Effect of microstructure on the mechanical properties and bendability of direct-quenched ultrahigh-strength steels

Abstract The effect of austenite pancaking in the non-recrystallisation regime on microstructure and mechanical properties, especially bendability, was investigated in direct-quenched ultrahigh-strength strip steels with martensitic-bainitic microstructures. Lowering the finishing rolling temperature (FRT) increased total reduction in the non-recrystallisation region (R tot). Niobium microalloying increased Rtot while variations in C, Mn and Mo did not affect Rtot to the same extent as Nb. A decrease in the FRT increased the incidence of softer microstructures such as ferrite and granular bainite in the subsurface layers. The microstructures at the centreline were comprised of auto-tempered martensite with some bainite. An increase in Rtot strengthens the intensities of the ~{554}<225>α and ~{112}<110>α texture components at the centreline and the components ~{112}<111>α and ~{110}<112>α - {110}<111>α at the strip subsurface. Bendability is poorer with the bend axis perpendicular rather than parallel to the rolling direction (RD) and is further impaired with increasing hardness below the sheet surface. An intense ~{112}<111>α shear texture combined with upper bainite containing MA islands in the subsurface region is shown to be detrimental to bendability when the bend axis is perpendicular to the RD. This anisotropy of bendability can be explained by the appearance of geometric softening in grain clusters belonging to this texture component when the bend axis is perpendicular to the RD. Shear localisation is prevented, however, by the presence of a sufficiently thick subsurface microstructure having adequate work hardening capacity, i.e., ferrite + granular bainite rather than ferrite + upper bainite. The strain required to initiate strain localisation can be increased and good bendability thereby achieved—even in the presence of detrimental texture components—by ensuring the presence of a sufficiently soft subsurface layer extending to a depth of approximately 5% of the total sheet thickness. The above beneficial microstructures can be obtained and good bendability ensured in direct-quenched strip steel with a yield stress above 900 MPa together with good impact toughness, provided a suitable combination of chemical composition and processing parameters is selected and sufficient attention is paid to steelmaking operations to obtain a proper inclusion structure.Tiivistelmä Austeniitin muokkauksen vaikutusta mikrorakenteeseen ja mekaanisiin ominaisuuksiin, erityisesti särmättävyyteen, tutkittiin suorasammutetuilla martensiittis-bainiittisilla suurlujuusnauhateräksillä. Kuumavalssauksen lopetuslämpötilan lasku kasvatti austeniitin kokonaisreduktiota ei-rekristallisaatioalueella. Mikroseostus niobilla kasvatti myös kokonaisreduktiota, kun taasen muutokset C-, Mn- ja Mo -pitoisuuksissa eivät vaikuttaneet yhtä voimakkaasti. Valssauksen lopetuslämpötilan lasku kasvatti pehmeämpien mikrorakenteiden, kuten ferriitin ja granulaarisen bainiitin, määrää nauhan pintakerroksessa. Terästen keskilinjan mikrorakenteet koostuivat pääasiassa itsepäässeestä martensiitista sekä pienestä määrästä bainiittia. Kokonaisreduktion kasvu voimisti ~{554}<225>α - ja ~{112}<110>α -tekstuurikomponentteja keskilinjalla sekä ~{112}<111>α- ja ~{110}<112>α - {110}<111>α -komponentteja nauhan pintakerroksessa. Särmättävyys oli huonompi särmän ollessa poikittain valssaussuuntaan nähden kuin pitkittäin. Pintakerroksen kovuuden kasvu heikensi särmättävyyttä. Pintakerroksen voimakas ~{112}<111>α -leikkaustekstuuri, yläbainiitin ja MA-saarekkeiden läsnä ollessa, osoittautui haitalliseksi särmän ollessa poikittain valssaussuuntaan nähden. Särmättävyyden anisotrooppisuus voidaan selittää geometrisella pehmenemisellä rakeissa, joissa kyseinen tekstuurikomponentti on voimakas. Leikkausmyötymän paikallistuminen estyy, kun pinnassa on riittävän paksu hyvän muokkauslujittumiskyvyn omaava kerros, mikä sisältää esim. ferriittiä ja granulaarista bainiittia, mutta ei ferriittiä ja yläbainiittia. Särmättävyys osoittautui pysyvän hyvänä huolimatta haitallisesta tekstuurikomponentista, kun pehmeä pintakerros ulottui noin 5 % syvyydelle levyn paksuudesta. Edellä mainitut mikrorakenteet ja hyvä särmättävyys voidaan saavuttaa suorasammutetuilla yli 900 MPa myötölujuuden nauhateräksillä yhdessä hyvän iskusitkeyden kanssa, kunhan valitaan sopiva kemiallisen koostumuksen ja valmistusparametrien yhdistelmä sekä kiinnitetään huomiota teräksen sulkeumapuhtauteen

