MXPA97002792A - Procedure for manufacturing steel tubes without cost - Google Patents

Procedure for manufacturing steel tubes without cost

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Publication number
MXPA97002792A
MXPA97002792A MXPA/A/1997/002792A MX9702792A MXPA97002792A MX PA97002792 A MXPA97002792 A MX PA97002792A MX 9702792 A MX9702792 A MX 9702792A MX PA97002792 A MXPA97002792 A MX PA97002792A
Authority
MX
Mexico
Prior art keywords
temperature
billet
laminator
steel tube
punching
Prior art date
Application number
MXPA/A/1997/002792A
Other languages
Spanish (es)
Other versions
MX9702792A (en
Inventor
Kondo Kunio
Okada Yasutaka
Tanimoto Seiji
Original Assignee
Sumitomo Metal Industries Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to JP6-255217 priority Critical
Priority to JP25521794 priority
Priority to JP6-255,217 priority
Application filed by Sumitomo Metal Industries Ltd filed Critical Sumitomo Metal Industries Ltd
Priority to PCT/JP1995/002155 priority patent/WO1996012574A1/en
Publication of MXPA97002792A publication Critical patent/MXPA97002792A/en
Publication of MX9702792A publication Critical patent/MX9702792A/en

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Abstract

The present invention relates to a method for manufacturing the seamless steel tube comprising the following steps from 1) to 8) which are specified in the following series: (1) a step of producing a billet having a circular cross section by continuous casting, (2) a step of cooling the billet to a temperature no greater than the transformation temperature Arl, (3) a step of heating the billet cooled to a temperature no greater than the transformation temperature Arl up to a temperature which it allows punching the billet, (4) a punching step, at a tension value not greater than 200 / sec. the billet heated to a temperature that allows said punching of the billet to obtain a hollow cylindrical body, (5) a step of obtaining a steel tube by stretching and finishing rolling of the hollow cylindrical body using a continuous stretching laminator and a continuous rolling mill of termination, which are directly connected to each other, at an average voltage value not less than 0.01 / sec, a reduction ratio of not less than 10%, and a termination temperature between 800 and 1,050 ° C., (6 ) a step of recrystallization of the steel tube at a temperature not less than the transformation temperature Ar3, (7) a step of quenching the steel tube obtained in step (6) from a temperature not lower than the transformation temperature Ar3 , and (8) a step of tempering the steel tube tempers

Description

"PROCEDURE FOR MANUFACTURING STEEL TUBES WITHOUT SEAMING" The present invention relates to a process for manufacturing seamless steel tubes and to an arrangement used to carry out said process. More particularly, the present invention relates to a method and an arrangement for manufacturing seamless steel tubes having excellent strength, toughness and corrosion resistance using a simple and continuous manufacturing process and equipment, thereby obtaining tubes of efficient seamless steel at a reduced cost. The tubular articles of the oil field, duct tubes, heat exchanger tubes, tubes in general and tubes for bearing bearings, are usually manufactured from seamless steel tubes. The seamless steel tubes used for such purposes are usually made of carbon steel, low alloy steel which have alloy components such as Cr and Mo and high Cr alloy stainless steel tubes. The seamless steel tubes are normally manufactured by the Mannesman mandrel rolling method. However, this method is often complex due, for example, to the fact that the hot processes are carried out by means of a mandrel and high levels of characteristics are required of the resulting product. Figure 1 illustrates an example of a manufacturing process using the Mannesman mandrel rolling method. There are a number of steps in this to form an initial steel ingot to a final tubular product. The material to be processed is subjected to several types of processes, heating and cooling repeatedly. The dashed lines in Figure 1 indicate line changes, which implies a transfer of materials between stages and processes such as temporary storage. In a manufacturing process using Mannesman's mandrel rolling method, numerous lines are used, which requires different types of equipment that have advanced functions and that consume a significant amount of energy. The above results in an inevitable increase in costs. In order to reduce manufacturing costs, it is necessary to increase productivity, reduce equipment costs and reduce operating costs. More specifically, in the manufacture of seamless steel tubes it is desirable to simplify the manufacturing stages and equipment and also obtain products that have characteristics superior to conventional tubes. For this purpose a variety of techniques for the manufacture of seamless steel tubes has been developed.
