JP2007231411A - Method of manufacturing machine structure component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a machine structure component capable of further improving fatigue strength compared to a conventional technique. <P>SOLUTION: In the manufacturing method, a steel material is subjected to a surface hardening process by quenching and subsequently subjected to tempering under a condition satisfying a prescribed relation. The steel material has a composition comprising 0.3-0.7 mass% C, ≤1.1 mass% Si, ≤2.0 mass% Mn, ≤0.05 mass% Al, ≤0.01 mass% B and ≤0.1 mass% Ti in a ratio satisfying a prescribed relation and further comprising at least one of ≤1.0 mass% Mo and ≤2.0 mass% W and the balance being Fe and unavoidable impurities. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a machine structural component and a method for manufacturing the same, which is partially provided with a surface hardened layer by quenching such as induction hardening. Here, examples of the mechanical structure component include a drive shaft, an input shaft, an output shaft, a crankshaft, an inner ring and an outer ring of a constant velocity joint, a hub, and a gear for an automobile.

Conventionally, mechanical structural parts such as automobile drive shafts and constant velocity joints, etc. are processed by hot forging, hot forging, cutting and cold forging into a predetermined shape, and then induction hardening. -By tempering, it is common to ensure fatigue strength such as torsional fatigue strength, bending fatigue strength, rolling fatigue strength, and sliding rolling fatigue strength, which are important characteristics for machine structural parts.
On the other hand, in recent years, there has been a strong demand for weight reduction of automobile parts due to the weight reduction of automobiles caused by environmental problems. From this viewpoint, further improvement of fatigue strength in automobile parts has been demanded.

As means for improving the fatigue strength as described above, various methods have been proposed so far.
For example, in order to improve the torsional fatigue strength, it is necessary to sufficiently increase the hardness of the surface portion of the steel material when performing surface hardening treatment by induction hardening or the like. However, excessively increasing the hardness leads to induction of brittle fracture such as grain boundary fracture, and there is a problem that the fatigue strength does not increase as the hardness increases.

In order to solve this problem, it is known to strengthen grain boundaries by adding B (for example, Patent Document 1). However, there is an extremely high demand for increasing the strength of machine structural parts for the applications described above, and even the technique of Patent Document 1 is not sufficient.
JP 2000-2229 A

  The present invention has been developed in view of the above-described present situation, and an object of the present invention is to propose a method for manufacturing a machine structural component capable of further improving fatigue strength as compared with the conventional one.

  Now, as a result of earnestly examining the surface-hardened material by induction hardening or the like in order to effectively improve the above-described fatigue characteristics, the inventors have achieved high by performing an appropriate tempering treatment according to the component composition. The present inventors have found that fatigue strength can be obtained and have completed the present invention.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.3 to 0.7 mass%, Si: 1.1 mass% or less, Mn: 2.0 mass% or less, Al: 0.05 mass% or less, B: 0.01 mass% or less, and Ti: 0.1 mass% or less (1 ) In a range that satisfies the formula, and further containing one or two of Mo: 1.0 mass% or less and W: 2.0 mass% or less, the balance is a steel material having a component composition of Fe and inevitable impurities by quenching A method for producing a machine structural component excellent in fatigue characteristics, characterized by performing a surface hardening treatment and then performing a tempering treatment under conditions satisfying the following expression (2).
B- (11 / 14N-11 / 48Ti)> 0.0003 --- (1)
10000 + 10000 (C + Mo / 2 + W / 4) ≧ (T + 273) × (log (t / 3600) +30) ≧ 10000 + 5000 (C + Mo / 2 + W / 4) --- (2)
Where T: tempering temperature (° C.)
t: Tempering time (s)

Here, the tempering temperature is the highest temperature reached during tempering heating.
Further, the tempering time is a time during which the tempering time stays at a maximum attained temperature of -20 ° C or higher.

