LU86467A1 - CONTINUOUS PROCESS OF MANUFACTURING STEEL WIRE - Google Patents

Description

1- · ·: .Λ! '* μ ·. · Ι: · j!

\ *! I

* I

ί ί ι; ί ί ι ι ί! ; ί j ι ι.

! (ι! I *.! 1! "

Continuous process for manufacturing steel wire.

The present invention relates to a continuous process for manufacturing steel wire, in particular concrete prestressing wire, made of steel having a carbon content greater than 0.4%.

Currently, steel wire is usually produced by hot rolling an ingot first in billets and then in wire rod whose diameter is between 5.5 mm and 14 mm. You can also get the wire rod from blooms or continuous casting billets. This wire rod is then drawn in several passes, through dies of decreasing diameter, to the desired final diameter.

15 After drawing, the wire undergoes an aging treatment under tension intended to give it good resistance to relaxation

. . I

'it ·. · I

• "i v I '' \ * i - 2 - i i i of the operating voltages.

i t

In addition to its relaxation properties, the quality of the final product (is characterized by two parameters, the breaking load and the necking. J! In the unilaminated raw state, the unalloyed wire rod has a structure such that the finished wire does not after the treatment described above, does not have sufficient strength.

10

Different methods are used to remedy this situation, methods which all have varying degrees of drawbacks, technical or economic: 1) increase in the rate of work hardening. This method makes it possible to increase the resistance of the wire, but deteriorates its ductility (reduction in necking).

2) Increase in the carbon content. This method has the drawback of increasing the difficulty of processing the wire rod and in particular of rendering random the use of semi-finished products for continuous casting (axial segregation).

3) Addition of chromium and silicon. This method increases manufacturing costs.

4) Patenting treatment.

25 This last treatment constitutes the most effective method of wire improvement, because it allows the increase in resistance and necking; it consists in passing the wire rod slowly through a reheating furnace in order to bring its temperature beyond the transformation point A ^ and thus to make it entirely austic-nitic. The wire rod is then cooled either by cooling

\ I

natural in air (air patenting) or by immersion in a lead bath (lead patenting). After patenting, the wire rod is then subjected to pickling and surface treatment before being drawn. The surface treatment essentially comprises the deposition of a layer of a substance, such as borax or lime, intended to facilitate the attachment of the drawing lubricant.

i i ï 1 1 4 - 3 - All of the patenting, pickling, surface treatment and wire drawing operations are generally carried out in wire drawing workshops, to which the wire rod is delivered in the raw rolling state.

5

Currently, all these operations are separated by more or less long periods of storage of the wire rod, which involves numerous manipulations and requires the development of storage areas at the various stages of wire preparation.

10

It would obviously be highly desirable if all these operations could be combined so as to be carried out continuously from patenting to drawing. Manufacturers could thus benefit from the advantages attached to continuous processes, namely the reduction of handling and storage costs, the reduction of the size of the installations by the elimination of storage areas, the reduction of energy consumption, reducing manufacturing times and increasing the regularity of processing.

20

In the wire drawing field, the serialization of the above operations in a single continuous process comes up against a fundamental difficulty, namely the very large speed difference between patenting and wire drawing. In practice, the speed ratio of the wire 25 between these two operations can reach and even exceed 15.

For example, for a wire rod 12 mm in diameter, the speeds are respectively 0.063 m / s for patenting and 0.986 m / s at the entry of the drawing, i.e. a speed ratio greater than 30 15 With such a wire, patenting with lead at a speed of 0.986 m / s - compatible with wire drawing - would require a lead bath with a length of more than 60 m, which obviously cannot be envisaged in an application. industrial.

t 35 ’. _ i - 4 -

A first object of the present invention consists precisely in proposing a method for manufacturing steel wire, comprising patenting and drawing the wire, in which the wire moves, at each i stage of manufacture, at speeds compatible with the input speed of the drawing.

Another object of: the present invention is to reveal a process for the serialization, in a continuous process, of the patenting, surface treatment and wire drawing operations of a steel wire.

