JP2022090676A - Pipe material manufacturing method - Google Patents

Abstract

To provide a technique for reducing an energy consumption amount for manufacturing a highly strong pipe material and improving recyclability of the pipe material.SOLUTION: A long and solid original material made of a steel material including C of 0.05-0.25% by weight is prepared. Then, after a solid billet 910 is formed by cutting the prepared original material, the billet 910 is machined into a hollow blank 920. The hollow blank 920 is molded into a pipe material 950 through warm extrusion molding.SELECTED DRAWING: Figure 1

Description

The present invention relates to a technique for manufacturing a steel pipe having high strength.

In recent years, there has been an increasing demand for improved fuel efficiency of automobiles, and there is a strong demand for further weight reduction. Therefore, in the parts used for automobiles, conventional parts that were molded from steel wire rods and bar materials are molded from steel pipe materials (hereinafter, also referred to as "steel pipes" or simply "pipe materials"). It is being replaced with the parts that have been removed. In addition, parts that have been conventionally manufactured by molding pipe materials are also required to be thinned in order to reduce the weight, while maintaining sufficient strength. Such a request is not limited to pipe materials used for manufacturing automobile parts, but is common to pipe materials used for manufacturing parts for moving bodies such as railway vehicles and aircraft, and parts used for various mechanical devices. is doing.

Generally, in order to match the outer diameter and inner diameter (pipe diameter) of the pipe material used for molding parts to the size of the part, it is cold to a thick pipe material (bare pipe) whose pipe diameter is larger than the pipe material. Manufactured by drawing. However, it is not always easy to sufficiently increase the strength of the pipe material obtained by the cold drawing process. Therefore, the composition is appropriately adjusted, and the pipe material previously formed into a desired shape by cold drawing is subjected to heat treatment such as quenching or tempering to increase the strength. For example, in Patent Document 1, in order to manufacture a pipe material for an airbag, a steel pipe to which chromium (Cr), molybdenum (Mo) or the like is added is cold-pulled, and then quenched and quenched under predetermined temperature conditions. It has been proposed to perform tempering.

Further, Patent Document 1 describes that an element such as titanium (Ti) or niobium (Nb) is added to a steel pipe to be cold drawn to finely granulate the crystal structure. Generally, it is known that the yield stress of a steel material increases as the crystal grain size becomes smaller. Therefore, it can be expected that the strength of the tube material will be further increased by adding elements such as Ti and Nb to finely granulate the crystal structure.

Japanese Unexamined Patent Publication No. 2004-76034

However, when heat treatment such as quenching or tempering is performed, pickling and neutralization are required with the heat treatment, and baking is required with the pickling. Therefore, although the energy consumption can be reduced in the cold drawing process in which the pipe diameter is reduced, a large amount of energy is consumed in the heat treatment and the baking, so that the energy of the entire pipe material manufacturing process is consumed. The amount of consumption will increase. Further, in order to further increase the strength of the pipe material, if an element such as Ti or Nb that finely granulates the crystal structure is added, the recyclability of the pipe material is lowered.

The present invention has been made to solve the above-mentioned conventional problems, and to provide a technique for reducing energy consumption when manufacturing a high-strength pipe material and improving the recyclability of the pipe material. The purpose.

In order to achieve at least a part of the above object, the present invention can be realized as the following form or application example.

[Application Example 1]
A method for manufacturing a pipe material, which consists of a steel material containing 0.05 to 0.25% by weight of C, and a step of preparing a long solid raw material and a step of cutting the raw material to form a solid material. A method for manufacturing a pipe material, comprising a raw material cutting step for forming a billet, a hollowing process for processing the billet into a hollow blank, and a warm extrusion step for warmly extruding the blank into a tubular shape.

According to this application example, the crystal structure of the obtained tube material can be finely granulated to increase the yield stress and lower the brittle transition temperature. Therefore, it is possible to omit performing heat treatment such as quenching on the obtained pipe material, and it is possible to reduce the energy consumption of the entire manufacturing process of the pipe material having high strength. Further, since the fine graining of the crystal structure is expressed without the addition of the element for finely granulating the crystal structure, it is possible to suppress the deterioration of the recyclability of the tube material due to the addition of the element.

[Application example 2]
The method for producing a pipe material according to claim 1, wherein the raw material is spheroidized and annealed. According to this application example, the ductility of the obtained pipe material can be satisfactorily improved.