In particular, different proposals were made concerning the formation of a billet from a steel ingot, hot punching, stretching, finishing lamination and the subsequent thermal treatments to provide the product with certain characteristics after the final lamination. With respect to the step of manufacturing a billet having a circular cross-section from a steel ingot, a proposal was made in which a circular bar is produced by continuous casting in order to avoid a preparation or forging lamination. By way of example, published Japanese patent application (kokai) No. 63-157705 describes a method for manufacturing a seamless steel tube in which a billet having a circular cross-section is punched and the latter being stretched. In the process described in this publication the technical problems with respect to the heating conditions for the punching of the billet and the punching conditions of a punch (ie lamination / punching with oblique cylinders) have not been sufficiently solved. Consequently, a punched material according to this method tends to crack formation during punching. further, from the point of view of the continuous forming processing stages, the publication "Iron and Steel", Vol. 71 (1985), No. 8, pgs. 965-971, describes a manufacturing arrangement in which a mandrel laminator (which is a continuous elongation laminator) and a gauge (which is constituted by a finishing laminator) are directly connected. The reason for directly connecting a continuous laminating laminator and a laminating finishing laminator as described in this publication is only to ensure a cooling temperature. As a natural consequence, the material that has been subjected to a finishing lamination is subjected to a cooling while still at an elevated temperature, which results in a thickening of the grains and a reduction in the toughness of the resulting tubular product. With respect to the stages of heat treatment, different improvements have been proposed to give the material certain characteristics that the final product must possess. Seamless steel tubes are required to have high quality and excellent characteristics. Consequently, as illustrated in FIG. 1, thermal treatments, including rapid cooling or quenching and tempering, which are critical to the quality of the product, are normally carried out outside the line since it allows strict control of the product. the production line The foregoing means that a hardening apparatus and an annealing furnace are normally provided independently of the pipe forming line. Such off-line processes affect the simplification of the manufacturing arrangement and the saving of energy consumption. To solve this problem, it has been tried in a recent manufacture of seamless steel tubes to carry out the in-line tempering by means of a so-called direct cooling method taking advantage of the heat of a pipe that has been subjected to the finishing laminate. Said direct hardening method is advantageous in that it renders the annealing equipment unnecessary and means the manufacturing stages, achieving a considerable reduction in costs. For example, published Japanese patent applications (kokai) Nos. 56-166324, 58-120720, 58-224116, 56-020423, 60-033312, 60-075523, and 62-151523 describe a method for manufacturing steel tubes. seamless comprising a direct tempering in which the steel tube is forcedly cooled immediately after it passes through the finishing lamination stage. Unfortunately, the products obtained through a process that includes a direct tempering do not have the same qualities as those products obtained through the process in which the off-line tempering is carried out. In other words, the grains in the microstructure are thicker than those obtained by conventional methods and as a result a lower toughness and corrosion resistance is obtained. In order to refine the grains, an online thermomechanical treatment has been proposed. For example, published Japanese patent application (kokai) No. 56-003626 describes a process in which a cooling stage and a reheat stage are incorporated between a stretch laminator and a finishing laminator. The published Japanese patent applications (kokai) Nos. 58-091123, 58-104120, 63-011621 and 04-358023 describe a method in which a combined reheat cooling treatment is carried out after the finishing laminate. The published Japanese patent application (kokai) No. 58-117832 discloses a method in which cooling and reheating are carried out twice, the first time during the course of rolling (between the stretch laminate and the laminate). termination) and for the second time after the finishing laminate. Any of these methods employs an in-line combination of cooling and reheating and represents a total of two or more iterations of transformation from austenite to ferrite and transformation of ferrite to austenite. Any of the above methods requires that the material of the tube be processed by forced cooling to a temperature range at which the transformation begins or is completed and is then reheated to a temperature range at which austenitization is complete. In consecuense, these methods consume large amounts of energy, causing high energy costs. Additionally, they require complicated manufacturing equipment, increasing manufacturing costs for manufacturing facilities. Moreover, mechanical characteristics such as resistance, etc. etc. The seamless steel tubes manufactured by a direct hardening method are strongly inconsistent. The above is due to the fact that the tempering temperature is not uniform in the longitudinal direction of the steel tube or because the temperature differs between different batches of manufacturing. As a result, there were still problems to be solved in order to efficiently produce seamless steel tubes in mass that have a uniform quality. Consequently, the methods mentioned above require improvements in equipment costs and operating costs, and also in the characteristics of the products obtained compared to conventional methods that comprise off-line tempering. If in the manufacture of seamless steel tubes, the respective stages are arranged independently out of line, the problem arises that more space is required to store the bars and similar materials to be processed because the processing speeds differ. from stage to stage. For example, since a beach is required to store billets to be punched and a place to temporarily store steel tubes before their thermal treatment, a large surface area is generally required. Also, in order to transport the materials stage by stage, a number of conveyors are required that include means such as cranes, trucks, etc. As described above, none of the conventional methods described were successful for the manufacture of seamless steel tubes that exhibit excellent characteristics using simplified manufacturing stages and equipment with high productivity and reduced manufacturing costs. The present invention has the purpose of solving the aforementioned problems and one of the objects of the present invention is to provide a process for manufacturing seamless steel tubes that have characteristics superior to those conventional products manufactured by simple manufacturing steps and equipment to reduced costs with good productivity, and in addition to a manufacturing provision to carry out the procedure. An object of the present invention is to provide a process for manufacturing seamless steel tubes that have superior characteristics to those of conventional products manufactured by simple manufacturing processes and equipment at reduced costs with good productivity, and manufacturing provisions to carry out The procedure. The manufacturing process of the present invention comprises the following operative steps 1) to 8) which are carried out in series on the same manufacturing line: (1) producing a billet having a circular cross-section by continuous casting; (2) cooling the billet to a temperature no greater than the transformation temperature A; (3) heating the cooled billet to a temperature no greater than the transformation temperature Arl to a temperature that allows the punching of the billet; (4) punching the billet heated to a temperature that allows said punching so as to obtain a hollow body or jacket, at a deformation rate not greater than 200 / sec; (5) Obtaining a steel tube by stretching and rolling the body or hollow sleeve using a stretch laminator and a finishing laminator, which are directly connected to each other, at an average deformation rate of not less than 0 , 01 / sec, a reduction ratio of not less than 10%, and a termination temperature between 800 and 1,050 ° C; (6) recrystallization of the steel tube at a temperature not less than the transformation temperature Ar3; (7) tempering the steel tube obtained in Step 6 from a temperature not lower than the transformation temperature Ar3; and (8) tempering of the hardened steel tube. The manufacturing arrangement employing the method of the present invention includes equipment corresponding to the aforementioned steps 1) to 8), said equipment being connected in series for continuous and sequential operation. According to the process and arrangement for manufacturing seamless steel pipes according to the present invention, it is possible to obtain seamless steel pipes having characteristics superior to those of conventional seamless steel pipes at reduced manufacturing costs and with good productivity. Accordingly, the present invention contributes significantly to the manufacture of seamless steel tubes on an industrial scale. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a flow diagram showing an example of a conventional process for manufacturing seamless steel tubes. Figure 2 illustrates a flow diagram showing steps for manufacturing seamless steel tubes according to the present invention.
