Coating Degradation in Hot Press Forming

Abstract

The thermo-mechanically induced microstructure changes occurring in a type 1 aluminized coating on hot press forming (HPF) steel were studied in detail. The formation of intermetallic phases at the soaking temperature prior to die quenching revealed that the coating matrix consists mainly of FeAl 2 intermetallic phase by the time the press forming carried out. Kirkendall void formation was observed to take place. The thermal oxidation of aluminized coating during HPF was found to be limited, with the coating acting as an effective barrier for the oxygen during heating. The deterioration caused by the high temperature plastic deformation was shown to lead to coating cracking without loss of adhesion. The steel surface was oxidized where it was exposed between the coating segments.KEY WORDS: hot press forming; aluminized; intermetallic; diffusion; Kirkendall voids. 561© 2010 ISIJ steels are coated with a 25 mm thick Al-10wt%Si type 1 coating. Type 2 coating consists of pure aluminum, and is used mainly for high reflectivity surface appearance, such as building cladding panels and ventilation systems. The typical thickness of type 2 coating is 38-60 mm. 9) ExperimentalThe material used in the present study was cold rolled and continuous hot dipped Al-10wt%Si aluminized steel sheet of 1.6 mm thickness supplied by the POSCO Technical Research Laboratories. The steel chemical composition is given in Table 1. The steel is a CMn steel with additions mainly of Cr and B. Carbon, an austenite stabilizer, is the most effective strengthening elements for martensite via the mechanism of interstitial solute strengthening. Weldability requirements limit the carbon equivalent to a maximum of about 0.5. A carbon content of 0.2 wt% C ensures the decomposition of austenite into a high strength lath martensite microstructure when relatively low cooling rates are used, providing B-additions are made. Mn is an effective hardening addition as it retards most austenite decomposition reactions. Cr is another strong hardenability agent, which effectively suppresses the bainite transformation. B is added to HPF steels to increase the hardenability by etarding the heterogeneous nucleation of ferrite at the austenite grain boundaries. Additions of Al and Ti are often used to avoid the formation of BN. AlN or TiN precipitates are formed as instead. TiN also suppresses grain growth. 10,11) Rectangular specimens with dimension of 40ϫ150 mm were cut. Their weight was approximately 100 g. The specimens had large enough surfaces to minimize the effect of the increased weight caused by the oxidation of the cut edges during high temperature tests. The specimens were transferred to a box furnace with a stationary air atmosphere and kept at 930°C for 2-120 min. The specimens were then removed from the furnace and cooled in air to room temperature. The specimens were weighted before and after the heat treatment. The specimens were grinded and polished with a 1 mm diamond suspension. The cross sectional specimens were not etched in order to avoid the me...

State‐of‐the‐Knowledge on Coating Systems for Hot Stamped Parts

“…The first layer has been identified as Fe 2 Al 5 in the work of Grigorieva and Drillet and co‐workers,15 Suehiro et al,16 and Im et al 18 Fan et al,19 however, identified it as a FeAl 2 layer resulting from the transformation of Fe 2 Al 5 due to the gradually increasing Fe content in the coating. The second layer has been identified as τ 1 Fe 2 SiAl 2 .…”

“…The high temperature behavior of type 1 aluminized coating on hot stamped steel was studied by Drillet,11Grigorieva et al,15 Takagi et al,12 Suehiro et al,16 Maki et al,17 Im et al,18 Fan et al,19 Jenner et al,20 and Borsetto et al21…”

Direct Resistance Joule Heating of Al-10 pct Si-Coated Press Hardening Steel

“…These voids are due to a mass imbalance caused by the large difference in the diffusivity of Fe and Al. [7,14] This microstructure is similar to that of Al-10 pct Si coatings heat treated using a conventional furnace heating. [15,16] The difference in microstructure between direct resistance Joule heating and furnace heating is the absence of the Si-containing layer in the case of direct resistance Joule heating.…”

“…Different coatings have been tested to satisfy these requirements such as Al-alloy coatings, Zn, and Zn alloy coatings. [7,8] Zn-based coatings can in principle provide cathodic protection to PHS. Zn and Zn alloy coatings make the PHS susceptible to liquid metal embrittlement (LME) due to the low melting temperature of Zn and the formation of Zn-Fe intermetallic compounds.…”

Surface Oxide Formation during Rapid Heating of Zn-coated Press Hardening Steel

“…The coating does however not provide cathodic protection, and the corrosion of the underlying steel can occur in areas where the coating layer has been damaged during the diequenching. 15,16) Zn and Zn-alloy coatings have been developed to address this issue, as Zn and Zn-alloys have the advantage of providing cathodic protection to steel. The use of a Zn coating results in a mild form of liquid metal induced embrittlement (LMIE), characterized by microcrack formation and the presence of a thick ZnO layer at the surface after die-quenching.…”