Acta Metall Sin  2020, Vol. 56 Issue (6): 863-873    DOI: 10.11900/0412.1961.2019.00352
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Effect of the Interface Microstructure of Hot-Dip Galvanizing High-Strength Automobile Steel on Its Tensile Fracture Behaviors
YU Jiaying1, WANG Hua1, ZHENG Weisen1, HE Yanlin1(), WU Yurui2, LI Lin1
1.School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
2.Saic Motor Corporation Limited Passenger Vehicle Co. , Shanghai 201804, China
Abstract

Hot-dip galvanizing is the most economical way to improve the corrosion resistance of advanced high strength automobile steel. With the high trend of developing of automobile steel towards light-weight and high-strength, the contents of Si, Mn, Al alloy elements in steel increase accordingly. These alloy elements would be selectively oxidized during hot-dip galvanizing process, which affect in turn the wettability of steel surface and form different interface microstructures. However, its effect on the mechanical behavior of steel has never been known as clear as desired. Base on this point, the thermodynamics of the surface oxide and its effect on the interface microstructure of three high-strength automobile steels were studied after the same hot-dip galvanizing treatment as well as their tensile fracture behavior under different deformation conditions was in situ analyzed. Combined with the microstructure analysis and thermodynamic calculation, it can be concluded that different compositions of steel would produce different kinds of oxide on its surface. When Mn2SiO4 and SiO2 were formed as thermodynamic stable phases, it was difficult to form a continuous Fe2Al5Zn0.4 inhibition layer at the interface, zinc liquid could penetrate into the iron substrate and then form the brittle phase ζ-FeZn13, where the crack was easily to be obtained and expanded to the substrate, resulting in the decrease of mechanical properties. When MnO and Mn2SiO4 with a small amount were formed as thermodynamic stable phase, the Fe2Al5Zn0.4 inhibition layer can be obtained. Under the tensile stress, this crack generated at the interface and extended to the zinc layer. So, the fracture of the experimental steel was mainly resulted from the failure of substrate. When MnO and FeO were formed as metastable phase, Fe, as formed by aluminothermic reduction during hot-dip galvanizing, would reacted with zinc liquid to form Γ-Fe11Zn40 phase in the zinc layer. The crack generated under the tensile stress and expanded in the zinc layer. Since the unreduced MnO layer at the interface exhibited a higher bonding strength with substrate, tensile fracture of the experimental steel was caused by the failure of substrate.

 ZTFLH: TQ153.1
Fund: National Key Research and Development Program of China(2017YFB0304402)
Corresponding Authors:  HE Yanlin     E-mail:  ylhe@t.shu.edu.cn
 Table 1  Chemical compositions of three high-strength steels Fig.1  Schematic of tensile sample size for in situ analysis (unit: mm) Fig.2  SEM images and EDS analyses of the inhibition layer in steels No.1 (a), No.2 (b) and No.3 (c) corroded by hydrochloric acid, and steel No.2 corroded by sulphuric acid (d) Fig.3  Cross-sectional TEM analyses of the steel/Zn coating interface of the galvanizing steel No.1 annealed at 870 ℃ in a gas atmosphere with a dew point of +10 ℃Color online(a) TEM image and EDS elemental distribution maps of interface(b, c) dark field TEM images and corresponding SAED patterns (insets) of Fe2Al5Zn0.4 and MnO in inhibition layer, respectively(d) dark field TEM image and corresponding SAED pattern (inset) of Fe substrate Fig.4  Cross-sectional TEM analyses of the steel/Zn coating interface of the galvanizing steel No.2 annealed at 870 ℃ in a gas atmosphere with a dew point of +10 ℃(a) TEM image and EDS elemental distribution maps of interface(b, c) dark field TEM images and corresponding SAED patterns (insets) of Fe2Al5Zn0.4 and Al2O3 in inhibition layer, respectively (d, e) dark field TEM images and corresponding SAED patterns (insets) of ferrite and ζ-FeZn13 in Fe substrate, respectivelyColor online Fig.5  Cross-sectional TEM analyses of the steel/Zn coating interface of the galvanizing steel No.3 annealed at 870 ℃ in a gas atmosphere with a dew point of +10 ℃Color online(a) TEM image and EDS elemental distribution maps of interface(b) dark field TEM image and corresponding SAED pattern (inset) of MnO layer(c) dark field TEM image and corresponding SAED pattern (inset) of Γ-Fe11Zn40 in Zn coating(d, e) dark field TEM images and corresponding SAED patterns (insets) of ferrite and Al2O3 in Fe substrate, respectively Fig.6  Contents of surface oxide on high-strength steels No.1 (a), No.2 (b) and No.3 (c) under different O2 partial pressures ($pO2$), and comparison among the three steels at $pO2$=4.69×10-21 MPa (d) Fig.7  In situ analyses of cracking of steel No.1 under different deformationsColor online(a) EDS line scan analysis (along dash arrow, inset) of the steel/coating interface, respectively(b, c) the cracking in coating under 6% and 9% deformations(d) fracture zone under 12% deformation Fig.8  In situ analyses of cracking of steel No.2 under different deformationsColor online(a) EDS line scan analysis (along dash arrow, inset) of the steel/coating interface(b) the cracking in coating under 6% deformation(c) EDS analysis of Fe-Zn compounds and the cracking in coating under 12% deformation(d) fracture zone under 14% deformation Fig.9  In situ analyses of cracking of steel No.3 under different deformationsColor online(a) EDS line scan analysis (along dash arrow, inset) of the steel/coating interface(b, c) the cracking in coating under 3% and 9% deformations, respectively(d) fracture zone under 17% deformation Fig.10  Engineering stress-engineering strain curves of high-strength coated and uncoated steels No.1 (a), No.2 (b) and No.3 (c) Fig.11  Ultimate tensile strength (UTS) (a), elongation (b) and product of strength and elongation (PSE) (c) of three high-strength coated and uncoated steels Fig.12  Schematics of cracking mechanism at the coating and steel interface of high strength steels No.1 (a), No.2 (b) and No.3 (c)