EP0885975A1 - Process for continuous heat treating metal wires and strips - Google Patents

Abstract

A process for continuous metal wire or strip substrate heat treatment involves passing the substrate through a heating system which consists of a molten metal or alloy (preferably copper or silver) bath of melting temperature at least equal to the heat treatment temperature, the substrate dwell time in the bath being such that the metal or alloy of the bath can completely dissolve from the substrate surface.

Description

The present invention relates to a method of treatment continuous thermal of a metal wire or ribbon, whereby the wire or ribbon is passed through heating means protected from elements liable to react with the metal of the wire or ribbon, to obtain a transformation of its microstructure.

By heat treatment is meant the operations of annealing of metals or alloys including in particular austenitization in the case of carbon steel. For others metals or alloys, annealing can be used to change the microstructure to improve the drawing for example or recrystallize the microstructure after the drawing. Others complete steel annealing operations can be used to dissolve carbides in order to give it a better mechanical resistance.

EP-A1-0 329 611 has already proposed a method of continuous coating of a threadlike steel substrate that we preheat in a neutral atmosphere, then we passed through a copper or copper alloy bath to put it on. Given the melting temperature of the copper, the steel wire is brought to its austenitization temperature, so that the wire thus coated and cooled with controlled manner according to the well-known TTT curve, transforms austenite into perlite in order to preserve over its malleability. Given the necessary conditions the coating of the wire and the adhesion of the copper or of copper alloy, the wire is preheated to a temperature relatively close to that of metal melting coating. This preheating is carried out by induction in a neutral atmosphere.

The disadvantage of this preheating process is in the fact that it is very difficult, by this means, to determine accurately preheat time and temperature wire thus heated. However, austenitization is depending on the temperature as well as the time during which the steel was brought to and maintained at this temperature. In addition, heating in an oven is relatively slow, so that if you want to have a working process at an economically profitable industrial speed, by example 30 m / min which is a usual speed, it is necessary to use a very long oven to heat up and maintain the wire at the desired temperature for several seconds again.

The inventors of the present invention have discovered that it was possible to solve both the problem of control of the heating time, as well as that of the stability and temperature control, using the present process.

To this end, the continuous heat treatment process a metal wire or ribbon mentioned above, according to the present invention, is characterized in that said means heating to bring said filiform metallic substrate at its heat treatment temperature are made up of a bath of a molten metal or alloy whose temperature of fusion is at least equal to the temperature of said treatment thermal and the duration of stay of said substrate is fixed in said bath so that the metal or alloy of this bath is either practically completely dissolved from the surface of said substrate.

The present invention also relates to a use of this proceed for austenitization and coating of steel-like substrates by copper or an alloy copper with higher melting temperature at conventional austenitization temperature of steel of the substrate.

The invention particularly relates to the austenitization of steel wire or ribbon preferably using silver or a molten silver alloy to make the bath steel wire heating. It turns out that silver is one of the few metals that does not react with steel and whose melting temperature (960 ° C) is close the usual autenitization temperature (950 ° C). In in addition, silver has excellent thermal conductivity, so it is also, from this point of view, one of the best possible candidates to reduce the heating time.

As can also be seen in the light of the description which follows, this same processing method can be used for other heat treatments of annealing or releasing internal tensions, for others metals than carbon steel and with other baths of treatment that money

The tests carried out during the tests on steel used steel wire of 1.3mm in diameter, of quality AISI 1070 whose composition by weight is as follows: VS: 0.70-0.75 Mn: 0.45-0.55 Yes: 0.10-0.25

According to the diagrams of Orlich (J.), Rose (A.), and Wiest (P.), taken from: Atlas zur Wärmebehandlung der Stähle, Flight. 3, 1973, Verlag Stahleisen m.b.H., it appears that during continuous heating, the heating rate is first importance because it determines the temperature austenitization at which a microstructure is obtained homogeneous. It appears that the higher the heating speed higher, the higher the heating temperature to obtain homogeneous austenite. For 0.7% steel carbon and a heating rate of 400 ° C / s, we obtain a homogeneous structure with no residence time at temperature heating, when the temperature is 1000 ° C or more. At lower temperatures and with the same heating rate, a stay at isothermal temperature is necessary in order to obtain the transformation of austenite non-homogeneous in homogeneous austenite. The stay of steel at high temperatures, for too long a time, generates growth of austenite grains.

