PURPOSE: To raise tenacity of steel at low temperatures by forming a constitution in which beinaito or tempered martensite coexists with ferrite base by subjecting Ni- containing low temperature steel to a specific heat treatment.

CONSTITUTION: A low temperature steel containing 0.2% C and 2 to 9% Ni is heated to the temperature of A1-A3 transformation point or to a temperature above the A3 transformation point and then cooled gradually down to the A1 transformtion point. The constitution thus obtained is made up of a two-phase coexisting constitution of ferrite and austenite. The steel is then cooled at a cooling rate of 350°C/hr at lowest. The ferrite is excellent in low temperature tenacity as well as stable at low temperatures, and the austenite has a good quenchability because C is concentrated and also is changed into martensite or beinaito. This is stabilized by heating it to a temperature from (A1 transformation point -150°C) to the A1 transformation point temperature followed by cooling. Thus, the ferrite encloses beinaito or tempered martensite, eliminating its brittleness but manifesting higher tenacity.