Effects of Bainitic Phase on Mechanical Properties of Bainite−Aided Multiphase Steels

• Published in: Steel research international 2017-09, Vol.88 (9), p.n/a
• Main Authors: Apimonton, Chanakarn, Sungthong, Chaninee, Luksanayam, Siwaphan, Suranuntchai, Surasak, Uthaisangsuk, Vitoon
• Format: Article
• Language: English
• Subjects: Alloys; Austenite; Automobile industry; Automobiles; Automotive engineering; Automotive parts; Bainite; Bainitic transformations; Damage tolerance; Dual phase steels; Duplex stainless steels; edge–flangeability; Elongation; Ferrite; Formability; Fracture toughness; Hardness measurement; Heat treatment; High strength steels; hole expansion ratio; Mechanical properties; Metal sheets; Microhardness; microstructures; Multiphase; multiphase steels; Quenching; Retained austenite
• ISSN: 1611-3683; 1869-344X
• DOI: 10.1002/srin.201700004

Advanced high strength steels play an important role in the automotive industry. However, they still show inferior formability such as low stretch−flangeability. Therefore, in this work, various heat treatment processes are performed for generating multiphase steels with enhanced mechanical and forming behavior. The bainitic phase with different amounts is included in dual phase (DP) steels by controlling oil−quenching temperatures. In other steels, varying bainitic transformation temperatures are applied. Microstructures and phase fractions of the investigated multiphase (MP) steels are characterized by optical and scanning electron microscope. Tensile test, hole expansion test, and micro−hardness measurement are carried out for the steel sheets. It is found that increased oil temperature leads to larger amount of bainite occurred in the DP steels. The bainite−aided DP steels exhibit lower strength, but considerably higher elongation, improved hole expansion ratio, and larger damage tolerance value. In case of MP steels with retained austenite, increasing the bainitic transformation temperature causes ferrite in the steels to be replaced by bainite and retained austenite with various amounts. It seems that the MP steels with higher bainitic phase fraction show deteriorated edge–flangeability and fracture resistance. DP steels and MP steels with different bainitic fractions are generated by intercritical annealing processes. Increasing amount of bainite in DP steels provides higher strength and hole expansion ratio. In contrast, the edge–flangeability of MP steels is basically controlled by ferritic fraction. The determined damage tolerance values are well correlated with obtained hole expansion ratios.