Modeling of Grain Size Distributions during Single Hit Deformation of a Nb-Containing Steel
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MECHANICAL properties of microalloyed steel plates are strongly influenced by the ferrite grain structure. In particular, a fine and uniform ferrite grain structure is desired in the final microstructure as it provides a balance between high strength and toughness. However, it has been found that in some Nb-microalloyed steel plates, a bimodal grain structure consisting of abnormally large grains surrounded by small grains can develop. It has been reported that certain processing conditions and compositions are more prone to the formation of bimodal grain structures.[1] A bimodal ferrite grain size is known to have a detrimental effect on impact toughness in thermomechanical controlled rolled (TMCR) microalloyed steel plates[2] resulting in significant scatter in toughness values. In particular, toughness has been related to the area fraction of the coarse sized grains present in the grain size distribution.[3] However, strength is largely unaffected by any bimodality present in the grain size distribution.[4] The final plate grain structure and any precipitate population are influenced by the rolling schedule—typically this could be hot rolling or TMCR—through the interaction between recrystallization and precipitation effects.[5,6] Hot deformation at a temperature where recrystallization will not be completed for that strain within the hold time results in a partially recrystallized microstructure, which can be inhomogeneous with large unrecrystallized grains and small recrystallized grains resulting in a spread in toughness properties.[7] The upper and lower limits of AMRITA KUNDU, Ph.D. Student, CLAIRE DAVIS, Professor, and MARTIN STRANGWOOD, Senior Lecturer, are with the School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 19, 2009. Article published online February 20, 2010 994—VOLUME 41A, APRIL 2010
the partial recrystallization regime are commonly known as the recrystallization limit temperature and recrystallization stop temperature, respectively. Both of these temperatures depend on several factors[8] such as material chemistry, especially Nb content, which determines the solute drag and precipitation potential, strain imposed, and strain rate. The model most frequently referred to for strain-induced precipitation, austenite recrystallization, and recrystallization-precipitation interaction in Nb-microalloyed steels is that proposed by Dutta and Sellars.[9–11] Dutta and Sellars[9] suggested the following equations to predict the static-recrystallization start time, Rs (i.e., 5 pct recrystallization), and strain-induced Nb(C, N) precipitation start time, Ps (i.e., 5 pct precipitation), after deformation and isothermal holding of Nb-microalloyed steels at different temperatures: 270; 000 Ps ¼ 3 106 ½Nb1 e1 Z0:5 exp RT ! ½1 10 2:5 10 exp T3 ðln KS Þ2
300; 000 Rs ¼ 6:75 10 exp RT 2:75 105 185 ½Nb exp T 20
D20 e4
½2
where [Nb] is the amount of Nb
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