Metallurgical investigation for producing deep-drawable continuously annealed Al-killed sheet steels through low-temperature coiling
1) Kimitsu R&D Lab., Metallurgical factors for realizing the low temperature coiling at around 650'C of hot-rolled bandshave beeninvestigated in producing deep-drawable continuously annealed AI-killed sheet steels with a high yield rate. The following conditions are proposed: the control of the chemicai compositions as i) O O1 "/• C~; 0.02 "/•, ii) Mn ;~O 15 •/• and iii) Al~~0.05 •/., and the choice of iv) Iow slab reheating temperatures befow 1 150'C. Conditions i) and ii) contribute to the
... ontribute to the coarsening of cementites in hot-rolled bands and to the decrease in the amount of Mn-O complex during annealing, whilst condition iii) contributes to the promotion of AIN precipitation. Theseare considered to play a significant role in improving the r-value of the annealed sheets. Furthermore condition iv) not only has an effect on promoting grain growth due to the coarsening of precipitates in hot-rolled bands, but also contributes to the prevention of hot-shortness causedby the segregation of S in austenite grain boundaries. A mechanism concerning the influence of the C and Mncontents on cementite coarsening in the hot-rolled bands is discussed, and it is concluded that the growih rate of cementite particle8 during coiling was retarded by the presence of Mn. Moreover the metallurgical meanig for obtaining the excellent r-values is also rationalized A mill test based on the knowledgeobtained in laboratory examination with a coiling temperature of 650'C resulted in a desired level of high r-value and less variation through a whole length of finished coil.