JP2009102684A - Steel for ball cage of cvj - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a ball cage of a CVJ, which shows the sufficient useful life even when being used for a CVJ that has been miniaturized and lightened, and even though being applied to the ball cage on which a bigger load stress than a conventional one works. <P>SOLUTION: The steel for the ball cage of a CVJ comprises, by mass%, 0.1 to 0.25% C, 0.05 to 0.7% Si, 0.05 to 1.2% Mn, 0.3 to 1.2% Cr, 0.005% or less S, 0.03% or less P, 0.02% or less N, 0.01% or less O (oxygen), 0.005 to 0.1% sol. Al and the balance Fe with impurities. The steel may further contain one or more elements among Mo, Ni, Ti, V and Zr in place of a part of Fe. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to steel used for a ball cage component of a constant velocity joint (CVJ) such as an automobile.

  As shown in FIG. 1, a constant velocity joint (hereinafter abbreviated as CVJ) that transmits power from a driving axle of an automobile to a driven axle includes an inner race (inner ring) 3 fixed to the driving shaft 1 and a driven shaft. In this structure, a plurality of balls 5 are inserted between an outer race (outer ring) 4 fixed to 2 and these balls are held by a ball cage 6. Since the ball cage 6 and the inner race 3 are intended to transmit a very large rotational force in terms of mechanism, high strength, toughness, and fatigue strength are required.

  Accordingly, case hardening steel to which high surface hardness and rolling fatigue are imparted by carburizing and quenching is applied to these parts. Conventionally, Cr-based, Cr-Mo such as JIS SCr415, SCM415, SNC415, SNCM415. , Ni—Cr and Ni—Cr—Mo alloy steels for machine structural use have been used as case hardening steels.

  However, recently, there is a large demand for reduction in size and weight with respect to the CVJ, and the stress acting on the ball cage is larger than in the past. In conventional case-hardened steel, since static strength, toughness and fatigue strength are not sufficient, for example, the following Patent Documents 1 to 5 propose a technique for solving the above-described problems by devising an alloy composition. Yes.

  In Patent Document 1, a steel in which Cr—Mo amount is optimized and Nb or Ti is added is added, and in Patent Document 2, Nb or Ti is added while Cr—Mo amount is optimized, and B and Steels with added Ni are each disclosed. Patent Documents 3 and 4 disclose steels in which the amount of Cr—Mo is optimized and B is added. Patent Document 5 discloses steel in which B is added to Cr steel.

JP-A-1-39351 JP-A-5-117806 JP-A-9-53150 Japanese Patent Laid-Open No. 9-53169 JP 2005-105379 A

  However, all the case-hardened steels proposed in these patent documents have a problem that the alloy cost is high and manufacturability is lowered as compared with steels defined in JIS. In addition, there is a problem that the lifetime is reduced compared to the conventional one.

  An object of the present invention is to provide a steel for a CVJ ball cage that can provide a sufficient service life even when applied to a ball cage in which a larger load stress is applied to a CVJ that is smaller and lighter than the conventional one. There is to do.

  The inventors of the present invention investigated in detail a case in which a CVJ that is smaller and lighter than the conventional one, and a ball cage that is subjected to a larger load stress than the conventional one, cannot be used in a short period of time. We analyzed the factors that affect the life span and examined measures to extend its lifespan. As a result, the following findings (a) to (h) were obtained.

  (a) When the joint cut angle of the CVJ is increased during high-speed rotation, the stress applied to the ball cage exceeds the material strength, and the ball cage breaks. Therefore, it is important to improve the material strength.

  (b) A CVJ ball cage was made using a high-strength material and a durability test was conducted. Even when a high-strength material was used, when the joint break angle of the CVJ was increased during high-speed rotation with an actual CVJ. In addition, the failure of the ball cage could not be suppressed, and the correspondence between whether the ball cage would break and the simple material strength did not correlate.

  (c) Although it was investigated whether the fracture of the ball cage was suppressed by increasing the surface hardness or increasing the toughness, none of them led to an improvement in the fracture strength, and in many cases, it decreased.

  (d) Based on the investigation results of the correspondence between the CVJ joint break angle increasing during high-speed rotation in an actual CVJ and the ball cage breaking, the ball cage breakage is caused by the corner of the window for fixing the ball. It was found that it occurred from the stress concentration part of the part. Therefore, it was found that the strength by the notched tensile test shows a good correspondence.

  (e) Therefore, it has been found that the material design index for improving the breaking strength when the case-hardened steel is used for a member having a characteristic shape called a ball cage is to increase the notch tensile strength. In other words, when the ball cage is reduced in size and weight, as a result, the stress to be applied increases, but in that case, it is necessary to improve the tensile strength with notches when it is desired to secure a breaking strength equal to or higher than the conventional one. I understood.

