JPH1129836A - Steel for machine structural use for induction hardening - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel excellent in quenching crack resistance as well as in torsional strength and torsional fatigue strength after induction hardening and tempering by preparing a steel having a specific composition increased in Al content. SOLUTION: A steel, having a composition consisting of, by weight, 0.45-0.70% C, 0.01-0.40% Si, 0.2-2.5% Mn, <0.020% P, <0.06% S, >0.05-0.25% Al, 0.0020-0.025% N, <=0.0020% O, 0.0003-0.0060% B, and the balance Fe with inevitable impurities, is prepared. If necessary, 0.005-0.05% Ti can be incorporated. Further, one or 22 kinds among 0.05-1.0% Mo, 0.01-1.0% Cr, <=1.0% Cu, and 0.1-3.5% Ni can be incorporated. By increasing Al content as mentioned above, austenite grains can be refined at the time of induction hardening heating and the structure of a hardened layer can be refined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for a machine structure, and more particularly to a steel for a machine structure suitably applied to a drive shaft or a constant velocity joint for an automobile which is used after being subjected to induction hardening.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for automobile members to be lighter due to environmental regulations. In order to reduce the weight, it is necessary to increase the strength of the members, and from this point, there is a demand for improvement in torsional strength and torsional fatigue strength of mechanical structural members (shaft parts) constituting a power transmission system of an automobile. .

Conventionally, shaft parts such as drive shafts or constant velocity joints for automobiles are manufactured by hot forging, cutting, cold forging, etc., into a predetermined shape, and then subjected to induction hardening and tempering. The torsional strength and torsional fatigue strength, which are important characteristics of shaft parts, are secured. For example, JP-A-7-90484 discloses that C: 0.35
~ 0.70%, S: 0.005 ~ 0.15%, Al: 0.0005 ~ 0.05%,
Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.00
2 to 0.02% containing specific amounts of Si and Mn, regulating P, Cu, and O to a specific amount or less, and further or a specific amount of Cr, Mo, Ni,
A high-strength induction hardened shaft part containing Nb, V, etc., having an average hardness in a cross section defined by a specific formula of 560 or more, preventing quench cracks and having a torsional strength of 160 kgf / mm ² or more has been proposed. I have.

[0004] Japanese Patent Application Laid-Open No. 8-253842 discloses that C: 0.
35-0.65%, Si: 0.35-2.5%, Mn: 1.0-1.8%, M
o: 0.05-0.8%, S: 0.01-0.15%, Al: 0.015-0.0
5%, Ti: 0.005-0.05%, B: 0.0005-0.005%,
N: contains 0.002 to 0.01%, restricts P, Cu, and O to a specific amount or less, and further or a specific amount of one or two of Nb and V
Induction-hardened shaft with excellent torsional fatigue strength, containing one or two or more specific amounts of Cr and Ni, and having a ferrite structure fraction of 35% or less and a ferrite crystal grain size of 20 μm or less. Steel members have been proposed.

[0005]

SUMMARY OF THE INVENTION
In the technology described in Japanese Patent Application Laid-Open No. 7-90484, although cracking and static torsional strength are mentioned, no consideration is given to torsional fatigue strength. Also, JP-A-8-
In the technique described in Japanese Patent No. 253842, static torsional strength and torsional fatigue strength are mentioned, but consideration is not given to quenching crack resistance during induction hardening, which is important as a member for machine structures. According to the study of the present inventors,
In the steel described in Japanese Patent Application Laid-Open No. 8-253842, no consideration is given to quenching. If the effective hardened layer depth after induction hardening / tempering (see JIS G 0559) exceeds 5 mm, many The problem remains that even when the effective hardened layer depth is about 5 mm, the occurrence of sintering cracks is unavoidable.

The present invention advantageously solves the above problems,
An object of the present invention is to propose a steel for machine structural use for induction hardening having excellent torsional strength, torsional fatigue strength and resistance to quenching cracking after induction hardening and tempering, and a method for producing the same.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies in order to achieve the above-mentioned object, and as a result, have determined the torsional strength, torsional fatigue strength and torsional cracking resistance of steel for machine structural use after induction hardening and tempering. It has been found that increasing the Al content is extremely effective in improving both. This is because the Al content increases due to the increase in Al content, the austenite grains become finer during induction quenching heating, and the structure of the quenched hardened layer becomes finer. Increases, the amount of segregation of P, S, Cu, O causing grain boundary embrittlement decreases at the grain boundaries, and the resistance to sintering cracks improves. O is fixed as Al ₂ O ₃ , causing grain boundary embrittlement. Reduces the amount of oxygen, improves torsional strength and torsional fatigue strength. Fixes N as AlN, reduces the amount of Ti added for N fixation, and as a result, adversely affects torsional fatigue strength.
It is presumed that this is because N can be reduced, torsional fatigue strength can be improved, and a decrease in toughness can be prevented.

