Effect of cold rolling and annealing on the grain refinement of low alloy steel

A low alloy steel containing 0.06 wt% C, 1.5 wt% Mn and 0.1 wt% V was given 85% cold rolling reduction. The aim of the rolling reduction was to induce energy due to stresses and distribution of carbides, used for grain refinement in subsequent annealing. The rolled specimens were heat treated at various temperatures from 590°C to 650°C for different lengths of soaking times ranging from five minutes to two hours, to promote the process of re-crystallization. At temperatures with long soaking times the re-crystallization process is expected to be completed with minimum of grain coarsening due to carbide distribution, especially vanadium carbides. A smooth drop in hardness with increase in annealing times was observed which may be due to recovery from stressed conditions during process of re-crystallization. Texture observations supported the re-crystallization process as the preferred orientation of (200) plane in rolled condition was successively reduced with annealing temperatures. Tensile properties observations of two hour annealing times at 590°C to 650°C clearly demonstrated that ductility increased at all annealing temperatures with maximum gain at 625°C and strength is decreased.


Introduction
The improvement in mechanical properties, especially the yield strength creates certain attraction in grain refinement. It is well known that yield strength, σy, is increased by refining the grain size according to Petch relationship [1]: σy = σ0 + kyd -1/2 (1) where σ0 is yield strength for material containing no grain boundaries, d is the grain diameter and ky is an experimentally determined coefficient. Severe deformation with high plastic deformation techniques has gained interest in recent years for recrystallization leading to grain refinement in low alloy steels and aluminum alloys [2][3][4][5]. Transformational grain refinement (TGR), in which austenite to ferrite transformation is used to produce refined ferrite structure in steel, was first introduced by Hodgson et al [6][7][8] and got a refined ferrite microstructure of 1 µm in a substantial fraction of its volume. Priestner and Ibraheem [9] used similar process to achieve grain sizes less than 1.5 µm through thickness of a Nb-microalloyed steel rolled to approximately 2.5 mm thickness. They also applied cold rolling after TGR process followed by annealing to promote re-crystallization for the achievement of ultrafine grain refinement. In the present work starting with ferrite and pearlite microstructure a heavy cold rolling reduction of 85% was given to get maximum stored energy used for re-crystallization on subsequent annealing. A heavy deformation like cold rolling provides store energy which acts as driving force for re-crystallization on annealing [10]. In cold rolling the dispersed cementite and vanadium carbides are expected to control the grain coarsening of the re-crystallized grains.

Experimental Work
The vanadium micro-alloyed steel was provided in the form of hot rolled and annealed slab with chemical compositions listed in the Table 1. The initial optical metallographic observations revealed that microstructure was mainly ferrite and pearlite, with minimum banding of prior deformation, shown in Fig.1.
After pickling a cold rolling reduction of 85 percent was given to get final thickness of 1 mm steel plate. Four sets of annealing at temperatures of 590°C, 600°C, 625°C and 650°C with different holding (soaking) times ranging from 5 minutes to two hours were given to promote re-crystallization. For Vickers hardness measurement after annealing average of five reading at different positions of the specimens were taken. For optical metallographic observations the specimens after grinding polishing were etched in 2% natal solution. The re-crystallized microstructures of the annealed specimens were examined on Olympus 3X-51 microscope equipped with image analyzer and DP-70 digital camera. Line intercept method was employed for grain size measurement. In this method concentric circles of known diameter were superimposed on the image of the microstructure to get the intercepts on the grain boundaries.
For the texture measurement X-ray diffractometer, model "Dmax-III A" of Rigaku Company of Japan, equipped with monochromator and Cu Kα radiation was used. Tensile testing was performed on SANS Universal testing machine with cross head speed of 2mm/min.

Cold rolling and annealing for re-crystallization
In cold rolling ferrite and pearlite (or ferrite plus carbide aggregate) microstructure developed previously dissociates and carbides are scattered to play roll in prevention of grain coarsening during subsequent heat treatment. The microstructure revealed after cold rolling is shown in Fig. 2.
The aspect ratio of the grains changed along the rolling direction providing a banded morphology with the strain energy used for re-crystallization. A large stored energy resulting from deformation provides large density of sites for nucleating re-crystallization [10]. The energy of deformation is expected to play role in re-crystallization during annealing and scattered carbide especially vanadium carbide (VC) may control the grain coarsening. The grain refinement by static recrystallisation after cold rolling depends upon recrystallisation temperature, degree of prior deformation and the grain size prior to deformation [11]. Starting with grain size of 8.4 µm of as received material, the cold rolled specimens were annealed at different temperatures to promote static re-crystallization. The effect of annealing on hardness for different holding times after 85% reduction is shown in Fig. 3.  The hardness dropped smoothly with soaking times at all annealing temperatures and this effect is more enhanced at 650°C. The decrease in hardness with annealing times and temperatures are due to the process of recovery and at higher temperature of 650°C it may be completed in 20 minutes. Apart from time and temperature of annealing complete recovery from deformation depends upon the type of deformation applied prior to annealing process. Although equal channel angular extrusion is more effective than cold rolling but accumulated strains introduced by these processes were effectively used in the intercritical annealing for grain refinement of ferrite and martensite [12][13]. The microstructures developed after two hours of annealing at 590°C, 600°C, 625°C and 650°C are shown in Fig. 4.
At low annealing temperature of 590°C the process of re-crystallization has started but still some elongated particles are also visible even after two hours of annealing, shown in Fig. 4(a). There is no appreciable drop in hardness between one and two hours of annealing, Fig. 3, predicting the recovery process has completed within or before this period of time. Therefore, the annealing temperature of 590°C may be low enough to mobilize the uncompleted process of re-crystallization by the store energy within the specified period of recovery time. At 600°C the microstructure Fig. 4(b) is mostly equi-axed in nature with some small grains of approximately 1µm are visible. Similarly the grain refinement process looks more efficient with some coarsening of grains in limited localized areas. However, the average grain size obtained after two hours of annealing at all temperatures of rolled specimens was 4.4 µm and minimum of 3.9 µm grain size at 650°C. A noticeable grain size reduction of about 46% was obtained after re-crystallization during process of heat treatment.

