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Role of Prior Martensite in a 2.0 GPa Multiple Phase Steel
Author(s) -
Cui Xixi,
Northwood Derek O.,
Liu Cheng
Publication year - 2018
Publication title -
steel research international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.603
H-Index - 49
eISSN - 1869-344X
pISSN - 1611-3683
DOI - 10.1002/srin.201800207
Subject(s) - martensite , austenite , materials science , isothermal process , quenching (fluorescence) , bainite , ferrite (magnet) , ultimate tensile strength , microstructure , metallurgy , phase (matter) , isothermal transformation diagram , composite material , thermodynamics , chemistry , organic chemistry , physics , quantum mechanics , fluorescence
A 55Mn2SiCr steel is developed by a novel multiple‐step process, which involves austenitizing at 900 °C for 30 min, rapid quenching to 210 °C, then holding at 170 °C for 5 min, and isothermally holding at 250 °C for different times, and finally cooling in air. The mixed microstructure consists of lenticular prior martensite (PM), fine needle bainitic ferrite (BF), and filmy retained austenite (RA). The results show that the highest tensile strength of 2030 MPa with a bending strength of 4000 MPa is achieved at 250 °C for 120 min. This is attributed to a synergistic multi‐phase strengthening effect. The presence of martensite formed during the quenching process prior to the isothermal treatment, accelerates the kinetics of subsequent nano‐scaled super bainitic transformation by bainitic laths nucleating quickly at the martensite‐austenite interfaces. The product of austenite fraction and its carbon content is found to be another important factor for controlling the strength. In addition, the phase evolution as well as carbon partitioning mechanism during isothermal treatment is discussed.