EP2640532B1 - Method for producing seamless hot-rolled pipes in continuous pipe rolling mills - Google Patents

Description

The invention relates to a method for the economic production of seamless hot-rolled tubes in Rohrkontiwalzwerken according to the preamble of claim 1, as known from EP 1 872 878 A1 is known. Various methods used to make hot rolled seamless tubes are in the Steel tube manual (Vulkan-Verlag, Essen, 12th Edition 1995, pp. 107-111 ).

In recent years, it has become increasingly necessary to produce products close to the consumer, because consumer countries want to be involved in job creation and by adding value. This inevitably entails a limitation of the sales market.

Typical products for such cases are, for example, pipes for the energy sector, for exploration and for the extraction of oil and gas.

The common dimension range is between approx. 60 and 273 mm in diameter and with wall thicknesses of approx. 5 to 15 mm.

The required capacity for tube rolling mills is approximately between 100,000 and 250,000 tonnes / year for these products.

• 
  Lochen des massiven Blocks zum Hohlblock,
• 
  Strecken des Hohlblocks zur Luppe und
• 
  Fertigwalzen der Luppe auf die warmfertige Rohrabmessung.

Der Begriff "Luppe" wird gleichbedeutend mit dem Begriff "Mutterrohr" angesehen. Da für den ersten Fertigungsschritt bis auf Sonderfälle Lochschrägwalzwerke zum Einsatz kommen und das Fertigwalzen ausschließlich über Streckreduzierwalzwerke oderSeamless hot-rolled tubes are usually produced in three process steps after heating the round primary material:

• 
  Punching the massive block to the hollow block,
• 
  Stretching the hollow block to the Luppe and
• 
  Finishing of the billet on the ready-to-use pipe dimension.

The term "Luppe" is synonymous with the term "mother tube". Since for the first production step except for special cases, hole-type cross-rolling mills are used and the finish-rolling exclusively by stretch-reducing mills or

Made to measure rolling mills, the tube rolling mills are named as a whole according to the coming to use stretch rolling mill.

Rolling mills having an annual capacity in the above-mentioned range are impact bench mills, Assel mills and Diescherwalzwerke. In the latter two, cross-rolling mills are used for stretching.

The operation of such rolling mills requires a high level of know-how, as it is not easy to produce pipes without external defects or internal defects. Typical pipe defects are z. B. small cracks with sometimes shallow depth. The risk of error increases with thinner wall, which is why the diameter / wall thickness ratio is limited. For example, this is around 20: 1 for Assel rolling mills. In these rolling mills, internal cracks can hardly be avoided, which is why the pipes must be reworked.

This quality disadvantage and the high requirements of the oil and gas industry prohibit the use of these rolling processes for the production of premium products without the need for time-consuming mechanical processing of the inside and outside surfaces of the pipe.

For reasons of quality, longitudinal rolling for the stretching of the hollow block with the continuous tube method has prevailed for these demanding products, in which the hollow block undergoes a reduction in cross section of up to 75% in up to nine closely arranged stands, resulting in an extension to four times the length. The cross-section decrease takes place continuously on the required for the finish rolling Luppenabmessung. Such a method is for example from the EP 1 764 167 B1 known.

• Schrägwalzwerk zum Lochen mit maximalen Hohlblocklängen zwischen 11 und 12,5 m,
• Streckaggregat (z. B. 2- oder 3-Walzen-Rohrkontiwalzwerk mit zurückgehaltener Stange) mit 5 oder 6 Gerüsten,
• Stangenumlauf des Streckaggregates mit 5 bis 8 Stangen, Stangenlängen um 20 m, hiervon etwa die Hälfte als Arbeitsteil zum Walzen, die andere Hälfte um die Distanz zwischen dem eigentlichen Walzwerk und dem Stangenrückhaltesystem zu überbrücken,
• Ausziehwalzwerk, bestehend aus 3 Gerüsten mit jeweils 3 Walzen, um die Luppe von der Walzstange abzuziehen,
• Nachwärmofen,
• Maß- oder Streckreduzierwalzwerk,
• Kühlbett

The possible range of dimensions in the tube contour approach is approximately between 25 and 498 mm in outside diameter, whereby this diameter range can not be covered by a single rolling mill. Apart from the furnace for heating the starting material, today an overall tube rolling mill typically looks like this:

