TWI534274B - Method of Annealing Process for High Strength Steel - Google Patents

Description

Annealing process method for high strength steel

The invention relates to an annealing process method for steel materials, in particular to an annealing process method for high-strength steel materials.

In recent years, in response to the demand for energy saving and carbon reduction, the automotive industry is committed to reducing the weight of the car body to reduce fuel consumption and achieve energy saving and carbon reduction. The effective way to reduce the weight of the car body is to thin the thickness of the steel plate for the car body. However, when the thickness of the steel plate is thinned, the safety of the car body cannot be sacrificed. Therefore, the strength of the steel plate for the car must be improved. While the strength of the steel sheet for the vehicle is improved, the ductility of the steel sheet cannot be sacrificed. Therefore, it is necessary to develop a steel sheet steel for vehicles with high strength and high ductility.

In the past few years, the steel industry has developed so-called first-generation (1st generation) and second-generation (2nd generation) high-strength automotive high strength steel (AHSS). The first generation of high-strength steel plate steels mainly refers to phase-induced plastic steels (TRIP-assisted steels) with tensile strengths between 600 and 1000 MPa and elongations between 20 and 40%. The product (i.e., the product of tensile strength and elongation) is less than 20 GPa%. Since the tensile strength and elongation of the phase-induced plasticity steel are lower than those of the automotive industry, there is a second generation of high-strength steel sheet steel for vehicles.

The second generation of high-strength steel plate steels mainly refers to TWIP steels, which are high-manganese alloy steels with a manganese content of about 20-30% by weight. The twin-induced plastic steel has excellent strength, the tensile strength is between 600 and 1100 MPa, and the elongation can be maintained between 60 and 95%, so that the strong extension can be as high as 60 GPa%. Although twinning induction The main reason that plastic steel has been developed for nearly 10 years, but still not accepted by the automotive industry is that its required manganese content is too high and does not meet commercial cost considerations.

Because the strong extension of the first generation of high-strength steel plate steel can not meet the steel plate property requirements and the second-generation high-strength steel plate steel is too high to meet the commercial cost requirements, the automotive industry It has turned to the development of the third generation of high-strength steel sheet steel for vehicles.

Referring to Figure 1, it is a view showing the range of the target area of the third-generation high-strength steel sheet steel for vehicles. As shown in Fig. 1, the strong extension of the third-generation high-strength steel sheet steel for vehicles is in the range of 20 to 40 GPa%. However, the automobile industry is still in the development stage for the third generation of high-strength steel plate steel for vehicles, especially how the annealing process of steel should be designed to achieve the goal of strong steel product, which is a very important topic in the automotive industry. .

Therefore, it is necessary to provide an innovative and progressive high-strength steel annealing process to produce steel that meets the properties of the third-generation high-strength automotive steel.

The invention provides an annealing process method for high-strength steel, comprising the steps of: providing an alloy steel material, the composition of the alloy steel comprising 0.1-0.4 wt% carbon, 1-3 wt% manganese, 1-2 wt% bismuth, 0.1-0.2 wt. % titanium and the rest of the iron and unavoidable impurities; heating the alloy steel to the Worthite iron formation temperature, so that the alloy steel forms the Worthfield iron phase; cooling the alloy steel to the ferrite iron formation temperature, so that The alloy steel forms an interface nano-precipitate and a ferrite-iron phase; the alloy steel is cooled to a tough iron formation temperature to form a toughened iron phase of the alloy steel; and the alloy steel is cooled to a normal temperature to obtain a complex phase Microstructured high strength steel.

The annealing process of the present invention can produce a high-strength steel having a tensile strength of 815 MPa, an elongation of 26%, and a strong elongation of 21.2 GPa%, and the properties of the steel meet the requirements of the third-generation high-strength steel plate steel.

In order to be able to understand the technical means of the present invention more clearly, it can be in accordance with the specification. The objects, features, and advantages of the invention will be apparent from the description and appended claims appended claims

Ac1‧‧‧When the steel is heated, the temperature of the Worthite iron begins to form.