(2) C: 0.3 to 0.7 mass%, Si: 1.1 mass% or less, Mn: 2.0 mass% or less, Al: 0.05 mass% or less, B: 0.01 mass% or less, and Ti: 0.1 mass% or less (1 ) In a range that satisfies the formula, and further containing one or two of Mo: 1.0 mass% or less and W: 2.0 mass% or less, the balance is a steel material having a component composition of Fe and inevitable impurities by quenching Excellent fatigue properties characterized by performing surface hardening treatment and then performing the tempering treatment under the condition that the following formula (2) is satisfied and the temperature rising rate is 20 ° C. or more and the holding time is 60 seconds or less. Manufacturing method for machine structural parts.
B- (11 / 14N-11 / 48Ti)> 0.0003 --- (1)
10000 + 10000 (C + Mo / 2 + W / 4) ≧ (T + 273) × (log (t / 3600) +30) ≧ 10000 + 5000 (C + Mo / 2 + W / 4) --- (2)
Where T: tempering temperature (° C.)
t: Tempering time (s)

(3) As said component composition, Cr: 2.5 mass% or less, Cu: 1.0 mass% or less, Ni: 2.5 mass% or less, and V: 0.3 mass% or less contain 1 type or 2 types or more The manufacturing method of the components for machine structures excellent in the fatigue characteristics of Claim 1 or 2.

(4) As said component composition, as described in said (1), (2) or (3) which contains further 1 type or 2 types chosen from Nb: 0.1 mass% or less and Zr: 0.1 mass% or less A method for manufacturing machine structural parts with excellent fatigue characteristics.

(5) The method for producing a machine structural component having excellent fatigue characteristics according to any one of (1) to (4), wherein the surface hardening treatment is performed under a condition of a surface heating temperature of 850 to 1050 ° C.

(6) The mechanical structure having excellent fatigue characteristics according to any one of the above (1) to (5), wherein the mechanical structural component is a shaft component, and a cured layer ratio during the surface curing treatment is 0.3 to 0.7. Method of manufacturing parts.

  According to the present invention, it is possible to stably obtain a machine structural component having excellent fatigue characteristics such as bending fatigue characteristics, rolling fatigue characteristics, and sliding rolling fatigue characteristics as well as torsional fatigue characteristics. As a result, it is a great achievement for the demand for weight reduction of automotive parts.

The present invention will be specifically described below.
The mechanical structural parts targeted by the present invention have various shapes and structures for each part, such as drive shafts, input shafts, output shafts, crankshafts, inner and outer rings of constant velocity joints, hubs, and gears for automobiles. In any case, a hardened layer that has been subjected to surface hardening treatment by quenching, for example, induction hardening, in a part or all that particularly requires fatigue strength, has an appropriate temperature according to the component composition. It is important that tempering is performed. In particular, tempering by high-temperature tempering and short-time holding treatment, such as induction tempering, is effective in improving fatigue characteristics. In this tempering, it is important to appropriately regulate the tempering temperature and the tempering time, and details of the tempering conditions will be described later.

First, the method of the present invention will be described in order from the component composition of the starting material.
That is, the components of steel used for machine structural parts are as follows +.
C: 0.3-0.7 mass%
C is an element having the greatest influence on the hardenability, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. However, if the content is less than 0.3 mass%, the required surface hardness cannot be obtained, so 0.3 mass% or more is added. On the other hand, if the content exceeds 0.7 mass%, the grain boundary strength decreases, and accordingly, the fatigue strength also decreases, and the machinability, cold forgeability, and fire crack resistance also decrease. For this reason, C was limited to the range of 0.3 to 0.7 mass%. Preferably it is the range of 0.35-0.6 mass%.

Si: 1.1 mass% or less
Si not only acts as a deoxidizer, but also effectively contributes to grain boundary strengthening and improvement of temper softening resistance. However, if the content exceeds 1.1 mass%, machinability and forgeability are reduced. Therefore, the amount of Si is 1.1 mass% or less. In addition, in order to exhibit said effect, it is preferable to set it as 0.15 mass% or more. More preferably, it is 0.35 mss% or more.

