Annealing process for dynamically adjusting yield strength of DP780

Abstract

The invention discloses an annealing process for dynamically adjusting yield strength of DP780. The annealing process comprises a preheating section, a heating section, a soaking section, a slow cooling section, a rapid cooling section, an aging section and a final cooling section. The slow cooling section comprises five cooling fans, and different opening degrees can be set for each cooling fan; the temperature of the slow cooling section is 600-650 DEG C, and different yield strengths are realized by adopting a positive cooling mode, a balanced mode or a reverse cooling mode. According to the process, through dynamic adjustment of a slow cooling mode, control over different yield strengths of DP780 under a same-component system is achieved, and production of low-cost DP780 products with different yield strengths is achieved; the technological process is easy to control, and different yield strength values can be achieved through the same component. According to the process, multi-stage adjustment of the yield strength is achieved only by regulating and controlling a single factor, namely a slow cooling system, the production cost is greatly reduced, the method has the advantages of being simple in process regulation and control and convenient to operate, process switching can be conducted at any time in the batch production process of DP780 of different grades, and the production efficiency is greatly improved.

Description

Technical Field

[0001] The invention relates to a steel heat treatment process, in particular to an annealing process for dynamically adjusting the yield strength of DP780. Background Art

[0002] As the requirements for automobile lightweighting become increasingly stringent, the proportion of ultra-high-strength steel used in automobile bodies is increasing. While considering lightweighting and improving strength, automobile manufacturers must also take into account reasonable incoming material costs. Therefore, the cost of the material production process becomes particularly important. Without changing the composition and upstream process, the use of dynamic adjustment of the downstream production process can significantly reduce production costs.

[0003] Under the current green and low-carbon background, dual-phase strip steel for automobiles is produced using a continuous annealing process. The annealing process purifies the structure and eliminates internal stress. While improving plasticity and strength, the product also maintains excellent surface quality. Dual-phase steel is widely favored by stamping manufacturers for its excellent continuous yield characteristics. The 780MPa-level dual-phase steel is widely used in high-strength supporting parts such as vehicle body anti-collision beams, door sill beams, door reinforcements, A-pillars, B-pillars, etc., and it covers different strengths and yield strength ratios, which makes the material particularly restricted by the component system and upstream processes during the production process, which is very unfavorable for stable production and energy saving and consumption reduction. Therefore, the present invention flexibly regulates from the perspective of annealing process, breaks through the component restrictions, and realizes dynamic adjustment of yield strength, so as to improve production stability and improve production efficiency.

[0004] After searching, the application number is: 202110113635.2, the name is: a method for producing cold-rolled DP780 dual-phase steel with simple control of multi-level yield strength. The multi-level control of yield strength adopts multi-means of adjusting heat and leveling, and the process control points are complex and not conducive to stable production. Application number: 201811260884.9, the name is: a method for producing economical hot-rolled DP780 steel with different yield strength levels, which is essentially different from the present invention. It is hot-rolled DP780 and does not involve annealing process. Application number: 202211282713.2, the name is: a 1180MPa graded continuous annealing dual-phase steel with different yield ratios and its preparation method. The annealing process is still relatively complicated. The different yield ratios are achieved through multi-level control of hot rolling process and annealing process, which is not as convenient as the process method of the present invention. Application number: 201910256190.6, name: A 780MPa grade hot-dip galvanized duplex steel with different yield strength levels and a production method thereof. Although the three non-yield strength levels are the same, the yield strength values ​​are quite different from those of the present invention. Secondly, the process also adopts multi-point control, which is not conducive to reducing process costs and stabilizing production. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide an annealing process for dynamically adjusting the yield strength of DP780 which is simple to control and convenient to regulate.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: the annealing process includes a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an aging section and a final cooling section; the slow cooling section includes five cooling fans, each of which can be set to a different opening degree; the temperature of the slow cooling section is 600°C to 650°C, and different yield strengths are achieved by using a positive cooling mode, a balanced mode or a reverse cooling mode;

[0007] In the positive cooling mode, the opening degrees of the five cooling fans are set to 20±20%, 35±20%, 50±20%, 65±20%, and 80±20% in sequence;

[0008] In the balanced mode, the opening degrees of the five cooling fans are set to 50±10%, 50±10%, 50±10%, 50±10%, 50±10% in sequence;

[0009] In the reverse cooling mode, the opening degrees of the five cooling fans are set to 80±20%, 65±20%, 50±20%, 35±20%, and 20±20% respectively.

