Effect of bainite on the mechanical properties of dual-phase steels

Abstract

The bainite-martensite-ferrite steels (tri-phase steels) were made in the laboratory by intercritical annealing, bainite transformation and oil quenching in sequence. With bainite inclusions, ductility was improved substantially without significant reduction of tensile strength. The ductility increase was found to be due to large deformation after necking and increased workhardening.EinfluB des Bainits auf die mechanischen Eigenschaften von Dualphasen-Stiihlen. Bainitisch-martensitisch-ferritische Stahle (Dreiphasen Stahle) wurden durch interkritisches GIGhen im Zweiphasen-Gebiet (ct+y) mit nachfolgender isothermer Teilumwandlung in der Bainitstufe und abschlieBendem Abschrecken der Proben im Olbad erzeugt. Steigende Gehalte an Bainit in der zweiten Phase fGhren zu einer hoheren Zahigkeit, ohne daB die Zugfestigkeit wesentlich abgesenkt wird. Die verbesserte Zahiqkeit ist zuruckzufuhren auf eine erhohte Verfestigung und ein hoheres Einschnurvermoqen. Table 1. Chemical composition (wt.-%) of the steel investigated Tafel 1. Chemische Zusammensetzung (Gew.-%) des untersuchten StahlsDual-phase steels consist of martensite particles dispersed in ferrite matrix. They can be produced either by intercritical annealing or controlled rolling. The cooling rate in controlled rolling processes tends to become slower as the steel temperature approaches bainite transformation temperature. Very often, therefore, bainite is included in as-rolled dualphase steels. When bainite was purposely introduced by adjusting the cooling schedule, ductility and fatigue strength were reported to improve while specimens retained some characteristics of dual-phase steels I).In this paper the effect of bainite inclusion on strength, ductility and work-hardening behaviour of dual-phase steels will be reported from investigation results with laboratory prepared steels. Experimental procedure C 0.057 a) Mn 1.38 Si 1.51 Cr 1.12The steel composition is given in table 1. A laboratory melted ingot was forged before being hot drawn to 12 mm diameter bars. The temperature range of bainite-martensite transformation was identified by use of a time-tern peraturetransformation diagram as between 600 and 400°C. Cylindrical standard tensile specimens (B 8 x 40) were machined from the 12 mm bars. The specimens were austenitized for 30 minutes at 900°C in nitrogen atmosphere, and then maintained at 800°C for 30 minutes in a salt bath. Then, the intercritically annealed specimens were transferred to another salt bath of 550°C. After maintaining at 550°C for varying periods of time, the specimens were quenched in a stirred oil bath, the cooling rate of which was measured as 28°C/s on an average between 650 and 350°C. The microstructure of the bainite-martensite-ferrite mixture was revealed by etching in Lel'era-solutiorr'). In this paper the bainite-martensite-ferrite mixture is refered to as "tri-phase steel". b) Figure 1. Light micrographs of tri-phase steels. LePera etch a) TP6, 16% martensite and 84% ferrite, 3 minutes at 550°C b) TP5, 8% martensite, 8% bainite...

Effect of Martensite on the Mechanical Behavior of Ferrite-Bainite Dual Phase Steels

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The microstructure, tensile response and fracture behavior of a high-performance structural steel: influence of orientation

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Ultrahigh Strength Steel: Development of Mechanical Properties Through Controlled Cooling

“…It is possible to obtain optimum combination of strength and toughness by a control process parameters of thermomechanical treatment such as slab reheating temperature, deformation temperature, deformation per pass, cooling rate, etc (Kim et al, 1987). In the present study, it was attempted to optimise some of the process parameters like slab reheating temperature, deformation temperatures and the cooling rate of the cooling medium, etc which are discussed in the following section.…”