Since the establishment of the factory in 2008, pitted and pitted surface defects have been a problem that affects the improvement of surface quality of Hanbao Company. In order to reduce the correction rate of first-grade products with pitted and pitted surfaces, improve the surface quality of strip steel, and reduce quality loss, research has been carried out to solve the problem. This problem also enables the normal use of high-speed steel rolls.
The hot rolling mill of Hanbao Company of Handan Iron and Steel Group has a 2250mm conventional hot continuous rolling strip production line with a contemporary international advanced level, with an annual designed production capacity of 4.5 million tons. The hot furnace is designed by the French Stein Company, and the main rolling line is designed by the German SMS Company and equipped with an electrical control system by the Japanese TMEIC Company. The entire production line adopts advanced technologies such as digital combustion of the heating furnace, large side pressure of the fixed-width machine, edge heating of the intermediate billet, multi-means plate shape control of the finishing rolling unit, and high-power AC-DC variable frequency transmission. It has a large rolling capacity and production technology. It has the characteristics of advanced equipment, reasonable equipment configuration, large rolling mill capacity and complete control means.
The products can cover automobile steel, ship hull structural steel, high weather-resistant structural steel and high-strength pipeline steel with thicknesses ranging from 1.2 mm to 25.4 mm and widths up to 2130 mm. The main features of the products are wide sections, high Precision, variety, high strength, etc.
Since the 2250mm was put into production, especially with the increase in cold rolling material preparation, pitting and oxide scale defects have been problems that have plagued the improvement of the first-grade product rate. In the next process, it was also repeatedly pointed out that there were pitting quality problems in the material preparation in our factory, which could not be corrected during the cold rolling process, resulting in a downgrade and a change in judgment.
In 2012, there were 9 objections raised by external users about pitting defects. According to the two-item process capability analysis, the correction rate of first-grade products with pitting defects in 2012 was 0.68%, causing losses to the company. Facing the current severe market situation, the company has made great efforts to transform its production and operations, adhere to the path of “variety, quality, and efficiency”, vigorously reduce unplanned products, and comprehensively improve process control capabilities and product physical quality. It is imperative to solve the pitting and pitting defects.
2. Characteristics of pitted surface defects
Based on a statistical analysis of the information on all pitted defective steel coils last year, it was found that their distribution has the following obvious characteristics:
2.1 is basically produced in the SS400 thin-gauge rolling plan. Most of them are cold-rolled materials, the specifications are thin, and the thickness is mostly distributed between 2.0-3.5mm.
2.2 More than 75% of pitting defects occur on the lower surface.
2.3 After off-line, the surface of the rolls has obvious oxide film peeling off and the surface is rough, especially the lower roll.
Using the fishbone diagram, we sorted out the influencing factors that caused pitted and pitted surface defects in the finished product and found a total of 11 main factors. In order to find out the key factors, we conducted an on-site production process quality investigation and research and used statistical tools to analyze the incoming materials. Eleven factors including composition, heating temperature, gas calorific value, descaling water pressure, descaling nozzle, roll wear, roll gap spray water, horizontal spray water purge, and plan arrangement were demonstrated one by one to find out the chemical composition and Roller wear is the main cause of pitting defects.
3. Theoretical analysis of pitted surface defects
3.1 Research on the chemical composition of slab
Analyze and explore through experiments the correlation between the chemical composition of the slab and the occurrence of pitting. Adjust the Si content of the SS400 slab to above 0.055% to arrange the production and follow up for confirmation. According to some data and literature, the pitting defects on the strip surface are closely related to the surface condition of the rolls, but the wear and oxide film damage on the roll surface is largely caused by the friction of the small and loose secondary iron oxide scale Fe3O4 on the surface of the strip during the rolling process. Damage, the amount of secondary iron oxide scale generated and the structural condition vary with the chemical composition of the rolled strip.
Therefore, there is a certain relationship between pitting defects on the surface of the strip and the chemical composition of the rolling blank, especially AI-Si-killed steel. When the Si content exceeds 0.05%, the Fe2SiO4 dense layer formed on the surface of the strip will prevent the steel from cracking. The formation of Fe3O4 from secondary oxidation plays a significant inhibitory role.
3.2 Research on roll material and wear
After testing and observation, it was found that when pitting defects occurred, the defects disappeared after replacing the fine rolls, and the oxide film peeled off obviously on the surface of the rolls after off-line, and the surface was rough, especially on the lower roll. When producing thin specifications, pitting is more likely to occur, which indicates that it is closely related to the wear of the rollers. On the one hand, the production of thin specifications causes more serious wear on the rollers. On the other hand, the temperature requirements are higher, which will accelerate the wear of the rollers.
