EP2089552B1 - Method for the continuous production of steel wire or bar - Google Patents

Description

Technical area

The invention relates to a method for the continuous production of wire or bar steel.

State of the art

In the production of wire and bar steels there is a continuing need for improved processes, which are optimized in particular in terms of cost, logistics but also with respect to the properties of the products. An important approach for process optimization lies in the shortening of the production process, which can be achieved, for example, by saving heat treatment (see Ball, J. et al., "Process Shortening by Saving Heat Treatments in the Production of Wire and Rod", Stahl and Eisen 11 (1997) No. 4, pp. 59-67 ). In particular, it has been shown that temperature-controlled rolling in certain steels can save on the heat treatment for GKZ annealing or that a better quality annealing structure is available, and that in certain steels the tempering treatment can be omitted. "Energy saving production of medium carbon steel wires with spherical cementite", 22.06.194, discloses a method in which steels having a medium carbon content of 0.2 to 0.5% by weight are processed as follows: a) first, a first hot working by rolling carried out at a temperature between A _c3 + 1000 and A _c3 , wherein the Gesamtumformgrad is at least 50%; b) then the rolling stock is cooled to a temperature between A _c3 + 1000 and A _c3 and a second hot working is carried out, the total degree of deformation being at least 20%; c) thereafter cooled + the rolling stock to a temperature between 1000 and A _c3 A _r1 and performed a third hot forming to the final dimension, wherein the cumulative reduction ratio is at least 25%; d) Finally, the rolling stock is placed in an oven with a temperature between A _c1 and A _c1 - 1000 and held there for at least 30 minutes, after which it is cooled to room temperature.

1. a) zunächst wird eine erste Warmumformung durch Walzen bei einer Temperatur zwischen Ac₃ + 100°C und Ac₃ durchgeführt, wobei der Gesamtumformgrad mindestens 50% beträgt;
2. b) danach wird das Walzgut auf eine Temperatur zwischen Ar₃ + 100°C und Ar₃ abgekühlt und eine zweite Warmumformung durchgeführt, wobei der Gesamtumformgrad mindestens 20% beträgt;
3. c) danach wird das Walzgut auf eine Temperatur zwischen Ar₃ und Ar₁ abgekühlt und eine dritte Warmumformung auf die Endabmessung durchgeführt, wobei der Gesamtumformgrad mindestens 25% beträgt;
4. d) schliesslich wird das Walzgut in einen Ofen mit einer Temperatur zwischen Ac₁ und Ac₁ - 100°C gebracht und dort mindestens 30 Minuten gehalten, um danach auf Raumtemperatur abgekühlt zu werden.

In the CN 1088265 A a generic method is described in which steels with a medium high carbon content of 0.26 to 0.5 wt .-% are processed as follows:

1. a) first, a first hot working by rolling at a temperature between Ac ₃ + 100 ° C and Ac _{3 is} performed, the Gesamtumformgrad is at least 50%;
2. b) then the rolling stock is cooled to a temperature between Ar ₃ + 100 ° C and Ar ₃ and a second hot working carried out, the Gesamtumformgrad is at least 20%;
3. c) thereafter the rolling stock is cooled to a temperature between Ar ₃ and Ar ₁ and a third hot working is carried out to the final dimension, the total degree of deformation being at least 25%;
4. d) Finally, the rolling stock is placed in an oven with a temperature between Ac ₁ and Ac ₁ - 100 ° C and held there for at least 30 minutes, after which it is cooled to room temperature.

The above method is characterized in that the final forming always takes place in the two-phase region and that reheating or, at best, holding at the final rolling temperature is always necessary after final shaping. The transition to the heat retention temperature is thus not effected by the cooling process (see transition from (6) to (7) in FIG Fig. 2 from CN 1088265 A ).

Presentation of the invention

The object of the invention is to provide an improved process for the continuous production of rod or wire steel, which allows a good control of the properties of the process products and at the same time simplifies the process management.

This object is achieved by the method defined in claim 1.

