EP0169827A1 - Method for producing high-carbon steel wire rod - Google Patents

Abstract

In the production of medium carbon steel wire rod, upon leaving the hot rolling mill, the rod is cooled in two phases. The first phase is operative as the rod moves at end-of-rolling speed along a cooling line disposed between the finishing block and feed rollers disposed at the entry of a head for placing the rod in overlapping turns on a conveyor, the cooling line being continuous-i.e. being devoid of air cooling breaks between consecutive intensive cooling sections, the length and capacity of the cooling line being such that the surface temperature of the rod at the end of the first phase is between, on the one hand, the start-of-martensitic-transformation temperature for the particular steel concerned and, on the other hand, the latter temperature plus 200 DEG C. The second cooling phase is operative upon the rod once it has been placed in overlapping non-concentric turns on the conveyor, the time which elapses between the end of the first phase and the start of the second phase being less than the time needed for the percentage of transformed austenite to exceed 5%. Austenite transformation is at least 95% at the departure from the second phase.

Description

The present invention relates to a method for manufacturing wire rod of hard steel, that is to say of steel having a carbon content greater than 0.4%; this process according to the invention comprises an original heat treatment phase, applied to the wire as soon as it leaves the hot rolling mill.

This process makes it possible to confer on the wire rod mechanical properties and a homogeneity of properties equivalent to those which one obtains when one practices the patenting operation with lead.

The process according to the invention has the further advantage of eliminating the drawbacks arising from central segregation resulting from the continuous casting operation of the steels; this central segregation is particularly troublesome when the wires are intended to be used after drawing in the form of active reinforcements for prestressed concrete. In fact, since the average carbon content can reach in this case 0, B%, the content of this element in the segregated zones can exceed 1.1%; during normal cooling on modern trains, there is a precipitation of cementite which has a detrimental effect on the wire drawing.

It is known that the final properties of a wire depend essentially on the state in which the wire is found before tackling the last cold deformation operations such as wire drawing. In fact, it is well known that in order to obtain the structure which is ideally suited for drawing, it is necessary to lower the temperature of the start of allotropic transformation and on the other hand to limit the heating due to the allotropic transformation itself which occurs and which carries the recalescence name. The classic way to do this is to subject the wire to lead patenting.

Thanks to this operation and to an appropriate work hardening, it is possible to obtain wires, even of very small diameter, having the desired final mechanical characteristics.

The main drawback of these methods consists in the fact that patenting and possible prepatentage with lead are particularly expensive operations, on the one hand in themselves and on the other hand having regard to the manipulations and the loss of productivity that they cause in a wire drawing.

In order to remedy these drawbacks, various methods have already been devised; very generally, these methods either do not lead to a product whose properties are comparable to those of wires which have undergone a lead patenting or still have implementation disadvantages which do not allow their industrial application.

The methods proposed to achieve the aim (elimination of patenting by heat treatment in the rolling mill) can be classified into two categories depending on whether the controlled cooling is applied online, before the formation of turns, or on the turn conveyor spread.

With regard to the treatment at the level of the water-cooling ramp before the coil-forming head, there is generally encountered a problem which resides in the fact that the time available for applying the imagined means is extremely low, for example of the order of half a second in rolling mill trains with high output speed; it follows that one is forced to apply very intense cooling to lower the temperature in the targeted lines, and this leads to the creation of significant thermal gradients in the section of the wire and a risk of martensitic quenching on the surface.

Among the various technical solutions which have already been proposed to solve this problem, two can be cited which deserve particular attention. The first consists in cooling the wire to a temperature below 600 ° C, by passing through water cooling boxes between which sections of air are inserted allowing a rise in the temperature at the surface of the wire; the entire installation is calculated so as to obtain the desired reduction in the average temperature of the wire while avoiding the formation of martensite on the surface.