  (f) Next, a study was conducted to improve the notched tensile strength of case-hardened steel for ball cages. As a result, as will be described later, it was found that the notched tensile strength particularly depends greatly on the S content. On the other hand, the tensile strength did not depend on the S content. Case-hardened steel is used in various applications such as gears, but is often cut. However, from the viewpoint of ensuring machinability, S is usually actively added. However, when the S content in the steel is reduced by changing the concept this time, the tensile strength with notches is significantly improved. I found out. That is, in order to improve the notch tensile strength, it has become clear that reducing the S content is the only method even if some reduction in machinability is expected.

  (g) The improvement in notched tensile strength when S is reduced is considered to be due to the number of elongated MnS inclusions by observation of the fracture surface after the tensile test. In a notched tensile test, stress concentrates in the notch part during the test, and if MnS stretched directly under the stress concentrated part exists, it becomes the starting point of fracture, and the carburized part causes brittle fracture and is not carburized. Since the toughness of the central portion is also reduced by the elongated MnS, once a crack is generated at an early stage, the crack growth is not stopped and the fracture occurs with low stress. In a normal tensile test, even if elongated MnS is present, there is no notch where stress is concentrated, so that brittle fracture of the carburized portion is unlikely to occur, and the fracture stress is considered to increase. Therefore, when the S content is reduced and the amount of MnS inclusions is reduced, the normal tensile strength hardly increases, but the notched tensile strength is greatly improved. Since the breaking strength during use of the ball cage, which is an application of the present invention, is governed by this tensile strength with notches, reducing the amount of S provides a highly reliable CVJ that is difficult to break down even when downsized. It becomes possible. This effect is a technique suitable for a part with a specific shape called a ball cage, and the effect of reducing the amount of S can be almost expected for other applications controlled by normal tensile strength and fatigue strength. However, it is not a technique that can be applied because it reduces the machinability.

  (h) FIGS. 2 to 17 are plots of the S content on the X axis and the mechanical properties on the Y axis for steels having various chemical compositions used in the examples described later. As the mechanical properties, normal tensile strength (smooth tensile strength) and notched tensile strength were used as indicators.

  2 and 3 are graphs in which the smooth and notched tensile strengths after carburizing of steels (steel Nos. 1 to 8) in which the S content is changed based on the SCr415 system are arranged. As shown in FIG. 2, even if the S content is reduced, the smooth tensile strength hardly changes. However, as shown in FIG. 3, it can be seen that the notched tensile strength varies greatly depending on the S content, and that the strength increases significantly when the S content in the steel is reduced.

  Next, FIGS. 4 and 5 are steels (steel Nos. 9 to 16) in which the S content is changed based on the SCM415 system, and FIGS. 6 and 7 are steels in which the S content is changed based on the SCr420 system. 8 and 9 are for steel (steel No. 25-32) in which the S content is changed based on the SCM420 system, and FIGS. 10 and 11 are S-containing based on the SAE8617 system. For steels with changed amounts (steel Nos. 33 to 41), FIGS. 12 and 13 show steels with changed S content (steel Nos. 42 to 49) based on steels with Ti added to the SCr415 system. FIGS. 14 and 15 show a steel (steel No. 50 to 57) in which the S content is changed based on a steel in which Nb is added to the SCM420 system, and FIGS. 16 and 17 show that Ti and Nb are added to the SAE8617 system. S content is changed based on steel For the allowed steel (steel Nanba58～65), respectively, it is a graph organizing smooth and notch tensile strength after carburizing. Similar to FIGS. 2 and 3, the smooth tensile strength hardly changes even when the S content is reduced, but it can be seen that the notched tensile strength increases significantly when the S content in the steel is reduced.

  The present invention has been completed based on the above findings, and the gist thereof is the steel for CVJ ball cage according to the following (1) to (4).

  (1) By mass%, C: 0.1-0.25%, Si: 0.05-0.7%, Mn: 0.05-1.2%, Cr: 0.3-1.2% , S: 0.005% or less, P: 0.03% or less, N: 0.02% or less, O (oxygen): 0.01% or less and sol.Al: 0.005 to 0.1% And CVJ ball cage steel, the balance being Fe and impurities.

  (2) The steel for a CVJ ball cage according to the above (1), further containing Mo: 0.7% or less instead of a part of Fe in mass%.

  (3) The steel for a CVJ ball cage according to the above (1) or (2), wherein the steel further contains Ni: 1% or less instead of a part of Fe in mass%.

  (4) By mass%, in place of part of Fe, Ti: 0.3% or less, Nb: 0.3% or less, V: 0.3% or less, and Zr: 0.3% or less The steel for a CVJ ball cage according to any one of (1) to (3) above, which contains one or more kinds.