[Application example 3]
The method for manufacturing a pipe material according to claim 1 or 2, wherein the hollowing processing step processes the billet into the blank by cold forging. By cold forging the billet into a blank, it is possible to suppress the consumption of energy required for heating the billet.

[Application example 4]
The method for producing a pipe material according to any one of claims 1 to 3, wherein the steel material further contains Mn of 0.60 to 1.5% by weight. By adding Mn to the steel material as the raw material, the tensile strength of the obtained pipe material can be further increased.

[Application Example 5]
The steel material further contains 0.30 to 0.85% by weight of Mn, 0.85 to 1.25% by weight of Cr, and 0.15 to 0.35% by weight of Mo. The method for manufacturing a pipe material according to any one of claims 1 to 3. By adding Cr and Mo to the steel material as the raw material, the obtained pipe material is improved in tensile strength by work hardening. Therefore, the strength of the parts obtained by processing the pipe material can be further increased.

The present invention can be realized in various aspects. For example, it can be realized by a method of manufacturing a pipe material, a pipe material manufactured by the manufacturing method, various parts using the pipe material, and the like.

The process diagram which shows the manufacturing process of the pipe material in 1st Embodiment. Explanatory drawing which shows the structure of the mold used for warm extrusion molding, and the state that warm extrusion molding is performed by the mold. An electron micrograph showing the result of evaluating the crystal grain size of the tube material. An electron micrograph showing the result of evaluating the crystal grain size of the compressed test piece. An electron micrograph showing the result of evaluating the crystal grain size of the compressed test piece. An electron micrograph showing the result of evaluating the crystal grain size of the compressed test piece. The graph which shows the result of having evaluated the crystal grain size of the test piece which performed compression processing. The flowchart which shows the manufacturing process of the pipe material in 2nd Embodiment. An electron micrograph showing the results of evaluating the effect of spheroidizing annealing on the crystal grain size. Explanatory drawing which shows the result of having evaluated the influence on the ductility of spheroidizing annealing.

Hereinafter, embodiments for carrying out the present invention will be described in the following order.
A. First Embodiment:
A1. Manufacturing process of pipe material in the first embodiment:
A2. Warm extrusion:
A3. Example of the first embodiment:
B. Second embodiment:
B1. Manufacturing process of pipe material in the second embodiment:
B2. Example of the second embodiment:

A. First Embodiment:
A1. Manufacturing process of pipe material in the first embodiment:
FIG. 1 is a process diagram showing a manufacturing process of a pipe material as the first embodiment of the present invention. 1 (a) to 1 (d) show the form of the workpiece processed in each step, that is, the billet 910, the hollow blank 920, the extruded material 950, and the final pipe material 960. Further, in FIGS. 1 (a) to 1 (d), the alternate long and short dash line CC'indicates the axis of the billet 910, the hollow blank 920, the extruded material 950, and the pipe material 960.

In the manufacturing process of the pipe material of the first embodiment, first, a solid and long steel material (raw material) which is a raw material of the pipe material is prepared. As the raw material, a coil material obtained by coiling a linear steel material may be used, or a bar material (bar steel) which is a bar-shaped steel material may be used. The material of the raw material may be a steel material based on carbon steel having a carbon (C) content of 0.05 to 0.25% by weight. By cutting the raw material prepared in this way to an appropriate length, a substantially columnar billet 910 can be obtained as shown in FIG. 1 (a).

The billet 910 thus obtained is processed into a hollow blank 920 as shown in FIG. 1 (b). In the example of FIG. 1 (b), the hollow blank 920 is provided with a circular hole 929 in the center, and the inner peripheral portion 922 is formed in a substantially annular shape thicker than the outer peripheral portion 921. Such processing from the billet 910 to the hollow blank 920 (hollowing processing) can be performed by a general cold forging technique using a parts former or the like. The hollowing process is not limited to cold forging, but can also be performed by warm forging or hot forging. However, the hollowing process is preferably performed by cold forging in that the energy consumption required for heating the billet 910 can be suppressed.

In the example of FIG. 1, the hollow blank 920 is formed so that the inner peripheral portion 922 is thicker than the outer peripheral portion 921, but the shape of the hollow blank can be variously changed. The hollow blank may have a shape having a uniform thickness from the outer peripheral portion to the inner peripheral portion, or may have a shape in which the outer peripheral portion is thicker than the inner peripheral portion.

