High strength thin steel sheet excellent in resistance to delayed fracture after forming and method for preparation thereof , and automobile parts requiring strength manufactured from high strength thin steel sheet

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 . High-strength steel sheet having excellent post-forming delayed-fracture resistance, characterized by: 
 containing, in mass %,    C : 0.05 to 0.3%,    Si: not more than 3.0%,    Mn: 0.01 to 3.0%,    P : not more than 0.02%,    S : not more than 0.02%,    Al: 0.01 to 3.0%,    N : not more than 0.01% and    Mg: 0.0002 to 0.01%, 
 with the remainder comprising iron and unavoidable impurities,  
 having the residual austenite in the structure of steel being not more than 7 vol. %,  
 including one or more of the oxides, sulfides, composite crystallized products and composite precipitates of Mg having means particle diameter d in the range of 0.01 to 5.0 μm, density ρ in the range of 100 to l00000/mm 2 , and distribution satisfying the ratio between the standard deviation a from mean particle diameter and mean particle diameter d, σ/d≦1.0, and  
 having the volume percentage Vγ(%) of residual austenite and tensile strength TSI (MPa) satisfying equation (A)  
   1000( Vγ− 0.1) − 5.5+α(Mg−40) 2 −50( d− 0.2) 2 +1.1 lnp+700( TS− 680) 0.9 ≧10   Equation (A):  
   where α=−0.005(Mg≦40), α=−0.002(Mg>40)    Vγ: volume percentage of residual austenite (%)    Mg: the amount of Mg (mass ppm)    d : particle diameter (μm)    ρ: density (particle/mm 2 )    TS: tensile strength (MPa) and, furthermore,    (i) 1000(V−0.1) −5.5 =10 when 1000(V−0.1) −5.5 ≧10,    (ii) 2≦Mg≦100 ppm    (iii) (d−0.2) 2 =0.2 when 0.01≦d≦5.0 μm and (d−0.2) 2 =0.2    (iv) 100≦ρ≦100000 particle/mm 2  and    (v) 780 MPa≦TS    
   
   
       2 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by: 
 containing, furthermore, in mass %, one or more of    V : 0.005 to 1 mass %,    Ti: 0.002 to 1 mass %,    Nb: 0.002 to 1 mass % and    Zr: 0.002 to 1 mass %.    
   
   
       3 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by: 
 containing, furthermore, in mass %, one or more of    Cr: 0.005 to 5 mass %,    Mo: 0.005 to 5 mass % and    W : 0.005 to 5 mass %.    
   
   
       4 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in any of claims  1 , characterized by: 
 containing, furthermore, in mass %,    Cu: 0.005 to 2.0 mass %.    
   
   
       5 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by: 
 containing, furthermore, in mass %, one or more of    Ni: 0.005 to 2.0 mass % and    Co: 0.005 to 2.0 mass %.    
   
   
       6 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by: 
 containing, furthermore, in mass %,    B: 0.0002 to 0.1 mass %.    
   
   
       7 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by: 
 containing furthermore, in mass %, one or more of    REM: 0.0005 to 0.01 mass %,    Ca: 0.0005 to 0.01 mass % and    Y : 0.0005 to 0.01 mass %.    
   
   
       8 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by that: 
 the high-strength steel sheet is hot-rolled or cold-rolled steel sheet.    
   
   
       9 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 1 , characterized by that 
 the high-strength steel sheet is zinc-coated on the surface.    
   
   
       10 . High-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 8 , characterized by that: 
 the high-strength steel sheet is also film-laminated.    
   
   
       11 . Method for manufacturing high-strength steel sheet having excellent post-forming delayed-fracture resistance, characterized by comprising the steps of: 
 preparing slab of the composition described in  claim 1 ,    hot rolling said slab with a finishing temperature not lower than the Ar 3  point,    coiling the hot-rolled strip at a temperature between 500° C. and 800° C.,    cold rolling with a draft of 30 to 80% after applying pickling,    applying recrystallization annealing by soaking at not lower than 600° C. and not higher than 950° C., and then    applying temper rolling.    
   
   
       12 . Method for manufacturing high-strength steel sheet having excellent post-forming delayed-fracture resistance described in  claim 11 , characterized by comprising, furthermore, the step of: 
 holding the strip in the temperature range of 200 to 700° C. for 1 minute to 10 hours after annealing.    
   
   
       13 . Automotive structural member, characterized by being manufactured of high-strength steel sheet having the excellent post-forming delayed-fracture resistance described in  claim 1.