鋼の焼入性改善に寄与する最適ボロン量

抄録

The effect of N, Ti and B on the hardenability of low and middle carbon steels has been studied, in order to make clear the optimum condition for obtaining the maximum hardenability of boron(B)-steels. The main results are as follows:
(1) The hardenability of B-containing steel is expressed by the following equation:
In the case of [B] ≤5 ppm, logV_C-90=logV_C-90°-0.14 [B], and when [B] ≥5ppm, V_C-90=0.2V_C-90°.where V_C-90 is the hardenability defined as critical cooling rate (°C/s) at which 90% of martensite is formed during cooling in B-containing steel, and V_C-90°is the hardenability as shown above in the base steel (B-free steel), and [B] is soluble B content (ppm). This means that the effect of B becomes maximum and constant when soluble B content exceeds 5 ppm.
(2) The B-precipitate observed by α-autoradiography can be considered as BN because that the resolve temperature of the B-precipitate coincidents well with that of BN. Therefore, the amount of BN and/or resolved B can be calculated by widely used solubility product of B-N.
(3) Boron-factor (f_B), defined by GROSSMANN, is a function of the hardenability of the base steel (B-freesteel) as well as [B] as shown below.
[B] ≤5ppm, f_B=1-F/b·D_I°
where, F is a function of [B], and b is constant, D_I° is the hardenability expressed by ideal critical diameter of the base steel.