JP2007119865A - Steel pipe for machine structural member and manufacturing method thereof - Google Patents

Abstract

A steel pipe for a mechanical structural member suitable for mechanical parts such as gears and cylinders and a structural member such as a shaft, and a manufacturing method thereof that can be manufactured at low cost without requiring expensive alloy or tempering heat treatment.
SOLUTION: Steel pipe, in mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005-0.03%, Al: 0.08% The remainder is composed of Fe and inevitable elements, the thickness is 5 to 22 mm, the length is 5 times or more of the outer diameter, the metal structure is a mixed structure of ferrite and pearlite, hardness The average value of Vickers hardness is 185 to 260, and the difference between the maximum value and the minimum value of hardness is 20 or less in terms of Vickers hardness. Moreover, the manufacturing method is accelerated cooling from the outer surface side while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface of the steel pipe of the component system at a temperature of 700 ° C. or higher, and 550 Accelerated cooling is stopped in a temperature range of ˜700 ° C.
[Selection] Figure 1

Description

  The present invention relates to a steel pipe suitable for machine structural members, particularly machine parts such as gears and cylinders, and hollow structural members such as shafts, and a method for manufacturing the same.

  Machine parts used in automobiles and industrial machines are often used after being processed into a predetermined shape by forging or cutting using steel bar as a raw material and then given predetermined mechanical properties by tempering heat treatment. On the other hand, due to the demand for reducing the cost of parts, the use of steel pipes already provided with the mechanical properties required for hollow-shaped parts as materials increases the number of cases for shortening the forging process and omitting the heat treatment process. It is coming. However, since the steel pipe material is generally more expensive than the steel bar material, the cost reduction effect due to the steel pipe may not be obtained even if it is a hollow shaped part.

  Therefore, in order to further reduce the cost of manufacturing steel pipes, a number of so-called non-tempered mechanical parts / structural steel pipes that omit the tempering heat treatment after hot pipe making as described in Patent Documents 1 to 7 are disclosed. Or has been proposed. Patent Documents 1 to 6 all seek to obtain a predetermined strength by improving the hardenability and precipitation strengthening ability by adding a large amount of alloy elements. For this reason, not only an increase in alloy costs is unavoidable, but also there may be difficulties in the steelmaking process. Patent Document 7 intends to refine the metal structure and improve the strength by rolling at a considerably low temperature as a hot rolling temperature of 600 ° C. to 750 ° C. However, although low temperature rolling is now a common technique in thick plate rolling, there are problems such as the occurrence of wrinkles and seizures due to contact with tools when rolling steel pipes. It is greatly restricted.

Patent Documents 8 to 10 disclose a technique for improving strength by performing accelerated cooling immediately after a hot tube. Patent Document 8 obtains high strength by combining low-temperature rolling with a reduction rate of 30% or more and accelerated cooling of 1 to 35 ° C./second in an unrecrystallized region, and the intended application is a cargo pipe of a crude oil tanker. is there. Therefore, the carbon content is as low as 0.03 to 0.07%. Further, Patent Document 9 was allowed to cool inner surface of the steel pipe after final finishing rolling, cooling after cooling the outer surface to from 500 to 400 ° C. at 60 ° C. / sec from 10 ° C. / sec from Ar ₃ point or more temperature To do. The intended use is oil well pipes, and the carbon content is defined as 0.1 to 0.3. Patent Document 10 is also an oil well pipe having a carbon content of 0.15 to 0.4%, and is directly quenched or acceleratedly cooled while being hot-rolled, and then tempered.

  However, in the case of steel pipes for machine structures, the surface is often subjected to induction hardening after parts processing to impart fatigue properties and wear resistance, but the surface hardness by induction hardening is determined by the amount of carbon. In general, a carbon content exceeding 0.3% is required. If such a high-carbon steel pipe is accelerated and cooled without any consideration, not only the surface of the steel pipe is locally hardened, but the subsequent cutting and machining become difficult, and in some cases, cracking occurs. May occur.

