Description

Technical Field

[0001] The invention relates to a process for isothermally patenting steel wire.

More specifically, according a first aspect, the invention relates to the process of patenting or annealing of a steel wire in a lead free process i.e. the invention concerns the controlled cooling of a carbon steel filament. [0002] According to a second aspect, the invention relates to a carbon steel wire resulting from such a process and a steel filament cold drawn from such a carbon steel wire. [0003] According to a third aspect, the invention relates to an installation for continuous controlled cooling of a carbon steel wire.

Background Art

[0004] Cold drawn carbon steel filaments are known in the art. The use of such steel filaments is ubiquitous: steel wire ropes in elevators, steel cord for tyres, wires for sawing hard and brittle materials and so. Typically the final diameter of those cold drawn carbon steel wires ranges from 0.02 mm up to 1.5 mm. Cold drawing is applied to obtain the final diameter and to increase the tensile strength of the steel filament. The degree of drawing is, however, limited. The higher the degree of drawing is, the more brittle the steel filament becomes and the more difficult to reduce further the diameter of the steel filament without causing too much filament fractures.

Too far drawn filaments break like glass. Commercially available wire rod diameters are typically 5.50 mm or 6.50 mm. Direct drawing from wire rod diameters to very fine diameters is not possible. [0005] The above-mentioned limited degree of drawing is the reason why the various drawing steps are alternated with one or more intermediate heat treatments. These heat treatments restore the internal metal structure of the steel filaments to the original structure so that further deformation is possible without increase in the frequency of filament fractures or increased bhttleness of the wire. The heat treatment is mostly a patenting treatment, i.e. heating until above the austenisation temperature (about 1000 ⁰ C) followed by cooling the steel filament down to between 500 C and 680 ⁰ C thereby allowing transformation from austenite to pearlite. Note that the technical term 'patenting' is typically reserved for steel wires that undergo this specific heat treatment.

[0006] The austenite phase of steel is the phase wherein the carbon is dissolved into the face centred cubic cells of the iron. Pearlite is a mixture of cementite - ironcarbide Fβ3C - in the form of very fine lamella that are separated by a ferritic phase. Pearlite forms during the cooling down of the austenite phase when the carbon is expelled out of the iron cubic cells.

[0007] There are other thermal treatments possible on steel wires that induce different properties on the steel wire. Stress relieving for example is used to increase the overall ductility of the wire. In such process the wire is heated from ambient to about 400 ⁰ C to 500°C during which a re- crystallisation of the grains occurs that leads to favourable properties.

[0008] The prior art has provided several ways for imposing quickly and accurately a specific temperature on a steel wire. For economy reasons, such processes have to be done in a continuous way: from spool-to-spool. In such processes the heat exchange between the wire and its surroundings must be quick and intense. Most important is that the temperature is kept constant during the phase transformation. This is not so obvious as it seems in that the phase transformation on itself may generate heat. The transition from austenitic to pearlitic phase e.g. is exothermal. [0009] The parameters determining the cooling of a wire can be simply described. The heat content 'ΔQ' (in J) of a piece of steel wire having a diameter 'd' (in m) with specific heat capacity 'c _p ' (in J K ^"1 kg ^"1 ) at temperature T' (in K) submerged over a length 'L' (in m) in an environment (with large heat capacity) at Tfmai is:

ΔQ = Csteel Psteel (π d ² L/4) (T- Tf _lna l)

The loss rate of heat per unit time through the surface of the same wire is: (dΔQ/dt) = - g (π d L) (T-T _fιna ,)

wherein 'g' is the heat transfer coefficient (W m ^"2 K ^"1 ) between the steel wire and the medium that surrounds it (radiative losses are not considered). As a result, the temperature of the steel wire section will decrease in an exponential way with a decay constant:

T = Csteel Psteel d/(4 g)

