WO2018180342A1

WO2018180342A1 - Shaft member

Info

Publication number: WO2018180342A1
Application number: PCT/JP2018/009027
Authority: WO
Inventors: 健宇佐美; 浩行水野; 康弘福田
Original assignee: 愛知製鋼株式会社
Priority date: 2017-03-30
Filing date: 2018-03-08
Publication date: 2018-10-04

Abstract

A shaft member includes 0.20 – 0.45% C, 0.03 – 1.50% Si, 0.070% or less S, 0.50 – 2.00% Mn, 0.30 – 2.50% Cr, 0.010 – 0.100% Al, 0.0070 – 0.0200% N, 0 – 0.30% V, 0 – 0.50% Mo, with the remainder formed from Fe and unavoidable impurities, and satisfies formula 1: 19[C] + 2.3[Mn] + 1.4[Cr] + 13[Mo] + 25[V] > 11. The shaft member is provided with a first hardened layer for which the concentration of C is 0.50% or greater and the hardness is 700 HV or greater, a second hardened layer for which the hardness is 450 HV or greater, and an inner layer for which the hardness is 300 HV or greater. The thickness of the first hardened layer is 0.20 mm or greater, and the inside edge of the second hardened layer is positioned at a depth of 0.1 parts or greater of the diameter of the shaft member from the outermost surface. The shaft member can include more than 0% to 0.10% or less Ti and more than 0% to 0.0050% or less B as necessary, thereby improving fatigue strength.

Description

Shaft member

The present invention relates to a shaft member used for a transmission, for example.

In recent years, there is an increasing need for miniaturization and weight reduction of transmissions, which are a kind of automobile parts. In order to satisfy the demand, it is necessary to reduce the diameter of the shaft member applied to the transmission by increasing the strength. Since a torsional torque is applied to the shaft member that transmits the rotational force generated by the engine, increasing its strength means improving torsional strength. Specific requirements for improving torsional strength include ensuring a high proportional limit to prevent plastic deformation due to high torque that is applied shockfully, and repeated with relatively low torque. There are two ways to prevent cracking due to fatigue, that is, to improve torsional fatigue strength. The torsional stress generated when the torsional torque is applied becomes higher as the distance from the axial center increases, becomes highest on the surface, and becomes lower as the position is closer to the axial center. Therefore, in order to achieve high strength, it is important that the necessary strength of each part is appropriately taken into account in consideration of the distance from the axis.

Conventionally, for shaft members that require high torsional strength, it is appropriate to use, for example, carburized steel such as SCM420 and carburized steel, and high-frequency hardened medium carbon steel such as SCr440. It is known. However, when these conventional shaft members are applied to parts with extremely high levels of demand, the former shaft member has a harder hardness at the innermost surface than the carburized layer, although the outermost carburized layer is hardened. . Immediately below the carburized layer where the effect of improving the strength by carburization is not achieved, a considerably higher stress is applied, although the load stress is slightly lower than the surface. For this reason, the part directly under the carburized layer cannot withstand the stress and becomes the weakest part, causing a problem that plastic deformation first occurs and the proportional limit is exceeded. Further, the latter shaft member has a problem that the surface has a higher hardness than the carburizing treatment and cannot withstand a high stress load on the surface. For these reasons, the conventional shaft member has a problem that the expected performance cannot be obtained, it is difficult to increase the torsional strength, and it is difficult to reduce the diameter. Further, the shaft member often has to be provided with oil holes for the purpose of lubrication or the like. For example, in the case of a shaft part having a hole or the like as shown in Patent Document 1, there is a portion where stress is concentrated. For this reason, in the case of a shaft component having a hole or the like, it is necessary to consider that it is easy to break from a stress concentration site, and there is a problem that it is more difficult to reduce the diameter than a solid case.

On the other hand, Patent Document 2 discloses a manufacturing method in which after induction hardening is performed at least once after gas carburizing and quenching, this is tempered, and thereafter an abnormal carburizing layer is removed. According to this manufacturing method, it is supposed that it has excellent impact strength and bending strength by the effect of refining crystal grains by repeatedly performing induction hardening and the effect by removing the carburized abnormal layer.

Japanese Patent Laid-Open No. 2000-2229 JP 2009-299165 A

However, the manufacturing method described in Patent Document 2 requires that the abnormal carburizing layer be removed, and ideally repeats induction hardening, which takes time for processing and is disadvantageous in manufacturing cost. Can be said. Further, in Patent Document 2, the strength of the carburized layer is increased by refining the crystal grains of the carburized layer, but the internal characteristics of the carburized layer are not particularly mentioned and are unknown. Yes, it remains unclear whether a higher proportional limit and excellent fatigue strength can be obtained when the internal state from the surface to the shaft center is adjusted.

In order to improve the torsional strength and reduce the diameter of the shaft member, it is essential not only to increase the hardness (strength) of the outermost surface but also to increase the proportional limit of the entire shaft. For that purpose, it is necessary to define the range of chemical components effective for improving the overall proportional limit and clarify how the hardness profile from the outermost surface to the inside should be set.

The present invention has been made in view of such a background, and an object of the present invention is to provide a shaft member that can increase the proportional limit more reliably than ever and can be reduced in diameter.

The first aspect of the present invention is, in mass%, C: 0.20 to 0.45%, Si: 0.03 to 1.50%, S: more than 0% and 0.070% or less, Mn: 0 50 to 2.00%, Cr: 0.30 to 2.50%, Al: 0.010 to 0.100%, N: 0.0070 to 0.0200%, V: 0 to 0.30%, Mo: 0 to 0.50% contained, with the balance being Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 19 [C] +2.3 [Mn] +1.4 [Cr] +13 [Mo] +25 [V]> 11
(However, [C], [Mn], [Cr], [Mo] and [V] in the formulas indicate the contents (mass%) of C, Mn, Cr, Mo and V, respectively).
A first hardened layer located on the outermost surface and having a C concentration of 0.50% or more and a hardness of 700HV or more;
A second hardened layer located inside the first hardened layer and having a hardness of 450 HV or more;
An inner layer located on the inner side of the second hardened layer and having a hardness of 300 HV or more,
The thickness of the first hardened layer is 0.20 mm or more,
The inner end of the second hardened layer is at a depth position of 0.1 times or more the diameter of the shaft member from the outermost surface.
Located on the shaft member.

