CN110878401A - A kind of preparation method of 1300MPa grade rare earth reverse transformation Q&P steel - Google Patents

Abstract

The invention discloses a preparation method of 1300MPa grade rare earth reverse transformation Q&P steel. Its chemical composition by mass percentage is: C: 0.12‑0.18%, Mn: 2.0‑3.0%, Si: 0.8‑1.2%, Al: 0.6‑1.0%, V: 0.05‑0.08%, B: 0.001‑0.003%, P < 0.02%, S < 0.01%, and the balance is Fe and inevitable impurities. According to the composition design of the present invention, a certain amount of mixed rare earth ferroalloy is added to smelt and prepare the cast billet. Combine the reverse transformation process with the Q&P process to increase the austenite stability. The lower Si content ensures the surface quality and hot rolling performance, and improves the hardenability through Al and B. A certain amount of V is added to introduce a dispersed second phase. The combination of V and Si enhances the work hardening ability of ferrite. By adding an appropriate content of rare earth, it can play a metamorphic role, refine the grains, and improve the comprehensive mechanical properties of the steel plate. The microstructure of the steel sheet produced by the invention is martensite, ferrite, residual austenite and the second phase of dispersion distribution. The tensile strength is greater than 1300MPa, the elongation after fracture is greater than 15%, and it has a good combination of strength and plasticity.

Description

A kind of preparation method of 1300MPa grade rare earth reverse transformation Q&P steel

technical field

The invention relates to the technical field of iron and steel metallurgy, in particular to a preparation method of 1300MPa grade rare earth reverse transformation Q&P steel.

Background technique

In recent years, my country's automobile industry has developed rapidly, but it also faces a series of problems such as energy saving, emission reduction and safety. The application of the third-generation advanced high-strength steel can effectively improve the lightweight level of automobiles. The traditional quenching and partitioning (Q&P) process is to fully austenitize the steel plate and then quench it to a temperature between _Ms and _Mf , perform carbon partitioning to improve the stability of retained austenite, and finally quench to room temperature. The microstructure of Q&P steel is martensite and retained austenite. According to the design principles of multi-phase, metastable and multi-scale high-strength steel, the microstructure of ferrite, martensite, retained austenite, and the second phase of dispersive precipitation has a good application prospect. But the difficulty lies in the control of the organization through the selection of ingredients and the formulation of process parameters.

The retained austenite can be regulated by adjusting the content of C element. Increasing the content of C element is beneficial to increase the stability of retained austenite, but too high content of C will reduce the weldability of the steel plate. Si element is beneficial to improve the hardenability of steel, but it affects the surface quality and reduces the hot rolling performance. The selection of low-carbon steel, alloying with various elements, and the distribution of Mn elements can also increase the stability of austenite. Austenite reverse transformation means that after austenitizing the steel, it is quenched to room temperature to obtain the precursor of lath-like martensite, which is reheated to the two-phase region for heat preservation, and the austenite is reshaped between the martensite laths. Nucleation, growth, and enrichment of alloying elements such as C and Mn. The comprehensive distribution process of different alloying elements is beneficial to improve the comprehensive mechanical properties of the steel plate.

Rare earth additives have the effect of purification and metamorphism, and can refine grains. In the prior art, Q&P steels are mostly selected without adding rare earths, and are produced by traditional quenching and partitioning processes. By adding a certain amount of rare earth, the comprehensive mechanical properties of steel can be further improved. However, the addition of more rare earths will greatly increase the cost.

SUMMARY OF THE INVENTION

In order to further improve the comprehensive mechanical properties of the steel plate and improve the welding performance and surface quality of the steel plate, the present invention adopts a reasonable composition design of the steel, adds a relatively low content of rare earth, uses a variety of elements for alloying, and divides the austenite inversion process and quenching. Combined with the process, the two-phase area is kept warm after the reverse transformation, and the C and Mn concentrations of the reverse transformation austenite are controlled by the one-step quenching-partition (Q&P) process to improve the stability of the austenite. The microstructure of the obtained steel is 50-60% martensite, 30-40% ferrite and 8-12% retained austenite, and there are nano-scale second-phase particles dispersedly distributed.

