CN105950970B - Tough automobile steel of a kind of compound bainite high-strength of Ultra-fine Grained and preparation method thereof - Google Patents

Abstract

Tough automobile steel of a kind of compound bainite high-strength of Ultra-fine Grained of the invention and preparation method thereof, belongs to metal material processing technical field.The material is that cold-rolled steel sheet is thermally treated resulting in through continuous annealing, and its chemical component weight percentage is C：≤0.30%、Si：1.05~1.75%、Mn：1.25~2.45%、V：0.15 ~ 0.25%, surplus is Fe and inevitable impurity, average grain size is less than 5 μm, materials microstructure is made up of lath-shaped and granular compound bainite, ferrite, retained austenite, ferrite bainite is 0.8 ~ 1.0 with the ratio between volume shared by granular bainite, the yield strength of material is more than 600MPa, tensile strength is more than 1100MPa, percentage of total elongation is more than 30%, strength and ductility product is more than 30GPa%, the performance requirement of third generation automobile steel is reached, with good strong plasticity energy, while cost is cheap and preparation technology is simple.

Description

A kind of ultrafine-grain composite bainite high-strength and toughness automobile steel and its preparation method

technical field

The invention belongs to the technical field of metal material processing, and relates to a room-temperature structure of lath-like and granular composite bainite, ferrite, and residual austenite multi-phase structure steel and vanadium microalloyed ultra-fine-grained steel with high-strength plastic product Organization of automotive steels and methods of manufacture.

Background technique

The development of the automobile industry has always played an important role in the industrial economy and even the national economy. Since the 1960s and 1970s, the automobile industries in North America, Japan and Western Europe have entered a stage of rapid development, and the technical concepts of the automobile industry have continued to develop and innovate. , and in the 21st century, China's auto industry ushered in a golden stage of rapid and stable development. As of 2015, China's auto production and sales both exceeded 24 million, exceeding more than a quarter of global production and sales, and auto production and sales ranked first in the world. In the face of the continuous development of the current automobile industry, the problems of energy consumption, environmental protection and safety of automobiles have become increasingly prominent, and the lightweight of automobiles has become a new direction for the development of the industry. Steel is a viable way to achieve lightweight vehicles. By improving the strength and plasticity of steel for automobiles, it can not only reduce the consumption of materials to achieve the purpose of reducing the weight of the vehicle, but also ensure the strength and safety performance of the car body, and has great potential for development and application.

Since the addition of alloying elements in low-alloy high-strength automotive steel is relatively small, in order to improve the strength-plastic product performance of the material, it is necessary to reasonably design and control its preparation process and microstructure to obtain a certain proportion of ferrite. , bainite, martensite and retained austenite dual-phase or multi-phase coexistence ultra-fine grain structure steel, to ensure that the material has good comprehensive properties of high strength, high plasticity, high toughness, low density and high fatigue resistance. Chinese patent application CN201110280804.8 introduces a method for preparing cold-rolled TRIP steel above 1000 MPa. The composition of the material is designed with compound addition of Nb and V, and 0.8 to 1.0 wt% of Al is added. Higher Al It will hinder complete austenitization and increase the Ms point. The operating range of the two-phase zone temperature and the bainite zone temperature is small. The room temperature structure of the material is ferrite, bainite and retained austenite. The elongation is more than 18%, and the comprehensive performance is lower than that of the composite bainite high-strength tough steel in the present invention. Chinese patent application CN201210411320.7 introduces a method for preparing a high-strength plastic product TRIP steel plate. The material is compounded with Nb, V, Ti, Cr, and Mo microalloying elements, and a cold-rolled plate is annealed in a bell annealing furnace to obtain a Steel with martensite, austenite and a small amount of ferrite structure, its strength and plasticity performance exceeds 30GPa·%. However, the alloy system of the material belongs to complex and increases the difficulty of smelting. In addition, the longer annealing holding time reduces the production efficiency And increased energy consumption. Chinese patent application CN200910046581.1 introduces a manufacturing method of ultra-fine-grained bainite high-strength steel. Cr, Nb, Ti, Zr microalloying elements are added to the material, and a super The fine-grained bainite structure steel has a tensile strength of about 800 MPa and an elongation rate of less than 20%, and its overall performance is lower than that of the composite bainite high-strength tough steel in the present invention.

