High-yield-ratio high-strength galvanized steel sheet and method for producing the same

Abstract

The high-yield-ratio high-strength galvanized steel sheet includes a steel sheet having a specific composition and a metallographic structure including, by area percentage, 20% or less ferrite, 40% or more in total of bainite and tempered martensite, and 60% or less as-quenched martensite, the bainite having an average grain size of 6.0 μm or less, and a coated layer arranged on the steel sheet, the coated layer having a coating weight per side of 20 to 120 g/m2 and having a Mn content of 0.05 g/m2 or less, in which the high-yield-ratio high-strength galvanized steel sheet has a yield ratio of 65% or more and a tensile strength of 950 MPa or more.

1 . A high-yield-ratio high-strength galvanized steel sheet comprising:
 a steel sheet including:
 a composition containing, on a percent by mass basis: 
 C: 0.12% or more and 0.25% or less; 
 Si: less than 1.0%; 
 Mn: 2.0% or more and 3% or less; 
 P: 0.05% or less; 
 S: 0.005% or less; 
 Al: 0.1% or less; 
 N: 0.008% or less; 
 Ca: 0.0003% or less; and 
 0.01% to 0.1% in total of one or more of Ti, Nb, V, and Zr, 
 the balance being Fe and unavoidable impurities, and 
   a metallographic structure containing, by area percentage, 20% or less of ferrite, 40% or more in total of bainite and tempered martensite, and 60% or less of as-quenched martensite, the bainite having an average grain size of 6.0 μm or less; and   a coated layer arranged on the steel sheet, the coated layer having a coating weight per side of 20 to 120 g/m 2  and having a Mn content of 0.05 g/m 2  or less; and   a yield ratio of 65% or more and a tensile strength of 950 MPa or more.   
     
     
         2 . The high-yield-ratio high-strength galvanized steel sheet according to  claim 1 , wherein the composition further contains, on a percent by mass basis, 0.1% to 0.5% in total of one or more of Mo, Cr, Cu, and Ni, and/or B: 0.0003% to 0.005%. 
     
     
         3 . The high-yield-ratio high-strength galvanized steel sheet according to  claim 1 , wherein the composition further contains, on a percent by mass basis, Sb: 0.001% to 0.05%. 
     
     
         4 . A method for producing a high-yield-ratio high-strength galvanized steel sheet, comprising:
 a hot-rolling step of heating a steel material having the composition according to  claim 1  to 1,100° C. or higher and 1,350° C. or lower, hot-rolling the steel material at a finish rolling temperature of 800° C. or higher and 950° C. or lower, and coiling the resulting hot-rolled steel sheet at a temperature of 450° C. or higher and 700° C. or lower;   a cold-rolling step of cold-rolling the hot-rolled steel sheet obtained in the hot-rolling step;   an annealing step of annealing the cold-rolled steel sheet obtained in the cold-rolling step under conditions that a heating temperature T is in a temperature range of an Ac3 point to 950° C., an annealing furnace atmosphere has a temperature in the temperature range and a hydrogen concentration H of 5% or more by volume, a dew point D in the annealing furnace having a temperature in the temperature range satisfies expression (1) described below, a heating time in the temperature range of the Ac3 point to 950° C. is 60 seconds or less, and a retention time in a temperature range of 450° C. to 550° C. is five seconds or more;   a galvanizing step of subjecting the annealed steel sheet obtained in the annealing step to coating treatment and cooling the resulting coated steel sheet to 50° C. or lower at an average cooling rate of 5° C./s or more to form a coating having a coating weight per side of 20 to 120 g/m 2 ; and   a skin-pass rolling step of subjecting the coated steel sheet obtained in the galvanizing step to skin-pass rolling at an elongation percentage of 0.1% or more,
   −40≤ D ≤( T− 1137.5)/7.5  (1)
 
   where in expression (1), D represents the dew point (° C.) in the annealing furnace, and T represents a temperature (° C.) in the annealing furnace.   
     
     
         5 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 4 , wherein the coating treatment is hot-dip galvanizing treatment or treatment in which hot-dip galvanizing is performed and then alloying is performed at a temperature of 450° C. or higher and 600° C. or lower. 
     
     
         6 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 4 , wherein the composition further contains, on a percent by mass basis, 0.1% to 0.5% in total of one or more of Mo, Cr, Cu, and Ni, and/or B: 0.0003% to 0.005%. 
     
     
         7 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 4 , wherein the composition further contains, on a percent by mass basis, Sb: 0.001% to 0.05%. 
     
     
         8 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 6 , wherein the composition further contains, on a percent by mass basis, Sb: 0.001% to 0.05%. 
     
     
         9 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 6 , wherein the coating treatment is hot-dip galvanizing treatment or treatment in which hot-dip galvanizing is performed and then alloying is performed at a temperature of 450° C. or higher and 600° C. or lower. 
     
     
         10 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 7 , wherein the coating treatment is hot-dip galvanizing treatment or treatment in which hot-dip galvanizing is performed and then alloying is performed at a temperature of 450° C. or higher and 600° C. or lower. 
     
     
         11 . The method for producing a high-yield-ratio high-strength galvanized steel sheet according to  claim 8 , wherein the coating treatment is hot-dip galvanizing treatment or treatment in which hot-dip galvanizing is performed and then alloying is performed at a temperature of 450° C. or higher and 600° C. or lower. 
     
     
         12 . The high-yield-ratio high-strength galvanized steel sheet according to  claim 2 , wherein the composition further contains, on a percent by mass basis, Sb: 0.001% to 0.05%.