When Ac3‧‧‧ steel is heated, all ferrites are converted to the temperature of Worthite

S21~S25‧‧‧Steps

1 shows a range of the target area of the third-generation high-strength steel sheet steel; FIG. 2 shows a flow chart of the annealing process of the high-strength steel of the present invention; and FIG. 3 shows the Ac1 of the present invention at a heating rate of 5 ° C/sec. When the steel is heated, the temperature of the Worthite iron is formed) and the temperature measurement curve of Ac3 (when the steel is heated, all ferrites are converted to the temperature of the Worthite iron); Figure 4 (a) and (b) respectively show The temperature-time-phase transition curve and the continuous cooling phase transition curve of the two stages of the steel of the present invention; FIG. 5 shows the temperature-time curve of the annealing process of the invention example; FIG. 6 shows the photomicrograph of the steel of the invention example; A photomicrograph of titanium carbide nano-precipitate in the ferrite iron of the steel of the invention is shown; and FIG. 8 shows a range of properties of the steel of the invention.

2 is a flow chart showing the annealing process of the high strength steel of the present invention. Referring to step S21 of FIG. 2, an alloy steel material is provided, which comprises 0.1-0.4 wt% carbon, 1-3 wt% manganese, 1-2 wt% bismuth, 0.1-0.2 wt% titanium and the rest of the iron and is inevitable Impurities. In this step, 0.1-0.4wt% carbon can change the steel strengthening and residual Worthite iron to increase the elongation of the steel; 1-3wt% manganese can improve the hardening energy of the steel and reduce the cooling rate during annealing, and relax the cooling rate Process conditions; 1-2wt% 矽 can inhibit the formation of stellite carbon-iron carbides in the toughened iron zone, thereby increasing the ductility of the steel, while solid solution strengthening increases the strength of the steel; 0.1-0.2wt% titanium can form interface nano-precipitation Carbonitride, to strengthen the steel, or, in another embodiment, the titanium may be replaced by vanadium, niobium, molybdenum or tungsten.

In addition, in this step, the alloy steel may be hot rolled or cold rolled to form a rolling Extend the sheet.

Referring to step S22, the alloy steel material is heated to a Worthite iron formation temperature to form the alloy steel material into a Worthfield iron phase. In the present embodiment, the Worthite iron generation temperature is 800 to 1100 ° C, and the temperature holding time is 60 to 300 seconds.

Referring to step S23, the alloy steel material is cooled to a ferrite-grain formation temperature, so that the alloy steel material forms an interface nano-precipitate and a ferrite-grain iron phase. In this step, the preferred cooling rate is 5 to 40 ° C / sec, and the ferrite iron formation temperature is 580 to 750 ° C, and the holding time is not more than 60 seconds. In addition, the interface nano-precipitate formed in this step is a carbonitride.

Referring to step S24, the alloy steel material is cooled to a tough iron formation temperature to form the alloy steel material into a toughened iron phase. In this step, a preferred cooling rate is 5 to 40 ° C / sec, and a tough iron generation temperature is 300 to 500 ° C, and the holding time is not more than 300 seconds.

Referring to step S25, the alloy steel material is cooled to a normal temperature to obtain a high-strength steel material having a complex phase microstructure. In this step, a preferred cooling rate is 0.5 to 40 ° C / sec, and the multiphase microstructure comprises 60 to 80% ferrite iron phase, no more than 20% toughened iron phase, and no more than 40% residual wolf Sitian iron phase and no more than 20% Ma Tian scattered iron phase.

The invention is illustrated by the following examples, which are not intended to be limited to the scope of the invention.

Figure 3 shows the temperature of Ac1 (the temperature at which the Worthite iron begins to form when the steel is heated) and the temperature of Ac3 (when the steel is heated, all ferrites are converted to the temperature of the Worthite iron) at a heating rate of 5 ° C / sec. Measurement curve. Figures 4(a) and (b) show the temperature-time-phase transition curve and the continuous cooling phase transition curve of the two stages of the steel of the present invention, respectively.

The present invention is exemplified by an iron-0.11 wt% carbon-1.5 wt% manganese-1.44 wt% 矽-0.1 wt% titanium alloy steel, although Figure 3 shows a complete Worthfield ironization temperature of 970 ° C, however, to further ensure steel Complete Worthfield ironification, selected 1000 to 1100 ° C as the Wolsfield ironification and precipitation solution temperature. After fertilization in Vostian, the steel is cooled at a cooling rate of 20 ° C / sec to the ferrite-forming iron formation temperature range (ie, 580 to 750 ° C) as shown in Figure 4, due to the temperature. The lower the time, the longer the time between the Worthite iron and the ferrite phase is changed. Therefore, the strengthening effect of the interface nano-precipitate can be simultaneously precipitated during the formation of the ferrite iron.