To sum up, the composite bainitic high-strength tough steel in the present invention has the characteristics of lath and granular materials at room temperature after annealing in the two-phase zone and isothermal heat treatment in the bainite zone by adjusting the process parameters of the heat treatment process. Composite multiphase structure of bainite, ferrite and retained austenite. The results show that when the volume fraction of each structure in the material is in a certain range, especially when the volume ratio of lath bainite to granular bainite is in the range of 0.8 to 1.0, the strength and plasticity of the material will be obtained. Significantly improved, at the same time, the structure is uniformly distributed and the grain size is fine, and the average grain size of the material is less than 5 μm. During the deformation process, the various structures coordinate and cooperate with each other. The composite bainite structure can improve the strength and toughness of the material, and the metastable retained austenite undergoes a phase transformation-induced plastic effect, so that the material can withstand large loads and deformations, and has great Good plasticity and impact resistance, yield strength R _p ≥ 600MPa, tensile strength R _m ≥ 1100MPa, elongation after fracture A ≥ 30%, strong plastic product ≥ 30GPa·%. Due to the low alloy composition content of this composite bainite high-strength steel, it can make full use of the advantages of rich vanadium resources in foreign countries. At the same time, its production process is simple and its comprehensive performance is excellent. It is an ideal material for new automotive steel.

Contents of the invention

Aiming at the requirements of the new type of automotive steel for material strength and plasticity, the invention provides an ultra-fine-grain composite bainite high-strength automotive steel and its preparation method. Composite bainite, ferrite, and retained austenite cooperate with each other and have good formability, and its strength and plasticity have reached the performance requirements of the third-generation automotive steel.

The technical solution of the present invention is: an ultra-fine-grain composite bainite high-strength and toughness automobile steel, the chemical composition weight percent of which is: C: ≤0.30%, Si: 1.05-1.75%, Mn: 1.25-2.45%, V : 0.15 to 0.25%, the balance being Fe and unavoidable impurities.

Another object of the invention of this method is to provide a preparation method for ultra-fine-grain composite bainite high-strength and tough automotive steel, which specifically includes the following steps:

Step 1: Weigh the raw materials according to the designed composition, carry out smelting and continuous casting to obtain hot-rolled plates, pickle the hot-rolled plates and then cold-roll them. mm cold rolled sheet;

Step 2: The cold-rolled sheet prepared in step 1 is heated to a temperature of 780-880°C in the austenite-ferrite two-phase region at 5-25°C/s, kept for 2-3min, and set aside;

Step 3: cooling the steel plate processed in step 2 to a temperature of 350-430° C. in the bainite transformation temperature region at 30-60° C./s, and keeping it warm for 6-7 minutes;

Step 4: cooling the steel plate treated in step 3 to room temperature at a cooling rate of 20-40° C./s to obtain ultra-fine-grain composite bainite high-strength and toughness automobile steel.

Further, the structure of the ultra-fine-grain composite bainite high-strength and tough automobile steel is lath bainite + granular bainite + ferrite + retained austenite, and the volume ratio of each structure is: lath bainite 20% to 30% of ferrite, 20% to 30% of granular bainite, 30% to 50% of ferrite, and 10% to 18% of retained austenite; the ultra-fine grain composite bainite high strength and toughness automotive steel The average grain size is less than 5 μm.

Further, the volume ratio of the lath bainite and the granular bainite is 0.8-1.0.

Further, the ultrafine-grain composite bainite high-strength and tough automotive steel has a yield strength R _p ≥ 600 MPa, a tensile strength R _m ≥ 1100 MPa, an elongation after fracture A ≥ 30%, and a strength-plastic product ≥ 30 GPa·%.

The effect of main element in the material of the present invention is:

C is an important element for the formation of austenite and the main element for stabilizing austenite. During the transformation process of austenite to bainite, the diffusion of C element occurs at the same time, and the enrichment of C element in the retained austenite makes it in the It can exist stably at room temperature, and the increase of C content can also improve the strength of the material.

Si can inhibit the precipitation of carbides such as cementite, reduce the solid solution C in ferrite, and increase the C content in austenite, so as to ensure sufficient stable retained austenite at room temperature.