Within 3xτ seconds the temperature of the wire will be close to the final temperature Tf _ina ι i.e. the temperature of the cooling medium (provided the wire remains long enough in this medium). From this the most important controllable parameters in the patenting process are clear: the diameter of the wire 'd', the heat transfer coefficient 'g' and the Tf _ina ι of the medium. The cooling phase or transformation phase may be carried out in a bath of lead or a lead alloy, such as disclosed in GB-B-1011972 (filing date 14 November 1961 ). From a metallurgical point of view, this is the best way for obtaining a proper metal structure for enabling further drawing of the steel wire. The reason is that having regard to the good heat transfer between the molten lead and the steel wire (high 'g') and the temperature of the lead bath (T _fιna ι) that can be controlled between 400 and 600 ⁰ C (lead melts at 600.5 K, 327°C), the transformation from austenite to pearlite is fast and fairly isothermal for all practical wire diameters 'd'. This gives a small size of the grains of the thus transformed steel wire, a very homogeneous metallographic structure and a low spread on the intermediate tensile strength of the patented wire. A lead bath, however, may cause considerable environmental problems. More and more, legislation is such that lead is forbidden because of its negative impact on the environment. In addition, lead may be dragged out with the steel wire causing quality problems in the downstream processing steps of the steel wire. Hence, since a number of years, there has been an increasing need to avoid lead in the processing of steel wires and to have alternative transformation or cooling methods. [0011] Alternatives to pure lead have been suggested in US 4944174 and US 1916407, wherein the use of lead alloys at eutectic or near eutectic mixtures with lowered melting points are suggested such as 80 to 90 % lead, 5 to 10 % tin and cadmium 10 to 5% or with an alloy of lead 33%, tin 15.5% and bismuth 51.5%. These are compositions by weight. With these alloys the environmental problem is of course not solved as the alloys still contain a considerable amount of lead.

[0012] In " Austen ite transformation and structure formation during patenting of - carbon- and cobalt steel wires" by Grachev, S.V. et al. which appeared in "Fizika Metallov i mettalovodiene", Vol. 52, Nr. 1 , 1981 , pages 172-177, it is described how short pieces of 50 mm carbon wire were drop-quenched in a liquid tin tank. No considerations on the composition of the tin or on how to implement this in a continuous way are given. This method is clearly not suitable for continuous processing of steel wire especially if they have to be processed further.

[0013] EP-A-O 181 653 (priority date 19 October 1984) and EP-B1 -0 410 501 disclose the use of a fluidized bed for the transformation from austenite to pearlite. A gas which may be a combination of air and combustion gas fluidizes a bed of particles. These particles take care of the cooling down of the steel wires. A fluidized bed technology may give the patented steel wire a proper metal structure with fine grain sizes and a relatively homogeneous metallographic structure. In addition, a fluidized bed avoids the use of lead. A fluidized bed, however, requires investment costs for the installation and operating or maintenance costs. Moreover the cooling time T is longer than that in lead patenting due to the heat conductivity of the fluidizing gas which is not so intense as in a lead bath ('g' is about a third of that of lead patenting).

[0014] The austenite to pearlite transformation may also be done in a water bath such as disclosed in EP-A-O 216 434 (priority date 27 September 1985). In contrast with fluidized bed technology, water patenting has the advantage of low investment costs and low operating costs. Water patenting, however, may give problems for wire diameters smaller than 2.8 mm. The wire cools down rapidly (small T due to small 'd') and quickly nears the Tf _ina ι temperature which is about 120°. The consequence is that fine steel wires are cooled too fast to too low temperatures in comparison to thick wires. As a consequence, brittle bainite or martensite structures will form.

[0015] EP-O 524 689 (priority date 22 July 1991 ) discloses a solution to the above-mentioned problem with water patenting. The cooling is done by two or more water cooling periods alternated with one or more air cooling periods. The cooling speed in air is not that high as in water. By alternating water cooling with air cooling the formation of bainite or martensite is avoided for steel wires with a diameter greater than about 1.10 mm. As with water patenting, this water / air / water patenting is cheap in investment and cheap in maintenance costs. However, a water / air / water patenting method also has its inherent limitations. A first limitation is that for very fine wire diameters, the smallest water bath may also cause risk for bainite or martensite formation. A second limitation is that the water / air / water patenting result in a metal structure which is too soft, i.e. with grain sizes which are greater than the grain sizes obtainable with lead patenting or with fluidized bed patenting. This soft structure is featured by a reduced tensile strength. In addition, the metallographic structure is not so homogeneous and the spread on the tensile strength of the patented wire may be high.

