The demand of high strength steel for ship and offshore structure application has been changed from conventionally heat treated steel to TMCP steel, because the latter provides the improved strength - toughness balance and weldability. Enhanced property of TMCP steel comes from the effective conditioning of austenite with controlled rolling and pertinent selection of microstructure by accelerated cooling process. This new thermo-mechanical control process, named PILAC(POSCO In-Line Accelerated Cooling) has been developed to produce the high qual1ty structural steel. The production conditions of thermo-mechanical control process were studied to obtain the appropriate combination of strength and toughness of base metal. The effects of carbon to manganese ratio and Ti addition on the heat affected zone toughness were evaluated to optimize the chemical composition of steel. Based on these studies, the 50kg/mm2 tensile strength grade steels for ship and offshore structure were produced with PILAC process. These steels showed good heat affected zone toughness, even in heat input of 18kJ/mm, and good weld :rack resistance.
Recently, the demand of structural steel has moved from mild steel to high strength steel and the consumed quantity of high strength steel has continuously increased. During last five years, the amount of high strength steel used in Korean industry have continuously increased as shown in Fig. I. From this figure, it could be found that there is a big jump in consumption rate of high strength steel from 36% in 1986 to 50-55% in 1989. Recent increase in consumption rate of high strength steel is largely dependent on a change in the demand of steel user and a quality renovation of high strength steel through TMCP technology. In ship building industries, as a measure of energy saving, the use of high strength steel is increased to reduce the total weight of ship.