Effect of Mo on Dynamic Recrystallization Behavior of Nb-Mo Microalloyed Steels

Abstract

The influence of the Mo addition on the dynamic recrystallization behavior of Nb microalloyed steels was studied. The initial austenite grain size, the amount of microalloying elements in solid solution and the deformation conditions (temperature and strain rate) affect dynamic recrystallization kinetics of both Nb and Nb-Mo steels. Continuous torsion tests were carried out to characterize the dynamic recrystallization behavior of the microalloyed austenite, after reheating the specimens at different temperatures between 1 100 and 1 460°C, this brought a wide range of initial grain sizes, from 22 to 805 mm. It was observed that decreasing the values of the Zener-Hollomon parameter and of the initial grain size promotes dynamic recrystallization. Mo in solid solution produces a large retardation effect in the dynamic recrystallization and brings higher values in the characteristic critical, e c , and peak, e p , strains, being this effect independent of the Mo content. A corrective factor has been applied to quantify the retardation produced by Mo in solid solution. This means that it was possible to propose a unique relationship to predict the peak strain for Nb, Nb-Ti and Nb-Mo steels.KEY WORDS: dynamic recrystallization; hot direct rolling; Nb and Nb-Mo microalloyed steels.ISIJ International, Vol. 47 (2007), No. 6, solute effect on the dynamic recrystallization kinetics leading to a significant retardation in the process. [1][2][3][4] However, this effect has not been yet quantified. This is an important issue, taking into account that any retardation of dynamic recrystallization could affect the design of the appropriate thin slab rolling sequences, above all for the initial stands, so that a correct refinement of the as-cast austenite can be achieved before niobium precipitation occurs. 19) This paper studied the DRX events for those steels containing Nb and Mo at the same time. A study was carried out to determine the effect of the initial austenite grain size, within a wide range of grain sizes, and the deformation conditions on DRX kinetics. Material and Experimental ProcedureFour Nb-Mo microalloyed steels were studied and the results compared to a Nb microalloyed steel. A 0.035 % Nb microalloyed base steel and two levels of Mo, 0.15 and 0.31, were considered. All the Nb-Mo steels were experimental casts. The chemical compositions of the steels studied are listed in Table 1. The results obtained with the Nb steel were described in detail in a previous work 18) but they will be used here for assessing the individual and combined effects. Isothermal torsion tests were performed after soaking the specimen at different temperatures in the range of 1 100-1 460°C for 15 min in an argon atmosphere to analyze the DRX behavior. Torsion samples were machined from 40 mm thickness plates in the case of Nb, Nb-Mo31b and Nb-Mo16 steels and from 40 mm diameter bars in the case of Nb-Mo31a and Nb-Mo15 steels, respectively. Coarse austenite microstructures (Ϸ600-800 mm) were produced by reheating at high temperatures, in ...

Effect of Mo Concentration on Dynamic Recrystallization Behavior of Low Carbon Microalloyed Steels

“…The temperature effect seemed to be more marked at the higher strain rates. The measured values of 0.5−1.1 are in line with literature data . Note that at 950 and 1000°C, peak strain was not reached at the higher strain rate.…”

“…Note that at the higher strain rate, peak stress was not reached for temperatures below 1050°C; see also flow stress curves in Figure . The measured values of 80−170 MPa are in the same range as those reported by others for steels of a similar composition.…”

“…The average value of ϵ c / ϵ p over all the tests where peak strain was reached was 0.42 ± 0.13 (mean ± SD), which is in the range of reported value for similar steels of 0.4−0.8, and the model from Siciliano and Jonas, which predicts 0.42−0.49 for the investigated compositions.…”

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

“…The most commonly used microalloying elements in steels, e.g. in linepipe steels, are Nb, V and Ti [10,13,14]. These microalloying elements can retard the recrystallization of austenite either by solute drag or by combining with C and N to form precipitates of carbide, nitride or carbonitrides [15].…”

“…These microalloying elements can retard the recrystallization of austenite either by solute drag or by combining with C and N to form precipitates of carbide, nitride or carbonitrides [15]. These fine precipitates play an important role in recrystallization mechanisms during and after hot deformation [10,13,16] to control the final microstructure and hence the product properties. Therefore, to examine the effect of precipitation and/or solute drag on the grain refinement, it becomes an imperative to heat the alloy to a high temperature so as to take all or most of the alloying elements into the austenite solid solution and allow precipitation during the process.…”