Steel having improved processing properties for working at elevated temperatures

Abstract

A flat steel product for hot forming, a formed shaped sheet metal part and methods of production of the same. The flat steel product and the shaped sheet metal part have improved properties, especially in conjunction with an aluminum-based anticorrosion coating.

1 .- 17 . (canceled) 
     
     
         18 . A flat steel product for hot forming, comprising:
 a steel substrate composed of steel comprising iron and by weight comprising:
 C 0.06-0.5%, 
 Si: 0.05-0.6%, 
 Mn: 0.4-3.0%, 
 Al: 0.06-1.0%, 
 Nb: 0.001-0.2%, 
 Ti: 0.001-0.10%, 
 B: 0.0005-0.01%, 
 P: ≤0.03%, 
 S: ≤0.02%, 
 N: ≤0.02%, 
 Sn: ≤0.03%, 
 As: ≤0.01%, and 
 unavoidable impurities ≤0.2%, 
 where an Al/Nb ratio of Al content to Nb content is: 
 Al/Nb≤20.0 when Mn≤1.6% by weight; and 
 Al/Nb≤30.0 when Mn≥1.7% by weight. 
   
     
     
         19 . The flat steel product of  claim 18 , wherein the steel further comprises one or more elements by weight comprising:
 Cr: 0.01-1.0%,   Cu: 0.01-0.2%,   Mo: 0.002-0.3%,   Ni: 0.01-0.5%,   V: 0.001-0.3%,   Ca: 0.0005-0.005%, and   W: 0.001-1.0%.   
     
     
         20 . The flat steel product of  claim 19 , wherein 0.7%<Mn+Cr<3.5% by weight. 
     
     
         21 . The flat steel product of  claim 18 , wherein Ti<3.42 N. 
     
     
         22 . The flat steel product of  claim 18 , further comprising an anticorrosion coating on at least one side of the steel substrate. 
     
     
         23 . The flat steel product of  claim 22 , wherein the anticorrosion coating is an aluminum-based anticorrosion coating and has an alloy layer and an Al base layer. 
     
     
         24 . The flat steel product of  claim 23 , wherein
 the alloy layer comprises 35-60% by weight of Fe, constituents limited to a total of not more than 5.0% by weight, and balanced aluminum, and   the Al base layer comprises 1.0-15% by weight of Si, 2-4% by weight of Fe, up to 5.0% by weight of alkali metals or alkaline earth metals, up to 10% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.   
     
     
         25 . The flat steel product of  claim 18 , further comprising one or more properties of:
 a yield point with a continuous progression (Rp0.2) or a yield point with a difference (ΔRe) between an upper yield point limit (ReH) and a lower yield point limit (ReL) of not more than 45 MPa;   a uniform expansion Ag of at least 10%; and   an elongation at break A80 at least 15% or at least 20%.   
     
     
         26 . A shaped sheet metal part formed from a flat steel product, comprising:
 a steel substrate composed of steel comprising iron and by weight comprising:
 C: 0.06-0.5%, 
 Si: 0.05-0.6%, 
 Mn: 0.4-3.0%, 
 Al: 0.06-1.0%, 
 Nb: 0.001-0.2%, 
 Ti: 0.001-0.10%, 
 B: 0.0005-0.01%, 
 P: ≤0.03%, 
 S: ≤0.02%, 
 N: ≤0.02%, 
 Sn: ≤0.03%, 
 As: ≤0.01%; and 
 unavoidable impurities ≤0.2%, 
 where an Al/Nb ratio of Al content to Nb content is: 
 Al/Nb≤20.0 when Mn≤1.6% by weight; and 
 Al/Nb≤30.0 when Mn≥1.7% by weight; and 
   an anticorrosion coating.   
     
     
         27 . The shaped sheet metal part of  claim 26 , the steel further comprises one or more elements by weight comprising:
 Cr: 0.01-1.0%,   Cu: 0.01-0.2%,   Mo: 0.002-0.3%,   Ni 0.01-0.5%,   V: 0.001-0.3%,   Ca: 0.0005-0.005%, and   W: 0.001-1.0%.   
     
     
         28 . The shaped sheet metal part of  claim 26 , wherein the steel substrate of the shaped sheet metal part has a microstructure having at least in part more than 80% or more than 90% martensite and/or lower bainite, and wherein former austenite grains of the martensite have an average grain diameter of less than 14 μm, less than 12 μm, or less than 10 μm. 
     
     
         29 . The shaped sheet metal part of  claim 28 , further comprising fine precipitates in the microstructure of the steel substrate in a form of niobium carbonitrides and/or titanium carbonitrides. 
     
     
         30 . The shaped sheet metal part of  claim 26 , at least in part comprising one or more properties of:
 a yield point of at least 950 MPa, at least 1100 MPa, at least 1300 MPa, or at least 1500 MPa;   a tensile strength of at least 1000 MPa, at least 1100 MPa, at least 1300 MPa, or at least 1800 MPa;   an elongation at break A80 of at least 4%, at least 5%, or at least 6%; and   a bending angle of at least 30°, at least 40°, or at least 50°.   
     