Figure 3 is a schematic illustration illustrating the arrangement of the equipment for manufacturing seamless steel tubes according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention carried out extensive studies in an attempt to simplify the manufacturing process of seamless steel tubes and to find the optimal treatment conditions at each stage of the process. Based on their new discoveries, the inventors were successful in creating the following manufacturing procedure and the following disposition, which do not present the aforementioned problems. Figure 2 illustrates the manufacturing process of the present invention. The present invention is based on the following ideas and technical proposals. (1) A billet having a circular cross section is produced by continuous casting. This proposal eliminates the preparation (blooming), the lamination, and the forging stages that are required when using a steel ingot or a block of square section cast continuously. (2) The billet that has passed through the casting stage is cooled to a temperature no higher than the transformation temperature Arl before it is heated for punching. By means of this cooling process the granule etria can be effectively reduced in the subsequent heating stages. The refinement of the granulometry prevents cracks from forming in the billet even during severe hot punching processing. (3) After the billet has been cooled to a temperature no higher than the transformation temperature Arl it is heated to a temperature that allows punching. At this point, the heating of the billet is preferably initiated from a temperature that is as high as possible, as long as it is cooled down from the transformation temperature Arl so as to fully utilize the heat accumulated in the casting stage. By this proposal, the energy required for heating the billet can be reduced considerably. Moreover, by comparing the methods in which steel ingots or square blocks cast continuously are used, in the present case the size of the storage beaches can be considerably reduced. (4) An oblique roll mill / punch is used for punching. Once a billet is perforated, a suitably selected deformation regime is applied thereto so as to avoid forming cracks in the billet. (5) For the continuous stretch lamination and finishing laminate which is carried out after the punching, a stretch laminator and a series finishing laminator are placed next to each other on an equiaxial line. By means of this arrangement, the reduction of the temperature of the material to be rolled is suppressed, the traction induced by the processing effectively accumulating. This proposal is effective to achieve a remarkable refinement of the grains in a subsequent stage of recrystallization. (6) After the finishing lamination and before tempering, it is applied to the steel tube to process a recrystallization treatment. That is, when the steel tube is transferred from the rolling stage of. Upon completion of the tempering step, the tube is slowly cooled by retaining the heat or heated to cause recrystallization. Due to the accumulations of stresses induced by the processing in the previous stage and the treatment in this stage, the grains can be effectively refined. In this step, a heating furnace can also be used if desired to adjust the temperature of the steel pipe. By such adjustment of the temperature of the steel tube, the differences between the tempering temperatures in the longitudinal direction of the tube and between the batch of tubes manufactured can be minimized. Even moreBy increasing or decreasing the temperature of the tube, the precipitation of carbon nitrides or similar substances can be controlled. As a result, the strength of a material having a certain composition can be controlled and additionally the thickening of the recrystallized grains can be suppressed. (7) A steel tube that has been adjusted to present an appropriate range of grain size and the appropriate amount of precipitates is tempered immediately from a temperature no lower than the Ar3 transformation temperature. Note that this stage does not exclude the gentle cooling of the tube at a temperature higher than the transformation temperature Ar3. (8) The tube is then tempered in a tempering furnace provided in the same processing line. The series of processes from (6) to (8) improves the toughness, corrosion resistance and other characteristics compared to conventional products. The present invention was carried out based on the technical concepts described above. Figure 3 is a schematic drawing illustrating an arrangement of manufacturing equipment for carrying out the method of the present invention. Referring 20 to Figures 2 and 3, the present invention will be described below in detail. Stage 1) - Production of a billet. A billet having a circular cross section is produced using a continuous casting equipment with a mold with a molten steel inlet with a circular cross section. The internal diameter of said mold was selected according to the external diameter of a billet, which is determined according to the outer diameter of the steel tube to be manufactured. Accordingly, a billet having an outer diameter and a predetermined length is continuously cast. Reference 1 in Figure 3 indicates a continuous casting equipment with a mold with an inlet having a circular cross section. Said continuous casting equipment has a structure that allows to change the molds according to the outer diameter of a billet to be produced. By using this continuous casting equipment, a round billet having an outer diameter that fits the tube forming program is continuously cast. At the downstream end of the billet casting section there is provided a disconnector device for severing a billet once the core of the billet has solidified almost completely or completely. The continuous casting equipment may include a cylinder frame to apply a slight reduction to the billet for the purpose of reforming the etalographic structure of the cast billet, etc. In this case the cylinder frame is located on the upstream side of the billet switch device. Stage 2) - Refining of a billet The cast billet is cooled to a temperature between the ambient temperature and the temperature of fc? - Arl transformation. The purpose of this treatment is to give the billet characteristics of hot workability so that it can withstand the heavy work applied by the oblique roll laminator / punch press (hereinafter referred to as punching) in the subsequent punching step. In order to improve the capacity of hot workability of a billet, it is necessary to refine the metallographic structure of it. From According to this strategy of the present invention, the billet is temporarily cooled to a temperature no higher than the transformation temperature Arl, at which the transformation of the austenite phase to the ferrite phase is completed, the metallographic structure then being refined. of the billet by exploiting the applied heat for the purpose of punching the billet. The cooling temperature at this time is preferably close but is not greater than the transformation temperature Arl so as to minimize the energy required to heat the billet in the stage next. The lower limit of the cooling temperature, however, can nevertheless be just above room temperature. To cool the billets you can determine the distance between the continuous casting equipment and a billet heating furnace that is current below so as to allow the billet to be cooled to a temperature no higher than the Ari transformation temperature.
