A method for controlling surface black ash defects of 1200mpa grade cold rolled dual phase steel

Abstract

The present application relates to a kind of methods for controlling 1200MPa grade cold rolled dual-phase steel surface black ash defects, comprising:1) hot rolling process control hot rolling final rolling temperature and coiling temperature, laminar cooling uses concentrated cooling mode, coiling after hot coil stacking cooling;2) cold rolling process uses shallow tank turbulent pickling to remove iron oxide scale in pickling stage;Control of straightening elongation, pickling temperature, new acid into acid quantity and pickling speed;Cold rolling stage control emulsion temperature, emulsion iron powder content and conductivity;3) annealing cleaning process controls the concentration of cleaning solution used in spraying section and steam temperature, and the concentration of degreasing alkali used in electrolytic cleaning section, steam temperature and voltage.Through the process control of whole flow, multiple processes, avoid or reduce product surface black ash defects, without other auxiliary equipment and special process equipment can be industrialized application, simplify production process, reduce production cost.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled duplex steel production technology, and in particular to a method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel through hot rolling, pickling and cleaning processes. Background Technology

[0002] Currently, cold-rolled duplex steels are widely used in automotive bodies. Steel grades such as DP1180, DP980, DP780, and DP590 are extensively used in reinforcement components like B-pillars and door frame reinforcement plates, as well as complex body structural components such as front and rear longitudinal beams. With the increasing strength requirements of automotive structural components, the production processes of hot rolling and cold rolling are becoming more complex, requiring higher alloy content. These factors can lead to black and gray defects on the surface of the finished steel sheets, directly affecting their subsequent application in automotive production. Therefore, reducing surface defects in high-strength duplex steels has become an important research direction.

[0003] 1200MPa grade cold-rolled duplex steel has a high phase transformation sensitivity due to its high alloy content. To ensure its stability during the cold rolling process, it is generally necessary to perform high-temperature coiling during the hot rolling stage to improve the uniformity of the microstructure of the hot-rolled plate. However, this also promotes the growth of iron oxide scale on the surface, which is not conducive to the surface removal in the subsequent pickling process. After the annealing process, black and gray defects are prone to appear on the surface, which is not conducive to improving the product yield and leads to surface quality problems during application.

[0004] For the surface black and gray defects of 1200MPa grade cold-rolled duplex steel, the control method of a single process is not very effective. It is necessary to suppress them based on the coupled design of the whole process. Therefore, it is of great significance to explore a multi-dimensional method for suppressing surface black and gray defects based on hot rolling, cold rolling and cleaning processes.

[0005] Chinese patent application No. 202010671264.5 discloses "a 1180MPa cold-rolled hot-dip galvanized duplex steel and its preparation method". The 1180MPa cold-rolled hot-dip galvanized duplex steel includes a steel substrate, a zinc plating layer, and a nickel plating layer located between the steel substrate and the zinc plating layer. The chemical composition of the steel substrate is as follows: C: 0.09%~0.15%, Si: 0.1%~0.4%, Mn: 1.9%~2.7%, P: ≤0.01%, S: ≤0.006%, with the remainder being Fe and unavoidable impurities. A nickel plating layer with a thickness of 50nm~100nm is introduced between the steel substrate and the zinc plating layer, which has good adaptability to continuous hot-dip galvanizing, no surface defects such as incomplete plating, and good coating performance. This is beneficial to improving the coating quality of the car body and the corrosion resistance of the automobile, and also has good local forming performance. It improves surface quality by introducing a nickel plating layer, but there is no effective control method for the surface of strip steel that does not use the zinc plating process, and the introduction of a nickel plating layer requires the support of special production equipment, which leads to increased costs.

[0006] Chinese patent application No. 201922220244.1 discloses "A device for controlling surface defects of strip steel in a continuous annealing furnace." This device solves the problem of impurities falling between the strip steel and the furnace rolls, causing surface defects such as dents, by setting up a slag collection trough. This reduces the amount of strip steel carried out due to dents in the furnace area, ensuring stable and continuous operation of the production line. However, while it reduces surface defects by preventing impurities from falling between the strip steel and the furnace rolls, it does not fundamentally solve the cause of surface defects and only addresses dents. Furthermore, equipment modifications are still needed to improve surface quality.

