A low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel and its preparation method and application

Abstract

The present invention belongs to the technical field of automobile steel preparation, and specifically relates to a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel and its preparation method and application. The method comprises the following steps: 1) preparing a steel billet through raw materials; 2) forging the steel billet obtained in step 1); 3) hot rolling the forged steel billet; 4) keeping the obtained steel at high temperature, then quenching it to room temperature, and then tempering it for carbon partitioning to obtain a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel. The structure of the hot-rolled quenched and tempered partitioned steel obtained by the process of the present invention is a multiphase structure of high-temperature ferrite, martensite and residual austenite, and its density is 7.24g / cm 3 , which is 7.8g / cm2 higher than ordinary automobile steel 3 It is reduced by about 7%, its tensile strength is ≥1200MPa, and its elongation is ≥9%.

Description

Technical Field

[0001] The invention belongs to the technical field of automobile steel preparation, and specifically relates to a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel and a preparation method and application thereof. Background Art

[0002] With the rapid development of the automobile industry, its lightweight and safety have received more and more attention and higher requirements. Therefore, high-strength steel materials that can reduce vehicle weight, improve fuel efficiency and reduce carbon dioxide emissions have gradually become the focus of people's attention.

[0003] As a representative of the third generation of high-strength steel, quenching and partitioning (Q&P) steel has excellent mechanical properties due to its TRIP effect and is widely used in the automotive industry. In recent years, a lot of research has been done on its process and microstructure performance at home and abroad. It includes first heating the steel to near Ac3 for annealing, then quenching to between Ms and Mf (generally higher than room temperature), keeping it at the quenching temperature (one-step process) or above the quenching temperature (two-step process), and allowing carbon to partition from the supersaturated martensite to the retained austenite during the holding process to improve its stability at room temperature and obtain as much retained austenite as possible. In order to improve the TRIP effect, the main function of the heat treatment of traditional Q&P steel is to obtain a suitable volume fraction of retained austenite and improve its stability. The quenching temperature (QT) has a decisive influence on the microstructure and performance of the steel and is a very critical and strict parameter. However, it is very difficult to control the temperature of the quenching medium in an industrial environment, and the fluctuation of the quenching temperature can easily affect the microstructure and performance of Q&P steel. For example, the Chinese patent document with publication number CN 110093559A discloses "Quenching-tempering partitioned steel plate and preparation method based on carbon-manganese synergistic partitioning". Through reasonable alloy composition design, the Mf temperature of steel is reduced to below room temperature and the Ms is above room temperature. This makes it no longer necessary to control the temperature of the quenching medium during the heat treatment process in an experimental or industrial environment. Instead, a large amount of residual austenite can be obtained by directly quenching it to room temperature, which greatly simplifies the heat treatment process and saves energy. However, the density and strength of the steel mentioned therein cannot meet the increasingly high lightweight and safety requirements for automotive steel.

[0004] At present, in most low-density high-strength steels, the increase in density comes at the expense of strength, and it is not possible to maintain high strength while reducing density. However, low density and high strength are two important evaluation factors for the development of future automotive steel. From the current published patents, it is not possible to achieve a density lower than 7.24g / cm 3 , and a method for preparing and heat treating TRIP steel with a strength greater than 1200MPa. Summary of the invention

[0005] In view of the demand for automobile steel, the purpose of the present invention is to obtain low-density, high-strength, and low-cost automobile steel. To this end, the present invention proposes a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel and its preparation method and application. The structure of the hot-rolled quenched and tempered partitioned steel obtained by the process of the present invention is a multiphase structure of high-temperature ferrite, martensite and residual austenite, and its density is 7.24g / cm 3 , which is 7.8g / cm2 higher than ordinary automobile steel 3 It is reduced by about 7%, its tensile strength is ≥1200MPa, and its elongation is ≥9%.

[0006] The technical solution provided by the present invention is as follows:

[0007] A method for preparing a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel comprises the following steps:

[0008] 1) preparing a steel billet from a raw material, wherein the steel billet comprises the following chemical components in mass percentage: C: 0.25-0.35%, Si: 0.20-1.60%, Mn: 4.50-5.50%, Al: 4.00-8.00%, Cr: 0.20-1.10%, Ni: 0.20-1.60%, Mo: 0.20-0.30%, Nb: 0.02-0.03%, P≤0.010%, S≤0.002%, and the balance is Fe and unavoidable impurities;

[0009] 2) forging the steel billet obtained in step 1);

[0010] 3) hot rolling the forged steel billet;

[0011] 4) The obtained steel is kept at high temperature, then quenched to room temperature, and then tempered for carbon partitioning to obtain a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel.

