Abstract: Ti-microalloyed medium carbon steel plate of 20 mm thickness was prepared by controlled rolling-controlled cooling-quenching-tempering processes. The effect of controlled cooling process (cooling rate) on effective grain size and precipitates of the steel was studied, and the strengthening and toughening mechanisms were also discussed. The results show that the faster the cooling rate, the smaller the effective grain size is, the narrower the martensite lath width is, and the finer the Ti-containing precipitates are, so that the steel has both high strength, good ductility and toughness. This is mainly because the rapid cooling can retain the crystal defects and deformation energy obtained during rolling, making the austenite refined during reheating. Moreover, the rapid cooling inhibits the precipitation of Ti in the cooling process, making Ti in the super-saturated state. In the reheating process, fine Ti-containing precipitates gradually precipitate, which can effectively prevent the growth of austenite grains. Effective grain refinement and nano Ti precipitates make the steel plate having good mechanical properties.

Key words: Ti-microalloyed medium carbon steel plate, cooling process, Ti precipitates, grain size, mechanical properties