JP2019141864A - Squeezing method for end of duplex stainless steel pipe, and production method for duplex stainless steel pipe - Google Patents

Abstract

To provide a squeezing method for the original pipe end of a duplex stainless steel pipe preventing crack without carrying out a solution heat treatment, and to provide a production method for the duplex stainless steel pipe using the squeezing method.SOLUTION: There is provided a squeezing method for the original pipe end of a duplex stainless steel including in mass%, 0.008-0.03% C, 0-1% Si, 0.1-2% Mn, 20-35% Cr, 3-10% Ni, 0-5% Mo, 0-6% W, 0-3% Cu, 0.15-0.40% N, and the balance Fe with impurities. The squeezing method includes a heating process and a squeezing process. In the heating process, the original pipe end is heated to 650°C or more but less than 800°C. In the squeezing process, the pipe end is heated so that the pipe end temperature Tf at squeezing process ending is 400°C or higher and satisfies an expression (1) with respect to a temperature To at squeezing process starting: Tf≥4.4To-2680 (1), and then is squeezed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for drawing a base pipe made of a duplex stainless steel and a method for producing a duplex stainless steel pipe.

  As a method of processing a steel pipe into a predetermined dimension, there is a drawing process. The general drawing process is as follows. First, one end portion of a base pipe of a steel pipe to be drawn is heated. One end of the heated element tube is squeezed down to be thin (reduced diameter) so that it can be easily gripped. The process of reducing the diameter by reducing one end of the raw tube is called “mouth drawing process”. One end portion reduced in diameter by the mouth drawing process is referred to as a “mouth drawing portion”. The mouth restrictor is passed through a die. The aperture stop is gripped by a gripping mechanism called a gripper. After gripping the mouth restrictor with the gripper, the gripper is moved in the pulling direction and the base tube is pulled out. The drawing process is performed through the above steps. After the drawing process, the mouth restrictor is cut into a product.

  In recent years, with the development of deep well oil wells and natural gas wells, demand for oil well pipes having high strength and excellent corrosion resistance is increasing. Therefore, a duplex stainless steel pipe having a ferrite phase and an austenite phase may be used for the oil well pipe.

  When a base tube made of duplex stainless steel is heated to a predetermined temperature range, a σ phase may precipitate. When the sigma phase precipitates, the toughness of the raw tube decreases. For this reason, cracking may occur when the raw tube on which the σ phase is precipitated is drawn. Even if no cracking occurs in the mouth drawing process, a crack may occur in the drawing process. Therefore, a solution treatment is performed in order to remove the σ phase of the raw pipe that has been drawn before the drawing process. In the solution treatment, for example, the raw tube is reheated to a predetermined temperature and rapidly cooled. Thereby, even if a base pipe made of high-strength duplex stainless steel is subjected to mouth drawing and drawing, cracking can be suppressed.

  A mouth-drawing method that suppresses cracking is disclosed in, for example, Japanese Patent Application Laid-Open No. 2011-194469 (Patent Document 1).

  In the mouth-drawing method disclosed in Patent Document 1, a high-strength ERW steel pipe having a tensile strength of 980 MPa or more is used. This ERW pipe is made of carbon steel. In the squeezing method disclosed in Patent Document 1, the squeezed part is heated to 450 to 550 ° C. before the squeezing process. The heated ERW steel pipe is wrung with a die. Thereby, it is described in patent document 1 that the crack of the seam part of an ERW steel pipe can be suppressed.

JP 2011-194469 A

  However, the squeezing method disclosed in Patent Document 1 is intended for an electric resistance welded steel pipe made of carbon steel in which the σ phase hardly precipitates. Therefore, the mouth drawing method of Patent Document 1 cannot be simply applied to the duplex stainless steel pipe. Moreover, an electric resistance steel pipe makes a steel plate circular, and joins the edge parts of a steel plate by welding. Therefore, even if the carbon steel of Patent Document 1 is replaced with a duplex stainless steel, a σ phase is easily generated by welding heat. That is, solution treatment is essential for the duplex stainless steel pipe processed by the mouth drawing method of Patent Document 1. If the solution treatment is performed, the number of work steps is increased and thus the production efficiency is low.

  Furthermore, the σ phase is precipitated when the material is held for a certain time in a predetermined temperature range. The predetermined temperature range in which the σ phase precipitates (σ phase precipitation temperature range) depends on the composition of the material. Therefore, it is conceivable that the σ phase precipitation temperature range of the material to be squeezed is investigated in advance, and squeezing is performed outside the σ phase precipitation temperature range to suppress cracking of the raw tube. However, in actual operation, even if the squeezing process is performed outside the σ phase precipitation temperature range, cracks may occur in the raw tube.

