Demand and share of lightweight materials on the market have been growing in the last few decades. Reduced energy consumption, reduced emissions and enhanced production chain efficiency are the main drivers of such phenomenon. Therefore, steelmaking industry has focused on lightweight steel development and Fe-Mn-Al-C steels have been put under the spotlight. In fact, such alloy system has been shown to be able to lower common steel density by more than 20% (keeping the same mechanical properties) and, in addition to structural purposes, it may be also suitable for cryogenic applications, corrosion and high-temperature oxidation resistance, wear resistance. Anyway, such flexibility implies complex metallurgy and still consistent lack of knowledge about application properties. That is why Fe-Mn-Al-C low-density steels are still confined on the R&D scale. The aim of this project has been to identify boundary conditions and involved reactions for the microstructural evolution of Fe-Mn-Al-C system. 24 alloys with 3-6-9-12% Al, 10-20-30% Mn, 0.4-1% C (wt%) have been hot-rolled and tempered at 400-600-800 °C for 30 min. The obtained microstructures have been characterized through optical and SEM microscopy, with the support of Vickers hardness tests, phase volume fraction diagrams and SEM-EDS analysis. Results have shown that chemical composition has been the main controller of α/γ balance. Regarding γ phase, discontinuous precipitation has occurred at 600 °C and 10% Mn. At the same conditions, but for 12% Al alloys, precipitation transformation has taken place. Cellular transformation has developed at 800 °C among 9-12% Al compositions. At the same conditions, but with 1% C, spinodal decomposition has been triggered at the expenses of cellular transformation. k-carbide precipitation within ferrite has occurred only at 9-12% Al, but it has been stopped at 1% C. Also, manganese has shown to enhance such reaction at 9Al-0.4C. In the end, it has been possible to draw some microstructural evolution trends of Fe-Mn-Al-C system and some effects related to chemical composition and tempering.
Negli ultimi decenni la domanda del mercato per materiali leggeri ha visto una crescita significativa. Ridurre emissioni e consumi energetici ed incrementare l'efficienza delle catene di produzione sono i motori principali di tale fenomeno. L'industria siderurgica si è pertanto focalizzata sullo sviluppo di acciai leggeri e la lega Fe-Mn-Al-C ha catturato l'attenzione. Infatti, è stato mostrato che tale sistema può raggiungere una densità di oltre il 20% inferiore a quella degli acciai comuni (a parità di proprietà meccaniche) e, oltre ad impieghi strutturali, può essere adatto per applicazioni criogeniche, di resistenza alla corrosione e all'ossidazione a caldo e di resistenza all'usura. Tale flessibilità implica una metallurgia complessa ed ancora poco note proprietà applicative. Pertanto, gli acciai Fe-Mn-Al-C rientrano ancora esclusivamente nella scena R&D. L'obiettivo di questo progetto è stato quello di individuare le condizioni al contorno e le reazioni coinvolte nell'evoluzione microstrutturale del sistema Fe-Mn-Al-C. 24 leghe al 3-6-9-12% Al, 10-20-30% Mn e 0.4-1% C (wt%) sono state laminate a caldo e rinvenute a 400-600-800 °C per 30 min. Le microstrutture sono state caratterizzate attraverso microscopio ottico e SEM, con il supporto di misure di durezza Vickers, diagrammi di fase e analisi EDS. I risultati hanno mostrato che la composizione chimica ha avuto l'effetto principale sul bilancio α/γ. Riguardo l'austenite, a 600 °C e 10% Mn è avvenuta la precipitazione discontinua. Alle stesse condizioni, nelle leghe al 12% Al, la trasformazione per precipitazione ha preso il sopravvento. A 800 °C e al 9-12% Al si è sviluppata la trasformazione cellulare. Alle stesse condizioni, ma all'1% C, la decomposizione spinodale è stata innescata a spese della trasformazione cellulare. La precipitazione di carburi k all'interno della ferrite è stata individuata solo al 9-12% Al ed è stata interrotta all'1% C. L'aggiunta di manganese ha incrementato tale reazione nelle leghe 9Al-0.4C. Infine, sono stati tracciati alcuni andamenti dell'evoluzione microstrutturale del sistema Fe-Mn-Al-C ed alcuni effetti legati a composizione chimica e rinvenimento.
Investigation about Fe-Mn-Al-C system for the production of lightweight steels
BIZZOZERO, MARCO
2016/2017
Abstract
Demand and share of lightweight materials on the market have been growing in the last few decades. Reduced energy consumption, reduced emissions and enhanced production chain efficiency are the main drivers of such phenomenon. Therefore, steelmaking industry has focused on lightweight steel development and Fe-Mn-Al-C steels have been put under the spotlight. In fact, such alloy system has been shown to be able to lower common steel density by more than 20% (keeping the same mechanical properties) and, in addition to structural purposes, it may be also suitable for cryogenic applications, corrosion and high-temperature oxidation resistance, wear resistance. Anyway, such flexibility implies complex metallurgy and still consistent lack of knowledge about application properties. That is why Fe-Mn-Al-C low-density steels are still confined on the R&D scale. The aim of this project has been to identify boundary conditions and involved reactions for the microstructural evolution of Fe-Mn-Al-C system. 24 alloys with 3-6-9-12% Al, 10-20-30% Mn, 0.4-1% C (wt%) have been hot-rolled and tempered at 400-600-800 °C for 30 min. The obtained microstructures have been characterized through optical and SEM microscopy, with the support of Vickers hardness tests, phase volume fraction diagrams and SEM-EDS analysis. Results have shown that chemical composition has been the main controller of α/γ balance. Regarding γ phase, discontinuous precipitation has occurred at 600 °C and 10% Mn. At the same conditions, but for 12% Al alloys, precipitation transformation has taken place. Cellular transformation has developed at 800 °C among 9-12% Al compositions. At the same conditions, but with 1% C, spinodal decomposition has been triggered at the expenses of cellular transformation. k-carbide precipitation within ferrite has occurred only at 9-12% Al, but it has been stopped at 1% C. Also, manganese has shown to enhance such reaction at 9Al-0.4C. In the end, it has been possible to draw some microstructural evolution trends of Fe-Mn-Al-C system and some effects related to chemical composition and tempering.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/139979