Journal of University of Science and Technology Beijing
Volume 14, Number 2, April 2007, Page 130
Corresponding author: Wangyue Yang, E-mail: wyyang@mater.ustb.edu.cn Also available online at www.sciencedirect.com
Materials
Modeling of microstructural evolution during dynamic recrystallization
in coarse Nb microalloyed austenite
Ling Zhang
1)
, Wangyue Yang
1)
, and Zuqing Sun
2)
1) Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, China
2) The State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
(Received 2006-04-17)
Abstract: The aim of the current study was to investigate the microstructural evolution during dynamic recrystallization in coarse Nb
microalloyed austenite in thin slab direct rolling (TSDR) processing. A model was developed to predict the change of the austenite
grain size during the dynamic recrystallization, by using the law of mixtures. The equations initially developed for partial static re-
crystallization were used for partial dynamic recrystallization, by adjusting the value of the constant. The results show that the change
of the austenite grain size can be reasonably described by using the equations developed according to the law of mixtures.
Key words: austenite; grain size; TSDR; partial recrystallization; dynamic recrystallization
[This work was financially supported by the Major State Basic Research Development of China (“973” Program, No.2004CB619102)
and CITIC Metal Co. Ltd.]
1. Introduction
In recent time, “thin slab direct rolling”, “thin slab
casting”, “hot direct rolling”, and “hot charge rolling”
(TSDR or TSCR) have become more and more com-
mon in steel processing for the production of hot rolled
flat or long products. The integration of casting with
rolling technology can not only save substantial energy,
but also reduce environmentally undesirable emission
[1-3].
However, this technology shows significant dif-
ferences with the conventional rolling processes. To
begin with, the metallurgical differences are mainly
related to the coarse grain size (~1000 m) and the high
supersaturation level of microalloying elements in the
as-cast austenite in comparison to the as-reheated ma-
terial. Next, there are differences in the processing
parameters, for example, a reduced number of passes
are applied during thin slab direct rolling, but the strain
in the initial stands are higher [4-7], so the conditions
for the onset of dynamic recrystallization (DRX [8])
may be achieved. And dynamic recrystallization has
been proposed as one of the procedures that can be used
to refine the as-cast microstructure during thin slab
direct rolling [8].
The dynamic events are the microstructural changes
occurring during the deformation. Since 1965, it has
been confirmed that dynamic recrystallization has been
occurring during deformation, and much attention has
been paid to the critical strain
c
(the onset of dynamic
recrystallization during hot deformation occurs when
c
is reached), recrystallized fraction (X
drx
), and re-
crystallized austenite grain size (D
drx
). There are few
reports about the austenite microstructure where the
coarse initial austenite undergoes partial recrystalliza-
tion that could have occurred during the first stand in
TSDR processing. In TSDR processing, the initial
austenite grain size is so coarse when compared with
that of the conventional thermomechanical processing
(TMCP) that the difference between unrecrystallized
grain size and recrystallized grain size is more signif-
icant than that in TMCP. Hence, it is very important to
figure out the austenite microstructure during dynamic
recrystallization.
In this article, the microstructural evolution during
dynamic recrystallization in the first stand in TSDR
technology is investigated. Also, a model is applied, to
predict the austenite mean grain size evolution during
deformation.
2. Experimental procedure and materials
Two low carbon steels bearing with Nb were cast as
40 kg in a vacuum induction furnace and the chemical
compositions are shown in Table 1. The specimens had
a gauge length of 20 mm and a diameter of 8 mm ma-