  However, since such a problem does not arise in the low carbon amount and application as in Patent Documents 8 and 9, in the preceding examples, consideration is not given to the accelerated cooling in terms of process or material. Moreover, although patent document 10 prescribes | regulates the upper limit of carbon amount as 0.4%, only the maximum carbon amount of 0.3% is described in the Example, and it is substantially to 0.3% C. In addition, in this patent document, tempering is essential after accelerated cooling.

  Based on the above background, Patent Document 11 provides a method of manufacturing a steel pipe for machine structural members at a low cost without adding an expensive alloy and without performing a tempering heat treatment. In order to achieve the above object, when a steel pipe containing C in an amount exceeding 0.3% is accelerated and cooled, if a decarburized layer is formed on the cooling surface in advance, surface hardening can be suppressed. In order to ensure workability, ferrite and pearlite are desirable as the structure, and it is necessary to accelerate cooling under limited conditions that can achieve the required strength. To uniformly cool, the steel pipe is rotated from the outer surface. It is said that cooling is effective.

  However, when the steel pipe described in Patent Document 11 is machined, variations in hardness at arbitrary positions in the circumferential direction and the longitudinal direction are recognized, and a problem arises in obtaining stable machining accuracy. As a result of diligent investigation of the cause, it became clear that it was caused by non-uniform ferrite formation.

JP 05-202447 A Japanese Patent Laid-Open No. 10-130783 Japanese Patent Laid-Open No. 10-204571 JP-A-10-324946 Japanese Patent Laid-Open No. 11-036017 JP 2004-292857 A JP 2001-247931 A Japanese Patent No. 3252651 Japanese Patent No. 3503211 Japanese Unexamined Patent Publication No. 07-041856 Japanese Patent Application No. 2005-092045

  The present invention provides a steel pipe for a machine structural member suitable for structural parts such as gears, cylinders, and other mechanical parts that solve the above-mentioned problems, and an expensive alloy added to the steel pipe. Therefore, the present invention provides a manufacturing method that can be manufactured at low cost without performing tempering heat treatment.

  In order to achieve the above object, the present inventors have studied various relations between the accelerated cooling conditions and the metal structure and hardness with respect to the steel pipe shape at that time. As a result, if the whole or part of the metal structure becomes a bainite structure, subsequent cutting and machining become difficult, so a ferrite + pearlite structure is obtained, and the hardness and the amount of ferrite produced are controlled, and these structures We found limited accelerated cooling conditions to obtain

  Moreover, when accelerated cooling of a steel pipe containing C in an amount exceeding 0.3%, which can ensure induction hardenability while suppressing the addition of alloy elements, a decarburized layer is formed on the cooling surface in advance. It was found that the local significant hardening of the surface can be suppressed.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005 to 0.03%, Al: 0.08% or less, with the balance having chemical components composed of Fe and inevitable elements, thickness 5 mm to 22 mm, length 5 times or more the outer diameter Accelerated cooling while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface side of the steel pipe at a temperature of 700 ° C. or higher, and stops accelerated cooling in the temperature range of 550 to 700 ° C. The manufacturing method of the steel pipe for machine structural members characterized by the above-mentioned.
(2) By mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005 to 0.03%, Al: 0.08% or less, with the balance having chemical components composed of Fe and inevitable elements, with a thickness of 5 mm or more and 22 mm or less in the hot drawing step, length Is accelerated and cooled while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface side of a steel tube at a temperature of 700 ° C. or more that is piped 5 times or more of the outer diameter, A method for producing a steel pipe for a machine structural member, characterized in that accelerated cooling is stopped in a temperature range of ° C.
(3) By mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005 to 0.03%, Al: 0.08% or less is contained, and the balance is obtained by hot piercing, rolling and stretching processes using a cylindrical bloom having a chemical component composed of Fe and inevitable elements. Rotate in the circumferential direction at a cooling rate of 0.5 to 10 ° C / sec from the outer surface side of a steel tube at a temperature of 700 ° C or higher, where the wall thickness is 5 mm or more and 22 mm or less and the length is 5 times the outer diameter. A method for producing a steel pipe for machine structural members, characterized in that accelerated cooling is performed while stopping and the accelerated cooling is stopped in a temperature range of 550 to 700 ° C.
(4) By mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005 to 0.03% Al: 0.08% or less, with the balance being a cylindrical bloom having a chemical component composed of Fe and inevitable elements at a temperature and time satisfying the following formula (1). Thickness from the outer surface of the steel pipe at a temperature of 700 ° C. or higher, which is formed by hot drilling, rolling and stretching processes, and having a wall thickness of 5 mm or more and 22 mm or less and a length of 5 times or more of the outer diameter. A decarburized layer of 100 to 500 μm is formed in the direction, accelerated cooling from the outer surface side while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second, and accelerated cooling in a temperature range of 550 to 700 ° C. A method of manufacturing a steel pipe for a machine structural member, characterized by stopping.

