JPH08117814A - Manufacture of seamless steel pipe and manufacturing equipment train for executing the same method - Google Patents

Abstract

PURPOSE: To provide a manufacturing method of a seamless steel pipe of excellent performance at high productivity with inexpensive equipment and manufacturing equipment train therefor. CONSTITUTION: A round billet is directly manufactured by use of a round-shaped casting mold by continuous casting, this round billet is soaked to a prescribed temp. in a heating furnace 3 while its temp. is over the transformation temp. Ar1, piercing and rolling are executed at a strain rate of 5 <=50/sec with a piercer 5 and, then, elongation and rolling for sizing are executed at an average strain rate of >=0.01/sec and at the finishing temp. of 800-1000 deg.C by imparting the working ratio of >=10% with the rolling mills of both continuous elongator and finishing mill which are directly connected in series on the same line. In the transporting route 4 on the outlet side of the finishing mill, recrystalization treatment is given in which either of slow cooling at a cooling rate under air-cooling and keeping the billet at the finishing temp. for a prescribed time, soaking/keeping it at the prescribed temp. for the prescribed time or temp. rising/heating/keeping is executed and, successively, direct quenching and tempering are on-line executed. To execute this method, respective equipment and device are arranged in direct connection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a seamless steel pipe and a series of manufacturing equipment, and more specifically, it effectively slims a seamless steel pipe excellent in strength, toughness and corrosion resistance by performing thermomechanical treatment. Method for manufacturing online at low cost using the mill line described above, and a manufacturing equipment line for implementing the method.

[0002]

2. Description of the Related Art In the production of steel products requiring huge equipment, simplification of the process by applying online thermomechanical treatment is being considered from the viewpoint of process saving and energy saving. In the manufacturing field, production by off-line quenching and tempering has considerably decreased, and online processed materials have become the majority.

On the other hand, in the field of manufacturing seamless steel pipes, since there is a strong demand for high reliability and high quality, most products are still manufactured by off-line quenching and tempering. The actual situation is that it is necessary to install a quenching furnace and a tempering furnace separately from the pipe manufacturing line to operate offline, which is an obstacle to energy saving.

For this reason, in recent years, even in the field of manufacturing seamless steel pipes, there is a movement to introduce a so-called direct quenching process in which quenching is immediately performed by utilizing the heat possessed by the material after hot working. A quenching furnace is no longer needed, and a large cost reduction is being industrially achieved.

For example, Japanese Patent Laid-Open Nos. 56-166324 and 58-
No. 120720, No. 58-224116, No. 59-020423, No. 60-033312, No. 60-075523, No. 62-
As seen in 151523, etc., in the manufacturing process of seamless steel pipes, a process of forcibly cooling immediately after hot working and directly quenching has been proposed and put into practical use, but it was manufactured through these direct quenching processes. The crystal grain size of the product is generally coarse, as compared with the product manufactured by quenching and tempering off-line, and there is a defect that the toughness and corrosion resistance are inferior.

Therefore, for the purpose of refining the crystal grains, cooling and reheating are combined online so that the transformation from austenite and the reverse transformation to austenite are performed twice or more in total, whereby the grains are refined. A method has been proposed, for example, JP-A-56-003626 discloses a method of incorporating a cooling and reheating process between rough rolling and finish rolling, JP-A-58-091123 and 58-58. -104120, 63-011621 and JP-A-04-358023, a method of combining cooling and reheating after final finishing rolling, JP-A-58-117832, during and after rolling. A method of cooling and reheating twice has been proposed.

All of the above methods require on-line forced cooling to a temperature range where transformation is started or completed and reheating to a temperature range where reverse transformation is completed again, resulting in remarkable energy consumption and complicated. In addition, it requires equipment with high construction cost, so compared to offline quenching equipment, there is not much difference in equipment cost and operating cost, and there are few merits, so inexpensive equipment with high productivity and excellent performance It was not a satisfactory method from the viewpoint of producing seamless steel pipe.

Further, JP-A-62-139815 and 63-22.
No. 3125, 64-055335, from the viewpoint of refining the crystal grains, processed in the unrecrystallized region, after obtaining fine crystal grains by the process of further recrystallization, direct quenching, tempering Although the methods have been proposed, all of these methods are intended for manufacturing steel sheets and require a large reduction at a non-recrystallization temperature range, that is, a relatively low temperature, which is different from sheet rolling. It is difficult to apply to rolling steel pipes with complicated plastic deformation. Explaining the reason specifically, for example, when the rolling process using a mandrel mill, which is a continuous drawing rolling mill, is carried out at a temperature of 1000 ° C. or lower, which is a non-recrystallizing region temperature, the rolling capacity of the mill is usually exceeded. This is because even if the mandrel bar can be rolled, defects such as surface flaws frequently occur, and it becomes extremely difficult to pull out the mandrel bar.

Further, Japanese Patent Application Laid-Open No. 61-238917 proposes a method of refining crystal grains by utilizing recrystallization after pipe making in a process for producing a seamless steel pipe.
Since the hot working conditions are not specified, the actual mill line may rather promote the coarsening.

