Performance Comparison of Zn-Based and Al-Si Based Coating on Boron Steel in Hot Stamping.

Long Chen,Miao Cao,Xin Li,Wei Chen

doi:10.3390/ma14227043

Abstract

The coatings of boron steels play an important role in affecting the quality of hot stamping parts, so it is important to evaluate the hot stamping performance of coatings before designing processes. Taking the U-type hot stamping part of boron steel as research objects, the surface quality, microstructure and temperature variation of samples with GA (galvannealed), GI (galvanized) and Al–Si coatings were observed and analyzed to evaluate the anti-oxidation, forming and quenching performances of different coatings. The results show that all the GA, GI and Al–Si coatings could provide good oxidation protection and also act as the lubricants for avoiding the friction damage of sample substrates and die-surface. But the different compositions of GA, GI and Al–Si coatings will contribute the different colors. Under the same deformation degree, the Al–Si coating can provide the best substrate protection and the GI coating will induce cracks in the substrate because of the liquid metal-induced embrittlement phenomenon. There is no significant difference between the quenching performances of GA, GI and Al–Si coatings, and the thermal conductivity of the GI coating is slightly better than Al–Si and GA coatings.

Highlights

Ultra-high strength structural parts have been widely used in automotive, aircraft, buildings and so on for energy-saving and emission reduction [1]
The boron steel hot stamping process is as follows: the quenchable boron steel sheets are heated in a furnace at approximately 930 ◦ C for several minutes to transform the sheets into austenite; subsequently, the austeniteized sheets are quickly transferred from the furnace to the stamping tool and deformed into the required shape with simultaneous rapid cooling in pressurized-closed-die; the ultra-high strength part with a tensile strength of 1500 MPa is taken out from a die after the full martensite transformation of the part during cooling [3]
The typical hot-dip Al–Si coating, used for USIBOR1500 boron steels developed by Arcelor is the eutectic Al–Si alloy that consist of 10 wt.% Si and 90 wt.% Al [7]

Summary

Introduction

Ultra-high strength structural parts have been widely used in automotive, aircraft, buildings and so on for energy-saving and emission reduction [1]. The hot stamping process is a popular technology for producing the ultra-high strength parts from boron steels [2]. The surface oxidation and carbonization of the boron steel sheets during the heating process will severely reduce the final performance of hot stamping parts [4]. The typical hot-dip Al–Si coating, used for USIBOR1500 boron steels developed by Arcelor is the eutectic Al–Si alloy that consist of 10 wt.% Si and 90 wt.% Al [7]. The microcracks, originate from the coating surface and end at the diffusion zone, and this will occur in Al–Si coating even without plastic deformation due to the difference in thermal expansion coefficients of coating components [8]. Increasing the heating temperature or extending the holding time will promote the formation of the iron-rich phase in an Al–Si coating, which will increase the ratio of ductile Fe–Al phase and reduce the amount and depth of cracks in coating [9]

Methods

Discussion

Conclusion