600MPa-grade heat treatment dual-phase steel and treatment process thereof

Abstract

The invention provides 600MPa-grade heat-treated dual-phase steel and a treatment process of the 600MPa-grade heat-treated dual-phase steel. The heat-treated dual-phase steel is obtained by taking Q235B as a raw material and adopting a mode of subcritical quenching and then low-temperature tempering. According to the heat-treated dual-phase steel prepared through the method, by optimizing heat treatment process parameters, the mechanical property of the heat-treated dual-phase steel can be effectively controlled, and good cooperation of high strength and good plasticity is achieved. The optimal heat treatment dual-phase steel treatment process of Q235B is optimized by changing the heat preservation temperature and the heat preservation time, analyzing the structure after heat treatment and carrying out a mechanical property experiment on a test plate after heat treatment. According to the obtained dual-phase steel, ferrite accounts for about 80.2%, and martensite accounts for about 19.8%. Compared with an original material, the mechanical property is remarkably improved, the tensile strength of the dual-phase steel subjected to heat treatment reaches 666Mpa and is improved by 38%, and the percentage elongation after fracture is improved to 38% from 30%.

Description

Technical Field

[0001] The present invention relates to dual-phase steel and its treatment technology, and particularly to a 600 MPa grade heat-treated dual-phase steel and its treatment process. Background Art

[0002] With the development of modern automobiles towards weight reduction, energy conservation, high safety, etc., it has promoted the research and development of high-strength thin steel sheets for automobiles. Dual-phase steel is divided into heat-treated dual-phase steel and hot-rolled dual-phase steel, which is mainly composed of 70 - 80% ferrite and 30 - 20% martensite. Dual-phase steel has the advantages of a low yield ratio, a high initial work-hardening rate, and a good combination of high strength and high plasticity. It has developed into a new type of high-strength stamping steel with good formability and is one of the preferred steel grades for modern automobiles. Ordinary carbon steel Q235B is inexpensive and is widely used as an engineering component material, but it has not been used in the development and use of dual-phase steel. The purpose of this technology is to obtain the process parameters of heat-treated dual-phase steel of Q235B through research on the heat treatment process, improve its mechanical properties, broaden the application range of Q235B in the automotive field, replace expensive alloy steel, and reduce production costs. Summary of the Invention

[0003] The main purpose of the present invention is to provide a 600 MPa grade heat-treated dual-phase steel and its treatment process. By using ordinary carbon steel Q235B with a wide range of use and low cost as raw materials to produce dual-phase steel, it solves the problems of high cost of alloy dual-phase steel and inaccurate performance control of hot-rolled dual-phase steel in the prior art. The dual-phase steel prepared by this method can effectively control its mechanical properties through optimizing the heat treatment process parameters, achieving a good combination of high strength and good plasticity.

[0004] The raw material used for the dual-phase steel is Q235B steel plate, and its chemical composition by mass percentage is as follows: the contents of C, Si, Mn, P, and S are 0.14%, 0.07%, 0.42%, 0.019%, and 0.011% respectively, and the balance is Fe.

[0005] The heat treatment process: the quenching and holding temperature is 785 - 815 °C, the holding time is 2 - 8 min, then water quenching is carried out, and the sample after water quenching is tempered at 160 °C - 200 °C for 1 - 2 h.

[0006] Preferred Embodiment 1: The heat treatment process parameters: the quenching and holding temperature is 785 - 795 °C, the holding time is 4 - 8 min, then water quenching is carried out, and the sample after water quenching is tempered at 160 °C - 200 °C for 1 - 2 h.

[0007] Preferred Embodiment 2: The heat treatment process parameters: the quenching and holding temperature is 795 - 805 °C, the holding time is 2 - 6 min, then water quenching is carried out, and the sample after water quenching is tempered at 160 °C - 200 °C for 1 - 2 h.

[0008] Preferred Embodiment 3: The heat treatment process parameters are as follows: the quenching holding temperature is 805 - 815°C, the holding time is 2 - 6 minutes, followed by water quenching, and the water-quenched sample is tempered at 160°C - 200°C for 1 - 2 hours.

[0009] The best embodiment of the present invention is as follows: The heat treatment process parameters are as follows: the quenching holding temperature is 800°C, the holding time is 4 minutes, followed by water quenching, and the water-quenched sample is tempered at 200°C for 1.5 hours.

