A flexible annealing method for cold-rolled high-strength steel

Abstract

The present invention discloses a flexible annealing method for cold-rolled high-strength steel, which is specifically as follows: inputting the strip speed adjustment amount M of the i-th strip speed adjustment i , based on the strip speed adjustment M i Adjust the target strip temperature T at the rapid cooling outlet i and target overaging temperature H i , to reduce product performance fluctuations caused by speed changes in the strip steel within the furnace. The present invention adaptively adjusts the rapid cooling outlet temperature and overaging temperature within the furnace based on the speed of the cold-rolled high-strength steel. This allows for both speed reduction to address unexpected accidents, reducing costs and increasing efficiency, and fully tapping production capacity to achieve speed increases and production increases. Furthermore, after speed adjustment, the product performance of the cold-rolled high-strength steel exhibits minimal fluctuations, and the difference in product performance before and after speed adjustment is minimal, resulting in good forming performance, as measured by elongation after fracture and hole expansion rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of automobile plate processing, and more particularly, to a flexible annealing method for cold-rolled high-strength steel. Background Art

[0002] Steel has long been a cornerstone material in the automotive industry. Automotive sheet steel is the most widely used type of steel in automotive applications, accounting for approximately 70%. Compared to conventional steel sheet, automotive sheet steel requires a longer production process, requires higher process requirements, and offers higher product margins. To adapt to the changing steel consumption structure, major steel companies have actively developed the automotive sheet market. Currently, major automotive sheet manufacturers include international companies such as ArcelorMittal, ThyssenKrupp, Nippon Steel & Sumitomo Metal, POSCO, and Swedish Steel, as well as domestic companies such as Baowu Steel Group, Anben Steel Group, Shougang Group, Hesteel Group, and Hunan Iron and Steel Group.

[0003] With the continuous advancement of lightweight vehicles, automotive sheet metal, represented by high-strength steel, has become the preferred material for body structures. In an increasingly competitive market, achieving optimal cost and efficiency are two key priorities. In industrial production, cold-rolled high-strength steel is typically heat treated in a continuous annealing furnace. The specific process includes uncoiling, welding, cleaning, annealing, flattening, surface inspection, and coiling. To ensure smooth production, an entry loop is installed between cleaning and annealing, an exit loop is installed between annealing and flattening, and an inspection loop is installed between flattening and surface inspection to ensure a sufficient strip storage capacity.

[0004] Current annealing processes typically utilize a rigid design rather than a flexible one, with on-site control based on target strip speed values. Under large-scale production conditions, this rigid design has the following drawbacks: 1) Relying on a looper to adjust the strip is insufficient to handle all emergencies, necessitating urgent action to reduce the strip speed within the furnace. First reducing the speed to address the incident and then increasing it to resume normal production often results in abnormal strip performance, leading to downgrades and reduced enterprise profitability. 2) When production is running smoothly, the looper cannot be fully utilized to adjust the strip and increase the strip speed within the furnace, hindering the company's ability to expand its profitability. Summary of the Invention

[0005] The present invention provides a flexible annealing method for cold-rolled high-strength steel, aiming to reduce product performance fluctuations caused by speed changes of strip steel in a furnace.

[0006] The present invention is achieved by providing a flexible annealing method for cold-rolled high-strength steel, the method being specifically as follows:

[0007] Input the strip speed adjustment amount M for the i-th furnace strip speed adjustment i , based on the speed adjustment of the strip steel in the furnace M i Adjust the target strip temperature T at the rapid cooling outlet i and target overaging temperature Hi , in order to reduce the fluctuation of product performance caused by the speed change of strip steel in the furnace.

[0008] Furthermore, the target strip temperature T at the rapid cooling outlet i The calculation formula is as follows:

[0009]

[0010] Among them, T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; M i is the speed adjustment amount of the strip steel when the speed of the strip steel in the furnace is adjusted for the i-th time; T0 is the starting strip steel temperature at the rapid cooling outlet before the speed adjustment of the strip steel in the furnace; X i Y is the slow cooling rate influence coefficient when the speed of the strip steel in the furnace is adjusted for the i-th time; i is the influence coefficient of the rapid cooling rate when the strip speed in the furnace is adjusted for the i-th time; Z1 is the influence coefficient of the chemical composition; Z2 is the influence coefficient of the rapid cooling outlet temperature.