[0016] Cancelling all water baths and using only air patenting is an option with the advantage that the risk for formation of bainite or martensite is not existent or very limited. However, air patenting leads to even softer and more inhomogeneous metal structures than water patenting or water / air / water patenting.

[0017] The above prior art illustrates that there is a need for an environmentally friendly way of continuous and controlled cooling or heating of steel wires which gives intermediate steel wires with an optimum intermediate level of tensile strength of the patented wire, a small grain size and a homogeneous metallographic structure. Disclosure of Invention

[0018] It is a general object of the present invention to avoid the drawbacks of the prior art. It is a first object of the present invention to provide a cooling or heating method and an installation which is not harmful to the environment. It is a second object of the present invention to provide a patenting method and installation which gives a metal structure to the steel wire comparable to the metal structure obtained by lead patenting or fluidized bed patenting. It is a third object of the present invention to avoid quality problems in the downstream processing of the steel wire after patenting. It is a fourth object of the present invention to provide a method of controlled and continuous cooling of a steel wire, independent of the steel wire diameter. [0019] According to a first aspect of the present invention, there is provided a method of continuous controlled cooling or heating of a carbon steel wire. Cooling of the carbon steel wire is e.g. needed in a patenting process of carbon steel wire. Heating of the carbon steel wire is e.g. needed in a stress-relieving operation such as in stress relieving bead wire in order to increase its elongation at break. The method comprises the step of contacting the steel wire with molten tin metal or molten tin alloy during the cooling or heating phase.

[0020] The terms "carbon steel wire" refer to a steel wire with a plain carbon steel composition where the carbon content ranges between 0.10% and 1.20%, preferably between 0.45% and 1.10%. The steel composition may also comprise between 0.30% and 1.50 % manganese and between 0.10% and 0.60% silicon. The amounts of sulphur and phosphorous are both limited to 0.05% each. The steel composition may also comprise other elements such as chromium, nickel, vanadium, boron, aluminium, copper, molybdenum, titanium. The remainder of the steel composition is iron. The above-mentioned percentages are all percentages by weight. [0021] With molten tin metal is meant the metal tin in its molten state without any other intentional metals added. In particular no or very little lead should be present in the tin metal: at the most 1.3 atomic percent (or 2.25% by weight) but more preferred is less than 0.7 atomic percent or even lower than 0.1 atomic percent. [0022] With a molten tin alloy is meant an alloy that comprises at least 47 atomic percent of tin, the remainder being an intentionally added metal that is also present in molten state. Again lead is explicitly excluded: at the most 1.3 atomic percent but more preferred is less than 0.7 atomic percent or even lower than 0.1 atomic percent. It is estimated that keeping the lead content lower than 0.05 atomic percent is possible under regular production circumstances. This corresponds to about 0.55 litre of lead in a 1000 litre bath.

[0023] The limited presence - or even better - complete exclusion of lead is given in by its harmfulness to the health and the environment.

[0024] Tin is a silvery, lustrous grey metal with a low melting temperature

(231.93 ⁰ C). A comparison of the properties of lead versus tin is given in table I: Table I:

[0025] Although being a heavy metal, tin is recognized as one of the safest elements from an environment and health point of view. Tin is non- carcinogenic and non-toxic. Hence, using tin avoids the typical environmental problems one has when using lead. Illustrative to this is the abundant use of tin baths for the production of float glass.

[0026] Next to tin different tin alloys can also be used to heat or cool the carbon steel wire. Particularly envisaged are the following:

- Tin-silver alloys: the eutectic composition is at 96.1 atomic percent of tin with a melting temperature of 221 ⁰ C. Any Sn-Ag alloy with more than 96.1 at% of tin is suitable.

- Tin-bismuth alloys: the eutectic composition is at 56 atomic percent of tin with a melting temperature of 139°C. Any Sn-Bi alloy with more than 56 at% of tin is suitable. This composition is less preferred as it wets metals very well.

