JP6612295B2 - Tubing material and method for producing the tubing - Google Patents

Description

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

  In recent years, demands for improving fuel efficiency have been increasing for automobiles, and further weight reduction of automobiles has been strongly demanded. For this reason, in parts used in automobiles, conventional parts formed from steel wires and rods are formed from steel pipes (hereinafter also referred to as “steel pipes” or simply “pipe materials”). Replacing these parts with other parts is underway. In addition, parts that have been manufactured by forming a pipe material from the past are also required to be thin in order to reduce weight, while maintaining sufficient strength. Such requests are not limited to pipe materials used in the manufacture of automobile parts, but are common to pipe materials used in the manufacture of parts for moving bodies such as railway vehicles and aircraft, and parts used for various mechanical devices. doing.

  In general, pipes used for forming parts are cold-thinned to thicker pipes (elementary pipes) whose pipe diameter is larger than that of the pipe in order to match the outer diameter and inner diameter (pipe diameter) to the size of the part. Manufactured by drawing. However, it is not always easy to sufficiently increase the strength of the pipe material obtained by cold drawing. Therefore, the strength is increased by applying heat treatment such as quenching and tempering to a pipe material that has been appropriately adjusted in composition and previously formed into a desired shape by cold drawing. For example, in Patent Document 1, in order to manufacture a tube material for an air bag, a steel pipe to which chromium (Cr), molybdenum (Mo), or the like is added is subjected to cold drawing, and thereafter quenched at a predetermined temperature condition. It has been proposed to perform tempering.

JP 2004-76034 A

  However, when heat treatment such as quenching or tempering is performed, pickling and neutralization are required as the heat treatment is performed, and baking is required as the pickling is performed. Therefore, in the cold drawing process for reducing the pipe diameter, the energy consumption can be reduced, but a large amount of energy is consumed in the heat treatment and baking. Consumption increases.

  This invention is made | formed in order to solve the conventional subject mentioned above, and it aims at providing the technique which reduces the energy consumption at the time of manufacturing a steel pipe with high intensity | strength.

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

[Application Example 1]
0.05-0.25 wt% C, 0.15-0.35 wt% Si, 0.3-1.5 wt% Mn, 0.035 wt% or less S, 0.030 wt% A tube material comprising the following P, the balance being made of a steel material that is Fe and inevitable impurities, and the ferrite structure being equiaxed crystals having an average crystal grain size of 1.5 μm or less.

  In the steel material used in this application example, the ferrite structure that has become an equiaxed crystal having an average crystal grain size of 1.5 μm or less is subjected to warm reduction processing on the raw material material (element tube) made of the steel material. can get. Thus, the strength of the tube material can be sufficiently increased by making the ferrite structure an equiaxed crystal having an average crystal grain size of 1.5 μm or less. Therefore, in the manufacture of the pipe material of this application example, since heat treatment can be omitted, heat treatment that consumes a large amount of energy, pickling / neutralization, and baking can be omitted. As a result, although the energy required for warm shrinkage processing increases, the energy consumption of the entire pipe manufacturing process can be reduced, and the energy consumption when manufacturing pipes that are steel pipes with high strength Can be reduced.

[Application Example 2]
The pipe according to Application Example 1, wherein the steel further includes at least one of 0.005 to 0.05 wt% Nb and 0.01 to 0.08 wt% Ti.

  Nb and Ti are elements that promote the refinement of the ferrite structure. Therefore, by adding at least one of Nb and Ti, the average crystal grain size of the ferrite structure can be made smaller, and the strength of the tube material can be made higher.

[Application Example 3]
It is a pipe material of application example 1 or 2, Comprising: The said steel materials are pipe materials containing 0.040-0.10 weight% V further.

  V is an element that improves the yield strength. Therefore, the proof stress of the pipe material can be further increased by adding V.

[Application Example 4]
The pipe material is manufactured by subjecting a raw pipe having an inner diameter and an outer diameter larger than the pipe material and a wall thickness thicker than the pipe material to a warm reduction process with a surface reduction rate of 85% or more. The tube material according to any one of Application Examples 1 to 3.

  By setting the area reduction rate in the warm reduction process to 85%, the average crystal grain size of the ferrite structure of the tube material can be more reliably set to 1.5 μm or less.

[Application Example 5]
The tube material according to application example 4, wherein the warm diameter reduction processing is performed by rolling under strong rolling.

  In rolling under strong rolling, it is easy to increase the area reduction rate in a single process. For this reason, the area reduction rate can be 85% or more with a smaller number of machining operations, so that the manufacturing process of the pipe material can be shortened.

