METHOD FOR PRODUCING HIGH-STRENGTH GALVANNEALED STEEL SHEETS
First Claim
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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Abstract
A method for producing high-strength galvannealed steel sheets having excellent workability and fatigue resistance. The method comprises subjecting a steel sheet to an oxidation treatment, the oxidation treatment including heating the steel sheet in 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 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 1000 ppm by volume and a H2O concentration of not less than 1000 ppm by volume. The method further comprises subjecting the steel sheet to reducing annealing, hot-dip galvanizing treatment and alloying treatment.
6 Citations
8 Claims
-
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; andthereafter 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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
-
Specification