JP3236883B2 - Case hardening steel and method for manufacturing steel pipe using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-toughness case hardening steel having a high breaking load after carburizing and quenching, and a method of manufacturing a high-strength and high-toughness case hardening steel pipe using the same.

[0002]

2. Description of the Related Art For example, as a joint for transmitting power from a drive axle of a motor vehicle to a driven axle, an outer race (outer ring) fixed to the drive shaft 1 as shown in FIG.
2 and an inner race (inner ring) 4 fixed to the driven shaft 3
A plurality of (for example, six) balls 5 are interposed between the two and a structure in which these balls are held by a ball cage 6 (a bar-field type joint) is generally used. It is heavily used. Since the ball cage 6 and the inner race 4 are intended to transmit a very large unsteady rotational force in terms of mechanism, they need to have high strength and toughness against an impact tensile load. In addition, on the contact surface with the ball 5, excellent wear resistance and rolling fatigue resistance are required. Therefore, "hardened steel" is applied to these parts to impart high surface hardness and rolling fatigue by carburizing and quenching.
S's SCr415, SCM415, SNC415, SNC
Cr type such as M415, Cr-Mo type, Ni-Cr type, Ni-Cr-
Mo-based alloy steels for machine structural use were used as case hardening steels.

[0003] In recent years, however, the use conditions of the above-mentioned "steel parts used after carburizing and quenching" have become more and more severe, and there has been a demand for case hardening steel as a material. In addition to high surface hardness and flexural fatigue strength after carburizing and quenching, it should also have better wear resistance and rolling fatigue characteristics, and even higher fracture strength and toughness against impact load. Is becoming desirable.
In particular, many steel parts used after carburizing and quenching have notched shapes, and the importance of notched tensile strength after carburizing and quenching is more strongly recognized among "fracture strength characteristics". It has come to be.

[0004] On the other hand, carburizing and quenching of each of the above steels specified by JIS causes brittleness of crystal grain boundaries, and therefore, a method of carburizing and quenching is devised. However, there is a problem that it is difficult to obtain the necessary effect of improving various strengths.

Accordingly, the present inventors have developed a trace amount of B as a case hardening steel which can prevent the grain boundary from becoming brittle due to carburizing and quenching.
A high-strength steel was proposed in which additions were made and alloy elements such as Cr, Mo, and Ni were modified (Japanese Patent Laid-Open No. 2-170944,
JP-A-5-117806). However, due to the careful examination that was continued thereafter, the case hardening steels proposed as
Fracture strength and toughness against impact load are not enough enough, and in some cases "insufficient quenching in carburized layer"
Therefore, it is considered that further improvement is desired in terms of surface hardness, wear resistance, rolling fatigue characteristics and the like.

On the other hand, for example, the ball cage and the like of the drive axle coupling shown in FIG. 1 are made of case hardened steel pipe (this material steel pipe is generally made of hot-hardened case hardened steel and then subjected to a predetermined process. It is manufactured by cold drawing for obtaining dimensional accuracy and annealing for improving workability). In addition to the problems mentioned above, various machine workability such as the life of cutting and punching tools when processing various parts, the roughness and dimensional accuracy of the cut surface when performing punching, and the clogging of the grinding wheel in polishing etc. It is required to be excellent. However, since these characteristics vary in a complicated manner depending on the chemical composition of the steel pipe and the manufacturing conditions,
It has been considered very difficult to stably impart these required properties to steel pipes. Above all, "coarsening due to abnormal growth of crystal grains", which tends to occur during carburizing and quenching after steel pipes are processed into parts, "impact strength reduction" and "dimensions""Degradation of accuracy" has been a major problem in case-hardened steel pipes.

[0007] In view of the above, the object of the present invention is not only to ensure high strength and abrasion resistance in the carburized portion even under relatively mild carburizing and quenching conditions, but also to provide the carburized portion with a surface. A high-strength, high-toughness case-hardening steel that exhibits a sufficiently high impact fracture load even when notches are present has been realized. The aim is to establish a means for accurately and stably producing a “high-strength, high-toughness case-hardened steel pipe” exhibiting the following characteristics.

