Steel having improved processing properties for working at elevated temperatures

Abstract

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

1 .- 18 . (canceled) 
     
     
         19 . A flat steel product for hot forming, comprising:
 a steel substrate composed of steel comprising iron and by weight comprising:   C: 0.30-0.50%,   Si: 0.05-0.6%,   Mn: 0.5-3.0%,   Al: 0.10-1.0%,   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%,   wherein an Al/Nb ratio of Al content to Nb content is ≤20.0.   
     
     
         20 . The flat steel product of  claim 19 , wherein the steel by weight further comprises one or more elements 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%.   
     
     
         21 . The flat steel product of  claim 20 , wherein at least one of following conditions is applicable to the steel:
 Ti<3.42N; and   0.7% by weight<Mn+Cr<3.5% by weight.   
     
     
         22 . The flat steel product of  claim 19 , 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 comprises 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 15% Zn, constituents limited to a total of not more than 2.0% by weight, and balanced aluminum.   
     
     
         25 . The flat steel product of  claim 19 , comprising one or more of following properties:
 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 elongation Ag of at least 10%; and   an elongation at break A80 of at least 15% or at least 20%.   
     
     
         26 . The flat steel product of  claim 19 , further comprising fine precipitates in a microstructure of the steel substrate in a form of niobium carbonitrides and/or titanium carbonitrides. 
     
     
         27 . The flat steel product of  claim 26 , wherein the fine precipitates in the microstructure are round precipitates having a diameter of up to 20 nm. 
     
     
         28 . 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: 0.30-0.50%, 
 Si: 0.05-0.6%, 
 Mn: 0.5-3.0%, 
 Al: 0.10-1.0%, 
 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%; and 
   an anticorrosion coating, wherein an Al/Nb ratio of Al content to Nb content is ≤20.0.   
     
     
         29 . The shaped sheet metal part of  claim 28 , wherein the steel by weight further comprises one or more elements 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%.   
     
     
         30 . The shaped sheet metal part of  claim 28 , wherein the steel substrate of the shaped sheet metal part has a microstructure having:
 at least in part more than 80% martensite and/or lower bainite, or   at least in part more than 90% martensite and/or lower bainite, and wherein a 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.   
     
     
         31 . The shaped sheet metal part of  claim 28 , further comprising one or more characterizations of:
 at least in part having a yield point of at least 1200 MPa or at least 1300;   at least in part having a tensile strength of at least 1400 MPa or at least 1600 MPa;   at least in part having an elongation at break A80 of at least 3.5%, at least 4%, at least 4.5%, or at least 5%;   at least in part having a bending angle of at least 30°, at least 40°, or at least 45°; and   at least in part having a yield point ratio of at least 60% and at most 85%.   
     
     
         32 . The shaped sheet metal part of  claim 28 , further comprising fine precipitates in a microstructure in a form of niobium carbonitrides and/or titanium carbonitrides. 
     
     
         33 . The shaped sheet metal part of  claim 28 , wherein the shaped sheet metal part at least partly has a Vickers hardness of at least 500 HV1 or at least 540 HV1. 
     
     
         34 . A process for producing a shaped sheet metal part, comprising steps of:
 a) 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: 0.30-0.50%, 
 Si: 0.05-0.6%, 
 Mn: 0.5-3.0%, 
 Al: 0.10-1.0%, 
 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%, wherein an Al/Nb ratio of Al content to Nb content is ≤20.0; 
 
   b) heating the sheet metal blank such that at least in part an 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;   c) inserting the heated sheet metal blank into the 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;   d) 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   e) removing the shaped sheet metal part cooled to the target temperature T target  from the forming tool.   
     
     
         35 . The process of  claim 34 , wherein the steel steel by weight further comprises one or more elements 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%.   
     
     
         36 . The process of  claim 34 , wherein a temperature at least partly obtained in the sheet metal blank in the step b) is between the AC3 temperature and 1000° C., or between 850° C. and 950° C. 
     
     
         37 . The process of  claim 34 , wherein the target temperature T target  of the shaped sheet metal part is at least partly below 400° C. or below 300° C. 
     
     
         38 . The process of  claim 34 , wherein the step a) of providing the sheet metal blank made of the flat steel product, further comprising:
 providing a slab or thin slab comprising 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.;   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.; and   coating the flat steel product cooled to the dipping temperature with an anticorrosion coating by hot dip coating in a melt bath with a melt temperature (T7) of 660-800° C. or 680-740° C.; and   cooling the coated flat steel product to 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.   
     
     
         39 . The process of  claim 38 , further comprising:
 pre-rolling the through-heated slab or thin slab to 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 at most 700° C.;   descaling the hot-rolled flat steel product;   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.   
     
     
         40 . The process of  claim 38 , wherein the anticorrosion coating on the flat steel product is applied to the flat steel product in liquid form and comprises one or more of:
 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 15% Zn,   constituents limited to a total of not more than 2.0% by weight, and   balanced aluminum.