  An object of the present invention is to provide a method for drawing a base pipe of a duplex stainless steel pipe that suppresses cracking without performing a solution treatment and a method for producing a duplex stainless steel pipe. Another object of the present invention is to provide a method for producing a duplex stainless steel pipe and a method for producing a duplex stainless steel pipe that suppresses cracking in consideration of actual operation.

The mouth-drawing method of this embodiment is mass%, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 5%, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.40%, with the balance being Fe and impurities This is a method of squeezing the end of a duplex stainless steel pipe. The squeezing method includes a heating step and a squeezing step. In the heating step, the end of the raw tube is heated to 650 ° C. or higher and lower than 800 ° C. In the squeezing process, the end temperature Tf at the end of the squeezing process is 400 ° C. or higher, and the end heated to satisfy the formula (1) with respect to the end temperature To at the start of the squeezing process. Mouth drawing part.
Tf ≧ 4.4 To-2680 (1)

The method for producing the duplex stainless steel pipe of the present embodiment includes a preparation process, a heating process, a mouth drawing process, and a cold drawing process. In the preparation step, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo in mass%. : 0 to 5%, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.40%, with the balance being elemental of duplex stainless steel made of Fe and impurities Prepare the tube. In the heating step, the end of the raw tube is heated to 650 ° C. or higher and lower than 800 ° C. In the squeezing process, the end temperature Tf at the end of the squeezing process is 400 ° C. or higher, and the end heated to satisfy the formula (1) with respect to the end temperature To at the start of the squeezing process. Mouth drawing part. In the cold drawing process, the drawn pipe is cold drawn.
Tf ≧ 4.4 To-2680 (1)

  According to the mouth-drawing method and the method for producing a duplex stainless steel pipe according to the present invention, cracking of the base pipe of the duplex stainless steel pipe can be suppressed without solution treatment. Moreover, according to the mouth-drawing method and the method for producing a duplex stainless steel pipe according to the present invention, it is possible to suppress cracking of the base pipe of the duplex stainless steel pipe in actual operation.

FIG. 1 is a diagram showing the relationship between the temperature of the blank tube before and after the squeezing process and the process cracks. FIG. 2 is a cross-sectional view showing a mouth drawing process. FIG. 3 is an overall configuration diagram of the drawing device.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  In this specification, the “element tube” means a duplex stainless steel tube before drawing. Moreover, the duplex stainless steel pipe of this embodiment is intended for a seamless steel pipe.

  The precipitation behavior of the σ phase depends on complex factors such as the heating temperature, the temperature of the raw tube at the end of the squeezing process, and the temperature history of the raw tube after the squeezing process. Once the σ phase is precipitated in the structure of the raw pipe after the squeezing process, the possibility that the squeezed part of the raw pipe breaks in the cold drawing process in the next step increases. Therefore, it is desirable to completely prevent the precipitation of the σ phase in the entire region of the raw tube after the squeezing process.

  The inventors of the present invention have examined the cause of cracking of the base pipe of a duplex stainless steel pipe in mouth drawing. As a result, it was found that if the temperature of the raw pipe before and after the mouth drawing process is lower than the σ phase precipitation temperature, the precipitation of the σ phase is suppressed and the cracking of the raw pipe of the duplex stainless steel pipe can be suppressed. From this point of view, if the σ phase precipitation temperature range of the material to be squeezed is previously examined and squeezed outside the sigma phase precipitation temperature range, cracking of the raw tube can be suppressed. The σ phase precipitation temperature range is obtained by applying an appropriate amount of strain to the raw tube to be drawn and then heating the material to various temperatures and observing the structure.

  However, in actual operation, cracks may occur in the raw tube even if the squeezing process is performed outside the σ phase precipitation temperature range. The present inventors examined the cause of cracking of the raw tube due to squeezing outside the σ phase precipitation temperature range. As a result, this cause is considered as follows.

  Generally, the squeezing process is performed in a plurality of times. At the time of squeezing, the raw tube comes into contact with the tool, so the temperature of the raw tube decreases due to heat removal from the tool. This becomes more prominent as the number of times of squeezing is increased. In addition, the temperature of the raw tube also decreases due to cooling. For this reason, the movement of lattice defects introduced into the material structure is stagnated due to plastic deformation of the raw pipe by the squeezing process, and the material recovery phenomenon is less likely to occur. As a result, the accumulated strain of the raw tube increases and the ductility of the material accompanying work hardening decreases. Thereby, it is thought that a crack may generate | occur | produce in a raw tube. In short, the present inventors appropriately control the temperature of the raw tube in consideration of the actual operation conditions caused by various factors such as tool heat removal even in the squeezing process outside the σ phase precipitation temperature range. If it is not, it has been found that cracks can occur in the raw pipe in the mouth drawing process.