- Tin-indium alloys: the eutectic composition is at 47.2 atomic percent of tin with a melting temperature of 117°C. Any Sn-In alloy with more than 47.2 at% of tin is suitable. Although the melting temperature is extremely low, the alloy is less preferred due to the cost of indium.

- Tin-zinc alloys: the eutectic composition is at 85.1 atomic percent of tin with a melting temperature of 198°C. Any Sn-Zn alloy with more than 85.1 at% of tin is suitable. This composition is more preferred as it wets metals poorly. However, it tends to form dross in the bath.

- Tin-magnesium alloys: the eutectic composition is at 90.4 atomic percent of tin with a melting temperature of 203 ⁰ C. Any Sn-Mg alloy with more than 90.4 at% of tin is suitable.

- Other binary alloys of tin with antimony or copper can be used as well. As they generally will not contain much of the alloying element (as the melting point increases with increased alloy element content) they are less preferred.

[0027] Ternary alloys such as Sn-Zn-Bi can of course also be considered. [0028] Using tin metal or a tin alloy instead of lead for patenting of a steel wire result in a comparable isothermal transformation from austenite to pearlite.

Hence, the favourable properties such as a small grain size (due to the cooling rate), a very homogeneous metallographic structure and a intermediate tensile strength of the patented wire are also found back when tin metal or tin alloy cooling or heating is used. Moreover, the tin patenting can be done at very fine intermediate wire diameters. Hence, very fine final filament diameters and related final tensile strengths can be obtained after final wire drawing.

[0029] It is quite surprising that molten tin metal or a molten tin alloy can be used for continuously leading a steel wire through, as tin metal or tin alloys are primarily known as soldering materials due to their very good wettability to metals. The inventor found to his surprise that a molten tin bath or a molten tin alloy bath can still be used for patenting without excessive drag- out of the melt. By taking additional and appropriate measures, as will be explained hereinafter, the drag out of tin or tin alloy can be limited to very small amounts. As a result, there are no disadvantageous effects of tin on the downstream stream processing steps of the steel wire. [0030] Drag out of tin can be prevented in a number of ways:

1. By retaining drawing residues from previous steps on the wire. These residues contain organic substances that burn in the furnace and form a carbon layer that is not easily wet by the tin.

2. Alternatively, the wire can be coated with carbon containing compound (an oil e.g.) that is subsequently burned in the furnace. As the atmosphere in the furnace is a reducing atmosphere, the carbon will not escape as carbon dioxide, but will form a carbon layer on the steel wire surface.

3. By intentionally growing an oxide layer on the wire. This can be done by shortly exposing the hot wire to atmosphere at the exit of the furnace prior to entry into the molten tin metal or tin alloy bath. 4. By preventing wear of the protective layer. Any friction point between the moving wire and a guiding piece must be avoided as it will damage the protective layer. A damaged protective layer results in immediate wetting by tin and tin drag-out.

5. By properly sweeping of the molten tin metal or molten tin alloy upon exit from the bath. A reducing gas atmosphere can help to prevent the formation of a tin oxide skin. Leading the wires through a bed of fine coal has been found to be very well usable in this respect. The coal pebbles sweep of the tin meniscus gently and the coal gas isolates the wires from atmospheric oxygen. Not all measures 1 to 5 must be used together: combinations are possible.

In particular preventive measure 3 has been found to be very practical [0031] According to a second aspect of the present invention, there is provided a carbon steel wire having on its surface traces of tin. The terms "on its surface" refer to the uppermost 1 - 3 monolayers. The term "traces" means that the amounts are there but are that limited that they have no function other than being a remainder of a previous cooling or heating operation in a molten tin metal or molten tin alloy bath. As such the 'traces of tin' are in indicator, a fingerprint for the process used for heating or cooling the steel wire. The presence of traces of tin is a strong indicator that a molten tin metal or molten tin metal alloy has been used for cooling or heating the wire. [0032] For example a bead wire, stress relieved by heating it up in a molten tin metal or tin metal alloy will show such traces prior to coating with bronze. [0033] By cold drawing of a carbon steel wire patented (i.e. cooled) in a molten tin metal or molten tin alloy bath a filament with a higher tensile strength can be obtained. Again traces of tin are detectable at the surface of the drawn filament, indicative for the use of a molten tin metal or molten tin alloy bath upstream in the processing. The cold drawing of the wire will not eliminate these traces. As a matter of a first example, such a cold drawn carbon steel filament can be used as a sawing wire for sawing hard and brittle materials.

