US20200087761A1 - Steel substrate for painted parts - Google Patents

Steel substrate for painted parts Download PDF

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US20200087761A1
US20200087761A1 US16/340,299 US201716340299A US2020087761A1 US 20200087761 A1 US20200087761 A1 US 20200087761A1 US 201716340299 A US201716340299 A US 201716340299A US 2020087761 A1 US2020087761 A1 US 2020087761A1
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roughness
max
strip
sheet
stand
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Maxim Peter AARNTS
Job Anthonius Van Der Hoeven
Edgar Matthijs TOOSE
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Tata Steel Ijmuiden BV
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Tata Steel Ijmuiden BV
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Assigned to TATA STEEL IJMUIDEN B.V. reassignment TATA STEEL IJMUIDEN B.V. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: AARNTS, MAXIM PETER, TOOSE, Edgar Matthijs, VAN DER HOEVEN, JOB ANTHONIUS
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron

Definitions

  • the invention relates to a steel strip, sheet or blank used for painted parts, e.g. for automotive purposes.
  • the invention also relates to a method for producing such a strip, sheet or blank.
  • Painted steel parts e.g. for the outer panels of automobiles, such as the hood and the doors, are subject to stringent requirements by the producers thereof. One of these requirements relates to the paint appearance of the painted part.
  • the steel substrate for producing the painted parts is usually coated with a metal coating, e.g. zinc based coating.
  • a manufacturer forms the (coated) substrate in a press into the desired shape for a panel. After pressing, the panels are usually painted using one or more layers of paint.
  • Outer panels with a very good paint appearance are highly valued, i.e. when the panels have a mirror-like surface that reflects light without distortion leading to sharp reflected images.
  • the paint appearance is influenced by the quality of the paint, but also by the surface of the (coated) substrate. This surface consists of in-plane structures of variable size and amplitude. The smaller structures are captured by the surface roughness, whereas the larger structures are captured by the so-called surface waviness.
  • the larger surface structures e.g. the surface waviness
  • the waviness of the surface of the (coated) substrate is to a certain extent still present at the surface of the outer paint layer.
  • the paint appearance of the painted part can be measured and is expressed by different measured values, e.g. Long Waviness LW in case it is measured using a BYK Wavescan Dual. Due to the transmission effect the Long Waviness, or a similar value, of the painted part is related to the surface waviness of the non-painted formed part.
  • the surface waviness of a formed part is the result of the surface waviness of the undeformed, e.g. flat part, and the waviness increase introduced by the forming step.
  • the difference between the waviness of the formed part and the waviness of the undeformed part is referred to as the delta waviness, e.g. ⁇ Wsa.
  • the delta waviness e.g. ⁇ Wsa.
  • the formed surface shows a line like pattern, in which the lines are perpendicular to the rolling direction.
  • An implication of this observation is that the delta waviness is higher in the rolling direction than in other directions. This directional effect is strongly present in the paint appearance values as well and therefore it is of importance that delta waviness in the rolling direction should is as much as possible.
  • a steel strip, sheet or blank used for painted parts, which strip, sheet or blank is optionally metallic coated, is provided of which the steel is an Ultra Low Carbon (ULC) steel type having a composition of (in weight %):
  • ULC Ultra Low Carbon
  • Ultra Low Carbon steels are necessary for producing a part having a low delta Waviness ⁇ Wsa, in particular a ⁇ Wsa ⁇ 0.12 ⁇ m.
  • ULC steels are often meant for applications demanding high formability.
  • Carbon in Ultra Low Carbon steels should be kept low because for deep drawing any Carbon in solid solution has a deleterious effect on the preferred recrystallisation texture.
  • IF (interstitial free) steel which is a special type of ULC steel
  • all Carbon is precipitated to avoid any Carbon in solid solution.
  • BH (bake hardenable) steel which is also a special type of ULC steel
  • a limited level of Carbon is kept in solid solution to benefit from a strength increase during baking, and the remaining Carbon should also be precipitated.
  • the total level of Carbon should not be more than 0.007 wt % otherwise the amount and size of formed precipitates will hamper formability.
  • Manganese is a solid solution strengthening element and can therefore be added to increase the strength but it has a negative effect on deep drawability. For this reason the Mn level should be kept to max 1.2 wt %. Furthermore, the formation of MnS might hamper the formation of the preferred Ti4C2S2 precipitates. For the latter reason, and to not compromise formability too much, it is preferred to have max 1.0 wt % Mn, or even more preferred to have max 0.8 wt % Mn.
