Metallurgy and heat treatment of medium manganese quenching & partitioning steels

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Journal Title
Journal ISSN
Volume Title
Kemian tekniikan korkeakoulu | Master's thesis
Date
2018-04-03
Department
Major/Subject
Sustainable Metals Processing
Mcode
CHEM3026
Degree programme
Master's Programme in Chemical, Biochemical and Materials Engineering
Language
en
Pages
143 + 15
Series
Abstract
Since emissions of the transportation sector contribute significantly to climate change, the longterm strategy of several auto manufacturers comprises of enhancing fuel efficiency without compromising safety. The automotive industry acts, thus, as a driving force for developing lightweight 3rd generation AHSS with both high strength and ductility. The 3rd generation AHSS with Rm × A > 20 GPa% include medium manganese steels and, Q&P and ART heat treated steels.These steels have recently gained attention due to their advantageous properties in previous studies reported in literature. The aim of this work was to study the final microstructure and mechanical properties of three Q&P and ART heat treated alloys Fe–0.2C–5Mn, Fe–0.2C–5Mn–1.7SiAl and Fe–0.2C–5Mn–2.8SiAl. The annealing temperatures were varied depending on alloying, whereas the quenching and partitioning temperatures were kept unchanged. In addition, the effect of cold rolled, martensitic and batch annealed starting microstructures was studied. The material production and heat treatments were conducted in laboratory scale. The effect of the heat treatments was studied with tensile tests, dilatometer tests, EBSD measurements, FEGSEM and SEM. Based on microstructural analysis, the Q&P heat treatments resulted in multi-phase steels with varying volume fractions of martensite, tempered martensite, bainite and precipitated carbides. The ART heat treatment, on the other hand, resulted in a microstructure with intercritical ferrite and martensite. In addition, some samples contained retained austenite. Therefore, it was stated that 0.2 wt. % C and 5.0 wt. % Mn were sufficient alloying amounts for retaining austenite to room temperature. However, the findings also implied that Si and Al were necessary additions as they enhance the stabilization of austenite by suppressing the formation of carbon consuming cementite. In addition, an increasing amount of Si and Al increased the PAG size, decreased strength, increased elongation and stabilized ferrite by increasing the A3 temperature. The cold rolled starting microstructure was the least favorable compared to the martensitic and batch annealed starting microstructures. Most of the heat treatments resulted in Q&P and ART steels with Rm × A25 < 20 GPa%, which was below the initial target. As an exception, the ART heat treated sample of alloy Fe–0.2C–5Mn–2.8SiAl was the most promising steel exhibiting Rm × A25 > 20 GPa%. Hence, future research should focus on optimizing the heat treatment parameters. The possibilities of implementing the production of such steels to a continuous industrial line should also be studied further.

Transportsektorns utsläpp bidrar till klimatförändringen, vilket har lett till att den långsiktiga strategin hos flera fordonstillverkare siktar på förbättring av fordonens bränsleeffektivitet utan att kompromissa med säkerheten. Fordonstillverkare är därför den drivande kraften i utvecklingen av tredje generationens avancerade höghållfasta stål med hög duktilitet. Tredje generationens stål med Rm × A > 20 GPa% inkluderar Q&P och ART värmebehandlade ståltyper samt medium manganese stål. Dessa ståltyper har fått mycket uppmärksamhet på grund av deras förmånliga egenskaper vilka rapporterats i tidigare studier inom forskningsområdet. Målet med detta arbete var att studera mikrostrukturen och de mekaniska egenskaperna hos tre Q&P och ART värmebehandlade legeringar: Fe–0.2C–5Mn, Fe–0.2C–5Mn–1.7SiAl och Fe–0.2C– 5Mn–2.8SiAl. Glödningstemperaturen i värmebehandlingarna varierades med avseende på legeringens egenskaper medan quenching- och partitioningtemperaturerna behölls konstanta. Effekten av begynnelsemikrostrukterna studerades genom att variera mellan kallvalsad, martensitisk och mjukglödad begynnelsemikrostruktur. Legeringarna tillverkades och värmebehandlingarna utfördes i laboratorieskala. Effekten av värmebehandlingarna studerades med dragprov, dilatometerprovning, EBSD mätningar, FEGSEM och SEM. På basis av mikrostrukturanalyserna visades Q&P stålen innehålla flera faser som martensit, tempererad martensit, bainit och karbider. ART värmebehandlingen resulterade i stål med ferrit och martensit. Restaustenit hittades i några specimen. Det visade sig därav att 0.2% C och 5.0% Mn är tillräckliga mängder för att stabilisera restaustenit till rumstemperatur. Tillsatser av Si och Al i legeringarna visade sig att vara viktigta för att stabilisera austenit eftersom de hindrar kolkonsumerande cementit från att formas. Si och Al ökade kornstorleken hos austenit, minskade hållfastheten, ökade duktiliteten och stabiliserade ferrit genom att öka A3 temperaturen. Kallvalsad begynnelsemikrostruktur visade sig mindre gynnsam i jämförelse med martensitisk och mjukglödad. Värmebehandlingarna ledde till Q&P och ART stål med Rm × A25 < 20 GPa%. Avvikande från det här visade sig ART värmebehandlade stålet av legering Fe–0.2C–5Mn–2.8SiAl med Rm × A25 > 20 GPa% vara det mest lovande. Kommande forskning borde alltså fokusera på att optimera värmebehandlingsparametrarna. Dessutom borde man forska i hur dessa nya ståltyper kunde produceras med kontinuerliga industriella processer.
Description
Supervisor
Jokilaakso, Ari
Thesis advisor
Oja, Olli
Jussila, Petri
Keywords
quenching and partitioning, austenite reverted transformation, 3rd generation AHSS, medium manganese, retained austenite
Other note
Citation