Effect of coiling temperature on the mechanical properties of Timicroalloyed high strength steel

Keeping chemical composition and other parameters unchanged, the effects of coiling temperature (625 and 579℃) on the microstructure and properties of Ti-microalloyed high strength steel were investigated during compact strip production (CSP). Experimental results of mechanical properties show that compared with steel strips coiled at 625℃, the yield strength of steel strips coiled at 579℃ decreases by 205 MPa, but the impacting energy at -20℃ increases from 11.7 J to 47 J. Optical microscopy and electron microscopy were used to study the microstructure and precipitates in the steel. It is found that the primary microstructural constituent of steel strips coiled at 625℃ is ferrite grains, but the microstructure of steel strips coiled at 579℃ is finer and characterized with bainite grains. The volmne fraction of uanometer carbides in steel strips significantly reduces with decreasing coiling temperature, which weakens the precipitation hardening effect and causes a marked reduction of strength. However, the toughness is improved due to grain refinement and volume fraction decreasing of precipitates. Coiling temperature needs to be strictly controlled, because it is a more important parameter of producing Ti-microalloyed high strength steel.