WO2024251972A1

WO2024251972A1 - Isothermal patenting of steel wires

Info

Publication number: WO2024251972A1
Application number: PCT/EP2024/065781
Authority: WO
Inventors: Gregory Lapeire; Christophe Mesplont; Wim VAN HAVER; Renaat Vandemeulebroecke; Wesley VER EECKE
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2023-06-09
Filing date: 2024-06-07
Publication date: 2024-12-12
Anticipated expiration: 2025-12-09

Abstract

A method and an equipment for isothermal patenting steel wire without lead or molten metal, comprises controlled cooling of steel wires to a predetermined temperature range, and controlled heat removal during the transformation from austenite to pearlite. A steel wire with excellent strain hardening is obtained.

Description

ISOTHERMAL PATENTING OF STEEL WIRES

Description

Technical Field

[1] The present invention relates to a method and an equipment for controlled cooling steel wires to a predetermined temperature range, and for controlled heat removal during the transformation from austenite to pearlite. A steel wire with excellent strain hardening is also provided.

Background Art

[2] Heat treatment of steel wires plays an important role in the production process. Usually the first step in wire-making starts with drawing a wire rod to a desired intermediate diameter. 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 of time for generally isothermal decomposition of the austenite to be completed. The temperature is usually in the region between 500°C and 680°C, more specifically close to 550°C, the “pearlitic nose” in the characteristic transformation diagrams used for steel, with the intention being generally to provide a fine pearlite structure.

[3] While several methods exist for re-heating a pre-drawn wire to a temperature at which the microstructure is fully austenitic, e.g. 1000°C, the cooling and isothermal transformation to pearlite is usually done in liquid lead (Pb) of which the temperature is maintained close to the nose of the pearlitic transformation, e.g. 550°C.

[4] Traditionally, the cooling step in patenting of steel wires is performed in a molten lead bath, which allows isothermal transformation of the austenite in fine pearlite because lead melts at relatively low temperature (320°C), has a high thermal stability and a high heat transfer coefficient. However, because of environmental and health issues, lead patenting is more and more replaced by alternative cooling techniques; of which the use of water based coolant baths is one example.

[5] EP0524689A1 discloses a process of patenting at least one steel wire with a diameter less than 2.8 mm. The cooling is alternatingly 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. The speed of cooling in water is high, while the speed of cooling in air is much lower. The high speed of cooling in water poses a serious risk of forming undesired, hard microstructures for wires with a diameter less than 2.8 mm. Cooling in air in between cooling in water sections is performed in order to slow down the cooling of the steel wires. The number of the water cooling periods, the number of the air cooling periods and the length of each water cooling period are so chosen so as to avoid the formation of martensite or bainite.

[6] EP2951327A1 discloses a forced cooling process on straight steel wires having a diameter larger than 5 mm. An impinging liquid immersed inside a coolant bath is directed to the steel wire to accelerate the cooling speed of the heated steel wire. This “forced” cooling zone in the coolant bath is followed by a cooling zone in which an undisturbed (this means without impinging liquid on the boiling film around the wire) boiling film cools the wires further.

[7] EP3568500A1 discloses an improved method of controlled cooling of one or multiple previously heated and substantially straight steel wire/wires of diameter larger than 2.8 mm to a predetermined temperature range. In contrast with EP2951327A1 the previously heated and substantially straight steel wire/wires along individual path/paths out of the first coolant bath/baths, i.e. at the exit of the “forced” cooling zone, are further cooled down in air, and then guided along individual path/paths through one or multiple second coolant bath/baths. In the method, the substantially straight steel wire/wires are subjected to a cooling transformation from austenite to pearlite.

[8] The above-mentioned methods have shown to improve significantly the tensile properties thanks to a controlled - faster cooling from the austenite region to the start of the austenite-to-pearlite transformation. However, a tensile strength equal to or higher than that of a lead patented wire could not be reached consistently, in particular for steel wire with diameter above 2.8mm. Additionally, it was found by the inventors that, the strain hardening measured after drawing a wire patented according to the above methods was lower than the strain hardening of a lead-patented wire of equivalent tensile strength.

Disclosure of Invention

[9] The first aspect of the invention is a method of continuous controlled cooling and controlled heat transformation removal during the austenite-to pearlite transformation for of a plurality of heated steel wires having an austenite microstructure. The plurality of steel wires comprises - and preferably consists out of - steel wires having a diameter larger than 3.0 mm and lower than 20.0 mm, e.g. steel wires having a diameter between 4.0 mm and 16.0 mm, e.g. between 5.0 mm and 14.0 mm. Steel wires are preferably plain carbon steel wires, i.e. their chemical composition comprise between 0.02wt%C and 1.2wt%C. Preferably, steel wires have a near-eutectoid composition, i.e. the carbon content is comprised between 0.60wt%C and 0.95wt%C.