• Cross-rolling mill for punching with maximum hollow block lengths between 11 and 12.5 m,
• Stretching aggregate (eg 2- or 3-roll rod-reticulated rod mill) with 5 or 6 stands,
• Bar circulation of the stretching unit with 5 to 8 bars, rod lengths of 20 m, of which about half as a working part for rolling, the other half to bridge the distance between the actual rolling mill and the bar restraint system,
• Extractor, consisting of 3 stands with 3 rollers each, to remove the billet from the roll bar,
• reheating furnace,
• Dimensional or stretch reducing mill,
• cooling bed

• die Walzstange in den Hohlblock einzufädeln,
• den Hohlblock mit Walzstange in das 1. Gerüst des Walzwerks einzustoßen,
• die Walzstange während des Walzens so zurückzuhalten, dass sich diese mit einer konstanten Geschwindigkeit, die unter der Einlaufgeschwindigkeit des Hohlblocks in das 1. Gerüst liegt, vorwärts bewegt,
• die Walzstange nach dem Walzende auf die Walzwerkseinlaufseite zurückzufahren.

The roll bar restraint system has the task here

• thread the rolling rod into the hollow block,
• to push the hollow block with rolling bar into the 1st stand of the rolling mill,
• to restrain the rolling rod during rolling in such a way that it moves forward at a constant speed, which is below the entry speed of the hollow block into the 1st framework,
• to return the rolling rod to the rolling mill inlet side after the end of rolling.

Thereafter, the roll bar for cooling and lubrication is ejected laterally in the bar circulation and passed on the return side of the bar circulation a "new" rolling bar in the roll bar restraint system.

The Ausziehwalzwerk has in the known plant to the end of the last frame of the 2- or 3-Walzen-Rohrkontiwalzwerks a distance of approximately 10 to 12 m. The removal of the billet from the roll bar begins as soon as the billet with its point runs into the 1st frame of the extracting mill. At this point in time, the stag is still partially in the Rohrkontiwalzwerk. Once the billet has left the pullout mill, the roll bar is retracted. The top of the roll bar is at this time just before the 1st stand of the Ausziehwalzwerks.

Frequently, the lumps must be reheated prior to finish rolling. There are two reasons for this. On the one hand, the temperature is different for different Luppenwanddicken different levels. Thin-walled pipes cool faster than thick-walled pipes. With the same diameter in the outlet of the stretch-reducing or sizing mill, this influences the cold finished pipe diameter due to the different shrinkage dimension. A second reason is that a cooling of the billets below about 600 ° C allows a normalization of the material, when subsequently reheated to temperatures above Ac3 in the reheating furnace.

In addition to the ability of these known rolling mills to roll tubes in premium quality, such plant concepts have a very high production capacity. Depending on the size and duration of production, this is between 300,000 and 900,000 t per year.

This method is particularly economical due to the ability to roll multiple lengths continuously, d. H. according to the required tube length, a hollow block length is used, which results in stretching a multiple length, which is then divided after finish rolling with minimal scrap metal to the required single tube length.

Disadvantage, however, is that with an annual capacity of only 100,000 to 250,000 tonnes for premium products such high annual capacity designed and built with high investment costs rolling mills are not economical to operate.

The EP 1 764 167 B1 Although there are indications for increasing the economic efficiency, in particular by omission of the extraction mill. By a controlled movement of the mandrel bar against the rolling direction in the continuous rolling mill, this is largely removed from the billet after completion of the rolling process, whereby a separate extraction mill is superfluous.

However, the described technical measures proposed for completely removing (stripping) the rolling rod from the billet are associated with disadvantages in practice. Thus, stripping with the aid of a scraper, at least in the case of thin wall thicknesses, is always associated with flaring of the end of the scraper, which must be sawn off before the subsequent sizing or stretching reduction. Also, stripping using a roller table is technologically critical, since the time in which the process takes place, is not to control.

In addition, the measures described here are not yet sufficient to substantially improve the economic efficiency of the process, since despite the elimination of the extraction mill, the investment costs for the rolling mill are still very high. To increase the economic efficiency, therefore, further cost-cutting measures must be added.

From the EP 1 764 167 B1 discloses a method for producing wire, rods or seamless tubes on a rolling mill, wherein to achieve an optimized operation with reduced investment costs in the main stage, a 3-Walzen-Kontivalzwerk both for piercing in tube production as well as solid rolling in the production of rods , Wire or the same one is used.

