JPH07138697A - Hypo-eutectoid graphite precipitated steel excellent in fatigue strength and cold workability - Google Patents

Abstract

PURPOSE:To obtain a hypo-eutectoid graphite precipitated steel which is used by being subjected to quenching-and-tempering after cold working and is improved in fatigue strength and cold workability by finely dispersing graphite, in particular. CONSTITUTION:This steel is a hypo-eutectoid graphite precipitated steel composed essentially of 0.35-0.65% C, 0.30-0.50% Si, 0.3-1.0% Mn, 0.002-0.020% P, 0.015-0.10% S, 0.01-0.03% Al, 0.001-0.O05% N, <=0.0002% B, and <=0.0003% Ca, having 0.35-0.65% graphite of 0.5-5mum average grain size, and excellent in fatigue strength and cold workability. By this method, the hypo-eutectoid graphite precipitated steel, excellent in fatigue strength and cold workability can be obtained.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is a graphite-precipitated steel that is used after quenching and tempering after cold working. In particular, it is a hypoeutectoid graphite precipitate in which fatigue strength and cold workability are improved by finely dispersing graphite. It is related to steel.

[0002]

2. Description of the Related Art Graphite-precipitated steel is a steel in which graphite and, in some cases, cementite are dispersed in a ferrite structure.
Regarding the excellent cold forgeability of this steel,
It is disclosed in Japanese Examined Patent Publication No. 54-30366. Further, regarding that the machinability is remarkably improved by the lubricating action of graphite, Japanese Patent Publication Nos. 53-15450 and 53-154.
No. 51, Japanese Patent Publication No. 53-46774, Japanese Patent Publication No. 54-53
67, Japanese Patent Publication No. 54-11773, JP-A-2-111.
No. 842.

On the other hand, the fatigue limit of hypoeutectoid graphite-precipitated steel decreases as the average graphite particle size increases. It is noted that the Metallurgical Society of Japan, Vol. 48, No. 10 (1984) 965-9.
71. The currently proposed hypoeutectoid graphite precipitated steel has an average graphite particle size of approximately 5 to 10 μm. Hypo-eutectoid graphite-precipitated steel is used after being cold forged and / or cut and then quenched and tempered in order to secure its strength. The place where was present becomes a hole. It has been pointed out that the size of the pores is almost equal to the average graphite particle size and the diameter is about 5 to 10 μm, which is a large starting point for fatigue cracks. Although hypoeutectoid graphite-precipitated steel has excellent workability, it has not been industrially used because of its poor fatigue strength.

[0004]

DISCLOSURE OF THE INVENTION The present invention is intended to provide a hypoeutectoid graphite-precipitated steel excellent in workability and fatigue characteristics by reducing the average graphite grain size, that is, the size of pores.

[0005]

The present invention has been made to solve the above-mentioned problems, and the gist thereof is that the chemical component value is% by weight.
As an indication, (1) C: 0.35 to 0.65%, Si: 0.30 to
0.50%, Mn: 0.3 to 1.0%, P: 0.002
~ 0.020%, S: 0.015 to 0.10%, Al:
0.01-0.03%, N: 0.001-0.005
%, B: 0.0002% or less, Ca: 0.0003% or less as a basic component, and having 0.35 to 0.65% graphite having an average particle diameter of 0.5 to 5 μm, fatigue strength, cold Hypo-eutectoid graphite precipitated steel with excellent workability. (2) C: 0.35 to 0.65%, Si: 0.30
0.50%, Mn: 0.3 to 1.0%, P: 0.002
~ 0.020%, S: 0.015 to 0.10%, Al:
0.01-0.03%, N: 0.001-0.005
%, B: 0.0002% or less, Ca: 0.0003% or less, Zr: 0.05 to 0.20%, V: 0.
05 to 0.20% of one or more kinds, and an average particle size:
A hypoeutectoid graphite-precipitated steel excellent in fatigue strength and cold workability, having 0.55 to 5 μm graphite 0.35 to 0.65%. It is in.