As described above, energy saving and space saving can be achieved when each manufacturing equipment is directly connected. For example, Japanese Patent Laid-Open No. 63-157705 discloses that
A method for producing a seamless steel pipe in which a round billet is directly produced by continuous casting without passing through a slabbing or forging process, and the round billet is perforated and stretch-rolled is also proposed. 71st (1985) No.8, pp.965-971 "
Discloses an equipment row in which a mandrel mill that is a continuous drawing and rolling machine and an extracting sizer that is a finishing rolling machine are directly connected to each other.

However, the method proposed in Japanese Patent Application Laid-Open No. 63-157705 specifies the temperature condition of a round billet before charging in a heating furnace and the piercing and rolling conditions in a piercer which is an inclined roll piercing and rolling machine. In addition, the equipment row disclosed in "Iron and Steel" is for the purpose of insuring the sizer to pull out the mandrel bar, which is an inner surface control tool, from the pipe rolled by the mandrel mill, and to simply secure the quenching temperature. A method of organically arranging each manufacturing apparatus for the purpose of highly efficiently manufacturing a seamless steel pipe having a fine crystal grain structure by online processing, and an existing manufacturing for performing this method. The actual situation is that there is no equipment line and little has been considered.

On the other hand, a typical manufacturing process of hot seamless steel pipe by the inclined roll piercing and rolling method is as follows. A round billet is pierced and rolled by an inclined roll piercing and rolling machine typified by Mannesmann piercer. This is a process of drawing and hollowing this hollow shell with a rolling mill such as a plug mill and a mandrel mill and a finishing mill such as a sizer and a stretch reducer. Looking at the process until completion, (a) the manufacturing process of the billet, which is the material for piercing and rolling, (b) the hot piercing and stretching, the constant diameter rolling process, (c) the heat treatment, that is, the heat treatment process. It can be roughly divided into three steps, and usually, the above steps (a) to (c) are separate and independent steps. Of these, above (b)
Regarding the steps of (c) and (c), as described above, there is a movement to continuously carry out the manufacturing online, and the direct hardening process is a typical one.

However, comparing a simple direct quenching type thermomechanical treatment with a conventional tempering treatment (quenching and tempering treatment), there is a problem that the crystal grains become coarse as described above.
The effects of labor saving and cost reduction due to direct quenching cannot be fully exerted. In addition, the mechanical properties of the seamless steel pipe produced by the direct quenching process have large strength variations due to temperature variations between the axial direction and manufacturing lots.
It has been difficult to produce a large quantity of homogeneous seamless steel pipes.

Further, if each process is independent, it is necessary to secure a place for stocking the material due to the difference in processing speed between the steps. For example, a billet for storing a billet which is a material for piercing and rolling. A yard and a place to temporarily store the seamless steel pipe before heat treatment are necessary, and there was a problem that a conventional manufacturing equipment row (factory layout) had to secure a considerably large space. Furthermore, it is necessary to convey the material between each process, the crane,
There is a problem that many auxiliary transportation facilities and means such as trucks are required.

[0015]

SUMMARY OF THE INVENTION The object of the present invention was made in view of the above-mentioned circumstances, and a method for manufacturing a seamless steel pipe having excellent performance with a low cost equipment with high productivity, and a manufacturing equipment line therefor, Specifically, by directly connecting each equipment and equipment to make the whole equipment as compact as possible and setting the optimum operating conditions appropriately, a seamless steel pipe with the same or higher performance as the product manufactured by the conventional method. It is to provide a method capable of manufacturing a steel sheet at a low cost and a manufacturing equipment line for carrying out the method.

[0016]

Means for Solving the Problems The inventors of the present invention have made extensive studies in order to solve the above problems, and as a result, have obtained the following knowledge (A) to (H), and a manufacturing process based on the knowledge. It was found that not only is it possible to manufacture seamless steel pipes by the ultimate continuous process, but also a remarkable improvement in performance can be obtained by performing optimal thermomechanical treatment.

(A) The billet is produced by using a continuous casting machine having a round mold, so that a round billet can be obtained as it is solidified. As a result, the agglomerates necessary when using a continuous continuous casting material as a raw material, Rolling or forging steps can be omitted.

(B) The as-cast round billet is immediately charged into a heating furnace, heated to a predetermined temperature, and then pierced and rolled by an inclined roll piercing and rolling machine so that the heat retained therein can be fully utilized. In addition to being able to greatly save the heating energy in the heating furnace, the casting bloom, agglomeration, and rolling that were necessary when using a continuous cast material as a raw material to make it into a round billet through the agglomeration, rolling or forging process Alternatively, there is no need for a storage area for round billets after forging, and space can be saved.

(C) The as-cast round billet is inferior in hot workability to the round billet obtained by slabbing, rolling or forging, and defects occur during piercing and rolling in the above-mentioned inclined roll piercing and rolling machine. Although it is easy, the occurrence of this defect can be prevented by appropriately setting the strain rate during piercing and rolling.

(D) After piercing and rolling with an inclined roll piercing and rolling machine, a mandrel mill which is a drawing and rolling machine and a continuous drawing and rolling machine consisting of a plurality of roll stands, and a sizer which is a finishing rolling machine and consisting of a plurality of roll stands, or Stretching with a stretch reducer and constant-diameter rolling, but the arrangement of both rolling mills should be a close integrated type in series on the same line with a distance less than the length of the rolling tube in the mandrel mill that is a continuous rolling mill. With this, it is possible to suppress the temperature drop of the material and increase the accumulation of processing strain, and the crystal grains obtained by the subsequent recrystallization treatment are larger than those when the separation distance between both rolling mills is large. The particles become remarkably fine, and performance such as toughness and corrosion resistance can be improved.