Mn: 2.0 mass% or less
Mn can be added because it is a component useful for improving the hardenability and ensuring the depth of hardening during quenching. If the content is less than 0.2 mass%, the effect of addition is poor, so 0.2 mass% or more is preferable. Furthermore, it is preferably 0.3 mass% or more. On the other hand, if the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the fatigue strength decreases. Therefore, Mn is preferably 2.0 mass% or less. It should be noted that if the content of Mn is large, the base material is hardened, which may be disadvantageous in machinability. Therefore, it is preferable to set the content to 1.2 mass% or less. More preferably, it is 1.0 mass% or less.

Al: 0.05 mass% or less
Al is an element effective as a deoxidizer, but if it exceeds 0.05 mass%, there is a possibility of increasing unnecessary inclusions, so the Al amount is 0.05 mass% or less.

B: 0.01 mass% or less B is an element that remarkably strengthens the grain boundary. Preferably, 0.0003 mass% or more is added, but even if added over 0.01 mass%, the effect is saturated, so 0.01 mass% or less. To do.

Ti: 0.1 mass% or less
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and the effect of improving the grain boundary strength due to B from disappearing. However, if Ti exceeds 0.1 mass%, a large amount of TiN precipitates and the fatigue strength deteriorates.

Here, from the viewpoint of securing B that contributes to grain boundary strengthening, the above-described components are further expressed by the above formula (1), that is, B- (11 / 14N-11 / 48Ti)> 0.0003.
Need to be satisfied.

Furthermore, in addition to the above five components, one or two of Mo: 1.0 mass% or less and W: 2.0 mass% or less are contained.
Mo: 1.0 mass% or less
Mo refines the prior austenite grains of the hardened hardened layer and at the same time refines the carbides precipitated at the grain boundaries when tempering, and remarkably increases the grain boundary strength, so 0.05 mass% or more is preferable However, it exceeds 1.0 mass%, leading to hardening of the base material and disadvantageous machinability, so it is set to 1.0 mass% or less.

W: 2.0 mass% or less Since W refines the prior austenite grains of the quenched hardened layer and simultaneously refines carbides precipitated at the grain boundaries when tempering, it significantly increases the grain boundary strength. Although 0.1 mass% or more is preferable, it exceeds 2.0 mass%, which causes the base material to harden and is disadvantageous in machinability, so is 2.0 mass% or less.

Although the basic components have been described above, in the present invention, one or more of the four components described below can be appropriately contained.
Cr: 2.5 mass% or less
Cr is effective for improving the hardenability and is a useful element for ensuring the depth of the hardened phase. However, if contained excessively, the carbides are stabilized to promote the formation of residual carbides, and the grain boundary strength may be lowered to deteriorate the fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 2.5 mass% is acceptable. Preferably it is 1.5 mass% or less.

Cu: 1.0 mass% or less
Cu is effective in improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves fatigue strength. Furthermore, by suppressing the formation of carbides, a decrease in grain boundary strength due to carbides is suppressed, and fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracks occur during hot working, so it is preferable to add 1.0 mass% or less. More preferably, it is 0.5 mass% or less.

Ni: 2.5 mass% or less
Since Ni is an element that improves hardenability, Ni is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves fatigue strength. However, Ni is an extremely expensive element, and adding more than 2.5 mass% increases the cost of the steel material. Therefore, it is preferable to add 2.5 mass% or less. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Furthermore, it is preferably 0.1 to 1.0 mass%.

V: 0.3 mass% or less V combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.5 mass%, it is preferable to make it 0.5 mass% or less. In addition, since the improvement effect of fatigue strength is small when added less than 0.01 mass%, it is desirable to add 0.01 mass% or more. More preferably, it is 0.03-0.3 mass%.

