BRASS COATED STEEL CORD WITH INCREASED IRON CONTENT AT THE SURFACE

Abstract

A steel filament for twisting into a steel cord for the reinforcement of rubber articles, which contains a steel substrate that is coated with a coating comprising brass. The coating is different in that the amount of iron at the surface is distinctively higher than that prior steel filaments. The coating has an average iron content of 4 or more atomic percent compared to the total of iron, zinc and copper atoms in the layer extending from the surface to a depth of 3 nanometer below the surface. The steel filaments show an improved adhesion retention under hot and humid conditions and in organic cobalt compound containing rubbers as well as rubbers that are substantially free of cobalt. The lifetime of the rubber article is extended.

Claims

1. A steel filament with diameter ‘d’ expressed in millimeter for reinforcing rubber articles comprising a steel substrate coated with a coating comprising brass, said brass consisting of copper and zinc; said coating having an average thickness of 450×d nanometer or more as determined by the total mass of copper and zinc of said brass; said brass having a mass of copper of between 61 to 75 mass percent compared to the total mass of copper and zinc in said brass, said thickness of said coating and said amount of copper in said brass being determined by wet chemical analytical methods, wherein said coating has an average iron content of 4 or more atomic percent in the first layer, said first layer extending from the surface of said filament to a depth of 3 nanometer below said surface, said iron content being expressed as an atomic percent related to the total of iron, zinc and copper in said first layer, said iron, zinc and copper content being determined by X-Ray photo electron spectroscopy, said average being taken over the depth of said first layer and over 4 different spots on the surface of said steel filament.

2. The steel filament according to claim 1, wherein said brass coating has an average iron content of 5 atomic percent or more in a second layer, said second layer extending from the surface of said filament to a depth of 9 nanometer below said surface.

3. The steel filament according to claim 2, wherein said brass coating has an average iron content of 6 atomic percent or more in a third layer, said third layer extending from the surface of said filament to a depth of 20 nanometer below said surface.

4. The steel filament according to claim 1, wherein said brass coating has an average iron content of 20 atomic percent or less in a third layer, said third layer extending from the surface of said filament to a depth of 20 nm below said surface.

5. The steel filament according to claim 1, wherein said brass coating has an average thickness of 1350×d nanometer or less.

6. The steel filament according to claim 1, wherein said coating shows alternating thin brass stripes and thick brass stripes oriented along the length of the steel filament, said stripes being discernable in a Scanning Electron Microscope operating in Back Scattered Electron mode, wherein thick brass stripes appear relatively light and said thin brass stripes appear relatively dark, wherein in said thick brass stripes an average iron content of 4 or more atomic percent on the total of iron, copper and zinc is present, said average being taken over the depth within the top 3 nanometer from the surface of said thick brass strip, said iron, zinc, and copper content as determined by Scanning Auger electron Spectroscopy.

7. The steel filament according to claim 6, wherein in said thick brass stripes, an average iron content of 6 or more atomic percent on the total of iron, copper and zinc is present, said average being taken over the depth within the top 3 nanometer from the surface of said thick brass strip, said iron, zinc, and copper content as determined by Scanning Auger electron Spectroscopy.

8. The steel filament according to claim 7, wherein in said thick brass stripes, an average iron content is 8 or more atomic percent on the total of iron, copper and zinc is present, said average being taken over a depth within the top 9 nanometer from the surface of said thick brass strip, said iron, zinc and copper content as determined by Scanning Auger electron Spectroscopy.

9. A steel cord comprising one, two or more steel filaments according to claim 1.

10. A rubber product reinforced with the steel cord according to claim 9, said rubber product being a tire, a hose, a belt or any other rubber based article reinforceable with steel cord.

11. The rubber product according to claim 10, wherein said rubber is substantially free of cobalt or cobalt containing compounds.

12. A method to produce a steel filament according to claim 1, comprising the steps of: a. selecting a wire rod; b. dry drawing said wire rod to an intermediate wire; c. patenting said intermediate wire to a patented wire; d. pickling said patented wire; e. electrolytically coating said patented wire with a copper layer; f. electrolytically coating said patented and copper coated wire with a zinc coating; g. diffusing said copper coating and zinc coating on said patented wire to form a brass coating thus forming a brass plated wire; h. wet wire drawing said brass plated wire into the steel filament; wherein said intermediate wire has a circumferential roughness R.sub.a that is larger than 0.40 micrometer.