[10] The method comprises the steps of: a) guiding the previously heated and substantially straight steel wire/wires along individual path/paths through a first cooling section. The first cooling section comprises a bath liquid, wherein the bath liquid comprises water and a stabilizing additive. The role of the stabilizing additive is to create a steam film around each steel wire itself along each individual path. Preferably, the first bath liquid in the first cooling section has a temperature of 80 °C or more. This step, called precooling step can be very short. The pre-cooling zone may have a predetermined length L1 ; b) from a length L1 from the exit of the heating medium (e.g. a gas heated or electrically heated furnace, or an inductor or a fluidized bed), an impinging liquid introduced inside the first cooling section is directed towards the previously heated and substantially straight steel wire/wires over a certain length L2 along individual path/paths. The impinging liquid is preferably introduced inside the first cooling section directly in the bath liquid, i.e. it is preferably immersed. The impinging liquid decreases the thickness of the steam film or destabilizes the steam film, thereby increasing the speed of cooling over the length L2 along individual path/paths. The intensity of the impinging liquids is individually set and/or controlled for each individual steel wire or for subsets of the plurality of steel wires. This step is the controlled cooling step. At the exit of L2 the substantially straight steel wire/wires are still fully austenitic, i.e. no phase transformation has started; c) The previously heated and substantially straight steel wire/wires are guided along individual path/paths out of the first cooling section to be further cooled down in one or more bath liquids and/or in air over a length L3. The length L3 comprises the full length between the exit of the controlled cooling zone with length L2 and the entry into the controlled transformation zone with length L4. In a first, preferred embodiment, the previously heated and substantially straight steel wire/wires leave the first cooling section and enter a second cooling section. In that case the length L3 mainly consists in a length in air. In a second embodiment the first and the second cooling section are the same. In that case the length L3 mainly consists in a length in coolant bath without impinging liquid, i.e. wherein a stable steam film is formed around the previously heated and substantially straight steel wire/wires. Optionally, the previously heated and substantially straight steel wire/wires can be guided along individual path/paths out of the bath liquid of the first cooling section to be further cooled down in air and re-enter in the same bath liquid. This step is the homogenisation step. It allows better control of the temperature of previously heated and substantially straight steel wire/wires until the start of the austenite-to-pearlite transformation; d) The previously heated, substantially straight steel wire/wires are guided along individual path/paths through a second cooling section. The second cooling section comprises a bath liquid, wherein the bath liquid comprises water and a stabilizing additive, wherein the bath liquid and the multiple previously heated and substantially straight steel wires create a steam film around each steel wire itself along each individual path and wherein an impinging liquid introduced inside the second cooling section is immediately directed towards the previously heated and substantially straight steel wire/wires over a certain length L4 along individual path/paths, wherein the impinging liquid decreases the thickness of the steam film or destabilizes the steam film, thereby removing the excess heat generated during the transformation from austenite to pearlite (i.e. the latent heat of transformation, or recalescence). The impinging liquid is preferably introduced inside the second cooling section directly in the bath liquid, i.e. it is immersed. Preferably, the second bath liquid in the second cooling section has a temperature of 75 °C or more. When the second cooling section is separated from the first cooling section, the bath liquid may comprise a stabilizing additive different from the stabilizing additive of the first cooling section. The temperature of the bath liquid in the second cooling section may differ from the temperature of the bath liquid in the first cooling section. Preferably, the temperature difference between the bath liquid of the first and the second cooling sections is more than 5°C, i.e. the temperature of the bath liquid of the first first cooling section is at least 5°C higher or 5°C lower than the temperature of the bath liquid of the second cooling section.

In a second embodiment, the second cooling section may be the same as the first cooling section, i.e. the first cooling section and the second cooling section are connected, e.g. the recipient containing the bath liquid is the same. In that case the bath liquid has the same stabilizing additive and the same temperature in both the first and the second cooling sections. This step is the controlled transformation step. e) The previously heated and substantially straight steel wire/wires are guided along individual path/paths out of the second cooling section to be further cooled down in air.