The second aspect of the present invention is, in mass%, C: 0.20 to 0.45%, Si: 0.03 to 1.50%, S: more than 0% and 0.070% or less, Mn: 0 50 to 2.00%, Cr: 0.30 to 2.50%, Al: 0.010 to 0.100%, N: more than 0% and 0.0200% or less, V: 0 to 0.30%, Mo: 0 to 0.50%, B: 0% to 0.0050% or less, Ti: 0% to 0.10% or less, with the balance being Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 19 [C] +2.3 [Mn] +1.4 [Cr] +13 [Mo] +25 [V]> 11
(However, [C], [Mn], [Cr], [Mo] and [V] in the formulas indicate the contents (mass%) of C, Mn, Cr, Mo and V, respectively).
A first hardened layer located on the outermost surface and having a C concentration of 0.50% or more and a hardness of 700HV or more;
A second hardened layer located inside the first hardened layer and having a hardness of 450 HV or more;
An inner layer located on the inner side of the second hardened layer and having a hardness of 300 HV or more,
The thickness of the first hardened layer is 0.20 mm or more,
The inner end of the second hardened layer is at a depth position of 0.1 times or more the diameter of the shaft member from the outermost surface.
Located on the shaft member.

The shaft member has a chemical component composition such that the content range of each element is in the specific range and further satisfies Formula 1, and the first cured layer and the second cured layer are sequentially formed from the outer surface. And the inner layer. And while the thickness of the said 1st hardening layer is 0.20 mm or more, the said 2nd hardening layer located under it exists to the depth of 0.1 times or more of the diameter of the said shaft member, and also under it In addition, there is an inner layer cured to a predetermined hardness or higher. By having all these requirements, the shaft member can withstand the load stress applied to each part from the shaft center to the surface, such as the hardness profile extending from the outermost surface to the inner part where there is no sudden hardness reduction part. It will be in a state suitable for. For this reason, the shaft member can greatly increase the proportional limit when torsional stress is applied compared to the conventional case, and can improve torsional strength. In addition, as a result of appropriate strengthening for each part according to the distance from the axis, it is possible to obtain a high strength improvement effect even for a shaft member in which a stress concentration part such as a corner such as an oil hole exists. it can. As a result, it is possible to meet the demand for reducing the diameter of the shaft member.

Explanatory drawing which shows the shaft member produced in Example 1 of an embodiment. FIG. 2 is a cross-sectional view taken along line AA in FIG. 1. Explanatory drawing which shows the hardness profile of Example 1 and the prior art example 1. FIG. Explanatory drawing which shows the relationship between the 2nd hardened layer depth and proportional limit in Embodiment Example 2. FIG. Explanatory drawing which shows the relationship between the distance from the surface and hardness in Example 3 of an embodiment. Explanatory drawing which shows the relationship between the presence or absence of V addition and the temper softening resistance in Embodiment Example 4. Explanatory drawing which shows the relationship between the presence or absence of V addition and the temper softening resistance in Embodiment Example 4.

The reason for limitation of the chemical component of the shaft member of the first aspect will be described.
C: 0.20 to 0.45%,
C (carbon) is an element necessary for securing internal hardness, and is contained in an amount of 0.20% or more. On the other hand, if C is added excessively, machinability, cold forgeability and toughness are reduced, so the upper limit of the C content is set to 0.45%.

Si: 0.03-1.50%,
Si (silicon) is an indispensable element for deoxidation, and is contained by 0.03% or more. On the other hand, if Si is added excessively, the toughness and hot workability are reduced, so the upper limit of the Si content is 1.50%.

S: more than 0% and 0.070% or less,
S (sulfur) is an element inevitably contained, and is contained in an amount exceeding 0%. However, when it is contained in a large amount, the fatigue strength may be reduced. Therefore, the upper limit of the S content is set to 0.070%.

Mn: 0.50 to 2.00%,
Mn (manganese) is an important element for securing internal hardness. Therefore, Mn is contained at 0.50% or more. On the other hand, if Mn is added excessively, the machinability is lowered, so the upper limit of the Mn content is 2.00%.

Cr: 0.30 to 2.50%,
Cr (chromium) is effective for securing the internal hardness, and 0.30% or more is contained in order to obtain the effect. On the other hand, even if Cr is added excessively, the effect is saturated and the cost is increased, so the upper limit of the Cr content is 2.50%.

Al: 0.010 to 0.100%,
Al (aluminum) is an effective element for suppressing coarsening of crystal grains after carburization, and is contained in an amount of 0.010% or more in order to obtain the effect. On the other hand, even if Al is added excessively, the above effect is saturated, hard alumina inclusions increase, and fatigue strength may be lowered. Therefore, the upper limit of Al content is set to 0.100%. .

N: 0.0070 to 0.0200%,
N (nitrogen) is also an effective element for obtaining the effect of suppressing the coarsening of crystal grains after carburization in the same manner as Al, and 0.0070% or more is contained for obtaining the effect. On the other hand, since the above effect may be saturated even if N is added excessively, the upper limit of the N content is set to 0.0200%.

V: 0 to 0.30% (including 0%)
V (vanadium) is an optional additive element, but it can be expected to have a crystal grain refining effect and a temper softening resistance improving effect. Therefore, V (vanadium) is preferably added when it is desired to enhance these effects. In particular, V greatly increases the hardness of the tempered zone by increasing the temper softening resistance by precipitation strengthening, that is, the position slightly inside the maximum depth where the temperature rises above the transformation point during high-frequency heating. it can. Since the temperature does not rise above the transformation point in the burned zone, the structure obtained by carburizing and quenching will be tempered by high-frequency heating and the hardness will be reduced, but if V is added, precipitation strengthening Since the hardness after high-frequency heating is greatly improved, the fatigue strength can be greatly improved. When adding V, it is preferable to set it as 0.20% or more, and, thereby, the above-mentioned effect can be acquired reliably. On the other hand, even if V is added excessively, the above effect is saturated and the cost may be increased. Therefore, the upper limit when V is contained is 0.30%.

