JPWO2008123397A1 - Case-hardened steel pipe with excellent workability and its manufacturing method - Google Patents

Abstract

A case-hardened steel pipe having a hardness of 72 to 80 HRB and having a carburized layer exhibiting high strength and wear resistance and sufficiently high impact fracture load when formed into a final product under relatively mild carburizing and quenching conditions after molding. % By mass, C: 0.1 to 0.25%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.9%, P: 0.02% or less, S: 0.3% 001 to 0.15%, Cr: 0.5 to 0.9%, Mo: 0.15 to 1%, Al: 0.01 to 0.1%, B: 0.0005 to 0.009%, N A tube is made from a steel having a steel composition containing less than 0.006% and the balance being essentially Fe, and the obtained steel tube is soaked to a temperature of 880 to 980 ° C. and then 70 ° C./min or less. Normalizing by cooling at a cooling rate, performing cold working on the normalized steel pipe, and annealing the cold worked steel pipe at a temperature of 700 to 820 ° C. More is produced.

Description

  The present invention relates to a case-hardened steel pipe having a high fracture load after carburizing and quenching (steel pipe made of case-hardened steel) and a method for producing the same. The present invention particularly relates to a case-hardened steel pipe excellent in workability and a manufacturing method thereof.

  Conventionally, it has been difficult to manufacture various machine structural parts for automobiles and industrial machines, especially surface hardened parts such as shafts, CVJ (constant velocity joint), CVT (continuously variable transmission continuously variable transmission), and gears. Hardened steel is used. The raw case-hardened steel is formed into a desired part shape by hot forging, cold forging, and further machining. For the purpose of improving the wear resistance and fatigue strength, the manufactured part is subjected to surface hardening treatment such as carburizing treatment or carbonitriding treatment before use.

  The properties required for such mechanical structural parts are becoming increasingly sophisticated. In other words, in addition to the conventionally required characteristics of high surface hardness and bending fatigue strength after carburizing and quenching, more excellent wear resistance and rolling fatigue characteristics, and even higher impact fracture resistance characteristics against shock load And it has become desirable to have toughness.

  Case-hardened steel may have problems such as a decrease in impact fracture strength due to abnormal growth of crystal grains during carburizing and quenching, a decrease in fatigue characteristics, a decrease in dimensional accuracy, and the like. In particular, from the viewpoint of rationalizing the carburizing treatment, when so-called high-temperature carburizing is performed in a temperature range of 990 to 1090 ° C. in order to shorten the carburizing time, coarse grains are generated, and necessary fatigue characteristics, rolling fatigue characteristics, etc. The problem that it cannot be obtained occurs.

  Japanese Patent Application Laid-Open No. 2005-240175 (Patent Document 1) proposes to suppress the generation of coarse grains in high-temperature carburizing of case-hardened steel by controlling steel components and Ti-based precipitates.

  Steel materials that can ensure high strength and wear resistance in the carburized layer under the relatively mild carburizing and quenching conditions without using high-temperature carburizing have been studied. Japanese Patent Application Laid-Open No. 9-53150 (Patent Document 2) discloses a high-strength, high-toughness case-hardened steel exhibiting a sufficiently high impact fracture load even when a notch is present on the surface of the carburized layer, and using the same Also disclosed is a method for stably producing a high-strength, high-toughness case-hardened steel pipe that has excellent workability and exhibits excellent impact fracture strength after carburizing and quenching.

  According to Patent Document 2, one of the causes of the problems of the prior art is the generation of an incompletely quenched structure, and the greatest cause of the generation of the incompletely quenched structure is on the austenite grain boundary that occurs when the carburized material is quenched. In the precipitation of carbides. Therefore, a component design is adopted in which B is added to prevent the above carbide precipitation and N is suppressed as much as possible so that the effect of B is sufficiently expressed.

  However, the high-strength, high-toughness case-hardened steel pipe disclosed in Patent Document 2 has excellent characteristics, particularly as a seamless steel pipe for case-hardened steel, but because of its relatively high hardness, forging at the customer stage, etc. There may be a problem with the workability.