[Application Example 6]
6. The pipe material according to Application Example 4 or 5, wherein the pipe material is manufactured by further performing cold diameter reduction processing on the pipe material subjected to the warm diameter reduction processing.

  The ferrite structure that has become the equiaxed crystal refined by the warm shrinking process is further refined by maintaining the state of the equiaxed crystal by performing the cold shrinking process. The elongation, squeezing and low temperature brittleness are well maintained. On the other hand, by performing cold diameter reduction processing, work hardening occurs and the strength and hardness of the pipe material increase. Therefore, it becomes possible to further increase the strength and hardness of the pipe material while maintaining the ease of post-processing.

Note that the present invention can be realized in various modes. For example, it is realizable with aspects, such as a pipe material, the manufacturing method of a pipe material, the pipe material manufactured by the manufacturing method, various parts using those pipe materials, and a manufacturing method .

The flowchart which shows the pipe manufacturing process for manufacturing a high intensity | strength pipe. Explanatory drawing which shows the specific example of warm diameter reduction processing. The flowchart which shows an example of a conventional process. Photograph of crystal structure of small diameter tube. The graph which shows the result of a tension test. The graph which shows the result of a Charpy impact test.

Hereinafter, modes for carrying out the present invention will be described in the following order.
A. Embodiment:
A1. Tube manufacturing process:
A2. Warm diameter reduction:
A3. Steel composition:
A4. Conventional pipe manufacturing process:
B. Example:
B1. Summary of evaluation:
B2. Evaluation results:
C. Variation:

A. Embodiment:
A1. Tube manufacturing process:
FIG. 1 is a flowchart showing a pipe manufacturing process for manufacturing a high-strength pipe as an embodiment of the present invention. In each step of FIG. 1, the numerical values shown on the right side, CO ₂ emissions per unit in each step, i.e., CO ₂ emitted during the tubing (product tubing) a ton (t) processed as a product Is the numerical value expressed in kilograms (kg), and represents the energy consumption. Also, at the end of the manufacturing process shown in FIG. 1, the numerical values shown in the lower part represents the total emissions of CO ₂ emitted in tubing manufacturing process of this embodiment CO ₂ in emission intensity.

  In the pipe manufacturing process of the present embodiment shown in FIG. 1, first, in step S100, a steel pipe (raw pipe) that is a raw material of the pipe is prepared. As the raw pipe, a seamless steel pipe or the like manufactured by hot rolling such as Mannesmann method is used. Since such a seamless steel pipe is usually provided in a long state, the long seamless steel pipe is cut into an appropriate length when preparing the raw pipe.

  Next, in step S110, the prepared pipe is immersed in an acidic solution to remove scales and the like attached to the pipe, and the pickled pipe is immersed in an alkaline solution and used for pickling. Neutralizing the acidic solution. Instead of pickling, the scale and the like may be removed by a physical method such as shot blasting. Moreover, it is also possible to perform the rust preventive film process which forms a rust preventive film in connection with removal of a scale. Depending on the state of the raw tube, the pickling / neutralization in step S110 or the removal of scale by other methods can be omitted.

  In step S120, diameter reduction processing (warm diameter reduction processing) is performed in a state where the temperature of the raw tube is maintained in the recrystallization temperature range (600 to 650 ° C.). Here, the diameter reduction processing is so-called processing for reducing the inner diameter and the outer diameter (hereinafter collectively referred to as “tube diameter”) of the pipe material. In the present embodiment, the warm diameter reduction process is performed by heating the raw tube and subjecting the heated raw tube to pilger rolling. By performing the warm diameter reduction processing, a pipe material (small diameter pipe) whose diameter is reduced from the raw pipe is obtained. The specific contents of the warm diameter reduction processing will be described later.

  In step S130, the bending of the small diameter pipe caused by the warm diameter reduction processing in step S120 is corrected. In addition, when the length of the small-diameter pipe is sufficiently short and bending is not caused by warm shrinking, or when the bending of the small-diameter pipe obtained by warm shrinking is within an allowable range, It is also possible to omit the bending correction in step S130.

  In step S140, in order to adjust the dimensions according to the shape of the product pipe material, cold rolling (finish cold rolling) is applied to the small diameter pipe for finishing, and in step S150, the bending caused by the finish cold rolling is corrected. To do. The finish cold rolling can be performed by, for example, a three-roll die type rolling device. In addition, when dimension adjustment is unnecessary, steps S140 and S150 can be omitted. After the finish cold rolling, in step S160, the small-diameter pipe is cut according to the length of the product pipe. Thereby, a high-strength pipe material processed into a desired shape is obtained as a product pipe material.

