JP5583352B2 - Induction hardening steel and induction hardening parts with excellent static torsional fracture strength and torsional fatigue strength - Google Patents

Description

  TECHNICAL FIELD The present invention relates to induction hardening steel and induction hardening parts, and in particular, steel suitable as a material for shaft parts such as shafts used in automobiles and various industrial equipment induction hardened at a frequency of 30 KHz or less and the steel as a material. It relates to induction hardening parts.

  Of the parts manufactured by induction hardening and tempering in automobiles and industrial machines, most of the parts that cause static torsional fracture or torsional fatigue breakage have parts with shafts.

  For example, shaft parts such as a drive shaft, an axle shaft, a steering shaft, and a crankshaft, a motor shaft, a transmission gear shaft, and the like also correspond.

  In recent years, in order to cope with the downsizing of products, a method of increasing the strength of parts and reducing the size has been taken, and the same applies to these induction-hardened parts. Therefore, various measures are taken to increase the strength.

  Patent Document 1 relates to a steel for induction hardening and an induction hardening component having excellent torsional fatigue characteristics, controls the shape of MnS that is expected to be the starting point of fracture, and further, the ferrite area ratio and grains that cause unevenness in induction hardening. The diameter is specified.

  Patent Document 2 relates to induction hardening parts, in order to improve impact bending characteristics and impact torsional characteristics, the hardening depth by induction hardening is specified within a specific range, and the induction hardening structure has a uniform martensite ratio of 90% or more. It describes that it is prescribed in the martensite organization.

JP 2002-069566 A JP-A-10-36937

  However, in the induction hardening steel described in Patent Document 1, any of rare earth metals Ca, Zr, Mg, and Y must be added in order to change the shape of MnS, resulting in poor economy and manufacturability. Moreover, since the ferrite content is closely related to the carbon content in the steel and the cooling rate during the formation of the structure, precise control is required for each chemical component.

  Further, in the refinement of ferrite grains, it is necessary to increase the number of ferrite nucleation sites and uniformly disperse them, and it is necessary to control the forging and rolling method and the precipitation state of fine precipitates, but it is not easy to select optimum conditions.

  The induction-hardened component described in Patent Document 2 has a predetermined thickness and depth in the induction-hardened region and a structure obtained by induction hardening. It must be selected in relation to the component composition, and the economic efficiency and work efficiency are reduced.

  Then, an object of this invention is to provide the components which are excellent in the static torsion fracture strength and the torsional fatigue strength which use the steel for induction hardening and the said steel as a raw material.

In order to solve the above-mentioned problems, the present inventors conducted intensive research and obtained the following knowledge.
1. It is possible to improve static torsional fracture strength and torsional fatigue strength by prescribing the internal hardness, hardened layer depth and crystal grain size of the quenched part obtained after induction hardening and optimizing the hardness distribution. .
2. If the structure of the steel material before induction hardening is mainly pearlite, the depth of the hardened layer by induction hardening becomes stable, and it becomes possible to suppress variations in hardness after induction hardening and tempering.

The present invention has been made on the basis of the above findings and has been further studied. In mass%, C: 0.40 to 0.55%, Si: 0.05 to 0.50%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.005 -0.060%, Cr: 0.05-0.50%, Mo: 0.35% or less, B: 0.0005-0.0080%, Ti: 0.010-0.035%, N: 0 A steel for induction hardening characterized by comprising a component composition comprising .0050-0.0100%, the balance being Fe and inevitable impurities, and a microstructure of pearlite mainly having a proeutectoid ferrite volume fraction of less than 15%.
2. 2. The steel for induction hardening according to 1, wherein the component composition further includes one or more of V: 0.005 to 0.4% and Nb: 0.01 to 0.10% by mass%.
3. The steel for induction hardening according to 1 or 2, wherein the component composition further satisfies the following formulas (1) and (2):
0.80 ≦ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2 + V / 2 ≦ 1.00 (1)
1.11 ≦ 0.36 × C + 0.18 × Si + 0.27 × Mn + 0.30 × Cr + 0.43 × Mo + 135 × B + 0.24 ≦ 1.31 (2)
In these formulas, each element indicates a content (mass%), and an element not included is 0.
After using the steel for induction hardening according to any one of 4.1 to 3 as a raw material to form a part having a shaft portion by rolling, forging, machining, or a combination thereof, the shaft portion is induction hardened and tempered. Thus, the induction hardened component having excellent static torsional fracture strength and torsional fatigue strength, in which the effective hardened layer depth / radius ratio is 0.4 to 0.6 and the crystal grain size of the quenched portion is 7.0 or more.
5. Furthermore, the induction hardened component having excellent static torsional fracture strength and torsional fatigue strength according to 4, wherein the hardness distribution after induction hardening and tempering satisfies the following formula (3):

6). 4. The induction-hardened component having excellent static torsional fracture strength and torsional fatigue strength according to 4 or 5, wherein the part subjected to induction hardening and tempering is further subjected to shot peening.

