連鋳二次冷却中の微細炭化物の析出と鋼の高温脆化

抄録

In order to reduce frequency of surface cracks in continuously cast low carbon steel slabs containing niobium, relation between morphology of carbonitride precipitates and high temperature ductility has been examined under the condition simulating thermal cycle of steel slabs passing through secondary cooling and bending zones of a vertical bending type continuous caster. Through experiments, it is found that precipitation of niobium carbonitrides is mainly affected by thermal cycle in the temperature range from 973 to 1073K, and fine precipitates at γ grain boundary make high temperature ductility worse. Number of fine precipitates containing niobium increases when a specimen stays in the temperature range longer. It is found that grain boundary embrittlement is becoming dominant in case when ductility becomes less, which means deformation takes place mainly at γ grain boundary because grain itself is strengthened by fine precipitates.
For improvement of high temperature ductility, it is recommended to modify fine niobium carbonitrides to coarser ones containing titanium and niobium by microalloying of titanium.