Effect of Proeutectoid Ferrite Morphology on the Microstructure and Mechanical Properties of Hot Rolled 60Si2MnA Spring Steel

Experimental process

The 60Si2MnA spring steel wire rod used in this work had a chemical composition of 0.59 %C–1.68 %Si–0.77 %Mn–0.15 %Cr (in wt %). The steel wire rod was machined into cylindrical specimens with a working section size of Φ6 × 15 mm for dynamic thermal simulation test on a Gleeble-1500 simulator from American DSI, and then the specimens were machined into standard size for tensile and impact test. The continuous cooling transformation (CCT) curve was obtained, and the microstructures and mechanical properties of spring steel under different cooling rates (0.5, 1, 2, 3, 4 and 5 °C/s) after finish rolling as were tested as shown in Figure 1.

Figure 1:

Schematic drawing of hot compression simulation tests.

Microstructural analysis and mechanical property measurements

The specimens were metallographically polished and then etched with 4 % nital for microstructural observation by optical microscopy (NEOPHOT-21) and scanning electron microscopy (JSM-6480LV). The bulk content of proeutectoid ferrite V_f, interlamellar spacing S_P and pearlitic colony d_p were measured by the software Image Tool using point counting technique and random intercept methods, respectively. The mechanical properties of the specimens at room temperature were determined by tensile and impact tests following the international standards ISO 6892-1:2009 and ISO 148-1:2010.

Effect of proeutectoid ferrite morphology on microstructure

The specimens were heated and deformed on a Gleeble-1500 simulator, and then cooled to room temperature continuously at different cooling rates as shown in Figure 1; the microstructures and microhardness HV of the specimens at different cooling rates were tested, and the CCT curve is given in Figure 2.

Figure 2:

The CCT curve of 60Si2MnA spring steel.

Figure 2 shows the CCT curve of 60Si2MnA spring steel; when the cooling rate was 0.5–5 °C/s, the microstructures of the specimens at room temperature were all proeutectoid ferrite and pearlite, but when the cooling rate increased to 6 °C/s, martensite began to form. As shown in Figure 3 and Table 1, cooling rate plays an important part in controlling the proeutectoid ferrite morphology of spring steel. When the cooling rate increased from 0.5 to 5 °C/s, the bulk content of proeutectoid ferrite V_f decreased from 24 % to 2 %; proeutectoid ferrite in steel transformed from the reticular morphology to the small, dispersed and blocky morphology, and the microhardness of microstructure increased from 311 HV to 397 HV. When the cooling rate after finish rolling was 0.5–2 °C/s, the corresponding undercooling ΔT was so low that the ferrite along the boundary of austenite grain grew up faster than the intragranular ferrite, so the allotriomorphic ferrite was formed, which was also called reticular proeutectoid ferrite. Moreover, the bulk content of the reticular proeutectoid ferrite increased with the decrease in cooling rate [13]. When the cooling rate increased from 3 to 5 °C/s, the undercooling ΔT increased; a lot of ferrite was too late to form along the prior austenite grain boundary, so the small, dispersed and blocky proeutectoid ferrite occurred with the increase in cooling rate. What is more, as shown in Table 1 and Figure 4, when the cooling rate increased from 0.5 to 5 °C/s, the undercooling ΔT also increased. As the morphology of proeutectoid ferrite changed, the stability of the deformed austenite increased. This could postpone the phase transition from austenite to pearlite, then the uniform and fine pearlite could be formed with the phase transition and nucleation of deformation band in austenite at the lower temperature, the interlamellar spacing decreased, and these made a good effect on refining pearlite and reducing the grain size [4, 5].

Figure 3:

Microstructures of the samples at different cooling rates: (1) 0.5, (2) 1, (3) 2, (4) 3, (5) 4 and (6) 5 °C/s.

Figure 4:

SEM micrographs showing the lamellar pearlite of the samples at different cooling rates: (a) 0.5 °C/s, (b) 1 °C/s, (c) 3 °C/s and (d) 5 °C/s.