[0006]

In other words, as a result of various investigations by the present inventors, the graphite of the present hypoeutectoid graphite-precipitated steel is precipitated in the ferrite crystal grain boundaries, and the grain boundaries are hexagonal crystals B like graphite.
It has been found that the presence of N, calcium oxides, phosphorus compounds, etc. serves as nucleation sites for graphite and facilitates the precipitation and growth of graphite, resulting in a large average particle size of 5 to 10 μm. In order to reduce the average particle size of graphite, it was clarified that there should be no grain boundary precipitates known as elements or precipitates that promote graphitization. The present inventors have accomplished the present invention by reducing the elements that segregate at grain boundaries to form precipitates, and disperse and precipitate them in ferrite grains. Since it is feared that the precipitation time of graphite will be long due to this, the quenching treatment was performed before the graphitization annealing to sufficiently dissolve the carbide in austenite.

The reasons why the claims of the steel of the present invention are defined as above are as follows. As for the item (1), when the C content is 0.35% or less, since the roughness is small, the ferrite-perlite structure remains sufficient without graphitization and has sufficient cold workability. The cold workability is remarkably improved by graphitization because the C content is 0.35% or more.
% And above. The upper limit is set to 0.65% or less in order to prevent deterioration of workability during hot rolling or hot forging. S
i is an essential element because it has a small bonding force with carbon atoms in steel and is one of the powerful elements that promote graphitization.
In order to precipitate graphite by quenching + annealing,
Is required, the lower limit of which is 0.30%
Must. If it is 0.50% or more, it is solid-dissolved in the ferrite phase to increase the size and the effect of reducing the size due to graphitization is offset, so the upper limit was limited to 0.50%. The amount of Mn needs to be the amount required to fix and disperse sulfur in steel as MnS and the amount required to form a solid solution in the matrix to secure strength, and the lower limit value is 0.3%. Is. When the amount of Mn becomes large, the graphitization time becomes extremely long, so the upper limit was made 1.0%.

[0008] P precipitates as a phosphorus compound in the grain boundaries in the steel and becomes a nucleation site of graphite, which promotes graphitization, but increases the average graphite particle size, so the upper limit is 0.02.
It must be%. The average graphite particle size is saturated at 0.002% and the particle size does not become smaller even if the P content is further reduced, so the lower limit was limited to 0.002%. S is M
It is present as MnS inclusions in combination with n. The S content was specified mainly from the viewpoint of workability. As the amount of MnS inclusions in steel increases, the chances of contact between the tool and MnS inclusions increase, and the MnS inclusions plastically deform on the tool rake face to form a film. As a result, adhesion is suppressed due to the reduced chance of contact between the ferrite and the tool. In order to suppress the adhesion, the lower limit value of S is 0.015%. The upper limit was 0.10% from the viewpoint of cold workability.

Al is required to be 0.01% or more in order to deoxidize the steel and prevent surface defects during rolling. The effect of deoxidation is saturated at 0.03%, so the upper limit is 0.03%. did. N
Reacts with a small amount of B present in the steel to precipitate BN at the crystal grain boundaries and serves as a nucleation site for graphite precipitation, which facilitates the precipitation of graphite, but increases the average particle size of graphite.
The upper limit must be 0.005% or less. Even if the N content is less than 0.001%, the graphite does not become finer, so the lower limit was made 0.001%. The upper limit must be 0.0002% because B reacts with N to precipitate BN at the crystal grain boundaries and serves as a nucleation site for graphite precipitation, promoting graphite growth and increasing the average crystal grain size. Since the crystal structure of BN is a hexagonal system like graphite, it tends to become a precipitation nucleus of graphite. Ca is dispersed as an oxide in the steel and becomes a nucleation site for graphite precipitation like BN, which significantly increases the average particle size of graphite.
The upper limit should be 0.0003%.

