Aluminized steel sheet and method for producing the same

Abstract

The invention relates to aluminum-silicon coated steel sheet, comprising successively: - a steel substrate (1) wherein the Sn content in the steel substrate is from 0.0005 to 0.1 wt. %, - an intermetallic layer (2) having a thickness from 3.5 to 11.0 µm, wherein the Sn content in the intermetallic layer is from 0.0005 to 0.0200, - a coating layer (3) having a thickness from 10 to 35 µm, said coating layer comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, unavoidable impurities, the balance being aluminum, wherein the Sn content in the coating layer is from 0.0020 to 0.050 wt. %, wherein the lightness difference is above 2.1 % as measured according to the description. The invention also relates to a manufacturing for said steel sheet.

Description

[0001] Aluminized steel sheet and method for producing the same

[0002]

[0001] The present invention relates to an aluminized steel sheet. The present invention also relates to the method to manufacture said aluminized steel sheet.

[0003]

[0002] Aluminized steel sheets are used in various fields of application. They are usually preferred to galvanized steel sheets for their better resistance to high temperature as Aluminium has a higher melting point than Zinc. For example, aluminized steel sheets are used to manufacture heat exchangers and various kinds of furnaces. In the automotive industry, they are implemented in exhaust gas systems, fuel tanks or heat shields among other parts.

[0004]

[0003] Aluminium based coatings are produced in conventional hot dip coating industrial facilities. This facility may be combined with a continuous annealing furnace.

[0005]

[0004] 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]

[0005] 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]

[0006] The hot dip coating bath is prepared with ingots containing the elements to be deposited in the coating. The ingots are melted to form the bath. During the hot dip coating process, additional ingots are melted in the coating bath to feed and renew it.

[0008]

[0007] The grain size of the coating determines many in-use properties of the surface of an aluminized steel sheet, like the crack resistance, the friction or the abrasion resistance.

[0008] There is a need for an aluminized steel sheet provided with a grain size which easy to control.

[0009]

[0009] The aim of the present invention is to provide an aluminized steel sheet having a grain size which is easily measurable.

[0010]

[0010] For this purpose, a first object of the present invention consists in a steel sheet according to claim 1.

[0011]

[0011] The steel sheet may also comprise the features of claim 2 to 5, taken individually or as a combination.

[0012]

[0012] The object of the invention is also a method according to claim 6. The method may also comprise the features of claims 7 to 9, taken individually or as a combination.

[0013]

[0013] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to: Figure 1, which shows a cross-section of a steel sheet according to the invention with reference to the following labels:

[0014] 1: Steel substrate

[0015] 2: Intermetallic layer

[0016] 21: First intermetallic sublayer

[0017] 22: Second intermetallic sublayer

[0018] 3: Coating layer

[0019]

[0014] Composition of the steel substrate.

[0020]

[0015] A steel substrate (1) is provided in the invention. It has the following chemical composition comprising, by weight percent:

[0021] C: 0.005 - 0.40 %

[0022] Sn: 0.0005 - 0.100 %

[0023] Mn: 0.1 - 1.8 %

[0024] Si: 0.1 - 1.25 %

[0025] Al: 0.01 - 0.1 %

[0026] Ti: 0.01 - 0.3 %

[0027] P ≤ 0.11 %

[0028] S ≤ 0.025 %N ≤ 0.010 %

[0029] and comprising optionally one or more of the following elements, by weight percent:

[0030] Ni ≤ 0.5 %

[0031] Cr ≤ 1.0 %

[0032] Mo ≤ 0.40 %

[0033] Nb ≤ 0.09 %

[0034] Ca ≤ 0.1 %

[0035] B ≤ 0.004 %

[0036] V ≤ 0.004 %

[0037] the remainder of the composition being iron and unavoidable impurities.

[0038]

[0016] The steel grade is not particularly limited. For example, it can be deep drawing steel with high formability and having the following chemical composition comprising, by weight percent:

[0039] C: 0.005 - 0.12 %

[0040] Sn: 0.0005 - 0.100 %

[0041] Mn: 0.1 - 0.6 %

[0042] Si: 0.1 - 0.5 %

[0043] Al: 0.01 - 0.1 %

[0044] Ti: 0.01 - 0.3 %

[0045] P ≤ 0.11 %

[0046] S ≤ 0.025 %

[0047] N < 0.010 %

[0048] the remainder of the composition being iron and unavoidable impurities.

