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Effect of decarburization on the bendability of martensitic steel, The
Schools, Jordan
Schools, Jordan
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2022
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Abstract
The use of advanced high strength steels (AHSS) has grown due to the necessity of lightweighting automotive vehicles. Certain grades that are used in anti-intrusion and other parts in vehicles are martensitic steels due to their high strength, allowing for thinner gauges and therefore lighter parts. In the case of an automobile collision or when roll formed, martensitic steels are often subject to bending. Therefore, improved bendability could allow for more effective application of martensitic steels. This study determined the effect of decarburization on the bendability of cold rolled martensitic (CRM) 1500 and press hardenable steel (PHS) 2000, with the numeric designation of each grade representing the minimum as received (AR) UTS in MPa. Decarburization was completed in a box furnace at 900 °C for times between 5 and 60 min. Uniaxial tensile testing, bend testing with digital image correlation (DIC), and microhardness testing were used to evaluate the effects of decarburization on the bendability of CRM1500 and PHS2000. The Van Ostrand Dewey model was used to model decarburization and correlated well with both through-thickness hardness measurements and 2-D static secondary ion mass spectroscopy (SIMS) carbon analysis.
Increasing decarburization time led to a greater decarburization depth as well as a decrease in UTS. The decarburization times were capped at 15 min for the CRM1500 and 30 min for the PHS2000, as longer times resulted in UTS values lower than the minimum as received UTS. Bendability showed improvement with increasing decarburization time. The best bendability achieved through decarburization was 95.0° for the CRM1500 and 62.9° for the PHS2000. These values represent an improvement of 2.3° and 32.8° relative to the respective baseline conditions. For both steels the strain at fracture for the outer tensile “fiber” increased with increased decarburization time. The neutral plane at the point of fracture, as determined by DIC images, is shifted toward the inner radius of the bend, and with increased decarburization the PHS2000 exhibited an increase in magnitude of this shift of the neutral plane as a result of the improved bending angle increasing strain of the outer tensile “fiber”.
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