2.7335 × 10 ⁶² T ^-19.509 >t> 2.4726 × 10 ⁵⁷ T ^-18.121 (1)
Where T: temperature (° C.) t: time (minutes)
(5) The mechanical structure according to any one of (1) to (4) above, wherein the steel pipe further contains Cr: 0.05 to 0.25% by mass%. Manufacturing method of steel pipe for members.
(6) The mechanical structure according to any one of (1) to (5) above, wherein the steel pipe further contains Nb: 0.001 to 0.1% by mass%. Manufacturing method of steel pipe for members.
(7) The end temperature of the rolling is 850 ° C. or higher, and the temperature to be inserted into the reheating furnace before the stretching step is 750 ° C. or higher, any one of (3) to (6) above The manufacturing method of the steel pipe for machine structural members of 1 item | term.
(8) Manufacture of steel pipe for machine structural members according to any one of (1) to (7) above, further subjected to stress relief annealing at 500 to 600 ° C. for 10 to 60 minutes after cooling. Method.
(9) By mass%, C: 0.3-0.6%, Si: 0.05-0.4%, Mn: 0.5-1.0%, P: 0.03% or less, S: 0.005 to 0.03%, Al: 0.08% or less, with the balance having chemical components composed of Fe and inevitable elements, thickness 5 mm to 22 mm, length 5 times or more the outer diameter The steel pipe for machine structural members of which the metal structure is a mixed structure of ferrite and pearlite, the average hardness is 185 to 260 in terms of Vickers hardness, and the difference between the maximum and minimum hardness values Is a steel pipe for machine structural members, characterized in that the Vickers hardness is 20 or less.
(10) The steel pipe for machine structural members according to (9), further comprising Cr: 0.05 to 0.25% by mass.
(11) The steel pipe for machine structural members according to (9) or (10) above, further containing, by mass%, Nb: 0.001 to 0.1%.
(12) The steel pipe for machine structural members according to any one of (9) to (11) above, wherein an area ratio of the ferrite is 15 to 35%.

  In the present invention, in a steel pipe having a wall thickness of 5 mm or more and 22 mm or less and a length of 5 times or more of the outer diameter, it is accelerated and cooled from a temperature range of 700 ° C. or more at a cooling rate of 0.5 ° C./second to 10 ° C./second, By stopping accelerated cooling at 550 ° C. to 700 ° C., a ferrite with a pearlite structure and uniform ferrite is generated, so that a steel pipe with small hardness variation can be obtained.

  Moreover, in this invention, it has a layer which is easy to process on the surface by having a 100-500 micrometers decarburization layer from the outer surface to the thickness direction. Further, this decarburized layer can be obtained by maintaining soaking at a temperature and time satisfying at least the following formula.

2.7335 × 10 ⁶² T ^-19.509 >t> 2.4726 × 10 ⁵⁷ T ^-18.121
Where T: temperature (° C.) t: time (minutes)

  The reason why the chemical composition of the steel pipe is limited in the present invention will be described. Note that “%” shown below means “% by mass” unless otherwise specified.