  Subsequently, the present inventors investigated in more detail the cause of the breakage of the duplex stainless steel pipe in mouth drawing or drawing. As a result, the following knowledge was obtained. With regard to the boundary of whether or not cracking occurs in the duplex stainless steel pipe in the drawing or drawing process, the heating temperature of the blank, that is, the temperature of the blank before the drawing (at the start) and the mouth There is a predetermined relationship with the temperature of the tube after drawing (at the end), and it is not necessary that the temperature of the tube is just outside the sigma phase precipitation temperature range before and after mouth drawing. Therefore, it is necessary to control the temperature of the raw tube. In short, the temperature of the tube before squeezing and the temperature of the tube after squeezing that can suppress cracking of the stub tube are not constant values, and the temperature of the tube after squeezing is the same as that before squeezing. Corresponds to tube temperature. This point will be specifically described.

  As shown in Examples described later, the present inventors heated the end of a base pipe of a duplex stainless steel pipe (heating process), and squeezed the end of the heated base pipe (mouth squeezing process). Process). Thereafter, the drawn pipe was cold drawn (drawing process) to produce a duplex stainless steel pipe. Then, it was confirmed by visual observation and microstructural observation whether cracks occurred in the raw tube during the mouth drawing process or the drawing process. Details of the test conditions will be described in the examples described later, and will be omitted here.

  FIG. 1 is a diagram showing the relationship between the temperature of the blank tube before and after the squeezing process and the process cracks. FIG. 1 shows the test results of examples described later. In FIG. 1, the horizontal axis indicates the temperature To (° C.) at the start of squeezing, and the vertical axis indicates the temperature Tf (° C.) at the end of squeezing. In FIG. 1, white circles indicate the results of no cracks occurring in the pipes during the squeezing and drawing processes, and black circles indicate the results of cracks occurring in the pipes during the squeezing or drawing processes. In FIG. 1, the white triangle mark indicates that no cracking occurred in the raw tube if it was less than 20 minutes from the start of the squeezing process to the end of the squeezing process. Shows the results that occurred.

  Referring to FIG. 1, the temperature range in which no cracks occurred in the raw tube based on the results of examples described later is a region surrounded by straight lines 1 to 4. Each straight line will be described.

The straight line 1 represents the boundary between the result of no cracks in the blank (white circles and white triangles) and the result of cracks (black circles). In the results indicated by white triangle marks, cracks occurred in the raw tube after 20 minutes or more from the start of squeezing, but in actual operation, the time from the start of squeezing to the end of squeezing is less than 20 minutes. Therefore, the white triangle mark is also included in the result of the crack not occurring in the element tube. When the straight line 1 is expressed as a function of the temperature Tf (° C.) of the raw tube (end) at the end of the squeezing process and the temperature To (° C.) of the raw pipe (end) at the start of the squeezing process, It becomes an expression.
Tf = 4.4 To-2680

  A straight line 2 indicates a line where the temperature Tf of the raw tube at the end of the squeezing process is 400 ° C. Since the duplex stainless steel pipe has high strength, if the temperature Tf of the raw pipe at the end of the drawing process is less than 400 ° C., the temperature at the start of the drawing process can be taken into account even if the temperature drop of the raw pipe is taken into consideration. The temperature To of the raw tube does not exceed 650 ° C. If the temperature To of the raw pipe at the start of squeezing is less than 650 ° C., the workability of the raw pipe is low, and it is difficult to reduce the diameter of the raw pipe to a desired outer diameter. Therefore, the temperature Tf of the raw tube at the end of the squeezing process is never 400 ° C. or lower in the squeezing process of the duplex stainless steel pipe. Line 2 represents this.

  A straight line 3 indicates a line in which the temperature To of the raw tube at the start of squeezing is 650 ° C. Similarly to the above, if the temperature To of the raw tube at the start of squeezing is less than 650 ° C., the workability of the raw tube is low, and it is difficult to reduce the diameter of the raw tube to a desired outer diameter. Even if the tube To temperature at the start of squeezing is less than 650 ° C, squeezing is not impossible, but a high-power press is required, and the die of the press is likely to be consumed quickly. Become. Therefore, the lower limit of the raw tube temperature To at the start of squeezing is 650 ° C.