[0008]

In order to achieve the above object, the present inventors have set forth the following: a) In general, carburized parts have a tempering temperature of 180 to 20 after carburizing.
It is about 0 ° C., so if the hardenability is excessive, the toughness of the core is impaired, and if the hardenability is insufficient, the strength is impaired. It is necessary to design the components. B) In addition, elements that cause grain boundary embrittlement due to carburization are reduced as much as possible, and the addition of elements that improve the grain boundary strength is aimed at, so that the impact load strength can be reduced. It is necessary to try to minimize the occurrence of grain boundary fracture that causes the reduction. As a result of earnest research while repeating many experiments based on the basic idea that the following findings are obtained. Was completed.

In recent years, the cooling rate during quenching has been reduced by changing the oil type of the cooling medium or increasing the oil temperature in order to minimize the deformation strain of the workpiece during carburizing and quenching. There is a growing tendency to do so. Therefore, if the hardenability of the carburized layer and the core of the component is not high, an incompletely hardened structure is likely to be generated. Against this background, quality problems that have been pointed out with conventional case hardening steel, such as “unsatisfactory impact load strength, abrasion resistance, rolling fatigue resistance, etc. cannot be ensured,” have been identified as “granulation occurring during carburizing. An abnormal carburizing layer is generated by the interfacial oxidation, which causes a decrease in the hardenability of the steel near the grain boundaries, which is caused by the formation of an incompletely hardened structure. "

However, the present inventors have found that the "precipitation of carbides on austenite grain boundaries generated when quenching a carburized material" has a greater adverse effect than the adverse effect of "grain boundary oxidation during carburization". It has been found that this phenomenon causes the formation of an incompletely quenched structure in the surface layer of the part during the carburizing and quenching treatment. The "precipitation of carbides at the grain boundaries during the quenching treatment of the carburized material"
It has also been found that addition can be prevented.

[0011] The conventionally proposed B-added case hardening steel includes:
In order to prevent grain boundary oxidation during carburizing, the alloying elements are reduced, and the decrease in hardenability due to the reduction in alloying elements is reduced by B
Some have adopted the component design philosophy to be supplemented by addition.
Has a tendency to decrease as the C content in steel increases. Therefore, in such a B-added case hardening steel based on the idea that the hardenability is supplemented by B, if the C content in the surface layer is increased by carburization, the effect of improving the hardenability of B disappears in the surface layer. As a result, an incompletely quenched structure is apt to be formed in the carburized surface layer, and it has not been possible to sufficiently obtain impact load strength, wear resistance and rolling fatigue. In addition, it was considered that this tendency gradually became more prominent due to the "treatment for reducing carburizing and quenching distortion" described above.

On the other hand, it is possible to prevent the "precipitation of carbides at the grain boundaries occurring during the quenching treatment of the carburized material", thereby suppressing the formation of an incompletely quenched structure in the carburized and quenched surface layer. In case hardening steels to which B is added under the idea of よ う, it is not necessary to worry particularly about the problem of "disappearance of" the effect of improving the hardenability of B "in the portion where the amount of C is increased by carburization"
It is possible to stably improve the impact load strength, abrasion resistance and rolling fatigue resistance without being greatly influenced by the C content of the carburized portion.

However, the effect of preventing the precipitation of carbides at the grain boundaries during the quenching treatment of the carburized material by the addition of B is described.
It is essential to reduce the N content in the steel to a specific region in order to secure the content. That is, the present inventors have found that 0.0%
Even when N of about 07% by weight (usually about the lower limit of steel) is contained, the addition of B stably secures the "effect of preventing carbide precipitation at grain boundaries during quenching of carburized material". It is difficult to do so, but when the N content in the steel is reduced particularly to a region of less than 0.006% by weight, the above-mentioned effect by adding B becomes remarkable, and various strengths such as impact load strength characteristics and toughness are sufficiently improved. I was able to clarify that it would be.

It has also been found that when the N content in the steel is less than 0.006% by weight, the rolling fatigue characteristics are remarkably improved. That is, regarding the rolling fatigue characteristics, conventionally, Ti is always added at the same time in B-added steel, so Ti
It was thought that N deteriorated the rolling fatigue characteristics, but it was confirmed that N in the steel had an adverse effect on the rolling fatigue characteristics even in the steel without Ti added, and this adverse effect was caused by N-containing steel. It has been found that the amount can be substantially eliminated by controlling the amount to less than 0.006% by weight.