Furthermore, in this invention, 1 type (s) or 2 or more types selected from Nb: 0.1 mass% or less and Zr: 0.1 mass% or less can be contained.
Nb: 0.1 mass% or less
Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, it is preferable to make it 0.1 mass% or less. In addition, if less than 0.005% is added, the effect of improving precipitation strengthening and tempering softening resistance is small, so it is desirable to add 0.005 mass% or more. Furthermore, Preferably it is 0.01-0.05 mass%.

Zr: 0.1 mass% or less
Zr not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, it is preferable to make it 0.1 mass% or less. In addition, if less than 0.005% is added, the effect of improving precipitation strengthening and tempering softening resistance is small, so it is desirable to add 0.005 mass% or more. Furthermore, Preferably it is 0.01-0.05 mass%.

Furthermore, in the present invention, S: 0.1 mass% or less, Pb: 0.1 mass% or less, Bi: 0.1 mass% or less, Se: 0.1 mass% or less, Te: 0.1 mass% or less, Ca: 0.01 mass% or less, Mg : 0.01 mass% or less and REM: 0.1 mass% or less can be contained.
S: 0.1 mass% or less S is a useful element that forms MnS in steel and improves the machinability, but if it exceeds 0.1 mass%, it segregates at the grain boundary and lowers the grain boundary strength. , S is preferably 0.1 mass% or less. Furthermore, it is preferably 0.04 mass% or less.

Pb: 0.1 mass% or less
Bi: 0.1 mass% or less
Both Pb and Bi can be added for this purpose because they improve machinability by melting, lubrication and embrittlement during cutting. However, adding Pb: 0.1 mass% and Bi: exceeding 0.1 mass% not only saturates the effect, but also increases the component cost. In order to improve machinability, it is preferable to contain Pb in an amount of 0.01 mass% or more and Bi in an amount of 0.01 mass% or more.

Se: 0.1 mass% or less
Te: 0.1 mass% or less
Se and Te combine with Mn to form MnSe and MnTe, respectively, which act as a chip breaker to improve machinability. However, if the content exceeds 0.1 mass%, the effect is saturated and the component cost is increased. In order to improve machinability, it is preferable to contain 0.003 mass% or more in the case of Se and 0.003 mass% or more in the case of Te.

Ca: 0.01 mass% or less
REM: 0.1 mass% or less
Ca and REM each form a sulfide with MnS, which improves the machinability by acting as a chip breaker. However, even if Ca and REM are added in amounts exceeding 0.01 mass% and 0.1 mass%, respectively, the effect is saturated and the component cost is increased. In order to improve the machinability, it is preferable to contain 0.0001 mass% or more of Ca and 0.0001 mass% or more of REM.

Mg: 0.01 mass% or less
Mg is not only a deoxidizing element but also serves as a stress concentration source and has an effect of improving machinability, and can be added as necessary. However, if added excessively, the effect is saturated and the component cost increases, so 0.01 mass% or less is preferable. In order to improve machinability, Mg is preferably contained in an amount of 0.0001 mass% or more.

The balance other than the elements described above is Fe and inevitable impurities, and among the inevitable impurities, P is particularly preferably 0.02 mass% or less. This is because P is an element that lowers the grain boundary strength.
Inevitable impurities other than P include O and N. N: 0.01 mass% and O: 0.008 mass% can be allowed, respectively.

Next, the manufacturing conditions of the present invention will be described.
That is, after hot forging, further cutting or cold forging, etc. are applied to the hot rolled steel bar produced from the above-mentioned steel of the predetermined component and processed into a predetermined shape, surface hardening treatment, for example, quenching, especially induction hardening is performed. After tempering, tempering is performed.

  As surface hardening treatment conditions, it is preferable to carry out under the conditions of surface heating temperature: 850 to 1050 ° C. That is, when the surface heating temperature is less than 850 ° C., quenching becomes insufficient, while when the surface heating temperature exceeds 1050 ° C., the growth of austenite grains is promoted, and the hardened layer after quenching has coarse old austenite grains. This is to become an organization. Specifically, induction hardening is suitable.