  C: C has a lower limit of 0.3% in order to obtain a cured layer of Hv550 or higher by induction hardening. However, if it exceeds 0.6%, the toughness and machinability are remarkably lowered, so the upper limit was set to 0.6%.

  Si: Si not only has a deoxidizing action but also has an action of strengthening ferrite by solid solution, and in order to obtain the effect, the lower limit was made 0.05%. However, the upper limit was limited to 0.4% because excessive addition would impair toughness.

  Mn: Mn expands the austenite region, reduces the proeutectoid ferrite and increases the pearlite fraction, and lowers the pearlite transformation start temperature and narrows the pearlite lamellar spacing, thereby contributing to the strength improvement of the ferrite + pearlite structure. . In order to obtain the effect, addition of 0.5% or more is necessary. However, if added excessively, the pearlite transformation is suppressed and the bainite transformation is easily generated, so the upper limit was made 1.0%.

  P: P lowers toughness, so its upper limit was made 0.03%. From the viewpoint of securing toughness, the addition amount is desirably as small as possible, and 0.02% or less is more preferable.

  S: S is an element effective for improving the machinability, and 0.005% or more must be added to obtain the effect. However, excessive addition causes a decrease in toughness, so the upper limit was made 0.03%.

  Al: Al is a deoxidizing element, and 0.01% or more is necessary to obtain the effect. However, if added excessively, coarse Al oxide is generated and toughness is caused, so the upper limit was made 0.08%.

  Nb: In the present invention, Nb is an effective element for uniformly forming ferrite for the purpose of suppressing hardness variation. For uniform formation of ferrite, it is necessary to suppress the partial abnormal growth of austenite before transformation and make the austenite grain size as uniform as possible. In order to suppress abnormal growth, it is extremely effective to generate fine Nb carbonitrides in the austenite region and suppress grain boundary migration. To obtain this effect, 0.001% or more must be added. It is. However, excessive addition produces coarse Nb carbonitrides and causes toughness reduction, so the upper limit was set to 0.1%.

  Cr: Cr contributes to improving the strength of the ferrite + pearlite structure in the same way as Mn, and in order to obtain the effect, addition of 0.05% or more is necessary. However, excessive addition is not preferable because the hardenability is increased and the structure changes mainly to bainite, and the upper limit is made 0.25% or less.

  In the present invention, the addition of alloying elements that causes an increase in cost is suppressed as much as possible, but as other elements having the same effect as Mn and Cr, 0.05% to 0.5% Ni, 0.05% to 0% .5% Mo and 0.0002 to 0.005% B can be added as necessary. Further, in the present invention, normal impurity level N is allowed, but in order to prevent toughness deterioration, the amount is preferably not more than 0.02%.

  The metal structure was a mixed structure of ferrite and pearlite from the viewpoint of tool life and machined surface roughness, and the hardness was limited to Hv 185 to 260 in machining to become a mechanical structural component. If the hardness is smaller than Hv185, the strength as a mechanical structural component is insufficient, so the lower limit is set to Hv185. However, since the wear of the tool used at the time of machining becomes remarkable when it exceeds Hv260, the upper limit is set to Hv260. When the hardness is Hv 185 to 260, in addition to the mixed structure of ferrite and pearlite, bainite, tempered martensite, mixed structure of ferrite and spheroidized cementite, etc. can be considered, but the mixed structure of ferrite and pearlite is more than any structure. However, the metallographic structure was limited to ferrite and pearlite in the future. Further, in order to suppress variations in hardness as much as possible, the area ratio of ferrite needs to be 15 to 35%, and the metal structure is a mixed structure of ferrite and pearlite, and the area ratio of ferrite is 15 to 35. %.

  Next, the reason why the manufacturing process is limited in the present invention will be described.