  According to the present invention, excellent static torsional fracture strength and torsional fatigue characteristics have been used without changing the conditions of induction hardening and tempering that have been conventionally used in automobiles, industrial machines, etc., and generally from 5 KHz to 30 KHz. It is extremely useful industrially because it is possible to manufacture the parts provided and to provide the materials.

The figure which shows the shape of the test piece for a static torsion fracture test and a torsional fatigue test.

The steel for induction hardening according to the present invention defines the component composition and the microstructure before induction hardening (the microstructure after hot rolling).
[Ingredient composition] In the description, “%” means “mass%”.
C: 0.40 to 0.55%
C is necessary for ensuring hardness in induction hardening, and determines the surface hardness after induction hardening and tempering. In addition, it affects the structure formation in the state of rolling or forging before induction hardening.

  If the C content is less than 0.40%, the structure before induction hardening is not pearlite, but a ferrite-pearlite two-phase structure, and the ferrite fraction is 15% or more, so that the variation in the quenching depth only increases. In addition, the surface hardness after induction hardening and tempering decreases, and the strength as a part cannot be secured. On the other hand, if it exceeds 0.55%, the hardenability becomes too high, so that cracks are likely to occur, so 0.40 to 0.55%.

Si: 0.05 to 0.50%
Si is a deoxidizing element. When the amount of Si added is less than 0.05%, a sufficient deoxidation effect cannot be obtained, and oxide remains in the interior, resulting in a decrease in fatigue strength. On the other hand, if it exceeds 0.50%, the hardness is increased and the toughness is lowered, so that the static torsional fracture strength is lowered. Therefore, the Si addition amount is made 0.05 to 0.50%.

Mn: 0.60 to 2.00%
Mn is an element that enhances hardenability. In order to ensure hardenability, 0.60% or more is necessary. On the other hand, if added over 2.00%, the hardenability is too high to cause quench cracking, so the Mn addition amount is 0.60-2. 0.000%.

P: 0.030% or less P has an action of segregating at the grain boundaries and embrittles the grain boundaries. When it exceeds 0.030%, it becomes remarkable, so 0.030% or less. Since it is an unavoidable impurity, it is desirable to reduce it within a range that does not adversely affect the manufacturing cost.

S: 0.060% or less S combines with Mn to form MnS and improves machinability. However, excessive MnS reduces hardenability and serves as a starting point for fracture in torsional fatigue, thus reducing fatigue strength. Therefore, it is made 0.060% or less. Since it is an unavoidable impurity, it is desirable to reduce it within a range that does not adversely affect the manufacturing cost.

Cr: 0.05 to 0.50%
Cr is a hardenability improving element, and 0.05% or more is added to obtain its effect. On the other hand, if it exceeds 0.50%, the hardenability becomes too high and it is easy to cause a burning crack, so 0.05 to 0.50%.

Mo: 0.35% or less Mo is a hardenability-improving element and is added. However, if it exceeds 0.35%, a cracking is likely to occur, so the content is made 0.35% or less.

Ti: 0.010 to 0.035%
Ti has a strong bonding force with N and is bonded in preference to B to form Ti nitride or Ti carbonitride. When Ti is less than 0.010%, solid solution N remains to form BN, and the effect of improving the hardenability of B cannot be obtained. On the other hand, even if added over 0.035%, the effect is saturated, so 0.010 to 0.035%.

B: 0.0005 to 0.0080%
B is a hardenability improving element and increases the depth of the hardened layer in the induction hardening. To obtain this effect, 0.0005% or more is necessary. However, if it exceeds 0.0080%, the effect is saturated, so 0.0005 to 0.0080%.

N: 0.0100% or less N needs to be bonded to Ti to form TiN and to secure solid solution B that contributes to hardenability. When N is high, solid solution N that remains without being bonded to Ti exists and bonds with B preferentially, so that it is difficult to secure solid solution B. In order to ensure the solid solution B stably, N needs to be 0.0100% or less. Therefore, the N content is set to 0.0100% or less.

  The above is the basic component composition of the steel of the present invention. After induction hardening and tempering, parts with excellent static torsional fracture strength and torsional fatigue strength can be obtained. When it is used, at least one of Nb and V is added.