  Furthermore, when the machine structural component is a shaft component, the surface hardening treatment preferably has a cured layer ratio of 0.3 to 0.7. Here, the hardened layer refers to a region having a Vickers hardness of Hv450 or more, and the hardened layer ratio is defined by hardened layer thickness / shaft component radius. The reason why the hardened layer ratio is 0.3 to 0.7 is that if it is less than 0.3, the fatigue strength is not sufficiently improved by quenching, and if it exceeds 0.7, the compressive residual stress is lowered and the fatigue strength is also lowered.

Next, tempering is performed by the above formula (2), that is,
10000 + 10000 (C + Mo / 2 + W / 4) ≧ (T + 273) × (log (t / 3600) +30) ≧ 10000 + 5000 (C + Mo / 2 + W / 4)
It is important to carry out under conditions that satisfy the above.

That is, FIG. 1 shows the results of arranging fatigue strengths by C + Mo / 2 + W / 4 and (T + 273) × (log (t / 3600) +30). Here, FIG. 1 shows that 0.4 to 0.7 mass% C-0.2 to 0.5 mass% Si-0.01 to 0.03 mass% Ti-0.002 mass% B steel is obtained by induction hardening at 900 ° C. at a temperature of 900 ° C. It is the result of investigating by changing the tempering temperature T and the tempering time t for a part having a hardened layer ratio of 0.45. As can be seen from this result, if the tempering condition satisfies (T + 273) × (log (t / 3600) +30), the high fatigue strength can be obtained. The fatigue strength in FIG. 1 is a strength of 3 × 10 ⁵ hours by a torsional fatigue test. In FIG. 1, C + Mo / 2 + W / 4 on the horizontal axis means an index representing the temper softening resistance by the component, and (T + 273) × (log (t / 3600) +30) on the vertical axis indicates the degree of tempering. This means an index (temper parameter) representing.
That is, if the tempering temperature is too low or if the tempering time is too short, the effect of improving the toughness is poor, and if the tempering temperature is too high or the tempering holding time is too long, the hardness of the hardened hardened layer will decrease too much. In this case, sufficient strength cannot be secured.

  Furthermore, high strength and high toughness can be achieved at a higher level by high-speed heating and short-time heat treatment such as induction tempering. Fig. 2 shows a 0.46 mass% C-0.3 mass% Mo-0.025 mass% Ti-0.002 mass% B steel that was tempered at 900 ° C (hardened layer ratio 0.45), tempering temperature T, and tempering time t. The graph which arranged the fatigue strength at the time of performing tempering by taking (T + 273) × (log (t / 3600) +30) on the horizontal axis is shown. Here, the tempering rate at the time of tempering is increased and the holding time is shortened (equivalent to induction tempering), and the one having a small temperature rising temperature and a long holding time (equivalent to tempering by furnace heating) are shown separately .

  From FIG. 2, in the case of induction tempering, specifically, (T + 273) × ((T + 273) × () by adjusting the heating temperature so that the heating temperature is set to 20 ° C./s or higher and the processing time is set to 60 seconds or less. It can be seen that even if the value of log (t / 3600) +30) is about the same, the fatigue strength can be further increased. Furthermore, rapid cooling by water cooling after heating is effective for obtaining the above effect.

  As a machine structural component of the present invention, a shaft simulating a drive shaft, output shaft, input shaft or the like of an automobile was manufactured. That is, a steel material having the composition shown in Table 1 was melted by a converter and made into a slab by continuous casting. The slab size was 300 × 400mm. This slab was rolled into a 150 mm square billet through a breakdown process, and then hot rolled into a steel bar having a diameter of 40 mm.

