Special features of air-water quenching process of structural steel large-size forgings are analyzed. The process includes two-stage accelerated controlled quenching followed by high tempering the steel with relatively low hardenability. Some peculiarities of decomposition of supercooled austenite under continuous cooling are considered. It is found that the controlled cooling in the air-water medium provides a set of high mechanical properties of structural steel (42CrMo4, AISI4140).