This paper introduced the investigation of low temperature ferrite steel plate with 355MPa yield strength (YS355) used for Liquefied Petroleum Gas (LPG) ships. The results showed that good combination of high strength and excellent low temperature toughness was obtained by cooperation of low carbon design, Nb microalloying and TMCP (Thermo-Mechanically Controlled Processing) technology. Mechanical properties was stable over the whole plate with yield strength greater than 355MPa, tensile strength between 490~630MPa, and total elongation of 26.5~29%. The DBTT (ductile-brittle transition temperature) and NDTT (Nil-Ductility Transition temperature) of experimental steel was between −80~-100°C and −80°C, respectively. The impact energy of experimental steel was greater than 250J at −80°C after 5% strain aging at 250°C for one hour. The mechanical properties of developed steel plates meet the requirements of LPG ships quite well.
Liquefied Petroleum Gas was widely used all over the world because of its high efficiency, non-pollution and convenient transportation (Du P, 2000). With high speed economic development in decades, China has become one of the largest LPG consumption countries. Recently, LPG shipping market has been developed very quickly in our country because domestic production cannot meet the increasing demands any more. LPG ships usually transport cargoes with low temperature such as propane, butane, and other gas production from petroleum. The working temperature of LPG ships is generally below −55°C, such that the steel plates are required to possess high strength and excellent low temperature toughness.
In recent years, the domestic investigation on low temperature steel plates just began. The studies were focused on low carbon chemical design and advanced TMCP processing, and obtain gratifying achievement (YE QB, 2015). According to the standard of classification society, F grade steel plates possess great toughness at −60°C and lowest service temperature of −40°C, which cannot meet the demand of LPG ships (ABS, 2008). In this paper, we present one YS355 low carbon ferrite steel processed through TMCP with excellent low temperature toughness at −75°C. The design thoughts, mechanical properties and microstructures are specifically presented in the following sections.
