Production Process Of Q550High Strength Steel Under Ultra Fast Cooling

This paper’s background is based on a domestic steel mills "2800mm ultra fast cooling after rolling equipment development and process development" project. The object of study is high strength steel for engineering machinery. The austenite deformation behavior at high temperature, phase transformation during discontinuous cooling conditions, and bainite transformation were investigated. Different cooling process based on UFC and ACC were researched. The main works contents are as follows:(1) Using single press test, the influence of the deforming temperature, deforming degree and deforming rate to the stress of the steel are researched, and the mathematical model of resistance of deformation is established.(2) Using the Formastor-F Ⅱ and MMS-300thermal simulation machine, The heat expansion method is taken to test the CCT curves on both deformed and undeformed condition. The influence of cooling rate and structure of the steel can be got from the metallographic. The result shows deformation makes CCT diagram move leftward and upward, promoting the transformation from austenite to ferrite. Increasing the cooling rate favors the formation of lath bainite; deformation beneficial grain refinement.(3) Hot rolling test on Lab’ s Φ450mill, combined with thermal simulation tests results, formulated the first stage rolling temperature is controlled between1050～1100℃, the second stage of the rolling temperature is controlled between850～950℃,the final cooling temperature is controlled at about550℃hot rolling system. To research the effects of UFC, ACC and UFC+ACC and other technology for micro-alloyed high strength steel organizational performance. The results showed that:In these three process conditions, the properties of Q550D steel are up to the standard requirements. The UFC process conditions whose cooling rate is controlled at40～50℃/s, Its products organization based mainly bainite lath and lath is more refinement, the performance of products is also excellent.(4) By the experiment in steel mill, t the results showed that:the scene mill at lower load conditions, the use of technology UFC, finishing temperature controlled at about800～850℃, the final cooling temperature controls at about500℃～550℃, produced550MPa grade high strength steel and production.