Study on the Ti Micro-Alloyed Ultra-High Strength Steel Produced by Thin Slab Casting and Direct Rolling

摘要

New 700MPa hot rolled ultra-high strength steels were successfully developed and produced in large scale by using Ti micro-alloying technology in CSP line. Experimental methods, such as SEM, TEM and chemical phase analysis,were used to study the microstructure and precipitates of the experimental steel. It is shown that the microstructure is composed of quasi-polygonal ferrite grains with average grain sizes ranging from 2.9～3.8μm. Small amount of titanium nitrides with size about several hundreds nanometers and a large fraction of titanium carbides are found in the steels. High dislocation density and the configuration of dislocation tangles can be observed. The major strengthening mechanism of 3mm ultra-high strength steel is grain refinement strengthening, which contributes more than 300MPa to the yield strength. The contribution of precipitation strengthening and solid solution strengthening are calculated as 158MPa and 117MPa,respectively. The contribution of dislocation hardening is estimated to be about 164MPa.