Fatigue performance of notched and hot-dip galvanized laser and mechanically cut S960 steel components considering local defects with the theory of critical distances

Abstract Experimental fatigue tests were performed for a S960 steel grade, including hot-dip galvanized (HDG) round base material specimens, and laser cut, and machined notched component-sized specimens made of t = 6 mm S960 ultra-high-strength steel (UHSS) plates. Cracking after the HDG was found to have a major influence on fatigue strength and thus reducing the effect of surface quality on the fatigue performance. Design guidelines for notched HDG components are proposed, and HDG with UHSSs was found suitable for structures with geometrical notches. Multiparametric TCD-based 4R method application was introduced, and it was found to be applicable for the fatigue strength assessment of structural details with initial cracks

Fatigue strength assessment of cut edges considering material strength and cutting quality

Abstract In the present study, statistical analysis for previously reported cut edge fatigue test results is performed. Experimental fatigue tests are conducted for machined, plasma, and fiber laser-cut S960 edges to verify the effect of yield strength and cut edge quality, and to study the effect of the cutting method on fatigue performance. Experimental fatigue tests were complemented with hardness and residual stress measurements and metallurgical analyses with electron backscatter diffraction (EBSD) to characterize cut edge fatigue properties and to verify statistical analysis findings. The results show that cut edges can be divided into high- and low-quality categories. On the basis of these high- and low-quality categories, material strength, and applied cutting methods, FAT classes and recommended fatigue design practices are proposed

Thrust Performance and Heat Load Modelling of Pulse Detonation Engines

Pulse Detonation Engines (PDEs) are propulsion systems that use repeated detonations to generate thrust. Currently in early stages of development, PDEs have been theorised to have advantages over current deflagration based engines. Air-breathing PDEs could attain higher specific impulse values and operate at higher Mach numbers than today's air-breathing engines, while Pulse Detonation Rocket Engines (PDREs) could become a lighter, cheaper, and more reliable alternative to traditional rocket engines. There are still however, many technological hurdles to overcome before PDEs can be developed into practical propulsion systems, one major barrier being management of their immense heat loads. This thesis outlines the development of a numerical model for determining thrust performance and heat load characteristics of PDEs. The model is based on a set of analytical equations which characterise the gas dynamics inside the engine throughout it's cyclic process. Being numerically light -when compared to CFD analysis- the model allows for fast turnaround of results and the ability to sweep through parameters to determine optimum operating conditions to maximise engine performance and reduce heat load. In this study, the working principles of the model are described and it's outputs are validated against data from published experimental and numerical studies. The model is then used to conduct a comprehensive parametric study on the effects of various reactant combinations, operating conditions, and engine geometries on engine thrust, specific impulse and heat load. Lastly, a brief study is conducted on the feasibility of regenerative cooling for PDEs, using model outputs to determine if a heat balance can be achieved and the performance losses and complications that would result

Global formability and bendability of ultra-high strength steels:effect of mechanical properties on the strain distribution and behaviour in air-bending