In addition, in patent document 2, regarding the manufacturing method of the case-hardened steel pipe, (i) a method of performing stress relief annealing on the steel pipe obtained by hot pipe making through cold working [Example 3], and (ii) It describes a method [Examples 4 and 5] in which a steel pipe obtained by hot pipe making is subjected to primary annealing and then subjected to stress processing annealing (secondary annealing) through cold working.
JP-A-2005-240175 JP-A-9-53150

  The present invention is excellent in workability, specifically, is a case-hardened steel pipe having a hardness of 72 to 80 in HRB (Rockwell B scale hardness), and is carburized and quenched under relatively mild conditions after forming. The present invention provides a case-hardened steel pipe and a method for producing the case-hardened steel pipe, in which the carburized layer exhibits high strength and wear resistance and sufficiently excellent impact fracture resistance when it is made into a final product.

The present invention is based on the following findings.
(1) In the heat treatment method after hot pipe making described in Patent Document 2, even if annealing is performed after cold working, the hardness is often 85 or more in HRB, and it is hard and is molded by the customer. Is not easy.

  (2) In the method of performing the primary annealing before the cold working of (ii) described in Patent Document 2 in particular, if the annealing temperature at this time is about 700 ° C., the Patent Document after the cold working Even if the secondary annealing is performed under the conditions described in 2, it is difficult to soften the steel. In this case, if the secondary annealing temperature is about 730 ° C., a bainite structure is obtained.

(3) In the above (2), when the heat treatment temperature is raised to about 930 ° C. in the secondary annealing after the cold working and the steel is gradually cooled, the steel can be softened around HRB = 75. However, under this temperature condition, the effect of cold working is eliminated by the phase transformation in the region of Ac ₃ or more points. Therefore, the hardness is set to HRB = by combining the cold working degree and the heat treatment condition in the secondary annealing. It becomes impossible to control freely in the range of 72-80. In addition, due to high temperature heating after cold working, dimensional accuracy may be reduced and surface decarburization may occur. The steel structure becomes a ferrite + pearlite structure and is easy to coarsen.

(4) Patent Document 2 specifically shows 870 ° C. as a temperature condition of primary annealing before cold working. Under this heat treatment condition, since the material is once heated to a temperature higher than the Ac ₃ point, the structure after the primary annealing becomes a ferrite + pearlite structure. However, since the primary annealing temperature is low, the softening of the steel cannot be expected unless the annealing is sufficiently long in the secondary annealing after the cold working.

  (5) Unlike the method disclosed in Patent Document 2, normalization is performed at a temperature of 880 ° C. or more (cooling rate is 70 ° C./min or less) as the heat treatment before cold working, and preferably the cross-sectional area reduction rate is 20 When annealing is performed at 700 to 820 ° C. after ˜50% cold working, part of the pearlite is spheroidized. Hereinafter, unless otherwise specified, “perlite spheroidization” means spheroidization of cementite in pearlite. Thereby, the desired hardness reduction can be achieved, and at the same time, by adjusting the cooling rate after normalization and the temperature of the secondary annealing, preferably by further adjusting the reduction rate of the cross-sectional area in cold working The hardness can be adjusted.

  That is, in the present invention, a steel pipe having a steel composition that enables carburizing and quenching is manufactured by first normalizing the steel pipe manufactured by hot pipe making, and then performing cold working. Stress relief annealing is performed. This annealing causes spheroidization of at least a part of pearlite in the ferrite + pearlite structure formed by normalization (that is, spheroidization of cementite in pearlite), softening the steel, and skin hardening with excellent workability. Steel pipe is manufactured.

  Further, in the present invention, pearlite that is spheroidized during annealing by adjusting the degree of processing in the cold working of the subsequent process and the heat treatment conditions in the subsequent annealing for the ferrite + pearlite structure generated in the normalization. Thus, the steel material hardness can be finely adjusted.

  In one surface, the present invention is in mass%, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: 0.02 %: S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005 -0.009%, N: less than 0.006%, the tube is made from steel having a steel composition consisting essentially of Fe, the temperature of the resulting steel tube being 880-980 ° C After holding, normalization is performed by cooling the temperature range of 880 to 400 ° C. at a cooling rate of 70 ° C./min or less, and cold-working is performed on the normalized steel pipe, and 700 to 700 to the cold-worked steel pipe It is a manufacturing method of a case hardening steel pipe characterized by performing annealing at the temperature of 820 ° C.

