A surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems

Abstract

The present invention relates to the field of stainless steel cold-rolled strip production. A surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems comprises the following steps: (1) a rolling mill section; (2) a degreasing section; and (3) a pickling section. The invention improves the surface pass rate of exhaust ultra-pure ferritic stainless steel produced on a continuous rolling line from 85.2% to 96.8%. This reduces defective products and improves the uniformity and consistency of the exhaust ultra-pure surface. Based on the electrolytic control characteristics of different component materials, the process procedures for producing exhaust ultra-pure ferritic stainless steel cold-rolled strips of different specifications are refined.

Description

Technical Field

[0001] The present invention relates to the field of stainless steel cold-rolled strip production, including a 5-stand 6-roller cold rolling mill, a heat treatment process, a pickling process and the like, and in particular to a surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems. Background Art

[0002] The Shanxi Taigang Stainless Steel Cold Rolling Mill's cold-rolled strip fully continuous production line (RAP line for short, hereinafter referred to as "RAP line" instead of "fully continuous production line") integrates six major processes, including cold rolling mill, annealing furnace, pickling, skin pass mill, straightening mill, and trimming shears, into one production line. The products are finished products as soon as they come off the line and can be directly delivered to users.

[0003] The RAP line produces ultra-pure ferritic stainless steels for exhaust systems, including 441, 436, and 439 series stainless steels. Widths range from 1000 to 1650 mm, and thicknesses range from 0.5 to 3.0 mm. Due to the high-efficiency production requirements of the RAP line, surface quality control is challenging. With the development of the automotive industry, surface requirements for ultra-pure ferritic stainless steel used in exhaust systems are becoming increasingly stringent. The original process design, which produced strips with light emulsion marks, watermarks, and acid stains, could no longer meet customer demands.

[0004] The present invention formulates different rolling emulsion concentration configurations, roller roughness configurations, and neutral salt electrolysis current process designs according to different specification materials, thereby achieving joint control of the surface quality of the entire line and ensuring that the surface of the finished steel plate is free of defects such as emulsion marks and electrolytic marks. Summary of the Invention

[0005] The purpose of the present invention is to solve the above problems and provide a surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems.

[0006] The object of the present invention is achieved as follows: A surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems, comprising the following steps: (1) rolling mill part: surface control of ultra-pure ferritic stainless steel for exhaust purposes, by special process design through special configuration of roll roughness and rolling lubricant concentration configuration; (2) degreasing part: degreasing liquid concentration is controlled by conductivity, the conductivity of 1# degreasing system is controlled at 6-8mS / cm, and the conductivity of 2# degreasing system is controlled at 3-5mS / cm, to eliminate residual emulsion on the surface of steel plate during rolling and avoid the formation of emulsion marks; (3) pickling part: exhaust ultra-pure ferritic stainless steel is difficult to pickle due to the presence of Si and Mo, and process design needs to be carried out from two aspects: electrolytic pickling and mixed acid pickling; sodium sulfate electrolysis belongs to neutral salt electrolysis, which relies on electrochemical reaction to remove iron scale on the surface of steel strip. Due to the particularity of surface oxide scale after heat treatment, different electrolytic processes need to be designed for steel grades with different Si contents.

[0007] Roller roughness in the rolling mill section: According to the deformation amount of the five stands, 0.8μm, 0.8μm, 0.3μm, 0.2μm, and 0.2μm are used on the five stands respectively, and the roll roughness range is ±0.02μm; the rolling lubricant concentration of the first three stands is configured to 3.2%-3.5%, and the rolling lubricant concentration of the 4# and 5# stands is configured to 1.5%-2.5%. At the same time, during the rolling process, a boundary lubrication state is formed in the deformation zone, so that the surface roughness of the rolled steel plate can reach 0.12-0.15μm.

[0008] In the pickling part, for ultra-pure ferritic stainless steel with a silicon content of more than 0.25%, the electrolysis current is set to 4500A-4700A, and the electrolysis time is controlled at 2-4s. Too long electrolysis time will cause electrolytic marks on the surface of stainless steel with high Si content; for ultra-pure ferritic stainless steel with a Si content of 0-0.25%, the electrolysis current is set to 4500-4700A, and the electrolysis time is controlled at 1-2S to ensure a uniform pickling surface.

