A method to produce a znaimg coated steel sheet and said steel sheet

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

[0001] A method to produce a ZnAIMg coated steel sheet and said steel sheet

[0002] The present invention relates to a method for manufacturing a steel sheet provided with a coating comprising from 3.50 to 4.10 wt.% of Aluminium, from 1.90 to 3.20 wt.% of Magnesium, the remainder being Zn. Such a steel sheet is well suited for industrial applications requiring high corrosion resistance.

[0003] It is known to add aluminium and magnesium to zinc-based coatings. Such zinc alloy coatings show a corrosion resistance that is much higher than the traditional galvanized or galvannealed coatings. Most steel companies are now marketing such so called ZnAIMg coatings because they provide an improved corrosion resistance to the customer. Such coatings are also of interest because they can be used with a coating thickness that is thinner than traditionally used while still providing an improved corrosion resistance, thereby saving in the use of zinc. Such ZnAIMg coatings are also frequently used for organic coated steel substrates, both for use inside buildings and for exterior use.

[0004] These ZnAIMg coatings are produced in conventional hot dip coating industrial facilities. This facility may be combined with a continuous annealing furnace.

[0005] Flat steel making delivers a slab that is first hot rolled to achieve a hot rolled coil. Then the hot rolled coil is optionally cold rolled. The cold rolled steel is annealed to determine the final microstructure the steel. The cold or hot rolled steel sheet in form of a coil is then processed at the hot dip coating facility.

[0006] During the hot dip coating process, the steel sheet is dipped into a molten metal bath. The excessive liquid metal is wiped by mean of gas knives. The wiping operation allows determining the thickness of the coating. After wiping, the coated steel sheet is cooled.

[0007] The hot dip coating bath is prepared with ingots containing the elements to be deposited in the coating. During the hot dip coating coating process, additional ingots are melted in the coating bath to feed and renew it. Orange peel is a cosmetic defect associated with a rough surface appearance. It is called orange peel because the surface has the appearance of the surface of an orange. Orange peel is detrimental for applications that are exposed to the eyes of the final customer. The deformation of the steel sheet can reveal orange peel. As well, successive layers of paint can decrease the appearance quality because of the orange peel defect.

[0008] The aim of the present invention is to provide a method to manufacture a steel sheet, provided with a coating free of orange peel defect.

[0009] For this purpose, the object of the invention is a method according to claim 1.

[0010] The method may also comprise the features of claims 2 to 8, taken individually or as a combination.

[0011] The object of the invention is also a steel sheet according to claim 9.

[0012] The steel sheet may also comprise the features of claim 10 to 12.

[0013] The object of the invention is also an ingot according to claim 13. The ingot may also comprise the features of claim 14.

[0014] The object of the invention is achieved by a method for manufacturing a steel sheet provided with a coating comprising from 3.50 to 4.10 wt.% of Aluminium, from 1.90 to 3.20 wt.% of Magnesium, from 1.0 to 15.0 weight ppm of Bi, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Zr, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn and unavoidable impurities, the method comprising the following successive steps:

[0015] A) providing a steel sheet,

[0016] B) depositing said coating by dipping said steel sheet in a molten bath,

[0017] C) wiping the liquid metal on the steel sheet,

[0018] D) cooling the metallic coating at a cooling rate complying with one of the following conditions: i) Vcooiing > 2.8 °C / s if the coating comprises from 1 .0 to 4.0 weight ppm of Bi, or ii) if the coating comprises more than 4.0 weight ppm and up to 15.0 weight ppm of Bi, where [Bi] is the Bismuth content in the metallic coating expressed in weight ppm, and Vcooling is the cooling rate expressed in °C / s.

