Effect of Cooling Rate on Microstructures and Properties of Directly Quenched Steel

The microstructure of two kinds of directly quenched steel specimens was observed,CCT curves of them were measured,and tensile test was performed with tempered specimens at different tempering temperatures after rolling.The bainite start point (Bs) decrease when boron was added and bainite ferrite with large acicular or lathy shape led to low strength.However,for the boron free steel at a slower cooling rate,grain bainite is apt to be transformed because Bs was much higher than that of boron bearing steel.And this kind of bainite mixed with martensite can achieve good strength.Therefore,to avoid of large bainite transformation region,fast cooling rate is appropriate for the boron bearing steel,but for the boron free steel,lower cooling rate is preferable.