Steel coils for ERW casing and tubing pipes had been used with low carbon HSLA for better toughness but, there had been several difficulties such as surface defect, lower tensile strength occasionally due to higher yield ratio, and higher mechanical properties variation. Considerable efforts have been made to improve all the above mentioned weak points. The surface defect classified as edge imperfection was originated from surface crack in slab mainly related with peritectic range of carbon. Edge imperfection was minimized by increasing carbon content and processing control. Hot rolling process parameters were optimized through considering precipitation and recrystallization behaviour. The effect of number of pass in rough rolling was compared to get better austenite condition during hot rolling. By optimizing hot rolling process parameters, proper range of tensile properties with lower yield ratio and good impact toughness are obtained with moderate range of mechanical properties deviation along coil length. API 5CT-J55 steel has been successfully modified in its chemical compositions as well as processing parameters to fulfil the requirements by the customers.