The present invention has been made based on the above findings. That is, the present invention provides a method for preparing C:
0.45 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.2 to 2.5%,
P: 0.020% or less, S: 0.06% or less, Al: more than 0.05 to 0.25
%, N: 0.0020 to 0.025%, O: 0.0020% or less, B: 0.
A steel for machine structural use for induction hardening having excellent torsional strength, torsional fatigue strength and resistance to quenching cracking after induction hardening and tempering, characterized by containing 0003 to 0.0060% and the balance being Fe and inevitable impurities. In addition to the main composition, it may further contain Ti: 0.005 to 0.05% by weight, and also in addition to the main composition, Mo: 0.05 to 1.
0%, Cr: 0.01-1.0%, Cu: 1.0% or less, Ni: 0.1-
One or more selected from 3.5% may be contained, and in addition to the above main composition, in weight%,
V: 0.05 to 0.5%, Nb: One to two selected from 0.01 to 0.05% may be contained. Further, in addition to the above main composition, Ti: 0.005 to 0.05%, Mo: 0.05 to 1.0%, C
r: 0.01 to 1.0%, Cu: 1.0% or less, Ni: 0.1 to 3.5%
One or two or more selected from the above may be contained. Further, in addition to the above main composition, Ti: 0.005 to 0.05%,
V: 0.05 to 0.5%, Nb: 0.01 to 0.05%, may contain one or two selected from the group. Further, in addition to the above main composition, Mo: 0.05 to 1.0%, Cr: 0.01 to 1.0%, Cu: 1.
0% or less, Ni: 0.1 to 3.5%, one or more selected from among them, V: 0.05 to 0.5%, Nb: 0.01 to 0.05%
One or two selected from the above may be contained.
Further, in addition to the above main composition, Ti: 0.005 to 0.05%, and Mo:
0.05-1.0%, Cr: 0.01-1.0%, Cu: 1.0% or less, N
i: may contain one or more selected from 0.1 to 3.5%, and one or two selected from V: 0.05 to 0.5% and Nb: 0.01 to 0.05%. .

Further, in the present invention, the following equation (1): Ms value = 538-317 (% C) -11 (% Si) -33 (% Mn) -28 (% Cr) -11 (% Mo) -17 (% Ni) -11 (% Cu) +6 (% V) +10 (% Al) (1) (where% C,% Si,% Mn% Cr,% Mo,% Ni,%
It is preferable that the Ms value defined by Cu,% V, and% Al: the content (% by weight) of each element is 315 or more.

[0010] The present invention is also directed to a steel material having any one of the above-mentioned compositions, which is formed into a predetermined shape by cutting or cold working, subjected to induction hardening, and then tempered.
A method for producing induction hardening steel having excellent torsional strength, torsional fatigue strength and resistance to quenching cracking after induction hardening and tempering, characterized in that heating is performed at a temperature of 175 ° C. or more for 10 minutes or more.

[0011]

First, the reasons for limiting the composition of the steel of the present invention will be described. C: 0.45 to 0.70% C is an element that has the greatest effect on induction hardening, and increases the hardness and depth of the hardened hardened layer and improves the torsional strength. However, if it is less than 0.45%, the hardness of the quench hardened layer is low, and it is necessary to dramatically increase the quench hardening depth to obtain the required torsional strength. The occurrence of is remarkable. On the other hand, when the content exceeds 0.70%, the machinability and the cold forgeability deteriorate, and brittle fracture occurs during the torsional test, so that the torsional strength decreases and the sintering crack resistance also decreases. For these reasons, C is limited to the range of 0.45 to 0.70%.

Si: 0.01 to 0.40% Si acts as a deoxidizing element, and also forms a solid solution in ferrite to strengthen the matrix, and also improves the tempering softening resistance after induction hardening and improves the torsional strength. is there. Such an effect is observed when the content is 0.01% or more. However, when the content exceeds 0.4%, the hardness increases due to solid solution hardening of the ferrite, and the machinability and the cold forgeability decrease. For this reason, Si was limited to the range of 0.01 to 0.40%. In addition, Preferably, it is the range of 0.01 to 0.30%.

