Evaluation of hole expansion ratio of ultra-high strength martensitic steels produced with various processing routes
Hannula, J.; Plosila, P.; Kaijalainen, A.; Suikkanen, P.; Kömi, J. (2023-06-22)
J. Hannula et al 2023 IOP Conf. Ser.: Mater. Sci. Eng. 1284 012036, https://doi.org/10.1088/1757-899X/1284/1/012036
© The Author(s) 2023. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
https://creativecommons.org/licenses/by/3.0/
https://urn.fi/URN:NBN:fi-fe2023062759333
Tiivistelmä
Abstract
Hole expansion ratio of hot rolled ultra-high strength martensitic steels with tensile strength around 1000 MPa was evaluated. Steels were produced with direct quenching and traditional reheat and quenching processes with final thickness of 3 mm. Achieved yield strength values (Rp0.2) varied between 918–1068 MPa depending on the processing route. Mean hole expansion ratios (HER) in direct-quenched (DQ) and direct-quenched and tempered (DQT) conditions were between 22–36 %, and no clear improvement was seen after tempering treatment compared to quenched variant. HER values for reheat and quenched (RQ) and reheat, quenched and tempered (RQT) variants were between 31–49 %, and the highest values were achieved with RQT steels, which were tempered at 600 °C. Based on the field emission scanning electron microscope with electron backscatter scanning diffraction (FESEM-EBSD) analysis/characterization of martensite grain size, more uniform grain structure was discovered in RQ steels, which could be the reason for improved HER properties. HER values were also compared to tensile test results (uniform elongation, true thickness strain), and formability maps were constructed. However, only minor correlation was found between HER and true thickness strain values.
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