[0015] However, in the case of a steel which has been improved in various properties as a case hardening steel as described above, it is necessary to use the steel to produce a pipe material in a normal process of "hot pipe making → cold drawing → annealing". In the past, "unsatisfactory in machinability" was pointed out in case hardening steel, and "Carburization and quenching performed after processing steel pipe into parts" caused by abnormal growth of crystal grains. "And the resulting" decrease in impact fracture strength "and" deterioration in dimensional accuracy "tend to appear more prominently. In particular, in the case of a steel pipe as an intermediate material for manufacturing the ball cage of the drive axle coupling shown in FIG. 1, the surface punched by press punching becomes the rolling surface of the ball, so the unevenness of the drilling surface Is an important factor that has a significant effect on product life, but poor machinability has a significant adverse effect on this.

[0016] However, these disadvantages in terms of machinability and other material properties are that if the microstructure (particularly ferrite grain size and carbide grain size) of the steel pipe is too fine, it is likely to occur, It became clear that the degree of cold drawing and annealing conditions had a significant effect on the microstructure, and as a result of research, this inconvenience was improved by improving the conditions for cold working and annealing after hot pipe making of steel pipes. It turns out that it can be almost solved.

The present invention has been completed on the basis of the above findings and the like. "The high strength and high toughness case hardening steel is prepared in the following manner: C: 0.1 to 0.25% (hereinafter,% representing the component ratio is% by weight. ), Si: 0.2 to 0.4%, Mn: 0.3 to 0.9%, P: 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 0.009%, N: Less than 0.006%, or Ni: 0.3% to 4.0%, with the balance being relatively mild by constituting the composition substantially consisting of Fe. high breaking load by carburizing and quenching treatment conditions, characterized in the point "can be ensured a high surface hardness and excellent wear resistance and rolling fatigue characteristics, even" the steel, or further Ti : 0.01~ 0.3%, Nb: 0.01~ 0.3%, V: 0.01~ 0.3%, Zr: 0.01~ 0.3% of one or even comprise the balance substantially of two or more Consisting of Fe to
50% reduction in cross-sectional area after hot- rolling steel into tube material
By performing the following cold working and annealing at 650-950 ° C., it has good machinability,
A high-strength, high-toughness steel pipe for case hardening that does not cause any significant reduction in impact fracture strength or dimensional accuracy degradation even when quenching is performed.

[0018]

The reasons for limiting the chemical composition of the case hardening steel and the manufacturing conditions of the case hardening steel tube as described above in the present invention are as follows.
A description will be given together with the operation. (A) Chemical composition a) C C is a basic component that secures the hardness and strength of steel. The steel must have a hardness of Hv250 or more to secure enough strength not to be deformed during use as a carburized / quenched part. To secure the required hardness, the C content is set to 0.1% or more. There is a need. On the other hand, if the content of C exceeds 0.25%, the core toughness of the steel deteriorates. Therefore, the C content is 0.1
0.20.25%.

B) Si Conventionally, in case-hardened steel, the amount of Si is often suppressed because Si is an element that contributes to grain boundary embrittlement due to grain boundary oxidation during carburization. However, in the steel of the present invention, the effect of improving the hardenability of Si is positively used in order to secure the hardenability of the carburized layer and achieve high impact fracture strength. If the Si content is less than 0.2%, the desired hardenability of the carburized layer cannot be ensured. On the other hand, if the Si content exceeds 0.4%, the Since the "vulnerability" becomes remarkable, the Si content is set to 0.2 to 0.4%.

C) Mn Mn, like Si, is often reduced in amount to suppress grain boundary embrittlement due to grain boundary oxidation during carburization. However, when Mn is reduced, the hardenability of the carburized layer decreases. It has been found that the action is increased and it is difficult to secure the high impact fracture strength aimed at by the present invention. That is, in the steel of the present invention, if the Mn content is less than 0.3%, the desired hardenability of the carburized layer cannot be secured. It was found that "weakening of Mn near the grain boundaries during carburization by oxidation of Mn" poses no practical problem even if the Mn content exceeds 0.9%. Deterioration of punching workability and grindstone grindability becomes remarkable. Therefore, Mn
The content was determined to be 0.3-0.9%.

