800MPa-grade heat treatment dual-phase steel and treatment process thereof

Abstract

The invention provides 800 MPa grade heat treatment dual-phase steel and a treatment process thereof. The heat-treated dual-phase steel is obtained by taking Q345 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 Q345 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 80.4%, and martensite accounts for about 19.6%. Compared with an original material, the mechanical property is remarkably improved, the tensile strength of the dual-phase steel subjected to heat treatment reaches 870 MPa and is improved by 58.1%, and the percentage elongation after fracture is improved to 36% from 32%.

Description

Technical Field

[0001] The present invention relates to the heat treatment technology of dual-phase steel, and particularly relates to an 800MPa-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 refers to that obtained by subjecting low-carbon steel to dual-phase treatment or controlled rolling, which mainly consists of 70-80% ferrite and 30-20% martensite. It 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. As a representative of domestic low-alloy structural steel, Q345 steel shows good weldability and basic mechanical properties (yield strength ≥ 345MPa, tensile strength ≥ 470MPa) in fields such as bridges and pressure vessels. However, its ferrite-pearlite structure limits its mechanical properties and it is difficult to meet the requirements of modern industry for material lightweight and high strength and toughness. The present technology intends to develop dual-phase steel using Q345 as the raw material, obtain the process parameters of the heat-treated dual-phase steel of Q345 through research on the heat treatment process, improve its mechanical properties, and broaden the application range of Q345 in the automotive field. Summary of the Invention

[0003] The main purpose of the present invention is to provide an 800MPa-grade heat-treated dual-phase steel and its treatment process, using Q345 steel as the raw material to produce dual-phase steel. By adjusting the heat treatment process parameters, the obtained dual-phase steel has about 80.4% ferrite and about 19.6% martensite; the tensile strength of this dual-phase steel is increased to 870MPa, which is increased by 58.1% compared with the raw material, and the elongation is increased to 36%, which is increased by 12.5% compared with the raw material; the tensile curve of this dual-phase steel is a smooth and continuous stress-strain curve without a yield phenomenon. It can effectively achieve the combination of high strength and high toughness, and has excellent surface quality.

[0004] The heat treatment process: the quenching holding temperature is 795-835°C, the holding time is 2-6 minutes, then water quenching is carried out, and the sample after water quenching is tempered at 160°C - 200°C for 1-2 hours.

[0005] Preferred Embodiment 1: The heat treatment process parameters: the quenching holding temperature is 795-805°C, the holding time is 2-6 minutes, then water quenching is carried out, and the sample after water quenching is tempered at 160°C - 200°C for 1-2 hours.

[0006] Preferred Embodiment 2: The heat treatment process parameters are as follows: the quenching holding temperature is 805 - 815°C, the holding time is 2 - 6 min, followed by water quenching, and the sample after water quenching is tempered at 160°C - 200°C for 1 - 2 h.

[0007] Preferred Embodiment 3: The heat treatment process parameters are as follows: the quenching holding temperature is 815 - 825°C, the holding time is 2 - 6 min, followed by water quenching, and the sample after water quenching is tempered at 160°C - 200°C for 1 - 2 h.

[0008] Preferred Embodiment 4: The heat treatment process parameters are as follows: the quenching holding temperature is 825 - 835°C, the holding time is 2 - 6 min, followed by water quenching, and the sample after water quenching is tempered at 160°C - 200°C for 1 - 2 h.

[0009] The best embodiment of the present invention is: The heat treatment process parameters are as follows: the quenching holding temperature is 820°C, the holding time is 4 min, followed by water quenching, and the sample after water quenching is tempered at 200°C for 1.5 h.

[0010] The beneficial effects of the present invention are as follows: An 800 MPa - grade heat - treated dual - phase steel and its treatment process provided by the present invention can control the martensite content by controlling the holding temperature and time, so as to precisely control its tensile strength and elongation. The obtained dual - phase steel has ferrite accounting for about 80.4% and martensite accounting for about 19.6%; the tensile strength of the dual - phase steel is increased to 870 MPa, an increase of 58.1% compared with the raw material, and the elongation is increased to 36%, an increase of 12.5% compared with the raw material; the tensile curve of the dual - phase steel is a smooth and continuous stress - strain curve without yield phenomenon. It can effectively achieve the combination of high strength and high toughness, and has excellent surface quality. Description of the Drawings

[0011] Figure 1 It shows the tensile strength and elongation of the Q345 steel plate in Example 1 of the present invention after water quenching at 800°C for different holding times and then tempering at 200°C for 1.5 h.