1. a) in einem ersten Verfahrensschritt wird ein Stahl mit einem Gewichtsanteil von 0.02 bis 0.65% Kohlenstoff und bis zu 0.15% Silizium, 0.25 bis zu 1.5% Mangan, bis zu 0.035% Phosphor, bis zu 0.004% Schwefel, bis zu 0.5% Molybdän, bis zu 1.7% Chrom, bis zu 0.25% Kupfer, bis zu 0.008% Bor, bis zu 0.2% Nickel und bis zu 0.25% Vanadium sowie weiteren stahlüblichen Beimengungen auf 1'050 bis 1'200°C erhitzt, anschliessend wird eine erste Warmumformung durch Walzen bei einer Temperatur oberhalb der Rekristallisationstemperatur durchgeführt, wobei der Gesamtumformgrad bei der ersten Warmumformung mindestens 60% beträgt;
2. b) in einem zweiten Verfahrensschritt wird das Walzgut unterhalb die Rekristallisationstemperatur abgekühlt und danach eine Endumformung bei einer Temperatur im Bereich des nicht-rekristallisierten Austenits vorgenommen, wobei das Abkühlen derart durchgeführt wird, dass im Walzgut vor der Endumformung ein mögliches Kornwachstum des Austenits verhindert wird und wobei der Gesamtumformgrad bei der Endumformung mindestens 30% beträgt; und
3. c) in einem dritten Verfahrensschritt wird das Walzgut ohne Wiedererwärmung bis zu einer vorbestimmten Warmhaltetemperatur um die Ar₁ Temperatur abgekühlt und während einer vorbestimmten Warmhaltezeit bei der besagten Warmhaltetemperatur belassen.

The process according to the invention comprises the following process stages:

1. a) in a first process step, a steel with a weight fraction of 0.02 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.5% Manganese, up to 0.035% phosphorus, up to 0.004% sulfur, up to 0.5% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.25% vanadium as well further admixtures customary in the steel are heated to from 1050 to 1200 ° C., after which a first hot working is carried out by rolling at a temperature above the recrystallization temperature, the total hot working degree being at least 60% in the first hot working;
2. b) in a second process step, the rolling stock is cooled below the recrystallization temperature and then a final deformation at a temperature in the region of the non-recrystallized austenite, wherein the cooling is carried out such that in the rolling stock before the final forming a possible grain growth of austenite is prevented and the total degree of deformation in the final forming is at least 30%; and
3. c) in a third process step, the rolling stock is reheated without reheating to a predetermined holding temperature by the Ar ₁ temperature and left for a predetermined holding time at said holding temperature.

Surprisingly, it has been found that the products produced by the process according to the invention after cooling to room temperature are suitable for further cold processing without annealing. Unlike the conventional method, it is therefore not necessary to reheat the wire or bar steel produced. In addition, it has been found that the heat retention time to be applied in the third method step of the method according to the invention is significantly shorter than the duration of the subsequent heat treatment in the case of the conventional methods. As a result, the process duration and the process chain in the process according to the invention are thus significantly shorter than in the conventional processes.

Unlike the in CN 1088265 A described method only two instead of three forming steps are required in the inventive method. This first of all ensures that only one controlled intermediate cooling is required instead of two such cooling operations between the forming steps. This simplifies the process and the required facility. Moreover, in the process according to the invention, the transition to the holding temperature takes place directly in the course of the cooling process. Reheating to the holding temperature is therefore not required, which not only simplifies the process, but also allows energy savings.

Preferred embodiments of the method are defined in the dependent claims.

In particular, the method according to the invention is suitable, on the one hand, for steels having a comparatively low proportion by weight of 0.02 to 0.2% carbon and up to 0.25% molybdenum and up to 0.008% boron; On the other hand, it is also suitable for steels with a medium to high weight content of 0.2 to 0.65% carbon and 0.25 to 0.50% molybdenum and up to 1.7% chromium. The holding temperature must be selected as explained below.

In the preferred process of claim 2, the steel has a weight fraction of 0.02 to 0.2% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.025% phosphorus, up to 0.004% sulfur, up to 0.25% molybdenum, bis to 0.1% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel, and up to 0.1% vanadium, the heat hold temperature being essentially at Ar ₁ ^e .

