JPH09157786A - Steel containing uniformly dispersed graphite, excellent in toughness, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel containing uniformly dispersed graphite, excellent in machinability and cold forgeability, by specifying the composition of a steel, incorporating specific numbers of composite grains of specific grain size, also incorporating graphite of specific grain size, and regulating the ratio of graphitization to a specific value. SOLUTION: A steel, having a composition consisting of, by weight, 0.10-1.0% C, 0.50-2.0% Si, 0.10-1.0% Mn, <=0.03% P, 0.001-0.10% S, 0.0001-0.01% B, 0.0002-0.02% N, <=0.1% Mg, and the balance Fe with inevitable impurities, is used. This steel is heated to a temp. between the Ar3 point and the solution heat treatment temp. of BN, hot-rolled, subjected to forced cooling from a temp. not lower than the Ar1 point down to a temp. not higher than the Ms point at (30 to 100) deg.C/sec average cooling rate, further subjected to natural cooling, and then to graphitizing treatment at 620-740 deg.C. By the above procedure, oxide and MnS grains of 0.1-5μm grain size are incorporated by (10<3> to 10<6> ) pieces/mm<2> and also graphite of <=5.0μm average grain size is incorporated, and further, the ratio of graphitization is regulated to >=80%. By this method, the steel containing uniformly dispersed graphite, excellent in toughness after quench-and-temper treatment, can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel bar that is used after quenching and tempering after cold working (forging, cutting), and in particular, by quenching the steel by finely and uniformly dispersing graphite.
The present invention relates to a graphite homogeneously dispersed steel excellent in toughness after tempering and a manufacturing method thereof.

[0002]

2. Description of the Related Art The improvement of cold forgeability and machinability by improving the ferrite + pearlite structure of a medium carbon steel into a two-phase structure of ferrite + graphite is described in Journal of the Japan Institute of Metals, vol. 53 (1989) p. 206, Journal of the Japan Institute of Metals, vol. 52 (1988) P.I. Reported in 1285 research papers. In addition, industrially, Japanese Patent Publication Sho 63-9580
No. 53-450
No. 1, JP-B-53-46774, JP-B-53-536
7, Japanese Patent Publication No. 54-11773, JP-A No. 2-1184
It is disclosed in each gazette of No. 2. The graphite particle size obtained by these prior arts is as coarse as about 30 μm, as described in JP-B-53-46774, and at such a graphite particle size, graphite is not decomposed during quenching and heating. However, there remains a problem that a portion where graphite is present due to diffusion of carbon remains as pores of about 30 μm. In order to solve this drawback, graphite may be finely dispersed. Japanese Patent Application Laid-Open No. 2-111842 discloses that, as a specific method of miniaturizing graphite, it is effective to use BN as a precipitation nucleus of graphite and to reduce the oxygen content to 30 ppm or less. However, in this method, since the dispersed state of BN is not controlled, the obtained graphite particle size is about 5 to 10 μm, but after the quenching, voids still remain and the toughness is low. The reason for this is that, as is well known, BN segregates at the crystal grain boundary, so that graphite, which precipitates BN as a nucleation site, also precipitates at the grain boundary, and the graphite dispersion becomes non-uniform. The fluctuation increases and its maximum value also increases. In particular, when the heating and holding time is as short as several seconds such as in induction heating and quenching, diffusion of carbon becomes insufficient and a uniform austenite structure is hard to be obtained when the maximum value of the distance between graphites becomes large. In such a steel, even if the graphite particle size is reduced to shorten the graphite decomposition time, a diffusion-controlled structure and a mixed structure of martensite + ferrite is formed. Such conventional steel has a fracture toughness value (impact value) due to the mixed structure and pores.
Has a low drawback.