[11 ] The required parameters for the controlled cooling and controlled heat removal in patenting processes depend on the diameter of the steel wire and on the steel alloy. When cooling using a lead bath, the parameters are less critical, as the transformation from austenite to pearlite occurs isothermal, thanks to the properties of the lead bath. When using water based cooling media, this is no longer the case. Therefore, parameter setting becomes much more critical in order to obtain proper transformation to fine pearlite when treating at the same time steel wires of different diameter and/or different steel alloy. It is a benefit of the invention that steel wires of different diameter and/or of different steel alloys can be patented at the same time; each to an optimum microstructure. It is a further benefit that the microstructure of each of the steel wires is more constant over the length of the wire.

[12] The method is also suitable for wires having a non-round section, e.g. flat wires, rectangular wires, or profiled wires, solving the inherent environmental issue caused by the large amount of lead drag-out generated during the classical lead-patenting process.

[13] In the method, the steel wires can comprise a plurality of subsets, parallel to each other. Each subset of wires can consist of wires of specific diameter and/or profile and specific alloy. By setting and/or controlling the intensity of the impinging liquids for each subset of the plurality of steel wires; the steel wires in each subset can be optimally patented.

[14] It is even possible to set and control the intensity of the impinging liquids for each individual steel wire in both the first cooling section and the second cooling section. Such embodiments allow high flexibility of the method in that more steel wires of different diameter and/or profile and/or alloy can be patented at the same time. In addition, each steel wire - even steel wires of the same diameter or profile and same alloy - can be optimally patented taking differences in wire positions in the equipment and in previous process steps (e.g. in the heating furnace, in pickling... ) into account.

[15] The stabilizing additive is provided to increase the stability of the vapor/steam film around the steel wires. The stabilizing additive may comprise surface active agents such as soap, stabilizing polymers such as polyvinyl pyrrolidone, polyvinyl alcohol and/or polymer quenchants such as alkali polyacrylates or sodium polyacrylate or combinations thereof. The additives are used to increase the thickness and stability of the vapor film around the steel wire.

[16] Preferably, the impinging liquid in the first cooling section has the same composition as the bath liquid of the first cooling section.

[17] Preferably, the impinging liquid in the second cooling section has the same composition as the bath liquid of the second cooling section.

[18] Preferably, the length and the position of the first cooling section and of the second cooling section are adjustable.

[19] Preferably the intensity of the impinging liquid is steered independently in the first cooling section and in the second cooling section.

[20] Preferably, in both the first cooling section and the second cooling section the intensity of the impinging liquid is individually set and/or controlled for each individual steel wire or for subsets of the plurality of steel wires by means of setting and/or controlling the flow rate of the liquid flows creating the impinging liquids. This can e.g. be implemented by controlling the flow rate of the pump or pumps creating the liquid flows for the impinging liquids; or by controlling or setting one or a plurality of valves or orifices. [21 ] More preferably, one or a plurality of sensors are provided. Control of the intensity of the impinging liquids for each individual steel wire or for subsets of the plurality of steel wires is provided by means of a measurement by the one or the plurality of sensors for or at each individual steel wire; or for or at subsets of the plurality of steel wires. Setting of or feedback control of the flow rate of the liquid flows creating the impinging liquids is performed using the measured signals and a controller.

[22] Even more preferably, the sensor or sensors comprise or consist out of pressure sensors. The pressure sensors are provided for measurement of the liquid pressure at the nozzles creating the impinging liquids; and the sensor measurements are used for setting of or feedback control of the flow rate of the liquid flows creating the impinging liquids. As an alternative - or in addition to - pressures sensors, the sensor or sensors comprise or consist out of flow sensors. The flow sensors are provided for measurement of the flow at the nozzles creating the impinging liquids; and the sensor measurements are used for setting of or feedback control of the flow rate of the liquid flows creating the impinging liquids.

[23] Preferably, the intensity of the impinging liquid is steered by pressure sensors because keeping a constant pressure ensures a more stable cooling speed or a constant amount of heat removal, especially when some nozzles are worn, or clogged partly or fully by dirt.

[24] Preferably, one or a plurality of magnetic sensors are provided to measure the magnetic response of one or of subsets of the steel wires; and to provide feedback to adapt in a closed loop control the impinging liquids in the first cooling section and/or in the second cooling section.

[25] In contrast with EP2951327A1 and EP3568500A1 , wherein the emphasis is given to the controlled cooling step to reach the desired start temperature of transformation from austenite to pearlite, the present method allows a full control of the heat generated during the phase transformation.