This process has so far not been very successful industrially because it presents some implementation difficulties; thus it is necessary to use several water boxes whose length decreases rapidly in the direction of advancement of the wire; on the other hand since it is imperative to stop the water at the outlet of each box in order to allow the heating of the surface, it is necessary to use jet-breaking devices whose use is neither easy nor effective. On the other hand, the optimum configuration of such a cooling installation depends on the diameter of the wire, its carbon content and the properties targeted; it follows that if even the optimum ramp could be achieved, it would only be for one quality and one wire diameter. As modern wire trains laminate a very wide range of products, for most of them, the installation would therefore not have the optimum characteristics desired. Finally, to circumvent the drawbacks which have just been mentioned, the ramp could be constructed in such a way that the surface temperature of the wire is much higher than the point Ms and this for all the diameters and all the qualities of the range of products, but the result of this obligation would be that the length of the cooling ramp would be very much greater than its maximum practical value. This would lead to very high investment costs and difficult operation of the train.

The second recommended system also consists in cooling the wire before depositing, but here admitting the formation of a surface layer of martensite of very limited thickness; this way of proceeding, if it makes it possible to shorten the cooling line and eliminate the drawback of making an installation comprising different cooling boxes and separated by air cooling zones, does not bring any solution to the other disadvantages which have just been mentioned, namely the difficulty of adapting the ramp to the whole range of products to be manufactured; on the other hand, this process involves an additional disadvantage in the continuation of the heat treatment because the recalescence which it is necessary to fight when the wire is spread on the conveyor is all the more important as the temperature has been lowered more sharply from the start of transformation.

If we now consider the second family of proposed solutions which consist in applying the treatment while the wire rod is extended in non-concentric turns on a conveyor, we observe that among the methods which have already been recommended, the. some increase the cooling efficiency by an appropriate choice of the fluid used, others attack the recalescence at the place where it takes place.

Among these different processes, mention may be made of those in which cooling is carried out with blown or aspirated air through turns. According to these methods, there has certainly been a marked improvement in the average value of the properties of the wire and in the dispersion of the measurements around this average, as well as an improvement in the structure sufficient to avoid prepatentage. In the case of wire rod, however, it was not possible to avoid the final lead patenting.

With this in mind, it has also been imagined to carry out the treatment of the wire placed in the form of turns spread out in a fluidized bed. This process brings some additional improvement compared to the previous one, but it presents other difficulties, essentially of a technological nature.

The cooling treatment of the wire by immersion in a bath of molten salt or in a concentrated aqueous solution also has its own drawbacks, in particular the essential subsequent washing of the wire and the need to use fairly specific installations.

In summary, the current technique has not made it possible to recource, this value, the problem posed, namely to obtain a good average value of the properties of a hard wire in each coil of this wire, as well as a dispersion follows the properties of the wire around the mean value. The reason is - except for the supply of air - that the processes considered are often economically and / or technically unusable.

The other methods, applied to the wire spread in non concentric turns if a conveyor, have the objective of eliminating the recalescence by accelerating the cooling at the place of the conveyor where this recalescence takes place; various types of cooling have thus been recommended, for example by water mist, by spraying, by immersion in a tin, etc .; here either the proposed processes have not been the subject of an industrial application, because it has not been possible in practice of app quer in the determined place where the recalescence took place, a selective cooling which is at the same time intense and homogeneous.

The subject of the present invention is precisely a process for the manufacture of hard steel wire rod, by means of which a wire is obtained whose mechanical properties are similar to those obtained when the additional lead patenting operation is carried out, and whose the dispersion of the properties around the mean value, in a coil, is so low that we can consider that these properties are homogeneous.

The process which is the subject of the present invention is essentially characterized in that, on leaving the hot rolling mill, the wire is subjected to cooling comprising two phases, the first being applied to the wire while the latter crosses at the speed of end of rolling a cooling line located between the finishing unit and the roller drive located at the entrance to the deposition head, said cooling line being continuous, that is to say having no air cooling intervals between successive sections of intense cooling, the length of said cooling line and its power being adjusted in such a way that the surface temperature of the wire at the end of this first phase is between the start temperature of the martensitic transformation for the steel considered and this temperature + 200 ° C, in that the second cooling phase is applied to the wire as soon as it is laid out in non-concentrated spreads ique on a conveyor, the time delay between the end of the first phase and the start of the second phase being less than that for which the percentage of transformed austenite exceeds 5%, and in that the transformation of austenite is at least 95% at the end of the second phase.