Finally, it is still out of the EP 1 102 033 A1 the production of seamless steel tubes in three forming stages, which consist of a punching in a cross rolling mill, the stretching in an Assel, Konti or other rolling mill and a finish rolling in a stretch-reducing mill.

Object of the present invention is to provide a method and a rolling mill, with the use of a Rohrkontiwalzwerks pipes produced economically even at relatively low annual demand and the aforementioned disadvantages of known Rohrkontiwalzwerke be avoided. In particular, the mill should be simpler and less expensive to build.

This object is achieved according to the preamble in conjunction with the characterizing features of claim 1. Advantageous developments are the subject of dependent claims.

The great advantage of the invention is that with the proposed method a very economical production of seamless tubes in premium quality with low annual demand is now possible in Rohrkontiwalzwerken, with their capacities are adapted to the needs.

An important factor influencing the plant costs is the pipe length. Known rolling mills for the production of seamless tubes are designed for economic reasons that Luppen between 28 and 30 m in length, d. H. Multiple lengths, can be rolled.

This requires that all units from the furnace to the cooling bed and including all transport systems, such as roller tables, etc., must be designed so that they can handle these lengths.

If one limits oneself to single lengths, d. H. to a Luppenlänge of about 14 to 15 m, so the lengths of the rolling mill components and thus the costs can be significantly reduced.

For a rolling mill, which can produce, for example, tubes with outer diameters between 108 and 273 mm, the following differences result: Standard tube rolling mill Rolling mill according to the invention Vormateriallänge 5,0m 3.6 m Hollow block length 12.0 m 9.0 m shell length 29.0 m 14.5 m Cooling bed length 42.0 m 16.0 m

This reduces the required length of the cooling bed to less than 40% of the normal length.

Limiting to single lengths also makes it possible to reduce the wall thickness decrease in the second step of the drawing stage. In known continuous rolling mills wall decreases of about 10 to 15 mm are usual, depending on the diameter range and number of frames.

If the wall thickness decrease is limited to values below 9 mm, instead of the five to six stands used in the standard construction, according to the invention only three stands are required. In an advantageous embodiment of the invention, therefore, the wall thickness decrease is limited to less than 9 mm.

Ideally, the ring cross-sectional area of the incoming hollow block is reduced to a smaller in cross-sectional diameter and wall thickness cross-sectional area in Rohrkontiverfahren. The wall thickness of the billet is identical to the wall thickness of this smaller cross-sectional area. In this case, it is typical for the method that the difference between the two cross-sectional areas is independent of the size of the wall thickness of the column.

The decision on the uniformity of the wall thickness during rolling as well as on the stability, that is the reproducibility as well as the susceptibility to rolling defects, determines how this cross-sectional acceptance takes place and is distributed over the frameworks.

• Gerüst 1 (Einlaufgerüst): 50 - 60%
• Gerüst 2 (Zwischengerüst): 35 - 40%
• Gerüst 3 (Auslaufgerüst): 5 - 7,5%

For the mentioned reduction in cross-section, according to the invention, the following divisions have proved to be advantageous for the geometric design of the deformation in the three rolling stands:

• Scaffold 1 (inlet frame): 50 - 60%
• Framework 2 (intermediate framework): 35 - 40%
• Scaffolding 3 (outlet stand): 5 - 7.5%

In the standard design of a continuous rolling mill, usually the first three stands render a high forming work and the following two to three stands a much smaller one, which is why two groups of sizes are used.

The limitation on small wall thickness decreases according to the invention therefore makes it possible to use a framework instead of three in the case of the large assembly. The small assembly with the lower forming work then has accordingly only two scaffolding.

Premium products for the exploration and production of oil and gas are generally subjected to heat treatment in the form of hardening and tempering after rolling. This eliminates the usual normalization and thus the investment of a reheating furnace.

An omission of the reheating furnace according to the invention allows to take off and finish rolling in one step. The otherwise necessary distance from Ausziehwalzwerk a 2- or 3-Walzen-Rohrkontiwalzwerks for finishing mill in the form of a stretch-reducing or sizing therefore completely eliminated, since the point of Ausziehwalzwerks now takes the Maß- or Streckreduzierwalzwerk itself.