(E) It is necessary to recrystallize after rolling with a sizer or a stretch reducer, which is a finish rolling mill. This is the cooling rate of the material between the finish rolling mill and the direct quenching device provided in the subsequent stage by air cooling. The structure composed of fine grains can be easily obtained by adjusting the temperature below or by appropriately adjusting the temperature of the raw material by providing a heating furnace to promote recrystallization.

(F) Further, when the material temperature is adjusted by providing the above-mentioned heating furnace, it is possible not only to reduce the variation in the quenching temperature in the longitudinal direction of the pipe axis and between manufacturing lots but also to control the temperature. By doing so, precipitation of carbides and the like can be controlled, the strength can be arbitrarily changed with the same material, and coarsening of recrystallized grains can be suppressed.

(G) Immediately directly quenching the material whose crystal grain size and amount of precipitates have been properly adjusted by the above treatment from a temperature of Ar3 transformation point or higher, and then tempering it in a tempering furnace provided on the same line. Therefore, performance such as toughness and corrosion resistance can be improved.

(H) By connecting all the processes from solidification of molten steel to pipe making and heat treatment online, and by defining new thermomechanical treatment conditions, a product having performance equal to or better than the conventional one and less variation in performance can be obtained. It can be manufactured at low cost.

The gist of the present invention based on the above findings lies in the following (1) to (7).

(1) A method for producing a seamless steel pipe, in which a round billet is directly and continuously cast using a round mold, and the round billet is kept at a temperature above the Ar1 transformation point. The process of charging into a heating furnace and soaking uniformly to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow shell With an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged,
Finishing temperature 800 to 10 by giving a working degree of 10% or more
Stretching at 00 ° C, the process of finishing the constant diameter rolling, and this stretching,
The process of cooling the steel pipe after the constant-diameter rolling to a temperature above the Ar3 transformation point at a cooling rate lower than air cooling and then directly quenching from this temperature, and then the process of tempering the directly quenched steel pipe are successively online. How to do in.

(2) A method for producing a seamless steel pipe, in which a round billet is directly and continuously cast using a round mold, and the round billet is kept at a temperature above the Ar1 transformation point. The process of charging into a heating furnace and soaking uniformly to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow shell With an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged,
Finishing temperature 800 to 10 by giving a working degree of 10% or more
Stretching at 00 ° C, the process of finishing the constant diameter rolling, and this stretching,
Steel pipes that have undergone constant-diameter rolling have a finishing temperature of 30 seconds ~
After holding for 30 minutes, immediately or immediately after cooling to a temperature not lower than the Ar3 transformation point at a cooling rate lower than air cooling, the step of directly quenching, and then the step of tempering the directly quenched steel pipe are successively online in sequence. How to do.

(3) A method for producing a seamless steel pipe, in which a round billet is directly and continuously cast using a round mold, and the temperature of the round billet is higher than the Ar1 transformation point. The process of charging into a heating furnace and soaking uniformly to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow shell With an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged,
Finishing temperature 800 to 10 by giving a working degree of 10% or more
Stretching at 00 ° C, the process of finishing the constant diameter rolling, and this stretching,
Steel pipe that has undergone constant-diameter rolling is heated to 850 to 980 ° C for 10 seconds
Immediately after holding for 30 minutes soaking or reheating, or immediately after cooling to a temperature not lower than the Ar3 transformation point at a cooling rate not higher than air cooling, the step of directly quenching, and then the step of tempering the directly quenched steel pipe, A method of performing online sequentially.

(4) A seamless steel pipe manufacturing equipment line for carrying out the method of (1) above, which is a continuous casting machine for directly manufacturing round billets, and billet heating for uniformly heating the cast round billets. Furnace, inclined roll piercing and rolling machine for piercing and rolling a uniformly billet-heated round billet, continuous drawing and rolling machine for drawing and rolling hollow tube, finish rolling machine for sizing and rolling the drawn and rolled hollow tube , Equipped with a direct quenching device for quenching in-line tempered pipes and a tempering furnace for tempering in-line tempered pipes, and successively arranging them at least satisfying the following conditions The equipment row.

The continuous casting machine conveys the round billet into the billet heating furnace with a separation distance that allows the round billet to be charged into the billet heating furnace after the completion of solidification of the molten metal in the cast round billet while the temperature does not fall below the Ar1 transformation point. It should be provided at the front stage position of the billet heating furnace through a passage or a conveying passage having a heat retaining means or a heating means for keeping the temperature at a temperature not lower than the Ar1 transformation point.

The continuous drawing and rolling mill and the finishing rolling machine are installed in series on the same line with a distance less than the length of the tube drawn and rolled by the continuous drawing and rolling mill.

The direct quenching apparatus should be provided at the subsequent stage of the finish rolling mill through a conveying path having a means for gradually cooling the drawn and constant-diameter rolled pipe at a cooling rate lower than air cooling.

(5) A seamless steel pipe manufacturing equipment line for carrying out the method of (2) or (3), wherein the conditions in (4) above are set as follows:
A heat-retaining furnace that keeps the temperature of a tube that has undergone constant-diameter rolling at that temperature,
Alternatively, it should be provided at a subsequent stage position of the finish rolling mill via a conveyance path having a heating furnace for holding the temperature so that it is uniformly heated or heated at a predetermined temperature. The equipment line.

