upper and lower rolling mill rolls is 4, to obtain a cold rolled plate.

[0043] By wire cutting, the experimental steel after high differential ratio rolling is processed into hot tensile specimens according to ASTM-E8-E8M standard and along the rolling direction, and then the tensile performance test is carried out at a tensile rate of 1.5mm / min. In this implementation case, the tensile strength of the medium manganese steel after high differential ratio rolling is 986MPa, the yield strength is 767MPa, and the elongation is 33.1%.

[0044] Step 5, Q&P treatment: The cold-rolled sheet obtained in step 4 was heated to 1250°C and kept at this temperature for 60 min to be completely austenitized, then cooled to 200°C for quenching and partitioning, and kept at this temperature for 30 min, and then water-cooled to room temperature to obtain experimental steel.

[0045] The obtained experimental steel was processed into hot tensile specimens along the rolling direction according to ASTM-E8-E8M standard by wire cutting, and then the tensile performance test was carried out at a tensile rate of 1.5mm / min. The tensile strength of the experimental steel was 1079MPa, the yield strength was 863MPa, and the elongation was 37.4%

[0046] The obtained experimental steel was hydrogenated in a 0.1 mol / L sodium hydroxide (NaOH) + 0.5 g / L thiourea (CH4N2S) aqueous solution at 800°C for 1 hour, and then the hydrogenated experimental steel was processed into hot tensile specimens along the rolling direction according to the ASTM-E8-E8M standard by wire cutting, and then the tensile properties were tested at a tensile rate of 1.5 mm / min. After hydrogenation, the tensile strength of the experimental steel was 1070 MPa, the yield strength was 858 MPa, and the elongation was 31.3%.

[0047] Example 3

[0048] A hydrogen embrittlement resistant heterogeneous structure medium manganese steel, the composition of which is calculated by mass percentage: C is 0.2%, Mn is 3.5%, Al is 5%, Nb is 0.15%, V is 0.07%, Mo is 0.09wt.%, and the balance is Fe and inevitable impurities.

[0049] The preparation steps of the above-mentioned medium manganese steel include:

[0050] Step 1, smelting: in a vacuum induction furnace, smelting and casting are performed to obtain steel ingots according to the chemical composition ratio of medium manganese steel.

[0051] Step 2, forging: heat the steel ingot to 1100°C, keep it warm for 3 hours, and forge it into a steel billet with a cross-sectional area of ​​100mm×40mm.

[0052] Step 3, high differential speed ratio warm rolling: heat the steel billet to 800°C, keep it warm for 2 hours, and perform 6 passes of high differential speed ratio warm rolling. The rolling speed ratio of the upper and lower rolling mills is 4, the rolling temperature range is 500°C to 700°C, and then air-cool it to room temperature to obtain a 16mm thick warm-rolled plate.

[0053] Step 4, high differential speed ratio cold rolling: the high differential speed ratio warm rolled plate is passed through a cold rolling mill for cold rolling with a reduction rate of 85%, wherein the rolling speed ratio of the upper and lower rolling mill rolls is 5, to obtain a cold rolled plate.

[0054] By wire cutting, the experimental steel after high differential ratio rolling is processed into hot tensile specimens according to ASTM-E8-E8M standard and along the rolling direction, and then the tensile performance test is carried out at a tensile rate of 1.5mm / min. In this implementation case, the tensile strength of the medium manganese steel after high differential ratio rolling is 953MPa, the yield strength is 712MPa, and the elongation is 31.4%.

[0055] Step 5, Q&P treatment: The cold-rolled sheet obtained in step 4 was heated to 925°C and kept at this temperature for 60 min to be completely austenitized, then cooled to 250°C for quenching and partitioning, and kept at this temperature for 60 min, and then water-cooled to room temperature to obtain experimental steel.

[0056] The obtained experimental steel was processed into hot tensile specimens along the rolling direction according to ASTM-E8-E8M standard by wire cutting, and then the tensile performance test was carried out at a tensile rate of 1.5mm / min. The tensile strength of the experimental steel was 1115MPa, the yield strength was 816MPa, and the elongation was 35.8%

[0057] The obtained experimental steel was hydrogenated in a 0.1 mol / L sodium hydroxide (NaOH) + 0.5 g / L thiourea (CH4N2S) aqueous solution at 800°C for 1 hour, and then the hydrogenated experimental steel was processed into hot tensile specimens along the rolling direction according to the ASTM-E8-E8M standard by wire cutting, and then the tensile properties were tested at a tensile rate of 1.5 mm / min. After hydrogenation, the tensile strength of the experimental steel was 1098 MPa, the yield strength was 811 MPa, and the elongation was 27.3%.

[0058] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications are also within the protection scope of the present invention.

Claims

1. A medium manganese steel with a hydrogen embrittlement resistant heterogeneous structure, characterized in that: In terms of mass percentage, its composition is: 0.05-0.3% C; 3-12% Mn; 0-8% Al; 0.05-0.3% Nb; 0.05-0.1% V; 0.02~0.4% Mo, the rest is Fe and unavoidable impurities.

2. The hydrogen embrittlement resistant heterogeneous structure medium manganese steel according to claim 1, characterized in that: According to the tensile performance test conducted in accordance with the STM-E8-E8M standard and a tensile rate of 1.5 mm / min, the manganese steel in the novel hydrogen embrittlement resistant heterogeneous structure has a tensile strength of 1028-1134 MPa, a yield strength of 753-813 MPa, and an elongation of 33.4-39.3%.

3. The method for preparing a hydrogen embrittlement resistant heterogeneous structure manganese steel according to claim 1 or 2, characterized in that: The following steps are involved: (1) Proportioning, smelting and casting according to the composition to obtain a steel ingot; (2) heating the obtained steel ingot to 1100° C. to 1200° C., keeping the temperature for 3 to 4 hours, and forging it into a steel billet; (3) subjecting the obtained steel billet to multiple passes of high differential speed ratio warm rolling at 500° C. to 700° C. and a rolling speed ratio of the upper and lower rolls of ≥4 to obtain a warm rolled plate; (4) subjecting the obtained warm rolled sheet to high differential speed ratio cold rolling under the condition that the rolling speed ratio of the upper and lower rollers is ≥3 to obtain a cold rolled sheet; (5) The obtained cold-rolled sheet is heated to the austenite region and kept warm to be completely austenitized, and then quenched and partitioned at a temperature of 100 to 350° C., and then cooled to room temperature to obtain a new type of hydrogen embrittlement-resistant heterogeneous medium manganese steel.

4. The preparation method according to claim 3, characterized in that: In step (3), the number of high differential speed ratio warm rolling passes is 6 to 7 times, and the total reduction rate is 60% to 70%.

5. The preparation method according to claim 3, characterized in that: In step (4), the reduction rate of high differential speed ratio cold rolling is 85% to 95%.

6. The preparation method according to claim 3, characterized in that: In step (5), the holding time of quenching and partitioning is 10 to 60 minutes.

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