Mo: 0 to 0.50% (including 0%),
Although Mo (molybdenum) is an optional additive element, it is effective to improve hardenability, so it is preferable to add it as necessary. On the other hand, if Mo is added excessively, the cost may increase and the workability may decrease, so the upper limit of the Mo content is set to 0.50%.

In addition, the optional elements V and Mo are inevitably contained as impurities when scrap is used as a raw material and manufactured in an electric furnace, and there are trace amounts of detection in actual analysis results. There are many. Usually, V may be inevitably contained as an impurity even if not actively added up to 0.01% and Mo up to about 0.06%.

Next, Formula 1: 19 [C] +2.3 [Mn] +1.4 [Cr] +13 [Mo] +25 [V]> 11 (where [C], [Mn], [Cr] in the formula, [Mo] and [V] indicate the contents (mass%) of C, Mn, Cr, Mo, and V, respectively, on the premise of the above-mentioned chemical components, and in the inner layer, 300 HV or more. It is necessary to ensure hardness and has been derived from many experiments. Therefore, it is important for the improvement of the torsional strength of the shaft member to further satisfy the formula 1 in consideration of the basic chemical component range described above. If this formula 1 is not satisfied at least, it is difficult to ensure sufficient hardness in the inner layer.

Next, the reason for limitation of the chemical component of the shaft member of the second aspect will be described.
The reasons for limitation other than N, B, and Ti in the second aspect are basically the same as those in the first aspect.

N: more than 0% and 0.0200% or less,
In the first aspect, N, as in the case of Al, ensures 0.0070% or more as an essential element in order to obtain the effect of suppressing grain coarsening after carburizing. However, in this second aspect, as will be described later, the effect of improving fatigue strength by improving grain boundary strength by using Ti and B as essential elements exceeds the effect of improving strength by crystal grain refinement by containing N, and N Since the lower content rate can improve the effect of addition of B, the lower limit of the N content rate is not particularly set. However, considering that N is usually inevitably contained, the lower limit is set to exceed 0%. The upper limit is as described above.

B (boron): more than 0% and 0.0050% or less,
B is an element contributing to the improvement of the grain boundary strength in the metal structure, and the fatigue strength is further improved by the effect. And in order to obtain the above-mentioned effect reliably, 0.0005% or more of content is necessary, but even if it is contained in a very small amount, it is possible to obtain the same or higher characteristics as compared with Ti and B non-added steel. So, no lower limit is set. On the other hand, since the above effect is saturated even if B is added excessively, the upper limit of the B content is set to 0.0050%. Note that the effect of addition of B is very large, and as shown in the examples described later, even when the test piece has a stress concentration site such as an oil hole, the effect of improving the corner strength by the effect of addition of B is the starting point of fracture. Has been confirmed to move from the hole corners to the fired zone.

Ti (titanium): more than 0% and 0.10% or less,
In order to obtain the above-described effect of improving the grain boundary strength due to B, it is necessary to suppress consumption of B due to bonding with N in steel and precipitation as BN. This is because when B is used, the effect of improving the grain boundary strength of B cannot be obtained. As a countermeasure, it is effective to positively add Ti, preferentially precipitate TiN, and secure an amount of solute B that contributes to improvement of B grain boundary strength. In order to obtain this effect, it is preferable to add 0.01% or more of Ti while adding 0.0005% or more of B. However, even if it is added in a small amount, the same or better characteristics can be obtained in comparison with Ti and B non-added steel, so no specific lower limit value is set. On the other hand, when Ti is excessively added, inclusions such as TiN are excessively generated, and the fatigue strength may be lowered. Therefore, the upper limit of the Ti content is set to 0.10%.

As described above, in the second aspect, it is essential to add Ti and B in a positive manner, and the fatigue strength of the shaft member is further improved by improving the grain boundary strength, while the lower limit value of N is relaxed. ing.

Next, the shaft member includes the first hardened layer, the second hardened layer, and the inner layer sequentially from the outermost surface. As will be described later, the shaft member is prepared by performing induction hardening after carburizing and quenching immediately thereafter, and the first hardened layer is a layer cured by carburizing and induction hardening. , And a C concentration of 0.50% or more and a hardness of 700 HV or more can be defined as a layer located on the outermost surface. The thickness can be grasped by grasping a profile of C concentration and hardness, and specifying a range where the C concentration is 0.50% or more and the hardness is 700 HV or more.

It is necessary to secure 0.20 mm or more for the first hardened layer. This part has a thickness that is necessary to obtain a hard martensite structure by quenching after the required amount of C has penetrated and diffused into the surface by carburization, and then is again quenched by induction quenching and then rapidly cooled. The securing can be substantially realized by securing a range where the C concentration is 0.50% or more from the surface by 0.20 mm or more during the carburizing process. Note that if the thickness of the first hardened layer, that is, the effective hardening depth by C diffusion by carburizing treatment is increased, the strength of the surface layer can be further increased, but the carburizing treatment time required for that purpose becomes longer. Since the productivity is lowered, it is preferable to adjust the thickness to about 0.80 mm or less, which is a realistic thickness considering that point. The upper limit value of the C concentration of the first hardened layer is preferably 0.95% because it is necessary to prevent cementite precipitation and suppress embrittlement of the surface layer. In addition, the carburizing process may be a plurality of processing methods such as gas carburizing, vacuum carburizing, etc., but in the present invention, as long as carbon can penetrate into the surface, the necessary performance can be obtained, The method is not particularly limited.

The second hardened layer is provided inside the first hardened layer. A 2nd hardened layer is a layer which is located inside the 1st hardened layer in which the effect of carburizing has reached, was hardened by induction hardening, and is a layer whose hardness is 450HV or more. Since the part heated during induction hardening is heated for a short time, the crystal grains are clearly finer compared to the part where the effect of induction heating inside is not exerted. can do. Specifically, since it can be confirmed by structure observation that the level of crystal grains is clearly different from the inner layer described later, those skilled in the art can easily determine the boundary between the second hardened layer and the inner layer described later. You can grasp the position.