  • Silicon is also a solid solution strengthening element and can therefore be added to increase the strength.
  • the Si level is too high the coating adhesion might deteriorate due to the forming of Mn2SiO4 spinel type oxides, and/or SiO2.
  • the maximum Si level is 0.5 wt %, more preferred max 0.25 wt %.
  • Phosphorus is a very potent solution strengthening element but high levels of P might increase the Ductile-to-Brittle-Transition-Temperature (DBTT) too much, in particular in IF steels. Adding Boron can counteract this, nevertheless the P level should be maximum 0.15 wt %. Furthermore, high levels of P will increase the change to the formation of Fe—Ti—P precipitates which are not desired. For this reason it is preferred to keep maximum P level at 0.10 wt %.
  • DBTT Ductile-to-Brittle-Transition-Temperature
  • Aluminium is mainly added to bind any remaining Oxygen, but it can also be used to precipitate with Nitrogen.
  • a minimum Aluminium level of 0.01 wt % is preferred.
  • the risk for clogging during casting also increases. For this reason the maximum level of Al is set at 0.1 wt %.
  • Nitrogen in solid solution is present as an interstitial element which hampers formability. It should therefore be fully precipitated.
  • Ti, Al or B are added to make sure all N has precipitated. Nevertheless the N level should not exceed 0.01 wt % and the amount of N should preferably be not more than 0.006 wt %.
  • Titanium, Niobium and Molybdenum are strong grain refiners and the presence of at least one of these elements is essential for the current invention.
  • Nb and Mo are even more potent as grain refiners than Ti; based on the observations by the inventors, Nb and Mo are about 2 times more effective (when given in wt %).
  • Ti and Nb are both present, they enhance each other such that their combined presence is about 4 ⁇ more effective as grain refiner compared to only Ti.
  • These elements work because they precipitate with N and/or C and the precipitates formed hinder recrystallisation and grain growth; Nb is also known to hinder recrystallisation and grain growth when in solid solution. Vanadium might also work, but Vanadium precipitates can dissolve at the temperatures used for annealing after cold rolling which renders these precipitates less effective.
  • the amount of Carbon in solid solution is important and needs to be controlled. Because Ti, Nb Mo and V precipitate with Carbon they are also important to control the amount of C in solid solution.
  • the balance between C, N, Ti, Mo, V and Nb needs to be tuned with care. In IF steels some excess Ti or Nb can be allowed. This limits Ti between 0.06 and 0.60 wt %, or Nb between 0.03 and 0.30 wt % or Mo between 0.03 and 0.30 wt %; combinations of these three elements are also possible in which case 4 ⁇ (Ti+Nb)+2 ⁇ Mo should be from 0.06 to 0.6 wt %.
  • the amount of Ti, Nb and Mo is especially important.
  • the amount of Ti or 2 ⁇ Nb or 2 ⁇ Mo must be at least 0.06 wt %, or when these elements are combined the amount of 4 ⁇ (Ti+Nb)+2 ⁇ Mo must be at least 0.06 wt %.
  • the ULC steel will not result in a steel part having the right ⁇ Wsa.
  • Copper is allowed up to 0.10 wt %. It can lead to the formation of CuS which with the right dimensions might hinder recrystallisation and grain growth but it is also in competition with the more desirable Ti4C2S2. Therefore, a maximum level of 0.04 wt % is more preferred.
  • Chromium and Nickel are basically impurities but a maximum of 0.06 and 0.08 wt % respectively does not harm. Nevertheless, a maximum of 0.04 wt % for each is more preferred.
  • Boron is an interstitial element so Boron in solid solution should be kept as low as possible, restricting B to maximum 0.0015 wt %. Boron can be added to reduce the chance for a too high DBTT, in particular in P alloyed IF steels. It can also be added to make sure all N is precipitated. On the other hand more than 0.0008 wt % B might lead to surface defects, so the more preferred range is 0.0005-0.0008 wt % B.
  • Cobalt and Tin are basically impurities but maximum 0.04 wt % for both can be allowed.
  • Calcium is sometimes added up to 0.005 wt % in steels for deoxidation and/or desulphurisation. A level up to 0.01 wt % can be allowed without deteriorating the properties.
  • the amounts of Ti, Nb and Mo are as follows (in weight %):
  • the upper limit for the formula for the combination of Ti, Nb and Mo is 0.30, because it is unusual that these elements are needed in such high amounts.
  • the more preferred upper level is 0.1 wt %.
