Transformation behavior of high-strength steel plates with low welding crack sensitivity

The dilatometric curves of Q690 steel at different cooling rates were tested using Gleeble 3800 to study the continuous cooling transformation behavior of high-strength steel with low welding crack sensitivity.The continuous cooling transformation curves of Q690 steel were constructed according to the transformation tem-peratures determined using dilatometric curves and corresponding microstructures at different cooling rates.The results show that the microstructure of Q690 steel remarkably changed with the increase in the cooling rate.The matrix of the steel was ferrite and pearlite when the cooling rate was between 0.1 K/s and 0.5 K/s.With increasing cooling rate, the microstructure of the steel was considerably refined, and the Vickers hardness of the steel increased.When the cooling rate reached 1 K/s, the microstructure of the steel was further refined, and bainite transformation occurred.Polygonal ferrite almost disappeared from the steel when the cooling rate reached 10 K/s, and the matrix was mainly composed of bainite and a small amount of lath martensite.With the increase in the cooling rate, the ferrite decreased and martensite increased in the steel, resulting in higher Vickers hardness.When the cooling rate reached 30 K/s, the microstructure of the tested steel was full martensite matrix.