The condition of being the second hardened layer does not include the C concentration. However, since the C diffusion due to the carburizing treatment is substantially smaller than the carburized layer, the C concentration is 0. The range is less than 50%. And this 2nd hardened layer needs to ensure thickness so that the deepest inner edge part may be located in the depth of 0.1 times or more of the diameter of a shaft member. In order to ensure this thickness, when induction hardening is performed, in order to heat up to austenitizing temperature from the surface in the depth direction up to 0.1 times the diameter of the shaft member to the austenitizing temperature, It is necessary to adjust the frequency, the current value or voltage value flowing through the heating coil, the moving speed of the heat-treated material relative to the heating coil, and the like.

If the second hardened layer cannot secure a thickness that exists to a depth of 0.1 or more times the diameter of the shaft member, it may be difficult to obtain a sufficient proportional limit improvement. Therefore, for example, if the diameter of the shaft member is 20 mmφ, it is necessary to provide the second hardened layer to a depth of at least 2.0 mm from the surface. The depth of the second hardened layer means not the thickness of only the second hardened layer but the depth from the surface to the boundary with the internal layer. And since the depth of a 2nd hardened layer has large influence on a proportional limit improvement, it is more preferable to set it as the depth of 2.0 mm or more from the surface irrespective of the diameter of a shaft member.

The inner layer is provided inside the second hardened layer. The inner layer is a layer having a hardness of 300 HV or more, and after being hardened by quenching after carburizing, the inner layer does not have a hardening effect by induction hardening. That is, since heating during induction hardening is performed only to a specific distance range from the surface, the region serving as the inner layer is not heated to a temperature higher than the transformation point by heating during induction hardening. Therefore, the inner layer is cured by quenching after heating at the time of carburizing, which is the entire heating. And as above-mentioned, since the part heated by the high frequency heating can be specified by structure | tissue observation after heat processing, it can be judged that the part near an axial center from the position is an internal layer.

And the said inner layer can ensure the hardness of 300HV or more by making it into the component which satisfies the said Formula 1. The maximum thickness of the inner layer is the range up to the shaft center of the shaft member or the inner peripheral surface in the case of being hollow, but at least from the outer peripheral surface to a position of 1/4 of the diameter. In the range, the hardness is preferably 300 HV or more. When torsion is applied, a greater stress is applied closer to the outer periphery, and a higher stress load is not applied near the shaft center. Therefore, if a hardness of 300 HV or more is secured from the outer peripheral surface to a quarter of the diameter, This is because a high proportional limit and fatigue strength can be obtained without necessarily limiting the hardness to the axis. In addition, since the raise of the hardness of an inner layer contributes to the improvement of a proportional limit, it is good to set it as 350 HV or more preferably.

In particular, in the vicinity of the boundary between the second hardened layer and the inner layer, that is, the position closest to the surface (in the burned area) of the inner layer, it is not heated more than the transformation point during high-frequency heating, but is considerably higher than the transformation point. Therefore, the structure obtained by quenching after carburizing is tempered, and the hardness is thereby reduced. However, if V is contained in the steel at this time, a decrease in hardness can be suppressed by precipitation strengthening, so that a great effect of improving fatigue strength can be obtained as shown in Examples described later. This is because the burned-in portion is closest to the surface in the inner layer and becomes a portion having a high stress, and this may be the weakest portion.

Next, a method for manufacturing the shaft member will be described. First, molten steel adjusted to a desired chemical composition manufactured in an electric furnace is cast to produce an ingot. The ingot is subjected to hot working such as rolling and forging to form a bar material before roughing. Then, the bar material is subjected to an annealing process for improving the machinability as necessary, and then roughing is performed to cut a shaft member that is substantially close to a desired shape. Then, after carburizing and quenching, tempering is performed as necessary. The purpose of this tempering process is to prevent delayed fracture before induction hardening, which is a subsequent process, and therefore is not necessarily required when induction hardening is performed immediately. Furthermore, after subjecting the outer peripheral surface of the shaft member to induction hardening, tempering is performed. Finally, finish processing such as chamfering is performed to obtain a desired shape.

In the above-described manufacturing method, in the carburizing and quenching process, the carburizing conditions and the like are adjusted so that the thickness of the carburized layer with the C content of 0.50% or more is 0.20 mm or more. Further, in the subsequent induction hardening process, the heating condition is adjusted so that a sufficient quenching effect can be obtained up to a depth of 0.1 times the diameter or more.

The shaft member may have an inner hole along the central axis and a lateral hole provided in the radial direction so as to communicate with the inner hole from the outer surface. In this case, the inner hole and the lateral hole are preferably provided during the roughing process.

(Example 1)
Examples of the shaft member will be described together with comparative examples. First, a plurality of steel types (E1 to E32, C1 to C6, conventional materials 1 and 2) having chemical components shown in Tables 1 and 2 were prepared. In Tables 1 and 2, “-” in the column of V means that the analysis result is less than 0.005%, and “-” in the columns of B and Ti in Table 2 is positive during production. It means the case where it did not add.

These steels are melted in an electric furnace to produce a steel ingot, and the steel ingot is forged and processed into a bar material before roughing. And after hold | maintaining this bar at 900 degreeC for 1 hour, the annealing process which air-cools after cooling to 600 degreeC over 4 hours was performed. After that, rough machining was performed to cut the shape nearly to the desired shape to obtain a shaft member. The shaft member was subjected to heat treatment such as carburization and induction hardening. Thereafter, finishing was performed to finish the shaft member into a final shape.

As shown in FIGS. 1 and 2, in this example, in order to evaluate the strength of the shaft member having a shape in which the influence of stress concentration such as oil holes cannot be ignored, the inner surface 2 has an inner hole 2 along the central axis and the outer surface 11. A shaft member 1 having a lateral hole 3 penetrating in the radial direction so as to communicate with the inner hole 2 was prepared and evaluated. The outer diameter D1 is 20 mm, the inner diameter D2 is 7 mm, and the inner diameter D3 of the lateral hole 3 is 4 mm. The opening end of the horizontal hole 3 is chamfered with C = 0.5 mm. The shaft member 1 is cut to the final shape by the above-described pre-heat treatment. In an actual test, since it is necessary to make the shape easy to load the torsion torque with a torsion tester, the fixed portions (not shown in FIG. 1) at the both ends of the test piece are thicker than those in FIG. A circular cross section having a flat surface was prepared, and only the processing for adjusting the dimensions of the flat surface portion was performed in the final finishing process.