13. The method according to claim 12, wherein in the wire drawing of said brass plated wire one or more dies comprising diamond are used in the one or more last passes.

14. The method according to claim 12, wherein in step ‘d’ pickling is performed in a hydrochloric acid, said hydrochloric acid having a concentration of iron(III) ions that is between 6 and 15 gram per liter.

15. The method according to claim 12, wherein after step ‘f’ and prior to step ‘g’ the wire is guided through an acidic bath containing iron(II) cations for exchanging zinc atoms with iron atoms at the top layer of said zinc coating.

16. The method according to claim 15, wherein said acidic bath is one from the group consisting of: ferrous chloride solutions; ferrous sulfate solutions; ferrous ammonium sulfate solutions; ferrous fluoroborate solutions; ferrous sulfamate solutions; and mixed sulfate-chloride baths.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0094] FIG. 1: atom concentration profile (Fe).sub.i of Fe relative to the sum of Fe, Cu, and Zn atoms for the reference sample and different samples according the invention;

[0095] FIG. 2: atom concentration profile (Fe).sub.i of Fe relative to the sum of Fe, Cu, and Zn atoms for the reference sample and different samples according the another embodiment of the invention;

[0096] FIG. 3: BSE SEM picture showing the thick brass stripes appearing relatively light and the thin brass stripes appearing relatively dark;

[0097] FIG. 4: Shows an enlarged and 180° turned part of FIG. 3 by means of secondary electron imaging in the Scanning Auger Microscope wherein the analysis points for elemental analysis are indicated;

[0098] FIG. 5: shows the difference in iron distribution in the thin brass stripes (i.e. appearing darker) for the reference sample and two inventive samples as measured by Auger depth analysis;

[0099] FIG. 6: shows the difference in iron distribution in the thick brass stripes (i.e. appearing lighter) for the reference and two inventive samples as measured by Auger depth analysis;

MODE(S) FOR CARRYING OUT THE INVENTION

[0100] For making samples of the invention the inventors departed from a high carbon steel wire rod of carbon class 0.80 wt % C with diameter 5.5 mm nominal.

[0101] The wire was dry drawn to an intermediate diameter of 1.85 mm. Care was taken to obtain a sufficiently high circumferential roughness R.sub.a of about 0.90 μm. The circumferential roughness of the intermediate wire R.sub.a can be increased by increasing the drag-in of soap powder in the dry drawing step, by reducing the reduction of the last die in the dry drawing step, by reducing the drawing speed in dry drawing, by decreasing the die angle in dry drawing or by a combination of any of the above. On the intermediate wire a circumferential roughness R.sub.a of between 0.80 to 1.00 μm was thus obtained.

[0102] Subsequently the wire was cleansed by pickling a method known per sé by the skilled person. The generally used acid for pickling purposes is hydrochloric acid. However, by keeping the concentration of the iron(III) cations above 6 gram per liter and below 15 gram per liter—which are unusual conditions for the skilled person—the circumferential roughness can be further increased.

[0103] Thereafter the wire was coated with copper by electrolytic deposition out of a copper pyro sulphate deposition bath. After proper rinsing and drying the wire is electrolytically coated with zinc deposited out of a zinc sulphate bath. These are techniques known to the skilled person.

[0104] Following zinc deposition, iron may be deposited out of an acidic electrolytic solution containing iron(II) cations by an exchange reaction with the zinc. As the zinc is less noble than the iron, zinc cations will go into solution and the iron(II) cations will deposit in order to preserve charge neutrality. A ferrous sulphate solution seems most appropriate to deposit the iron, as the acid is compatible to that of the zinc electrolyte. The amount of iron deposited will depend on the immersion time of the wire.

[0105] Thereafter the copper and zinc is diffused by heating the wire by resistive heating or by mid frequency induction heating, the exchanged iron remaining present at the surface.

[0106] Patenting, copper plating, zinc plating, iron deposition and diffusion are performed in line on a run through installation wherein a spool of intermediate wire is unwound, guided through the installation and the resulting brass plated wire is wound on a take-up spool.

[0107] In a subsequent step the wire is wet wire drawn into a steel filament of diameter 0.28 mm. By using diamond dies in the one or more of the last passes i.e. the dies situated towards the exit of the wet wire drawing bench, the wire is well drawable. In particular drawability is a problem when the brass is enriched with iron through the zinc—iron exchange reaction.