Mn: 0.2 to 2.5% Mn is an element which improves the hardenability and deepens the hardening depth during induction hardening, but its effect is small when added less than 0.2%. On the other hand, the addition exceeding 2.5% increases the retained austenite during induction hardening,
Decreases surface hardness. For this reason, torsional strength and torsional fatigue strength decrease. Therefore, Mn is 0.2 ~
Limited to the 2.5% range. Preferably, 0.2 to 2.0
%.

P: not more than 0.020% P segregates at austenite grain boundaries, reduces grain boundary strength, reduces torsional strength and torsional fatigue strength, and promotes quenching cracking. For this reason, it is desirable to reduce the P content as much as possible. However, since the P content is acceptable up to 0.020%, the upper limit is made 0.020%.

S: 0.06% or less S forms MnS in steel and improves machinability.
If added in excess of 06%, segregation occurs at the grain boundaries and the grain boundary strength decreases. Therefore, the upper limit of S is set to 0.06%. In addition, it is preferably 0.01 to 0.04%. Al: more than 0.05 to 0.25% Al is an important element in the present invention and is positively added. Al
Acts as a deoxidizing element for lowering oxygen, and N
To form AlN and suppress the growth of austenite grains during induction quenching heating, thereby improving torsional strength and torsional fatigue strength. If the content is 0.05% or less, the above-described effect is small. On the other hand, if the content exceeds 0.25%, the effect is saturated and the cost is increased. In addition, from a point of cost, it is preferably 0.05 to 0.10%.

N: 0.0020 to 0.025% N combines with Al to form AlN, suppresses the growth of austenite grains during induction hardening heating, and improves torsional strength and torsional fatigue strength. If it is less than 0.0020%, this effect is not recognized, while if it exceeds 0.025%, the hot deformability is reduced and the surface defects of the slab are significantly increased during continuous casting. For this reason, N is limited to the range of 0.0020 to 0.025%. Incidentally, the content is preferably in the range of 0.0030 to 0.010%.

O: 0.0020% or less O exists as hard oxide-based nonmetallic inclusions and segregates at the grain boundaries to lower the grain boundary strength. Further, an increase in the amount of O coarsens oxide-based nonmetallic inclusions. The coarser oxide-based nonmetallic inclusions reduce the torsional fatigue strength, so it is desirable to reduce as much as possible.
Up to 0.0020% is acceptable. Because of this, O
Limited to 0.0020% or less.

B: 0.0003% to 0.0060% B is an element that improves the hardenability with a small amount of addition, and has the effect of increasing the quenching depth during induction hardening. In addition, B preferentially segregates at the grain boundaries, has the effect of reducing the grain boundary concentration of P, which tends to segregate at the grain boundaries, and increasing the grain boundary strength. In the present invention, since the increase in torsional strength due to the increase in the quenching depth and the increase in torsional strength and torsional fatigue strength due to the increase in the grain boundary strength can be expected. Less than 0.0003% has little effect, and 0.0060%
%, The effect corresponding to the added amount cannot be expected and the cost is increased. Therefore, B is limited to the range of 0.0003 to 0.0060%.

Ti: 0.005 to 0.05% Ti is more easily bonded to N than B, and can be added to prevent B from being bonded to N and exhibit the effect of improving the hardenability of B. If less than 0.005% is added, the effect is not observed. On the other hand, TiN formed by combining Ti and N
Becomes the starting point of fatigue fracture and lowers the torsional fatigue strength. If more than 0.05% of Ti is added, a large amount of TiN is formed, and the torsional fatigue strength is significantly reduced. For this reason, Ti
Is limited to the range of 0.005 to 0.05%.

Mo: 0.05-1.0%, Cr: 0.01-1.0%, C
u: 1.0% or less, Ni: one or more selected from 0.1 to 3.5% Mo: 0.05 to 1.0% Mo is an element that improves the hardenability and the tempering softening resistance. It can be added to adjust the hardenability of steel. Mo has the effect of reducing impurity elements such as P segregated at the grain boundaries, increasing the grain boundary strength, and suppressing grain boundary destruction. Mo improves torsional strength and torsional fatigue strength by these actions. If less than 0.05% is added, the effect is not recognized. On the other hand, if it is added in excess of 1.0%, carbides that are difficult to dissolve in austenite by short-time heating such as high-frequency heating are formed, and the torsional fatigue strength deteriorates. For this reason, Mo was limited to the range of 0.05 to 1.0%.