  From another aspect, the present invention provides, in mass%, C: 0.1 to 0.25%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.9%, P: 0 0.02% or less, S: 0.001 to 0.15%, Cr: 0.5 to 0.9%, Mo: 0.15 to 1%, Al: 0.01 to 0.1%, B: 0 .0005 to 0.0009%, N: less than 0.006%, the balance is essentially composed of Fe, and the steel structure is a mixed structure of ferrite + pearlite + spheroidized cementite or ferrite + spherical A cold-finished case-hardened steel pipe characterized by having a mixed structure of cementitious cementite.

The steel composition may further contain one or more elements selected from the following (1) and (2):
(1) By mass%, Ni: 0.3-4.0%
(2) By mass%, Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, Zr: 0.01-0.3% 1 type or 2 types or more selected from

In the steel composition, the B content is preferably B: 0.0005 to 0.003%.
“Skin-hardened steel” and “Skin-hardened steel pipe” are processed into a predetermined shape of a product (for example, the above-mentioned machine structural parts) and finally carburized and hardened to increase the surface layer (carburized layer). Steel and steel pipe used after being hardened. Needless to say, the above-described hardness is the hardness of the case-hardened steel, that is, the hardness before molding into a part shape (naturally before carburizing and quenching). Molding to a predetermined part shape and carburizing and quenching are usually performed at the customer (on the customer side).

  “The balance consists essentially of Fe” means that the balance can contain inevitable impurities.

It is a microscope picture of the case hardening steel pipe concerning the present invention obtained in the example.

The reason why the steel composition of the case-hardened steel pipe is limited as described above in the present invention will be described together with its action. In this specification, “%” representing the steel composition is “mass%”.
C:
C is a basic component that ensures the hardness and strength of steel. Hardness of Hv250 or higher is required to secure the steel with a strength that does not cause deformation during use of the carburized and quenched parts. In order to ensure the required hardness, the C content is set to 0.1% or more. On the other hand, when C is contained exceeding 0.25%, the core toughness of steel deteriorates. Therefore, the C content is 0.1 to 0.25%, preferably 0.12 to 0.20%.

Si:
In order to realize high impact fracture resistance by quenching the carburized layer, the effect of improving the hardenability of Si is actively used. If the Si content is less than 0.2%, the desired high carburized layer hardenability cannot be ensured. On the other hand, when Si is contained exceeding 0.4%, the grain boundary becomes brittle due to oxidation of Si in the vicinity of the grain boundary during carburizing. Therefore, the Si content is set to 0.2 to 0.4%.

Mn:
Mn is also added to improve the hardenability of the carburized layer and realize high impact fracture resistance. If the Mn content is reduced to less than 0.3%, the hardenability of the carburized layer is lowered, and the desired high impact fracture resistance cannot be ensured. It has been found that the weakening of the grain boundary due to the oxidation of Mn near the grain boundary during carburization has no practical problem even if the Mn content exceeds 0.9%. However, when Mn is contained exceeding 0.9%, the deterioration of the punching workability and the grindstone grindability becomes remarkable. Properties such as punchability and grindability are particularly important for efficient machining such as CVJ. Therefore, the Mn content is set to 0.3 to 0.9%.

P:
P is an extremely harmful impurity element in case-hardened steel because it significantly promotes the weakening of grain boundaries due to precipitation of cementite on austenite grain boundaries during carburizing and quenching. Therefore, it is preferable to reduce the P content as much as possible. However, since the reduction of P is accompanied by an increase in costs in the raw materials and the refining process, an allowable value is designed from the balance between the target performance and the cost. In the present invention, the upper limit of the allowable P content is set to 0.02% in consideration of the effect of B described later.

S:
S causes a deterioration in the toughness of the steel, but is also a component that should be positively added in terms of improving the machinability (machinability and punchability). When the S content is less than 0.001%, the machinability improving effect is not remarkable, and when the S content exceeds 0.15%, the toughness of the steel is significantly deteriorated. Therefore, the S content is set to 0.001 to 0.15%. In the case of usage that requires less machinability, it is a good idea to keep the S content low.