By long-term tracking of roll temperature and offline roll surface conditions, we explore the use of rolls with high hardness, wear resistance and low roughness, which is to effectively improve the quality and service life of the rolls. Drawing on the experience of similar companies, we try to use high-speed steel rolls in F2-F5 to improve the wear resistance of the rolls and improve the surface quality of the rolls and strips.
4. Solving the problem of pitted surface defects and putting high-speed steel rolls into use
4.1 Test of chemical composition
First, the chemical composition of the slab is adjusted. The correlation between the chemical composition of the slab and the occurrence of pitting was found through analysis and experiments. Adjust the Si content of the SS400 slab to above 0.055% to arrange the production and follow up for confirmation. According to some data and literature, the pitting defects on the strip surface are closely related to the surface condition of the rolls, but the wear and oxide film damage on the roll surface is largely caused by the friction of the small and loose secondary iron oxide scale Fe3O4 on the surface of the strip during the rolling process. Damage, the amount of secondary iron oxide scale generated and the structural condition vary with the chemical composition of the rolled strip.
Therefore, there is a certain relationship between pitting defects on the surface of the strip and the chemical composition of the rolling blank, especially AI-Si-killed steel. When the Si content exceeds 0.05%, the Fe2SiO4 dense layer formed on the surface of the strip will prevent the steel from cracking. The formation of Fe3O4 from secondary oxidation plays a significant inhibitory role. Through organizing two tests with the plating chamber, the research personnel conducted on-site tracking. From the flat quality of the slab, heating, rolling, and coiling to the surface quality of the finished product, process quality control was carried out in each process. After being compared with conventional products The effect is not obvious.
4.2 Testing and research on high-speed steel rolls
Because the 2250mm production line mainly produces thin specifications and varieties of steel, the rolls, especially the finishing rolls, wear quickly, making it difficult to ensure the surface quality of the product at the end of the rolling plan. When the surface quality of the rolls is poor, pitting defects are prone to occur. In order to improve the service life and surface quality of the rolls, high-speed steel rolls are used instead.
4.2.1 Optimization of cooling system
Good cooling of the roll is the most effective way to reduce the excessive thickening of the oxide film on the roll surface and avoid exposing the roll surface to high temperatures that produce oxidation. This can maintain the good adhesion of the oxide film without falling off, and will not cause the skin and surface to peel off. Therefore, increase and stabilize the water pressure to ensure sufficient water pressure for the cooling water and ensure that the work roll cooling system is effective. The most suitable water pressure is between 7 and 15 bar.
4.2.2 Analysis and Improvement of the “Meteor Spot” Problem
First, high-speed steel rolls were used on the F5 stand, and then high-speed steel rolls were put into use. In the initial stage of use, “meteor spots” defects appeared on the surface of the rolls, and as the number of times the machine was used increased, the meteor-shaped spots would gradually increase, and sometimes the depth would appear. Large meteor-shaped spots seriously affect the quality of the roll surface, and the roll has to be ground in advance to improve the surface quality. However, this greatly increases the consumption of high-speed steel rolls, and the meteor spot defects will also increase as the service life increases. , seriously affecting the surface quality of the strip.
By measuring the roll temperature on site, consulting information, collecting data, finding the reasons, and discussing the reasons for this phenomenon, we finally found that the meteor spot defect is closely related to the rolling rhythm and roll temperature. Because the graphite content in the structure of high-speed steel is lower than that of cast iron rollers, and the wear-resistant mechanism of high-speed steel does not rely on carbides to increase wear resistance, the friction with the steel plate is greater during the rolling process, and an excessively fast rolling rhythm will cause The surface temperature of the roll rises sharply, causing popular spots to form under the action of friction and thermal fatigue.
Focusing on the defect formation mechanism, the research team formulated a reasonable rolling rhythm to fully cool the rolls and reduce thermal fatigue effects. In view of the high friction characteristics of high-speed steel rolls, they reviewed relevant information and decided to use a roll gap system to solve this problem. Problem, after using rolling oil, the rolling force is effectively reduced, and the friction between the roll and the slab is reduced. Meteor spot defects on the surface of high-speed steel rolls have been significantly reduced, and the roll service life has been increased from the previous 2 times to 4 times, doubling the roll service life. While saving roller consumption, it also improves the surface quality of strip steel, especially the surface quality of high-end automobiles such as Grade 05.
5. Application effects
After the implementation of the measures, the data for the four months from January to April 2013 were analyzed. It was found that the first-grade product reclassification rate for pitted noodles dropped from 0.68% to 0.26%, and the number of roll grindings was reduced by about 48 times. With an annual output of 4.5 million tons, it can create an annual profit of nearly 3.8 million yuan.