  Moreover, when the component design is performed so as to satisfy the following parameter formulas (1) and (2), the torsional fatigue strength can be further improved as compared with the case where one or more of Nb and V are added. The effects of the parameter formulas (1) and (2) can be obtained in the basic component composition in the same manner as when one or more of Nb and V are added to the basic component composition.

One or more of Nb: 0.01 to 0.10%, V: 0.0005 to 0.4% Nb and V combine with C to precipitate finely, thereby improving internal hardness and static torsion fracture Contributes to improvement of strength and torsional fatigue strength. The effect can be obtained when Nb: 0.01% or more, V: 0.0005% or more, while Nb: more than 0.10% and V: more than 0.4%, the effect is saturated. Range.

0.80 ≦ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2 + V / 2 ≦ 1.00 (1)
1.11 ≦ 0.36 × C + 0.18 × Si + 0.27 × Mn + 0.30 × Cr + 0.43 × Mo + 135 × B + 0.24 ≦ 1.31 (2)
In these formulas, each element indicates a content (mass%), and an element not included is 0.

Equation (1) indicates a component design guideline for controlling the internal hardness. If the internal hardness is low, the torsional fatigue strength tends to decrease, and if it is high, the workability deteriorates. In order to obtain the best characteristics in consideration of the balance between strength and workability, the value of equation (1) is set to 0.80 to 1.00 .

Equation (2) is an equation that is an index of effective hardened layer depth under general induction hardening conditions using a frequency of 30 KHz or less, and is used to obtain an optimum effective hardened layer depth / radius for improving torsional fatigue strength. , (2) values are set to 1.11 to 1.31 . In order to improve the machinability of the steel of the present invention, free cutting elements such as Pb and Ca may be included.

[Previous organization]
In the steel of the present invention, the microstructure before hot quenching and before induction hardening is made to be a pearlite-based structure having a proeutectoid ferrite volume fraction of less than 15%. When the ferrite fraction is 15% or more, not only the variation in the quenching depth increases, but also the surface hardness after induction hardening and tempering decreases, and the strength as a part cannot be secured.

  When producing induction-hardened parts using the induction-quenched steel according to the present invention as a raw material, the steel is quenched at a frequency of 30 KHz or less, which is used for general induction hardening. 1. Effective hardened layer depth / radius ratio of 0.4 to 0.6; Hardened part crystal grain size: When the grain size is 7.0 or more, especially when improving the static torsional fracture strength, the relationship between the hardness distribution and the shaft dimension (shaft diameter) so as to satisfy the parameter formula (formula (3)) described later Adjust. In addition, although this invention steel is suitable as a raw material of the components which have an axial shape, it is not limited. When the part is not a part having an axial shape, the radius is defined as the depth in the thickness direction in the following description. However, the formula (3) is applied only when the shaft-shaped member is targeted.

  The torsional fatigue strength varies depending on the hardened layer depth. When the hardened layer depth / radius value is less than 0.4, the hardened layer is thin and fatigue cracks are likely to occur. If it exceeds 0.6, fatigue cracks are likely to occur from the inside of the part, so 0.4 to 0.6.

  When the grain size of the quenched part is less than 7.0, the grain boundary becomes brittle due to impurities in the steel, and fatigue cracks starting from the grain boundary are likely to occur. 7.0 or higher.

  In order to improve the static torsional fracture strength, it is effective to optimize the hardness distribution profile from the surface layer to the center, and the induction hardening conditions are controlled so as to satisfy the expression (3). Further, when it is desired to further improve the torsional fatigue strength of the part, shot peening may be performed on the portion subjected to induction hardening and tempering. Hereinafter, the present invention will be compared with comparative examples by way of examples, and the effects thereof will be described in detail.

  Steel having the chemical components shown in Table 1 was melted, melted, and hot-rolled by a conventional method to prepare a round steel bar having a diameter of 40 mm. In the table, No. 1-31 are developed steels having a component composition within the scope of the present invention. 32 to 46 are comparative examples having component compositions outside the scope of the present invention. No. 47 is a conventional steel and is JIS S45C, which is a component composition outside the scope of the present invention.

  The obtained round steel bar was cut into a length of 800 mm, both ends were forged up to 100 mm length positions to 60φ, and further machined to a parallel portion of 40 mmφ and a length of 300 mm at 40 mmφ. A round bar test piece was subjected to induction hardening and tempering treatment to obtain a test piece for static torsional fracture test and torsional fatigue test. FIG. 1 shows the shapes of test pieces for a static torsional fracture test and a torsional fatigue test.