  According to the present invention, a steel for a CVJ ball cage that can provide a sufficient service life even when applied to a ball cage that is subjected to a larger load stress than a conventional CVJ that is smaller and lighter than the conventional one. There is to do.

  The reason for limiting the chemical composition constituting the CVJ ball cage steel according to the present invention will be described below. In addition, “%” of the content of each element represents “mass%”.

C: 0.1-0.25%
Since C is a basic element that determines the hardness or strength of steel, it is an element to be added according to the required strength. In order to ensure the minimum strength as a ball cage, it is necessary to contain 0.10% or more. On the other hand, if the C content is too high, the toughness decreases, so the upper limit was made 0.25%.

Si: 0.05-0.7%
Si is effective in deoxidation of steel, and is an element that enhances hardenability and strengthens steel, and needs to be contained by 0.05% or more. However, if the Si content exceeds 0.7%, grain boundary oxidation is promoted during carburizing and quenching, and the notched tensile strength is reduced, so 0.7% was made the upper limit. From the viewpoint of notched tensile strength, the Si content is preferably low. Although there is a balance with the deoxidation level, 0.5% or less is preferable and 0.3% or less is more preferable.

Mn: 0.05-1.2%
Mn, like Si, is effective in deoxidizing steel, and is an element that enhances hardenability and strengthens steel, and it is necessary to contain 0.05% or more. However, if the Mn content exceeds 1.2%, grain boundary oxidation is promoted during carburizing and quenching, and the notched tensile strength is reduced. From the viewpoint of notched tensile strength, the Mn content is preferably low. Although there is a balance with the deoxidation level, 1.0% or less is preferable.

Cr: 0.3-1.2%
Cr has a large affinity with C, and is an element necessary for advancing carburization in a short time. It is also an important element for ensuring hardenability during carburizing and quenching. In order to acquire the effect, it is necessary to contain 0.3% or more. On the other hand, if the Cr content exceeds 1.2%, Cr carbide precipitates and the toughness is lowered. Therefore, the Cr content is set to 0.3 to 1.2%.

S: 0.005% or less Keeping the S content low is the most important feature of the steel according to the present invention. If the content exceeds 0.005%, the necessary notched tensile strength is reduced. It cannot be secured. Therefore, the content of S is set to 0.005% or less. More preferably, it is 0.003% or less, and further preferably, 0.0015% or less. The lower the value, the higher the notched tensile strength. However, since there is a problem that the machinability is lowered with the reduction of S and the steel making cost increases when S is reduced, the preferable lower limit of S is more than 0.0009% when considering these. Amount.

P: 0.03% or less P is contained as an impurity in the steel. P is an element that reduces the bond strength at the grain boundary and lowers the toughness, and is preferably as small as possible. 0.03% or less. If it is 0.03% or less, sufficient toughness as a ball cage can be obtained.

N: 0.02% or less N is contained as an impurity in the steel. If the N content exceeds 0.02%, the toughness decreases, so the content was set to 0.02% or less. A more preferable upper limit is 0.01%, and a further preferable upper limit is 0.007%.

O (oxygen): 0.01% or less O is contained as an impurity in the steel. If the O content exceeds 0.01%, the toughness decreases, so the content was set to 0.01% or less.
sol.Al: 0.005 to 0.1%
Al is an element necessary for deoxidation of steel, and should be contained by 0.005% or more. On the other hand, if it exceeds 0.1%, cluster-like nonmetallic inclusions are generated and the toughness is lowered, so the content was made 0.005% to 0.1%.

  The high-strength case-hardened steel pipe for CVJ ball cage of the present invention may further contain one or more of Mo, Ni, Ti, Nb, V and Zr in addition to the above components. In this case, it is the same that P, N, and O need to be kept low as described above.

Mo: 0.7% or less Mo has an effect of improving the hardenability of the carburized portion and improving the strength of the steel. Therefore, when this effect is desired to be expressed, Mo can be contained. However, even if the content exceeds 0.7%, the effect is saturated and the cost only increases. Therefore, even when Mo is contained, the upper limit of the content is set to 0.7%. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Ni: 1% or less Ni has the effect of improving the strength and toughness of steel. Unlike Cr and Mo, it is an element that has little adverse effect even when added in a large amount, and is therefore an effective component for designing a particularly strong material. Therefore, when this effect is desired to be expressed, Ni can be contained. However, if the content exceeds 1%, the effect is saturated and the cost only increases. Therefore, even when Ni is contained, the upper limit of the content is set to 1%. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Ti, Nb, V, and Zr: each 0.3% or less These elements have the effect of suppressing coarsening of crystal grains during carburizing and quenching and improving toughness. Therefore, when this effect is desired to be expressed, one or more of Ti, Nb, V and Zr can be contained. However, if each content exceeds 0.3%, the toughness is rather lowered. Therefore, even when at least one of Ti, Nb, V and Zr is contained, the upper limit of the content is 0. .. 3% or less In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.003% or more.