Table I below indicates the size of austenite grains as a function of the heating temperature and the time during which the wire remains in the silver bath. The wire temperature was measured using a thermocouple and recorded as a function of time. After the heating period, the wire is immediately soaked in a bath of molten lead for its isothermal transformation at 650 ° C, in order to determine the size of the austenite grains. Sample No. Temp. Ag bath (0 ° C) Temperature holding time (s) Grain size (µm) 1 1100 5.2 34.6 2 1050 4.0 24.0 3 1050 5.6 32.8 4 1000 4.2 22.4 5 1000 4.5 30.8 6 1000 7.6 33.8 7 980 4.9 21.8 8 980 7.0 22.2

It appears from this table I that for a holding time at a given temperature, an increase in temperature austenitization causes growth in size seeds. For a residence time of 5 seconds and a ΔT (Ex.1-Ex.7) of 120 ° C, increasing the grain size is ≈59%. Likewise, at a given austenitization temperature, a longer residence time also generates an increase the grain size. For T = 1000 ° C, Δt (Ex.6-Ex.4) by 3.4s, the grain size increases by ≈51%. At 980 ° C, this phenomenon is not as important as the growth grain takes longer.

Observation of the microstructure of transformed wires isothermally showed the influence of the residence time at each austenitization temperature. Each of them exists a minimum time after which the microstructure becomes more homogeneous. At times below this, we observe a larger amount of small perlite colonies probably unresolved.

Based on the two criteria of the smallest grain size (leading to better stretchability) and the homogeneity of the microstructure, it can be concluded that the temperature holding times, in addition to the heating time, are The following: 1100 ° C ≈2.9s 1050 ° C ≈4.3s 1000 ° C ≈6.6s 980 ° C ≈11.2s

In Table II which follows, the same temperatures of the molten silver bath are repeated, accompanied by different times of temperature maintenance which are added to the time of temperature rise of the wire which is of the order of 2 seconds, and the reduction rate of the cross section of the wire is given after a tensile test. Sample No. Austenitization temperature (° C) Temperature holding time (s) Section reduction (%) 1 1100 2.1 43.6 2 2.9 43.6 3 4.9 46.4 4 1050 2.2 51.9 5 4.3 51.9 6 7.0 49.2 7 1000 4.0 54.5 8 3.6 54.5 9 9.8 51.9 10 980 1.8 57.1 11 4.7 57.1 12 7.2 54.5 13 10.0 57.1

We can see that in the majority of cases, at a given temperature, the section reduction rate decreases strongly depending on the increase in hold time in temperature. It is confirmed that this reduction rate of section is essentially a function of the dimension of grains of austenite, itself dependent on the holding time in temperature and temperature.

• bain d'austénitisation: argent en fusion
• temps de chauffage du fil: 2s
• température d'austénitisation:1050°C ou un peu moins
• temps de maintien en température: 4,1-4,3s

Basically based on the observations of the microstructure and the reduction rate of the wire cross-section, the following parameters can be selected for the austenitization of the aforementioned steel wire:

• austenitization bath: molten silver
• wire heating time: 2s
• austenitization temperature: 1050 ° C or slightly lower
• temperature holding time: 4.1-4.3 s

The above examples have all been made from non-continuously on samples of determined length, in order to define the parameters necessary for the implementation of the method according to the invention. It is understood however, that one of the applications particularly interesting of this process is its use in a process continuous, on an endless steel wire or ribbon continuously. We will now explain, using from the attached schematic drawing, how can, for example, implement the process which is the subject of the present invention continuously.

La figure 1 est une vue en coupe du dispositif de chauffage du fil d'acier dans le métal liquide;

La figure 2 est une vue partielle en coupe de la figure 1 où la sortie du dispositif de chauffage est associée à un four;

La figure 3 est une vue en coupe transversale selon la ligne III-III de la figure 1.

It should be noted that the passage time of the wire or ribbon in the bath of molten metal is called to vary according to the surface condition of this wire at the time of treatment. If this wire has been previously cleaned in order to rid it of its oxides, it is necessary to increase this passage time to allow the silver to redissolve so that it emerges practically without silver. On the other hand, if the wire is oxidized, the silver does not adhere and there is no need to lengthen the time of passage in the bath beyond the time of temperature rise.

Figure 1 is a sectional view of the device for heating the steel wire in the liquid metal;

Figure 2 is a partial sectional view of Figure 1 where the outlet of the heating device is associated with an oven;

Figure 3 is a cross-sectional view along line III-III of Figure 1.