  Induction hardening and tempering were performed on all test steels by induction hardening at a frequency of 5 KHz and a moving speed of 9 mm / second, and then tempering at 160 ° C. for 2 hours. The conditions used are those conventionally used for parts made of S45C and having the same shaft diameter.

  The developed steel No. For the steel material No. 32, in addition to the test piece (No. 32-1) prepared under the induction hardening conditions, a torsion test piece with different induction hardening conditions was also prepared.

  Specimen No. 31-2, 31-3, and 31-4 had frequencies of 10 KHz, 30 KHz, and 32 KHz, respectively, and the feed rate was adjusted to have a hardness distribution equivalent to that produced under the induction hardening conditions.

Specimen No. In the case of 31-5, the ratio of the hardened layer depth / radius was made smaller than the range of the present invention by increasing the feed rate to 12 mm / second while maintaining the frequency at 5 KHz.
Specimen No. In the case of 31-6, the ratio of the hardened layer depth / radius was made larger than that of the present invention by reducing the feed rate to 6 mm / second while keeping the frequency at 5 KHz.

  Specimen No. No. 32-2 increased the maximum temperature during quenching heating at a frequency of 32 KHz and a feed rate of 7 mm / second, and increased the crystal grains while maintaining the same hardness distribution as that prepared under the induction hardening conditions.

  Specimen No. In No. 32-3, the feed rate was increased to 12 mm / sec with the frequency kept at 5 KHz, and the ratio of the cured layer depth / radius was made smaller than the range of the present invention.

  Specimen No. In the case of 32-4, the ratio of the hardened layer depth / radius was made larger than that of the present invention by reducing the feed rate to 6 mm / second while keeping the frequency at 5 KHz.

  After induction hardening and tempering, the crystal grain size of the quenched layer was measured. Further, the induction-quenched shaft portion was cut, and the surface hardness, internal hardness, and cross-sectional hardness distribution were measured to the center using a Vickers hardness tester.

  The effective hardened layer depth is obtained by calculating the average of the results obtained by examining the depth obtained at 450 HV in Vickers hardness and changing the position by 45 ° on the circumferential surface of the cross section in 8 directions. did. The cured layer depth unevenness was compared with the effective cured layer depth required in each of the eight directions, and when the difference between the maximum value and the minimum value was 0.2 mm or more, the cured layer depth unevenness was determined. Moreover, (3) Formula was calculated using the measurement result of hardness distribution.

  In the static torsion fracture test, one end of one end of the test piece (60Φ as it was forged) was fixed, and the other grip was rotated at 1 ° / min to break, and the maximum torque was determined and evaluated.

  The torsional fatigue test was performed by fixing the test piece, twisting both ends in the opposite direction by the same displacement and applying torque, and evaluating the maximum torque that does not break even after repeated 10 million torsion as the fatigue strength.

  Table 2 shows the properties of the test pieces after induction hardening and tempering and the results of static torsional fracture test and torsional fatigue test. Note that the structure of the test steel before induction hardening satisfied the provisions of the present invention for any of the test steels.

  No. It was confirmed that the target effective hardened layer depth was obtained for the test pieces 1 to 31-3, and high static torsional fracture strength and torsional fatigue strength were obtained.

  In particular, No. 1 satisfying the expressions (1) and (2). The test pieces of 1, 2, 4, 7, 10-13, 15-18, 23-31-3 had higher torsional fatigue strength, and No. 1 satisfying the expression (3) was obtained. The test pieces of 1 to 3, 5, 6, 8, 10 to 18, and 20 to 31-3 had higher static torsional fracture strength.

  No. 1 satisfying all of the expressions (1), (2) and (3). The test pieces of 1, 2, 10-13, 15-18 and 23-31-3 were higher in both static torsion fracture strength and torsional fatigue strength than the other test pieces.

  On the other hand, the test piece No. No. 32 had a C content lower than the range of the present invention, unevenness in the cured layer depth was observed, and the surface hardness was also low. Furthermore, the depth of the hardened layer was also shallow. Therefore, the static torsional fracture strength was low and the torsional fatigue strength was also low.

Specimen No. No. 33 has a C content exceeding the range of the present invention, so that the hardenability is too high and a crack is generated, and the static torsional fracture strength and torsional fatigue strength cannot be investigated.
Specimen No. In No. 34, since the Si content was lower than the range of the present invention, a large amount of oxide generated during production in steelmaking remained and became a fatigue starting point, and the torsional fatigue strength was lowered.