Alternatively, a cooling means that cools the billet can be provided. As illustrated in Figure 3, the equipment provided for carrying out this step comprises a transverse conveyor 2 and a billet heating furnace 3. The transverse conveyor 2 preferably has the required length to cool the cast billet at a temperature not is greater than the transformation temperature Arl. If the design of the The arrangement or other conditions do not allow such distance, the billet can be cooled through a forced cooling means arranged in the conveyor 2. Stage 3 - Heating a billet In this stage, the billet is calendared and uniformed, Sufficiently, in the heating furnace 3 at a temperature that allows punching with a punch 5 in the subsequent punching stage. The optimum temperature depends on the material and has been determined taking into account the characteristics of the punching material, including the ductility at high temperature and high temperature resistance. A heating temperature is generally in the range between 1,100 and 1,300 ° C. The billet heating furnace 3 is preferably of the type which advances the billets in the direction transversal. Since the heating efficiency of a billet can be improved by recourse to raising the charge ratio of billets in the heating furnace, said billets preferably being as long as possible. Accordingly, the length of a billet loaded in the heating furnace is preferably a multiple of the length of the billet which is subjected to punching. In this case the billet is sectioned with a disconnecting device 4a such as a gas cutter or a saw provided between the billet heating furnace 3 and the punch 5. In case the temperature of the billet pieces drops excessively during the operation of cutting, they can be heated using an auxiliary heating apparatus 4b, such as a tunnel-type induction heater which is provided on the downstream side of the disconnecting device and which is capable of heating the billet in a short time. Step 4 - Puncture In the present invention, a cast billet that has not been hot rolled is punched using a punch 5 to produce a sleeve or a hollow cylindrical body. Since punching represents an extremely heavy work, the material subjected to punching tends to crack during punching. A countermeasure employed in the present invention to prevent crack formation consists of a combination of refining the billet metallographic structure and punching the billet at a reduced rate of deformation of not more than 200 / sec. Consequently, a deformation regime not greater than 200 / sec. during punching is an essential feature of the present invention. The deformation regime is defined by the following equation: (SB-SA) / (SBxT) being, SB: Area of the cross section of the material before being worked SA: Area of the cross section of the material after being worked T: Time necessary for said work (seconds). When the material has poor characteristics of hot workability, it is preferably punched at a temperature as high as possible. For this purpose, an auxiliary heating device 4b is provided, such as a tunnel-type induction heater device, immediately in front of the punch 5 for raising the billet temperature. The deformation rate should not be greater than 200 / sec. In itself a critical lower limit for said deformation regime is not given. Since a deformation rate of less than 0.1 / sec significantly reduces the useful life of the tools, such as the punch head and shoe of the punch 5, the deformation rate is preferably selected so that it is not less than 0, 1 / sec The punch 5 can be of any type as long as it is an oblique roll punch mill. In order to carry out the method of the present invention, an oblique cylinder punch with a high crossing angle is particularly suitable, which is capable of manufacturing thin-walled tubes and of executing a high rate of expansion of the tube. The reason for this selection is given by the fact that a round billet of a single diameter is sufficient to produce different hollow cylindrical liners or bodies that have a large variety of diameters, thus reducing the required number of different billets. The temperature of the sleeve or of the hollow cylindrical body that has passed through the punching stage normally imports between 1,050 and 1,250 ° C, although it varies according to the material, punching conditions, etc. Step 5 - Stretching and finishing rolling The resulting hollow cylindrical sleeve or body is transported by a transverse conveyor 6 to a feed table of hollow cylindrical bodies of a continuous stretch laminator (punch laminator) 7, which is arranged in the exit end of the conveyor 6. On said table a mandrel bar is inserted into the hollow cylindrical body while the rear end of the mandrel bar / * - is fixed to a rod retainer device. With a continuous stretch laminator 7 and a finishing laminator 8, the hollow cylindrical body is then stretched and finished by rolling at an average rate of deformation of not less than 0.01 / sec, a reduction ratio of not less than 10%, preferably not less than 40%, and a termination temperature between 800 and 1,050 ° C, thus obtaining a steel tube having the size default. The stretch laminator is preferably a mandrel laminator which is preferably constituted by a continuous type stretch laminator comprising a plurality of cylinder frames. To carry out the finishing laminate is used 2 a stretch gauge or reducer, both comprising a plurality of cylinder frames similar in shape to a mandrel laminator. The operation in this stage is carried out at a lower temperature than that in the previous stage of punching as a consequence that the material is cools between both stages. The present invention exploits this relatively low temperature to effect a thermomechanical treatment, which is a feature of the present invention. Accordingly, this is an important step of the present invention. Agree to the present invention, a mandrel laminator 7 (which is a continuous stretch laminator) and a gauge 8 (which is constituted by a finishing laminator) or a stretch reducer are not disposed very far from each other, on the contrary they are directly connected to one another. Specifically, the two laminators are arranged in series on the same process line with a smaller separation than the length of the drawn steel tube by a continuous stretching laminator between said laminators. With this arrangement it is possible to immediately subject the tube to additional work with a finishing laminator before the deformations induced by said work performed with the continuous stretch laminator are recovered. A process that satisfies said purpose can effectively achieve a refinement of the grains of the steel tube in a subsequent recrystallization step. In more detail, even when using the same run program, in case a continuous stretch laminator and a finishing laminator are arranged independently with a greater separation between them, the grains will become larger after the tube has gone through the recrystallization step. In order to obtain the objective of the present invention, that is to obtain steel tubes with an improved quality superior to conventional tubes, this feature of having in series a continuous stretching laminator and a finishing laminator next one immediately behind the another is