The hollow blank 920 obtained by the hollowing process is substantially tubular by warm extrusion molding (details will be described later) in which extrusion is performed from the rear side (C direction side) to the front side (C'direction side). It is formed into an extruded material 950 (FIG. 1 (c)). In the following, the direction of extrusion (C'direction) is referred to as the front, and the opposite direction (C direction) is referred to as the rear.

The extruded material 950 formed by warm extrusion molding has a tubular tip portion 951 and a rear end portion 952 deformed along the shape of a mold (described later). Then, as shown in FIG. 1 (d), a cylindrical tube material 960 is obtained by cutting out the tip portion 951 from the extruded material 950.

In the example of FIG. 1, the manufacturing process of the pipe material includes the cutting of the pipe material of FIG. 1 (d). However, since the extruded material 950 shown in FIG. 1 (c) also has a substantially tubular shape, it may be considered that the production of the pipe material is completed at the stage of the warm extrusion molding shown in FIG. 1 (c). It is possible. Therefore, in the present invention, the process up to warm extrusion molding is referred to as a method for manufacturing a pipe material.

A2. Warm extrusion:
FIG. 2 is an explanatory diagram showing a configuration of a mold used for warm extrusion molding and a state in which warm extrusion molding is performed by the mold. 2 (a) to 2 (c) show how the hollow blank 920 is deformed by warm extrusion molding to form an extruded material 950 (FIG. 1 (c)). In addition, in FIGS. 2A to 2C, a punch 100, a counter punch 200 and a die 300 constituting a mold for warm extrusion molding, a hollow blank 920 and an intermediate material 930 which is a work in the middle of molding, A cross section of 940 and 940 cut along the axis CC'is shown.

The punch 100 is formed from a flat plate-shaped base 110 attached to a ram of a press device used for warm extrusion, a substantially cylindrical intermediate portion 120 extending forward (C'direction) from the base 110, and an intermediate portion 120. It has a cylindrical tip 130 that extends forward and enters the gap between the counter punch 200 and the die 300.

The counter punch 200 is a rod-shaped member, which is located on the rear side and has a columnar small diameter portion 210 having a small outer diameter and is located on the front side of the small diameter portion 210, and the outer diameter expands toward the front. It has a tapered portion 220 to be formed, and a large diameter portion 230 located on the front side of the tapered portion 220 and having an outer diameter larger than that of the small diameter portion 210.

The die 300 is a substantially cylindrical member, and has a diameter-expanded portion 310 having a large inner diameter, a tapered portion 320 located on the front side of the diameter-expanded portion 310 and whose inner diameter decreases toward the front, and a tapered portion. It is located on the front side of the 320 and has a reduced diameter portion 330 having an inner diameter smaller than that of the enlarged diameter portion 310.

As shown in FIG. 2, in the counter punch 200 and the die 300, the large diameter portion 230 of the counter punch 200 and the reduced diameter portion 330 of the die 300 face each other, and the counter punch 200 and the inner surface 309 of the die 300 are coaxial with each other. It is arranged so as to be. Further, the inner diameter of the tip portion 130 of the punch 100 is set to be substantially the same as the outer diameter of the small diameter portion 210 of the counter punch 200, and the outer diameter thereof is substantially the same as the inner diameter of the enlarged diameter portion 310 of the die 300. It is set to be. Then, the tip portion 130 of the punch 100 and the inner surface 309 of the counter punch 200 and the die 300 are arranged so as to be coaxial with each other, and the punch 100 is moved forward to perform extrusion molding.

In the warm extrusion molding, first, the hollow blank 920 is heated to a preset blank temperature and the counter punch 200 and the die 300 are preset so that the extrusion molding proceeds in the warm temperature region (described later). The temperature is raised to the temperature of the molded mold. Such a blank temperature and a mold temperature can be set by simulation or the like in consideration of heat generation (plastic heat generation) due to plastic deformation so that extrusion molding proceeds in a warm temperature region. In general, the warm temperature range is 600 to 650 ° C., and the temperature rise due to plastic heat generation is about 100 to 150 ° C., so that the blank temperature and the mold temperature are set to 450 to 550 ° C.