[0011] Furthermore, the slow cooling rate influence coefficient X i , Rapid cooling rate influence coefficient Y i The specific methods for determining the values ​​of the chemical composition influence coefficient Z1 and the rapid cooling outlet temperature influence coefficient Z2 are as follows:

[0012] Average cooling rate R i Satisfy 6.5≤R i <8.0℃ / s, X i =0.8; satisfying 3.5≤R i <6.5℃ / s, X i =0.4; satisfying 1.0≤R i <3.5℃ / s, X i =0;

[0013] Average fast cooling rate K i Satisfy 30≤K i <60℃ / s, Y i =1.0; satisfying 60≤K i <100℃ / s, Y i =0.5; flat meets 100≤K i <120℃ / s, Y i =0;

[0014] When the mass fraction of Nb and Ti satisfies Nb+Ti≤0.025%, Z1=1.0; when Nb+Ti>0.025%, Z1=1.5;

[0015] When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, Z2=1.0; when Nb+Ti>0.025%, Z2=2.0.

[0016] Furthermore, the average cooling rate R i The calculation formula is: R0 is the initial average slow cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment;

[0017] Average fast cooling rate K i The calculation formula is: K0 is the initial average rapid cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment.

[0018] Furthermore, the target overaging temperature H i The formula is as follows:

[0019] H i =H0+T i -T0

[0020] Among them, H i is the target over-aging temperature when the speed of the strip steel in the furnace is adjusted for the i-th time, H0 is the initial over-aging temperature before the speed adjustment of the strip steel in the furnace; T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; T0 is the starting strip temperature at the rapid cooling outlet before the strip speed in the furnace is adjusted.

[0021] Furthermore, the speed adjustment of the strip steel in the furnace M i The speed is in the range of -10m / min to 10m / min.

[0022] Furthermore, the target strip speed V in the furnace i Meet: 60m / min≤V i ≤300m / min.

[0023] Furthermore, the cooling rate R i Meet: 1.0℃ / s <R i <8℃ / s, and the rapid cooling outlet temperature T i Satisfaction: T i ≤360℃.

[0024] Furthermore, the cold-rolled high-strength steel structure is a dual-phase structure of ferrite + martensite, or a multi-phase structure of at least one of ferrite + martensite + bainite, retained austenite, cementite, and second phase precipitates.

[0025] The present invention adaptively adjusts the rapid cooling outlet temperature and over-aging temperature in the furnace based on the speed of cold-rolled high-strength steel, which can not only reduce the speed to cope with sudden accidents, reduce costs and increase efficiency, but also fully tap the production capacity and achieve speed increase and production increase; in addition, after speed adjustment, the product performance of the cold-rolled high-strength steel has little fluctuation, and the difference in product performance before and after speed adjustment is small, and the forming performance with elongation after fracture and hole expansion rate as indicators is good. DETAILED DESCRIPTION

[0026] The specific implementation methods of the present invention are further explained in detail below through the description of embodiments, so as to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0027] The raw material of the strip steel is hard rolled coil, which is prepared according to the process flow of smelting → continuous casting → hot rolling → pickling and cold rolling → continuous annealing. The process flow of continuous annealing includes: heating, soaking, slow cooling, rapid cooling and over-aging. During the slow cooling period, austenite transforms into ferrite, and during the rapid cooling period, austenite transforms into martensite or bainite. During the over-aging period, tempering and further transformation of the retained austenite occur. Changing the speed of the strip steel in the furnace will inevitably change the time of each continuous annealing process, which will affect the structural ratio of the soft and hard phases and the hardness difference between the soft and hard phases. In order to obtain excellent product performance, the matching optimization design of the annealing process needs to be carried out during the speed regulation. Since the strip steel composition, slow cooling rate, rapid cooling rate, and rapid cooling outlet temperature have an impact on the microstructure and properties, the rapid cooling outlet temperature and chemical composition influence coefficient Z1, rapid cooling outlet temperature influence coefficient Z2, slow cooling rate influence coefficient X i , Rapid cooling rate influence coefficient Y i The present invention synchronously adjusts the rapid cooling outlet temperature and the over-aging temperature, which can better offset the influence of speed regulation on the microstructure and performance.

[0028] Based on production experience, when the slow cooling rate is ≥8°C / s, the ferrite transformation during the slow cooling period is significantly reduced; when the slow cooling rate is ≤1.0°C / s, the ferrite transformation during the slow cooling period is significantly increased, making it difficult to ensure the final microstructure and strength. Therefore, the slow cooling rate after speed increase is limited to >1.0°C / s and <8°C / s.

[0029] Based on this, the method for increasing speed and yield of cold-rolled dual-phase steel provided by the present invention is as follows:

[0030] The flexible annealing method for cold-rolled high-strength steel provided by the present invention is specifically as follows:

[0031] 1) Parameter collection, including the niobium and titanium mass fraction of the strip raw material, the initial strip speed V0 in the furnace before speed regulation, the initial average slow cooling rate R0, the initial average rapid cooling rate K0, the initial rapid cooling outlet temperature T0, and the initial overaging temperature H0;

[0032] 2) Use formula (1) to calculate the target strip speed V in the furnace during speed regulation i , the formula is as follows:

[0033] V i =V0+ΣM i (1)

[0034] Among them, i is the number of times the strip speed in the furnace is adjusted, V0 is the initial strip speed in the furnace before the strip speed is adjusted, V i is the target strip speed in the furnace during the i-th strip speed adjustment, M i is the strip speed adjustment amount during the i-th strip speed adjustment in the furnace.

