MXPA97010237A - Method and apparatus for manufacturing steel tube sincost - Google Patents

Abstract

A method for manufacturing a seamless steel tube, with excellent properties with high productivity in online processing, and an apparatus in which the equipment for melting and heat treating, etc., is available online. In the method the following steps (1) to (6) are carried out successively. (1) A round billet is melted with a casting process machine with continuous solidification 1. (2) The billet is heated and soaked in furnace 3 after it cools to a temperature no higher than point Ar1, (3) the soaked billet is perforated with a perforation 5 at a degree of formation not greater than 200 / sec to form a hollow laminar structure. (4) The hollow laminar structure is lengthened and the last hand lamination is made, at a degree of formation not less than 0.01 / sec., With a working formation of not less than 40% and a finishing temperature between 800 and 1000øC for form a seamless steel tube. (5) The tube is cooled to a temperature no higher than point Ar3, with cooling speed not less than 80 ° C / sec by means of a cooling apparatus 9. (6) The tube cooled to 850-1000 ° C, for 10 seconds-30 minutes then temper and reverse

Description

METHOD AND APPARATUS FOR MANUFACTURING STEEL TOBO WITHOUT SEWING TECHNICAL FIELD The present invention relates to a seamless steel tube manufacturing technology, more particularly, the invention relates to a method and apparatus for manufacturing a seamless steel tube with Excellent strength, toughness and resistance to corrosion. The aforementioned apparatus is not only adapted to perform said manufacturing method, but is also suitable for being widely used to manufacture various types of seamless steel tubes. TECHNICAL BACKGROUND In the steel industry, which is characterized by huge facilities and high energy consumption, a combination of steps has been created in the manufacturing process, the so-called "online processing", in order to simplify the process and reduce energy consumption. In the manufacture of steel sheet (steel plate or thick sheet), a thermomechanical treatment in the online process has been widely adopted. The treatment with conventional heat (for example, quenching, tempering, etc.) has been considerably reduced by using an apparatus that is placed separately from the rolling line. On the other hand, in the field of the manufacture of seamless steel tubes, most of the tubes are still treated in the so-called "off-line process", where the apparatus, for example, a heating furnace, a Cooling equipment and a tempering furnace are installed in a separate line, with respect to the pipe making line because the reliability and quality of the pipe is very severe. Naturally, it is difficult to reduce the energy consumption in said manufacturing process. A considerably large space is necessary for the off-line process to be carried out in the conventional factory layout, because the processing speed between the different independent process steps requires spaces such as a billet yard to maintain the billet before perforate it, a space for temporary storage of the tube before heat treatment, etc. In addition, the cost of manufacture inevitably increases due to transportation, because the materials must be transported from one step to another, and therefore means of transportation such as a conveyor belt, crane, truck and loading and unloading operations are required. . Recently, in the field of the manufacture of seamless steel tubes, there is a tendency to introduce a so-called "direct tempering process", where the tubes are annealed immediately after hot work with the use of the heat retained during the hot work By using this process, it is possible to dramatically reduce the cost of manufacturing, because the heating furnace for tuning is unnecessary. For example, as disclosed in the Japanese Patent Application Publication (referenced as PJPA hereinafter) Nos. 56-166324, 58-120720, 58-224116, 56-020423, 60-033312, 60-075523 and 62-151523, methods for manufacturing seamless steel pipes are proposed, including the direct tempering process in which steel pipes are forced to cool immediately after the last-hand rolling. Unfortunately, the products obtained through the direct tempering process have a disadvantage, that the tenacity and corrosion resistance of those is lower than that of conventional products, which are tempered and retained in the off-line process, due to that the products treated by direct tempering generally have a coarser grain size in the microstructure than that of conventional products treated by tempering and tempering in the off-line process. In the field of steel sheet manufacturing, as mentioned above, various methods of direct heat treatment (in-line) are proposed in the sheet after hot rolling. For example, PJPA Numbers 62-139815, 63-223125 and 64-055335 present methods for directly tempering and reventing steel sheets, which have been worked in the non-recrystallization state and then recrystallized in order to refine the grain structure. Because these methods require high reduction laminates in a relatively low temperature area, that is, the non-recrystallization area, it is difficult to apply to the lamination of the steel tube, which is accompanied by a more complex plastic deformation than that of the laminate of sheet. For example, the rolling of steel tubing at a temperature of less than 1000 ° C, which is the non-recrystallization temperature, through a mandrel burr, that is, a continuous lengthening burr, can not be performed in general due to that the working voltage exceeds the capacity of the milling cutter. Even if lamination could be carried out, many problems occur, for example, surface defects of the tube and difficulty in removing the mandrel rod, which do not appear in the laminate of the sheet. By applying the on-line heat treatment to the seamless steel tube manufacturing