JP3288563B2 - 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 machine structural steel having excellent machinability and resistance to quenching cracking, and more particularly to a drive shaft for automobiles having a torsional strength of 1400 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 predetermined processes such as cutting and cold forging. After being processed into a shape, induction hardening and tempering are generally performed to ensure 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 having a mechanical strength of 1400 MPa or more and satisfying the machinability and the resistance to burning cracking, and a method for producing the same.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, increasing the strength of the core (unhardened portion) after induction hardening and tempering increases the torsional strength. We noticed that it was extremely advantageous to make it work. Conventionally, in steel, the structure after hot rolling, forging or normalizing was ferrite + pearlite. Therefore, the present inventors have come to think that the structure of the core is ferrite + bainite or ferrite + pearlite + bainite in order to increase the strength of the core. On the other hand, it has been 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 machinability is significantly improved. The present invention was newly found and constituted the present invention.

That is, the first invention of the present invention provides a
And C: 0.35% or more and 0.60% or less, Si: 0.05% or less, Mn:
0.65% or more and 1.70% or less, P: 0.020% or less, S: 0.005
% To 0.035%, 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 having excellent machinability and resistance to fire cracking, characterized by containing 50% or less, the balance being Fe and inevitable impurities, and having a structure containing a bainite phase in an area ratio of 5 to 30%. It is.

[0008] In a second aspect of the present invention, in addition to the first aspect, Ms = 538-317 (% C) -33 (% Mn) -28 (% Cr)
Ms value defined by -17 (% Ni) -11 (% Si) -11 (% Mo) is 360 or more, and is a steel material for machine structure excellent in machinability and resistance to fire cracking. . Further, a third invention of the present invention provides a method of the present invention, in which, in mass%, C: 0.35% or more and 0.60% or less;
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 0.035% 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
A mechanical structure excellent in machinability and resistance to quenching cracking, characterized by comprising a structure containing at least one or more species, the balance being Fe and unavoidable impurities, and having a structure containing a bainite phase in an area ratio of 5 to 30%. Steel.

[0009] In a fourth aspect of the present invention, in addition to the third aspect, Ms = 538-317 (% C) -33 (% Mn) -28 (% Cr)
Ms value defined by -17 (% Ni) -11 (% Si) -11 (% Mo) is 360 or more, and is a steel material for machine structure excellent in machinability and resistance to fire cracking. . Further, the fifth invention of the present invention is characterized in that C: 0.35% to 0.60% in mass%;
i: 0.05 or less, Mn: 0.65% or more and 1.70% or less, P: 0.020
%, S: 0.005% to 0.035%, Cr: 0.15% or less, Mo: 0.05% to 0.50%, Ti: 0.01% to 0.05%
Below, 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 the heating, the bainite phase is cooled at a cooling rate of 0.2 to 2 ° C./sec.
This is a method for producing a steel material for machine structural use, which is excellent in machinability and resistance to fire cracking, characterized by having a structure containing 30%.

[0010] In a sixth aspect of the present invention, the steel material is such that, in mass%, C: 0.35% to 0.60%, Si: 0.05%
Mn: 0.65% or more and 1.70% or less, P: 0.020% or less,
S: 0.005% or more and 0.035% 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% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, Ms = 538-317 (% C) -33 (% Mn)-
Machine with excellent machinability and resistance to fire cracking, characterized in that the Ms value defined by 28 (% Cr) -17 (% Ni) -11 (% Si) -11 (% Mo) is 360 or more. This is a method for producing structural steel.

In a seventh aspect of the present invention, the steel material according to the fifth aspect is characterized in that the steel material is mass%, and C: 0.35% to 0.60%.
Below, Si: 0.05% or less, Mn: 0.65% or more and 1.70% or less,
P: 0.020% or less, S: 0.005% or more and 0.035% or less, C
r: 0.15% or less, Mo: 0.05% to 0.50%, Ti: 0.01
% To 0.05%, Al: 0.01% to 0.05%, 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
Machinability and fire-resistant cracking characterized by being a steel material containing one or more selected from 0.30% or less and Nb: 0.005% or more and 0.050% or less, with the balance being Fe and unavoidable impurities. This is a method for producing a steel material for machine structures having excellent properties.

An eighth invention of the present invention is the steel invention according to the sixth invention, wherein the steel material is mass%, and C: 0.35% to 0.60%.
Below, Si: 0.05% or less, Mn: 0.65% or more and 1.70% or less,
P: 0.020% or less, S: 0.005% or more and 0.035% or less, C
r: 0.15% or less, Mo: 0.05% to 0.50%, Ti: 0.01
% To 0.05%, Al: 0.01% to 0.05%, 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
0.30% or less, Nb: One or more selected from 0.005% to 0.050% or less, the balance consisting of Fe and unavoidable impurities, Ms = 538-317 (% C) -33 (% Mn )
Excellent in machinability and fire cracking resistance, characterized in that the Ms value defined by -28 (% Cr), -17 (% Ni), -11 (% Si), -11 (% Mo) is 360 or more. This is a method for producing steel for machine structural use.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The steel material of the present invention has a torsion strength after induction hardening and tempering of 1400 MPa or more, a tool life of 2000 mm or more in machinability, and a rate of occurrence of crazing cracks equal to or more than conventional steel. It is. Hereinafter, the present invention will be described in detail. First, the reasons for limiting the composition will be described.

