Abstract
To apply the duplex type low-carbon medium-manganese steel to the hot/warm-forging and -stamping products, the influence of cooling process routes immediately after intercritical annealing such as air-cooling (AC) and isothermal transformation (IT) processes on the impact toughness of 0.2%C-1.5%Si-5%Mn (in mass %) duplex type medium-Mn (D-MMn) steel was investigated. Moreover the microstructural and tensile properties were also investigated. The AC process increased the volume fraction of reverted austenite but decreased the thermal and mechanical stability in the D-MMn steel, compared to the IT process. The AC process increased the tensile strength but decreased the total elongation. The Charpy V-notch impact value and ductile-brittle transition temperature were deteriorated by the AC process, compared to the IT process. This deterioration of the impact toughness was mainly related to the reverted austenite characteristics and fracture mode.
Highlights
The first, second, and third-generation advanced high strength steels (AHSSs) recently developed are expected to apply to the automotive sheet products [1,2,3] and wire-rod products [4,5,6,7]
The tensile properties and impact toughness of the duplex type medium Mn (D-MMn) steels produced with the air cooling (AC) process are not exactly compared with those subjected to the isothermal transformation (IT) process
The tensile properties and impact toughness of the D-MMn steels produ2coefd15with the AC process are not exactly compared with those subjected to the IT process
Summary
The first-, second-, and third-generation advanced high strength steels (AHSSs) recently developed are expected to apply to the automotive sheet products [1,2,3] and wire-rod products [4,5,6,7]. The D-MMn steel can achieve a higher product of tensile strength and total elongation (TS × TEl) than 30 GPa% through intercritical annealing and air cooling (AC) or quenching in a water/oil bath [16,17,18,19,20,21]. The D-MMn steel subjected to the AC process possesses a high-impact absorbed value [27,28,29,30]. Tanino et al [21] proposed that an isothermal transformation (IT) process immediately after intercritical annealing enhances further the TS × TEl and impact absorbed value through the TRIP effect or strain-induced martensite transformation (SIMT) hardening of a large amount of metastable reverted austenite in 0.2%C-1.5%Si-5%Mn (in mass %) D-MMn steel. The tensile properties and impact toughness of the D-MMn steels produced with the AC process are not exactly compared with those subjected to the IT process
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