process, we present a method to refine the grain by using recrystallization after tube formation in PJP Number 61-238917. However, because hot working conditions are not specified in this method, there is a possibility that favorable grain growth is promoted when it is performed on a practical laminating line. Grain refining methods are presented through the combination of cooling and reheating (see, for example, PJPA Numbers 56-3626, 63-11621, 58-91123 and 58-1041209). In these methods, two or more cycles of a normal transformation from austenite to ferrite and an inverse transformation of ferrite to austenite are applied. Another method of refining grain, where reheating takes place twice during rolling and after rolling, is proposed in the PJPA Number 58-117832. However, when the transformation occurs in the first step of last-hand lamination as in the methods of the PJPA Number 56-3626 and 63-11621, the grain becomes thicker because the heating temperature should be selected in a range relatively, high in order to play the last-hand laminate. If the heating temperature is not high enough, last-hand rolling can not be carried out. In the PJPA Numbers 58-91123, 58-104120 and 4-358023, the conditions for the last-hand rolling are not specified. Therefore, the "normal-reverse transformation" treatment can not be applied to obtain a sufficient effect to refine the grains of the steel tube. Fine grains are provided safely through a method shown in PJPA Number 58-117832, where two reheats are carried out. However, the cost of this method is higher than that of the conventional off-line tempering and tempering process, because the cost of the equipment and the treatment become higher. From the point of view of the continuation of the manufacturing steps, there are several proposals in order to reduce cost and save space by combining several types of turn. For example, PJPA Number 63-157705 presents a method for manufacturing a seamless steel tube, in which a billet having a round cross-section (referred to as "round billet" onwards) is drilled and then stretched without passing it through the step of devastation or formed. In addition, "Tetsu-to-Hagane" vol. 71 (1985), Number 8, pages 965-971 presents a manufacturing apparatus in which a mandrel cutter, that is, a continuous milling cutter, and an extraction gauge, that is, a cutting edge milling cutter , they connect directly. In the method presented in the PJPA Number 63-157705, however, the temperature of the round billet, before being loaded in the furnace and the rolling conditions in the perforator, which is the cut-off drill bit, is not specified. In the apparatus described in the aforementioned "Tetsu-to-Hagane", the two reamers only connect directly without any metallurgical consideration. The objects of the connection in the aforementioned apparatus are only for extracting the bar from the mandrel of the rolled tube, by means of the calibrator, and maintaining the tempering temperature of the tube. Under the present circumstances, an efficient manufacturing method for seamless steel pipe with fine-grained structure, including the in-line process, and suitable apparatus for the manufacturing method in which the treatment equipment does not exist and has been little studied. be available organically. In a typical fabrication process of the seamless steel tube by hot working, a round billet is drilled and rolled into a hollow lamellar structure through the so-called Mannesmann driller, which is typical among the cut-off drilling drills, the The hollow laminar structure is lengthened to form a tube with a lengthening rolling mill (referred to as an "extension" hereafter), for example a plug cutter, a mandrel cutter, etc., and then the tube is finished, that is, dimension is given to the tube with an external diameter previously determined through a milling cutter of last hand, for example, a calibrator, a reducer of stretching, etc. The steps of this conventional manufacturing process, from the casting of an ingot or billet to the manufacture of the final product, are roughly classified as follows: 1) A step of billet manufacture as raw material for a hollow laminar structure, ) A hot working step that includes drilling, lengthening and last-hand rolling, 3) One step of tempering and reventing, that is, one step of heat treatment. Each step of the previous ones from i) to 3) is usually independent of the others. As mentioned above, there are some proposals to combine steps 2) and 3) continuously, and carry out the so-called online process. The typical is the direct tempering process. However, steel grains, which are thermomechanically treated in a simple and direct tempering process, tend to be thicker than those of steel that was treated with conventional off-line heat treatment (tempering and tempering). In addition, it is difficult in the direct tempering process to mass produce a homogeneous seamless steel tube with constant properties. The reason is that the mechanical properties, for example, the resistance of the tube, which is manufactured in the on-line process, vary greatly depending on the sites in the direction of the circumference or in the longitudinal direction, due to the fluctuation of temperature between the positions of a tube or tubes of different batches of manufacture. The inventors have proposed, in Japanese Patent Application No. 6-255088 and PCT / JP95 / 02155, a method for manufacturing a seamless steel tube, characterized by specific hot working conditions and manufacturing fine grains by a treatment of recrystallization after tube formation. This method is making an epoch because, in spite of an online process for the manufacture of the tube, the quality of the steel tube manufactured through this method is comparable to or superior to that of the tube manufactured through the conventional off-line heat treatment. . Occasionally, however, the method does not fully satisfy the requirement