Referring to Figure 5, there is shown a graph of the annealing process temperature-time of the inventive example. Taking FIG. 5 as an example, the temperature of the ferrite iron is selected to be 600 ° C and the temperature is maintained for 12 to 22 seconds to obtain about 70% of the interface nano-precipitation enhanced ferrite iron structure. Subsequently, it is cooled to a tough iron phase change zone (450 ° C) and held for 200 seconds, and finally cooled to room temperature to obtain the multiphase transformation of the ferrite iron, the toughened iron and the Worthite iron, and the residual phase When the material is plastically deformed, the steel is transformed into a new generation of granulated iron, and then the stress induces the phase transformation to increase the ductility of the steel.

Figure 6 shows a photomicrograph of the steel of the inventive example. Fig. 7 is a photomicrograph showing the titanium carbide nanoprecipitate in the ferrite of the steel of the invention.

Referring to Figures 6 and 7, the microstructure of the steel is a multiphase microstructure comprising 70% ferrite iron, 15% toughened iron, 12% 麻田散铁, and 3% residual Worthite iron. The steel elongation (El) can be increased to 26%, while the titanium carbide nanoprecipitate shown in Figure 7 can strengthen the ferrite iron, thereby increasing the tensile strength of the steel to 815 MPa. In addition, its strong extension can reach 21.2GPa%.

Referring to Figure 8, there is shown a map of the nature of the steel of the inventive example. As shown in Fig. 8, the properties of the steel of the invention are in conformity with the properties of the third generation high-strength steel sheet steel for vehicles.

The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the scope of the present invention. The scope of the invention should be as set forth in the appended claims.

S21~S25‧‧‧Steps

Claims (12)

An annealing process method for high-strength steel, comprising the steps of: (a) providing an alloy steel comprising 0.1-0.4 wt% carbon, 1-3 wt% manganese, 1-2 wt% bismuth, 0.1-0.2 wt. % titanium and the remaining iron and unavoidable impurities; (b) heating the alloy steel to the Worthite iron formation temperature so that the alloy steel forms the Worthfield iron phase; (c) cooling the alloy steel to the ferrite iron The temperature is generated such that the alloy steel material forms an interface nano-precipitate and a ferrite-grain iron phase; (d) cooling the alloy steel material to a toughening iron formation temperature, and the temperature holding time is not more than 300 seconds to form the alloy steel material Toughening the iron phase; and (e) cooling the alloy steel to room temperature to produce a high strength steel having a complex phase microstructure. The annealing process method of claim 1, wherein the step (a) further comprises hot rolling or cold rolling the alloy steel to form a rolled sheet. An annealing process for a high-strength steel material according to claim 1, wherein the Worstian iron formation temperature of the step (b) is 800 to 1100 °C. The annealing process method of claim 1, wherein the holding time of the step (b) is 60 to 300 seconds. An annealing process for a high strength steel material according to claim 1, wherein the cooling rate of the step (c) is 5 to 40 ° C / sec. An annealing process for a high-strength steel material according to claim 1, wherein the ferrite-iron formation temperature of the step (c) is 580 to 750 °C. The annealing process method of the high strength steel material of claim 1, The holding time of step (c) is not more than 60 seconds. An annealing process for a high-strength steel material according to claim 1, wherein the interface nano-precipitate of the step (c) is a carbonitride. The annealing process of claim 1, wherein the cooling rate of the step (d) is 5 to 40 ° C / sec. The annealing process of claim 1, wherein the toughening iron forming temperature of the step (d) is 300 to 500 °C. The annealing process of claim 1, wherein the cooling rate of step (e) is from 0.5 to 40 ° C / sec. The annealing process method of claim 1, wherein the multiphase microstructure of the step (e) comprises 60 to 80% of the ferrite phase, no more than 20% of the toughened iron phase, and no more than 40% of the residual Voss. Tian Tiexiang and no more than 20% Ma Tian scattered iron phase.