EP1378577B1 - Process for heat treating cold rolled formable steel strip and steel strip thus obtained - Google Patents

Abstract

Heat treatment of a cold-rolled steel strip involves annealing at a temperature located in the austenitic phase of the steel, cooling to a temperature located in the inter-critical region between the eutectoid and austenitic temperatures, stabilization at the final temperature of cooling, and tempering to ambient temperature. Heat treatment of a cold-rolled steel strip involves heating to an annealing temperature above the A3 temperature, stabilization at the annealing temperature, and tempering at a temperature below the critical temperature of martensitic transformation (Ms). The annealing temperature is above that of austenitic transformation (A3). Between the stabilization and tempering stages, the steel strip is cooled to an intermediate temperature between the A1 and A3 transformation points, and is stabilized at the intermediate temperature. Independent claims are given for: (a) a process for the production of a formable steel strip; and (b) a formable steel strip obtained by the invented processes.

Description

Object of the invention

The present invention relates to the manufacture of cold rolled steel strips, particularly steel strips for forming processes, for example deep drawing.

The invention relates more particularly to a method for the heat treatment of a cold-rolled steel strip, in order to give it a high strength and high formability.

State of the art

In general, the steel strips for forming operations, in particular of high formability, are cold-rolled strips. This type of strip has in fact favorable properties, and in particular good ductility, for the operations envisaged. In general, these properties are the result of well-known thermal annealing and cooling treatments.

It is also known that it is possible to produce steels having on the one hand a high strength and on the other hand a high ductility by giving them a two-phase structure (also called, in English, "dual phase" structure). This structure biphase consists essentially of a soft ferritic matrix, in which are dispersed fine particles of martensite. In schematizing, it is considered that the ferritic matrix ensures the ductility of the steel, while the resistance is given by martensite, which is a phase of high hardness, obtained by a quenching operation.

From a metallurgical point of view, forming the two-phase structure defined above in a steel strip by subjecting it to a suitable heat treatment in several stages including annealing and quenching. According to a known method, the annealing is carried out by heating the steel strip to an annealing temperature located in the intercritical range of the equilibrium diagram of the steel, between the transformation points A1 (eutectoid temperature ) and A3 (minimum temperature at which the austenitic phase γ is the only stable phase of the steel). The steel strip is then maintained at this annealing temperature for a time sufficient to convert the initial ferritic structure of the steel into a mixed ferrite and austenite structure. Quenching is then carried out by quenching the steel strip to a temperature below the transformation temperature of austenite to martensite.

A method of this type is in particular described and claimed in the document WO-A-02/00947 . In this known method, the annealing temperature, located in the intercritical range between the equilibrium points A1 and A3 of the steel, is selected so as to ensure the formation of at most 20% (preferably from 10 to 15%). %) in volume of austenite in the mixed structure of ferrite and austenite, in order to give the steel a high strength without significantly affecting its ductility. We quote temperatures from 725 to 825 ° C. For the quenching temperature, below the critical temperature Ms, the temperatures below 350 ° C., for example the temperatures of 120 to 340 ° C. or the ambient temperature, are mentioned.

The heat treatment of the steel strips according to the method described above is preferably carried out in a continuous process, in an on-line installation. Reasons for investment and congestion on the ground make it necessary to limit the duration of the different steps of the process. Thus, in the known process described in the aforementioned patent application, the heating of the steel strip to the annealing temperature is carried out at a speed of at least 150 ° C./s, advantageously from 150 to 350 ° C. C / s, the maintenance of the steel strip at this temperature does not exceed 20 s and the cooling rate up to the critical quenching temperature Ms is at least 300 ° C / s (for example 300 at 1000 ° C / s).