(6) The inclined roll piercing and rolling mill is provided at the rear stage position of the billet heating furnace through a conveying path having billet cutting means for cutting the round billet that has been soaked and heated to a predetermined length. ) Or (5) the equipment sequence.

(7) The transport path interposed between the billet heating furnace and the inclined roll piercing and rolling mill is provided with the billet heating means after the billet cutting means provided in the transport path. The equipment row according to any one of (4) to (6) above.

[0036]

[Action]

[Production of round billet]: Round billet, using a continuous casting machine having a round mold of various inner diameter, to produce a round billet of outer diameter, length according to pipe making setup, normal slab, After the rolling or forging process, the billet heating furnace in the latter stage is charged while the temperature is higher than the Ar1 transformation point. This is because when soaking and heating in a billet heating furnace, the lower the billet temperature before the charging can refine the crystal grains due to the microstructural transformation, but the tilt roll piercing and rolling machine described below (hereinafter simply referred to as piercer) As the billet to be used for, it is sufficient if the solidification of its central portion has been completed, the higher the temperature before charging to the billet heating furnace, the more heating energy can be saved, so in the present invention, importance is placed on energy saving. It was decided to charge the billet heating furnace at a temperature above the Ar1 transformation point, which is the austenite / ferrite transformation end temperature.

Therefore, the continuous casting machine has a separation distance which can be charged into the billet heating furnace after the completion of solidification of the center of the cast round billet and before the temperature does not fall below the temperature of the Ar1 transformation point, or the temperature thereof can be changed. At a pre-stage position of the billet heating furnace via a conveying path having a heat retaining means capable of maintaining a temperature above the Ar1 transformation point or a suitable heating means such as a tunnel type induction heating furnace capable of being reheated and maintained in a short time. Set up.

The temperature before charging the continuously cast round billet into the billet heating furnace may be a temperature above the Ar1 transformation point, and it is not necessary to set the upper limit. However, from the viewpoint of performance such as toughness and corrosion resistance of the material, it is preferable that the temperature is below the Ar3 transformation point at which the ferrite transformation starts, and the temperature of the billet before insertion into the heating furnace is from the Ar1 transformation point to the Ar3 transformation point or less. Is preferred.

Further, the continuous casting machine may have a roll stand to which light rolling is applied for the purpose of modifying the casting structure of the cast round billet.

[Heating of billet]: The heating temperature for soaking and heating the billet may be a temperature at which hot piercing and rolling can be performed by a piercer in the subsequent stage. The optimum temperature varies depending on the material, and the high temperature of the material to be piercing and rolled. Although appropriately determined in consideration of ductility and high temperature strength, heating is usually performed between 1100 and 1300 ° C.

As the billet heating furnace, a rotary hearth type so-called rotary furnace is preferably used. Further, in order to increase the charging rate of the billet to the heating furnace and to achieve high efficiency billet heating, it is necessary to charge the billet in a long state in which the billet length to be perforated and rolled by a piercer described later is multiple times longer. Often,
In this case, an appropriate cutting machine such as a gas cutting machine or a hot saw may be provided in the transfer path provided between the billet heating furnace and the piercer, and the piercer may be supplied while cutting into a predetermined length. Further, when the temperature of the material decreases during cutting, an appropriate auxiliary heating means such as a tunnel type induction heating furnace is provided in the subsequent stage of the cutting means, similar to that provided in the subsequent transfer path of the continuous casting machine. You may make it heat up and heat.

[Drilling rolling]: Since a round billet having a coarse grain structure is subject to piercing and rolling as it is cast, flaws occur when severe processing is applied, but as a result of various experimental studies, a strain rate of 50 is obtained.
It was found that when it was less than or equal to / second, it was possible to perforate without causing flaws. Therefore, the strain rate during piercing and rolling is set to 50 / sec or less.

In order to pierce and roll a material having inferior hot workability, it is preferable to work at a temperature as high as possible, and an appropriate auxiliary heating such as a tunnel type induction heating device similar to the one described above is provided at a position immediately before the piercer. It is preferable that a means is provided to heat and heat the material before piercing and rolling.

The strain rate may be 50 / sec or less, and it is not necessary to set the lower limit. However, if it is less than 0.1 / sec, the tool life of the plugs, guide shoes, etc. is remarkably shortened. It is desirable that it is 1 / second or more.

Further, the piercer may be of any type as long as it is an inclined roll piercing and rolling mill, but a cross type inclined roll piercing and rolling mill capable of piercing with a thin wall and / or a high expansion ratio is used. , It is preferable for integrating and integrating the outer diameter size of the round billet.

[Stretching, Constant Diameter Rolling]: A mandrel mill that is a stretching rolling machine and is a continuous stretching rolling machine that includes a plurality of roll stands, and a finishing mill that is a sizer that has a plurality of roll stands and that is the same as the mandrel mill. Compared to the processing with the piercer in the previous stage, the processing with the stretch reducer has a lower temperature of the material in the piercer, so it will be processed in a relatively low temperature range, but when considering the heat treatment, it gives effective processing This is an important step.
For this reason, the mandrel mill that is a continuous drawing and rolling machine and the sizer or stretch reducer that is a finishing rolling machine are not directly separated but placed in an integrated direct connection type, specifically, are drawn and rolled by a mandrel mill that is a continuous drawing and rolling machine. It is necessary to place them in series on the same line with a distance less than the length of the pipe that has been set, so that the work strain imparted by the mandrel mill, which is a continuous drawing and rolling machine, can be immediately finished. Further processing can be performed with a sizer or a stretch reducer, which is a rolling mill, and subsequent refinement of recrystallized grains can be realized.