From the viewpoint of fatigue strength, the upper limit of the average particle size of graphite must be 5 μm. Even if the thickness is smaller than 0.5 μm, the fatigue strength is not further improved, so the lower limit value was made 0.5 μm. Since almost all C in steel is graphitized, the amount of graphite is almost equal to the C content. Therefore, the limitation of the amount of graphite is exactly the same as the reason described in the reason for limiting C.

Next, the reasons for limiting the components of the item (2) of the present invention will be described. C, Si, Mn, P, S, Al,
N, B, Ca, the average particle diameter of graphite, and the weight of graphite are exactly the same as in (1). Zr and V are mainly dispersed as oxides in ferrite crystal grains and serve as nucleation sites for graphite. Since the number of nucleation sites increases, it is effective in reducing the average particle size of graphite. When added in an amount of 0.20% or more, carbides are formed to inhibit graphitization, so the upper limit was made 0.20%. If it is less than 0.05%, the number of nucleation sites is reduced and the effect of refining graphite is small, so the lower limit was made 0.05%.

Reference will now be made to the means for producing the steel of the present invention. The steel of the present invention can be easily manufactured by a conventional steel manufacturing method and rolling process. Immediately after the end of rolling, the water cooling device installed on the line uniformly sprays water on the surface of the steel material for rapid cooling,
After that, it is a method of annealing for a long time in a heating furnace.

[0013]

EXAMPLES Next, the effects of the present invention will be more specifically illustrated by the following examples. Table 1 shows the chemical composition of the sample steel and the average particle size of graphite, and Table 2 shows the cold workability and fatigue limit. The average particle diameter of graphite was determined by measuring the diameter of graphite contained in a square section of a steel material in the rolling direction and dividing the total by the total number of graphite. Individual graphite diameters were measured using an optical microscope at 200x magnification. Cold forgeability was evaluated by a cold upsetting test. The test conditions are as follows. Test piece shape: 20 mmφ × 30 mmh, test piece state: ferrite-graphite structure, compressibility: 40 to 65%, 6 in 5% increments
Stage, number of test repetitions: 10 times, crack occurrence rate is 50%
The maximum compressibility that is less than the critical compressibility was defined as the critical compressibility, and whether the cold forgeability was good or bad was determined based on the value. As the fatigue test piece, ferrite + graphite structure steel was heat-treated under the following conditions. Quenching: 8
25 ° C x 60 min-oil cooling, tempering: 600 ° C x 60 mi
n-oil cooled. After cutting a test piece having a parallel portion of 8 mmφ, the fatigue limit was measured by a rotary bending fatigue tester.

[0014]

[Table 1]

[0015]

[Table 2]

The cold forgeability of hypoeutectoid graphite-precipitated steel having a small average grain size of 5 μm or less of the graphite of the present invention has an average grain size of 6 to
It is significantly superior to the comparative steel having a large size of 9 μm and has a high fatigue limit.

[0017]

As is clear from the above examples, according to the present invention, it is possible to provide a hypoeutectoid graphite-precipitated steel which is extremely excellent in fatigue strength and cold workability. Is extremely prominent.

Claims (2)

[Claims] 1. C: 0.35 to 0.65%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002-0.020%, S: 0.015-0.10%,
Al: 0.01 to 0.03%, N: 0.001 to 0.0
05%, B: 0.0002% or less, Ca: 0.0003
% Or less as a basic component, and an average particle diameter: 0.5 to 5 μ
A hypoeutectoid graphite-precipitated steel excellent in fatigue strength and cold workability, having 0.35 to 0.65% m graphite. 2. C: 0.35 to 0.65%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002-0.020%, S: 0.015-0.10%,
Al: 0.01 to 0.03%, N: 0.001 to 0.0
05%, B: 0.0002% or less, Ca: 0.0003
% Or less as a basic component, Zr: 0.05 to 0.20%,
V: containing 0.05 to 0.20% of 1 or more, and having 0.35 to 0.65% of graphite having an average particle size: 0.5 to 5 μm, excellent in fatigue strength and cold workability. Hypo-eutectoid graphite precipitation steel.