HIGH-STRENGTH STEEL MATERIAL HAVING OUTSTANDING ULTRA-LOW-TEMPERATURE TOUGHNESS AND A PRODUCTION METHOD THEREFOR

The present invention relates to a steel material containing manganese and nickel which is used as a structural material for ultra-low-temperature storage containers for LNG (Liquefied Natural Gas) or the like, and relates to a production method therefor. More specifically, the object of the present invention is to provide: a steel material having high strength and also outstanding ultra-low-temperature toughness, because of the addition of the optimum proportion of inexpensive Mn or the like instead of expensive Ni, refinement of the structure through controlled rolling and cooling, and the precipitation of residual austenite by means of tempering; and a production method for the steel material. In order to achieve this object, the technical essence of the present invention comprises a production method for a steel material comprising: a heating step involving heating, in a temperature range of between 1,000 and 1,250°C, a steel slab comprising, as percentages by weight, from 0.01 to 0.06% of carbon (C), from 2.0 to 8.0% of manganese (Mn), from 0.01 to 6.0% of nickel (Ni), from 0.02 to 0.6% of molybdenum (Mo), from 0.03 to 0.5% of silicon (Si), from 0.003 to 0.05% of aluminium (Al), from 0.0015 to 0.01% of nitrogen (N), no more than 0.02% of phosphorus (P), no more than 0.01% of sulphur (S) and a balance of Fe and impurities; a rolling step in which the slab is rolled at a temperature of no higher than 950°C; a cooling step in which the rolled slab is cooled to a temperature of no higher than 400°C at a cooling temperature of at least 2°C/s; and a tempering step involving tempering for between 0.5 and 4 hours in a temperature region of between 550 and 650°C after the cooling step. La présente invention se rapporte à un matériau en acier qui contient du manganèse et du nickel, qui est utilisé comme matériau structurel pour des récipients de stockage à des températures ultra-basses pour le gaz naturel liquéfié (GNL) ou analogue, et se rapporte à un procédé de production de ce matériau. De façon plus précise, le but de la présente invention est de proposer : un matériau en acier qui présente une résistance élevée et également une excellente ténacité à des températures ultra-basses en raison de l'ajout de la proportion optimale de manganèse, ou analogue, qui est bon marché, à la place de nickel qui est onéreux, de l'affinage de la structure au moyen d'un laminage et d'un refroidissement régulés, et de la précipitation de l'austénite résiduelle au moyen d'un r