  • Bake Hardenable ultra low carbon steel strip, sheet or blank wherein the amount of Ti, Nb and Mo are tuned with respect to the C, N and S levels as follows (all in wt %):
  • Csol free carbon
  • the steel has grains with an essentially equi-axed median grain size smaller than 11.0 micrometer.
  • the inventors have found that the grain size is an important determining factor for waviness, and especially for determining ⁇ Wsa.
  • the grain size and the ⁇ Wsa of numerous steel samples the inventors have been able to determine a relationship between grain size and ⁇ Wsa, with essentially equi-axed grains with median grain size smaller than 11.0 micrometer results in a ⁇ Wsa ⁇ 0.12 ⁇ m of the surface of the strip, sheet or blank.
  • Wsa is defined in standard SEP 1941.
  • the relationship between grain size and ⁇ Wsa makes it possible to produce steel strips, sheets and blanks having a desired ⁇ Wsa ⁇ 0.12 ⁇ m when the grain size of the steel substrate is controlled.
  • the grain size is the size of the grains after continuous annealing and optionally metallic coating.
  • the inventors have found that the grain size should be smaller than 11.0 micrometer when combined with Ultra Low Carbon steel types, which are mainly used for painted parts such as outer panels of automobiles, such that providing grains with the right size that is an average size of less than 11.0 micrometer as essentially equi-axed grains is combined when the composition of the steel is as indicated above.
  • equi-axed means that, in a cross section (RD/ND plane), the number of grain boundaries intersecting with a straight line parallel to RD, divided by the number of grain boundaries intersecting with a straight line of equal length in ND is at least 0.66; the straight line should be long enough to yield at least 200 intersects in RD as well as in ND, or the procedure is repeated with several equally distributed lines such that the sum of all intersects in RD as well as in ND is at least 200. In the latter case the number of intersects in RD and ND is totalled over the lines before they are divided.
  • RD/ND plane In a cross section (RD/ND plane) the number of grain boundaries intersecting with 10 straight lines, equally distributed over ND (normal direction) and parallel to RD (rolling direction) were measured. Also the numbers of grain boundaries intersecting with 10 straight lines, equally distributed over RD, and parallel to ND were measured.
  • the lines in RD and ND were of equal length and long enough to yield at least 20 grain boundary intersects per line. The total number of intersects over all lines in RD was divided by the total number of intersects over all lines in ND, and in all cases this number was ⁇ 0.66.
  • Having essentially equi-axed grains with median grain size smaller than 11.0 micrometer is an important condition but other conditions are important as well to get the best results.
  • the roughness at the last stand of the cold mill, as well as the roughness of the temper mill, and the reductions given at the last stand of the cold mill and at the temper mill are parameters that need to be controlled; this is known for the person skilled in the art.
  • the essentially equi-axed grains have a median size smaller than 10.0 micrometer.
  • a small median grain size can result in a ⁇ Wsa of 0.10 or even smaller.
  • the undeformed steel surface of the strip, sheet or blank has a waviness Wsa ⁇ 0.35 ⁇ m where Wsa is measured in the rolling direction, preferably a waviness Wsa ⁇ 0.32 ⁇ m, even more preferably Wsa ⁇ 0.29 ⁇ m and even more preferably Wsa ⁇ 0.26 ⁇ m.
  • the waviness of the undeformed steel surface in combination with ⁇ Wsa determined the Wsa of the formed part.
  • the strip, sheet or blank is coated with a zinc based coating, a Zn—Al—Mg based coating, or an aluminium based coating.
  • the zinc based coating consists of 0.1-1.2 wt % aluminium and up to 0.3 wt % of other elements, the remainder being unavoidable impurities and zinc
  • the Zn—Al—Mg based coating preferably consists of 0.2-3.0 wt % aluminium and 0.2-3.0 wt % magnesium, up to 0.3 wt % of other elements, the remainder being unavoidable impurities and zinc
  • the aluminium based coating preferably consists of 0.2-13 wt % silicon, up to 0.3 wt % of other elements, the remainder being unavoidable impurities and aluminium.
  • These coating are used in the automotive industry and are therefore preferably used to coat the steel strip, sheet or blank.
  • the other elements mentioned can be Si, Sn, Bi, Sb, Ln, Ce, Ti, Sc, Sr and/or B.
  • a method for producing a steel strip according to the first or second aspect of the invention wherein the steel strip is hot rolled and cold rolled, and wherein the last stand or the only stand for the cold rolling contains work rolls having a roughness Ra between 0.5 ⁇ m and 7.0 ⁇ m.