[0108] By varying the different parameters mentioned above a series of samples was made that showed increasingly higher iron concentrations at the surface. As a reference a regular brass wire was used that followed the same routing but of which the surface roughness of the intermediate wire was below 0.40 μm, pickling conditions were normal (iron(III) cation concentration between 4 and 7 grams per liter), no supplemental addition of iron took place and drawing was done in regular widia dies. This sample is indicated with ‘Ref’ for Reference.

[0109] By means of XRFS the mass of copper and zinc per mass unit of steel cord was determined according the method known by the skilled artisan. Out of the total mass of copper and zinc the average thickness of the coating (expressed in nm) can be calculated. The mass percentage of copper is calculated through the ratio of copper mass over total mass of copper and zinc.

[0110] The iron distribution at the top layer of the brass coating was measured by means of X-ray photo-electron spectroscopy. The equipment used was a K-Alpha X-Ray Photoelectron Spectrometer (XPS) system obtainable from Thermo Fisher Scientific. Typically the depth of analysis of the sample is 2 to 5 nm over a beam area of about 8 000 μm.sup.2. From the kinetic energy distribution of the emitted electrons, information of the atoms probed can be obtained in terms of element number (energy position of peak) and number of atoms present (height of the peak). While many elements can be probed, only the counts on copper, zinc and iron are retained. By stepwise removing the top layer of atoms by means of an argon gun, a depth profile of the atomic distribution on top of the layer is obtained. Examples of traces can be found back in FIGS. 1 and 2 that shows the (Fe).sub.i amount in atomic percent in total of the Cu, Zn and Fe atoms detected at different total sputtering times. For a calibrated argon beam 10 seconds of sputtering amounts to about 1 nm of material removed. It is clear from the traces that the amount of iron present at the surface increases much more in the inventive wires compared to the reference wire (indicated with ‘Ref’).

[0111] By means of the trapezoidal rule, the average iron content is calculated over a layer extending from the surface ‘0’ to a depth of ‘x’ nanometer, ‘x’ taking the values of 3, 9 and 20 nm, corresponding to first, second and third layer. This procedure is repeated on four traces taken at different spots on the surface of one steel filament in order to prevent that measurements are biased through local variations in the coating.

[0112] In this way the following TABLE I is constructed (the numbers in italic fall under the terms of the claims):

TABLE-US-00001 TABLE 1 Average iron concentration over depth Sample 0-3 nm 0-9 nm 0-20 nm t(nm) wt % Cu Die Ref 1.3 1.7 2.2 254 63.2 W S2 3.3 3.6 4.3 249 64.3 D S3 4.4 4.7 5.5 244 64.9 D S4 6.5 7.2 8.2 238 66.5 D S5 7.7 8.6 9.9 234 68.6 D S6 10.5 11.2 12.6 220 70.2 D S7 6.8 7.3 8.2 225 69.6 D S9 2.5 3.0 3.8 253 64.3 W S10 3.3 3.2 3.8 249 64.9 W S11 5.4 6.0 6.6 241 66.7 W S12 7.6 8.0 8.8 236 68.5 W S13 6.5 6.0 6.4 230 69.7 W

[0113] In TABLE I, the only difference between samples (S2,S9), (S3, S10), (S4, S11), (S5, S12) and (S6, S13) is that the first member of the pair is drawn by means of diamond die (‘D’), while the second member is drawn by means of a conventional tungsten carbide die (‘W’). It is to be noted that the use of diamond containing dies always appear to lead to an increased presence of iron at the surface which is in line with the invention, namely to increase the presence of iron at the surface.

[0114] FIG. 3, shows a picture of the surface of brass filament formed by backscattered electrons in the BSE mode of a scanning electron microscope (FEI Inspect model). In this picture the coating shows alternating lighter and darker stripes in the direction of the wire. The lighter stripes correspond to thicker brass stripes—of which a point is indicated with the ‘+2’ reference—while the darker stripes relate to a relatively thin coating for example at the point indicated by ‘+1’. A scratch was deliberately made in order to allow the analysis of the same spot in a Scanning Auger Microscope.

[0115] FIG. 4 shows the same region but in ‘secondary electron imaging’ (SEI) mode as observed in a Scanning Auger Microscope of PHI. Note that the orientation is turned 180° and the magnification is times four compared to FIG. 3. At the indicated spots ‘1’ and ‘2’ Auger electron profiles were measured by means of a ‘PHI-700 Scanning Auger Nanoprobe’ obtainable from ULVAC-PHI. By this procedure, the depth profiles obtained in both regions can be compared to one another for different samples. Auger analysis differs from XPS in that the analysis region is very small, typically below 100 nm.sup.2 whereas for XPS this is several square micrometer.

