Dual phase steel sheet and method of manufacturing the same

Abstract

The present disclosure relates to a dual phase steel sheet and a method of manufacturing the same. The steel sheet comprises C: 0.05˜0.10% wt %, Si: 0.03˜0.50 wt %, Mn: 1.50˜2.00 wt %, P: greater than 0 wt %˜0.03 wt %, S: greater than 0 wt %˜0.003 wt %, Al: 0.03˜0.50 wt %, Cr: 0.1˜0.2 wt %, Mo: 0.1˜0.20 wt %, Nb: 0.02˜0.04 wt %, B: greater than 0 wt %˜0.005 wt %, N: greater than 0 wt %˜0.01 wt %, and the balance of Fe and other unavoidable impurities. To impart excellent formability, bake hardenability, dent resistance, high Ri value and plating characteristics to the steel sheet for exterior and interior panels of automobiles, the steel sheet is processed to have a dual phase structure through cold rolling, annealing, and hot-dip galvanizing.

What is claimed is: 
     
         1 . A dual phase steel sheet for interior and exterior panels of automobiles, comprising: C: 0.05˜0.10 wt %, Si: 0.03˜0.50 wt %, Mn: 1.50˜2.00 wt %, P: greater than 0 wt %˜0.03 wt %, S: greater than 0 wt %˜0.003 wt %, Al: 0.03˜0.50 wt %, Cr: 0.1˜0.2 wt %, Mo: 0.1˜0.20 wt %, Nb: 0.02˜0.04 wt %, B: greater than 0 wt %˜0.005 wt %, N: greater than 0 wt %˜0.01 wt %, and the balance of Fe and other unavoidable impurities, the steel sheet having a tensile strength (TS) of 440˜590 MPa, and containing ferrite as a main phase, martensite as a secondary phase in a volume fraction of 5˜20% and bainite in a volume fraction of greater than 0% and less than 5%. 
     
     
         2 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has a yield strength (YS) of 270 MPa or more. 
     
     
         3 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has an elongation (El) of 28% or more. 
     
     
         4 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has a work hardening index (n) of 0.15˜0.2. 
     
     
         5 . The dual phase steel sheet of  claim 1 , wherein the steel sheet has an Ri value of 1.0˜2.0. 
     
     
         6 . A method of manufacturing a dual phase steel sheet for interior and exterior panels of automobiles, comprising:
 reheating a steel slab, the steel slab comprising C: 0.05˜0.10% by weight (wt %), Si: 0.03˜0.50 wt %, Mn: 1.50˜2.00 wt %, P: greater than 0 wt %˜0.03 wt %, S: greater than 0 wt %˜0.003 wt %, Al: 0.03˜0.50 wt %, Cr: 0.1˜0.2 wt %, Mo: 0.1˜0.20 wt %, Nb: 0.02˜0.04 wt %, B: greater than 0 wt %˜0.005 wt %, N: greater than 0 wt %˜0.01 wt %, and the balance of Fe and other unavoidable impurities;   hot-rolling the steel slab to prepare a hot-rolled steel sheet;   coiling the hot-rolled steel sheet to prepare a hot-rolled coil;   picking and cold-rolling the steel sheet after uncoiling the hot-rolled coil to prepare a cold-rolled steel sheet; and   annealing the cold-rolled steel sheet to prepare an annealed steel sheet having a dual phase, wherein the annealing is performed on a continuous annealing line, the continuous annealing line including an annealing line on which the steel sheet is heated to a temperature of 750˜850° C. at 10˜20° C./sec and annealed for 100˜110 seconds, a cooling line on which the annealed steel sheet is cooled to 460˜540° C. at 3˜15° C./sec, and an over-aging line on which the cooled steel sheet is subjected to over-aging at 460˜540° C. for 100˜200 seconds.   
     
     
         7 . The method of  claim 6 , wherein the steel slab is produced by preparing molten steel through a steel making process, followed by making an ingot using the molten steel or continuous casting the molten steel. 
     
     
         8 . The method of  claim 6 , wherein the reheating is performed at 1150˜1250° C. for 1.5˜3.5 hours. 
     
     
         9 . The method of  claim 6 , wherein the hot rolling is five-pass hot rolling performed at 800˜900° C. 
     
     
         10 . The method of  claim 6 , wherein the coiling is performed at 550˜650° C. 
     
     
         11 . The method of  claim 6 , wherein the cold-rolling is performed at a reduction ratio of 50˜80%. 
     
     
         12 . The method of  claim 6 , further comprising: hot-dip galvanizing the annealed steel sheet at 480˜560° C. 
     
     
         13 . The method of  claim 6 , wherein the continuous annealing line is operated at a line speed (L/S) of 80˜200 mpm.