1. GRAIN ULTRA-REFINEMENT IN A LOW CARBON STEEL AT SUB-CRITICAL
TEMPERATURE THROUGH THERMOMECHANICAL PROCESSING
Silva Neto1, O. V. and Balancin2, O.
1,2
Departamento de Engenharia de Materiais - Universidade Federal de São Carlos - DEMa/UFSCar
Rodovia Washington Luis, Km 235, Cx.P. 676, CEP 13385-000 - São Carlos - São Paulo - Brasil.
1
e-mail: pvillar@iris.ufscar.br
Heavy plastic straining greater than ε = 2.0 plays an important role in grain ultra-refinement mechanism in
steels. However, the amount of plastic strain often is more intense at some position of the each component, such as in
the surfaces of rolled plates or forged pieces, and hinders a uniform microstructure to be obtained in all the extent of the
material [1]. The precipitation of fine cementite particles (Fe3C) during thermomechanical process can produce a more
stable and homogeneous microstructure. For instance, precipitation of fine cementite particles on ferrite grain and sub-
grain boundaries decreases the ferrite grain-growth rate due to boundary pinning effect [2,3]. Nevertheless, the
refinement of the ferrite microstructure caused by the precipitation of carbides is the most common mechanism
associated with the increase of strength and decrease of ductile-to-brittle transition temperature of metallic materials
[1,2,3,4].
The spheroidization process promoted by long-time annealing occurs on the sub-critical (below A1) and inter-
critical (between A1 and A3) temperature ranges. The application of straining during the spheroidization annealing
becomes the process more effective, in other words, there is an increase of spheroidite fraction and decrease of time
needed for carbides precipitation. Thermomechanical treatment carried out at temperatures below the critical has
showed more effectiveness to increase the spheroidite fraction than at inter-critical region [5].
The purpose of this work was to investigate the effects of cementite precipitation, which takes place after an
extensive straining by warm torsion tests, in grain refinement of the ferrite matrix in low-carbon steel. The chemical
composition of the steel investigated is given in Table 1. The thermomechanical treatments applied aimed to produce
ferrite grains with ultra fine size (<2µm) attended by carbides precipitation. Torsion tests were accomplished at
temperatures near A1, after sub-critical annealing. Two different groups of samples were strained: in the first samples
were heated to 1100 °C, kept at this temperature for 10 minutes, water quenched and spheroidized at 670 °C during 7
hours. After this, they were strained at 700 °C after hold time indicated in Table 2. The other set was spheroidized
assisted by straining as depicted in Table 2. The metallographic observation of the strained and air-cooled samples was
done by optical microscopy. The effects of annealing time, strain rate and amount of strain on flow stress curves and on
ferrite grain refinement were investigated.
In both processing routes, large precipitates of Fe3C and ultra-fine ferrite grains (<2µm) were observed, as
showed in Figure 1. Figures 1a and 1b depicted homogeneous microstructure with ferrite grains size of 1,5µm, which
were developed on the surface of the samples annealed and strained at 700oC, in both routes. The curves of the Figure 2
indicated that spheroidized samples by conventional heat treatment had less ductility than the quenched and strained.
Also, it is worth noting that the decrease of strain rate increases the ductility, indicating that the recrystallization of
ferrite is the dominant phenomenon of grain refinement, which is maximized by increase of nucleation sites developed
by cementite precipitates along grain boundaries.
The routes applied were sufficient for producing ferrite grain size smaller than 2µm. It was verified the
possibility of accelerating the precipitation of Fe3C, as well as grain refinement, at sub-critical temperatures.
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] H. Mabuchi, T. Hasegawa and T. Ishikawa, ISIJ International 39 (1999) 477-485.
[3] D. H. Shin, K.-T. Park. and Y.-S. Kim, Metallurgical and Materials Transactions 32A (2001) 2373-2381.
[4] D. B. Santos, R. K. Bruzszek, P. C. M. Rodrigues and E. V. Pereloma, Mat. Sci. Engineering A346 (2003) 189-195.
[5] J. M. O’Brien and W. F. Hosford, Metallurgical and Materials Transactions 33A (2002) 1255-1261.

2. Table 1 - Chemical composition of the investigated steel (wt%).
C Mn Si Al S P V Cr B Ni Cu
0,162 1,343 0,459 0,038 0,009 0,019 0,030 0,011 0,0002 0,230 0,012
Table 2 – Thermomechanical processing route applied.
Austenitising Soak Conventional Delay
Schedule Temperature Time Quenching Spheroidization Time ε ε
&
(oC) (min) (670oC) (min) (700oC) (s-1)
E7 1100 10 water 420min 10 5.0 0.5
EC 1100 10 water 420min 10 2.3 1.0
E4 1000 10 water X 60 2.0 1.0
E6 1000 10 water X 60 5.0 0.1
E8 1000 10 water X 60 5.0 0.5
(a) (b)
Figure 1 – Ultra-fine ferrite grains of the strained specimens. (a) E7 and (b) E4.
300
E4
E6
250 E7
E8
EC
200
Stress [MPa]
150
100
50
0
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0
Strain
Figure 2 – True stress-strain curve of the investigated steel.