Next, the heated hollow blank 920 is charged into a mold as shown in FIG. 2 (a). Specifically, the hollow blank 920 is arranged between the small diameter portion 210 of the counter punch 200 and the enlarged diameter portion 310 of the die 300.

In the example of FIG. 2A, the inner diameter of the hollow blank 920 is set to the outer diameter of the small diameter portion 210 of the counter punch 200 so that a large gap does not occur between the hollow blank 920 and the counter punch 200 or the die 300. The outer diameter is substantially the same as the inner diameter of the enlarged diameter portion 310 of the die 300. However, the inner and outer diameters of the hollow blank can be variously changed as long as the hollow blank 920 can be arranged between the small diameter portion 210 of the counter punch 200 and the enlarged diameter portion 310 of the die 300. However, as shown in FIG. 2A, the hollow blank 920 and the counter punch 200 are suppressed in that voids are suppressed from being generated in the extruded material 950 (FIG. 1 (c)) formed by warm extrusion molding. It is preferable that a large gap is not formed between the extruder and the die 300.

As shown in FIG. 2A, when the hollow blank 920 is charged into the mold and then the punch 100 is moved forward, the hollow blank 920 is deformed, and the deformed hollow blank causes the small diameter portion of the counter punch 200. The gap between the 210 and the tapered portion 220 and the enlarged diameter portion 310 and the tapered portion 320 of the die 300 is filled. Then, when the punch 100 is further moved forward, extrusion is started as shown in FIG. 2B, and the deformed hollow blank, that is, the intermediate material 930 has a rear end portion 931 and a large diameter portion 230 of the counter punch 200. It is extruded into a gap (narrowed portion) between the dice 300 and the reduced diameter portion 330, and a tubular tip portion 932 is formed in the narrowed portion.

Further, as shown in FIG. 2B, when the punch 100 is moved forward to advance the extrusion after the extrusion is started, the rear end portion 931 of the intermediate material 930 becomes the large diameter portion 230 of the counter punch 200. It is extruded into the gap between the dice 300 and the reduced diameter portion 330. Therefore, as shown in FIG. 2C, the volume of the rear end portion 941 of the intermediate material 940 is reduced, the constricted portion is filled with the intermediate portion 942 of the intermediate material 940, and the position in front of the die 300 is reached. In, a molded tubular portion 943 is formed.

As described above, in the state where extrusion is in progress, a large number of dislocations are introduced into the intermediate portion 942 of the intermediate material 940 by plastic deformation, and a deformed structure is formed. Further, the temperature of the intermediate portion 942 rises to a warm temperature region (600 to 650 ° C.) where recrystallization proceeds due to plastic heat generation. Then, when the temperature of the intermediate portion 942 becomes a warm temperature region, the deformed structure into which the dislocations have been introduced undergoes a recovery process in which the dislocations disappear or are rearranged, and primary recrystallization due to nucleation and growth proceeds. It becomes a fine crystal structure (subgrain).

In the first embodiment, since the molded tubular portion 943 is exposed to the outside of the die 300, the temperature of the tubular portion 943 is lower than the warm temperature region, and the grain growth of the subgrain is suppressed. Therefore, the crystal structure of the tubular portion 943 is maintained in a fine state (crystal particle size is 1.5 μm or less). Then, by finely granulating the crystal structure of the tubular portion 943, the crystal structure of the tip portion 951 of the extruded material 950 (FIG. 1 (c)) and the tube material 960 (FIG. 1 (d)) corresponding to the tubular portion 943. Also finely granulated.

As described above, according to the first embodiment, the crystal structure of the obtained tube material 960 is atomized by performing the warm extrusion molding in which the extrusion molding proceeds in the warm temperature region. In general, the yield stress of steel materials increases as the crystal grain size decreases, and the brittle transition temperature decreases as the crystal grain size decreases (Hall-Petch relationship). Therefore, according to the first embodiment, since the crystal structure of the obtained pipe material 960 is atomized, the yield stress of the pipe material 960 can be made higher and the brittle transition temperature can be made lower. Therefore, it is possible to omit performing heat treatment such as quenching on the obtained pipe material 960, and it is possible to reduce the energy consumption of the entire manufacturing process of the pipe material having high strength.