That is, even when pipes are produced on the same pass schedule, after recrystallization, there is a case where the continuous drawing rolling mill and the finish rolling mill are separately arranged independently with a distance larger than the above-mentioned separation distance, and a case where they are not. There is a difference in the particle size of
It has been found that fine particles can be obtained by placing them in series close to each other within the distance defined by the present invention. Therefore, the separation distance is determined as described above.

At this time, if the average strain rate (Vε) defined by the following equation (a) is less than 0.01 / sec, recrystallization between each pass does not occur and strain is not accumulated. Does not provide the refinement effect, and if the strain amount is less than 10% in terms of the workability (area reduction rate), the recrystallization does not proceed smoothly and the refinement effect cannot be obtained. Furthermore, the temperature of the finished material on the finishing rolling mill is important, and the temperature is 800 to 10
It was found that when the temperature was 00 ° C., the refining effect by the subsequent recrystallization was extremely large. Therefore, the average strain rate, the workability, and the finishing temperature in the finishing rolling mill are 0.01 / sec or more, 10% or more, and 800 to 1 respectively.
It was set at 000 ° C.

The average strain rate and workability are 0.0
1 / sec or more and 10% or more are sufficient, and it is not necessary to set the upper limit, but the average strain rate is 10 /
If it exceeds 2 seconds, the tool life of the mandrel bar, which is an inner surface control tool of the mandrel mill that is a continuous drawing and rolling machine, will be significantly reduced, so 10 / sec or less is desirable, and the working ratio exceeds 95%. Since the occurrence of defects becomes remarkable, it is desirable to set it to 95% or less.

Vε = (Mε + Sε) / Mt ... (a) However, Mε: processing strain in the continuous drawing rolling mill Sε: processing strain in the finishing rolling mill Mt: Time required for the end of the hollow shell to get out of the finishing mill after being bitten into the continuous stretching mill (seconds) Furthermore, for a mandrel mill that is a continuous stretching mill, the rear end of the mandrel bar that is an inner surface regulation tool Any mandrel bar restraining means (bar retainer) can be used for restraining and pulling back the mandrel bar through the hole type roll row to the mill entrance side after the completion of stretching and rolling, and any type can be used. Use a mandrel mill whose bar restraint has a function of controlling the moving speed of the mandrel bar at a speed independent of the rolling moving speed of the tube during the drawing and rolling of the hollow shell. Desirable. The sizer or stretch reducer, which is a finish rolling mill, may be of any type as long as it does not have an inner surface regulation tool, but the pipe is pulled out from the mandrel bar in the pipe rolled by the continuous drawing and rolling mill. It is desirable to use a so-called extractor-type sizer or stretch reducer that has the function of separating.

[Recrystallization treatment]: In the present invention, a continuous stretching and rolling mill (hereinafter, simply referred to as a mandrel mill) and a finishing rolling mill (hereinafter, simply referred to as a sizer) are used for stretching, constant-diameter rolling, and then a sizer and a direct quenching device. It is one of the major characteristics to perform recrystallization treatment between the two, and by this recrystallization treatment, recrystallization is effective by combining processing with a mandrel mill and sizer and slow cooling, heat retention or heating. Is induced, it becomes possible to make the crystal grains finer.

[In case of gradual cooling]: This is a case where the material after finishing rolling is gradually cooled to a predetermined quenching temperature not lower than the Ar3 transformation point, and recrystallization is completed by the start of quenching to obtain a fine structure. Therefore, the slower the cooling rate, the better, but a cooling rate lower than air cooling is sufficient. However, if the cooling rate is higher than that of air cooling, coarse grains or a mixed grain structure is formed and the toughness decreases. Therefore, the cooling rate in the case of removing the heat is determined to be less than air cooling. The rate is preferably 0.5 ° C./second or less.

The cooling rate is, for example, a cover having a suitable insulating material such as glass wool lined in the conveying path from the exit of the sizer to the entrance of the hardening device, and a mirror having a mirror surface for reflecting radiant heat. It can be obtained by covering an appropriate material such as with a cover lined with an appropriate material, but other appropriate means may be adopted, and the specific method thereof is not limited.

[In the case of heat retention]: This is a case where the temperature of the raw material after the finish rolling is kept at the finish temperature, and if the heat retention time is less than 30 seconds, recrystallization does not start, and it takes 30 minutes. The effect will not change even if it is kept over, and the energy cost will increase and the production efficiency will further decrease. Therefore, the heat retention time is set to 30 seconds to 30 minutes.

[In case of soaking or heating at elevated temperature]:
This is a case where the material after finishing rolling is soaked or heated and held in the following predetermined temperature range for a predetermined time, and the temperature is 850
Recrystallization does not proceed if the temperature is less than 0 ° C and the holding time is less than 10 seconds,
On the contrary, if the temperature exceeds 980 ° C. and the holding time exceeds 30 minutes, crystal grains grow and the structure becomes coarse. Therefore, change the temperature to 85
The holding time was set to 0 to 980 ° C. and the holding time was set to 10 seconds to 30 minutes. The soaking here also includes the operation of charging the material into a heating furnace in which the furnace temperature is set to a temperature within the above range, which is lower than the finishing temperature of the material, and soaking the material.

