Microstructure and properties of ultrafine grained low carbon steel with different initial structure by cryorolling
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Date
2019-07
Authors
Lew Mei Pei
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Abstract
Common low strength steel cannot meet the requirements of most manufacturing and production industry in modern society. In recent years, ultrafine-grained structure steel with grain size below 1 μm has shown the prospect of high strength and toughness. The aim of this research work is to investigate the effect of different initial structure of low carbon steel on the formation of ultrafine grained and their properties after processed by cryorolling. There are two initial microstructures employed to fabricate UFG low carbon steel; ferrite-pearlite (annealed at 550°C with 60 minutes followed by furnace cooling) and ferrite-martensite (annealed at 750°C with 10 minutes followed by ice water quenching). Then, the two different starting microstructures underwent cryorolling process with 70% thickness reduction. Prior to cryorolling, the samples were dipped in liquid nitrogen for 10 minutes. After cryorolling, all samples were characterized by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), Vicker microhardness test, tensile test, potentiodynamic polarization, electrochemical impedance spectroscopy and immersion test. Comparing as received and annealed sample, sample annealed at 750°C has the highest grain aspect ratio (2.2), smallest crystallite size (43.6 nm), highest hardness (123.18 HV), highest UTS (534.91 MPa), highest YS (413.92 MPa) and lowest elongation (23.48%). The cryorolled sample with initial structure of ferrite and martensite has the highest grain aspect ratio (12.86), highest hardness (157.04 HV), highest UTS (927.36 MPa), highest YS (920.05 MPa), smallest crystallite size (4.8 nm) and highest elongation (8.08%) compared with sample with ferrite and pearlite structure. Moreover, sample with initial structure of ferrite and martensite also has the highest corrosion resistance compared with cryorolled sample with ferrite and pearlite structure.