Further, in the first embodiment, a solid and long steel material is used as a raw material in the production of the pipe material. Therefore, according to the first embodiment, the storage space of the raw material is smaller than that of a general method for manufacturing a pipe material in which a pipe material (raw pipe) as a raw material is processed to manufacture a pipe material having a desired shape. can do.

Further, in the first embodiment, even when carbon steel having a C content of 0.05 to 0.25% by weight is used as the material of the pipe material 960 (that is, the material of the raw material), the crystals of the pipe material 960 are crystallized. The structure can be sufficiently finely granulated. Therefore, it is possible to omit the addition of alloying elements such as titanium (Ti) and niobium (Nb) that promote fine graining of the crystal structure, so that the recyclability of the pipe material 960 can be improved and the price of the pipe material 960 can be reduced. Can be done.

The material of the raw material (that is, the material of the pipe material) for manufacturing the pipe material by applying the first embodiment can be appropriately changed according to the mechanical properties required for the final pipe material. For example, in order to increase the tensile strength, it is also possible to use a raw material to which manganese (Mn), which is an alloying element for increasing the tensile strength, is added as in C. In this case, the Mn content is preferably adjusted by the presence or absence of chromium (Cr) and molybdenum (Mo) selectively added as described later. When Cr and Mo are added, the Mn content is preferably 0.30 to 0.85%, and when Cr and Mo are not added, the Mn content is 0.60 to 0.85%. It is preferably 1.5%. Further, in order to promote the improvement of tensile strength by work hardening, it is also possible to use a raw material to which Cr and Mo are added. In this case, the Cr content is preferably 0.85 to 1.25%, and the Mo content is preferably 0.15 to 0.35%.

A3. Example of the first embodiment:
[Evaluation of crystal grain size of pipe material]
In order to confirm the effect of the first embodiment, a pipe material was prepared from a raw material of carbon steel (S15C) to which no alloying element was added, and the crystal grain size of the prepared pipe material was evaluated. Specifically, a coil material of S15C was prepared as a raw material. The chemical composition of the prepared coil material is as shown in Table 1 below. In Table 1, Si, Cu, and Ni represent silicon, copper, and nickel, respectively.

Next, using a parts former, the prepared coil material was cut into billets, and the cut billets were cold forged to create a hollow blank. Then, the temperature of the hollow blank was raised to 550 ° C, and the temperature of the counter punch and the die was raised to 472 ° C, and extrusion molding was performed. With respect to the tip end portion (that is, the pipe material) of the extruded material (see FIG. 1 (c)) thus obtained by extrusion molding, a scanning electron microscope (that is, a cross section perpendicular to the axial direction (CC'direction) is obtained. Hereinafter, the observation was simply performed with an "electron microscope"). Observation with an electron microscope was performed on the outer diameter side of the pipe material, the inner diameter side of the pipe material, and the intermediate portion (intermediate part).

FIG. 3 is an electron micrograph showing the result of evaluating the crystal grain size of the tube material. 3 (a) and 3 (b) show the crystal structure on the outer diameter side, and FIGS. 3 (c) and 3 (d) show the crystal structure in the middle portion, and FIGS. 3 (e) and 3 (e) show. 3 (f) shows the crystal structure on the inner diameter side. Further, FIGS. 3 (a), 3 (c), and 3 (e) show the observation at a magnification of 5000 times, and FIGS. 3 (b), 3 (d), and 3 (f) show the observation. It shows the observation at a magnification of 10,000 times.

As can be seen from FIGS. 3 (a), 3 (c), and 3 (e), in the evaluated pipe material, the meat of the pipe material is obtained on any of the outer diameter side, the intermediate portion, and the inner diameter side. Throughout the thickness, recrystallization formed a crystalline structure. Further, as can be seen from FIGS. 3 (b), 3 (d), and 3 (f), it was confirmed that the crystal grain size was 1.5 μm in the entire wall thickness of the pipe material.

From the above, when the tube material is manufactured by applying the first embodiment of warm extrusion molding, and S15C to which an element that promotes fine graining of the crystal structure is not added is used as the raw material. It was also found that it is possible to finely granulate the crystal structure of the obtained tube material.