that the seamless steel tube have greater strength and greater toughness. PRESENTATION OF THE INVENTION The first objective of the present invention is to provide a method of a continuous in-line process for manufacturing a seamless steel tube with properties that are comparable to or superior to those of the tube manufactured through conventional heat treatment outside of line. The second objective of the present invention is to provide a manufacturing apparatus; wherein each equipment for the aforementioned steps 1) to 3) is arranged in a line (a line of equipment) in order to make the entire compact apparatus; where it is possible to reduce the manufacturing cost by saving space and energy consumption; and in which several thermomechanical treatments can be carried out in accordance with the requirement of the properties of the products. The present invention relates to a manufacturing method following (1), and a manufacturing apparatus following (2) for the seamless steel tube. (1) a method for manufacturing a seamless steel tube characterized in that the following steps (1) to (6) are carried out in a series on a manufacturing line; (1) A step to produce a billet, which has a round cross-section, by casting with continuous solidification, (2) a step of cooling the billet to a temperature not higher than that of the transformation point Ar ,, and then heating and Soak the billet at a suitable temperature to drill it (3) a step of perforating the soaked billet to form a hollow lamellar structure at a degree of deformation not exceeding 200 / sec. (4) the step of laminating in the hollow laminar structure in a seamless steel pipe to a degree of average reformation not less than Ar2 with a reduction ratio of not less than 40%, and finishing the laminate at a temperature of 800 ° C at 1050 ° C, through a milling cutter (a set of milling cutters), in which a milling cutter with continuous lengthening and a milling cutter of last-hand are arranged. (5) One step of cooling the seamless steel tube to a temperature no higher than that of the Ar3 transformation point at a cooling rate of 80 ° C / min. (6) One step of reheating the seamless steel tube to a temperature in the range of 850 ° C to 1000 ° C for a time of 10 seconds to 30 minutes, tempering and then reworking. The above average degree of deformation in step (4) is expressed by the following equation (a). Ve = (Me + Se) / Mt (a) where Me: is the deformation induced by the rolling mill of continuous elongation, Se: deformation induced by the milling cutter of last hand, and Mt: a period of time ( second), from the moment when the upper end of a hollow laminar structure enters the first roll of the continuous stretching mill until the moment when the last roller of the last-hand rolling mill leaves. (2) An apparatus for manufacturing a seamless steel tube, characterized in that the following equipment (A) to (G) is arranged in a series to compose a manufacturing line: (A) a continuous solidification melting process machine to produce a billet with a round cross-section, (B) a billet heating furnace to heat and soak the billet that has been melted, (C) a cut-out rotary drill to pierce the soaked billet to form a hollow laminar structure , (D) a continuous extruder laminator to lengthen the hollow lamellar structure and form a tube, (E) a last-hand laminator for the last-hand rolling of a tube to form a seamless steel tube with a pre-sized determined, (F) a supplementary heating furnace for heating, maintaining or decreasing the cooling of the seamless steel tube after the last-hand lamination, (G) a water treatment equipment heat to temper and repair the seamless steel tube. The following (3) to (5) are preferred embodiments of the apparatus in accordance with the above (2). (3) An apparatus in accordance with (2), wherein the distance between the continuous-length rolling mill (D) and the last-hand rolled mill (E) is less than the length of the steel tube that has been elongated by means of the continuous lengthening rolling mill. (4) An apparatus according to (2), wherein a tube cooling apparatus is installed between the last-hand rolling mill (E) and the complementary heating furnace (F). (5) An apparatus in accordance with (2), wherein the distance between the continuous-length rolling mill (D) and the last-hand rolled mill (E) is less than the length of the steel tube that has been elongated by the continuous elongation rolling mill, and a tube cooling apparatus that is installed between the last-hand rolling mill (E) and the supplementary heating furnace (F). (6) An apparatus, according to (2), (3), (4) or (5), characterized in that the supplementary billet heating equipment is installed between the billet heating furnace (B) and the perforated roller drilling machine (C). It is desirable, additionally, in these installations, that a cooling mechanism is installed for the molten billet, which is capable of cooling it to a temperature no higher than the point Ars, between the continuous solidification casting machine (A) and the heating furnace of billets (B). These apparatuses are particularly suitable for carrying out the manufacturing method (1) of this invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a conception diagram showing an arrangement of equipment, furnace, etc., of the apparatus in accordance with the present invention. Figure 2 is a chart showing the chemical arrangements of steels used as a raw material in the examples of the present invention. Figure 3 is a table showing the relationship between the degree of deformation and the depth of cracks of the hollow sheet structures in a perforation test. Figure 4 is a table showing the conditions of hot work and heat treatment in the examples of the present invention. Figure 5 is a table showing the conditions of hot work and heat treatment of reference and conventional examples. Figure 6 is a table showing the results of the tests of