[0034] As a matter of a second example, such a cold drawn carbon steel filament can be used in steel cords for reinforcement of rubber products or of polymeric products.

[0035] In both applications, as sawing wire or as steel filament in a steel cord, the steel filaments may be coated with a metal coating providing corrosion resistance or with a metal coating leading to improved adhesion with rubber or with polymers. [0036] According to a third aspect of the present invention, there is provided an installation for continuous and controlled cooling or heating of a carbon steel wire filament. The installation comprises a bath capable of containing molten tin metal or molten tin alloy. The steel wire comes into contact with the molten tin metal or molten tin alloy in the bath during the cooling or heating phase. By preference the bath is made of grey cast iron.

[0037] Grey cast iron has a graphitic microstructure, which causes fractures of the material to have a grey appearance, hence the name. Most cast irons have a chemical composition of 2.5 to 4.0% carbon, 1 to 3% silicon the remainder being iron. Grey cast iron has an excellent resistance to tin rich molten alloys. This is attributed to the formation of a graphite layer at the interface. Grey cast iron is also relatively inexpensive. Alternatively a ceramic lined bath can also be used but this is more expensive. A pure iron bath is not recommended as the molten tin metal or molten tin alloy gradually dissolves the iron. [0038] In a preferable embodiment of the invention, the immersion bath has two or more zones allowing for separate temperature monitoring and/or control. As the entry bath - where the hot wire first enters the bath - receives a lot of heat, no or little heating is required. In a second phase - when the exothermal phase transition from austenite to pearlite occurs - little or no heating will be required either. Heating will be required further on in order to control the temperature of the bath till exit. [0039] In another preferred embodiment of the invention, efforts are done to reduce the amount of molten tin metal or molten tin alloy in the installation. The reason is that, in comparison with lead, tin is relatively expensive.

One of the ways to reduce the volume of tin is to introduce so-called dead bodies into the bath. The term dead bodies refer to bodies which have no other function than reducing the amount of tin. By preference these bodies sink into the molten tin metal or molten tin alloy, as otherwise they have to be attached to the bottom of the bath. Pure iron bodies are preferred for these but then they have to be coated with a graphite layer to prevent attack by the molten tin metal or molten tin alloy. Grey cast iron sink bodies are somewhat less preferred here as the density is at par with that of molten tin metal or molten tin alloy. Other metals or metal alloys (brass, copper, nickel) can be considered provided they have a higher density than that of the molten tin metal or molten tin metal alloy and are properly protected against the attack by tin.

Brief Description of Figures in the Drawings [0040] Figure 1 shows a longitudinal section of one embodiment of a tin bath; [0041] Figure 2 shows a transversal section of another embodiment of a tin bath.

Mode(s) for Carrying Out the Invention

[0042] Figure 1 illustrates the cooling step in the patenting treatment of a steel wire 10. A carbon steel rod has first been cold drawn to an intermediate steel wire at an intermediate steel wire diameter. This intermediate steel wire diameter may vary within a large range (0.50 mm to 3.00 mm) since the cooling in molten tin metal or molten tin alloy: - is at the correct temperature as the tin bath can be maintained at the desired temperature of between 500 and 680 ⁰ C and;

- brings the wire very fast to the desired temperature as the cooling speed is much higher compared to e.g. fluidised bed or water-air patenting due to the high heat transfer coefficient 'g' between the wire and the molten tin metal or molten tin alloy which makes the process more flexible.

The intermediate steel wire diameter may be lower than 0.70 mm or equal to it, which makes the process particularly suitable to produce very fine high tensile wires.