[26] The transformation from austenite to pearlite starts before the end of the homogenization step, e.g. 5cm before the end of length L3, and finishes before the end of the controlled transformation step with length L4. The position of the second cooling section, the length L4 and intensity of the controlled transformation step provide a fine tuning of the microstructure during the phase transformation from austenite to pearlite. As illustrated below with the examples, the fineness of pearlite, measured by the average interlamellar spacing as well as the volume fraction of coarse pearlite and bainite can be controlled so that an optimized microstructure - similar to or better than the microstructure obtained by patenting in molten lead - can be obtained. The method of the invention provides a more isothermal patenting of steel wire without the use of harmful lead. As will be illustrated further below, the wires obtained according to the method of the invention exhibit excellent strain hardening behavior.

[27] The second aspect of the invention is an equipment for performing the method of the first aspect of the invention. The equipment is particularly adapted for isothermal patenting of one or multiple previously heated and substantially straight steel wires, by controlled cooling to a predetermined temperature range, and controlled heat removal during the transformation from austenite to pearlite. The equipment comprises: a) a first coolant bath for comprising a first coolant liquid; b) a second coolant bath for comprising a second coolant liquid; c) means for guiding the plurality of previously heated steel wires parallel to each other along individual paths through the coolant liquid contained in the first coolant bath; d) means for guiding the plurality of previously heated steel wires parallel to each other along individual paths through the coolant liquid contained in the second coolant bath; e) impinging liquid generator(s) immersed inside the first coolant bath(s), wherein the impinging liquid generator(s) are adapted to direct impinging liquid towards the steel wires over a certain length L2; f) impinging liquid generator(s) immersed inside the second coolant bath(s), wherein the impinging liquid generator(s) are adapted to direct impinging liquid towards the steel wires over a certain length L4; g) means for individually setting or controlling the intensity of the impinging liquids for each individual steel wire or for subsets of the plurality of steel wires; h) means for guiding the plurality of steel wires parallel to each other through air for further cooling.

As an alternative, the equipment may consist of one single coolant bath comprising at least two different impinging liquid generators immersed inside the single coolant bath, wherein the impinging liquid generators are adapted to direct impinging liquid towards the steel wires over a certain length L2 and a certain length L4 separated by a certain length L3 wherein no impinging liquid is directed towards the steel wires. In other words the first coolant bath and the second coolant bath are connected to form a single coolant bath, therefore comprising the same coolant liquid.

[28] The third aspect of the invention is a patented steel wire having a fully pearlitic microstructure and excellent strain hardening. The inventors have found that when patenting steel wire with lead-free methods from the prior art, although tensile properties close to tensile properties obtained with lead patenting could be reached, the strain hardening after drawing was lower compared to the strain hardening of lead-patented wire.

[29] Unexpectedly, when higher tensile strength could be reached after lead- free patenting compared with lead-patenting, the final strength after drawing was still lower because of lower strain hardening.

[30] The inventors found that thanks to the method and equipment of the present invention, in particular the controlled heat removal during the austenite-to-pearlite transformation, the microstructure could be optimized and the strain hardening after patenting could be improved.

[31 ] It was found that higher strain hardening was obtained in a eutectoid steel wire patented according to the present method, and having a low average interlamellar spacing, e.g. less than 110nm, a low volume fraction of coarse pearlite, e.g. less than 5vol% and a low volume fraction of bainite, e.g. less than 5vol. %.

[32] Moreover, due to the more isothermal character of the pearl itic transformation, the difference between the maximum average interlamellar spacing and the minimum average interlamellar spacing measured at different locations between the core and the surface of the eutectoid steel wire patented according to the present method was less than 15nm.

[33] A pearlitic steel wire with excellent strain hardening preferably comprises between 0.60wt%C and 0.95wt%C, has an average interlamellar spacing, ILS, lower than 105nm, e.g. lower than 100nm, comprises less than 3vol% of coarse pearlite and less than 3vol% of bainite. Moreover, the pearlitic steel wire with excellent strain hardening of the invention has a surface that does not contain any trace or residues of Pb or any other metal used for isothermal transformation (Bi, Sn,...). This aspect provides the possibility to further process the patented wire, e.g. by providing a metallic coating such as but not limited to Zn, Zn alloys (e.g. Zn-AI, Zn-AI-Mg), Cu, Cu alloys... without contamination by other metals, ensuring better corrosion resistance, easier recycling and better sustainability.

[34] A typical steel wire rod composition may comprise a carbon content between 0.60wt% and 0.95wt%, a manganese content between 0.20wt% and 0.90wt%, a silicon content between 0.10wt% and 1.40wt%, chromium from 0.05 to 0.40wt%, additional microalloying elements such as aluminum, vanadium and boron, the sum of which is between 0 and 0.20wt%, unavoidable impurities and the balance being iron.