Regarding the conventional manufacturing method for uniformly dispersing fine graphite, the conventional knowledge for promoting the graphitization by introducing the graphite nucleation site is vo.
l. 30 (1966) P.I. 279, Journal of the Japan Institute of Metals vo
l. 43 (1979) P.P. 640. It has been reported that martensitic transformation strain and work strain are used as graphite nucleation sites, and quenching and cold working are effective as pretreatment for graphitization. The Japan Institute of Metals, vol. 4
3 (1979) P.I. According to No. 640, it is reported that the graphite dispersion state greatly changes depending on the quenching temperature, and quenching from around the Ac1 transformation temperature of 740 ° C. is most effective for graphitization and refining. Further, industrially, it is disclosed in JP-A-49-67817 as a method of utilizing a carbon supersaturated state (martensite structure) and martensitic transformation strain. According to this, after hot rolling, a steel having a specific chemical composition is reheated to 750 to 950 ° C., quenched and martensite transformed, and then reheated again and annealed at 600 to 750 ° C. to obtain a graphite dispersed steel. Has been obtained. Further, in Japanese Patent Application Laid-Open No. 7-41851, a steel containing specific components is heated to a temperature of BN and AlN solid solution or higher, and then 800
Heat to ~ 950 ° C to normalize, then 300
Manufacture of graphitizing C in steel by temporarily holding it in the temperature range of 600 ° C to 600 ° C or by directly raising the temperature to the temperature range of 650 ° C to 740 ° C and holding it for 5 hours or more without going through this holding A method is disclosed. However, in these conventional manufacturing methods, the graphite particle size is as large as 5 μm or more and the toughness value after quenching and tempering is low, or because the graphitization treatment takes a long time and the heat treatment cost is high, etc. It has not yet been put to practical use.

[0004]

SUMMARY OF THE INVENTION The present invention has been devised to solve the above problems, and the average particle size of graphite (voids after quenching) can be improved by improving the components and manufacturing conditions. The present invention provides a graphite homogeneously dispersed steel excellent in toughness in which graphite is uniformly dispersed in ferrite grains without remaining in grain boundaries, and a manufacturing method thereof.

[0005]

Means for Solving the Problems As a result of various investigations by the present inventors, by adding a specific amount of Mg, the oxide and MnS complex function as a BN nucleation site, and ferrite crystal grains are obtained. It has been found that graphite is generated in the inside and uniform dispersion of graphite can be realized. Unlike the case where a grain boundary or a single BN serves as a graphite site, an oxide or a complex of an oxide and MnS is uniformly and finely dispersed in steel, and BN or graphite having these as production sites is also a steel. It is uniformly dispersed in the inside. Martensite like conventional steel + even when heating and holding time is short like high frequency heating
Since it does not have a ferrite mixed structure and no coarse pores remain after the graphite is decomposed, a high fracture toughness value can be secured.

The present inventors, based on such findings,
Uniform dispersion and miniaturization of graphite, which were difficult in the past,
The inventors have invented a graphite uniformly dispersed steel for cold working which is excellent in toughness and a method for producing the same. The summary is as follows. (1)% by weight, C: 0.10 to 1.0%, Si:
0.50 to 2.0%, Mn: 0.10 to 1.0%, P
: 0.03% or less, S: 0.001 to 0.10%,
B: 0.0001 to 0.01%, N: 0.0002
.About.0.02%, Mg: 0.1% or less as a basic component, the balance consisting of Fe and unavoidable impurities, and a particle diameter of 0.1.
Containing 10 ³ to 10 ⁶ particles / mm ² of an oxide or a composite of oxide and MnS having an average particle size of 5.0 μm or less and having a graphitization rate Is 80
% Or more, it is a graphite homogeneously dispersed steel excellent in toughness. (2) By weight%, C: 0.10 to 1.0%, Si:
0.50 to 2.0%, Mn: 0.10 to 1.0%, P
: 0.03% or less, S: 0.001 to 0.10%,
B: 0.0001 to 0.01%, N: 0.0002
~ 0.02%, Mg: 0.1% or less, Mo: 0.05 ~
10 ³ to 10 ⁶ particles of an oxide or a complex of oxide and MnS having a basic component of 0.5%, the balance of Fe and inevitable impurities, and a particle diameter of 0.1 to 5.0 μm. / Mm ² and a graphite having an average particle size of 5.0 μm or less, and a graphitization ratio of 80% or more. (3) Solubility product of Ar ₃ and BN: log [B] [N] =-13970 / T +
After heating within a temperature range between the BN solution temperature given by 5.24 and hot rolling, the cooling start temperature is from Ar ₁ point or higher to an average cooling rate of 30 to 100 ° C./sec. The average particle diameter is 5.0 μm by forcibly cooling to the cooling end temperature Ms point or less, further natural cooling, and then graphitizing at a heating temperature range of 620 ° C. to 740 ° C.
A method for producing a uniformly dispersed graphite steel having excellent toughness, which comprises the following graphite and has a graphitization ratio of 80% or more. (4) The BN solubility product: log [B] [N] =-13970 / T + 5.24, for the steel consisting of the components described in (1).
After heating above the BN solution temperature given by, hot rolling is performed, and after cooling to a cooling end temperature Ar ₁ point or less, Ar ₃
And BN solubility product: log [B] [N] =-13970
/T+5.24, after reheating within a temperature range between the BN solution temperature and the cooling start temperature from the temperature of Ar ₁ point or higher to the average cooling rate of 30 to 100 ° C / sec. Forced cooling to below the Ms point, further natural cooling, and then graphitization treatment at a heating temperature range of 620 ° C. to 740 ° C., whereby graphite having an average particle size of 5.0 μm or less and having a graphitization rate Is 80% or more and is a method for producing a graphite uniformly dispersed steel having excellent toughness. (5) Solubility product of Ar ₃ and BN: log [B] [N] =-13970 / T +
After heating within a temperature range between the BN solution temperature given by 5.24 and hot rolling, the cooling start temperature is from Ar ₁ point or higher to an average cooling rate of 30 to 100 ° C./sec. The average particle diameter is 5.0 μm by forcibly cooling to the cooling end temperature Ms point or less, further natural cooling, and then graphitizing at a heating temperature range of 620 ° C. to 740 ° C.
A method for producing a uniformly dispersed graphite steel having excellent toughness, which comprises the following graphite and has a graphitization ratio of 80% or more. (6) The BN solubility product: log [B] [N] =-13970 / T + 5.24 is applied to the steel composed of the components described in (2).
After heating above the BN solution temperature given by, hot rolling is performed, and after cooling to a cooling end temperature Ar ₁ point or less, Ar ₃
And BN solubility product: log [B] [N] =-13970
/T+5.24, after reheating within a temperature range between the BN solution temperature and the cooling start temperature from the temperature of Ar ₁ point or higher to the average cooling rate of 30 to 100 ° C / sec. Forced cooling to below the Ms point, further natural cooling, and then graphitization treatment at a heating temperature range of 620 ° C. to 740 ° C., whereby graphite having an average particle size of 5.0 μm or less and having a graphitization rate Is 80% or more and is a method for producing a graphite uniformly dispersed steel having excellent toughness.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for defining the claims of the chemical components of the present invention as described above are shown below.