[0035] 3) Use formula (2) to calculate the target strip temperature T at the rapid cooling outlet during speed regulation i , the formula is as follows:

[0036]

[0037] Among them, T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; T0 is the starting strip temperature at the rapid cooling outlet before the strip speed in the furnace is adjusted; X i Y is the slow cooling rate influence coefficient when the speed of the strip steel in the furnace is adjusted for the i-th time; i is the influence coefficient of the rapid cooling rate when the strip speed in the furnace is adjusted for the i-th time; Z1 is the influence coefficient of the chemical composition; Z2 is the influence coefficient of the rapid cooling outlet temperature.

[0038] The slow cooling rate influence coefficient X when the speed of the strip steel in the furnace is adjusted for the i-th time i , the rapid cooling rate influence coefficient Y when the strip speed in the furnace is adjusted for the i-th time i The specific methods for determining the values ​​of the chemical composition influence coefficient Z1 and the rapid cooling outlet temperature influence coefficient Z2 are as follows:

[0039] Average cooling rate R i Satisfy 6.5≤R i <8.0℃ / s, X i =0.8; satisfying 3.5≤R i <6.5℃ / s, X i =0.4; satisfies 1.0 <R i <3.5℃ / s, X i =0; average cooling rate R i The calculation formula is: R0 is the initial average slow cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment.

[0040] Average fast cooling rate K i Satisfy 30≤K i <60℃ / s, Y i =1.0; satisfying 60≤K i <100℃ / s, Y i =0.5; flat meets 100≤K i <120℃ / s, Y i =0; average rapid cooling rate K i The calculation formula is: K0 is the initial average rapid cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment.

[0041] When the mass fraction of Nb and Ti satisfies Nb+Ti≤0.025%, Z1=1.0; when Nb+Ti>0.025%, Z1=1.5;

[0042] When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, Z2=1.0; when Nb+Ti>0.025%, Z2=2.0.

[0043] 4) Using formula (3) the target over-aging temperature H during the speed regulation period i , the formula is as follows:

[0044] H i =H0+T i -T0 (3)

[0045] Among them, H i is the target over-aging temperature of the i-th speed adjustment of the strip in the furnace, H0 is the initial over-aging temperature before the speed adjustment of the strip in the furnace; T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; T0 is the starting strip temperature at the rapid cooling outlet before the strip speed in the furnace is adjusted.

[0046] 5) The speed of strip steel in the furnace is V i Control, quick cooling outlet temperature press T i Control, over-aging temperature according to H i Control, the production process of the strip steel raw material remains unchanged, and other annealing process parameters remain unchanged during the speed regulation period.

[0047] It is worth noting that in order to ensure the smooth operation of the strip during the speed adjustment period, the speed adjustment value M i Cannot exceed the range of -10m / min to 10m / min, that is, -10m / min≤M i≤10m / min, the time interval between each speed adjustment is not less than 10s. In order to obtain martensite or bainite that can undergo phase transformation strengthening, thereby ensuring product strength, the target strip speed V in the furnace during adjustment i Cannot be less than 60m / min, that is, 60m / min≤V i ≤300m / min. In order to ensure the calculation accuracy, the slow cooling rate R during the speed regulation period i Cannot exceed the range of 1.0℃ / s to 8℃ / s, i.e. 1.0℃ / s <R i <8℃ / s. When the rapid cooling outlet temperature exceeds 360℃, the microstructure and properties of cold-rolled high-strength steel will undergo a sudden change. Therefore, the rapid cooling outlet temperature T i Limited to 360℃, that is, T i ≤360℃.

[0048] In an embodiment of the present invention, the thickness of the hard rolled coil is 0.3 mm to 2.5 mm;

[0049] The cold-rolled high-strength steel structure produced by the flexible annealing method can be a typical ferrite + martensite dual-phase structure, or a multi-phase structure of at least one of ferrite + martensite + bainite, retained austenite, cementite, and second phase precipitates.

[0050] The chemical composition, parameter collection, calculation and mechanical properties of Examples 1 to 3 are shown in Tables 1 to 4, respectively.