  In the present invention, a steel pipe having a chemical component of 700 ° C. or higher is used. This steel pipe can be used as it is in-line as long as it is 700 ° C. or higher immediately after being formed by a hot piercing-rolling-stretching process at the final stage of the manufacturing process. It may be used in-line as it is in the cold forming-electricity welding process or the cold forming-electricity welding-stretching process. However, once the steel pipe manufacturing process is finished, there is no problem even if a steel pipe reheated offline is used. The reason why the temperature of the steel pipe is limited to 750 ° C. or more is that the metal structure at the start of accelerated cooling is an austenite single phase. If the temperature of the steel pipe is too high, the austenite grains become coarse and cause a reduction in toughness.

  Next, the speed of accelerated cooling of this steel pipe will be described. At 0.5 ° C./second or less, the growth of ferrite proceeds, and the average hardness becomes Hv 185 or less. On the other hand, if it exceeds 10 ° C./second, the generation of ferrite is not uniform depending on the circumferential direction and the longitudinal direction, and the variation in hardness increases. Therefore, the accelerated cooling rate was limited to 0.5 to 10 ° C./second.

  The stop temperature for accelerated cooling is 700 ° C. or more, since ferrite is generated much and the average hardness is smaller than Hv185. When the temperature is lower than 550 ° C., in addition to the generation of ferrite and pearlite, some bainite is generated and the variation in hardness increases, so the lower limit was set to 550 ° C.

  The method of accelerated cooling can be selected arbitrarily, such as applying water directly to the surface of the steel pipe, applying water in the direction of contact with the pipe, or mist cooling, but during accelerated cooling, the outer surface is rotated while rotating the steel pipe in the circumferential direction. It was stipulated that only cooling from. This is for cooling uniformly in the circumferential direction and the longitudinal direction. If the steel pipe is not rotated, the lower surface of the steel pipe will be excessively cooled, and even when cooling from the inner surface side, water will be stored on the lower surface and a sufficient cooling rate will be obtained. There is a problem that can not be.

  In addition, in the present invention, in order to suppress variation in hardness, when the pipe is formed by a hot piercing-rolling-stretching process, a final rolling end temperature and a reheating furnace insertion temperature before the stretching process are limited. The reason for this is that the nonuniformity of ferrite formation, which is a factor of hardness variation, is caused by partial grain growth of the prior austenite grains, and the grain growth of the former austenite grains depends on the rolling end temperature and the reheating furnace insertion furnace temperature. It is to do. When the final rolling temperature is 850 ° C. or higher, all austenite is recrystallized, so that some grains do not grow. Further, if the insertion temperature is 750 ° C. or higher, the grain growth is suppressed because it is not cooled to the two-phase region that promotes grain growth. Therefore, the final rolling end temperature is limited to 850 ° C. or higher and the reheating furnace insertion temperature is limited to 750 ° C. or higher from the viewpoint of suppressing hardness variation.

  In the present invention, applicable steel pipe shapes are limited to a wall thickness of 5 mm or more and 22 mm or less and a length of 5 times or more of the outer diameter. The reason is that when the thickness is 22 mm or more, the difference in cooling rate between the outer surface side and the inner surface side becomes large. On the other hand, if the thickness is 5 mm or less, the wall thickness is too thin when the machining allowance on the inner and outer surfaces is subtracted, which is not so common for machine parts. The reason why the length of the steel pipe is limited to 5 times or more is that a short pipe whose length is less than 5 times the outer diameter can be manufactured by the conventional technique of forging a steel bar as a raw material, but the length is 5 This is because a long pipe more than double is easily buckled by forging from a steel bar and is difficult to manufacture. In order to make the cost merit for the forged material more reliable by using the long steel pipe material, it is more preferable that the steel pipe length is 10 times or more of the outer diameter.

  Moreover, it is necessary to have a 100-500 micrometers decarburization layer in the thickness direction from an outer surface with respect to the steel pipe just before accelerated cooling. In order to obtain a predetermined decarburized layer thickness in the hot piercing, rolling and stretching processes by heating the cylindrical bloom, the bloom heating temperature and time were set to satisfy the following formula (1). The reason why the decarburized layer thickness and the Bloom heating condition are defined is that the steel pipe is accelerated and cooled from the outer surface, so that the vicinity of the outer surface pole is easily hardened, and the workability deterioration is suppressed. In order to obtain the effect, the decarburized layer depth needs to be 100 μm or more. If the decarburized layer exceeds 500 μm, it will affect the subsequent hardening by induction hardening, so the upper limit was set to 500 μm. A desirable decarburized layer depth is 100 to 200 μm.