Abstract Bendability is a key property for ultra-high strength steels, that affects their usability in many industrial applications. Previous research and efforts on improving the bendability of high-strength steels have focused mostly on the minimum bend radius. However, as the minimum bend radius has been deemed insufficient as a measure of bendability, a new approach may be necessary for further advancements in bendability research. In this paper, bendability of nine materials is investigated from a global formability perspective, through bending tests and tensile tests. Digital image correlation is used for strain measurement in both the bending and tensile tests. Linear regression is used for determining the relationships between the obtained tensile test results and bending strain distributions. The findings of this paper show that applying a “local/global formability” approach to bendability could be beneficial for future research, as better description of the bending behaviour can be obtained and the factors affecting certain bending behaviours can be thoroughly investigated

Effect of niobium and phase transformation temperature on the microstructure and texture of a novel 0.40% C thermomechanically processed steel

Abstract Field emission scanning electron microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) have been employed to investigate the effect of niobium and phase transformation temperature on the evolution of microstructure and texture in a novel thermomechanically processed, medium‑carbon, low-alloy steel intended for slurry pipeline applications. Thermomechanical processing consisted of hot-rolling in the austenitic region with deformation both above the recrystallization limit temperature and below the recrystallization stop temperature. Immediately after rolling, specimens were directly quenched in water to two different temperatures of 560 °C and 420 °C and subsequently furnace cooled from those temperatures to simulate the cooling of coiled strip on a hot strip mill. The microstructure of samples quenched to 560 °C mostly comprised of upper bainite, whereas the samples quenched to 420 °C mainly consisted of lath-type lower bainite. The transformation texture of all samples at the mid-thickness position consisted of α, γ and ε-fibers with high intensities close to the transformed copper, transformed brass and rotated cube components. The addition of 0.013 wt% Nb refined the microstructure and sharpened the texture. The texture of the small fraction of retained austenite present in the final microstructures indicated that the main bcc texture components result from the brass and copper components in the parent austenite

Evaluation of mechanical properties and microstructures of molybdenum and niobium microalloyed thermomechanically rolled high-strength press hardening steel

Abstract This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly re-austenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength >1500 MPa and ~ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel

The effect of tempering temperature on microstructure, mechanical properties and bendability of direct-quenched low-alloy strip steel

Abstract The tempering of re-austenized, quenched and tempered (RAQT) martensitic steels is an extensively studied and well understood field of metallurgy. However, a similar understanding of the effect of tempering on direct-quenched (DQ) high-strength steels has been lacking. Now, for the first time, the effect of tempering in the range of 250–650 °C on the strength, toughness, bendability, microstructure, crystallography and dislocation density of a DQ steel is reported. In the case of tempering at 570 °C, the effects of having a RAQ or DQ starting condition are compared. For the composition and thermal cycles studied, it was found that a peak tempering temperature in the range of 570–600 °C resulted in a DQT steel with an optimal balance of strength, bendability and toughness, i.e. a yield strength greater than 960 MPa, a minimum usable bending radius of 2 times the sheet thickness and T28J of − 50 to − 75 °C depending on the test direction. Crystallographic texture, dislocation density and the distribution of carbides are important factors affecting the bendability of DQT strip. Tempering had no effect on texture, but strongly influenced the size and distribution of carbides thereby resulting in differences in bendability and impact toughness transition temperature

High-stress abrasive wear characteristics of ultra-high strength press-hardening steel

Abstract Ultra-high strength steels are widely utilized in many applications operating in harsh abrasive wear conditions. For instance, the machineries used in mining and mineral handling or in agricultural sector require robust, but cost-effective wear-resistant materials. Steels provide excellent combination of mechanical properties and usability. This study encompasses mechanical and wear testing of an experimental medium-carbon press-hardening steel. The as-received material was austenitized at two different temperatures and quenched in water. Additionally, low-temperature tempering was applied for one variant. In total, three variants of the press-hardening steel were produced. Microstructural characterization and mechanical testing were conducted for the steel samples. The wear testing was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed granite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The experimental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of 750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the improved impact toughness by tempering did not significantly affect the wear resistance. The reference steel had nearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness measurements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. It was concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. Therefore, hardness was determined to be the most significant factor affecting the wear performance of studied steels