C has a lower limit of 0.1% in order to secure the strength after quenching. The upper limit is 1.0% in order to prevent quench cracks in the heat treatment after cold working.

Si is an essential element because it has a small bonding force with carbon atoms in steel and is one of the powerful elements for promoting graphitization. In order to deposit sufficient graphite by quenching + annealing to obtain a high graphitization rate, it is necessary to add Si, and the lower limit value was set to 0.5%. However, when the Si content increases, the ferrite layer solid-solution hardens and causes deterioration in cold workability. Therefore, the upper limit is set to 2.0%.

Mn is a necessary element because it binds to S and exists as MnS inclusions, forms a complex with an oxide, and serves as a site for producing graphite or BN. Therefore, its lower limit value is 0. It was set to 1%. However, if the amount of Mn becomes large, graphitization is significantly hindered, so the upper limit is 1.0%.
And

Since P causes grain boundary segregation and center segregation in steel and causes deterioration of toughness, the upper limit of P is set to 0.03%.

S binds to Mn and exists as MnS inclusions, forms an oxide complex, and is effective as a nucleation site for graphite or BN. However, if it is less than 0.001%, the effect as the BN nucleation site does not appear, so the lower limit was made 0.001%. If the S content is too large, the cold workability deteriorates, so the upper limit was set to 0.1%.

B combines with N to form BN and functions as a precipitation site for graphite. Therefore, it is effective for refining graphite. In order to obtain the miniaturization effect, 0.0001% or more must be added. However, if 0.01% or more is added, on the contrary, the refining effect is reduced, so the upper limit was made 0.01%.

N combines with B to produce BN. 0.0
The amount necessary for converting 001% to 0.01% B into BN is 0.0002% to 0.02%.

Mg forms an oxide, and particles of the oxide or a complex of the oxide and MnS are uniformly dispersed in the steel,
Nucleation sites of graphite or BN are given to miniaturize the graphite. However, if 0.1% or more is added, the refining effect is saturated and the toughness is deteriorated.

Mo is added as necessary to ensure hardenability. In order to obtain a sufficient hardenability effect, the lower limit of the amount added is set to 0.05%, and if added in excess of 0.5%, the hardness of the ferrite base increases and cold workability is impaired. The upper limit was 0.5%.