the present invention, the reference and conventional examples. SUITABLE MODE FOR CARRYING OUT THE INVENTION The apparatus of this invention will be explained first, and then the method thereof will be described. I. Apparatus of this invention for manufacturing seamless steel pipe. A design diagram of the seamless steel tube making apparatus of the present invention is shown in Figure 1. The apparatus comprises the equipment (A) to (G) mentioned above. These are explained in order from now on. (A) Continuous Solidification Casting Process Machine In Figure 1, a continuous solidification casting machine 1 with a mold with a round cross section produces a round billet continuously. Various billets with different outside diameters that are required in accordance with the specific pipe manufacturing program can be melted by selecting molds of different internal diameters. Because the cross section of the molten billet is round, no devastating or deformed step is necessary. The step is essential when the round billet is manufactured with castings for example of an ingot or a round billet drilled in the center with continuous solidification with a rectangular cross section. The continuously solidified casting process machine 1 may include one or more mill boxes to apply light reduction to the molten billet in order to improve its metallographic structure. The billet (round billet) is cut to a previously determined length after the core of the billet has solidified for the most part completely. (B) Bulk Heating Furnace. The billet from the smelting process machine 1 is heated and soaked in a billet heating furnace 3 to adjust the temperature of the billet before drilling it by means of a cut-out drilling roll milling cutter 5 (referred to as a "drilling machine" in FIG. ahead) . The billet as it melts is charged in the furnace 3 through the path of the conveyor 2. In order to save energy, it is desirable that the billet coming from the casting process machine be loaded at a temperature as high as possible. the furnace 3. As will be described later, however, when the billet is cooled once at a temperature no higher than that of the transformation point Ar ,, before being heated and soaked to be drilled, the billet grains become thin, and therefore defects in the surface of the hollow sheet structure can be prevented, even if the billet is hot worked in a severe manner in the next perforation step. In order to apply the cooling treatment, it is desirable that cooling mechanisms are installed for the billet in the transport line. The cooling mechanism can be, for example, an extension of the conveyor path 2, and an installation of a cooling apparatus on the conveyor 2. In the billet heating furnace, the billet is heated and soaked at a temperature suitable for perforation. Energy is reduced to heat the billet by making full use of the heat retained during casting. The preferable billet heating furnace 3 is a pilgrim pitch furnace of a type that advances the billet in a transverse direction, or a so-called rotary kiln having a rotary kiln floor. In order to improve the efficiency by increasing the bilge-charge ratio in the heating furnace, it is preferable that the length of the billet loaded in the furnace is a multiple length of the point 8 which will be drilled by the drilling machine. In this case, the billet is cut with a cutting equipment 4a, for example an oxymer or a hot saw, which is installed in the path of the conveyor 4 between the billet heating furnace 3 and a perforator 5, and the pieces of The resulting billet is supplied to the drilling machine. Furthermore, it is desirable to install supplementary heating equipment 4b, for example, a tunnel-type induction heater, downstream in the cutting equipment, in order to compensate for heat loss during the transport or cutting operation. (C) Strawberry Punched Roller Drill (Punch) The round billet of furnace 3 is drilled with a cut-out drilling (boring) drill 5. Because the hot work susceptibility of round billet as cast is poor compared with that of the billet that has been worked by devastation or forging, a hollow lamellar structure fabricated as a cast billet tends to induce surface defects during drilling. It is possible to eliminate the tendency to induce surface defects by refining the grain before drilling and selecting a suitable degree of formation for drilling as will be described later. The grain refining effect can be obtained through an operation in which the billet, after being melted, is cooled once at a temperature not higher than the transformation point Ar1 # then reheated in the billet heating furnace . Although the punching machine can be of any type as long as it is a cut-off drilling bur, it is desirable to use a cut-out drilling rig with a pronounced nail angle, which is capable of producing a thin-walled tube and achieving a high expansion rate of the pipe. tube. When using the stripped roller punch with pronounced nail angle, the number of billets of different sizes can be decreased because the hollow sheet structures, which have various sizes, can be made of billets of the same diameter by this punch. (D) Continuous Elongation Reamer (Elongation) A continuous elongation reamer (referred to as "elongation" onwards) 7, which has several laminator boxes, laminates the perforated hollow laminar structure to form a tube. The typical lengthener is the mandrel mill. Any mandrel cutter can be used as an extension cord as long as it has a retainer for a mandrel rod (bar retainer), which holds the back of the mandrel bar, and pulls the bar back through a series of rollers. caliber of the strawberry, in order to reuse the bar after the elongation laminate. Especially, it is preferable that the mandrel mill be equipped with the bar retainer, with a function of controlling a transfer rate of the mandrel bar regardless of the rolling speed of the material during the rolling of the hollow web structure.

