Austenite Recrystallization–Precipitation Interaction in Niobium Microalloyed Steels

Abstract

A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at lower temperatures by solute drag or by precipitation pinning. In this study, the recrystallization behavior of four Nb-microalloyed model alloys which were designed to show either extensive or almost no precipitation, was compared by multi-hit torsion tests and double hit compression tests. A good consistency between the different types of tests was found and the results were verified by optical micrographs. Further, by construction of softeningtime-temperature diagrams the recrystallization behavior was linked to the precipitation state of the material which was investigated by thermodynamical equilibrium calculations and by experimental observations from TEM-EDX, Inductively Coupled Plasma Mass Spectroscopy and X-ray Diffraction. Quantitative agreement between the experimental measurements and the calculations for precipitated mass fraction and precipitate composition as a function of temperature and steel composition is demonstrated.KEY WORDS: recrystallization; microalloyed steels; precipitation; solute drag. 911© 2009 ISIJ tions with experimental data from Craven et al. 13)The objective of the present paper is to separate both retarding mechanisms, i.e. solute drag and precipitation pinning, by investigating model alloys designed to show either extensive or almost no precipitation. Moreover, the recrystallization kinetics of four Nb-microalloyed steels during hot deformation is linked to the morphology and composition of the precipitates and the amount of Nb-solutes found in these materials. The recrystallization kinetics were investigated combining different hot deformation testing techniques while information on the precipitation state of the material was obtained from thermodynamic equilibrium calculations and from a combination of experimental observation techniques. With this work, a contribution to the understanding of the fundamental mechanisms responsible for the retardation of austenite recrystallization in Nb-microalloyed steels is achieved. Experimental ProcedureFour model alloys were designed and casted as 100 kg ingots in a Pfeiffer vacuum furnace operated under argon gas atmosphere. The chemical composition of these alloys can be found in Table 1. The C-Mn-reference alloy, without additional Nb, represents a reference steel. The second alloy, a lowC-Mn-Nb alloy containing only a few ppm C, allowed to study the effect of Nb in solid solution. The third alloy (C-Mn-Nb) was designed to study the effect of NbC-precipitates on the recrystallization kinetics. To have the highest fraction of NbC precipitates possible, a stoichiometric Nb/C-ratio of 8/1 was chosen. Finally, the fourth alloy (C-Mn-Nb-N) was designed to study the influence of N on the recrystallization and precipitation behavior. The cast blocks were thermomechanically processed under conditions comparable to those during industrial steel plate rolli...

Control of the Austenite Recrystallization in Niobium Microalloyed Steels

“…In that way, a stoichiometry of NbC 0.25 N 0.75 was found from the X-ray spectrum. The found stoichiometries of the precipitates are in good agreement with those calculated under equilibrium conditions [9]. This confirms the statement of Pandit et al [11] who declared that deformation will greatly enhance the kinetics of precipitation but that the nature and the chemical composition of the precipitates is expected to remain the same as in the case without deformation.…”

“…In order to quantify the amount of softening between these extremes, the 2% offset method was used. With that method the softening effects by recovery can be neglected [8] and consequently the calculated softening fraction is linearly related to the statically recrystallized volume fraction. The evolution of the recrystallized fractions with time for all these materials is displayed in Figure 1, showing in all cases that decreasing the temperature slows down the recrystallization kinetics.…”

“…At temperatures of 1075°C and 1050°C, the amount of Nb precipitated in the C-Mn-Nb-N alloy was remarkably higher than the amount of Nb precipitated in the C-Mn-Nb alloy. Thermodynamic equilibrium calculations [9] ascribed the higher fraction of Nb precipitated in the C-Mn-Nb-N alloy to the formation of NbN and/or NbCN.…”

Effect of deformation temperature on niobium clustering, precipitation and austenite recrystallisation in a Nb–Ti microalloyed steel

“…%) has shown an increase in the recrystallisation stop temperature, T nr , indicating a retardation of recrystallisation as the Nb content of the steel increases [2]. The Nb(C,N) precipitates were shown to pin grain boundaries more effectively than the Nb solute atoms [3,4]. The maximum pinning force was reported for precipitates sized between 1 and 20 nm [5][6][7].…”

Thermomechanical controlled processing to achieve very fine grains in the ISO 5832-9 austenitic stainless steel biomaterial

“…7e) and 20 s (Fig. 7f) small when compared to grain sizes obtained for niobium-microalloyed steels and austenitic stainless steels without precipitates under the same soaking conditions [25][26][27][28]. The precipitation analysis showed that only TiNbN particles with large sizes remained insoluble after soaking.…”