The steel of the present invention contains a specific range of number of particles of an oxide or an oxide / MnS composite having a specific range of size. The particles of the oxide or the complex of the oxide and MnS are uniformly dispersed in the steel to provide nucleation sites of graphite or BN, and the graphite is miniaturized. However, if the number of oxides or MnS and the number of oxides is less than 10 ³ / mm ² , the graphite is not uniformly dispersed and the average particle size is 5.0 μm.
More than m graphite is formed. The upper limit of the number is 10 ⁶ / m
The reason for setting m ² is that it is difficult to increase the number of oxides of 0.1 μm or more. Here, the particle size of the particles of the oxide or the complex of MnS and the oxide is 0.1 to 5.0.
The reason why the thickness is limited to μm is that if it is 0.1 μm or less, it is ineffective as a nucleation site, and if it exceeds 5.0 μm, the oxide itself becomes a starting point of fracture and causes deterioration of toughness.

The upper limit of the average particle diameter of graphite must be 5 μm from the viewpoint of toughness (impact characteristics). If it exceeds 5 μm, the quenched structure becomes a mixed structure of ferrite and martensite, and the impact value decreases.

The graphitization ratio was calculated by the following equation.

(Graphite content in steel / carbon content in steel) × 100 (%) In order to sufficiently soften the ferrite base material, the graphitization ratio must be 80% or more.

The manufacturing conditions of the present invention are set forth in claim 3 and claim.
The reason why the limitation is made as in No. 5 is shown below. Hot rolling
As for heating temperature, quenching after rolling to martensite structure
For Ar _ThreeIt must be above the point. However,
If the heating temperature is raised above the solution temperature of BN, the graphite precipitation
When BN disappears and is graphitized, coarse particles of about 10 μm
Large graphite is formed. Therefore, the upper limit of the heating temperature is BN
Was set as the solid solution temperature. Here, the solid solution temperature of BN is T (K) obtained from the solubility product log [B] [N] =-13970 / T + 5.24. [B] and [N] are
Weight% of B and N respectively. Cooling start temperature is Martensa
Graphite transformation site
And secure carbon and iron diffusion paths during graphitization
Therefore, Ar₁Must be above the point. Cooling end temperature
Obtains sufficient martensitic transformation structure to facilitate graphite formation
Must be less than or equal to the Ms point. Average cooling speed
The lower limit of 30 ° C / s is the reason for the martensite change.
Easy to graphitize by leaving processing strain to obtain the microstructure
This is because the upper limit value is 100 ° C / s.
The martensitic transformation amount does not increase even if it is rapidly cooled above this temperature.
This is because Lower limit of graphitization annealing temperature is 620 ℃
The limit value of 740 ℃ is graphitization in this temperature range
This is because the time is the shortest.

The reason why the manufacturing conditions of the present invention are limited as in claim 4 and claim 6 is as follows. Assuming that the BN precipitation treatment is performed in an offline heating furnace, the BN or B segregation zone is once solid-dissolved in austenite by heating to a solutionizing temperature of BN or higher and hot rolling. The lower limit of the heating temperature at that time must be higher than the solid solution temperature of BN.
Here, the solid solution temperature of BN is T (K) obtained from the solubility product log [B] [N] =-13970 / T + 5.24. [B] and [N] are% by weight of B and N, respectively. After that, it is cooled to below Ar ₁ by water cooling or air cooling. In order for MnS to function as an effective nucleation site for BN, the interface between MnS and iron must be inconsistent. Since the lattice matching of the MnS / austenite interface is good, it is necessary to once cool to the ferrite single phase region. That is, the cooling end temperature must be below the Ar ₁ point. Desirably, in order to secure a larger number of BN precipitation sites, cooling by water cooling is performed up to the Ms point or lower. Then, it is reheated to precipitate BN. The heating temperature must be at or above the Ar ₃ point in order to rapidly cool the structure after heating to form a martensite structure. During this heat treatment, that is, during heating and temperature increase or during retention in a heating furnace, BN is formed in which fine and uniform dispersion is made by using non-matching MnS or oxide or γ grain boundaries as precipitation sites. When the heating temperature exceeds the solution temperature of BN, BN in the graphite precipitation site disappears, and when graphitized, 10 μm
Coarse graphite is formed. Therefore, the upper limit of the heating temperature is the solid solution temperature of BN. Here, the solid solution temperature of BN is T (K) obtained from the solubility product log [B] [N] =-13970 / T + 5.24. [B] and [N] are% by weight of B and N, respectively. The cooling start temperature must be at least Ar ₁ point in order to simultaneously generate martensitic transformation strain and rolling strain to secure a graphite precipitation site and a diffusion route of carbon and iron during graphitization. The cooling end temperature must be below the Ms point in order to obtain a sufficient martensitic transformation structure and facilitate graphite formation. The lower limit of the average cooling rate is set to 30 ° C./s in order to facilitate the graphitization by leaving the processing strain in order to obtain the martensitic transformation structure, and the upper limit is set to 100 ° C./s. Is because the amount of martensitic transformation does not increase even if the material is rapidly cooled further. The lower limit of the graphitization annealing temperature is 620 ° C. and the upper limit is 740 ° C. because the graphitization time is the shortest in this temperature range.