The hollow laminar structure, obtained by perforation, is transported through the conveyor 6, for example, transverse conveyor or a longitudinal roller conveyor, to the entrance table of the extension 7. The mandrel, whose rear end is retained by the retainer , it is inserted into the laminar structure on the table and then the lamination of the hollow laminar structure begins. (E) Ultimate Hand Laminating Reamer A last-hand rolling mill 8, comprising several mill boxes, is referred to as a gauge or a stretch reducer. The roll of last-hand rolling is used in order to make the elongated tube, from the extender 7, and form the tube with a predetermined outside diameter. It is desirable that the extension bit 7 of the last-hand rolling mill 8 be arranged closely in a series on the same process line, with a distance less than the length of the elongated steel tube with the extension. In greater detail, it is desirable that these two cutters be arranged in such a way that the upper part of the steel tube, which left the extension 7, can be bitten and rolled by the laminator boxes of the last-hand rolling mill 8, while that the rear end of the tube remains and continues to be rolled in some boxes of the laminator of the extender. Therefore, the temperature drop of the steel tube during work can be suppressed, and the storage of the deformation can be increased. In the tube thus laminated, the effect of refining grain in the subsequent heat treatment is obtained and remarkably improves the properties of the tube, for example, toughness, resistance to corrosion, etc. The last-hand rolling mill, for example a calibrator or a stretch reducer, can be of any type, as long as it does not have any tool to regulate the inner surface of the tube. Particularly, it is preferable to use an extraction type or stretch reducing gauge, in which the bar of the mandrel in the elongated tube can be separated and removed therefrom. A group of milling cutters comprising the extension 7 and the last-hand rolling mill 8 described above is referred to as the "milling train" M hereinafter. (F) Complementary Heating Furnace A supplementary heating furnace 10 is used for the heat treatment in order to provide the steel pipe with the properties required after the last-hand rolling. One of the remarkable features of the apparatus of the present invention is that the complementary oven is installed in the same manufacturing line, including the milling cutters, etc. In step 6 of the method of this invention, as described below, this supplemental heating furnace is used as a reheat furnace prior to quenching. The reheating not only controls the tempering temperature, but also decreases a temperature difference between the positions along the longitudinal or circular directions of a pipe, and between the pipes of the same manufacturing batch. The decreased temperature difference results in the improvement of the dispersion of properties between positions in a steel tube or between tubes of the same manufacturing lot. The dispersion of properties originates the variation of the heat treatment conditions. In addition, the supplementary heating furnace 10 can be used for various types of treatment in the rolled tube, for example slow cooling, maintaining temperature, etc. Consequently, the installation of the supplementary heating furnace 10 makes it possible for many types of heat treatment to be carried out in the on-line process, in order to satisfy the various requirements of the properties of the pipes. A cooling apparatus 9 can be installed before the supplementary heating furnace. For example (the details will be described later), the rolled steel tube with the last-hand rolling mill 8 is cooled in order to transform it once, by means of the cooling apparatus 9, at a temperature not higher than that of the point of transformation Ar3, preferably not greater than that of point Ar. and then reheating at a temperature no lower than that of Ar3 in order to transform it inversely, in the supplementary heating furnace 10, when quenching. As a result of this processing, a steel tube having an extremely fine grain structure is obtained, even if it is treated in the line process. The properties of the steel tube are comparable to or higher than those of the tube that is tempered in the conventional off-line process. (G) Tempering and Tempering Equipment A tempering equipment 11 is used to temper the use of steel after last-hand rolling in a condition or rolled or reheated. Generally, water cooling equipment is used. In order to quench the cooling speed fast enough for a thick-walled steel pipe, it is desirable that the cooling mechanism be able to cool both the inner and outer parts of the pipe at the same time. For example, a team that has features of water jet cooling for the inner part and water-laminar flow cooling for the outer part, is preferred. A tempering equipment 12, which can usually be a heating furnace, is placed in the downstream flow of the tempering equipment on the same line. It is recommended that a rectifier 13 be installed in order to straighten the steel tube after quenching. In addition, auxiliary apparatuses can be installed, for example a cutter for cutting the end of the tube to an equal length in the same line for the manufacture of tubes, although they are not illustrated in Figure 1. As described above, the apparatus of the present invention invention makes it possible for all the processes of manufacturing steel tubes, from casting the billet to drilling, rolling and heat treatment, to be carried out in the on-line process. Because the apparatus is compact, not only space can be reduced, but the transport of materials between the steps can be simplified and an amount in energy consumption can be reduced. Next, the method of manufacturing a seamless steel tube with superior mechanical properties and excellent corrosion resistant, by using the aforementioned apparatus, that is, the method of (1) above, is described. II. METHOD OF MANUFACTURE OF THE PRESENT INVENTION Each step of the manufacturing method of the invention is explained below. (1) Round Loop Manufacturing Step A round cross-sectional billet is produced using the continuous solidification casting machine 1 with round cross-section molds with various inner diameters. The cast round billet, with a specified outside diameter and length, in accordance with the hot work schedule for the pipe, is sent to the drilling step without passing through the usual devastating or formed step. (2) Step of Cooling and Reheating of the Casting The round cast billet, after being cut to the required length, if necessary, is cooled to a temperature no higher than that of the transformation point Ar, preferably in a temperature range that it is located between the no greater than Ar, but not lower than the ambient temperature. After that, the billet is loaded in the subsequent billet heating furnace. The following are reasons for performing the mentioned cooling. The billet is fed to the punch 5 in a condition when the portion of its core has solidified. Therefore, as long as the solidification is complete, the higher the billet temperature, the more it is loaded in the furnace, the more the energy consumption amount is reduced to heat it. In the method of the present invention, however, the billet is intentionally cooled in order to refine the grains to improve its susceptibility to hot work. The refined grains make it possible to perform an extremely heavy deformation of the material during drilling, for example, a thin-walled perforation or a high-expansion-rate perforation, or both.