10

The process which is the subject of the present invention is based on the development by the Applicant of a patenting operation with water of the steel wire traveling at high speeds, of the order of magnitude of the speeds practiced at the entrance to the wire drawing.

15

This rapid patenting process with water is, moreover, yet another object of the present invention.

The method of the invention applies to a wire generally found in the form of a coil, such as pickled machine wire or wire having undergone a first wire drawing. As regards the pickled wire rod, pickling can either have been carried out separately, for example following rolling, or be carried out continuously immediately before the process of the invention.

25

It will also be emphasized that, in the process of the invention, the steel wire is unwound from the initial coil and it is subjected continuously, that is to say without intermediate manipulations or interruption of its running movement, to various successive operations of pre-paring and drawing, then it is again wound into a final coil.

Under these conditions, the process which is the subject of the present invention, for the continuous manufacture of steel wire, in which the steel wire is initially unwound from a spool of wire. * 1. ’- 5 - at ambient temperature and is finally wound up again in a coil of wire, is essentially characterized in that, successively, (a) the wire is heated rapidly to a temperature above ^ at point A ^; (b) the wire is suddenly cooled, in a first step, to a temperature below 700 ° C but above the point Ms, which marks the beginning of the martensitic transformation of said steel; (C) the wire is cooled more slowly, in a second step, so as to ensure the transformation of most of the austenite into perlite; (d) a layer of a substance promoting the attachment of the wire drawing lubricant is deposited on the surface of the wire; and (e) the wire is drawn to its final dimension.

Usually, the wire delivered to wire drawing workshops has a decarburized layer on the surface. In this case, the point Ms mentioned above is that which corresponds to the base steel, that is to say not decarburized. It can then happen that the method of the invention causes, during the first cooling step (point b), the martensitic transformation of this decarburized layer, due to the higher value of the point Ms in this layer. This does not, however, constitute a drawback since, during the second cooling stage (point c), the surface of the wire heats up to a temperature of at least 580 ° C., which ensures an income of said layer. martensite.

According to a first feature of the invention, the wire is heated under a protective atmosphere, preferably a slightly reducing atmosphere composed for example of ^ - 5% H2, in order to prevent or suppress any oxidation of the surface of the wire.

The heating conditions are adjusted so that at the outlet of the oven, the wire has an austenitic structure at any point 35 of its section and that all the carbides, in particular cementite, present in the wire before heating have been returned. in solution.

- 6 - • 1. .

.. ’1

This condition implies that the wire is brought to a temperature higher than its transformation temperature Ac ^.

i

Heating the wire above the temperature Ac ^ is known per se in the conventional technique of lead or air patenting. In the known technique, however, this is very slow heating, which requires a long time and is not compatible with the high speeds recommended by the present invention.

In the context of the present invention, the speed of heating of the wire is greatly increased, compared with conventional practice, in order to take account on the one hand of the speed of circulation of the wire and on the other hand of the necessary limitation dimensions of the oven.

π also appeared that it was not enough to raise the heating rate, but that it was also necessary to increase the heating temperature · to ensure rapid dissolution of the carbides.

20

According to the invention, the heating temperature is advantageously between 850 ° C and 1050 ° C.

This temperature is reached in a period of time which can vary depending on the speed of the wire but which, also according to the invention, is preferably not more than 5 mirâtes, in order to limit the size of the oven. .

According to another feature of the invention, the wire which is at a temperature above the point A ^ is subjected to a first cooling step intended to remove part of the sensible heat from the wire without causing it to the allotropic transformation of austenite.

This first step is based on the observation that it is possible to act on the final properties of the weakly drawn wire! - 7 - sant vary the temperature at which the pearlitic transformation occurs in the wire. The first cooling step of the process of the invention therefore consists of intense and brief cooling, which preserves the austenitic structure of the wire despite the establishment of high thermal gradients in its section.

The cooling intensity, i.e. the density of heat flux exchanged between the cooling agent and the wire during this first step, must be such that the cooling curve is located in front of the nose of the transformation curves of the steel constituting the wire.

The duration of the first cooling step is determined by the fact that the temperature of the wire cannot at any point reach the temperature Ms at the start of the martensitic transformation.

