JP3320958B2 - Steel for mechanical structure excellent in machinability and resistance to fire cracking and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structural use having excellent machinability and resistance to quenching cracking, and more particularly to a drive shaft for automobiles having a torsional strength of 1100 MPa or more after induction hardening and tempering. The present invention relates to a steel material for machine structure suitable for application to a joint or the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, members for mechanical structures such as drive shafts and constant velocity joints for automobiles are subjected to hot forging or further normalizing hot-rolled steel bars, and are subjected to cold cutting, cold forging, etc. In general, after processing into a predetermined shape, induction hardening and tempering are performed to secure torsional strength, which is an important property as a member for a machine structure.

On the other hand, in recent years, there has been a strong demand for weight reduction of parts for automobile parts due to environmental problems, and from this point, an increase in torsional strength of automobile parts has been required. In order to increase the torsional strength, it has been considered to increase the quench hardening depth by induction hardening. However, in order to increase the quench hardening depth, it is conceivable to change the induction hardening conditions or to increase the amount of alloying elements in the steel material, but both have economic problems.

Japanese Patent Application Laid-Open No. Hei 4-218641 proposes a technique for limiting the amount of alloying elements so as to simultaneously satisfy the torsional strength, machinability and resistance to burn-out cracking of automotive members.
However, if only the chemical composition is limited, the range of the chemical composition that simultaneously satisfies the machinability, the resistance to quenching cracking and the torsional strength is narrow, and the quality level still has a problem.

[0005]

SUMMARY OF THE INVENTION The present invention is used for automobile parts, and has a high torsional strength after induction hardening and tempering.
An object of the present invention is to provide a steel material for a machine structure which has a hardness of 1100 MPa or more and simultaneously satisfies machinability and resistance to fire cracking.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, increasing the strength of the core (uncured portion) after induction hardening and tempering increases the torsional strength. We noticed that it was extremely advantageous.
Conventionally, in steel, the structure after hot rolling, forging or normalizing was ferrite + pearlite. Then, in order to increase the strength of the core, he came to think that the structure of the core was ferrite + bainite or ferrite + pearlite + bainite. On the other hand, it was known that the machinability generally depends on the microstructure in addition to the hardness, but the present inventors have found that by slightly including the bainite phase,
The present inventors have found that the machinability is significantly improved, and constituted the present invention.

That is, the first invention of the present invention provides a
And C: 0.25% or more and less than 0.35%, Si: 0.05% or less, Mn:
0.65% or more and 1.70% or less, P: 0.020% or less, S: 0.005
% To 0.030%, Cr: 0.15% or less, Mo: 0.05% or more
0.50% or less, Ti: 0.01% or more and 0.05% or less, Al: 0.01% or more and 0.05% or less, N: 0.01% or less, B: 0.0005% or more and 0.00
A steel material for machine structural use with excellent machinability and resistance to fire cracking characterized by containing 50% or less, the balance being Fe and unavoidable impurities, and having a structure containing bainite in an area ratio of 5 to 30%. is there.

[0008] The second invention of the present invention provides a method of
C: 0.25% or more and less than 0.35%, Si: 0.05% or less, Mn: 0.65
% To 1.70%, P: 0.020% or less, S: 0.005% to 0.030%, Cr: 0.15% or less, Mo: 0.05% to 0.50
%, Ti: 0.01% to 0.05%, Al: 0.01% to 0.
05% or less, N: 0.01% or less, B: 0.0005% or more and 0.0050%
Cu: 1.0% or less, Ni: 3.5% or less, V: 0.01% to 0.30%, Nb: 0.005% to 0.05
Contains one or more selected from 0% or less,
A steel for machine structural use having excellent machinability and resistance to fire cracking, characterized by having a balance of Fe and inevitable impurities and a structure containing bainite in an area ratio of 5 to 30%.

