Method for producing a steel product with a Zn coating and a tribologically active layer deposited on the coating, and a steel product produced according to said method
11078573 · 2021-08-03
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Abstract
A process for producing a steel product having a protective coating based on zinc and a tribologically active layer applied to the protective coating where the process includes providing a flat steel product provided with the protective coating, and applying an aqueous solution comprising ammonium sulfate and demineralized water to the protective coating of the steel product. The concentration of the ammonium sulfate based on the SO.sub.4.sup.2− ions is 0.01-5.7 mol/L, the pH of the aqueous solution is 4-6, and the near-surface reaction time between the aqueous solution and the protective coating is more than 0 seconds and not more than 5 seconds. After a drying operation conducted without preceding rinsing, a tribologically active layer consisting of ammonium zinc sulfate is formed atop the protective zinc coating. Also, a steel product having a protective coating based on zinc and a tribologically active layer of ammonium zinc sulfate.
Claims
1. A process for producing a steel product having a protective coating based on zinc and a tribologically active layer applied to the protective coating, comprising the following steps: providing a steel product provided with the protective coating; applying an aqueous solution comprising ammonium sulfate and demineralized water to the protective coating of the steel product; wherein the concentration of the ammonium sulfate based on the SO.sub.4.sup.2− ions is 0.01-5.7 mol/L, wherein the pH of the aqueous solution is 4-6, and wherein the near-surface reaction time between the aqueous solution and the protective coating is more than 0 seconds and not more than 5 seconds, such that, after a drying operation conducted without preceding rinsing, there is a tribologically active layer consisting of ammonium zinc sulfate atop the protective coating.
2. The process as claimed in claim 1, wherein the concentration of the ammonium sulfate in the aqueous solution based on the SO.sub.4.sup.2− ions is 0.1-3 mol/L.
3. The process as claimed in claim 2, wherein the concentration of the ammonium sulfate in the aqueous solution based on the SO.sub.4.sup.2− ions is 0.4-0.7 mol/L.
4. The process as claimed in claim 1, wherein the pH of the aqueous solution is 4.2-5.7.
5. The process as claimed in claim 1, wherein the steel product is a flat steel product.
6. The process as claimed in claim 1, wherein the aqueous solution is applied to the protective coating by a chemcoater or coil coater.
7. The process as claimed in claim 1, wherein the steel product, after the aqueous solution has been applied, is dried at a temperature of 70-90° C.
8. The process as claimed in claim 1, wherein the surface of the steel product to be coated is subjected to alkaline cleaning prior to the coating operation.
9. A steel product comprising a steel substrate and a protective coating based on zinc borne by the steel substrate, wherein a tribologically active layer consisting of ammonium zinc sulfate has been formed atop the protective coating.
10. The steel product as claimed in claim 9, wherein the dry coat weight of the tribologically active layer is 1-100 mg/m.sup.2 based on the sulfur content.
11. The steel product as claimed in claim 10, wherein the dry coat weight of the tribologically active layer is 10-50 mg/m.sup.2 based on the sulfur content.
12. The steel product as claimed in claim 11, wherein the dry coat weight of the tribologically active layer is 10-20 mg/m.sup.2 based on the sulfur content.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention is elucidated in detail hereinafter by working examples. The figures show:
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DESCRIPTION OF THE INVENTION
(6) The efficacy of the invention was tested using series of experiments that have been conducted a) under laboratory conditions and b) on the industrial scale on a coating line.
(7) First of all, in a general form, the common features of the experiments and the test methods employed in the respective case for verification of the experimental results are elucidated hereinafter. There then follows a description of the experimental conditions specifically employed in the experiments in each case and of the experimental results obtained in specific terms.
(8) For the coating experiments, an of a typical of automobiles under the DX51D name known (materials number 1.0226) consisting cold-rolled steel strip was used, which has been coated by conventional hot-dip galvanization with a 7 μm zinc layer that consisted of 1% by weight of aluminum, the balance being Zn and technically unavoidable impurities.
(9) An application solution that consisted of ammonium sulfate dissolved in demineralized water (conductivity <0.05 μS/cm) was applied to the steel substrate coated in this way with Zn. The ammonium sulfate was completely in solution. No further substances were added. The application solution was consequently a completely aqueous, solids-free solution.
(10) After the aqueous solution has been applied to the Zn-coated flat steel product samples, the samples have been dried.
(11) Subsequently, the samples thus coated in accordance with the invention have been used to conduct various tests in order to verify the effect of the ammonium zinc sulfate layer produced in accordance with the invention.