Cr: 0.01 to 1.0% Cr is an element that improves the hardenability and can be added to adjust the hardenability of steel. However, Cr is an expensive element, and it is economically disadvantageous to add more than 1.0%. Cu: 1.0% or less Cu is an element that improves hardenability and can be added to adjust the hardenability of steel. Further, Cu forms a solid solution or precipitates in the matrix to increase the strength of the steel and increase the torsional strength. However, if added in excess of 1.0%, the hot workability deteriorates, so Cu was limited to 1.0% or less.

Ni: 0.1-3.5% Ni is an element for improving the hardenability, and can be added to adjust the hardenability of the steel material. However, the addition of less than 0.1% does not provide the expected effect. On the other hand, Ni is an extremely expensive element, so that large amount addition is economically disadvantageous. Therefore, the addition of Ni is limited to 3.5% or less.

V: 0.05 to 0.5%, Nb: one or two selected from 0.01 to 0.05% V: 0.05 to 0.5% V increases the strength of the steel by precipitation hardening and also has a resistance to temper softening. Is an element that increases torsional strength. However, when the addition is less than 0.05%, the increase in strength is small, while when the addition exceeds 0.5%, the increase in strength corresponding to the added amount is not observed. Therefore, V is 0.05-0.5%
Limited to the range.

Nb: 0.01-0.05% Nb is an element that increases the strength of steel by precipitation hardening and also increases the tempering softening resistance, and increases the torsional strength. However, if the addition is less than 0.01%, the increase in strength is small, while if it exceeds 0.05%, the increase in strength is saturated. For this reason, V was limited to the range of 0.01 to 0.05%.

In the present invention, the following equation (1): Ms value = 538-317 (% C) -11 (% Si) -33 (% Mn) -28 (% Cr) -11 (% Mo) -17 (% Ni) -11 (% Cu) +6 (% V) +10 (% Al) (1) (where% C,% Si,% Mn% Cr,% Mo,% Ni,%
It is preferable that the Ms value defined by Cu,% V, and% Al: the content (% by weight) of each element is 315 or more.

The Ms value has a good correlation with the resistance to burning cracking.
If the s value is 315 or more, the resistance to burning cracking is remarkably improved. Further, in the present invention, a steel material having any one of the above-described compositions is processed into a predetermined shape by cutting or cold working, induction hardening is performed, and then tempering is performed at 175 ° C.
Heat at the above temperature for 10min or more.

If the tempering is insufficient and the hardness of the quenched hardened layer is high, the material is brittlely fractured in a torsion test, and the torsional strength is reduced. In order to prevent brittle fracture in such a torsion test, it is necessary to perform a tempering treatment at a temperature of 175 ° C. or more for a holding time of 10 minutes or more. 17 tempering
If the temperature is less than 5 ° C., the tempering of the quenched and hardened layer proceeds slowly, and the productivity is deteriorated. On the other hand, if the tempering treatment time is less than 10 minutes, the decomposition of the carbide will not proceed sufficiently, and the tempering will be insufficient.

[0028]

EXAMPLES Steel having the composition shown in Table 1 was melted by a converter and cast into a slab having a cross section of 300 × 400 mm by a continuous casting method. After making this slab into a 150mm square billet in the breakdown process,
Rolled to φ26 mm steel bars. From these bars, the parallel part 20mm
A torsion test piece with a notch of φ (a stress concentration coefficient of the notch portion α = 1.5) was collected. The test pieces were quenched using a 15 kHz frequency induction hardening apparatus.
Tempering treatment was performed under the conditions shown in (1), and a torsional strength test and a torsional fatigue test were performed. The quenching depth was adjusted by the output of the induction hardening device.

In the torsional strength test, the maximum torsional shear stress was determined by using a torsional tester of 500 kgf · m, and the torsional strength was determined. The torsional fatigue test was carried out using a 500 kgf · m torsional tester while changing the stress conditions in both swings, and the stress at which the life was 1 × 10 ⁶ times was evaluated as the fatigue strength. In addition, a round bar test piece of 22 mmφ having an axial V-shaped groove on the surface thereof was sampled from these steel bars and subjected to a quench cracking test.

[0030] The sintering cracking test was conducted by applying a frequency of 15 kHz to this test piece.
After the quenching treatment was performed using the induction hardening apparatus of z, the cross section was polished and observed at 10 sections (C section) perpendicular to the axial direction of the round bar test piece, and the number of cracks generated was measured. The burning cracking resistance was evaluated based on the total of the measured number of cracks generated at 10 places. Table 2 shows the results.

