Alloyed hot-dip galvanized steel sheet and production method thereof

Abstract

Disclosed is an alloyed hot-dip galvanized steel sheet containing 2.0 to 3.5 percent by mass of Mn. The steel sheet includes a base steel sheet and a galvanized zinc-coat layer thereon, in which MnO particles are present in an average number of 10 or less per micrometer on a straight line lying in an interface between the galvanized zinc-coat layer and the steel sheet, an Fe—Al—O alloy layer is present at the interface between the MnO particles and the steel sheet, and the length of the Fe—Al—O alloy layer is less than 10% of the overall length of the interface. The alloyed hot-dip galvanized steel sheet, even though having a high Mn content, is resistant to uneven alloying and excels in surface appearance, because the amounts of the MnO particles and the Fe—Al—O alloy layer that cause uneven alloying are controlled.

1 . An alloyed hot-dip galvanized steel sheet, including a base steel sheet and a galvanized zinc-coat layer on the steel sheet, the galvanized zinc-coat layer and the steel sheet being alloyed, a steel constituting the base steel sheet comprising, on the mass basis, 0.02% to 0.2% of carbon (C), 2.0% to 3.5% of manganese (Mn), 0.03% to 0.5% of chromium (Cr), 0.01% to 0.15% of aluminum (Al), 0.04% or less (including 0%) of silicon (Si), 0.03% or less (including 0%) of phosphorus (P), and 0.03% or less (including 0%) of sulfur (S), with the remainder including iron (Fe) and inevitable impurities,
 wherein MnO particles are present on an arbitrary straight line lying in an interface between the galvanized zinc-coat layer and the steel sheet in an average number of 10 or less per micrometer of the straight line,   wherein an Fe—Al—O alloy layer is present at an interface between the MnO particles and the steel sheet, and   wherein the length of the Fe—Al—O alloy layer is less than 10% of the overall length of the interface, each of the length and the overall length being measured on the arbitrary straight line.   
     
     
         2 . The alloyed hot-dip galvanized steel sheet according to  claim 1 , wherein the steel further comprises, on the mass basis, a total of 0.003%, to 1.0% of at least one member selected from the group consisting of 0.003% to 0.5% of copper (Cu), 0.003% to 1.0% of nickel (Ni), and 0.003% to 1.0% of titanium (Ti). 
     
     
         3 . The alloyed hot-dip galvanized steel sheet according to  claim 1 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.003% to 1.0% of vanadium (V), 0.003% to 1.0% of niobium (Nb), 0.0002% to 0.1% of boron (B), and 0.003% to 1.0% of molybdenum (Mo). 
     
     
         4 . The alloyed hot-dip galvanized steel sheet according to  claim 2 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.003% to 1.0% of vanadium (V), 0.003% to 1.0% of niobium (Nb), 0.0002% to 0.1% of boron (B), and 0.003% to 1.0% of molybdenum (Mo). 
     
     
         5 . The alloyed hot-dip galvanized steel sheet according to  claim 1 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.0005% to 0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg). 
     
     
         6 . The alloyed hot-dip galvanized steel sheet according to  claim 2 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.0005% to 0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg). 
     
     
         7 . The alloyed hot-dip galvanized steel sheet according to  claim 3 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.0005% to 0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg). 
     
     
         8 . The alloyed hot-dip galvanized steel sheet according to  claim 4 , wherein the steel further comprises, on the mass basis, at least one member selected from the group consisting of 0.0005% to 0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg). 
     
     
         9 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 1 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         10 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 2 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         11 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 3 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         12 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 3 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         13 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 4 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         14 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 5 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         15 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 6 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         16 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 7 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.   
     
     
         17 . A method for producing the alloyed hot-dip galvanized steel sheet of  claim 8 , the method comprising the steps of:
 carrying out annealing of the steel sheet under such conditions that an oxygen partial pressure PO 2  (in units of atmospheric pressure (atm)) satisfies the following condition: −log(PO 2 )≧20;   galvanizing the annealed steel sheet to form the galvanized zinc-coat layer on the surface of the steel sheet; and   alloying the steel sheet bearing the galvanized zinc-coat layer.