In a preferred implementation of the method of the invention, the first cooling is carried out by means of a fluid applied using devices allowing a cooling intensity characterized by an average density of heat flow to be achieved. between 3 and 7 MW / m ² .

In a particular way of implementing the method of the invention, the intensity of the cooling during the second phase is between 0.1 MW / m ² and 0.4 MW / m2.

According to the invention, the cooling of the wire spread on the conveyor, during the second phase of the heat treatment, can be obtained either by blowing air, by immersion in boiling water or by any other known means.

The value targeted by the surface temperature (Ts) at the outlet of the first phase is obtained, according to the invention, by choosing an appropriate combination between the values of the length (L) of the cooling line (or the duration) and the average heat flux density (ϕ).

The couple (ϕ- L) chosen, according to the process, will be such that the desired mechanical properties are obtained after the treatment of the second phase. The target breaking load (TS) will be close to the value given by the formula:

• - vitesse de fin de laminage (V) : 22,88 m/s
• - longueur de rampe installée dans le bloc (L₁) : 4 m
• - longueur de la rampe principale de refroidissement à l'eau (L₂) : 39,13 m
• - densité moyenne de flux calorifique au cours de la première phase (ϕ) : 3,58 MW/_m ²
• - coefficient d'échange calorifique dans le dispositif de refroidissement au cours de la deuxième phase (a) : 0,27 kW/m² °C
• - MTT surface : 601°C
• - MTT contre : 626°C
• -MTT : 606°C

In FIG. 1 are represented the cooling curves at the surface (I) and at the center (II) of a wire 12 mm in diameter made of steel at 0.63% C 0.65% Mn, to which the process has been applied of the invention under the following conditions:

• - end of rolling speed (V): 22.88 m / s
• - ramp length installed in the block (L ₁ ): 4 m
• - length of the main water cooling ramp (L ₂ ): 39.13 m
• - average heat flux density during the first phase (ϕ): 3.58 MW / _m ²
• - heat exchange coefficient in the cooling device during the second phase (a): 0.27 kW / m ² ° C
• - MTT surface: 601 ° C
• - MTT against: 626 ° C
• -MTT: 606 ° C

n (a) and (a ') the amount of austenite transformed is 2% while it is 98% in (b) and (b'), that is to say at the end of the second phaae.

This figure also shows that the second objective, elimination of the pro-eutectoid cementite at the core of the segregated wires, is automatically achieved by applying the method. In fact, the temperature at the start of transformation of the heart is lowered below 600 ° C., which prevents precipitation of the pro-eutectoid cementite.

As a practical example of the process of the invention, we will examine below the case of an application in a new rolling mill for which it is a question of determining the distance to be expected between the outlet of the block and the trainer to pebbles; Table I below lists the main data for the installation and the product, while Figure 2 schematically shows the location of the equipment.

We distinguish in this figure in (1) the finisher comprising, following the cages (2), a cooling line (3) of length (L ₁ ) located in place of unused cages, the "break out box""(4), the continuous cooling line (5) of length (L ₂ ), the head (6) for winding and depositing on the conveyor (7), which is provided with a cooling device (8 ) of length (L ₃ ) to ensure the second phase of the treatment.

We can, from this application in special conditions, make the following remarks.

In order to minimize the space requirement of the installation, a cooling section of length L ₁ was installed in place of the unused cages; therefore, L ₁ is a function of the diameter.

It is assumed that the wire is not cooled in the "break-out box". However, given the short length of this device, the interruption of cooling has no metallurgical effect and the cooling ramp behaves like a continuous ramp of length L ₁ + L ₂ .