The reciprocal of interlamellar spacing SP−1, the reciprocal of pearlitic colony dp−1 and the reciprocal of proeutectoid ferrite bulk content Vf−1 all follow a linear relationship with the undercooling ΔT, and the calculation model is illustrated in eq. (1) by Zhang et al. [11, 12, 14–16]. In addition, the previous study [17] showed that the cooling rate also followed a nearly linear relationship with the undercooling ΔT at a certain range. Therefore, the interlamellar spacings, pearlitic colonies and proeutectoid ferrite bulk contents of the four specimens at the cooling rates of 0.5, 1, 3 and 5 °C/s were measured in order to explore the coexistence relationship between proeutectoid ferrite and pearlite in this study. As shown in Figure 5, the reciprocal of proeutectoid ferrite bulk content Vf−1 also follows a nearly linear relationship with the reciprocal of interlamellar spacing SP−1 and the reciprocal of pearlitic colony dp−1, which accorded with the results of Zhang. Hence, with the transformation of proeutectoid ferrite from the reticular morphology to the small, dispersed and blocky morphology, the bulk content of proeutectoid ferrite decreased, the interlamellar spacing and colony size of pearlite coexisting with the proeutectoid ferrite also both decreased accordingly, and the relationships could be illustrated in eqs (2) and (3):

where A and B are material constants.

Figure 5:

The coexistence relationship between proeutectoid ferrite and pearlite.

Effect of proeutectoid ferrite morphology on mechanical properties

The results of tensile and impact tests are shown in Table 2 and Figures 6 and 7.

Figure 6:

Effect of proeutectoid ferrite on tensile strength and percentage of area reduction.

Figure 7:

Effect of proeutectoid ferrite on impact energy and microhardness.

The previous studies [10, 11] show that the tensile strength R_m, percentage of area reduction Z, impact energy A_KU and microhardness HV all follow a linear relationship with SP−1/2, which all accord with a Hall–Petch-type relationship. At the given ranges of the proeutectoid ferrite bulk content and interlamellar spacing in Table 1, it was found that Vf−1/2 also can follow a linear relationship with SP−1/2 as in eq. (4). So as shown in Table 2 and Figures 6 and 7, mechanical properties of the spring steels with microstructures of different proeutectoid ferrite morphologies were determined by the tensile and impact tests at room temperature in this study. As expected, the tensile strength R_m, percentage of area reduction Z, impact energy A_KU and microhardness HV all follow a linear relationship with Vf−1/2, which can also accord with a Hall–Petch-type relationship; the relationships are illustrated in eqs (5)–(8). Therefore, with the transformation of the reticular proeutectoid ferrite to the small, dispersed and blocky proeutectoid ferrite and the decreasing of proeutectoid ferrite bulk content, the tensile strength R_m, percentage of area reduction Z, impact energy A_KU and microhardness HV of hot rolled spring steel all increased obviously, and the strength and plasticity of spring steel were both improved:

The room temperature tensile fractures of spring steels (a), (b), (c) and (d) with microstructures of different proeutectoid ferrite morphologies are shown in Figure 8. It is shown that the tensile fractures of specimens (a) and (b) with reticular proeutectoid ferrite are intergranular and cleavage fractures containing a lot of obvious tearing ridges; however, the tensile fractures of specimens (c) and (d) with small, dispersed and blocky proeutectoid ferrite are cleavage and dimple fractures.

Figure 8:

The tensile fractures of the spring steels at different cooling rates: (a) 0.5 °C/s, (b) 1 °C/s: the reticular proeutectoid ferrite; (c) 3 °C/s, (d) 5 °C/s: the blocky proeutectoid ferrite.

The room temperature impact fractures of spring steels with microstructures of different proeutectoid ferrite morphologies are shown in Figure 9. Similarly, it is shown that the impact fractures of specimens (a) and (b) with reticular proeutectoid ferrite are intergranular fractures containing more tearing ridges that are more pronounced than that of corresponding tensile fractures; however, the impact fractures of specimens (c) and (d) with small, dispersed and blocky proeutectoid ferrite are cleavage and more dimple fractures that are smaller and deeper than that of corresponding tensile fractures.

Figure 9:

The impact fractures of the spring steels at different cooling rates: (a) 0.5 °C/s, (b) 1 °C/s: the reticular proeutectoid ferrite; (c) 3 °C/s, (d) 5 °C/s: the blocky proeutectoid ferrite.

The differences of tensile and impact fractures of spring steels with different proeutectoid ferrite morphologies can be analyzed that the reticular proeutectoid ferrite forms along the prior austenite grain boundary of hot rolled spring steel; its strength is much lower than that of sorbite around, so the small crack can occur in the weak link of grain boundary by the external force at first, then expand and spread quickly, at last, the spring steel breaks down, and these make the tensile strength and percentage of area reduction of spring steel to both decrease. While, with the formation of small, dispersed and blocky proeutectoid ferrite, the proeutectoid ferrite bulk content and interlamellar spacing both decrease and a lot of fine sorbite can be obtained, these can avoid the intergranular fracture effectively and ensure the steady improvement of strength and plasticity of spring steel [18].