This first cooling step is preferably carried out by passing the wire through a stream of coolant circulating in a direction substantially parallel to the longitudinal axis of the wire.

In particular, the wire can be made to circulate substantially along the axis of a tube into which the coolant is injected under pressure into the cylindrical space between the wire and the inner wall of the tube.

The value of the parameters giving the intensity and the duration of the cooling depends on the diameter and the chemical composition of the wire, as well as on its temperature at the entry of this first cooling step.

For example, in the case of a wire rod 5.5 mm to 12 mm in diameter, made of steel containing 0.6% to 0.9 carbon and heated to 850 ° C - 1050 ° C,, the duration of the first cooling step

OC

00 ra between 0.3 and 5 seconds; this duration corresponds to a speed of the wire from 0.5 to 4 m / s and a length of the cooling device 1 t I - slowing between 0.5 and 2.5 m. If the coolant is water, the desired cooling intensity can be reached with a water temperature between 45 ° C and 85 ° C and a specific flow rate between 50 1 / m2 s and 90 1 / m2 s.

5

According to yet another feature of the invention, the duration of the second cooling step is adjusted so that at the end of this step, at least 95% of the austenite is transformed into perlite.

10

It appeared that the intensity of the cooling during this second stage had to respect two conditions of a contradictory nature; on the one hand, it must be high enough to ensure the evacuation of the heat of transformation and limit the phenomenon of recalescence so as to allow obtaining fine perlite throughout the section of the wire; and on the other hand, it must be low enough to avoid the formation of bainite or. marten-site in the thread.

^ For this purpose, this second cooling step advantageously consists of immersing the wire in an aqueous solution brought to a temperature between 90% and 100% of its boiling point, and preferably maintained at boiling point.

25

In this context, the intensity of the cooling can be modified during the second step by varying the composition and / or the temperature of the aqueous solution in which the wire is immersed.

The cooling time can also be adjusted during the second step by varying the residence time of the wire in the aqueous solution; for a given wire speed, this is equivalent to varying the length of wire immersed in the solution.

As indicated above, this period must be sufficient to ensure the transformation of at least 95% of the austenite into perlite.

35 i i - 9 -

During the tests of implementation of the process, it appeared that it was also possible to carry out this second stage of the treatment in calm air. In this case, the cooling intensity of the first phase must be increased: the calorific flux density and / or the cooling time must be increased. In practice, this consists in modifying in the direction indicated one or more of the parameters listed below: - increasing the water flow rate, - decreasing the water temperature, 10 - decreasing the thread running speed, - increasing the length of the cooling device.

Still according to the invention, the wire is then subjected to a surface treatment whose purpose, known per se, is to deposit on its surface a layer of a substance promoting the subsequent attachment of the drawing lubricant. Among the substances generally used for this purpose and which, moreover, are not part of the subject of the invention, are borax and lime.

In a manner also known per se, this surface treatment comprises a first deposition phase and a second phase of drying of the bonding layer of the lubricant.

In the context of the present invention, the deposition phase consists in immersing the wire in an aqueous solution of said substance having a conventional composition but being brought to its boiling point.

^ According to a feature of the invention, the wire enters said aqueous solution at a temperature above the boiling temperature of the solution.

The process of cooling a body immersed nc in an aqueous bath is well known.

O O

- 10 - of the submerged body. If the body temperature at the time of immersion in the bath is above a certain critical temperature, a vapor film is formed on the surface of the body which prevents direct contact of the body with the bath and therefore brakes strongly cooling it. This period is known as the "caléfaction phase". When the body, as it cools, crosses this critical temperature, the vapor film is broken and the body-bath system is then in "nucleated boiling phase" during which the body can be wetted by the bath aqueous; IQ the cooling exerted during this last phase is faster than during the caulking phase. It may be recalled that this critical temperature depends in particular on the surface condition of the product as well as on the composition and the temperature of the aqueous bath.

15

According to the invention, the wire leaves said aqueous solution at a temperature below the critical temperature defined above.