  The formula (1) shows the billet heating conditions such that the thickness of the decarburized layer to be formed is 100 to 500 μm, although there are some differences depending on the degree of processing when the cylindrical billet is heated to form a pipe. 1 is an equation obtained by regression analysis from the experimental results shown in FIG.

2.7335 × 10 ⁶² T ^-19.509 >t> 2.4726 × 10 ⁵⁷ T ^-18.121 (1)
Where T: temperature (° C.) t: time (minutes)
In the present invention, the thickness of the decarburized layer is not particularly defined with respect to the inner surface side. The reason is that in the case of cooling from the outer surface side, the cooling rate becomes slower and the hardening becomes smaller toward the inner surface side, so that a decarburized layer is unnecessary on the inner surface side. Therefore, it is better to keep the decarburized layer on the inner surface side as thin as possible. The decarburized layer in the present invention is defined as a region where the area ratio of granular ferrite is 80% or more.

  The steel pipe in the present invention is mainly a seamless steel pipe manufactured by hot-drilling-rolling-stretching, but it is manufactured by hot-extrusion press by cold-drilling or hot-drilling. In addition, a seamless steel pipe or a welded steel pipe manufactured by welding a hot coil into a tubular shape with a roll by cold or hot and then welding both end faces is also included.

  In addition, it was prescribed | regulated as one form of this invention to give the stress removal annealing for 10 to 60 minutes at 500 to 600 degreeC. This is because when accelerated cooling is applied, residual stress is generated due to slight cooling unevenness. If there is residual stress, the accuracy of machine parts after processing deteriorates, so it is desirable to remove the residual stress as much as possible. The limitation of the heat treatment condition is that the residual stress is set to 150 MPa or less, and it takes a long time if it is less than 500 ° C., and softening becomes remarkable if it exceeds 600 ° C. Desirably, it is best to set the temperature at 530 to 570 ° C. for 20 to 40 minutes.

  Steels having chemical components shown in Table 1 were melted and a 170 mm diameter bloom was cast by a converter-continuous casting process. These blooms were heated to 1240 ° C., pierced and rolled by the Mannesmann-plug mill method, then reheated to 950 ° C. and subjected to shrinkage rolling, and then water-cooled from the outer surface side by ring cooling. The steel pipe size after the reduction rolling was set to an outer diameter of 120 mm and a wall thickness of 12 mm. Furthermore, in order to investigate the influence of the wall thickness, a tube having an outer diameter of 150 mm and a wall thickness of 25 mm was also manufactured. In order to change the depth of the decarburized layer, the heating temperature and heating time of the bloom were adjusted. The manufactured steel pipe was observed for metal structure at arbitrary positions in the circumferential direction, the longitudinal direction, and the thickness direction, and the Vickers hardness was measured at 10 kgf. As the metal structure, a scanning electron microscope and an optical microscope were used.

  In the normal case, the average thickness of the decarburized layer on the outer surface of the pipe is 200 μm. In order to investigate the effect of the thickness of the decarburized layer, the heating temperature and time of the bloom are adjusted, and the thickness of the decarburized layer on the outer surface of the pipe is adjusted. Steel pipes having a thickness of 600 μm and 80 μm were also produced.

  The metal structure was observed at an arbitrary position 1 mm inside from the outer surface and the inner surface of the prototype tube, and the Vickers hardness was measured at 10 kg. The metal structure was observed by magnifying up to 5000 times with a scanning electron microscope, and was determined to be ferrite + pearlite, banite (sometimes including ferrite), and tempered martensite.

  The evaluation of toughness was carried out by conducting a Charpy test at + 20 ° C. using a half-size 2 mm U notch test piece and measuring the impact value.

