Heredity Characteristic From Hot Rolled Microstructure to Annealed Microstructure in High Strength TRIP Steels

The influence of hot rolled process on microstructure in TRIP (transformation induced plasticity) steel and the heredity characteristic from the hot rolled microstructure to annealed microstructure are investigated. The results show that there are two kinds of hot rolled microstructures at different coiling temperatures. One is composed of coarse grains of ferrite, pearlite and bainite, and the other is composed of small grains of ferrite, bainite and austenite. After annealing, the first kind of hot rolled microstructure is greatly refined, and volume fraction, and carbon content of austenite increase significantly. However, it has little changes in grain size, volume fraction and carbon content of austenite after the second kind of hot rolled sheet is annealed. There are also differences in distribution of retained austenite between the two annealed microstructures observed by EBSD and TEM technology. Retained austenite in the first annealed microstructure is distributed mainly on the inside of the polygonal ferrite in the form of spot, only little retained austenite is located on bainte ferrite boundary, however retained austenite in the second annealing microstructure is located in several places, such as inside of polygonal ferrite, polygonal ferrite boundary, and bainte ferrite boundary.