[0007] Chinese patent application No. 202210675544.2 discloses a "High-Grade Surface Control Method for a 316 Series Stainless Steel Cold-Rolled Strip Production Line," comprising the following steps: the first part is the rolling mill section; the second part is the heat treatment section; the third part is the pickling section; the fourth part is the leveling section; and the fifth part is the management of the entire strip channel. After adopting this method, the first-pass yield of 316 series high-grade surface-grade stainless steel on the continuous rolling production line increased from 47% to 99.8%, reducing defective products. While this method achieves the improved yield through multi-stage surface quality control, it suffers from significant differences in the production processes of stainless steel and cold-rolled duplex steel. Furthermore, the causes of surface defects in stainless steel and the strip composition design are completely different from those in cold-rolled duplex steel, limiting its reference value. Summary of the Invention

[0008] This invention provides a method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel. Through full-process and multi-step process control, black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel products are avoided or reduced. It can be industrialized without other auxiliary equipment or special process equipment, which simplifies the production process and reduces production costs.

[0009] To achieve the above objectives, the present invention employs the following technical solution: A method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel involves controlling the following processes during production: 1) Hot rolling process; the final hot rolling temperature is 900-950℃, laminar flow cooling is set to centralized cooling mode, the coiling temperature is 500-550℃, and the coils are cooled by hot coil stacking after coiling. 2) Cold rolling process; In the cold rolling pickling stage, shallow-tank turbulent pickling is used to remove iron oxide scale; the elongation is controlled at 0.5%–1.0% during tension leveling, the pickling temperature is 85±5℃, and the fresh acid feed rate is ≥3.5m. 3 / h, pickling rate ≤100m / min; During the cold rolling stage, the emulsion temperature is controlled at 55–60℃. For stands 1–4, the iron powder content in the emulsion is ≤250 ppm, and the emulsion conductivity is ≤350 μs / cm. For stand 5, the iron powder content in the emulsion is ≤100 ppm, and the emulsion conductivity is ≤15 μs / cm. The residual oil content on one side of the strip is guaranteed to be <200 mg / m². 2 The residual iron content on one side of the strip is <50mg / m. 2 ; 3) Annealing and cleaning process; the NaOH concentration of the cleaning solution used in the spray section is 3.5% to 5%, and the steam temperature is 70 to 75℃; the concentration of the degreasing alkaline solution used in the electrolytic cleaning section is 50 to 60 g / L, the steam temperature is 70 to 75℃, and the voltage is 20 to 30V.

[0010] Furthermore, the chemical composition of the steel, by mass percentage, is: C: 0.15%–0.20%, Si: 1.00%–1.50%, Mn: 1.50%–2.50%, P≤0.020%, S≤0.0005%, Al S 0.025%~0.055%, N≤0.0050%, Ti: 0.02%~0.05%, balance is Fe and unavoidable impurities.

[0011] Compared with the prior art, the beneficial effects of the present invention are: (1) Compared with the previous method of surface quality control relying on a single process, the present invention adopts full-process, multi-process coupled control, which effectively avoids or reduces black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel products. (2) The method described in this invention does not require additional manufacturing equipment and has the characteristics of low cost and high efficiency. It has been applied to the industrial mass production of 1200MPa grade cold-rolled duplex steel and the implementation effect is good. Detailed Implementation

[0012] This invention relates to a method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel. The corresponding product is 1200MPa grade cold-rolled duplex steel with the following chemical composition by mass percentage: C: 0.15%–0.20%, Si: 1.00%–1.50%, Mn: 1.50%–2.50%, P≤0.020%, S≤0.0005%, Al… S 0.025%~0.055%, N≤0.0050%, Ti: 0.02%~0.05%, balance is Fe and unavoidable impurities; the tensile strength of the finished strip steel is above 1180MPa.

[0013] The black and gray defect described in this invention refers to a black strip-shaped defect covering the surface of the finished strip steel, mainly concentrated in the middle of the strip steel along the width direction. The defect is reduced after wiping. The main component of the defect is Fe powder. Further analysis of the defect revealed that its formation mechanism is as follows: due to the mismatch in the hot rolling process design of 1200MPa grade cold-rolled duplex steel, the local iron oxide scale is relatively thick. During the cold rolling process, there are a small amount of iron oxide scale residue and a small amount of iron-containing emulsion residue. In the annealing process, the iron oxide scale is reduced to iron powder by the high-temperature reducing atmosphere in the continuous annealing furnace. The residual iron powder is not effectively removed. The two work together to cover the surface of the strip steel to form the black and gray defect.

