フェライト鋼のひずみ時効特性に及ぼすボロン添加の効果

抄録

Effect of B addition on strain aging properties of ferritic steels was examined by Fe-B-C alloys by strain aging tests together with computer simulations. By the addition of B, the strain aging curve of the Fe-C alloy shifted to the longer aging time and the increase in the yield stress, Δσ, decreased at any aging time under the present conditions. The activation energy for the strain aging of the Fe-C alloy was similar to those previously reported and the activation energy for bulk diffusion of C in α-Fe. By the addition of B, the activation energy became larger than that of the Fe-C alloy. It suggests that B addition makes C diffusion slower. From the analysis by Yoshinaga's model, it is found that the maximum concentration of C directly below the extra half planes decreases and the formation of the C atmosphere is delayed with decreasing C concentration. These tendencies are the same as those obtained in the strain aging tests. Thus, it is concluded that the amount of interstitial solute atoms C and N is decreased by B addition. Thermodynamic calculations with Thermo-Calc showed that the C solubility was not affected by the B addition, and that the N solubility was decreased due to the precipitation of BN. From these results, it is concluded that the effect of B addition on the strain aging characteristics in ferritic steels is explained by the reduction of C diffusion rate and the reduction of the total amount of solute elements, C and N.