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Structural banding, carbon distribution, and orientation effects on the observed properties of austenite containing steels
Thrun, Melissa M.
Thrun, Melissa M.
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2023
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2024-10-18
Abstract
Third generation advanced high strength steels, such as quenched and partitioned (Q&P) steels are a favorable candidate for downgauging of high strength components in the automotive industry for fuel savings. Having complex microstructures, Q&P steels contain metastable austenite that will go through the deformation-induced transformation during straining. Metastable austenite in these steels will go through a transformation during deformation to martensite, providing an increase in instantaneous strain hardening rate, increasing strength, and delaying localization. This transformation during deformation is dependent on the stability of the austenite, with stability being tied to intrinsic and extrinsic variables of the system. Investigation into the effect of prior processing, such as starting microstructure and cold reduction, was performed as a way to change the transformation behavior of the austenite present in advanced high strength steels.
A new approach using thermodynamic modeling was performed to assess the effects of macroscopic banding present in Q&P steels, due to their Mn content, on the effect of the amount of retained austenite predicted in an as-heat treated microstructure. Accounting for Mn banding that is present after solidification, the informed model predicts lower retained austenite fractions, with a range of temperatures producing the theoretical maximum amount of retained austenite. Likewise, the model predicts two chemical distributions of austenite, indicating a distribution in stabilities.
Leveraging different microstructures prior to Q&P heat treatment was performed to change the austenite behavior during deformation. Two microstructures were used for Q&P processing, a ferrite-pearlite microstructure, and a martensitic microstructure. By changing the length scale of diffusion, and homogenizing carbon through the initial microstructure, mechanical properties can be varied, with ultimate tensile strengths ranging from 980 - 1070 MPa, and uniform elongations ranging from 7 - 20 pct.
Synchrotron X-Ray diffraction was used as a method to monitor austenite fraction in situ during straining of a 304 stainless steel, to evaluate transformation behavior in absence of microstructure neighborhood stability effects. This was coupled with ex situ electron backscatter diffraction imaging to evaluate the texture present. It was observed that the transformation behavior between the tensile axis in the rolling direction and transverse direction of the sheet differed, despite little difference in their mechanical properties. It was also found that regardless of starting condition, the (111) pole was preferentially aligned with the tensile axis after deformation, indicating that this orientation may have a stabilizing effect on the austenite present.
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