Research On The Thermo Mechanical Control Process Of High Strength Steel Q550for Engineering Machinery

The main structure of loaders, excavators, crane, large equipments and other engineering machinery bears complex and changeable cycle load. So the structure materials need to be with high yield strength and fatigue strength, good impact toughness, and good weld ability. With the development of large-scale and light-duty engineering machinery, these performances are more and more required. The development of controlled cooling technology gives the strength of the steel plate a great advance space. So the amount of micro alloying elements added could be reduced when these performances reach the standard. It also makes the reduction of cost possible.This article’s background is "AnSteel Ultra Fast Cooling technology development--research on products and process of high preferment plate" 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 involved 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. The behavior of dynamic softening of the steel to be tested in austenitic area is also researched. The activation energy of dynamic recrystallization was calculated. Micro alloying elements Mo can improve the deformation resistance and the activation energy of dynamic recrystallization, and expand the area of unrecrystallization temperature range.(2) The heat expansion method is taken to test the CCT curves on deformed condition. The influence of cooling rate and structure of the steel can be got from the metallographic. The microstructure under different cooling rates was all bainite. The greater cooling rates were, the smaller the grain size was. When cooling rate changes from0.5℃/s to30℃/s, the micro hardness got bigger and bigger.(3) By RAL’s hot rolling experiment, the steel plate used UFC can meet the engineering machinery Q550and Q690standards. The final rolling temperature is controlled at800℃～850℃, the final cooling temperature is controlled at450～500℃, the cooling rate of UFC is controlled at30℃/s.(4) By the experiment in steel mill, the steel plate (20mm,40mm thick) used the cooling process of UFC+ACC can meet the engineering machinery Q550standard. The final rolling temperature is controlled at800癈～850℃, the final cooling temperature is controlled at400～450℃.Reducing final cooling rolling temperature appropriately can increase the strength without decreasing the low temperature toughness too much.