In the known process which has just been described, it has proved particularly difficult, if not impossible, to reach the thermodynamic equilibrium of the mixed ferrite and austenite structure at the end of the annealing, which makes the unstable process. In practice, it has indeed appeared that this equilibrium could be reached only after a very long period of maintenance of the steel strip at the annealing temperature. This long duration requires the construction of a very long line, representing significant investments in industrial exploitation.

ou au-delà en vitesse moyenne dans un intervalle de température de 600-300°C et à la vitesse de l'équation : $$\mathrm{logCR}\left(\mathrm{°C}\right)\mathrm{=}\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{73}\left[\mathrm{Mn}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{26}\mathrm{Si}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{5}\mathrm{P}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{1}\mathrm{,}\mathrm{3}\mathrm{Cr}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{67}\mathrm{Mo}\left(\mathrm{\%}\right)\right]\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{95}$$

ou au-delà, dans le cas d'ajout de Si, etc. Cependant, la valeur 3,95 de l'équation (2) est modifiée en 3,40 dans le cas d'ajout de B.The document JP-A-60 100 630 discloses a process in which a steel strip containing, by weight, 0.02 to 0.15% C, 0.2 to 3.5% Mn, 0.01 to 0.15% P and not more than 0 , 10% Al and, in addition, at least one or two, selected from group A, consisting of 1-1.5% Si, 0.1 to 1% Cr and Mo respectively and 5-100 ppm B, and the group B consisting of 0.01 to 0.20% V and Ti respectively and 0.01 to 0.10% Nb, the remainder being iron, is subjected to a quench of 3-60 seconds in a temperature range from transformation point Ac3 to Ac3 + 50 ° C. The steel strip is then cooled in the biphasic region of ferrite and austenite in the temperature range from transformation point Ar3 to transformation point Ar1 and is maintained for at least 20 seconds at this temperature. The strip is then cooled to the critical cooling rate CR (° C / sec) of the equation: $$\mathrm{logCR}\left(\mathrm{°\; C}\right)\mathrm{=}\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{73}\left[\mathrm{mn}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{5}\mathrm{P}\left(\mathrm{\%}\right)\right]\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{95}$$

or above in average velocity in a temperature range of 600-300 ° C and at the speed of the equation: $$\mathrm{logCR}\left(\mathrm{°\; C}\right)\mathrm{=}\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{73}\left[\mathrm{mn}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{26}\mathrm{Yes}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{5}\mathrm{P}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{1}\mathrm{,}\mathrm{3}\mathrm{Cr}\left(\mathrm{\%}\right)\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{67}\mathrm{MB}\left(\mathrm{\%}\right)\right]\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{95}$$

or beyond, in the case of adding Si, etc. However, the value 3.95 of equation (2) is modified in 3.40 in the case of the addition of B.

Goals of the invention

The aim of the invention is to remedy this drawback of the known method described above, by providing a new process for the heat treatment of a cold-rolled steel strip, which confers on it reproducible mechanical strength properties, particularly high ductility and high strength, in online industrial installations of reduced size.

Main characteristic elements of the invention

The invention relates to a method for heat treating a cold-rolled steel strip, comprising heating to an annealing temperature above A3, stabilizing at said annealing temperature and quenching to a temperature below the Martensitic transformation critical temperature Ms, the method being characterized by selecting an annealing temperature higher than the austenitic transformation temperature A3 and in that between the annealing temperature stabilization and quenching the steel strip is cooled to an intermediate temperature in the intercritical range between transformation points A1 and A3 and is stabilized at said intermediate temperature.

In the process according to the invention, the temperatures A1 and A3 are well known in the iron and steel environment. The temperature A1 corresponds to the temperature of the eutectoid of the equilibrium diagram of the iron-carbon alloy. It is about 725 ° C but may vary depending on the alloying elements of the steel and the heating rate of the steel strip to the annealing temperature. The temperature A3 is the minimum temperature at which the phase γ is the only stable phase of the steel. It depends on the composition of the steel, particularly its carbon content, and the heating rate up to the annealing temperature.

The critical temperature Ms of martensitic transformation is also well known in the iron and steel industry. It is the limit temperature under which, during rapid cooling of the steel from the annealing temperature, the austenite is transformed into martensite without formation of perlite. This critical temperature and the cooling rate required for transformation to martensite depend on the steel composition and the annealing temperature.

