High-strength steel sheets with excellent resistance to delayed fracture after forming, method for manufacturing the same, and high-strength automotive part manufactured of the same

Abstract

Steel sheets containing residual austenite of not more than 7 vol. %, crystallized and/or precipitated compounds with particle diameters of 0.01 to 5.0 μm of 100 to 100000 particle/mm 2 and C of 0.05 to 0.3 mass %, Si of not more than 3.0 mass %, Mn of 0.01 to 3.0 mass %, P of not more than 0.02 mass %, S of not more than 0.02 mass %, Al of 0.01 to 3.0 mass %, N of not more than 0.01 mass % and Mg of 0.0002 to 0.01 mass %, with the remainder comprising iron and unavoidable impurities.

1 - 13 . (canceled) 
     
     
         14 . A method for manufacturing high-strength steel sheet, having excellent post-forming delayed-fracture resistance, the method comprising the steps of:
 preparing steel slab containing, in mass percent,   C: 0.05 to 0.3 percent,   Si: not more than 3.0 percent,   Mn: 0.01 to 3.0 percent,   P: not more than 0.02 percent,   S: not more than 0.02 percent,   Al: 0.01 to 3.0 percent,   N: not more than 0.01 percent and   Mg: 0.0002 to 0.01 percent, and,   
       wherein, optionally, the slab further contains one or more of
 V: 0.005 to 1 mass percent, 
 Ti: 0.002 to 1 mass percent, 
 Nb: 0.002 to 1 mass percent, 
 Zr: 0.002 to 1 mass percent, 
 Cr: 0.005 to 5 mass percent, 
 Mo: 0.005 to 5 mass percent, 
 W: 0.005 to 5 mass percent, 
 Cu: 0.005 to 2.0 mass percent, 
 Ni: 0.005 to 2.0 mass percent, 
 Co: 0.005 to 2.0 mass percent, 
 B: 0.0002 to 0.1 mass percent, 
 REM: 0.0005 to 0.01 mass percent, 
 Ca: 0.0005 to 0.01 mass percent and 
 Y: 0.0005 to 0.01 mass percent, 
 
       with a remainder of iron and unavoidable impurities by continuous casting with a casting speed of 1.0 to 3.0 m/min;
 hot rolling the steel slab with a finishing temperature not lower than the slab Ar 3  point to provide a hot-rolled steel strip; 
 coiling the hot-rolled steel strip at a temperature between 500° C. and 800° C.; 
 pickling the coiled hot-rolled steel strip prior to cold rolling with a draft of 30 to 80 percent; 
 recrystallization annealing the cold-rolled steel strip by soaking at a temperature of from 600° C. to 950° C.; and then 
 temper rolling the annealed cold-rolled steel strip. 
 
     
     
         15 . The method according to  claim 14 , further comprising holding the steel strip in a temperature range of 200° to 700° C. for 1 minute to 10 hours after annealing. 
     
     
         16 . The method according to  claim 14 , further comprising applying a zinc coating to the temper-rolled steel strip. 
     
     
         17 . The method according to  claim 14 , wherein the slab contains one or more of:
 V: 0.005 to 1 mass percent,   Ti: 0.002 to 1 mass percent,   Nb: 0.002 to 1 mass percent,   Zr: 0.002 to 1 mass percent,   Cr: 0.005 to 5 mass percent,   Mo: 0.005 to 5 mass percent,   W: 0.005 to 5 mass percent,   Cu: 0.005 to 2.0 mass percent,   Ni: 0.005 to 2.0 mass percent,   Co: 0.005 to 2.0 mass percent,   B: 0.0002 to 0.1 mass percent,   REM: 0.0005 to 0.01 mass percent,   Ca: 0.0005 to 0.01 mass percent and   Y: 0.0005 to 0.01 mass percent.   
     
     
         18 . The method according to  claim 14 , wherein the slab contains one or more of:
 V: 0.005 to 1 mass percent,   Ti: 0.002 to 1 mass percent,   Nb: 0.002 to 1 mass percent, and   Zr: 0.002 to 1 mass percent.   
     
