The effect of Nb on recrystallization behavior of a Nb micro-alloyed steel

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Abstract

By the analysis of true stress–strain curves and the observation of microstructures at different deformation stages, the dynamic recrystallization (DRX) of a Nb micro-alloyed steel and a plain carbon steel is studied, and the critical stress and critical strain are determined under different deformation conditions. By the method of stress relaxation, the post-dynamic recrystallization of two steels is studied. The energy to induce recrystallization of austenite of Nb steel is higher than that of the plain carbon steel. The critical stress of DRX for Nb steel is higher than that for the carbon steel. The apparent activation energy of static recrystallization Qsrx of Nb steel and the plain carbon steel is 318 and 231 kJ/mol, respectively. It is obvious that Nb in solid solution increases the value of apparent activation energy (Qsrx) dramatically. The precipitation and recrystallization are in competition because of stored deformation energy. The work shows that the ratios of ɛp to ɛc and that of ɛ* to ɛc are 1.6 and 2.2, respectively, for the steels.

Introduction

With the development of Nb micro-alloying technology, the element of Nb is used widely in the production of new III grade ribbed bar to improve the strength of steels. It is well known that the recrystallization of plain carbon steel is influenced by the addition of microquantity of Nb. The retardation of recrystallization is caused by the solute drag effect of Nb and the pinning effect of fine precipitates such as NbC. By the retardation of recrystallization, strain can be accumulated and the microstructure can be refined after phase transformation [1].

The peak in the stress–strain curve is the major criterion for the occurrence of dynamic recrystallization (DRX). The peak represents the point where work hardening and dynamic softening are in balance. A method to determine the critical strain of DRX based on the thermodynamics of irreversible process was developed by Poliak and Jonas [2]. The onset of DRX can be identified by an inflection point in the θσ plot (θ = dσ/dɛ), corresponding to the appearance of an additional thermodynamic degree of freedom in the system. The method has been confirmed for many kinds of steels by some researchers [3], [4].

The kinetics of post-dynamic recrystallization of austenite can be described by an equation of Avrami in the way:X=1exp0.693tt0.5nwhere X is the fraction of recrystallized volume and t0.5 is the time corresponding to half of recrystallized volume, which depends on the deformation conditions and original austenite microstructures and follows a law of the type:t0.5=Ad0mεpε˙qexpQrexRTwhere ɛ is the strain and ε˙ is the strain rate, d0 the grain size of austenite and Qrex is the apparent activation energy of recrystallization. Recently, the newly established technique of stress relaxation has been applied to measure the kinetics of static and metadynamic recrystallization of austenite in plain carbon steel and micro-alloyed steels [5], [6], [7], [8].

The present work aims at investigating the behaviors of DRX and post-dynamic recrystallization of Nb micro-alloyed steels and a plain carbon steel employed for reference purposes by the methods mentioned above. By comparison, the effect of Nb on recrystallization is discussed.

Section snippets

Experimental

In the present work, a plain carbon steel and Nb micro-alloyed steel were obtained from Long Steel in Shanxi Province of China. The chemical composition is shown in Table 1. The specimens were machined directly from ribbed bar. In a hot working simulator (type: Thermecmastor-Z), the compression specimens of 8 mm in diameter and 12 mm in height were heated to 1200 °C and held for 300 s. The original austenite of the two kinds of steel is very similar and the grain size is about 135 μm, shown in Fig. 1

Flow stress curves and deformation resistance

The effect of strain rate and deformation temperature on strain–stress curves of the two kinds of steels is shown in Fig. 2. The curves exhibit the peaks and then softening occurs to a steady state. As we know, stress peak indicates the occurrence of DRX. The stress is dependent on deformation conditions. The peak stress σp and peak strain ɛp increase with increasing strain rate and decreasing deformation temperature. The values of σp and ɛp of Nb steel are higher than those of plain carbon

Conclusions

In the present work, static, dynamic, metadynamic recrystallization of a Nb micro-alloyed steel and a plain carbon steel were investigated by the comparison. Nb in solid solution retards the dynamic recrystallization and increases the apparent deformation activation energy dramatically. Although the ratio of ɛc to ɛp is similar for the Nb steel and plain carbon steel, the ratio of σc/σp of former is higher than that of latter. Energy to induce recrystallization of austenite of the Nb steel is

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