The wire is then dried in a conventional manner, then coated with the lubrication-drawing bru-20 and finally drawn.

It should however be emphasized that if the drawing constraints allow the wire to exit from the aqueous solution at a relatively high temperature, although below the aforementioned critical temperature, for example at 150-200 ° C, the drying of the wire is greatly facilitated and one can avoid resorting to blowing hot air.

According to a particularly advantageous implementation of the method of the invention, the second cooling step and the first phase of surface treatment of the wire are combined in a single operation.

To this end, the wire, which is at a temperature above the point Ms of steel, is immersed in an aqueous solution containing said substance and maintained at its boiling temperature.

35 - 11 -

This particular implementation has in particular the important advantage of using only one aqueous bath, which leads to a simplification of the installation and consequently a reduction in its cost.

5

In the particular implementation of the invention, the second step of cooling the wire takes place in the surface treatment solution. The wire enters it at a temperature close to 600 ° C., which is the temperature reached by the wire at the end of the inten-10 se and brief cooling of the first step. Upon immersion, a layer of vapor is formed on the surface of the wire which prevents direct contact of the wire with the solution and which regulates heat exchange. It is the period of "caléfaction" recalled above; it thus coincides with the second stage of cooling.

As soon as the temperature of the wire falls below the critical value defined above, the "nucleated boiling" regime appears almost instantaneously and the surface of the wire is brought into contact with the solution. As soon as this phenomenon occurs, the substance contained in the solution is deposited on the wire and the wire can be immediately removed from the solution.

According to the invention, the wire is introduced into said aqueous solution 25 at a temperature in the region of 600 ° C. and it is removed therefrom when its temperature is below the critical temperature corresponding to the passage from heat build-up to nucleated boiling.

In this implementation, the composition and the temperature of the aqueous solution are governed by the conditions of the surface treatment.

In particular, the solution is maintained at its boiling temperature, which ensures a strictly constant temperature and facilitates the evacuation of the heat extracted from the wire by simple condensation of the vapor released.

- 12 -

Since the wire can be removed from the bath immediately after crossing the above critical temperature, the duration of the first phase of surface treatment can be very short. The temperature of the wire at the end of this first phase is significantly higher than the temperature of the solution and the bonding layer can therefore form under very favorable conditions. In addition, drying can be carried out without having to resort to blowing hot air.

Other features and advantages of the process of the invention will become apparent on reading the description which follows, which refers to the appended drawings, in which FIG. 1 schematically illustrates the discontinuous process for manufacturing a drawn wire according to the technique anterior; Figure 2 schematically illustrates the continuous process, according to the invention, for the manufacture of a high speed drawn wire; FIG. 3 shows the evolution of the breaking load, as a function of the temperature of the water in the first cooling step, for two values of the injection pressure, for a wire treated according to the process of the invention. By way of comparison, it also indicates the average level of the breaking load obtained by lead patenting on the one hand and in the raw rolling state on the other hand; Figure 4 illustrates the distribution of hardness along a radius of the section of a wire treated under the conditions of Figure 3; and FIG. 5 illustrates the improvement in the properties of the wire brought by the water patenting according to the invention compared to the raw rolling state.

With reference to FIG. 1, it will be recalled succinctly that the usual process for manufacturing a drawn wire comprises operations of patenting, pickling, surface treatment and finally drawing, separated each time by periods of storage. In this process, the wire is at room temperature - 13 - for all operations performed after patenting. As already indicated in the introduction, the speed ratio between the drawing and the patenting is very high - greater than 15 in the case illustrated in FIG. 1 - and is opposed to any continuation of the process. .

FIG. 2 schematically represents an installation for implementing the continuous process which is the subject of the invention. In practice, the installation will of course include the usual ha-10 equipment for continuous lines, such as welders, input and output accumulators, lubricant applicators, etc., which, for the sake of clarity, are not not shown in Figure 2.