essential. In this stage the average deformation rate (Ve) defined by the following equation (a) should not be less than 0.01 / sec. If the average deformation rate is less than 0.01 / sec, a recrystallization takes place between the passes and consequently the accumulation of deformations is affected. Under such conditions, a further recrystallization step can not provide a sufficient level of grain refinement. The working relationship in this stage should be no less than 10%, preferably 40%. If the calculated strain value in the working ratio (reduction ratio of the cross-sectional area) is less than 10%, recrystallization will not easily take place and consequently the desired refinement effect of the grains can not be obtained. The final temperature of the material that has been subjected to the finishing laminate imports between 800 and 1,050 ° C. This temperature range has been selected because with it a significant refinement of the grains is achieved. In consecuense, at this stage the average deformation rate is not less than 0.01 / sec, the working ratio is not less than 10%, and the final temperature in a finishing mill is between 800 and 1,050 ° C. It is not necessary to provide particular upper limits for the average deformation ratio and the working regime. However, since an average deformation rate in excess of 10 / sec significantly decreases the useful life of tools, such as the mandrel bar or laminator, the average rate of deformation is preferably selected so that it is not less that 10 / sec Because a working ratio in excess of 95% causes a considerable amount of cracking, the working ratio is preferably no greater than 95%.
See «(Me + Se) / Mt (a) being Me: deformation induced by the continuous stretch laminator, Se: deformation induced by the finishing laminator; Mt: time (seconds) after the front end of a hollow cylindrical body enters the continuous stretch laminator until it leaves the finishing laminator (sec). The mandrel laminator that can be used in the present invention for continuous stretching can be of any type as long as it has a retainer means for the mandrel rod (a bar retainer), which holds the rear end of the bar mandrel to regulate the inner sleeve of the hollow cylindrical body and which allows to reuse the bar by extracting it through a series of calibrating cylinders once it has been completed with the stretch lamination. It is especially preferred that the mandrel laminator be equipped with a bar retainer capable of controlling the speed of the mandrel rod in motion at a speed independent of the speed of the hollow cylindrical body transferred by rolling during the rolling lamination of the hollow cylindrical body. . The finishing laminator (a calibrator or a stretch reducer) can be of any type as long as it does not use an internal adjustment tool. In particular, it is preferred to use an extraction gauge or an extraction stretch reducer capable of extracting the hollow cylindrical body to separate the mandrel rod surrounded by the hollow body that has passed through a continuous stretch laminator.
The conveyor 6 can not only be of the transverse type, but also of the longitudinal type. Step 6 - Recrystallization Prior to the tempering step, a recrystallization treatment at a temperature not lower than the transformation temperature is carried out according to the present invention.
Ar3 after the stretch lamination and the termination lamination. In this step, the deformation induced by the continuous elongation and the finishing laminate in the previous stage is combined with an annealing, heat retention or heating carried out in the present stage to effectively produce a recrystallization and to refine the size of the grains. . The treatment carried out by the combination of these two stages is unique to the present invention and represents a very effective thermomechanical treatment to improve the quality of the products obtained. The recrystallization is carried out using a conveyor 9, which is located on the downstream side of a calibrator (finishing laminator) 8 and which is capable of slowly cooling the steel tubes. Alternatively, the recrystallization can be carried out using a heat holding furnace or a heating furnace, or in a heat holding furnace / heating furnace disposed in the path of displacements of the conveyor. (Slow cooling method) The steel tube that has passed through the finishing laminator is slowly cooled to a predetermined tempering temperature not lower than the transformation temperature Ar3. At this stage it is necessary that the recrystallization has been completed to refine the grains before the tempering step is initiated, whereby lower cooling rates are preferred. If the cooling rate exceeds the air cooling, coarse grains or mixed grains are obtained which cause a reduction in the toughness of the steel. Accordingly, the cooling rate is determined to be less than the air cooling regime. Preferably, the cooling rate is not greater than 0.5 ° C / sec. In order to slowly cool the steel tubes in this stage, the conveyor 9 arranged between the exit of or the finishing mill and the inlet of the tempering apparatus may be enclosed with a cover that is coated with an insulating material such as glass wool or with a plate having a mirror surface capable of reflecting the radiant heat in order to avoid rapid cooling. (Heat retention method) The heat retention method is a method to maintain the temperature of the steel tube after it has been subjected to the termination lamination at the termination temperature. 0 If the retention time is less than 30 seconds no recrystallization takes place. On the other hand, even when the temperature is maintained more than 30 minutes, no increased effects have been obtained in terms of recrystallization. Since the retention for long periods increases the energy costs, simultaneously affecting the production efficiency, it has been determined that the retention time must be reencountered in the range between 30 seconds and 30 minutes. (Heating method and temperature maintenance) A steel tube once subjected to the termination lamination is maintained at a temperature between 850 and 980 ° C for 10 seconds to 30 minutes. If the temperature is lower than 850 ° C and the retention time is less than 10 ° C, no crystallization takes place. On the other hand, if the temperature is higher than 980 ° C and the retention time exceeds 30 minutes, 0 the size of the grains increases. Accordingly, in the present invention the steel tube is maintained at a temperature between 850 and 980 ° C for 10 seconds to 30 minutes. In this specification, the term "temperature maintenance" has the purpose of encompassing an operation in wherein the steel tube is maintained in a heating furnace at a temperature lower than the termination temperature of the steel tube in the previous stage. The heat retention, the heating accompanied by elevation of temperature, or the maintenance of the temperature, described above, can be carried out using a heat retention furnace, a heating furnace and a heat / heating retention furnace which are commonly used in this technical field. The use of these furnaces is in itself recommended since in this way the desired temperature of the material to be hardened can easily be obtained.