[0009] Mixed acid pickling is used in the pickling part, which is divided into 3 tanks. The first tank has high concentration and the pickling concentration is gradually reduced. The concentrations of nitric acid and hydrofluoric acid in the first tank are: 140-180g / l, 16-22g / l respectively; the concentrations of nitric acid and hydrofluoric acid in the second tank are: 120-150g / l, 12-18g / l respectively; the concentrations of nitric acid and hydrofluoric acid in the third tank are: 110-130g / l, 10-15g / l respectively.

[0010] The beneficial effects of the present invention are as follows: 1. The first pass rate of the surface of the exhaust ultra-pure ferritic stainless steel produced on the continuous rolling line is increased from 85.2% to 96.8%, thereby reducing defective products.

[0011] 2. The uniformity and consistency of the exhaust ultra-pure surface are improved.

[0012] 3. According to the electrolysis control characteristics of materials with different components, the process procedures for exhaust ultra-pure ferritic stainless steel cold-rolled strips with different specifications are refined. DETAILED DESCRIPTION

[0013] Ultra-pure ferritic stainless steel cold-rolled strip coils are continuously rolled on a 5-stand tandem mill, followed by continuous annealing and pickling. Finally, they are continuously trimmed and re-coiled online according to customer requirements to ensure that they are finished products as soon as they roll off the line. The composition of ultra-pure ferritic stainless steel cold-rolled strip coils, by weight, is as follows: C ≤ 0.025%, Mo ≤ 2.00%, Ti ≤ 0.25%, Nb ≤ 0.50%, Si ≤ 1.00%, Mn ≤ 1.00%, P ≤ 0.045%, S ≤ 0.030%, Cr 15-23%, Ni ≤ 0.50%, N ≤ 0.015%, with the remainder being Fe and unavoidable impurities.

[0014] The technical solutions of the present invention are as follows:

[0015] 1. Rolling mill part

[0016] 1. Surface control of ultra-pure ferritic stainless steel for exhaust applications is achieved through special process design for the specific configuration of roll roughness and the concentration of rolling lubricant.

[0017] 2. Roller roughness: According to the deformation of the five stands, the values ​​are 0.8μm, 0.8μm, 0.3μm, 0.2μm, and 0.2μm, respectively, and the roll roughness range is ±0.02μm.

[0018] 3. The rolling lubricant concentration for the first three stands is set at 3.2% to 3.5% to ensure adequate surface lubrication during low-speed rolling and eliminate surface defects in the raw material. The rolling lubricant concentration for stands 4# and 5# is set at 1.5% to 2.5% to prevent slippage during high-speed rolling. Furthermore, boundary lubrication is achieved in the deformation zone during rolling, resulting in a surface roughness of 0.12 to 0.15 μm for the rolled steel plate.

[0019] 2. Degreasing part

[0020] 1. Degreasing is a crucial step in eliminating mill emulsion marks. The degreasing solution concentration is controlled by conductivity. The conductivity of the 1# degreasing system is controlled at 7 mS / cm, and the conductivity of the 2# degreasing system is controlled at 4 mS / cm. This eliminates residual emulsion on the steel plate surface during the rolling process, preventing the formation of emulsion marks.

[0021] 3. Pickling part

[0022] 1. Exhaust ultra-pure ferritic stainless steel contains Si and Mo, which makes it difficult to pickle. Therefore, process design needs to be carried out from two aspects: electrolytic pickling and mixed acid pickling.

[0023] 2. Sodium sulfate electrolysis is a neutral salt electrolysis method that mainly relies on electrochemical reactions to remove scale from the surface of the steel strip. Due to the particularity of the surface oxide scale after heat treatment, different electrolysis processes need to be designed for steel grades with different Si contents.

[0024] 3. For ultra-pure ferritic stainless steel with a silicon content of more than 0.25%, the electrolysis current is set at 4500A~4700A, and the electrolysis time is controlled at 2-4s. Too long an electrolysis time will cause electrolytic marks on the surface of the stainless steel with a high Si content.

[0025] 4. For ultra-pure ferritic stainless steel with a Si content of 0-0.25%, the electrolysis current is set at 4500-4700A and the electrolysis time is controlled at 1-2S to ensure a uniform pickling surface.

[0026] 5. The mixed acid pickling process is divided into three tanks. The first tank is high-concentration, and the pickling concentration is gradually reduced. The nitric acid and hydrofluoric acid concentrations in the first tank are 140-180 g / l and 16-22 g / l, respectively. The nitric acid and hydrofluoric acid concentrations in the second tank are 120-150 g / l and 12-18 g / l, respectively. The nitric acid and hydrofluoric acid concentrations in the third tank are 110-130 g / l and 10-15 g / l, respectively. The mixed acid etching and passivation process ensures that the surface does not become excessively corroded and increase in roughness.