Rare earth additives, as strong deoxidizers and desulfurizers in molten steel, play a purifying role; rare earth reacts with oxygen and sulfur in molten steel to form oxides and sulfides, which are partially retained in molten steel. Due to the high melting point of these inclusions in molten steel, they can be used as non-homogeneous nucleation centers during molten steel solidification, and play a metamorphic role, thereby refining grains.

The composition of the automobile sheet of the present invention is: C: 0.12-0.18%, Mn: 2.0-3.0%, Si: 0.8-1.2%, Al: 0.6-1.0%, V: 0.05-0.08%, B: 0.001 -0.003%, P<0.02%, S<0.01%, the balance is Fe and inevitable impurities.

The preparation method of the 1300MPa grade rare earth reverse transformation Q&P steel of the present invention, the selection basis of each element in the composition design is as follows:

C: C element has a key influence on the strength and plasticity of Q&P steel, and is conducive to the formation and stability of reverse transformation austenite. However, when the C content is high, a large number of twins may appear in the martensite, which affects the plasticity of the steel plate; excessive carbide precipitation is caused during the C partitioning process, which reduces the C content of the retained austenite; the heat affected zone is increased. And lead to welding cracking, reduce welding performance. Therefore, the upper limit of the C element in the present invention is 0.18%. When the C content is lower than 0.12%, the C distribution effect is poor, the austenite is depleted in carbon, and the stability of the retained austenite at room temperature is reduced. In the present invention, the content of C element is selected as 0.12-0.18%.

Mn: Mn element is an austenite forming element, which can expand the austenite phase region and reduce the M _s point. The enrichment of Mn element in austenite is beneficial to improve the stability of austenite. The distribution of Mn element in the process of austenite reverse transformation is of great significance to improve the comprehensive properties of the steel plate. Besides, Mn is also a good desulfurizer. However, if the Mn element content is too high, it will increase the production cost and affect the welding performance of the steel plate. Based on the above considerations, the Mn element composition selection is close to the lower limit of medium manganese steel, which is 2.0-3.0%.

Al: Al element can expand the ferrite region, and it is easy to obtain ferrite during heat treatment. Al element can inhibit the precipitation of cementite during heat treatment and improve the hardenability. At the same time, Al element also has the effect of refining grains. In addition, Al also contributes to the weight reduction of the steel sheet. However, too high Al will make the molten steel viscous, and there are problems such as easy clogging of the nozzle, which reduces the production efficiency. The content of Al element in the present invention is selected to be 0.6-1.0%.

Si: Si element can improve the hardenability of steel, inhibit the formation of carbides during heat treatment, and facilitate the diffusion of C atoms from martensite to retained austenite, but excessive Si content may reduce hot rolling properties. cause thermal cracking, etc. At the same time, the surface quality may be reduced, thereby affecting the coatability, etc. The Si element content of the present invention is selected to be 0.8-1.2%.

V: The second phase can be precipitated by microalloying of V. The steel sheet produced by the present invention has a relatively high content of ferrite. The combination of V and appropriate Si content can improve the strength of ferrite and reduce the strength gradient between martensite and ferrite. A part of the retained austenite exists around the ferrite in the form of a thin film. The TRIP effect is more obvious when the structure strength around the retained austenite is high. Therefore, the content of V element is selected as 0.05-0.08%.

B: The hardenability of steel is further improved. The content of B element is selected as 0.001-0.003%.

The present invention includes the following steps:

Step 1, smelting and casting of the steel plate: smelting the component powder of the present invention in an electric arc furnace, transferring it to an IF furnace for refining, and adding 5% cerium, 2.5% lanthanum, and the rest Fe and other components by the wire feeding method. The mixed rare earth iron alloy with inevitable impurities is obtained as a cast billet.

Step 2, heat preservation, hot rolling, quenching:

The slab is heated to 1000-1050°C, kept for 1-1.5h, and then hot rolled at a rolling temperature of 950-1000°C, finishing rolling at 800-900°C, rolling to obtain a steel plate with a thickness of 1.8 mm, and quenching. to room temperature;

Step 3, the first reverse transformation heat preservation and quenching: the steel plate cooled to room temperature after rolling is heated to 20-30 °C above A _C3 at a rate of 20-30 °C/s, and the temperature is kept for 3-10 minutes. 80℃/s cooling rate, water quenched to room temperature;

Step 4, cold rolling: after pickling the steel plate, multi-pass cold rolling is performed to obtain a 1.2 mm thick steel plate.