The distance between the Rohrkontiwalzwerk and the removal of the pipe from the rod with accepting Maß- or stretch-reducing mill can be reduced below the usual 10 - 12 m. Simply limiting to a single length of the billet allows shortening to about half. Further shortening becomes possible when the speed of the rolling rod is reduced. A lower limit for the distance results from the selected type of scaffolding change (lateral change or change in the rolling line) in the drafting unit.

If the scaffolding in the rolling line pulled out so a corresponding space must be available, which is shorter than the standard construction due to the reduced number of scaffolds. In the case of a side frame change, the maximum travel of the rod specifies the required minimum clearance.

In addition, because the rolling rod is subjected to a lower heat load due to the reduced wall thickness decrease and the reduced skeleton number, the expensive part of the rolling rod subject to wear can be correspondingly shorter, which adds to a large cost saving.

In addition, the temperature differences are significantly reduced because of the shorter contact time to the inner tool. Thus, the second reason for installing a reheating furnace in a (total) Rohrkontiwalzwerk is obsolete.

As the above comparison shows, the cooling bed also becomes significantly shorter. With the part saws can also be saved. When using a draft-reducing mill for finish rolling, two part saws are sufficient instead of the usual four saws.

If you are working with a sizing mill, the part saw can even be completely eliminated, since the required end cuts are made in the area of nondestructive testing which has one or more cutting units to cut out errors and to take samples.

In addition, the roller tables for transporting the tubes can be designed significantly shorter. If a finishing line is attached directly to the stretch-reducing mill or sizing mill, a saw is enough to cut the head, so that the nondestructive testing of the pipes is not destroyed by unclean pipe ends.

The usual disadvantage in deploying single lengths in known overall tube rolling mills can advantageously be improved, for example, by rolling "sharpened" tube wall ends. Because of this, the wall buckling during subsequent stretching reduction, which would usually lead to a pipe end thickening outside the tolerance limit compensated.

In addition, tubes produced in particular in 3-Walzen-Rohrkontiwalzwerken have a very good concentricity, whereby the disadvantage of any lower application is compensated.

According to the invention, therefore, the rolling mill is designed as a 3-stand rolling mill with three rollers per scaffold.

Due to the special procedure using single-length dolls and the consequent elimination of otherwise necessary expensive rolling mill units, the disadvantages of a relative to the annual capacity low utilization of the rolling mill are significantly compensated or overcompensated.

For the safe and economical operation of a rolling mill, the suitable choice of the starting material formats and the number of different calibers for the production of different tube finished diameters are of great importance. The goal is to keep the necessary number of formats and calibres to be kept as low as possible.

With Format the outer diameter of the pre-material block is named here. Caliber refers to the outside diameter of the billet behind the 3-roll continuous tube rolling mill. Different sizes and calibers are required to manufacture different tube diameter.

mit:

• D-Rohr-max: maximaler Rohrfertigdurchmesser in mm
• D-Rohr-min: minimaler Rohrfertigdurchmesser in mm,

mit folgenden Werten der Konstante C1, welche die sinnvolle Umfangsverkleinerung des zugehörigen Walzaggregates beschreibt: $$1\le \mathrm{C}1\le 4$$

für Streckreduzierwalzwerke $$\mathrm{1,2}\le \mathrm{C}1\le \mathrm{1,45}$$

für MaßwalzwerkeIn order to manage with as little as possible held different formats and calibers, therefore, in a first step, the minimum number N caliber needed in an advantageous embodiment of the invention via the following formula: $$\mathrm{N}=\mathrm{Round\; up\; to\; integer}:\mathrm{}\left(\log \left(\mathrm{D\; Pipe\; max}/\mathrm{D-tube\; min}\right)/\log \left(\mathrm{C}1\right)\right)$$

With:

• D-tube max: maximum tube diameter in mm
• D-tube min: minimum tube diameter in mm,

with the following values of the constant C1, which describes the reasonable circumferential reduction of the associated rolling aggregate: $$1\le \mathrm{C}1\le 4$$

for stretch-reducing mills $$1.2\le \mathrm{C}1\le 1.45$$

for sizing mills

mit: $$\mathrm{1,04}\le \mathrm{C}2\le \mathrm{1,12}$$

$$22\le \mathrm{C}3\le 28$$

$$-\mathrm{0,03}\le \mathrm{C}4\le \mathrm{0,15}$$

If only 1 caliber is required, the range for the block diameter DB results according to the invention via the following formula (data in mm): $$\mathrm{DB}=\left(\mathrm{D\; Pipe\; max}\times \mathrm{<figure-callout\; id="2"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}2+\mathrm{C}3\right)/\left(1+\mathrm{C}4\right)$$