The above heat retaining, soaking or temperature rising heating operation is
A dedicated heat-retaining furnace or heating furnace having an appropriate structure known to those skilled in the art can be provided, or a heat-retaining / combining furnace can be provided. In this case, the quenching temperature can be easily secured, and the pipe shaft It is possible to easily assure uniform heat distribution in the long direction and between production lots, and it is possible to greatly reduce variations in product performance. In addition, the heat retention temperature or soaking temperature or elevated heating temperature is set to a higher value to re-dissolve the carbides and the like precipitated during drawing and constant-diameter rolling to improve the temper softening resistance, or conversely set to a lower value. However, it is also possible to prevent the coarsening of the crystal grains by the grain boundary pinning effect by positively depositing the precipitate.

[Quenching]: Quenching must be carried out at a temperature not lower than the Ar3 transformation point in order to obtain strength and sufficient toughness. Further, even a thick steel pipe needs to be rapidly cooled at a sufficient cooling rate, and in this case, it is desirable to cool it by using a cooling means having an appropriate structure capable of simultaneously cooling the inner and outer surfaces.

[Tempering]: Tempering is carried out by a tempering furnace provided on the line after the direct quenching apparatus, but it is an important process for determining the performance of the final product, and is appropriately selected according to the performance to be obtained. It is necessary to set a proper tempering temperature and carry out soaking at that temperature sufficiently, and the variation in temperature is at most ± 10 ° C, preferably ± 5 ° C.
It is essential that As a result, the proof stress (Y
S), variation in tensile strength (TS) ± 5 kg of target strength
It can be suppressed within the range of f / mm ² .

[0059]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail below with reference to the accompanying drawings.

FIG. 1 is a schematic view showing an example of an equipment row for carrying out the method of the present invention. In the figure, 1 is a continuous casting machine having a round mold, and is appropriately changed to a mold having a predetermined inner diameter. By doing so, a round billet with an outer diameter corresponding to the pipe making setup is continuously cast, and solidification of the center part is almost completed,
Or, cut it to a specified length after it is completely completed. Further, if necessary, a light reduction process is applied by a roll stand provided at the front stage or the rear stage of the cutting means to improve the casting structure.

The round billet cut into a predetermined length is charged into the billet heating furnace 3 using the transverse feed type conveying path 2 while the temperature is higher than the Ar1 transformation point. Therefore, the length of the conveying path 2 is such that the round billet temperature after casting is Ar1.
There is a distance that can be charged into the billet heating 3 while the temperature does not fall below the transformation point. If there is no choice but to use a transport path that exceeds the above distance due to the site of the factory, so that the temperature can be maintained above the Ar1 transformation point in the transport path, for example, a tunnel-type induction heating device, etc. By providing an appropriate auxiliary heating means, or an appropriate heat retaining means such as a cover lined with a heat insulating material such as glass wool or a reflection plate such as a mirror having a mirror surface that reflects radiant heat with the lined cover, The temperature may be maintained above the Ar1 transformation point.

The round billet, which has been soaked and heated to a predetermined temperature in the billet heating furnace 3, passes through the conveying path 4 and the piercer 5
And is perforated and rolled at a strain rate of 50 / sec or less to form a hollow shell having no defects such as flaws. At this time, the length of the round billet charged into the billet heating furnace 3 is a multiple of the length used for piercing and rolling in the piercer 5 in order to increase the charging and filling rate into the furnace and improve the heating efficiency. It is desirable to insert the billet cutting machine 4a such as a gas cutting machine or a hot saw in the conveying path 4 provided between the billet heating furnace 3 and the piercer 5 in this case. It is preferable that the piercer 5 is fed after being cut to a predetermined length. If there is a concern that the temperature will drop during this cutting, an auxiliary heating device 4b, such as a tunnel-type induction heating device, which can heat up and heat in a short time is provided at the subsequent stage of the billet cutting machine 4a. The temperature can be raised by heating.

The hollow shell formed by piercing and rolling by the piercer 5 is conveyed through the transverse feed type conveying path 6 and is fed to the entry side table of the mandrel mill 7 provided at the end of the hollow tube. After inserting a mandrel bar whose rear end is restrained and held by a bar retainer into the tube, the mandrel mill 7 stretches and rolls the tube with a separation distance less than the length of the tube to form a close proximity integrated type on the same line in series. The mandrel mill 7 and the sizer 8 which are directly connected to each other are stretched at a strain rate of 0.01 / sec or more and a workability of 10% or more, are stretched, are subjected to constant-diameter rolling, and are finished at a finishing temperature of 800 to 1000 ° C. After being subjected to the various recrystallization treatments described below by the transfer device 9 provided in the above, the transfer device 9 is transferred to the quenching device 11 arranged near the sizer 8. The transport path 6
May be a vertical feed type such as a roller conveyor instead of the horizontal feed type.

