This document investigates the effect of cementite particles on microstructure refinement and formation of high angle boundaries in low carbon steel. Warm deformation tests were conducted on a 0.16C steel and an ultra-low carbon steel up to a strain of 5 at 685°C and 0.1 s-1 strain rate. Electron backscatter diffraction analysis showed that cementite precipitation and ferrite recrystallization led to the formation of high angle boundaries and grain refinement during subcritical deformation. Flow stress was higher for the 0.16C steel due to cementite particles anchoring grain boundaries.

1. EFFECT OF CEMENTITE PARTICLES IN FORMATION OF HIGH ANGLE
BOUNDARIES AND OF ULTRAFINE GRAINS IN LOW CARBON STEEL. Otavio Villar
da Silva Neto(1) and Oscar Balancin(1). (1)Department of Materials Engineering, Federal
University of São Carlos (DEMa/UFSCar),Via Washington Luiz, Km 235, 13.565-905, São
Carlos, São Paulo State, Brazil. Email: pvillar@iris.ufscar.br
Nowadays, great efforts have been destining to obtain steels with ultrafine grains through viable
industrially routes. These efforts are justified by costs reduction with the alloy elements and the
improvement properties from plain carbon steels, which increase the aggregated value and its
commercial range application. However, to obtain ultrafine grains steels with stable
microstructure represents a hard task, owing a strong tendency for grains growth [1]. For this
reason, some fine grains microstructures are inherently unstable which turns necessary to promote
mechanisms that restrict grain boundaries movement to stabilize these microstructures [2,3,4].
The cementite particles precipitation during the thermomechanical processing can produce a
stable and homogeneous microstructure [5,6,7,8,9,10]. In this work, the influence of cementite
precipitation in microstructure refinement of a low carbon steel, as well, the high angle
boundaries generation during the warm processing were investigated. During the accomplishment
of this work, two steels were used; a 0,16C steel (Cosar) and another of ultra-low carbon (IF), as
reference. The subcritic field deformation in quenched and tempered samples, was previously
imposed by torsion test. The use of the EBSD (Electron Backscattering Diffraction)
technique, accomplished in a Philips microscope of high resolution, XL30 FEG (30KV)
model, enabled the attainment of data related to the misorientation amongst grains and/or
sub-grains after isothermals torsion test. Heavy deformations ( ε = 5 ) were applied through
warm torsion tests at 685ºC at equivalent strain rate of 0.1 s-1. They also carried through
tests with interruptions after pre-defined deformation amounts – 0, 1, 2, 3, 4 and 5 – in
which the samples were water cooled, allowing the study of the microstructural evolution
on the condition of 0.1 s-1 deformation. The 0,16C steel deformation results were compared
with the IF steel, which allowed verify the influence of the cementite particles precipitate during
the processing. 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.
References
[1] M. A. F. Oliveira, A.M. Jorge Jr. and O. Balancin, Scripta materialia (2004) 50, 1157-1162.
[2] P. A. Manohar; T. Chandra and C. R. Kilmore, ISIJ int. (1996) 36, 1486-1493.
[3] X. Liu; J. K. Solberg and R. Gjengendal, Mater. sci. technol. (1996) 12, 345-350.
[4] M. Niikura et al., Journal of materials processing technology (2001) 117, 341-346.
[5] D. H. Shin; Y. –S. Kim and J. Lavernia, Acta Mater. (2001) vol. 48, 2387-2393.
[6] D. H. Shin; K.-T. Park and Y.-S. Kim, Metall. and Mat. Transactions (2001) 32A, 2373-2381.
[7] X. J. Hao et al., Materials Science and Technology (2001) vol. 17, 1347-1352.
[8] Y. D. Huang et al., Journal of Materials Processing Technology (2003) 134, 19-25.
[9] O. V. Silva Neto and O. Balancin, in Proceedings CONAMET/SAM Cong. (2004) 237-242.
[10] O. V. Silva Neto and O. Balancin, in 8th Inter American Congress of Electron Microscopy,
La Habana, Cuba (2005).

2. (a) (b) (c)
Figure 1 – Ultrafine ferrite grains in strained specimens (0,16C steel): a) ε = 0,0 , b) ε = 3,0 e c) ε = 5,0 .
100
90
80
High Angle Boundary [%]
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6
Strain
(a) (b) (c)
Figure 2 – a) Inverse polo figure- ε = 4 ; b) colors code [001]; c) strain x % disorientation angle - 0,16C.
24 0
20 0
16 0
Stress [MPa]
12 0
80
-1
0,1s
C osar
40 IF
0
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0
D eform ação
(a) (b)
Figure 3 – (a) Flow stress curves; (b) Fe3C precipitated anchoring grains boundaries (Cosar steel).
100
90
80
High Angle Boundary [%]
70
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16
Strain
(a) (b) (c)
Figure 4 –a) Inverse polo figure - ε = 0 ; b) colors code [001]; c) strain x % disorientation angle – IF.