Effect of Boron on Recrystallization Behavior in Ultra-low Carbon Martensitic Steels

The microstructural change of lath martensite during tempering was investigated in ultra-low carbon 9%Cr-1%Ni-(080) ppm B martensitic steels, and then the effect of boron (B) content on the recrystallization behavior was discussed in terms of the relation between driving force and restraining force for grain boundary bulging. The addition of small amount of B is very effective for the increase in dislocation density of martensite, but excess addition more than 10 ppm is not so effective. Besides, the B addition works to reduce the spacing of carbide particles precipitated on grain boundaries with the increase in volume of the particles. The condition for the occurrence of the recrystallization is decided by the energy balance between the driving force (dislocation density) and the restraining force (grain boundary pinning). The recrystallization of martensite is promoted in the low B steel (040 ppm B) because the driving force is enlarged owing to the increase in the dislocation density. On the contrary, in the high B steel (4080 ppm B), the restraining force becomes higher than the driving force because of the reduction of carbide spacing on grain boundaries, and this leads to a retardation of recrystallization.