Cr:
Cr is an indispensable component for ensuring the hardenability of the steel substrate (the portion of the steel excluding the carburized layer on the surface) and achieving the carbon concentration necessary for the carburized layer in a short time. A Cr content of 0.5% or more is required. However, at the same time, Cr significantly promotes the weakening of the grain boundary due to the precipitation of cementite on the austenite grain boundary during carburizing and quenching, so the content is limited to 0.9% or less. However, if the Cr content is limited to 0.9% or less, the hardenability of the steel, particularly the hardenability of the carburized layer with a high C content, becomes insufficient. Therefore, in this invention, hardenability is supplemented by addition of B, Mo, and Ni which does not cause the grain boundary to become brittle. Thus, the Cr content is set to 0.5 to 0.9%, but is preferably adjusted to 0.5 to 0.65%.

Mo:
Mo is an essential component for improving the strength and toughness of the steel substrate and the carburized layer and achieving the carbon concentration required for the carburized layer in a short time. Since the effect of improving the hardenability of Mo is hardly affected by the C content of the steel substrate, the effect of improving the hardenability is stably exhibited even in a carburized layer having a high carbon content.

  As described above, in the present invention, the Cr content is reduced in order to suppress the weakening of grain boundaries accompanying carburization, and the hardenability is replenished by adding B. In such steel, the hardenability is remarkably lowered even when the carbon content becomes high. Therefore, the hardenability compensation of the carburized layer by Mo is very important. If the Mo content is less than 0.15%, not only sufficient hardenability compensation can be performed, but also the amount of C that is infiltrated by a short time carburizing treatment is reduced. From the viewpoint of imparting the above effects, it is preferable that the Mo content is large, but a sufficient effect can be obtained by adding up to 1%, and addition of Mo in an amount exceeding this is not economically advantageous. Therefore, the Mo content is 0.15 to 1%, preferably 0.2 to 0.7%, more preferably 0.2 to 0.6%.

Al:
Al is an effective component for deoxidation and grain refinement of steel. If the content is less than 0.01%, the effect is not sufficient. On the other hand, when Al is contained exceeding 0.1%, inclusions harmful to toughness increase. Therefore, the Al content is set to 0.01 to 0.1%.

B:
B suppresses the precipitation of carbides (Cr carbides, etc.) on the austenite grain boundaries generated when quenching the carburized material, thereby preventing the formation of incompletely quenched structures in the carburized layer and preventing grain boundary embrittlement. Thus, it is an indispensable component for securing sufficient impact fracture resistance, wear resistance, rolling fatigue characteristics and the like for the carburized and quenched material. In particular, in the present invention, the Cr content is limited in order to prevent the adverse effect of Cr, which significantly promotes the weakening of the grain boundaries by precipitation of carbides on the grain boundaries during carburizing and quenching. B also contributes to the effect of securing the hardenability of the steel core part by compensating for the hardenability deterioration of the steel base resulting from the reduction of the Cr content.

  If the B content is less than 0.0005%, the desired effect due to the above action cannot be obtained. On the other hand, if B exceeds 0.0009%, grain boundary embrittlement occurs due to B. Therefore, the B content is set to 0.0005 to 0.009%.

In the present invention, as will be described later, heat treatment (normalization) is performed at a temperature of Ac ₃ points or higher, specifically, 880 ° C. or higher before cold working. This heat treatment assumes that B is once dissolved in order to achieve the purpose of reducing the hardness by annealing after cold working. When the amount of B is large, it takes a long time for the solid solution of B, and therefore the heat treatment in normalization, so the B content is desirably low in the above range. Specifically, the B content is particularly preferably 0.003% or less (that is, within a range of 0.0005 to 0.003%).

N:
As described in Patent Document 2, the amount of N in steel is very important for making the action of B effective. That is, only when the N content in the steel is reduced to a region of less than 0.006%, the effect of preventing carbide precipitation at the grain boundaries that occurs during the quenching treatment of the carburized material due to the addition of B becomes significant and sufficient. Not only the impact load strength is ensured, but also the rolling fatigue characteristics are remarkably improved. The smaller the N content in steel, the better. However, in industrial production in the atmosphere, it is extremely difficult to reduce the N content to less than 0.001% with the current steelmaking technology.

