The document summarizes a study on grain refinement in 0.16C steel through subcritical processing involving heavy deformation. Samples underwent hot torsion deformation at 685°C with strain rates of 1.0, 0.5, and 0.1 s-1. Microstructural analysis found the final microstructure was recrystallized with an average grain size of 1 μm. Three factors were found to contribute to the ultrafine grains: continuous dynamic recrystallization, rotation and breaking of initial boundaries, and cementite particles interfering with subgrain rotation and growth.

1. FORMATION OF HIGH ANGLE BOUNDARIES AND GRAIN REFINING DURING THE
SUBCRITICAL PROCESSING OF 0,16C STEEL
Silva Neto, O.V. 1,* ; Balancin, O.
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: pvillar@iris.ufscar.br
A promising way to enhance, simultaneously, strength and toughness of steel without altering its chemical
composition is promoting the refinement of its microstructure, especially when the average size of common
steels ferritic grain can be reduced to 1 µm [1-4]. Such levels of refinement have been obtained from the
application of large levels of deformation, separately or in a combination sequence of these events [4-6]. The
preferable sites of ferrite nucleation are enhanced, according to the increase of the defects produced during
the deformation. The deformation creates complex fields of obstacles, and then a huge increase in the
activity of the dislocations emerges in the interior of the sub-grains, enhancing the density of dislocations in
order to reach the critical energy for initial nucleation [7-8]. Thus, like the heavy deformation, large strain
rates promote the increase of the defects quantities and of the deformation bands, which contribute to the
occurrence of the dynamic recrystallization and ultrafine ferrite formation [3]. The continuous dynamic
recrystallization refines the microstructure, and the presence of thin dispersed particles possesses the effect
of pinning, anchoring the grains boundaries [4-5]. In this work, we study the grain refinement in low carbon
steel, using samples with thin particles of cementite dispersed in the ferritic matrix submitted to the lukewarm
deformation process. A 0.16C 1.34Mn common steel with dispersion of globular cementite was submitted to
the heavy deformation in the ferritic domain. The initial microstructure was obtained from the formation of
globular cementite through the thermal treatments of quench and tempering. To characterize the
microstructure of deformation, tests with pre-determined interruptions in certain levels of deformation were
carried through. The samples were reheated at 900ºC, they were kept at this temperature for 10 minutes,
and then they were water-quenched. These samples were tempered at 685ºC for 1 hour. Heavy
deformations ( ε = 5 ) were applied through hot torsion tests at 685ºC at equivalent strain rates of 1.0, 0.5 e
-1
0.1 s . 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
-1
condition of 0.1 s deformation. From these specimens, samples for electronic and optical microscopy
analysis were prepared, with which they measured the average sizes of the grains/sub-grains. 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. The final microstructure revealed itself recrystallized and composed by ultrafine
grains, with final average size of 1 µm. Three phenomena are more likely to be responsible for the presence
of the ultrafine grains: the process of ferrite dynamic softening (continuous dynamic recrystallization), the
rotation and break of the initial microstructure boundaries, and the cementite particles, which interfered with
the process of rotation of the sub-grains, and the formation and grains growth.
Acknowledgements:
The authors acknowledge the Brazilian Research Funding Agencies CAPES, CNPq and FAPESP for
the financial support.
References:
[1] K. Nagai, Journal of Materials Processing Technology 117 (2001) 329-332.
[2] D. B. Santos et al., Materials Science Engineering A346 (2003) 189-195.
[3] Y. D. Huang et al., Journal of Materials Processing Technology 134 (2003) 19-25.
[4] O. V. Silva Neto and O. Balancin, in Proceedings CONAMET/SAM Congress, La Serena, Chile (2004)
237-242.
th
[5] O. V. Silva Neto and O. Balancin, in 8 Inter American Congress of Electron Microscopy, La Habana,
Cuba (2005).
[6] J. Baczynski and J. J. Jonas, Metallurgical and Materials Transactions A 29A (1998) 447-462.
[7] A. M. Jorge Jr., W. Regone and O. Balancin, J. of Materials Processing Technology 142 (2003) 415-421.
[8] D. Chu and J. W. Moris Jr., Acta Materialia 44 (1996) 2599-2610.

2. 300 2,00 75,0
275 74,5
1,75 Grain size average
250 High angle boundary
74,0
225 1,50
High Angle Boundary [%]
Grain Average Size [µm]
73,5
200
1,25
73,0
Stress [MPa]
175
150 1,00 72,5
125 72,0
o 0,75
ttemp =1H; Ttemp=685 C
100 -1
0,1s 71,5
-1
75 0,5s 0,50
-1
1,0s 71,0
50
0,25
70,5
25
0,00 70,0
0
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
-1
Strain Strain Rate [s ]
(a) (b)
o
Figure 1 – (a) Flow stress curve - tempering (685 C); (b) Average grain size and high angle boundary.
(a) (b) (c) (d)
−1 −1 −1
Figure 2 – EBSD maps: (a-c) Inverse polo figure – (a) 1.0 s ; (b). 0.5 s ; (c) 0.1 s ; (d) colors code - [001].
250 100
225 90
200 80
High Angle Boundary [%]
175 70
Stress [MPa]
150 60
125 50
-1
0.1 s
100 40
75
ε=1 30
ε=2
50 ε=3 20
ε=4
25 ε=5 10
0 0
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 0 1 2 3 4 5 6
Strain Total Strain
(a) (b)
Figure 3 - (a) Flow curve – microstructural evolution; (b) % high angle boundary x total strain.
(a) (b)
Figure 4 - Ultrafine ferrite grains - strained specimens to 0.1 s − 1 : (a) ε = 1. 0 ; (b) ε = 5.0 .