     
         31 . The shaped sheet metal part of  claim 26 , wherein an electrochemical potential of a surface of the shaped sheet metal part in a corrosive medium is at least −0.50 V. 
     
     
         32 . The shaped sheet metal part of  claim 26 , wherein
 the anticorrosion coating is an aluminum-based anticorrosion coating comprising an alloy layer and an Al base layer, and   in a cross section of the alloy layer, over a measurement length of 500 μm, an area occupied by pores in the alloy layer is less than 250 μm 2 , and wherein a proportion of the area occupied by pores has a diameter of not less than 0.1 μm is less than 10%.   
     
     
         33 . The shaped sheet metal part of  claim 32 , wherein a Nb content in the alloy layer is greater than 0.010% by weight, greater than 0.015% by weight, or greater than 0.018% by weight. 
     
     
         34 . The shaped sheet metal part of  claim 26 , further comprising a welding range of at least 0.9 kA. 
     
     
         35 . A process for producing a shaped sheet metal part, comprising:
 providing a sheet metal blank made of a flat steel product, comprising:
 a steel substrate composed of steel comprising iron and by weight comprising:
 C 0.06-0.5%, 
 Si: 0.05-0.6%, 
 Mn: 0.4-3.0%, 
 Al: 0.06-1.0%, 
 Nb: 0.001-0.2%, 
 Ti: 0.001-0.10%, 
 B: 0.0005-0.01%, 
 P: ≤0.03%, 
 S: ≤0.02%, 
 N: ≤0.02%, 
 Sn: ≤0.03%, 
 As: ≤0.01%, and 
 unavoidable impurities ≤0.5%, 
 where an Al/Nb ratio of Al content to Nb content is: 
 Al/Nb≤20.0 when Mn≤1.6% by weight; and 
 Al/Nb≤30.0 when Mn≥1.7% by weight; 
 
   heating the sheet metal blank such that at least in part a AC3 temperature of the sheet metal blank is exceeded and a temperature T ins  of the sheet metal blank on insertion into a forming tool provided for hot press forming at least in part has a temperature above Ms+100° C., wherein Ms is a martensite start temperature;   inserting the heated sheet metal blank into a forming tool, wherein a transfer time t trans  required for removal from a heating device and insertion of the sheet metal blank is not more than 20 s or not more than 15 s;   hot press forming the sheet metal blank to the shaped sheet metal part, wherein the sheet metal blank during the hot press forming is cooled down to a target temperature T target  over a period t tool  of more than 1 s at a cooling rate r tool  of at least in part more than 30 K/s and kept at the target temperature T target ; and   removing the shaped sheet metal part cooled to the target temperature T target  from the forming tool.   
     
     
         36 . The process of  claim 35 , wherein the steel further comprises one or more elements by weight comprising:
 Cr: 0.01-1.0%,   Cu: 0.01-0.2%,   Mo: 0.002-0.3%,   Ni: 0.01-0.5%,   V: 0.001-0.3%,   Ca: 0.0005-0.005%, and   W: 0.001-1.0%.   
     
     
         37 . The process of  claim 35 , wherein the producing the shaped sheet metal part further comprises producing the flat steel product for hot forming having an anticorrosion coating, comprising:
 providing a slab or thin slab composed of the steel;   through-heating the slab or thin slab at a temperature (T1) of 1100-1400° C.;   hot rolling the slab or thin slab to produce a hot-rolled flat steel product, wherein a final rolling temperature (T3) is 750-1000° C.;   descaling the hot-rolled flat steel product;   annealing the flat steel product at an annealing temperature (T5) of 650-900° C.;   cooling the flat steel product to a dipping temperature (T6) of 650-800° C. or 670-800° C.;   coating the flat steel product cooled to the dipping temperature with the anticorrosion coating by hot dip coating in a melt bath at a melt temperature (T7) of 660-800° C. or 680-740° C.; and   cooling the coated flat steel product to a room temperature, wherein a first cooling time t MT  in a temperature range between 600° C. and 450° C. is more than 10 s or more than 14 s, and a second cooling time t LT  in a temperature range between 400° C. and 300° C. is more than 8 s or more than 12 s.   
     
     
         38 . The process of  claim 37 , further comprising:
 pre-rolling the through-heated slab or thin slab to produce an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;   coiling the hot-rolled flat steel product, wherein a coiling temperature (T4) is not more than 700° C.;   cold-rolling the flat steel product, wherein a degree of cold rolling is at least 30%; and   skin pass-rolling the coated flat steel product.   
     
     
         39 . The process of  claim 37 , wherein the melt bath used in the hot dip coating comprises a anticorrosion material to be applied to the flat steel product in liquid form, and wherein the anticorrosion material comprises up to 15% by weight of Si, 2-4% by weight of Fe, up to 5% by weight of alkali metals or alkaline earth metals, up to 10% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.