It is necessary, in order to refine the billet grains, to cool the billet once in a temperature range no greater than the Ar point, in order to transform it into a ferrite structure of an austenite structure. Because the goal of this cooling is to induce the transformation of austenite to ferrite, it is not necessary that the cooling temperature be too low, as long as it is not greater than the point Ar,. In order to reduce the reheat energy, it is preferable that the billet be cooled to a temperature as high as possible, as long as it is not greater than the Ar point, for example a range of 400 ° C to Ar is recommended, . For the purpose of the aforementioned treatment, a consideration on the disposition of the equipment is necessary. The temperature of the molten round billet, after solidification, must be less than the Ar point, (and not less than the ambient temperature) by the time it is loaded in the heating furnace. This control temperature can be achieved as follows: the length of the conveyor path (conveyor path 2 shown in Figure 1) from the melting process machine with continuous solidification to the heating furnace is suitably determined to allow the billet to be cooled to a temperature not higher than Ar, or a cooling device could be installed by force such as a water sprinkler in the path of the conveyor.

After this, the billet is cooled, reheated and soaked sufficiently at a suitable temperature to be drilled with the punch in the next step. The temperature of heating and soaking is determined in consideration of the high temperature ductility and the strength of the material to be drilled, because the optimum temperature for hot working varies depending on the materials. In general, the optimum temperature is in a range of 1100-1300 ° C. After reheating the billet, if the temperature of the billet falls during operations, for example the cutting of the billet with a cutter, to a required length, etc., the billet can be heated again with the auxiliary heating apparatus 4b. (3) Drilling Step When drilling a billet that is molten and has a hard grain structure the surface defects are usually induced in a hollow lamellar structure by heavy hot work, that is, drilled. In the method of the present invention, it is possible to obtain a crack-free laminar structure by using the billet having fine-grained structure obtained in step (2) above and the perforated under a condition of deformation degree not greater than 200 / sec. There is no critical lower limit in the degree of training as long as it is not higher than 200 / sec. However, the degree of deformation is preferably not less than 0.1 / sec. When a degree of deformation is less than 0.1 / sec, the service life of the tools, for example plug, guide shoes, etc. , it is significantly shortened due to the increased temperature of the tool as a result of long contact with the laminated material. As described above, for this step it is desirable to use a pronounced roll angle piercing hole punch. If the susceptibility to hot work of the material is poor, the perforation should be carried out at a temperature as high as possible. In order to keep the material at a high temperature during drilling, it is advisable to heat the billet by auxiliary heating equipment 4b, for example, the aforementioned tunnel type induction heater installed in the front of the drilling machine. (4) Elongation Step and Last Hand Laminate In this step, the laminar structure is continuously lengthened and its last-hand rolling is made to form a tube as final product through the continuous lengthening mill, that is, the lengthener ( mandrel mill) comprising several mill boxes. Although the hot work is done at a low temperature compared to the drilling in the previous step, due to the increase in temperature, it is important that the material be sufficiently deformed in this step in order to obtain the thermomechanical treatment effect to refine the grain. In the method of the present invention, the milling train M shown in FIG. 1 is used, wherein the continuous extension mill 7 and the last-hand milling mill 8 are not disposed independently far apart, but which are arranged very closely like a built-in strawberry. More specifically, in the milling train M, the two reamers 7 and 8 are arranged in series in a process line, with a distance less than the length of the elongated steel tube by the continuous extension mill 7. In this arrangement, It is possible to immediately apply additional work to the tube through the last-hand rolling mill, that is, calibrator or stretch reducer, before recovering the induced deformation by working on the stretcher (mandrel mill), and it is updated a grain refinement effect of the "normal-reverse transformation" in the subsequent heat treatment. Even if the tubes are hot-worked with the same program of rolling passes, there is a difference in grain sizes, after the normal-reverse transformation treatment, between the tube, which has been worked by an extension and a milling cutter last hand independently arranged with a greater distance than the previously mentioned distance, and the tube worked in the milling M train. Apparently, the grains of the steel tube, worked in the two milling cutters arranged close to each other (M milling train), were they become thinner than those of the tube worked in strawberries arranged independently. When working with milling train M, the average deformation degree (Ve) defined by Equation (a) above should not be less than 0.01 / sec. If the average degree of formation is less than 0.01 / sec., The effect of grain refinement in subsequent steps is insufficient because the deformation of the material is released by the work, due to the recrystallization during the intervals between the passes of laminate. The rolling work speed in the milling train M must not be less than 40% in the ratio of reduction of the cross-sectional area. If the proportion of work is less than 40%, the effect of grain refining, after the normal, inverse transformation, is sufficient. It is also very important the effect of the finish temperature on the tube, on the last-hand laminate through the milling train M. When the temperature is in the range of 800 ° C to 1050 ° C, the grain is refined significantly through the normal-reverse transformation in subsequent steps. It is unnecessary to provide particular upper limits for the average degree of training and for the proportion of work, as long as it is not less than 0.01 / sec and not less than 40%, respectively. However, the average degree of deformation is preferably not less than 10 / sec because the degree of more than 10 / sec results in a short service life of the tools such as the mandrel bar or the mandrel mill. In addition, the working ratio is preferably not greater than 95% because a work ratio greater than 95% causes surface defects in the tube. (5) Cooling Treatment Step It is a remarkable feature of the method of this invention that the heat treatment, ie, the normal / inverse transformation, is applied to the steel tube between the last-hand rolling mill (calibrator) and the direct tempering equipment, after lengthening and making the last-hand rolling by milling train M. Because the grains of the steel tube are effectively refined by a combination of hot working by the milling train M and the cooling-reheating treatment, the properties of steel tube become comparable or superior to those of a steel tube, which was produced by