In this example, as shown in Tables 3 and 4, as examples, comparative examples, and conventional examples, experiments were performed by partially changing the heat treatment conditions using the steel types shown in Table 1.

First, the carburizing and quenching of Examples 1 to 32, Comparative Examples 1 to 6, 8 to 10, and Conventional Example 1 is a gas carburizing process using RX gas and enriched gas based on propane gas, and the holding temperature : 950 ° C., carburizing period: 75 minutes, diffusion period: 75 minutes, treated at a temperature of 850 ° C., then quenched into 130 ° C. oil. This condition is a condition aimed at the depth of the carburized layer where the C concentration is 0.6% or more being 0.30 mm or more. Further, tempering after carburizing and quenching in these examples was performed under the condition of 150 ° C. × 1 hr.

In Comparative Example 7 and Conventional Example 2, carburizing and quenching and subsequent tempering are not performed. In Comparative Examples 8 and 9, C.I. P. (Carbon potential) and treatment time were changed, and the influence on the proportional limit and fatigue strength when the depth of the carburized layer with C concentration of 0.50% or more was less than 0.20 mm was investigated. . The tempering after carburizing and quenching was the same as the other examples.

Next, induction hardening was performed using a workpiece moving induction hardening apparatus under the condition that induction hardening was performed on the outer peripheral surface in all axial directions at a frequency of 200 kHz. The cooling was water cooling. The hardened layer depth was adjusted by the coil voltage and the moving speed of the test material relative to the coil. Tempering after induction hardening was performed under the condition of holding at 180 ° C. for 1 hour. In Examples 1 to 32 and Comparative Examples 1 to 5 and 7 to 9, the hardened layer depth (depth at which austenite is formed by high frequency heating) was set to 3.0 mm. On the other hand, Comparative Example 10 was intentionally adjusted to be less than 2.0 mm, which does not satisfy the conditions of the present invention, in order to confirm the influence of the cured layer depth. Comparative Example 6 was evaluated without performing induction hardening in order to confirm the effect of not performing induction hardening. Further, Conventional Example 1 is an example of a shaft material reinforced by a conventional carburizing process, and therefore, induction hardening is not performed. In addition, about the prior art example 2, although induction hardening was performed without performing a carburizing process, the conditions are mentioned later.

The following evaluations were performed on the shaft members of the examples, comparative examples, and conventional examples obtained by the above manufacturing method, and the results are shown in Tables 5-7.

<C concentration measurement>
As shown in FIG. 2, in the cross section at the position where the horizontal hole 3 was provided, the C concentration was measured at a measurement point pitch of 1 μm using EPMA from the outer peripheral end toward the inside in the cross section along the measurement line L. Table 5 shows the C concentration at the outer peripheral edge as the surface C concentration. Further, the depth of the innermost measurement point having a C concentration of 0.50% or more was measured, and it was determined whether or not the point was 0.20 mm or more. The values are shown in Table 5 when it is 0.20 mm or more, and the values are shown in Table 5 when it is less than 0.20 mm. In addition, all the values in the circles were numerical values within the range of 0.30 to 0.45 mm.

<Hardness measurement>
The Vickers hardness at the outermost surface position on the measurement line L shown in FIG. 2, a depth of 0.20 mm position, a position of 2.0 mm, and a position of 5.0 mm (corresponding to a depth of ¼ of the diameter) Measurement was performed with a measurement load of 300 gf.

<Evaluation of first hardened layer>
The hardness of the outermost layer of the first hardened layer and the hardness at a position of 0.20 mm are shown in Tables 5 to 7 as numerical values when the value is 700 HV or more and less than 700 HV. Moreover, about the test material which did not carburize partly, it entered in Table 5 that it was not implemented.

<Second cured layer evaluation>
Although the hardness of the second hardened layer varies slightly, the depth from the outermost surface is 0.1 times the diameter of the test piece in consideration of the fact that the hardness tends to decrease as the depth from the surface increases. The hardness at a position of 2.0 mm from the surface was measured as a representative value. By confirming that the hardness at a position of 2.0 mm where the depth from the surface is 0.1 times the test piece diameter is 450 HV or more, the conditions of the second cured layer of the present invention are satisfied. You can check whether or not.

Also, some test materials that were not induction hardened were entered in Table 5 as not yet implemented. As described above, induction hardening is adjusted so that the curing depth is 3.0 mm, except for some test materials, and the cross-sectional structure of the test materials adjusted to 3.0 mm is used. Was observed, and it was confirmed that the result was as adjusted. Further, the comparative material 10 which was intentionally adjusted so that the curing depth was shallower had a quenching depth of 1.8 mm as shown in Table 5. here. As described above, the boundary between the second hardened layer and the inner layer was determined by observing the difference in crystal grain size. From this observation result, it was confirmed whether the thickness of the 2nd hardened layer was more than predetermined thickness. In addition, although the comparison material 10 has a hardening depth of 1.8 mm by induction hardening, since it had a hardness of 450 HV or more even at a position 2.0 mm from the surface, it was added to the column of the hardness of the comparison material 10. Evaluation was described as ○.

<Inner layer evaluation>
Considering the fact that the hardness of the inner layer is closer to the axis, the cooling rate during quenching after carburization becomes slower and the hardness tends to be lower, and the depth is 5.0 mm, which is a position of 1/4 of the diameter. The position value was evaluated. From this result, it can be confirmed that the hardness is a predetermined value or more from the surface to a depth of 1/4 of the diameter. Tables 5 to 7 show the hardness when the hardness is 300 HV or more, ◎ when the hardness is 350 HV or more, and numerical values when the hardness is less than 300 HV, as shown in Tables 5 to 7. Conventional examples 1 and 2 are shown numerically.