  The prototype steel pipe was cut into a length of 50 mm and cut by 1 mm on the inner and outer surfaces, and then the outer surface was heated at high frequency until it reached 950 ° C. and then cooled with water to perform induction hardening. Thereafter, the surface hardness was measured. The results are shown in Table 2.

  No. which is an example of the present invention. 1 to 8 are steel pipes manufactured under appropriate accelerated cooling conditions, having a proper metal structure and hardness and toughness required as machine parts, easy to cut and cut, and excellent in high frequency hardenability. .

  No. No. 9 is an example in which the amount of C is too high and the average value of the hardness is high, so that the machinability is difficult.

  No. No. 10 is an example in which the amount of C was too low and the surface hardness after induction hardening was insufficient.

  No. No. 11 is an example in which the average value of hardness is too low because the amount of Mn is insufficient and the cooling rate is too slow.

  No. No. 12 is an example in which the average value of hardness was insufficient because the amount of Mn was too high and the toughness was poor, and the cooling rate was too low and the cooling rate was too low.

  No. No. 13 is an example in which the average hardness was high because the amount of Cr was too high and the cooling rate was too high.

  No. No. 14 is an example in which the variation in hardness was large because the cooling stop temperature was low, in addition to the poor toughness because the Nb amount was too high.

  No. No. 15 is an example in which the cooling rate was too slow, so the ferrite fraction was high and the average hardness was too low.

  No. No. 16 is an example in which the cooling rate was too fast and the stop temperature was too low, the average hardness was large, and the structure was bainite, so the toughness was too low.

  No. 17 is an example in which the variation in hardness was large because the cooling stop temperature was low.

  No. No. 18 is an example in which since the rolling end temperature and the cooling start temperature were too low, the generation of ferrite was uneven and the hardness variation was large.

  No. manufactured in the examples. 4 was subjected to stress relief annealing under various heat treatment conditions, and the relationship between the residual stress in the circumferential direction of the outer surface and the processing accuracy was investigated. The pipe after stress-relief annealing is cut into a length of 50 mm, the inner and outer surfaces are cut by 1 mm each, the inner surface is cut by machining, and the inner teeth are induction hardened at 10 kHz × 10 seconds to form a gear shape. A mechanical structural member was prepared. The machining accuracy is measured every 15 ° as the distance between the tooth vertices facing the center of the inner diameter of the member, and the difference between the maximum value and the minimum value of the six measured values is defined as “ellipticity”. Asked. The results are shown in Table 3. No. No. 4 had a residual stress of 150 MPa or less before the stress-relieving annealing and satisfied an ellipticity of 100 μm or less classified as a good product. No. 4 was subjected to stress removal annealing at 450 ° C. for 60 minutes. No. 20 is an example in which the heat treatment temperature was too low and there was almost no effect of stress removal. No. 5 annealed at 500 ° C. for 5 minutes. No. 22 is an example in which the degree of ellipticity, which is classified as the best product, did not reach 50 μm or less because the time was short. No. 1 subjected to stress relief annealing at a temperature of 500 to 600 ° C. for 10 to 60 minutes. 21 and no. Since Nos. 23 to 26 were appropriate heat treatments, the ellipticity could be reduced to 50 μm or less while satisfying the required hardness. No. No. 27 is an example in which the ellipticity can be remarkably improved, but the required heat is too low because the heat treatment temperature is too high.

It is a figure which shows the regression formula which demarcates the billet heating conditions experimentally obtained so that the thickness of the decarburized layer formed may be 100-500 micrometers, and the scope of the present invention.