[0010] Furthermore, the temperature of the preheating section is 230°C to 250°C; the temperature of the heating section is 650°C to 800°C; the temperature of the soaking section is 760°C to 820°C; the temperature of the rapid cooling section is 240°C to 320°C; the temperature of the aging section is 240°C to 280°C; and the temperature of the final cooling section is 100°C to 150°C.

[0011] Furthermore, the thickness of the DP780 steel is 0.5-3.0 mm, and the belt speed during the annealing process is 60 m / min-160 m / min.

[0012] Furthermore, the yield strength of the DP780 steel obtained by adopting various modes in the slow cooling section is as follows: in the forward cooling mode, the yield strength is 420-470 MPa; in the equilibrium mode, the yield strength is 450-500 MPa; and in the reverse cooling mode, the yield strength is 500-550 MPa.

[0013] The technical principle of the present invention is that cold-rolled strip steel needs to be annealed, especially ultra-high strength steel for automobiles, and its excellent mechanical properties benefit from the process control during the annealing process. Two-phase annealing is a necessary condition for strip annealing. During the annealing heating process, the original ferrite structure is transformed into austenite, and during the annealing cooling process, austenite is transformed into bainite and martensite, accompanied by the precipitation of ferrite and carbide. However, the actual cooling process is divided into high-temperature interval cooling and low-temperature interval cooling. The present invention realizes the difference in the precipitation size and purity of ferrite during the slow cooling process by controlling the cooling rate of different intervals during the cooling process. According to the principle of metallurgy, the ferrite precipitated in the high-temperature interval is larger and purer, and the yield strength is lower; the ferrite precipitated in the low-temperature interval has finer grains and is rich in carbides, and the yield strength is higher. Therefore, the precipitation of ferrite in different temperature sections is achieved by controlling the cooling rate of the high and low temperature intervals by the slow cooling fan. In the positive cooling mode, the high temperature section is cooled slowly and ferrite precipitates at high temperature; in the balanced mode, the rate is the same and ferrite precipitates evenly; in the reverse cooling mode, the high temperature section is cooled at a high speed and ferrite can only precipitate at low temperature, thereby achieving control of different yield strengths in the three slow cooling modes.

[0014] The beneficial effects of the above technical solution are: the present invention realizes the control of different yield strengths of DP780 under the same composition system through dynamic adjustment of the slow cooling mode, and realizes the production of DP780 products with low cost and different yield strengths; the process control is simple, and different yield strength values ​​can be achieved with the same composition; it breaks through the cognition of traditional production technology, regulates the slow cooling system according to the function of the equipment, combines the equipment and the process, and realizes the optimization of the equipment function. The present invention only regulates the single factor of the slow cooling system, and realizes the multi-level adjustment of the yield strength, which greatly reduces the production cost. It has the characteristics of simple process control and easy operation. The process can be switched at any time during the batch production of DP780 at different levels, which greatly improves the production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0016] Figure 1 is a schematic diagram comparing the cooling capacities of the five slow cooling fans in different modes according to the present invention;

[0017] Figure 2 This is a metallographic structure diagram of a product obtained by adopting a positive cooling mode in the slow cooling section of the present invention;

[0018] Figure 3 It is a metallographic structure diagram of the product obtained by adopting the equilibrium mode in the slow cooling stage of the present invention;

[0019] Figure 4This is a metallographic structure diagram of a product obtained by adopting a reverse cooling mode in the slow cooling section of the present invention. DETAILED DESCRIPTION

[0020] The annealing process for dynamically adjusting the yield strength of DP780 is applicable to DP780 dual-phase steel, wherein the chemical composition and mass percentage of the DP780 dual-phase steel are as follows: C 0.070-0.110%, Si 0.25-0.55%, Mn 1.90-2.20%, Cr 0.15-0.35%, Nb 0.015-0.040%, Mo 0.15-0.25%, P≤0.020%, N≤30ppm, and the remainder is Fe and some unavoidable impurities.