[0049]

[0017] The remainder of the composition of the steel substrate is iron and impurities resulting from the elaboration process. The level of impurities resulting from the elaboration process will depend on the production route used and of the level of scrap used in the steel melt. For example, when using a Basic oxygen furnace route with a low level of steel scrap (recycled steel), the level of impurities will remain very low. It is however also possible to adda high amount of scrap in the converter to the pig iron produced in the basic oxygen furnace, which will increase the level of impurities. Furthermore, when elaborating the steel using an electric furnace for example, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased, reaching up to the following levels:

[0050] Cu ≤ 0.2 %

[0051] Sb ≤ 0.03%

[0052] As ≤ 0.03%

[0053] Pb ≤ 0.03%

[0054]

[0018] Other impurities can be for example Ni, Li, Zr, Ag, P, Bi and Mg in a lesser content. The total amount of unavoidable impurities is up to 1.0 wt. %. Advantageously, the total amount of unavoidable impurities in up to 0.5 wt. %, or even less than 0.2 wt. %.

[0055]

[0019] The steel substrate comprises from 0.0005 to 0.100 wt. % of Sn. At least 0.0005 wt. % of Sn is induced using for example recycled scrap. If the steel comprises more than 0.100 wt. % of Sn, the Sn content in the bath will excessively increase by dissolution of the steel sheet in the molten bath.

[0056]

[0020] Preferably, the steel substrate comprises from 0.001 to 0.05 wt. % of Sn. Advantageously, the steel substrate comprises from 0.005 to 0.02 wt. % of Sn.

[0057]

[0021] An intermetallic layer is formed when the steel substrate is in contact with the bath. The intermetallic layer has a thickness from 2.5 to 11.0 pm and comprises from 20 to 60 wt. % Fe, from to 3.0 to 10.0 wt. % Si, from 0.0005 to 0.0200 wt. % of Sn, the remainder being Al unavoidable impurities. Sn in the intermetallic comes from the steel sheet by diffusion and from the coating layer.

[0058]

[0022] Preferably, the intermetallic layer comprises from 0.0005 to 0.0100 wt. % of Sn.

[0059]

[0023] Preferably, the intermetallic layer comprises two sub layers:

[0060] A first sublayer in contact with the steel substrate, having a thickness from 0.5 to 2.5 pm and comprising by weight,comprising from 30 to 60 wt. % Fe, up to 5.0 wt. % Si, from 0.0005 to 0.0500 wt. % Sn, the remainder being Al and unavoidable impurities,

[0061] A second sublayer on top of the first sublayer having a thickness from 2.0 to 7.5 μm and comprising by weight, from 20 to 50 wt. % Fe, from 6.0 to 15.0 wt. % Si, up to 0.0005 wt. % Sn, the remainder being Al and unavoidable impurities.

[0062]

[0024] Coating layer.

[0063]

[0025] The coating layer (3) comprises from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0020 to 0.050 wt. % of Sn, the balance being aluminum and unavoidable impurities.

[0064]

[0026] Silicon in the coating improves the resistance to heat. If the coating comprises less than 6.0 wt. % silicon, the coating is sensitive to high temperatures. If the coating comprises more than 12 wt.% silicon, surface defects like black spots are generated which may impair the coating appearance.

[0065]

[0027] Preferably, the coating layer comprises from 8.5 to 11 wt. % silicon.

[0066]

[0028] Up to 3.0 wt. % Iron comes in the coating from dissolution of the steel sheet.

[0067]

[0029] The coating layer comprises from 0.0020 to 0.050 wt. % of Sn. If the coating comprises less than 0.0020 wt. % of Sn, it won’t be possible to distinguish the grain boundaries. If the coating comprises more than 0.050 wt. %, Sn could concentrate on the surface and form a layer on the top of all the grains. This would make to grain boundaries invisible.

[0068]

[0030] Preferably, the coating layer comprises from 0.0040 to 0.020 wt. % of Sn.

[0069]

[0031] Advantageously, the coating layer comprises from 0.0050 to 0.010 wt.

[0070] % of Sn.

[0071]

[0032] The remainder of the composition of the coating layer is aluminium and impurities resulting from the elaboration process. Unavoidable impurities present in the metal bath may come from the steel sheet by dissolution orfrom the ingots melted to form the bath. Impurity elements are for example Sb, As, Pb, Mg, Ca, La, Ce, Y, Ti, Ni, Mo, Cr, Co, V, Nb, Mn, Sr, B, Li, Zr, W, Ag, Bi and Zn. The total amount of all impurities in the bath doesn’t exceed 0.4 wt. %, preferably 0.2 wt. %.