Claims (12)

  In mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.4%, Mn: 0.5 to 1.0%, P: 0.03% or less, S: 0.005 ~ 0.03%, Al: not more than 0.08%, the balance having chemical components consisting of Fe and inevitable elements, thickness 5 mm or more and 22 mm or less, length 700 ° C. which is 5 times the outer diameter or more Accelerated cooling while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface side of the steel pipe at the above temperature, and the accelerated cooling is stopped in a temperature range of 550 to 700 ° C. A method for manufacturing a steel pipe for machine structural members.   In mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.4%, Mn: 0.5 to 1.0%, P: 0.03% or less, S: 0.005 ~ 0.03%, Al: 0.08% or less, with the balance having chemical components composed of Fe and inevitable elements, with a thickness of 5 mm or more and 22 mm or less in the hot drawing step, and the length is the outer diameter Accelerated cooling while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface side of the steel tube at a temperature of 700 ° C. or more which is made 5 times higher than the temperature of 550 to 700 ° C. A method of manufacturing a steel pipe for machine structural members, characterized in that accelerated cooling is stopped in a region.   In mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.4%, Mn: 0.5 to 1.0%, P: 0.03% or less, S: 0.005 .About.0.03%, Al: 0.08% or less, the balance is 5 mm by hot piercing, rolling and stretching processes using a cylindrical bloom having a chemical component consisting of Fe and inevitable elements. Accelerating while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second from the outer surface side of a steel tube at a temperature of 700 ° C. or higher and having a length of 22 mm or less and a length of 5 or more times the outer diameter. A method for producing a steel pipe for a machine structural member, characterized by cooling and stopping accelerated cooling in a temperature range of 550 to 700 ° C. In mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.4%, Mn: 0.5 to 1.0%, P: 0.03% or less, S: 0.005 ~ 0.03%, Al: 0.08% or less, with the balance being soaked at a temperature and time satisfying the following formula (1), a cylindrical bloom having a chemical component composed of Fe and inevitable elements Then, 100 mm in the thickness direction from the outer surface of the steel pipe at a temperature of 700 ° C. or higher, which was piped to a thickness of 5 mm to 22 mm and whose length was 5 times or more of the outer diameter by hot drilling, rolling and stretching processes. Form a decarburized layer of ˜500 μm, accelerate cooling from the outer surface side while rotating in the circumferential direction at a cooling rate of 0.5 to 10 ° C./second, and stop the accelerated cooling in the temperature range of 550 to 700 ° C. The manufacturing method of the steel pipe for machine structural members characterized by these.
2.7335 × 10 ⁶² T ^-19.509 >t> 2.4726 × 10 ⁵⁷ T ^-18.121 (1)
Where T: temperature (° C.) t: time (minutes)   The said steel pipe is a mass%, and contains Cr: 0.05-0.25%, The manufacturing method of the steel pipe for machine structural members of any one of Claims 1-4 characterized by the above-mentioned. .   The method for manufacturing a steel pipe for machine structural members according to any one of claims 1 to 5, wherein the steel pipe further contains Nb: 0.001 to 0.1% by mass%. .   The machine according to any one of claims 3 to 6, wherein an end temperature of the rolling is 850 ° C or higher, and a temperature to be inserted into a reheating furnace before the stretching step is 750 ° C or higher. Manufacturing method of steel pipe for structural members.   The method for producing a steel pipe for machine structural members according to any one of claims 1 to 7, wherein stress-relieving annealing is further performed at 500 to 600 ° C after cooling for 10 to 60 minutes.   In mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.4%, Mn: 0.5 to 1.0%, P: 0.03% or less, S: 0.005 ~ 0.03%, Al: 0.08% or less, the balance is a chemical structure composed of Fe and inevitable elements, the thickness is 5 mm or more and 22 mm or less, and the length is 5 times or more of the outer diameter. It is a steel pipe for members, and its metal structure is a mixed structure of ferrite and pearlite, the average value of hardness is 185 to 260 in terms of Vickers hardness, and the difference between the maximum value and the minimum value of hardness is Vickers hardness A steel pipe for machine structural members, wherein the steel pipe is 20 or less.   Furthermore, the steel pipe for machine structural members of Claim 9 containing Cr: 0.05-0.25% by the mass%.   The steel pipe for machine structural members according to claim 9 or 10, further comprising Nb: 0.001 to 0.1% by mass.   The steel pipe for machine structural members according to any one of claims 9 to 11, wherein an area ratio of the ferrite is 15 to 35%.

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