Figure 1 illustrates the device used for setting continuous use of the process which is the subject of the present invention. The cylindrical crucible is made of a graphite tube 1, the two ends of which are fixed to plugs in graphite 2, 3 respectively closing the inlet and the outlet from the crucible. Each graphite plug 2, 3 is crossed by an inlet conduit 4, respectively outlet 5, parallel to the axis of revolution of cylinder 1, but located at a higher level. Each cap 2, 3 is housed in a cover of refractory material 6, respectively 7, all two crossed by conduits 4, respectively 5. These covers 6, 7 are also used for mounting a second tube 8 alumina, surrounding the graphite tube 1 concentrically by leaving a space 9 between them. The level of inlet and outlet ducts 4, 5 defines the level of the cash 10 inside the crucible 1.

Three openings 11, 12, 13 are formed along the upper part of tubes 1 and 8, one 11 located towards entrance 4 of crucible 1, the second 12 in the center and the third 13 to the outlet duct 5. A piston, formed a graphite rod 14 housed in an alumina tube 15 is mounted in each opening 11, 12 and 13. The end of the graphite rod has an inverted V-shaped section, whose line formed by the junction of the two branches of the V is parallel to the longitudinal axis of tubes 1 and 8. These pistons used to hold the steel wire to austenitize below the level of molten silver 10.

FIG. 2 illustrates a variant implementation of the process to prevent any coating of the steel wire austenitized with silver from the heating bath 10. At this effect, a wiping tube 16, the internal diameter of which is a few hundredths of a millimeter larger than that of the steel wire to be austenitized, is housed in the outlet duct 5 passing through the graphite plug 3 and the cover alumina 7.

In yet another variant, the crucible 1 can be used for heating steel wire to its temperature austenitization, maintaining the wire at this temperature being obtained in an oven 17, supplied with neutral gas, by example of argon, communicating with the outlet of crucible 1 by a quartz tube 18. This variant is not necessarily associated with the wiping tube 16, in particular if one increases the residence time or in the case of an oxidized wire. Similarly, this wiping tube is not necessarily linked to the use of an oven 17. It should be noted that the tube wiping 16 is especially useful in the conditions of laboratory in which the tests were carried out. he aims to prevent more or less solidified money is not entrained by the wire, between the exit of the bath 10 of molten silver and the exit from crucible 1. We could however remedy this drawback by modifying the crucible and / or by modifying the heating conditions. Instead of to heat the whole crucible by induction, as we did in the laboratory prototype, we could heat a certain portion of the crucible, adjacent to the outlet end 3, using an electric oven, capable of giving more homogeneous heating conditions, thereby reducing the risk of solidification of the silver on the surface of the wire.

In the examples made using the device that we just described, the wire speed is 4m / min., the diameter of the crucible is 28 mm, its length 420 mm, the height of the silver bath of 20 mm and the distance between openings 11 and 13 being 265 mm for a crucible 420 mm long. The crucible 1 is heated by a high-frequency heating, power 25 kW, 300 kHz.

The following examples are representative of results obtained continuously using the process according to the invention, implemented with the laboratory equipment described above.

Example 14

We ran a steel wire, cleaned beforehand, of the above-mentioned composition, 1.32 mm in diameter in crucible 1 at the speed of 3.37 m / min. The temperature of the inlet plug is 993 ° C, that of the silver in the entry area is 989 ° C, in the middle, 986 ° C, in the outlet area of 1060 ° C and that of the outlet plug 3, 1066 ° C. The outlet duct 5 is provided with the wiping tube 16. In this example, an oven 17 was not used in the out of the silver bath, that is to say that the whole process austenitization took place in molten silver. The temperature, 1 cm after leaving the crucible is 994 ° C and at 20 cm, from 758 ° C. Air cooling rate is 66.3 ° C / s.

The microstructure of this sample is homogeneous despite the fact that the temperature in the center of the crucible is low (986 ° C). This low temperature is probably compensated by a higher temperature in the exit area and in the outlet conduit 5 (1060-1066 ° C).

The measured hardness of 374 Hv is increased by the fact that the air cooling speed increases the content of bainite. The tensile strength is 1188 MPa and the elastic limit is 741 MPa. Elongation at break is 8.3% and the section reduction rate is 49.2%. No money was found on the wire.

