Patent Application
20150361536
The present invention relates to a method and an equipment for controlled cooling of steel wires.
Heat treatment of steel wires usually plays an important role in the art of wire-making. The first step in wire-making starts with drawing a wire rod to a desired intermediate diameter which can vary from 1.0 to 5.0 mm or more. At this stage of work-hardening the drawn wires are heat treated to pearlite by a patenting process to enable further plastic deformation. Subsequently, the patented steel wires are drawn to a smaller size, either a second intermediate size or a final diameter. Patenting involves heating carbon steel wires into the austenitic phase, generally above 800° C. and then cooling the wires to a chosen temperature held for a sufficient period for generally isothermal decomposition of the austenite to be completed. The temperature is usually in the region of 550° C., with the intention being generally to provide a fine pearlite structure.
As is well known, steel wire rod made by hot rolling from ingots or billets is applied to practical use in the rolled state but after having been subjected to a controlled cooling. In order to cool the high carbon rod immediately after having been hot-rolled to have an excellent cold-workability, it was proposed to dip the high carbon rod into a warm water bath as described in GB 1 276 738. The method of heat-treating a steel wire rod with a wire rod diameter ranging between 5.5 mm and 6.5 mm disclosed in this document comprises dipping the wire rod maintained at a temperature of from 600° C. to 1100° C. into a warm water bath containing a surface active agent. The water is held at a temperature higher than 45° C. thus generating a steam film uniformly on the wire rod surface and thereby controlling the cooling velocity of the wire rod. The essential point of this heat-treating method is to generate the steam film uniformly on the wire rod surface and to keep this state for some period of time until pearlite transformation has finished. Such a method has various merits when used in the direct cooling of hot rolled rods transported in spiral coils on a horizontal conveyor. However, this method has been regarded as being less suitable or unreliable for treatment of wires with other diameters.
Regarding the heat treatment of drawn wires having a desired intermediate diameter which can vary from 1.5 to 5.0 mm, EP 0 216 434 discloses another suitable and reliable method of controlled cooling of previously heated steel wire to an austenite temperature: the wire is transported continuously through a coolant bath containing substantially pure water of at least 80° C. and is immersed in the bath so as to effect a cooling to pearlite without producing martensite or bainite. The wire is subjected to uniform and stable film-boiled cooling along its entire immersion length by contacting the wire with a continuous non-turbulent flow of the substantially pure water. The water patented wires feature a sufficiently uniform pearlitic microstructure with excellent drawability records.
EP 0 524 689 also makes use of water of at least 80° C. as the coolant for the steel wire having a diameter which is less than 2.8 mm, but not continuously through a coolant bath as the aforementioned method disclosed in EP 0 216 434. Austenite to pearlite transformation may also be done in a water bath, however, if there is only one water bath provided, it may give problems for wire diameters smaller than 2.8 mm and even becomes impossible for wire diameters smaller than about 1.8 mm as the cooling velocity/speed of such a steel wire is too fast, which further causes unfavourable metallic structure of the patented steel wire. Thus, as an example desclosed in this EP patent, there are two water baths with air cooling in-between. The cooling is alternating done by film boiling in water during one or more water cooling periods and in air during one or more air cooling periods. A water cooling period immediately follows an air cooling period and vice versa, which is named as a “water-air-water patenting” process. The number of the water cooling periods, the number of the air cooling periods, the length of each water cooling period and the length of each air cooling period are so chosen so as to avoid the formation of martensite or bainite.
As explained in EP 0 524 689, all other technical parameters such as steel composition, coolant bath composition, temperature . . . being kept equal, the diameter of the wire plays a crucial part in the cooling speed. The smaller the diameter the greater the cooling speed, the greater the diameter the smaller the cooling speed.
WO2007/023696 relates to a direct heat treatment method of a loose coil-like rolled wire rod having a diameter more than 11.0 mm. The coil-like rolled wire rod are cooled by immersing them into refrigerant or exposing them to refrigerant flow.
Up to now prior attempts to use the aforementioned methods for the purpose of effecting a cooling-transformation of drawn and austenitized thick steel wires to pearlite, have been largely unsuccessful in many respects. The results of the heat treatment are too often unreliable and the treated thick wires show too high a variation in properties such as inconsistent drawability and frequently unexpected brittle behaviour because of numerous undesirable metallic structures. The exact metallic structure of the patented wire not only determines the absence or presence of wire fractures during the subsequent wire drawing but also determines to a large extent the mechanical properties of the wire at its final diameter. In this way, transformation conditions must be such that martensite or bainite are avoided even at very local spots on the steel wire surface. On the other hand, the metallic structure of the patented steel wire must not be too soft, i.e. it must not present too coarse a pearlite structure or too great a quantity of ferrite, since such a metallic structure would never yield the desired ultimate tensile strength of the steel wire. According to the statement of the previous paragraph, the essential point of carrying out a reliable thick wires' transformation-cooling is to accelerate cooling intentionally based on a conventional wire heat treatment process.