With: $$1.04\le \mathrm{<figure-callout\; id="2"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}2\le 1.12$$

$$22\le \mathrm{C}3\le 28$$

$$-0.03\le \mathrm{C}4\le 0.15$$

Herein, the constants describe the limits of the abilities relevant to the diameter changes of the aggregate mill (C5) or stretch-reducing mill (C2), continuous-tube rolling mill (C3) and cross-rolling mill (C4).

mit: $$\mathrm{1,4}\le \mathrm{C}5\le \mathrm{1,45}$$

$$22\le \mathrm{C}3\le 28$$

$$-\mathrm{0,03}\le \mathrm{C}4\le \mathrm{0,15}$$

If more than 1 caliber is required, the other block diameter ranges result from the equation: $$\mathrm{DB}=\left(\mathrm{D-tube\; min}\times \mathrm{<figure-callout\; id="5"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}5\exp ,\mathrm{Caliber\; number\; n}\left(\mathrm{with\; n}=\mathrm{1,}\mathrm{}\mathrm{2,}\mathrm{}3...\right)+\mathrm{C}3\right)/\left(1+\mathrm{C}4\right)$$

With: $$1.4\le \mathrm{<figure-callout\; id="5"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}5\le 1.45$$

$$22\le \mathrm{C}3\le 28$$

$$-0.03\le \mathrm{C}4\le 0.15$$

The constant C5 describes the maximum formability of the sizing mill and replaces the constant C2.

The following two examples show the calculation and decision path.

Example 1:

$$\mathrm{D-Rohr-min}=\mathrm{60,3}\mathrm{mm}$$

It should be produced tubes with the following diameters: $$\mathrm{D\; Pipe\; max}=139.7\mathrm{mm}$$

$$\mathrm{D-tube\; min}=60.3\mathrm{mm}$$

The pipe dimensions are thus in the typical range for a stretch-reducing mill.

This results according to formula 1 with the corresponding limits of C1, the following

$$\mathrm{DB\; max}=\left(\mathrm{108,0}\times \mathrm{1,451}+28\right)/\mathrm{0,97}=\mathrm{190,3}\mathrm{mm}$$

und für Kaliber 2 ergibt sich: $$\mathrm{DB\; min}=\left(\mathrm{108,0}\times \mathrm{1,42}+22\right)/\mathrm{1,15}=\mathrm{203,2}\mathrm{mm}$$

$$\mathrm{DB\; max}=\left(\mathrm{108,0}\times \mathrm{1,452}+28\right)/\mathrm{0,97}=\mathrm{263,0}\mathrm{mm}$$

For caliber 1 results: $$\mathrm{DB\; min}=\left(108.0\times 1.41+22\right)/1.15=150.6\mathrm{mm}$$

$$\mathrm{DB\; max}=\left(108.0\times \mathrm{1,451}+28\right)/0.97=190.3\mathrm{mm}$$

and for caliber 2 results: $$\mathrm{DB\; min}=\left(108.0\times 1.42+22\right)/1.15=203.2\mathrm{mm}$$

$$\mathrm{DB\; max}=\left(108.0\times \mathrm{1,452}+28\right)/0.97=263.0\mathrm{mm}$$

mit: $$\mathrm{1,4}\le \mathrm{C}5\le \mathrm{1,45}$$

$$22\le \mathrm{C}3\le 28$$

$$-\mathrm{0,03}\le \mathrm{C}4\le \mathrm{0,15}$$

These three block diameter ranges result in a gapless coverage of the diameters between 108 and 273 mm. If gaps are permitted, then instead of the minimum diameter of the total area, the minimum finished pipe diameter of the respective caliber number n must be taken into account: $$\mathrm{DB}=\left(\mathrm{D-tube\; min}\left(\mathrm{Caliber\; number\; n}\right)\times \mathrm{<figure-callout\; id="5"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}5+\mathrm{C}3\right)/\left(1+\mathrm{C}4\right)$$