[0009] The third invention of the present invention provides a method of
C: 0.25% or more and less than 0.35%, Si: 0.05% or less, Mn: 0.65
% To 1.70%, P: 0.020% or less, S: 0.005% to 0.030%, Cr: 0.15% or less, Mo: 0.05% to 0.50
%, Ti: 0.01% to 0.05%, Al: 0.01% to 0.
05% or less, N: 0.01% or less, B: 0.0005% or more and 0.0050%
A steel material containing the following, the balance being Fe and unavoidable impurities, is hot-worked into a predetermined shape by hot rolling and / or hot forging, and after the completion of the hot working or after an intermediate treatment heating, 0.2 to 2.0 ° C / A method for producing a steel material for machine structural use having excellent machinability and resistance to quenching cracking, characterized by having a structure containing bainite in an area ratio of 5 to 30% by cooling at a cooling rate of sec.

[0010] In a fourth aspect of the present invention, the steel material has a mass% of C: 0.25% or more and less than 0.35%, Si: 0.05%
Mn: 0.65% or more and 1.70% or less, P: 0.020% or less,
S: 0.005% or more and 0.030% or less, Cr: 0.15% or less, Mo:
0.05% or more and 0.50% or less, Ti: 0.01% or more and 0.05% or less, A
l: 0.01% or more and 0.05% or less, N: 0.01% or less, B: 0.000
5% to 0.0050%, Cu: 1.0% or less, Ni: 3.5% or less, V: 0.01% to 0.30%, Nb:
One or two selected from 0.005% or more and 0.050% or less
4. The method for producing a steel material for machine structural use according to claim 3, which is a steel material containing at least one seed and the balance being Fe and unavoidable impurities.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The steel material of the present invention satisfies a torsional strength of 1100 MPa or more after induction hardening and tempering, a tool life of 5000 mm or more as a machinability, and a rate of occurrence of quenching cracking of less than 10%. Hereinafter, the present invention will be described in detail. First, the reasons for limiting the composition will be described.

C: 0.25% or more and less than 0.35% C is an element that has the greatest effect on induction hardening.
It is useful for increasing the hardness and depth of the quenched hardened layer to secure the torsional strength of 1100 MPa or more after induction hardening and tempering. At least 0.25% to achieve that effect
It is necessary. On the other hand, when 0.35% or more is added, the machinability is reduced and the resistance to fire cracking is also reduced. Therefore, the amount of C is 0.25
% To less than 0.35%.

Si: 0.05% or less Si is an element which forms a solid solution in ferrite and strengthens it, and is reduced as much as possible in the present invention. The allowable upper limit for the softening of ferrite is 0.05%, and the content of Si is 0.05% or less.
By reducing Si, the ferrite phase softens and machinability improves. Further, the effect of improving the machinability by the bainite phase of the second phase is exhibited when the ferrite phase of the first phase is sufficiently soft.

Mn: 0.65% or more and 1.70% or less Mn is an element useful for improving the hardenability, and at the same time, an element which fixes S in steel and prevents hot brittleness. At least 0.65% is required to achieve the effect, but 1.70%
If added in excess of, the pearlite fraction increases and the machinability decreases. Therefore, the Mn content is set to 0.65% or more and 1.70% or less. Preferably it is 0.65 to 1.3%.

P: not more than 0.020% P segregates at austenite grain boundaries during quenching and promotes quenching cracking. Therefore, its content should be reduced as much as possible, and the upper limit is made 0.020%. S: 0.005% or more and 0.030% or less S forms MnS in steel and improves machinability. For that purpose, 0.005% or more is required. On the other hand, since MnS tends to be a starting point of cracks and causes a decrease in strength and toughness, the upper limit of S is set to 0.030%.