The wire, unwound from the initial coil 1, enters a heating furnace 2 under a reducing atmosphere, where it is brought to a temperature above the transformation point A ^. As soon as it leaves the oven 2, the wire is subjected to a first, intense and brief cooling step, in a device 3 which is preferably constituted by a tube traversed by water. This first cooling step brings the wire temperature down to a value, greater than the Ms point, leading to the desired mechanical properties. This value of the wire temperature is generally between 580 ° C and 650 ° C. The device designated by the reference numeral 4 essentially consists of a tank containing an aqueous solution brought to its boiling temperature. This solution contains, in proportions known per se, a substance intended to facilitate the attachment of the drawing lubricant. This arrangement corresponds to the particular implementation of the invention, in which the second cooling step, comprising the pearl transformation, and the phase 30 of depositing the abovementioned substance are combined in a single operation. The wire enters the aqueous solution at a temperature in the region of 600 ° C as indicated above, yes is clearly higher than the temperature of the solution. It therefore contributes to maintaining the solution at boiling point and thus limiting the consumption of external energy for heating the solution.

* D * - 14 - *

In this solution, the wire is cooled at a relatively low speed during the heat-up period, which allows the transformation of the austenite into fine perlite and the evacuation of the heat of transformation thereby avoiding the phenomenon of recalescence. Shortly after the establishment of the nucleated boiling period mentioned above, which allows the deposition of the substance contained in the solution, the wire leaves the device 4 and is introduced into the drying chamber 5. L The drying operation can be carried out, in a conventional manner, by blowing hot air onto the wire. However, if the wire drawing constraints allow for temperature 1 ′, the wire can leave the device 4 at a temperature of 150-200 ° C. which provides it with natural drying without blowing hot air. After having been supplied with the drawing lubricant in a conventional device not shown, the wire is drawn in 6 to the desired dimension and then wound in 7 into a final coil.

From its entry into the heating oven 2 to the exit of the drying device 5, the wire is kept away from atmospheric air in order to avoid its oxidation ·. For this purpose, it goes through tra-20 to sealed devices 8 disposed between the various items of equipment 2, 3, 4 and 5. It therefore does not require any pickling before it is brought into contact with an aqueous cooling agent.

It should however be emphasized that, during the first cooling-down step (3), the wire undergoes surface oxidation by contact with water vapor. A very thin layer of FeO iron oxide is formed on this occasion, which however must not be removed because, combined with the substance (borax, lime) deposited subsequently, it constitutes an excellent bonding layer for the lubricant of tréfi-30 läge.

Still in FIG. 2> the reference numeral 9 designates a heat exchanger making it possible to adjust the temperature of the cooling water of the first stage; at 10 is shown a condensate ensuring the condensation of the water vapor given off by the boiling solution during the combined operation of cooling and surface treatment. Finally, the pump 11 injects pressurized water into the device 3 of the first step.

Figures 3, 4 and 5 illustrate an example of processing a steel wire in accordance with the present invention.

The treatment concerned a steel wire for prestressing, with a diameter of 12 mm and containing 0.78 C and 0.67% Mn. In its initial state (coil 1, Figure 2), the wire was hot rolled and pickled. The wire was first made austenitic by heating to 880 ° C in 5 min., Under an atmosphere of - 5% l · ^, the transformation point of this steel being about 725 ° C. The first cooling step consisted of quenching for 1.21 s in a stream of water at different temperatures injected under two different pressures. After quenching, the wire temperature equalized to about 620 ° C and the wire was immersed at this temperature in a boiling solution containing about 100 g of borax per liter. After a stay of 25 s in this solution, the wire came out at a temperature of 190 ° C. The drying was carried out by natural evaporation, then the wire was drawn to a diameter of 7 mm, ie with a reduction in section of 66%. The treatment ended with a stress relief annealing at 400 ° C under a tensile stress equal to 40% of the steel breaking load.

FIG. 3 shows that the water patenting process, which constitutes one of the objects of the present invention, makes it possible to achieve a breaking load equivalent to that which is achieved by lead patenting. It also appears that this breaking load, of the order of 1185 MPa to 1240 MPa, is much higher than that which corresponds to the raw rolling state, for which it is on average around 1050 MPa. The two curves in FIG. 3 also show that to reach the desired level of breaking load, the temperature of the cooling water must be higher when the injection pressure increases. For example, a breaking load of 1200 MPa is reached by injecting water at 53 ° C under a pressure of a - 16 - * "Λ V ^ 0.5 bar, while the water temperature must go to 62 ° C if the injection pressure is 1 bar.

