罩式退火铌微合金化汽车钢的强化机理

 通过比较相同冷轧与罩式退火工艺下Mn-Si系和铌微合金化2种汽车用低合金高强钢的显微组织与力学性能，研究微量铌在冷轧罩式退火低合金高强钢中的强化机理。利用OM、SEM、TEM和拉伸试验机分别对2种钢的显微组织与力学性能进行了表征。对比分析表明：相对热轧板来说，2种钢冷轧退火板的铁素体晶粒和第二相析出物的尺寸都有所长大，导致了强度降低。相对Mn-Si钢而言，铌微合金化钢热轧板和冷轧退火板中的铁素体晶粒和第二相析出物尺寸更细小，细小第二相析出物的数量也更多，在相同的伸长率水平下明显提高了强度。冷轧罩式退火板的强化机理分析表明，铌微合金化低合金高强钢的主要强化方式是细晶强化和NbC的沉淀强化；研究认为添加质量分数为0.025%的铌时细晶强化更强烈。

Strengthening mechanism of batch annealed Nb microalloyed steel for automobile

Two series of cold rolled low-alloy and high-strength automotive steel， i.e. Mn-Si series and Nb microalloyed series， have been investigated to study the microstructure and mechanical properties through industrial trial with the same process， including batch annealing. The strengthening mechanism of the Nb microalloyed steel has been investigated by comparing with Mn-Si series cold rolled batch annealed low-alloy and high-strength steel. The microstructures of the hot-rolled and cold-rolled annealed sheets of those two kinds of steels were characterized by means of OM， SEM and TEM. The mechanical properties were also measured by tensile testing machine. The results were compared and showed that the sizes of the ferrite grains and nano precipitates of the two kinds of cold rolled annealed sheets were larger than those of the hot rolled sheets，which led to the reduction of the strength. Comparing with the Mn-Si steel， the sizes of ferrite grains and nano precipitates of hot-rolled and cold-rolled annealed sheets of Nb microalloyed steel were smaller， and the amount of the nano precipitates was larger. Thus the strength of the Nb microalloyed steel was higher with similar elongation. The strengthening mechanism analysis of cold rolled batch annealed sheet indicates that， the main strengthening method for Nb microalloyed low-alloy and high-strength steel is grain refinement strengthening and NbC precipitation strengthening. In the present study， grain refinement strengthening is stronger under 0.025% Nb microalloying.