D) PP is a very harmful impurity element in case hardening steel since it significantly promotes "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 reducing P results in an increase in the cost of 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 allowable upper limit is set to 0.02% in consideration of the effect of B described later.

D) SS S is a component that is desired to be positively added in terms of improving machinability (machinability and punching properties) while deteriorating the toughness of steel. When the S content is less than 0.001%, the effect of improving machinability does not become noticeable,
If the content of S exceeds that, the toughness of the steel deteriorates remarkably. Therefore, in the present invention, the S content is set to 0.001 to 0.15%. However, it is advisable to keep the S content low if it is used in such a way that less machinability is required.

E) Cr Cr is an indispensable component for securing the hardenability of the steel base and for securing the carbon concentration of the carburized layer in a short time. For that purpose, a content of 0.5% or more is required. It is. However,
At the same time, Cr significantly promotes "weakening of grain boundaries due to precipitation of cementite on austenite grain boundaries during carburizing and quenching", so it is necessary to limit the content to at most 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 portion having a high C content, becomes insufficient, so that the present invention does not cause the grain boundary to become brittle. , Mo, and Ni were added to compensate for this.
For this reason, the Cr content is set to 0.5 to 0.9%, but is preferably adjusted to 0.5 to 0.65%.

F) Mo Mo is an essential component for improving the strength and toughness of the steel base and the carburized portion and ensuring the carbon concentration of the carburized layer in a short time. In particular, since the effect of improving the hardenability of Mo is hardly affected by the amount of carbon in the steel base, the effect of improving the hardenability is stably exhibited even in a carburized portion having a high carbon content. In addition, in steels in which the Cr content is reduced and the hardenability is replenished with B in order to suppress the brittleness of the grain boundaries due to carburization, the hardenability is remarkably reduced even at high carbon. The hardenability compensation of the carburized part is very important. In this case, when the Mo content is less than 0.15%, not only sufficient hardenability compensation cannot be performed, but also the amount of C that enters by short-time carburizing treatment decreases. From the viewpoint of imparting the above effects, it is preferable that the Mo content is large, but 1%
Since sufficient effects can be obtained with the addition up to, addition beyond this is considered to be uneconomical. Therefore, the Mo content is 0.
It was determined to be 15 to 1%.

G) Al Al is an effective component for deoxidizing steel and refining crystal grains. If its content is less than 0.01%, its effect is not sufficient. On the other hand, it is contained in excess of 0.1%. The Al content was determined to be 0.01 to 0.1% because the inclusions harmful to the toughness increased.

H) BB B suppresses "precipitation of carbides (Cr carbides and the like) on the austenite grain boundaries generated when quenching the carburized material", whereby the incomplete quenching structure of the carburized portion, It is an indispensable component for preventing grain boundary embrittlement and ensuring sufficient impact load strength, wear resistance, rolling fatigue characteristics, etc. for carburized and hardened materials. Also,
In the present invention, the Cr content is limited in order to prevent "the harmful effect of Cr that carbides are precipitated on the grain boundaries during carburizing and quenching to greatly promote the brittleness of the grain boundaries".
The steel also plays a role in ensuring the hardenability of the steel core by compensating for the "reduced hardenability of the steel matrix" resulting from the reduction of the Cr content. However, if the B content is less than 0.0005%, the desired effect due to the above effect cannot be obtained.
If B is contained in excess of B, the grain boundary embrittlement due to B will occur conversely, so the B content is set to 0.0005 to 0.009%. Even if the steel contains B to suppress "precipitation of carbides at the grain boundaries during carburizing and quenching", the above-mentioned effect of B can be obtained if the N content in the steel is about 0.007% of normal steel. As described above, it cannot be secured sufficiently.

I) N As described above, the amount of N in steel is very important to make the effect of B effective. That is, the amount of N in steel is 0.006%
For the first time, the effect of preventing the precipitation of carbides at the grain boundaries at the time of quenching of the carburized material becomes remarkable when B is reduced to a range of less than not only, and sufficient impact load strength is secured, Rolling fatigue properties are also significantly improved. It is desirable that the N content in steel be as small as possible. However, in industrial production in the atmosphere, the N content is 0.001% with current steelmaking technology.
% Is extremely difficult to achieve.

