New 700MPa hot rolled ultra-high strength steels were successfully developed by using Ti micro-alloying technology in CSP line. Experimental methods, such as OM, TEM and chemical phase analysis, were used to study the experimental steel. The microstructure is composed of quasi-polygonal ferrite grains, whose average grain size is about 4μm. Large number of nanometer TiC particles distribute along dislocations. The mass fraction of MX phase is 0.0793wt%, in which the particles smaller than 10nm account for 33.7%. The contribution of precipitation hardening resulting from nanometer particles is calculated as approximate 158MPa.

Precipitates in ultra-high strength Ti micro-alloyed strips with 700MPa yield strength on TSCR process and in the thermal simulation experimental specimen are observed via Scanning Electronic Microscope (SEM) and Transmission Electron Microscope (TEM), the results show that: the precipitates in the Ti micro-alloyed cast slab thermal simulation experiment can be divided into three categories: 1) micron-sized liquation TiN, 2) 100-200nm sized TiC formed along original austenite grain boundaries or along the dendrite segregation band, 3) solid precipitated 50-100nm sized TiN. Deformation induced spherical TiC (about 10nm-30nm) homogeneously distributed in the matrix after the austenite deformation .After simulated coiling, dispersed TiC (about 5-15nm ) precipitated from ferrite are found in the specimen. TiN with hundreds of nanometers size are commonly found in Ti micro-alloyed strips in industrial production. Ti4C2S2and Ti (CN) are complex precipitated, Ti4C2S2size is less than 30nm; physical and chemical phase analysis shows that the nanosized TiC precipitates are characterized by high volume fraction and small dimensions.

High strength steel ZJ700MC with yield strength grade of 700 MPa was successfully developed by Ti micro-alloyed technology in the EAF-CSP line at Zhujiang Steel. The composition design, smelting process, continuous casting and rolling process of the steel, as well as the mechanical properties of the hot-rolled product are introduced in the paper. An analysis on the strengthening mechanism of the high strength steel shows that the enhanced strength and toughness of the steel were caused by the refinement of ferrite grain and the precipitation strengthening of TiC.