A Study On Direct-quenching Process Of700MPa Micro-alloyed High Strength Steel

Compared with the traditional reheat-quenching (RQ) process, direct-quenching (DQ) process needn’t re-heating. So it can reduce the cost of production and shorten the process; at the same time, the carbon equivalent can be reduced because of the less content of alloying elements, thus the combination of strength and toughness and the welding performance can be improved.In this paper, direct-quenching process of700MPa micro-alloyed high strength steel is carried out. The essential difference between the direct-quenching and reheat-quenching is studied. Meanwhile, in order to further enhance the combination of strength and toughness of direct-quenching tested steel after rolling, a series of process parameters are set up. At last, the best experimental process is obtained by studying the microstructures and properties.Detailed experiments are as follows:(1) The effect rules of deformation temperature, deformation degree and deformation rate on the resistance of deformation of this material are researched by single-pass pressing experiments. And the resistance of deformation and mathematical models of the dynamic crystallization deformation were built.(2) Using thermal simulation experiment to determine the critical temperatures of tested steel, draw the static and dynamic CCT curves and research the impact of cooling rate and deformation on the continuous cooling transformation. The result shows that the beginning transformation temperature decreases with the increasing of cooling rate; and the beginning transformation temperatures increases with deformation.(3) Reheat-quenching (RQ) and direct-quenching (DQ) heat treatment experiments after rolling were carried out on the700MPa micro-alloyed high strength steel in order to study the difference between reheat-quenching and direct-quenching steel. The effect of heat treatment processes after rolling on microstructures and properties of the tested steels are investigated. And the toughening and strengthening mechanisms of direct-quenching steel is analyzed.(4) Direct-quenching process of the tested steel is carried out. The result shows that, the combination of strength and toughness of tested steels is the best when the start-quenching temperature is890℃, and the final-quenching temperature is210℃.(5) Tempering process of the tested steel is carried out. The result shows that, the combination of strength and toughness of tested steel is the best when the tempering temperature is620℃, and the tempering time is50min.