The main function of Mn is to enhance the stability of austenite, prolong its transformation and incubation period, and make the transformation of ferrite and bainite easy to control. At the same time, it also promotes the reduction of Ms and forms a certain volume of carbon-rich retained austenite.

In the process of heating and heat preservation, V can inhibit the grain growth and increase the grain coarsening temperature, thereby refining the grain structure to a certain extent; in addition, V is also a strong carbide constituent element, and it mainly dissolves in a trace amount in steel It exists in two forms of ferrite or the second phase of vanadium carbonitride, which plays a role of precipitation strengthening.

The invention prepares a kind of multi-phase structure high-strength toughness automobile steel with ultra-fine grain structure through reasonable element ratio through rolling forming and heat treatment process, and the volume ratio of lath bainite and granular bainite It is controlled between 0.8 and 1.0 to ensure that the material has high strength and toughness. The metastable retained austenite-induced phase transformation effect can further improve the plasticity of the material, and the precipitated phase formed by V and C can also produce a precipitation strengthening effect. In addition, the fine-grain strengthening effect can improve the comprehensive performance of the material to varying degrees.

Description of drawings

FIG. 1 is a scanned structure image of high-strength steel in Example 1 of the present invention.

Fig. 2 is a scanned structure image of the high-strength steel in Example 1 of the present invention.

Fig. 3 is a scanned structure image of the high-strength steel in Example 1 of the present invention.

Fig. 4 is a transmission electron microscope structure image of the high-strength steel in the embodiment of the present invention.

Fig. 5 is an image of the XRD detection result of the high-strength steel in the embodiment of the present invention.

detailed description

The technical solutions of the present invention will be further described below in conjunction with specific embodiments.

Example 1:

First, smelting and continuous casting are carried out according to the above composition range, and then the composition of the slab is detected, see Table 1.

Table 1 Composition of billets (wt.%)

The specific heat treatment process parameters are shown in Table 2.

Table 2 Heat treatment process parameters

The mechanical properties of the high-strength automotive steel prepared according to Table 1 and Table 2 are shown in Table 3.

Table 3 Mechanical Properties

The high-strength automotive steel prepared according to Table 1 and Table 2 was measured by XRD to measure the residual volume fraction, and at the same time calculate the volume fraction of other phases. The results are shown in Figure 5 and Table 4.

Table 4 Contents of different tissues

It can be seen from Table 3 that each mechanical property has reached the required performance index, the yield strength is 692MPa, the tensile strength is 1116MPa, the elongation is 31%, and the strong-plastic product is 34.6GPa·%. The scanning photo is shown in Figure 1, and the transmission photo is shown in Figure 4. The structure is lath bainite + granular bainite + ferrite + retained austenite, and lath and granular bainite respectively account for the total volume 21% and 25%, the volume ratio of the two is 0.84, the ferrite volume fraction is 40%, and the retained austenite volume fraction is 14%, mainly distributed in the ferrite grain boundary in block form or in film form Between the bainite laths.

Example 2:

First, smelting and continuous casting are carried out according to the above composition range, and then the composition of the slab is detected, see Table 5.

Table 5 Composition of billets (wt.%)

The specific heat treatment process parameters are shown in Table 6.

Table 6 Heat treatment process parameters

Table 7 shows the mechanical properties of the high-strength plastic product automotive steel prepared according to Table 5 and Table 6.

Table 7 Mechanical Properties

The high-strength automotive steel prepared according to Table 5 and Table 6 was measured by XRD to measure the paralyzed volume fraction, and at the same time calculate the volume fraction of other phases. The results are shown in Figure 5 and Table 4.

Table 8 Contents of different tissues

It can be seen from Table 7 that all mechanical properties have reached the required performance indicators, the yield strength is 669MPa, the tensile strength is 1136MPa, the elongation is 30%, and the strong-plastic product is 34.1GPa·%. The scanning photo is shown in Figure 2, and the transmission photo is shown in Figure 4. The structure is lath bainite + granular bainite + ferrite + retained austenite, and lath and granular bainite respectively account for the total volume 24% and 26%, the volume ratio of the two is 0.92, the volume fraction of ferrite is 35%, and the volume fraction of retained austenite is 15%, which are mainly distributed in the ferrite grain boundary or in the form of film Distributed between bainite laths.

