A process to limit the sn content in an aluminium based liquid bath

Abstract

The invention relates to A process to limit the Sn content in an aluminium bath for the hot dip coating of a steel substrate comprising the following steps: A) Selecting a steel substrate, said steel substrate comprising Sn: 0.0005 to 0.0200 wt. % B) Dipping said steel substrate in an aluminium bath comprising from 6.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminium and unavoidable impurities, C) Wiping said liquid metal to set a thickness t of a coating layer from 25 to 45 µm / side, wherein t is limited in function of the Sn content in the steel substrate. D) Cooling the wiped metal to obtain a steel substrate provided with a solid coating layer.

Description

[0001] A process to limit the Sn content in an aluminium based liquid bath

[0002]

[0001] The present invention relates to a process to limit the Sn content in an aluminium bath for the hot dip coating of a steel substrate. The present invention also relates to the aluminium coated steel substrate with Sn limited in the coating.

[0003]

[0002] Aluminium coated steel sheets are used in various fields of application.

[0004] 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, aluminium coated 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.

[0005]

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

[0006]

[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.

[0007]

[0005] During the hot dip coating process, the steel sheet is dipped into a molten metal bath. The excessive liquid metal is wiped. The wiping operation allows determining the thickness of the coating. After wiping, the coated steel sheet is cooled.

[0008]

[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.

[0009]

[0007] The hot dip bath comprises various elements which are unavoidable impurities from the manufacturing process. Tin (Sn) is known as a residualelement that has a negative influence in the aluminium based coating. It can be detrimental for its quality, especially its appearance. A wrong appearance is not desired by the end-user. Sn in the aluminium coating has also an influence on the in-use properties like the friction coefficient or the abrasion resistance.

[0010]

[0008] The aim of the present invention is to provide a process to limit the Sn content in an aluminium bath for the hot dip coating of a steel substrate.

[0011]

[0009] For this purpose, a first object of the present invention consists in a process according to claim 1.

[0012]

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

[0013]

[0011] The object of the invention is also an aluminium coated steel substrate according to claims 9 to 11.

[0014]

[0012] 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:

[0015] - Figure 1, which shows the simulated relation between the Sn content in the aluminum bath and the coating layer thickness.

[0016] - Figure 2, which shows a cross-section of a steel substrate according to the invention with reference to the following labels:

[0017] 1: Steel substrate

[0018] 2: Intermetallic layer

[0019] 3: Coating layer

[0020]

[0013] 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 - 0.0200 %

[0023] Mn: 0.1 - 1.8 %

[0024] Si: 0.1 - 1.25 %

[0025] Al: 0.01 - 0.1 %Ti: 0.01 - 0.3 %

[0026] P ≤ 0.11 %

[0027] S ≤ 0.025 %

[0028] 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]

[0014] 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 - 0.005 %

[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]

[0015] 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 usinga 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 add a 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]

[0016] 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]

[0017] The steel substrate comprises from 0.0005 to 0.0200 wt % of Sn. At least 0.0005 wt. % of Sn is induced using for example recycled scrap. If the steel comprises more than 0.0200 wt. % of Sn, the Sn content in the bath will excessively increase by dissolution of the steel substrate in the molten bath.

[0056]

[0018] Preferably, the steel substrate comprises from 0.0005 to 0.0145 wt. % of Sn.

[0057]

[0019] The steel substrate consists of several coils. Said coils are joined with each other. For example, they are joined by welding. The joined coils are continuously dipped in the aluminium bath, continuously wiped and cooled.

[0058]

[0020] In a hot dip coating process, the composition and dimension of the steel substrate is the same all along the steel coil. The settings of the coating process parameters, notably the coating thickness are usually the same all along each coil.

[0059]

[0021] The steel substrate is dipped in a molten bath of liquid metal. The bath is an aluminum bath, meaning the bath comprises more than 50 wt. aluminum.

[0022] The aluminum bath comprises from 6.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminium and unavoidable impurities,

[0060]

[0023] Silicon in the aluminum bath improves the resistance to heat of the aluminium coating. If the aluminium bath comprises less than 6.0 wt. % silicon, the coating is sensitive to high temperatures. If the aluminium bath comprises more than 12.0 wt.% silicon surface defects like black spots are generated which may impair the coating appearance.

[0061]

[0024] Preferably, the aluminium bath comprises from 8.5 to 11.0 wt. % silicon.

[0062]

[0025] Up to 4.0 wt. % Iron in the aluminium bath comes from dissolution of the steel substrate.

[0063]

[0026] The aluminium bath comprises Sn from the steel substrate by dissolution or from the ingots melted to form the bath. The aluminium bath comprises up to 0.0035 wt. % of Sn. If the bath comprises more the 0.0035 wt. % of Sn, the quality of the aluminium coating will decrease.

