超快冷对碳素钢中渗碳体析出强化行为的影响*

用超快速冷却技术并控制轧后冷却温度, 研究了3种碳含量不同的碳素钢热轧后组织中渗碳体的析出行为和强化机制。结果表明, 在超快速冷却条件下0.04%C和0.5%C(质量分数, 下同)实验钢的主要强化方式分别是细化晶粒和细化珠光体片层间距, 没有纳米级渗碳体颗粒析出, 而在0.17%C实验钢的组织中则有大量弥散的纳米级渗碳体析出, 颗粒直径范围为10-100 nm, 通过超快速冷却技术实现了在不添加微合金元素的条件下纳米级渗碳体的析出。随着超快速冷却终冷温度的降低纳米渗碳体的析出强化作用使0.17%C钢的屈服强度提高110 MPa, 强化效果明显。在超快速冷却的工艺基础上若继续采用形变热处理工艺, 可进一步提高0.17%C实验钢的位错密度, 促进渗碳体均匀形核, 实现纳米级渗碳体颗粒在整个组织中更加均匀弥散的分布, 达到更好的均匀强化效果。在超快速冷却和形变热处理工艺条件下0.17%C钢的屈服强度可达到650 MPa以上, 强化效果提高300 MPa以上。

Effect of Ultra Fast Cooling on Precipitation Behavior of Cementite in Carbon Steels and Its Strengthening Effect

The effect of ultra fast cooling (UFC) during hot strip rolling on the precipitation behavior of cementite in carbon steels and its subsequent strengthening effect have been investigated by controlling the cooling temperatures for three carbon steels with 0.04%,0.17% and 0.5%C respectively. The results show that the refinement of ferrite grains and the reduction of pearlite lamellar spacing might mainly be responsible for the strengthening of the two steels containing 0.04%C and 0.5%C respectively, while no nano-scaled cementite precipitation formed. On the other hand, a large number of nano-scaled cementite precipitates with the size of 10~100 nm formed in the steel with 0.17%C. Therefore, the precipitation of the nanoscaled cementite precipitates could be realized by the UFC process for the plain carbon steel with 0.17%C but with no request for the addition of microalloying elements. Due to the precipitation strengthening of the nanoscaled cementite, the yield strength of the experimental steels with 0.17%C increased with the lowering the finish temperature of the UFC process gradually and typically reached an increment higher than 110MPa. A further thermo mechanical treatment (TMT) after UFC can increase evidently the dislocation density for cementite nucleation, and it will be a feasible way to realize the uniform precipitation of nano-scaled cementite entirely in the microstructure of the steel, thereby further enhancing the strengthening effect. After hot rolling with the UFC and TMT process, the yield strength of the 0.17%C steel may reach a level greater than 650 MPa, in other words, a net increment larger than 300 MPa may be ascribed to the precipitation strengthening effect of nano-scaled cementite.