Effect of alloying elements on Z-direction property of high strength low alloy H-beam steels

To study the effect of alloying elements, Nb, B and Ni, on lamination crack resistant property (Z-direction property) of high strength low alloy H-beam steels, contrast experiment was designed. Microstructure, composition and mechanical properties, especially the Z-direction property of the experimental steels were investigated by means of SEM, XRD, TEM, EDS analysis and tensile test. The results indicate that the microalloy elements of Nb, B and small amount of Ni do harm to the lamination crack resistant property, and the experiment steel of Q345E, which contains Nb, B, Ni and lower inclusions, exhibits lower Z-direction property after ladle refining than the steel of Q345B, which does not contain Nb, B, Ni and does not experience the refining process. Mechanism of this phenomenon is mainly ascribed to the effect of Nb, B and Ni on obstruction of dislocations at grain boundaries and inside grains.