Chemistry effects on the crack susceptibility of structural steels during continuous casting

This work is a review of research results, which have been determined in recent years in order to reveal the effects of the chemical compositions on the high temperature properties of structural steels. Special emphasis has been laid on the solidification structure, phase reactions and precipitates. For this comparison exemplary different structural steel grades have been chosen. The effect of the chemical composition on the solidification structure is shown by increasing Ni mass contents up to 10 %, the influence on the phase transformation is illustrated on the basis of a steel with a Mn mass content of 1.6 % and varying carbon contents. The influence of precipitates has been investigated both on the basis of the Mn/S‐ratio and by microalloyed structural steels with different Nb, V and Ti additions. For all these steel grades the laboratory testing conditions were the same. The high temperature properties of steels can be investigated by high temperature tensile tests; the range of good ductility is determined by the measurement of the reduction of area, e.g. RoA > 50 % or >70 %. The upper limit of the ductility range is the temperature of zero ductility, T_ZD%. The lower limit of the good ductility range is marked by the transition to the ductility minimum II. The experiments for a series of different structural steels show that the hot ductility properties are affected by the metallurgical phenomena mentioned before.     

First results of process‐studies to enhance the ductility of ferritic steels containing chromium mass contents of more than 8%

Otherwise their drawback is the tendency towards cold brittleness expressed by their low impact energy values. They obtain the high range of ductility only at termperatures higher than room temperature. It was found that this low ductility at room temperature is caused by a high‐temperature embrittlement besides the well‐known low‐temperature 475°C embrittlement. The high‐temperature embrittlement is provoked by submicroscopic precipitates but can be remedied by a special annealing treatment. The results with enhanced ductility of the ferritic microstructure are confirmed under manufacturing conditions for semifinished products such as slabs, billets, bars, wire rod and strips, depending on alloying contents of mainly chromium and aluminium. In the field of near‐net‐shape casting, first results also confirm the same beneficial effect. Using the heat‐content of the casting and rolling for the annealing process, moreover, results in lower process costs.     

Talbrücke Elben – Eine Stahlverbundbrücke mit einteiligem Querschnitt

Viaduct Elben – A steel‐composite bridge with one‐piece cross‐section. In the north of Siegen a non existing section of the German motorway Autobahn A 4 and of the federal highway B 54n, also called “Hüttentalstraße”, was completed during the last years. The 432 m long viaduct – called “Talbrücke Elben”, as an important element in the construction for the extension of the A 4, is in particular interesting because of its design and its construction itself. During the last years about 10 similar constructions have been implemented, others are currently planned. Therefore, it seems appropriate to report about the experiences with steel‐composite bridges in general and particular, always having the bridge “Talbrücke Elben” in mind. Furthermore, the paper intends to give an overview on further fields of application in the near future.     

Residual analysis for ARCH(p)-time series

In this paper we provide consistency and distributional convergence results for functions of residuals from an ARCH(p)-time series. An application to a goodness-of-fit problem is also discussed.     

Das Verformungsverhalten mittengekerbter Großzugproben

Aufgrund der Geometrie und der überwiegenden Zugbeanspruchung weisen die mittengekerbten Großzugproben ein Verformungsverhalten auf, das sich von dem anderer fehlerbehafteter Proben deutlich unterscheidet. Der eingebrachte Fehlerführt dazu, daß die plastischen Verformungen in V-förmigen Bereichen ablaufen. Probenbreite, Fehlerlänge und die Form der Spannungs-Dehnungskurve haben keinen Einfluß auf die Form und die bevorzugte Ausbreitungsrichtung der plastischen Zonen. Die Abhängigkeit der gemessenen Spannungen und Dehnungen der Großproben von der Probenbreite kann anhand von begleitenden FE-Rechnungen durch das Verformungsverhalten erklärt werden. Die FE-Rechnungen ergaben auch, daß das Gesamtverhalten der Großproben unter der Annahme des ebenen Spannungszustands beschrieben werden kann, an der Fehlerspitze jedoch ein dreiachsiger Spannungszustand vorliegt.     