[0021] The annealing process for dynamically adjusting the yield strength of DP780 includes an annealing process and a leveling process. The annealing production line is an ultra-high strength steel continuous annealing line, and the slow cooling section is equipped with 5 cooling fans. The process is as follows:

[0022] (1) Annealing process: Using cold hardened steel strip as raw material, it is produced through continuous annealing process, including preheating section, heating section, soaking section, slow cooling section, rapid cooling section, aging section and final cooling section in sequence; soaking section is divided into heating section 1 and heating section 2 in sequence. The temperature of the preheating section is 230℃~250℃; the temperature of the heating section 1 is 650℃~700℃; the temperature of the heating section 2 is 750℃~800℃; the temperature of the soaking section is 760℃~820℃; the temperature of the slow cooling section is 600℃~650℃; the temperature of the rapid cooling section is 240~320℃; the temperature of the aging section is 240~280℃; the temperature of the final cooling section is 100~150℃. The thickness of the DP780 steel is 0.5~3.0mm, and the belt speed of the annealing process is 60m / min~160m / min. It is best to adjust the belt speed according to the thickness of the plate.

[0023] The slow cooling section includes five cooling fans, each of which can be set to a different opening degree, and the opening degree is 0-100%. The opening degree is controlled by power to achieve different cooling capacities. Three slow cooling modes are achieved by different opening degrees of the five cooling fans, namely, positive cooling mode, balanced mode or reverse cooling mode. The fan opening degree control of each slow cooling mode is as follows:

[0024] The positive cooling mode: according to the moving direction of the plate, the opening degrees of the cooling fans from the first to the fifth rack are set to 20±20%, 35±20%, 50±20%, 65±20%, and 80±20% respectively;

[0025] The balance mode: according to the moving direction of the plate, the opening degrees of the first to fifth cooling fans are set to: 50±10%, 50±10%, 50±10%, 50±10%, 50±10% and 50±10% in sequence;

[0026] The reverse cooling mode: according to the moving direction of the plate, the opening degrees of the cooling fans from the first to the fifth rack are set to: 80±20%, 65±20%, 50±20%, 35±20%, 20±20% respectively.

[0027] The cooling capacity comparison of the five slow cooling fans in different modes is shown in Figure 1 ; Among them, the slope of each line segment in the figure represents the opening degree of the fan. The larger the opening degree, the larger the absolute value of the slope of the fan, and the stronger the cooling capacity. Conversely, the smaller the opening degree, the smaller the absolute value of the slope, and the weaker the cooling capacity. When the absolute values ​​of the slopes are the same, the cooling capacities of the five fans are the same.

[0028] (2) Leveling process: The leveling elongation is 0.1% to 0.5%.

[0029] (3) After adopting the above process, the longitudinal elongation after fracture A80 of the obtained DP780 steel is ≥18%, the ultimate hole expansion rate is ≥25%, and the tensile strength is 780MPa~900MPa; by controlling the above slow cooling mode, different yield strengths and metallographic structures are obtained: in the positive cooling mode, the yield strength is 420~470MPa, and the structure is island martensite + block ferrite with a scale of 6~10μm; in the equilibrium mode, the yield strength is 450~500MPa, and the structure is island martensite + block ferrite with a scale of 5~8μm; in the reverse cooling mode, the yield strength is 500~550MPa, and the structure is island martensite + block ferrite with a scale of 4~6μm.

