Method of manufacturing a steel strip and coated steel sheet obtainable thereby

Abstract

A method manufacturing a steel strip, including the subsequent steps of hot rolling the strip into a hot rolled strip, cold rolling the hot rolled strip and hot dip coating the cold rolled strip with a Zn based coating by leading the strip through a bath including molten zinc and wiping the strip after the coating using a gas knife having a knife slot from which a wiping gas is projected and the steel strip is cold rolled to a final cold rolled thickness of between 0.40 mm and 1.00 mm in a multi-stand cold rolling mill, and the coated steel sheet includes a steel substrate provided with a hot dip metal coating.

1 . The coated steel strip of claim  11 , wherein the coated steel sheet was made by a method of manufacturing, comprising the subsequent steps of hot rolling the strip into a hot rolled strip, cold rolling the hot rolled strip and hot dip coating the cold rolled strip with a Zn based coating by leading the strip through a bath comprising molten zinc and wiping the strip after said coating using a gas knife having a knife slot from which a wiping gas is projected, wherein:
 the steel strip is cold rolled to a final cold rolled thickness of between 0.40 mm and 1.00 mm in a multi-stand cold rolling mill wherein cold rolling in the last stand takes place such that:   
       
         
           
             
               
                 SRF 
                 AWR 
               
               ≥ 
               
                 21000 
                 ⁢ 
                     
                 kN 
                 / 
                 
                   m 
                   2 
                 
               
             
           
         
         wherein SRF is the specific rolling force expressed in kN/m calculated as the rolling force in kN divided by the strip width in m, and AWR is the average work roll radius in m of the top and bottom work roll at mid roll position, and wherein GKD is the average distance between the knife slot from which the wiping gas is projected and the surface of the coated strip that is being wiped, wherein GKD≤10 mm; 
         wherein the bath of molten metal has a composition comprising Zn, Al and Mg, wherein the strip after coating and wiping is cooled in a cooling section between the location where the strip is wiped and a downstream location where the strip is first contacted by a guiding roll; 
         wherein an active cooling gas flow Q in m3/hr is used which is required to maintain the strip temperature within a bandwidth of 20 degrees of a target strip temperature in the range between 200° C. and 300° C. at said guiding roll; 
         wherein the cooling gas flow in the second half of the cooling section is a percentage p of Q and the cooling gas flow in the first half of the cooling section is a percentage of (100−p) of Q, wherein p is set at 70% or more; and 
         wherein cold rolling in the last stand takes place using work rolls that have a roughness Ra which is 7 μm or less but in all cases 1.0 μm or more. 
       
     
     
         2 . The coated steel strip according to  claim 1 , wherein 
       
         
           
             
               
                 
                   SRF 
                   AWR 
                 
                 ≥ 
                 
                   22000 
                   ⁢ 
                       
                   kN 
                   / 
                   
                     m 
                     2 
                   
                 
               
               , 
             
           
         
       
     
     
         3 . The coated steel strip according to  claim 1 , wherein cold rolling in the last stand takes place using work rolls that have a roughness Ra which is 6 μm or less, but in all cases 1.0 μm or more. 
     
     
         4 . The coated steel strip according to  claim 1 , wherein GKD is the average distance between the knife slot from which the wiping gas is projected and the surface of the coated strip that is being wiped wherein GKD≤9 mm. 
     
     
         5 . The coated steel strip according to according to  claim 1 , wherein p is set at 80% or more. 
     
     
         6 . The coated steel strip according to according to  claim 1 , wherein the bath consists of 0.6-4.0 weight % aluminium and 0.3-4.0 weight % magnesium, optionally up to 0.2 weight % each of an element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the remainder being unavoidable impurities and zinc. 
     
     
         7 . The coated steel strip according to  claim 6 , wherein the aluminium content is 0.6-3.0 weight % and/or the magnesium content is 0.3-2.0 weight %. 
     
     
         8 . The coated steel strip according to  claim 1 , wherein the bath consists of 0.20-0.90 weight % aluminium, and up to 0.2 weight % each of an element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the remainder being unavoidable impurities and zinc. 
     
     
         9 . The coated steel strip according to according to  claim 1 , wherein the hot dip coated strip is temper rolled with an elongation of 0.5% or more, using a temper work roll with an average diameter of 400 mm or more. 
     
     
         10 . The coated steel strip according to  claim 9 , wherein a temper work roll roughness Ra is used of 4.5 μm or less. 
     
