This work studies the microstructure and tensile properties of a cold-rolled Fe-0.20C-4.86Mn (mass %) steel after short intercritical annealing (IA) times using scanning and transmission electron microscopy, and uniaxial tensile tests. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The experimental results show that IA temperature has a strong influence on the final microstructure and tensile properties while IA time has less. The fractions of retained austenite are much higher after IA at 650 and 675 degrees C than the other IA temperatures, and thus improving elongation. Simulations using the DICTRA software and constitutive modelling are further performed to assist the understanding of the microstructure evolution and stress-strain curves.

Medium Mn steels (~ 3–10 mass % Mn), a new category of advanced high strength steels, attracted worldwide research interests recent years due to their excellent mechanical properties and low cost. These steels have fine microstructures and contain large fraction of metastable retained austenite (~ 30 volume %), therefore exhibit excellent strength and elongation. The fine microstructure is mainly introduced by an intercritical annealing process.

To accelerate the design of such steels, materials design is applied. The materials design concept is a systematic method. Contrary to conventional methods largely based on trial and error, it is based on the classical processing–structure–properties relationships and a quantitative knowledge of each relation represented by a mathematical model, so-called linkage model. Such models are thus an essential part in materials design.

The present thesis aims to develop a framework used for materials design of medium Mn steel. The development of models which serve as linkage tools is thus the focus. Tensile properties, i.e. strength and elongation, are set as the design objectives driven by the industrial application.

The major part is concentrated on the linkage tools of processing–structure, i.e. models and simulations to predict the microstructure evolution associated with processing. These linkage tools are based on thermodynamic calculations and kinetic simulations using the commercially available Thermo-Calc and DICTRA software. To be specific, the processing involves austenitization and quenching as well as intercritical annealing and quenching; while the associated structure involves transformation of austenite to martensite and reversion of martensite to austenite. Therefore the following aspects have been studied:

1. martensite fraction with undercooling;
2. austenite reversion during intercritical annealing;
3. influence of austenite grain size on martensite start temperature;
4. mechanical stability of retained austenite.

Besides these, prediction of tensile properties is studied in the last part, which serves as an example of a linkage tool of structure–properties.

Via integrating these models, to achieve certain tensile properties, the required microstructure and the associated processing can be traced back.

Medium Mn stål (~ 3–10 mass% Mn), en ny typ av avancerade höghållfasta stål, har varit föremål för stort  globalt forskningsintresse de senaste åren på grund av stålens utmärkta mekaniska egenskaper och låga kostnader. Dessa stål har en fin mikrostrukturer och innehåller en stor fraktion av metastabil restaustenit (~ 30 volymprocent). Det uppvisar därför utmärkta värden på styrka och förlängning. Den fina mikrostrukturen åstadkommes huvudsakligen genom en sk interkritisk glödgningsprocess.

För att påskynda utvecklingen av sådana stål utnyttjas materialdesign. Materialdesignkonceptet är en systematisk metod. I motsats till konventionella metoder, som i stor utsträckning bygger på “trial and error”, baseras den på de klassiska relationerna process–struktur–egenskaper och en kvantitativ kunskap om varje relation representerad av en matematisk modell, en så kallad länkmodell. Sådana modeller är därför en väsentlig del i materialdesign.

Föreliggande avhandling syftar till att utveckla ett ramverk för materialdesign av medium Mn stål. Utvecklingen av modeller som fungerar som länkar är i fokus. Egenskaper vid enaxligt dragprov, dvs styrka och brottförlängning, formuleras som designkriterier i den givna industriapplikationen.

Avhandlinges huvuddel  koncentreras på länkmodeller mellan process och struktur, dvs modeller och simuleringsmetoder för att förutsäga mikrostrukturutvecklingen under värmebehandling. Dessa länkmodeller bygger på termodynamiska beräkningar och kinetisk simulering med hjälp av de kommersiellt tillgängliga Thermo-Calc- och DICTRA-koderna. Mer specifikt involverar värmebehandlingen austenitisering och släckning såväl som interkritisk glödgning och släckning; medan den associerade strukturen innebär omvandling av austenit till martensit och den omvända omvandlingen av martensit till austenit. Följaktligen har följande aspekter studerats:

1. martensitfraktion som funktion av underkylning;
2. austenit bildning under interkritisk glödgning;
3. inverkan av austenitkornstorlek på martensitens starttemperatur;
4. mekanisk stabilitet av restaustenit.

Förutom dessa aspekter analyseras förutsägelse av dragegenskaper i den sista delen, som exempel på länkmodell mellan struktur och egenskaper.

Genom att integrera dessa modeller, för att uppnå vissa dragegenskaper, kan den erforderliga mikrostrukturen och den därtill hörande behandlingen spåras tillbaka.

As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.

