Effect of Coiling Temperatue and Cold-rolling Reduction on the Properties of High Strength IF Steel

Two industrial produced hot-rolled high strength IF steel strips were cold rolled in laboratory cold rolling mill,and then annealed in salt bath according to the process of a continuous hot-dip galvanizing line.Optical microscopy and tensile tests were used to study the effect of coiling temperature(670℃ and 710℃,respectively) and cold-rolling reduction(55%,65%,75% and 85%,respectively)on their properties.The results showed that,lower coiling temperature refines the grain size of the hot-rolled strip,but the grain size of the cold-rolled and annealed steel is relatively coaser and the r_m value is also relatively lower.the ferrite grain size of the cold-rolled and annealed steel remains fine.However,the cold-rolled and annealed steel has much lower yield strength as a result of scavenging of interstitial carbon atoms by means of NbC ripening during annealing.The coiling temperature has little effect on the strength,elongation and n_m value of the cold-rolled and annealed steels.An increase of cold rolling reduction from 55% to 75% almost doesn't affect the ferrite grain size of the annealed steel.A further increase of cold rolling rduction to 85% only refines the ferrite grain a little.The cold rolling reduction has no obvious effect on strength and elongation and has no effect on n_m value.But r_m value increases when the cold-rolling reduction increases.