Effects of Rolling and Cooling Conditions on Microstructure of Umbrella-Bone Steel

Dynamic CCT diagrams of umbrella-bone steel

The starting and ending temperatures of phase transformation were determined according to the dilatometric curve and microstructure at various cooling rates. The beginning and ending temperatures of the austenite phase transformation, A_c1 and A_c3, respectively, were obtained from the dilatometric curve at a heating rate of 0.05 °C/s. The results show that Ac1 and Ac3 are 732 and 808 °C, respectively. The theoretical temperature of martensite transformation calculated using an empirical formula [12], is 394.7 °C. Based on these results, the dynamic CCT diagrams of umbrella-bone steel were plotted, as shown in Figure 3. Since the phase transformation was obtained according to the dilatometric curve, the tests and calculation was run by 3 times each to reduce the operating errors.

Figure 3:

Dynamic CCT diagrams of umbrella-bone steel at deformation temperatures of (a) 850 °C and (b) 900 °C.

As shown in the figure, the transformations of different phases occurs under different conditions. When the deformation temperature was 850 °C, the transformations for ferrite and pearlite occured in supercooled austenite for cooling rates blow 40 °C/s. The martensite transition is observed at above 40 °C/s. Similar results were obtained for a deformation of 900 °C, but the critical cooling rate of martensite decreased to 30 °C/s.

Microstructure evolution

The strain at the center of the compressed samples is higher than surface. [13]. In order to keep the same strain for all samples, the microstructure of the range at a distance of 0.5 R (radius of the samples) from the center was analyzed using an optical microscopy. Figure 4 shows the microstructure at a deformation of 850 °C. When the cooling rate was less than 10 °C/s, the microstructure at room temperature consisted of quasi-polygon ferrite and pearlite. When the cooling rate was between 10 and 30 °C/s, allotrimorphic ferrite and perlite, along with few Widmanstätten ferrite, appeared in the micrographs. When the cooling rate was 40 °C/s, martensite appeared. The microstructure at a deformation temperature of 900 °C is shown in Figure 5. The microstructure is similar to that at a deformation temperature of 850 °C for low cooling rates. However, martensite was found at a cooling rate of 30 °C/s.

Figure 4:

Optical micrographs of samples at deformation temperature of 850 °C and cooling rates of (a) 0.1 °C/s, (b) 1 °C/s, (c) 10 °C/s, (d) 20 °C/s, (e) 30 °C/s, (f) 40 °C/s, and (g) 50 °C/s.

Figure 5:

Optical micrographs of samples at deformation temperature of 900 °C and cooling rates of (a) 0.1 °C/s, (b) 1 °C/s, (c) 10 °C/s, (d) 20 °C/s, (e) 30 °C/s, (f) 40 °C/s, and (g) 50 °C/s.

The microstructure proportions corresponding to various cooling rates for deformation temperatures of 850 and 900 °C are shown in Figure 6(a) and (b), respectively. The proportion of ferrite decreased with larger cooling rate. The proportion of pearlite initially increased and then decreased. The proportion of martensite rapidly increased at high cooling rates. An increase in deformation temperature led to a decrease in the critical cooling rate of martensite.

Figure 6:

Microstructure proportion corresponding to various cooling rates for deformation temperatures of (a) 850 °C and (b) 900 °C.

The curves of Vickers hardness of the samples and microstructure versus cooling rate for deformation temperatures of 850 and 900 °C are presented in Figure 7. The Vickers hardness of the samples increased with increasing cooling rate due to the decrease in ferrite and increase in pearlite. Furthermore, the hardness increased rapidly when martensite appeared. The Vickers hardness of pearlite initially increased and then remained constant with increasing cooling rate except the one below 1 °C/s. For a given cooling rate, a decrease of the deformation temperature resulted in a decrease in the Vickers hardness.

Figure 7:

Variation of Vickers hardness with cooling rate.

Figure 8 shows the variation of the starting temperature of ferrite transformation with cooling rate. As shown, the starting temperature of ferrite transformation falls with increasing cooling rate while a reduction occurred with low deformation temperature for a given cooling rate.

Figure 8:

Variation of starting temperature of ferrite transformation with cooling rate.

Effects of cooling rate on microstructure

Fast cooling, which improves supercooling and increases the density of nucleation sites for ferrite, results in a decrease in the starting temperature of ferrite transformation and ferrite grain refinement [14, 15]. As previously presented, the morphology of ferrite changes from quasi-polygon to allotrimorphic and the grain is gradually refined at a high cooling rate. Fast cooling can suppress the γ-α transformation during cooling attributing to less time for diffusional transformation [10], namely the region of medium-temperature transformation is obtained quickly. As a result, proeutectoid ferrite is suppressed and the proportion of pearlite is improved. However, the appearance of martensite decreases the proportion of pearlite when the cooling rate exceeds the critical cooling rate of martensite. Due to the differences in the Vickers hardness among ferrite, pearlite, and martensite, fast cooling usually increases the Vickers hardness of a sample. Fast cooling results in high supercooling, which increases the density of nucleation sites for pearlite and reduces carbon diffusion. The interlamellar spacing of pearlite is determined mainly by the driving force for pearlite transformation, which is controlled by supercooling [16]. Decreasing interlamellar spacing promotes the strength and plasticity of the microstructure, which explains the variation of Vickers hardness in pearlite described above.

Effects of deformation temperature on microstructure

Austenite rolling, consisting of deformation and recrystallization, is divided into working hardening and softening processing. There are two stages in working hardening. Under a low strain stage, the dislocation density increases quickly, resulting in a rapid generation of low-angle boundaries. With increasing strain, ultrafine grains surrounded by new high-angle grain boundaries are gradually produced, which is attributed to deformation and dynamic recrystallization (DRX) [17, 18, 19]. The size of new grains in DRX is controlled by the Zener-Hollomon parameter [20], which can be expressed as:

where ε˙ is the strain rate, T is the absolute temperature, R is the gas constant, and Q denotes the apparent activation energy for softening during deformation. The size of the new grains in DRX thus decreases with decreasing deformation temperature.

Softening processing after deformation is also critically affected by deformation temperature. At high temperature, recovery and static recrystallization occur simultaneously, named as the region of austenite recrystallization in rolling. Recovery only occurs in the region of austenite non-recrystallization at low temperature. For a given cooling rate after deformation, lower temperature weakens the effect of softening processing caused by recovery or recrystallization, enhancing the effect of strain accumulation in austenite.

Due to the low temperature and small strain in this experiment, the samples can be considered to be rolled in the region of austenite non-recrystallization. Lower temperature enhances the effect of working hardening and subsequently strain accumulation in the cooling process prior to the γ-α transformation. Therefore, the density of nucleation sites for ferrite increases at lower temperature, which decreases the hardenability of austenite. As a result, the starting temperature of ferrite and the critical cooling rate of martensite both increase.