[0116] FIG. 5 shows the presence of iron relative to the total of iron, copper and zinc in an area that appears dark in BSE mode of SEM. As expected—as there the brass coating is thin—the amount of iron present sharply rises when the steel substrate is reached, which typically occurs within 10 nm. Both the samples according the invention and the reference may show a marked presence of iron with the first few nanometers.

[0117] In contrast therewith when analysing the presence of iron in a region that appears light in BSE mode of SEM on the Reference sample, there is only a very limited presence of iron relative to the total of iron, copper and zinc in the first 3, 10 or even 15 nm from the surface: on average this remains below 0.03 or 3 atom percent. However, in the inventive samples, S3 and S4 there is already a marked presence of iron detected even in the thick brass area even at very low depth. When calculating the average atom percentage of iron in the different samples the following results are obtained (TABLE II):

TABLE-US-00002 TABLE II 0 to 3 nm 0 to 9 nm 0 to 15 nm Sample (Fe at %) (Fe at %) (Fe at %) Ref. 1.3 2.0 2.2 S3 4.1 3.1 3.2 S4 12.2 10.2 9.9

[0118] The presence of an increased amount of surficial iron results in an improved adhesion performance in cobalt salt containing as well as cobalt free compounds as will be demonstrated hereinafter.

[0119] Three filaments of 0.28 mm of each type of Reference and sample filaments were twisted together to form a 3×0.28 mm steel cord. These steel cords were used for performing adhesion tests in a large number of different adhesion compounds basically falling apart in two groups: [0120] Group I contains 6 different compounds with the common feature that they do contain intentionally added organic cobalt salts, [0121] Group II contains 6 different compounds that are all free of intentionally added cobalt.

[0122] For each one of the twelve compounds the conditions for regular cure (RC) were set as the TC90 time plus 5 minutes, TC90 being that time where the particular rubber reaches 90% of its maximum torque on a rheometer curve taken at the vulcanisation temperature.

[0123] In order to establish the adhesion retention the following aging conditions are applied to RC cured samples: [0124] After Cured Humidity aging (CH): RC samples are held at 93° C. in a 95% relative humidity environment for 14 days [0125] After Steam Aging (SA): in which RC samples are steam cooked at 120° C. for 2 days.

[0126] In what follows each of the vulcanisation conditions RC, CH or SA will be referred to as a ‘Condition’.

[0127] Adhesion results are pull-out forces as determined according to the ASTM D2229-04 standard, as further detailed in the BISFA (“The International Bureau for Standardisation of Man-made fibres”) brochure ‘Internationally agreed methods for testing of steel tyre cord’ 1995 Edition, “D12 Determination of static adhesion to rubber compounds”. In this test steel cords are embedded in a block shaped rubber and pulled out of the rubber along the axial direction after vulcanisation. The maximum force (in N) attained is noted. The average of several (at least four) measurements of individual maximum forces (in N) is noted as the ‘Pull-Out Force’ (POF) for one sample, one Group, one Condition combination.

[0128] The results of the adhesion tests are represented in the TABLE III and TABLE IV below as a Z-score relative to a Reference Average (‘RA’). The Reference Average RA is equal the weighted average of the ‘Ref’ sample i.e. a regular brass coating drawn in tungsten carbide dies in all cobalt containing compounds of Group I and this for the particular Condition as per the heading of the column. The Reference Standard Deviation (‘RSTD’) is equal to the statistical standard deviation of all results obtained on the Reference sample in the Group I compounds in the particular Condition. In short: the deviations in positive or negative are calculated relative to the known brass steel cord drawn in tungsten carbide dies tested in a cobalt containing rubber for each of the different Conditions.

[0129] For each of the Groups I and II and for a selection of samples (‘Samples’) of Table II the Pull-Out Force has been determined for each Condition. The Pull-out Forces are weight averaged to a Sample Average (‘SA’) and the statistical standard deviation calculated, referred to as the Sample Standard Deviation, (‘SSTD’) for that Family and Condition.

