JP3093577B2 - Fine graphite uniformly dispersed steel bar with excellent toughness and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to cold working (forging, cutting).
The present invention relates to a steel bar which is later quenched and tempered, and particularly to a fine graphite uniformly dispersed steel bar excellent in toughness after quenching and tempering by dispersing graphite finely and uniformly.

[0002]

2. Description of the Related Art When a ferrite + pearlite structure in a medium carbon steel is changed to a two-phase structure of ferrite + graphite, its hardness decreases from Hv160 to Hv110 by Vickers hardness. Therefore, when the graphite ratio is increased, the cold forgeability exceeds that of sulfur free-cutting steel.
ol. 53 (1989) p. It has been reported in 206 research papers. Industrially, as disclosed in JP-B-63-9580, it is introduced that when the microstructure is made into a two-phase structure of ferrite + graphite, the cold forgeability is remarkably improved.

It is also well known that the machining performance is improved by the lubricating action of graphite. Journal of the Japan Institute of Metals vo
l. 52 (1988) p. According to 1285, when the graphitization rate increases, the main force of the cutting force and the cutting force feed force are almost halved, the shear angle increases to reduce the shear stress, the friction coefficient decreases, and It is said that the curl radius is reduced and the processability is improved. Industrially, JP-B-53-15450, JP-B-53
No.-15451, JP-B-53-46774, JP-B-5
No. 3-5367, Japanese Patent Publication No. 54-11773,
It is disclosed that the machinability is improved as seen in the publications of -111842.

On the other hand, according to Japanese Patent Application Laid-Open No. 2-111842, graphite-dispersed steel is excellent in cold forgeability and machinability, but has a low decomposition rate of graphite during quenching and heating and does not sufficiently dissolve in austenite. Therefore, there is a drawback that quenching hardness is insufficient. Therefore, the gazette states that in order to solve this drawback, it is only necessary to make graphite finer,
As a specific method, it is disclosed that it is effective to use BN as a precipitation nucleus of graphite and to reduce the oxygen content to 30 ppm or less. Although the size of graphite obtained by the prior art has been refined, for example, as described in JP-B-53-46774, the average particle size of graphite is approximately 30 μm. However, at such a graphite particle size, the graphite remains undecomposed during quenching and heating, and the place where graphite is present due to the diffusion of carbon remains as pores of about 30 μm. There was a problem.

In order to obtain a uniform austenite structure during quenching and heating, it is necessary to diffuse carbon after graphite is decomposed and to make the carbon concentration uniform.
There is a problem with graphite dispersed steel in which N is a precipitation nucleus of graphite. That is, as is well known, BN finely precipitates graphite. However, since BN segregates at crystal grain boundaries, graphite having BN as a nucleation site similarly segregates at ferrite grain boundaries and disperses unevenly. As a result, the variation in the distance between graphites increases, and the maximum value also increases. In particular, when the heating holding time is as short as several seconds, such as induction hardening, when the maximum value of the distance between graphites is large, diffusion of carbon becomes insufficient and a uniform austenite structure is hardly formed. In such a steel, even if the graphite particle size is reduced and the decomposition time of graphite is shortened, it becomes diffusion-controlled and becomes a mixed structure of martensite and ferrite. Such a conventional steel has a defect that the fracture toughness value (impact value) is low due to the mixed structure and the voids.

Next, a conventional manufacturing method for uniformly dispersing fine graphite will be described. Conventional findings for promoting graphitization by introducing graphite nucleation sites are described in Journal of the Japan Institute of Metals, vol. 30 (1966) p. 279 and v
ol. 43 (1979) p. 640.
That is, it is described that carbon supersaturation, martensitic transformation strain, and work strain in ferrite are effective as graphite deposition sites.

Japanese Patent Application Laid-Open No. 49-67817 discloses a method utilizing the above knowledge and utilizing a carbon supersaturation state (martensite structure) and a martensitic transformation strain. According to this, C (Total): 0.45-1.
5%, graphite: 0.45 to 1.50%, Si: 0.5 to
2.5%, Mn: 0.1-2.0%, P: 0.02-
0.15%, S: 0.001 to 0.015%, N: 0.
008-0.02%, Ni: 0.1-2.0%, Al,
0.015 to 0.5% of one or two kinds of Ti, Ca:
After hot rolling a steel containing 0.0005 to 0.030%,
This is a production method of reheating to 750 to 950 ° C. and quenching to transform to martensite, reheating it again and annealing at 600 to 750 ° C. However, this manufacturing method has a problem in manufacturing cost because the annealing time for graphitization is long because no processing strain is added, and the heating step is required twice after hot rolling.

