Formation Mechanism And Control Of Surface Local Coarse Grain Defect Of Low Carbon Al-killed Steel

Surface quality is an important quality measurement of cold rolled strip.With the rapid development of automobile and home appliance manufacturing industry,the demand for high quality cold rolled strip is growing,and the customer demand for the surface quality of cold rolled products is increasing.So the product surface quality control ability becomes the key of the efficiency of steel enterprises and the market competitiveness.Due to the long production process and many processes of the cold rolled products,the causes of surface defects are very complicated,and it is very difficult to qualify.The cold rolled products in Shougang Jingtang Company face surface quality problems such as pressing orange peel,bright spot after electroplating,surface micro crack,etc.It is especially serious with low carbon and Al-killed steel.Through analysis,it is found that in the above-mentioned defect area,the surface of the substrate exist dark spot along the strip.The microstructure analysis shows that there are abnormal surface coarse grains.This paper aims at solving the problem of surface roughness of cold-rolled strip steel,and the research work has been carried out with the Shougang technology research institute.The main research work and conclusions are summarized as follows.After a lot of the division of sampling analyses and bulging experiments,the results showed that the defect appeared on the cold hard roll strip.The rolling process has only experienced acid surface oxidation iron removal and cold deformation.There is no possibility of recovery or recrystallization resulted from grain boundary migration and the grain growth.It can be concluded that low-carbon Al-killed steel surface coarse crystal defects appear in the hot rolling process.The present work was also focused on the investigation of the mechanism for surface local coarse grain formation by analyzing the composition,dimensions and production technology,by measuring the CCT curve of low carbon Al-killed steel,and by carrying out laboratory rolling simulation.In order to control the surface local coarse grain of the low carbon Al-killed steel,it was proposed that some adjustments for the process and equipment should be made in the hot rolling line.Thus,it significantly reduces the surface coarse grain defect in low carbon Al-killed steel.This thesis provides good theoretical guidance for the production of low carbon Al-killed steel products.It is also a strong guarantee for the production of high quality cold rolled strip steel for Shougang Company.