Cold Rolling and Annealing Microstructures and Textures of Low Carbon Steels

M. Almojil1,a, P. S. Bate1,b 1School of Materials, Materials Science Centre, The University of Manchester,

Grosvenor Street, Manchester, M1 7HS, UK amarwan.almojil@postgrad.manchester.ac.uk, bpete.bate@manchester.ac.uk

Keywords: Low carbon steels, recrystallisation, texture, PSN, EBSD Abstract. The development of crystallographic textures of IF and HSLA steels after 20, 50, 70 and 90% cold rolling reductions and subsequent recrystallisation have been investigated using Electron Backscattered Diffraction (EBSD). The HSLA steel was initially processed to give a volume fraction of about 0.2 of fine pearlite colonies, which acted as mechanically hard particles. Both cold rolling and recrystallisation textures are shown to be largely dependent on the rolling reduction for both steels. With increasing rolling reduction, the texture shows gradual intensification of α and γ fibre components. Although PSN was the dominant nucleation site in the HSLA steel during annealing, the α and γ fibres also exist in the recrystallisation textures, but with lower density.

Introduction

The presence of large particles (d>1µm) in the matrix tends to promote recrystallisation through the Particle Stimulated Nucleation (PSN) mechanism [1,2]. They give rise to high local concentrations of stored energy and large misorientations in the surrounding microstructure when the particle-containing material is deformed. Although these particles are known to weaken the textures [3], it is not clear to what extent they can control the textures produced when PSN is the dominant nucleation site. This paper is focused on this in low carbon steels using electron microscopy.

Experimental Procedures

Interstitial free (IF) steel and High Strength Low Alloy (HSLA) steel were used in this study, and the chemical compositions are given in Table 1. Both steels have low carbon content, with no free interstitial atoms in solution in the IF steel. The steels were obtained in the form of hot rolled plate with a thickness of about 4.2mm for the IF steel and about 12mm for the HSLA steel. As the aim of this work was to study the texture produced by PSN, initial heat treatment was carried out on the HSLA steel to produce a structure with fine, spheroidal, pearlite colonies. The steel was heated to 1250oC for one hour to dissolve the (Nb, Ti)(C, N) carbonitrides, followed by water quenching. It was then reheated to 945oC for 30 minutes to give Nb(C, N) precipitation and then water quenched. Finally, the steel was aged at 712oC for 24 hours to spheroidise the pearlite colonies (i.e. fine pearlite particles). These particles are coarse enough to act as nucleation sites for recrystallisation. The particles’ volume fraction were measured to be about 20~24% with an average diameter of 3µm. Table 1 Chemical composition (in wt %) of the steels investigated.

Material C N Mn Al Ti Nb IF 0.003 0.003 0.15 0.05 0.08 -

HSLA 0.07 0.006 1.39 0.05 0.003 0.059

Both steels were then cold rolled to four different reductions; 20%, 50%, 70% and 90%, using a two-high rolling mill with 250mm diameter rolls operating with a roll surface speed of 150 mm/s.

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Rolling was followed by recrystallisation annealing at different times and temperatures. Rolled samples were placed in a pre-heated LMF Carbolite high temperature furnace. A copper rig housing the sample with an attached thermocouple was used. Transverse cross-sections of the as-rolled, partially recrystallised and fully recrystallised samples were ground and polished, and then the structures were revealed using a 2% Nital etch at room temperature. The samples used for texture analysis were further electropolished using a solution of 8% Perchloric acid and 92% acetic acid at 50 volts for 30 seconds, at room temperature.

The EBSD technique for texture analysis was conducted using the CamScan MaXim, FEI-Sirion and Philips XL-30 FEGSEM microscopes equipped with channel 5 EBSD acquisition software by HKL. The working distance ranged from 13 to 27mm depending on the microscope used while the accelerating voltage and the sample tilt with respect to the electron beam were 20KeV and 70o, respectively. In-house software was employed for representation of microstructure and texture analysis.

Results and Discussion

The cold rolling textures of both steels investigated, shown in Figure 1, consist mainly of the α and γ fibres texture components, and are largely dependent on the rolling reduction. With increasing reduction, the texture shows gradual intensification of both fibres, with weaker intensities in the HSLA steel where particles are present. The maximum orientation density along the α fibre, in the case of IF steel, was at {001}<110>, {115}<110>, {112}<110> and {223}<110> for 20%, 50%, 70% and 90% reductions, respectively. The uniformly distributed orientation density along the γ fibre in 90% cold rolled IF steel can be explained also by crystallographic rotations along another path. The weak {554}<225> observed in the 20% reduction rotate towards {111}<112> resulting in a strong {111}<112> orientation component after 50% reduction. With higher reductions, the grains having this orientation are rotated to {111}<110> and finally to {223}<110>. The crystallographic rotations along both fibres seem to follow the two paths given by Inagaki [4].

