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US-12624407-B2 - Method of manufacturing a steel strip and coated steel sheet obtainable thereby

US12624407B2US 12624407 B2US12624407 B2US 12624407B2US-12624407-B2

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.

Inventors

  • Guido Cornelis Hensen
  • Roel Marinus Maria MALLENS
  • Freek SLUIS
  • Jean-Paul GRAVEMAKER
  • Edgar Matthijs TOOSE

Assignees

  • TATA STEEL IJMUIDEN B.V.

Dates

Publication Date
20260512
Application Date
20210629
Priority Date
20200630

Claims (20)

  1. 1 . A method of manufacturing a steel strip for use in an automobile body, 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 a top work roll and a 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 a 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 20° C. 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. 2 . The method according to claim 1 , wherein SRF AWR ≥ 22000 ⁢ kN / m 2 .
  3. 3 . The method 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. 4 . The method 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. 5 . The method according to according to claim 1 , wherein p is set at 80% or more.
  6. 6 . The method 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. 7 . The method 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. 8 . The method 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. 9 . The method 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. 10 . The method according to claim 9 , wherein a temper work roll roughness Ra is used of 4.5 μm or less.
  11. 11 . The method according to claim 1 , performed with the purpose of producing 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 or lower.
  12. 12 . The method according to claim 1 , wherein the steel strip comprises 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: the steel substrate has a composition, all in weight %: C max 0.04; Mn 0.01-1.20; Si 0.001-0.50; AI 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; the steel strip 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 the steel strip, 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, 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∧ 2 ); AB is defined by Wsa=(0.2686)−(0.0543*Sc)+(0.0105*Sc∧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.
  13. 13 . The method according to claim 12 , 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∧ 2 ); A′F is defined by Wsa=(0.2276)−(0.0266*Sc)+(0.0054*Sc∧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′.
  14. 14 . The method according to claim 12 , 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∧2); A″G is defined by Wsa=(0.208)−(0.0118*Sc)+(0.0027*Sc∧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″.
  15. 15 . The method according to claim 1 , wherein the steel strip has a total coating weight on both sides together of 60-175 g/m 2 .
  16. 16 . The method according to claim 1 , wherein the steel strip has a surface roughness Ra between 0.9 μm and 1.8 μm.
  17. 17 . The method according to claim 1 , wherein the steel strip comprises 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: the steel strip 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 the steel strip, 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, 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∧2); AB is defined by Wsa=(0.2686)−(0.0543*Sc)+(0.0105*Sc∧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.
  18. 18 . The method according to claim 1 , wherein SRF AWR ≥ 27000 ⁢ kN / m 2 .
  19. 19 . The method according to claim 1 , wherein 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.
  20. 20 . The method according to claim 1 , wherein the Zn based coating is a Zn—Al—Mg coating.

Description

This invention relates to a method of manufacturing a steel strip, 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, as well as to a coated steel sheet comprising a steel substrate provided with a hot dip metal coating obtainable by the method. Methods of this kind and the resulting products are widely known throughout the steel industry. A steel strip suitable for hot dip coating is produced by hot rolling a steel slab into a hot rolled strip, which is subsequently pickled and cold rolled into a cold rolled strip, in a multi-stand cold rolling mill. The cold rolled strip is subsequently coated in a continuous hot dip coating line. Continuous hot dip coating lines are widely used and employed everywhere in the world. Hot dip coating was originally developed for galvanising i.e. zinc-coating, but is now also used to apply other metals or metal alloys to the steel sheet. In continuous hot dip coating the cold rolled steel strip is passed as a continuous ribbon through a bath of molten metal at high speeds. In the molten metal bath the steel strip reacts with the molten metal and the coating bonds onto the strip surface. The strip passes one or more submerged rolls and exits the bath in a vertical direction. Above the exit point a set of gas knives wipes off excess molten metal allowing a controlled thickness of coating usually expressed as weight of coating per unit area on the strip surface. After cooling the strip feeds into the exit end of the hot dip coating line often comprising a temper mill, also called skin pass mill. As wiping gas normally air or nitrogen gas is used. For producing high quality coated products normally nitrogen gas is used. Originally hot dip coated steel sheets were used for applications that did not demand a high quality finish or a high degree of formability, but in recent times they are increasingly used for more demanding applications such as for automotive hoods, fenders and doors. The surface quality of the coated steel sheets is influenced by defects of several types. The main types of defects are dross type defects, furnace defects and coating defects, the latter being related to solidification and oxidation of the liquid metal during the hot dip coating process. For the improvement of the surface quality it is important not only to find a way to reduce dross type and furnace defects, but also to find a way to reduce these coating defects. If such an improvement is found, this immediately leads to further improvement of the product since the other types of defects become more prominent and can be eliminated in a targeted way. Further, it also enables the declassification of problematic sheets because other defects are no longer missed, so that on balance a product with better surface quality is sold to the market. Several ways to improve the surface quality of the subject products have been proposed, especially also as regards reduction of coating defects as mentioned above. One proposed solution is to reduce the level of oxygen in the atmosphere surrounding the steel strip after hot dipping. Another proposed solution is to vary amounts of certain elements such as Al and or Mg in the hot dip bath, or to add very specific elements such as Be or Ga to it. Both solutions to improve the surface quality of the coated sheets have their downsides. The first one requires the use of a confinement box shielding the coated strip. Such a box limits the visibility of the strip and limits the room for positioning the wiping device and any further devices including skimming equipment, all required for optimal control of the hot dip coating process. The second one is often unsatisfactory as the in-use application properties such as sensitivity to filiform corrosion or corrosion resistance are compromised. It is an objective of the invention to provide an improved method for manufacturing a hot dip coated steel sheet with a high surface quality, in which the number of defects is low, and which has a low waviness in the final product, e.g. a visible part of an automobile body. It is also an objective to provide an improved hot dip coated steel sheet, that is suitable especially for use in a visible part of an automobile body. These objectives are achieved according to the independent claims. Preferred embodiments are defined in the respective dependent claims. It should be noted that the features listed in the claims can be combined in any technically meaningful manner to describe further embodiments of the invention. The following specification explains the features of the inventions and contains additional embodiments of the invention. Further, it should be noted that features described in