A method for increasing speed and yield of cold rolled dual phase steel

Abstract

The application discloses a method for increasing speed and yield of cold-rolled dual-phase steel, and specifically as follows: when (V1-V0)X1<(T c -T0)X2, a formula T1=T0+(V1-V0)X1 / X2 is used to calculate the target strip temperature T1 at the fast cooling outlet after speed-up; when (V1-V0)X1≥(T c -T0)X2, a formula T1=T c +(V1-V0)X1 / X3-(T c -T0)X2 / X3 is used to calculate the target strip temperature T1 at the fast cooling outlet after speed-up. Under the condition that the strip raw material process is unchanged, the fast cooling outlet temperature and overaging temperature are quickly determined, and then the matching design of the strip speed in the furnace and the annealing process of the cold-rolled dual-phase steel is quickly realized, so that the speed-up and yield increase of the cold-rolled dual-phase steel are quickly realized. The calculation model of the fast cooling outlet temperature and the overaging temperature is simple, and the product performance difference before and after the speed-up is small, and the forming performance with the elongation after fracture and the hole expansion rate as indexes is good.

Description

Technical Field

[0001] This invention belongs to the field of automotive sheet processing technology, and more specifically, this invention relates to a method for accelerating and increasing the production of cold-rolled duplex steel. Background Technology

[0002] For a long time, steel has been a fundamental material in the automotive industry. Automotive steel sheet is the largest category of steel used in automobiles, accounting for approximately 70%. Compared to ordinary steel sheets, automotive steel sheet production has a longer process, higher technological requirements, and better profit margins. To adapt to changes in steel consumption patterns, major steel companies have vigorously developed automotive steel sheet production. Currently, major automotive steel sheet manufacturers include international companies such as ArcelorMittal, ThyssenKrupp, Nippon Steel & Sumitomo Metal, POSCO (South Korea), and Swedish Steel, as well as domestic companies such as Baowu Steel Group, Anshan Iron & Steel Group, Shougang Group, Hebei Iron & Steel Group, and Hunan Iron & Steel Group.

[0003] Duplex steel possesses an excellent strength-ductility balance, overcoming the poor formability of traditional low-alloy high-strength steels with a ferrite + pearlite microstructure. It is widely used in automotive structural components, reinforcements, and crash barriers, holding a crucial position in high-strength automotive sheet metal. Faced with an increasingly competitive market environment, accelerating production of duplex steel is a vital research topic in the automotive sheet metal industry, as it can reduce costs and improve production efficiency. However, a search of existing technologies reveals almost no disclosed solutions for accelerating the production of cold-rolled duplex steel. Summary of the Invention

[0004] This invention provides a method for increasing the production speed of cold-rolled duplex steel. By matching the strip speed, annealing temperature and over-aging temperature of cold-rolled duplex steel, the goal of increasing production and efficiency is achieved.

[0005] This invention is implemented as follows: a method for accelerating and increasing the production of cold-rolled duplex steel, the specific method of which is as follows:

[0006] When (V1-V0)X1<(T) c When (T0)X2, the target strip temperature T1 at the fast cooling outlet after speed-up is calculated using formula (1); when (V1-V0)X1≥(T c When -T0)X2, the target strip temperature T1 at the fast cooling outlet after speed-up is calculated using formula (2):

[0007] T1=T0+(V1-V0)X1 / X2 (1)

[0008] T1 = T c +(V1-V0)X1 / X3-(T c -T0)X2 / X3 (2)

[0009] Where T0 is the initial strip temperature at the rapid cooling outlet before speed increase; V1 is the target strip speed in the furnace after speed increase; V0 is the initial strip speed in the furnace before speed increase; T c X1 is the critical temperature for the change of strip properties, X2 is the influence coefficient of the strip speed in the furnace, X3 is the influence coefficient of the early stage of the rapid cooling outlet temperature, and X4 is the influence coefficient of the later stage of the rapid cooling outlet temperature.

[0010] Furthermore, the specific methods for determining the values ​​of the following factors are as follows: X1 (influence coefficient of strip speed in furnace), X2 (initial influence coefficient of rapid cooling outlet temperature), and X3 (later influence coefficient of rapid cooling outlet temperature):

[0011] When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X1=1.0; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X1=1.5.

