Seznam abstraktů 2024: SEKCE C - OCELI, VLASTNOSTI, VÝROBKY Z OCELÍ, TESTOVÁNÍ

For many years automotive market pushed the researchers in the development of high strength steels. High Mn TWIP steels have very good mechanical behaviour, but their elaboration path is quite complex and costly. Hence, studies of medium Mn steels with the Mn content in between 4 and 12 wt.% are taking more and more importance nowadays. Recent studies showed that intercritical annealing of such medium Mn steels can offer an exclusive combination of strength and ductility.
The objective of this work was to study the dependence of microstructure, especially retained austenite fraction, and of mechanical properties on the intercritical annealing temperature. Two medium Mn steels with different C level were studied. Samples were annealed at various
intercritical temperatures using gradient batch annealing furnace after preliminary austenitization and quench. Mechanical properties were measured using a standard tensile test. Microstructure observations were performed and retained austenite fraction was measured using the X-ray diffraction. Relationship between microstructure and mechanical properties was analysed and discussed.

The work concerns the analysis of the effect of deformation under static and dynamic conditions of high-manganese X42 steel with the SFU value of 24 mJ/m2 on energy absorption capacity and microstructure. X42 steel is graded as a steel that strengthens by mechanical twinning (TWIP effect). TWIP steels have an excellent combination of strength and ductility properties, making them an excellent material for use in controlled crumple zone components for vehicles. A static tensile test and a flywheel machine tensile test at a speed corresponding to dynamic conditions were carried out for the tested steel. Microstructure studies were carried out using light microscopy and scanning electron microscopy techniques. From the stress-strain curves, the value of the energy absorbed during plastic deformation was calculated. By analysing the results of the tensile tests, it can be concluded that the material strengthens with the increase in strain rate without losing its good plastic properties. Analysis of the microstructure revealed deformation effects in the form of austenite grains elongated in the direction of the tensile force, within which mechanical twins and deformation bands are formed and the intensity of the generation of twins is correlated to the strain rate. The value of absorbed energy during plastic deformation of the tested steel is at a similar level compared to other TWIP steels expected to be used for structural components absorbing energy during impact.

The ferritic-martensitic microstructure is typical for the dual-phase steels such as DP600. This microstructure can be obtained during the hot rolling process followed by interrupted water cooling with short isothermal pause in the intercritical region. In manufacturing of the DP steel by hot rolling process the final deformation takes place slightly above Ar3, where the material is composed of austenite alone. Upon completion of plastic deformation, the hot strip is rapidly cooled to the temperature between Ar3 and Ar1, where supercooled austenite undergoes phase transformation into ferrite. Since both the temperature and time spent in this temperature region are not sufficient for complete phase transformation, remaining austenite is quenched during the second step of cooling process, which leads to the formation of a martensite. By knowing the relation between the temperatures used in the process and resulting microstructure - especially martensite volume fraction, it is possible to design industrial process so that the requirements regarding mechanical properties of a material can be ensured. Two experiments were carried out to analyze the mentioned process parameters. First, the quenching experiment, where samples with two different initial microstructures, ferritic-pearlitic and ferritic-martensitic, were annealed in wide range of temperatures and then quenched. Second, the two stage cooling from austenite region, where samples were first annealed at different temperatures just above Ar3, then cooled and held for a few seconds at different intercritical temperatures and finally quenched. The resulting microstructures allowed evaluation of phase composition and its morphology in relation to temperatures used in experiments.

Dual-phase (DP) steels, commonly employed in the automotive industry, are characterized by their ferrite and martensite microstructure. The martensite can be seen to be banded in the rolling direction. This banded microstructure arises from micro-segregation during solidification, primarily of manganese (Mn), followed by subsequent rolling. The banding can lead to anisotropy in mechanical properties and poor formability. In this study, the concept of altering the morphology of martensite has been employed to enhance the formability of DP steel. In this context, the influence of Mn segregation on the distribution of the second phase has been neutralised by redesigning the steel composition compared to conventional DP grades. The new DP steel grade has been introduced as ‘segregation-neutralised (SN)’ DP steel. A combination of finite element modelling and in-situ tensile tests was utilised to study local formability via microstrain distribution and void nucleation/ coalescence in SN-DP compared to a conventional banded DP grade. Local formability was improved in the SN-DP grade which was attributed to a lower void nucleation / coalesence rate compared to the benchmark grade. This agreement was strengthened with finite element modelling where strain-field measurement revealed a more homogeneous shear band formation in SN-DP, resulting in a decrease in the strain intensity of high-localised strain regions compared to the benchmark DP grade.
Uniaxial tensile test was also employed to compare the global formability of SN-DP and the benchmark DP grade. Around 5% higher total elongation in SN-DP compared to the benchmark DP grade was observed while maintaining a similar strength.