Additionally, the use of such furnaces is advantageous in that the temperature can be easily uniformed in the longitudinal direction of a steel tube and other manufactured batches, and consequently it is possible to remarkably minimize the differences in the quality of the products. Likewise, if the temperature at which the steel tube is maintained or to which the steel tube is heated with or without temperature rise is adjusted slightly higher, it is possible to dissolve the carbides that have precipitated during the stretching or the finishing laminate and consequently the resistance to sweetening by annealing is improved by a hardening by secondary precipitation. On the contrary, if the temperature is adjusted slightly lower, the precipitations can be accelerated to avoid a thickening of the grains by the effect of needle formation in the grain boundary. Step 7 - Tempering Once the recrystallization treatment has been completed, the steel tube is transported to a hardening device 11 by means of a conveyor 9. During transport, the temperature of the steel tube must not fall below the transformation temperature Ar3. Accordingly, the finishing laminator 8 and a hardening device 11 are connected in line through a conveyor 9 or similar means. In the tempering device 11 the steel tube which is at a temperature not lower than the transformation temperature Ar3 is cooled. It is necessary that the tempering is carried out from a temperature not lower than the transformation temperature Ar3 at a high speed so that the steel tube has sufficient strength and tenacity. Even when a steel tube having a thick wall is treated, the cooling rate has to be fast enough. To achieve the aforesaid, use is made of a device that has a structure capable of cooling both the inner surfaces, as well as the outer surfaces of the steel tube simultaneously. Step 8 - Restored The steel tube that has passed through the quenching is transferred to an annealing furnace 12 disposed adjacently and on the downstream side with respect to the quenching device 11 on the same production line. Consequently, the hardening device and the tempering furnace 12 are connected on the same production line through a conveyor. The tempering of the steel tubes is carried out by heating and uniforming at a prescribed temperature. Since tempering is an important process that affects the characteristics of the final product, it is necessary that an optimum tempering temperature is determined according to the desired characteristics of the final product and that the tube Steel is fully maintained and uniformed at the temperature thus determined. The differences in tempering temperatures can be at most + 10 ° C, and preferably within + 5 ° C. By this treatment the differences in the tensile strength (YS) and the apparent limit of elasticity (TS) can be suppressed so that they are contained within +5 kp / mm2 of the desired strength. After tempering, the steel tube is straightened with a straightening device 13, thereby obtaining a finished steel tube. MODES OF EMBODIMENT The manufacturing process of the present invention was carried out by means of the following two trials. (Test 1) In the present case the relationship between the deformation regime when a billet is punched and the occurrence of cracks in the hollow punched body was investigated. The billets 2nd test were obtained by casting two types of molten steel that had compositions A and B illustrated in Figure 4 attached in a mold having an internal diameter of 90 mm. Compositions A and B correspond to AISI 1524 and AISI 4130, respectively. Once the molten steel has solidified, the billets were removed immediately from the molds. Both billets were cooled to 600 ° C (steel A) and 500 ° C (steel B), respectively, said temperatures being lower than the transformation temperature Arl illustrated in Figure 4. The billets were then maintained at a temperature of 1.250. ° C for 1 hour in a heating furnace, followed by a punching test using a punch for experimental use, obtaining hollow bodies or shirts. The hollow bodies were checked for the occurrence of cracks and the maximum depth of these cracks. Figure 5 illustrates the measurements made regarding the maximum depths of the fissures in the hollow bodies or shirts. As can be seen from Figure 5, none of the steels A or B produced cracks in the body or resulting hollow shell when the deformation rate was not greater than 200 / sec. However, at a rate of deformation exceeding 200 / sec, crack formation could be observed. Accordingly, it could be confirmed that if the punching of the billet was carried out in a situation in which the molten billet is cooled to a temperature not higher than the transformation temperature Arl and is subsequently heated to a temperature that allows punching, the regime of deformation during punching should not be greater than 200 / sec. (Test 2) The outer diameters and chemical compositions of the billets used in this Test were exactly the same as those used in Test 1. Once the billets were completely solidified they were immediately removed from the molds and cooled to a temperature not greater than the transformation temperature At3. Then, they were maintained at a temperature of 1250 ° C for 1 hour in a heating oven, after which a hot pressing test was carried out under the conditions illustrated in Figure 6 and, illustrating the Table 3 the conditions for punching, stretching and finishing lamination, and Figure 7 a diagram of the recrystallization conditions in test 2. The hot pressing test was designed to simulate punching (operation with a punch), Stretching (operation with mandrel laminators) and finishing laminate (operation with a gauge). As illustrated in Figures 6 and 7, test tubes 1 to 8 represent products of the present invention, while test tubes 19 to 24 represent comparative products formed outside the range of the present invention. The test tubes No. 25 and 26 represent steel tubes manufactured according to a conventional procedure illustrated in Figure 1. In carrying out the conventional procedure, the rate of deformation rate of the billets during punching was slightly higher that the range allowed by the present invention, and additionally, the workings of stretching work and finishing rolling was not carried out continuously. Moreover, the test tubes were cooled to room temperature between the termination laminate and the temper. The test tubes of the present invention, the tubes of the comparative procedure and conventional tubes were manufactured in each case with two types of steel A and B. The cooling regime illustrated in Figure 7 indicates the rate determined when the test tubes that have been subjected to punching and lamination of termination under the conditions illustrated in Figure 6 were gradually cooled during the termination temperature to temperatures not lower than the transformation temperature Ar3. Taking into account that steels A and B tended to have different resistances if they are subjected to the same heat treatment and that their tensile strength and tenacity could not be compared, two different tempering temperatures have been used so that the comparison was carried out to almost the same resistance of the test tubes. The test pieces after processing were selected for material strength, primitive austenite grain size, toughness (vTrs), and corrosion resistance (Se).
