Hot-dip Zn-Al-Mg coated steel sheet excellent in corrosion resistance and surface appearance and process for the production thereof

A hot-dip Zn—Al—Mg-system plated steel sheet good in corrosion resistance and surface appearance that is a hot-dip Zn-base plated steel sheet obtained by forming on a surface of a steel sheet a plating layer composed of Al:

• 4.0-10 wt. %, Mg:
• 1.0-4.0 wt. %, Ti:
• 0.002-0.1 wt. %, B:
• 0.001-0.045 wt. % and the balance of Zn and unavoidable impurities, the plating layer having a metallic structure including a primary crystal Al phase and a Zn single phase in a matrix of Al/Zn/Zn2Mg ternary eutectic structure.

A method of producing hot-dip Zn—Al—Mg plated steel sheet good in corrosion resistance and surface appearance that is a method of producing a hot-dip Zn—Al—Mg plated steel sheet using a hot-dip plating bath composed of Al:

• 4.0-10 wt. %, Mg:
• 1.0-4.0 wt. %, Ti:
• 0.002-0.1 wt. %, B:
• 0.001-0.045 wt. % and the balance of Zn and unavoidable impurities, characterized in controlling a bath temperature of the plating bath to not lower than the melting point and lower than 410° C. and a post-plating cooling rate to not less than 7° C./s.

A method of producing hot-dip Zn—Al—Mg plated steel sheet good in corrosion resistance and surface appearance that is a method of producing a hot-dip Zn—Al—Mg plated steel sheet using a hot-dip plating bath composed of Al:

• 4.0-10 wt. %, Mg:
• 1.0-4.0 wt. %, Ti:
• 0.002-0.1 wt. %, B:
• 0.001-0.045 wt. % and the balance of Zn and unavoidable impurities, characterized in controlling a bath temperature of the plating bath to not lower than 410° C. and a post-plating cooling rate to not less than 0.5° C./s.

A Mg-containing hot-dip Zn-base plated steel sheet formed with a plated surface whose steepness is not more than 0.1% by, during continuous extraction of a steel strip from a hot-dip plating bath in which it is continuously immersed, which bath is composed of Al:

• 4.0-10 wt. % and Mg:
• 1.0-4.0 wt. %, and, as required, Ti:
• 0.002-0.1 wt. % and B:
• 0.001-0.045 wt. %, and the balance of Zn and unavoidable impurities, controlling a morphology of a Mg oxide-containing coating forming on a surface of a plating layer up to solidification of the surface layer,
• provided that the steepness (%) is a value calculated by Equation (1) from an undulating shape curve of a unit length of a measured undulating shape of the plating surface in a sheet passage direction (lengthwise direction of the strip)
• Steepness (%)=100×Nm×(M+V)/L (1),
• where:
• L=Unit length (set to a value not less than 100×103 μm such as 250×103 μm),
• Nm=Number of mountains within unit length,
• M=Average mountain height within unit length (μm),
• V=Average valley depth within unit length (μm).