The automobile contributes significantly of around 24% to the total CO2 emissions globally. For a typical passenger car, 10% of CO2 emission is from production and 90% from the tail end emissions combine to the total life cycle of 30-35 T of CO2 emission. The materials used in automobile consists of aluminum & magnesium alloys, plastics, polymers and around 54% is made of steel. The conventional steel production process heavily relies on energy-intensive blast furnaces, leading to substantial greenhouse gas emissions of around 1.94 T of CO2/Ton of Hot Metal. For the steel industry to follow a trajectory compatible with the goals of the Paris Agreement, its direct CO2 emissions must fall by more than 50% by 2050 relative to today. Our Sustainable Development Scenario outlines a pathway compatible with achieving these goals, which requires a reduction in the direct emission intensity of steel production as well as the tail end emission of the automobile. In this presentation we will discuss the development of Advanced High Strength Steel for automobiles which can reduce the carbon foot print by reducing the specific steel consumption and improved fuel economy by weight reduction. Around 19% of the today’s conventional steel can be replaced with Advanced High Strength Steels in Passenger vehicle segment. This leads to 10% reduction in weight where the fuel economy improves by 6-8%. Also 13% improvement in freight efficiency for 6% reduction in weight in Commercial/ Heavy duty vehicles.

Small punch test is an advantageous method for evaluation of mechanical properties of components bcause much less material is required for sampling and subsequential material testing. This is especially valuable in cases where not enough bulk material is available i.e. power industry. The testing material is sampled by a special sampling device that ensures no component damage and the sampled volume is so small that no component repair is necessary after sampling. Small punch testing is viable not only for evaluation of mechanical properties, but can also be used for evaluation of brittle fracture properties (transition temperature, toughness) which play integral part in lifetime of some power components and structures. This paper introduces and compares two different approaches to transition temperature evaluation. Also a way to evaluate J-integral value using small disc specimen with side notch which is suggested by EN 10 371 standard is introduced and compared to J-integral evalution via standard fracture mechanics approach using multiple CT specimen and plotting crack growth resistance curve.

In this work, the influence of strain and strain aging parameters on the strength and low temperature toughness of low-carbon microalloyed Nb-V-Ti and Nb-Ti steels is investigated. Industrial samples of 23 mm heavy plates after TMCP (Thermo-Mechanical Controlled Processing) and additional sPWHT (subsequent Post-Weld Heat Treatment) were used for the research.
Theoretical foundations of steel aging were considered, and its primary parameters and mechanisms were described. One of the main challenges with steels used in Arctic areas is to ensure adequate material toughness to prevent brittle fracture. Therefore, a parameter indicating the susceptibility of steels to aging was determined. The change in the basic strength properties under 5%, 7.5% and 10% pre-strain was determined. Subsequent aging was carried out by heating to 250°C for 60 minutes. Impact tests were conducted at temperatures as low as -80°C to determine the reliability and durability of constructions, especially in Arctic conditions.
The presented results have been used with a positive outcome in the certification tests for production of heavy plate products in quality categories S355MLO, S420MLO, S460MLO according to the standards EN10225-1:2019 and NORSOK M-120, including Arctic editions (Annex F).

The oil and gas pipeline industries continuously demand steel with improved strength and ductility at lower costs. Thus, the current experimental work investigated the role of thermomechanical processing and coiling temperature (CT) on various strengthening mechanisms in a low-carbon Nb + V microalloyed steel and its mechanical properties. The samples were hot-rolled (50 % thickness reduction) at 1100 °C and then air-cooled directly to the different coiling temperatures. Further, isothermal holding was performed at various CTs (650-500 °C) for 1hr, followed by water-quenching. The result indicates that yield strength for samples coiled at 650, 600, 550 and 500 °C is 454, 463, 476 and 481 MPa, respectively, with a large amount of elongation (~25-30 %). This is mainly due to the presence of pearlite and ferrite in the sample microstructure. The minimal amount of nucleated0 Widmanstatten ferrite or degenerate pearlite (~5 %) does not show much impact on strength-ductility for 500 °C coiling temperature. The precipitation strengthening offers a substantial amount of strengthening (26-35 MPa) compared to grain refinement (126-156 MPa) and dislocation strengthening (115-150 MPa). The SEM analysis indicates the segregation of MnS inclusions at the phase interfaces of ferrite and pearlite, which act as the potential nucleation sites for crack formation during tensile tests.