The values of resistance to corrosion (Se) were measured in accordance with the standards TM01-77-92, method B provided by the NACE INTERNATIONAL standard. The grain size of primitive austenite was determined by obtaining the average length of the grains that exceeded a distance of 1 mm. The results are illustrated in Figure 8. The test tubes of the present invention were first compared to the test tubes Nos. 25 and 26 manufactured by a conventional method. In the tests in which steel A was used that was processed at an annealing temperature of 600 ° C, the test tubes N ° 1 to N ° 6 of the present invention showed smaller grains and exhibited comparable toughness and corrosion resistance or better than a conventional tube (No. 25).
Also, in the tests where processed steel B was used at an annealing temperature of 720 ° C, the comparison between the two groups, namely test tubes No. 7 to 18 of the present invention and test tube No. 26 manufactured by a conventional procedure, it revealed results analogous to those obtained in the case of steel A. Test tubes No. 19 to 24, which represent comparative products and which were manufactured under conditions that were outside the ranges of the present invention, had larger grains and a lower tenacity and corrosion resistance with respect to the test tubes manufactured according to the present invention. The reason for this result is that the refinement of the grains due to the work to which they were subjected and the recrystallization were insufficient. As is evident from the results of the aforementioned tests, it could be confirmed that the seamless steel tubes manufactured by the process of the present invention exhibited excellent mechanical characteristics and corrosion resistance, which were comparable to or even better than the tubes Seamless steel manufactured by conventional procedures. As described above, by the manufacturing process according to the present invention, seamless steel pipes can be obtained using a simple process and simple equipment in a single manufacturing line connected from the billet to the final product under stable manufacturing conditions. . Accordingly, the resulting seamless steel tube manufactured according to the method of the present invention and using the manufacturing arrangement according to the present invention has excellent comparative qualities or qualities superior to conventional products.
Moreover, since the costs of construction and operation can be reduced, the manufacturing costs of seamless steel pipes can also be reduced.
Additionally, seamless steel tubes can be effectively produced in mass. Accordingly, the method and arrangement for the manufacture of seamless steel pipes according to the present invention is particularly suitable for the manufacture of seamless steel pipes on an industrial scale.

Claims (9)

  1. CLAIMS It has been described that it has been the nature of the present invention and the way to put it into practice, it is declared that what is claimed as invention and exclusive property, is: 1.- Procedure for manufacturing seamless steel tubes, CHARACTERIZED because it comprises the following operative steps 1) to 8), which are carried out in series: (1) producing a billet having a circular cross section by continuous casting; (2) cooling the billet to a temperature no greater than the transformation temperature ArJ; (3) heating the cooled billet to a temperature no greater than the transformation temperature Arl to a temperature that allows the punching of the billet; (4) punching the billet heated to a temperature that allows said punching so as to obtain a hollow body or jacket, at a deformation rate not greater than 200 / sec; (5) Obtaining a steel tube by stretching and rolling the body or hollow sleeve using a stretch laminator and a finishing laminator, which are directly connected to each other, at an average deformation rate of not less than 0 , 01 / sec, a reduction ratio of not less than 10%, and a termination temperature between 800 and 1,050 ° C; (6) recrystallization of the steel tube at a temperature not less than the transformation temperature Ar3; (7) tempering the steel tube obtained in Step 6 from a temperature not lower than the transformation temperature Ar3; and (8) tempering of the hardened steel tube.
  2. 2. Process for manufacturing seamless steel tubes as described in claim 1, wherein the step 6 which induces recrystallization comprises the cooling of the steel tube formed in step 5 at a temperature not less than a transformation temperature Af3 at a cooling rate milder than air cooling.
  3. 3. Process for manufacturing seamless steel tubes as described in claim 1, wherein the step 6 that induces recrystallization comprises maintaining the steel tube formed in step 5 at the termination temperature indicated in step 5 during 30 seconds to 30 minutes.
  4. 4. Process for manufacturing seamless steel tubes as described in claim 1, wherein the step 6 that induces recrystallization comprises maintaining or reheating / maintaining the steel tube formed in Step 5 at a temperature between 850 and 900 ° C for 10 seconds to 30 minutes.
  5. 5. - Process for manufacturing seamless steel tubes as described in claims 3 or 4, CHARACTERIZED because it further comprises cooling the steel tube that has been subjected to the recrystallization treatment according to Step 6 at a temperature not lower than the transformation temperature Ar3 .
