Low-si 1600mpa grade uncoated warm forming medium manganese steel and preparation method thereof

Abstract

The present application relates to a kind of low Si 1600MPa level free-coating warm forming medium manganese steel and preparation method, the chemical composition in steel is as follows by mass percent: C 0.20%~0.25%, Si≤0.1%, Mn 6%~9%, Al 0.2%~0.6%, Cr 3%~5%, the rest is Fe and inevitable impurity element.The warm forming process condition of the present application no longer uses Al-Si coating, and after austenitizing warm forming, tensile strength≥1600MPa, elongation A50≥7%.The medium manganese steel of the present application is designed with low Si content, and there is no need for atmosphere protection during warm forming.The oxidation increase is greatly reduced while ensuring that the oxidation layer is dense and not easy to fall off, and there is no need for shot blasting treatment after warm forming.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of high-strength steel, in particular to a low-Si 1600MPa-grade coating-free warm forming medium-manganese steel and a preparation method. BACKGROUND

[0002] The application of ultra-high-strength steel hot stamping components can reduce the weight of a whole vehicle while ensuring the strength and safety of the vehicle body, and is an important way to realize the light weight of the vehicle. The hot forming steel has a high ultimate tensile strength (UTS) of more than 1500MPa, is widely used in modern automobile parts, and can improve the anti-invasion ability and impact energy absorption. At present, the most widely used steel is 22MnB5 steel, which has a full martensite structure after hot forming. However, the 22MnB5 steel plate needs a protective Al-Si coating to prevent the formation of an oxide skin during the hot forming process at a high temperature of 850-1000 DEG C. In addition, during the heating in the furnace, the residues falling from the Al-Si coating accumulate on the rollers, causing the movement of the slab to be difficult and even the rollers to be broken. Moreover, when the Al-Si coating becomes hard and brittle after cooling, cracks are easily generated. Therefore, people strongly hope to have an economical alternative method to prevent the oxidation of the hot forming steel. At present, the coating-free hot forming steel appears on the market, and due to the easy oxidation and decarburization during heating, the performance of the steel is affected, and the additional atmosphere protection and shot blasting treatment will increase the cost and process.

[0003] The patent document with the publication number CN103255340B proposes a high-strength and high-toughness hot forming steel plate for automobiles and a preparation method thereof. The chemical composition of the steel plate includes C: 0.1-0.5%, Si: 0.5-1.5%, Mn: 1.2-2.4%, Ti: 0.01-0.05%, B: 0.001-0.005%, S: ≤0.01%, and P: ≤0.01%. The tensile strength of the steel plate after hot forming reaches 1600MPa, and the elongation reaches 16%. However, the steel plate needs to be deformed during the heating process, and then needs to be quenched twice. The hot forming process is complex, and at present, it cannot be realized by the existing equipment. In addition, the heating process needs gas protection, and the steel plate needs to be subjected to shot blasting treatment after hot forming. Moreover, the steel grade adds a high Si element, which is not conducive to the welding performance and surface quality control.

[0004] The patent document with the publication number CN104846274A realizes the reduction of the austenitizing heating temperature to 780 DEG C based on the carbon and manganese components in the steel, so that the galvanized steel plate can prevent the liquefaction and serious oxidation of zinc in the hot stamping process of the hot stamping steel, and avoid the cracking caused by liquid zinc. The method uses the galvanizing or other coating treatment process to prevent oxidation during the hot stamping process of the steel, improves the composition, and has a certain adverse effect on the die.

[0005] The patent document with publication number CN110643909A proposes an antioxidant ultra-high strength steel sheet for hot stamping forming and a low-temperature hot forming process thereof. The chemical composition of the steel sheet is C: 0.20-0.35%, Mn: 6-8%, Cr: 2-5%, Nb+V: 0.05-0.3%, Si: 0.3-0.8%, Al: 0.1-0.8%, and the balance Fe. After heating and forming at 780-900℃, the steel sheet has an oxidation weight gain of ≤6.5g / m 2 , an oxidation layer thickness of ≤18.7μm, a tensile strength of ≥1880MPa after forming, and a total elongation of ≥14%. To obtain better high-temperature oxidation resistance, a large amount of Si element is added to the chemical composition, which adversely affects the welding performance and is not conducive to obtaining good surface quality.