If the duration of stay of the wire in the air between the two phases of accelerated cooling must be minimal, it cannot however be canceled for technological reasons (roller drive, removal head, fall of the turns, ...) ; it was considered that the minimum stay of the air line corresponded to a straight path of 20 m.

It is assumed that the phase I cooling device is characterized by a constant value of the heat flux density (ϕ). It follows that the exchange coefficient taken into account for the calculations will be of the form:

où T_s est la température superficielle et T_m la température du fluide de refroidissement.

where T _s is the surface temperature and T _m the temperature of the coolant.

It has been found that this hypothesis is in reasonable agreement with experience, at least for values of T _s greater than 350-400 ^D C.

For the cooling during phase II, we assume a Newtonian type cooling (a = const.) And we therefore neglect the differences in cooling intensity between the centers and the nodes of the water table. turns. This assumption is very close to reality in the case Cooling is done by immersion of turns in boiling water. However, these conclusions remain valid at first approximation in the case where the cooling during the second phase is done by blowing air.

• - la valeur minimale de L_Z permettant d'appliquer le procédé à l'ensemble du product mix (tableau I);
• - la valeur de (ϕ) pour chaque diamètre et chaque teneur en carbone du product mix;
• - la longueur minimale de traitement dans la phase II (L₃).

The problem is to determine:

• - the minimum value of L _Z allowing the process to be applied to the entire product mix (Table I);
• - the value of (ϕ) for each diameter and each carbon content of the product mix;
• - the minimum length of treatment in phase II (L ₃ ).

at. Length L ₂

The calculation procedure used is as follows:

• For a given diameter, we consider the most difficult case which corresponds:
  • - at the maximum deposition temperature (105D ° C in the example),
  • - the minimum C content (0.6% in the example), because it is indeed for the minimum value of% C that the point M _s is higher and, therefore, that the minimum surface temperature that can be reached during the first phase is the highest.

We know the relation which exists between T _S and the average temperature of transformation (MTT), this one being defined in a point considered as being the average of the curve T = f (z) giving in this point the variation of the temperature (T) and the percentage of transformed austenite (z):

Now it is clear that MTT is a function of the point considered in the section and, assuming cylindrical symmetry, we can consider that MTT is constant along a circle of radius (r) located in the straight section of the wire, c 'is to say that MTT = MTT (r); on the other hand, experience shows that the breaking load is a linear function of MTT (ie y = a MTT + b).

en posant x = r² et en remplaçant y par sa valeur en fonction de MTT, on obtient :

d'ou

avec

Finally, experience has also shown that in a wire of radius R in which MTT varies with r [and, consequently, y = y (r)], we can apply the additivity rule to find the macroscopic breaking load some thread :

by setting x = r ² and replacing y by its value as a function of MTT, we obtain:

from where

with

This last relation defines MTT for the whole section of the wire.

The last two relations show that MTT is the constant value of MTT in the section and which would give the macroscopic value of TS of the breaking load of the wire.

(désignée par

).The above shows that we can therefore calculate the target value for

(designated by

).

In the example chosen:

If we arbitrarily choose a value of L ₂ , to this value corresponds one and only one value of (ϕ) giving at the end of the transformation, that is to say after the second phase,

FIG. 3 represents all of the couples (L ₂ , ϕ) making it possible to produce MTT in the case of the manufacture of a wire of 7 mm in diameter, made of steel at 0.63% C and 0.65% Mn, laminated with an end-of-rolling temperature (T ₀ ) of 1050 ° C and an exit speed (V) of 82.8 m / s, where the ramp length (L ₁ ) installed in the block is 1 m. This figure 3 also gives, for each pair (L ₂ , ϕ) the value of the minimum surface temperature (T) reached during the first phase. It is clear that the higher the minimum permitted surface temperature, the greater the length L ₂ necessary 5.

This minimum value allowed depends on the quality of the control installed on the train.

