Computational Metallurgy Laboratory Graduate Institute of Ferrous Technology (GIFT)

Cold Deformation of Martensite in Steels

H. F. Lan, W. J. Liu and X. H. Liu

Abstract

Ultrafine grains, as small as a few hundred nanometers were obtained, without severe plastic deformation, by tempering cold-rolled martensite in a low carbon and microalloyed steel. A multilevel subdivision mechanism responsible for the formation of ultrafine ferrite grains in cold-rolled martensite was discussed. It involves subdividing firstly a prior austenite grain into several martensite packets by phase transformation and then further subdividing the martensite structure into ultrafine cell blocks by plastic deformation. The relatively large misorientation between the ultrafine cell blocks achieved at a moderate strain level in martensite may be attributed to the interaction between the transformation induced and the deformation induced dislocations. Ultrafine ferrite grains were developed from the cell blocks during tempering at temperatures in the ragne 500-600^oC for 60 minutes. It was also demonstrated that the microalloying precipitates can effectively pin down the movement of dislocations and grain boundaries and thus, can increase the thermal stability of the fine grained microstructure.

Download paper, Ultrafine ferrite grains produced by tempering cold-rolled martensite in low carbon and microalloyed steels, H. F. Lan, W. J. Liu and X. H. Liu

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Additional Information

Reversing cold-mill, with electro-hydraulic screwdown system and hydraulic bending roll system, which is developed for processing research during cold rollingin the State key Laboratory of Rolling and automation, Northeastern University, China	Reversing cold-mill, with electro-hydraulic screwdown system and hydraulic bending roll system, which is developed for processing research during cold rollingin the State key Laboratory of Rolling and automation, Northeastern University, China	Reversing cold-mill, with electro-hydraulic screwdown system and hydraulic bending roll system, which is developed for processing research during cold rollingin the State key Laboratory of Rolling and automation, Northeastern University, China
Reversing hot flat-mill with accelerated cooling and high rate device cooling system in the State key Laboratory of Rolling and automation, Northeastern University, China	ensite lath structure of as-quenched hot-rolled sheets - Steel 1	Detailed martensite lath structure of as-quenched hot-rolled sheets - Steel 2
Morphology of cold-rolled martensite. (a) along rolling direction, (b) severely bent, (c) perpendicular to the rolling direciton.	Ultrafine grained microstructure of Steel 1	Ultrafine-grained microstructure of Steel 2.
Electron backscattered diffraction image of Steel 1, heat treated at 550 C	Electron backscattered diffraction image of Steel 1, heat treated at 600 C	Electron backscattered diffraction image of Steel 1, heat treated at 650 C
Electron backscattered diffraction image of Steel 2, heat treated at 550 C	Electron backscattered diffraction image of Steel 2, heat treated at 600 C	Electron backscattered diffraction image of Steel 2, heat treated at 550 C.

It is evident from these results that the steel with microalloying elements (Steel 1) has finer grain size than that without microalloying elements (Steel 2). Microalloying precipitates retard recrystallization process by pinning grain boundary movement.