[0012] When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X2=1.0; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X2=2.0;

[0013] When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X3=2.5; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X3=3.0.

[0014] Furthermore, the rapid cooling outlet temperature and over-aging temperature are adjusted synchronously to offset the impact of strip speed increase on microstructure and properties.

[0015] Furthermore, the specific formula for calculating the target over-aging temperature H1 after the acceleration is as follows:

[0016] H1=H0+T1-T0 (3)

[0017] Where H0 is the initial over-aging temperature before speed-up; T1 is the target strip temperature at the rapid cooling exit after speed-up; and T0 is the initial strip temperature at the rapid cooling exit before speed-up.

[0018] Furthermore, the critical temperature T for strip steel properties c It is 360℃.

[0019] Furthermore, after the speed increase, the target strip speed V1 in the furnace satisfies: V0 <V1≤200m / min。

[0020] Furthermore, the strip steel raw material is a hard-rolled coil with a thickness of 0.3mm to 2.5mm.

[0021] Furthermore, the slow cooling rate after the acceleration is 8.0℃ / s~16.5℃ / s.

[0022] Furthermore, the microstructure of cold-rolled dual-phase steel is a dual-phase structure of ferrite + martensite, or a multiphase structure of at least one of ferrite + martensite + bainite, retained austenite, cementite, and second-phase precipitates.

[0023] The method of this invention uses a conventional continuous annealing furnace. Under the condition that the strip steel raw material process remains unchanged, it rapidly determines the rapid cooling outlet temperature and the over-aging temperature, thereby quickly achieving the matching design of the strip speed and annealing process in the furnace for cold-rolled duplex steel, thus rapidly increasing the production capacity of cold-rolled duplex steel. The calculation model for the rapid cooling outlet temperature and the over-aging temperature is simple, and the difference in product performance before and after the speed increase is small. The forming performance, indicated by elongation after fracture and expansion rate, is good. Detailed Implementation

[0024] The following description of the embodiments will provide a more detailed explanation of the specific implementation of the present invention, in order 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.

[0025] The raw material for strip steel is hard-rolled coil. The production process includes smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing. The continuous annealing process includes heating, homogenization, slow cooling, rapid cooling, and over-aging. During slow cooling, austenite transforms into ferrite, and during rapid cooling, austenite transforms into martensite or bainite. During over-aging, tempering and further transformation of the retained austenite occur. If the strip speed in the continuous annealing furnace is changed, the time of each process in the continuous annealing process will inevitably be changed, which will seriously affect the structural ratio of soft and hard phases and the hardness difference between soft and hard phases. To achieve superior product performance, the annealing process needs to be optimized and matched when accelerating production. While the process remains unchanged, the impact of the strip speed on microstructure and properties varies depending on the strip composition. Therefore, the determination of the rapid cooling exit temperature is related to the influence coefficient X1 of the strip speed in the furnace. The influence of the rapid cooling exit temperature on microstructure and properties is not only limited by the strip composition but also by the rapid cooling exit temperature itself. Therefore, the determination of the rapid cooling exit temperature is related to two influence coefficients X2 and X3. This invention simultaneously adjusts the rapid cooling exit temperature and the over-aging temperature to better improve the microstructure and properties of the product and offset the deteriorating effect of increased strip speed on microstructure and properties.

[0026] Based on this, the method for accelerating and increasing production of cold-rolled duplex steel provided by the present invention is as follows:

[0027] 1) Collect the niobium-titanium mass fraction of the strip steel raw material, the initial strip steel speed V0 in the furnace before speed increase, the initial strip steel temperature T0 at the fast cooler outlet before speed increase, and the initial over-aging temperature H0 before speed increase.

[0028] 2) Set the target strip speed V1 in the furnace after speed-up. When (V1-V0)X1 < (T cWhen (T0)X2, the target strip temperature at the fast cooling outlet after speed-up is calculated using formula (1); when (V1-V0)X1≥(T c When -T0)X2, the target strip temperature at the fast cooling outlet after speed-up is calculated using formula (2), where the expressions for formulas (1) and (2) are as follows:

[0029] T1=T0+(V1-V0)X1 / X2 (1)

[0030] T1 = T c +(V1-V0)X1 / X3-(T c -T0)X2 / X3 (2)

[0031] Where T1 is the target strip temperature at the rapid cooling outlet after speed increase, in °C; T0 is the initial strip temperature at the rapid cooling outlet before speed increase, in °C; V1 is the target strip speed in the furnace after speed increase (set value), in m / min; V0 is the initial strip speed in the furnace before speed increase, in m / min; T c The critical temperature for the change in strip steel properties is T. c The value is 360, and the unit is ℃. The above formulas (1) and (2) are based on numerical patterns.