[0072]

[0033] The coating layer has the same composition as the metal bath.

[0073]

[0034] The coating has a thickness from 5 to 35 pm per side.

[0074]

[0035] Preferably, the coating thickness is from 5 to 20 pm per side.

[0075]

[0036] Measurement of the surface lightness difference.

[0076]

[0037] The lightness of the surface is defined in the in the CIELAB color space (L*a*b). In this space, the lightness value, L* defines black at 0 and white at 100. f the lightness is different from one point to the next one, this generates a visual defect for the end-user. This defect can be measured thanks to a sphere spectrophotometer, by computing the lightness difference:

[0077] Lightness difference(%) = 100 * (10% High − 10% Low) / 10% High

[0038] Where:

[0078] 10% Low = the mean of the 10% L* lowest values, and

[0079] 10% High = the mean of the 10% L* highest values

[0080]

[0039] The inventors have observed that when the surface has a lightness difference up to 2.1 %, it is impossible to distinguish the grain boundaries from the grains on the surface.

[0081]

[0040] Without to be bound by theory, it is believed that Sn remains in higher concentration in the last liquid phase during solidification. For this reason, Sn seems to concentrate in the solidification shrinkages at the grain boundaries.

[0082]

[0041] Manufacturing process

[0083]

[0042] 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.

[0084]

[0043] The steel grade is not limited.

[0085]

[0044] The molten metal bath in step B) is at a temperature from 630 to 770 °C, preferably from 640 to 670 °C.

[0045] The thickness of the liquid metal film remaining on the steel sheet can be set by any mean. For example, the liquid metal is wiped on the steel sheet by gas knives. Electromagnetic wiping means may be also used. The thickness of the coating layer is from 10 to 35 pm, preferably from 20 to 30 pm.

[0086]

[0046] In the case where the liquid metal is wiped by gas knives, 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.

[0087]

[0047] Cooling of the liquid metal to a solid coating layer can occur by any mean. For example, it is cooled by forced air convection. Preferably, the cooling speed is from 5 to 25°C / s, advantageously from 15 to 20°C / s.

[0088]

[0048] As molten metal leaves the bath and remains on the steel sheet, the bath must be refilled. Refilling occurs with ingots. For example, ingots can be progressively molten in the bath. Sn in the bath comes thus from both the refilling ingots and the steel sheet by dissolution.

[0089]

[0049] Preferably, refilling ingots comprise from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0005 to 0.050 wt. % of Sn, the balance being aluminium and unavoidable impurities.

[0090]

[0050] Advantageously, the refilling ingots comprise from 0.0020 to 0.0200 wt. % Sn.

[0091]

[0051] Optionally, a temper rolling step, also called skin pass, is performed on the coated steel sheet. This operation allows to erase some punctual surface defects and to adjust the final mechanical properties of the steel sheet. The temper rolling elongation is from 0.1 to 2.0 %, preferably from 0.1 to 1.0 %, advantageously from 0.2 to 0.5 %.EXAMPLES

[0092]

[0052] Manufacturing of samples.

[0093]

[0053] Table 1: steel substrate composition in wt. %, the remainder being iron and unavoidable impurities.

[0094] Steel

[0095] C Sn Mn Si Al Ti Cr Mo Ni Cu B substrate

[0096] A 0.22 0.022 1.19 0.224 0.038 0.035 0.177 0.006 0.029 0.036 0.0024 B 0.225 0.016 1.198 0.248 0.047 0.035 0.196 0.018 0.051 0.151 0.0024 C 0.23 0.005 1.14 0.258 0.031 0.035 0.186 0.002 0.018 0.029 0.0024 D 0.225 0.016 1.198 0.248 0.047 0.035 0.196 0.018 0.051 0.151 0.0024 E 0.218 0.008 1.147 0.24 0.041 0.035 0.174 0.014 0.047 0.177 0.0024 F 0.216 0.022 1.19 0.222 0.038 0.035 0.176 0.006 0.029 0.036 0.0024 G 0.216 0.022 1.19 0.222 0.038 0.035 0.176 0.006 0.029 0.036 0.0024 H 0.232 0.004 1.18 0.254 0.028 0.035 0.182 0.002 0.019 0.022 0.0024

[0097]

[0098] 1 0.219 0.023 1.19 0.223 0.038 0.035 0.177 0.006 0.028 0.036 0.0024

[0099]

[0054] Steel grades A to I with a composition according to table 1 were coated in a molten bath comprising from 8.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0020 to 0.0500 wt. % of Sn, the balance being aluminium and unavoidable impurities. The bath temperature was from 645 to 670 °C. The molten metal was wiped with air knives to the desired coating thickness.

