Patent ID: 8491734

Claim:
A process of production of a high strength galvannealed steel sheet, comprising pickling a hot-rolled steel sheet having a content of Si of 0.4 to 2.0 wt %, then, cold-rolling the steel sheet, then, continuously hot-dip galvanizing the steel sheet in an all radiant tube type annealing furnace with a reducing atmosphere and without an oxidizing zone, wherein the all radiant tube type annealing furnace comprises a heating zone ( 2 ), a soaking zone ( 3 ), a cooling zone ( 4 ), a reducing gas pipe ( 14 ) connecting the heating zone ( 2 ), soaking zone ( 3 ), and cooling zone ( 4 ), and a combustion gas pipe ( 17 ) connecting the heating zone ( 2 ) and soaking zone ( 3 ), and in the all radiant tube type annealing furnace, a reducing gas ( 15 ) flows into the heating zone ( 2 ), soaking zone ( 3 ), and cooling zone ( 4 ), and a combustion gas ( 18 ) flows into the heating zone ( 2 ) and soaking zone ( 3 ), and the atmosphere is made not to cause iron to oxidize, during the continuously hot-dip galvanizing: introducing the combustion gas ( 18 ) containing CO 2 in an amount of 1 to 100 wt % and a balance of N 2 , H 2 O, O 2 , CO, and unavoidable impurities into the heating zone ( 2 ), and soaking zone ( 3 ) of the annealing furnace, making the atmosphere of the annealing furnace an atmosphere containing H 2 to 1 to 60 wt % and the balance being N 2 , H 2 O, O 2 , CO 2 , CO, and unavoidable impurities, controlling, in the atmosphere, (i) the log(PCO 2 /PH 2 ) of the carbon dioxide partial pressure and hydrogen partial pressure to log(PCO 2 /PH 2 )≦−0.5, (ii) the log(PH 2 O/PH 2 ) of the water partial pressure and hydrogen partial pressure to log(PH 2 O/PH 2 )≦−0.5, and (iii) the log(P T /PH 2 ) of (a) the total partial pressure P T of the carbon dioxide partial pressure PCO 2 , the water partial pressure PH 2 O, and the hydrogen partial pressure PH 2 and (b) the hydrogen partial pressure PH 2 to −3≦log(P T /PH 2 )≦−0.5, performing annealing in the reducing atmosphere in a ferrite-austenite two-phase temperature region at 720° C. to 880° C., then cooling by a plating bath and performing molten zinc plating so as to form a hot-dip galvanizing layer on the surface of the steel sheet, and then heating for alloying the steel sheet on which the hot-dip galvanizing layer is formed at 460 to 550° C., so as to produce a high strength galvannealed steel sheet.