the tempered and tempered treatment in the conventional off-line process. The cooling of this treatment is carried out using the previous cooling equipment shown in Figure 1. The cooling rate of the aforementioned treatment should not be less than 80 ° C / min because the ferrite grains, induced by austenite transformation, become coarse grains when the cooling speed is too slow. The cooling finishing temperature should not be higher than point Ar3 in order to refine the grain by normal-reverse transformation. More preferably, the finish temperature is not greater than the Ar point, in order to maximize the refining effect. Although the finishing temperature may be room temperature, it is desirable to maintain a temperature as high as possible (for example about 500 ° C) to save energy in the next reheat step. (6) Reheat Treatment Step; In this step, the steel tube, which was cooled and transformed into the ferrite phase, after last-hand rolling, is reheated and maintained at a temperature no lower than that of the Ar3 point, such that the ferrite phase of the Steel tube is inversely transformed into the Austenite phase. Additionally, in this step, the dispersion of the properties of the steel tube, after quenching and reworking, is minimized, by sufficient heating, to ensure a suitable tempering temperature and uniform soaking. The superheat is carried out by the complementary heating furnace 10 shown in Figure 1. When the reheat temperature is lower than 850 ° C, the temperature is maintained in time in less than 10 seconds, the inverse transformation is insufficient. On the other hand, when the reheat temperature is higher than 1000 ° C, or the holding time is greater than 30 minutes, the grains of the tube become thick. Therefore, it has been determined that the temperature range should be 850 ° C to 1000 ° C and the retention time should be 10 seconds to 30 minutes. A tempering temperature must not be less than the transformation point Ar3 in order to obtain sufficient toughness and strength. In the method of this invention, tempering is effected by rapid cooling of the aforesaid temperature, 850-1000 ° C. In order to quench at a sufficiently rapid cooling rate, even if the steel tube is considered thick, it is desirable to use the above cooling mechanism in which both the inner and outer surface of the tube can be cooled at the same time. The steel tube, after being tempered, is tempered by tempering equipment, which is placed downstream in the tempering equipment on the same line. Because tempering is an important step that affects the properties of the tubes, it is also necessary to select at a suitable temperature in accordance with the required properties, and soak the tube after annealing for a sufficient period at this temperature. The deflection of the previously determined tempering temperature must be within ± 10 ° C at most, preferably within ± 5 ° C. By this treatment, the dispersion of the elastic limit (YS) and the tensile strength (TS) is able to be suppressed within ± 5 gf / mm2 of the values sought. After hardening, the steel tube is straightened, cut at the end, and other treatments that usually accompany it are applied, then sent as the final product. EXAMPLE [Example 1] Steel A and steel B having chemical compositions shown in Figure 2 were melted into billets by a continuous solidification casting machine with a round mold with an inner diameter of 90 mm. After solidification, some billets of each steel were loaded at 900 ° C, at a higher temperature of the transformation and Ar3, in an oven of 1250 ° C and kept for 1 hour, and other billets of each steel were cooled to 550 ° C or 420 ° C, then loaded in the oven at the same temperature as the previous one and kept for the same period of time. After being heated, these billets were drilled to form hollow sheet structures for the perforation test with various degrees of deformation with a drill for experimental use. The results of the test are shown in Figure 3.

As apparent in Figure 3, surface defects were observed in the hollow sheet structures prepared by drilling at 100 / sec. Deformation grades from billets, which were loaded in an oven from 1250 ° C to 900 ° C without cooling unless from point Ar3 after melts. On the other hand, when the billets were reheated after having been cooled below the Ar3 point, good laminar structures were made without surface defects with these billets at a degree of formation up to 200 / sec, while surface defects were observed in the hollow lamellar structures perforated to a degree of deformation of 250 / sec or greater. It can be concluded from the above results that hollow sheet structures without surface defects are produced under severe drilling conditions, as long as the degree of formation is not greater than 200 / sec. , provided that the billets have been cooled in a temperature range lower than point Ar, after heating them. [Example 2] Steel A and steel B shown in Figure 2 were cast into billets by a continuous solidification casting machine with a round mold with an inner diameter of 90 mm. After solidification, these billets were cooled in a lower temperature range of Ar, then loaded in a heating oven of 1250 ° C and maintained for 1 hour. Seamless tubes were fabricated with these billets, under various conditions for each tube making step as shown in Figures 4 and 5. Figure 6 shows the results of strength test, previous grain size of austenite (grain size) before the transformation) and tenacity (vTrs) of the tubes obtained. The tempering temperatures were modified, depending on the type of steel, in such a way that the resistance of the tubes manufactured with the same steel can be almost at the same level. The test tubes Numbers 33 and 34 of the Figure 5 were hot rolled with the conventional milling cutter in which the lengthening and last-hand rolling mills are placed separately, and heat treated in the conventional off-line process, comprising reheating, tempering and tempering steps. When comparing the results shown in Figure 6 of the tubes manufactured with steel A and steel B respectively, it is apparent that the tubes of Numbers 1-20 have smaller grain sizes and better tenacity, compared to those of the tubes Numbers 33 or 34, which were manufactured with the conventional method (the so-called QT treatment). The tubes Numbers 21, 22, 27 and 28 that were manufactured under unsuitable working conditions in the mandrel and caliper bur, and tube numbers 23-26 and 29-32, which were manufactured under cooling or reheating conditions not suitable, have lower tenacity, because the refining effect of the normal-inverse transformation treatment is not sufficient. INDUSTRIAL APPLICABILITY In accordance with the manufacturing method of the present invention, not only can the steps from billet casting to hot work and heat treatment in series in the in-line process be performed, but also steel pipes with excellent qualities comparable to or superior to those of the tubes manufactured in the conventional off-line process. This method can be carried out at low cost using the apparatus of the present invention. The apparatus of the present invention is a compact one, in which all the necessary equipment is installed. Therefore, it has many advantages to take advantage of factory space and simplify the manufacturing process. In addition, the apparatus is applicable to changing heat treatment conditions for various required properties of the product.

