1. Precipitated Cementite Effect In The Formation Of Ultrafine High Angle Boundaries
In Low Carbon Steel
Silva Neto, O. V. (1)* and Balancin, O.(2)
(1)* (2) Department of Materials Engineering, Federal University of São Carlos - DEMa/UFSCar, Via
Washington Luiz, Km 235, 13.565-905, São Carlos, SP, Brazil, e-mail: (1)* pvillar@iris.ufscar.br
and (2) balancin@power.ufscar.br
Abstract – The microstructural refinement is an attractive approach to improve the strength of low carbon steel, mainly when
is obtained ultrafine grains about 1 µm. A low carbon steel with cementite dispersion was submitted to deformation in ferritic domain by
warm torsion tests. Carbide dispersion was introduced through thermal treatments.
The ultrarefinement of ferritic grain attributes considerable improvement in the mechanical properties of the
common structural steels. The increase in the mechanical resistance and toughness are the main advantages
obtained from grain refinement, referring to the mechanical behavior of these materials, specially when the
average ferritic grains size of common steels can be reduced to close sizes to 1 µm without addition of
microalloying elements [1-3]. Though, the severe deformations applications have been very difficult to be reproduced
in industrial scale. Thus, the ferrite deformation with cementite globular particles appears as route alternative [1]. The
preferential nucleation sites of ferrite increase with defects produced during deformation. The deformation creates
complex obstacles fields and a great amount of dislocations activity happens inside of subgrains, increasing the
dislocations density that is the critical energy to reach the initial nucleation [4]. So much the heavy deformation as
high strain rate promotes increase in amount of defects and deformation bands, which contributes for occurrence
of dynamic recrystallization and the ultrafine ferrite formation [5]. The continuous dynamic recrystallization refines
the microstructure and the finely dispersed particles have showed pinning effect, anchoring the grain boundaries
[2]. In this work, the ultrafine grains formation with high angle boundaries in a low carbon steel (LC) was studied.
Samples with cementite particles finely dispersed in the ferritic matrix of a common steel 0,16C 1,34Mn was
submitted to warm deformations process. Carbide dispersion was introduced through quench and tempering
thermal treatments. An ultra-low carbon steel (IF) was tested in the same conditions, which allowed verify the
influence of cementite precipitated particles during the processing. To characterize the microstructural evolution
during the deformation, tests were performed with interruption of the deformation in certain pre-determined levels.
The deformations were applied by hot torsion tests at 685 oC and at equivalent strain rate of 0.1 s-1. Also, in other
tests, after each pre-certain deformation amount – 0, 1, 2, 3, 4 and 5 - the samples were water cooled, allowing
the microstructural evolution study. The flow stress curves of steel with spheroidzed carbides showed that the flow
stress rises rapidly as a hump at the beginning of the straining, followed by extensive flow-softening region and,
after a large straining steady-state levels were attained. Differently, in IF steel the steady-state happens starting
from the maximum stress. The use of the EBSD technique, accomplished in a Philips XL30 FEG (30KV) model,
enabled the attainment of data related to the misorientation amongst grains and/or sub-grains. The final
microstructures were shown recrystallized and composed by ultrafine grains, with close average size to 1 µm. It
was evidenced that the cementite precipitation and the ferrite dynamic recristallyzation are responsible for the
formation of high angle boundaries, as well as by intense grain refinement during the subcritic deformation.
240
200
160
Stress [MPa]
120
80
-1
0 .1 s
40 LC
IF
0
0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0 3 .5 4 .0 4 .5 5 .0
S tra in g
o
Figure 1: Flow stress curves of materials deformed at 685 C. Figure 2: Cementite carbides precipitated in ultrafine ferrite grains.
References
[1] M. Niikura et al. Journal of Materials Processing Technology 117 (2001) 341-346.
[2] O. V. Silva Neto and O. Balancin, in Proceedings CONAMET/SAM Congress, La Serena, Chile, November 2004, 237-242.
[3] K. Nagai, Journal of Materials Processing Technology 117 (2001) 329-332.
[4] D. Chu and J. W. Moris Jr., Acta Materialia 44 (1996) 2599-2610.
[5] Y. D. Huang et al., Journal of Materials Processing Technology 134 (2003) 19-25.