Hot-Rolled Flat Steel Product and Method For the Production Thereof

Abstract

A hot-rolled flat steel product having a product of Rm and A80 of ≧18,000 MPa*%, a composition including (in wt.) C:0.10-0.60%, Si:0.4-2.0%, Al:≦2.0%, Mn:0.4-2.5%, Ni:≦1%, Cu:≦2.0%, Mo:≦0.4%, Cr≦2%, Ii:≦0.2%, Nb:≦0.2%, V:≦0.5%, remainder Fe and unavoidable impurities, and a microstructure of bainite and residual austenite, wherein the microstructure includes ≧60 vol.% bainite, and wherein at least some of the residual austenite is in block form and ≧98% of the residual austenite blocks have a size of ≦5 μm. Also, a method where a slab, thin slab or a cast strip having the aforementioned composition is hot-rolled at a hot-rolling end temperature of ≧880° C., cooled with a cooling rate of ≧5° C./s to a coiling temperature between the martensite start temperature and 600° C., coiled, and cooled in the coil while being held between the bainite start temperature and the martensite start temperature until ≧60 vol.% of the hot strip microstructure is bainite.

1 . A hot-rolled flat steel product, with the mathematical product of tensile strength (Rm) and elongation (A80) being at least 18,000 MPa*% and comprising, in addition to iron and unavoidable impurities (in % by weight):
 C: 0.10-0.60%,   Si: 0.4-2.0%,   Al: up to 2.0%,   Mn: 0.4-2.5%,   Ni: up to 1%,   Cu: up to 2.0%,   Mo: up to 0.4%,   Cr: up to 2%,   Ti: up to 0.2%,   Nb: up to 0.2%,   V: up to 0.5%,   wherein the flat steel product has a microstructure dominated by two phases, one dominating constituent of the microstructure being bainite and the second dominating constituent of the microstructure being residual austenite, wherein the microstructure of the flat steel product consists of bainite to an extent of at least 50% by volume and of residual austenite as the remainder, wherein optionally up to 5% by volume ferrite and up to 10% by volume martensite may be present in the microstructure of the flat steel product, and wherein at least part of the residual austenite is present in block form and at least 98% of the blocks of the residual austenite being present in block form have a mean diameter of less than 5 μm.   
     
     
         2 . The flat steel product according to  claim 1 , wherein the microstructure of said flat steel product contains at least 10% by volume residual austenite. 
     
     
         3 . The flat steel product according to  claim 1 , wherein the Cu content of said flat steel product is at least 0.15% by weight. 
     
     
         4 . The flat steel product according to  claim 1 , wherein the C content of said flat steel product is at least 0.3% by weight. 
     
     
         5 . The flat steel product according to  claim 1 , wherein the Mn, Cr, Ni, Cu and C contents of said flat steel product satisfy the following condition:
   1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2,   where % Mn: respective Mn content in % by weight,
 % Cr: respective Cr content in % by weight, 
 % Ni: respective Ni content in % by weight, 
 % Cu: respective Cu content in % by weight, 
 % C: respective C content in % by weight. 
   
     
     
         6 . The flat steel product according to  claim 1 , wherein the Si and Al contents of said flat steel product satisfy the following condition:
 % Si+0.8% Al>1.2% by weight,   where % Si: respective Si content in % by weight,
 % Al: respective Al content in % by weight. 
   
     
     
         7 . The flat steel product according to  claim 1 , wherein the diameter of the block residual austenite is 1-3 μm. 
     
     
         8 . A method for producing a flat steel product, said method comprising:
 providing a preliminary product in the form of a slab, thin slab or a cast strip, which, in addition to iron and unavoidable impurities, contains (in % by weight): 0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2.5% Mn, up to 1% Ni, up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up to 0.5% V;   hot-rolling the preliminary product to form a hot strip in one or more rolling passes, the hot strip obtained having a hot-rolling end temperature of at least 880° C. when it leaves the last rolling pass;   accelerated cooling of the hot strip obtained with a cooling rate of at least 5° C./s to a coiling temperature lying in the range between the martensite start temperature (MS) and 600° C.;   coiling the hot strip to form a coil;   cooling the coil, the temperature of the coil being held, during the cooling to form bainite, in a temperature range having an upper limit which is the same as the bainite start temperature (BS), from which bainite forms in the microstructure of the hot strip, and having a lower limit which is the same as the martensite start temperature (MS), from which martensite forms in the microstructure of the hot strip, until at least 50% by volume of the microstructure of the hot strip comprises bainite.   
     
     
         9 . The method according to  claim 8 , wherein the end temperature of the hot-rolling is at least 900° C. 
     
     
         10 . The method according to  claim 8 , wherein the cooling rate is at least 10° C./s. 
     
     
         11 . The method according to  claim 8 , wherein the cooling rate is at most 150° C./s. 
     
     
         12 . The method according to  claim 8 , wherein the cooling rate is at most 50° C./s. 
     
     
         13 . The method according to  claim 8 , wherein the lower limit of the coiling temperature at which the cooling begins in the coil is about 20° C. higher than the martensite start temperature (MS). 
     
     
         14 . The method according to  claim 8 , wherein the upper limit of the coiling temperature at which the cooling begins in the coil is 550° C. 
     
     
         15 . The method according to  claim 8 , wherein the coiling temperature at least corresponds to the temperature HTopt determined by the following formula:
     HT opt=MS+(BS−MS)/3.