Flux and production method of steel product with hot-dip Zn—Al—Mg coating using said flux

A flux for hot dip Zn—Al—Mg-based alloy plating, comprising:

• ZnCl2;
• and
• a low-reactive chloride having lower reactivity with Mg in a plating bath than the ZnCl2, the low-reactive chloride containing at least two chlorides selected from the compound group consisting of alkali metal chlorides and alkaline-earth metal chlorides, the low-reactive chloride being a chloride having free energy of formation that is lower than that of MgCl2,
• said flux being in a pellet form and substantially not containing NH4Cl and substantially not containing NaF,
• a composition of the ZnCl2 and the low-reactive chloride being adjusted so that a liquidus temperature of said flux is 450° C. or lower even in a case where all of the ZnCl2 contained in said flux has been substituted by MgCl2,
• the low-reactive chloride containing NaCl, and KCl, the flux containing an auxiliary chloride in an amount of more than 0% by mass and not more than 7.5% by mass in total, the auxiliary chloride including at least one chloride among a chloride of Sn, a chloride of Pb, and a chloride of Bi, and
• where a total mass of (i) the ZnCl2 and (ii) the low-reactive chloride is assumed to be 100 parts by mass,
• a contained amount of ZnCl2 being not less than 52.5% by mass and not more than 74.1% by mass,
• a total contained amount of the NaCl and KCl being not less than 25.9% by mass and not more than 47.5% by mass, and
• a ratio of a mass of KCl to a mass of NaCl (KCl/NaCl) being 0.15 or more and 11.5 or less.