A2. Warm diameter reduction:
FIG. 2 is an explanatory view showing a specific example of warm diameter reduction processing. In the warm diameter reduction process, first, as shown in FIG. In the example of FIG. 2, the element tube 10 is heated by induction heating using the high-frequency heating device 100. Specifically, after inserting the raw tube 10 into the coil 110, the high-frequency power is supplied from the high-frequency power source 120 to the coil 110, whereby the raw tube 10 is heated. The heating temperature is set so that the temperature of the tube material is maintained in the recrystallization temperature range at the time when the warm diameter reduction processing is completed. However, the heating temperature is preferably 710 ° C. or lower in order to suppress decarburization and coarsening of the ferrite structure (crystal grains).

  Next, as shown in FIG. 2B, pilger rolling is performed on the heated raw tube 10. FIG. 2B shows a pilger rolling apparatus 200, the raw tube 10, the small-diameter tube 20, and a tube material (intermediate tube material) 90 in the process of reducing the diameter, including the axis c and a surface perpendicular to the roll shaft 214 (described later). It shows how it was cut at.

  As shown in FIG. 2B, the pilger rolling device 200 includes a pair of roll dies 210, a mandrel 220, and a rod 230. The roll die 210 has a roll body 212 and a roll shaft 214, and the roll body 212 is rotatable around the roll shaft 214. The roll body 212 is provided with a substantially semicircular groove (caliber) 219 provided from the outer peripheral side toward the roll shaft 214. The caliber 219 is formed so that its inner diameter gradually changes in the circumferential direction of the roll body 212.

  The mandrel 220 is a truncated cone-shaped member formed so that the outer diameter decreases from the raw tube 10 side (upstream side) toward the small diameter tube 20 side (downstream side). The mandrel 220 is attached to the rod 230 on the upstream side thereof, and the mandrel 220 is also rotated by rotating the rod 230 about the axis c.

  As shown in FIG. 2B, the roll main body 212 is disposed on the outer peripheral side of the intermediate tube member 90, and the mandrel 220 is disposed on the inner peripheral side of the intermediate tube member 90. Then, with the inner surface of the caliber 219 pressed against the outer peripheral surface of the intermediate tube member 90, the pair of roll dies 210 are reciprocated simultaneously in both directions, upstream and downstream. Thereby, the intermediate tube material 90 is deformed until its inner surface comes into contact with the mandrel 220. In this way, the roll die 210 is reciprocated and the mandrel 220 is rotated to rotate the base tube 10, the intermediate tube 90, and the small diameter tube 20 (workpiece). By repeating the reciprocating movement of the roll die 210 and the rotation of the workpiece, the small-diameter pipe 20 is obtained.

  At the position (contact position) p where the inner surface of the caliber 219 and the outer peripheral surface of the intermediate tube 90 are in contact with each other during pilger rolling, the outer periphery of the intermediate tube 90 is indicated by the white arrow in FIG. Strong compressive stress is applied to both the side and the inner peripheral side. By applying a strong compressive stress to the intermediate tube member 90 in this way, the crystal grains of the intermediate tube member 90 are refined, and the crystal particles of the small-diameter tube 20 that has been subjected to the diameter reduction process are reduced. In the present embodiment, since the Pilger rolling is performed in a warm state, that is, in a state where the temperature of the intermediate tube material 90 is maintained in the recrystallization temperature range, the crystal grains become equiaxed crystals, and the reduced diameter processed small diameter tube. The strength of 20 can be made isotropic.

Further, such refinement of crystal grains is promoted by increasing the area reduction rate in pilger rolling. Therefore, in this embodiment, the average grain size (crystal grain size) of the ferrite structure in the small-diameter tube 20 after the pilger rolling is sufficiently small (1.5 μm or less) by setting the area reduction rate in the pilger rolling to 85% or more. I have to. Note that the area reduction rate is a so-called reduction rate of the cross-sectional area with respect to the cross-sectional area of the tube material before processing, and uses the cross-sectional area S0 of the tube material before processing and the cross-sectional area S1 of the tube material after processing. Given by.
Area reduction ratio = (S0−S1) / S0

  In the present embodiment, by sufficiently reducing the crystal grain size as described above, the strength (tensile strength and proof stress) and hardness can be sufficiently increased without performing heat treatment or the like. Moreover, by making the crystal grain size sufficiently small, the toughness can be increased, and the brittleness at an extremely low temperature (−100 ° C.) can be improved. Furthermore, since the crystal grains become equiaxed crystals and the strength becomes isotropic, when the manufactured product tube is further processed to form the part as the final product, the difference in strength and the like depending on the direction is taken into consideration. Since it is not necessary, it is possible to more easily form a part as a final product.