[0043] The intermediate steel wire 10 is first heated in a furnace (not shown) until above the austenitizing temperature, e.g. at about 900 ⁰ C for a 0.80 wt % carbon steel. Immediately after leaving the furnace the steel wire 10 is guided in a bath 12 of molten tin metal or molten tin alloy 14. [0044] Unfortunately tin dissolves many metals and the existing lead baths may not be used as tin bath. A refractory lining must be introduced into the steel bath shell, or the bath has to be replaced with a bath of grey cast iron.

[0045] The bath 12 of molten tin metal or molten tin alloy 14 may comprise dead bodies such as a dummy iron block 16 coated with graphite. The function of these dead bodies is nothing else than reducing the required amount of molten metal. [0046] Figure 2 illustrates another embodiment of an installation 20 where efforts have been made to reduce the required amount of tin 14. A number of parallel steel wires 10 run in a small bath of molten tin metal or molten tin alloy 14 which is positioned by means of supporting elements 24 in a larger bath of a molten salt or of lead 22. [0047] The length of the tin bath 12 can be divided into two or more zones with individual and separate monitoring and/or control of the temperature. As a matter of example only, the bath may be divided into two zones. A first zone contains mains for heating and cooling. The second zone contains means for heating only, since the steel wires 10 have already been cooled down to a large extent. [0048] Heating of the tin bath may be done by means of outside burners, by means of electrical immersion coils or by induction. Local cooling of the tin bath may be done by means of air or gas running in tubes in and around the bath. [0049] In order to prevent oxidation of the molten tin metal or molten tin alloy surface some measures can be envisaged:

- Either an atmosphere of nitrogen with 5 to 8 % of hydrogen is maintained above the bath to prevent oxidation or

- The bath is covered with a grain type of ceramic material or anthracite or even with aluminium shavings in order to prevent oxidation of the surface.

[0050] In a series of trials, a 0.80 wt% carbon steel wire, cold drawn from 5.5 mm to 2.3 mm was patented in the above described installation. The speed of the line was such to obtain a 'vxd' (velocity times diameter) of 50 mm m/min. The wire was heat soaked at 995°C thereby converting the steel into the austenitic phase. A small, elongated bath was held immersed in an existing lead bath (as depicted in FIGURE 2). The small bath was filled with high purity tin (i.e. more than 99.7 wt% of tin). The temperature of the lead bath was held at 500, 540, 580 and 620 ⁰ C in a series of trials. The trials were performed

- under 75%N ₂ /25%H ₂ shield gas in the furnace and

- without any shield gas in the furnace

[0051] Before entering the austenising furnace, the wire was not cleaned in order to leave on as much as possible organic (carbon containing) soap residues. In addition, prior to submersion in the molten tin metal, the wire was shortly exposed to air (20 cm). At the exit of the patenting bath the wire was lead through anthracite coal to keep the tin in the bath and to prevent formation of a tin oxide skin. Samples of the wire were taken at different patenting temperatures which are summarised in table II. [0052] It can be concluded that:

- Contrary to expectations, the tin drag remained controllable only leaving minor traces of tin in subsequent steps. The iron oxides - which are a mixture of FeO, Fe ₂ Os and Fβ3θ ₄ - are sufficient to prevent the reaction of tin with iron. - However, damage to the oxide film results in Fe-Sn alloy formation. So the oxide film must be closed.

- A small grain size, homogeneous metallic structure can be obtained in this way comparable to the ones obtained with lead patenting when patenting in molten tin metal at about 600 ⁰ C. This is indicative for a true isothermal patenting. The minor traces of tin can be detected on patented steel wire even when no tin drag occurs. If this steel wire is further drawn, traces will be still detectable by advanced surface analysis techniques such as Time-of- Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). ToF-SIMS provides information on the atomic and molecular composition of the uppermost one to three monolayers with sensitivities at ppm level and lateral resolution down to 100 nm. ToF-SIMS is not an inherently quantitative technique because the detected intensities depend on the chemical composition of the ambient material (there is a 'matrix effect'). Semiquantitative information can be obtained if the chemical environment of the samples compared is similar.

Table Il