  A straight line 4 shows an isothermal line between the temperature Tf of the raw tube at the end of the squeezing process and the temperature To of the raw tube at the start of the squeezing process. In the squeezing process, a die presses down the end of the raw tube. At that time, the temperature of the die is lower than the temperature of the heated raw tube. Therefore, the raw tube is extracted by mouth drawing. In other words, it is impossible in principle that the temperature Tf of the raw tube at the end of the squeezing process becomes higher than the temperature To of the raw tube at the start of the squeezing process. Therefore, the straight line 4 is a line determined for physical reasons. If the die is operated at high speed or the output of the press machine is remarkably increased during the squeezing process, the temperature of the tube at the end of the squeeze process will be reduced by the heat generated by the process. May be higher than temperature. However, such high-speed and strong machining is excluded in the mouth drawing method and the method of producing a duplex stainless steel pipe according to this embodiment.

  Here, pay attention to the straight line 1. As described above, it is known that the precipitation of the σ phase that causes cracks in the raw tube depends on the temperature of the raw tube. Therefore, in general, whether or not the raw tube is broken is not the relationship between the temperature of the raw tube before the squeezing process and the temperature of the raw tube after the squeezing process, but the raw tube before, during, and after the squeezing process. It was thought that the temperature of the sigma was not in the σ phase precipitation temperature range. However, in practice, as described above, due to a decrease in ductility of the material of the raw tube due to the squeezing process, the raw tube may be cracked even in the squeezing process outside the σ phase precipitation temperature range. As a result of detailed examinations by the present inventors, the boundary of whether or not cracking occurs in the raw pipe is a function of the temperature Tf of the raw pipe at the end of squeezing and the temperature To of the raw pipe at the start of squeezing. Was found to be.

  For example, looking at the case where the temperature To of the raw tube at the start of squeezing is 700 ° C., in this case, the temperature Tf of the raw tube at the end of the squeezing process is 400 to 700 ° C. ° C. On the other hand, when the temperature To of the raw tube at the start of the squeezing process is 750 ° C., the temperature Tf of the raw tube at the end of the squeezing process is about 620 ° C. to prevent cracking in the raw tube. About 750 ° C. In this way, if the temperature of the raw pipe before, during and after the squeezing is within a certain temperature range outside the σ phase precipitation temperature range, the raw pipe will not crack, but before and after the squeezing process. It has been found that there is a correlation with the temperature of the raw tube, and the temperature of the raw tube at the end of the drawing process, which can suppress the occurrence of cracking, changes depending on the temperature of the raw tube at the start of the drawing process.

The squeezing method of this embodiment based on the above knowledge is mass%, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 -35%, Ni: 3-10%, Mo: 0-5%, W: 0-6%, Cu: 0-3%, and N: 0.15-0.40%, the balance Is a method of squeezing the end portion of a duplex stainless steel pipe made of Fe and impurities. The squeezing method includes a heating step and a squeezing step. In the heating step, the end of the raw tube is heated to 650 ° C. or higher and lower than 800 ° C. In the squeezing process, the end temperature Tf at the end of the squeezing process is 400 ° C. or higher, and the end heated to satisfy the formula (1) with respect to the end temperature To at the start of the squeezing process. Mouth drawing part.
Tf ≧ 4.4 To-2680 (1)

  As described above, cracks may occur in the raw pipe even in the squeezing process outside the σ phase precipitation temperature range. The present inventors have found that this cause is due to a decrease in ductility of the material during squeezing. For cracking of the pipe due to the ductility reduction of this material, the temperature at the end of the pipe at the start of squeezing and the temperature at the end of the pipe at the end of squeezing have a predetermined relationship (formula (1)). Based on this knowledge, if the temperature of the base tube of the duplex stainless steel pipe before and after the squeezing process is controlled, the sigma phase is changed to perform the squeezing process outside the sigma phase precipitation temperature range as shown in the examples described later. There is no need for a solution treatment to be removed. Furthermore, since the ductility reduction of the pipe during the squeezing process in actual operation is also considered, the occurrence of cracks in the pipe is further suppressed.

  Preferably, in the above-described squeezing method, the time from the start of squeezing to the end of squeezing is within 15 minutes.

  The precipitation of the σ phase depends on the temperature of the material and the holding time of that temperature. In the squeezing process, it is practically difficult to accurately grasp the temperature of the entire end portion of the raw tube. For this reason, it is conceivable that the temperature of a part of the end portion of the raw tube reaches the σ phase precipitation temperature range. Therefore, if the squeezing process is completed before the σ phase is precipitated, cracking of the raw tube can be suppressed even if part of the end of the raw tube is in the σ phase precipitation temperature range.

  Preferably, in the step of squeezing, the outer diameter processing degree of the end portion is 32% or less.

  Since the duplex stainless steel has high strength, if the outer diameter processing degree is too large, it is necessary to increase the output of the press machine, and the mouth drawing tool (dies) is consumed quickly. Therefore, it is preferable that the outer diameter processing degree is not more than a certain value.