  In the present embodiment, as shown in FIG. 1, after the warm diameter reduction processing (step S120) is performed, the finish cold rolling (step S140) and bending are performed until the production of the product tube material is completed. Is corrected (steps S130 and S150). However, even if these processes are performed, the crystal grain shape is maintained, but the crystal grain shape is maintained in an equiaxed crystal state. Therefore, also in the tube material as a product, that is, the product tube material, the ferrite structure is maintained in an equiaxed crystal having a crystal grain size of 1.5 μm or less.

  Furthermore, when processing the product tube material to create a part that will be the final product, cold processing such as drawing, flattening, and serration processing, joining processing such as welding, cutting processing, etc. (hereinafter collectively referred to as “ Also called “post-processing”. However, even if these processes are performed, the crystal grains are maintained in a refined state, so the mechanical properties such as strength and toughness achieved by the warm diameter reduction process are maintained in the final product parts. Is done.

Moreover, in this embodiment, since the pilger rolling is performed warmly, the deformation resistance of the intermediate tube 90 during pilger rolling is small. Therefore, even if the area reduction rate in pilger rolling is increased, the occurrence of cracks and the like is suppressed. Therefore, as shown in FIG. 1, by performing a single hot shrinking process (Pilger rolling), the raw pipe is processed so as to obtain a desired area reduction ratio, and a high-strength product pipe is manufactured. Can do. In addition, it is good also as what lowers the area reduction rate in one warm diameter reduction process, performs a warm diameter reduction process 2 times or more, and processes a raw tube so that it may become a desired area reduction rate. However, since the process can be shortened and the amount of CO ₂ emission can be reduced, it is preferable to form the small-diameter pipe from the raw pipe by a smaller number of warm diameter reduction processes.

  Furthermore, in this embodiment, since the pilger rolling is performed warmly, the work hardening in the pilger rolling is very small as compared with the cold processing. Therefore, when the finish cold rolling (step S140 in FIG. 1) and the subsequent bending correction (step S150) are omitted, the elongation and ductility (drawing) of the product tube material is increased, and post-processing becomes easier.

  In addition, as described above, even when finish cold rolling or bending correction is performed, since the crystal grains are maintained in an equiaxed crystal state, the elongation, drawing and low temperature brittleness of the product pipe are Maintained well. On the other hand, by performing the finish cold rolling, work hardening occurs, and the strength and hardness of the product pipe are increased. Therefore, by performing finish cold rolling, it becomes possible to further increase the strength and hardness of the product tube while maintaining the ease of post-processing. In addition, since finish cold rolling is also a cold diameter reduction process, it can also be called a cold diameter reduction process.

  In the example of FIG. 2, the element tube 10 is heated by the high-frequency heating device 100, but the element tube 10 can also be heated by using another type of heating device such as a tunnel-type coil heating device. is there. The heating device used for heating the blank tube 10 is preferably arranged near the pilger rolling device 200 in order to suppress a temperature drop of the blank tube 10 after heating.

  Moreover, in order to suppress the temperature drop of the intermediate tube material 90, the temperature of the mold (the roll body 212 or the mandrel 220) in contact with the intermediate tube material 90 in the pilger rolling apparatus 200 may be appropriately adjusted. Furthermore, in order to make the surface skin of the small-diameter pipe 20 and the product pipe material in a good state and to protect the roll body 212, it is more preferable to supply lubricating oil in the vicinity of the contact position p. In this case, in order to suppress the temperature drop of the intermediate tube material 90, the temperature of the lubricating oil may be appropriately adjusted.

A3. Steel composition:
As described above, in order to form parts of the final product, the pipe material as a product is subjected to post-processing such as cold processing, joining processing, and cutting processing. Therefore, the composition of the steel material used for the pipe material is determined so that the work deformability, weldability, and machinability are sufficiently high. In determining the composition, it is also considered that the refinement of the ferrite structure by pilger rolling is promoted depending on the type of the contained element. Specifically, the steel material contains the contained elements as shown below, and the balance is iron (Fe) and inevitable impurities. In addition, below, content of each containing element is represented by weight%.

  Carbon (C) is an element that increases the tensile strength, but work hardening tends to occur when the content increases. However, in this embodiment, since the diameter reduction process is performed warmly, the C content can be made relatively high. Therefore, the content of C is set to 0.05 to 0.25%. However, since the weldability decreases as the C content increases, the upper limit of the C content is preferably 0.20%. Moreover, since the ductility fall by work hardening at the time of post-processing will become remarkable when C content becomes high, it is more preferable that the upper limit of C content shall be 0.15%.

