Improved Model for Static Recrystallization Kinetics of Hot Deformed Austenite in Low Alloy and Nb/V Microalloyed Steels.

Abstract

Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (V, Nb) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol Ϫ1 , which is similar to other values found in the literature for the grain boundary self-diffusion energy of pure Fe g . Static recrystallization kinetics follow Avrami's law and expressions are given for the parameter t 0.5 and the exponent n.KEY WORDS: austenite; static recrystallization; low alloy steels; microalloyed steels.compositions are shown in Table 1. The twenty-six steels include different combinations of C, N and metallic precipitate-forming elements such as V and Nb. Given that the nitrides, carbides or carbonitrides of Nb are less soluble in austenite than those of V, the limit imposed on C and N contents has been that the solubility temperature should not exceed 1 300°C. In this sense, some compositions, such as steel N9, with a very low Nb content and high C content, are not currently standard compositions, but the interest in studying them lies in ascertaining the influence of low Nb contents on recrystallization.Torsion specimens were prepared with a gauge length of 50 and 6 mm diameter. The reheating temperature prior to torsion deformation was different depending on whether the steel was microalloyed with V or with Nb (Table 1), as the solubility temperature of the precipitates depends on their nature and on the precipitate-forming element content. For steels containing vanadium, designated by the letter V, the reheating temperature was 1 230°C for steels V1, V2 and V3 and 1 200°C for the rest, which is sufficient to dissolve vanadium nitrides and carbides. In the case of niobium steels, designated by the letter N, the reheating temperature depended on the C, Nb and N contents, but was always above the solubility temperature of niobium carbonitrides. 17)All the C-Mn/Si steels (C1, C2, C3, M1, M2, S1, S2), C-Mo steels (Mo1, Mo2) and the microalloyed steels V1, V2, V3, N1 and N2 have been studied previously 14,15) in relation with static recrystallization, but their results are taken into account in order to achieve greater universalization of the model proposed in this work.To ensure that the testing temperatures corresponded to the austenitic phase, critical transformation temperatures were measured by d...

The Static and Metadynamic Recrystallization Behaviour of an X60 Nb Microalloyed Steel

“…(1) that is an estimate for a range of steels, 12) it was predicted that the time to 50 % softening was 2 s. By confirming this value (in some cases we confirm the new value in others we use an approximated value for the next step) the time to 50 % softening at the next strain of 0.6 was estimated to be between 5 and 10 s. This estimate was based on a guess that the strain exponent in Eq. (2) was between Ϫ2 and Ϫ3.…”

“…(2) was between Ϫ2 and Ϫ3. 12,13) Once the new value was obtained a plot of log t 0.5 versus log was used to join the two data points and a linear fit assumed to provide estimates to t 0.5 at lower strains. This process was continued to obtain Fig.…”

From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

“…After the plateau, the superiority of driving forces for recrystallisation over pinning forces is about two orders of magnitude [6]. In Nb-microalloyed steels the Nb atoms/NbCN precipitates retard the recrystallisation of deformed austenite [7][8][9][10]. However, V-microalloyed steels also exhibit grain refinement through intragranular nucleation of ferrite on VN precipitates partly due to low lattice mismatch of VN with ferrite [11,12].…”

“…Bearing in mind that the nose of the P s curve corresponds approximately to a 50% recrystallised fraction [9], the dislocation density corresponding to the curve nose will be given by 10 12 +0.5 (Table 3). Table 4.…”

New form of equation for deformation resistance prediction under hot rolling industrial conditions

“…Of those probably the most powerful and promising, but at the same time relatively simple, for investigating steel and predicting its behavior under hot rolling service conditions, are the torsion plastometer test [1][2][3][4][5] and the compression plastometer test, utilizing either ordinary compression [6][7][8] or stress relaxation methods [9][10][11].…”