PHASE TRANSFORMATION AND STRENGTH–TOUGHNESS OF A FGBA/BG DIPHASE STEEL CONTAINING 0.02%Nb
FENG Chun; FANG Hongsheng; BAI Bingzhe; ZHENG Yankang
Key Laboratory of Advanced Materials of Ministry of Education; Department of Material Science and Engineering; Tsinghua University; Beijing 100084
Cite this article:
FENG Chun FANG Hongsheng BAI Bingzhe ZHENG Yankang. PHASE TRANSFORMATION AND STRENGTH–TOUGHNESS OF A FGBA/BG DIPHASE STEEL CONTAINING 0.02%Nb. Acta Metall Sin, 2010, 46(4): 473-478.
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 25 μm in length and 10 μm in width. The average size of intragranular ferrite is about 6—8 μm. For FGBA/BG steel with 0.02%Nb, the average size of FGBA is 10 μm in lengtand 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 decease 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 of γ→α, refines the size of grain boundary allotriomorphic ferrite, promotes the granular bainitic transformation, lowers the bainitic start temperature (Bs), 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 strengtening phases improve the strength of the FGBA/BG steel with 0.02%Nb. Compared wih ypicaNb ree FGBA/BG steel after hot rolling and air cooling, it is found that the tensile strengtand yielstrength of the FGBA/BG steel wit0.02Nb% rise to 157 MPa and 97 MPa respectively. Moreover, some possible reasons of structure refinement and increase of stregthening phases induced by 0.02%Nb addition have been disscussed in this paper.
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