Optimization Research On Secondary Cooling System Of Slab Continuous Casting At Nanjing Iron & Steel Group Co.

During the producing process of slab caster, secondary cooling system is a very important factor which influences slab quality. Almost the familiar slab quality defects have relation to secondary cooling system. As for the most producers, how to choose a rational secondary cooling system has became a focal problem. In this paper, the problem of triangular area cracks occurred on continuous casting slab at Nanjing Iron & Steel Co. Ltd. was analyzed and for the purpose of solving the exist problem of slab quality defects, the secondary cooling system was optimized and the corresponding optimization result was put forward finally.At first, the cold and hot state characteristic of the four kinds of nozzles for slab caster at Nangang was systemically tested in this paper. The formula of water flow density distribution and heat transfer coefficient of nozzle was attained by experiment to offer the comparative accurate boundary condition for computer simulation. According to the exist shortcoming of the current nozzle testing equipment for heat transfer coefficient in lab, testing equipment and testing methods was improved. Then on the basis of non-stabilization state method, the new nozzle testing equipment for heat transfer coefficient was revamped and by means of advanced programming language Visual Basic 6.0, the software named PZ-202 nozzle hot state characteristic testing software was finally developed. At the same time in order to get the measurable simulated results, the heat transfer function was discrete by using the finite difference method. And combined with the current particular condition of slab caster at Nangang, the heat transfer mathematics model of secondary cooling process on slab continuous casting at Nangang was established. Then in view of the disadvantage of current computer simulation software of secondary cooling on slab continuous casting, the software was modified and improved, and the corresponding new software named CCSCCS-3.0 was developed. Through the macroscopic structure,sulphur print test of slab and quality statisticresults, the quality status of slab at Nangang was systematically analyzed. From the three points of nozzle style,nozzle arrangement manner and water distribution schedule at secondary cooling area, the current secondary cooling system of slab at Nangang was evaluated. The conclusion thinked that the direct reason which leaded to the triangular area crack on continuous casting slab at Nangang was that the water capacity at slab side zone was excessive large. But at the same time the unfitness of nozzle style on slab side and the irrationality of nozzle arrangement were also the most important factors that influence formation of triangular area crack. Aim at the problem of the current triangular area crack at Nangang, by use of the experiment result of nozzle characteristic test and the simulation software for secondary cooling process, and combined with the current status of secondary cooling system at Nangang, the optimization scheme of secondary cooling system was finally put forward. The scheme considered that the circular style nozzle arranged at the second line of slab side should be remodeled as the flat style nozzle, and the nozzle arrangement and water distribution schedule used for the first and second line of slab arc area should be utilized for different slab section. Then the water scheme for different casting speed was also offered. Finally the optimization scheme was partly validated at Nangang steel factory, taking example for steel 20g, the ratio of slabs occurring triangular area crack in the fifth month of year 2002 has been decreased from 32.29% to 2.33%, the work for reducing the ratio of steel grade changed has taken out great effect obviously.