The conveying device 9 is composed of, for example, a roller conveyor, and is covered with a heat insulating cover lined with a heat insulating material such as glass wool or a cover lined with a reflection plate such as a mirror having a mirror surface for reflecting radiant heat. The rolled material is recrystallized while being gradually cooled to a temperature not lower than the Ar3 transformation point at a cooling rate lower than air cooling and fed to the quenching device 11 in the subsequent stage. Then, the material fed to the quenching device 11 is directly subjected to quenching treatment at a temperature of Ar3 transformation point or higher. Further, the transport path 9 may have a heat-retaining furnace, a heating furnace, or a heat-retaining furnace 10 in place of the slow-cooling transport path, and in this case, finish rolling is performed. The material is kept at its finishing temperature for 30 seconds to 30 minutes to be recrystallized, or 850 to 980
After soaking at 10 ° C. to 30 minutes for 30 seconds or at elevated temperature for recrystallization treatment, it is fed to the quenching device 11 in the subsequent stage. Then, the material fed to the quenching device 11 is directly quenched here at a temperature not lower than the Ar3 transformation point, as in the case of the slow cooling. In FIG. 1, the transfer furnace 9 is shown as a slow-cooling transfer path or having a heat-retaining furnace, a heating furnace, or a combined heat-retaining furnace. It is also possible to dispose three of the paths and the heating furnace intervening transfer path in parallel, or to dispose two of the slow cooling transfer path and the transfer path interposing the furnace for heating and heating in parallel.

The material directly quenched by the quenching device 11 is soaked and heated to a predetermined appropriate temperature by a tempering furnace 12 disposed in the vicinity of the subsequent stage of the quenching device 11 and then tempered, and then a straightening machine. Bending is corrected by 13 to obtain a product.

Experimental Example 1 Steel A having the chemical composition shown in Table 1,
B is cast into a 90φ mold and immediately removed from the mold after solidification,
Table 2 shows the results of carrying out a piercing test by holding in a heating furnace at 1250 ° C. for 1 hour and changing the strain rate variously with an experimental piercer. The temperature charged into the billet heating furnace was
Steel A has a temperature of 690 ° C and Steel B has a temperature of 760 ° C. For Steel A, the temperature between the Ar1 transformation point and the Ar3 transformation point, Steel B
Then, the temperature was above the Ar3 transformation point.

[0067]

[Table 1]

[0068]

[Table 2]

As is clear from the results shown in Table 2, when the solid billet is continuously cast and is not cooled to a temperature below the Ar1 transformation point, the round billet is heated and pierce-rolled with a piercer, the strain rate is 50%. Good hollow shells have been obtained for those having a flow rate of not more than / sec. On the other hand, if the strain rate exceeds 50 / sec, a flaw is generated.

[Experimental Example 2] Steels A and B shown in Table 1 have a diameter of 90φ.
Immediately after solidification by casting into the mold, remove from the mold at 1250 ° C
In the heating furnace of 1 hour, press working is performed thereafter, and the strength of the material, the former austenite crystal grain size, the toughness (vTrs), the corrosion resistance (Sc) when the pipe manufacturing process is simulated under the conditions shown in Table 3 and Table 4 are performed. The values are shown in Table 5.

The strengths of the materials were made uniform by varying the tempering temperature depending on the steel type. Also, the Sc value of the corrosion resistance index is NACE (American Corrosion Association)
-TM01-77-92, determined by the method specified in METHOD-B.

[0072]

[Table 3]

[0073]

[Table 4]

[0074]

[Table 5]

As is clear from the results shown in Table 5, when the performances are compared according to the steel type, when the QT materials of the conventional manufacturing method are used as the trial Nos. 25 and 26, the examples of the present invention (Trial No. In Nos. 1 to 18), the crystal grain size is small and the toughness and corrosion resistance are equal to or higher than those of the conventional QT material. On the other hand, in Comparative Examples outside the scope of the present invention (Trial Nos. 19 to 24), the effect of refining the crystal grains by processing and recrystallization is small, and the toughness and corrosion resistance are poor.

[0076]

EFFECTS OF THE INVENTION According to the present invention, a product having performance equal to or higher than that of the conventional product and stable and less variation can be obtained by reducing the billet casting, pipe making, and heat treatment all online with compact equipment without waste. It can be manufactured at low cost, and its industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of an equipment row of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Conveying path 3 Billet heating furnace 4 Conveying path 4a Billet cutting machine 4b Auxiliary heating device 5 Piercer 6 Conveying path 7 Mandrel mill 8 Sizer 9 Conveying device 10 Heat-retaining furnace or heating furnace or combined heating furnace 11 Quenching device 12 Tempering furnace 13 Straightening machine

Claims (7)

