ZINC-PLATED STEEL SHEET FOR HOT PRESSING HAVING OUTSTANDING SURFACE CHARACTERISTICS, HOT-PRESSED MOULDED PARTS OBTAINED USING THE SAME, AND A PRODUCTION METHOD FOR THE SAME

摘要

In one aspect of the present invention, provided is a zinc-plated steel sheet for hot pressing having outstanding surface characteristics, comprising: a steel foundation plate comprising a surface diffusion layer of a metal of which the Gibbs free energy reduction per mole of oxygen during oxidation is less than that of Cr, within a depth of 1 µm from the surface; an aluminium-rich layer containing at least 30 wt.% of aluminium formed on the surface diffusion layer of the metal of which the Gibbs free energy reduction per mole of oxygen during oxidation is less than that of Cr; and a zinc plating layer formed on the aluminium-rich layer; wherein annealing oxides having a mean thickness of no more than 150 nm are distributed in a non-continuous fashion between the surface diffusion layer and the aluminium-rich layer, and there is a content of at least 0.1 wt.% of the metal of which the Gibbs free energy reduction per mole of oxygen during oxidation is less than that of Cr, within a depth of 1 µm from the surface of the steel foundation plate. In this way, a metal having a low affinity for oxygen is coated to an effective thickness prior to annealing and thus the creation of annealing oxides at the surface of the steel sheet is suppressed and a uniform zinc plating layer is formed, and alloying of the zinc plating layer is promoted during press-processing heat treatment such that the melt temperature of the zinc plating layer is increased within a short time, thereby making it possible to prevent deterioration of the plating layer and to minimise the occurrence of internal oxides formed after hot press moulding. Also, according to another aspect of the present invention, an oxide layer able to prevent deterioration of the zinc plating layer is formed on the surface of the plating layer during hot press heating, and a three element phase of Zn, Fe and the metal is formed in the plating layer such that the zinc plating layer can be kept stable, a good surface condition is ensured and the phosphatability is outstanding, the plating properties during electrodeposition plating and film adhesion can be ensured even without carrying out a separate phosphating process, and workability can be improved as cracking in the steel foundation plate during hot press moulding is prevented.