Japanese Patent Application Laid-Open No. Sho 63
No. -9580. According to this, C: 0.01
5 to 0.140%, Mn: 0.3% or less, Sol. A
l: 0.02 to 0.30%, N: 0.006% or less,
P: not more than 0.01%, S: not more than 0.010%, and satisfy the formula P (%) × S (%) ≦ 10 × 10 ⁻⁶ ,
Further, Si: 0.03 to 2.50%, Ni: 0.1 to
After hot-rolling a steel containing 4.0%, one or more of Cu: 0.03 to 1.00%, and the balance being Fe and impurities, cold rolling is performed at a rolling reduction of 30% or more. This is a manufacturing method in which strain is introduced and then annealed. However, it is not a realistic manufacturing method because a new step of cold rolling the steel bar after hot rolling at a rolling reduction of 30% is required. As described above, since knowledge on the chemical composition and production method for obtaining fine graphite uniformly dispersed steel bars with excellent toughness has not been found, except for thin and medium plates that do not require fracture toughness values, Steel bars have not yet been used on an industrial scale.

[0009]

SUMMARY OF THE INVENTION The present invention has been devised to solve the above-mentioned problems. The present invention has been made to improve the average particle size (pores after quenching) by improving the components and production conditions. (Average size of graphite) is reduced, and graphite is uniformly dispersed in ferrite grains as well as in grain boundaries.

[0010]

DISCLOSURE OF THE INVENTION The present invention relates to a fine graphite uniformly dispersed steel bar excellent in toughness which can advantageously eliminate the above-mentioned disadvantages of the conventional method, and a method for producing the same. By weight%, C: 0.30 to 1.0%, Si: 0.6
１1.3%, Mn: 0.4-1.0%, P: ≦ 0.02
0%, S: 0.015 to 0.055%, Al: 0.01
-0.10%, B: 0.001-0.004%, N:
0.002 to 0.008%, Mo: 0.05 to 0.20
% As a basic component, the balance being Fe and unavoidable impurities, an average particle size of 4.0 μm or less, and a number of particles of 3000 / mm ^2.
A fine graphite uniformly dispersed steel bar having excellent toughness, characterized by having the above graphite of 0.3 to 1.0%. By weight%, C: 0.30 to 1.0%, Si: 0.6
１1.3%, Mn: 0.4-1.0%, P: ≦ 0.02
%, S: 0.015 to 0.055%, Al: 0.01 to
0.10%, B: 0.001 to 0.004%, N: 0.
002 to 0.008%, Mo: 0.05 to 0.20%,
With the water cooling device installed on the rear surface of the hot rolling line, the cooling start temperature is _{Ar 1} point or more, and the cooling end temperature is M _S point below 30 the average cooling rate
After cooling at 100 ° C./s, it is further cooled naturally, and then graphitized at a heating temperature of 600 to 720 ° C., characterized by an average particle size of 4.0 μm or less and a number of particles of 3000 / mm.
^A method for producing a fine graphite uniformly dispersed steel bar having excellent toughness, characterized by having 0.3 to 1.0% of ^two or more graphites.

[0011]

Hereinafter, the present invention will be described in detail. The present inventors have conducted various studies, and as a result, when Mo is added, the number of graphite particles is significantly increased and the graphite particle size is reduced, and the precipitation location becomes both within the ferrite crystal grains and at the grain boundary, and the graphite becomes It has been newly found that they are uniformly dispersed. However, conventionally, as introduced in Japanese Unexamined Patent Application Publication No. Hei 2-111842 (particularly, page 7, upper left column, line 7 to line 12),
Was thought to be an element that forms a solid solution in cementite to delay the decomposition of cementite, and thus, like Cr, inhibits graphitization. As a generation mechanism of graphite particles contrary to the common sense, since Mo ₂ C has the same hexagonal crystal structure as BN, it is supposed that Mo ₂ C becomes a deposition site of graphite, which is also hexagonal. . Further, it is considered that the reason why the graphite is uniformly dispersed is related to the fact that Mo ₂ C is uniformly dispersed irrespective of the grain boundaries and the inside of the grains.