Ni:
In the case-hardened steel pipe of the present invention, when used for an inner race or a ball cage of a general drive axle joint of an automobile, Ni, Ti, Nb, V, or Zr described below is added. Even if not, properties such as strength and toughness are sufficient. However, when used in applications where the conditions are more severe, it is effective to improve the strength and toughness by containing one or more of these elements.

  Ni is an effective component for improving the strength and toughness of the steel substrate, and contributes greatly to improving the strength and toughness of the carburized layer in cooperation with Mo. If the Ni content is less than 0.3%, the effect is insufficient. On the other hand, the effect is saturated even if it contains Ni exceeding 4.0%. Therefore, when adding Ni, the content is made 0.3 to 4.0%.

Ti, Nb, V and Zr:
These elements have the effect of improving the toughness by refining the crystal grains of steel. Therefore, when severe use conditions are expected, it is preferable to contain one or more of these. If the content of each of these components is less than 0.01%, the above effects are insufficient. On the other hand, if each content exceeds 0.3%, the toughness and rolling fatigue characteristics of the steel are deteriorated. Therefore, the contents of Ti, Nb, V and Zr are set to 0.01 to 0.3%, respectively.

Next, the manufacturing conditions of the case-hardened steel pipe of the present invention will be described in the order of steps.
Pipe making:
From the steel having the above-described steel composition (skin-hardened steel), a pipe to be a raw pipe is produced by an appropriate pipe making method. The base pipe is preferably a seamless steel pipe that has been hot-made. However, as will be described below, since the heat treatment is performed at a temperature of Ac ₃ or higher once during normalization, the processing history in the previous process is not affected. Accordingly, the pipe making method is not particularly limited, and for example, an electric resistance welded steel pipe can be used as a raw pipe. There are no particular restrictions on the seamless pipe making of seamless steel pipes, but the steel having the above steel composition is made into a billet form by hot forging a steel ingot, for example, and then billet → Mannesmann piercing rolling → mandrel What is necessary is just to make a seamless steel pipe by drawing rolling in a mill → constant diameter rolling.

Normalization:
Prior to cold working, the steel pipe (element pipe) manufactured by the above-described method is subjected to normalization rather than primary annealing as disclosed in Patent Documents 1 and 2. The normalization is performed by, for example, heat treatment (soaking) in which a steel pipe is appropriately charged in a heating furnace and maintained at a predetermined temperature, and then cooling. The purpose of this normalization is to make the steel structure a mixed structure of ferrite and pearlite. Once the steel structure is made ferrite + pearlite and then annealed in a specific temperature range after cold working, desirable characteristics are manifested in the case-hardened steel pipe.

  The heat treatment temperature for normalization is 880 ° C. or higher and 980 ° C. or lower. If the heat treatment temperature exceeds 980 ° C, decarburization may proceed. The lower limit temperature of 880 ° C. is a temperature necessary to make B dissolve in austenite in a short time to make the structure uniform. Due to the solid solution of B, the hardness of the steel substrate can be reduced. If the heat treatment temperature during normalization is lower than 880 ° C., sufficient solid solution of B cannot be achieved, and even if the temperature is maintained for a long time, the hardness of the steel substrate is not reduced.

  The soaking time may be as short as 30 seconds as long as the temperature reaches all the portions of the steel pipe, but it is preferably 1 minute or longer from the viewpoint of suppressing variation in characteristics. If the soaking time exceeds 30 minutes, decarburization may proceed, so 30 minutes or less is desirable.

  The cooling after the heat treatment (soaking) may be air cooling, but the range from the heat treatment temperature to 400 ° C. (therefore, the range of at least 800 to 400 ° C.) is 70 ° C./min or less. If the cooling rate is higher than this, bainite is generated, and the effect of the present invention cannot be obtained. Since the lower limit of the cooling rate is premised on normalization, it is not particularly limited as long as it is about air cooling or higher, but it is preferably 20 ° C./min or higher in consideration of economics such as processing time.

Cold working:
A steel pipe obtained by hot pipe making is preliminarily subjected to cold working. In general, cold working is necessary to secure predetermined dimensions and dimensional accuracy in a steel pipe. In the present invention, spheroidization of cementite in pearlite is performed in the secondary heat treatment stage by annealing following cold working ( Accordingly, there is an effect of generating pearlite spheroidization.