+30°. Dans ces conditions, la figure 3 permet de trouver (L₂ min) et le (ϕ) correspondant pour le diamètre de 7 mm, soit en l'occurrence L₂ min = 45,2 m et ϕ = 6,77 MW/m^2. En répétant la même procédure pour tous les diamètres laminés, on établit la variation de L₂ min avec le diamètre (figure 4). On constate que la plus grande valeur de L₂ min correspond, pour l'exemple choisi, au diamètre de 12 mm et c'est cette valeur qui sera choisie pour L₂ (49,1 m). La figure 4 donne également la variation de en fonction du diamètre à laminer.For precise control by computer and thanks to the single ramp, very great governability cannot

+ 30 °. Under these conditions, Figure 3 makes it possible to find (L ₂ min) and the corresponding (ϕ) for the diameter of 7 mm, that is to say L ₂ min = 45.2 m and ϕ = 6.77 MW / m ^2. By repeating the same procedure for all the rolled diameters, the variation of L ₂ min is established with the diameter (FIG. 4). It is found that the largest value of L ₂ min corresponds, for the example chosen, to the diameter of 12 mm and it is this value which will be chosen for L ₂ (49.1 m). FIG. 4 also gives the variation of as a function of the diameter to be laminated.

b. Calculation of ϕ.

Knowing L ₂ , we can calculate, for the other diameters, the values of (ϕ) and the minimum surface temperature:

It can be seen that the security vis-à-vis M _s increases as the diameter decreases; it becomes considerable for the diameter 5.5 mm, because for the latter, the instantaneous mass flow of the train is reduced.

It is also noted that in order to apply the process of the invention under the conditions of the example, it is necessary to be able to produce on the same installation flows of between 3.5 and 6.5 MW / m ² .

The above calculations can be carried out for the higher carbon wires with completely comparable results except with regard to the security with respect to the M _s which will be greater.

vs. Determination of (L ₃ ).

The table below gives for a density of turns of 25 m, the speed of the conveyor, the duration of the treatment necessary for the transformation to be completed at 98% and the corresponding length (L ₃ ):

The largest of the values calculated for L ₃ (7 m) will be retained. If the cooling during the second phase is done by blowing air, it will have to be taken into account that, in this case, the cooling of the nodes is much slower.

The technology used to apply the method of the invention is, as a whole, known.

In the first phase, for example "water cannons" are used of a conventional type and the (ϕ) is adjusted by acting on the supply pressure of these cannons. Water-air cannons can also be used and in this case the adjustment is made by action on the air flow.

Claims (4)

1. A method for manufacturing hard steel wire rod, characterized in that at the exit from the hot rolling mill, the wire is subjected to cooling comprising two phases, the first being applied to the wire while the latter passes through the end of rolling speed a cooling line located between the finishing unit and the roller drive located at the entrance to the head for depositing the wire in coils spread on a conveyor, the said cooling line being continuous, it that is to say having no air cooling intervals between successive sections of intense cooling, the length of said cooling line and its power being adjusted in such a way that the surface temperature of the wire at the end of this first phase is between the temperature at the start of the martensitic transformation for the steel in question and this temperature + 200 ° C., in that the second cooling phase is applied to the wire as soon as it is set. n turns spread non-concentrically on the conveyor, the time delay between the end of the first phase and the start of the second phase being less than that for which the percentage of transformed austenite exceeds 5%, and in that the transformation of austenite is at least 95%, at the end of the second phase. 2. A wire rod manufacturing method according to claim 1, characterized in that the first cooling is carried out by means of a fluid applied using devices allowing to reach a cooling intensity characterized by an average flux density. calorific between 3 and 7MW / m. 3. Method according to either of Claims 1 and 2, characterized in that the intensity of the cooling during the second phase is between 0.1 MW / m ² and 0.4 MW / m ² . 4. Method according to either of claims 1 to 3, characterized in that the cooling of the wire spread on the conveyor, during the second phase of the heat treatment, is ensured by immersion in boiling water.

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