Cr: 0.15% or less Cr is a harmful element that promotes layering of pearlite lamellar and reduces machinability. Furthermore, Cr concentrates in cementite during heating before induction hardening and stabilizes it,
Even after heating before induction hardening, residual carbides that do not form a solid solution in austenite are formed, which become the starting point of fatigue cracks, especially torsional fatigue cracks, and reduce the fatigue strength. acceptable. Although the upper limit of the Cr content is 0.15% or less, it is desirably in the range of 0.05% or less.

Mo: 0.05% or more and 0.50% or less Mo is not only useful for improving hardenability, but also promotes formation of bainite and improves machinability. 0 for that.
05% or more is required. On the other hand, excessive addition causes a large amount of hard bainite to be generated and reduces machinability, so the upper limit is 0.
50%. For the formation of bainite, 0.05 to 0.25% is suitable.

Ti: 0.01% or more and 0.05% or less Ti combines with N to form a nitride and is necessary to secure free B useful for improving hardenability. For that, 0.01
% Is required. On the other hand, if added in excess, the toughness is impaired, so the upper limit is made 0.05%. In the case of a normal melting method in relation to the N content, 0.01 to 0.03% of Ti is suitable.

Al: 0.01% or more and 0.05% or less Al is a strong deoxidizing element and is essential for reducing O in steel. The addition amount must be 0.01% or more, but if added over 0.05%, huge alumina is formed. This becomes the starting point of fatigue fracture and lowers the fatigue strength. Therefore, the upper limit of Al is 0.05%. And

N: 0.01% or less N combines with Ti to form TiN, and is useful for improving fatigue strength by reducing the austenite grain size during high-frequency heating. However, over 0.01%, coarse TiN
Not only reduces the fatigue strength but also forms BN to reduce the free B amount effective for hardenability. Therefore, the upper limit was set to 0.01%. For austenite grain refinement, 0.0040 to 0.0080% is preferable in relation to the amount of Ti.

B: 0.0005% or more and 0.0050% or less B enhances the hardenability and increases the quenching depth during induction hardening to increase the torsional strength. For that, 0.00
Addition of at least 05% is necessary, but if it exceeds 0.0050%, the toughness is reduced, so the upper limit was made 0.0050%. Cu: 1.0% or less Cu is an element useful for improving hardenability and machinability. However, if added in excess of 1.0%, hot embrittlement occurs, so the upper limit of the Cu content is 1.0%. The preferred content is 0.4 to 1.0%.

Ni: 3.5% or less Ni is an element useful for improving hardenability. However, if added in excess of 3.5%, the machinability decreases, so the Ni content should be 3.5% or less. The preferred content is 0.5 to 2.0
%. V: 0.01% or more and 0.30% or less V forms carbonitrides, refines austenite grains, and contributes to improvement in strength. For that purpose, 0.01% or more is required. On the other hand, if added in excess, coarse precipitates are formed and the toughness is impaired, so the upper limit is made 0.30%.

Nb: 0.005% or more and 0.050% or less Nb forms carbonitrides, refines austenite grains, and contributes to improvement in fatigue strength. For that purpose, 0.005% or more is required. On the other hand, if added in excess, coarse precipitates are formed and the toughness is impaired, so the upper limit is made 0.050%. Next, the limitation of the organization will be described.

It is necessary that bainite be contained in an area ratio of 5 to 30%. In the present invention, the structure after hot rolling, forging or normalizing is defined as bainite + ferrite or ferrite + pearlite + bainite. The machinability dramatically increases due to the presence of the bainite phase. For this reason, the present inventors consider as follows. Chips during cutting are formed separately from the base material by expansion and connection of voids generated in shearing. In a ferrite-pearlite steel, voids occur at the interface between ferrite and pearlite or at the interface between ferrite and cementite in pearlite. However, cementite in the ground of pearlite has a small effect because it is regularly arranged in a lamellar shape, whereas the bainite phase has more irregular carbides than the pearlite phase, and the interface between the carbide and the ferrite phase is poor. Since it acts most effectively as a void generation site during cutting, machinability is dramatically improved when a bainite phase is present in steel. In order to exert an effect on machinability, bainite must be present in an area ratio of 5% or more. However, if it exceeds 30%, the hardness increases greatly, and the machinability is rather reduced. Therefore, the bainite ratio is set in the range of 5 to 30% in area ratio.