Brief Description of Figures in the Drawings

[35] Figure 1 shows an isothermal patenting concept according to the present invention.

[36] Figure 2 shows a second embodiment of an isothermal patenting concept according to the present invention.

[37] Figure 3 shows cooling curves of previously heated steel wires according to different routines. [38] Figure 4 shows strain hardening curves obtained during drawing steel wires that have been cooled according to the different routines of figure 3.

Mode(s) for Carrying Out the Invention

[39] For sake of clarity, lengths L1 to L4 above-mentioned in the description and in the claims, correspond to lengths L11 to L41 in figure 1 , and to lengths L12 to L42 in figure 2, respectively.

[40] Figure 1 schematically illustrates an isothermal patenting of one substantially straight steel wire according to the present invention. As shown in figure 1 , a steel wire 10 is led out of a furnace 12 having a temperature T of about 1000 °C. The wire running speed can be adjusted according to the diameter of the wire, e.g. between 5m/min and 80m/min, e.g. about 20 m/min. A first cooling section comprises a first coolant bath

14 of an overflow-type and is situated immediately downstream the furnace 12. A plurality of jets 16 from the holes 20 of a perforated plate 22 immersed inside the first coolant bath 14 are forming an impinging liquid, which flow rate is controlled by a circulation pump and control system 18 outside the first coolant bath. The impinging liquid under pressure from the holes 20 is jetting towards the steel wire 10. As illustrated in figure 1 , the first length L11 is the distance away from the exit of furnace 12 to the impinging liquid. The second length L21 indicates the length used for forced coolant cooling process - forced coolant cooling length - in the first coolant bath. The steel wire 10 is then led out of the first coolant bath and subjected to a slow cooling comprising non-forced cooling by immersion in one or more coolants and/or an air gap region. The slow cooling region has a length L31 as indicated in figure 1. Thereafter, the steel wire 10 is guided into a second cooling section comprising a second coolant bath 15 and immediately subjected to forced cooling. A plurality of jets 17 from the holes 21 of a perforated plate 23 immersed inside the second coolant bath

15 are forming an impinging liquid, which flow rate is controlled by a circulation pump and control system 19 outside the first coolant bath. The impinging liquid under pressure from the holes 21 is jetting towards the steel wire 10. The immersion length of the steel wire 10 in the second coolant bath 15 is indicated as L41. Finally, the steel wire 10 is led out of the second coolant bath and subjected to air cooling or any other type of cooling to room temperature or any desired intermediate temperature.

[41 ] The first length L11 defines the pre-cooling zone, it is the distance away from the exit of furnace 12 to the impinging liquid and is used to build a stable steam film around the steel wire 10.

[42] The second length L21 defines the controlled cooling zone and indicates the length used for forced coolant cooling process. The pressure and flow rate of the impinging liquid determine the intensity of the jets. The aim of this section is to control the speed of cooling a previously heated wire 10 from austenite to a desired temperature before the start of the transformation to pearlite.

[43] The third cooling length L31 defines the homogenisation zone and comprises zones where the wire 10 is still immersed in the first coolant bath 14 of the first cooling section, but outside the controlled cooling length, optionally cooling zones in the air, and zones inside the second cooling bath 15 before reaching the controlled transformation zone. The aim of this section is to precisely control the temperature at which the austenite-to-pearlite transformation starts.

[44] The fourth cooling length L41 defines the controlled transformation zone and indicates the length used for forced coolant cooling control. The pressure and flow rate of the impinging liquid determine the intensity of the jets. The aim of this section is to remove the heat generated during the austenite-to-pearlite transformation and obtain a more isothermal transformation. The length L41 should be long enough to cover the full phase transformation, i.e. when the wire 10 leaves the controlled transformation length the microstructure is fully pearlitic.

[45] The pre-cooling zone may be short and may have a fixed predetermined length L11 , e.g. less than 1 m or less than 50cm.

[46] The controlled cooling zone may have a variable predetermined length L21 depending on the wire diameter and compositions, or may be adjustable, e.g. by increasing the number of jets directing an impinging liquid under the wire 10. Preferably, the length L21 is fixed. [47] The homogenisation zone preferably has a variable length L31 , which is adjusted by moving the second cooling bath 15 in a direction parallel to the running direction of the wire 10.