[0100]

[0055] After coating and wiping, some trials were temper rolled in a Skin Pass operation, some weren’t. For the purpose of further investigation, samples were cut in each trial. Samples have a circular shape of 63 mm diameter.

[0101]

[0056] The coating aspect was further investigated to determine the lightness difference. This operation was performed on a portable spectrophotometer CM-2300d from Konica Minolta integrating sphere. Spectrophotometers are used for color control in the CIELAB color space (L*a*b). In this space, the lightness value, L* defines black at 0 and white at 100. Before each series of measurement, it was calibrated. 50 measurement points are performed on each sample. The measurements are done in a way to cover all the sample, from its contour to the center. The lightness difference is calculated as follows from the:

[0102]

[0057] Lightness difference(%) = 100 * (10% High −10% Low) / 10% High

[0058] Where:

[0103] 10% Low = the mean of the 10% L* lowest values, and

[0104] 10% High = the mean of the 10% L* highest values

[0105] of 50 values measured on a sample having a diameter of 63 mm.

[0106]

[0059] The mean grain area on the surface of the coating was then measured.

[0107] This operation was performed with an optical microscope including an image processing software. The microscope was a Keyence VHX-7000 with a magnification of 20x. Over samples of circular shape of 63 mm diameter, the boundaries of closed grains can be detected by coloring each pixel from white to black and detecting sharp transitions of color.

[0108]

[0060] The mean grain area value is obtained according to the following steps:

[0109] - Plot the contour of each grain on the sample based on the color transition displayed as described above,

[0110] - Count the grains on the sample thanks to the image processing software,

[0111] - Divide the surface area of the sample (in mm2) by the overall grain number to obtain the mean grain area in mm2.

[0112]

[0061] To measure the intermetallic thickness and the Sn content in the intermetallic, samples were cut in cross-section. The cross-section was embedded in resin and then polished for observations. Intermetallic thickness was measure with an optical microscope with a magnification of 500x.

[0113]

[0062] The Sn content in the intermetallic was measured by mean of EPMA (Electron Probe MicroAnalyzer) on WDS (Wavelength Dispersive Energy) spectrometers of model JXA 8530F Plus from equipment manufacturer JEOL.

[0114]

[0063] The Sn content in the intermetallic and in the coating are respectively noted Sn, INTER and the Sn, COAT.

[0115]

[0064] Table 2: Mean grain areaBATH INTERMETALLI SKIN

[0116] STEEL COMPO COATING LAYER COATING ASPECT COMPO C LAYER PASS

[0117] Sn

[0118] Sn Sn Lightne Mean Tri Steel conte Thickn

[0119] Sn (wt. INTER COAT Thickness / Elongat ss grain al substr nt ess

[0120] (wt. (wt. side (|im) ion (%) differe area Nr ate (wt. PPm) (pm)

[0121] %) %) nee (%) (mm2) %)

[0122] not 0.022 0.002 <0.00

[0123] 1 A <10 7.5 18.0 0.0 1.0 measura 0 0 10

[0124] ble not 0.016 0.001 <0.00

[0125] 2 B <10 7.0 22.6 0.5 1.9 measura 1 4 10

[0126] ble not 0.005 0.002 <0.00

[0127] 3 C <10 7.5 25.0 0.0 1.9 measura 0 0 10

[0128] ble not 0.016 0.001 0.001

[0129] 4 D 12 7.0 30.9 0.5

[0130] 4 2.0 measura 1 2

[0131] ble 0.007 0.002 0.002

[0132] 5* E 20 10.0 32.6 0.5 2.7 25.8

[0133] 5 0 0

[0134] 0.022 0.002 0.003

[0135] 6* F 37 7.5 18.0 0.0

[0136] 0 2.2 13.3 0 7

[0137] 0.022 0.002 0.003

[0138] 7* G 37 7.5 25.0 0.0

[0139] 0 2.7 7.8 0 7

[0140] 0.004 0.002 0.008

[0141] 8* H 81 10.5 18.0 0.0 3.2 9.1 0 4 1

[0142] g* 0.023 0.002 0.008

[0143] 1 81 10.5 25.0 0.0 4.9 5.0

[0144]

[0145] 0 4 1

[0146] *T rials according to the invention

[0147] Underlined values are not according to the invention.

[0148]

[0065] Comparative trials 1 to 4 according to the invention have a lightness difference below 2.1 % and it is not possible to distinguish the grain on their surface.