[0014] This invention employs the following measures to control the generation of black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel: (1) Hot rolling process; In the hot rolling process, the black and gray defects are suppressed by inhibiting the growth of iron oxide scale; The specific control process and parameters include: the hot rolling final rolling temperature is 900-950℃; the laminar flow cooling is set to centralized cooling mode, and the coiling temperature is 500-550℃; after coiling, the hot coil stacking method is used for cooling.

[0015] (2) Cold rolling process; During the cold rolling pickling stage, shallow-tank turbulent pickling is used to remove iron oxide scale, thereby suppressing black and gray defects by improving the iron oxide scale removal efficiency. Specific process control parameters include: stretching elongation 0.5%~1.0%, pickling temperature 85±5℃, and fresh acid feed rate ≥3.5m. 3Pickling speed ≤ 100 m / min. During the cold rolling stage, residual iron and emulsion residue in the emulsion are controlled to achieve a residual oil content of < 200 mg / m² on the strip surface. 2 (Single-sided), residual iron content on the surface <50mg / m 2 (Single-sided); Specific control parameters include: emulsion temperature 55~60℃, emulsion iron powder content ≤250ppm for frames 1 to 4, emulsion conductivity ≤350μs / cm; emulsion iron powder content ≤100ppm for frame 5, emulsion conductivity ≤150μs / cm.

[0016] (3) Annealing and cleaning process; In the cleaning stage, residual iron on the surface of the strip steel is removed efficiently. The specific process and parameters include: the concentration of NaOH in the cleaning solution used in the spray section (usually two sections) is 3.5% to 5%, and the steam temperature is 70 to 75℃; the concentration of the degreasing alkaline solution used in the electrolytic cleaning section is 50 to 60 g / L, the steam temperature is 70 to 75℃, and the voltage is 20 to 30V.

[0017] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0018]

Example

[0019] Table 1 Hot rolling process parameters

[0020] Table 2 Pickling process parameters

[0021] Table 3 Emulsion Parameters

[0022] Table 4 Cleaning process parameters

[0023] The 1200MPa grade cold-rolled duplex steel produced in each embodiment had no black or gray defects on its surface.

[0024] It is evident that the black and gray defect control method described in this invention can effectively control the black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel products, significantly improving the yield and surface quality of the products.

[0025] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel, characterized in that, The following processes are controlled during production: 1) Hot rolling process; The final hot rolling temperature is 900-950℃, laminar flow cooling is set to centralized cooling mode, the coiling temperature is 500-550℃, and the coils are cooled by hot coil stacking after coiling. 2) Cold rolling process; During the cold rolling pickling stage, shallow-tank turbulent pickling is used to remove iron oxide scale; the elongation is controlled at 0.5%–1.0% during tension leveling, the pickling temperature is 85±5℃, and the fresh acid feed rate is ≥3.5m. 3 / h, pickling rate ≤100m / min; During the cold rolling stage, the emulsion temperature is controlled at 55–60℃. For stands 1–4, the iron powder content in the emulsion is ≤250 ppm, and the emulsion conductivity is ≤350 μs / cm. For stand 5, the iron powder content in the emulsion is ≤100 ppm, and the emulsion conductivity is ≤150 μs / cm. The residual oil content on one side of the strip is guaranteed to be <200 mg / m². 2 The residual iron content on one side of the strip is <50mg / m. 2 ; 3) Annealing and cleaning process; the NaOH concentration of the cleaning solution used in the spray section is 3.5% to 5%, and the steam temperature is 70 to 75℃; the concentration of the degreasing alkaline solution used in the electrolytic cleaning section is 50 to 60 g / L, the steam temperature is 70 to 75℃, and the voltage is 20 to 30V.

2. The method for controlling black and gray defects on the surface of 1200MPa grade cold-rolled duplex steel according to claim 1, characterized in that, The chemical composition of the steel, by mass percentage, is: C: 0.15%–0.20%, Si: 1.00%–1.50%, Mn: 1.50%–2.50%, P≤0.020%, S≤0.0005%, Al S 0.025%~0.055%, N≤0.0050%, Ti: 0.02%~0.05%, balance is Fe and unavoidable impurities.

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