Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel.

Mohammad Nishat Akhtar,Asif Afzal,Mohammad Mursaleen Butt,Abdul Rahim Othman,Muneer Khan,Elmi Abu Bakar,Sheikh Nazir Ahmad,Ram Subbiah,Sher Afghan Khan,Murtuja Husain

doi:10.3390/ma14102639

Abstract

In the present investigation, the non-recrystallization temperature (TNR) of niobium-microalloyed steel is determined to plan rolling schedules for obtaining the desired properties of steel. The value of TNR is based on both alloying elements and deformation parameters. In the literature, TNR equations have been developed and utilized. However, each equation has certain limitations which constrain its applicability. This study was completed using laboratory-grade low-carbon Nb-microalloyed steels designed to meet the API X-70 specification. Nb- microalloyed steel is processed by the melting and casting process, and the composition is found by optical emission spectroscopy (OES). Multiple-hit deformation tests were carried out on a Gleeble® 3500 system in the standard pocket-jaw configuration to determine TNR. Cuboidal specimens (10 (L) × 20 (W) × 20 (T) mm3) were taken for compression test (multiple-hit deformation tests) in gleeble. Microstructure evolutions were carried out by using OM (optical microscopy) and SEM (scanning electron microscopy). The value of TNR determined for 0.1 wt.% niobium bearing microalloyed steel is ~ 951 °C. Nb- microalloyed steel rolled at TNR produce partially recrystallized grain with ferrite nucleation. Hence, to verify the TNR value, a rolling process is applied with the finishing rolling temperature near TNR (~951 °C). The microstructure is also revealed in the pancake shape, which confirms TNR.

Highlights

Microalloyed (MA) steels have become increasingly popular, in the plate and pipeline steel applications where larger diameter pipes are being developed with the need for increased strength, formability, and joinability [1,2]
In combination with processing parameters, small additions of V, Nb, and Ti to HSLA steels are designed to achieve higher strength while minimizing required plate thicknesses for the desired application
Nb added to the MA steel composition produces carbonitride and niobium carbide in the austenite region, which retards the recrystallization of the austenite due to increasing the austenite’s non-recrystallization temperature (TNR )

Summary

Introduction

Microalloyed (MA) steels have become increasingly popular, in the plate and pipeline steel applications where larger diameter pipes are being developed with the need for increased strength, formability, and joinability [1,2]. In combination with processing parameters, small additions of V (vanadium), Nb (niobium), and Ti (titanium) to HSLA (high strength low alloy) steels are designed to achieve higher strength while minimizing required plate thicknesses for the desired application. Bauer et al [5] discussed that the most common mechanical properties required for optimum pipeline performance are high strength and high toughness. Sophisticated processing routes as TMCP (thermomechanical controlled processing) allow achieving the skelp’s desired property levels by optimizing the final microstructure and crystallographic texture [5]. The steel’s critical parameters to obtain the anticipated mechanical properties are the final crystallographic texture and microstructure. The crystallographic texture, grain shape, and grain size distribution of a finished skelp are the result of a sequence of events occurring during the thermomechanical processing of the steel [4]

Objectives

Results

Conclusion