  As a cold working means, cold drawing, cold rolling or the like can be adopted, and there is no particular limitation. The degree of cold working is preferably 20 to 50%, more preferably 25 to 50%, in terms of the cross-sectional area reduction rate. If the degree of processing is less than 20%, it is difficult to spheroidize part of the pearlite in the next step. If the degree of work exceeds 50%, seizure between the tool and the material tends to occur during cold working, and the accumulation of strain in the steel substrate increases, causing abnormal growth of austenite grains during carburizing heat treatment. Causes the hardened structure to become coarse and mixed. Furthermore, when the cold work degree exceeds 50%, the hardness of the steel pipe is remarkably increased by work hardening, and it becomes difficult to soften by annealing performed thereafter, and the workability of the steel pipe deteriorates.

Annealing:
The annealing after the cold working is generally performed in order to release the strain accumulated in the steel base by the cold working to soften the steel base and ensure the workability required by the customer. There is also an object of spheroidizing at least a part of the cementite in the pearlite. For this purpose, the annealing temperature after cold working is set to a range of 700 to 820 ° C. When the annealing temperature is lower than 700 ° C or higher than 820 ° C, pearlite spheroidization does not proceed sufficiently.

  When pearlite (cementite in pearlite) is all spheroidized by annealing, the steel structure becomes a mixed structure of ferrite and spheroidized cementite. On the other hand, when a part of pearlite is spheroidized, the steel structure becomes a mixed structure of ferrite + pearlite + spheroidized cementite. The case-hardened steel pipe according to the present invention can be characterized by this steel structure and the above steel composition.

  Thus, the hardness of a steel pipe falls because at least one part of pearlite spheroidizes. By adding this effect to the softening by annealing, according to the present invention, it is possible to manufacture a case-hardened steel pipe having a hardness of 72 to 80 and having a hardness of 72 to 80. This hardness can be adjusted to a desired value by changing the ratio of spheroidization of pearlite depending on the degree of work during cold working and the annealing conditions.

  As described above, in case-hardened steel, usually, a consumer performs a forming process and a carburizing and quenching process to produce a target part. In the case of producing a part from the case-hardened steel pipe according to the present invention, there is no limitation on the forming process and carburizing and quenching conditions, but it is preferable to do so because relatively mild carburizing and quenching conditions can be adopted. An example of the carburizing and quenching conditions is carburizing by soaking at 920 ° C. for 2 hours, followed by quenching from 870 ° C.

  The following examples are intended to illustrate the present invention and do not limit the present invention in any way. Various changes and modifications can be made by those skilled in the art within the scope of the present invention.

  A steel ingot (1 ton) with the steel composition shown in Table 1 was obtained by casting molten steel that was vacuum-melted, then hot forged into round steel pieces, further pierced and rolled, stretched and rolled by a mandrel mill, and stretched. -A raw pipe (steel pipe) having an outer diameter of 80 mm (diameter) and a wall thickness of 6.1 mm was produced by hot pipe making with constant diameter rolling by a reducer.

  The steel pipe was subjected to primary heat treatment (normalization) and subsequent cooling under the conditions shown in Table 2, followed by cold drawing with a cross-sectional area reduction rate of 28.4%, an outer diameter of 66.2 mm (diameter), A seamless steel pipe having a wall thickness of 5.3 mm was finished. This steel pipe was annealed under the conditions shown in Table 2 as secondary heat treatment. A test piece was collected from the steel pipe after the secondary heat treatment (annealing), and the Rockwell B scale hardness (HRB) in the pipe cross section was measured. The results are shown in Table 2.

In No. 1 and No. 2 in Table 2, the heat treatment temperature in normalization before cold working was 700 ° C., which was lower than Ac ₁ point, and a hard finish of HRB = 87 or more was obtained. On the other hand, in No. 3 to No. 7 where the heat treatment temperature before cold working exceeds Ac ₃ points, when the heat treatment temperature is lower than 880 ° C., except for No. 3, the hardness is HRB = 82 or more. The purpose of softening the HRB to 80 or less could not be achieved. No. 3 with a cooling rate as low as 10 ° C./min after heat treatment (soaking) had an HRB of 77 and was able to achieve the purpose of softening, but the heat treatment time including the cooling process became longer and continuous. When processing is assumed, it is obvious that the heat insulation equipment is long and is not economical.