[0012] Figure 2 The microstructures (500 times) of Q345 in Examples 1 - 4 of the present invention

[0013] (a) Quenched after holding at 800°C for 5 min; (b) Quenched after holding at 810°C for 4 min; (c) Quenched after holding at 820°C for 4 min; (d) Quenched after holding at 830°C for 3 min

[0014] Figure 3 It shows the martensite content of Q345 in Examples 1 - 4 of the present invention after quenching at 800°C, 810°C, 820°C, and 830°C for different holding times.

[0015] Figure 4 This is the optimal heat treatment process curve in Example 1 of the present invention.

[0016] Figure 5 This is the tensile strength and elongation after fracture of Q345 in Example 2 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.

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

[0018] Figure 7 This is the tensile strength and elongation after fracture of Q345 in Example 3 of the present invention after quenching in water after holding at 820 °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 Example 3 of the present invention.

[0020] Figure 9 This is the tensile strength and elongation after fracture of Q345 in Example 4 of the present invention after quenching in water after holding at 830 °C for different times and then tempering at 200 °C for 1.5 h.

[0021] Figure 10 This is the optimal heat treatment process curve in Example 4 of the present invention. Detailed implementation manners

[0022] In the embodiment of the present application, by providing an 800 MPa grade heat-treated dual-phase steel and its treatment process, using structural steel Q345 as the raw material to produce dual-phase steel can increase the tensile strength of Q345 to the 800 MPa - 870 MPa grade (an increase of more than 50%), and the elongation rate remains above 30%, effectively achieving the advantages of high strength and good plasticity.

[0023] To better understand the above technical solutions, the above technical solutions 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 embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of the present application, rather than limitations on the technical solutions 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.

[0024] An 800 MPa grade heat-treated dual-phase steel provided by an embodiment of the present invention, the chemical composition of the dual-phase steel by mass percentage: the contents of C, Si, Mn, P, S, Cr, Ni, V, and Cu are 0.18%, 0.23%, 1.45%, 0.017%, 0.007%, 0.07%, 0.06%, 0.004%, and 0.09% respectively, and the balance is Fe.

[0025] The structure of the dual-phase steel includes: soft-phase ferrite and hard-phase martensite;

[0026] The volume fraction of the ferrite is 65-85%, and the volume fraction of the hard-phase martensite is 15-35%.

[0027] The tensile strength of the dual-phase steel is 800-870 MPa.

[0028] The total elongation after fracture of the dual-phase steel is 32-36%.

[0029] This embodiment also provides a production method for 800 MPa grade heat-treated dual-phase steel. The chemical composition of Q345 by mass percentage is as follows: the contents of C, Si, Mn, P, S, Cr, Ni, V, and Cu are 0.18%, 0.23%, 1.45%, 0.017%, 0.007%, 0.07%, 0.06%, 0.004%, and 0.09% respectively, and the balance is Fe.

[0030] The heat treatment process for the dual-phase steel of Q345 includes the following steps:

[0031] Process the above Q345 steel plate into a 250×37×6 steel plate for tensile test (GB / T228-2002); put the processed steel plate for tensile test into a box-type resistance furnace at 795-835 °C and hold for 2-6 min, 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 h and then air cool to room temperature.

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Example 1

[0034] Put the 250×37×6 steel plate for tensile test into a box-type resistance furnace at 795-805 °C and hold for 2-6 min, 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 h. As

[0035] shown, for the Q345 steel plate water quenched after holding at 800 °C for different times and tempered at 200 °C for 1.5 h, the tensile strength and elongation. As the holding time extends, the tensile strength and elongation first increase and then decrease. The maximum tensile strength and elongation are reached at 5 min. The maximum tensile strength is 823 MPa, and the total elongation after fracture is 36%. The microstructure of the sample held at 800 °C for 5 min is as Figure 1 shown in (a), which consists of approximately 83% ferrite and 17% martensite. Figure 2 (a)

[0036] The martensite content of Q345 steel plate after quenching at 800°C for different holding times is shown in Figure 3 。

[0037] The optimal heat treatment process curve in Example 1 of the present invention is shown in Figure 4 。

[0038] Example 2

[0039] Put the 250×37×6 tensile test steel plate into a box-type resistance furnace at 805 - 815°C, hold for 2 - 6 minutes and 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 5 shown, the tensile strength and elongation after water quenching at 810°C for different holding times and then tempering at 200°C for 1.5 hours are presented. As the holding time extends, the tensile strength and elongation first increase and then decrease, reaching the maximum at 4 minutes. The maximum tensile strength is 846 MPa, and the total elongation after fracture is 35%. The microstructure of the steel plate held at 810°C for 4 minutes is shown in Figure 2 (b), which consists of approximately 82% ferrite and 18% martensite.