J) Ni When the steel of the present invention is used for an inner race or a ball cage of a joint for a drive axle of a generally used automobile,
Even without adding Ni or Ti, Nb, V or Zr described below, the properties such as strength and toughness are sufficient, but if more severe use is expected, one or two of these elements can be used. It is effective to contain more than one species. Ni is an effective component for improving the strength and toughness of the steel base, and also greatly contributes to improving the strength and toughness of the carburized portion in cooperation with Mo. But Ni
When the content is less than 0.3%, the above effect is insufficient. On the other hand, when the content is more than 4.0%, the effect is saturated.
%.

K) Ti, Nb, V and Zr These elements have the effect of refining the crystal grains of steel and improving the toughness.
It is preferred to include one or more species. However, if the content of each of these components is less than 0.01%, the above effect is insufficient. On the other hand, if the content exceeds 0.3%, on the contrary, the toughness and rolling fatigue characteristics of the steel are deteriorated. The content of Ti, Nb, V or Zr is 0.01 to 0.3 respectively.
%.

(B) Manufacturing conditions for case hardening steel pipe using the steel of the present invention a) Degree of cold working after hot pipe making When manufacturing the steel pipe for case hardening using the steel of the present invention, ordinary heat treatment is performed. The reason why the cross-sectional area reduction rate of the cold working added to secure predetermined dimensions and dimensional accuracy after completion of the inter-pipe forming is limited to 50% or less is that when the cross-sectional area reduction rate exceeds 50%, In the subsequent carburizing heat treatment, abnormal growth of austenite crystal grains occurs, resulting in a coarse quenched structure.
Causes granulation. On the other hand, if the cold work degree exceeds 50%, the hardness of the steel pipe is significantly increased due to work hardening, so that it is difficult to soften by subsequent annealing, and the workability of the steel pipe deteriorates. Not only does the dimensional accuracy in such cases deteriorate, but the life of the working tool also decreases. For this reason, the working ratio of the cold working performed after the hot pipe making is limited to 50% or less.

B) Heating temperature for annealing The heating temperature for annealing performed after the cold working is set to 650 to 9
The reason why the temperature is set to 50 ° C. is as follows. That is, if the annealing temperature is lower than 650 ° C., the accumulation of strain in the steel base due to the cold working described above is not sufficiently released, so that not only the hardness is high and the punching workability is deteriorated, but also the subsequent carburization. The heat treatment tends to cause abnormal growth of austenite crystal grains, resulting in deterioration of impact load characteristics and rolling fatigue characteristics. on the other hand,
If the annealing temperature exceeds 950 ° C., abnormal growth of austenite crystal grains occurs during annealing, and this mixed state is further expanded by the subsequent carburizing heat treatment, so that the impact load characteristics and rolling fatigue characteristics are also deteriorated. Invite.

Next, the present invention will be described with reference to examples.

【Example】

Example 1 First, a 150 kg ingot having the chemical composition shown in Tables 1 and 2 was obtained by vacuum smelting and casting, and then subjected to hot forging and normalizing treatment. “Smooth round bar tensile test specimen with parallel part diameter: 8.0mmφ” by processing
And "notched tensile test piece test piece" shown in FIG.

[0033]

[Table 1]

[0034]

[Table 2]

Next, each of the test pieces was carburized under the conditions shown in FIG.
Tensile tests were performed on the quenched and tempered steels, and the smooth tensile strength and the notched tensile strength were measured to evaluate the breaking load. The results are shown in Tables 1 and 2. For comparison, Tables 1 and 2 also show the results of a tensile test when the test pieces prepared by machining were subjected to a carburizing, quenching, and tempering treatment under the conventional conditions shown in FIG. .

From the results shown in Tables 1 and 2,
The steels according to the present invention are all carburized under relatively mild conditions.
It can be seen that the steel has excellent breaking strength, such as exhibiting a smooth tensile strength of 120 kgf / mm ² or more after quenching and tempering, and a notch tensile strength of 130 kgf / mm ² or more.

Example 2 A disk-shaped test piece having a diameter of 60.0 mm and a thickness of 5.0 mm was prepared from each steel having the chemical composition shown in Tables 1 and 2 above, and was subjected to the conditions shown in FIG. Carburizing with
After quenching and tempering, the average surface roughness (Ra) is 0.05μ
# 60 mirror-polished finish
A thrust-type rolling fatigue test was performed at room temperature with spindle oil lubrication. In addition, the evaluation of the rolling fatigue characteristics, the test was performed by changing the surface pressure, the number of stress loads repeated until the peeling life was obtained,
A method was employed in which the surface pressure at the rolling fatigue limit was determined from the SN diagram and compared. Tables 1 and 2 also summarize these results.

