Method for producing high-strength hot-dip galvanized steel sheet

Abstract

A method for producing a high-strength hot-dip galvanized steel sheet is disclosed. In the method, in a direct-fired furnace, in an early stage, a steel sheet is heated to a temperature of not less than 400° C. and not more than 670° C. in an atmosphere containing 1000 ppm by volume or more of O 2 and 1000 ppm by volume or more of H 2 O, and in a later stage, the steel sheet is heated to a temperature of not less than 600° C. and not more than 700° C. in an atmosphere containing 500 ppm by volume or less of O 2 , and in an annealing furnace including a radiant tube-type heating and holding furnace, the steel sheet is held at a temperature of not less than 650° C. and not more than 900° C. for at least 90 seconds in an atmosphere which satisfies certain conditions.

1 . A method for producing a high-strength hot-dip galvanized steel sheet, comprising: a hot rolling step of hot-rolling a slab containing, in % by mass, C: not less than 0.05% and not more than 0.30%, Si: not less than 0.45% and not more than 2.0%, and Mn: not less than 1.0% and not more than 4.0%, and coiling the hot-rolled sheet at a temperature equal to or lower than a temperature T C  (° C.) calculated from the following equation (1), followed by pickling; a cold rolling step of cold-rolling the hot-rolled sheet obtained in the hot rolling step; a step of continuously annealing the cold-rolled steel sheet, obtained in the cold rolling step, in a direct-fired furnace and in an annealing furnace comprising a radiant tube-type heating and holding furnace; and a step of hot-dip galvanizing the annealed steel sheet,
 wherein in the direct-fired furnace, in an early stage, the steel sheet is heated to a temperature of not less than 400° C. and not more than 670° C. in an atmosphere containing 1000 ppm by volume or more of O 2  and 1000 ppm by volume or more of H 2 O, and in a later stage, the steel sheet is heated to a temperature of not less than 600° C. and not more than 700° C. in an atmosphere containing 500 ppm by volume or less of O 2 , and 
 wherein in the annealing furnace comprising the radiant tube-type heating and holding furnace, the steel sheet is held at a temperature of not less than 650° C. and not more than 900° C. for at least 90 seconds in an atmosphere which satisfies the following conditions: the H 2 O concentration is not less than 5000 ppm by volume and not more than 40000 ppm volume, the H 2  concentration is not less than 2% by volume and not more than 20% by volume, and the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −1.1 and not more than 0.5: 
 
       
         
           
             
               
                 
                   
                     
                       T 
                       C 
                     
                     = 
                     
                       
                         
                           - 
                           3 
                         
                         ⁢ 
                         0 
                         ⁢ 
                         
                           ( 
                           
                             
                               [ 
                               Si 
                                 
                               ] 
                             
                               
                             + 
                               
                             
                               [ 
                               Mn 
                                 
                               ] 
                             
                           
                           ) 
                         
                       
                       + 
                       775 
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         where [Si] is the Si content (mass %) of the steel sheet, and [Mn] is the Mn content (mass %) of the steel sheet. 
       
     
     
         2 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 1 , wherein the steel sheet after the hot-dip galvanization is subjected to an alloying treatment. 
     
     
         3 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 1 , further comprising a cooling and heating step of cooling the steel sheet, which has undergone the heating and holding in the radiant tube-type heating and holding furnace, from the final holding temperature during the annealing to a temperature of 150 to 350° C. at an average cooling rate of at least 10° C./sec, and then heating the steel sheet to a temperature of 350 to 600° C. and holding it at that temperature for 10 to 600 seconds. 
     
     
         4 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 2 , further comprising a cooling and heating step of cooling the steel sheet, which has undergone the heating and holding in the radiant tube-type heating and holding furnace, from the final holding temperature during the annealing to a temperature of 150 to 350° C. at an average cooling rate of at least 10° C./sec, and then heating the steel sheet to a temperature of 350 to 600° C. and holding it at that temperature for 10 to 600 seconds. 
     
     
         5 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 1 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.99 and not more than 0.5. 
     
     
         6 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 2 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than 0.99 and not more than 0.5. 
     
     
         7 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 3 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.99 and not more than 0.5. 
     
     
         8 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 4 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.99 and not more than 0.5. 
     
     
         9 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 1 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.9 and not more than 0.5. 
     
     
         10 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 2 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.9 and not more than 0.5. 
     
     
         11 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 3 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.9 and not more than 0.5. 
     
     
         12 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 4 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.9 and not more than 0.5. 
     
     
         13 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 1 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.7 and not more than 0.5. 
     
     
         14 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 2 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.7 and not more than 0.5. 
     
     
         15 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 3 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.7 and not more than 0.5. 
     
     
         16 . The method for producing a high-strength hot-dip galvanized steel sheet according to  claim 4 , wherein the logarithm of the ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2  (P H2 ), i.e. log(P H2O /P H2 ), is not less than −0.7 and not more than 0.5.