采用Gleeble-1500D热模拟机进行热模拟实验和轧制实验,研究了添加0.02%Nb对仿晶界型铁索体(FGBA)/粒状贝氏体(BG)复相空冷钢相变及力学性能的影响.结果表明,0.02%Nb使该钢的连续冷却转变曲线右移,淬透性增加;0.02%Nb抑制了γ→α相变,细化了仿晶界铁索体,促进了粒状贝氏体转变,细化了粒状贝氏体及其内部的铁索体片条及马氏体-奥氏体(M-A)岛.与不含Nb的F_GBA/S_G复相钢相比,由于组织细化及强化相体积分数的提高,含0.02%Nb的复相钢经轧后空冷后抗拉强度上升了157 MPa,屈服强度增加了93 MPa,强化效果显著.分析了添加0.02%Nb使复相钢的组织细化及强化相体积分数增加的原因.
In order to produce Mn-series low carbon steels using alloying elements with low cost and following the conventional production line without controlled cooling facilities and not needing special heat-treatment, Mn-series low carbon air cooling grain boundary allotriomorphic ferrite (F_(GBA))/granular bainite (B_G) steels, alloying with Si-Mn-Cr, have been developed. The effect of 0.02%Nb on the phase transformation and strength-toughness of a F_(GBA)/B_G steel have been investigated by using Gleeble-1500D machine and hot rolling test in this paper. Two specimens with and without Nb addition were deformed by 21% at 850 ℃ and then air cooling to room temperature. For Nb free steel, the average size of F_(GBA) is 25 μm in length and 10 μm in width. The average size of intragranular ferrite is about 6-8 μm. For F_(GBA)/B_G steel with 0.02%Nb, the average size of F_(GBA) is 10 μm in length and 4 μm in width. The average size of intragranular ferrite is less than 4 μm.Compared with Nb free steel, the size of bainitic ferrite and martensite/austenite (M-A) island decrease and their corresponding volume fractions increase in the F_(GBA)/B_G steel with 0.02%Nb. The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the F_(GBA)/B_G steel, suppresses the transformation of γ→α, refines the size of grain boundary allotriomorphic ferrite,promotes the granular bainitic transformation, lowers the bainitic start temperature (B_s), and refines the granular bainite including its bainitic ferrite and M-A islands. It is suggested that the effects of structure refinement and increase of strengthening phases improve the strength of the F_(GBA)/B_G steel with 0.02%Nb. Compared with typical Nb free F_(GBA)/B_G steel after hot rolling and air cooling, it is found that the tensile strength and yield strength of the F_(GBA)/B_G steel with 0.02Nb% rise to 157 Mpa and 97 Mpa respectively. Moreover, some possible reasons of structure refinement and increase of strengthening phases induced by 0.02%Nb addition have been disscussed in this paper.
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