In contrast, the steel in the preferred method according to claim 3, a weight fraction of 0.2 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, 0.25 to 0.50% molybdenum up to 1.7% chromium, up to 0.25% copper, up to 0.2% nickel and up to 0.1% vanadium, wherein the holding temperature is substantially in the range of Ar ₁ ^b to Ar ₁ ^e .

Here, Ar ₁ ^b and Ar ₁ ^e denote in a known manner the beginning (upper index "b") and the end (upper index "e") of the austenite-pearlite transformation in the cooling direction (where "r" stands for "refroidissement" ).

According to a preferred embodiment, the final deformation is carried out at a temperature above Ar ₃ . In particular, according to claim 4, the final transformation is carried out at most Ar ₃ + 60 ° C. As a result, the required holding time can be reduced to about half compared to the known rolling process.

According to a further embodiment, the final forming is carried out at a temperature just below Ar. ₃ In particular, according to claim 5, the final deformation is carried out at a temperature in the range of Ar ₁ to Ar ₃ , wherein the final deformation temperature does not fall below 690 ° C. This allows a further shortening of the required holding time to about one-eighth of the known rolling process.

Brief description of the drawings

Fig. 1

eine Vorrichtung zur kontinuierlichen Herstellung von Draht- oder Stabstahl, in schematischer Darstellung, in Draufsicht;

Fig. 2

ein Temperatur-Zeit-Profil für ein Walzverfahren nach dem Stand der Technik;

Fig. 3

ein Temperatur-Zeit-Profil für eine erste Ausführungsform des erfindungsgemässen Walzverfahrens;

Fig. 4

ein Temperatur-Zeit-Profil für eine zweite Ausführungsform des erfindungsgemässen Walzverfahrens; und

Fig. 5

Gefüge abgeschreckter Proben nach dem letzten Stich in Abhängigkeit von der Endwalztemperatur (Stahl C).

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1

a device for the continuous production of wire or bar steel, in a schematic representation, in plan view;

Fig. 2

a temperature-time profile for a rolling process according to the prior art;

Fig. 3

a temperature-time profile for a first embodiment of the inventive rolling process;

Fig. 4

a temperature-time profile for a second embodiment of the inventive rolling process; and

Fig. 5

Structure of quenched samples after the last pass depending on the final rolling temperature (steel C).

Ways to carry out the invention

The in the Fig. 1 The rolling device shown has unspecified propulsion means for a steel strand 2, which convey it in a propulsion direction V through the device. As will be explained in more detail below, certain assemblies of the rolling apparatus are used only for certain final dimensions, or depending on the final dimension, a certain function is taken over by one or the other assembly. First, however, the rolling process will be described generally.

Starting from a furnace 4 with a temperature of 1'050 to 1'200 ° C, the steel strand 2 passes to a first hot-forming device 6, where a first hot working takes place by rolling at a temperature above the recrystallization temperature. Depending on the final dimension, the first hot deformation takes place by six to twenty stitches at a temperature above 950 ° C.

Subsequently, the steel strand passes through a first cooling device 8 (for bar steel) or 8a (for wire), by means of which it is cooled below the recrystallization temperature. Thereafter, the steel strand passes into a second hot working apparatus 10 (for bar steel) and 10a (for wire), where a final forming is carried out at a temperature in the region of the non-recrystallized austenite.

Finally, the beam strand is optionally supplied to a wire station 14 or to a bar station 16. These stations are equipped with second cooling devices 18 (for wire) and 20 (for bar steel) and with not shown holding devices for the cut-to-length rolled product in the form of wire rings or rods.

In the production of bar steel, the first hot rolling device 6 is formed by a first stand group 22. In the case of a comparatively thick steel bar, the first cooling device 8 is a direct water cooling system 24 in the area of the first stand group 22. For comparatively thin bar steel, a waiting loop 26 with continuous cooling 28 is used as the first cooling device 8. If necessary, both cooling variants can be used. The second hot rolling apparatus 10 may be formed by a downstream second stand group 30 as in the arrangement shown here. Subsequently, the rolling stock reaches the bar station 16, which as mentioned includes a second cooling device 20.