[0040] The martensite content of Q345 steel plate after quenching at 810°C for different holding times is shown in Figure 3 。

[0041] The optimal heat treatment process curve in Example 2 of the present invention is shown in Figure 6 。

[0042] Example 3

[0043] Put the 250×37×6 tensile test steel plate into a box-type resistance furnace at 815 - 825°C, hold for 2 - 6 minutes and 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 7 shown, the tensile strength and elongation after water quenching at 820°C for different holding times and then tempering at 200°C for 1.5 hours are presented. As the holding time extends, the tensile strength and elongation first increase and then decrease, reaching the maximum at 4 minutes. The maximum tensile strength is 870 MPa, and the total elongation after fracture is 36%. The microstructure of the steel plate held at 820°C for 4 minutes is shown in Figure 2 (c), which consists of approximately 80.4% ferrite and 19.6% martensite.

[0044] The martensite content of Q345 steel plate after quenching at 820°C for different holding times is shown in Figure 3 。

[0045] In Example 3 of the present invention, the best heat treatment process curve is shown in Figure 8 .

[0046] Example 4

[0047] Put the 250×37×6 steel plate for tensile test into a box-type resistance furnace at 825 - 835°C, hold for 2 - 6 min, and then water quench. When water 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

[0048] 1 - 2 h. As Figure 9 shown, after holding at 820°C for different times and then water quenching, and tempering the water-quenched samples at 200°C for 1.5 h, the tensile strength and elongation are presented. As the holding time extends, the tensile strength and elongation first increase and then decrease, reaching the maximum at 3 min. The maximum tensile strength is 862 MPa, and the total elongation after fracture is 34%. The microstructure of the sample held at 830°C for 3 min is shown in Figure 2 (d), which consists of approximately 77% ferrite and 23% martensite.

[0049] The martensite content of Q345 steel plate after quenching at 830°C for different times is shown in Figure 3 .

[0050] In Example 4 of the present invention, the best heat treatment process curve is shown in Figure 10 .

[0051] Conclusion

[0052] An 800 MPa grade heat-treated dual-phase steel and its treatment process are provided. The chemical composition of the dual-phase steel is as follows: the contents of C, Si, Mn, P, S, Cr, Ni, V, and Cu are 0.18%, 0.23%, 1.45%, 0.017%, 0.007%, 0.07%, 0.06%, 0.004%, and 0.09% respectively, and the balance is Fe. The best heat treatment process for the dual-phase steel is: the quenching and holding temperature is 820°C, the holding time is 4 min, then water quench, and the water-quenched sample is tempered at 200°C for 1.5 h (as shown in 8). The microstructure of the dual-phase steel obtained by this process is 80.4% ferrite and 19.6% martensite. The tensile strength of the dual-phase steel reaches 870 MPa, an increase of 58.1%, and the elongation after fracture increases from 32% to 36%.

[0053] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention in other forms. Any person skilled in the relevant art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with 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. An 800MPa grade heat-treated dual-phase steel and its treatment process, characterized in that: The chemical composition of the dual-phase steel, by mass percentage, is as follows: the contents of C, Si, Mn, P, S, Cr, Ni, V, and Cu are 0.18%, 0.23%, 1.45%, 0.017%, 0.007%, 0.07%, 0.06%, 0.004%, and 0.09% respectively, and the balance is Fe. The heat treatment process of the Q345 dual-phase steel includes the following treatment steps: Process a 6-mm-thick Q345 original steel plate into a 250×37×6 steel plate for tensile testing (GB / T228.1-2021). Heat the box-type resistance furnace to 795-835 °C, then place the steel plate for tensile testing into the box-type resistance furnace. After holding for 2-6 minutes, take out the steel plate for tensile testing and quench it. The quenching method is water quenching. Then place the quenched steel plate into the box-type resistance furnace and temper it at 160-200 °C for 1-2 hours, and then take it out for air cooling.

2. The 800MPa grade heat-treated dual-phase steel and its treatment process according to claim 1, characterized in that, The microstructure of the dual-phase steel obtained by this process is: the volume fraction of ferrite is 65-85%, and the volume fraction of martensite is 15-35%.

3. The 800 MPa grade heat-treated duplex steel and its treatment process according to claim 2, characterized in that, The mechanical properties of the dual-phase steel obtained by this process are: the tensile strength is 800-870 MPa, which is 45%-58.1% higher than that of the raw material, and the elongation is increased from 32% of the raw material to 36%.

Images