According to the invention, the annealing temperature is higher than the temperature A3 of the steel, so that the treated steel is then in the austenitic range of the phase equilibrium diagram of the steel. The choice of the optimum temperature of the annealing will depend on the composition of the steel, in particular its carbon content. In the case of a low alloy steel, comprising from 0.08 to 0.15% by weight of carbon, the annealing temperature is generally greater than 830 ° C and is advantageously between 850 and 880 ° C.

The heating of the steel strip to the annealing temperature is advantageously carried out at high speed, so as to reduce the size of the annealing installation and to optimize the crystal structure of the steel. Speeds above 100 ° C / sec are recommended, with speeds of 100 to 300 ° C / sec being preferred. The heating of the steel to the annealing temperature can be carried out in an oven heated by any conventional heating means. This may for example include natural gas heating, oil heating or induction electric heating. Such ovens are well known in the art.

After heating to the annealing temperature, the steel strip is subjected to a stabilization step. During this period, the steel strip is maintained at the annealing temperature for a time sufficient to convert the steel to austenite and form a homogeneous austenite. The duration of this stabilization step depends on the temperature at which it is carried out and the composition of the steel. This stabilization step being performed in a thermostatically controlled oven, it is advantageous to make it as short as possible, for reasons of space and investment. In practice, it is generally maintained under 1 minute, the durations of 1 to 20 s generally suitable.

According to the invention, stabilization at the annealing temperature is followed by cooling to an intermediate temperature in the intercritical range. By definition, the intercritical domain is the equilibrium domain of steel, located between the temperature A1 and the temperature A3. It corresponds to the coexistence domain of an α ferritic phase and a γ austenitic phase. The choice of the optimum value for the intermediate temperature will depend on the desired properties for the steel strip subjected to heat treatment. In general, it is chosen so that at equilibrium there corresponds at most 20% (for example from 5 to 20%) by weight of austenite, the values of 10 to 15% by weight of austenite generally being suitable. good. This optimum temperature depends on the temperature A3 of the steel in question and, consequently, on the composition thereof and can easily be determined from the equilibrium diagram of the steels. In practice, temperatures of 600 to 750 ° C are suitable in most cases.

The cooling to the intermediate temperature is generally carried out using gas cooling ("gas jet cooling"). We do it advantageously at a speed greater than 50 ° C / s, for example 100 to 150 ° C / s.

The cooling of the steel strip to the intermediate temperature is followed by stabilization at this temperature. Stabilization at the intermediate temperature consists in maintaining the steel strip at the selected intermediate temperature for a time sufficient to transform a fraction of the austenite into ferrite and reach the thermodynamic equilibrium of the two phases in the presence (ferritic and austenitic). The duration of stabilization at the intermediate temperature is generally greater than 2 s. In practice, it is usually at least 5 s. It is not beneficial for it to exceed 25 seconds. The durations of 5 to 15 s are generally well suited. The stabilization phase at the intermediate temperature is preferably carried out in a low inertia furnace, also called a tunnel furnace.

After stabilization at the intermediate temperature, the steel strip is quenched. This consists in cooling the steel strip to a temperature below the critical temperature Ms, with a cooling rate sufficient to convert the austenitic phase to martensite without forming perlite. The choice of the optimum values for the quenching temperature and the cooling rate will depend in particular on the composition of the steel and the intermediate temperature. These optimum values can easily be determined by laboratory tests. In practice, a quenching start temperature is chosen between 600 and 750 ° C. and a cooling rate greater than 100 ° C./s. The quenching may for example be carried out by spraying a mist of water and compressed air or by immersion in a hot water bath (for example example at a temperature of 70 to 90 ° C). For cooling to the quenching temperature, speeds of 300 to 1000 ° C / s are especially recommended.

In one embodiment of the process according to the invention, quenching is carried out by immersing the steel strip in a galvanizing bath, ideally a molten zinc bath at a temperature of 460 to 500 ° C.

In another particular embodiment of the process according to the invention, after quenching, the steel strip is subjected to an overaging operation. compatible with galvanization, intended to precipitate the soluble carbon formed during the transformation of austenite to martensite. This operation reduces the sensitivity of the steel to aging and improves its ductility.

