Ultrahigh hole expansion steel and method for manufacturing therefor

Abstract

The present invention provides an ultrahigh hole expansion steel and a method for manufacturing therefor. The steel comprises the following components in percentage by mass: C: 0.03-0.09%; Si≤0.2%; Mn: 0.5-2.0%; P≤0.02%; S≤0.003%; Al: 0.2-1.2%; N≤0.004%; Ti: 0.05-0.20%; Mo: 0.05-0.5%; Mg≤0.005%; O≤0.003%; B≤0.001%; and the balance being Fe and inevitable impurities. wherein C, Mn, Mo and B in the steel satisfy the following formula: 0.25≤2×C+Mn/3+Mo+150×B≤1.5; wherein each chemical element in the formula represent the numerical value before the percentage sign of the percentage by mass of corresponding chemical elements. The steel according to the present invention has excellent matching of strength, plasticity and hole expansion performance, and can be applied in passenger vehicle chassis parts that require high strength and thickness reduction, such as a control arm and a subframe.

1 . A steel, comprising the following components in percentage by mass:
 C: 0.03-0.09%; Si≤0.2%; Mn: 0.5-2.0%; P≤0.02%; S≤0.003%; Al: 0.2-1.2%; N≤0.004%; Ti: 0.05-0.20%; Mo: 0.05-0.5%; Mg≤0.005%; O≤0.003%; B≤0.001%; and the balance being Fe and inevitable impurities, wherein C, Mn, Mo and B in the steel satisfy the following formula:   
       
         
           
             
               
                 0.25 
                 ≤ 
                 
                   
                     2 
                     × 
                     C 
                   
                   + 
                   
                     Mn 
                     / 
                     3 
                   
                   + 
                   Mo 
                   + 
                   
                     150 
                     × 
                     B 
                   
                 
                 ≤ 
                 1.5 
               
               , 
             
           
         
         wherein each chemical element in the formula represent the numerical value before the percentage sign of the percentage by mass of corresponding chemical elements. 
       
     
     
         2 . The steel as claimed in  claim 1 , characterized in that, the steel further comprises one or more elements selected from Nb, V, Cu, Ni and Cr, wherein Nb≤0.06%, V≤0.10%, preferably ≤0.05%, Cu≤0.5%, preferably ≤0.3 wt %, Ni≤0.5%, preferably ≤0.3%, Cr≤0.5%, preferably ≤0.3% in percentage by mass. 
     
     
         3 . The steel as claimed in  claim 1 , characterized in that, the components of the steel further satisfy at least one of the following: Si≤0.15 wt %, Mn: 1.0-1.6 wt %, S≤0.0015 wt %, Al: 0.5-1.0 wt %, N≤0.003 wt %, Ti: 0.07-0.11 wt %, Mo: 0.15-0.45 wt %, Ni≤0.03 wt %, B≤0.0005 wt %. 
     
     
         4 . The steel as claimed in  claim 1 , characterized in that, the steel has a yield strength of ≥700 MPa, a tensile strength of ≥780 MPa, a transverse elongation A50 of ≥17%, and a hole expansion rate ≥80%. 
     
     
         5 . The steel as claimed in  claim 1 , characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides. 
     
     
         6 . A method for manufacturing the steel as claimed in  claim 1 , comprising the following steps:
 1) Smelting and casting;   Smelting a molten steel in a converter or an electric furnace according to the composition as claimed in  claim 1 , then secondary refining in a vacuum furnace, and casting into a billet or an ingot;   2) Reheating the billet or the ingot;   Heating temperature≥1200° C., holding time: 1-2 hours;   3) Hot rolling and cooling the billet or the ingot;   wherein initial rolling temperature: 1050-1150° C., rough rolling of 3-5 passes is carried out under high pressure at 1050° C. or more to a cumulative deformation of ≥50%, obtaining an intermediate billet, thereafter, the intermediate billet is air-cooled or water-cooled to 950-1000° C., and finishing rolling of 5-7 passes is carried out to a cumulative deformation of ≥70%, a final rolling temperature is 850-950° C., obtaining a steel strip;   wherein cooling adopts laminar flow cooling; after final rolling, water cooling the steel strip to 550-650° C. at a cooling speed of ≥10° C./s and coiling, after coiling, cooling to room temperature at a cooling speed of ≤50° C./h, obtaining a hot-rolled strip steel.   
     
     
         7 . The method as claimed in  claim 6 , characterized in that, the method further comprises step 4) Pickling, wherein a pickling operating speed of the hot-rolled strip steel is 30-140m/min, a pickling temperature is 75-85° C., a straightening rate is ≤3%, rinsing is carried out at 35-50° C., and surface drying and oiling are carried out at 120-140° C. 
     
     
         8 . The method as claimed in  claim 6 , characterized in that, the steel further comprises one or more elements selected from Nb, V, Cu, Ni and Cr, wherein Nb≤0.06%, V≤0.10%, preferably ≤0.05%, Cu≤0.5%, preferably ≤0.3 wt %, Ni≤0.5%, preferably ≤0.3%, Cr≤0.5%, preferably ≤0.3% in percentage by mass. 
     
     
         9 . The method as claimed in  claim 6 , characterized in that, the components of the steel further satisfy at least one of the following: Si≤0.15 wt %, Mn: 1.0-1.6 wt %, S≤0.0015 wt %, Al: 0.5-1.0wt %, N≤0.003 wt %, Ti: 0.07-0.11 wt %, Mo: 0.15-0.45 wt %, Ni≤0.03 wt %, B≤0.0005 wt %. 
     
     
         10 . The method as claimed in  claim 6 , characterized in that, the steel has a yield strength of ≥700 MPa, a tensile strength of ≥780 MPa, a transverse elongation A50 of ≥17%, and a hole expansion rate ≥80%. 
     
     
         11 . The method as claimed in  claim 6 , characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides. 
     
     
         12 . The steel as claimed in  claim 2 , characterized in that, the steel has a yield strength of ≥700 MPa, a tensile strength of ≥780 MPa, a transverse elongation A50 of ≥17%, and a hole expansion rate ≥80%. 
     
     
         13 . The steel as claimed in  claim 3 , characterized in that, the steel has a yield strength of ≥700 MPa, a tensile strength of ≥780 MPa, a transverse elongation A50 of ≥17%, and a hole expansion rate ≥80%. 
     
     
         14 . The steel as claimed in  claim 2 , characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides. 
     
     
         15 . The steel as claimed in  claim 3 , characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides. 
     
     
         16 . The steel as claimed in  claim 4 , characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.