MICROSTRUCTURAL EVOLUTION OF THE NORMALIZING PLATE ROLLING OF NIOBIUM MICROALLOYED STEELS

Antonio Augusto Gorni,Marcos Roberto Soares Da Silva

doi:10.4322/2176-1523.0750

Abstract

There are concerns about the use of Nb microalloyed steels in the normalizing rolling of plate steels, as this thermomechanical treatment is finished at a temperature range where Nb(CN) precipitation can promote partial recrystallization of austenite. The aim of this work was to verify the effectiveness of Nb in retarding austenite recrystallization and to determine conditions under which Nb microalloyed steels can be processed through normalizing rolling fulfilling, as much as possible, the metallurgical recommendations proposed to this thermomechanical treatment, mainly the full austenite recrystallization between consecutive rolling passes.

Highlights

Some grades of structural steel plate are supplied in the normalized condition
The first thing to do was to determine the microstructural evolution of a plain C-Mn steel submitted to the normalizing rolling process, a case where there is no restriction of austenite recrystallization due to precipitation effects
This situation applies to the C-Mn steels processed by this route, but for Nb microalloyed steels, where austenite recrystallization is strongly retarded by Nb in solution and totally suppressed as NbCN precipitation begins

Summary

Introduction

Some grades of structural steel plate are supplied in the normalized condition. In this case, the rolled product is austenitized in a furnace and allowed to cool in still air. The continuous need for process rationalization and cost reduction paved the way for the production of normalized plates directly from the rolling heat. This is possible through the use of a thermomechanical treatment called normalizing rolling, a variant of the recrystallization controlled rolling. The ideal normalizing rolling process is characterized by full recrystallization between hot rolling passes and a finishing temperature that is equal to the austenitizing temperature originally used in the heat treating furnace. The final aim is a ferrite-pearlite microstructure with fine and homogeneous grain size

Objectives

Methods

Results

Conclusion