[0019] Without willing to be bound theory, it is believed that the method according to the present invention allows a steel sheet provided with a coating comprising from 3.50 to 4.10 wt.% of Aluminium, from 1.90 to 3.20 wt.% of Magnesium, the remainder being Zn, to get a surface free from orange peel. Indeed, it seems that the methods of the prior art were unable to prevent from orange peel when the purity of the molten metal bath was degraded, especially when the bath comprises Bismuth. The inventors have found that not only the Bismuth content in the molten bath, but also the cooling speed have an impact on the apparition of orange peel.

[0020] It seems that the orange peel defect is related to the grain size of the coating: a coarser grain size tends to increase the orange peel appearance.

[0021] In a preferred embodiment, in method step D), the cooling rate further complies with one of the following conditions: iii) Vcooling > 7. 1 °C / s if the coating comprises from 1 .0 to 4.0 weight ppm of Bi, or

[0022] \ ■£ . i iv) if the coating comprises more than 4.0 weight ppm and up to 15.0 weight ppm of Bi, where [Bi] is the Bismuth content in the metallic coating expressed in weight ppm, and Vcooling is the cooling rate expressed in °C / s.

[0023] It has been observed that complying with the conditions (iii) or (iv) in addition to complying with the conditions (i) or (ii) allows to reduce further the orange peel appearance. During hot-dip coating, the aluminium present in the bath will first react with the steel to create a so-called inhibition layer composed of intermetallic elements made of aluminium and iron. Such inhibition layer is usually composed of Fe2Alsand FeAh and has a thickness varying from 20 to 80 nm. The coating layer containing from 3.50 to 4.10% by weight of aluminum and from 1.90 to 3,20 % by weight of magnesium, as described above, is formed on this inhibition layer.

[0024] The Bi content in the molten metal bath or in the coating can be measured by Inductively coupled plasma mass spectrometry (ICP-MS) performed on samples taken from the bath.

[0025] The Bi content in the ingots may be controlled by any means. For example, it is controlled by spark emission spectroscopy.

[0026] In step A), any steel sheet can be provided. Preferably, the steel sheet is hot rolled and has a thickness from 1.5 to 6.0 mm. In another embodiment, the steel sheet is cold rolled and has a thickness from 0.4 to 2.5 mm.

[0027] The steel grade is not limited. For example, it can be Interstitial Free steel (IF-steel). It refers to the fact, that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. As another example, it can be Aluminium-killed mild steel.

[0028] In step B), the molten metal bath can comprise up to 5.0 wt% iron coming from the steel sheet by dissolution.

[0029] Advantageously, the molten metal bath comprises from 3.60 to 4.10 wt% of Aluminium and from 2.90 to 3.10 weight % of Magnesium.

[0030] Preferably, the coating comprises from 3.50 to 3.90 wt.% of Aluminium, from 2.80 to 3.20 wt.% of Magnesium.

[0031] In an advantageous embodiment, the molten metal bath comprises from 3.60 to 3.80 wt% of Aluminium and from 2.90 to 3.10 weight % of Magnesium.

[0032] The molten metal bath according to the invention comprises from 1 .0 to 15.0 weight ppm of Bi. A coating comprising less than 1 .0 weight ppm Bi would require a too high purity of the ingots for bath preparation. If the coating comprises more than 15.0 weight ppm of Bi, it will have an inadmissible orange peel aspect on its surface.

[0033] Preferably, the molten metal bath comprises from 1.0 to 10.0 weight ppm of Bi, advantageously from 1 .0 to 7.0 weight ppm of Bi, particularly from 1 to 5 weight ppm.

[0034] The molten metal bath may comprise unavoidable impurities.

[0035] The bath may also contain up to 0.3% by weight of optional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Zr. These different elements may allow improvement in the resistance to corrosion of the coating or else its brittleness or its adhesion, for example.

[0036] The one skilled in the art who is aware of their effects on the characteristics of the coating will know how to use them according to the sought additional purpose.

[0037] The temperature of the metal bath is from 380°C to 460 °C. Depending on the metal bath temperature, the time during which the coating remains liquid on the steel sheet varies. The localization of the solidification also varies and the grain size as well. Preferably, the temperature of the metal bath is from 400°C to 430°C.