  Manganese (Mn) is an element that increases the tensile strength like C, and complements the effect of C. Therefore, to make the tensile strength sufficiently high, the Mn content is set to 0.3 to 1.5%. In the present embodiment, since the C content is set low, the lower limit of the Mn content is preferably 0.40%. Moreover, since work hardening will be accelerated | stimulated when content of Mn becomes high, it is preferable that the upper limit of content of Mn shall be 1.0%.

  Silicon (Si) is an element that promotes ferrite transformation, and the work deformability can be secured by addition. Therefore, the lower limit for the Si content is 0.15%. On the other hand, when the Si content increases, ductility decreases. Therefore, the upper limit of the Si content is 0.35%.

  Phosphorus (P) and sulfur (S) are effective elements for improving machinability. However, since P and S may reduce toughness and ductility and cause processing limit cracking or the like during post-processing, the P content is 0.030% or less, and the S content is 0.2%. It is preferable to set it to 035% or less.

  Titanium (Ti) and niobium (Nb) are elements in which nitride suppresses the growth of crystals in the ferrite structure, and by adding at least one of Ti and Nb, the refinement of the ferrite structure is promoted and ferrite is added. Variation in the grain size of the structure can be reduced (homogenized). When either one of Ti and Nb is added, it is preferable to add Ti from the viewpoint of availability and reduction in the price of the steel material. In order to express the effect of refining and homogenizing the ferrite structure, when Ti is added, the content is 0.01 to 0.08%, and when Nb is added, the content is 0. 0.005 to 0.05%.

  Vanadium (V) is an element that improves yield stress (yield strength). V also suppresses the growth of crystals in which the carbide is a ferrite structure. Therefore, by adding V, the ferrite structure can be refined and the ferrite structure can be homogenized. When adding V, in order to express these effects, the V content is set to 0.040 to 0.10%.

  Nb, Ti, and V are not necessarily added. However, Nb, Ti, and V promote the refinement of crystal grains as described above. Therefore, when none of Nb, Ti and V is added, the crystal grain size is slightly (0.2 to 0) compared to the case where any of these is added and warm shrinkage processing is performed under the same conditions. About 5 μm). Further, as the crystal grain size is slightly increased, the strength (tensile strength and proof stress) is 20 to 50 MPa, the elongation is 2 to 3%, and the hardness (Vickers hardness HV) is 8 to 12 HV. Therefore, it is preferable to add at least one of Nb, Ti, and V from the viewpoint that the strength, elongation, and hardness can be further increased. On the other hand, depending on the strength and the like of the target product tube material, it is preferable to omit these additions in that the steel material price can be reduced.

  When copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) are added, the price of the steel material increases, and the recyclability of the steel material decreases. Therefore, it is preferable not to add Cu, Ni, Cr and Mo. Therefore, it is preferable that Cu, Ni, Cr, and Mo are not contained more than the inevitable content, and the upper limit of the content is 0.30% or less.

A4. Conventional pipe manufacturing process:
FIG. 3 is a flowchart showing an example of a pipe material manufacturing process (conventional process) for manufacturing a high-strength pipe material using a conventional technique. In FIG. 3, as in FIG. 1, the CO ₂ emission basic unit at the step is shown on the right side of each step, and the total emission amount of CO ₂ emitted in the conventional process is shown at the lower stage of the terminal.

  As shown in FIG. 3, in the conventional process, the diameter reduction processing is mainly performed cold (steps S222 and S224), the quenching / tempering is performed (step S232), and the quenching / tempering is performed. The point of pickling / neutralization accompanying the implementation (step S234), the point of baking accompanying the pickling / neutralization (step S262), and the finish cold rolling (step S140) are omitted. In this respect, it differs from the manufacturing process of the pipe material of this embodiment shown in FIG. Other points are the same as the manufacturing process of the pipe material of this embodiment.

  As shown in FIG. 3, in the conventional process, a small-diameter pipe is obtained by performing cold drawing (also referred to as “drawing”) on the base pipe (steps S222 and S224). In this cold drawing process, since the dimensional accuracy can be increased, the finish cold rolling (step S140) can be omitted. However, small-diameter pipes obtained by cold drawing are low in strength (tensile strength of 600 MPa or less) and hardened in small-diameter pipes to obtain high strength (tensile strength of 1000 MPa or more). -It is necessary to perform a tempering process (step S232). Moreover, in order to suppress the hydrogen embrittlement which arises by pickling and neutralization (step S234) which removes the scale which arose by quenching and tempering by performing quenching and tempering, and also by pickling and neutralization It is necessary to perform baking (step S262).