[0006] To solve these problems, the present application discloses a low-Si content tensile strength exceeding 1600MPa free-coating warm forming medium manganese steel, which has good high-temperature oxidation resistance and a dense oxidation layer on the surface of the component that is not easy to fall off, etc. Therefore, no atmosphere protection is required during warm forming, and no shot blasting treatment is required after forming. SUMMARY

[0007] The purpose of the present application is to provide a low-Si 1600MPa grade free-coating warm forming medium manganese steel and a preparation method. In the warm forming process conditions of the present application, Al-Si coating is no longer used, and the tensile strength after austenitizing warm forming is ≥1600MPa, and the elongation A50 is ≥7%. The warm forming medium manganese steel of the present application uses low-Si content design, no atmosphere protection is required during warm forming, the oxidation gain is greatly reduced, and the oxidation layer has the characteristics of being dense and not easy to fall off, and no shot blasting treatment is required after warm forming.

[0008] To achieve the above-mentioned purposes, the present application adopts the following technical solutions:

[0009] The low-Si 1600MPa grade free-coating warm forming medium manganese steel has the following chemical composition in mass percentage: C 0.20%-0.25%, Si≤0.1%, Mn 6%-9%, Al 0.2%-0.6%, Cr 3%-5%, and the balance Fe and unavoidable impurity elements.

[0010] The effects of alloying elements are as follows:

[0011] C: As an austenite stabilizing element in medium manganese steel, it can effectively expand the austenite phase region and improve the volume fraction and stability of austenite in medium manganese steel. Increasing the content of C in medium manganese steel can effectively improve the tensile strength and elongation of the material, but too high carbon content can easily form hard and brittle phase cementite in the austenite reverse transformation annealing process of medium manganese steel, which becomes a crack initiation and expansion channel during tension, thereby deteriorating the mechanical properties. The selected carbon content range of the present application is 0.2-0.25%, and too high carbon content seriously deteriorates the welding performance of medium manganese steel.

[0012] Al: As a ferrite stabilizing element, it can increase the Ms point temperature and effectively improve the high temperature oxidation resistance of medium manganese steel. The selected Al content range of the present application is Al 0.2%-0.6%, and too much Al element added in medium manganese steel is not conducive to the increase of austenite content.

[0013] Si: As a ferrite stabilizing element, in the research of medium manganese steel, Si and Al have similar effects to some extent, both can inhibit the formation of cementite, thereby improving the stability of austenite. At the same time, the addition of Si element can improve the high temperature oxidation resistance of medium manganese steel, but too high Si content will affect the welding performance and increase the difficulty of pickling, therefore, the Si content is controlled to ≤0.1% in the present application.

[0014] Mn: As an austenite stabilizing element, it can expand the austenite phase region, increase the volume fraction, stability and stacking fault energy of austenite in medium manganese steel. At the same time, with the increase of Mn content, the A3 point temperature of medium manganese steel is reduced, which promotes the formation of austenite at low temperature, and can realize quenching treatment at relatively low temperature, Mn 6%-9%.

[0015] Cr: As a ferrite stabilizing element, it can significantly improve the ductile-brittle transition temperature of medium manganese steel and improve the hardenability of medium manganese steel. A small amount of Cr added in medium manganese steel can significantly improve the oxidation resistance of steel and increase the corrosion resistance of steel. In order to obtain excellent high temperature oxidation resistance, the Cr content is controlled to 3%-5% in the present application.

[0016] The tensile strength of the formed steel is more than 1600MPa, and the elongation A50 is greater than 7%. The thickness of the oxidation layer is less than 3μm.

[0017] The preparation method of low Si 1600MPa grade free-coating warm forming medium manganese steel comprises:

[0018] 1) The continuous casting blank is heated to 1150-1250℃, and after heat preservation for 2-3h, hot rolling is carried out, the opening rolling temperature is 1100-1150℃, and the final rolling temperature is 850-1000℃, and then air cooling to room temperature to obtain a hot rolled plate with a thickness of 3mm;

[0019] 2) hot-rolled steel strip is cut into structural blank, heated to 780-850 DEG C in heating furnace, and kept for 3-10 min, so that the structural blank is completely austenitized;

[0020] 3) then the blank after the heating treatment is transferred to a stamping die, stamping forming is carried out in the range of 720-800 DEG C, the stamping part is kept with the die, quenching cooling rate is greater than or equal to 10 DEG C / s, the keeping time is 15-30 s, and the target geometric shape of the quenched member is obtained;

[0021] 4) after the member is cooled to room temperature, it is heated to 170-200 DEG C again, kept for 15-20 min, the baking hardening process is completed, and finally the high-temperature-oxidation-resistant high-strength-ductility structural member is obtained.

[0022] The room-temperature structure of the warm forming stamping part is typical martensite + residual austenite + a small amount of carbide structure.