[0051] Table 1 Chemical composition of Examples 1-3

[0052]

[0053] Table 2 Parameter collection of Examples 1-3

[0054] serial number Example 1 Example 2 Example 3 Thickness / mm 0.8 1.8 1.5 <![CDATA[V0 / (m / min)]]> 180 100 150 <![CDATA[T0 / ℃]]> 300.0 290.0 280.0 <![CDATA[R0 / (℃ / s)]]> 5.45 3.03 5.45 <![CDATA[K0 / (℃ / s)]]> 73.13 41.67 62.50 <![CDATA[H0 / ℃]]> 290.0 290.0 280.0

[0055] Table 3 Parameter values ​​and calculations of Examples 1-3

[0056]

[0057]

[0058]

[0059] Table 4 Mechanical properties of Examples 1-3

[0060]

[0061] Note 1: The mechanical properties are determined according to the national standard GB / T 228.1-2021, the specimen type is P6, and the specimen direction is longitudinal. The hole expansion rate is determined according to the national standard GB / T 15825.4-2008, using punching and conical punches.

[0062] The results show that the method of the present invention can quickly realize the flexible speed change of cold-rolled high-strength steel. After the speed adjustment, the performance fluctuation of the coiled product is small. The speed can be reduced to deal with emergencies, and the speed can be increased to increase production.

[0063] The present invention has been described exemplarily. Obviously, the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A flexible annealing method for cold-rolled high-strength steel, characterized in that: The method is specifically as follows: Input the strip speed adjustment amount M for the i-th furnace strip speed adjustment i , based on the speed adjustment of the strip steel in the furnace M i Adjust the target strip temperature T at the rapid cooling outlet i and target overaging temperature H i , to reduce product performance fluctuations caused by speed changes of the strip in the furnace; Target strip temperature T at the rapid cooling outlet i The calculation formula is as follows: Among them, T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; M i is the strip speed adjustment amount of the i-th furnace strip speed adjustment; T0 is the starting strip temperature at the rapid cooling outlet before the strip speed adjustment; X i Y is the slow cooling rate influence coefficient when the speed of the strip steel in the furnace is adjusted for the i-th time; i is the influence coefficient of the rapid cooling rate when the strip speed in the furnace is adjusted for the i-th time; Z1 is the influence coefficient of the chemical composition; Z2 is the influence coefficient of the rapid cooling outlet temperature; Target overaging temperature H i The formula is as follows: H i =H0+T i -T0 Among them, H i is the target over-aging temperature when the speed of the strip steel in the furnace is adjusted for the i-th time, H0 is the initial over-aging temperature before the speed adjustment of the strip steel in the furnace; T i is the target strip temperature at the rapid cooling outlet when the strip speed in the furnace is adjusted for the i-th time; T0 is the starting strip temperature at the rapid cooling outlet before the strip speed in the furnace is adjusted.

2. The flexible annealing method for cold-rolled high-strength steel according to claim 1, characterized in that: Slow cooling rate influence coefficient X i , Rapid cooling rate influence coefficient Y i The specific methods for determining the values ​​of the chemical composition influence coefficient Z1 and the rapid cooling outlet temperature influence coefficient Z2 are as follows: Average cooling rate R i Satisfy 6.5≤R i <8.0℃ / s, X i =0.8; satisfying 3.5≤R i <6.5℃ / s, X i =0.4; satisfying 1.0≤R i <3.5℃ / s, X i =0; Average fast cooling rate K i Satisfy 30≤K i <60℃ / s, Y i =1.0; satisfying 60≤K i <100℃ / s, Y i =0.5; flat meets 100≤K i <120℃ / s, Y i =0; When the mass fraction of Nb and Ti satisfies Nb+Ti≤0.025%, Z1=1.0; when Nb+Ti>0.025%, Z1=1.5; When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, Z2=1.0; when Nb+Ti>0.025%, Z2=2.

0.

3. The flexible annealing method for cold-rolled high-strength steel according to claim 2, characterized in that: Average cooling rate R i The calculation formula is: R0 is the initial average slow cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment; Average fast cooling rate K i The calculation formula is: K0 is the initial average rapid cooling rate before the strip speed is adjusted in the furnace, V i is the target strip speed in the furnace during the i-th strip speed adjustment, and V0 is the initial strip speed in the furnace before the strip speed adjustment.

4. The flexible annealing method for cold-rolled high-strength steel according to claim 1, characterized in that: Speed ​​adjustment of strip steel in furnace M i The speed is in the range of -10m / min to 10m / min.

5. The flexible annealing method for cold-rolled high-strength steel according to claim 3, characterized in that: Target strip speed V in the furnace i Meet: 60m / min≤V i ≤300m / min.

6. The flexible annealing method for cold-rolled high-strength steel according to claim 3, characterized in that: Average cooling rate R i Meet: 1.0℃ / s <R i <8℃ / s, and the target strip temperature T at the rapid cooling outlet i Satisfaction: T i ≤360℃.

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