Production method of low-alloy high-strength automobile steel by cover annealing process

Abstract

The invention relates to the field of steel rolling, in particular to a production method of low-alloy high-strength automobile steel for a cover annealing process, which comprises the following chemical components in percentage by weight: 0.05 to 0.09 percent of C, 0.28 to 0.38 percent of Si, 0.80 to 1.10 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.04 percent of S, 0.027 to 0.037 percent of Nb, 0.02 to 0.032 percent of Cr and the balance of Fe and inevitable impurities. The production method comprises a hot rolling process, a pickling cold rolling process, a cold rolling cover annealing process and a leveling process. The method has the advantages that the feeding components are optimized, the problem of low strength caused by weak solution strengthening and precipitation strengthening in the cover annealing process is solved, the content of Si, Mn and Nb is increased, the strengthening mechanism of the low-alloy high-strength steel is enhanced, and the overall strength is improved; by optimizing product components and controlling hot rolling and cold rolling processes, the performance stability of low overall strength, high head and tail strength compared with intermediate strength and the like of the low-alloy high-strength automobile strip steel is improved, and the stamping use requirement and the product strength requirement of automobile users are met.

Description

Technical Field

[0001] The invention relates to the field of steel rolling, and in particular to a method for producing low-alloy high-strength automobile steel by annealing process. Background Art

[0002] Cold-rolled annealed low-alloy high-strength steel has high strength as well as good plasticity, low-temperature toughness and weldability. Therefore, it is widely used in automotive structural parts and building structural parts. At present, low-alloy high-strength steel is mainly achieved by combining solid solution strengthening and precipitation strengthening, which is obtained by adding elements such as Mn, Si, B, V and Nb to the steel and C and N elements in the steel and coordinating the hot and cold rolling process control. In order to better control the precipitation strengthening of low-alloy high-strength steel, a continuous annealing process is generally adopted to achieve more precise control of the cooling of the strip. However, when the hood annealing process is adopted, there is no control advantage of the continuous annealing process, and there are usually problems such as low overall strength of the strip and higher strength at the head and tail compared to the middle.

[0003] In the prior art, the announcement number is CN111575592A, and the patent name is "A low-alloy high-strength steel with a yield strength of 460MPa and a production method", "The weight percentage of its chemical composition is: C: 0.07-0.09%, Si≤0.05%, Mn: 1.20-1.50%, P: 0.03%-0.05%, A1s: 0.025-0.060%, Nb: 0.045-0.055%, Ti: 0.005-0.015%, Ca: 0.001-0.003%, S≤0.006%, N≤0.0025%, O≤0.0025%, and the rest is Fe and unavoidable impurities. Further, the weight percentage of its chemical composition is: C: 0.07-0.09%, Si≤0.05%, Mn: 1.20-1 .50%, P: 0.03%~0.05%, A1s: 0.025~0.060%, Nb: 0.045~0.055% Ti: 0.005~0.015%, Ca: 0.001~0.003%, S≤0.005%, N≤0.0020%, 0≤0.0015%, the rest is Fe and unavoidable impurities. A method for producing low-alloy high-strength steel with a yield strength of 460MPa according to any of the above items, including hot rolling, pickling cold rolling, continuous annealing and leveling processes, and also including smelting processes and continuous casting processes before hot rolling. The smelting process adopts LF-RH double refining process. This invention is mainly aimed at low-alloy high-strength steel with a yield strength of 460MPa. The matching cold rolling annealing process is a continuous annealing process path, and the process control technology of the hood annealing process path is not pointed out. Summary of the invention

[0004] The purpose of the present invention is to provide a production method of low-alloy high-strength automobile steel by annealing process. The method is mainly based on the annealing process to study the production of low-alloy high-strength automobile steel, and solves the problems of unstable product performance such as low overall strength of the strip and higher strength in the middle compared with the head and tail strength by optimizing the composition and process.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A method for producing low-alloy high-strength automobile steel by annealing process, wherein the chemical composition of the steel is C: 0.05%-0.09%, Si: 0.28%-0.38%, Mn: 0.80%-1.10%, P: ≤0.02%, S: ≤0.04%, Nb: 0.027%-0.037%, Cr: 0.02%-0.032%, and the remainder is Fe and unavoidable impurities; the production method comprises hot rolling process, pickling cold rolling, cold rolling annealing process and leveling process.

[0007] Hot rolling process: The coiling temperature is 670-690°C, and the coiling temperature is controlled in a U shape.

