Study Of Cold Rolling Process And Deep Drawing Ability Of Sheets Hot-Rolled By CSP

It has a huge economic benefit to produce cold-rolled sheets for automobile hot-rolled bycompact strip production (CSP), the short smelting technology of thin bands. But the CSP-coldrolled bands always have low deep drawability. Many factors, such as the type of the steel andproduction process parameters, influence the deep drawing ability. Therefore, it is meaningful tostudy the influencing factors, especially the carbon content, cold-rolled reduction and annealingprocess, for designing the corresponding parameters of CSP-cold rolled line.For confirming cold-rolled line parameters, the low carbon aluminum killed steel hot-rolledby CSP is used in the experiments. The cold-rolled sheets were annealed on different temperature,from400℃to750℃, and the recrystallization temperature was calculated by the temperature-hardness (HV) carve as well as the microstructure variation. CSP hot-rolled bands werecold-rolled in reduction from45%to85%, and then after batch-annealed the final mechanicalpropertites of the sheets were tested in order to design the favorable cold-rolled reduction.According to parameters confirmed in the experiment, a trial production of deep drawing sheetsin DC04digree were produced by CSP-cold rolled line in the steel works, and themicrostructure, texture, precipitate etc were analized by SEM, TEM and EBSD.The results showed that the recrystallizing temperature was550℃and700℃at the end.The favorable cold-rolled reduction is about70%as the final sheets had the best mechanicalproperties. The equiaxed grains of the CSP-hot bands turned to be fiber microstructure ofcold-rolled bands and pancake-shaped grains of annealed bands, and the annealed bands have thefavorable {111} texture. All of the deep-drawing sheets produced in the tiral production meet theexpect targets.The r-value, one of the indexs of deep drawing ability, is found higher when the annealedsheet microstructure is of pancake-shaped grains elongated in the rolling direction (RD). Amathematical model (as called the PG model) has been developed between the r-value andpancake-shaped grains flatness:The PG model was then verified by takingsamples. Besides, two different annealing schedules with different heating rate were designed toanalyze the cause of the pancake-shaped grains formation and the effect of the annealing. It isconcluded that the pancake-grains formed by bulging of initial cold-rolled fiber grain boundaries,and finished to be pancaked-shaped because the the new grain growth is inhibited in normaldirection while the grain growth is still occurring in RD.