<Proportional limit>
A static torsion test was performed on each shaft member, and the proportional limit was measured. Specifically, a strain gauge is attached to the surface side of the position on the cross section L in FIG. 2 of each sample, a torsion torque is applied, and a strain-torque diagram is obtained from the output from the strain gauge and the applied torsion torque. I asked for each. Then, the torsional torque at the time when the linear relationship in the elastic deformation region was removed was obtained as the value of the proportional limit. The proportional limit is determined as a ratio with respect to the case of Conventional Example 2 as 100%, x when less than 100%, Δ when 100% or more and less than 110%, ○ when 110% or more and less than 120%, Table 5 shows the case of 120% or more as ◎.

<Fatigue strength>
A fatigue torsion test was performed on each shaft member, and the strength was measured 100,000 times. Specifically, when the fatigue life is 100,000 times while repeatedly varying the torque within the range of torque that the same maximum torque is applied to the right twist side and the left twist side as viewed from one axial direction, The maximum torsion torque was evaluated based on the value. The strength at 100,000 times is determined as a ratio with respect to the value in the case of Conventional Example 1 as 100%, x when less than 100%, Δ when 100% or more and less than 110%, and 110% or more and less than 130%. The results are shown in Tables 5 to 7 with ◯ as the case, ◎ as the case of 130% or more, ☆ as the case of 140% or more, and ★ as the case of 150% or more.

As shown in Table 7, in Examples 27 to 32, after photographing the fracture starting point in the sample broken by the fatigue torsion test with SEM, the photographed image is visually confirmed, and the cross-sectional hardness in the burned area is confirmed. Measured. The “marginal zone” means a position where the hardness is lowest in the vicinity of the depth that is slightly inside the limit depth that is heated to a temperature equal to or higher than the transformation point temperature during high-frequency heating. In other words, it is a position closer to the shaft core than the maximum depth position that is heated above the transformation point during high-frequency heating and less than the transformation point during high-frequency heating, but is heated to a considerably high temperature, so that the hardness is improved by the tempering effect. Means the position where the drop occurs.

Here, in FIG. 3, the hardness profiles of Example 1 and Conventional Example 1 are shown. In the figure, the horizontal axis represents the distance (mm) from the surface, the vertical axis represents the hardness (HV), and the hardness is plotted with Example 1 as a and Conventional Example 1 as b. . As can be seen from the figure, the conventional example 1 has not been induction-hardened, and thus has a carburized layer of 700 HV or more on the surface, but suddenly drops to 400 HV or less at a depth of about 0.5 to 1.0 mm. It has a hardness profile that reduces the hardness.

On the other hand, in Example 1, since carburizing quenching and induction hardening are performed so that the hardening depth is suitable for obtaining strength with excellent strength, it is 700 HV or more and is cured by carburization and induction hardening. The first cured layer is provided with at least 0.2 mm or more, and the second cured layer, which is 450 HV or more and hardened by induction hardening, has a depth corresponding to 0.1 or more times the diameter. 1.0 mm or more (in this embodiment, adjusted to be aimed at 3.0 mm), and a hardness profile having an inner layer hardened to a hardness sufficiently exceeding 300 HV by quenching after carburizing inside You can see that it has.

As shown in Tables 1, 2, and 5 to 7, Examples 1 to 32 have the specific chemical composition described above, and all the values of Formula 1 exceed 11. Then, by performing an appropriate heat treatment, as shown in FIG. 3, the second carbon was hardened by carburization and induction hardening so that the C concentration is 0.50% or more and the hardness is 700 HV or more. The 1st hardening layer has the thickness of 0.20 mm or more, and the 2nd hardening layer hardened by induction hardening so that hardness may become 450HV or more, the inner end is 0.1 of the diameter of a shaft member from the outermost surface. It has an inner layer that has a depth more than doubled and is hardened by quenching after carburizing so that the hardness is 300 HV or more.

When a torsional torque is applied, the closer to the surface, the higher the stress state is. However, the shaft member adjusted to the above state has a strength suitable for each part to withstand the actually applied stress. It has become a thing. As a result, it was confirmed that Examples 1 to 32 have a significantly improved proportional limit and superior fatigue strength as compared with Conventional Examples 1 and 2. Particularly, in Examples 1, 3 to 7, 9, 11, 13, 14, 16, 17, 19, 22 to 24, and 26 in which the hardness of the inner layer is 350 HV or more, the proportional limit is larger than that of Conventional Example 2. Improved by more than 20%.

In particular, Examples 11 to 24, which are examples in which Ti and B are added in a preferable range, are clearly superior in fatigue strength compared to Examples 1 to 10 which are examples of steels to which Ti and B are not added. In particular, Examples 21 to 24, in which the hardness of the burned zone was improved by adding V, showed particularly excellent fatigue strength.

Examples 25 and 26 show examples in which Ti and B were added but the addition was not sufficient, but even in this case, the effects of addition of Ti and B were not sufficient, It was confirmed that the Ti and B non-added steels shown in Examples 1 to 10 showed almost the same performance.

In Examples 27 to 32, steel types E27 to 29 and E30 to 32 contain components other than Ti, B, and V in order to clarify the difference between the effects of addition of Ti and B and the effect of the presence or absence of V addition. Steel types E27 and E30 are Ti, B non-added steel, steel types E28 and E31 are Ti and B added steel, V non-added steel, steel types E29 and E32 are all added with Ti, B and V Table 7 shows the evaluation results. From the results shown in Table 7, it was confirmed that the addition of Ti and B greatly improved the strength of the oil hole corners by improving the grain boundary strength, and the fracture starting point moved from the oil hole corners to the burned area. It was confirmed that when V was further added, the hardness of the fired zone was improved by about 50 to 60 HV, and the fatigue strength was further improved.

In Tables 5 to 7, the hardness and quenching depth are indicated by ○ and ◎ in the sense that the conditions of the present invention are satisfied. Specific measured values are the hardness of the outermost layer of the carburized layer. Was 742 to 833 HV, 0.2 mm position hardness was 702 to 788 HV, and the second hardened layer hardness was 453 to 700 HV. Further, the inner layer hardness ◎ was 352 to 446 HV, and ◯ was 303 to 346 HV (including the comparative example and the conventional example ○).