[0032] X1 is the influence coefficient of the strip speed in the furnace. When the mass fractions of Nb and Ti satisfy: Nb+Ti≤0.025%, X1=1.0; when the mass fractions of Nb and Ti satisfy: Nb+Ti>0.025%, X1=1.5.

[0033] Because the properties of strip steel differ significantly between temperatures above and below the critical temperature change, the influence coefficient of the rapid cooling exit temperature is divided into an early-stage influence coefficient X2 and a later-stage influence coefficient X3. The specific methods for determining the values ​​of influence coefficients X2 and X3 are as follows:

[0034] When the mass fractions of Nb and Ti satisfy the following conditions: Nb + Ti ≤ 0.025%, X2 = 1.0; when the mass fractions of Nb and Ti satisfy the following condition: Nb + Ti > 0.025%, X2 = 2.0.

[0035] When the mass fractions of Nb and Ti satisfy the following conditions: when Nb+Ti≤0.025%, X3=2.5; when the mass fractions of Nb and Ti satisfy the following condition: when Nb+Ti>0.025%, X3=3.0.

[0036] 3) The target over-aging temperature H1 after the acceleration is calculated using formula (3). The specific calculation formula is as follows:

[0037] H1=H0+T1-T0 (3)

[0038] Where H0 is the initial over-aging temperature before speed-up; T1 is the target strip temperature at the rapid cooling exit after speed-up; and T0 is the initial strip temperature at the rapid cooling exit before speed-up.

[0039] 4) The strip speed in the furnace is controlled by V1, the rapid cooling outlet temperature is controlled by T1, and the over-aging temperature is controlled by H1. The production process of the strip raw material remains unchanged before and after the speed increase, and other annealing process parameters remain unchanged.

[0040] In this embodiment of the invention, the target strip speed V1 in the furnace after the speed increase satisfies: V0 <V1≤200m / min;

[0041] In this embodiment of the invention, the thickness of the hard-rolled coil is 0.3 mm to 2.5 mm;

[0042] When the slow cooling rate is greater than 16.5 m / min, the ferrite transformation during slow cooling is significantly reduced. When the slow cooling rate is less than 8.0 m / min, the ferrite transformation during slow cooling is significantly increased. Therefore, after increasing the speed, the slow cooling rate is limited to 8.0℃ / s~16.5℃ / s.

[0043] The microstructure of cold-rolled duplex steel generated by the above-mentioned speed-up and production-increasing methods can be a typical ferrite + martensite duplex structure, or a multiphase structure consisting of at least one of ferrite + martensite + bainite, retained austenite, cementite, and second-phase precipitates.

[0044] The chemical composition, initial process and model parameters, target process after speed-up, and mechanical properties of Examples 1-4 are shown in Tables 1-3, respectively. In numerical order, the chemical composition of Comparative Examples 1-4 is the same as that of Examples 1-4, and the strip steel raw material process is the same. The difference is that during continuous annealing, the annealing process before speed-up is followed. The annealing process and mechanical properties are shown in Table 4. The chemical composition of Comparative Example 5 is the same as that of Examples 2 and 2, and the strip steel raw material process is the same. The difference is that Comparative Example 5 simply increases the speed. The strip steel speed in the furnace after speed-up is the same as that in Example 2 after speed-up, but the rapid cooling outlet temperature and over-aging temperature are not designed for matching. The annealing process and mechanical properties are shown in Table 5.