[0023]

EXAMPLES The effects of the present invention will be more specifically described below with reference to examples.

Table 1 shows the chemical composition and graphitization ratio of the test steel,
1 shows the number of oxide particles having a particle size of 1 to 5.0 μm or a composite of MnS and an oxide, the average particle diameter of graphite, the maximum distance between graphite particles, and the Charpy absorbed energy when quenching and tempering after graphitization. The production conditions at this time are: slab-rolling, air-cooling, reheating to 950 ° C., hot rolling, and 0.3 to 0.5 per unit area on the entire surface by a water cooling device installed on the extension line of the hot rolling line.
Tons of cooling water were uniformly sprayed to cool. After that, a graphitization treatment was performed at 700 ° C. in an off-line annealing furnace. The graphitization rate was calculated by the following formula.

(Graphite content in steel / carbon content in steel) × 100 (%) The carbon content in steel was quantified by chemical analysis. The graphite content was calculated from the average graphite particle size, density and the number of graphite particles.
Regarding the number of particles of the oxide or the complex of MnS and the oxide, an extraction replica sample was prepared, and a 5000 times photograph was taken in 20 fields of view with an electron microscope to measure the number of particles. The graphite particle diameter was measured by irradiating the graphite particles with an electron beam, binarizing the intensity of the reflected electron beam, and using an image analysis system. The maximum distance between the graphite grains was an arc including an area including no graphite on the optical micrograph, and the maximum value of the diameter was defined as the maximum distance between the graphite particles. The Charpy impact value was obtained by using a steel bar in the state of graphite precipitation, quenching (850 ° C × 30 minutes → water cooling), tempering (600 ° C × 60 minutes → water cooling), and then a 2 mm U-notch Charpy impact test piece (JIS Z2202 No.
3) was sampled and subjected to a Charpy impact test at a test temperature of 20 ° C.

As shown in Table 1, all of the steels 1 to 5 satisfying the composition range of the steel of the present invention have a graphite average particle size of 5 μm or less, while the addition amount of Mg is less than the claimed range.
The steel 7 in which the addition amount of 8 or S is less than the claimed range is 7 μm or more. Further, it is understood that the maximum distance between graphites is smaller in Steels 1 to 5 satisfying the composition range of the steel of the present invention than in Steels 6 to 8 in Comparative Examples, and the graphite is uniformly and finely dispersed. The Charpy absorbed energy after quenching and tempering is remarkably large in Steels 1 to 5 as compared with Steels 6 to 8 in Comparative Examples, indicating that the steels of the present invention have excellent toughness.

[0027]

[Table 1] Table 2 shows the manufacturing conditions when the steel of the present invention was manufactured under various conditions. Table 3 shows the graphitization ratio, the average graphite particle size, the maximum distance between graphites, and the Charpy impact value after quenching and tempering.

The cooling start temperature and the cooling end temperature are values obtained by measuring the surface temperature of the steel material with a radiation thermometer, and the average cooling rate was obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. . As shown in Table 3, No. 1 satisfying the manufacturing conditions of the present invention. 1 to 5 all have an average graphite particle size of 5μ
No. which does not satisfy the manufacturing conditions, although it is less than m. 6 ~
All 8 are 5 μm or more. In addition, the maximum distance between graphites is No. 1 that satisfies the manufacturing conditions of the present invention. Nos. 1 to 5 are comparative examples. It can be seen that graphite is smaller than that of Nos. 6 to 8 and that graphite is uniformly dispersed. Charpy absorbed energy after quenching and tempering is No. Nos. 1 to 5 of Comparative Example. 6-8, which is remarkably large as compared with Nos. 6 to 8, and it can be seen that the production conditions of the present invention can produce a graphite uniformly dispersed steel excellent in toughness.

