Process for the phosphate chemical conversion treatment of a steel material

A process of phosphate conversion treatment for a steel material comprising bringing a steel material into contact with a phosphate chemical conversion treatment liquid containing mixed anions consisting of phosphate ions and at least one other type of active anions, at least one type of metal ions capable of forming a chemical conversion coating layer, and an oxidizing agent, to provide a phosphate chemical conversion coating layer on the surfaces of the steel material, wherein the temperature of the phosphate chemical conversion treatment liquid is controlled to a level of 40.degree. C. or less without external heating of the liquid, and the composition of the phosphate chemical conversion treatment liquid is controlled in a manner such that the ratio )P/An) of a weight (P) of the phosphate ions to a total weight (An) of the mixed anions therein is 1/2 or less, the pH thereof is in the range of from 0.5 to 4.5 and the oxidation-reduction potential (hydrogen standard electrode potential) of at least 300 mV wherein, the process as claimed in claim 12, wherein, in the control of the ratio P/An, the pH and the oxidation-reduction potential of the phosphate chemical conversion treatment liquid respectively to a predetermined level.

• (A) when the pH of the treatment liquid becomes higher than a predetermined maximum level, a principal feed including the chemical conversion coating layer-forming metal ions and the active anions is added to the treatment liquid, and when the pH of the treatment liquid becomes lower than a predetermined minimum level, an aqueous alkali solution is added to the treatment liquid;
• (B) when the ORP of the treatment liquid becomes higher than a predetermined maximum level, an aqueous solution containing Fe.sup.2+ ions is added to the treatment liquid, and when the ORP of the treatment liquid becomes lower than a predetermined minimum level, an aqueous solution containing an oxidizing agent is added to the treatment liquid, and
• (C) when the electroconductivity (EC) of the treatment liquid becomes lower than a predetermined minimum value, the above-mentioned principal feed is added to the treatment liquid, and when the EC of the treatment liquid becomes higher than a predetermined maximum level, the principal feed is not added to the treatment liquid even if the value of the pH is higher than the predetermined maximum level.