[0031]

[Table 1]

[0032]

[Table 2]

From Table 2, it can be seen that the examples of the present invention (test Nos. 1 to 12).
It can be seen that each of them is superior in the resistance to fire cracking, the torsional strength and the torsional fatigue strength as compared with the comparative examples out of the range of the present invention. On the other hand, in tests No. 13 and No. 14 of the comparative examples in which the tempering conditions are out of the range of the present invention, the torsional strength and the torsional fatigue strength are reduced.

In Comparative Examples No. 15 and No. 16 in which the Al and Si contents are out of the range of the present invention, the resistance to burning cracking, torsional strength and torsional fatigue strength are reduced. Test No.16
Is a shallower quenching depth of 4 mm compared to Test No. 15, which is advantageous for quenching. However, since the chemical composition is far out of the range of the present invention, the number of quenching cracks is smaller than that of Test No. 15. Is decreased, but the fire cracking resistance is deteriorated as compared with the examples of the present invention.

In the test No. 17 of the comparative example in which the Al and Si contents are out of the range of the present invention, the resistance to burning cracking, the torsional strength and the torsional fatigue strength are reduced. In Test No. 18 of Comparative Example in which the contents of C and Al were out of the range of the present invention, the resistance to burning cracking, torsional strength and torsional fatigue strength were reduced. Further, in Test No. 19 of Comparative Example in which the contents of C, S, and N are out of the range of the present invention, the resistance to sintering cracking, torsional strength and torsional fatigue strength are reduced. In particular, since the Ms value is less than 315, the resistance to burn cracking is significantly reduced.

Further, in Test No. 20, which is a comparative example in which the contents of O and Mn are out of the range of the present invention, the resistance to burning cracking, the torsional strength and the torsional fatigue strength are reduced. In particular, if the Ms value is 31
Since it is less than 5, the deterioration of resistance to burning cracking is remarkable. Also,
Test No. 2 of Comparative Example in which C and P contents are out of the range of the present invention
In No. 1, the fire cracking resistance, torsional strength and torsional fatigue strength are reduced. In particular, since the Ms value is less than 315,
Deterioration of fire cracking resistance is remarkable.

In Test No. 22, which is a comparative example having a large Ti content and deviating from the range of the present invention, the torsional fatigue strength is low. It can be seen that the steel for machine structural use within the scope of the present invention is a steel material having excellent quench cracking resistance after induction hardening, torsional strength after induction hardening and tempering, and torsional fatigue strength.

[0038]

According to the present invention, steel for machine structural use having excellent torsional strength, torsional fatigue strength and resistance to quenching cracking after induction quenching and tempering can be easily produced, and it has an industrially outstanding effect.

Claims (6)

[Claims] C. 0.45 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.2 to 2.5%, P: 0.020% or less, S: 0.06% or less, Al: more than 0.05 to 0.25% by weight%. N: 0.0020-0.025%, O: 0.0020% or less, B: 0.0003-0.0060%, characterized by the balance of Fe and inevitable impurities, torsional strength, torsional fatigue strength, and torsion cracking after induction hardening and tempering. Steel for machine structural use for induction hardening with excellent heat resistance. 2. The steel according to claim 1, further comprising 0.005 to 0.05% by weight of Ti. 3. Mo%: 0.05-1.0%, C:
r: 0.01 to 1.0%, Cu: 1.0% or less, Ni: 0.1 to 3.5%
The steel for machine structural use for induction hardening according to claim 1, comprising one or more selected from the group consisting of: 4. V: 0.05-0.5% by weight, N
The steel for machine structural use for induction hardening according to any one of claims 1 to 3, wherein the steel contains one or two kinds selected from b: 0.01 to 0.05%. 5. The Ms value defined by the following equation (1) is 315.
The steel for machine structural use for induction hardening according to any one of claims 1 to 4, which is as described above. Ms value = 538-317 (% C) -11 (% Si) -33 (% Mn) -28 (% Cr) -11 (% Mo) -17 (% Ni) -11 (% Cu) +6 ( % V) +10 (% Al) (1) where% C,% Si,% Mn% Cr,% Mo,% Ni,% C
u,% V,% Al: Content of each element (% by weight) 6. A steel material having a composition according to any one of claims 1 to 5, which is formed into a predetermined shape by cutting or cold working, induction hardening is performed, and then tempering is performed at a temperature of 175 ° C. or more. A method for producing steel for machine structural use for induction hardening having excellent torsional strength, torsional fatigue strength and resistance to quenching cracking after induction hardening and tempering, characterized in that heating is performed for 10 minutes or more.