Claims (1)

1. CLAIMS 1. A method for manufacturing a seamless steel tube comprising the following steps (1) to (6) which are carried out continuously in a series; (1) a step of producing a billet, having a section in round cross-section, by continuous casting, (2) a step of cooling the billet to a temperature no greater than that of the transformation point Ar,, and then heating and to soak the billet at a suitable temperature to perforate the same, (3) a step of perforating the soaked billet in a hollow laminar structure at a degree of formation not greater than 200 / sec. (4) a step of laminating the laminar structure in a seamless steel tube to an average degree of deformation not less than 0.01 / sec, with a reduction ratio of not less than 40%, and finishing the laminate at a temperature of 800 ° at 1050 ° C, by means of a milling train and in which a milling cutter with continuous elongation and a milling cutter of last-hand lamination are arranged in close proximity, (5) a step of cooling a seamless steel pipe of the temperature not higher than the transformation point Ar3 at a cooling rate not lower than 80 ° C / min, (6) a step of reheating the seamless steel tube to a temperature in a range of 850 ° C to 1000 ° C , for a period of time from 10 seconds to 30 minutes, temper and then recover. 2. An apparatus for manufacturing a seamless steel tube comprising the following equipment (A) to (G), which are arranged in a series to compose a manufacturing line: (A) a continuous melting and solidification process machine to produce a billet having a cross section in round cross section, (B) a billet heating furnace to heat and soak the billet that has been melted, (C) a perforated roller milling cutter to pierce the soaked billet to form a hollow laminar structure, (D) a milling cutter of continuous elongation to lengthen the hollow laminar structure and form a tube, (E) a milling cutter of last hand for the lamination of last hand of the tube and form a steel tube without seams with a previously determined size, (F) a complementary heating furnace to heat, maintain or cool down the seamless steel tube slowly after the last-hand lamination, (G) a treatment equipment with heat to temper and repair the seamless steel tube. 3. An apparatus, according to claim 2, characterized in that the distance between the rolling mill of continuous elongation (D) and the milling cutter of last hand (E) is less than the length of the tube that has been lengthened by means of the milling cutter of continuous elongation. 4. An apparatus, in accordance with the claim 2, characterized in that a pipe cooling apparatus is installed between the last-hand rolling mill (E) and the complementary heating furnace (F). 5. An apparatus, according to claim 2, characterized in that the distance between the continuous elongation rolling mill (D) and the last-hand rolling mill (E) is less than the length of a steel tube without process that has been lengthened with the continuous lengthening rolling mill, and a tube cooling apparatus is installed between the last-hand rolling mill (E) and the supplementary heating furnace (F). 6. An apparatus, according to Claim 2, 3, 4, 5, characterized in that the supplementary billet heating equipment is installed between the billet heating furnace (B) and the cut-off roller milling cutter (C). EXTRACT DB THE INVENTION A method for manufacturing a seamless steel tube, with excellent properties with high productivity in online processing, and an apparatus in which the equipment for melting and heat treating, etc., is available online. In the method the following steps are carried out (1) a (6) successively. (1) A round billet is melted with a casting process machine with continuous solidification 1. (2) The billet is heated and soaked in the furnace 3 after it is cooled to a temperature no higher than the Ar point,, (3) the soaked billet is drilled with a perforator 5 at a degree of formation not greater than 200 / sec to form a hollow laminar structure. (4) The hollow laminar structure is lengthened and the last-hand lamination is done, at a degree of formation not less than 0.01 / sec., With a working formation of not less than 40% and a finishing temperature between 800 and 1000 ° C to form a seamless steel tube. (5) The tube is cooled to a temperature no higher than point Ar3, with a cooling rate of not less than 80 ° C / sec by means of a cooling apparatus 9. (6) The cooled tube is reheated to 850-1000 ° C, for 10 seconds-30 minutes, then it is tempered and heated.