[0064]

[0027] Preferably, the aluminium bath comprises up to 0.0025 wt. % of Sn.

[0065]

[0028] Preferably, the aluminium bath comprises at least 0.0005 wt. % of Sn.

[0066]

[0029] Preferably, the aluminium bath comprises at least 0.0010 wt. % of Sn.

[0067]

[0030] The remainder in the composition of the aluminium bath is aluminium and unavoidable impurities resulting from the elaboration process. Unavoidable impurities present in the metal bath may come from the steel substrate by dissolution or from 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. %.

[0068]

[0031] Preferably, the aluminium bath in step is at a temperature from 630 to 770 °C, advantageously from 640 to 670 °C.

[0069]

[0032] The thickness of the liquid metal film remaining on the steel substrate can be set by any mean. For example, the liquid metal is wiped on the steel substrate by gas knives. Electromagnetic wiping means may be also used.

[0033] In the case where the liquid metal is wiped by gas knives, the thickness is controlled by the wiping pressure. In this case, 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.

[0070]

[0034] The thickness by side is expressed as the mean values of the thickness on each side.

[0071]

[0035] The inventors have found that limiting the thickness of the coating layer in function of the Sn content in the steel substrate allows to limit the Sn content in an aluminium bath.

[0072]

[0036] They have found that the limitation needed for the thickness t of the coating layer set by wiping decreases when the Sn content in the steel substrate increases.

[0073]

[0037] Preferably, the thickness of the coating layer satisfies the following equation:

[0074] — b + √b2— 4ac

[0075]

[0076] - Ta -(1)

[0077] where:

[0078] t = thickness of the coating layer per side in pm

[0079] a = SnSTEEL* 8.2 + 0.017

[0080] b = — SnSTEEL* 420 — 0.87

[0081] c = SnSTEEL* 6300 — 12.4

[0082] and where SHSTEEL is the Sn content in the steel substrate expressed in weight %.

[0083]

[0038] It has been observed that satisfying equation (1 ) allows reducing the Sn content in the aluminium bath.

[0084]

[0039] In an even more preferred embodiment, the thickness per side in pm of the coating satisfies the following equation:— b + √b2— 4ad

[0085] (2)

[0086]

[0087] 2a

[0088] where:

[0089] t = thickness of the coating layer per side in pm

[0090] a = SnSTEEL* 8.2 + 0.017

[0091] b = — SnSTEEL* 420 — 0.87

[0092] d = SnSTEEL* 6300 — 2.4

[0093]

[0040] It has been observed that satisfying equation (2), in addition to satisfying equation (1) allows reducing further the Sn content in the aluminium bath.

[0094]

[0041] An intermetallic layer 2 is formed on the surface of the steel substrate 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 0.0100 wt. % of Sn, the remainder being Al unavoidable impurities. Sn in the intermetallic comes from the steel substrate by diffusion and from the coating layer.

[0095]

[0042] Preferably, the intermetallic layer comprises from 0.0005 to 0.0050 wt. % of Sn.

[0096]

[0043] A coating layer 3 is formed on top of the intermetallic by cooling the liquid metal. The composition of the coating layer 3 is the same as the composition of the aluminum bath.

[0097]

[0044] The thickness of the intermetallic layer, as well as the thickness of the coating layer can be measured by any mean. For example, they can be measure by mean of an optical microscope on a cross-section of the coated steel substrate. The interface between the intermetallic layer and the coating layer is easy to distinguish by color difference.

[0098]

[0045] Another measurement method can be used, for example the glow discharge optical emission spectroscopy (GDOES). In this case, the interface between the intermetallic layer and the coating layer can be detected by the difference in iron content.

[0099]

[0046] The aluminum coated steel substrate, comprising successively from the steel to the surface of said coated steel substrate:- a steel substrate (1)

[0100] - an intermetallic layer (2)

[0101] - a coating layer (3)

[0102]

[0047] Examples

[0103]

[0048] For all samples, steel substrates used are 22MnB5. The composition by weight of the steel substrates is as follows: C = 0.23 %; Mn = 1.2%; Si = 0.25%; %; Cr = 0.2 %; Al = 0.04%; Ti = 0.04%; B = 0.003 %, Sn from 0.0005 to 0.0200 %, the remainder being iron and unavoidable impurities.

[0104]

[0049] The trials were dipped in a coating bath comprising from 8.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminium and unavoidable impurities.

[0105]

[0050] Subsequently, the trials were wiped by setting the proper wiping pressure to achieve the desired coating thickness. The Sn content in the aluminum bath was regularly controlled by sampling small a quantity of liquid, which was analyzed X-ray fluorescence (XRF) after solidification.