In this study, we conducted a comprehensive examination of two copper-modified steels, analyzing their phase composition and mechanical behavior under both as-received (AR) and quenched-and-tempered (Q&T) conditions. A detailed electron microscopic investigation was carried out to examine precipitates, and the MatCalc simulation was utilized to predict mean radius, number density, and volume fraction of these precipitates. The Continuous Cooling Transformation (CCT) curve, generated through JMAT Pro software, indicated higher hardenability for HCu steel compared to LCu steel. The dominance of co-operative/short-range diffusion in HCu steel was justified, leading to a significant degenerate pearlite formation (approximately 71 vol. pct.), while LCu steel exhibited a lesser extent (around 3 vol. pct.). The study delves into the tempering softening mechanism and aging-related hardening mechanisms, correlating these phenomena with changes in the tetragonality of quenched structures and the transition of Cu precipitates from BCC (coherent) to 9R (semi-coherent) and eventually to FCC (incoherent) crystal structures. Analysis of engineering stress-strain curves revealed superior yield strength (YS) and ultimate tensile strength (UTS) for HCu samples. However, LCu steel exhibited excellent total elongation, tensile toughness, and strain hardening ability. After tempering for 0.75 hours, a noteworthy combination of strength, ductility, and hardness (811 MPa, 19.35%, and 267±4 HV, respectively) was observed for LCu steel, along with a tensile toughness of 146*105 KJ/m3. The microstructure-mechanical property correlation was established by employing fractographic analysis, considering the extent of coherency of different precipitates. This comprehensive investigation contributes to a deeper understanding of the performance of copper-modified steels, offering insights into their microstructural evolution and mechanical responses under varying conditions.

High-strength low-alloy (HSLA) steels are low-carbon steels, making them an excellent construction material for automotive applications, transportation of media such as oil and gas, or responsible marine structures often operating in the harsh weather conditions of northern seas. These steels gain unique properties from micro-additives of Nb, V or Ti (up to 0.15 wt%) and can, for example, reduce the weight of structures by up to 40% compared with the classic S235JR steel. The only place in Poland where HSLA steel strips can be produced is the ArcelorMittal Poland Hot Rolling Mill in Krakow. These steels are investigated by many research institutes, but due to the restricted number of places where they are produced and the high cost of industrial research, there is still a need for the investigations related to mass-scale production. In this study, the authors examined two selected steels produced at the ArcelorMittal Hot Rolling Mill in Krakow. Dilatometric analyses and strain dilatometry investigations were carried out within this study. The research enabled a good characterization of the material in terms of the phase transformations taking place during controlled cooling as well as the analysis of the material microstructural changes. The obtained results may contribute to more precise control of the material cooling conditions and, consequently, to obtaining the expected mechanical properties meeting customer expectations.

High-silicon grain oriented steels require strict requirements for chemical composition. The steelmaking process must therefore be extremely tightly controlled due to the need to achieve elemental compactness within strictly defined ranges. Particularly important in this regard is the Al/N ratio. The hot-rolling process is also subject to special requirements, being carried out at high temperatures due to the favorable effect of temperature on the solubility of AlN which is an inhibitor of grain growth in the secondary recrystallization process occurring in the subsequent stages of the electrical steelmaking process. The processes of precipitation and solubility of non-metallic inclusions such as AlN play a key role in the formation of the desired texture. In order to determine the occurrence of precipitation processes of non-metallic inclusions, studies on precipitation processes were performed: thermocalk under equilibrium conditions, dilatometric studies on heating and cooling and cooling according to the technological process. It was found that despite the potential for the occurrence of austenitic region, in practice, there is no entry into the austenitic range. On the other hand, the effects of ALN precipitation can be observed.