On the other hand, regarding the manufacturing method, the present inventors
The steel bar immediately after the hot rolling is cooled by a water cooling device installed on the rear surface of the hot rolling line so that the cooling start temperature is _{Ar 1} point or more.
The cooling end temperature below M _S point, the average cooling rate 30 to 10
After cooling at 0 ° C./s, it was further found that graphite was refined by further natural cooling and then graphitizing at a heating temperature of 600 to 720 ° C. This is because the rolling strain remaining in martensite is added by rapid cooling after hot rolling in addition to the martensitic transformation strain, so the total amount of strain included in martensite increases, and as a result, the number of graphite occurrence locations increases it is conceivable that.

The present invention has been made based on the above findings. The reasons for defining the scope of the present invention as described above will be described below. Regarding claim 1, C has a lower limit of 0.1 to secure the strength after quenching and to secure the amount of graphite necessary for obtaining sufficient machinability.
30%. The upper limit is set to 1.0% in order to prevent burning 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 precipitate a sufficient amount of graphite by quenching + annealing treatment to obtain a high degree of graphitization, it is necessary to add Si, and its lower limit must be 0.6%. When the content exceeds 1.3%, the graphitization ratio increases, but Si forms a solid solution with the ferrite phase.
Since the hardness increases with an increase in the content, the cold working performance deteriorates. Since the effect of reducing the hardness due to graphitization is offset, the upper limit is limited to 1.3%. Mn requires an amount obtained by adding an amount necessary to fix and disperse sulfur in steel as MnS and an amount necessary to secure strength by solid solution in a matrix, and the lower limit is 0.4. %. If the amount of Mn is large, graphitization is significantly inhibited, so the upper limit is set to 1.0%.

Since P exists as a phosphorus compound precipitated at grain boundaries in steel and exists as a solid solution in ferrite, machinability is improved, but hot workability is significantly impaired.
The upper limit was set to 0.020%. S bonds with Mn and exists as MnS inclusions. As the amount of MnS inclusions in the steel increases, the chance of contact between the tool and the MnS inclusions increases, and the MnS inclusions plastically deform on the rake face of the tool to form a coating. As a result, the chance of contact between the ferrite and the tool is reduced, so that adhesion is suppressed and the properties of the cut surface are improved. In order to suppress the adhesion, the lower limit of S needs to be 0.015%. Since S impairs cold forgeability, the upper limit is set to 0.055%.

Al removes oxygen in steel as oxide-based inclusions. Further, in order to adjust the crystal grain size, 0.01
% Or more is required. If the amount of the oxide-based inclusions is too large, the toughness is impaired. Therefore, the upper limit is set to 0.10%. B and N
Generates BN and shortens the graphitizing annealing time. In order to obtain a sufficient shortening effect, 0.001% or more of B must be added. If B exceeds 0.004%, the shortening effect is saturated, so the upper limit was made 0.004%. N
Is an amount necessary for converting B to BN from 0.001 to 0.004%, that is, 0.002 to 0.008%. Mo is
In order to precipitate graphite in ferrite grains, it must be added at 0.05% or more. If it exceeds 0.20%, the hardness of the ferrite ground increases, so the upper limit is 0.20%.
%.

The upper limit of the average particle size of graphite must be 4 μm from the viewpoint of toughness (impact characteristics). If it exceeds 4 μm, the quenched structure becomes a mixed structure of ferrite and martensite, and the impact value decreases. 300 graphite particles
If the number is less than 0 / mm ² , the distance between graphites becomes large and the diffusion distance of carbon becomes large, so that the quenched structure becomes an incomplete quenched structure of martensite + ferrite. For this purpose, the lower limit must be 3000 pieces / mm ² . In order to graphitize almost all of C in steel, the lower limit of graphite is C
The lower limit of the content should be 0.30% and the upper limit should also be equal to the upper limit of 1.0% of the C content.

C, Si, Mn, P, S, A
1, B, N, and Mo are exactly the same as in claim 1. Regarding the production conditions, the steel bar immediately after hot finish rolling is forcibly cooled by a water cooling device installed on an extension of the hot rolling line, because the rolling strain caused by hot rolling remains in the quenched martensite structure. It is. According to this method, the heat energy of the red-hot steel bar after hot rolling can be used for quenching and reheating is not required, and as a result, the cost of heat treatment can be reduced.

The cooling start temperature measured on the surface of the steel bar must be equal to or higher than the _Ar1 point in order to increase the number of graphite forming sites by simultaneously generating martensitic transformation strain and rolling strain. 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 obtain a martensitic transformation structure and to maintain the processing strain to facilitate graphitization.
C / s is because the amount of martensitic transformation does not increase even if cooling is performed more rapidly. The lower limit of the annealing temperature is limited to 600 ° C. and the upper limit is set to 720 ° C. because the graphitization time in this temperature range is the shortest.

