Effect of Tempering on Microstructure and Mechanical Properties of Ultra Low Carbon Heavy Steel Plate with Intercritical Quenching

The Effect of tempering on microstructure and mechanical properties of ultra low carbon heavy steel plate with intercritical quenching were studied by utilizing SEM, TEM, tensile and impact tests. Mobile dislocations decreased, plenty of carbides and secondary phase particles precipitated dispersedly in the matrix, and then some dislocations were pinned by these precipitations, which made the YS increase when tempered from 180 to 450 degrees C. However, the bainite islands, formed after intercritically quenched, were still existed, the boundaries of which were clear but just laths broadening in the interior, so the UTS had a slight variation. With increasing tempering temperature from 450 to 630 degrees C, carbides became coarse, the bainite islands were gradually polygonized, even the boundary of which became fuzzy, dislocations transferred and formed the cellular sub-grain, which caused synthetically both YS and UTS decreasing evidently, the low temperature toughness gradually increasing, and the mixed fracture of impact specimens converting into ductile fracture.