  The CVJ ball cage steel according to the present invention is used as a CVJ component after being carburized and tempered after being processed into a ball cage shape.

  After melting steel materials having chemical compositions shown in Tables 1 and 2 and performing hot forging and hot rolling to produce plate materials of 25 mm thickness, 120 mm width and 550 mm length, and normalizing at 920 ° C. And tempering at 720 ° C. for softening. Next, after removing the surface scale, it was cold worked to a thickness of 16 mm, and tempered again at 720 ° C. to be softened.

  A smooth tensile test piece having a parallel part diameter of 8 mm and a notched tensile test piece having a parallel part diameter of 8 mm and a depth of 1 mm from the above material are perpendicular to the rolling direction. Each sample was collected from the T direction. And the carburizing process and heat processing were implemented to the test piece on the following conditions.

  First, carburization was performed at a carbon potential of 0.9% and 930 ° C. for 130 minutes, followed by carburization at a carbon potential of 0.8% and 870 ° C. for 30 minutes, followed by oil quenching. Subsequently, it tempered at 160 degreeC for 2 hours, measured the smooth tensile strength and the notched tensile strength, and evaluated the mechanical property. Tables 1 and 2 show the measurement results of the smooth tensile strength and the notched tensile strength.

  2 to 17, using the data in Table 2, for each steel type, the S content is plotted on the X axis, and the mechanical properties (smooth tensile strength, notched tensile strength) are plotted on the Y axis. As described above, it is a graph. It can be seen that even if the S content is reduced, the smooth tensile strength hardly changes, but the notched tensile strength increases significantly when the S content in the steel is reduced.

  When the CVJ ball cage steel according to the present invention is applied to a ball cage in which a larger load stress acts in a CVJ reduced in size and weight, a sufficient service life can be obtained. Therefore, it greatly contributes to the weight reduction and fuel consumption improvement of automobiles.

It is sectional drawing which shows an example of a constant velocity joint. The relationship between the S content and the smooth tensile strength in the SCr415 steel type is shown. The relationship between the S content and the notched tensile strength in the SCr415 steel type is shown. The relationship between the S content and the smooth tensile strength in the SCM415 type steel is shown. The relationship between the S content and the notched tensile strength in SCM415 steel types is shown. The relationship between the S content and the smooth tensile strength in the SCr420 steel type is shown. The relationship between the S content and the notched tensile strength in the SCr420 steel grade is shown. The relationship between the S content and the smooth tensile strength in the SCM420 steel type is shown. The relationship between the S content and the notched tensile strength in the SCM420 steel type is shown. The relationship between the S content and the smooth tensile strength in the SAE8617 steel type is shown. The relationship between the S content and the notched tensile strength in the SAE8617 steel grade is shown. The relationship between the S content and the smooth tensile strength in the steel type in which Ti is added to the SCr415 system is shown. The relationship between the S content and the notched tensile strength in the steel type in which Ti is added to the SCr415 system is shown. The relationship between the S content and the smooth tensile strength in the steel type in which Nb is added to the SCM420 system is shown. The relationship between the S content and the notched tensile strength in the steel type in which Nb is added to the SCM420 system is shown. The relationship between the S content and the smooth tensile strength in a steel type in which Ti and Nb are added to the SAE8617 system is shown. The relationship between the S content and the notched tensile strength in steel types obtained by adding Ti and Nb to the SAE8617 system is shown.

Explanation of symbols

1 Driving shaft 2 Driven shaft 3 Inner race 4 Outer race 5 Ball 6 Ball cage

Claims (4)

  In mass%, C: 0.1-0.25%, Si: 0.05-0.7%, Mn: 0.05-1.2%, Cr: 0.3-1.2%, S: 0.005% or less, P: 0.03% or less, N: 0.02% or less, O (oxygen): 0.01% or less and sol.Al: 0.005-0.1%, the balance Is a steel for a CVJ ball cage characterized by comprising Fe and impurities.   The steel for a CVJ ball cage according to claim 1, wherein the steel further contains Mo: 0.7% or less in mass% instead of part of Fe.   The steel for a CVJ ball cage according to claim 1 or 2, further comprising Ni: 1% or less instead of a part of Fe in mass%.   1% or more of Ti: 0.3% or less, Nb: 0.3% or less, V: 0.3% or less and Zr: 0.3% or less The steel for a CVJ ball cage according to any one of claims 1 to 3, characterized by comprising:

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