Recrystallization of deep drawing columbium (Nb)-treated interstitial-free sheet steels
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AND
H. NYO
7he softening response after isochronal annealing of cold-rolled, Cb-treated, vacuum decarburized-deoxidized, interstitial-free steels was investigated by hardness m e a s u r e ments, tension tests, and optical and transmission electron microscopy. Softening after annealing, following cold reduction, occurs by r e c o v e r y - r e c r y s t a l l i z a t i o n and a reduction in precipitation hardening due to the coarsening of CbC precipitates. The r e c r y s t a l l i z a tion start temperature increases markedly with increasing amounts of Cb in solid solution in ferrite. Fine CbC precipitates r e t a r d recrystallization to a much l e s s e r degree, and their effects can be eliminated by coarsening prior to cold reduction. Recrystallization initiates at free surfaces. Partially recrystallized structures consist of two layers of recrystallized matrix separated by an unrecrystallized zone. The r e c r y s t a l l i z e d layers thicken with increasing temperature or time. This phenomenon is an inherent c h a r acteristic of these steels and not a result of external influences.
T H E r e c o v e r y and recrystallization of iron and dilute iron-based alloys is influenced by composition, s t r u c ture, and prior thermal history, and the extensive body of literature on thi~ subject has been reviewed by L e s lie e t a l . 1 Abrahamson and Blakeney 2 investigated the effect of binary solid solution additions of transition elements on the recrystallization temperature of iron, assessed from isochronal annealing-hardness data. They noted a correlation between the rate of change of r e c r y s t a t lization temperature with atomic percent solute and the electron configuration of the solute element. Columbium in solid solution was found to strongly r e t a r d recrystallization. Kubota, e t a l 3 investigated the annealing response after cold reduction of carbon steels containing 0.1 wt pct carbon and two levels of columbium, 0.03 and 0.06 wt pct. In these steels all of the columbium was precipitated as columbium carbonitrides. R e c r y s t a l lization was greatly retarded and the effect is ascribed to the precipitates present. The hardening caused by the precipitates was not diminished by r e c r y s t a l l i z a tion, up to 700~ (973 K). Jones and West 4 investigated the r e c o v e r y and r e crystallization of three cold-rolled and annealed dilute iron-columbium alloys. One was a solid-solution alloy containing 0.15 wt pct columbium and 0.01 wt pct c a r bon. The other two were a F e + Fe2Cb alloys containing 0.5 and 1.8 wt pct columbium and 0.01 and 0.006 wt pct carbon, respectively. They state that based on their results that "it may be inferred that niobium (columbium) in solid solution in ferrite does not greatly affect r e c o v e r y or recrystallization in the presence of a very low carbon content." This statement is completely contrary to the results of Abrahamson and Blakeney. 2 The present study concerns itself with the isochroR. E. H O O K and H. N Y O are R e s e a r c h Associate a
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