On the other hand, in Comparative Example 1, the chemical component did not satisfy Formula 1, and thus the hardness of the inner layer was insufficient. As a result, the proportional limit was lowered.

In Comparative Example 2, since the C content was too low, the hardness of the second cured layer after induction hardening was lowered, and the second cured layer satisfying the conditions of the present invention was not formed. As a result, the proportional limit was lowered.

In Comparative Example 3, since the Mn content was too low, the effect of improving the hardness of the inner layer by quenching after carburizing was not sufficiently obtained. As a result, the proportional limit was lowered.

In Comparative Example 4, since the S content was too high, the number of sulfide inclusions starting from fatigue strength increased, and as a result, fatigue strength decreased.

In Comparative Example 5, since the Cr content was too low and Formula 1 was not satisfied, the hardness improvement effect of the inner layer by quenching after carburization could not be sufficiently obtained. As a result, the proportional limit was lowered.

Since Comparative Example 6 was not induction hardened, the second hardened layer satisfying the conditions of the present invention was not formed as in Conventional Example 1 above, and the hardness profile suddenly decreased in hardness directly under the carburized layer. As a result, the proportional limit became lower.

In Comparative Example 7, the first hardened layer was not formed because carburizing and quenching was not performed. As a result, the surface hardness was lower (550 HV) than when carburizing and quenching, the fatigue strength was lower than that of Conventional Example 1, and the proportional limit was not improved as expected.

Comparative Example 8 is C.I. P. This is a comparative example for grasping the effect of terminating the process before sufficient carbon diffuses and penetrates by carburization by adjusting the carburizing process time. For this reason, the formation of the hardened carburized layer is incomplete, the fatigue strength improving effect cannot be obtained sufficiently, and the proportional limit improving effect is also insufficient.

Comparative Example 9 is an example in which the carburized layer depth is intentionally adjusted to be shallow by adjusting the carburizing time, and as a result, the depth of the first hardened layer is less than 0.2 mm. The improvement effect of both the proportional limit and fatigue strength is insufficient.

In Comparative Example 10, the hardening depth by the induction hardening treatment is shallow, and the second hardened layer that satisfies the conditions of the present invention was not formed, so that the effect of improving the proportional limit was insufficient. In Comparative Example 10, since the heating depth at the time of induction hardening was shallow, the hardness at the depth of 2.0 mm position was assured to be 450 HV or more, but from that position, the hardness was directed toward the inner layer. It was confirmed that the level dropped rapidly to the HV level of 350 to 360.

Conventional Example 1 is a gas carburized treatment using JIS SCM420, which has been used as a conventional high-strength shaft material, and is a standard for fatigue strength evaluation for grasping the fatigue strength level of a conventional shaft material. The test results are shown. This test material is not subjected to induction hardening, and the second hardened layer that satisfies the conditions of the present invention is not formed. Therefore, the proportional limit is greatly inferior.

Similarly to Conventional Example 1, Conventional Example 2 was also subjected to induction hardening using JIS SCr440 that has been used as a conventional high-strength shaft material, in order to grasp the proportional limit level of the conventional shaft material. The standard of proportional limit evaluation and the test result are shown. In addition, the high frequency heating conditions were adjusted so that the depth of the second hardened layer was 2.0 mm in consideration of the specifications of many conventional shaft materials. The results are shown in Table 5.
This test material was not carburized and quenched, and the first hardened layer having a high hardness was not formed. Therefore, it was found that the fatigue strength was greatly inferior.

Embodiment 2
In this example, an experiment was conducted to examine the relationship between the depth of the second hardened layer and the proportional limit in more detail. First, the chemical composition is fixed to the steel type E1 in Example 1, the carburizing quenching and the subsequent tempering conditions are the same as in Example 1, the heating coil voltage during induction quenching, and the moving speed of the test material during heating are set. A plurality of shaft members in which the depth of the second hardened layer was changed by adjustment or the like were prepared. By making the chemical components the same, the hardness of all the inner layers was adjusted to about 400 HV due to the hardness improvement effect by quenching after carburizing, so the influence on the proportional limit due to the difference in hardness of the inner layers is I can judge that there is nothing. Other conditions are the same as in the first embodiment. And the static torsion test similar to Example 1 was implemented, and the proportional limit was calculated | required.

The proportional limit was obtained as a ratio with respect to the case of Conventional Example 2 described above as 100%, and the numerical values are shown in Table 8. In FIG. 4, the second hardened layer depth (mm) is plotted on the horizontal axis, and the proportional strength ratio (based on Conventional Example 2) is plotted on the vertical axis, and the data in Table 8 is plotted.

As can be seen from Table 8 and FIG. 4, by setting the depth of the second hardened layer to 2.0 mm or more, which is 0.1 times the diameter of the shaft member, a high proportional limit has been conventionally obtained. It can be seen that the proportional limit can be reliably improved by 10% or more as compared to the conventional example 2 that has been adopted. In particular, it can be seen that when the depth of the second hardened layer is 3.0 mm or more, the proportional limit can be increased by 20% or more.

(Embodiment 3)
In this example, an experiment was conducted to examine in more detail the relationship between the hardness of the inner layer (the hardness at a position at a depth of 1/4 of the diameter) and the proportional limit. First, as shown in Table 9, in order to change the hardness of the inner layer, the chemical composition was adjusted, and investigation materials 1 to 6 which were six different steel types with different hardenability were prepared. A shaft member was produced in the same manner. When changing this component, if the C content changes, the hardness of the martensite, that is, the hardness of the second hardened layer may change, which may affect the value of the proportional limit. As shown in Table 9, the C content of 6 was set to be almost the same. Carburizing quenching and subsequent tempering, induction quenching and subsequent tempering conditions were the same as in Example 1. In addition, the induction heating conditions were adjusted so that the induction hardening depth was 3.0 mm, as in Example 1. Other conditions are the same as in the first embodiment. And the static torsion test similar to Example 1 was implemented, and the proportional limit was calculated | required.