The method for producing the duplex stainless steel pipe of the present embodiment includes a preparation process, a heating process, a mouth drawing process, and a cold drawing process. In the preparation step, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo in mass%. : 0 to 5%, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.40%, with the balance being elemental of duplex stainless steel made of Fe and impurities Prepare the tube. In the heating step, the end of the raw tube is heated to 650 ° C. or higher and lower than 800 ° C. In the squeezing process, the end temperature Tf at the end of the squeezing process is 400 ° C. or higher, and the end heated to satisfy the formula (1) with respect to the end temperature To at the start of the squeezing process. Mouth drawing part. In the cold drawing process, the drawn pipe is cold drawn.
Tf ≧ 4.4 To-2680 (1)

  Hereinafter, the mouth drawing method of the present embodiment will be described. The mouth drawing method of this embodiment includes a heating step and a mouth drawing step.

[Heating process]
In the heating step, the end of the base pipe of the duplex stainless steel pipe is heated by a heating device. The heating method is not particularly limited, but is usually performed by an induction heater, gas heating, or the like. The end portion of the raw tube means a region having a predetermined length from the end surface of the raw tube. In short, the end portion of the raw tube corresponds to the mouth restrictor. Since the workability of the material is improved by heating the end of the blank tube, even a high-strength duplex stainless steel tube can be drawn.

  The heating temperature at the end of the raw tube is 650 ° C. or higher and lower than 800 ° C. As described above, if the temperature To of the raw tube at the start of squeezing is less than 650 ° C., the workability of the raw tube is low, and it is difficult to reduce the diameter of the raw tube to a desired outer diameter. If the temperature of the end portion of the raw tube is 650 ° C. or higher in the heating process, the temperature of the raw tube at the start of squeezing process is practically less than 650 ° C. Therefore, the lower limit of the heating temperature at the end of the raw tube in the heating step is 650 ° C. Note that even if the tube tube temperature To at the start of the squeezing process is less than 650 ° C., the squeezing process is not impossible, but a high-power press is required, and the press dies are quickly consumed. It becomes easy to do. Also from this viewpoint, the lower limit of the temperature To of the raw pipe at the start of the squeezing process is 650 ° C. Based on an example described later, if the temperature To of the raw tube at the start of squeezing is 800 ° C. or higher, the σ phase is likely to precipitate. If the temperature of the end portion of the raw tube is less than 800 ° C. in the heating process, in practice, the temperature of the raw tube at the start of squeezing does not become 800 ° C. or more in principle. Therefore, the upper limit of the heating temperature at the end of the raw tube in the heating process is 800 ° C. Preferably, the upper limit of the heating temperature at the end of the raw tube in the heating step is 750 ° C. This is because the lower the temperature To of the raw tube at the start of squeezing, the less the σ phase is precipitated.

[Mouth drawing process]
The mouth drawing process is performed after the heating process. In the squeezing process, the end of the heated raw tube is squeezed. More specifically, the end portion of the raw tube is reduced by a squeezing device to reduce the diameter. The mouth drawing device is, for example, a press device, a swaging device, or the like.

  FIG. 2 is a cross-sectional view showing a mouth drawing process. FIG. 2 shows a cross section perpendicular to the tube axis CA direction of the raw tube. Referring to FIG. 2, the end of the raw tube 2 is fixed to the mouth drawing device 1 so as to be rotatable. The aperture drawing apparatus 1 has a plurality of dies 3. FIG. 2 shows a case where the squeezing apparatus 1 has eight dice 3 arranged at equal intervals in the circumferential direction of the raw tube 2. However, the number of dies 3 is not limited to eight. The number of dies 3 may be set as appropriate.

  The die 3 reciprocates in the radial direction of the raw tube 2 (the arrow direction in FIG. 2) while rotating the raw tube 2 around the tube axis CA. Thereby, the end portion of the raw tube 2 is squeezed to reduce the diameter. That is, the mouth restrictor is formed. Note that, unlike the above description, the die 3 may move along the circumferential direction of the raw tube 2 to squeeze the end of the raw tube 2. The cross-sectional shape of the aperture stop may be circular or may not be circular. In short, the cross-sectional shape of the mouthpiece portion may be any shape that can be gripped by the gripper during drawing.

  The squeezing process may be repeated a plurality of times. That is, the end portion of the raw tube may be squeezed in multiple steps. By dividing the squeezing process into a plurality of times, it is possible to reduce the degree of outer diameter processing at one time, and it is possible to further suppress the cracking of the raw pipe due to the squeezing process.