[0106]

[0051] The results are gathered in table 1 with the following abbreviations:

[0107] SnsTEEi: Sn content in the steel substrate, and

[0108] SnBATH: Sn content in the hot dip coating bath.

[0109]

[0052] After having been produced, the samples were observed by naked eye and their appearance quality was rated as follows:

[0110] 1: excellent appearance

[0111] 2: good appearance

[0112] 3: intermediated appearance, acceptable for low quality applications 4: wrong appearance

[0113]

[0053] Results with the rated appearance of some of the samples analyzed are gathered in table 2.

[0054] Table 1

[0114] coating

[0115] TRIAL Sn, STEEL -b + √b2- 4ac Equation (1) —b + √b2— 4ad Equation (2) Sn, BATH thickness / side

[0116] Nr in wt. % satisfied satisfied in wt. % t in pm 2n 2a

[0117] 1* 0.0064 29 42 YES 36 YES 0.0013 2 0.0065 42 41 NO 36 NO 0.0036 3* 0.0065 40 41 YES 36 NO 0.0032 4* 0.0067 30 41 YES 36 YES 0.0012 5* 0.0075 33 40 YES 35 YES 0.0020 6* 0.0075 36 40 YES 35 NO 0.0027 7* 0.0075 33 40 YES 35 YES 0.0020 8* 0.0085 30 39 YES 34 YES 0.0012 9* 0.0085 36 39 YES 34 NO 0.0029 10 0.0085 40 39 NO 34 NO 0.0037 11 0.0100 40 38 NO 32 NO 0.0040 12* 0.0100 30 38 YES 32 YES 0.0023 13* 0.0102 28 37 YES 32 YES 0.0019 14* 0.0109 29 37 YES 31 YES 0.0024 15* 0.0114 28 36 YES 31 YES 0.0023 16* 0.0121 30 36 YES 30 YES 0.0025 17 0.0120 38 36 NO 30 NO 0.0040 18* 0.0145 25 34 YES 27 YES 0.0025 19* 0.0145 32 34 YES 27 NO 0.0031 20 0.0145 37 34 NO 27 NO 0.0044 21* 0.0176 29 32 YES - - 0.0030 22 0.0176 34 32 NO - - 0.0039 23* 0.0198 25 31 YES - - 0.0030

[0118]

[0119] 24 0.0198 32 31 NO - - 0.0037*trials according to the invention

[0120] Underlined values are not according to the invention.

[0055] Trials 2, 10, 11, 17, 20, 22, 24 not according to the invention, have an excessive coating thickness in function of the Sn content in steel grade. Consequently, the Sn content in the coating layer of these trials is above 0.0035 wt. %.

[0121]

[0056] Trials 3, 6, 9, 19, 21 and 23 satisfy to equation (1) and do not satisfy to equation (2). They have a Sn content in the coating layer of more than 0.0025 and up to 0.0035 wt. %.

[0122]

[0057] 1, 4, 5, 7, 8, 12 to 16 and 18 satisfy to equation (1) and to equation (2).

[0123] They have a Sn content in the coating layer up to 0.0025 wt. %.

[0124]

[0058] Table 2

[0125] TRIAL Sn, STEEL coating

[0126] -b + y'b2- Appearance

[0127] Nr in wt. % thickness / side 4ac

[0128] t in pm 2a quality rating

[0129] 1* 0.0064 29 42 1

[0130] 6* 0.0075 36 40 3

[0131] 8* 0.0085 30 39 1

[0132] 9* 0.0085 36 39 3

[0133] 10 0.0085 40 39 4

[0134] 13* 0.0102 28 37 1

[0135] 14* 0.0109 29 37 2

[0136] 16* 0.0121 30 36 2

[0137] 17 0.0120 38 36 4

[0138] 22 0.0176 34 32 4

[0139]

[0140] 23* 0.0198 25 31 3

[0141] *trials according to the invention

[0142] Underlined values are not according to the invention.

[0143]

[0059] The trials according to the invention have an appearance rated from 1 to 3, i. e. from excellent to acceptable for low quality applications.

[0144]

[0060] Trials not according to the invention have a wrong appearance, which is not desired by the end-user.

Claims

CLAIMS1. A process to limit the Sn content in an aluminium bath for the hot dip coating of a steel substrate comprising the following steps:A) Selecting a steel substrate, said steel substrate comprising, by weight percent:C: 0.005 - 0.40 %Sn: 0.0005 to 0.0200 %Mn: 0.1 - 1.8 %Si: 0.1 - 1.25 %Al: 0.01 - 0.1 %Ti: 0.01 - 0.3 %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 %V < 0.004 %the remainder of the composition being iron and unavoidable impurities,B) Dipping said steel substrate in an aluminium bath comprising from 6.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminium and unavoidable impurities,C) Wiping said liquid metal to set a thickness t of a coating layer from 25 to 45 pm / side, wherein t is limited in function of the Sn content in the steel substrate, and wherein the limited thickness tof the coating layer set in step C) decreases when the Sn content in the steel substrate increases.D) Cooling the wiped metal to obtain a steel substrate provided with a solid coating layer.

