Effects of Tempering Treatment on the Microstructure and Mechanical Properties of High-Strength Weathering Bridge Steel
Two kinds of weathering bridge steels with lath-like bainite(Q470 steel)and granular bainite(Q550 steel)were prepared by controlling the cooling temperature after hot rolling.The effects of tempe-ring treatment on the microstructure and mechanical properties of these two kinds of steels were investiga-ted.Physical-chemical phase analysis method,X ray diffractometer and universal testing machine were used to characterize the evolution of microstructure and mechanical properties during tempering.The results showed that the yield strength and-40℃impact energy of Q470 steel was higher than that of Q550 steel in hot rolling state.After tempering at 500-650℃,the yield strength of Q470 steel decreased first and then increased with the increase of tempering temperature(665-705 MPa),and the-40℃impact energy gradually decreased from 300 J to 220 J.Under the same tempering conditions,the yield strength of Q550 steel increased with the increase of tempering temperature(500-650 MPa),and the-40℃impact energy maintained at 210-230 J.The main strengthening mechanisms of these two kinds of steels were dislocation strengthening and fine grain strengthening.In addition,the strengthening of Nb precipitation gradually in-creases with increasing tempering temperature,compensating for the strength loss caused by the decrease in dislocation density,which was the main reason for the increase in yield strength of experimental steels.
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