High strength steel sheet having excellent formability and method for manufacturing the same

Abstract

A thin high strength steel sheet having excellent formability has a composition which includes, by mass %, 0.08 to 0.15% of C, 0.5 to 1.5% of Si, 0.5 to 1.5% of Mn, 0.01 to 0.1% of Al, and 0.005% or less of N to form a hot rolled sheet is conducted.

1 . A high strength steel sheet having excellent formability which has a composition comprising, by mass %:
 0.08 to 0.15% of C, 0.5 to 1.5% of Si,   0.5 to 1.5% of Mn, 0.1% or less of P,   0.01% or less of S, 0.01 to 0.1% of Al,   0.005% or less of N,   
       and the balance Fe with inevitable impurities, the steel sheet having a microstructure composed of a ferrite phase which is a main phase and a second phase including at least pearlite, wherein an area fraction of the ferrite phase is 75 to 90% and an area fraction of the pearlite is 10 to 25% with respect to the entire microstructure, an average grain size of the pearlite is 5 μm or smaller, and an area fraction of the pearlite is 70% or greater with respect to the total area of the second phase. 
     
     
         2 . The high strength steel sheet according to  claim 1 , wherein the composition further comprises one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo by mass %. 
     
     
         3 . The high strength steel sheet according to  claim 1 , wherein the composition further comprises one or two selected from the group consisting of 0.01 to 0.1% of Ti and 0.01 to 0.1% of Nb by mass %. 
     
     
         4 . The high strength steel sheet according to  claim 1 , wherein the composition further comprises 0.0003 to 0.0050% of B by mass %. 
     
     
         5 . The high strength steel sheet according to  claim 1 , wherein the composition further comprises one or both of 0.05 to 0.5% of Ni and 0.05 to 0.5% of Cu by mass %. 
     
     
         6 . The high strength steel sheet according to  claim 1 , wherein the composition further comprises one or both of 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM by mass %. 
     
     
         7 . A method for manufacturing a high strength steel sheet having excellent formability, comprising:
 a hot rolling step of hot-rolling a steel having a composition which comprises, by mass %,   0.08 to 0.15% of C, 0.5 to 1.5% of Si,   0.5 to 1.5% of Mn, 0.1% or less of P,   0.01% or less of S, 0.01 to 0.1% of Al,   0.005% or less of N,   
       and the balance Fe with inevitable impurities to form a hot rolled sheet;
 a continuous annealing step including an annealing treatment of pickling the hot rolled sheet, and holding the pickled hot rolled sheet in a first temperature region of an A c1  transformation point to an A c3  transformation point for 5 to 400 s; and 
 a cooling treatment of cooling the sheet at an average cooling rate of 5° C./s or higher from a first temperature region to 700° C. after the annealing treatment and adjusting a residence time in a second temperature region of 700° C. to 400° C. in the range of 30 to 400 s by using a continuous annealing line. 
 
     
     
         8 . The method according to  claim 7 , wherein the hot rolling step includes heating the steel at a temperature of 1100° C. to 1280° C., hot-rolling the heated steel with a finish hot rolling temperature of 870° C. to 950° C. to form a hot rolled sheet, and coiling the hot rolled sheet at a coiling temperature of 350° C. to 720° C. upon completion of the hot rolling. 
     
     
         9 . The method according to  claim 7 , wherein a cooling time in a temperature region of 700° C. to 550° C. in the second temperature region is 10 s or longer. 
     
     
         10 . The method according to  claim 7 , wherein the composition further comprises one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo by mass %. 
     
     
         11 . The method according to  claim 7 , wherein the composition further one or two selected from the group consisting of 0.01 to 0.1% of Ti and 0.01 to 0.1% of Nb by mass %. 
     
     
         12 . The method according to  claim 7 , wherein the composition further comprises 0.0003 to 0.0050% of B by mass %. 
     
     
         13 . The method according to  claim 7 , wherein the composition further comprises one or two selected from the group consisting of 0.05 to 0.5% of Ni and 0.05 to 0.5% of Cu by mass %. 
     
     
         14 . The method according to  claim 7 , wherein the composition further comprises one or two selected from the group consisting of 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM by mass %. 
     
     
         15 . The high strength steel sheet according to  claim 2 , wherein the composition further comprises one or two selected from the group consisting of 0.01 to 0.1% of Ti and 0.01 to 0.1% of Nb by mass %. 
     
     
         16 . The high strength steel sheet according to  claim 2 , wherein the composition further comprises 0.0003 to 0.0050% of B by mass %. 
     
     
         17 . The high strength steel sheet according to  claim 3 , wherein the composition further comprises 0.0003 to 0.0050% of B by mass %. 
     
     
         18 . The method for manufacturing a high strength steel sheet according to  claim 8 , wherein a cooling time in a temperature region of 700° C. to 550° C. in the second temperature region is 10 s or longer. 
     
     
         19 . The method for manufacturing a high strength steel sheet according to  claim 8 , wherein the composition further comprises one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo by mass %. 
     
     
         20 . The method for manufacturing a high strength steel sheet according to  claim 9 , wherein the composition further comprises one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo by mass %.