As can be confirmed from the rolling fatigue test results shown in Tables 1 and 2, each of the steels according to the present invention has a hardness of 30%.
0kgf / mm ² or higher high rolling fatigue limit.
It can be seen that it has rolling fatigue characteristics clearly better than 0 to 290 kgf / mm ² .

Example 3 Table 3 was obtained by vacuum melting and casting.
After obtaining a 1-ton steel ingot having the chemical composition shown in Table 1 below, it was hot forged into a round steel piece and passed through a manufacturing process of “mandrel mill pipe → cold drawing → stress relief annealing” to produce an outer diameter of 60 mm. Inner diameter 5
A 0 mm seamless steel pipe was obtained.

[0040]

[Table 3]

Next, after cutting these steel pipes into short pipes having a length of 50 mm, a ball cage simulated test piece of a drive axle joint shown in FIG. 5 was prepared by machining, and then gas carburized under the following conditions. -Quenching and tempering were performed. Heating at 930 ° C for 4 hours at a carbon potential of 0.9 to 1.0 → Cooling in a furnace at 840 ° C → Oil quenching → Tempering at 180 ° C → Polishing.

Next, each of the ball cage simulated test pieces subjected to the above-mentioned treatment was subjected to an impact tensile test using an electrohydraulic tensile tester (capacity: 10 tons) shown in FIG. In the impact tensile test, as shown in FIG. 6, a round bar was inserted into each ball cage simulation specimen, the upper crosshead was fixed, and the lower actuator was pulled at a speed of 120 mm /. The method used was to measure the breaking load while pulling down at s. In addition,
The test was performed on three ball cage simulated test specimens, and the results (average values of the breaking strengths of the three specimens) are also shown in Table 3.

Separately, in order to evaluate the workability, a short tube having a length of 50 mm was cut from each of the seamless steel pipes, and this was developed into a flat plate of 50 mm × 188 mm and each was cut into three plates. Was prepared. And the above 3
Workability was evaluated by a method in which each set of holed test specimens was polished on both sides with a grindstone and punched by hydraulic press punching. (Evaluation was performed based on the results of three test specimens.) The drilling test conditions were as follows. Punch material: high-speed steel, punch diameter: 15.7 mm, drilling speed: 2.5 mm / s. Table 3 also shows the evaluation results of the workability by the drilling test. In addition, the determination of good or bad workability was made based on "unevenness of the drilled surface".

As can be seen from the results shown in Table 3, the steel according to the present invention has good workability, and at the same time, the boring of the drive axle joint.
When applied to a cage, it can be confirmed that it shows high impact tensile strength after carburizing and quenching.

Example 4 A 1-ton ingot having each chemical composition corresponding to the steel of the present invention shown in Table 3 was obtained by vacuum melting and casting, and then these were hot forged into round steel pieces. After hot pipe production by the mandrel mill pipe production method, "primary annealing → cold drawing → secondary annealing" was performed under the conditions shown in Table 4 to obtain a seamless steel pipe.

[0046]

[Table 4]

Next, a short pipe having a length of 50 mm was cut from each of the obtained seamless steel pipes, and was developed into a 50 mm × 188 mm flat plate to prepare a holed test piece. Then, in the same manner as in Example 3, the workability was evaluated by a method in which the above-mentioned drilling test piece was polished on both sides with a grindstone and then punched by hydraulic press punching. The drilling test conditions were the same as in Example 3 as follows. Punch material: high-speed steel, punch diameter: 15.7 mm, drilling speed: 2.5 mm / s.

Tables 5 to 7 show the evaluation results of the workability by the drilling test. From the results shown in Tables 5 to 7, it is confirmed that the steel pipe manufactured according to the method of the present invention exhibits good workability.

[0049]

[Table 5]

[0050]

[Table 6]

[0051]

[Table 7]

Example 5 A 1-ton ingot having the chemical composition of each of the steel 49 of the present invention and the conventional steel 55 shown in Table 3 above was obtained by vacuum smelting and casting. After hot forging by hot forging by the mandrel mill manufacturing method, Table 8
"Primary annealing → cold drawing → secondary annealing"
To obtain a seamless steel pipe.