[0010] The beneficial effects of the present invention are as follows: The present invention provides a 600MPa grade heat-treated dual-phase steel and its treatment process, which selects low-cost steel grades and does not add precious elements, thereby reducing raw material costs; the product has a uniform microstructure and flexible process control; by controlling the holding temperature and time, the martensite content is controlled, effectively realizing the control of its mechanical properties. While the tensile strength of the dual-phase steel increases, the elongation rate does not decrease but increases. The tensile strength of the dual-phase steel reaches 666 Mpa, an increase of 38% compared to the raw material, and the elongation rate of the dual-phase steel increases from 30% of the raw material to 38%; the dual-phase steel has excellent cold forming performance, high surface quality, and no Lüder's bands. This invention has important development prospects for promoting the manufacture of low-cost high-strength steel plates in the steel industry and greatly expanding the application of Q235B steel in high-strength steel demand fields such as automobiles. Description of the Drawings

[0011] Figure 1 It shows the tensile strength and elongation rate of the Q235B steel plate in Example 1 of the present invention after water quenching at 790°C for different holding times and then tempering at 200°C for 1.5 hours.

[0012] Figure 2 The microstructures (500 times) of Q235B in Examples 1 - 3 of the present invention

[0013] (a) Quenched after holding at 790°C for 6 minutes; (b) Quenched after holding at 800°C for 4 minutes; (c) Quenched after holding at 810°C for 4 minutes.

[0014] Figure 3 It shows the martensite content of Q235B in Examples 1 - 3 of the present invention after quenching at 790°C, 800°C, and 810°C for different holding times.

[0015] Figure 4 It shows the best heat treatment process curve in Example 1 of the present invention.

[0016] Figure 5 It shows the tensile strength and elongation after fracture of Q235B in Example 2 of the present invention after water quenching at 800°C for different holding times and then tempering at 200°C for 1.5 hours

[0017] Figure 6 This is the optimal heat treatment process curve in Embodiment 2 of the present invention.

[0018] Figure 7 This is the tensile strength and elongation after fracture of Q235B in Embodiment 3 of the present invention after quenching in water after holding at 810°C for different times and then tempering at 200°C for 1.5 h.

[0019] Figure 8 This is the optimal heat treatment process curve in Embodiment 3 of the present invention. Detailed implementation manners

[0020] In the embodiment of the present application, by providing a 600 MPa grade heat-treated dual-phase steel and its treatment process, using ordinary carbon steel Q235B with a wide range of uses and low cost as the raw material to produce dual-phase steel, the problems in the prior art such as the high cost of alloy dual-phase steel and the inaccurate control of the performance of existing hot-rolled dual-phase steel are solved. The dual-phase steel prepared by this method can effectively achieve precise control of its mechanical properties by optimizing the heat treatment process parameters, and has the advantages of high strength and good plasticity.

[0021] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners. It should be understood that the specific features in the embodiments of the present invention and the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. Without conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0022] A 600 MPa grade heat-treated dual-phase steel provided by an embodiment of the present invention, the chemical composition of the dual-phase steel is as follows: by mass percentage, the contents of C, Si, Mn, P, and S are 0.14%, 0.07%, 0.42%, 0.019%, and 0.011% respectively, and the balance is Fe.

[0023] The structure of the dual-phase steel includes: soft-phase ferrite and hard-phase martensite;

[0024] The volume fraction of the ferrite is 65-85%, and the volume fraction of the hard-phase martensite is 15-35%.

[0025] The tensile strength of the dual-phase steel is 550-666 MPa.

[0026] The total elongation after fracture of the dual-phase steel is 31-38%.

[0027] This embodiment also provides a 600 MPa grade heat-treated dual-phase steel and its treatment process. The raw material used for the dual-phase steel is Q235B steel plate, and its chemical composition by mass percentage is as follows: the contents of C, Si, Mn, P, and S are 0.14%, 0.07%, 0.42%, 0.019%, and 0.011% respectively, and the balance is Fe.

[0028] A treatment process for a 600 MPa grade heat-treated dual-phase steel includes the following steps:

[0029] Process the above Q235B steel plate into a 250×37×6 tensile test steel plate (GB / T 228.1-2021); put the processed tensile test steel plate into a box-type resistance furnace at 785-815 °C and keep it warm for 2-8 minutes, then water quench. When quenching, keep the tensile test steel plate in a horizontal state to prevent stress concentration; temper the quenched steel plate at 160-200 °C for 1-2 hours and then air cool to room temperature.

[0030] The following further describes the present invention in detail with reference to the drawings and specific embodiments.

[0031] Example 1

[0032] Put the 250×37×6 tensile test steel plate into a box-type resistance furnace at 785-795 °C and keep it warm for 4-8 minutes, then water quench. When quenching, keep the tensile test steel plate in a horizontal state to prevent stress concentration. Temper the quenched steel plate at 160-200 °C for 1-2 hours. As Figure 1 shown are the tensile strength and elongation of the Q235B steel plate after water quenching at 790 °C for different holding times and then tempering at 200 °C for 1.5 hours. As the holding time prolongs, the tensile strength and elongation first increase and then decrease. The tensile strength and elongation reach the maximum at 6 minutes, the maximum tensile strength is 594 Mpa, and the total elongation after fracture is 34%. The microstructure of the steel plate held at 790 °C for 6 minutes is as Figure 2 (a) shown, which is composed of approximately 77% ferrite and 23% martensite.

