Method for producing a galvanized steel sheet

Abstract

A method for producing a galvanized steel sheet including (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is 2.1 to 2.8, and the balance being iron and unavoidable impurities, (b) hot rolling and cold rolling the steel from step (a) to provide a steel sheet, and (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point, wherein the galvanized steel sheet has a structure which includes a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.

1 . A method for producing a galvanized steel sheet comprising:
 (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is in a range of 2.1 to 2.8, and the balance being iron and unavoidable impurities,   (b) hot rolling in,a hot rolling step and cold rolling in a cold rolling step the steel from step (a) to provide a steel sheet, and   (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point,   wherein the galvanized steel sheet has a structure which consists of a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.   
     
     
         2 . A method for producing a galvanized steel sheet comprising:
 (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is in a range of 2.2 to 2.8, and the balance being iron and unavoidable impurities,   (b) hot rolling in a hot rolling step and cold rolling in a cold rolling step the steel from step (a) to provide a steel sheet, and   (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point,   wherein the galvanized steel sheet has a structure which consists of a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.   
     
     
         3 . A method for producing a galvanized steel sheet comprising:
 (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is in a range of 2.3 to 2.8, and the balance being iron and unavoidable impurities,   (b) hot rolling in a hot rolling step and cold rolling in a cold rolling step the steel from step (a) to provide a steel sheet, and   (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point,   wherein the galvanized steel sheet has a structure which consists of a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.   
     
     
         4 . A method for producing a galvanized steel sheet comprising:
 (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.35 to 0.8 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is in a range of 2.3 to 2.8, and the balance being iron and unavoidable impurities,   (b) hot rolling in a hot rolling step and cold rolling in a cold rolling step the steel from step (a) to provide a steel sheet, and   (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point,   wherein the galvanized steel sheet has a structure which consists of a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.   
     
     
         5 . The method of producing a galvanized steel sheet according to  claim 1 , wherein the steel composition further comprises one or more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass % or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb. 
     
     
         6 . The method of producing a galvanized steel sheet according to  claim 2 , wherein the steel composition further comprises one or more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass % or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb. 
     
     
         7 . The method of producing a galvanized steel sheet according to  claim 3 , wherein the steel composition further comprises one or more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass % or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb. 
     
     
         8 . The method of producing a galvanized steel sheet according to  claim 4 , wherein the steel composition further comprises one or more of 0.5 mass % or lower Mo, 0.5 mass % or lower V, 0.01 mass % or less B, 0.1 mass % or lower Ti and 0.1 mass % or lower Nb. 
     
     
         9 . The method for producing a galvanized steel sheet according to  claim 1 , wherein zinc which is used to plate the steel sheet is alloyed. 
     
     
         10 . The method for producing a galvanized steel sheet according to  claim 2 , wherein zinc which is used to plate the steel sheet is alloyed. 
     
     
         11 . The method for producing a galvanized steel sheet according to  claim 3 , wherein zinc which is used to plate the steel sheet is alloyed. 
     
     
         12 . The method for producing a galvanized steel sheet according to  claim 4 , wherein zinc which is used to plate the steel sheet is alloyed. 
     
     
         13 . The method for producing a galvanized steel sheet according to  claim 1 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         14 . The method for producing a galvanized steel sheet according to  claim 2 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         15 . The method for producing a galvanized steel sheet according to  claim 3 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         16 . The method for producing a galvanized steel sheet according to  claim 4 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         17 . The method for producing a galvanized steel sheet according to  claim 5 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         18 . The method for producing a galvanized steel sheet according to  claim 6 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         19 . The method for producing a galvanized steel sheet according to  claim 7 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         20 . The method for producing a galvanized steel sheet according to  claim 8 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         21 . The method for producing a galvanized steel sheet according to  claim 9 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         22 . The method for producing a galvanized steel sheet according to  claim 10 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         23 . The method for producing a galvanized steel sheet according to  claim 11 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         24 . The method for producing a galvanized steel sheet according to  claim 12 , wherein a steel sheet after the hot rolling step has a structure containing a low-temperature transformation phase at a volume fraction of 60% or higher. 
     
     
         25 . The method for producing a galvanized steel sheet according to  claim 1 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         26 . The method for producing a galvanized steel sheet according to  claim 2 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         27 . The method for producing a galvanized steel sheet according to  claim 3 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         28 . The method for producing a galvanized steel sheet according to  claim 4 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         29 . The method for producing a galvanized steel sheet according to  claim 5 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         30 . The method for producing a galvanized steel sheet according to  claim 6 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         31 . The method for producing a galvanized steel sheet according to  claim 7 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         32 . The method for producing a galvanized steel sheet according to  claim 8 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         33 . The method for producing a galvanized steel sheet according to  claim 13 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         34 . The method for producing a galvanized steel sheet according to  claim 14 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         35 . The method for producing a galvanized steel sheet according to  claim 15 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         36 . The method for producing a galvanized steel sheet according to  claim 16 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         37 . The method for producing a galvanized steel sheet according to  claim 17 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         38 . The method for producing a galvanized steel sheet according to  claim 18 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         39 . The method for producing a galvanized steel sheet according to  claim 19 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization. 
     
     
         40 . The method for producing a galvanized steel sheet according to  claim 20 , wherein zinc which is used to plate the steel sheet is alloyed after galvanization.