High-Strength Steel Sheet with Excellent Ductility and Crackless Edge Portion, Hot-Dip Galvanized Steel Sheet, and Manufacturing Method Thereof

Abstract

There are provided steel sheets which have a tensile strength of 780-980 MPa and an elongation of 28% or higher and are free of edge cracks. The high-strength steel sheet includes, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfying 1.75%≦Si+Al≦3.25%.

1 . A high-strength steel sheet comprising, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfying 1.75%≦Si+Al≦3.25%. 
     
     
         2 . A high-strength, hot-rolled steel sheet comprising, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfying 1.75%≦Si+Al≦3.25%, wherein the microstructure of the steel sheet after hot rolling comprises 30-70% martensite and a balance of ferrite. 
     
     
         3 . A high-strength, cold-rolled steel sheet comprising by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfying 1.75≦Si+Al≦3.25%, wherein the microstructure of the steel sheet after cold rolling comprises 5-15% retained austenite, 20-40% bainite and a balance of ferrite. 
     
     
         4 . The high-strength steel sheet of  claim 3 , wherein the cold-rolled steel sheet has a tensile strength of 780-980 MPa and an elongation of 28% or higher. 
     
     
         5 . A high-strength, hot-dipped galvanized steel sheet comprising, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfying 1.75≦Si+Al≦3.25%, the steel sheet having a galvanized layer. 
     
     
         6 . A method for manufacturing a high-strength, hot-rolled steel sheet, the method comprising:
 hot-rolling a steel slab, which comprises, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfies 1.75%≦Si+Al≦3.25%, at a temperature higher than an A 3  transformation point, and primarily cooling the hot-rolled steel sheet to a temperature ranging from 600° C. to 800° C. at a cooling rate of 30-200° C./sec;   air-cooling the primarily cooled hot-rolled steel sheet at a temperature ranging from 600° C. to 800° C.;   secondarily cooling the air-dried hot-rolled steel sheet to a temperature ranging from room temperature to 300° C. at a cooling rate of 50-200° C./sec; and   coiling the secondarily cooled hot-rolled steel sheet at a temperature ranging from room temperature to 300° C.   
     
     
         7 . A method for manufacturing a high-strength, cold-rolled steel sheet, the method comprising:
 hot-rolling a steel slab, which comprises, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfies 1.75≦Si+Al≦3.25%, at a temperature higher than an A 3  transformation point or higher, and primarily cooling the hot-rolled steel sheet to a temperature ranging from 600° C. to 800° C. at a cooling rate of 30˜200° C./sec;   air-cooling the primarily cooled hot-rolled steel sheet at a temperature ranging from 600° C. to 800° C.;   secondarily cooling the air-dried hot-rolled steel sheet to a temperature ranging from room temperature to 300° C. at a cooling rate of 50-200° C./sec;   coiling the secondarily cooled hot-rolled steel sheet at a temperature ranging from room temperature to 300° C.; and   cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 30-50% and annealing the cold-rolled steel sheet.   
     
     
         8 . A method for manufacturing a high-strength, hot-dipped, galvanized steel sheet, the method comprising:
 hot-rolling a steel slab, which comprises, by wt %, 0.1-0.25% C, 1.0-1.9% Si, 1.5-2.5% Mn, 0.5-1.6% Al, 0.005-0.03% Ti, 5-30 ppm B, 0.01-0.03% Sb and a balance of Fe and inevitable impurities, and satisfies 1.75%≦Si+Al≦3.25%, at a temperature of an A 3  transformation point or higher, and primarily cooling the hot-rolled steel sheet to a temperature ranging from 600° C. to 800° C. at a cooling rate of 30˜200° C./sec;   air-cooling the primarily cooled hot-rolled steel sheet at a temperature ranging from 600° C. to 800° C.;   secondarily cooling the air-dried hot-rolled steel sheet to a temperature ranging from room temperature to 300° C. at a cooling rate of 50-200° C./sec;   coiling the secondarily cooled hot-rolled steel sheet at a temperature ranging from room temperature to 300° C.;   cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 30-50% and annealing the cold-rolled steel sheet; and   galvanizing the annealed cold-rolled steel sheet.