[48] Figure 2 shows a second embodiment of the invention. The pre-cooling zone has a length L12, the controlled cooling zone has a length L22. Alternatively, the length L32 can be increased or decreased by means of e.g. guiding wheels lifting the wire 10 outside the first coolant bath 14 in the vertical direction. This embodiment is illustrated in figure 2 where a single coolant bath 14 is used instead of two separate coolant baths 14 and 15. In that case it is also possible to skip the air gap and use the slow cooling in the coolant bath as a stable steam film is formed around the steel wire 10. Although in figure 2 as an illustrative example guiding wheels are represented, any other mean to create an homogenisation zone can be envisaged, such as e.g. by blowing high pressure air or by creating a depression in the coolant bath.

[49] The controlled transformation zone may have a fixed predetermined length L41/L42, that is function of the wire diameter, speed and chemical composition, such that the length L41/L42 fully covers the transformation from austenite to pearlite. Preferably, the length L41/L42 is adjustable.

[50] The position of the controlled transformation zone and the jets intensity are important parameters to obtain the best microstructure and mechanical properties. This is illustrated by the examples below.

[51 ] Pilot trials have been successfully done on steel wire with a diameter ranging from 3mm to 13mm and with carbon content between 0.60wt% and 0.95wt%. To illustrate the working principle of the invention, examples below refer to the results of the trials done with a 6mm diameter wire having 0.80wt%C. A wire rod with 8mm diameter has been previously drawn to 6mm. The 6mm pre-drawn wire is heated in line in a resistive heating furnace under protective atmosphere comprising H2 and N2 to around 1000°C and submitted to different cooling paths according to the invention. The linear speed of the 6mm wire is 20m/min.

[52] One comparative sample A is patented in liquid lead at 550°C.

[53] Figure 3 illustrates the different cooling paths as simulated by a thermal model for the 0.80wt%C - 6mm wire pre-heated at 1000°C. [54] Sample A is cooled in molten lead at 550°C and is assumed to have the most isothermal transformation. However, it can be seen from the dotted curve in figure 3 that even in lead significant heat is generated during the austenite-to-pearlite transformation, causing the wire temperature to increase with approximately 50°C from about 555°C to 605°C.

[55] Samples B, C and D are obtained according to the second embodiment by varying the respective lengths (referring to figure 2) L22, L32 and L42, and the jet intensities in the controlled cooling zone and in the controlled transformation zone (i.e. relative to the length L22 and L42).

[56] Samples E and F are obtained according to the first embodiment by varying the respective lengths (referring to figure 1 ) L21 , L31 and L41 , and the jet intensities in the controlled cooling zone and in the controlled transformation zone (i.e. relative to the length L21 and L41 ).

[57] 3 levels of jet intensities are used:

[58] - very low, corresponding to a pump flow of 1 m³/h in the control system 19;

[59] - low, corresponding to a pump flow of 5 m³/h in control system 18 or 19;

[60] - high, corresponding to a pump flow of 10 m³/h in control system 18 or 19.

[61 ] The same length L1 (L11 or L12) of 50cm is used for all samples and is not represented in figure 3. The target start transformation temperature of 550°C +/-5°C is reached in samples B, C and D with a low intensity for a long length L22, or in samples E and F with a high intensity for a short length L21. For samples E and F, a longer, slow cooling I homogenization length L31 is needed before the start of the austenite-to-pearlite transformation. In other words, with a low intensity and long length L22 less time is available for homogenization before the transformation starts.

[62] Surprisingly, according to the simulations of figure 3, and confirmed by the pilot trials, the configuration of figure 1 , although comprising separate baths, requires a shorter total length compared to the single bath version of figure 2.

[63] Table 1 reports the cooling conditions, the microstructure and the tensile strength of samples A to F. [64] The microstructure is characterized by the amount of coarse pearlite (CP), and the volume fraction of bainite.

[65] The structure components should be evaluated in general microscopic way, e.g. by light optical microscopy. Coarse pearlite is evaluated at magnification 500x. Higher bainite is evaluated at 500x or at 1000x if determination is not clear enough.

[66] Pearlite is considered as coarse when lamellae can be seen at 500x microscope magnification, if no lamellae are visible then it is defined as fine lamellar pearlite. Samples containing coarse pearlite have been selected and the interlamellar spacing of coarse pearlite has been measured by scanning electron microscope with a x7500 magnification. It could be established that lamellae become visible at 500x magnification in an optical microscope when the mean true interlamellar spacing of pearlite, ILS, is higher than 200 nm.

[67] Higher bainite is a “feather-like” phase in a pearlite matrix, showing low contrast with the matrix phase.

[68] If there is sufficient phase present to evaluate, the magnification, number of fields, grid size is selected according ASTM E562.

[69] In samples A to F, both coarse pearlite and bainite volume fractions are measured by analysing 9 pictures taken by light optical microscope with a x500 magnification.

