Microstructural Evolution of 11Al-3Mg-Zn Ternary Alloy-Coated Steels During Austenitization Heat Treatment
- PDF / 2,607,654 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 49 Downloads / 189 Views
ODUCTION
WEIGHT reduction is one of the main trends in the automobile industry to reduce fuel consumption and carbon dioxide emission. Advanced high-strength steels (AHSS) have thus been widely used in body in white, which also enhances the passengers’ safety and crashworthiness.[1,2] Conventional cold-forming technology for the AHSS, however, has some disadvantages of springbacks and wrinkles because of their high strength.[3] The hot stamping process provides excellent formability.[3–5] Nevertheless, oxidation and decarburization that appear during austenitizaton are associated with the hot stamping process.[4,6] Oxidation-preventive oils have been developed for press-hardening steel without providing additional corrosion protection capability for automotive parts.[7] Surface coatings are thus a necessity for press-hardening steel, including hot-dip aluminizing, hot-dip galvanizing (GI) and galvannealing (GA), and electrogalvanizing Zn-Ni coating.[6,8,9] Among them, hot-dip aluminizing and galvanizing have been commercialized, such as Usibor AluSi, GI, and GA press-hardening steels.[10] Hot-dip aluminizing is the technology most widely employed for press-hardening steel.[9,11–15] Hot-dip aluminized coatings are sufficient to solve the oxidation and decarburization problem via the formation of a compact
JUN-KAI CHANG and CHAO-SUNG LIN are with the Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan. Contact e-mail: [email protected] Manuscript submitted December 28, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Al2O3 layer.[9,11,13,15,16] The resulting Fe-Al alloy layer offers good barrier protection, but it is not enough to withstand harsh environments because of the insufficient sacrificial protection.[17,18] Hot-dip galvanized coatings are generally recognized for providing both the barrier protection and the sacrificial protection on steels.[6,19,20] When a hot-dip galvanized steel is subjected to austenitizaton, Zn-saturated a-Fe (a-Fe(Zn)) and liquid Zn with an Fe content generally lower than approximately 10 wt pct coexist in the alloy coating layer.[6,21–24] When forming is in progress, the liquid Zn(Fe) penetrates the steel substrate and cracking occurs along a-Fe(Zn) grain boundaries and proceeds down to the steel substrate.[21,22] This is called ‘‘liquid metal-induced embrittlement (LMIE),’’[6,21,22,25,26] which markedly deteriorates the formability of steels. Upon cooling, liquid Zn(Fe) and a-Fe(Zn) react to form the C phase by a peritectic reaction occurring at 1055 K (782 °C). The resulting diffusion layer after hot stamping is thus composed of the a-Fe(Zn) and C. Because the a-Fe(Zn) and the C phase have a nobler corrosion potential and less corrosion current density compared with the pure Zn coating,[27,28] the phase transformation of the coating during hot stamping has a direct and, to some extent, positive effect on the corrosion resistance of the press-hardening steel parts. Dosdat et al. compared the corrosion resistance of p
Data Loading...