Thus, in the conventional process, since the diameter reduction process is performed by cold drawing, the CO ₂ emission basic unit in the diameter reduction process itself is 87.8 kg / t, which is 340 kg / t of this embodiment (see FIG. 1). Less than. However, in the conventional process, as described above, the quenching / tempering process (step S232) and the baking (step S262) are required, so the total CO ₂ emission amount is 1331.1 kg / t, It becomes more than 477.6 kg / t of this embodiment. As described above, according to the present embodiment in which the diameter reducing process is performed warmly, the total emission amount of CO ₂ can be reduced as compared with the conventional process in the manufacturing process of the high-strength pipe material.

  Furthermore, in the cold drawing process performed in the conventional process, the crystal grain size becomes as large as 10 to 20 μm. In addition, in order to reduce the crystal grain size, even if induction hardening is performed instead of the quenching / tempering process (step S232), the crystal grain size is about 6 to 12 μm. On the other hand, according to the present embodiment, by setting the crystal grain size to 1.5 μm or less, the strength and toughness of the small-diameter pipe can be sufficiently increased without performing heat treatment.

  In addition, the cold drawing process has a low process limit, and if the area reduction rate in one process is increased, there is a possibility that breakage or cracking may occur. Therefore, in general, when the diameter reduction processing is performed by cold drawing, the cold drawing is performed a plurality of times (twice in the example of FIG. 3) as shown in FIG. Therefore, when a pipe material is manufactured in a conventional process, the manufacturing process becomes longer than in this embodiment.

  Further, when the cold drawing process is performed a plurality of times, work hardening occurs by repeatedly performing the cold drawing process, and the hardness and tensile strength are increased, but the elongation and ductility (drawing) are reduced. This work hardening becomes more prominent as the overall area reduction by multiple drawing processes becomes larger, and the drawing process itself becomes difficult as work hardening progresses. In particular, in the cold drawing process, if the hardness exceeds 290 HV, there is a high possibility that breakage or cracking will occur. Therefore, it is extremely difficult to manufacture a tube material having a hardness of 290 HV or higher. On the other hand, according to the present embodiment, by setting the crystal grain size to 1.5 μm or less, the hardness can be sufficiently increased without performing heat treatment.

B. Example:
B1. Summary of evaluation:
In order to confirm the effect of the present invention, as examples and comparative examples, a raw pipe made of five types of steel materials (steel materials A to D, X) having different compositions was prepared, and diameter reduction processing was performed on the prepared raw pipe. A small diameter tube as a sample was prepared. In addition, the composition of the five types of steel materials used for the prepared blanks is as shown in Table 1 below.

  As shown in Table 1, among the steel materials (steel materials A to D, X) used for the preparation of the samples, the steel materials (steel materials A to D) used for the samples as examples are SAE 1005, SAE 1010, SAE 1015 and Nb, Ti and V were added based on SAE1025. As one of comparative examples, as will be described later, SCM435 having a high C content (0.35%) is included in a sample prepared in the conventional process of quenching and tempering after cold shrinking. used.

Among the pipes made of these five types of steel materials (steel materials A to D, X), the pipe made of steel materials A to D was subjected to warm reduction by pilger rolling, and four samples (Example 1). To 4). In Examples 1 and 2, the area reduction rate was 90.9%, and in Examples 3 and 4, the area reduction rate was 85.2%. Further, as a comparative example, a sample (Comparative Example 1) obtained by warm-reducing the raw pipe made of the steel material C used in Example 3 at a surface reduction rate of 70.2% and the raw pipe made of the steel material C were reduced. A sample (Comparative Example 2) that was cold-reduced at an area ratio of 20.2% and an element pipe made of steel X (SCM435) were cold-reduced at an area reduction rate of 20.2%, and then quenched and quenched. A tempered sample (Comparative Example 3) was prepared. For each of these samples, the steel material used, the tube diameter of the raw tube and the small-diameter pipe after the diameter reduction processing, the area reduction ratio in the diameter reduction processing, the temperature conditions during the diameter reduction processing, and the heat treatment (quenching / tempering) The presence or absence of) is as shown in Table 2 below. As can be seen from Table 2, the diameter of the small diameter pipe after the diameter reduction processing was the same in all the samples. Therefore, the area reduction ratio of the diameter reduction processing was adjusted by preparing a raw pipe having a pipe diameter appropriately set.