[0030] Example 1-15: The annealing process for dynamically adjusting the yield strength of DP780 is specifically described as follows.

[0031] (1) The annealing process parameters of each embodiment are shown in Table 1; the opening degree of each cooling fan of each frame in the slow cooling section of each embodiment is shown in Table 2.

[0032] Table 1: Annealing process parameters of Examples 1 to 15

[0033]

[0034]

[0035] Table 2: Opening degree of each cooling fan in Examples 1 to 15 (%)

[0036]

[0037]

[0038] (2) The composition of the DP780 steel plate is shown in Table 3; the mechanical properties of the obtained DP780 steel plate are shown in Table 4.

[0039] Table 3: Chemical composition of DP780 steel plates in Examples 1 to 15

[0040]

[0041] In Table 3, the balance is Fe and some inevitable impurities.

[0042] Table 4: Mechanical properties of DP780 steel plates obtained in Examples 1 to 15

[0043]

[0044]

[0045] As shown in Table 4, the tensile strength Rm of DP780 dual-phase steel prepared by this annealing process is 780MPa~900MPa; the elongation after fracture is A80≥18%; the ultimate hole expansion rate is ≥25%. The yield strength and microstructure are classified according to the slow cooling mode: positive cooling mode, low yield strength: 420~470MPa, the microstructure is island martensite + block ferrite with a scale of 6~10μm; balanced mode, medium yield strength: 450~500MPa, the microstructure is island martensite + block ferrite with a scale of 5~8μm; reverse cooling mode, high yield strength: 500~550MPa, the microstructure is island martensite + block ferrite with a scale of 4~6μm.

[0046] The metallographic structures of the dual-phase steel produced in Example 1, Example 6 and Example 11 are shown in FIG. Figure 2 , Figure 3 , Figure 4 From the metallographic organization diagrams 2-4, it can be seen that the metallographic organization of the DP780 dual-phase steel obtained by this annealing process is blocky ferrite + island martensite. The average grain sizes of ferrite are statistically 9.6μm, 7.8μm, and 4.8μm, respectively.

Claims

1. An annealing process for dynamically adjusting the yield strength of DP780, characterized in that: The annealing process includes preheating section, heating section, soaking section, slow cooling section, fast cooling section, aging section and final cooling section; the slow cooling section contains five cooling fans, each of which can be set to a different opening degree; the temperature of the slow cooling section is 600℃~650℃, and different yield strengths are achieved by using positive cooling mode, balanced mode or reverse cooling mode; In the positive cooling mode, the opening degrees of the five cooling fans are set to 20±20%, 35±20%, 50±20%, 65±20%, and 80±20% respectively; In the balanced mode, the opening degrees of the five cooling fans are set to 50±10%, 50±10%, 50±10%, 50±10%, 50±10% in sequence; In the reverse cooling mode, the opening degrees of the five cooling fans are set to 80±20%, 65±20%, 50±20%, 35±20%, and 20±20% respectively.

2. The annealing process for dynamically adjusting the yield strength of DP780 according to claim 1, characterized in that: The temperature of the preheating section is 230°C to 250°C; the temperature of the heating section is 650°C to 800°C; the temperature of the soaking section is 760°C to 820°C; the temperature of the rapid cooling section is 240°C to 320°C; the temperature of the aging section is 240°C to 280°C; and the temperature of the final cooling section is 100°C to 150°C.

3. The annealing process for dynamically adjusting the yield strength of DP780 according to claim 1, characterized in that: The thickness of the DP780 steel is 0.5-3.0 mm, and the belt speed during the annealing process is 60 m / min-160 m / min.

4. An annealing process for dynamically adjusting DP780 yield strength according to claim 1, 2 or 3, characterized in that: The yield strength of the DP780 steel obtained by adopting various modes in the slow cooling section is as follows: in the forward cooling mode, the yield strength is 420-470 MPa; in the equilibrium mode, the yield strength is 450-500 MPa; and in the reverse cooling mode, the yield strength is 500-550 MPa.

Images