Effect of Cooling Temperature in Controlled Rolling and Cooling on Microstructure and Tensile Properties of Q550D High Strength Steel

Controlled rolling and cooling process was conducted on hot-rolled Q550D steel plates. The effect of six groups of start cooling temperatures and final cooling temperatures on the microstructure and tensile properties of the test steel was studied. The results show that the matrix microstructures of the test steel all consisted of bainite+ferrite+martensite/austenite (M/A) island at six groups of cooling temperatures. At the same finish cooling temperature, when the start cooling temperature decreased, the ferrite content in the test steel increased significantly and the M/A island content increased slightly; the yield ratio decreased. At the same start cooling temperature, when the finish cooling temperature decreased, the ferrite conten decreased slightly while the M/A island content increased obviously; the yield ratio increased. When the start cooling and finish cooling temperatures were lowered at the same time, the content of ferrite and M/A island increased synchronously, but the increase in M/A island content was smaller; the yield ratio decreased. The test steel had the best tensile properties at the start cooling temperature of 755℃ and finish cooling temperature of 395℃.