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METHOD FOR PRODUCING HIGH-STRENGTH GALVANNEALED STEEL SHEETS

  • US 20160304982A1
  • Filed: 12/11/2014
  • Published: 10/20/2016
  • Est. Priority Date: 12/13/2013
  • Status: Active Grant
First Claim
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1. A method for producing high-strength galvannealed steel sheets, the method comprising:

  • subjecting a steel sheet to an oxidation treatment, the steel sheet comprising C;

    not more than 0.3%, by mass %, Si;

    0.1 to 2.5%, by mass %, Mn;

    0.5 to 3.0%, by mass %, and Fe and incidental impurities, the oxidation treatment including heating the steel sheet in (i) an upstream stage at a temperature in the range of 400°

    C. to 750°

    C. in an atmosphere having an O2 concentration of not less than 1000 ppm by volume and a H2O concentration of not less than 1000 ppm by volume and (ii) a downstream stage at a temperature in the range of 600°

    C. to 850°

    C. in an atmosphere having an O2 concentration of less than 100 ppm by volume and a H2O concentration of not less than 1000 ppm by volume;

    then subjecting the steel sheet to reducing annealing, the reducing annealing including heating the steel sheet at a temperature in the range of 650°

    C. to 90°

    C. in an atmosphere containing 5% to 30% of H2, by vol %, 500 ppm to 5000 ppm of H2O, by volume, and a balance of N2 and incidental impurities; and

    thereafter subjecting the steel sheet to a hot-dip galvanizing treatment and to an alloying treatment, the alloying treatment including treating the steel sheet at a temperature T (°

    C.) satisfying the following relationship for 10 to 60 seconds;




    50 log([H2O])+650

    T




    40 log([H2O])+680 

    where [H2O] represents the H2O concentration by volume ppm during the reducing annealing.

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