Step 5, the second reverse transformation and heat preservation: the steel plate is heated to 30-80 °C above A _C1 at a rate of 20-30 °C/s, and the temperature is maintained for 3-15 minutes to carry out reverse transformation austenitization, and promote the distribution of Mn elements;

Step 6, C element distribution: quench the steel plate after reverse transformation austenitization at a cooling rate of 50-80°C/s to between M _s -M _f , keep for 30-90 s, and finally quench to room temperature.

The beneficial effects of the present invention are:

(1) The distribution of C and Mn elements, in order to improve the welding performance, select a lower content of C element, the content of Mn element is close to 2.0-3.0% of the lower limit of medium manganese steel, and the Mn element is carried out in the reverse transformation austenitization process after quenching. Combined with the low-temperature carbon distribution in the quenching-partitioning process, the content and stability of room temperature retained austenite are improved.

(2) A mixed rare earth iron alloy with 5% cerium, 2.5% lanthanum added, and Fe and other unavoidable impurities. By adding a small amount of rare earth, it is used as a strong deoxidizer and desulfurizer for molten steel, and plays the role of refining grains, improving surface quality and rolling quality, improving overall performance, and less cost increase.

(3) The content of Si element is reduced. In order to improve the hardenability, an appropriate amount of Al and B elements are added. By adding Al, the ferrite transformation region is enlarged and ferrite is more easily obtained. Ferrite ensures the plasticity of steel, and the appropriate combination of Si and V can improve the work hardening ability of ferrite and enhance the TRIP effect of retained austenite.

(4) Compared with the heat preservation and annealing time of the traditional process, the two reverse transformation annealing time is shorter and the heat preservation temperature is lower, which improves the production efficiency and reduces the production cost to a certain extent.

(5) The microstructure is 40-60% martensite, 30-40% ferrite, and 8-12% retained austenite. By adding rare earth and V, etc., there are some dispersed nano-second phases. . tensile strength greater than

1300MPa, the yield strength is greater than 780MPa, and the elongation after fracture is greater than 15%. It has good strength and plastic matching.

Description of drawings

Fig. 1 is the rolling and heat treatment process diagram of the present invention.

FIG. 2 is a SEM image of the microstructure of Example 1 of the present invention.

Fig. 3 is the XRD pattern of Example 1 of the present invention.

Detailed ways

The present invention will be further described below with reference to the embodiments and the accompanying drawings.

Example 1

The present invention relates to a preparation method of 1300MPa grade rare earth reverse transformation Q&P steel, the preparation steps are: (1) smelting and casting of steel plate: the components are: C: 0.17%, Mn: 2.31%, Si: 1.10% by mass percentage %, Al: 0.80%, V: 0.06%, B: 0.002%, the balance is Fe and inevitable impurities, the component powder of the present invention is smelted in an electric arc furnace, transferred to an IF furnace for refining, and fed By silk method, a mixed rare earth iron alloy with a content of 5% cerium, 2.5% lanthanum, and the rest of Fe and other inevitable impurities is added to obtain a cast billet.

(2) Heat preservation, hot rolling and quenching: heat the cast slab to 1050°C, keep the temperature for 1.2h, and then perform hot rolling after heat preservation. The rolling temperature is 950°C, and finish rolling is performed at 850°C to obtain a steel plate with a thickness of 1.8 mm. , quenched to room temperature.

(3) The first reverse transformation heat preservation and quenching: the steel plate cooled to room temperature after rolling is heated to 840 ° C at a rate of 30 ° C/s, kept for 8 minutes, and the steel plate is quenched to room temperature after heat preservation;

(4) Cold rolling: After pickling the steel plate, it is cold-rolled in multiple passes to obtain a 1.2mm thick steel plate.

(5) The second reverse transformation and heat preservation: the steel plate is heated to 780 °C at a rate of 30 °C/s, and held for 5 minutes to perform reverse transformation austenitization to promote the distribution of Mn elements;

(6) C element distribution: The steel plate after reverse transformation austenitization is quenched to 260°C, kept for 60s, and finally quenched to room temperature.

The scanning electron microscope (SEM) structure of the steel sheet prepared in this example is shown in FIG. 2 , and it can be seen that the microstructure is martensite, ferrite and retained austenite. And there are nano second phase particles. The ferrite grains are fine and dispersed in the martensite matrix. The retained austenite content is 11%. The yield strength is 842.6MPa, the tensile strength is 1380.5MPa, the elongation after fracture is 16.7%, and the strength-plastic product is 23.0GPa·%.