In the production of wire with a diameter of, for example, up to 22 mm, the first hot rolling device 6 comprises the first stand group 22 and the second stand group 30. The first cooling device 8a is configured as a downstream waiting loop 32, which advantageously comprises a water cooling 34. The second hot rolling apparatus 10 a is formed by a downstream third skeleton group 36. Subsequently, the rolling stock reaches the wire station 14, which includes a second cooling device 18 as mentioned.

The Fig. 2 to 4 show on the basis of the relevant temperature-time profiles a comparison between a previously known rolling process ( Fig. 2 ) and two variants of the process according to the invention ( Fig. 3 and 4 ).

In the known method according to Fig. 2 The steel strand is first heated to about 1'150 ° C. Thereafter, a hot working is carried out by rolling at a temperature above 950 ° C, which is above the recrystallization temperature. In the example shown this is done by 6 stitches. Subsequently, the rolling stock, depending on the final size or rolling station, within 5 to 10 s at 820 to 920 ° C and then cooled at a cooling rate of 0.3 to 1.9 ° C / s to room temperature. The product thus obtained is not suitable for cold processing, but it requires a heat treatment lasting several hours. For example, the product must be reheated to a temperature of 680 to 720 ° C and typically held at that temperature for about 14 hours. The product treated in this way can finally be cooled to room temperature and is then available for cold further processing.

In the procedures of Fig. 3 and 4 First, a first hot working is carried out above the recrystallization temperature, wherein the degree of deformation in this process step is at least 60%. Subsequently, however, the rolling stock - unlike the previously described method of Fig. 2 - Initially cooled below the recrystallization temperature and then made a final deformation at a temperature in the region of the non-recrystallized austenite. The total degree of deformation in the final forming is at least 30% and is accomplished in the examples shown by two stitches. It is essential that the cooling below the recrystallization temperature is carried out in such a way that no grain growth of the austenite takes place in the rolling stock before the final shaping.

In the variant of Fig. 3 the final transformation takes place at a temperature which is above Ar ₃ ; in the variant of Fig. 4 the final rolling temperature is even lower and is just below Ar ₃ in the range between Ar ₁ and Ar ₃ . As in the Fig. 3 and 4 indicated schematically, could the Endumformung Depending on the rolling station, dimension or rolling speed cause re-heating of the rolling stock. This circumstance must be taken into account, ie it is necessary to avoid that the temperature temporarily rises again above the desired final rolling temperature.

In a third process step, the rolling stock is substantially cooled to the Ar ₁ temperature, for example at 680 to 720 ° C, and then left at this holding temperature for a certain holding time. A rewarming process is accordingly not required. Finally, the product is cooled to room temperature.

The keep-warm time is determined in preliminary tests. It is chosen so that the properties of the products after cooling to room temperature are comparable to those after conventional rolling and additional heat treatment. This ensures that the product is cold processed without further heat treatment.

As from the comparison of Fig. 3 and 4 can be achieved by lowering the final rolling temperature combined with a specific modified cooling process to achieve a shortening of the required holding time, which in the case of Fig. 3 about 7 hours and in the case of Fig. 4 only 2 to 4 hours.

It is understood that to set the described conditions, the rolling device is to be dimensioned or set up accordingly. In particular, the controlled cooling processes are realized by means of water cooling and possibly by means of waiting loops. In addition, depending on the type and dimension of the product to be produced (ie wire steel, steel bars) different numbers and positions of the rolling stands are selected. Keeping warm takes place - also depending on the type of product - in suitable holding facilities.

