Strain-induced precipitation of NbC in Nb and Nb–Ti microalloyed HSLA steels

doi:10.1016/S1359-6462(01)01214-3

Scripta Materialia

Volume 46, Issue 2, 18 January 2002, Pages 163-168

https://doi.org/10.1016/S1359-6462(01)01214-3 Get rights and content

Abstract

The precipitation start time (P_s) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel. The delay of precipitation of strain-induced NbC carbides is attributed to both the insufficient solution of Nb during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides.

Introduction

It is well established that the precipitation of carbides or carbonitrides in microalloyed steels containing one or more of the transition metals, such as Nb, Ti and V, plays a critical role in determining the properties of HSLA steels. During the last two decades, most of investigations for the precipitation behavior have been concerned about Nb, Ti, Nb–V, and Nb–Mo steels [1], [2], [3], [4], [5]. In contrast, in the Nb steel containing Ti, which is often added to enhance the toughness of HSLA steels, the precipitation behavior has not been well understood. Although it has been found that the Ti-rich carbonitrides can influence the precipitation behavior of NbC carbides [6], systematic investigation and/or understanding of the precipitation behavior in Nb–Ti steel has not been well established yet. The purpose of the present study is, thus, to identify precipitation behavior in Nb–Ti microalloyed steels, based on both the systematic hot deformation simulation experiments, high resolution electron microscopy, and chemical analyses of carbonitrides.

Section snippets

Experimental procedure

The chemical composition of the Nb and Nb–Ti steels examined in this study is given in Table 1. The specimens were prepared by vacuum induction melting to ingots, homogenized at 1250 °C for 2 h, and hot rolled to about 20 mm thick plate. The cylindrical specimens (Ø 10 mm × 12 mm) were then machined from hot rolled plate. In order to investigate the effect of Ti addition on the precipitation kinetics of NbC, two-stage interrupted compression tests were conducted using a hot deformation

Softening kinetics and precipitation-time-temperature diagrams

The softening parameter (X_y or X_A ) is usually obtained by detecting a change in either yield strength (X_y) or area under flow curve (X_A), as shown in Fig. 2.

Since the measurement of yield and flow strength is, generally, not very sensitive to a subtle change in precipitation, the area measurement method, which is more sensitive to the strain energy change due to precipitation, is adopted in the present study. Precipitation-time-temperature (PTT) diagrams were consequently obtained by using the

Conclusion

1.
PTT diagrams were obtained by measuring softening ratio at high temperatures using two-stage interrupted compression tests.
2.
The precipitation start time (P_s) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel.
3.
In the Nb–Ti steel, the supersaturation of Nb in austenite was reduced by both the insufficient solution of Nb due to stable (Ti,Nb)(C,N) carbonitrides remained during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides on

Acknowledgments

The authors thank Pohang Iron and Steel Co. (POSCO) for the financial support (1PL0190201).

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