The primary object of the invention is to provide an alternative controlled cooling process.
Another object of the present invention is to give patented steel wires with a proper metallic structure, i.e. a fine pearlite structure without any martensitic or bainitic spots.
It is still another object of the present invention to provide a process which is suitable for transformation from austenite to pearlite of steel wires with a diameter greater than 5.0 mm, e.g. greater than 8.0 mm.
According a first aspect of the invention there is provided a method of controlled cooling of one or multiple previously heated and substantially straight steel wire to a predetermined temperature range, the method comprises the steps of:
In the present invention, the controlled cooling method relates to one or multiple substantially straight lines of steel wires. These steel wires pass through the coolant bath along individual paths. In the other words, the paths in the coolant bath are substantially straight. Therefore, the paths of each steel wire are well defined. Normally, the coolant bath may have a rectangular shape and the paths of steel wires are substantially parallel to one side of the rectangular shaped coolant bath. This make it possible to direct an impinging liquid immersed inside the coolant bath towards the steam film on the steel wires. For instance, the imping liquid can come below the steel wires, towards said steel wires (or said steam film) and along the individual paths. Thus, the steam film can be destabilized or the thickness of the steam film is decreased. As a comparison, this advantage cannot be realized by the solution mentioned in WO2007/023696A1 where a loose coil-like hot rolled wire rods are cooled with refrigerant. The loose coil-like hot rolled wire rods are conveying by a conveyor through the refrigerant tank. A boiled water or gas-liquid mixture are injected to the refrigerant from nozzles immersed within the refrigerant tank, while making the refrigerant in the refrigerant tank flow and relieving the dispersion/variation in a coolant temperature. In WO2007/023696A1, it is trying to suppress the cooling nonuniformity of the wire by creating turbulence in the refrigerant tank by ejecting gas-liquid mixture into the refrigerant tank. The steam film on the steel wire is not really destabilized or at least is not uniformly destabilized over the whole length of the coil-like wire rods since the hot rolled wire rods are in a loose coil-like form. For the coil-like hot rolled wire rods, the closer to the nozzles the more destabilization of the steam film. The nozzles of WO2007/023696A1 are arranged in a line or in three lines. The distance of the coil-like steel wire rods from the nozzles depends on the location on the coil and thus the cooling of the coil-like steel wire rods are also location dependent. The effect of the turbulence in the refrigerant tank on the steam film of the steel wires are not comparable to direct an impinging liquid towards the steam films according to the present invention.
Another advantage of the present invention is that the controlled cooling method can be applied to multiple lines of steel wires. Preferably, the multiple lines of steel wires are parallel to each other. The pattern of impinging liquid immersed inside the coolant bath can be flexibly designed for each individual steel wires. For instance, each steel wire can have a same impinging liquid pattern. Alternatively, impinging liquid can be immersed partially below some of the multiple said previously heated and substantially straight steel wires along their individual paths. It makes possible that multiple steel wires can have different impinging liquid pattern and thus different cooling scheme as desired in a same coolant bath.
According to an embodiment of the invention, the previously heated steel wire/wires is/are subjected to a controlled cooling-transformation treatment from austenite to pearlite. Said steel wire/wires is/are previously heated above austenitizing temperature and cooled at a predetermined temperature range from 400° C. to 650° C. in order to allow transformation from austenite to pearlite, preferably at the temperature of 580° C.
When the steel wire has been heated above the austenitizing temperature the cooling stage comprises a pre-transformation stage, a transformation stage and a post-transformation stage. The lengths of the process, e.g. the forced water cooling length L and the length of conventional water cooling during the pre-transformation stage are preferably so chosen so as to start the transformation from austenite to pearlite at a temperature between 400° C. and 650° C., which allows a patented steel wire with suitable mechanical properties.
Preferably to carry out this method, the forced water cooling length L is smaller than the length of the coolant bath. Usually the pre-transformation stage consists of the whole forced water cooling period and of only a short length of subsequent conventional water cooling period. During this forced water cooling period the steel wire is initially cooled rapidly and then go through a short “soft” conventional water patenting length where this rapid cooling is slowed down so as to enter the “nose” of transformation curve at a proper place—following a predetermined cooling curve (TTT diagram).
Relating to the transformation stage, the complete transformation from austenite to pearlite may occur in the coolant bath, substantially after the wire leaves the forced water cooling process. Cooling in the post-transformation stage may be done in air. Preferably the cooling by air or in air is not a forced air cooling but a simple cooling in ambient air.