[0033] The martensite content of the Q235B steel plate after quenching at 790 °C for different holding times is referred to Figure 3 .

[0034] The best heat treatment process curve in Example 1 of the present invention is referred to Figure 4 .

[0035] Example 2

[0036] Put the 250×37×6 steel plate for tensile test into a box-type resistance furnace at 795 - 805°C, hold for 2 - 6 minutes and then water quench. When quenching, keep the steel plate for tensile test in a horizontal state to prevent stress concentration. Temper the quenched steel plate at 160 - 200°C for 1 - 2 hours. As Figure 5 Shown are the tensile strength and elongation after water quenching after holding at 800°C for different times and tempering the water-quenched samples at 200°C for 1.5 hours. As the holding time prolongs, the tensile strength and elongation first increase and then decrease, reaching the maximum at 4 minutes. The maximum tensile strength is 666 Mpa and the total elongation after fracture is 38%. The microstructure after holding at 800°C for 4 minutes is as Figure 2 (b) shows, which consists of about 80.2% ferrite and 19.8% martensite.

[0037] The martensite content of Q235B steel plate after quenching at 800°C for different times is referred to Figure 3 .

[0038] The best heat treatment process curve in Example 2 of the present invention is referred to Figure 6 .

[0039] Example 3

[0040] Put the 250×37×6 steel plate for tensile test into a box-type resistance furnace at 805 - 815°C, hold for 2 - 6 minutes and then water quench. When quenching, keep the steel plate for tensile test in a horizontal state to prevent stress concentration. Temper the quenched steel plate at 160 - 200°C for 1 - 2 hours. As Figure 7 Shown are the tensile strength and elongation after water quenching after holding at 810°C for different times and tempering the water-quenched samples at 200°C for 1.5 hours. As the holding time prolongs, the tensile strength and elongation first increase and then decrease, reaching the maximum at 4 minutes. The maximum tensile strength is 603 Mpa and the total elongation after fracture is 34%. The microstructure after holding at 810°C for 4 minutes is as Figure 2 (c) shows, which consists of about 68.8% ferrite and 31.2% martensite.

[0041] The martensite content of Q235B steel plate after quenching at 810°C for different times is referred to Figure 3 .

[0042] The best heat treatment process curve in Example 3 of the present invention is referred to Figure 8 .

[0043] Conclusion

[0044] A 600MPa grade heat-treated dual-phase steel and its treatment process are provided. The chemical composition of the dual-phase steel is as follows: by mass percentage, the contents of C, Si, Mn, P, and S are 0.14%, 0.07%, 0.42%, 0.019%, and 0.011% respectively, and the balance is Fe. The optimal heat treatment process of the dual-phase steel is: the quenching and holding temperature is 800°C, the holding time is 4 minutes, and then water quenching is carried out. The sample after water quenching is tempered at 200°C for 1.5 h (as Figure 6 shown). The structure of the dual-phase steel obtained by this process is that ferrite accounts for 80.2% and martensite accounts for 19.8%. The tensile strength of the dual-phase steel reaches 666 Mpa, an increase of 38%, and the elongation after fracture increases from 30% to 38%.

[0045] As mentioned above, it is only a preferred embodiment of the present invention and not a limitation of the present invention in other forms. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments of equivalent changes. However, any simple modifications, equivalent changes, and improvements made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A 600MPa grade heat-treated dual-phase steel and a processing process thereof, characterized in that: The raw material used for the dual-phase steel is Q235B steel plate, and its chemical composition, by mass percentage, is as follows: the contents of C, Si, Mn, P, and S are 0.14%, 0.07%, 0.42%, 0.019%, and 0.011%, respectively, and the balance is Fe. A 600MPa grade heat-treated dual-phase steel processing process includes the following processing steps: The 6mm thick Q235B original steel plate is processed into a 250×37×6 tensile test steel plate (GB / T228.1-2021). The box-type resistance furnace is heated to 785-815°C, and then the tensile test steel plate is placed in the box-type resistance furnace. After keeping warm for 2-8 minutes, the tensile test steel plate is taken out for quenching. The quenching method is water quenching. Then the quenched steel plate is placed in a box-type resistance furnace and tempered at 160-200°C for 1-2 hours, and then taken out for air cooling.

2. A 600MPa grade heat-treated dual-phase steel and a processing process thereof as claimed in claim 1, characterized in that: The microstructure of the dual-phase steel obtained by the process is as follows: the volume fraction of ferrite is 65-85%, and the volume fraction of martensite is 15-35%.

3. A 600MPa grade heat-treated dual-phase steel and a processing process thereof as claimed in claim 2, characterized in that: The best mechanical properties of the dual-phase steel obtained through this process are: the tensile strength reaches 666Mpa, which is 38% higher than the raw material, and the elongation is increased from 30% of the raw material to 38%.

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