  6. 6.- Arrangement for manufacturing seamless steel tubes and carrying out the method as described in claims 1 or 2, characterized in that it comprises the following equipment a) to g), which are arranged sequentially in series: a) device continuous casting to produce billets having a circular cross section; b) billet heating furnace to heat the billets that have been cast; c) oblique roll punching laminator for punching the hot billet to form a hollow body or shell; d) continuous stretching laminator for stretching the sleeve or hollow body; e) finishing laminator for the finished rolling of the elongated hollow body or shell to obtain a steel tube having predetermined dimensions; f) tempering device for hardening the finished rolled steel pipe in the same manufacturing line; and g) tempering furnace to repair the hardened steel pipe in the same manufacturing line; under the following conditions 1) to 3): 1) the separation between the continuous casting device and the billet heating furnace allows the billet to be fed into the billet heating furnace while it is at a temperature from the room temperature at the Arl transformation temperature, but not lower than the ambient temperature, alternatively a cooling device capable of cooling the billet forcedly to a temperature between the ambient temperature and the transformation temperature is provided.; 2) the spacing between the continuous stretch laminator and the finishing laminator is less than the length of a steel tube that has been stretched by the continuous stretch laminator, said continuous stretch laminator and the finishing laminator being arranged in series in the same line of manufactured; and 3) the finishing laminator and the tempering device are connected by means of a conveyor equipped with a means capable of cooling a steel tube which has been rolled to the final dimensions gradually at a lower cooling rate than air cooling. .
  7. 7. - Arrangement for manufacturing seamless steel pipes and carrying out the process as described in claims 4 or 5, characterized in that it comprises the following equipment a) to h), which are arranged sequentially in series: a) continuous casting device to produce billets that have a circular cross section; b) billet heating furnace to heat the billets that have been cast; c) oblique roll laminator / punching machine for punching the hot billet to form a hollow body or sheet; d) continuous stretch laminator for stretching to the hollow body or shell; e) finishing laminator for laminating the final dimensions to the sleeve or stretched hollow body for 25 obtain a steel tube having predetermined dimensions; f) heat holding furnace to keep the rolled steel pipe to the final dimensions at the termination temperature or at a predetermined temperature, or a heating furnace to keep the rolled steel pipe at the 20 final dimensions at a predetermined temperature after it has been heated to said temperature; g) tempering device to harden the rolled steel pipe to the final dimensions in the same manufacturing line; and 25 h) tempering furnace to repair the hardened steel pipe in the same manufacturing line; under the following conditions 1) to 3): 1) the separation between the continuous casting device and the billet heating furnace allows the billet to be fed into the billet heating furnace while it is at a temperature from ambient temperature up to the transformation temperature Arl, but not lower than the ambient temperature, alternatively providing a cooling device capable of cooling in a forced manner 10 to the billet at a temperature between the ambient temperature and the transformation temperature Arl; 2) distance between the continuous stretch laminator and the finishing laminator is less than the length of the steel tube that has been stretched by the 25 continuous stretch laminator, said continuous stretch laminating machine and the finishing laminator being arranged in series on the same manufacturing line; and 3) the finishing laminator and the tempering device are connected to each other by a conveyor equipment provided with a heat holding furnace to keep the rolled steel pipe in the final dimensions at the termination temperature or at a predetermined temperature, or a heating furnace to keep the rolled steel pipe to the final dimensions at a predetermined temperature 25 after it has been heated to that temperature.
  8. 8. - Provision to manufacture steel tubes without * - * c seam as described in claim 6 or 7, CHARACTERIZED because the oblique roll laminator / punch press and the billet heating furnace are connected 5 with a conveyor equipment provided with billet disconnecting devices to section a heated billet at the temperature that allows the punching thereof.
  9. 9.- Arrangement for manufacturing seamless steel tubes as described in any of claims 6, 10 7 or 8, CHARACTERIZED in that the bilge reheating device is provided between the billet disconnector device and the oblique roll mill / punch press. fifteen R E S U M E N The present invention has the purpose of providing a process for manufacturing seamless steel tubes having superior characteristics to the products. 5 by using a simple process and a simple arrangement at reduced costs; the manufacturing process of the present invention is characterized by comprising the ative stages 1) to 8), which are arranged sequentially, and the stages or devices 2o for the production of billets to finished products, are connected in the same continuous line of manufacturing: 1) produce a round billet by continuous casting; 2) cool the billet to a temperature no greater than the transformation temperature Arl; 15 3) heat the billet to a temperature that allows the punching of the billet; 4) punching the billet to obtain a shirt or hollow body at a deformation rate not greater than 200 / sec; 5) forming a steel tube by stretching and rolling 2o to the final dimensions of the sleeve or hollow body using a continuous stretching laminator and a finishing laminator which are directly connected to each other, at an average rate of deformation, a predetermined reduction ratio and a predetermined termination temperature 5; 6) recrystallize the steel tube at a temperature no lower, r1 46 that the transformation temperature Ar3; 7) temper the steel tube from a temperature not less than * £ - the transformation temperature Ar3; and 8) repair the steel tube.
MX9702792A 1994-10-20 1995-10-20 Method of manufacturing seamless steel pipes and manufacturing equipment therefor. MX9702792A (en)

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JP6-255217 1994-10-20
JP25521794 1994-10-20
JP6-255,217 1994-10-20
PCT/JP1995/002155 WO1996012574A1 (en) 1994-10-20 1995-10-20 Method of manufacturing seamless steel pipes and manufacturing equipment therefor

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MXPA97002792A true MXPA97002792A (en) 1998-02-01
MX9702792A MX9702792A (en) 1998-02-28

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