[0008] The cold rolling hood annealing process includes:

[0009] S1. Increase the hood annealing temperature and holding time

[0010] Annealing temperature is 700-720°C, holding time is 6-7h;

[0011] S2. The cooling of the automobile steel to be annealed is carried out by air cooling and water spray cooling in sequence.

[0012] Air cooling: The temperature of the steel plate is cooled from 700-720°C to 390-410°C;

[0013] Water spray cooling: The steel plate temperature is cooled from 390-410°C to 140-160°C.

[0014] During the air cooling process, when the steel plate temperature is between 600℃ and 710℃, the cooling time is 120 to 140 minutes; when the steel plate temperature is between 400℃ and 600℃, the cooling time is 120 to 140 minutes.

[0015] During the water spray cooling process, the cooling water is sprayed directly onto the top of the inner cover using the cooling cover buckled on the top of the inner cover. The cooling water is sprayed from the water outlet on the top of the cooling cover to the top of the inner cover, and the steel coil is cooled indirectly by cooling the inner cover.

[0016] U-shaped control means that the hot rolling coiling temperature is controlled in a U-shape, the coil head and tail are 10 to 50 meters long, and the coiling temperature is 50 to 70°C higher than the middle temperature target value.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] Optimize the feed composition, improve the low strength problem caused by weak solid solution strengthening and precipitation strengthening in the annealing process, focus on increasing the Si, Mn and Nb content, enhance the strengthening mechanism of low alloy high strength steel, and improve the overall strength; by optimizing the product composition and hot rolling and cold rolling process control, improve the overall low strength of low alloy high strength automotive steel strip and the comparison between the head and tail strength and the high strength in the middle and other performance stability, to meet the stamping requirements of automobile users and the demand for product strength. DETAILED DESCRIPTION

[0019] The present invention is described in detail below, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0020] The chemical composition of a low alloy high strength automotive steel produced by annealing process is shown in Table 1 in weight percentage.

[0021] Table 1 shows the chemical composition of low alloy high strength automobile steel by annealing process in weight percentage.

[0022] C Si Mn P S Nb Cr Before optimization 0.05％-0.09％ 0.10％-0.20％ 0.65％-0.95％ ≤0.02％ ≤0.04％ 0.02％-0.03％ 0.02％-0.032％ After optimization 0.05％-0.09％ 0.28％-0.38％ 0.80％-1.10％ ≤0.02％ ≤0.04％ 0.027％-0.037％ 0.02％-0.032％

[0023] The production method of low-alloy high-strength automobile steel by annealing process includes hot rolling process, pickling cold rolling, cold rolling annealing process and leveling process. Before hot rolling, it also includes smelting process and continuous casting process. The smelting process adopts LF-RH duplex refining process.

[0024] Hot rolling process: The coiling temperature is 670-690℃, and the coiling temperature is controlled in a U shape, that is, the head and tail coiling temperatures are 50-70℃ higher than the middle temperature target value, and the head and tail length of the steel coil is 10-50 meters.

[0025] The cold rolling hood annealing process includes:

[0026] S1. Increase the hood annealing temperature and holding time

[0027] The annealing temperature is increased from 690-710°C to 700-720°C, and the holding time is 6-7h.

[0028] S2. The cooling of the automobile steel to be annealed is carried out by air cooling and water spray cooling in sequence.

[0029] During the air cooling process, when the steel plate temperature is between 600℃ and 710℃, the cooling time is 120 to 140 minutes; when the steel plate temperature is between 400℃ and 600℃, the cooling time is 120 to 140 minutes.

[0030] During the water spray cooling process, the cooling water is sprayed directly onto the top of the inner cover using the cooling cover buckled on the top of the inner cover. The cooling water is sprayed from the water outlet on the top of the cooling cover to the top of the inner cover, and the steel coil is cooled indirectly by cooling the inner cover.

[0031] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[0032] [Example 1]

[0033] Optimize the feed composition, increase the content of Si, Mn and Nb, and enhance the strengthening mechanism, see Table 2.

[0034] Table 2 shows the chemical composition of low alloy high strength automotive steel by annealing process in percentage by weight.

[0035] serial number C Si Mn P S Nb Cr 1 0.07％ 0.33％ 0.98％ 0.02％ 0.004％ 0.032％ 0.026％ 2 0.06％ 0.30％ 0.85％ 0.02％ 0.003％ 0.030％ 0.023％ 3 0.08％ 0.35％ 1.05％ 0.02％ 0.003％ 0.035％ 0.030％

[0036] Hot rolling process: Optimize the hot rolling coiling temperature and increase the coiling temperature from 620℃ to 680℃, which can reduce the recrystallization resistance during cold rolling and reduce the yield and tensile strength. In conjunction with the optimization of the feed composition, the overall performance strength is avoided to be too high. The coiling temperature of the steel strip is controlled in a U shape, that is, the coiling temperature of the head and tail 10 to 50 meters is 50 to 70℃ higher than the target temperature in the middle, solving the problem of uneven coiling performance caused by the excessive cooling rate of the head and tail of the hot rolling coil.