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

As is apparent from the above examples, according to the present invention, it is possible to provide a graphite uniformly dispersed steel having excellent machinability and cold forgeability and excellent toughness after quenching and tempering. The industrial effects are extremely remarkable.

Claims (6)

[Claims] 1. By weight%, C: 0.10 to 1.0%, Si: 0.50 to 2.0%, Mn: 0.10 to 1.0%, P: 0.03% or less, S: 0.001 to 0.10%, B: 0.0001 to 0.01%, N: 0.0002 to 0.02%, Mg: 0.1% or less as a basic component, the balance being Fe and unavoidable. 10 ³ to 10 ⁶ particles / mm ^{2 of} oxides or composites of oxides and MnS that are composed of specific impurities and have a particle diameter of 0.1 to 5.0 μm.
Containing graphite having an average particle size of 5.0 μm or less,
Further, a graphite uniformly dispersed steel excellent in toughness characterized by having a graphitization ratio of 80% or more. 2. By weight%, C: 0.10 to 1.0%, Si: 0.50 to 2.0%, Mn: 0.10 to 1.0%, P: 0.03% or less, S: 0.001 to 0.10%, B: 0.0001 to 0.01%, N: 0.0002 to 0.02%, Mg: 0.1% or less, Mo: 0.05 to 0.5. % As a basic component, the balance consisting of Fe and unavoidable impurities, and 10 ³ to 10 ⁶ particles / mm ² of an oxide or a composite of oxide and MnS having a particle diameter of 0.1 to 5.0 μm.
Containing graphite having an average particle size of 5.0 μm or less,
Further, a graphite uniformly dispersed steel excellent in toughness characterized by having a graphitization ratio of 80% or more. 3. A steel comprising the composition according to claim 1,
_ThreeAnd BN solubility product: log [B] [N] = − 1397
Between the BN solution temperature given by 0 / T + 5.24
After heating within the temperature range, hot rolling is performed to set the cooling start temperature.
Ar₁From the temperature above the point, the average cooling rate is 30 to 100 ° C /
Forced cooling to below the cooling end temperature Ms point in the range of sec,
Further natural cooling, then heating temperature 620 ℃ ~ 740 ℃
By graphitizing in the temperature range of
It has graphite of 5.0 μm or less and the graphitization rate is 80% or less.
Graphite uniform dispersion steel with excellent toughness characterized by the above
Manufacturing method. 4. A steel comprising the composition according to claim 1,
Solubility product of: log [B] [N] =-13970 / T +
After heating above the BN solution temperature given by 5.24, hot rolling and cooling to the cooling end temperature Ar ₁ point or less, then the solubility product of Ar ₃ and BN: log [B] [N] =
After reheating within a temperature range between the BN solution temperature given by -13970 / T + 5.24, the cooling start temperature is from Ar ₁ point or higher to an average cooling rate of 30 to 100 ° C /
Forced cooling to below the cooling end temperature Ms point in the range of sec,
Further natural cooling, then heating temperature 620 ℃ ~ 740 ℃
Of graphite having an average particle size of 5.0 μm or less and having a graphitization ratio of 80% or more by performing graphitization in the temperature range of 1. Method. 5. A steel comprising the composition according to claim 2,
Solubility product of ₃ and BN: log [B] [N] =-1397
After heating within a temperature range between the BN solution temperature given by 0 / T + 5.24 and hot rolling, the cooling start temperature is from Ar ₁ point or higher to an average cooling rate of 30 to 100 ° C. /
Forced cooling to below the cooling end temperature Ms point in the range of sec,
Further natural cooling, then heating temperature 620 ℃ ~ 740 ℃
Of graphite having an average particle size of 5.0 μm or less and having a graphitization ratio of 80% or more by performing graphitization in the temperature range of 1. Method. 6. A steel comprising the composition according to claim 2,
Solubility product of: log [B] [N] =-13970 / T +
After heating above the BN solution temperature given by 5.24, hot rolling and cooling to the cooling end temperature Ar ₁ point or less, then the solubility product of Ar ₃ and BN: log [B] [N] =
After reheating within a temperature range between the BN solution temperature given by -13970 / T + 5.24, the cooling start temperature is from Ar ₁ point or higher to an average cooling rate of 30 to 100 ° C /
Forced cooling to below the cooling end temperature Ms point in the range of sec,
Further natural cooling, then heating temperature 620 ℃ ~ 740 ℃
Of graphite having an average particle size of 5.0 μm or less and having a graphitization ratio of 80% or more by performing graphitization in the temperature range of 1. Method.