Compared with the known method described above, the method according to the invention makes it possible to give cold-rolled steel strips optimum mechanical properties, which are reproducible, in online industrial installations, which do not require a space requirement and an excessive investment. This advantageous result of the process according to the invention, in particular the reproducibility of the desired properties, is attributable, on the one hand, to the choice of an annealing temperature which places the thermodynamic equilibrium point of the steel in the austenitic zone. and, on the other hand, to precede quenching with cooling to an intermediate temperature which places the representative point of the steel in the intercritical domain. The inventors have found that this advantageous result of the process according to the invention originates in the rate of transformation and diffusion of carbon in the steel during the annealing and during the intermediate cooling which follows it. In particular, the inventors found that in the process according to the invention, the direct conversion of a unit ferritic phase into a unit austenitic phase, by heating, followed by a transformation of said. austenite unit phase in a mixed phase of ferrite and austenite by cooling requires less time than a direct conversion of a unit ferritic phase into a mixed phase of ferrite and austenite by heating (known process described in document WO-A-02/00947 ).

The method according to the invention applies to all low carbon steel strips of the two-phase (or "dual phase") type defined above. It is especially suitable for 0.3 to 2 mm thick cold-rolled LC ('Low Carbon') steel strips for forming as used in the manufacture of components used in the automotive industry. More particularly, the heat treatment method according to the invention is particularly applicable to steel strips with low carbon and manganese contents, in particular to steel strips containing from 0.06 to 0.2% by weight of carbon and 0.6 to 2.0% by weight of manganese. Examples of LC steels for which the process according to the invention is well suited comprise from 0.08 to 0.15% by weight of carbon and from 0.6 to 1.5% by weight of manganese. The silicon content is preferably less than 0.5% by weight, for example between 0.01 and 0.4% by weight.

The cold rolled steel strip used in the heat treatment process according to the invention generally comes from a slab manufactured in a hot rolling mill.

Accordingly, the invention also relates to a method of manufacturing a steel strip suitable for forming, wherein a steel slab is subjected to hot rolling at a temperature in the zone. austenitic steel, the hot rolled sheet is subjected to a winding at a temperature of 680 to 750 ° C, then to a cold rolling with a reduction rate of 50 to 80% and then subject the sheet recovered from cold rolling to a heat treatment according to the invention.

According to the method of manufacturing steel strips suitable for forming according to the invention, steel strips are manufactured for the automotive industry.

Brief description of the figures

The figure 1 is a diagram schematizing the different operating steps of two distinct processes for the heat treatment of a mild steel strip with a two-phase structure, intended for forming.

The figure 2 is a diagram showing the mechanism of formation of austenite with ordinate the volume fraction indicated by P, from a homogeneous ferritic structure (curve 11) or from a homogeneous austenitic structure (curve 12).

In these figures, the same reference notations have identical meanings.

Detailed description of the invention

To the figure 1 , the abscissa represents the time and the ordinate scale represents the temperatures. The line designated as a whole by the reference numeral 1 represents the heat treatment process of the state of the art. The line designated as a whole by the reference numeral 5 represents a thermal treatment method according to the invention.

• un chauffage 2 de la bande d'acier jusqu'à une température de recuit T₁ située dans la zone intercritique (entre les températures A1 et A3) de l'acier traité;
• une stabilisation 3 de l'acier à la température de recuit T₁;
• une trempe 4 jusqu'à la température ambiante, à une vitesse supérieure à la vitesse critique de transformation de l'austénite en martensite.

The method according to the state of the art, schematized by line 1 to figure 1 includes the successive steps. following

• heating 2 of the steel strip to an annealing temperature T ₁ located in the intercritical zone (between the temperatures A1 and A3) of the treated steel;
• a stabilization 3 of the steel at the annealing temperature T ₁ ;
• quenching 4 to room temperature, at a rate above the critical transformation rate of austenite to martensite.

• un chauffage 6 de la bande d'acier jusqu'à une température de recuit T₂ située dans la phase austénitique (au-dessus de la température A3) de l'acier traité ;
• une stabilisation 7 de l'acier à la température de recuit T₂;
• un refroidissement 8 jusqu'à une température T₃ située dans la zone intercritique (entre les températures A1 et A3) de l'acier traité;
• une stabilisation 9 de l'acier à la température T₃;
• une trempe 10 jusqu'à la température ambiante, à une vitesse supérieure à la vitesse critique de transformation de l'austénite en martensite.