[Confirmation of fine grain expression conditions]
Following the evaluation of the crystal grain size of the tube material, the material of the raw material, the processing state of the raw material, and the temperature of the blank and the mold were changed, and the conditions under which fine particles were developed were confirmed. Specifically, as a processing method corresponding to extrusion processing in which plastic deformation progresses due to strong compressive stress, compression processing was performed in a heated state, and the crystal structure of the compressed test piece was observed using an electron microscope. For comparison, the crystal structure of the test piece compressed by cold compression processing was observed using an electron microscope.

As the material of the test piece to be compressed in a heated state, carbon steel (S15C) whose crystal grain size was evaluated in the pipe material and manganese steel (Q345B) capable of increasing the strength were used. For comparison, the material of the test piece to be cold-compressed is chrome molybdenum steel, which has substantially the same C content as S15C and Q345B used for compression in a heated state and is used as structural steel. (SCM415) and was used. The chemical composition of the steel material used as these test pieces is shown in Table 2 below. In Table 2, the underlined items indicate the elements added to the steel material.

As the test piece for compression processing, a flat plate having a thickness of 7.7 mm was prepared by cutting out the rods of S15C, Q345B and SCM415 on the plane perpendicular to the axial direction. Further, in order to confirm the presence or absence of the influence of the hollowing process performed prior to the warm extrusion process in the first embodiment, the rods of S15C and Q345B were subjected to axial stationary forging at a processing rate of 40%. After that, the material subjected to the embedding forging was cut out on a plane perpendicular to the embedding direction, and a flat plate-shaped test piece having a thickness of 7.7 mm was prepared. In the following, for the test piece obtained by cutting out the material subjected to the stationary forging in this way, the processing rate of the test piece is expressed as 40%, and the bar is obtained by cutting the bar in a plane perpendicular to the axial direction. For the test piece, the processing rate of the test piece is expressed as 0%.

In compression processing, the upper and lower dies of the convex shape are arranged so that the convex portions face each other, the test piece is placed on the lower mold, and then the upper mold is lowered, and between the convex portions of the upper mold and the lower mold. The compression process was performed so that the thickness was 1 mm (that is, the processing rate of the compression process was 87%). When compression processing was performed in a heated state, the temperature of the test piece and the upper and lower molds were raised in a heating furnace in which the heating temperature was set in advance, and the heated state was maintained for 15 minutes after the temperature rise. Then, the test piece was quickly compressed using the upper and lower dies taken out from the heating furnace. On the other hand, when the cold compression process was performed, the test piece at room temperature was compressed using the upper and lower dies at room temperature.

4 to 6 are electron micrographs showing the results of evaluating the crystal grain size of the compressed test piece. 4 (a) to 4 (c) show the crystal structure of the test piece subjected to cold compression processing for each test piece using S15C, Q345B and SCM415 as the materials. 5 (a) to 5 (f) show the crystal structure of the test piece subjected to compression processing in a heated state using S15C as the material, and FIGS. 6 (a) to 6 (f) show the crystal structure of the test piece as the material. The crystal structure of the test piece subjected to the compression processing in a heated state using Q345B is shown.

As can be seen from FIGS. 4 (a) to 4 (c), no fine particles were expressed in the test piece subjected to the cold compression process regardless of the material of the test piece. On the other hand, as can be seen from FIGS. 5 (a), 5 (b), 6 (a), and 6 (b), the test piece having a heating temperature of 450 ° C. is irrespective of the material and processing rate of the test piece. However, fine particles were expressed. On the other hand, in the test pieces having a heating temperature of 550 ° C. or 650 ° C. (FIGS. 5 (c) to 5 (f) and 6 (c) to 6 (f)), fine particles are expressed. However, the crystal grain size was larger than that of the test piece having a heating temperature of 450 ° C. It is considered that this is because the temperature of the test piece and the mold at the start of the compression process was excessively high, so that the grain growth proceeded and the crystal structure of the test piece became coarse.

FIG. 7 is a graph showing the results of evaluating the crystal grain size of the test piece subjected to compression processing. FIG. 7A shows the crystal grain size of the test piece containing the cold-compressed product, and FIG. 7B shows the crystal grain size of the test piece not containing the cold-compressed product.

As shown in FIG. 7A, in the test piece subjected to cold compression processing, the crystal grain size of the test piece whose material is SCM415 is about 20 μm, and the crystal grain size of the test piece whose material is S15C or Q345B is about 20 μm. Was about 50 μm. On the other hand, in the test piece subjected to warm compression processing, the crystal grain size of the test piece is close to 1.5 μm (broken line in FIG. 7A), which is the crystal grain size targeted for fine granulation. rice field.