In carrying out this cooling-transformation method as a substitute for a conventional heat treatment, i.e. WAP (water-air patenting, one over-flow water bath followed by ambient air), the steel wires are allowed to cool from austenitization temperature and then transform to pearlite. The principle is quite simple but needs to be well controlled. By way of example, the flow rate of the impinging liquid has to be carefully tuned in order to have a desired destabilization of steam film or reduction in thickness of steam film. Instead of using a separated system with cold water and impinging liquid like in many cooling or patenting installations, this new concept uses the same cooling solution as for WAP—the chemical composition of the impinging liquid and the liquid in the coolant bath is the same. This brings two major advantages: one is much lower cost of installation (uses the same tank and cooling liquid); the other is reducing the temperature gradient between the core and the surface of the wire (no direct contact with cold water, thinner steam film cooling), leading to still homogeneous patented structure.
More preferably, the impinging liquid is taken from the coolant bath itself and can be continuously recirculated, e.g. by means of a circulation pump, which further helps to generate a considerably homogeneous solution within the whole coolant bath, which brings a stable cooling system.
The term “liquid” refers to water where additives may have been added to. The additives may comprise surface active agents such as soap, polyvinyl alcohol and polymer quenchants such as alkalipolyacrylates or sodium polyacrylate (e.g. AQUAQUENCH 110®, see e.g. K. J. Mason and T. Griffin, The Use of Polymer Quenchants for the Patenting of High-carbon Steel Wire and Rod, Heat Treatment of Metals, 1982.3, pp 77-83). The additives are used to increase the thickness and stability of the vapour film around the steel wire. The water temperature is preferably more than 80° C., e.g. 85° C., most preferably above 90° C., e.g. around 95° C. The higher the water temperature, the higher the stability of the vapour film around the steel wire.
In a classical WAP installation the cooling speed of the wire depends mainly on its diameter (and to a lesser extent to the temperature and polymer concentration of the cooling liquid). With the forced water cooling process according to this invention, immersed impinging liquid reduces the thickness of the steam film, increases the cooling speed, and the forced water cooling length L can be adjusted to control the transformation temperature.
Forced water cooling is conveniently done in a coolant bath where the steel wire/wires is/are guided continuously along individual path/paths. A horizontal and rectilinear path is preferable to provide the travelling channel for each steel wire. The bath is usually of the overflow-type, the same as the conventional coolant bath. Preferably, there is provided an impinging liquid by a plurality of jets from holes immersed inside said coolant bath below the steel wire itself along each individual path. One of the advantages of having the jets below the steel wires is that one can easily reach and arrange the steel wires without being hindered by the jets.
A plurality of jets from the immersed holes are adapted to rectilinearly direct towards the steam films, e.g. perpendicular to the wire or wires so as to have an effective impact on the steam films—destabilize said steam films, or decrease the thickness of the steam films, further to increase the cooling speed of the thick steel wire or wires. The flow rate of the impinging liquid from the holes may be controlled by the pump. The pump flow rate has a direct influence on the destabilization of the steam films or the decreasing degree of the thickness and further the cooling speed. In general, the higher the pump flow rate, the more stinging the impinging towards the steam films, thus the higher the cooling speed. Of course, different pump flow rates can not only lead to different cooling speeds, but also different positions of the start of transformation ultimately.
According to the invention, the terms “thick wires” refer to wires with a diameter greater than 5.0 mm; preferably, the diameter ranges from 5.5 mm to 20 mm and more preferably, from 6.5 mm to 13.5 mm, e.g. 7.0 mm; 8.0 mm; 9.0 mm.
For steel wire diameters of about 5 mm and more, by way of example, 6 mm, the pump flow rate in the forced water cooling period may be not so high as a very fast cooling speed is not necessary for such not very thick steel wires. If the cooling speed is too fast, the cooling curve will pass by the nose of the transformation curve and bainite or martensite risks to be formed.
In extreme cases, for very thick diameters, e.g. more than 13 mm, the pump flow rate in the forced water cooling period is requested to be significantly high so as to obtain a sufficient destabilization or a much thinner steam film further to have a rapid cooling speed.
