TWO-PHASE STAINLESS STEEL AND MANUFACTURING METHOD THEREFOR

The present invention has as its object to expand applications to types of steel having corrosion resistances of SUS 329J1 or more and has as its challenge to obtain duplex stainless steel which has excellent corrosion resistance in an environment with a high chloride ion concentration close to brackish water or seawater and having a high economicalness.The inventors discovered that by reducing Mn to less than 2.0% and N to 0.25% or less, then adding a trace amount of Nb, the effect of raising the critical pitting temperature CPT is easily obtained. Further, they heated steel to which a trace amount of Nb was added for solution heat treatment, then examined the effects on the precipitation of Cr nitrides and Nb nitrides and developed duplex stainless steel raised in pitting resistance of the matrix material. That is, by slow cooling down to 800°C, then fast cooling down to 600, it is possible to control the precipitation of chromium nitrides and niobium nitrides and by making the ratio of the amount of Cr in the extraction residue [Cr] and the amount of Nb [Nb] 0.2 or more, it is possible to raise the corrosion resistance.