Hot Deformation and Recrystallization of 3% Silicon: Steel Part 2: Effect of Microstructural Variables on Static Recrystallization

Abstract

The recrystallization kinetics and the recrystallized grain size of 3 % Si steel after hot rolling at temperatures of 900 to 1 100°C and annealing at the rolling temperature have been investigated for a range of initial grain sizes from 140 to 850 mm and initial austenite contents from 0 to 16 %. It is shown that, in single phase ferrite, nucleation of recrystallization takes place only at grain boundaries, whereas some intragranular nucleation also occurs when austenite is present during rolling. Finer initial grain size and the presence of austenite therefore lead to faster recrystallization and to finer recrystallized grain size.Recovery during annealing causes a rapid decrease in stored energy, and therefore a decrease in the growth rate of recrystallizing regions with annealing time. This results in plateaux in the fraction recrystallized, which decrease in level with increase in initial grain size and with decrease in initial austenite content. Recrystallization kinetics approximate closely to site saturated nucleation and to negligible coarsening of recrystallized grains, leading to simple relationships between the time for 0.3 fraction of recrystallization and the fully recrystallized grain size.KEY WORDS: silicon steel; hot rolling; static recrystallization; quantitative metallography; grain size; austenite content.ing after hot rolling.Other slabs of Mat(A) were rolled and annealed to produce different grain sizes of 140, 235 (Mat(A1)), and 475 mm with 16 % austenite present, and of 180 (Mat(A3)), 340 and 580 mm with 0 % austenite present, for further rolling and annealing to investigate the influence of grain size on recrystallization behaviour. It is noteworthy that while there is any austenite present at a particular position in the slab used for metallographic investigation (1/4 thickness and the centre), the carbon content in solution is expected to remain at about 0.02 % C, whereas once single phase ferrite is produced, further annealing reduces the carbon in solution to very low levels. For comparison of results with decarburized Mat(A), a second steel Mat(B) of composition 3.31% Si, 0.016% C was produced. This had a grain size of 233Ϯ10 mm and remained single phase ferrite at all reheating temperatures. 1)Experimental rolling was carried out on a 50 t Hille rolling mill. For most of the investigation in this paper, a total equivalent strain of 0.82 was applied in two sequential passes of 30 % reduction. A chromel-alumel thermocouple embedded to the centre of the slab was used to record temperature during rolling, which included 10 s from withdrawal of the slab from the furnace to the first pass and about 15 s between passes. Rolling temperatures quoted are slab centre temperatures at entry to the first pass. As discussed elsewhere, 9) there was no detectable recrystallization in the coarse grained materials rolled by a single pass of 30 % reduction on annealing for at least an order of magnitude longer time that the 15 s interpass time, even at 1 100°C. For the smallest grain sizes, recrystalli...

Hot Deformation and Recrystallization of 3 % Silicon Steel: Part 3: Effect of Hot Deformation Variables on Static Recrystallization

“…2 for material of coarse initial grain size, 850 mm. It can be seen, as discussed elsewhere, 2) that the curves in Fig. 1 are of typical sigmoidal form, whereas those in Fig.…”

“…For rolling to a strain of 0.82 at 1 100°C, the dislocation density inside subgrains. estimated as described previously 2) from the flow stress, enables the driving pressure, P s0 , at 1 100°C to be estimated as 1.33ϫ10 5 Pa. Using this value, the intercept of the log t 0.3 vs. d* rex plots reported previously 2) of Ϫ1.45, the apparent activation energy for recrystallization of 230 kJ/mol, Fig.…”

“…(3) (11) where Q r is the effective activation energy for recovery, which takes into account the activation energy for self diffusion, Q s , and any temperature dependence of the jog density, c j . From the earlier discussion, (Q r ϪQ m )ϭ10.5 kJ/mol, and the value of C 6 can then be found from the slopes of the plots of log t 0.3 vs. d* rex reported previously for 1 100°C, 2) with the resulting values shown in Table 3. For rolling to a strain of 0.82 at 1 100°C, the dislocation density inside subgrains.…”

“…(A1), (A2) and (A3) in Ref. 2) that a direct effect should arise from the increase in grain boundary edge length (L v ) and in grain boundary area (S v ) with increasing strain. The effect of this increase on recrystallized grain size is illustrated for S v by the line for (S v /S v0 ) Ϫ1/3 at the top of Fig.…”

“…1,2) The present paper reports the results of quantitative metallographic measurements of the effects of the processing variables of strain, strain rate and temperature of deformation, and temperature of annealing on the kinetics of static recrystallization and on the recrystallized grain size. A physically based model developed for the effects of the initial microstructural variables 2) is applied to interpret the experimental observations.…”