According a second aspect of the invention, there is provided an equipment for controlled cooling of one or multiple previously heated and substantially straight steel wire to a predetermined temperature range according to the first aspect of the invention. This equipment preferably comprises:
The equipment may comprise means for conveying an austenitized thick steel wire or a plurality of austenitized thick steel wires continuously along individual path/paths to a coolant bath through which the wires are passed horizontally for a predetermined immersion length. This predetermined immersion length is equal to the sum of a length of forced cooling and a length of non-forced cooling or soft cooling. Within the length of forced cooling, the wires are contacted with a predominantly laminar flow of a water coolant having a constant temperature of more than 80° C. and processing a sufficient purity so as to achieve and to maintain stable film boiled cooling without inducing local nucleate boiling and quench martensite formation, while an impinging liquid is provided by a plurality of jets from the holes immersed inside said coolant bath directing towards said steam films over a certain length L, in order to destabilize said steam films or decrease the thickness of said steam films over the length L. This can be controlled by a circulation pump outside the coolant bath. Thereafter, within the length of non-forced cooling or soft cooling, the wires are cooled during immersion at the same coolant bath at a stage of conventional water patenting process to a desired temperature range of pearlite transformation.
This equipment has the advantage of low investment costs and low running costs. It is quite easy to adapt a conventional WAP equipment to a forced water cooling equipment according to this invention. The equipment according to this invention is not only applied to cool a plurality of previously heated steel wires with the same diameter but also a plurality of previously heated steel wires with different diameters, which is realized by means of adjusting the total immersion length separately and individually for each wire and/or by adjusting forced water cooling length L separately and individually along each individual path.
General description of influence of diameter on cooling speed with respect to TTT diagram of
Referring now to
The first length l1 is due to the positioning of the forced water cooling equipment. The forced water cooling equipment might be installed just at the exit of the furnace (l1=0) or a small distance away from the exit. The length l1 can be adjustable as required. The second length l2 indicates the length used for forced water cooling process—forced water cooling length. The third length l3 is the remaining cooling length in the same water coolant bath 14.
The magnetic point, indicating the start of the austenite to pearlite transformation was measured using a magnet and is indicated in table 1 (Magtrans—defined as the distance away from the exit of the furnace). The tensile strength was also measured and indicated in table 1 together with other four samples (S1 and S2 and S4 and S5, S1 is the reference wire through a conventional WAP while S2 to S5 are the wires through the inventive process—forced water cooling treatment).
For the present examples starting product is a plain carbon steel wire rod. This steel wire rod has following steel composition: a carbon content of 0.60%, a manganese content of 0.50%, a silicon content of 0.202%, a sulphur content of 0.013%, a phosphorus content of 0.085%, all percentages being percentages by weight.
A typical steel wire rod composition for high-tensile steel wire has a minimum carbon content of around 0.80 weight %, e.g. 0.78-1.02 weight %, a manganese content ranging from 0.30% to 1.10%, a silicon content ranging from 0.15% to 1.30%, a maximum sulphur content of 0.15%, a maximum phosphorus content of 0.20%, all percentages being percentages by weight. Additional micro-alloying elements may also be added, such as chromium from 0.20% to 0.40%, copper up to 0.20%, vanadium up to 0.30%.
Table 1 further illustrates the effect of low and high pump flow rates in the installation. The situation acted on the last sample S5 is extreme since in normal conditions the flow rate is between 6 and 10 m3/h. During the last two trials (S4, S5), with the same forced cooling length l2=0.6 m and the same “soft” water cooling length l3=2.6 m, the position of the start of transformation was measured respectively using a magnet for different pump flow rates. A clear correlation between the distance from the furnace to the transformation point and the flow rate was found as shown in
However, according to this invention, the parameter—the pump flow rate is calculated as the sum of the jets from all the holes. If the size of the holes is fixed, the more the holes, the higher the flow rate; if the number of the holes is fixed, the bigger the holes, the higher the flow rate. Further, the higher the pump flow rate, the higher the forced cooling speed.
Ideally the system should provide the same cooling speed irrespective of the travelling path of the steel wires. Indeed the steel wires may change somewhat from travelling path. In case only one set of holes is provided for one steel wire, a changing travelling path may cause changing cooling speeds and this is to be avoided. This can be avoided by providing various types of distributions of the holes. For example, there may be an at random distribution of holes.
In
The diameter of the holes preferably ranges from 0.5 mm to 5.0 mm, e.g. 1.0 mm, 2.5 mm, 4.0 mm, and the length between two adjacent holes along the same line are preferably larger than 5.0 mm, e.g. 6.8 mm, 8.2 mm, 10.6 mm. The holes 52 shown in
It is obvious that such a design is applied to cool a plurality of previously heated steel wires with different diameters at the same time.
As illustrated in
The tensile properties of other samples cooled with the prototype are significantly higher than those of reference S1 and are close to the expected tensile strength of a 10 mm lead-patented wire rod with 0.6 wt %C (target value 1010 N/mm2).
| Number | Date | Country | Kind |
|---|---|---|---|
| 13153642.7 | Feb 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/051407 | 1/24/2014 | WO | 00 |
