Hot-dip galvanized steel sheet and manufacturing method therefor

Abstract

Provided is a hot-dip galvanized steel sheet having both high strength and good workability, as well as excellent coating quality. The chemical composition of the base steel sheet is set within a specified range, the steel microstructure of the base steel sheet is a complex structure of ferrite, martensite and bainite, oxygen is present as oxides in the surface layer of the base steel sheet in an amount of 0.05 g/m 2 or more and 0.50 g/m 2 or less per surface, and the hot-dip galvanized layer contains Fe in an amount of 0.40 mass % or more.

1 . A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein
 the base steel sheet comprises   a chemical composition containing, in mass %,
 C: 0.09% or more and 0.17% or less, 
 Si: 0.3% or more and 1.1% or less, 
 Mn: 1.9% or more and 2.7% or less, 
 P: 0.10% or less, 
 S: 0.050% or less, 
 Al: 0.01% or more and 0.20% or less, and 
 N: 0.10% or less, 
 with the balance being Fe and inevitable impurities, and 
   a steel microstructure where
 ferrite has an area ratio of 30% or more and 85% or less, 
 martensite has an area ratio of 5% or more and 30% or less, 
 bainite has an area ratio of 10% or more and 60% or less, and 
 other metallic phases have an area ratio of 15% or less 
 with respect to the entire steel microstructure, 
   oxygen is present as oxides in a surface layer of the base steel sheet in an amount of 0.05 g/m 2  or more and 0.50 g/m 2  or less per surface, where the surface layer is an area from a surface of the base steel sheet to a position at a depth of 100 μm, and   the hot-dip galvanized layer contains Fe in an amount of 0.40 mass % or more.   
     
     
         2 . The hot-dip galvanized steel sheet according to  claim 1 , wherein the other metallic phases have an area ratio of 5% or less. 
     
     
         3 . The hot-dip galvanized steel sheet according to  claim 1 , wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less. 
     
     
         4 . The hot-dip galvanized steel sheet according to  claim 1 , wherein the hot-dip galvanized layer has a coating weight of 20 g/m 2  or more per surface. 
     
     
         5 . The hot-dip galvanized steel sheet according to  claim 1 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         6 . (canceled) 
     
     
         7 . A method of manufacturing a hot-dip galvanized steel sheet, comprising:
 a hot rolling process where a steel slab having the chemical composition as recited in  claim 1  is subjected to hot rolling to obtain a hot-rolled steel sheet,   a cold rolling process where the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet,   an annealing process where the cold-rolled steel sheet is heated to an annealing temperature, held at the annealing temperature, and then cooled, and   then a coating treatment process where the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, wherein   in the annealing process,
 an average heating rate in a temperature range from 500° C. to the annealing temperature is 1° C./s or higher and 7° C./s or lower, 
 the annealing temperature is (A C1  point+50° C.) or higher and (A C3  point+20° C.) or lower, 
 during the holding, a holding time is 1 second or longer and 40 seconds or shorter, 
 during the holding, an atmosphere has a dew point of −20° C. or higher and 5° C. or lower, and a hydrogen concentration is 3 mass % or more and 20 mass % or less, 
 an average cooling rate in a temperature range from the annealing temperature to a primary cooling stop temperature is 10° C./s or higher, 
 the primary cooling stop temperature is 450° C. or higher and 600° C. or lower, 
 a secondary cooling time is 20 seconds or longer and 100 seconds or shorter, and 
 a secondary cooling stop temperature is 400° C. or higher and 500° C. or lower, and 
   in the coating treatment process,
 a temperature of the cold-rolled steel sheet when it enters a coating bath is at least 10° C. higher than a coating bath temperature. 
   
     
     
         8 . The hot-dip galvanized steel sheet according to  claim 2 , wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less. 
     
     
         9 . The hot-dip galvanized steel sheet according to  claim 2 , wherein the hot-dip galvanized layer has a coating weight of 20 g/m 2  or more per surface. 
     
     
         10 . The hot-dip galvanized steel sheet according to  claim 3 , wherein the hot-dip galvanized layer has a coating weight of 20 g/m 2  or more per surface. 
     
     
         11 . The hot-dip galvanized steel sheet according to  claim 8 , wherein the hot-dip galvanized layer has a coating weight of 20 g/m 2  or more per surface. 
     
     
         12 . The hot-dip galvanized steel sheet according to  claim 2 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         13 . The hot-dip galvanized steel sheet according to  claim 3 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         14 . The hot-dip galvanized steel sheet according to  claim 4 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         15 . The hot-dip galvanized steel sheet according to  claim 8 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         16 . The hot-dip galvanized steel sheet according to  claim 9 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         17 . The hot-dip galvanized steel sheet according to  claim 10 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         18 . The hot-dip galvanized steel sheet according to  claim 11 , wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
 Nb: 0.040% or less,   Ti: 0.030% or less,   B: 0.0030% or less,   Cr: 0.3% or less,   Mo: 0.2% or less,   V: 0.065% or less, and   at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.   
     
     
         19 . A method of manufacturing a hot-dip galvanized steel sheet, comprising:
 a hot rolling process where a steel slab having the chemical composition as recited in  claim 5  is subjected to hot rolling to obtain a hot-rolled steel sheet,   a cold rolling process where the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet,   an annealing process where the cold-rolled steel sheet is heated to an annealing temperature, held at the annealing temperature, and then cooled, and   then a coating treatment process where the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, wherein   in the annealing process,
 an average heating rate in a temperature range from 500° C. to the annealing temperature is 1° C./s or higher and 7° C./s or lower, 
 the annealing temperature is (A C1  point+50° C.) or higher and (A C3  point+20° C.) or lower, 
 during the holding, a holding time is 1 second or longer and 40 seconds or shorter, 
 during the holding, an atmosphere has a dew point of −20° C. or higher and 5° C. or lower, and a hydrogen concentration is 3 mass % or more and 20 mass % or less, 
 an average cooling rate in a temperature range from the annealing temperature to a primary cooling stop temperature is 10° C./s or higher, 
 the primary cooling stop temperature is 450° C. or higher and 600° C. or lower, 
 a secondary cooling time is 20 seconds or longer and 100 seconds or shorter, and 
 a secondary cooling stop temperature is 400° C. or higher and 500° C. or lower, and 
   in the coating treatment process,
 a temperature of the cold-rolled steel sheet when it enters a coating bath is at least 10° C. higher than a coating bath temperature.