[0038] Advantageously, the temperature of the metal bath is from 420 to 430°C. A bath temperature from 420°C to 430°C allows for a better control of the coating grain size.

[0039] In step C), wiping can be performed by any gas. Preferably, it has an oxidizing power lower than that of an atmosphere consisting of 4% oxygen by volume and 96% nitrogen by volume. It may be advantageous to use pure nitrogen or pure argon, or else mixtures of nitrogen or argon and oxidizing gases such as, for example, oxygen, CO / CO2 mixtures or H2 / H2O mixtures. It is also possible to use CO / CO2 mixtures or H2 / H2O mixtures without the addition of an inert gas. Preferably, the wiping gas consists of nitrogen.

[0040] Preferably, the wiping operation is performed in a confined atmosphere. Said confinement allows for better control of the oxidation of the liquid coating before solidification. Advantageously, the atmosphere in the confinement zone has an oxidizing power lower than that of an atmosphere consisting of 4% oxygen by volume and 96% nitrogen by volume.

[0041] In step D), the cooling can occur by any mean. For example, cooling is performed by convection with the ambient air. Preferably, cooling speed is increased by forced convection. To this purpose, gas flows are oriented towards the steel sheet.

[0042] Quicker cooling can be achieved by introducing the steel sheet in a specific humid atmosphere or even by spraying droplets on the sheet.

[0043] The coated steel sheet according to the invention has an acceptable aspect for visible applications.

[0044] Preferably, the coated steel sheet has a premium aspect.

[0045] The coated steel sheet has a coating grain size below 200 pm, preferably below 100 pm.

[0046] The coating grain size is at least 40 pm. Indeed, a smaller grain size may not be achieved because the maximal cooling rate may be limited by the cooling technology used.

[0047] The coating weight is not particularly limited. For example, it can be from 70 to 700 g / m2for the sum of both faces. Preferably, the coating weight is from 190 to 310 g / m2for the sum of both faces.

[0048] In an advantaged embodiment, coating thickness is from 15 to 25 pm per face.

[0049] The invention will now be illustrated by tests given as an indication and not as a limitation.

[0050] Examples I F-Steel sheets with a thickness of 1 .36 mm were provided. They were coated with a hot dip coating bath comprising, by weight 3.0% of Mg, 3.7% of Al, the remainder being Zn and unavoidable impurities.

[0051] The Bi content of the hot dip bath was from 1 to 14 weight ppm.

[0052] The metal bath temperature was from 420 to 430°C.

[0053] Then the coating was wiped with nitrogen and cooled by forced convection of nitrogen.

[0054] The coating weight was from 190 to 310 mg / m2for the sum of both faces. The coating thickness was locally measured with a deltascope. The average coating thickness by face was from 20.6 to 21 .9 pm.

[0055] The coating composition was analyzed by Inductively coupled plasma mass spectrometry (ICP-MS).

[0056] The orange peel was assessed by naked eye. Quotation is as follows:

[0057] 0 = a premium aspect,

[0058] 1 = acceptable aspect for less demanding applications,

[0059] 2 = unacceptable aspect regarding orange peel,

[0060] 3 = degraded aspect,

[0061] 4 = sheet covered with lots of defects.

[0062] Samples were cut from the sheet to analyze the grain size of the coating on its surface. This operation was performed with an optical microscope including an image processing software. The microscope was a Keyence VHX-7000 with a magnification from 100x to 300x.

[0063] The grains are quasi equiaxed. They look like circles.

[0064] The diameter of a grain is obtained according to the following steps:

[0065] - Plot manually the contour of the grain

[0066] - Determine the length of the contour thanks to the image processing software,

[0067] - Calculate the diameter of each grain assumed as a circle by dividing the contour length by IT (Pi). The grain size displayed in the table below is a mean value of 15 grain diameters measured on 15 grains on the surface.