Basically, the method described for steels with a weight fraction of 0.02 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.5% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, up to 0.5% molybdenum, up Use 1.7% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.25% vanadium and other common steel admixtures. Three examples of such compositions are given in Table 1. <u> Table 1: </ u> Steel compositions in wt% stole C Si Mn P S Ni Cr Not a word al Cu B N A 12:09 12:08 12:36 0004 0005 12:06 12:06 0008 0026 12:07 - 0011 B 12:21 12:05 0.97 0005 0009 12:05 12:03 0007 0021 12:08 0003 0008 C 12:43 12:08 0.74 0005 0030 12:06 1:07 0215 0021 12:08 - 0009

Examples

The following examples were carried out on a pilot plant. In all cases, the minimum required heat treatment time or holding time was determined in preliminary tests, which leads to a cold processable product. However, since at the end no wire rings or long bars are laid down on the test facility, the necessary heat treatment times or holding times in the present examples are significantly shorter than in a corresponding production facility. Accordingly, the following heat treatment times or holding times are about 3.5 times shorter than on a production line.

Comparative Example 1 (prior art)

A steel with the chemical composition "A" according to Table 1 was heated to 1'150 ° C and then rolled in 6 passes, the initial diameter was 38 mm and the final diameter was 17.2 mm. This matches with a total degree of deformation of 80%. The final rolling temperature was approx. 980 ° C. Subsequently, the rolling stock was cooled within 6 seconds to 820 to 960 ° C and then with a cooling rate of 0.3 to 1.9 ° C / s (depending on the rolling station) to room temperature. The product thus obtained was then subjected to a heat treatment at 680 ° C for 4 hours.

example 1

A steel having the same composition as in Comparative Example 1 was heated to 1'150 ° C and then rolled in 4 passes, the initial diameter being 38 mm and the final diameter being 23.8 mm. This corresponds to a total degree of deformation at the first hot deformation of 60%. The temperature was after the 4th stitch at about 1'020 ° C. After intensive cooling, the rolling stock was then rolled within 13 seconds in a further 4 passes from 23.8 to 17.2 mm. This corresponds to a total deformation rate of 47% for the final forming. The final rolling temperature was 880 ° C, which is about 20 ° C above Ar ₃ . After the last pass, the rolling stock was cooled to 680 ° C within 6 seconds, which essentially corresponds to Ar ₁ ^e , and left at this temperature for 2 hours.

Example 2

A steel having the same composition as in Comparative Example 1 was heated to 1'150 ° C and then rolled in 4 passes, the initial diameter being 38 mm and the final diameter being 23.8 mm. The temperature was after the 4th stitch at about 1'020 ° C. After intensive cooling, the rolling stock was then rolled within 13 seconds in another 2 passes from 23.8 to 17.2 mm. The final rolling temperature was 850 ° C, which is just under 10 ° C below Ar ₃ . After the last pass, the rolling stock was cooled to 680 ° C in 6 seconds, which essentially corresponds to Ar ₁ ^e , and left at this temperature for 1 hour.

The mechanical properties of steel A rolled products are summarized in Table 2. It can clearly be seen that targeted level conversion to Ar ₃ combined with controlled cooling by Ar ₁ without prior reheating and holding at that temperature can achieve a level of properties comparable to that after conventional forming followed by expensive heat treatment. The required holding time was reduced to half to one quarter of the usual annealing time. The product can be processed cold in this condition. <u> Table 2 </ u>: Mechanical properties of steel rolled products A Comparative Example 1 example 1 Example 2 Tensile strength R _M [MPa] 375 395 392 Fracture Z [%] 76.0 75.8 74.2

Comparative Example 2 (prior art)

A steel with the chemical composition "C" according to Table 1 was heated to 1'150 ° C and then rolled in 6 passes, the initial diameter of 38 mm and the final diameter was 17.2 mm. This corresponds to a Gesamtumformgrad of 80%. The final rolling temperature was approx. 980 ° C. Subsequently, the rolling stock was cooled in 5 seconds to about 820 to 970 ° C and then with a cooling rate of 0.3 to 1.9 ° C / s to room temperature. The product thus obtained was then subjected to a heat treatment at 680 ° C for 4 hours.

Example 3

A steel having the same composition as in Comparative Example 2 was heated to 1'150 ° C and then rolled in 4 passes, the initial diameter being 38 mm and the final diameter being 23.8 mm. This matches with a total degree of deformation at the first hot deformation of 60%. The temperature was after the 4th stitch at about 1'020 ° C. After intensive cooling, the rolling stock was then rolled within 13 seconds in another 2 passes from 23.8 to 17.2 mm. This corresponds to a total deformation rate of 47% for the final forming. The final rolling temperature was 800 ° C, which is above Ar ₃ . After the last pass, the rolling stock was cooled to 680 ° C within 6 seconds, which is between Ar ₁ ^b and Ar ₁ ^e , and left at this temperature for 2 hours.