[0012] In the above technical solution:

[0013] The low-density steel of the present invention is based on C-Si-Mn-Al as the main alloying element, wherein C plays a role of solid solution strengthening in steel, promotes austenite formation and increases the stability of residual austenite, but the increase of C content will cause excessive precipitation of steel carbides and poor welding performance. Considering the comprehensive performance of steel, the C content in the steel of the present invention is 0.25-0.35%. Mn can infinitely solid dissolve with Fe to improve the strength of steel, and is also an austenite stabilizing element, ensuring good plasticity and increasing the hardenability of steel, but too high Mn content will introduce segregation in steel, so the content of Mn is 4.50-5.50%. Al is a lightweight element, and the density of steel will be reduced by about 1.3% for every 1% of Al added, but too high Al content will cause delta ferrite to exist stably at room temperature, and a large amount of coarse dendrite delta ferrite and microsegregation will seriously reduce the mechanical properties of the material, so the content of Al is 4.00-8.00%. Adding Si element can assist Al in reducing the density of steel, inhibit the formation of cementite, and can also improve the yield strength through solid solution strengthening, but excessive Si will affect the surface quality of steel, so the content of Si is 0.20-1.60%. The present invention also adds alloy elements Cr, Ni and Mo to improve the performance of steel. Considering the cost of steel, the content of Cr, Ni and Mo is the best at 0.20-0.30%, and 0.02-0.03% Nb is added to refine the grains and disperse carbides.

[0014] The above technical solution significantly reduces the material density while achieving a strength of 1200Mpa.

[0015] Preferably, the steel billet contains the following chemical components in mass percentage: C: 0.25-0.35%, Si: 1.40-1.60%, Mn: 4.50-5.50%, Al: 4.00-5.00%, Cr: 0.20-0.30%, Ni: 0.20-0.30%, Mo: 0.20-0.30%, Nb: 0.02-0.03%, P≤0.005%, S≤0.002%, and the balance is Fe and unavoidable impurities.

[0016] Specifically, in step 1), the raw materials are melted in a vacuum induction furnace, subjected to electroslag remelting treatment, and mold-casted in a vacuum or protective atmosphere to obtain a steel billet.

[0017] Specifically, in step 2), the steel billet is heated to 1100-1150°C, kept in vacuum or protective atmosphere for 1.5-2.5h, the initial forging temperature is 1050-1100°C, three-fire forging is performed, each fire forging ratio is greater than 2, and the strain rate is 0.1-1s -1 The final forging temperature is >850℃. After forging, it is air-cooled and solution treated at 1020℃~1150℃ in vacuum or protective atmosphere for 1~2h. Then it is water-cooled to room temperature and aged at 525~535℃ for 9~12h.

[0018] Preferably, in step 2): the mixture is kept warm for 2 hours in a vacuum or protective atmosphere; water-cooled to room temperature and subjected to aging treatment at 530° C. for 9 to 12 hours.

[0019] Based on the above casting and forging process, the original austenite grains can be refined, the initial dislocation density can be increased, and the strength of the steel can be improved.

[0020] Specifically, in step 3), the forged steel billet is kept at 1180-1220° C. for 1.5-2.5 hours, then rolled to a thickness of 15-17 mm, with a deformation greater than 60%, and air-cooled to room temperature.

[0021] Preferably, in step 3), the forged steel billet is kept at 1200° C. for 2 hours and then rolled to a thickness of 16 mm.

[0022] Based on the above hot rolling process, austenite grains and high-temperature ferrite grains can be alternately stacked in a lamellar structure, further increasing the initial dislocation density and strength.

[0023] Specifically, in step 4), the rolled steel is heated to 980°C-1180°C for 25-35 minutes, and then quenched to room temperature, with a tempering temperature of 280-320°C and a tempering time of 0.5-1.5 hours.

[0024] Preferably, in step 4), the insulation temperature is 1080° C. for 30 min; the tempering temperature is 300° C. for 1 h.