Example 3:

First, smelting and continuous casting are carried out according to the above composition range, and then the composition of the slab is detected, see Table 9.

Table 9 Composition of billets (wt.%)

The specific heat treatment process parameters are shown in Table 10.

Table 10 heat treatment process parameters

See Table 11 for the mechanical properties of the high-strength plastic product automotive steel prepared according to Tables 9 and 10.

Table 11 Mechanical Properties

The high-strength automotive steel prepared according to Table 9 and Table 10 was measured by XRD to measure the residual volume fraction, and at the same time calculate the volume fraction of other phases. The results are shown in Figure 5 and Table 4.

Table 12 Content of different tissues

It can be seen from Table 11 that each mechanical property has reached the required performance index, the yield strength is 654MPa, the tensile strength is 1163MPa, the elongation is 30%, and the strong-plastic product is 34.9GPa·%. The scanning photo is shown in Figure 3, and the transmission photo is shown in Figure 4. The structure is lath bainite + granular bainite + ferrite + retained austenite, and lath and granular bainite respectively account for the total volume 21% and 26%, the volume ratio of the two is 0.81, the volume fraction of ferrite is 38%, and the volume fraction of retained austenite is 15%, mainly distributed in the ferrite grain boundary in block form or in film form Distributed between bainite laths.

For the samples prepared according to the ingredients and process parameters of the above examples, all the mechanical properties reached the required performance indicators, the yield strength>600MPa, the tensile strength>1100MPa, the elongation after fracture A≥30%, and the strength-plastic product≥30GPa ·%. The transmission electron microscope photo is shown in Figure 4. It is composed of lath and granular composite bainite, ferrite, and retained austenite. The volume ratio of lath bainite to granular bainite is 0.8 to 1.0. The grains of the structure are uniform and fine, and the average grain size is less than 5 μm. Retained austenite is mainly distributed in the ferrite grain boundary in block form or in the bainite lath interface in film form, and the phase transformation induced plastic effect occurs during deformation to improve the deformability of the material, and the composite form of bainite The body structure can also improve the strength and toughness of the material. The material has excellent comprehensive mechanical properties, and its strength and plasticity can meet the requirements of the third-generation automotive steel.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all should be covered by the present invention. within the scope of the claims.

Claims (4)

1. A method for preparing ultra-fine-grain composite bainitic high-strength and tough automotive steel, the chemical composition weight percentage of the automotive steel is: C: ≤0.30%, Si: 1.05~1.75%, Mn: 1.25~2.45% , V: 0.15~0.25%, the balance is Fe and unavoidable impurities. It is characterized in that step 1: take raw materials according to the design composition, carry out smelting and continuous casting to obtain hot-rolled sheets, and pickle the hot-rolled sheets After cold rolling, the reduction rate of cold rolling is 45% to 65%, and a cold rolled sheet with a thickness of 1.5 to 2.0 mm is obtained; Step 2: The cold-rolled sheet prepared in step 1 is heated to a temperature of 780-880°C in the austenite-ferrite two-phase region at 5-25°C/s, kept for 2-3min, and set aside; Step 3: cooling the steel plate processed in step 2 to a temperature of 350-430° C. in the bainite transformation temperature region at a rate of 30-60° C./s, and keeping it warm for 6-7 minutes; Step 4: cooling the steel plate treated in step 3 to room temperature at a cooling rate of 20-40° C./s to obtain ultra-fine-grain composite bainite high-strength and toughness automobile steel. 2. The preparation method according to claim 1, characterized in that, the volume ratio of each structure of the ultra-fine-grain composite bainite high-strength and tough automotive steel is: lath bainite 20% ~ 30%, granular 20%-30% of bainite, 30%-50% of ferrite, and 10%-18% of retained austenite; the average grain size of the ultra-fine-grain composite bainite high-strength and tough automotive steel is less than 5 μm. 3 . The preparation method according to claim 2 , characterized in that, the volume ratio of the lath bainite and granular bainite is 0.8-1.0. 4. The preparation method according to claim 1, characterized in that, the yield strength R _p ≥ 600 MPa, the tensile strength R _m ≥ 1100 MPa, and the elongation after fracture of the ultra-fine-grain composite bainite high-strength and toughness automotive steel A ≥ 30%, strong plastic product ≥ 30GPa·%.