Example 4

A steel having the same composition as in Comparative Example 2 was heated to 1'150 ° C and then rolled in 4 passes, the initial diameter being 38 mm and the final diameter being 23.8 mm. The temperature was after the 4th stitch at about 1'020 ° C. After intensive cooling, the rolling stock was then rolled within 13 seconds in another 2 passes from 23.8 to 17.2 mm. The final rolling temperature was 690 ° C, which is below Ar ₃ . After the last pass, the rolling stock was cooled to 680 ° C within 6 seconds, which is between Ar ₁ ^b and Ar ₁ ^e , and left at this temperature for 0.5 hour.

Fig. 5 represents the microstructure of quenched samples after the last pass, depending on the final rolling temperature. It can be seen that the final forming in Comparative Example 2 (see Fig. 5a ) and Example 3 (see Fig. 5b ) in metastable austenite. In contrast, this was done in Example 4 (see Fig. 5c ) in the (ya) two-phase region.

The mechanical properties of steel C rolled products are summarized in Table 3. <u> Table 3 </ u>: Mechanical properties of steel rolled products C Comparative Example 2 Example 3 Example 4 Tensile strength R _M [MPa] 695 685 695 Fracture Z [%] 60.9 56.6 57.7

Closing remarks

The above comparison measurements show that the continuous rolling with targeted setting of the final rolling temperature by Ar ₃ combined with a specific modified cooling process and a deliberately selected holding temperature leads to products whose mechanical properties are comparable to those after a conventional heat treatment. In particular, the products can be processed cold without annealing.

Claims (5)

1. A method for the continuous production of steel wire or steel bar, which comprises the following process steps:

   a) in a first process step, a steel with a weight proportion consisting of 0.02 to 0.65% carbon, more than 0 and up to 0.15% silicon, 0.25 to 1.5% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, more than 0 and up to 0.5% molybdenum, more than 0 and up to 1.7% chromium, more than 0 and up to 0.25% copper, up to 0.008% boron, more than 0 and up to 0.2% nickel and up to 0.25% vanadium, more than 0 and up to 0.026% aluminium and further admixtures usually contained in steel is heated to 1'050 to 1'200°C, subsequently a first hot forming is carried out by rolling at a temperature above the recrystallization temperature, wherein the total deformation degree in the first hot forming is at least 60%;

   b) in a second process step, the rolled material is cooled to below the recrystallization temperature and subsequently an end forming at a temperature in the range of non-recrystallized austenite is carried out, wherein the cooling is carried out in such manner that in the rolled material a possible grain growth is prevented before the end forming and wherein the total deformation degree in the end forming is at least 30%; and

   c) in a third process step, the rolled material is cooled without reheating down to a soaking temperature in the range of Ar₁ ^b to Ar₁ ^e around the Ar₁ temperature, and is maintained at the said soaking temperature during a soaking time.
2. The method according to claim 1, characterized in that the steel comprises a weight proportion of 0.02 to 0.2% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.025% phosphorus, up to 0.004% sulfur, up to 0.25% molybdenum, up to 0.1% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel, up to 0.026 aluminium and up to 0.1% vanadium and that the soaking temperature is substantially at Ar₁ ^e.
3. The method according to claim 1, characterized in that the steel comprises a weight proportion of 0.2 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, 0.25 up to 0.50% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.2% nickel, up to 0.026 aluminium and up to 0.1% vanadium and that the soaking temperature is in the range of Ar₁ ^b to Ar₁ ^e.
4. The method according to one of claims 1 to 3, characterized in that the end forming is carried out at a temperature of at most Ar₃ + 60°C.
5. The method according to one of claims 1 to 3, characterized in that the end forming is carried out at a temperature in the range of Ar₁ to Ar₃ with the provision that the end forming temperature does not fall below 690°C.

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