Development of Q500q Steel Plate Via Ultra-fast Cooling and Accelerated Cooling System

After continuous cooling transformation test on Q500 q steel,a ultra-fast cooling and accelerated cooling technology had been proposed.For the new controlling rolling and cooling process,during the roughing rolling phase, start rolling temperature shall be over 1 050 ℃ and total reduction shall be greater than 60%,and the thickness of transferred bar is 2.5 times over that of finished product.For the finishing rolling phase,start rolling temperature is between 830～860 ℃and finishing rolling temperature is between 780～810 ℃.The cooling strategy of combined Ultra-fast Cooling and Accelerated Cooling System is used to achieve 630～650 ℃ of finishing cooling temperature with cooling rate of about 15 ℃/s.Microstructure observation show that microstructure of the test steel is composed of acicular ferrite and bainite,with average grain size at ASTM 13.Mechanical test results show yield strength between 538 MPa to 560 MPa,tensile strength between 538 MPa to 560 MPa,elongation≥21% and impact absorbed energy≥160 J at-40 ℃,the direction of thickness reduction of area≥75%.The tested steel has a satisfied combination of strength and toughness.