A thermodynamic-based model to predict the fraction of martensite in steels with undercooling has been developed. The model utilizes the thermodynamic driving force to describe the transformation curve and it is able to predict the fraction of athermal martensite at quenching to different temperatures for low alloy steels. The only model parameter is a linear function of the martensite start temperature (M (s)), and the model predicts that a steel with a higher M (s) has a lower difference between the martensite start and finish temperatures. When the present model is combined with a previously developed thermodynamic-based model for M (s), the model predictions of the full martensite transformation curve with undercooling are in close agreement with literature data.

Thermodynamics-based modelling of the fraction of martensite formed upon quenching in low-alloy steels is developed. The adopted modelling approach has two distinct features: 1) it applies the driving force of the transformation, i.e. the difference of Gibbs energy between austenite and martensite, from thermodynamic calculations; 2) it predicts the sigmoidal shape of transformation to capture also the initial 10-20% of martensite formation, which is distinct from some previous modelling using e.g. the Koistinen-Marburger equation. It is found that the general equation can describe the experimental data of martensite fraction versus quenching temperature for plain carbon steels and low-alloy steels well. Furthermore, the only model parameter that is needed is linearly proportional to the martensite start temperature of the steel, which opens the possibility for a thermodynamics-based simple but yet predictive model if it is coupled with the previously developed thermodynamics-based model for the Ms temperature.

The effect of solute silicon on the ferrite lattice parameter has been investigated using X-ray diffraction in cast ductile irons (DI) with nominal Si contents between 2.50 and 4.56 wt%. It was found that silicon changes the ferrite lattice parameter by –0.00185 Å per wt% Si. This contraction coefficient is three times larger than the most commonly used Si coefficient in the literature. Since substitutional solution by silicon contracts the ferrite lattice while the interstitial solution by carbon expands the lattice, the Si contraction coefficient found will have a significant effect on subsequent evaluation of the carbon content in austempered Si-alloyed ductile irons and steels.

In the present work the 475 degrees C embrittlement in binary Fe-Cr and ternary Fe-Cr-X (X=Ni, Cu and Mn) alloys have been investigated. The mechanical properties were evaluated using microhardness and impact testing, and the structural evolution was evaluated using atom probe tomography (APT). The APT results after aging at 500 degrees C for 10 h clearly showed that both Ni and Mn accelerate the ferrite decomposition. No evident phase separation of either the Fe-20Cr or Fe-20Cr-1.5Cu samples was detected after 10 h of aging and thus no conclusions on the effect of Cu can be drawn. Cu clustering was however found in the Fe-20Cr-1.5Cu sample after 10 h aging at 500 degrees C. The mechanical property evolution was consistent with the structural evolution found from APT. Samples aged at 450 and 500 degrees C all showed increasing hardness and decreasing impact energy. The embrittlement was observed to take place mainly during the first 10 h of aging and it could primarily be attributed to phase separation, but also substitutional solute clustering and possibly carbon and nitrogen segregation may contribute in a negative way.

Medium Mn steels have attracted worldwide interests recent years due to their excellent mechanical properties and low cost. These steels contain large fraction (~30%) of metastable retained austenite and exhibit good elongation due to transformation-induced plasticity (TRIP). In order to obtain the highest elongation, the mechanical stability of austenite, quantified using M_s^σ, needs to be optimized. M_s^σ is defined as the highest temperature at which martensite can form under stress without austenite yielding by slip. The present work aims to formulate a model of M_s^σ which can be used to design medium Mn steels with optimized elongation. In the present work, an Fe–0.18C–5.08Mn (mass %) steel was intercritically annealed at 650 °C. Based on tensile tests at different temperatures using a single specimen method, the M_s^σ temperatures were experimentally determined to about 0 °C regardless of intercritical annealing time between 15 min and 3 h. Microstructure observations showed that large austenite grains with a globular shape are more transformed than thin-film ones, and thus the former probably governs the determined M_s^σ. M_s^σ was further predicted at the crossing point of yielding by martensite formation and by austenite slip; the former was modeled by expanding an existing model of martensite start temperature and the latter by a constitutive model. The predicted M_s^σ showed reasonable agreement with the determined values. The model also indicated that a large and a small austenite grain have similar M_s^σ, which could partly explain why the determined M_s^σ is rather constant regardless of IA time.

This work studies the microstructure and tensile properties of a cold-rolled Fe–0.204C–4.86Mn (mass %) steel after short intercritical annealing (IA) times, i.e. 3 and 10 min. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The microstructure evolution is studied using scanning and transmission electron microscopy, and the tensile properties are obtained using uniaxial tensile tests. The experimental results show that IA temperature (600–700 °C) has strong, while IA time has less, influence on the final microstructure and tensile properties. The fractions of retained austenite are much higher after IA at 650 and 675 °C (~ 10 vol. %) than the other IA temperatures, and thus result in improved elongation (~ 20–30 %). Simulations using the DICTRA software and constitutive modeling are further performed to assist the understanding of the microstructure evolution and stress-strain curves.