Further, as shown in FIG. 7B, in the test piece having a heating temperature of 550 ° C or 650 ° C, the crystal grain size was 2 μm or more. On the other hand, in the test piece having a heating temperature of 450 ° C., it was confirmed that the crystal grain size was 1 μm or less, which was lower than the target crystal grain size of 1.5 μm.

In the compression process in the heated state, it is assumed that the temperature of the test piece during the compression process rises by about 150 ° C. due to plastic heating. Therefore, in the test piece whose heating temperature, that is, the temperature of the test piece and the mold at the start of the compression processing was 450 ° C., the crystal grain size was 1 μm or less because the compression processing proceeded in the warm temperature region. Conceivable. On the other hand, in the test piece having a heating temperature of 550 ° C. or 650 ° C., it is considered that the crystal grain size was 2 μm or more because the compression processing proceeded at a temperature higher than the warm temperature region.

As described above, according to the first embodiment, the crystal grain size of the obtained tube material is 1.5 μm or less by producing the tube material by warm extrusion molding in which the extrusion process proceeds in the warm temperature region. It is possible to make the crystal structure finer. Therefore, the yield stress of the obtained pipe material can be increased and the brittle transition temperature can be lowered, so that heat treatment such as quenching can be omitted and the energy consumption of the entire manufacturing process of the high-strength pipe material can be reduced. can do. Further, since such fine granulation is expressed without adding an element that promotes fine granulation of the crystal structure, it is possible to improve the recyclability of the obtained pipe material and reduce the price of the pipe material.

B. Second embodiment:
B1. Manufacturing process of pipe material in the second embodiment:
FIG. 8 is a flowchart showing a manufacturing process of the pipe material in the second embodiment. The second embodiment is different from the first embodiment in that it has a step of spheroidizing and annealing the raw material (step S1) prior to the cutting step of the raw material for forming the billet (step S2). There is. Since each step from cutting the raw material (step S2) to cutting out the pipe material (step S5) is the same as that of the first embodiment, the description thereof will be omitted here.

By spheroidizing and annealing the raw material, iron carbide (Fe _3C ), which was laminated with ferrite to form pearlite in the raw material, is dispersed in the raw material and precipitated as fine spherical cementite. Then, in the second embodiment, the temperature of the work does not become higher than the warm temperature region in the entire manufacturing process (steps S2 to S5) from the formation of the billet to the formation of the pipe material. Therefore, even in the final pipe material, the iron carbide is maintained in a spherical cementite state, so that the ductility of the obtained pipe material can be further increased.

On the other hand, in the second embodiment, the pipe material is formed by warm extrusion processing as in the first embodiment. Therefore, as in the first embodiment, the crystal structure is finely divided in the finally obtained tube material. Therefore, according to the second embodiment, the yield stress of the pipe material can be made higher, the brittle transition temperature can be made lower, and the ductility can be made higher.

In the second embodiment, the raw material is spheroidized and annealed in step S1 of the pipe material manufacturing process, but step S1 may be omitted by preparing the raw material that has been spheroidized and annealed in advance. It is possible. Further, the step of performing the spheroidizing annealing in step S1 can be regarded as the step of preparing the raw material to which the spheroidizing annealing has been performed.

B2. Example of the second embodiment:
In order to confirm the effect of the second embodiment, the effect of spheroidizing annealing on the expression of fine particles in the tube material and the mechanical properties of the tube material was evaluated. Specifically, a plurality of rods made of Q345B are prepared, and a part of the rods is spheroidized and annealed, and the rods not spheroidized and annealed and the spheroidized and annealed rods are the first embodiment. A test piece for warm compression processing and a billet for manufacturing a pipe material were prepared in the same manner as in the above embodiment. The chemical composition of the prepared bar is the same as that of the first embodiment shown in Table 2.

The test pieces for warm compression processing were subjected to warm compression processing at a heating temperature of 450 ° C., and the presence or absence of fine particles was confirmed and the mechanical properties were evaluated. On the other hand, for billets for manufacturing pipe materials, after hollowing, warm extrusion molding was performed to prepare pipe materials, and the obtained pipe materials were subjected to a flattening test. In addition, various conditions such as compression processing conditions and pipe material manufacturing conditions are the same as those in the first embodiment.