C: 0.35% or more and 0.60% or less 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 1400 MPa or more after induction hardening and tempering. At least 0.35% to achieve that effect
It is necessary. However, if it is added in excess of 0.60%, the machinability decreases and the resistance to fire cracking also decreases. Therefore, the amount of C is
0.35% or more and 0.60% or less.

Si: 0.05% or less Si is an element that forms a solid solution in ferrite and strengthens it. In the present invention, Si is reduced as much as possible to soften the ferrite and improve machinability. 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. In particular, 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: not less than 0.65% and not more than 1.70% Mn is an element useful for improving hardenability, and at the same time, an element for fixing S in steel to prevent 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: 0.020% or less P segregates at the austenite grain boundary 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 0.035% 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 0.035%.

Cr: 0.15% or less Cr is a harmful element that promotes layering of pearlite and reduces machinability. In addition, Cr concentrates in cementite during heating before induction hardening and stabilizes it. Therefore, by heating before induction hardening, a residual carbide that does not form a solid solution in austenite is formed, and this serves as a starting point of fatigue cracks, especially torsional fatigue cracks, and lowers fatigue strength. Therefore, its content should be reduced as much as possible,
Acceptable up to 15%. 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%. From the viewpoint of machinability, 0.05 to 0.25% is preferable.

Ti: 0.01% or more and 0.05% or less Ti combines with N to form a nitride, and is a solid solution B useful for refining austenite grains during high-temperature heating and improving hardenability.
It is necessary to secure For that purpose, 0.01% or more is necessary. 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 necessary for reducing O in steel. For this purpose, 0.01% or more is necessary, but 0.1% is required.
If it exceeds 05%, it will form huge alumina,
Since this becomes the starting point of the fatigue fracture, thereby reducing the fatigue strength, the content is set to 0.05% or less.

N: 0.01% or less N combines with Al or Ti to form a nitride, and is useful for improving the fatigue strength by reducing the austenite grain size during high-frequency heating. However, if it exceeds 0.01%, coarse nitrides are formed and the fatigue strength is reduced. Excessive
N forms BN and reduces the amount of free B effective for hardenability. Therefore, the upper limit was set to 0.01%. For austenite grain refinement, 0.0040 to 0.0080% is suitable.

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 is made 0.0050%. Cu: 1.0% or less Cu is an element useful for improving hardenability and machinability. However, adding more than 1.0% causes hot embrittlement, 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 a useful element that increases hardenability and contributes to improvement of strength. 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%. Further, the Ms value is set to 360 or more.

The Ms value is Ms = 538-317 (% C) -33 (%
Mn) −28 (% Cr) −17 (% Ni) −11 (% Si) −11 (% Mo) The content in parentheses is the content of each element (mass%)
Means The present inventors have conducted intensive studies on the resistance to burning cracking, and as a result, have found that the resistance to burning cracking depends on the Ms value (martensite transformation start temperature) of the material. In the steel type of the present invention, when Ms = 360 or more, the resistance to quenching cracking is remarkably improved.

Next, the limitation of the organization will be described. The ratio of the bainite phase in the structure is 5 to 30% in area ratio.
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 the 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 pearlite ground is regularly arranged in a lamella shape, and the effect as a void generation site during cutting is small. On the other hand, carbides are irregular in the bainite phase, and the interface between the carbide and the ferrite phase acts most effectively as a void generation site during cutting. Thereby, when the bainite phase exists in the steel, the machinability is remarkably improved. In order to improve machinability, the bainite phase needs to be present in an amount of 5% or more. However, if it exceeds 30%, the increase in hardness is large and the machinability is rather reduced. Therefore, the ratio of the bainite phase is in the range of 5 to 30% in area ratio.

The method of 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, cooling after austenitization such as after hot rolling or hot forging or normalizing is carried out at a rate of 0.2 ° C. /
The cooling rate is preferably in the range of sec to 2 ° 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.

In addition, the final induction hardening and tempering
0.2 to 1 sec at 120 kW output using induction hardening equipment of kHz
And then quenched, and the torsional strength when tempered at 170 ° C. for 1 hour was evaluated as a standard.

[0031]

【Example】

(Example 1) A steel having a chemical composition shown in Table 1 was melted in a converter and turned into a 400 x 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 air-cooled (0.7 ° C./min). Table 2 shows the ratio of the bainite phase in the structure of the straight rod after cooling. From the ratio of the bainite phase, the microstructure of the steel material was photographed by an optical microscope, and the area ratio of the bainite phase was measured from the photograph by an image analyzer. The following tests were performed using this straight rod, and the results are shown in Table 2.

