Werkstoff- und verfahrenstechnische Optimierung bei der Herstellung hochfester Schrauben

Abstract

Schrauben aus hochfesten Stählen und deren wirtschaftliche Fertigung unterliegen hohen Qualitätsansprüchen und werden ständig weiter verbessert. Für hochfeste Schrauben werden üblicherweise MnB-und 1 %Cr-Stähle im vergüteten Zustand verwendet. Als kostengünstige Alternativen gelten thermomechanisch gewalzte Stähle, Dualphasenstähle und AFP-Stähle, die ohne zusätzliche Wärmebehandlung (Vergütung) hergestellt werden können. Ausgehend von der Beschreibung der metallurgischen und walzbedingten Einflüsse bei der Drahtherstellung und deren Auswirkungen auf die Umformbarkeit bei der Schraubenherstellung werden die alternativen Stahlgüten vergleichend dargestellt. Ergänzend dazu werden die verfahrenstechnischen Optimierungsmöglichkeiten bei der Schraubenfertigung betrachtet, wobei primär der Vorzug, die Eigenspannungsentwicklung und die Wärmebehandlung untersucht werden. Durch FE-Simulationen werden die Fertigungsschritte analysiert und mittels Schädigungsmodellen die Grenzen der Umformbarkeit aufgezeigt. Materials and Processing Optimization for the Manufacturing of High Strength Fasteners.Fasteners made from high strength steels and their economical production are faced to high quality requirements and are improved continuously. High strength screws are usually made from MnB and 1 %Cr steels. Alternative cost-saving steel grades are thermomechanically rolled steels, dual phase steels and precipitation hardened ferritic-perlitic steels, which do not need an extra heat treatment (Q&T). Based on the description of the effect of metallurgical influences and the conditions during hot rolling of the wire on the formability, the alternative steel grades are compared. In addition, the measures to optimize the processing parameters during the manufacturing of fasteners are considered, taking into account pre-drawing, residual stresses and heat treatment. By finite element simulations, the fastener production steps and forming limits are analysed by damage models.

Microstructural, Chemical, and Crystallographic Investigations of Dynamic Strain-Induced Ferrite in a Microalloyed QT Steel

“…In order to investigate the crystallographic orientation of DSIT ferrite after route II in the direction of the compression, the data were rotated in the direction of the compression force. The results are shown in Figure 9 and show that a (111) [-110] and (111) [1][2][3][4][5][6][7][8][9][10] texture are present. This means that most of the <111> plane normals are parallel to the compression direction, with the <110> directions pointing towards the radial direction of the compressed sample.…”

“…In order to avoid this cost-intensive annealing step, micro-alloyed and thermomechanical controlled (TMC) processed QT steels have emerged as an alternative. The underlying idea is based on the good formability of fine ferritic microstructures, which can be adjusted using TMC processes and the addition of microalloying elements such as Nb and Ti [1]. A specific form of TMC processing is the formation of ferrite by dynamic strain-induced transformation (DSIT) [2][3][4].…”

Thermomechanical Treatment of Steels - A Real Disruptive Technology Since Decades

“…Depending on the processing conditions, high strength rods and wire as well as rods with improved ductility can be produced. For cold heading applications, an improved ductility can be achieved by lowering the finishing rolling temperature and a slow controlled cooling at the Stelmor conveyor, see Figure . By physical Gleeble simulation, the constitutive equations for the prediction of recrystallization, grain size development must be combined with general models for the prediction of TTT‐diagrams.…”

Virtual Process Design for Cold Formed Components

“…Virtual process design and optimization of combined processing chains have to be an optimization of all multistep operations. Through the coupling of material, design and process the product performance can be improved and the formability limit can be extended [16]. Inherited properties and state values have strong influence and accumulating behaviour on the subsequent processes and at least on the final product performance.…”

“…In many cases this approach seems to give satisfactory results, like information about material flow, folding, mould filling and die forces. But it is not possible this way including the complete component history (for example one or more foregoing wire drawing processes and their influence on the following forging stages [4,5]). It is also not possible to include effects, like elastoplastic and accumulated inherent characteristics such as residual stress-and damage evolution throughout the whole processing chain, which however may have significant influence on the final mechanical product properties [6].…”