With reference to FIG. 1, as described above, in the mouth drawing process of the base pipe of the duplex stainless steel pipe, the generation of cracks in the base pipe can be suppressed in the region surrounded by the straight lines 1 to 4. Based on the straight line 2 in FIG. 1, the temperature Tf at the end of the raw tube at the end of the squeezing process needs to be 400 ° C. or higher in order to suppress cracking at the end of the raw tube. In addition to this, based on the straight line 1 in FIG. 1, the temperature Tf at the end of the raw tube at the end of the squeezing process must also satisfy the following equation (1).
Tf ≧ 4.4 To-2680 (1)

  That is, the end temperature Tf at the end of squeezing is 400 ° C. or more, and the end of the base tube is squeezed by satisfying the formula (1) with respect to the temperature To of the base tube at the start of squeezing. If it processes, the crack of the pipe | tube resulting from precipitation of (sigma) phase can be suppressed, and also the crack of the pipe | tube based on the ductile fall of the material by mouth drawing process can also be suppressed. Further, since the temperature in the region surrounded by the straight lines 1 to 4 in FIG. 1 is outside the σ phase precipitation temperature range as shown in the examples described later, the precipitation of the σ phase is suppressed and the σ phase is removed. It is not necessary to carry out a solution treatment. Even when the squeezing process is performed a plurality of times, the end temperature Tf at the end of the squeezing process is 400 ° C. or more in each squeezing process, and the raw tube at the start of the squeezing process It is necessary to satisfy the formula (1) with respect to the temperature To.

  Based on the examples described later, in the squeezing process, it is preferable that the outer diameter processing degree of the end portion of the raw tube by one squeezing process is 32% or less. High-strength materials such as duplex stainless steel are difficult to plastically deform. For this reason, if the outer diameter processing degree of the raw pipe is larger than 32%, cracking or the like is likely to occur at the end of the raw pipe due to mouth drawing. Therefore, it is preferable that the outer diameter processing degree of the raw tube is 32% or less.

[Machining time]
The precipitation of the σ phase depends not only on the temperature of the raw tube but also on the holding time of the temperature of the raw tube. For example, it is assumed that a base pipe of a duplex stainless steel pipe is heated to a certain σ phase precipitation temperature and the temperature is maintained. The σ phase does not precipitate in the raw tube for a predetermined time after heating, but after the predetermined time, the σ phase precipitates in the raw tube. This predetermined time varies depending on the temperature of the raw tube. Therefore, in actual operation, the σ phase precipitation temperature range of the material of the raw pipe to be squeezed is investigated in advance, and the temperature of the raw pipe before and after the squeezing is controlled based on this to suppress the σ phase precipitation. .

  However, in the squeezing process, the temperature of the raw tube changes from moment to moment. In addition, the amount of strain introduced by the squeezing process varies depending on the location of the end of the raw tube and is not uniform. Therefore, grasping the precipitation condition of the σ phase at a certain temperature and a certain amount of strain does not completely take into account the conditions of squeezing in actual operation, but it is a guideline for determining the operation condition. Not too much. In addition, it is practically difficult to preliminarily investigate the precipitation conditions of the σ phase for all temperatures and strains. Therefore, it is conceivable that the temperature of a part of the end portion of the raw tube reaches the σ phase temperature range. Therefore, if the squeezing process is completed before the σ phase is precipitated, cracking of the raw tube can be suppressed even if part of the end of the raw tube is in the σ phase precipitation temperature range. In the squeezing process, the time from the start of the squeezing process to the end of the squeezing process may be less than 20 minutes based on the examples described later. However, in actual operation, it is preferable that there is some margin than immediately before the time when the σ phase precipitates, and therefore, it is preferable that the time from the start of the squeezing process to the end of the squeezing process is within 15 minutes.

[Duplex stainless steel]
The squeezing method of this embodiment is intended for squeezing of the end of the base pipe of a duplex stainless steel pipe. Hereinafter, the duplex stainless steel of this embodiment will be described. In the following description, “%” of the content of each component element means “mass%”.

C: 0.008 to 0.03%
Carbon (C) stabilizes the austenite phase in the steel. On the other hand, if the C content is too high, coarse carbides are likely to precipitate, and the corrosion resistance of the steel, particularly the SCC resistance, is reduced. Therefore, the C content is 0.008 to 0.03%.

Si: 0 to 1%
Silicon (Si) is an optional element. Si deoxidizes steel. Si further increases the corrosion resistance of the steel. If the Si content is too high, the stability of the austenite structure is lowered and the ductility is lowered. Therefore, the Si content is 0 to 1%.

Mn: 0.1 to 2%
Manganese (Mn) deoxidizes steel and stabilizes the austenite structure. Further, Mn increases the strength of the steel while suppressing the precipitation of the σ phase. On the other hand, if the Mn content is too high, the corrosion resistance of the steel decreases. Therefore, the Mn content is 0.1 to 2%.