[0025] Based on the above technical solution, the quenching process allows the steel to generate high-strength martensite in the original austenite grains, and the tempering process above the Ms point allows the carbon in the supersaturated martensite to be distributed to the retained austenite, thereby enhancing the stability of the retained austenite and the TRIP effect. Finally, the steel of the present invention is obtained at room temperature, and its structure is a multiphase structure of high-temperature ferrite, martensite and retained austenite.

[0026] The present invention also provides low-density 1200MPa-grade hot-rolled quenched and tempered partitioned steel prepared by the above preparation method.

[0027] The steel provided by the invention has a multiphase structure of high-temperature ferrite, martensite and residual austenite. Compared with other existing low-density high-strength steels, the steel developed by the invention has an excellent strength of 1200MPa under the premise of greatly reducing density.

[0028] The invention also provides the application of low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel as steel material for automobiles.

[0029] The steel material provided by the present invention has a density of 7.24 g / cm 3 , which is 7.8g / cm2 higher than ordinary automobile steel3 It is reduced by about 7%, its tensile strength is ≥1200MPa, and its elongation is ≥9%, which is suitable as automobile steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the heat treatment process design of samples 1 to 5 of Example 2 of the present invention, wherein part a is the iron-carbon phase diagram, part b is the phase fraction curve, and part c is the heat treatment process flow chart.

[0031] Figure 2 This is a metallographic structure diagram of the steel material produced by Sample 3 of Example 2 of the present invention.

[0032] Figure 3 This is an EPMA line scan data diagram of the carbon content distribution of the steel produced by samples 1, 3, and 5 of Example 2 of the present invention after partitioning, wherein part a is sample 1, part b is sample 3, and part c is sample 5. DETAILED DESCRIPTION

[0033] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0034] Table 1 below is a list of chemical composition values ​​of various embodiments of the present invention;

[0035] Table 2 below is a list of main mechanical properties test results of the embodiments of the present invention.

[0036] See also Figure 1 The method for preparing a low-density 1200MPa grade quenched and tempered partitioned steel according to an embodiment of the present invention comprises the following steps:

[0037] (1) Smelting and casting: using vacuum induction furnace melting, electroslag remelting, vacuum or protective atmosphere casting.

[0038] (2) Forging process: Heating to about 1150℃ and keeping in vacuum or protective atmosphere for 2h, the initial forging temperature is about 1100℃, three-fire forging, each fire forging ratio is greater than 2, strain rate 0.1-1s -1 The final forging temperature is >850℃, after forging, it is air-cooled and solution treated at 1100℃ in vacuum or protective atmosphere for about 2h, then water-cooled to room temperature and aged at 530℃ for about 12h.

[0039] (3) Hot rolling process: The forged steel billet is kept at 1200°C for 2 hours and then rolled to a thickness of 16 mm with a deformation greater than 60%, and then air-cooled to room temperature.

[0040] (4) Quenching and tempering process: The rolled steel is heated to 980°C, 1030°C, 1080°C, 1130°C, and 1180°C for 30 min, then quenched to room temperature, tempered at 300°C, and tempered for 1 h. Figure 1 Part (a) is the iron-carbon phase diagram calculated for the composition system 5Al-5Mn-1.5Si. Figure 1 Part (b) is the phase fraction curve at different annealing temperatures with a carbon content of 0.3%. Figure 1 Part (c) is a heat treatment process flow chart designed based on the information in Figures (a) and (b). The five sample annealing temperatures of 1, 2, 3, 4, and 5 are 980°C, 1030°C, 1080°C, 1130°C, and 1180°C, respectively.

[0041] Table 1 List of chemical composition values ​​of various embodiments of the present invention (wt%)

[0042] C Si Mn Al Cr Ni Mo Nb P S Fe Example 1 0.31 0.32 5.16 7.92 1.02 1.56 0.26 0.022 ≤0.010 ≤0.002 other Example 2 0.29 1.54 5.08 4.94 0.29 0.24 0.25 0.024 ≤0.010 ≤0.002 other

[0043] Table 2 List of main mechanical properties test results of the embodiments of the present invention

[0044]