[0045] Table 1 Chemical composition of Examples 1-4

[0046]

[0047] Table 2. Starting process and model parameters for Examples 1-4

[0048] serial number <![CDATA[V0 / (m / min)]]> <![CDATA[T0 / ℃]]> <![CDATA[H0 / ℃]]> <![CDATA[X1]]> <![CDATA[X2]]> <![CDATA[X3]]> Example 1 120 310.0 300.0 1.0 1.0 2.5 Example 2 120 320.0 300.0 1.0 1.0 2.5 Example 3 120 300.0 285.0 1.0 1.0 2.5 Example 4 100 295.0 285.0 1.5 2.0 3.0

[0049] Table 3 Target process and mechanical properties after speed-up in Examples 1-4

[0050]

[0051]

[0052] Table 4 Annealing process and mechanical properties of Comparative Examples 1-4

[0053]

[0054] Table 5 Annealing process and mechanical properties of Comparative Example 5

[0055]

[0056] Note 1: The method for determining mechanical properties adopts the national standard GB / T 228.1-2021, the sample type is P6, the sample direction is longitudinal, and the method for determining the hole expansion rate adopts the national standard GB / T 15825.4-2008, using punching and tapered punch.

[0057] The results show that the method for increasing the speed and production capacity of cold-rolled duplex steel using the present invention can quickly obtain a speed-up process scheme, rapidly achieve increased production capacity of cold-rolled duplex steel, and the difference in product performance before and after the speed-up is small, with product performance meeting requirements. However, simply increasing the speed without adjusting the rapid cooling outlet temperature and over-aging temperature will result in problems such as increased strength, decreased elongation after fracture, and decreased porosity. This will seriously affect product usability, increase the risk of losing market share and customers, and increase the company's economic losses.

[0058] The present invention has been described by way of example. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A method for accelerating and increasing the production of cold-rolled duplex steel, characterized in that, The method is as follows: When (V1-V0)X1<(T) c When (T0)X2, the target strip temperature T1 at the fast cooling outlet after speed-up is calculated using formula (1); when (V1-V0)X1≥(T c When -T0)X2, the target strip temperature T1 at the fast cooling outlet after speed-up is calculated using formula (2): T1=T0+(V1-V0)X1 / X2 (1) T1=T c +(V1-V0)X1 / X3-(T c -T0)X2 / X3 (2) Where T0 is the initial strip temperature at the rapid cooling outlet before speed increase; V1 is the target strip speed in the furnace after speed increase; V0 is the initial strip speed in the furnace before speed increase; T c X1 is the critical temperature for the change in strip properties, X2 is the influence coefficient of the strip speed in the furnace, X3 is the influence coefficient of the initial stage of the rapid cooling exit temperature, and X4 is the influence coefficient of the later stage of the rapid cooling exit temperature. The specific methods for determining the values ​​of the influence coefficients X1 (strip speed in the furnace), X2 (initial stage of the rapid cooling exit temperature), and X3 (later stage of the rapid cooling exit temperature) are as follows: When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X1=1.0; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X1=1.

5. When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X2=1.0; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X2=2.

0. When the mass fractions of Nb and Ti satisfy Nb+Ti≤0.025%, X3=2.5; when the mass fractions of Nb and Ti satisfy Nb+Ti>0.025%, X3=3.

0.

2. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, The rapid cooling outlet temperature and over-aging temperature are adjusted synchronously to offset the impact of strip speed increase on microstructure and properties.

3. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 2, characterized in that, The specific formula for calculating the target over-aging temperature H1 after the speed increase is as follows: H1 = H0 + T1 - T0 (3) Where H0 is the initial over-aging temperature before speed-up; T1 is the target strip temperature at the rapid cooling exit after speed-up; and T0 is the initial strip temperature at the rapid cooling exit before speed-up.

4. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, Critical temperature T for strip steel properties c It is 360℃.

5. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, After the speed increase, the target strip speed V1 in the furnace satisfies: V0 <V1≤200m / min。 6. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, The raw material for the strip steel is hard-rolled coil, and the thickness of the hard-rolled coil is 0.3 mm to 2.5 mm.

7. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, After the speed increase, the slow cooling rate is 8.0℃ / s ~ 16.5℃ / s.

8. The method for accelerating and increasing production of cold-rolled duplex steel as described in claim 1, characterized in that, The microstructure of cold-rolled duplex steel is a dual-phase structure of ferrite + martensite, or a multiphase structure of at least one of ferrite + martensite + bainite, retained austenite, cementite, and second-phase precipitates.

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