  • the roughness Ra of the work rolls in the last stand or the only stand is between 0.55 ⁇ m and 5.0 ⁇ m, more preferably between 0.6 ⁇ m and 4.0 ⁇ m, most preferably between 0.6 ⁇ m and 2.0 ⁇ m.
  • the inventors have found that work rolls with a roughness between these limits provide good results.
  • the work rolls should have a roughness Ra between 0.5 ⁇ m and 7.0 ⁇ m.
  • the work rolls of the first stand should have a roughness Ra between 0.6 ⁇ m and 3.0 ⁇ m, and the work rolls of the last stand should have a roughness Ra between 0.5 ⁇ m and 7.0 ⁇ m.
  • the work rolls of the first stand should have a roughness Ra between 0.6 ⁇ m and 3.0 ⁇ m
  • the work rolls of the intermediate stands should have a roughness Ra between 0.3 ⁇ m and 0.8 ⁇ m
  • the work rolls of the last stand should have a roughness Ra between 0.5 ⁇ m and 7.0 ⁇ m.
  • the work rolls used before the strip leaves the cold rolling mill always should have a roughness Ra between 0.5 ⁇ m and 7.0 ⁇ m.
  • the roughness thereof should be between 0.6 ⁇ m and 3.0 ⁇ m. If intermediate stands are present, these should have a low roughness: between 0.3 ⁇ m and 0.8 ⁇ m.
  • the cold rolled strip is skin passed, preferably after applying a metallic coating, using temper rolls having a roughness between 0.5 and 4.0 ⁇ m, preferably a roughness ⁇ 2.8 ⁇ m.
  • the roughness of the skin pass rolls is transferred on the strip, sheet of blank that is formed, and thereby has a strong influence on the waviness of the flat product.
  • a strip produced with the method according to the third aspect of the invention is produced, wherein the surface of the strip has a roughness Ra lower than 2.0 ⁇ m and a waviness Wsa lower than 0.6 ⁇ m in rolling direction of the strip for a strip coated with an aluminium based coating having a coating thickness between 4 and 12 ⁇ m.
  • the strip has a roughness Ra between 0.7 and 1.6 ⁇ m and a waviness Wsa between 0.15 and 0.35 ⁇ m in rolling direction of the strip.
  • the grain size was determined as well as the waviness Wsa before and after cupping.
  • the coils were continuously annealed, top temperature 810 ⁇ 20° C., and hot dip galvanised at 470 ⁇ 10° C. Air knives were used to adjust the coating thickness, and cooling was applied immediately after the air knives to solidify the coating. Finally, the strip was temper rolled. The roughness of the temper mill was EDT 1.9 ⁇ 0.1 ⁇ m.
  • RD-ND sections of the samples were mounted in conductive resin (so called polyfast) and mechanically polished to 1 ⁇ m. Care was taken to remove any surface deformation caused by the previous grinding and polishing steps. To obtain a fully deformation free surface, the final polishing step was conducted with colloidal silica.
  • the microstructure analysis was performed using a FEG-SEM (Field Emission Gun scanning electron microscope, Zeiss Ultra 55 FEG-SEM) equipped with an EDAX PEGASUS XM 4 HIKARI EBSD system.
  • EBSD Electro Backscatter Diffraction
  • the samples were placed under a 70° angle in the SEM.
  • the acceleration voltage was 15 kV, the high current option was on, the 120 ⁇ m aperture was used and typically the working distance was 17 mm during scanning.
  • the dynamic focus correction was used during scanning.
  • the EBSD scans were captured using software from firm EDAX (TSL OIM Data Collection version 7.0.1. (8-27-13)). Typically the following data collection settings were used: Hikari camera at 6 ⁇ 6 binning combined with standard background subtraction. The scan area was in all cases at most the sample thickness, and care was taken not to include non metallic inclusions in the scan area.
  • EBSD Scan size 500 ⁇ 500 ⁇ m, step size 0.5 ⁇ m. scan rate ca. 80 frames per second, phase included during scanning: Fe(a).
  • Hough settings used during data collections were: Binned pattern size 96; theta set size: 1; rho fraction max peak count: 13; min peak count: 5; Hough type: classic; Hough resolution: low; butterfly convolution mask: 9 ⁇ 9; peak symmetry: 0.5; min peak magnitude: 5 max peak distance: 15.
