A new YP500MPa grade high strength steel plate was developed to meet the requirements of BHS500 (BHS: bridge high performance steel). Microstructure control of extremely-low carbon bainite steel by thermo-mechanical controlled rolling and controlled cooling by the Super-OLAC were applied to realize production of an as-rolled (non-heat-treated) high yield point steel with low PCM by an on-line process. The developed steel satisfied the required mechanical properties and showed satisfactory toughness after cold working and no deterioration in mechanical properties after the line-heating process. The maximum hardness of the heat affected zone (HAZ) was less than 280 points in Vickers hardness (HV) under small heat input conditions such as arc-strike, showing excellent welding hardening resistance, and no cracking occurred in the y-groove weld cracking test, indicating that preheating is not required for welding. Evaluation of butt welded joints produced by electro gas welding (EGW) and submerged arc welding (SAW) also revealed that the developed steel plate has good weldability.