[0045] It can be seen from Table 2 that the steel obtained in Example 2 Sample 3 of the present invention has a yield strength ≥1100MPa, a tensile strength ≥1200MPa, and an elongation ≥9%. Compared with steel of the same strength level, the density is reduced by about 7%. In addition. The residual austenite and its carbon content in the steel were detected by XRD. The results showed that when the annealing temperature was in the range of 980℃-1180℃, as the annealing temperature increased, the volume fraction of residual austenite gradually decreased while the carbon content gradually increased, indicating that the higher the annealing temperature, the weaker the TRIP effect. At the same time, the volume fractions of martensite and high-temperature ferrite in the steel were statistically analyzed by EBSD. The results showed that the volume fraction of martensite that provides the main strength increases first and then decreases with the increase of annealing temperature. Figure 1 The change law of the fcc (original austenite) phase fraction curve in part (b) is consistent. This is because the addition of a large amount of Al makes it impossible to eliminate high-temperature ferrite, and thus a single austenite phase region cannot appear. When the annealing temperature is 1080℃, the martensite volume fraction of the steel at room temperature is the highest, and its strength is also the highest.

[0046] Figure 2 It is the metallographic structure diagram of the steel produced by sample 3 of embodiment 2 of the present invention. It can be seen from the figure that the test steel is mainly composed of a mixed structure of high-temperature ferrite, banded martensite and residual austenite, in which the martensite is distributed in the center of the original austenite grains, and the residual austenite is distributed outside the original austenite grains.

[0047] like Figure 3As shown, it is the EPMA line scan data of the carbon content distribution of the steel produced in Example 2 of the present invention after partitioning, and the height of the peak represents the carbon content. In each sample, during annealing in the heat treatment process, the carbon content in the original austenite and high-temperature ferrite is the same; when quenched to room temperature, the carbon in the steel is enriched to form supersaturated martensite, and part of the original austenite grains are transformed into martensite, and part is residual austenite; and during tempering, the carbon in the supersaturated martensite is partitioned into the residual austenite. It can be seen from the figure that the carbon content is the lowest in the high-temperature ferrite area, the carbon content in the residual austenite area is significantly higher than that in the high-temperature ferrite area, and the carbon content in the martensite area is slightly lower than that in the residual austenite area, which can prove that during tempering, the carbon in the supersaturated martensite is partitioned into the residual austenite.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel, characterized in that: The following steps are involved: 1) preparing a steel billet from a raw material, wherein the steel billet comprises the following chemical components by mass percentage: C: 0.29%, Si: 1.54%, Mn: 5.08%, Al: 4.94%, Cr: 0.29%, Ni: 0.24%, Mo: 0.25%, Nb: 0.024%, P≤0.010%, S≤0.002%, and the balance is Fe and unavoidable impurities; 2) forging the steel billet obtained in step 1); 3) hot rolling the forged steel billet; 4) The rolled steel is heated to 980℃-1080℃ and kept warm for 25~35min, then quenched to room temperature, tempered at 280~320℃, and tempered for 0.5~1.5h to obtain low-density 1200MPa grade hot-rolled quenched and tempered steel.

2. The method for preparing the low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 1, characterized in that: In step 1), the raw materials are melted in a vacuum induction furnace, subjected to electroslag remelting treatment, and mold-casted in a vacuum or protective atmosphere to obtain a steel billet.

3. The method for preparing the low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 1, characterized in that: In step 2), the steel billet is heated to 1100-1150℃, kept in vacuum or protective atmosphere for 1.5-2.5h, the initial forging temperature is 1050-1100℃, three-fire forging, each fire forging ratio is greater than 2, and the strain rate is 0.1-1s -1 The final forging temperature is >850℃. After forging, it is air-cooled and solution-treated at 1020℃~1150℃ in vacuum or protective atmosphere for 1~2h. Then it is water-cooled to room temperature and aged at 525~535℃ for 9~12h.

4. The method for preparing the low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 3, characterized in that: Keep warm in vacuum or protective atmosphere for 2 hours; water cool to room temperature and perform aging treatment at 530℃ for 9 to 12 hours.

5. The method for preparing low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 1, characterized in that: In step 3), the forged steel billet is kept at 1180-1220°C for 1.5-2.5h, then rolled to a thickness of 15-17mm with a deformation greater than 60%, and air-cooled to room temperature.

6. The method for preparing the low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 5, characterized in that: The forged steel billet was kept at 1200°C for 2h and then rolled to a thickness of 16mm.

7. A low-density 1200 MPa grade hot-rolled quenched and tempered partitioned steel prepared according to the preparation method according to any one of claims 1 to 6.

8. An application of the low-density 1200MPa grade hot-rolled quenched and tempered partitioned steel according to claim 7, characterized in that: As steel for automobiles.

Images