High strength cold-rolled steel sheet and manufacturing method therefor

Abstract

Provided is a high-strength cold-rolled steel plate. According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate includes: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities. According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate has a microstructure including, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (martensite-austenite composite phase) and the remainder being martensite.

1 . A high-strength cold-rolled steel plate, comprising: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities,
 wherein the high-strength cold-rolled steel plate has a microstructure comprising, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (martensite-austenite composite phase) and a remainder being martensite, and   the high-strength cold-rolled steel plate has a flatness (deformation height) of 3.0 mm or less.   
     
     
         2 . The high-strength cold-rolled steel plate according to  claim 1 , further comprising one or more of niobium (Nb), titanium (Ti) and vanadium (V),
 wherein a total content of niobium (Nb), titanium (Ti) and vanadium (V) is 0.05% or less (greater than 0).   
     
     
         3 . The high-strength cold-rolled steel plate according to  claim 1 , wherein the carbon concentration in the retained austenite is 1.1% to 1.4% in % by weight. 
     
     
         4 . The high-strength cold-rolled steel plate according to  claim 1 , wherein the high-strength cold-rolled steel plate has a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation index (EI) of 20% or more and a tensile strength*elongation index of 20,000 MPa % or more. 
     
     
         5 . The high-strength cold-rolled steel plate according to  claim 1 , wherein the martensite comprises fresh martensite and tempered martensite, and
 a value (FM/TM) obtained by dividing an area ratio of the fresh martensite (FM) by an area ratio of the tempered martensite (TM) ranges from 0.1 to 0.6.   
     
     
         6 . The high-strength cold-rolled steel plate according to  claim 1 , wherein an area ratio of retained austenite having an aspect ratio of 3 or more, which is obtained by dividing a major axis length of the retained austenite by a minor axis length thereof, ranges from 3% to 8%. 
     
     
         7 . The high-strength cold-rolled steel plate according to  claim 6 , wherein a value obtained by dividing a total area ratio of the retained austenite by the area ratio of the retained austenite having an aspect ratio of 3 or more ranges from 0.5 to 0.8. 
     
     
         8 . A method of manufacturing a high-strength cold-rolled steel plate, the method comprising:
 hot-rolling a steel comprising: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities to provide a hot-rolled steel plate;   cold-rolling the hot-rolled steel plate to provide a cold-rolled steel plate;   annealing the cold-rolled steel plate in a dual-phase temperature range of austenite and ferrite;   first cooling the annealed cold-rolled steel plate at a first cooling rate;   second cooling the first cooled cold-rolled steel plate at a cooling rate higher than the first cooling rate of the first cooling;   partitioning by re-heating the second cooled cold-rolled steel plate, and by third cooling at a cooling rate of 0.07° C./sec to 0.21° C./sec immediately when reaching a target temperature of 350° C. to 460° C.   
     
     
         9 . The method according to  claim 8 , wherein first cooling the annealed cold-rolled steel plate is at a first cooling rate in a first cooling end temperature of 650° C. to 800° C. 
     
     
         10 . The method according to  claim 9 , wherein the high-strength hot-rolled steel plate further comprises one or more of niobium (Nb), titanium (Ti) and vanadium (V),
 wherein a total content of niobium (Nb), titanium (Ti) and vanadium (V) is 0.05% or less (greater than 0).   
     
     
         11 . The method according to  claim 8 , wherein the second cooling comprises first rapid cooling and second rapid cooling,
 wherein the first rapid cooling is performed by cooling up to a first rapid cooling end temperature of Ms (martensite transformation start temperature) or higher at a cooling rate of 70° C./sec to 110° C./sec, and   the second rapid cooling is performed by cooling up to a second rapid cooling end temperature being lower than Ms and above Mf (martensite transformation end temperature) at a cooling rate of 30° C./sec or higher and lower than 70° C./sec.   
     
     
         12 . The method according to  claim 11 , wherein the first rapid cooling end temperature ranges from 320° C. to 350° C., and
 the second rapid cooling end temperature ranges from 200° C. to 260° C. 
 
     
     
         13 . The method according to  claim 8 , wherein the partitioning is performed for a time ranging from 30 sec to 600 sec. 
     
     
         14 . The method according to  claim 8 , wherein the dual-phase temperature range is 780° C. to 860° C. 
     
     
         15 . The method according to  claim 8 , wherein the first cooling end temperature ranges from 680° C. to 800° C. 
     
     
         16 . The method according to  claim 8 , wherein a third cooling end temperature during the third cooling ranges from 340° C. to 400° C.