Cr: 20 to 35%
Chromium (Cr) increases the corrosion resistance of steel. On the other hand, if the Cr content is too high, intermetallic compounds typified by the σ phase are remarkably precipitated and the hot workability of the steel is reduced. Therefore, the Cr content is 20 to 35%.

Ni: 3 to 10%
Nickel (Ni) stabilizes the austenite structure. Ni further enhances the corrosion resistance of the steel. On the other hand, if the Ni content is too high, the proportion of the ferrite phase in the duplex stainless steel decreases. Furthermore, intermetallic compounds represented by the σ phase are remarkably precipitated. Therefore, the Ni content is 3 to 10%.

Mo: 0 to 5%
Molybdenum (Mo) is an optional element. Mo increases the corrosion resistance and strength of steel. On the other hand, if the Mo content is too high, intermetallic compounds typified by the σ phase are significantly precipitated. Therefore, the Mo content is 0 to 5%.

W: 0-6%
Tungsten (W) is an optional element. W increases the corrosion resistance of steel. On the other hand, if the W content is too high, an intermetallic compound typified by the σ phase precipitates significantly. Therefore, the W content is 0 to 6%.

Cu: 0 to 3%
Copper (Cu) is an optional element. Cu stabilizes the austenite structure. Cu further suppresses the generation of the σ phase at the boundary between the ferrite phase and the austenite phase. Cu further increases the strength of the steel. On the other hand, if Cu content is too high, the hot workability of steel will fall. Therefore, the Cu content is 0 to 3%.

N: 0.15-0.40%
Nitrogen (N) stabilizes the austenite structure. N enhances the thermal stability, strength and corrosion resistance of the duplex stainless steel. On the other hand, if the N content is too high, blow holes that are welding defects are likely to occur. Furthermore, coarse nitrides are generated due to the heat effect during welding, and the toughness and corrosion resistance of the steel are reduced. Therefore, the N content is 0.15 to 0.40%.

  The balance of the duplex stainless steel according to this embodiment is made of iron (Fe) and impurities. The impurities referred to here are ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process. Impurities are, for example, phosphorus (P), sulfur (S), aluminum (Al), and the like.

  The mouth drawing method of the present embodiment has been described above. Next, a method for producing a duplex stainless steel pipe using the mouth drawing method of this embodiment will be described. The method for producing the duplex stainless steel pipe of the present embodiment includes a preparation process, a heating process, a mouth drawing process, and a drawing process.

[Preparation process]
In the preparation step, the above-mentioned duplex stainless steel pipe is prepared. The raw pipe is a seamless steel pipe. A base tube of a duplex stainless steel pipe manufactured by Mannesmann manufacturing method, Eugene manufacturing method, or the like is cut into a predetermined length to obtain a duplex stainless steel tube base tube for mouth drawing.

[Heating process]
A heating process is the same as the heating process of the mouth-drawing processing method of this embodiment mentioned above. Therefore, detailed description is omitted.

[Mouth drawing process]
The squeezing process is the same as the squeezing process step of the squeezing method of the present embodiment described above. Therefore, detailed description is omitted. That is, in the heating process and the squeezing process of the method for producing the duplex stainless steel pipe of the present embodiment, the above-described squeezing process method of the present embodiment is performed. After the squeezing process, the tube is cooled to room temperature. The cooling method is, for example, cooling at room temperature.

[Drawing process]
Cold drawing is performed on the cooled raw tube. Thereby, a duplex stainless steel pipe reduced in diameter to a predetermined size is obtained.

  FIG. 3 is an overall configuration diagram of the drawing device. The drawing device includes a gripper 4, a die 5, a carriage 6, and a moving device 7. In FIG. 3, the die 5 shows a cross-sectional view.

  The die 5 reduces the diameter of the raw tube 2 at the time of drawing to increase the roundness of the raw tube 2. The die 5 includes an approach portion 8 and a bearing portion 9 sequentially from the entry side (left side in FIG. 3) to the exit side (right side in FIG. 3). In the approach portion 8, the inner diameter gradually decreases from the entry side to the exit side of the die 5. That is, the approach portion 8 has a tapered shape. The bearing portion 9 is a cylinder. The inner diameter of the bearing portion 9 is constant and corresponds to the outer diameter of the manufactured duplex stainless steel pipe.

