HIGH STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT YIELD STRENGTH, DUCTILITY, AND HOLE EXPANDABILITY, HOT-DIP GALVANIZED STEEL SHEET, AND METHOD FOR PRODUCING SAME
Provided are a high strength cold rolled steel plate having excellent yield strength, ductility, and hole expandability, a hot dip galvanized steel plate, and a method for producing the same. The cold rolled steel plate of the present invention comprises 0.06-0.2 wt % of carbon (C), 1.5-3.0 wt % of manganese (Mn), 0.3-2.5 wt % of silicon (Si), 0.01-0.2 wt % of aluminum (Al), 0.01-3.0 wt % of nickel (Ni), 0.2 wt % or less of molybdenum (Mo), 0.01-0.05 wt % of titanium (Ti), 0.02-0.05 wt % of antimony (Sb), 0.0005-0.003 wt % of boron (B), 0.01 wt % or less (but not 0%) of nitrogen (N), with the remainder comprising Fe and unavoidable impurities, and the microstructure thereof comprises, in terms of area fraction, 50% or more of bainite, 10% or more of tempered martensite (TM), 10% or less of fresh martensite (FM), 20% or less of residual austenite, and 5% or less of ferrite.
1 . A high strength cold-rolled steel sheet having excellent yield strength, ductility, and hole expandability, comprising:
by wt %, 0.06 to 0.2% of carbon (C), 1.5 to 3.0% of manganese (Mn), 0.3 to 2.5% of silicon (Si), 0.01 to 0.2% of aluminum (Al), 0.01 to 3.0% of nickel (Ni), 0.2% or less of molybdenum (Mo), 0.01 to 0.05% of titanium (Ti), 0.02 to 0.05% of antimony (Sb), 0.0005 to 0.003% of boron (B), 0.01% or less of nitrogen (N), excluding 0, and a balance of Fe and inevitable impurities,
wherein a microstructure thereof comprises, by area fraction, bainite of 50% or higher, tempered martensite (TM) of 10% or higher, fresh martensite (FM) of 10% or less, residual austenite of 20% or less, and ferrite of 5% or less.
2 . The high strength cold-rolled steel sheet of claim 1 , wherein a TM/FM ratio exceeds 2.
3 . A hot-dip galvanized steel sheet, manufactured by hot-dip zinc plating a surface of the cold-rolled steel sheet of claim 1 .
4 . An alloyed hot-dip galvanized steel sheet, manufactured by alloy hot-dip zinc plating a surface of the cold-rolled steel sheet of claim 1 .
5 . A method of manufacturing a high strength cold-rolled steel sheet having excellent yield strength, ductility, and hole expandability, the method comprising:
reheating a steel slab comprising by wt %, 0.06 to 0.2% of carbon (C), 1.5 to 3.0% of manganese (Mn), 0.3 to 2.5% of silicon (Si), 0.01 to 0.2% of aluminum (Al), 0.01 to 3.0% of nickel (Ni), 0.2% or less of molybdenum (Mo), 0.01 to 0.05% of titanium (Ti), 0.02 to 0.05% of antimony (Sb), 0.0005 to 0.003% of boron (B), 0.01% or less, excluding 0, of nitrogen (N), and a balance of Fe and inevitable impurities, hot-rolling the steel slab, and performing a coiling process; and
cold-rolling and continuously Q&P annealing the coiled hot-rolled steel sheet,
wherein the continuous Q&P annealing comprises:
soaking the manufactured cold-rolled steel sheet to an Ac3 temperature or higher for 30 seconds or longer, and cooling the cold-rolled steel sheet to a quenching temperature (QT)±10° C. defined by Relational Expression 1 below at a cooling rate of 5 to 20° C./sec,
QT=493.497+36.2874×Al−394.0×C−45.0×Mn−11.4332×Mo−20.8772×Ni−13.0438×Si−12.8×Cr; [Relational Expression 1]
reheating the cooled steel sheet to a bainite temperature (PT)±10° C. defined by Relational Expression 2 below, maintaining the steel sheet within a temperature range of QT≥ or ≥QT−100° C. for 100 seconds, and cooling the steel sheet,
PT=599.088+11.5214×Al−225.2×C−35.0×Mn−19.9474×Ni−24.9385×Si−56.718×Mo−22.1×Cr. [Relational Expression 2]
6 . The method of claim 5 , wherein the steel sheet after the continuous Q&P annealing has a microstructure including, by area fraction, bainite of 50% or higher, tempered martensite (TM) of 10% or higher, fresh martensite (FM) of 10% or less, residual austenite of 20% or less, and ferrite of 5% or less.
7 . The method of claim 6 , wherein a TM/FM ratio exceeds 2.
8 . A method of manufacturing a hot-dip galvanized steel sheet having excellent yield strength, ductility, and hole expandability, the method comprising:
hot-dip zinc plating a surface of the continuously Q&P annealed cold-rolled steel sheet of claim 5 .
9 . A method of manufacturing an alloyed hot-dip galvanized steel sheet having excellent yield strength, ductility, and hole expandability, the method comprising:
alloy hot-dip zinc plating a surface of the continuously Q&P annealed cold-rolled steel sheet of claim 5 .