[0053]

[Table 8]

[0054]

[Table 9]

Next, a short pipe having a length of 50 mm was cut from each of the obtained seamless steel pipes, and developed into a 50 mm × 188 mm flat plate to prepare a holed test piece. Then, in the same manner as in Examples 3 and 4, the workability was evaluated by a method in which the drilling test piece was polished on both sides with a grindstone and punched by hydraulic press punching. The drilling test conditions were the same as in the case of Example 3 and Example 4.

Separately, 6.0 mm × 5.0 mm as shown in FIG.
A rectangular pillar tensile test piece having a cross section parallel (a simulated column part of a window frame of a ball cage) was prepared. Then, carburizing, quenching, and annealing treatments are performed under the conditions shown in FIG. 3, and the structure of the test piece after the heat treatment is observed to investigate the presence or absence of coarse particles and to measure the tensile strength. Was.

The results of these investigations are shown in Tables 8 and 9. From the results shown in Tables 8 and 9, it is confirmed that the steel pipe manufactured according to the method of the present invention not only shows good workability but also has excellent properties after carburizing and quenching. You.

[0058]

As described above, according to the present invention, not only high strength and wear resistance can be ensured in the carburized portion, but also a sufficiently high impact breaking load can be obtained even when a notch is present on the surface of the carburized portion. Can provide high strength and high toughness case hardening steel,
The use of this steel has made it possible to accurately and stably produce "high-strength, high-toughness case-hardened steel pipes that exhibit excellent workability and excellent impact fracture strength after carburizing and quenching". A more useful effect is brought about.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an example of a bar-field type joint having a ball cage and an inner race.

FIG. 2 is an explanatory view of a notched tensile test piece.

FIG. 3 is a graph showing conditions of carburizing, quenching and tempering treatments employed in Examples.

FIG. 4 is a graph showing the conventional carburizing, quenching, and tempering conditions tested in Examples for comparison.

FIG. 5 is an explanatory view of a ball cage simulated test piece subjected to a destructive test.

FIG. 6 is an explanatory diagram of a ball cage breaking test (impact tensile test) applied in the examples.

FIG. 7 is an explanatory view of a square pillar tensile test piece created in an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Drive shaft 2 Driven shaft 3 Outer race 4 Inner race 5 Ball 6 Ball cage

Continued on the front page (72) Inventor Kazuhiko Yoshida 1905-37, Hirooka, Fukuroi-shi, Shizuoka Prefecture (72) Inventor Tatsuhiro Goto 1031 Senden-do, Iwata-shi, Shizuoka Prefecture (72) Inventor Akira Wakita 1039-1, Kamiokada, Iwata-shi, Shizuoka Prefecture ( 56) References JP-A-58-120719 (JP, A) JP-A-2-85342 (JP, A) JP-A-8-295981 (JP, A) JP-A 8-218147 (JP, A) ) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10

Claims (4)

(57) [Claims] [Claim 1] In terms of weight ratio, C: 0.1 to 0.25%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.9%, P: 0.02% or less, S: 0.001 to 0.15%, Cr: 0.5 to 0.9%, A high-strength and high-toughness case hardening steel containing Mo: 0.15 to 1%, Al: 0.01 to 0.1%, B: 0.0005 to 0.009%, and N: less than 0.006%, with the balance substantially consisting of Fe. 2. The high-strength and high-toughness case hardening steel according to claim 1, further comprising Ni: 0.3 to 4.0% by weight. 3. The steel according to claim 1 or 2 is hot-pipe-formed.
Cold working with a cross-sectional area reduction rate of 50% or less
And annealing at 650 to 950 ° C.
Manufacturing method of high strength and high toughness case hardening steel pipe . 4. The method according to claim 1, wherein the weight percentage is
In addition to the components, a steel tube containing one or more of Ti: 0.01 to 0.3%, Nb: 0.01 to 0.3%, V: 0.01 to 0.3%, Zr: 0.01 to 0.3% After cold working to reduce the cross-sectional area to 50% or less,
A method for producing a high-strength and high-toughness case-hardening steel pipe, comprising annealing at 50 to 950 ° C.