Austenite stability in medium Mn Q&P steels
Penttilä, Jani (2021)
Penttilä, Jani
2021
Materiaalitekniikan DI-ohjelma - Master's Programme in Materials Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2021-02-01
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202101281784
https://urn.fi/URN:NBN:fi:tuni-202101281784
Tiivistelmä
There is a high demand in automotive industry for better fuel efficiency, which is a driving force for the development of 3rd generation advanced high strength steels (AHSS) e.g. quenching and partitioning (Q&P) steels. Medium Mn Q&P steels containing 5 – 10 % Mn have been broadly studied with promising results, but a high Mn content may impose problems in manufacturing process. One way to avoid these problems is to replace Mn with Si and/or Al.
The effects of different Q&P process parameters on the austenite stability and on the mechanical properties of Fe-0.26C-3.01Mn-2.19(Si+Al)-0.43(Mo+Cr)-0.04Nb steel have been investigated. Fine-grained multi-phase microstructures consisting ferrite and martensite with blocky and/or film-like retained austenite (RA) were obtained. Volume fractions of RA ranged from 5 to 34 % mostly depending on intercritical annealing temperature (IAT). Carbon partitioning from intercritical ferrite at low IAT was enough to stabilize RA fractions of 30 % or more at room temperature (RT). For high IAT samples, intercritical austenite was partially unstable and primary martensite formed during initial quenching. The RA fractions of samples containing significant amounts of primary/secondary martensite ranged from 5 to 29 %. It was concluded from the XRD results that RA fractions correlated with partitioning time (Pt). The RA fractions of the present steel grade was comparable to RA fractions of Q&P steels with higher Mn content. A conclusion was made that Si and Al are suitable replacement alloys for Mn. Tensile tests were conducted and results comparable to the current stage of Q&P steels were obtained. Ultimate tensile strengths (UTS) and tensile elongations (TE) varied between 1120 – 1580 MPa and 8 – 22 %, respectively. These results were explained by phase fractions and transformation induced plasticity (TRIP) effect. Dynamic strain aging (DSA) was observed with samples of low IAT. Instantaneous strain hardening exponent (n) curves were calculated, which showed differences in RA stabilities.
The effects of different Q&P process parameters on the austenite stability and on the mechanical properties of Fe-0.26C-3.01Mn-2.19(Si+Al)-0.43(Mo+Cr)-0.04Nb steel have been investigated. Fine-grained multi-phase microstructures consisting ferrite and martensite with blocky and/or film-like retained austenite (RA) were obtained. Volume fractions of RA ranged from 5 to 34 % mostly depending on intercritical annealing temperature (IAT). Carbon partitioning from intercritical ferrite at low IAT was enough to stabilize RA fractions of 30 % or more at room temperature (RT). For high IAT samples, intercritical austenite was partially unstable and primary martensite formed during initial quenching. The RA fractions of samples containing significant amounts of primary/secondary martensite ranged from 5 to 29 %. It was concluded from the XRD results that RA fractions correlated with partitioning time (Pt). The RA fractions of the present steel grade was comparable to RA fractions of Q&P steels with higher Mn content. A conclusion was made that Si and Al are suitable replacement alloys for Mn. Tensile tests were conducted and results comparable to the current stage of Q&P steels were obtained. Ultimate tensile strengths (UTS) and tensile elongations (TE) varied between 1120 – 1580 MPa and 8 – 22 %, respectively. These results were explained by phase fractions and transformation induced plasticity (TRIP) effect. Dynamic strain aging (DSA) was observed with samples of low IAT. Instantaneous strain hardening exponent (n) curves were calculated, which showed differences in RA stabilities.