     
         19 . The method according to  claim 14 , wherein the slab contains one or more of:
 Cr: 0.005 to 5 mass percent,   Mo: 0.005 to 5 mass percent, and   W: 0.005 to 5 mass percent.   
     
     
         20 . The method according to  claim 14 , wherein the slab contains Cu in an amount of 0.005 to 2.0 mass percent. 
     
     
         21 . The method according to  claim 14 , wherein the slab contains at least one of:
 Ni: 0.005 to 2.0 mass percent, and   Co: 0.005 to 2.0 mass percent.   
     
     
         20 . The method according to  claim 14 , wherein the slab contains B in an amount of 0.0002 to 0.1 mass percent. 
     
     
         23 . The method according to  claim 14 , wherein the slab contains at least one of:
 REM: 0.0005 to 0.01 mass percent,   Ca: 0.0005 to 0.01 mass percent, and   Y: 0.0005 to 0.01 mass percent.   
     
     
         24 . The method according to  claim 14 , further comprising crystallizing and/or precipitating at least one of carbides, oxides, nitrides, and sulfides to form H trap sites. 
     
     
         25 . The method according to  claim 24 , wherein particles formed by the crystallizing and/or precipitating have a mean particle diameter of from 0.1 to 5.0 μm. 
     
     
         26 . The method according to  claim 24 , wherein the carbides, oxides, nitrides, and sulfides are carbides, oxides, nitrides, and sulfides of at least one of Mg, Ti, Nb, C, Cr, Mo, REM, and Ca. 
     
     
         27 . A high-strength steel sheet, prepared by the method according to  claim 14 . 
     
     
         28 . A method for manufacturing high-strength steel sheet, having excellent post-forming delayed-fracture resistance, the method comprising the steps of:
 preparing steel slab containing, in mass percent,   C: 0.05 to 0.3 percent,   Si: not more than 3.0 percent,   Mn: 0.01 to 3.0 percent,   P: not more than 0.02 percent,   S: not more than 0.02 percent,   Al: 0.01 to 3.0 percent,   N: not more than 0.01 percent and   Mg: 0.0002 to 0.01 percent, and,   
       wherein, the slab further contains one or more of
 V: 0.005 to 1 mass percent, 
 Ti: 0.002 to 1 mass percent, 
 Nb: 0.002 to 1 mass percent, 
 Zr: 0.002 to 1 mass percent, 
 Cr: 0.005 to 5 mass percent, 
 Mo: 0.005 to 5 mass percent, 
 W: 0.005 to 5 mass percent, 
 Cu: 0.005 to 2.0 mass percent, 
 Ni: 0.005 to 2.0 mass percent, 
 Co: 0.005 to 2.0 mass percent, 
 B: 0.0002 to 0.1 mass percent, 
 REM: 0.0005 to 0.01 mass percent, 
 Ca: 0.0005 to 0.01 mass percent and 
 Y: 0.0005 to 0.01 mass percent, 
 
       with a remainder of iron and unavoidable impurities by continuous casting with a casting speed of 1.0 to 3.0 m/min;
 hot rolling the slab with a finishing temperature not lower than the Ar 3  point to provide a hot-rolled steel strip; 
 coiling the hot-rolled steel strip at a temperature between 500° C. and 800° C.; 
 pickling the coiled hot-rolled steel strip prior to cold rolling with a draft of 30 to 80 percent; 
 recrystallization annealing the cold-rolled steel strip by soaking at not lower than 600° C. and not higher than 950° C.; and then 
 temper rolling the annealed cold-rolled steel strip. 
 
     
     
         29 . The method according to  claim 28 , The method according to  claim 14 , further comprising crystallizing and/or precipitating at least one of carbides, oxides, nitrides, and sulfides to form H trap sites. 
     
     
         30 . The method according to  claim 29 , wherein particles formed by the crystallizing and/or precipitating have a mean particle diameter of from 0.1 to 5.0 μm. 
     
     
         31 . The method according to  claim 29 , wherein the carbides, oxides, nitrides, and sulfides are carbides, oxides, nitrides, and sulfides of at least one of Mg, Ti, Nb, C, Cr, Mo, REM, and Ca.