Study On Microstructure And Properties Of Hot Dip Zinc-Nickel Alloy Coating Of Dual Phase Steel

With the rapid development of new energy automobile industry,today’s material industry is also developing in the direction of producing advanced steel to meet the needs of manufacturing lightweight in modern automobile industry.Hot-dip galvanized high-strength steel due to its excellent formability is widely used in the production of auto parts,but it is easy to appear powdering and failure in the forming process.Therefore,improving the powdering resistance of galvanized high-strength steel can reduce the defective rate of formed parts in actual production.In this paper,the effects of substrate surface quality,Ni content in zinc bath and annealing holding time on the microstructure and properties of hot-dip alloy coating were studied by means of SEM,EDS,XRD,U-bending,the Cupping forming and unidirectional tension,and the following results were obtained:1.With the increase of the roughness of substrate surface,the thickness of Zn-0.06wt%Ni alloy coating decreases.Also,the powdering resistance of alloy coating increases gradually in the process of U-bending,while it increases first and then decreases in the process of the Cupping forming.2.With the increase of Ni content,theζphase thickness of Zn-x Ni(x=0.00wt%,0.03wt%,0.06wt%,0.10wt%,0.12wt%)alloy coating decreases first and then increases,while the thickness ofδphase keeps increasing steadily.When the Ni content is0.10wt%,the thickness ofζphase is the thinnest.It was also found that when the Ni content was 0.06wt%,the content ofδ_pphase occupies relatively more;When the content of Ni is 0.10wt%,the content of Ni in the alloy coatingδ_k phase occupies relatively more.With the increase of Ni content,the powdering resistance of alloy coating increases first and then decreases in the process of U-bending,while it increases first,then decreases and then rises again in the process of the Cupping forming,indicating that the powdering amount of alloy coating is related to the stress state in the forming process,but when the Ni content is 0.06wt%,the powdering resistance of alloy coating under the two kinds of forming methods is the best.In the uniaxial tensile test,with the increase of Ni content,the tensile strength,yield strength and elongation of alloy coated steel plate increased first and then decreased.And when the Ni content is0.06wt%,the mechanical properties of alloy coated steel are the best.3.With the extension of annealing holding time,the surface structure and cross-section structure of Zn-0.06wt%Ni alloy coating will appears phase transformation,the initial stageηphase gradually transforms intoζphase,δ_pphase andδ_k phase,finally forms theΓphase.At the same time,it is found that holes will be formed on the surface of the alloy coating when annealing time exceed 30 s,and the holes will gradually increase with the extension of annealing and holding time until the cracks run through the whole coating.In the unidirectional tensile process,the stress-strain curve of the annealed alloy coated steel shows an obvious yield platform,but compared with the untreated galvanized steel,the mechanical properties of the annealed alloy coated steel reduce.