Nb添加した炭素鋼の析出温度域と強度の関連について

抄録

The effect of austenitizing temperatures of 900°C to 1250°C and isothermal holding at 900°C after the solution treatment at 1250°C on the mechanical properties of a Nb-bearing carbon steel were studied. Electron microscopic observations by the replica method, optical microscopic observations and analysis of Nb contents in the residues extracted by 1:1 HCl were carried out on some of these specimens. The results obtained are as follows:
(1) The tensile strength after air cooling increases with increasing austenitizing temperature above 1100°C. This probably is the result of strengthening of the ferrite matrix due to the precipitation of fine NbC(N).
(2) The tensile strength of Nb-steel decreases sharply with increasing holding time at 900°C after the solution treatment and reaches nearly the same strength as the strength of C-steel. This variaiton in tensile strength corresponds well to the Nb content in the residues extracted by 1:1 HCl. The localized distribution of NbC(N) suggests that the precipitation occurred at the original austenite grain boundary, twin boundary, stacking fault or dislocation. Retempering of the steel which became nearly the same strength as C-steel shows no secondary hardening between 500°∼700°C and decreases with increasing tempering temperature.
The above results suggest that the Nb dissolved in solution over the austenitic range precipitates during or after transformation as fine NbC(N) precipitates coherent with the ferrite matrix and contribute to the increase in tensile strength.