[0149]

[0066] Trials 5 to 9 according to this invention show visible grain boundaries.

[0150] Their mean grain area could be observed easily and measured.

Claims

CLAIMS1. An aluminum-silicon coated steel sheet, comprising successively:- a steel substrate (1 ) having a steel sheet having the following chemical composition comprising, by weight percent:C: 0.005 - 0.40 %Sn: 0.0005- 0.100 %Mn: 0.1 - 1.8 %Si: 0.1 - 1.25 %Al: 0.01 - 0.3 %Ti: 0.01 - 0.1 %P ≤ 0.11 %S ≤ 0.025 %N < 0.010 %and comprising optionally one or more of the following elements, by weight percent:Ni < 0.5 %Cr < 1.0 %Mo < 0.40 %Nb < 0.09 %Ca < 0.1 %B < 0.004 %the remainder of the composition being iron and unavoidable impurities,- an intermetallic layer (2) having a thickness from 2.5 to 11.0 μm comprising from 20 to 60 wt. % of Fe, from to 3.0 to 10.0 wt. % of Si, from 0.0005 to 0.0200 wt. % of Sn and, the remainder being Al unavoidable impurities,- a coating layer (3) having a thickness t from 5 to 35 μm, said coating layer comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0020 to 0.0500 wt. % Sn, the balance being aluminum and unavoidable impurities,wherein the lightness difference on the surface of the aluminum-silicon coated steel sheet is above 2.1 % as measured according to the description.

2. An aluminium-silicon coated steel sheet steel sheet according to claim 1, wherein the coating layer (3) has a thickness from 5 to 20 μm.

3. An aluminium-silicon coated steel sheet according to claims 1 or 2, wherein the steel substrate comprises from 0.001 to 0.05 wt. % of Sn.

4. An aluminium-silicon coated steel sheet according to anyone of claims 1 to 3, wherein the coating layer comprises from 0.0050 to 0.0100 wt. % of Sn.

5. An aluminium-silicon coated steel sheet according to anyone of claims 1 to 4, wherein the intermetallic (2) layer comprises two sub layers:A first sublayer (21 ) in contact with the steel substrate, having a thickness from 0.5 to 3.5 μm and comprising by weight, comprising from 30 to 60 wt. % Fe, up to 5.0 wt. % Si, from 0.0005 to 0.0500 wt. % Sn, the remainder being Al and unavoidable impurities,A second sublayer (22) on top of the first sublayer having a thickness from 2.0 to 7.5 μm and comprising by weight, from 20 to 50 wt. % Fe, from 6.0 to 15.0 wt. % Si, up to 0.0005 wt. % Sn, the remainder being Al and unavoidable impurities.

6. A process to manufacture an aluminum coated steel sheet comprising the following steps:A) Providing a steel substrate (1) comprising, by weight percent:C: 0.005 - 0.40 %Sn: 0.0005 - 0.100 %Mn: 0.1 - 1.8 %Si: 0.1 - 1.25 %Al: 0.01 - 0.3 %Ti: 0.01 - 0.1 %P ≤ 0.11 %S ≤ 0.025 %N < 0.010 %and comprising optionally one or more of the following elements, by weight percent:Ni < 0.5 %Cr < 1.0 %Mo < 0.40 %Nb < 0.09 %Ca < 0.1 %B < 0.004 %the remainder of the composition being iron and unavoidable impurities,B) Dipping said steel substrate in a molten bath of liquid metal comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0020 to 0.0500 wt. % of Sn, the remainder being aluminium and unavoidable impurities,C) Wiping said liquid metal to set a thickness t of a coating layer, said thickness t is from 5 to 35 μm / side,D) Cooling the wiped metal to obtain a solid coating layer.

7. A process to manufacture an aluminum coated steel sheet according to claim 6, wherein in step B), the molten bath of liquid metal has a temperature from 640 to 670 °C.

8. A process to manufacture an aluminum coated steel sheet according to claims 6 or 7, wherein in step D) the average cooling speed until complete solidification of the coating is from 15 to 20°C / s.

9. A process to manufacture an aluminum coated steel sheet according to anyone of claims 6 to 8, wherein refilling the molten metal bath is performedby dissolution of ingots comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, from 0.0005 to 0.0500 wt. ppm of Sn, the balance being aluminium and unavoidable impurities.

10. A process to manufacture an aluminium coated steel sheet according to any one of claims 6 to 9, wherein after step D), a skin pass step is performed with an elongation from 0.1 to 1.0 %.

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