(12) The friction characteristics of the flat steel product samples coated in the manner of the invention with a tribologically active ammonium zinc sulfate layer have been determined using a strip-drawing test system. The strip-drawing experiment conducted with this system gives a significant simplification of the friction conditions at the binder during a deep-drawing forming operation.
(13) The termination criterion for the strip-drawing test is the occurrence of the stick-slip effect. This effect refers to the sticking and slipping of solids moved against one another. The result is a rapid sequence of sticking, stretching, separating and slipping mechanisms between the surfaces in relative movement and occurs in the event of inadequate separation of the surfaces (for example by a lubricant). The experimental setup is shown in schematic form in
(14) In order to be able to assess the effect on the bonding characteristics of a bodyshell, T-peel tests were conducted according to DIN EN 1465-2009. For this purpose, an adhesive supplied under the Betamate 1480.V203 trade name by Dow Automotive was used.
(15) The fracture area and the fracture type were then assessed visually according to the method of EN ISO 10365:1995. The fracture took place either in the adhesive itself or in the material of the part that has been joined. In the case of fractures within the adhesive, a distinction was made between a cohesion fracture in which the separation takes place in the adhesive, and an adhesion fracture in which the fracture takes place at the interface between the part that has been joined and the adhesive. It was additionally possible for the material of the sample itself to fail while the adhesive remains intact. A differentiation was made here between fracture of a part that has been joined and fracture through delamination.
(16) Subsequently, in welding experiments, the suitability for welding of the flat steel products coated in accordance with the invention in the manner elucidated was examined. For this purpose, resistance point welds were conducted, for which, in addition to the assessment of the electrode lifetime, the welding current range of two flat steel products lying one on top of the other was assessed. In the case of the electrode lifetime, what was examined was how many weld points can be produced with an electrode pair without going below the weld point diameter of 3.6 mm. The diameter here was examined after 100 points had been produced. The specifications with regard to the electrode lifetime are dependent on the manufacturer. However, at least 400 weld points should be achievable with one electrode pair since redressing of the electrodes is then necessary.
(17) In addition, a sufficiently large welding range is of essential significance in the automobile industry with regard to the suitability of the material for welding. It is necessary that this range is at least 1 kA in size. The lower limit results from the minimum lens diameter, the upper limit from spattering.
a) Experiment Under Laboratory Conditions
(18) An aqueous coating solution was produced. For this purpose, 92.5 g of ammonium sulfate were dissolved in 1 liter of demineralized water. No particular measures were taken here to adjust the pH of the coating solution; instead, the innate pH of the solution was used, which was about 5. More particularly, no bases or acids were added to adjust the pH.
(19) The hot dip-galvanized flat steel product samples were subjected to alkaline cleaning before the application of the coating.
(20) The treatment solution was distributed homogeneously by means of a conventional coil-coater on the hot dip-galvanized sheet.
(21) This was followed by the drying of the wet film applied in a tunnel oven at a drying temperature of 50-90° C.
(22) By appropriate adjustment of the coil-coater, the amount of aqueous coating solution applied was adjusted such that the dry coat weight of the (NH.sub.4).sub.2Zn(SO.sub.4).sub.2 layer obtained on the samples corresponded to the provisions of the invention.
(23) The coat weight applied in mgS/m.sup.2 was measured by means of mobile x-ray fluorescence analysis (RFA).
(24) Before the strip-drawing test, the test samples were coated with the pre-lube (Anticorit PL 3802-39S). The oil application rate was 1.5 g/m.sup.2.
(25) In order to represent the correlation between the dry coat weight applied in mgS/m.sup.2 and the resulting coefficient of friction as a function of areal pressure in MPa, various dry coat weights were applied by means of the coater and tested by means of the strip-drawing test.
(26) Table 3 summarizes the results of these experiments. It is clearly apparent that the forming performance is significantly improved with rising coat weight.
(27) TABLE-US-00003 TABLE 3 Occurrence of the stick-slip effect according to the dry (NH.sub.4).sub.2Zn(SO.sub.4).sub.2 coat weight (laboratory application) Area pressure before the Coat weight stick-slip effect occurs [mgS/m.sup.2] [MPa] 0 2 10 35 20 >100 30 >100
b) Experiment Under Industrial Conditions (Production Line Application)
(28) First of all, a coating solution having a concentration of 51 g/L ammonium sulfate in demineralized water was prepared. Left unchanged, the innate pH of the aqueous solution obtained was about 5.
(29) The aqueous ammonium sulfate solution was applied by means of an application unit arranged inline downstream of a conventional hot dip galvanization plant in which the solution was sprayed onto the galvanized flat steel product and then squeezed off in a manner known per se to establish the layer thickness.
(30) The drying was effected in a tunnel oven at 80° C.