Deformation Texture
The preferred orientation or textures developed by forming process with severe plastic deformation is called Deformation Texture. In a polycrystalline aggregate where grains have their own random preferred orientation rotate during plastic deformation. As a result grains undergo slip and rotate in a complex fashion to set a new nonrandom orientation which is determined by the imposed forces. The texture development after cold rolling and changes after process of annealing are shown in Fig. 5.
The deformation texture developed by plastic deformation or cold rolling can be assessed by the evaluation from the XRD pattern shown in the present case of 85% reduction sample as Fig. 5(a). The preferred orientation or texture was developed after cold rolling along the (200) plane. After two hours of annealing at 590°C, Fig. 5(b), no appreciable changes of intensities along (200) and (211) planes were observed. There is noticeable decrease in intensity along (200) plane after two hours of annealing at 650°C, Fig. 5(c). The intensity together with random intensity, texture coefficient of (200) plane and texture developed (%) are described in Table 2.  To calculate the texture coefficient (T.C) the normalized intensities were divided by the average of all the normalized intensities according to the equation: is the measured integral intensity of a given (hkl) reflection in the specimen (ℎ ) is the calculated theoretical intensity for the same (hkl) reflection in a randomly oriented specimen is the total number of rections When the cold worked metal is annealed and re-crystallized new grains have their own preferred orientation with re-crystallization or annealed texture [14]. A texture of 163% was developed after cold rolling, shown in Table 2 which gradually reduced by increasing the annealing temperature. At 590 °C after two hours of annealing the texture reduced to 104%, reflecting the partial recrystallization, observed in Fig.4 (a). The reduction in texture percentage along (200) is prominent at higher temperature assessed in the metallographic observations of Fig.4 (b) to (d) and texture reduced to 11% at 650°c for two hours of annealing.

Tensile Properties
The stress strain curves of the specimens after two hours of annealing at 590°C, 600°C, 625°C and 650°C are shown in Fig. 6.
Tensile testing results of as cold rolled specimen are also added for comparison. In cold rolled condition the tensile strength was increased to 946 MPa with only 4.4% of uniform strain. The expected increase in strength and hardness, as shown in Fig. 3 before the start of annealing process, may be due to introduction of dislocations in the microstructure during heavy plastic deformation in cold rolling. The interaction of these dislocations in tensile deformation raised the tensile strength at the expense of ductility.
After two hours of annealing all the specimens showed sharp decrease in tensile strength and increase in ductility. The decrease in strength may be due to recovery from stressed conditions and release of stored energy used for grain refinement. In annealed conditions when refined microstructure played its role in raising the yield strength, a maximum gain was achieved after annealing at 600°C. The micro-structural observation after two hours of annealing at 600°C, Fig. 4(b), clearly shows recrystallization and increased population of micron or submicron sizes of grains. The Petch relationship [1]: in equation 1 described an inverse relationship of yielding with grain size of the materials. In tensile testing the plastic deformation of all annealed specimens progressed with minimum of work hardening which is also observed previously by Hodgson et.al [7] in ultrafine ferrite steel.
The tensile strengths of the annealed specimens were not sharply decreased with increasing the annealing temperatures. This may be due to absence of abnormal grain growth during annealing observed previously by Priestner and Ibraheem [9] is steel containing niobium. The present studies the grain coarsening process looks to be effectively controlled by the vanadium precipitates.

Conclusions
A low alloy steel containing 6.06 wt% C was given 85% cold rolling reduction to induce effective strain used for grain refinement in subsequent annealing. The cold rolled specimens were annealed at different temperatures with different soaking times. The results are concluded as follow: • The optical metallography of the rolled specimen showed elongation of the grains and increase of aspect ratio along the rolling direction. The microstructural observations of the annealed specimens clearly revealed the re-crystallizations. An overall grain size reductions of 46% was obtained after annealing. No abnormal grain growth was observed by increasing time and temperature of annealing. • A texture along (200) plane, as preferred orientation after cold rolling, was developed. The cold rolling texture of 163% was gradually reduced with increase of annealing temperatures. Annealing at 650°C reduced the texture to 11%, reflecting the re-crystallization along the new preferred orientations. • The Annealed specimens with refined microstructure showed typical tensile properties of ultra-fined steels with lack of excessive work hardening and increased ductility. The tensile strength dropped after annealing and re-crystallization due recovery from stressed conditions. However, increased fracture strains reflect their improved formability essentially required for structural applications of steel.