[0032]

[Table 1]

[0033]

[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 4mm
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 25 mmφ straight bar, and applying the same high frequency as in (2). 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 1-1
1 is an example of the present invention. It can be seen that the machinability is higher when the ratio of the bainite phase falls within the range of the present invention, as compared with steels 12 to 19 of Comparative Examples. The Ms value is 360 even in the range of the present invention.
The above steels have improved cracking resistance as the number of occurrences of cracking is 20 or less. The steel 17 of the comparative example has a higher carbon content than the range of the present invention, has no remarkable improvement in torsional strength, and furthermore has deteriorated resistance to fire cracking and reduced machinability. In the comparative example with a low C content, steel 15, the torsional strength is significantly reduced.
Steel 16 having a high Si content does not change the ratio of the bainite phase, but has reduced machinability. Steel 14 with low Mo content
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 560 × 400 mm, a 25 mmφ round bar was formed by hot rolling. After the completion of the hot rolling, accelerated cooling was performed to obtain a material. Example 1 using these materials
A test similar to the above was performed, and the results are shown in Table 4.

From these results, it can be seen that by setting the range of the present invention,
Also excellent in machinability, torsional strength, and fire resistance. 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.

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

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.

Continuing on the front page (72) Inventor Toshiyuki Hoshino 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Yasuhiro Omori 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation In the laboratory (56) References JP-A-9-87801 (JP, A) JP-A-8-283910 (JP, A) JP-A-8-176733 (JP, A) JP-A-8-127844 (JP, A) JP-A-6-256892 (JP, A) JP-A-4-228519 (JP, A) JP-A-3-17757 (JP, A) JP-A-4-218641 (JP, A) JP-A-2- 145744 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] 1. mass%, C: 0.35% to 0.60%,
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 0.035% 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: excellent in machinability and fire-cracking resistance characterized by containing 0.0005% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, and having a structure containing a bainite phase in an area ratio of 5 to 30%. Machine structural steel. 2. mass%, C: 0.35% or more and 0.60% or less,
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 0.035% 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 structure containing 0.0005% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, an Ms value defined by the following formula of 360 or more, and a structure containing a bainite phase in an area ratio of 5 to 30%. Ms = 538 -317 (% C) -33 (% Mn) -28 (% Cr) -17 (% N
i) -11 (% Si) -11 (% Mo) 3. mass%, C: 0.35% or more and 0.60% or less,
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 0.035% 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
A mechanical structure which is excellent in machinability and quenching crack resistance, characterized by comprising a structure containing at least one or more species, the balance being Fe and unavoidable impurities, and comprising a bainite phase in an area ratio of 5 to 30%. For steel. 4. Mass%, C: 0.35% to 0.60%,
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 0.035% 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
Containing two or more species, the balance consisting of Fe and unavoidable impurities, the Ms value defined by the following formula is 360 or more,
A steel material for machine structural use having excellent machinability and resistance to fire cracking, characterized by having a structure containing a bainite phase in an area ratio of 5 to 30%. Ms = 538 -317 (% C) -33 (% Mn) -28 (% Cr) -17 (% N
i) -11 (% Si) -11 (% Mo) 5. Mass%, C: 0.35% to 0.60%,
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 0.035% 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 the heating, the bainite phase is cooled at a cooling rate of 0.2 to 2 ° C./sec.
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%. 6. In mass%, C: 0.35% or more and 0.60% or less,
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 0.035% 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: Ms containing 0.0005% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, and defined by the following formula
A steel material having a value of 360 or more is hot-worked into a predetermined shape by hot rolling and / or hot forging, and is cooled at a cooling rate of 0.2 to 2 ° C./sec after completion of the hot working or after heating during the intermediate treatment. Thereby, the bainite phase is 5 to 30 in area ratio.
A method for producing a steel material for machine structural use having excellent machinability and resistance to burn-out cracking, wherein Ms = 538 -317 (% C) -33 (% Mn) -28 (% Cr) -17 (% N
i) -11 (% Si) -11 (% Mo) 7. The steel material is mass%, C: 0.35% or more and 0.60% or less, 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 0.035% 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
6. A steel material containing one or more selected from the group consisting of Nb: 0.005% to 0.050% and Nb: 0.005% to 0.050%, with 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. 8. The steel material has a mass% of C: 0.35% to 0.60%, Si: 0.05% or less, Mn: 0.65% to 1.70%.
Below, P: 0.020% or less, S: 0.005% or more and 0.035% 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
% Or more and 0.30% or less, Nb: one or more kinds selected from 0.005% or more and 0.050% or less, the balance being Fe and inevitable impurities, and the Ms value defined by the following formula is 360 or more. 7. The method for producing a steel material for machine structural use according to claim 6, wherein the steel material is excellent in machinability and resistance to fire cracking. Ms = 538 -317 (% C) -33 (% Mn) -28 (% Cr) -17 (% N
i) -11 (% Si) -11 (% Mo)