Example 15

The running speed of the same wire, cleaned beforehand, is 12 m / min, its austenization time is 5.7s, i.e. 2.1s in crucible 1 and the rest of the time in the outlet duct 5, in the duct 18 connecting the outlet from the crucible to the oven 17 and in the latter. The temperatures in the inlet duct, in the inlet area, in the center of the crucible, in the outlet area and in the output are 1070, 1000, 971, 1056 and 1056 ° C. The temperature of the oven 17 is fixed at 1050 ° C. Increasing the speed of the thread, passing from 3.37 m / min in Example 14 to 12 m / min in this one, causes a decrease in the temperature of the silver bath 10 in the center of the crucible. This shows that it is then necessary increase the volume of money in order to maintain its temperature above the solidification point. We observed that at 10 m / min, the bath temperature can drop by 100 ° C in seconds.

The photos of the microstructure of this sample have showed large grains of perlite but grains of austenite smaller. It is possible that the desired microstructure is not completely obtained from the fact that the total time (heating time and holding time) is less than the time of 6.1-6.3 s defined during tests not previously mentioned. The hardness is 375 Hv. The tensile strength is 1122 MPa and the limit elastic is 685 MPa. The elongation at break is 9.4% and the section reduction rate is 60.3%. We have noted on this sample a slight deposit of money due to the reduction in residence time in the consecutive silver bath increasing the frame rate and bath temperature.

Example 16

The wire running speed is 10.90 m / min, from so that the heating and holding time the total is 6.2 s. Part of the holding time in temperature is obtained in an oven. Temperatures inlet plug 2, from the inlet zone, from the center of the crucible, of the outlet zone and the outlet plug 3, are 1076, 1059, 1019, 1056 and 1056 ° C respectively. The cooling temperature 1 cm and 20 cm from the outlet of the oven whose temperature is 1050 ° C are respectively from 961 and 894 ° C. The cooling rate is 56.3 ° C / s.

The sample has a homogeneous microstructure with a fine perlite, little fine proeutectoid ferrite around limits of the austenite grains and probably a little bit of upper bainite. The residence time at 1050 ° C in the oven 17 proved sufficient to guarantee complete dissolution of cementite and the homogenization of the structure.

The hardness of the sample is 373 Hv, its resistance the tensile strength is 1118 MPa and the elastic limit of 661 MPa. The elongation rate at break is 9.1% and the section reduction rate is 57.4%. This sample has virtually no deposit of money.

Of course, as those skilled in the art know, the parameters relating to the heating and holding times in temperature are expected to change depending on the composition of the steel, in particular according to its content made of carbon and the section of wire or ribbon. In what concerns the scrolling speed, it is a speed adopted for the realization of a prototype line, which is therefore expected to increase during industrialization of the process, to reach a speed comparable to that of state-of-the-art processes, or even better.

The wire patenting stage after the austenitization phase is not part of the present invention. For the tests carried out by soaking the samples in the bath silver, patenting with controlled cooling and keeping the wire at a temperature of 560 ° C to 650 ° C for a time depending on its composition and its carbon content, was made by passing the wire through a lead bath in fusion. In continuous testing, cooling has been done in the air.

As for the reduction rate criteria of the section wire by stretching, we can see that with the parameters selected mentioned above, one can get a rate reduction of around 52% which is entirely consistent industrial requirements.

Advantageously, the austenitization process can be used in the continuous coating process a steel-like substrate, for preheating of this substrate and simultaneously austenitize it, then bring it to the temperature necessary for coating the substrate, as described in EP-A1-0 329 611. In this case, it would obviously be necessary to take account of the passage time wire in copper or molten brass for austenitization time.

Besides the speed of the heating obtained by the process according to the invention, which makes it possible to reduce the space taken up by the entire installation required for preheating and the coating, the heating process also allows better control the heating temperature and maintain the wire away from air. Since we gain a operation by combining the method according to the invention with that described in EP-A1-0 329 611, since the heat communicated to the wire or ribbon to austenitize it is then used for preheating this wire or ribbon for the coating operation according to this EP-A1-0 329 611, we gain time, energy and space.

As mentioned above, other treatments annealing than austenitization, other metals than carbon steel and other treatment baths than silver in fusion can be considered as part of the present invention. Some examples will be given below to illustrate other applications of this invention.

Example 17

This annealing treatment is applied to a 0.5mm wire diameter in austenitic stainless steel to give it maximum corrosion resistance and softening by soaking it in a silver bath whose temperature was around 1050-1120 ° C and the treatment time between 30 and 60s. The speed was 0.4 to 0.6m / min.

Example 18

We applied the same treatment as in the example previous on a 3041 austenitic stainless steel tube cold drawn with an outside diameter of 1.1mm and a wall thickness of 0.2mm in a silver bath, the temperature was of the order of 1030 to 1110 ° C and the duration immersion from 10 to 20s. The speed was around 0.4 at 0.5m / min.