For the same wire, FIG. 4 shows the distribution of the hardness HV 5 along a radius, from the center to the surface of the wire, after patenting with water according to the invention. This distribution appears remarkably uniform, which clearly indicates the regularity of the structure of the wire and the great interest of the process of the invention.

FIG. 5 illustrates the improvement in the resistance properties of the above-mentioned wire treated by the method of the invention, compared with the same wire in the raw rolling state. In both cases, the wire was drawn with a reduction in cross-section of 66%, then subjected to a stress-relieving anneal under the conditions indicated above.

15

The arrow AB corresponds to the wire in the raw rolling state (point A) then drawn (point B), and the arrow CD refers to the wire patented with water according to the invention (point C) then drawn (point D ).

* 20 The points A, B, C and D express the values of the two properties indicated, namely the necking and the breaking load, obtained by a tensile test for the wire being in the state corresponding respectively to these four points . The minimum levels of necking (25%) and breaking load (1470 MPa) which are generally imposed on the drawn wire are also plotted in this figure 5.

FIG. 5 clearly shows that the raw rolling and drawn wire (arrow AB) substantially reaches the minimum breaking load required, but that its necking is clearly insufficient. On the contrary, the thread patented with water according to the invention and then drawn (arrow CD) has a necking and a breaking load which are clearly higher than the minimum values imposed.

35

Claims (10)

1. A continuous method of manufacturing steel wire, in which the steel wire is initially unwound from a coil of wire at room temperature and is finally wound up again into a coil of wire, is essentially characterized in that, successively, (a) the wire is heated rapidly to a temperature above its transformation point A ^; IQ (b) the wire is suddenly cooled, in a first step, to a temperature below 700 ° C but above the point Ms, which marks the beginning of the martensitic transformation of said steel; (c) the wire is cooled more slowly, in a second step, so as to ensure the transformation of most of the atenite into perlite; (d) a layer of a substance promoting the attachment of the wire drawing lubricant is deposited on the surface of the wire; and (e) the wire is drawn to its final dimension. 20 2. Method according to claim 1, characterized in that the wire is heated under a protective atmosphere, in particular a slightly reducing atmosphere. 3. Method according to either of claims 1 and 2, charac terized in that the wire is heated to a temperature between 850aC and 1050aC. 4. Method according to either of claims 1 to 3, charac-30 terized in that the wire is heated in a period of time which is not more than 5 min. i 5. Method according to either of claims 1 to 4, characterized in that during the first cooling step, the wire is cooled at a speed at least equal to the critical quenching speed of said steel . fi - 18 - 6. Method according to either of claims 1 to 5, characterized in that said first cooling step is carried out by passing the wire through a stream of coolant circulating in a direction substantially parallel to 5 the longitudinal axis of the wire. 7. Method according to either of claims 1 to 6, characterized in that the duration of the second cooling step is adjusted so that at the end of this step, at least 95% 10 of austenite are transformed into perlite. 8. Method according to either of claims 1 to 7, characterized in that said second cooling step consists in immersing the wire in an aqueous solution brought to a temperature between 90% and 100% of its boiling point, and preferably maintained at the boil. 9. Method according to either of claims 1 to 8, characterized in that the phase of deposition, on the surface of the wire, of a layer of a substance promoting the attachment of the lubricant of trephide. The age comprises a first step of immersing the wire in an aqueous solution of said substance, maintained at its boiling temperature, and a second step of drying the wire. 10. Method according to claim 9, characterized in that said second cooling step and the step of immersing said deposition phase are combined in a single operation of immersing the wire in an aqueous solution of said substance, maintained at its boiling point. 30 Drawings; ...... plates ... JLg ...... pages of which; ....... J ..... cover page .......- Uo description pages .. ........ $} · .. claim pages ......... JL- descriptive abstract Luxembourg, le The representative: Me Alain Rukavina