[0027] Taking steel grade 441 produced by 1#RAP line with specifications of 1.2×1280mm as an example, its composition is C=0.0098%, Mo=0.0019%, Ti=0.1776%, Nb=0.4137%, Si = 0.1381%, Mn= 0.1013%, P =0.021%, S =0.001%, Cr=17.93%, Ni= 0.0841%, N =0.0079%, and the rest are Fe and unavoidable impurities.

[0028] The roughness of the working rolls of the 1#-5# stands are 0.8μm, 0.8μm, 0.3μm, 0.2μm and 0.2μm respectively. The rolling lubricant concentration of the first three stands is configured to 3.4%, and the rolling lubricant concentration of the 4# and 5# stands is configured to 2.2%. The conductivity of the 1# degreasing system is 7mS / cm, and the conductivity of the 2# degreasing system is 4 mS / cm.

[0029] The sodium sulfate electrolysis current was 4500A, the electrolysis time was 1.4s, and the concentrations of nitric acid and hydrofluoric acid in the first mixed acid cell were 152g / l and 18g / l, respectively. The concentrations of nitric acid and hydrofluoric acid in the second mixed acid cell were 128g / l and 15g / l, respectively. The concentrations of nitric acid and hydrofluoric acid in the third mixed acid cell were 116g / l and 12g / l, respectively.

[0030] This solution has been applied to the 1#RAP continuous rolling line of TISCO Stainless Steel Cold Rolling Mill. It has been in operation for more than one year. The process and program design are running stably and the use effect is good.

[0031] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by any technician in this field within the scope of the present invention are included in the patent scope of the present invention.

Claims

1. A surface control method for a stainless steel cold-rolled strip production line for automobile exhaust systems, characterized by: The following steps are involved: (1) Rolling mill: Surface control of ultra-pure ferritic stainless steel for exhaust purposes, through special process design of special configuration of roll roughness and rolling lubricant concentration; (2) Degreasing part: The concentration of degreasing liquid is controlled by conductivity. The conductivity of 1# degreasing system is controlled at 6-8mS / cm, and the conductivity of 2# degreasing system is controlled at 3-5mS / cm to eliminate the residual emulsion on the surface of the steel plate during the rolling process and avoid the formation of emulsion marks; (3) Pickling part: Exhaust ultra-pure ferritic stainless steel contains Si and Mo, which makes it difficult to pickle. It is necessary to design the process from two aspects: electrolytic pickling and mixed acid pickling. Sodium sulfate electrolysis belongs to neutral salt electrolysis, which relies on electrochemical reaction to remove the iron scale on the surface of the steel strip. Due to the particularity of the surface oxide scale after heat treatment, different electrolysis processes need to be designed for steel grades with different Si contents. Roller roughness in the rolling mill section: Based on the deformation of the five stands, the roll roughness is set at 0.8μm, 0.8μm, 0.3μm, 0.2μm, and 0.2μm, respectively, and the roll roughness range is ±0.02μm. The concentration of the rolling lubricant for the first three stands is set to 3.2%-3.5%, and the concentration of the rolling lubricant for stands 4# and 5# is set to 1.5%-2.5%. At the same time, during the rolling process, a boundary lubrication state is formed in the deformation zone, so that the surface roughness of the rolled steel plate can reach 0.12-0.15μm. In the pickling part, for ultra-pure ferritic stainless steel with a silicon content of more than 0.25%, the electrolysis current is set to 4500A-4700A, and the electrolysis time is controlled at 2-4s. Too long an electrolysis time will cause electrolytic marks on the surface of the stainless steel with a high Si content. For ultra-pure ferritic stainless steel with a Si content of 0-0.25%, the electrolysis current is set to 4500-4700A, and the electrolysis time is controlled at 1-2s to ensure a uniform pickling surface. Mixed acid pickling is used in the pickling part, which is divided into 3 tanks. The first tank has high concentration and the pickling concentration is gradually reduced. The concentrations of nitric acid and hydrofluoric acid in the first tank are: 140-180g / l, 16-22g / l respectively; the concentrations of nitric acid and hydrofluoric acid in the second tank are: 120-150g / l, 12-18g / l respectively; the concentrations of nitric acid and hydrofluoric acid in the third tank are: 110-130g / l, 10-15g / l respectively.