[0019]

Next, the effects of the present invention will be described more specifically by way of examples. Table 1 shows the chemical composition, production conditions and graphitization ratio of the test steel. The steel bars used in this test are straight bars or burn-in coils having a diameter of 19 mm or more. Cooling was performed by uniformly spraying cooling water of 0.3 to 0.5 ton / m ² per unit area over the entire surface of the steel bar by a cooling device installed on an extension of the hot rolling line. The cooling device is 20m long
In a pipe having a plurality of holes for supplying cooling water on the circumference, the steel bar is cooled when passing on the center line of the pipe. The cooling start temperature and the cooling end temperature are values obtained by measuring the surface temperature of steel with a radiation thermometer.
It was determined by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. Then, it was cooled naturally and further graphitized in an offline annealing furnace. The graphitization rate was calculated by the following equation. (Graphite content in steel / Carbon content of steel) × 100 (%) The carbon content of steel was quantified by chemical analysis. The graphite content was calculated from the average graphite particle diameter, the density and the number of graphite particles.
The graphitization rate of the steel bars produced by this method is 100%, which is a remarkably excellent result, although the annealing time is as short as about 10 hours. In the case of the comparative production method, the graphitization ratio is as low as about 50%.

[0020]

[Table 1]

Table 2 shows the graphite particle size and the maximum distance between graphite. The graphite particle size was measured by the following method. By irradiating the graphite particles with an electron beam and binarizing the intensity of the reflected electron beam, the graphite was imaged on a SEM screen, and the particle size was measured and analyzed using an analysis system. The area of one visual field is 100
The number of fields of view is 25 μm × 100 μm, and the total measured area is 0.25 mm ² . The maximum distance of the graphite surface is 20 magnification.
It was measured on an optical microscope photograph of 0 times. An arc including only a portion where no graphite was present was drawn on the photograph, and the maximum value of the diameter was defined as the maximum distance between graphite. The graphite grain size and the maximum distance between graphites of the steel of the present invention are smaller than those of the comparative steel.

[0022]

[Table 2]

Table 3 shows the Charpy impact values. The Charpy impact test method is described below. After turning a steel bar having a diameter of 38 mm in a graphite precipitated state to a diameter of 25 mm, induction hardening (1000 ° C. × 3 sec → water cooling) and tempering (600 ° C. × 6 min → water cooling) were performed. A JIS No. 3 test piece of U notch was cut out from the quenched and tempered steel bar. The Charpy test temperature is 20 ° C. The Charpy impact value indicating the toughness of the steel of the present invention is significantly higher than that of the comparative steel.

[0024]

[Table 3]

[0025]

As is clear from the above examples, according to the present invention, it is possible to provide an ultrafine graphite uniformly dispersed steel bar excellent in toughness, and the industrial effect is extremely remarkable. is there.

(56) References JP-A-8-60295 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8 / 06,9 / 52

Claims (2)

(57) [Claims] C: 0.30 to 1.0% Si: 0.6 to 1.3% Mn: 0.4 to 1.0% P: ≦ 0.020% S: 0. 015 to 0.055% Al: 0.01 to 0.10% B: 0.001 to 0.004% N: 0.002 to 0.008% Mo: 0.05 to 0.20% as a basic component , The balance being Fe and unavoidable impurities, having an average particle size of 4.0 μm or less, and the number of particles of 3000 / mm ²
A fine graphite uniformly dispersed steel bar having excellent toughness, characterized by having the above graphite of 0.3 to 1.0%. 2. In% by weight, C: 0.30 to 1.0% Si: 0.6 to 1.3% Mn: 0.4 to 1.0% P: ≦ 0.020% S: 0. 015 to 0.055% Al: 0.01 to 0.10% B: 0.001 to 0.004% N: 0.002 to 0.008% Mo: 0.05 to 0.20% as a basic component , the balance being Fe and unavoidable impurities, the steel bar immediately after hot rolling, by installing water cooling unit to the rear surface of the hot rolling line, the cooling start temperature a _r1 or more points, the cooling end temperature below M _S point , Average cooling rate 30 ~
After cooling at 100 ° C./s, it is further cooled naturally, and then graphitized at a heating temperature of 600 to 720 ° C., characterized by an average particle size of 4.0 μm or less and a number of particles of 3000 / mm.
^A method for producing a fine graphite uniformly dispersed steel bar having excellent toughness and having 0.3 to 1.0% of ^two or more graphites.