[0130] The Z-score of a Sample in a Group of compounds for a certain Condition is then equal to the difference between the Sample Average for that Group and Condition minus the Reference Average for that Condition divided by the pooled standard deviation of the Reference Standard Deviation and Sample Standard Deviation. In short:

[00005]$Z=\frac{SA-RA}{\sqrt{\frac{\left(\left(N_S-1\right)SST{D}^2+\left(N_R-1\right)RST{D}^2\right)}{N_S+N_R-2}}}$

[0131] Wherein N.sub.S is the number of results pooled to obtain SA and SSTD and N.sub.R is the number of results pooled to obtain RA and RSTD.

[0132] The Z-score indicates in how for the deviations from the averages are statistically significant from the Reference Average i.e. the current state of the art in the particular Group and Condition the Sample has been tested: [0133] Z-scores that are below ‘−2’ indicate statistically significant deterioration compared to the Reference Average; [0134] Z-scores between −2 and −1 are indicative for a possible deterioration but are not statistically significant; [0135] Z-scores between ‘−1’ and ‘+1’ indicate that no statistical significant deterioration or improvement to the Reference Average can be inferred; [0136] Z-scores between +1 and +2 are indicative for a possible improvement that is not statistically significant; [0137] Z-scores above +2 represent a statistically significant improvement to the current state of the art.

[0138] TABLE III summarizes the Z-sore results obtained on the selected samples as obtained in Group I compounds. Conclusions are: [0139] In regular cure (RC) the overall results are neutral to non-significantly lower compared to the reference for average iron concentrations below 7.5 at % Fe within 0 to 3 nm. [0140] Samples with an average iron concentration above 7.5 at % Fe (S6, S12) show a significantly lower result to the Reference (double underlined). [0141] A too high iron content at the surface therefore has a detrimental effect on Regular Cure results; [0142] For cured humidity (CH) the results generally are better compared to the standard Reference (all positive signs). [0143] The results on samples (S4, S5, indicated bold) show a significant improvement in CH results in cobalt containing compounds. However, samples drawn with conventional tungsten carbide dies show lower results [0144] An average iron concentration of higher than 4 at % Fe at the surface of the steel filament therefore has a positive effect on the cured humidity aged adhesion retention; [0145] In steam aging (SA) a general positive improvement is found that is significant for an average iron concentration of higher than 4 at % Fe at the surface of the steel filament.

[0146] Samples drawn with tungsten carbide dies result in lower values. Samples having a 4 to 7.5 at % Fe in the top 0 to 3 nm of the steel filament perform best over the three Conditions in adhesion rubbers comprising cobalt compounds.

[0147] TABLE IV summarizes the Z-sore results obtained on selected samples of Group II compounds. Conclusions are: [0148] In regular cure (RC) the overall results are neutral to not significantly negative compared to the Reference for average iron concentrations below 7.5 at % Fe within 0 to 3 nm of the surface for samples drawn with dies comprising diamond. [0149] Samples drawn with tungsten carbide score significantly lower; [0150] In cured humidity aging (CH) all inventive samples score better than the Reference, and are significantly better than Reference for average iron concentrations above 4 at % Fe within 0 to 3 nm; [0151] In steam aged samples (SA) all inventive samples score significantly better than the Reference.

[0152] In conclusion: inventive samples with an average iron content above 4 at % Fe and below 7.5 at % Fe within the 0 to 3 nm of the surface show equal results for regular cure and improved results for cured humidity aged and steam aged conditions in adhesion compounds comprising cobalt salts. When tested in adhesion compounds that are substantially free of cobalt, the results after cured humidity aging and steam aging are significantly better, while regular cure results are only moderately lower. Using one or more dies comprising diamonds in the one or more last passes in wet wire drawing further improves these results.

TABLE-US-00003 TABLE III Sample RC CH SA Ref 0 0 0 S2 −0.3 1.0 1.7 S3 −0.1 1.8 2.2 S4 0.0 2.3 3.9 S5 −1.6 2.3 4.0 S6 −2.2 1.8 4.9 S7 −0.1 1.8 2.7 S11 −1.0 1.2 0.7 S12 −3.3 1.4 2.4

TABLE-US-00004 TABLE IV Sample RC CH SA Ref −3.4 −1.4 3.1 S2 −1.0 1.1 3.7 S3 −1.5 2.6 4.8 S4 −0.7 4.1 6.1 S5 −3.2 3.3 5.5 S6 −4.7 3.8 6.6 S7 −0.6 2.9 4.0 S11 −4.3 1.4 5.5 S12 −6.1 2.8 5.0