The process according to the invention, schematized by line 5 to figure 1 , includes the following operating steps:

• heating the steel strip to an annealing temperature T ₂ in the austenitic phase (above the temperature A3) of the treated steel;
• a stabilization 7 of the steel at the annealing temperature T ₂ ;
• a cooling 8 to a temperature T ₃ located in the intercritical zone (between the temperatures A1 and A3) of the treated steel;
• a stabilization 9 of the steel at the temperature T ₃ ;
• quenching to room temperature, at a rate above the critical transformation rate of austenite to martensite.

To the figure 2 the abscissa is time and the ordinate is the austenite content of steel. Line 11 corresponds to the heat treatment according to the state of the art (shown schematically by line 1 to figure 1 ) and line 12 corresponds to the heat treatment according to the invention (shown schematically by line 5 to figure 1 ).

In the heat treatment schematized by line 11, the steel initially in the equilibrium zone of the ferrite is heated to the annealing temperature T ₁ located in the intercritical zone of the equilibrium diagram (between the temperatures A1 and A3) . A fraction of the ferrite turns into austenite and a substantial time t ₁ is required to reach equilibrium.

In the heat treatment schematized by line 12, the steel is first heated to the annealing temperature T ₂ located in the equilibrium zone of the austenite (higher than the temperature A3 of the equilibrium diagram). The time required to obtain a transformation of all the ferrite into austenite is very short and represented by t ₂ . Then, the steel is cooled to the intermediate temperature T ₃ located in the intercritical zone of the equilibrium diagram and maintained at this temperature the time required to obtain a transformation of a fraction of austenite to ferrite.

We observe on the diagram of the figure 2 that with the process according to the invention, the total time t ₃ necessary to obtain the stable two-phase structure of ferrite and austenite is clearly less than the time t ₁ which is necessary in the method of the state of the art.

Description of some examples of execution

The examples described below will show the interest of the process according to the invention.

In each of the examples, a strip of low alloy steel (0.1% by weight of carbon and 1.25% by weight of manganese) 0.7 mm thick, from a cold rolling mill, was used. . The steel strip has been subjected to a heat treatment, in order to give it a two-phase structure (or "dual phase") comprising martensite grains dispersed in a ferrite matrix.

Example 1 (reference test)

The steel strip has been subjected to a heat treatment according to the state of the art, shown schematically by the line 1 to the figure 1 . This heat treatment consisted of rapid heating for a few seconds up to an annealing temperature of 800 ° C, stabilization at that temperature for 3 minutes and quenching to room temperature.

• YS désigne la limite élastique exprimée en mégapascal (MPa) ;
• TS désigne la charge de rupture à la traction, exprimée en mégapascal (MPa) ;
• Eltot désigne l'allongement total à la rupture par traction, exprimé en % de la longueur initiale de l'éprouvette ;
• F désigne une phase de ferrite ;
• M désigne une phase de martensite ;
• P désigne une phase de perlite.

At the end of the test, the mechanical properties of the steel strip were measured and the crystal structure of the steel was analyzed. The results are shown in Table 1 below, in which

• YS is the elastic limit expressed in megapascal (MPa);
• TS is tensile tensile strength, expressed in megapascals (MPa);
• Eltot denotes the total tensile elongation, expressed as a% of the initial length of the specimen;
• F denotes a ferrite phase;
• M denotes a martensite phase;
• P denotes a phase of pearlite.

Example 2 (reference test)

The steel strip has been subjected to a heat treatment according to the state of the art, shown schematically by the line 1 to the figure 1 . This heat treatment consisted of rapid heating for a few seconds up to an annealing temperature of 800 ° C, stabilization at this temperature for 3 minutes followed cooling in boiling water (2 seconds) to a temperature of 641 ° C and quenching to room temperature.

The mechanical properties and structure of the steel at the end of the test are shown in Table 1 below.