  The gripper 4 holds the mouth restrictor 10 of the raw tube 2 being drawn. In FIG. 3, the mouth restrictor 10 is hidden by the gripper 4 and is therefore indicated by a broken line. The gripper 4 is attached to the carriage 6. The carriage 6 is attached to the moving device 7. The moving device 7 is a chain, for example, and moves the carriage 6 and the gripper 4 in the drawing direction X at the time of drawing. Thereby, the raw tube 2 gripped by the gripper 4 is pulled out, and a duplex stainless steel tube is obtained.

  In order to confirm the effect of the squeezing method of the present embodiment, the squeezing process was performed by variously changing the heating temperature of the raw tube before the squeezing process. That is, the squeezing process was performed by changing the temperature To of the raw tube at the start of the squeezing process. Specifically, the end of the base pipe of the duplex stainless steel pipe was heated (heating process). Thereafter, the end portion of the heated raw tube was subjected to squeezing in the range of 5 to 10 times (squeezing process). In the heating step, the heating furnace was controlled, and the end of the raw tube was heated to various temperatures ranging from about 670 ° C to about 800 ° C. In the squeezing process, the amount of squeezing is changed from 2 to 10 times by changing the processing amount per time, and the time from the start of squeezing process to the end of squeezing process is between 3 minutes and 15 minutes Thus, the temperature of the end portion of the raw tube at the end of the squeezing process was changed to various temperatures ranging from about 420 ° C. to about 720 ° C. And after mouth-drawing processing, the presence or absence of the crack of a raw tube was confirmed by visual observation and microstructure observation.

[Test conditions]
Table 1 shows the composition of the duplex stainless steel used in this example. Table 2 shows the squeezing conditions.

In Table 2, “diaphragm diameter” means the outer diameter of the raw tube after squeezing. The “outer diameter processing degree” was calculated by the following equation. The heating method of the end of the raw tube in this example was direct heating by an induction heater.
(Outer diameter processing degree (%)) = (1− (Outer diameter of end of raw pipe after squeezing) / (Outer diameter of end of raw pipe before squeezing)) × 100

[Test results]
The test results are shown in FIG. Referring to FIG. 1, if the temperature To of the raw tube at the start of squeezing was 800 ° C., the raw tube was cracked. When the raw tube was observed in a test where the temperature To of the raw tube at the start of squeezing was 800 ° C., precipitation of the σ phase was observed. Further, the σ phase is likely to precipitate when the temperature of the raw tube becomes high. That is, it was found that if the temperature To of the raw tube at the start of squeezing was 800 ° C. or higher, it was in the σ phase precipitation temperature range.

  Even when the temperature To of the raw tube at the start of the squeezing process was less than 800 ° C., the lower the temperature Tf of the raw tube at the end of the squeezing process, the easier the cracking occurred in the raw tube. When the raw tube was observed in a test in which the temperature To of the raw tube at the start of squeezing was less than 800 ° C. and cracked in the raw tube, no precipitation of σ phase was observed. That is, in these tests, it is considered that the base pipe was not cracked due to the σ phase, and cracks were generated in the base pipe due to a decrease in the ductility of the material as described above.

  The embodiments of the present invention have been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be carried out by appropriately changing the above-described embodiment without departing from the spirit thereof.

1: Mouth drawing device 2: Raw tube 3: Die 4: Gripper 5: Die 6: Carriage 7: Moving device 8: Approach part 9: Bearing part 10: Mouth part

Claims (4)

In mass%, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 5 %, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.40%, with the balance being the end of a duplex stainless steel element tube made of Fe and impurities Squeezing method of
Heating the end of the base tube to 650 ° C. or higher and lower than 800 ° C .;
The temperature Tf of the end portion at the end of the squeezing process is 400 ° C. or higher, and the end portion heated to satisfy the formula (1) with respect to the temperature To of the end portion at the start of the squeezing process. A mouth drawing method comprising: a drawing process.
Tf ≧ 4.4 To-2680 (1) The mouth-drawing method according to claim 1,
The mouth drawing method, wherein the time from the start of the mouth drawing to the end of the mouth drawing is within 15 minutes. The squeezing method according to claim 1 or 2,
In the squeezing process, a squeezing method in which an outer diameter processing degree of the end portion is 32% or less. In mass%, C: 0.008 to 0.03%, Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 5 %, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.40%, and the balance is prepared as a base tube of duplex stainless steel made of Fe and impurities Process,
Heating the end of the base tube to 650 ° C. or higher and lower than 800 ° C .;
The temperature Tf of the end portion at the end of the squeezing process is 400 ° C. or higher, and the end portion heated to satisfy the formula (1) with respect to the temperature To of the end portion at the start of the squeezing process. Drawing process;
A method of producing a duplex stainless steel pipe comprising a step of cold drawing the raw pipe subjected to the mouth drawing.
Tf ≧ 4.4 To-2680 (1)

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