With: $$1.4\le \mathrm{<figure-callout\; id="5"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}5\le 1.45$$

$$22\le \mathrm{C}3\le 28$$

$$-0.03\le \mathrm{C}4\le 0.15$$

$$\mathrm{DB\; max}=\left(\mathrm{168,3}\times \mathrm{1,451}+28\right)/\mathrm{0,97}=\mathrm{280,4}\mathrm{mm}$$

Damit ergibt sich ein Überschneidungsbereich mit Kaliber 3 (266,1 bis 280,4 mm) und es kann auf ein zusätzliches Blockformat verzichtet werden.For example, if caliber 2 starts at a pipe diameter of 168.3 mm (caliber 1 theoretically ends at 108 x C5 = 108 x 1.45 = 156.6 mm), the following block diameter would result for this caliber: $$\mathrm{DB\; min}=\left(168.3\times 1.41+22\right)/1.15=224.0\mathrm{mm}$$

$$\mathrm{DB\; max}=\left(168.3\times \mathrm{1,451}+28\right)/0.97=280.4\mathrm{mm}$$

This results in an overlap area with caliber 3 (266.1 to 280.4 mm) and it can be dispensed with an additional block format.

Further features, advantages and details of the invention will become apparent from the following description of an embodiment shown in a drawing.

In the FIG. 1 the known layout of a Rohrkontiwalzwerkes is shown as a total mill for rolling multiple lengths.

In addition to the rotary hearth furnace (0), the total mill has a cross rolling mill (1) for punching a massive block not shown here to a hollow block, a drafting unit as 3-Walzen-Rohrkontiwalzwerk (2) for stretching the hollow block to a billet, a Ausziehwalzwalzwerk (3) for stripping the billet from the mandrel bar, a reheating furnace (4) for reheating the billet to rolling temperature, a stretch reducing mill (5) for rolling the billet to final dimension, a cooling bed (6), and a pipe end finishing saw field (7).

The pullout mill (3) consists of 3 stands with three rollers each to pull the billet off the roll bar.

FIG. 2 shows in comparison to the plant layout of the rolling mill according to the invention. In direct comparison, it can be seen that the plant concept according to the invention is characterized by a significantly reduced overall length.

Due to the consistent implementation of the concept "single-length rolling of single rolls" the total length of the continuous rolling mill is significantly reduced by omission of the extracting mill (3) and the reheating furnace (4) and length adjustment of the cooling bed (6) and saw field with pipe end machining (7).

The investment costs for the rolling mill according to the invention are correspondingly lower in comparison to the known continuous rolling mill.

In addition, the rolling mill is equipped with an in-line test unit, not shown here, which further reduces investment and operating costs. The in-line test unit consists of a nondestructive testing facility with an upstream straightening machine, a stray flux test for longitudinal and transverse defects and an ultrasonic wall thickness test, and connects directly to the cooling bed (6) in conjunction with a repair cycle for pipes to be reworked.

This eliminates a separate quality inspection outside the production line. In addition, this test allows a quick feedback on quality problems of the rolling mill, a minimal piping of the pipes, in which only that is sawn off what is necessary as well as the use of pre-tested tubes for the heat treatment and other finishing lines. As a result, the throughput times of the tubes and thus the efficiency of the production are significantly increased.

In plants for the production of pipes with ≥ 177.8 mm outside diameter, this has already proved its worth in tests. The meter performance in systems for the production of smaller pipe diameters, especially in the known Rohrkontistraßen, but is so large that the non-destructive test can not handle the amount of pending for testing pipes. Only the limitation to single-length Luppen allows this again.

The Ausziehwalzwerk (3) consists of 3 stands, each with three rollers to pull the mother tube from the roll bar.

FIG. 2 shows in comparison to the plant layout of the rolling mill according to the invention. In direct comparison, it can be seen that the plant concept according to the invention is characterized by a significantly reduced overall length.

Due to the consistent implementation of the concept of "single core tube rolling", the total length of the continuous rolling mill is significantly reduced by omitting the extracting mill (3) and the reheating furnace (4) and by length adjustment of the cooling bed (6) and saw field with pipe end machining (7).

The investment costs for the rolling mill according to the invention are correspondingly lower in comparison to the known continuous rolling mill.