AISI H13 Is very commonly used in the hot-work category of steels. Whether it is used as a forging die, a hot-cutting tool, or a mold in injection molding or die casting, it is always on the table, as the material of choice. However, its potential has not yet been fully realized. New manufacturing techniques such as additive manufacturing (AM) broaden the horizon of the material’s application, and promise improved performance, through optimized geometry, unobtainable by traditional means, and heightened mechanical properties. One of the more widespread AM processes is Powder Bed Fusion (PBF) where a laser or electron beam constructs the model, by meting a thin coating of metal powder applied to a base plate. By repeatedly applying and melting powder, the end result is a layer-by-layer produced part. However, the techniques for producing such parts are not yet refined enough and require further research. Problems like porosities, part deflection, and crack formation due to residual stress are commonplace, while comparably low mechanical properties in the as-processed state call for post-production treatments. Naturally, every technique has its boons and drawbacks, and that is what this work aims to analyse - How do the PBF parts compare to conventionally produced ones, and what are the difficulties in producing the later-mentioned parts.

This research presents the development on the methodology for numerical simulation of the low cycle fatigue life. The sensitivity of the number of components of the kinematic hardening parameters on the stress-strain hysteresis curve of the low cycle fatigue behaviour of the stainless-steel working under cyclic loading was investigated. Experimental and numerical investigation was carried out on AISI316L steel under 0.18% strain loading at 20°C for the prediction of the fatigue failure of the component. It is observed that the cyclic elasto-plastic repose of the low cycle fatigue loading is described accurately with the kinematic hardening parameters consisting of three components. On the comparison of the low cycle fatigue numerical simulation results with the experimental LCF test showed very close alignment with the shape of the stress-strain hysteresis curve. It can be concluded that the presented methodology can be employed for the estimation of the fatigue life of the stainless steel under repetitive strain loading.

Household appliances often utilize polymer-metal bonding, typically achieved through adhesive bonding or press-fitting techniques. Corrosion resistance is a major factor in household appliances, as corrosion can compromise the functionality and aesthetics of household appliances. Stainless steels are a natural choice. However, the nature of polymer-stainless steel bonds can cause susceptibility to crevice corrosion. Crevice corrosion, a localized attack on stainless steel in confined spaces. This study investigates crevice corrosion in mixer heads fabricated with two different grades of stainless steel (1.4016 and 1.4021). Metallographic analysis revealed a ferritic microstructure with higher chromium content for 1.4016, contributing to its superior corrosion resistance compared to the martensitic structure of 1.4021. Both materials exhibited a critical crevice width of 9-10 µm between the plastic head and the steel shaft, facilitating the entrapment of corrosive solutions (confirmed by EDS analysis) and subsequent crevice corrosion. The PREN (pitting resistance equivalent number) calculation further supported the observed behavior, with 1.4016 boasting a higher value (16-18) compared to 1.4021 (12-14). While PREN offers a valuable indicator, it highlights the importance of design considerations to minimize crevice formation. Here, eliminating the gap between plastic and steel through improved design or sealants emerges as a crucial preventive strategy. This study emphasizes the interplay between material selection and design in mitigating crevice corrosion in household appliances. By understanding the factors influencing corrosion susceptibility, manufacturers can optimize material choices and implement design solutions to ensure the longevity and performance of their products.

Today, fossil fuels used to produce electrical energy all over the world pose a threat to human health because they emit serious greenhouse gases.The costs of fossil fuels, which we use today because they are cheap, will increase over time as their production will decrease. Therefore, the demand for renewable energy resources that are easy to obtain, do not harm the environment, and do not require any raw materials for production is increasing day by day. Considering Turkey's potential for wind energy, the importance of this energy is highlighted. The necessary studies for the security of this energy are increasing rapidly. The current irregular change in wind also weakens the confidence in this energy. In order to minimize this insecurity, estimating the effective wind speed is an important issue. In this study, the eleven-year wind and meteorological values of the Mardin region, which is located in the southeast of Turkey and has the status of a metropolitan city, were forwarded according to the data obtained between 2000 and 2010 by using these values, radial basis neural networks, regression analysis methods, recurrent neural networks and artificial neural networks. An attempt has been made to create wind speed estimates that will be effective in wind speed energy. The best Average Absolute Percent Error data of the selected methods for the Mardin City Region were revealed and compared. As a result, these methods have achieved more effective results in wind forecasting than other methods.