Research Of Ultra-fast Cooling Process Of One Type High Strength Low Alloy Wear Resistant Steel

Compared with conventional quenehed process, ultra-fast cooling process eliminated off-line reheating and quenching, reduced processing costs and shortened process, in addition, ultra-fast cooling process increased hardenability, weldability, and mechanical balance by reducing alloy elements. This paper focused on the parameters of ultra-fast cooling process and investigated their effect on properties of test steel. In the thesis, thermal-mechanical simulating technology and hot rolling process were applied to study the ultra-fast cooling process of one type high strength low alloy wear resistant steel. The main content of the thesis was introduced as following:(1) The single-pass compression was tested for the experimental steels. Through the flow stress-strain curves, the influence of the deforming temperature, strain value and strain rate on the the austenite recrystallization were investigated, and the mathematical models of deformation resistance were also established. The double hit compression was exerted for the research of the softening behaviors during holding time at constant temperatures after deformation. The softening rate curves of the experimental steels were plotted. By the softening rate curves, the recrystallization stop temperature of test steel was950℃. Through the simulation of ultra-fast cooling process, we can see that if the starting temperature of finishing rolling was950℃and the pates were quenched within a short period of time after rolling, the hardness of test steel was high.(2) The controlled rolling test in experiment were carried out to study the effect of finishing rolling process on the microstructure and mechanical properties of tested steel by ultra fast cooling. The first hot rolling test in laboratory show that mechanical balance was got when the starting temperature of finishing rolling was950℃. The rough rolling temperature of second rolling test was1050℃, finishing rolling950℃, and the ultra-fast cooling was carried out within a short period of time after rolling. Tempered at300℃, the test steel had good mechanical properties balance. The transmission electron microscope was used to study the fine microstructure. The results showed that with the tempering temperature increasing, the dislocation density in the lath martensite reduced notablely, and martensite lath became unclear. Also s-carbides started to precipitate and grow out.(3) Different starting cooling temperature, finishing cooling temperature and cooling rate were carried out to study their effect on the mechanical properties of test steel in the cooling process. The results showed that the strength and toughness of steel were reduced first, then increased and hardness reduced with the start cooling temperature decreasing. Through the analysising of microstructure, the results showed that the recovery occurred in the work-hardened austenite and the dislocation density in the lath martensite reduced, while the start cooling temperature decreased. With the finishing cooling temperature increasing, the strength and hardness of test steel decreased, the toughness first increased and then decreased, then increased and then decreased. Different levels of wear-resistant flexible steels could be rolled by controlling the finishing cooling temperature. When the Finish cooling temperature was300℃, a certain amount of bainite generated between the martensite. At the same time, a higher degree of self-tempering took place, with excellent toughness. When the cooling rate was30℃/s, the microstructure generated more bainite, mechanical properties of test steel declined.