Copper-Alloyed PHFP Steel for Energy-Efficient and Distortion-Reduced Production of Cold-Formed, High-Strength Structural Components

Abstract

This article reports on the development of a hardenable PHFP steel for energy-efficient and distortion-reduced production of cold-formed, high-strength structural components. Based on an alloying concept containing 0.8 wt.-% copper, a technology for the production of screws has been developed to exploit the precipitation-hardening effect of copper for increasing strength by tempering while avoiding final quenching.

Full text

Copper-Alloyed PHFP Steel for Energy-Efficient and Distortion-Reduced
Production of Cold-Formed, High-Strength Structural Components
Stefan Meiler1,a*, Frank Hoffmann1, Hannah Schwich2, Rudolf Kawalla1,
Wolfgang Bleck2, Ulrich Prahl1
1Institute of Metal Forming, TU BA Freiberg, Bernhard-von Cotta-Str. 4, 09599 Freiberg, Germany
2Steel Institute, RWTH Aachen University, Intzestr. 1, 52072 Aachen, Germany
aStefan.Meiler@imf.tu-freiberg.de
Keywords: PFHP-steel, Copper, Precipitation-hardening, Cold forging, Screws
Abstract. This article reports on the development of a hardenable PHFP steel for energy-efficient
and distortion-reduced production of cold-formed, high-strength structural components. Based on an
alloying concept containing 0.8 wt.-% copper, a technology for the production of screws has been
developed to exploit the precipitation-hardening effect of copper for increasing strength by tempering
while avoiding final quenching.
Introduction
During steel recycling, undesirable trace elements that cannot be removed from the melt under
oxidizing conditions are an important issue. In particular, copper is in the spotlight as it noticeably
deteriorates the forming properties of steel, among other effects. In future, increasing use of copper
in the field of electro mobility will exacerbate the problem of rising copper contents in the steel
cycle [1]. Therefore, it is of importance to develop alloying and process concepts enabling to take
advantage of rising copper content in the cycle.
This production technology comprises the entire process chain including hot wire rolling, cold
component forging and temper treatment. Here, the hot forming strategy with controlled cooling aims
to completely dissolve the copper during austenitization, to achieve grain refinement by
recrystallization and to control cooling in such a way that a high copper fraction remains in solution
and at the same time a cold-formable microstructure with large fraction of ferrite/pearlite is created.
Subsequently, work hardening is carried out by cold drawing and forging in relative soft condition.
During final component tempering, the dissolved copper shall precipitate leading to a strength
increase by precipitation hardening. In contrast to a conventional quenching and tempering process,
temper annealing is not associated with phase changes. Thus, distortion of the component can be
reduced significantly, so that cost-intensive hard machining can be eliminated [2]. The process chain
is shortened, loss of material due to scale formation can be avoided and the use of protective
atmospheres while tempering is not necessary [3-5].
Alloy Development
Based on numerical simulations of the precipitation kinetics of copper precipitates and equilibrium
diagrams with Thermo-Calc and MatCalc, optimal copper contents for precipitation hardening were
determined. At >1.5 wt.-% Cu the formation of 9R precipitates is possible, which can be used for
hardening. To keep the copper in solution after hot forming, a precise and fast cooling rate is
necessary. However, as premature precipitation of copper is more and more difficult to prevent with
increasing copper contents of >1 wt.-% and preliminary precipitation behavior tests have shown that
copper contents of 1.5 wt.-% compared to copper contents of 0.8 wt.-% have no advantages in
precipitation hardening, the following alloy was selected (Table 1). It has been shown that the initial
hardness of 193 HV (≈ 618 MPa) after hot forming of alloy described in Table 1 is too high for various
application areas targeted in the project, such as the production of cold-formed structural components.
Materials Science Forum Submitted: 2018-06-01
ISSN: 1662-9752, Vol. 959, pp 161-165 Revised: 2019-01-02
doi:10.4028/www.scientific.net/MSF.959.161 Accepted: 2019-03-11
© 2019 Trans Tech Publications Ltd, Switzerland Online: 2019-06-21
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