Bending Behavior of Zinc‐Coated Hot Stamping Steels

Abstract

The contradictory requirements of increased passenger safety and a simultaneous mass reduction of the body-in-white drive the development of advanced high strength steels in the automotive industry. Especially, components for the safety cell, e.g., reinforcement of B-pillar, bumper, and roof rails, need to be rigid and impede intrusion in the case of a crash event to protect the occupants. [1] For these applications, ultrahigh strength steels with a tensile strength (TS) of up to 2000 MPa and a martensitic microstructure are used. [2] However, their exceptional high strength is accompanied by limited formability, which can be overcome by hot stamping. The combination of shaping by hot forming and the simultaneous microstructure adjustment by quenching in water-cooled dies enables the production of components with complex geometries. Besides the reduced press forces, this manufacturing process also offers the ability to produce tailored blanks and eliminate spring back, which increases with the sheet strength in cold-forming operations. [3,4] While hot stamping was initially used for built-in components, the expansion of their application to more exposed structures leads to the need for corrosion protection. [5] In contrast to AlSi coatings that only provide barrier protection, zinc coatings additionally offer cathodic corrosion protection and, therefore, maintain their protection effect even when the coating layer is breached, e.g., by stone chipping. The major drawback of zinc coatings is their susceptibility to liquid metal embrittlement (LME) during direct hot press forming due to the presence of liquid zinc during deformation. [6] Thus, zinc-coated press hardening steels are mainly produced via the "indirect" process, where the sheet is cold stamped and subsequently subjected to a quenching and calibration operation in the press after full austenitization. Another possibility to avoid LME is by elimination of liquid phases during hot forming. One idea is to increase austenitization time to fully transform the coating into a single-phase solid solution of α-Fe(Zn) ferrite. [7] The disadvantage of this approach, besides the longer process time, is the decreased corrosion protection due to the lower potential difference between matrix and α-Fe(Zn) ferrite compared with zinc-rich Γ-ZnFe. [8] A further possibility is to increase the transfer time [9] or add an additional precooling step [8,10] to solidify all zinc phases, i.e., Γ-ZnFe, prior to the stamping operation. Typical precooling temperatures range between 450 and 650 C. [11] To guarantee a stable process, the composition of the standard hot stamping alloy 22MnB5 has been slightly adapted to 20MnB8. An increased amount of

Experimental Investigation of Hot Forming of Zn‐Coated Hot‐Stamping Steel

“…The increase of annealing time can be used to enhance the Fe element content in coating layer. Hofer et al [ 21 ] analyzed the effect of deformation on the performance of hot‐stamped part with Zn‐coating layer. The crack initiates and progresses along the outside edge of bending specimen.…”

Effect of Heat Treatment on the Formability of Zn‐Coated Hot Stamping Steel

“…They found that the increase in annealing time can eliminate the liquid phase and prevent the LMIE effectively. Hofer et al [ 20 ] analyzed the bending performance of the hot stamping steel with Zn coating. The crack initiation and propagation until failure were studied.…”

Influence of Process Parameters on the Formability of Zn‐Coated Hot Stamping Steel

“…The mechanism of embrittlement crack induced by liquid metal was mainly analyzed in hot stamping process. Hofer et al [18] researched the bending performance of hot DOI: 10.1002/srin.202100474 Herein, the effect of process parameters on the formability of Zn-coated stamping steel is analyzed, including heating temperature, soaking time, forming temperature, deformation level, and strain rate. The studied heating temperature ranges from 850 to 930 C; the soaking time ranges from 3 to 7 min; the deformation level ranges from 10% to the break; and the strain rate ranges from 0.1 to 10 s À1 .…”

“…The mechanism of embrittlement crack induced by liquid metal was mainly analyzed in hot stamping process. Hofer et al [ 18 ] researched the bending performance of hot stamping steel with Zn coating layer. The effect of decarburization, second phase, grain size, and nonmetallic inclusions on the bendability was analyzed.…”