  Specimen No. No. 35 had a high Si content exceeding the range of the present invention, the internal hardness became too high, and the static torsional fracture strength decreased. Specimen No. In No. 36, the Mn content was lower than the range of the present invention, the hardenability was low, the effective hardened layer depth was too shallow, and both the static torsional fracture strength and the torsional fatigue fatigue strength were reduced.

  Specimen No. No. 37 had a high Mn content and therefore had high hardenability and caused cracking. Specimen No. In No. 38, since the P content was higher than the range of the present invention, the grain boundary became brittle, and both the static torsional fracture strength and the torsional fatigue strength decreased.

  Specimen No. No. 39 had an S content higher than the range of the present invention, so that MnS inclusions were excessive, thereby reducing the hardenability and reducing the static torsional fracture strength. In addition, since a large amount of MnS was present, the torsional fatigue strength decreased as a starting point of torsional fatigue strength.

  Specimen No. In No. 40, the Cr content was lower than the range of the present invention, so that the hardenability was lowered, the internal hardness was low, and the effective hardened layer depth was shallow, so that the static torsional fracture strength and torsional fatigue strength were reduced.

  Since test piece No41 had Cr content higher than the range of this invention, its hardenability was high and the burning crack generate | occur | produced. Specimen No. No. 42 had a Mo content higher than the range of the present invention. As a result, the depth of the hardened layer became too deep, and the torsional fatigue strength was lowered.

  Specimen No. In No. 43, the B content was lower than the range of the present invention, and the hardenability decreased, so that the hardened layer depth became shallow, and the static torsional fracture strength and torsional fatigue strength decreased.

  Specimen No. No. 44 had a Ti content lower than the range of the present invention, and solid solution B could not be secured, resulting in a decrease in hardenability. Therefore, the hardened layer depth became shallow, and static torsional fracture strength and torsional fatigue strength decreased.

  Specimen No. No. 45 has a Ti content higher than the range of the present invention, so that there were many TiN inclusions and became the starting point of fracture, and static torsional fracture strength and torsional fatigue strength were reduced.

  Specimen No. No. 46 had a higher N content than the range of the present invention, and could not secure solid solution B, but the hardenability decreased and the hardened layer depth became shallow, and the static torsional fracture strength and torsional fatigue strength decreased.

  Conventional steel No. 47 had no hardenability because B was not added, the hardened layer was shallow, and the crystal grains were large. As a result, static torsional fracture strength and torsional fatigue strength decreased.

  In addition, test piece No. No. 31 was induction hardened at a higher frequency than before. In 31-4, the crystal grain size was smaller than the range of the invention, and the torsional fatigue strength decreased because the crystal grains were large. No. with a hardened layer depth shallower than the scope of the invention. 31-5 and No. 31 in which the depth of the hardened layer was deeper than the scope of the invention. In 31-6, the static torsional fracture strength and torsional fatigue strength were reduced.

Claims (5)

In mass%, C: 0.40 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.005 -0.060%, Cr: 0.05-0.50%, Mo: 0.35% or less, B: 0.0005-0.0080%, Ti: 0.010-0.035%, N≤0 0.0100%, a composition comprising the balance Fe and inevitable impurities, and a pearlite-based microstructure with a proeutectoid ferrite volume fraction of less than 15%,
The steel for induction hardening , wherein the component composition further satisfies the following formulas (1) and (2) .
0.80 ≦ C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 2 + V / 2 ≦ 1.00 (
1)
1.11 ≦ 0.36 × C + 0.18 × Si + 0.27 × Mn + 0.30 × Cr + 0.43
× Mo + 135 × B + 0.24 ≦ 1.31 (2)
In these formulas, each element indicates a content (mass%), and an element not included is 0.   The component composition further comprises one or more of V: 0.005 to 0.4% and Nb: 0.01 to 0.10% by mass%, according to claim 1, steel. The steel for induction hardening according to claim 1 or 2 is used as a raw material, and after forming into a part shape having a shaft portion by rolling, forging or machining or a combination thereof, the effective hardening layer depth is obtained by induction hardening and tempering. Induction-hardened parts having excellent static torsional fracture strength and torsional fatigue strength with a thickness / radius ratio of 0.4 to 0.6 and a grain size of the quenched portion of 7.0 or more. Furthermore, the hardness distribution after induction hardening and tempering satisfies the following formula (3): The induction hardening component excellent in static torsional fracture strength and torsional fatigue strength according to claim 3 .

The induction-quenched part excellent in static torsional fracture strength and torsional fatigue strength according to claim 4, wherein shot peening is further performed on the portion subjected to induction hardening and tempering.