Example 3 (reference test)

The steel strip has been subjected to a heat treatment according to the state of the art, shown schematically by the line 1 to the figure 1 . This heat treatment consisted of rapid heating for a few seconds up to an annealing temperature of 800 ° C, stabilization at this temperature for 3 minutes followed by cooling in air (14 seconds) to a temperature of 516 ° C and quenched to room temperature.

The mechanical properties and structure of the steel at the end of the test are shown in Table 1 below.

Example 4 (in accordance with the invention)

The steel strip has been subjected to a heat treatment according to the invention, schematically represented by the line 5 to the figure 1 . This heat treatment consisted of rapid heating for a few seconds up to an annealing temperature of 865 ° C., stabilization at this temperature for 40 seconds, rapid intermediate cooling at a speed of greater than 100 ° C./s up to at a temperature of 650 ° C., stabilization at 650 ° C. for 10 seconds and quenching to room temperature.

The mechanical properties and structure of the steel at the end of the test are recorded in the table 1 below. Table 1 Example N ° YS TS YS / TS Eltot microstructure 1 576 951 0.61 9 55% F + 45% M 2 276 545 0.51 15 91% F + M 3% + 6% P 3 378 726 0.52 16 91% F + 9% H 4 313 633 0.58 17 91% F + 9% H

A comparison of the results of Examples 1 to 3 shows that in the method of the prior art a stable two-phase structure of ferrite and martensite requires a long-time stabilization at the annealing temperature (3 minutes), followed by a preliminary cooling slow (14 seconds) before quenching. In the absence of this slow preliminary cooling, the structure obtained is not stable or reproducible or contains an undesirable perlite phase, detrimental to the mechanical properties of the steel strip. The method of the invention makes it possible to obtain the desired characteristics in terms of stability of the microstructure and mechanical properties, without requiring a long annealing period or slow cooling before quenching.

Claims (10)

1. Method for heat-treating a cold-rolled steel strip with low carbon content in order to give it a dual phase structure with martensite grains dispersed in a ferrite matrix, comprising, by weight, from 0.06 to 0.2% carbon, from 0.6 to 2% manganese and less than 0.5% silicon, preferably between 0.1 and 0.4% silicon, said strip having a thickness of between 0.3 and 2 mm, said method comprising:

   - heating up to an annealing temperature higher than the temperature A3 and stabilising at said annealing temperature for a period of less than one minute;

   - cooling at a speed greater than 50°C/s to an intermediate temperature located in the intercritical range between the transformation points A1 and A3, and stabilising at said intermediate temperature for a period of 5 to 15 seconds; and

   - quenching to a temperature below the critical temperature Ms of martensite transformation in such a way that, during the quenching process of the steel strip, it is immersed in a galvanisation bath maintained at a temperature of between 460 and 500°C.
2. Method according to Claim 1, characterised in that a temperature of between 600 and 750°C, preferably 650°C, is selected as intermediate temperature.
3. Method according to Claim 1 or 2, characterised in that an annealing temperature higher than 830°C is selected.
4. Method according to Claim 3, characterised in that an annealing temperature of between 850 and 880°C is selected.
5. Method according to any one of Claims 1 to 4, characterised in that heating up to the annealing temperature is achieved at a speed greater than 100°C/s.
6. Method according to Claim 5, characterised in that heating up to the annealing temperature is achieved at a speed of between 100 and 300°C/s.
7. Method according to Claim 1, characterised in that stabilising at the annealing temperature takes from 1 to 20 s.
8. Method according to Claim 1, characterised in that cooling from the annealing temperature to the intermediate temperature is achieved at a speed of between 100 and 150°C/s.
9. Method according to any one of Claims 1 to 8, characterised in that an LC steel strip with from 0.08 to 0.15% by weight of carbon and 0.6 to 1.5% by weight of manganese is implemented.
10. Method for manufacturing a steel strip that is suitable for forming, according to which a steel slab is subjected to hot rolling at a temperature located in the austenitic zone of steel, the sheet obtained from hot rolling is subjected to winding at a temperature of 680 to 750°C, then to cold rolling with a reduction rate of 50 to 80% and the bar obtained from cold rolling is then subjected to a heat treatment according to any one of Claims 1 to 9.

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