ボロン鋼の焼入性におよぼすオーステナイト化温度および冷却速度の影響

抄録

As a direct quenching process has become into wide use, it becomes very important to investigate the effect of austenitizing conditions on the hardenability of a steel. In this study, in order to apply this process successfully to B-added steels, the effect of heating rate, austenitizing temperature and cooling rate in γ-phase on the hardenability of B-added steels has been carried out.
The results obtained are as follows :
1)The effect of B is an only function of free-B content [B] unless BN precipitates in γ-phase during cooling from a austenitizing temperature. In case of [B] ≤5 ppm, the effect of B increases with [B]. When [B] is more than 5 ppm, it becomes constant.
2)The hardenability of B-added steels does not depend on the γ-grain size as well as the heating rate.
3)When BN precipitates prior to γ→α transformation, the effect of B disappeares. The critical cooling velocity of precipitation of BN, V_BN (°C/S) is shown as follows :
log V_BN=0.0042[B][N]-0.25
where, the unit of [B] and [N] is "ppm".
4)Therefore, the necessary conditions for B being effective are as follows :
(1) When a B-added steel is austenitized, [B]must not be less than 5 ppm.
(2) V_γ>V_BN, where V_γ is the cooling rate of the steel in γ-phase after austenitizing.