[70] The mean interlamellar spacing (ILS) of pearlite is determined according to Underwood’s method. It consists in superimposing an intersection grid of three circumferences in such a manner that the lines intersect the pearlite lamellae randomly in all directions. The mean intercept I is obtained by dividing the total line length by the number of the lamellae intercepted of each grid line. The mean true value of interlamellar spacing ILS is given by the equation below:

ILS = I/2

[71 ] In the following sections of the description the terms “ILS” or “mean interlamellar spacing” or “average interlamellar spacing” or simply “interlamellar spacing” all refer to the mean true value of interlamellar spacing as defined above.

The space between the lamellae defines the interlamellar spacing and this distance can be measured by scanning electron microscope (SEM). The mean interlamellar spacing (ILS) is measured by analysing 30 pictures taken by SEM with a x7500 magnification.

[72] The tensile strength (TS) has been measured according to ISO 6892- 1 :2016. The tensile strength in N/mm² is reported in table 1 both after patenting and after further drawing at a logarithmic strain of 2.1 .

[73] Table 1 .

[74] When very low or low intensity is used in the controlled transformation zone, i.e. for samples B, C and E, the heat generated during the austenite- to-pearlite transformation is higher compared to the reference sample A patented in molten lead. As a consequence the average transformation temperature increases and more coarse pearlite is present in the microstructure. Moreover, the interlamellar spacing (ILS) is higher compared to the ILS measured in the lead patented sample A, causing the tensile strength to be lower than desired.

[75] In samples D and F, on the contrary, more heat has been removed during the austenite-to-pearlite transformation due to higher jet intensity with length L4. In both samples D and F the average transformation temperature is decreased compared to other samples, the tensile strength is even higher than in the lead patented sample A.

[76] In sample D, however, the length L3 is slightly too short, or the position of L4 too early, causing undercooling. As a consequence sample D contains a higher volume fraction of bainite, detrimental for strain hardening during further drawing. [77] Figure 4 shows the strain hardening curves for samples A, B, C, D and F during drawing from 6mm diameter to 2.25mm diameter in 10 passes. This deformation corresponds to a logarithmic strain of 2.1 . It can be seen that although sample D has a higher tensile strength than sample A after patenting, the strain hardening rate decreases and the drawn wire has a lower final tensile strength than the lead-patented sample A.

[78] Sample F has an optimized microstructure with ILS below 105nm, and less than 3vol% of coarse pearlite and less than 3vol% bainite. Figure 4 shows that optimized sample F has not only a higher tensile strength after patenting compared to the lead patented sample A, but sample F has also similar strain hardening. The tensile strength of the drawn sample F is the highest of all samples.

[79] In samples A to F, ILS was measured at several locations along a radius of the cross section, namely near the surface of the sample, at intermediate distance between the core and the surface of the sample, and at the core of the sample. The different locations for measurement can be summarized as R=0, R/4, R/2 and 3R/4 and R, with R the radius of the cross section. In sample F the difference between the maximum ILS and the minimum ILS was only 10nm. In comparison, in the lead patented sample A the difference between the maximum ILS and the minimum ILS was 17nm and in samples B, C and E the difference between the maximum and the minimum ILS was more than 30nm.

[80] The present invention allows a more isothermal patenting process without the use of molten metal detrimental for the environment. The obtained wires can reach a tensile strength equivalent or higher than lead-patented wires, have same or better strain hardening that lead-patented wires and do not contain traces or residues of lead or other metal on their surface or at their interface with a metallic coating.

Claims

1. A method for isothermally patenting one or multiple previously heated and substantially straight steel wires by controlled cooling to a predetermined temperature range, and controlled heat removal during the transformation from austenite to pearlite, said method comprising the steps of: b) Pre-cooling step: guiding the previously heated and substantially straight steel wire/wires along individual path/paths through a first cooling section, said first cooling section comprises a bath liquid, wherein the bath liquid comprises water and a steam film stabilizing additive, wherein the bath liquid and the multiple previously heated and substantially straight steel wires create a steam film around each steel wire itself along each individual path; c) Controlled cooling step: from a length L1 , directing an impinging liquid introduced inside the first cooling section towards the previously heated and substantially straight steel wire/wires over a certain length L2 along individual path/paths, to cool down the previously heated and substantially straight steel wire/wires, wherein the impinging liquid decreases the thickness of the steam film or destabilizes the steam film, thereby increasing the speed of cooling over the length L2 along individual path/paths; d) Homogenisation step: guiding the previously heated and substantially straight steel wire/wires along individual path/paths out of the first cooling section to be further cooled down in one or more bath liquids and/or in air over a length L3; e) Controlled transformation step: guiding the previously heated, substantially straight steel wire/wires along individual path/paths through a second cooling section, said second cooling section comprises a bath liquid, wherein the bath liquid comprises water and a steam film stabilizing additive, wherein the bath liquid and the multiple previously heated and substantially straight steel wires create a steam film around each steel wire itself along each individual path and immediately directing an impinging liquid introduced inside the second cooling section towards the previously heated and substantially straight steel wire/wires over a certain length L4 along individual path/paths, wherein the impinging liquid decreases the thickness of the steam film or destabilizes the steam film, thereby removing the excess heat generated during the transformation from austenite to pearlite; f) guiding the previously heated and substantially straight steel wire/wires along individual path/paths out of the second cooling section to be further cooled down in air.