The method for producing the steel material of the present invention is not particularly limited as long as it is produced according to a conventional method. As the smelting method, smelting may be performed in a converter or an electric furnace, and vacuum degassing such as RH degassing, refining in a ladle, or the like may be added. The molten steel is solidified by a continuous casting method or an ingot casting method, solidified, and then subjected to hot rolling or hot / warm forging to obtain a material having a predetermined shape. These materials are subjected to an intermediate heat treatment such as normalizing, spheroidizing annealing, softening annealing and the like as required, and finished to a desired shape by cold working such as cutting, forging, and rolling.

In the present invention, the cooling after austenitization such as after hot rolling or hot forging or normalizing is performed at a rate of 0.2 ° C./sec in order to make the structure of the steel material a predetermined amount of bainite.
Cooling rate within the range of ~ 2.0 ° C / sec. In particular, in the case of a large-diameter steel bar, it is preferable to perform accelerated cooling with cooling adjusted. Below the range of the cooling conditions, formation of the bainite phase is small, and if the cooling rate is higher than this, a hardened phase appears and the machinability is reduced.

[0027]

【Example】

(Example 1) A steel having a chemical composition shown in Table 1 was melted in a converter and turned into a 400 × 540 mm bloom by continuous casting.
It was made into a 150 mm square billet by hot rolling. The billet was heated to 1030 ° C., and then hot-rolled into a straight bar having a diameter of 25 mm. After rolling, it was cooled by air (0.7 ° C./min). Table 2 shows the ratio of the bainite phase in the structure of the straight rod after cooling. The following tests were performed using this straight rod, and the results are shown in Table 2.

[0028]

[Table 1]

[0029]

[Table 2]

(1) Machinability test A machinability test piece was taken from this straight bar. The machinability test is
SKH4, using a 4mmφ drill at 1500rpm
A 12 mm long hole was drilled, and the total drilling length (mm) until cutting became impossible was determined as the tool life and evaluated. (2) Torsion strength test A smooth round bar torsion test piece with a parallel portion of 20 mmφ was prepared from a straight bar, quenched using a 15 kHz frequency induction hardening device, and tempered at 170 ° C for 30 minutes. The test was performed. The quenching depth after induction hardening and tempering is 3mm
And In the torsion test, the maximum torsional shear strength was determined using a torsional tester of 500 kgf · m and defined as the torsional strength.

(3) Burning cracking test Burning cracking resistance was determined by processing a round bar (20 mmφ) with a V-shaped groove in the surface from the above-mentioned 25 mmφ straight bar. After quenching, 10 rounds of the C section of the round bar were polished and observed, and the number of cracks generated was evaluated. Steel 1,
2, 3, 4, 11, and 12 are examples of the present invention. Comparative Example Steel 5,
It can be seen that the machinability is higher when the ratio of the bainite phase is within the range of the present invention as compared with 6, 7, 8, and 10. The steel 8 of the comparative example has a higher carbon content than the range of the present invention, has no remarkable improvement in torsional strength, and has a reduced resistance to sintering cracking and reduced machinability. In the comparative example, steel 9 having a low C content, the torsional strength is significantly reduced. Steel 10 with a high Si content does not change the ratio of the bainite phase, but has reduced machinability.

(Example 2) Steel having the chemical composition shown in Table 3 was
After being melted in a converter to form a bloom of 400 × 540 mm, a 25 mmφ round bar was formed by hot rolling. After hot rolling 1.4 ~
The material was cooled at an accelerated rate of 3.0 ° C / sec. Using these materials, the same tests as in Example 1 were performed, and the results are shown in Table 4.