Figure 1 Textures of IF (a-d) and HSLA (e-h) steels (ODF sections at φ2=45o) cold rolled to (a) 20%, (b) 50%, (c) 70% and (d) 90%.

On the other hand, the rolling texture of the HSLA steel (Figure 1e-h) was found to be weaker than those in IF steel. Since rotation toward the stable end orientations occurs by dislocation slip, the present of pearlitic particles in this steel affects dislocation motion and, hence, create highly orientation-spread zones around them. The orientation density along the α and γ fibres shows a peak

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at {111}<110> for low reduction (i.e. 20%). With increasing reduction, this peak shifted toward {001}<110> and {111}<112> along α and γ fibres, respectively. It therefore appears that the rotation paths in this steel differ from those observed in the IF steel. These differences may have an effect on the recrystallisation behaviour due to the variation in stored energy with different orientation components, although PSN is dominant in the HSLA steel.

Figure 2 (a) EBSD map (band contrast) showing recrystallisation originating around the particles

(PSN) in HSLA steel and (b) Effect of reduction on the grain size of fully annealed samples of both steel investigated.

Nucleation of recrystallised grains in HSLA steel occurs mostly in the deformed zones

developed around the particles during cold rolling as shown in Figure 2a. In addition, Figure 2b shows the effect of reduction on the grain size of the fully recrystallised samples of both steels. As expected, the grain size decreases with increasing reduction as a result of the higher stored energy associated with large reductions and, hence, a higher nucleation density.

Texture development during the recrystallisation of IF steel, as shown in Figure 3a-d, is largely dependent on the rolling reduction. It can be concluded that the annealing texture is mainly α fibre at low reductions, as a result of grain boundary nucleation by SIBM. However, with increasing rolling reduction, the nucleation mechanism changed to subgrain growth in γ fibre grains since the stored energy in these grains becomes high and, hence, the annealing texture shows a gradual intensification of γ fibre components. This is in good agreement with much of the published work investigating the recrystallisation textures of IF steel [5,6].

On the other hand, the recrystallisation textures for the HSLA steel are somewhat different. These are given in Figure 3e-h, which shows that with increasing rolling reduction, gradual intensification of both fibres are observed. This behaviour differs from that of the IF steel where only the γ fibre becomes significant with increasing reduction. The existence of both fibres, even with low densities, in this type of steel where PSN is the preferential nucleation mechanism seems to be due to that the formation of nuclei which occur within the deformation zones in regions away from the particle surface. This leads to recrystallisation textures similar to that of the rolling texture and also, although with lower intensities, to recrystallisation textures in IF steel.

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Figure 3 Recrystallisation textures of IF (a-d) and HSLA (e-h) steels (ODF sections at φ2=45o) cold rolled to (a) 20%, (b) 50%, (c) 70% and (d) 90%.%.

Conclusions

The effect of pearlite particles in the rolling and recrystallisation textures of low carbon steel has been investigated. The cold rolling textures were found to be slightly weakened compared to particle-free steel (i.e. IF steel) as these particles hinder dislocation motion and, hence, affect the crystallographic rotation towards stable end orientations. Although PSN was the dominant nucleation mechanism in the HSLA steel during annealing, the α and γ fibres also exist in the recrystallisation textures, even though with lower density. The location of nucleation events, being away from the particle/matrix interfaces, may explain this behaviour. This seems to result in orientations which differ from those of the deformed material to a limited extent.

References

[1] N. Hansen: Mem. Sci. Rev. Met. Vol. 72 (1975), p. 189 [2] F. J. Humphreys: Acta Metall. Vol. 25 (1977), p. 1323 [3] F. J. Humphreys and M. Hatherly: Recrystallization and Related Annealing Phenomena,

(Pergamon, Oxford, UK, 2004) [4] H. Inagaki: Z. Metallkde. Vol. 78 (1987), p. 630 [5] W. B. Hutchinson: Int. Met. Rev. Vol. 29 (1984), p. 25 [6] F. Emren, U. von Schlippenback and K. Lücke: Acta Metall. Vol. 34, p. 2105

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PRICM7 10.4028/www.scientific.net/MSF.654-656 Cold Rolling and Annealing Microstructures and Textures of Low Carbon Steels 10.4028/www.scientific.net/MSF.654-656.214

DOI References

[2] F. J. Humphreys: Acta Metall. Vol. 25 (1977), p. 1323

doi:10.1016/0001-6160(77)90109-2 [3] F. J. Humphreys and M. Hatherly: Recrystallization and Related Annealing Phenomena, Pergamon,

Oxford, UK, 2004)

doi:10.1016/B978-008044164-1/50011-6 [3] F. J. Humphreys and M. Hatherly: Recrystallization and Related Annealing Phenomena, (Pergamon,

Oxford, UK, 2004)

doi:10.1016/B978-008044164-1/50011-6