2. The process to limit the Sn content in an aluminium bath according to claim 1, wherein the thickness of the coating layer satisfies the following equation:— b + V b2— 4act2a (1)wheret = thickness of the coating layer per side in pma = SnSTEEL* 8.2 + 0.017b = — SnSTEEL* 420 — 0.87c = SnSTEEL* 6300 — 12.4and where SnSTEELis the Sn content in the steel substrate expressed in weight %.

3. The process to limit the Sn content in an aluminium bath according to anyone of claims 1 or 2, wherein refilling the aluminium bath is performed by dissolution of ingots comprising from 6.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminium and unavoidable impurities.

4. The process to limit the Sn content in an aluminium bath according to anyone of claims 1, wherein the steel substrate comprises from 0.0005 to 0.0145 wt. % of Sn and wherein the aluminium bath comprises up to 0.0025 wt. % of Sn.

5. The process to limit the Sn content in an aluminium bath according to claim 4, wherein the thickness per side in pm of the coating satisfies the following equation:— b + V b2— 4adt < (2)2awheret = thickness of the coating layer per side in pma = SnSTEEL* 8.2 + 0.017b = — SnSTEEL* 420 — 0.87d = SnSTEEL* 6300 — 2.

46. The process to limit the Sn content in an aluminium bath according to claims 4 or 5, wherein refilling the aluminium bath is performed by dissolution of ingots comprising from 6.0 to 12.0 wt. % of silicon, up to 4.0 wt. % of iron, up to 0.0025 wt. % of Sn, the balance being aluminium and unavoidable impurities.

7. The process to limit the Sn content in an aluminium bath according to anyone of claims 1 to 6, wherein the aluminium bath of step B) comprises at least 0.0005 wt. % of Sn.

8. An aluminum coated steel substrate, comprising successively:- a steel substrate (1 ) having the following chemical composition comprising, by weight percent:C: 0.005 - 0.40 %Sn: 0.0005 to 0.0200 %Mn: 0.1 - 1.8 %Si: 0.1 - 1.25 %Al: 0.01 - 0.1 %Ti: 0.01 - 0.3 %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 %V < 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 pm comprising from 20 to 60 wt. % Fe, from to 3.0 to 10.0 wt. % Si, from 0.0005 to 0.0100 wt. % of Sn, the remainder being Al and unavoidable impurities,- a coating layer (3) having a thickness from 25 to 45 pm, said coating layer comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, up to 0.0035 wt. % of Sn, the balance being aluminum and unavoidable impurities,wherein the thickness per side in pm of the coating satisfies the following equation:— b + V b2— 4ac t2a (1)wheret = thickness of the coating layer per side in pma = SnSTEEL* 8.2 + 0.017b = — SnSTEEL* 420 — 0.87c = SnSTEEL* 6300 — 12.4and where SnSTEELis the Sn content in the steel substrate expressed in weight %.

9. An aluminum coated steel substrate according to claim 8, comprising successively:- a steel substrate (1 ) having the following chemical composition comprising, by weight percent:C: 0.005 - 0.40 %Sn: 0.0005 to 0.0145 %Mn: 0.1 - 1.8 %Si: 0.1 - 1.25 %Al: 0.01 - 0.1 %Ti: 0.01 - 0.3 %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 %V < 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 pm comprising from 20 to 60 wt. % Fe, from to 3.0 to 10.0 wt. % Si, from 0.0005 to 0.0050 wt. % of Sn, the remainder being Al and unavoidable impurities,- a coating layer (3) having a thickness from 25 to 45 pm, said coating layer comprising from 6.0 to 12.0 wt. % of silicon, up to 3.0 wt. % of iron, up to 0.0025 wt. % of Sn, the balance being aluminum and unavoidable impurities,wherein the thickness per side in pm of the coating satisfies the following equation:— b + V b2— 4adt < - - (2)2awheret = thickness of the coating layer per side in pma = SnSTEEL* 8.2 + 0.017b = — SnSTEEL* 420 — 0.87d = SnSTEEL* 6300 — 2.4and where SnSTEELis the Sn content in the steel substrate expressed in weight %.

10. An aluminum coated steel substrate according to claims 8 or 9, wherein the coating layer (3) comprises at least 0.0005 wt. % of Sn.

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