| With the development of the national economy, the requirement for the steel output has become more and the demands for the steel quality become higher. Due to the quality of slab will effect on the quality of final product greatly, the process controlling to continuous casting slab is very important for steel manufacture.The steelmaking branch of Benxi Steel has made great efforts to renewal and improves production equipment since 1993; its product category and product quality has increased greatly. However, compared with the foreign and some of domestic factories, the composition of product, performance, shape and dimensional accuracy still have certain shortcomings. In order to improve the quality of automobile sheet and high strength steel slab in Benxi Steel, this study is carried out based on the existing process condition. The main contents are as follows:(1) The main steel grades for continuous casting slab in Benxi Steel were selected such as SPHC, DC01, A36-2, S235-JR, CRA2, DC53D+Z, BG420CL, Q235. Their hot ductility was measured using Gleebe-2000 Hot Simulation Test, because hot ductility is a very important parameter to continuous casting slab. The stretch test were carried at temperature of 700℃,750℃,800℃,900℃,1000℃,1100℃,1200℃, 1300℃, and the actual stress-strain at top of temperature, and the critical strain of selected steel were obtained.The selected steel, include carbon steel, low carbon steel, ultra low carbon steel and microalloy steel, was studied with hot ductility simulation test. The experimental parameters were determined and analyzed, and then the appropriate slab mould and the temperature straightening, as well as the withdrawal speed which suited for the steel above were put forward.(2) In all of casting slab shortcomings, the proportion of the slab cracks is about 80%. Casting slab cracks not only affect the productivity of continuous casting machine, but also affect the product quality and increase the production costs. This study analyzed the characteristics of slab cracks and the relevant factors, proposed measures for reducing the slab and conducted a production test, the results show that: the content of element S in steel should be controlled less than 0.020%, w[Mn]/w[S]≥20, superheat should be about 30℃, the pulling speed should not more than 1.2m/min. The superheat and secondary cooling water allocation should be adjusted when casting speed exceeds the speed limit. Casting slab straightening temperature should be controlled more than 900℃. According to the results, the continuous casting process parameters for wheels steel (BG420CL) were optimized:the casting speed was selected at 1.0m/min and the cooling water volume of mould and secondary section were determined, and the suitable surface temperature for high strength steel slab was obtained. The ratio of spheroidal inclusions has raise to 87.21% after RE added.(3) The surface quality of slab is directly related to the quality of the follow-up products. The slab surface crack has existed since the device been put into operation. In this research work, the parameters that may influence quality of casting slab were analyzed by the six sigma management method, such as the chemical composition, mould vibration, mass area ratio, secondary cooling zone and continuous casting roller. Then the relevant measures to solve the problem were put forward in practice. The result shows the slab surface crack was effectively controlled through adjustment of the mould amplitude, the water quantity, the content controlling to w[N] and w[S]. The quality of slab was improved greatly.(4) In order to produce qualified automobile steel sheet, the existing production technique level and equipment condition of Benxi Steel was analysised and discussed in detail. Then, the technological process for advanced automobile sheet was be proposed, the quality standard and main parameters of each process were made.(5) In order to produce advanced automobile slab with high ductility, high r and excellent surface properties, the content of C, N, O in steel should be as low as possible. The factor that may influence on carbon pick up during continuous casting process of ultra low carbon steel, such as ladle brick, front powder, tundish powder is analyzed by the six sigma management method. The results show that the ladle brick, starter flux, covering slag are the most influence factors on the carbon pick up. According to the results, the carbon-free brick ladle, carbon-free starter flux and carbon-free powder were applied and produced an obvious result; the carbon increment of the ultra low-carbon steel was lower than 0.0003%.The factor that influenced on nitrogen pick up also been analyzed, the corresponding technical measures, such as improving the seal of seal ring and the seal of slag of tundish, were applied in practice. The average nitrogen increment is reduced to 2.8 X 10’6; The qualified ratio reached 90.7%. |
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