| 摘要 |
PROBLEM TO BE SOLVED: To provide an easy and inexpensive production method for a steel material for low temperature for raw material such as an LNG storage tank, having low Ni content of 7 mass% or less and low temperature toughness similar to that of 9% Ni steel.SOLUTION: The production method for low temperature steel is provided that has a composition containing, by mass%, C:0.02 to 0.10%, Si:0.10 to 0.50%, Mn:3.0 to 5.0%, P:0.010% or less, S:0.003% or less, Ni:4.5 to 7.0%, Al:0.005 to 0.10%, N:0.0015 to 0.0040% and the balance Fe with inevitable impurities and that has structure in which: a main phase is composed of a tempered martensite phase and a tempered bainite phase; retained austenite having an average circle equivalent diameter of 1.0 μm or less is contained in a an amount of 3.0% or more by volume rate; an aggregate structure with integration degree of 1.2 or more in a [211] face parallel to a sheet surface in a 50% or more of areas in a sheet thickness direction with a center of a sheet thickness center position is present and average particle diameter of areas surrounding large angle grain boundary with direction difference of neighboring crystal particles of 15° or more is 5.0 μm or less by circle equivalent diameter.SELECTED DRAWING: None |