(31) The coat weight applied in mgS/m.sup.2 was measured by means of mobile x-ray fluorescence analysis (RFA).
(32) The flat steel product samples coated in the manner of the invention that were obtained were oiled at an oiling rate of about 1 g/m.sup.2 with a thixotropic, barium-free anticorrosion oil supplied under the RP4107S trade name by FUCHS Europe Schmierstoffe GmbH.
(33) The friction-reducing effect of the ammonium zinc sulfate layer produced in accordance with the invention was characterized by means of the strip-drawing test.
(34) Table 4 states the areal pressure attained in MPa before the stick-slip effect occurred and the test had to be stopped. Here too, a significant improvement in the tribological properties of the ammonium zinc sulfate-coated substrates was found.
(35) TABLE-US-00004 TABLE 4 Occurrence of the stick-slip effect depending on the (NH.sub.4).sub.2Zn(SO.sub.4).sub.2 coat weight (production line application) Areal pressure before the Coat weight stick-slip effect occurs [mgS/m.sup.2] [MPa] 0 7 20 65 25 >100 36 >100 70 >100
(36) As well as the forming-promoting properties of the tribological coating, the subsequent process compatibility of such a coating plays an important role for the employability of such coatings in the automotive sector.
(37) Therefore, the T-peel test already elucidated in general terms above was conducted on the flat steel product samples obtained in the manner described above in order to verify adhesive compatibility in the case of use of conventional structural adhesives.
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(39) The reduction in the tensile lap-shear strength and in the peel resistance compared to the uncoated reference is acceptable and surprisingly does not restrict the use of this coating in the field of automobile bodywork. Moreover, the fracture type close to the substrate is cohesive.
(40) Welding tests according to Stahl-Eisen-Prüfblatt [Steel and Iron Test Sheet] 1220-2 showed that the resistance point welding of sheets coated in accordance with the invention was not impaired by the ammonium zinc sulfate layer applied in accordance with the invention when the dry coat weight did not exceed 25 mg S/m.sup.2, especially 20 mg S/m.sup.2. In the case of coat weights of more than 25 mg S/m.sup.2, there was increased surface spattering in the case of the flat steel product samples coated in accordance with the invention that were examined, which would make it difficult to use them in the automotive sector. On compliance with dry coat weights in the range of 10-15 mg S/m.sup.2, both the welding range ΔI ascertained according to Stahl-Eisen-Prüfblatt 1220-2 and the electrode lifetime were sufficiently high. Table 5 summarizes the suitability of the flat steel product samples produced in the experiment and coated in accordance with the invention for various subsequent processes.
(41) A dry applied coat weight of, for example, 100 mgS/m.sup.2 would have a severe adverse effect on resistance point welding and subsequent bonding. However, phosphation would not be a problem and would still be unproblematic since there are multiple rinsing and cleaning cascades just before the phosphation and the ammonium zinc sulfate layer of the invention is removed as a result.
(42) TABLE-US-00005 TABLE 5 Suitability of the fine sheets coated in accordance with the invention for various subsequent processes typical of automobiles Coat weight Suitability for [mg S/m.sup.2] resistance point welding bonding phosphation 10 very good very good very good 20 very good very good very good 30 moderate moderate very good 40 poor poor good 80 very poor very poor good 100 very poor very poor good
(43) In order to represent the suitability for adhesion of the ammonium zinc sulfate layer on a hot dip-galvanized fine steel sheet (99% by weight of zinc, 1% by weight of aluminum) by comparison with a zinc sulfate layer on a hot dip-galvanized fine steel sheet (99% by weight of zinc, 1% by weight of aluminum), a T-peel test in accordance with DIN EN 1464-2010 was conducted with the Elastosol M 105 adhesive (Bostik GmbH) at a test temperature of 130° C. The fracture area and fracture type were then assessed visually by the method of EN ISO 10365:1995. The advantageous properties of the ammonium zinc sulfate coating are clearly manifested here by comparison with a zinc sulfate coating:
(44) TABLE-US-00006 TABLE 6 Assessment of the fracture area after T-peel test in accordance with DIN EN 1464-2010 at a test temperature of 130° C. Variant 1 is a hot dip-galvanized fine steel sheet (99% zinc, 1% aluminum) coated with ammonium zinc sulfate at a dry applied coat weight of 20 mg S/m.sup.2. Variant 2 is a hot dip-galvanized fine steel sheet (99% zinc, 1% aluminum) coated with zinc sulfate at a dry applied coat weight of 20 mg S/m.sup.2. Unbonded region (visually assessed Variant according to EN ISO 10365: 1995) 1 <1% 2 >80%