The continues cooling transformation (CCT) of a low carbon Mn-Nb-B steel in the undeformed and deformed conditions were investigated, respectively. The CCT diagrams of the steel were constructed. The microstructures and microhardness were analysized. The results showed that the microsructures contains ferrite, pearlite, granular bainite, acicular ferrite and lath bainite depending on cooling rate; Deformation moved the CCT curve to the top left corner, increased the transformation start temperatures slightly, and promoted the formation of ferrite and pearlite. Furthmore deformation also fined the transformed microstructures.
The effects of direct quenching and tempering (DQ-T) process and conventional reheat quenching and tempering (RQ-T) processes on the microstructure and mechanical properties of a high strength low alloy steel were investigated. In the as-quenched DQ steel, prior austenite grains are elongated parallel to the rolling direction, whereas the as-quenched RQ steel mainly consists of equiaxed grains; The DQ process was found to enhance the hardenability of steel effectively. The tensile strength and yield strength of DQ specimen, were higher than that of RQ specimen. In contrast, low temperature toughness of DQ-T specimen was generally inferior to that of RQ-T specimen.
The effects of boron on the undercooled austenite transformation of low carbon Mn-Nb steels during continuous cooling were investigated in this study. Five kinds of steel specimens were fabricated by varying boron contents. Continuous cooling transformation (CCT) curves of the investigated steels under deformed conditions were constructed by a combination of deformation dilatometry and metallographic methods. The results indicated that a small amount of boron efficiently increased the hardenability of steels and lowered transformation temperature, which leading to a finer bainite microstructure and an increase in hardness.
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