[0023] Compared with the prior art, the beneficial effects of the present application are:

[0024] 1) the steel does not need to add micro-alloying elements, does not need to be protected by any anti-oxidation coating under the warm forming process condition, the tensile strength after forming is more than 1600 MPa, and the elongation A50 is greater than 7%.

[0025] 2) through the low Si content design, the member can still have high-temperature-oxidation resistance, and the surface oxidation layer is more dense and not easy to fall off.

[0026] 3) under the condition of higher warm forming temperature (780 DEG C), high strength and ductility can still be obtained, and the high-temperature-oxidation resistance is more excellent, and the oxidation layer thickness is less than 3 mu m. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is the warm forming process flow chart of the present application.

[0028] Figure 2 It is the metallographic structure chart of the hot-rolled example 1.

[0029] Figure 3 It is the structure chart of the warm forming of example 1.

[0030] Figure 4 It is the mechanical property chart of the warm forming of example 1.

[0031] Figure 5 It is the oxidation layer surface morphology chart of the warm forming of example 1.

[0032] Figure 6 It is the oxidation layer thickness chart of the warm forming of example 1. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application. Obviously, the described embodiments are only used as examples, and are not used to limit the present application.

[0034] The chemical components of the steels in the examples are shown in Table 1, the production processes of the examples are shown in Table 2, and the mechanical properties of the manganese steels in the warm forming of the examples are shown in Table 3.

[0035] Table 1 Chemical components of steels in examples (wt%)

[0036] Examples C Si Mn Al Cr 1 0.18 0.11 7.9 0.41 3.3 2 0.19 0.11 8.2 0.42 3.2 3 0.22 0.12 8.8 0.58 3.3 4 0.24 0.10 6.3 0.23 4.8

[0037] Table 2 Production processes of examples

[0038]

[0039]

[0040] Table 3 Mechanical properties of manganese steels in warm forming of examples

[0041]

[0042] The above description is only the preferred embodiments of the present application, but the protection scope of the present application is not limited to this. Any skilled person in the art, according to the technical solutions and the inventive concept of the present application, makes equivalent replacements or changes within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. Low-Si 1600MPa grade uncoated high-temperature forming medium-manganese steel, characterized in that, The chemical composition of the steel, by mass percentage, is as follows: C 0.20%–0.25%, Si ≤ 0.1%, Mn 6%–9%, ​​Al 0.2%–0.6%, Cr 3%–5%, with the remainder being Fe and unavoidable impurity elements; The method for preparing low-Si 1600MPa grade uncoated high-temperature forming medium-manganese steel includes: 1) The continuously cast billet is heated to 1150-1250℃, held for 2-3 hours and then hot rolled. The initial rolling temperature is 1100-1150℃ and the final rolling temperature is 850-1000℃. After rolling, it is air-cooled to room temperature. 2) Cut the hot-rolled steel strip into structural blanks, heat them in a heating furnace to 780℃~850℃, and hold for 3~10 minutes; 3) The heat-treated blank is then transferred to the stamping die and stamped in the range of 720℃~800℃. The stamped part is then held under pressure and quenched with the die. The quenching cooling rate is ≥10℃ / s and the holding time is 15s-30s. 4) After the components have cooled to room temperature, reheat them to 170℃~200℃ and hold for 15~20 minutes; Al-Si coating is not used under warm forming process conditions; After forming, the tensile strength of the steel plate exceeds 1600 MPa, and the elongation (A50) is greater than 7%. The room temperature microstructure of warm-formed stamped parts consists of martensite, retained austenite, and a small amount of carbides.

2. The low-Si 1600MPa grade uncoated high-temperature forming medium-manganese steel according to claim 1, characterized in that, The oxide layer thickness is less than 3 μm.

3. The method for preparing low-Si 1600MPa grade uncoated high-temperature forming medium-manganese steel as described in claim 1 or 2, characterized in that, include: 1) The continuously cast billet is heated to 1150-1250℃, held for 2-3 hours and then hot rolled. The initial rolling temperature is 1100-1150℃ and the final rolling temperature is 850-1000℃. After rolling, it is air-cooled to room temperature. 2) Cut the hot-rolled steel strip into structural blanks, heat them in a heating furnace to 780℃~850℃, and hold for 3~10 minutes; 3) The heat-treated blank is then transferred to the stamping die and stamped in the range of 720℃~800℃. The stamped part is then held under pressure and quenched with the die. The quenching cooling rate is ≥10℃ / s and the holding time is 15s-30s. 4) After the components have cooled to room temperature, they are reheated to 170℃~200℃ and held for 15~20 minutes.

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