Role of Vanadium Microalloying in Austenite Conditioning and Pearlite Microstructure in Thermomechanically Processed Eutectoid Steels

Abstract

Multipass torsion tests were carried out with two eutectoid steels, one microalloyed with vanadium, using different deformation sequences. The aim of the study was to investigate the potency of vanadium in the retardation of recrystallization for accumulating strain in the austenite. The study showed that at certain deformation conditions well defined non-recrystallization temperatures (T nr ) were observed in the vanadium microalloyed steel. As a consequence, an increase in the austenite grain boundary area per unit volume (S V ) was obtained which led to a refinement of the "ferrite unit" size in the pearlite.KEY WORDS: eutectoid steel; vanadium microalloying; austenite conditioning. 1615© 2009 ISIJ proved toughness.Nevertheless, there is a lack of knowledge concerning the potential use of vanadium for the accumulation of strain in eutectoid steels and the optimum deformation conditions to achieve this. This paper attempts to analyze the potency of vanadium to accumulate strain in the austenite through the application of thermomechanical treatments. Experimental ProcedureTwo eutectoid steels were considered for this study: a plain carbon one and another microalloyed with vanadium, both having the same base composition. The C-Mn steel was commercially produced in bar shape, while the C-Mn-V was cast in the laboratory by adding vanadium after remelting the above C-Mn steel. Afterwards the resulting slabs were hot worked to square ingots. The compositions of the materials are indicated in Table 1.Multipass torsion tests were carried out to determine the non-recrystallization temperature (T nr ). Specimens, having a gauge length of lϭ12 mm with a diameter of 2rϭ6 mm, were reheated for 15 min at 1 200°C. After reheating the samples were deformed applying 24 passes at decreasing temperature in the range 1 150-800°C. The measured torque G and the twist q were converted to von Mises equivalent stress (s) and strain (e) with the help of these equations:.................... (1) The tests were performed using different pass-strains: eϭ 0.2-0.3-0.4. The strain-rate used in all the tests was e˙ϭ 1.5 s Ϫ1, and the interpass time at each deformation stage was 10 s. During deformation a cooling rate of 1.5°C/s was applied. In a particular test, the strain per pass, strain-rate and interpass time were held constant. The values of the T nr were calculated by using the method proposed by Bai et al.,12) from the graphic representation of the mean flow stress (MFS) versus the inverse absolute temperature for each one of the passes.The initial austenite grain size prior to deformation was determined in specimens directly quenched after reheating treatment. In most tests, the specimens were also directly quenched after deformation for the analysis of the austenite microstructure. Some additional tests were carried out applying natural cooling (ϳ3°C/s) after deformation for the study of the pearlite microstructure.The metallographic observation of the samples was done on a surface parallel to the torsion axis at a distance close to ...

Influence of Thermomechanical Processing on the Austenite–Pearlite Transformation in High Carbon Vanadium Microalloyed Steels

“…4, Table 3, some previous results obtained for the plain C-Mn2 steel 12) in Table 1 and those obtained with steel 11VN in Ref. 13), the following expression was obtained for the f treat term of Eq. (5): ....... (8) where CR is in °C/s and S V in mm…”

“…In this figure two additional values obtained for steel 11VN in a previous work were included because they provided data in the range of larger S V values and thus could be helpful to improve the fitting. 13) From the data shown in Fig. 12, it is observed that as S V increases the microstructural D p unit size decreases.…”

“…As a result, it is necessary to determine the amount of VN precipitated in austenite during rolling in order to properly evaluate the strengthening contribution of V. Based on the analysis done in Ref. 13) the first hypothesis was that 30 % of the total possible equilibrium fraction was required to stop recrystallization and accumulate strain when the T nr temperature was reached. Applying this approach to the tests corresponding to schedule B, it was estimated that less than 10 % of the total nominal vanadium would be precipitated in austenite for the three steels.…”

“…13) As occurs in low carbon steels, the accumulation of strain produces a significant refinement in the austenite, this leads to high values in the specific grain boundary area (S V ) which may produce, after transformation, a refinement in the "ferrite unit" in pearlite. In the aforementioned paper the work was mainly focused on analyzing the relevance of VN precipitation on austenite conditioning and the subsequent influence on the size of "ferrite units" obtained after transformation compared with a plain C-Mn steel.…”

“…The procedure followed was similar to that described in Ref. 13). Figure 1 shows the stress-strain curves and the corresponding austenite microstructures obtained after the application of the different schedules described in Table 2 with steels 5V and 11VN.…”