  Next, after cutting this steel bar to a predetermined length, the diameter is adjusted by applying surface cutting and partial cold drawing, and at the same time, a spline is formed by rolling, and the dimensions shown in FIG. And the shaft 1 having the spline part 2 having a shape was produced. This shaft was induction-quenched by moving quenching using an induction hardening apparatus having a frequency of 15 kHz, and then tempered under various conditions. At this time, the depth at which a hardness of 450 Hv or more was obtained was measured to obtain the cured layer depth. Thereafter, the torsional fatigue strength was investigated.

The torsional fatigue strength was evaluated by the repeated stress value when the number of repeated fractures was 3 × 10 ⁵ in the torsional fatigue test of the shaft. The torsional fatigue test uses a hydraulic fatigue tester, and the shaft spline part 2 shown in Fig. 3 is incorporated in each disc-shaped gripping tool, and a torsional torque is repeatedly applied at a frequency of 1 to 2 Hz between the gripping tools. It was done by doing.

It is a graph which shows the adaptation conditions of tempering. It is a graph which shows the adaptation conditions of hardening. It is a figure which shows the shaft used for a torsional fatigue test.

Explanation of symbols

1 Shaft 2 Spline part

Claims (6)

C: 0.3-0.7 mass%
Si: 1.1 mass% or less,
Mn: 2.0 mass% or less,
Al: 0.05 mass% or less,
B: 0.01 mass% or less and
Ti: 0.1 mass% or less is included within a range that satisfies the following formula (1), and
Mo: 1.0 mass% or less and W: 2.0 mass% or less, one or two of them, with the balance being subjected to surface hardening treatment by quenching to the steel having the component composition of Fe and inevitable impurities. (2) A method for producing a machine structural component having excellent fatigue characteristics, characterized by performing a tempering treatment under a condition satisfying the expression (2).
B- (11 / 14N-11 / 48Ti)> 0.0003 --- (1)
10000 + 10000 (C + Mo / 2 + W / 4) ≧ (T + 273) × (log (t / 3600) +30) ≧ 10000 + 5000 (C + Mo / 2 + W / 4) --- (2)
Where T: tempering temperature (° C.)
t: Tempering time (s) C: 0.3-0.7 mass%
Si: 1.1 mass% or less,
Mn: 2.0 mass% or less,
Al: 0.05 mass% or less,
B: 0.01 mass% or less and
Ti: 0.1 mass% or less is included within a range that satisfies the following formula (1), and
Mo: 1.0 mass% or less and W: 2.0 mass% or less containing one or two types, the balance being subjected to surface hardening treatment by quenching to the steel having the component composition of Fe and inevitable impurities, 2) A method for producing a machine structural component having excellent fatigue characteristics, wherein the tempering treatment is performed under the conditions of satisfying the formula, a temperature rising rate of 20 ° C. or more and a holding time of 60 seconds or less.
B- (11 / 14N-11 / 48Ti)> 0.0003 --- (1)
10000 + 10000 (C + Mo / 2 + W / 4) ≧ (T + 273) × (log (t / 3600) +30) ≧ 10000 + 5000 (C + Mo / 2 + W / 4) --- (2)
Where T: tempering temperature (° C.)
t: Tempering time (s) As the component composition,
Cr: 2.5 mass% or less,
Cu: 1.0 mass% or less,
The method for producing a machine structural component excellent in fatigue characteristics according to claim 1 or 2, comprising one or more selected from Ni: 2.5 mass% or less and V: 0.3 mass% or less. As the component composition,
Nb: 0.1 mass% or less and
The method for producing a machine structural component excellent in fatigue characteristics according to claim 1, 2 or 3, comprising one or two selected from Zr: 0.1 mass% or less.   The method for producing a machine structural component having excellent fatigue characteristics according to any one of claims 1 to 4, wherein the surface hardening treatment is performed under a condition of a surface heating temperature: 850 to 1050 ° C.   The method for manufacturing a machine structure part having excellent fatigue characteristics according to any one of claims 1 to 5, wherein the machine structure part is a shaft part, and a ratio of a hardened layer during the surface hardening treatment is 0.3 to 0.7.

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