  After preparing the sample in this manner, the obtained sample was evaluated for crystal grain size, mechanical properties, and low temperature brittleness. The crystal grain size was measured by cutting a small-diameter tube as a sample in the axial direction (longitudinal direction) and the radial direction (transverse direction), and observing the cut surfaces (longitudinal section and transverse section) that appeared by cutting with a microscope. The mechanical properties were evaluated by a tensile test, and the low temperature brittleness was evaluated by a Charpy impact test.

B2. Evaluation results:
FIG. 4 is a photograph of the crystal structure of a sample (small diameter tube) showing the evaluation results of the crystal grain size. FIG. 4A shows a crystal structure of a cross section of a sample subjected to cold shrinking as a comparative example (Comparative Example 2), and FIG. 4B shows warm shrinking as an example. 2 shows the crystal structure of the cross section of the sample (Example 3).

  As shown in FIG. 4A, in the sample subjected to the cold shrinking process (Comparative Example 2), the refinement of crystal grains was not advanced, and the crystal grain size was about 20 μm. On the other hand, in the sample (Example 3) subjected to the warm diameter reduction, as shown in FIG. 4 (b), the refinement of crystal grains progressed, and the crystal grain size was 1.5 μm or less. Moreover, although the crystal structure photograph is not shown, in Example 3, the shape of the crystal grains is isotropic even in the longitudinal section, and it was confirmed that the crystal grains were equiaxed crystals. Furthermore, as shown in FIG. 4B, it was confirmed that pearlite was present between the crystal grains (ferrite structure). From this result, it was found that by performing warm shrinkage processing and refining the crystal grains, it is possible to improve the severability of chips at the time of cutting and to improve the machinability.

  FIG. 5 is a graph showing the results of a tensile test as a mechanical property evaluation. The horizontal axis in FIG. 5 represents strain (elongation), and the vertical axis represents stress. As shown in FIG. 5, the sample (Comparative Example 2) that was only subjected to cold shrinking and not quenched and tempered had an elongation of 35%, but the tensile strength remained at 550 MPa. It was. Further, in the sample (Comparative Example 3) in which the steel material X (SCM435) having a high C content is used and quenched and tempered after the cold diameter reduction processing, the tensile strength reaches 1100 MPa and the hardness is 340 HV. Beyond. Moreover, although elongation became 10% and exceeded 9% which is a standard of work deformability, fracture occurred when the strain exceeded 12%. For this reason, in a small diameter pipe manufactured by cold shrinking, if the C content is increased and quenching / tempering is performed in order to increase the strength, it is difficult to increase the work deformability. I was able to confirm. On the other hand, in the sample (Example 3) subjected to the warm diameter reduction, the tensile strength is 1100 MPa, and it can be confirmed that the strength can be sufficiently increased by performing the warm diameter reduction processing. It was. In Example 3, the elongation reached 11%, and no fracture occurred until the strain exceeded 20%. From this, it was found that by performing warm shrinkage processing and refining the crystal grains, it is possible to sufficiently secure the work deformability while sufficiently increasing the strength.

  FIG. 6 is a graph showing the results of a Charpy impact test as a low-temperature brittleness evaluation. The horizontal axis in FIG. 6 represents the temperature at which the Charpy impact test was performed (Charpy test temperature), and the vertical axis represents the brittle fracture surface ratio. As shown in FIG. 6, in the sample subjected to cold shrinking (Comparative Example 2), the brittle fracture surface ratio starts to increase when the temperature is 0 ° C. or lower, and the brittle fracture surface ratio is 95% at −100 ° C. became. From this, it was found that it is difficult to improve brittleness at a very low temperature (−100 ° C.) when cold diameter reduction is performed. On the other hand, in the sample (Example 3) subjected to warm diameter reduction, the brittle fracture surface ratio increased only at −150 ° C. or lower, and no brittle fracture occurred at −150 ° C. or higher. From this result, it was found that the brittleness at an extremely low temperature can be improved by carrying out warm shrinkage processing and refining the crystal grains.

4 to 6, the evaluation results of the crystal grain size, mechanical properties (tensile strength / elongation), and low temperature brittleness of the samples prepared as Example 3 and Comparative Example 2 have been described. FIG. 5 shows the evaluation results of tensile strength and elongation for the sample prepared as Comparative Example 3. Table 3 shows the evaluation results of mechanical properties and the evaluation results of crystal grain size for these Examples and Comparative Examples (Example 3, Comparative Examples 2 and 3) and other Examples and Comparative Examples. Shown in

  As shown in Table 3, the crystal grain size could be reduced to 1.5 μm or less by setting the area reduction ratio of warm reduction processing to 85% or more. In addition, by setting the crystal grain size to 1.5 μm or less, it is possible to achieve the same strength category 10.9 (JIS standard B1051, tensile strength: 1040 MPa or more, proof stress: 940 MPa or more) equivalent to a quenched / tempered tube material. I understood. Moreover, all became 10% or more, and exceeded 9% used as the standard of the quality of work deformability. Furthermore, it was found that the Vickers hardness HV was 330 HV or higher, and the hardness specified in the strength category 10.9 (320 HV or higher) was obtained. Although not shown in Table 3, all of Examples 1 to 4 were not brittle at -100 ° C.

