A segmented 3-D coupled electromagnetic-thermal solute transportation model, aimed to better understand the macro-segregation formation in the strand during a popular continuous casting (CC) process, has been developed. Based on the model validation by industrial tests, the effect of M-EMS and F-EMS running parameters on the segregation distribution were subsequently carried out. It is shown that the simulated solute segregation profile in the W-shape along the casting thickness direction is in a good agreement with the measured profile. In the initial solidification shell with thickness in 0.020 m, the solute segregation degree changes from a positive value to a negative with the increasing distance from strand surface because of the washing effect induced by the impact flow from the nozzle side port and M-EMS. Here, the minimum degree of carbon segregation decreases from 0.976 to 0.875 with the increasing stirring current from 100A to 550A. As the stirring current of F-EMS decreases from 630A to 200A, the minimum segregation degree locating at 0.109 m distance from strand surface increases from 0.805 to 0.967. The carbon segregation degree at the strand center first decreases from 1.10 to the minimum value of 1.06 at the case of 350 A/4 Hz because of the concentration equilibrium for the local decreasing negative segregation induced by F-EMS, and then increases to 1.16 due to the local poor stirring.

Experimental methods, such as OM, SEM and X-EDS, were used to study the transverse cracks appeared on the external arc of tensile region during pipe bending. The pipe line steel consists of acicular ferrite (AF) and granular bainite (GB) with good mechanical properties. Copper phase enrichment occurs at prior austenite grain boundaries near the cracks of bend pipe, which is the main reason for the surface cracks. In order to decrease the cracks ratio, the microstructure evolution and stress state during the bending process need to be researched deeply.

The effect of peak temperature on austenite grain size was investigated by using thermal simulation and OM, TEM analysis. Grains grow slowly below 1250°C, and dramatically grow up as the temperature exceeds 1350°C. The primary precipitates in X80 pipeline steel are TiN and NbC, TiN particles cannot absolutely dissolve in steel during holding for 2s at 1300°C.Although TiN particles exist in steel, they cannot hinder grain growth of CGHAZ during welding process. The phenomenon can be explained by Ostwald Ripening mechanism.

Experimental methods, such as OM, SEM and X-EDS, were used to study the microstructure of X80 pipeline steel. It mainly consists of fine acicular ferrite (AF). X80 pipeline steel possesses high strength and impact energy at-30°C approaches to 400J. Grain refinement and precipitation hardening are the main reasons for high strength, and toughness improvement can be attributed to grain refinement and particular microstructural characteristics of AF.

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.