Example 2

The present invention relates to a preparation method of 1300MPa grade rare earth reverse transformation Q&P steel, the preparation steps are: (1) smelting and casting of steel plate: the components are: C: 0.17%, Mn: 2.31%, Si: 1.10% by mass percentage %, Al: 0.80%, V: 0.06%, B: 0.002%, the balance is Fe and inevitable impurities, the component powder of the present invention is smelted in an electric arc furnace, transferred to an IF furnace for refining, and fed By silk method, a mixed rare earth iron alloy with a content of 5% cerium, 2.5% lanthanum, and the rest of Fe and other inevitable impurities is added to obtain a cast billet.

(3) Heat preservation, hot rolling, and quenching: heat the slab to 1050°C, keep the temperature for 1.2h, and then perform hot rolling after heat preservation. The rolling temperature is 950°C, and finish rolling is performed at 850°C to obtain a steel plate with a thickness of 1.8 mm. , quenched to room temperature.

(3) The first reverse transformation heat preservation and quenching: the steel plate cooled to room temperature after rolling is heated to 850°C at a rate of 30°C/s, kept for 5 minutes, and then the steel plate is quenched to room temperature after heat preservation;

(4) Cold rolling: After pickling the steel plate, it is cold-rolled in multiple passes to obtain a 1.2mm thick steel plate.

(5) The second reverse transformation and heat preservation: the steel plate is heated to 770 °C at a rate of 30 °C/s, and held for 5 minutes to perform reverse transformation austenitization to promote the distribution of Mn elements;

(6) C element distribution: the steel plate after reverse transformation austenitization is quenched to 260°C, kept for 90s, and finally quenched to room temperature.

The microstructure of the steel sheet prepared in this example is martensite, ferrite and retained austenite and there are a certain amount of nanosecond phase particles. The retained austenite content is 10%. The yield strength is 860.5MPa, the tensile strength is 1360.2MPa, the elongation after fracture is 16.2%, and the strength-plastic product is 22.0GPa·%.

Claims (3)

1. A method for preparing a 1300MPa grade rare earth reverse transformation Q&P steel, characterized in that its chemical composition is: C: 0.12-0.18%, Mn: 2.0-3.0%, Si: 0.8-1.2%, Al: 0.6- 1.0%, V: 0.05-0.08%, B: 0.001-0.003%, P<0.02%, S<0.01%, the balance is Fe and inevitable impurities, and a certain amount of mixed rare earth iron alloy is added during the smelting process. 2. a kind of 1300MPa grade rare earth reverse transformation Q&P steel preparation method described in claim 1, concrete preparation steps are as follows: (1) Smelting and casting of steel plate: according to the ingredients of the steel plate according to the present invention, it is smelted in an electric arc furnace, transferred to an IF furnace for refining, and mixed with a rare earth ferroalloy to obtain a billet. (2) Heat preservation, hot rolling, and quenching: heat to 1000-1050 °C, heat preservation for 1-1.5 h, hot rolling after heat preservation, rough rolling at 950-1000 °C, finish rolling at 800-900 °C, rolling to obtain 1.8 mm thick steel plate, quenched to room temperature; (3) The first reverse transformation heat preservation and quenching: the steel plate cooled to room temperature after rolling is heated to 20-30°C above A _C3 at a rate of 20-30°C/s, and kept for 3-10min. 80℃/s cooling rate, water quenched to room temperature; (4) Cold rolling: After pickling the steel plate, it is cold-rolled in multiple passes to obtain a 1.2mm thick steel plate. (5) The second reverse transformation and heat preservation: the steel plate is heated to 50-80 °C above A _C1 at a rate of 20-30 °C/s, and held for 3-15 minutes to perform reverse transformation austenitization to promote the distribution of Mn elements; (6) C element distribution: The steel plate after reverse transformation austenitization is quenched at a cooling rate of 50-80°C/s to between M _s and M _f , kept for 30-90 s, and finally quenched to room temperature. 3. The mixed rare earth iron alloy described in claim 1, the content is 5% cerium, 2.5% lanthanum, and the rest are Fe and other inevitable impurities. When refining in the IF furnace, it is added by the wire feeding method.

Images