[Claims] 1. A method for producing a seamless steel pipe, comprising the steps of continuously casting a round billet using a round mold and directly producing the round billet at a temperature above the Ar1 transformation point. In the meantime, the process of charging into a heating furnace and soaking uniformly to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow Finishing temperature of 800 to 100% by imparting a workability of 10% or more with an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged.
A process of finishing the stretching and constant-diameter rolling at 1000 ° C. and a steel tube that has undergone the stretching and constant-diameter rolling at a cooling rate of less than air cooling
A method for producing a seamless steel pipe, characterized in that a process of directly quenching from this temperature after cooling to a temperature of r3 transformation point or higher and a process of subsequently tempering the directly quenched steel pipe are successively carried out online. 2. A method for producing a seamless steel pipe, comprising the steps of directly casting a round billet by continuous casting using a round mold, and the temperature of this round billet at a temperature above the Ar1 transformation point. In the meantime, the process of charging into a heating furnace and soaking uniformly to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow Finishing temperature of 800 to 100% by imparting a workability of 10% or more with an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged.
The process of finishing the stretching and constant-diameter rolling at 1000 ° C., and the steel pipe which has been stretched and constant-diameter rolling to the finishing temperature of 30
After holding for 30 seconds to 30 minutes, a step of directly quenching immediately or immediately after cooling to a temperature not lower than the Ar3 transformation point at a cooling rate of air cooling or less, and then a step of tempering the directly quenched steel pipe are successively performed. A method for manufacturing a seamless steel pipe, which is performed online. 3. A method for producing a seamless steel pipe, comprising the steps of directly casting a round billet by continuous casting using a round mold, and the temperature of this round billet being higher than the Ar1 transformation point. In the meantime, the process of charging into a heating furnace and soaking and heating to a predetermined temperature, the process of piercing and rolling the soaked and heated round billet at a strain rate of 50 / sec or less to form a hollow shell, and this hollow Finishing temperature of 800 to 100% by imparting a workability of 10% or more with an average strain rate of 0.01 / sec or more by both rolling machines in which a continuous drawing rolling machine and a finishing rolling machine are directly connected and arranged.
The process of finishing the stretching and constant-diameter rolling at 1000 ° C. and the steel pipe which has been stretched and constant-diameter rolling at 850 to 980 ° C.
After soaking or holding for 30 seconds to 30 minutes, immediately or immediately after cooling to a temperature above the Ar3 transformation point at a cooling rate of less than air cooling, the step of directly quenching, and then the step of tempering this directly quenched steel pipe The method for producing a seamless steel pipe is characterized in that the steps are sequentially and online performed. 4. A seamless steel pipe manufacturing equipment line for carrying out the method according to claim 1, which is a continuous casting machine for directly producing round billets, and billet heating for uniformly heating the cast round billets. Furnace, inclined roll piercing and rolling machine for piercing and rolling a uniformly billet-heated round billet, continuous drawing and rolling machine for drawing and rolling hollow tube, finish rolling machine for sizing and rolling the drawn and rolled hollow tube , Equipped with a direct quenching device for quenching in-line tempered pipes and a tempering furnace for tempering in-line tempered pipes, and successively arranging them at least satisfying the following conditions A series of seamless steel pipe manufacturing equipment characterized by The continuous casting machine, after the completion of solidification of the molten metal in the cast round billet, while the temperature has a temperature not lower than the Ar1 transformation point, the round billet is passed through a conveying path having a separation distance capable of charging the billet heating furnace. Alternatively, it must be provided at the front stage position of the billet heating furnace through a conveying path having a heat retaining means or a heating means for keeping the temperature at a temperature not lower than the Ar1 transformation point. The continuous drawing and rolling mill and the finish rolling machine are installed in series on the same line with a distance less than the length of the tube drawn and rolled by the continuous drawing and rolling mill. The direct quenching device shall be provided at the subsequent stage of the finishing rolling mill via a conveying path having means for gradually cooling the drawn and constant-diameter rolled pipe at a cooling rate lower than air cooling. 5. A seamless steel pipe manufacturing equipment line for carrying out the method according to claim 2 or 3, wherein a continuous casting machine for directly manufacturing a round billet, and a soaking of the cast round billet. Billet heating furnace for heating, inclined roll piercing and rolling machine for piercing and rolling a soaked and heated round billet into a hollow shell,
A continuous drawing rolling machine for drawing and rolling a hollow shell, a finishing rolling machine for rolling a drawn hollow tube to a constant diameter, a direct quenching device and a quenching tube for quenching the rolled tube in a line. A series of equipment for manufacturing seamless steel pipes, characterized by comprising a tempering furnace for carrying out a tempering treatment in an in-line, and successively arranging them at least satisfying the following conditions. The continuous casting machine, after the completion of solidification of the molten metal in the cast round billet, while the temperature has a temperature not lower than the Ar1 transformation point, the round billet is passed through a conveying path having a separation distance capable of charging the billet heating furnace. Alternatively, it must be provided at the front stage position of the billet heating furnace through a conveying path having a heat retaining means or a heating means for keeping the temperature at a temperature not lower than the Ar1 transformation point. The continuous drawing and rolling mill and the finish rolling machine are installed in series on the same line with a distance less than the length of the tube drawn and rolled by the continuous drawing and rolling mill. The direct quenching equipment is a finishing rolling mill via a carrier path having a heat-retaining furnace that holds the temperature of the drawn or constant-diameter rolled pipe at that temperature, or a heating furnace that holds the temperature soaking or heats it to a predetermined temperature. Be provided in the rear stage position. 6. An inclined roll piercing and rolling mill is provided at a rear stage position of a billet heating furnace via a conveying path having a billet cutting means for cutting a round billet subjected to soaking and heating to a predetermined length. The seamless steel pipe manufacturing equipment line according to claim 4 or 5. 7. A conveying path provided between a billet heating furnace and an inclined roll piercing and rolling mill has a billet heating means after the billet cutting means provided in the conveying path. 7. The seamless steel pipe manufacturing equipment row according to any one of claims 4 to 6.