Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

Dynamic recrystallization microstructures under hot working conditions

Dynamic transformation of deformed austenite at temperatures above the Ae3

The recrystallization behaviour of a Nb microalloyed steel, under deformation conditions corresponding to the initial stands of a thin slab hot rolling process which is characterized by coarse initial austenite grain sizes and large strains per pass, has been investigated. A range of strain levels were studied ranging from below the critical strain for the onset of dynamic recrystallization, e c , well into the steady state for dynamic recrystallization, e > e ss . There is a transition strain, e c e T e ss , separating the post-dynamic softening strain-dependent range from that which is strain-independent. At strains above e T , the kinetics of recrystallization is independent of strain but strongly dependent on strain rate. Under these conditions, metadynamic recrystallization operates as the prime softening mechanism and the kinetics have been derived. At strains between e c and e T , both metadynamic and classical static recrystallization are involved in post-dynamic softening. A model is derived to predict the evolution of softening as a function of the holding time for different strain ranges, based on the contributions of the individual components to the overall softening.

Transition between static and metadynamic recrystallization kinetics in coarse Nb microalloyed austenite

Rolling processes are categorized by the lengths of their interpass times: those that involve interpass intervals significantly longer than 1 s ( e.g. reversing mills) are particularly suitable for conventional controlled rolling or recrystallization-controlled rolling; by contrast, those involving interpass times of 15–100 ms or more ( e.g. the finishing trains of tandem mills) are shown to be suitable for dynamic or metadynamic recrystallization-controlled rolling. This distinction relies on the minimum time required to permit strain-induced precipitation, the occurrence of which inhibits both static and dynamic recrystallization. The effect of metadynamic recrystallization can play an important strain is described. Examples are given of industrial processes in which dynamic recrystallization can play an important role. These include the stretch-reducing mill for the finishing of seamless tubes, the finishing stages of rod rolling, and the finishing of microalloyed steels in hot-strip mills. The practical consequences of dynamic recrystallization on the microstructures produced by these operations and on the modelling of rolling load are also described.

Dynamic recrystallization—scientific curiosity or industrial tool?

A 0.06 pct C-0.3 pct Mn and a 0.07 pct C-0.6 pct Mn-0.028 pct Nb steel were deformed in torsion at a constant strain rate of 2/s. Two schedules were used. In schedule A, seven roughing passes executed between 1260°C and 1130°C were followed by a single large finishing pass with a strain of 3.5 at constant temperatures between 1010°C and 840°C. The time between roughing and finishing was 200 seconds. In schedule B, the seven roughing passes were followed by 10 finishing passes, again applied isothermally, with strains of 0.3 and interpass times of 0.6, 2, and 10 seconds. The results indicate that for the Nb steel, low rolling temperatures (870°C) and strains above 2 are required for complete dynamic recrystallization, which results in austenite grain sizes under 6μm. Cooled at a rate of 10°C/s, the dynamically recrystallized austenite grain structures transform into ferrite with grain sizes under 4 μ. Extrapolations from the present data suggest that at industrial strain rates and cooling rates, ferrite grain sizes under 2 μm should be achieved.

J. W. Bowden

Papers

Effect of interpass time on austenite grain refinement by means of dynamic recrystallization of austenite