In addition, the rolling mill is equipped with an in-line test unit, not shown here, which further reduces investment and operating costs. The in-line test unit consists of a nondestructive testing facility with an upstream straightening machine, a stray flux test for longitudinal and transverse defects and an ultrasonic wall thickness test, and connects directly to the cooling bed (6) in conjunction with a repair cycle for pipes to be reworked.

This eliminates the need for a separate quality inspection outside the production line. In addition, this test allows quick feedback on quality problems of the rolling mill, minimal piping of the pipes, where only what is required is sawn off and the use of already pre-tested pipes for the heat treatment and other finishing lines. As a result, the throughput times of the tubes and thus the efficiency of the production are significantly increased.

In plants for the production of pipes with ≥ 177.8 mm outside diameter, this has already proved its worth in tests. The meter performance in systems for the production of smaller pipe diameters, especially in the known Rohrkontistraßen, but is so large that the non-destructive test can not handle the amount of pending for testing pipes. Only the limitation to mother tube single lengths allows this again. <B> LIST OF REFERENCES </ b> No. description 0 Rotary hearth furnace 1 Piercing mill 2 Continuous rolling mill in three-roller arrangement 3 Ausziehwalzwerk 4 reheating furnace 5 stretch 6 cooling bed 7 Saw field and pipe end machining

Claims (4)

1. Method for the production of seamless pipes, wherein a hot hollow block which has been generated beforehand in a piercing mill is stretched by means of a continuous rolling mill (2) on a mandrel bar to form a mother pipe and, dispensing with an extracting mill and reheating furnace, the mother pipe is fed directly to a stretch reducing mill or sizing mill (5) as finishing mill and is rolled therein to the required finished pipe diameter, characterized in that the length of the hollow block is predimensioned in such a way that only a single length of about 14 to 15 m is produced as required mother pipe length during stretching in the continuous rolling mill (2), and during the subsequent finish rolling the extraction of the mother pipe from the mandrel bar is carried out by the finish rolling, and the rolling is carried out with rolling mill components whose dimensions are designed for handling single lengths and the rolling in the continuous rolling mill (2) is carried out with three stands and three rolls per stand and the cross section reduction of the rolling stock between hollow block and mother pipe is divided between the three roll stands of the continuous rolling mill (2) as follows: Stand 1 (entry stand hollow block): 50 - 60 % Stand 2 (intermediate stand): 35 - 40 % Stand 3 (exit stand): 5 - 7,5 % and wall thickness reduction in the continuous rolling mill (2) is limited to ≤ 9 mm.
2. Method according to claim 1, characterized in that the minimum quantity N of pass diameters required for finishing different finished pipe diameters is calculated according to the following formula: $$\mathrm{N}=\mathrm{rounded\; up\; to\; whole\; number}:\left(\log \left(\mathrm{D-pipe-max}/\mathrm{D-pipe-min}\right)/\log \left(\mathrm{C}1\right)\right),$$

   

   where

   D-pipe-max: maximum finished pipe diameter in mm,

   D-pipe-min: minimum finished pipe diameter in mm,
   2 ≤ C1 ≤ 4 for stretch reducing mills, and
   1,2 ≤ C1 ≤ 1,45 for sizing mills
3. Method according to claim 2, characterized in that when finishing is carried out with one pass the range for the block diameter is calculated according to the following formula:

   block diameter DB in mm: $$\mathrm{DB}=\left(\mathrm{D-pipe-max}\times \mathrm{C}2+\mathrm{C}3\right)/\left(1+\mathrm{C}4\right)$$

   

   where $$\mathrm{1,04}\le \mathrm{C}2\le \mathrm{1,12}$$

   

   $$22\le \mathrm{C}3\le 28$$

   

   $$-\mathrm{0,03}\le \mathrm{C}4\le \mathrm{0,15}$$

   
4. Method according to claim 3, characterized in that when more than one pass is required, the additional ranges for the block diameter are calculated by means of the following formula: $$\mathrm{DP}=\left(\mathrm{D-pipe-min}\times \mathrm{C}5\exp .\mathrm{pass\; number\; n}+\mathrm{C}3\right)/\left(1+\mathrm{C}4\right)$$

   

   where $$\mathrm{1,4}\le \mathrm{C}5\le \mathrm{1,45}$$

   

   $$22\le \mathrm{C}3\le 28$$

   

   $$-\mathrm{0,03}\le \mathrm{C}4\le \mathrm{0,15}$$

   

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