Metallurgical investigation for producing deep-drawable continuously annealed Al-killed sheet steels through low-temperature coiling.

Abstract

Metallurgical factors for realizing the low temperature coiling at around 650°C of hot-rolled bands have been investigated in producing deep-drawable continuously annealed Al-killed sheet steels with a high yield rate. The following conditions are proposed: the control of the chemical compositions as i) 0.01% < C ≤ 0.02%, ii) Mn ≤ 0.15% and iii) Al ≥ 0.05%, and the choice of iv) low slab reheating temperatures below 1150°C. Conditions i) and ii) contribute to the coarsening of cementites in hot-rolled bands and to the decrease in the amount of Mn-C complex during annealing, whilst condition iii) contributes to the promotion of AIN precipitation. These are 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 caused by the segregation of S in austenite grain boundaries. A mechanism concerning the influence of the C and Mn contents on cementite coarsening in the hot-rolled bands is discussed, and it is concluded that the growth rate of cementite particles during coiling was retarded by the presence of Mn. Moreover the metallurgical meaning for obtaining the excellent r-values is also rationalized.A mill test based on the knowledge obtained 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.