The proportional limit was obtained as a ratio with respect to the case of the conventional example 2 as 100%, and the numerical values are shown in Table 10. In FIG. 5, the hardness of the inner layer (HV) is plotted on the horizontal axis, and the strength ratio at the proportional limit (based on Conventional Example 2) is plotted on the vertical axis, and the data in Table 10 is plotted.

As can be seen from Table 10 and FIG. 5, it can be seen that by setting the hardness of the inner layer to 300 HV or more, the proportional limit can be improved more than 10% more reliably than in the past.
The inner layer is closer to the axial center than the second hardened layer changed in FIG. 4, and the influence on the proportional limit due to the hardness change in the low stress region is, of course, the change in the hardness of the second hardened layer. The effect of increasing the hardness is not small, even if it is the hardness of the inner layer, but it is effective to increase the proportional limit by increasing the hardness. It became clear.

(Embodiment example 4)
In this example, an example in which a difference in temper softening resistance due to addition of V and non-addition of V is specifically examined is shown. As shown in Table 11, the investigation materials 41 and 42 which are Ti and B non-added steel and the investigation materials 43 and 44 which are Ti and B addition steel, with the components other than V being substantially the same, V-added steel and non-V-added steel were prepared, and this steel was subjected to heat treatment by the method described later, and the difference in hardness after heat treatment depending on the presence or absence of V addition was investigated.

Investigative materials 41 to 44 were machined after forging a material melted in an electric furnace and processed into a test piece having a diameter of 26 mm and a length of 40 mm. The test piece was subjected to a soaking treatment at 850 ° C. for 1 hour as a pretreatment, followed by a quenching treatment in which it was quenched into 130 ° C. oil. Then, it inserted in the tubular furnace heated up to each temperature (tempering temperature), and soaked for 1 hour, and then air-cooled and tempered. As tempering temperatures, four types of temperatures of 500 ° C., 550 ° C., 600 ° C., and 650 ° C. were used.

About each test piece which the tempering process was completed, it cut | disconnected in the center of the longitudinal direction, the cut surface was grind | polished, and hardness was measured by the Vickers hardness test. The hardness test was carried out under the condition that the D / 4 position (position of 1/4 depth of the diameter from the surface) was the measurement position and the measurement load was 10 kgf. Each test was repeated for each of three test pieces (n = 3), and the average value of the measured values was adopted as the measurement result. The measurement results are shown in FIGS. In these figures, the horizontal axis represents the tempering temperature (° C.), and the vertical axis represents the hardness (HV). The Vickers hardness was similarly measured for the test piece before tempering treatment, and plotted at a position of 150 ° C. in each figure.

As shown in FIGS. 6 and 7, the survey materials 41 and 43 that are V-added steels exhibit higher hardness at any tempering temperature than the survey materials 42 and 44 that are V-free steels. There was little decrease in hardness after tempering. This indicates that the positive addition of V brings about the effect of improving the temper softening resistance by the effect of precipitation strengthening. In the induction hardening, a certain depth is heated, and a deeper position is not heated more than the transformation point. Phenomenon that the structure obtained by heating at the time of carburizing and quenching before induction hardening is tempered because it is heated only below the transformation point at a position slightly inside the depth that is heated above the transformation point. Happens. At this time, since the temper softening resistance is increased by precipitation hardening in the V-added steel, the hardness is less likely to be lower than that in the V-unadded steel. And it has confirmed that this effect in V addition steel brought about the hardness improvement of a boundary part, and led to the great improvement of fatigue strength.

Claims

In mass%, C: 0.20 to 0.45%, Si: 0.03 to 1.50%, S: more than 0% and 0.070% or less, Mn: 0.50 to 2.00%, Cr : 0.30 to 2.50%, Al: 0.010 to 0.100%, N: 0.0070 to 0.0200%, V: 0 to 0.30% (including 0%), Mo: 0 Containing 0.5% (including 0%), the balance being Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 19 [C] +2.3 [Mn] +1.4 [Cr] +13 [Mo] +25 [V]> 11
(However, [C], [Mn], [Cr], [Mo] and [V] in the formulas indicate the contents (mass%) of C, Mn, Cr, Mo and V, respectively).
A first hardened layer located on the outermost surface and having a C concentration of 0.50% or more and a hardness of 700HV or more;
A second hardened layer located inside the first hardened layer and having a hardness of 450 HV or more;
An inner layer located on the inner side of the second hardened layer and having a hardness of 300 HV or more,
The thickness of the first hardened layer is 0.20 mm or more,
The inner end of the second hardened layer is at a depth position of 0.1 times or more the diameter of the shaft member from the outermost surface.
Shaft member.
In mass%, C: 0.20 to 0.45%, Si: 0.03 to 1.50%, S: more than 0% and 0.070% or less, Mn: 0.50 to 2.00%, Cr : 0.30 to 2.50%, Al: 0.010 to 0.100%, N: more than 0% to 0.0200% or less, V: 0 to 0.30% (including 0%), Mo: 0 0.50% (including 0%), B: more than 0% and 0.0050% or less, Ti: more than 0% and 0.10% or less, with the balance being Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 19 [C] +2.3 [Mn] +1.4 [Cr] +13 [Mo] +25 [V]> 11
(However, [C], [Mn], [Cr], [Mo] and [V] in the formulas indicate the contents (mass%) of C, Mn, Cr, Mo and V, respectively).
A first hardened layer located on the outermost surface and having a C concentration of 0.50% or more and a hardness of 700HV or more;
A second hardened layer located inside the first hardened layer and having a hardness of 450 HV or more;
An inner layer located on the inner side of the second hardened layer and having a hardness of 300 HV or more,
The thickness of the first hardened layer is 0.20 mm or more,
The inner end of the second hardened layer is at a depth position of 0.1 times or more the diameter of the shaft member from the outermost surface.
Shaft member.
The shaft member according to claim 1 or 2, wherein the shaft member has an inner hole along a central axis and a lateral hole provided in a radial direction so as to communicate with the inner hole from an outer surface.