FIG. 9 is an electron micrograph showing the results of evaluating the effect of spheroidizing annealing on the crystal grain size. FIG. 9A shows the crystal structure of the test piece that has not been spheroidized and annealed, and FIG. 9B shows the crystal structure of the test piece that has been spheroidized and annealed.

As can be seen from FIGS. 9 (a) and 9 (b), it was confirmed that the crystal structure became finer by performing the warm compression process regardless of the presence or absence of spheroidizing annealing. Further, it was confirmed that the crystal structure was sufficiently finely granulated, although the crystal grain size was slightly increased by performing spheroidizing annealing.

The mechanical properties of the test pieces subjected to warm compression processing were evaluated by tensile tests at normal temperature and low temperature (-40 ° C). Table 3 below shows the crystal grain size of each test piece and the evaluation results of the tensile test.

As shown in Table 3, in both normal temperature and low temperature tests, by applying spheroidizing annealing to the raw material, the tensile strength is slightly reduced (-10% at room temperature), but the elongation is significantly increased. It was found to be (+ 47% at room temperature). From this result, it was found that the ductility of the test piece subjected to the warm compression processing corresponding to the warm extrusion molding can be satisfactorily improved by subjecting the raw material to spheroidizing annealing.

FIG. 10 is an explanatory diagram showing the results of evaluating the effect of spheroidizing annealing on the ductility. FIGS. 10 (a) and 10 (b) are photographs of the upper surface and the side surface of the sample subjected to the flattening test for the pipe material which has not been spheroidized and annealed in the raw material. FIGS. 10 (c) and 10 (d) are photographs of the upper surface and the side surface of the sample subjected to the flattening test for the tube material obtained by spheroidizing and annealing the raw material.

As shown in FIGS. 10 (a) and 10 (b), in the pipe material not subjected to spheroidizing annealing, cracks were not generated on the side surface side, but cracks were generated on the upper surface side. On the other hand, as shown in FIGS. 10 (c) and 10 (d), in the spheroidized annealed pipe material, cracks did not occur on either the upper surface side or the side surface side. From this result, it was found that the ductility of the pipe material formed by warm extrusion can be satisfactorily improved by subjecting the raw material to spheroidizing annealing.

From the above results, by applying the second embodiment, the crystal structure of the tube material can be sufficiently finely granulated, so that the yield stress of the tube material can be made higher and the brittle transition temperature can be made lower. It was also found that the ductility of the pipe material can be further increased by spheroidizing annealing of the raw material.

100 ... Punch 110 ... Base 120 ... Middle part 130 ... Tip part 200 ... Counter punch 210 ... Small diameter part 220 ... Tapered part 230 ... Large diameter part 300 ... Die 309 ... Inner surface 310 ... Enlarged part 320 ... Tapered part 330 ... Reduced diameter Part 910 ... Billet 920 ... Hollow blank 921 ... Outer peripheral part 922 ... Inner peripheral part 929 ... Hole 930 ... Intermediate material 931 ... Rear end part 932 ... Tip part 940 ... Hollow blank 941 ... Rear end part 942 ... Middle part 943 ... Tubular part 950 ... Extruded material 951 ... Tip part 952 ... Rear end part 960 ... Pipe material

Claims (5)

It is a manufacturing method of pipe material,
The process of preparing a long, solid raw material made of steel containing 0.05 to 0.25% by weight of C, and
A raw material cutting step of cutting the raw material to form a solid billet,
A hollowing process for processing the billet into a hollow blank, and
A warm extrusion step in which the blank is warm-extruded into a tubular shape, and
To prepare
Manufacturing method of pipe material. The method for producing a pipe material according to claim 1, wherein the raw material is spheroidized and annealed. The method for manufacturing a pipe material according to claim 1 or 2, wherein the hollowing processing step processes the billet into the blank by cold forging. The method for producing a pipe material according to any one of claims 1 to 3, wherein the steel material further contains Mn of 0.60 to 1.5% by weight. The steel material further contains 0.30 to 0.85% by weight of Mn, 0.85 to 1.25% by weight of Cr, and 0.15 to 0.35% by weight of Mo. The method for manufacturing a pipe material according to any one of claims 1 to 3.

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