Example 19

0.25mm cold drawn molybdenum wire was annealed at 1200 ° C in a copper bath to recrystallize it to make it ductile. The duration of treatment was about 4 minutes. Annealing was done by raising the temperature wire in the copper bath and holding it then in temperature, after redissolution of the copper, in an oven. The speed was 0.2m / min.

Example 20

A 5mmx0.25mm molybdenum tape was annealed to remove internal tensions (without recrystallization) in a silver bath and then in an oven whose temperature is maintained at 990 ° C. Total duration of treatment was 3min. The ribbon speed was around 0.3m / min.

Example 21

0.5mm diameter nickel alloy wire sold under the trade name Inconel 600 has been fully annealing in a silver bath at around 1050 ° C. The weather annealing was between 1 and 2 min. The speed was from 0.2 to 0.3m / min. The same yarn has been processed for to remove internal tensions in a silver bath to 1010 ° C. The duration of treatment was approximately 0.6 s. Speed wire was 2m / min.

Example 22

A 1mm diameter wire made from a cobalt-based alloy, the cold-drawn Hayness X 750 has been subjected to treatment of dissolution in a silver bath at 1175 ° C for 12s. The wire speed was around 2m / min.

Alternatively, one could also consider, depending on the case, and to avoid a reaction between the metal or the alloy of the substrate and the metal or alloy bath, to protect the substrate by oxidizing it for example.

Claims (11)

Continuous heat treatment process for a substrate wire-like metallic, according to which we pass said substrate through sheltered heating means elements likely to react with the metal or alloy of the substrate, to obtain a transformation of its microstructure, characterized in that said heating means to bring said filiform metallic substrate to its temperature heat treatment consist of a bath of a molten metal or alloy whose melting temperature is at least equal to the temperature of said treatment thermal and the duration of stay of said substrate in said bath so that the metal or alloy of this bath is practically completely dissolved from the surface of said substrate. Method according to claim 1, characterized in that that said metal or alloy of said bath is chosen from among metals or alloys not reacting with said metal or alloying said substrate at said treatment temperature. Method according to one of claims 1 and 2, characterized in that said wire or ribbon is of steel and that this wire or ribbon is heated in said bath to its austenitization temperature and then kept at this temperature during the austenitization time of this wire or ribbon. Method according to claim 3, characterized in that only after heating said wire or ribbon to its temperature austenitization, it is kept at this temperature by passing through an oven supplied with neutral gas, heated approximately at said austenitization temperature. Method according to one of the preceding claims, characterized in that said heat treatment is an operation complete annealing on steel. Method according to one of claims 1 to 6, characterized in that said heat treatment is annealing applied to austenitic stainless steel to increase its corrosion resistance and soften it. Method according to one of the preceding claims, characterized in that the molten metal is copper. Method according to one of the preceding claims, characterized in that said metal is silver. Method according to claim 1, characterized in that that said heat treatment is an annealing operation performed on one of the following metals: Ni, Fe, Co, Mo, W, Ta, or an alloy of these. Use of the heat treatment method according to one of claims 1 to 8 for austenitizing and continuously coating a steel-like substrate with copper or a copper alloy having a melting temperature higher than the austenitizing temperature of the steel, by immersion of said substrate in a copper bath or of said molten copper alloy according to which, the steel substrate is preheated to a temperature lower than that of said bath, the preheating temperature of this substrate is maintained until it enters said metal or coating alloy bath, this steel substrate is passed through this metal or coating alloy bath to raise the temperature of said substrate at least to its austenitization temperature, the immersion time being of the order of 0.01 seconds and the tension exerted on the substrate being 15 MPa or less, said coated steel substrate is maintained at an elevated temperature for a time sufficient to produce a fine-grained ferrite-pearlite structure of the steel of said substrate and the coated steel substrate is cooled, characterized in that said steel substrate is preheated by passing it through a bath of a molten metal or alloy chosen from metals or alloys the melting temperature of which is at least equal to the austenitization temperature and which does not react with the steel of said substrate, the duration of stay of said substrate in said bath is fixed so that the metal or alloy of this bath is almost completely dissolved from the surface of said substrate, the temperature of said substrate is lowered to bring it just below that of the melting of said metal or coating alloy before passing it through this molten metal or coating alloy. Use according to claim 11, characterized in that the coated filiform substrate is refried without any other heat treatment, this re-drawing producing a section reduction of 0-95%.

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