[0068] Results are gathered in table 1. The two last columns regarding the conditions (i) or (ii) and the conditions (iii) or (iv) are noted Y for yes or noted N for no.

[0069] Table 1 The samples 4, 7, 10 and 18 complying only with the conditions (i) or (ii), have an acceptable aspect and an average coating grain size from 100 to 200 pm. The samples 2, 5, 8 and 11 , further complying with the condition (iii) or (iv), have a premium aspect and a coating grain size below 100 pm.

Claims

CLAIMS1 . A method for manufacturing a steel sheet provided with a coating comprising from 3.50 to 4.10 wt.% of Aluminium, from 1.90 to 3.20 wt.% of Magnesium, from 1.0 to 15.0 weight ppm of Bi, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Zr, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn and unavoidable impurities, the method comprising the following successive steps:A) providing a steel sheet,B) depositing said coating by dipping said steel sheet in a molten metal bath at a temperature from 380°C to 460 °C,C) wiping the liquid metal on the steel sheet,D) cooling the metallic coating at a cooling rate complying with one of the following conditions: i) Vcooiing > 2.8 °C / s if the coating comprises from 1 .0 to 4.0 weight ppm of Bi, or ii) if the coatingcomprises more than 4.0 weight ppm and up to 15.0 weight ppm of Bi, where [Bi] is the Bismuth content in the metallic coating expressed in weight ppm, and Vcooiing is the cooling rate expressed in °C / s.

2. A method according to claim 1 , wherein in step D), the cooling rate further complies with one of the following conditions: iii) Vcooiing > 7. 1 °C / s if the coating comprises from 1 .0 to 4.0 weight ppm of Bi, or. . r .. iv) if thecoating more than 4.0 weight ppm and up to 15.0 weight ppm of Bi, where [Bi] is the Bismuth content in the metallic coating expressed in weight ppm, and Vcooiing is the cooling rate expressed in °C / s.

3. A method according to claims 1 or 2, wherein the coating comprises from 3.50 to 3.80 wt.% of Aluminium and from 2.80 to 3.20 wt.% of Magnesium.

4. A method according to anyone of claims 1 to 3, wherein the coating comprises from 1 .0 to 7.0 weight ppm of Bi.

5. A method according to anyone of claims 1 to 4, wherein in step B), the metal bath has a temperature from 420°C to 430 °C,6. A method according to anyone of claims 1 to 5, wherein in step C), wiping occurs in a confinement zone where the atmosphere has an oxidizing power lower than that of an atmosphere consisting of 4% oxygen by volume and 96% nitrogen by volume.

7. A method according to claim 6, wherein wiping is performed with nitrogen.

8. A method according to anyone of claims 1 to 7, wherein in step D), the cooling is performed by forced convection.

9. A steel sheet provided with a coating comprising from 3.50 to 4.10 wt.% of Al, from 1.90 to 3.20 wt.% of Mg, from 1.0 to 15.0 weight ppm of Bi, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Zr, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn and unavoidable impurities, wherein the average grain size of said coating is below 200 pm, as measured by optical microscopy.

10. A steel sheet according to claim 9, wherein said coating comprises from 3.50 to 3.80 wt.% of Aluminium and from 2.80 to 3.20 wt.% of Magnesium.

11. A steel sheet according to claims 9 or 10, wherein the coating grain size of said coating is below 100 pm.

12. A steel sheet according to anyone of claims 9 to 11 , wherein the coating weight is from 190 to 310 g / m2for the sum of both faces.

13. An ingot to prepare and to feed the molten metal bath according to the method of anyone of claims 1 to 8 wherein, the ingot comprises from 3.50 to 4.10 wt.% of Al, from 1.90 to 3.20 wt.% of Mg, from 1.0 to 15.0 weight ppm of Bi, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Zr, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn and unavoidable impurities.

14. An ingot according to claim 13, wherein the ingot comprises from 1.0 to 7.0 weight ppm of Bi.