Effect of initial microstructures on the properties of Ferrite-Martensite Dual-Phase pipeline steels with Strain-Based design

Yueyue Hu,Rutao Li,Zhanzhan Zhang,Xiurong Zuo

doi:10.1590/s1516-14392012005000021

Abstract

This study aims to investigate the effect of initial microstructures on the properties of ferrite-martensite dual-phase pipeline steels with strain-based design. For this purpose, the as-received acicular ferrite steels were first austenitized at 920 ºC for 15 minutes followed by air cooling and water quenching to produce ferrite-pearlite and ferrite-martensite microstructure, respectively. Subsequently, the steels with ferrite-pearlite, ferrite-martensite and as-received acicular ferrite microstructure were intercritically annealed at 820 ºC for 10 minutes followed by water quenching to produce three different ferrite-martensite dual-phase microstructures. Tensile tests, Vickers hardness and Charpy impact tests were carried out to investigate the mechanical properties. Scanning electron microscope was used to analyze the microstructures and tensile fractographs. The results showed that all the tensile specimens of these three different ferrite-martensite dual-phase steels fractured in ductile mode, however, their microstructures and mechanical properties varied significantly. By contrast, the ferrite-martensite dual-phase steel derived from acicular ferrite initial microstructure had optimal combination of the strength, toughness and deformability, which provided a good candidate for the pipeline steels with strain-based design used in severe geological environments.

Highlights

Steels composed of hard-phase martensite or bainite imbedded in a soft-matrix of ferrite are usually referred to as dual-phase steels[1,2,3,4], which are widely used in the automobile industry to reduce weight and save fuel[5,6]
This kind of combination makes them possess continuous yielding, low yield strength, high tensile strength, high initial work-hardening rates, superior uniform and total elongation compared to other high-strength low alloy (HSLA) steels at a given strength level[7,8,9,10]
With an aim to investigate the effect of initial microstructures before intercritical annealing on the properties of the ferrite-martensite dual-phase (FMDP) steels, three different heat-treatment schedules were used: a) hold at 920 °C for 15 minutes, followed by air cooling, reheat to 820 °C for 10 minutes, and water quench; b) hold at 920 °C for 15 minutes, followed by water quenching, reheat to 820 °C for 10 minutes, and water quench; and c) hold at 820 °C for 10 minutes, and water quench

Summary

Introduction

Steels composed of hard-phase martensite or bainite imbedded in a soft-matrix of ferrite are usually referred to as dual-phase steels[1,2,3,4], which are widely used in the automobile industry to reduce weight and save fuel[5,6]. This kind of combination makes them possess continuous yielding, low yield strength, high tensile strength, high initial work-hardening rates, superior uniform and total elongation compared to other high-strength low alloy (HSLA) steels at a given strength level[7,8,9,10]. The acicular ferrite microstructure is widely used in high grade pipeline steels which are based on the stress design to offer

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Conclusion