0.02%Nb空冷仿晶界型铁素体/粒状贝氏体复相钢的相变及强韧性

PHASE TRANSFORMATION AND STRENGTH-TOUGHNESS OF A F_(GBA)/B_G DIPHASE STEEL CONTAINING 0.02%Nb

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对仿晶界型铁素体(F_(GBA))/粒状贝氏体(B_G)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,细化了仿晶界铁素体,促进了粒状贝氏体转变,细化了粒状贝氏体及其内部的铁素体片条及马氏体-奥氏体(M-A)岛.与不含Nb的F_(GBA)/B_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 （FGBA）/granular bainite（BG） steels,alloying with Si-Mn-Cr,have been developed.The effect of 0.02%Nb on the phase transformation and strength-toughness of a FGBA/BG 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 FGBA is 25μm in length and 10μm in width.The average size of intragranular ferrite is about 6-8μm.For FGBA/BG steel with 0.02%Nb,the average size of FGBA is 10μm in length and 4μm in width.The average size of intragranular ferrite is less than 4μm. 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 FGBA/BG steel with 0.02%Nb.The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the FGBA/BG steel,suppresses the transformation ofγ→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 FGBA/BG steel with 0.02%Nb.Compared with typical Nb free FGBA/BG steel after hot rolling and air cooling,it is found that the tensile strength and yield strength of the FGBA/BG 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.