1. FORMATION AND EVOLUTION OF ULTRAFINE FERRITE IN LOW CARBON STEEL
BY THERMOMECHANICAL TREATMENT IN FERRITE DOMAIN
Silva Neto, O. V. 1,*; Balancin, O.1
1
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: pvillar@iris.ufscar.br
Microstructural refinement is an attractive approach to improve the strength of low carbon steel, mainly when ultrafine
ferritic grains which size is about 1 μm can be obtained without addition of microalloying elements. Simulations in
laboratory indicate that this processing route forms ultrafine ferritic grains [1]. Though, the application of severe
deformations ( ε ≈ 5 ) healthy very difficult of they be reproduced in industrial scale. Like this, it appeared as route
alternative the deformation of the ferrite with globular particles of cementite [2]. In the current work, it was investigation the
obtainment of ultrafine grains applying deformations in the ferrite. A low carbon steel with a dispersion of globular
cementite was subjected to heavy deformation in the ferrite domain. Carbide dispersion was introduced through
quenching followed subcritical tempering and strain. Samples were reheated to 900 oC, for 10 minutes kept at this
temperature, and then water quenched. There samples were tempered at 685 oC during 1 hour. Heavy deformation
( ε = 5 ) was applied by hot torsion test at 685 oC and at equivalent strain rates of 1.0, 0.5 and 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 study
of the microstructural evolution in the deformed condition the 0.1 s-1. Regardless the routes to carbides spheroidization,
the flow stress curves display a peculiar shape: the flow stress rises rapidly to a hump at the commencement of the
straining, followed by extensive flow-softening region and, after large straining steady-state levels were attained. The
material’s ductility depends on the strain rate; at higher strain rate the samples failed before the steady state to be
attained. The microstructural evolution was observed by optical and scanning microscopy. The high resolution electron
backscattered diffraction (EBSD) has been used to analyze the boundary misorientations formed during deformation,
determining both high and low angle boundaries. The preferential nucleation sites of ferrite are increasing with defects
produced during deformation. So much the heavy deformation as high deformation rates, increase the amount of defects
and deformation bands, which contribute for occurrence of dynamic recrystallization and the formation of the ultrafine
ferrite [3]. This way, the obtaining of ultrafine ferritic grains can be associated to the process of ferrite dynamic
softening: continuous dynamic recrystallization [4]. As a result, finer and dispersive carbides could be verified on
samples submitted at lower tempering times. The cementite particles should interfere in the process of rotation of the
subgrains and to inhibit the growth of the formed grains. In addition, precipitate size caused little influence on the
ferritic grain refinement. The final microstructure consisted of a ferrite matrix with average grains size smaller than 2
μm. Observation of the microstructural evolution during deformation shown the formation of a set of uniform grains
inside the old ferrite, suggesting a continuous process of “dynamic recrystallization”. The occurrence of ferrite dynamic
recrystallization is due to high strain and strain rate applied during torsion test.
Acknowledgements:
The authors acknowledge the Brazilian Research Funding Agencies CAPES, CNPq and FAPESP for the
financial support.
References:
[1] A. Najafi-Zadeh, J. J. Jonas and S. Yue, Metallurgical Transactions 23A (1992) 2607-2616.
[2] M. Niikura, M. Fujioka, Y. Adachi, A. Matsukura, T. Yokota, Y. Shirota and Y. Hagiwara, Journal of Materials
Processing Technology 117 (2001) 341-346.
[3] Y. D. Huang, W. Y. Yang and Z. Q. Sun, Journal of Materials Processing Technology 134 (2003) 19-25.
[4] O. V. Silva Neto and O. Balancin, in Annals of the Congress CONAMET/SAM (2004) 237-242.

2. 300
-1
275 Strain Rate [s ]
0.1
250
0.5
225 1.0
200
Stress [MPa]
175
150
125
100
75
50
25
0
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0
Strain
(a) (b)
Figure 1 – a) Schematic representation of the rotes accomplished: until the fracture and interrupted isothermals tests; b)
True stress-strain curve of the investigated steel: tempering samples for 1 hour to 685 oC.
(a) (b) (c)
. . .
Figure 2 – Ultrafine ferrite grains of the strained specimens: a) ε = 1.0 s −1 ; b). ε = 0.5 s −1 ; c) ε = 0.1 s −1 .
(a) (b)
. .
Figure 3 – Cementites precipitated intragranular and on grain boundaries - a) ε = 1.0 s −1 ; b) ε = 0.1 s −1 .
(a) (b) (c)
Figure 4 – a) Inverse polo figure map; b) texture map; c) colors code map - according to the preferential direction [001].