  No. 8 to No. 18 are examples in which the heat treatment before cold working was performed by soaking at 880 ° C. or 930 ° C. In No. 9 and 13 in which the cooling rate after soaking exceeded 70 ° C./min and No. 14 and 16 in which the annealing temperature after cold working was too low or too high, the HRB exceeded 80 and was sufficiently soft Could not be converted. On the other hand, in the examples of the present invention in which the cooling rate after soaking was 70 ° C./min or less and the annealing temperature after cold working was in the range of 700 to 820 ° C., all achieved the softening purpose of HRB 80 or less. We were able to.

  When the microstructure of the steel pipe after the secondary heat treatment (annealing) was observed, No. 1 and No. 2 were bainite structures, and No. 3 to No. 7 were ferrite + pearlite structures. In No. 3, the tendency of coarsening was recognized.

  On the other hand, with respect to No. 8 to No. 18, in the present invention example with an HRB of 80 or less, it was confirmed that the ferrite + pearlite + spheroidized cementite structure was formed, and the cementite of the pearlite structure was partially spheroidized. It was. However, spheroidized cementite was not observed in No. 8 to No. 18 in which the hardness exceeded HRB84. In No. 9 and No. 13 where the cooling rate was 80 ° C./min, some bainite was recognized.

  Therefore, the steel pipe is soaked at 880 ° C. or higher in advance, normalized by cooling at a cooling rate of 70 ° C./min or less, and annealed at a temperature of 700 to 820 ° C. after cold working. The transition from a mixed structure to a mixed structure of pearlite + ferrite + spheroidized cementite progressed, and it is considered that the purpose of softening was achieved.

  When the drilling test (punch material: high-speed steel, punch diameter 15.7 mm, drilling speed: 2.5 mm / sec) was performed on the steel pipes of the present invention examples in Table 2, the irregularities and dimensional accuracy of the drilling surface were exceptional. There was no problem. Satisfactory results were also obtained in an impact tensile test using a ball cage simulated specimen. Furthermore, the characteristics after carburizing and quenching were also good.

  1 shows a micrograph of the steel pipe obtained in No. 11 of Table 2. It can be seen that carbide (cementite) is spheroidized in the ferrite + pearlite structure.

Claims (7)

  C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: 0.02% or less, S: 0.001 -0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: A tube is made from steel having a steel composition comprising less than 0.006%, the balance being essentially Fe, and the resulting steel tube is kept at a temperature of 880-980 ° C. and then in a temperature range of 880-400 ° C. Is subjected to normalization by cooling at a cooling rate of 70 ° C./min or less, cold-working is performed on the normalized steel pipe, and annealing is performed on the cold-worked steel pipe at a temperature of 700 to 820 ° C. A method for producing a case-hardened steel pipe. The method according to claim 1, wherein the steel composition further contains one or more elements selected from the following (1) and (2).
(1) By mass%, Ni: 0.3-4.0%
(2) By mass%, Ti: 0.01-0.3%, Nb: 0.01-0.3%, V: 0.01-0.3%, Zr: 0.01-0.3% 1 type or 2 types or more selected from   The method according to claim 1 or 2, wherein in the steel composition, B is 0.0005 to 0.003%.   C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: 0.02% or less, S: 0.001 -0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: Steel composition comprising less than 0.006%, the balance being essentially composed of Fe, (a) a mixed structure of ferrite + pearlite + spheroidized cementite and (b) a mixed structure of ferrite + spheroidized cementite A cold-finished case-hardened steel pipe characterized by having a structure.   The case-hardened steel pipe according to claim 4, wherein the steel composition further contains one or more elements selected from the following (1) and (2).   The case-hardened steel pipe according to claim 4 or 5, wherein the steel composition is B: 0.0005 to 0.003%.   The case hardening steel pipe according to any one of claims 4 to 6, wherein Rockwell B scale hardness (HRB) is 72 to 80.

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