[0037] The cold rolling hood annealing process includes:

[0038] S1. Increase the hood annealing temperature and holding time to improve the problem of low hood annealing temperature and insufficient local recrystallization; the annealing temperature is increased from 700℃ to 710℃, and the holding time is increased from 4h to 6h.

[0039] S2. The cooling of the automobile steel to be annealed is carried out by air cooling and water spray cooling in sequence.

[0040] During air cooling and water spray cooling, too fast air cooling will enhance precipitation strengthening and cause high performance; water spray cooling will not affect the performance of the final product, so the air cooling process needs to be controlled. Air cooling relies on two axial flow fans to achieve annealing temperature from 710℃ to 400℃. Each axial flow fan has a power of 7.5kW and a speed of 1500 (r / min); through field tests, when the steel plate temperature is between 6000℃ and 710℃, the cooling time is about 120 to 140 minutes; when the steel plate temperature is between 400℃ and 600℃, the cooling time is about 120 to 140 minutes. This can effectively control the cooling rate during precipitation strengthening, thereby controlling the strength of low-alloy high-strength steel in the annealing process.

[0041] Water spray cooling relies on the cooling hood, and the cooling temperature is cooled from 400℃ to 150℃. A water spray hood (cooling hood) buckled on the top of the inner hood is used to spray cooling water directly to the top of the inner hood. The cooling water is sprayed from the water outlet on the top of the cooling hood to the top of the inner hood, and the steel coil is cooled by cooling the inner hood.

[0042] The present invention optimizes the feed composition, improves the low strength problem caused by weak solid solution strengthening and precipitation strengthening in the annealing process, and focuses on increasing the Si, Mn and Nb contents, enhancing the strengthening mechanism of low-alloy high-strength steel, and improving the overall strength; by optimizing the product composition and hot rolling and cold rolling process control, the performance stability of the low overall strength of the low-alloy high-strength automobile steel strip and the high head and tail strength compared to the middle strength is improved, thereby meeting the stamping requirements of automobile users and the demand for product strength.

Claims

1. A method for producing low-alloy high-strength automotive steel by annealing process, characterized in that: Its chemical composition is calculated by weight as follows: C: 0.05%-0.09%, Si: 0.28%-0.38%, Mn: 0.80%-1.10%, P: ≤0.02%, S: ≤0.04%, Nb: 0.027%-0.037%, Cr: 0.02%-0.032%, and the balance is Fe and inevitable impurities; the production method includes hot rolling process, pickling cold rolling, cold rolling hood annealing process, and leveling process.

2. The method for producing low alloy high strength automobile steel by annealing process according to claim 1, characterized in that: The hot rolling process: the coiling temperature is 670-690°C, and the coiling temperature is controlled in a U shape.

3. The method for producing low alloy high strength automobile steel by annealing process according to claim 1, characterized in that: The cold rolling hood annealing process comprises: S1. Increase the hood annealing temperature and holding time Annealing temperature is 700-720°C, holding time is 6-7h; S2. The cooling of the automobile steel to be annealed is carried out by air cooling and water spray cooling in sequence. Air cooling: The temperature of the steel plate is cooled from 700-720°C to 390-410°C; Water spray cooling: The steel plate temperature is cooled from 390-410°C to 140-160°C.

4. The method for producing low alloy high strength automobile steel by annealing process according to claim 3, characterized in that: In the air cooling process, when the temperature of the steel plate is between 600°C and 710°C, the cooling time is 120 to 140 minutes; when the temperature of the steel plate is between 400°C and 600°C, the cooling time is 120 to 140 minutes.

5. The method for producing low alloy high strength automobile steel by annealing process according to claim 3, characterized in that: In the water spray cooling process, the cooling hood buckled on the top of the inner hood is used to directly spray cooling water to the top of the inner hood. The cooling water is sprayed from the water outlet on the top of the cooling hood to the top of the inner hood, and the steel coil is indirectly cooled by cooling the inner hood.

6. The method for producing low alloy high strength automobile steel by annealing process according to claim 2, characterized in that: The U-shaped control is that the hot rolling coiling temperature is controlled in a U-shape, the coiling temperature is 50 to 70° C. higher than the middle temperature target value, and the coiling temperature is 10 to 50 meters from the head to the tail of the steel coil.