  On the other hand, even in the case of using a steel material (steel material C) to which Nb, Ti, and V are added, the sample (Comparative Example 1) subjected to the warm diameter reduction with a surface reduction rate of 70.2% has a crystal grain size of 2 It was ˜3 μm. Thus, when the crystal grain size was 2 to 3 μm, the tensile strength was 900 MPa and the proof stress was 840 MPa, and sufficient strength (strength category 10.9) could not be obtained. From these facts, even when using a steel material to which Nb, Ti and V are added, it can be determined that the crystal grain size needs to be 1.5 μm or less in order to obtain sufficient strength.

C. Variation:
The present invention is not limited to the above-described embodiments and examples, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

C1. Modification 1:
In the above-described embodiment, warm diameter reduction processing is performed by pilger rolling. However, the warm diameter reduction processing is generally performed by a rolling method (strong rolling) in which a strong compressive stress is applied to the intermediate pipe material being processed. . Even in this case, the intermediate pipe material being processed is subjected to strong compressive stress on both the outer peripheral side and the inner peripheral side, and the refinement of the ferrite structure is promoted. Therefore, even in a small-diameter pipe that has been subjected to strong rolling, it is possible to obtain a sufficiently small-diameter pipe with a crystal grain size of 1.5 μm or less. In addition to the pilger rolling, three-roll die rolling or the like can be employed as a method of strong rolling. However, it is preferable to employ pilger rolling as the rolling reduction method because it is easier to manufacture small diameter pipes having different diameters and mass production of small diameter pipes is easier.

C2. Modification 2:
In the above embodiment, warm diameter reduction is performed by pilger rolling, which is strong rolling, but the warm diameter reduction can also be performed by a method other than strong rolling. For example, it is possible to produce a small-diameter pipe by drawing the base pipe warmly. In this case as well, if the area reduction ratio is 85% or more, the ferrite structure can be made into an equiaxed crystal having a crystal grain size of 1.5 μm or less, so that a sufficiently large small diameter tube can be obtained. it can. However, in the drawing process, it is difficult to increase the area reduction rate in one process even if it is warm. Therefore, in order to make the area reduction rate 85% or more by drawing, usually, drawing is performed a plurality of times, and the manufacturing process of the small-diameter pipe becomes long. On the other hand, in strong rolling, it is easy to increase the area reduction rate in one processing, and the manufacturing process of the small diameter pipe can be shortened. In this regard, it is preferable that the warm diameter reduction processing is performed by rolling under strong rolling.

DESCRIPTION OF SYMBOLS 10 ... Elementary tube 20 ... Small diameter pipe 90 ... Intermediate pipe material 100 ... High frequency heating device 110 ... Coil 120 ... High frequency power supply 200 ... Pilger rolling device 210 ... Roll die 212 ... Roll main body 214 ... Roll shaft 219 ... Caliber 220 ... Mandrel 230 ... Rod

Claims (6)

0.05-0.25 wt% C, 0.15-0.35 wt% Si, 0.3-1.5 wt% Mn, 0.035 wt% or less S, 0.030 wt% It consists of a steel material containing the following P, the balance being Fe and inevitable impurities,
The ferrite structure is an equiaxed crystal having an average crystal grain size of 1.5 μm or less,
Tube material. The pipe according to claim 1,
The steel material further includes at least one of 0.005 to 0.05% by weight of Nb and 0.01 to 0.08% by weight of Ti,
Tube material. The pipe according to claim 1 or 2,
The steel material further includes 0.040 to 0.10 wt% of V,
Tube material. A method of manufacturing a pipe material according to any one of claims 1 to 3,
Inner and outer diameters are larger than the previous SL tubing, and the mother tube wall thickness than the tube material, reduction of area is subjected to a diameter reduction warm above 85%, the production method of the tubing. The tube material manufacturing method according to claim 4, wherein the warm diameter reduction processing is performed by rolling under strong rolling. It is a manufacturing method of the pipe material according to claim 4 or 5,
A method for manufacturing a pipe material, wherein the pipe material subjected to the warm diameter reduction process is further subjected to a cold diameter reduction process.