2. Method as in claim 1 , wherein in both the first cooling section and the second cooling section the impinging liquid is introduced and directed towards each of the previously heated and substantially straight steel wire itself along each individual path; or wherein the impinging liquid is immersed partially below some of the multiple previously heated and substantially straight steel wires along their individual paths.

3. Method as in claim 1 or claim 2, wherein the length of the first cooling section and the length of the second cooling section are adjustable.

4. Method as in any of the previous claims wherein the intensity of the impinging liquid is steered independently in the first cooling section and in the second cooling section.

5. Method as in any of the previous claims wherein the intensity of the impinging liquid is steered by pressure sensors.

6. Method as in any of the previous claims wherein the steam film stabilizing additive comprised in the bath liquid of the first first cooling section is different from the steam film stabilizing additive comprised in the bath liquid of the second cooling section.

7. Method as in any of the previous claims wherein the temperature of the bath liquid of the first first cooling section is different from the temperature of the bath liquid of the second cooling section.

8. Method as in claim 7 wherein the temperature of the bath liquid of the first first cooling section is at least 5°C higher or 5°C lower than the temperature of the bath liquid of the second cooling section.

9. Method as in claim 1 wherein the bath liquid of the first cooling section is the same as the bath liquid of the second cooling section.

10. Method as in claim 9 wherein the first cooling section and the second cooling section are connected.

11 . An equipment for isothermally patenting one or multiple previously heated and substantially straight steel wires by controlled cooling to a predetermined temperature range, and controlled heat removal during the transformation from austenite to pearlite, said equipment comprising: a. a first coolant bath for comprising a first coolant liquid; b. a second coolant bath for comprising a second coolant liquid; c. means for guiding the plurality of previously heated steel wires parallel to each other along individual paths through the coolant liquid contained in the first coolant bath; d. means for guiding the plurality of previously heated steel wires parallel to each other along individual paths through the coolant liquid contained in the second coolant bath; e. impinging liquid generator(s) immersed inside the first coolant bath(s), wherein the impinging liquid generator(s) are adapted to direct impinging liquid towards the steel wires over a certain length L2; f. impinging liquid generator(s) immersed inside the second coolant bath(s), wherein the impinging liquid generator(s) are adapted to direct impinging liquid towards the steel wires over a certain length L4; g. means for individually setting or controlling the intensity of the impinging liquids for each individual steel wire or for subsets of the plurality of steel wires; h. means for guiding the plurality of steel wires parallel to each other through air for further cooling.

12. An equipment according to claim 11 , wherein the first coolant bath and the second coolant bath are connected to form a single coolant bath.

13. A pearlitic steel wire with excellent strain hardening; said pearlitic steel wire comprises between 0.60 and 0.95wt%C; between 0.20 and 0.90wt%Mn; between 0.10 and 1.40wt%Si; between 0.10 and 0.40wt%Cr, additional microalloying elements such as aluminum, vanadium and boron, the sum of which is between 0 and 0.20wt%, unavoidable impurities and the balance being iron;said pearlitic steel wire has an average interlamellar spacing, ILS, lower than 110nm, preferably lower than 105nm; said pearlitic steel wire comprises less than 5vol%, preferably less than 3vol% of coarse pearlite, said coarse pearlite being pearlite having an interlamellar spacing higher than 200nm locally and; said pearlitic steel wire comprises less than 5vol%, preferably less than 3% of bainite; and said pearlitic steel wire does not contain traces or residues of lead or other metal on its surface or at its interface with a metallic coating.

14. A pearlitic wire with excellent strain hardening as in claim 13 wherein the difference between the maximum average interlamellar spacing and the minimum average interlamellar spacing measured at different locations along a radius of the cross section between the core and the surface of said pearlitic steel is less than 15nm.