Thermal Rearrangements,XIII [1]. Thermal Isomerization of Exocyclic Allenes

The thermal isomerization of several exocyclic mono- and bis-allenes in a high-temperature flow system has been investigated. At 600°C vinylidenecyclobutane (10) rearranges cleanly to the allene dimer 1,2-bismethylenecyclobutane (12) by an allene-diene isomerization. The methylallene 16 behaves analogously (600°C), although the primary product 18 does not survive under the reaction conditions: the isolated cross-conjugated triene 20 is a product of a 1,5-hydrogen shift followed by an electrocylic ring-opening reaction. Whereas vinylidenecyclopentane (27) yields aromatic products only (800°C, toluene (36), benzene (38)), the five-ring 1,3-bisallene 30 isomerizes to the cyclopentene derivative 43 at 650°C. Vinylidenecyclohexane (46) affords only aromatic products again (760°C, o-xylene (57), 36, 38) and 1,3-bisvinylidenecyclohexane (49) isomerizes to the dienynes 60 and 61 by a series of hydrogen shifts at 550°C. On the other hand, the allene-diene isomerization is observed again, when a C–C bond is doubly activated by an allene and methylene group as in 52 or by two allene moieties as in 53: at 500°C the triene 63 and the exocyclic tetraene 65, respectively, are obtained in good yield.     

Beitrag zur Charakterisierung der Karbonitride in mikrolegierten Stählen

Zur genaueren Kennzeichnung von Karbonitriden, die in mikrolegierten Stählen auftreten, kann ein thermodynamisch abgeleitetes Mischungsmodell aus der Literatur zur Bestimmung der Stöchiometrie herangezogen werden. Im Fall von zwei metallischen Mikrolegierungselementen kann das Modell entsprechend erweitert werden, wenn eine starke thermodynamische Ähnlichkeit beider Mikrolegierungselemente vorhanden ist. Hiermit ist eine Beschreibung von vierkomponentigen Verbindungen in Stählen, die z. B. mit Nb und V legiert sind, möglich. Die Wechselwirkung der Karbonitride mit vorhandenen Aluminiumnitriden kann durch Betrachtung einer schrittweisen Aufteilung des Gesamtstickstoffgehaltes für jede erreichte Temperatur bearbeitet werden. Die Berechnung der Karbonitridverbindungen von drei mikrolegierten Stählen, die entweder nur mit Nb bzw. V sowie mit Nb und V legiert sind, zeigt bei Temperaturen oberhalb 900°C einen zunehmenden Austausch des Kohlenstoffs durch Stickstoff mit steigender Temperatur. Im Stahl, der mit Nb und V legiert ist und in dem vierkomponentige Nb,V-Karbonitride berechnet werden können, führt das gegenüber den reinen Nb-Karbonitriden zusätzlich vorhandene Vanadium zu einer Stabilisierung des Kohlenstoffgehaltes und – bei tiefen Temperaturen – des Karbonitrids allgemein. Zur weiteren Vereinfachung und eindeutigen Charakterisierung der Karbonitride können alle bei 900°C gebildeten Verbindungen als beständig angesehen werden. Durch sie wird das Kornwachstumsverhalten dreier unterschiedlicher Stähle sehr gut beschrieben, was insbesondere den Ansatz von vierkomponentigen Verbindungen im Stahl mit zwei metallischen Mikrolegierungselementen rechtfertigt.     

Feasibility of fluidized bed CVD for the formation of protective coatings

Attempts have been made to clarify the feasibility of a fluidized bed CVD (FBCVD) process as another method for the formation of cementing and diffusion coatings in order to improve wear, oxidation and corrosion resistance of metals. Treating agent powders consist of a donator, usually a powder of the metal, an alloy or a metal carbide or nitride of the coating forming metal, an activator, usually a halide or alkali metal halides, and a filler oxide, usually Al₂O₃, SiO₂ etc. The precursors of the coating forming metal are formed “in situ” the fluidized bed reactor by the reaction of the donator with the activator. The bed can be fluidized by inert or reactive gas or gas mixtures like Ar, N₂, Ar + H₂ etc. As the parts can be charged and withdrawn while the furnace is at treating temperature, this process can be integrated into the heat treatment cycles. The FBCVD process was applied for the formation of Ti-, V-, Cr-, Al-, and Si-bearing coatings. The produced coatings were dense and uniform over the whole specimen surface. According to the results, the use of the FBCVD process is promising since it can lead to a clean, flexible surface modification technology, combining adaptability with relatively low capital and operational costs.     

Toughness requirements for steel structural components with surface flaws

Fracture mechanic safety analyses consist of a comparison between material toughness and toughness requirement of the Investigated structure. For the selection of suitable materials the requirements must be known. The fracture mechanics toughness requirements for a structural component depend on several parameters. Load case, geometry of the component and of a postulated or detected flaw, strength of the steel and the chosen safety criterion are decisive for the toughness requirements. Performing three-dimensional elastic plastic finite element analyses the toughness requirements for plates of high strength structural steels with surface flaws have been determined as a function of the above mentioned parameters.