  • the EBSD scans were evaluated with TSL OIM Analysis software version 7.1.0 ⁇ 64 (30-14-14). Typically, the data sets were 90° rotated over RD to get the scans in the proper orientation with respect to the measurement orientation. A standard grain dilation clean up was performed (GTA 5, minimum grain size 5 and grain must contain multiple rows single iteration).
  • Cups were produced by pressing a blank of 145 mm ⁇ 145 mm in a press with a hollow punch with diameter 75 mm and a blankholder force such that any material movement of the (coated) substrate between the blankholder and die is completely suppressed.
  • the deformation of the cup is such that the thickness strain in the bottom is 9%+/ ⁇ 0.3%.
  • the thickness strain is defined as (t(original) t(deformed))/t(original) ⁇ 100%, with t(original) the undeformed thickness and t(deformed) the thickness after deformation.
  • the results are shown in table 2.
  • the table indicates that in order to increase the chance for ⁇ Wsa ⁇ 0.12 ⁇ m, the grain size of the material needs to be smaller than 11.0 ⁇ m.
  • Alloy 4A has a grain size ⁇ 11.0 ⁇ m which does lead to ⁇ Wsa ⁇ 0.12 although the “effectiveness of Ti/Nb/Mo” ⁇ 0.06. This indicates that even when the “effectiveness of Ti/Nb/Mo” is too low, good products are possible but good results are not usual.
  • ⁇ Wsa is indeed very much dependent on the median equi-axed grain size, both in regard to the upper limit as in regard to the lower limit of ⁇ Wsa.
  • FIG. 1 shows the ⁇ Wsa that was obtained in these four experiments; Ra values of the samples before cupping were between 1.05 and 1.2 ⁇ m, and the Rpc of the samples before cupping was between 80 and 105 cm ⁇ 1 . (Rpc is the peak count, that is the number of roughness peaks per given length).
  • FIG. 1 shows that the roughness of the last stand of the cold mill can have a significant influence on the ⁇ Wsa that is obtained.

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WO2024023553A1 (en) * 2022-07-28 2024-02-01 Arcelormittal Method for manufacturing a coated press hardened steel part having an improved appearance and corresponding steel part
US12325893B2 (en) 2020-11-16 2025-06-10 Arcelormittal Method for manufacturing a steel sheet with a ZnAlMg coating, corresponding coated steel sheet, part and vehicle
US12338533B2 (en) 2020-11-12 2025-06-24 Posco Co., Ltd High-strength hot-dip galvanized steel sheet with excellent formability and process for producing same
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US11371128B2 (en) 2009-05-14 2022-06-28 Arcelormittal Coated metal band having an improved appearance
US12359291B2 (en) 2020-11-05 2025-07-15 Posco Co., Ltd High-strength galvanized steel sheet having excellent distinctness of image after painting and manufacturing method therefor
US12338533B2 (en) 2020-11-12 2025-06-24 Posco Co., Ltd High-strength hot-dip galvanized steel sheet with excellent formability and process for producing same
US12325893B2 (en) 2020-11-16 2025-06-10 Arcelormittal Method for manufacturing a steel sheet with a ZnAlMg coating, corresponding coated steel sheet, part and vehicle
WO2022223719A1 (en) * 2021-04-21 2022-10-27 Tata Steel Ijmuiden B.V. Steel strip or sheet and method for producing it
WO2023006732A1 (en) * 2021-07-28 2023-02-02 Tata Steel Ijmuiden B.V. Hot rolled steel sheet and a method of manufacturing it
WO2024023690A1 (en) * 2022-07-28 2024-02-01 Arcelormittal Automotive vehicle with press hardened visible steel parts
WO2024023553A1 (en) * 2022-07-28 2024-02-01 Arcelormittal Method for manufacturing a coated press hardened steel part having an improved appearance and corresponding steel part
WO2024023552A1 (en) * 2022-07-28 2024-02-01 Arcelormittal Method for manufacturing a coated press hardened steel part having an improved appearance and corresponding steel part
WO2024023697A1 (en) * 2022-07-28 2024-02-01 Arcelormittal Automotive vehicle with press hardened visible steel parts

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MX2019004458A (es) 2019-06-17
JP7066698B2 (ja) 2022-05-13
CA3039083A1 (en) 2018-04-26
BR112019007262A2 (pt) 2019-07-09
CN109844158A (zh) 2019-06-04
JP2019534949A (ja) 2019-12-05
BR112019007262B1 (pt) 2022-12-06
KR102439914B1 (ko) 2022-09-05
KR20190070336A (ko) 2019-06-20
US20220064765A1 (en) 2022-03-03

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