Tribological performances of new steel grades for hot stamping tools

Abstract

Abstract. In the last years, the use of High Strength Steels (HSS) as structural parts in car bodyin-white manufacturing has rapidly increased thanks to their favourable strength-to-weight ratio and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coatings and on the analysis of hard PVD, CVD coatings and plasma nitriding applied on the tools. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. On this base, the paper deals with the analysis and comparison the tribological performances in terms of wear, friction and heat transfer of new tool steel grades for high-temperature applications, characterized by a higher thermal conductivity than the commonly used tools. Testing equipment, procedures as well as measurements analyses to evaluate the friction coefficient, the wear and heat transfer phenomena are presented. Emphasis is given on the physical simulation techniques that were specifically developed to reproduce the thermal and mechanical cycles on the metal sheets and dies as in the industrial practice. The reference industrial process is the direct hot stamping of the 22MnB5 HSS coated with the common Al-Si coating for automotive applications. IntroductionThe increasing demand for fuel-efficient vehicles, improved safety, and crashworthiness [1], has led automotive industry to introduce new alloys in car manufacturing, characterized by a high stiffness and strength-to-weight ratio. To meet the need to form these new materials, hot stamping has been widely applied in the automotive industry [2] to produce car body-in-white parts with a high strength-to-weight ratio, which allows a reduction of the fuel consumption [3] and preserves the passengers' safety. Despite the advantages that the hot stamping leads in terms of (i) shorter process chain, especially in the case of direct hot stamping, (ii) increase of mechanical properties just after the deformation process and without the need of additional heat treatments, (iii) reduction of the total weight of the vehicles thanks to thinner components maintaining the frame stiffness, (iv) high shape precision [4], (v) absence of lubricants that are often hazardous for the environment and require expensive cleaning operations [5], friction and wear become particularly critical for the sheet sliding in the dies cavities and their service life [6] and the heating phase, with the subsequent inevitable contact with air during transport and forming, causes the oxidation and decarburization of the metal sheets [7]. For these reasons, the 22MnB5 steel sheets that

Tribological interactions and modelling of friction in hot stamping

“…Furthermore, abrasive wear (Vilaseca, Pujante and Casellas, et al 2014), (J. Pujante, M. Vilaseca and D. Caellas, et al 2016), (Ghiotti, Sgarabotto and Bruschi 2013), (Medea, Ghiotti, et al 2015), (Pujante, Pelcastre, et al 2013), (Rahn and Schruff 2013), thermal fatigue and corrosive wear have all been shown to occur during hot stamping as well (Vilaseca, Pujante and Casellas, et al 2014).…”

“…The choice of tool material is important since the high temperatures induce severe thermal and mechanical cyclic stresses on the dies (Medea, Ghiotti, et al 2015). Besides high resistance to wear, also a high conductivity is desired for tool steels in hot stamping.…”

“…In most studies, a decreasing COF for increasing forming temperature is observed (for some, up to a certain temperature). Reduction in COF is probably due to reduction in shear strength of the coating and easier deformation of asperities at higher temperatures (Medea, Venturato, et al 2017), (Ghiotti, Bruschi and Borsetto 2011). Hardell et al (Hardell, Kassfeldt and Prakash 2008) explain the decrease in COF by formation of compacted layers of oxide and wear debris due to oxidation at elevated temperatures.…”

“…Besides adhesive wear and compaction galling, two and three-body abrasive wear is observed which is caused by hard Al-Si intermetallic compounds in the coating and/or third-body debris such as aluminum oxide particles at the tool-sheet interface (J. Pujante, M. Vilaseca and D. Caellas, et al 2016), (Vilaseca, Pujante and Casellas, et al 2014), (Ghiotti, Sgarabotto and Bruschi 2013), (Medea, Venturato, et al 2017), see Figure 2.6. In production tools, a dynamic process of wear build-up and detachment occurs, resulting in a net adhesion growth (Vilaseca, Pujante and Ramirez 2013).…”

“…Medea et al (Medea, Venturato, et al 2017) reported a change in the main wear mechanism depending on temperature: at 800 °C the main wear mechanism is abrasion, while at 600 °C the adhesion is predominant. The authors relate this change in mechanism to the shear strength of the coating.…”