     
         11 . A coated steel sheet, the sheet comprising a steel substrate provided with a Zn based hot dip coating, the steel substrate having a thickness of between 0.40 mm and 1.00 mm, wherein:
 i) the steel substrate has a composition, all in weight %:
 C max 0.04; 
 Mn 0.01-1.20; 
 Si 0.001-0.50; 
 Al 0.005-0.1; 
 P max 0.15; 
 S max 0.045; 
 N max 0.01; 
 Mo max 0.12; 
 Ti max 0.12; 
 Nb max 0.12; 
 Cu: max 0.10; 
 Cr: max 0.06; 
 Ni: max 0.08; 
 B: max 0.0025; 
 V: max 0.01; 
 Ca: max 0.01; 
 Co: max 0.01; 
 Sn: max 0.01; 
 the remainder being iron and unavoidable impurities; 
   ii) the coated steel sheet has a surface characteristic Sc, Sc being defined as: Sc=Sk/(0.7*t+0.3), wherein Sk in μm is defined according to NEN-EN-ISO 25178-2:2012 and t is the thickness of the steel substrate in mm being between 0.40 mm and 1.00 mm, and   iii) the coated steel sheet, after a 5% Marciniak bi-axial deformation, has a waviness Wsa which is the Wsa(1-5) value in μm, measured in rolling direction, according to SEP 1941,   iv) wherein the combination Sc and Wsa lies within an area defined by a contour ABCDEA in an XY-plot of Sc and Wsa respectively, wherein:
 A is defined as the intersection of Sc=3.00 and Wsa=(0.2686)−(0.0543*Sc)+(0.0105*Sc{circumflex over ( )}2); 
 AB is defined by Wsa=(0.2686)−(0.0543*Sc)+(0.0105*Sc{circumflex over ( )}2) from Sc=3.00 at A to Wsa=0.50 at B; 
 BC is defined by Wsa=0.50 from B to C, C having Sc=14.50; 
 CD is defined by Sc=14.50 for Wsa=0.50 at C to Wsa=0.10 at D; 
 DE is defined by Wsa=0.10 for Sc=14.50 at D to Sc=3.00 at E; and 
 EA closes the contour and is defined by Sc=3.00 from E to A. 
   
     
     
         12 . The coated steel sheet according to  claim 11 , wherein the combination Sc and Wsa lies within an area defined by a contour A′FCDEA′ in an XY plot of Sc and Wsa respectively, wherein:
 A′ is defined as the intersection of Sc=3.00 and Wsa=(0.2276)−(0.0266*Sc)+(0.0054*Sc{circumflex over ( )}2); 
 A′F is defined by Wsa=(0.2276)−(0.0266*Sc)+(0.0054*Sc{circumflex over ( )}2) for Sc=3.00 at A′ to Wsa=0.50 at F; 
 FC is defined by Wsa=0.50 from F to C, C having Sc=14.50; 
 CD is defined by Sc=14.50 for Wsa=0.50 at C to Wsa=0.10 at D; 
 DE is defined by Wsa=0.10 for Sc=14.50 at D to Sc=3.00 at E; and 
 EA′ closes the contour and is defined by Sc=3.00 from E to A′. 
 
     
     
         13 . The coated steel sheet according to  claim 11 , wherein the combination Sc and Wsa lies within an area defined by a contour A″GCDEA″ in an XY plot of Sc and Wsa respectively, wherein:
 A″ is defined as the intersection of Sc=3.00 and Wsa=(0.208)−(0.0118*Sc)+(0.0027*Sc{circumflex over ( )}2); 
 A″G is defined by Wsa=(0.208)−(0.0118*Sc)+(0.0027*Sc{circumflex over ( )}2) for Wsa=0.20 to Wsa=0.50 at G; 
 GC is defined by Wsa=0.50 from B to C, C having Sc=14.50; 
 CD is defined by Sc=14.50 for Wsa=0.50 at C to Wsa=0.10 at D; 
 DE is defined by Wsa=0.10 for Sc=14.50 at D to Sc=3.00 at E; and 
 EA″ closes the contour and is defined by Sc=3.00 from E to A″. 
 
     
     
         14 . The coated steel sheet according to  claim 11 , having a total coating weight on both sides together of 60-175 g/m2. 
     
     
         15 . The coated steel sheet according to  claim 11 , having a surface roughness Ra between 0.9 μm and 1.8 μm. 
     
     
         16 . The coated steel sheet according to  claim 11 , wherein the coated steel sheet is a hot dip coated steel sheet having in its end use, in deformed state, a guaranteed maximum waviness Wsa which is the Wsa(1-5) value, measured in rolling direction, according to SEP 1941, of 0.35 μm. 
     
     
         17 . The coated steel sheet according to  claim 1 , wherein 
       
         
           
             
               
                 
                   SRF 
                   AWR 
                 
                 ≥ 
                 
                   25000 
                       
                   
                     kN/m 
                     2 
                   
                 
               
               , 
               
                 
                   SRF 
                   AWR 
                 
                 ≥ 
                 
                   26000 
                       
                   
                     kN/m 
                     2 
                   
                 
               
               , 
               
                 
                   or 
                   ⁢ 
                       
                   even 
                   ⁢ 
                       
                   
                     SRF 
                     AWR 
                   
                 
                 ≥ 
                 
                   27000 
                   ⁢ 
                       
                   
                     
                       kN/m 
                       2 
                     
                     . 
                   
                 
               
             
           
         
       
     
     
         18 . The coated steel sheet according to  claim 11 , having a surface roughness Ra between 0.9 μm and 1.4 μm. 
     
     
         19 . The coated steel sheet according to  claim 11 , wherein the coated steel sheet is a hot dip coated steel sheet having in its end use, in deformed state, a guaranteed maximum waviness Wsa which is the Wsa(1-5) value, measured in rolling direction, according to SEP 1941, of 0.28 μm or lower.