From these results, by setting the composition within the range of the present invention, the machinability, the torsional strength, and the resistance to fire cracking are also excellent.
Although the bainite phase ratio changes depending on the cooling conditions after rolling, the machinability is inferior with a bainite amount outside the range of the present invention.

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

According to the present invention, a steel material having high torsional strength after induction hardening and tempering and having both machinability and resistance to quenching cracking at the same time is obtained, and its industrial value is great.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhiro Omori 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd. Technical Research Institute (56) References 8-127844 (JP, A) JP-A-6-340924 (JP, A) JP-A-5-209223 (JP, A) JP-A-4-318146 (JP, A) JP-A-4-218641 (JP, A A) JP-A-7-118794 (JP, A) JP-A-9-111401 (JP, A) Patent 2979987 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38 / 00-38/60

Claims (4)

(57) [Claims] (1) mass%, C: 0.25% or more and less than 0.35%;
Si: 0.05% or less, Mn: 0.65% or more and 1.70% or less, P: 0.02
0% or less, S: 0.005% or more and 0.030% or less, Cr: 0.15%
Mo: 0.05% to 0.50%, Ti: 0.01% to 0.05
% Or less, Al: 0.01% or more and 0.05% or less, N: 0.01% or less,
B: 0.0005% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, and bainite having an area ratio of 5%
A steel material for machine structural use having excellent machinability and resistance to fire cracking, characterized by having a structure containing up to 30%. 2. mass%, C: 0.25% or more and less than 0.35%,
Si: 0.05% or less, Mn: 0.65% or more and 1.70% or less, P: 0.02
0% or less, S: 0.005% or more and 0.030% or less, Cr: 0.15%
Mo: 0.05% to 0.50%, Ti: 0.01% to 0.05
% Or less, Al: 0.01% or more and 0.05% or less, N: 0.01% or less,
B: 0.0005% to 0.0050%, Cu: 1.
0% or less, Ni: 3.5% or less, V: 0.01% or more and 0.30% or less, Nb: 0.005% or more and 0.050% or less
For mechanical structures with excellent machinability and fire-cracking resistance, characterized in that they contain at least one species, the balance consists of Fe and inevitable impurities, and the structure contains bainite in an area ratio of 5 to 30%. Steel. 3. mass%, C: 0.25% or more and less than 0.35%,
Si: 0.05% or less, Mn: 0.65% or more and 1.70% or less, P: 0.02
0% or less, S: 0.005% or more and 0.030% or less, Cr: 0.15%
Mo: 0.05% to 0.50%, Ti: 0.01% to 0.05
% Or less, Al: 0.01% or more and 0.05% or less, N: 0.01% or less,
B: A steel material containing 0.0005% or more and 0.0050% or less, with the balance being Fe and unavoidable impurities, is hot-rolled and / or hot-forged into a predetermined shape, and after hot working or after intermediate processing. After heating, the bainite is cooled at a cooling rate of 0.2 to 2.0 ° C./sec, so that
A method for producing a steel material for machine structural use having excellent machinability and resistance to fire cracking, characterized by having a structure containing 30%. 4. The steel material is, in mass%, C: 0.25% or more and less than 0.35%, Si: 0.05% or less, Mn: 0.65% or more and 1.70%.
Below, P: 0.020% or less, S: 0.005% or more and 0.030% or less, Cr: 0.15% or less, Mo: 0.05% or more and 0.50% or less, Ti:
0.01% or more and 0.05% or less, Al: 0.01% or more and 0.05% or less,
N: 0.01% or less, B: 0.0005% to 0.0050%, Cu: 1.0% or less, Ni: 3.5% or less, V: 0.01
4. A steel material containing one or more selected from among Nb: 0.005% to 0.050% and Nb: 0.005% to 0.050%, the balance being Fe and unavoidable impurities. Method for producing a steel material for machine structural use having excellent machinability and resistance to fire cracking.