JP2955456B2 - Hypoeutectoid graphite precipitated steel with excellent fatigue strength and cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graphite-precipitated steel which is used after quenching and tempering after cold working. In particular, hypoeutectoid graphite precipitation having improved fatigue strength and cold workability 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.
Already about the excellent cold forgeability of this steel,
It is disclosed in Japanese Patent Publication No. 54-30366. The fact that the machinability is significantly improved by the lubricating action of graphite is described in JP-B-53-15450 and JP-B-53-154.
No. 51, JP-B-53-46774, JP-B-54-53
No. 67, JP-B 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 grain size increases, see Journal of the Japan Institute of Metals, Vol. 48, No. 10, (1984) 965-9.
71. The average graphite grain size of hypoeutectoid graphite-precipitated steel proposed at present is approximately 5 to 10 μm. Hypoeutectoid graphite-precipitated steel is cold forged and / or cut and then quenched and tempered to ensure strength. In that case, graphite diffuses into the ferrite phase and graphite is used. The hole where the existed becomes a hole. It has been pointed out that the size of the pores is substantially equal to the average graphite particle size, that is, about 5 to 10 μm in diameter, which is a starting point of fatigue cracks. Although hypoeutectoid graphite-precipitated steel has excellent workability, its fatigue strength is inferior, so it has not actually been used industrially.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hypoeutectoid graphite-precipitated steel which has a small average graphite particle size, that is, a pore size, and has excellent workability and fatigue characteristics.

[0005]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is that the chemical component value is expressed by weight%.
The indications are: (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: 0.020 % to 0.10%, Al:
0.01-0.03%, N: 0.001-0.005
%, B: 0.0002% or less, Ca: 0.0003% or less, and 0.35 to 0.65% of graphite having an average particle size of 0.5 to 5 μm. Hypoeutectoid graphite precipitated steel with excellent workability. (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: 0.020 % 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.
0.5 to 0.20%, and an average particle size:
A hypoeutectoid graphite precipitated steel having 0.35 to 0.65% of graphite of 0.5 to 5 μm and excellent in fatigue strength and cold workability. It is in.

[0006]

The present inventors have conducted various studies and found that the graphite of the present hypoeutectoid graphite-precipitated steel precipitates at the ferrite grain boundaries, and that the grain boundaries are hexagonal, like graphite.
It has been found that the presence of N, a calcium oxide, a phosphorus compound, etc., becomes a nucleus generation site for graphite, and the graphite easily precipitates and grows, and the average particle size becomes as large as 5 to 10 μm. In order to reduce the average particle size of graphite, it has been clarified that it is better not to have an element that promotes graphitization or a grain boundary precipitate known as a precipitate. The present inventors made the present invention by dispersing and precipitating in ferrite grains by minimizing the elements that segregate at the grain boundaries to form precipitates. Since there is a concern that the precipitation time of graphite may be prolonged, quenching treatment was performed before the graphitizing annealing so that the carbides were sufficiently dissolved in austenite.

The reasons for defining the scope of the steel of the present invention as described above will be described below. Regarding the item (1), when the C content is 0.35% or less, since the length is small, the ferrite-pearlite structure has sufficient cold workability without graphitization even if it is not graphitized. The remarkable improvement in cold workability due to the graphitization is caused by the fact that the C content is 0.35% or more, so that the lower limit is 0.35%.
% Or more. The upper limit is set to 0.65% or less in order to prevent a reduction in 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 and annealing, Si
Need to be added, the lower limit of which is 0.30%
Must. If the content is 0.50% or more, solid solution in the ferrite phase increases the size of the casta, thereby offsetting the effect of reducing the casta by graphitization. Therefore, the upper limit is limited to 0.50%. Mn requires an amount obtained by adding an amount required 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 0.3%. It is. If the amount of Mn is large, the graphitization time becomes extremely long, so the upper limit was set to 1.0%.

[0008] P precipitates as a phosphorus compound at the grain boundaries in steel and becomes a nucleus generating site for graphite, which promotes graphitization, but increases the average graphite particle size.
%. The average graphite particle size is saturated at 0.002%, and the particle size does not decrease even if the P content is further reduced. Therefore, the lower limit is limited to 0.002%. S is M
n and exists as MnS inclusions. The S content was specified mainly from the viewpoint of workability. 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, adhesion is suppressed because the chance of contact between the ferrite and the tool is reduced. In order to suppress the adhesion, the lower limit of S needs to be 0.020 %. The upper limit is set to 0.10% from the viewpoint of cold workability.

Al is required to be 0.01% or more in order to deoxidize steel and prevent surface flaws at the time of rolling. Since the effect of deoxidation is saturated at 0.03%, the upper limit is made 0.03%. did. N
Reacts with a small amount of B in steel to precipitate BN at the crystal grain boundaries and becomes a nucleation site for graphite precipitation, which facilitates graphite precipitation, 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%, graphite does not become finer than this, so the lower limit was made 0.001%. B reacts with N to precipitate BN at the crystal grain boundaries and becomes a nucleation site for graphite precipitation, thereby promoting the growth of graphite and increasing the average crystal grain size. Therefore, the upper limit must be 0.0002%. Since BN has a hexagonal crystal structure like graphite, it is particularly likely to become a precipitation nucleus of graphite. Ca is dispersed as an oxide in the steel and serves as a nucleation site for graphite precipitation as in the case of BN, thereby significantly increasing the average particle size of graphite.
The upper limit must be 0.0003%.

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

Next, the reasons for the limitation of the components (2) of the present invention will be described. C, Si, Mn, P, S, Al,
The average particle diameter of N, B, Ca, graphite and the weight of graphite are exactly the same as those in the item (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, there is an effect of 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%.

Here, means for producing the steel of the present invention will be described. The steel of the present invention can be easily produced by ordinary steel making and rolling processes. Immediately after the end of rolling, water is evenly sprayed on the steel surface by a water cooling device installed on the line and rapidly cooled,
After that, annealing is performed for a long time in a heating furnace.

[0013]

Next, the effects of the present invention will be described more specifically by way of examples. Table 1 shows the chemical composition of the test steel and the average particle size of graphite, and Table 2 shows the cold workability and the fatigue limit. The average particle size of graphite was determined by measuring the diameter of graphite contained within 1 square mm in the cross section in the steel rolling direction, and dividing the sum by the total number of graphite. Individual graphite diameters were measured using an optical microscope at 200 magnification. The cold forgeability was evaluated by a cold upsetting test. The test conditions are as follows. Specimen shape: 20 mmφ × 30 mmh, state of specimen: ferrite-graphite structure, compressibility: 6 from 40 to 65% in 5% increments
Steps, number of test repetitions: 10 times, crack occurrence rate 50%
The maximum compression ratio of less than the maximum compression ratio was defined as the critical compression ratio, and the quality of the cold forgeability was judged based on the value of the value. As a fatigue test piece, a ferrite + graphite structure steel was heat-treated under the following conditions. Quenching: 8
25 ° C × 60min-oil cooling, tempering: 600 ° C × 60mi
n-oil cooling. 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 the hypoeutectoid graphite-precipitated steel having a small average particle size of 5 μm or less according to the present invention is as follows.
It is remarkably superior to the comparative steel as large as 9 μm, and the fatigue limit is also high.

[0017]

As is clear from the above embodiments, according to the present invention, it is possible to provide a hypoeutectoid graphite-precipitated steel having remarkably excellent fatigue strength and cold workability. Is extremely remarkable.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/06 C22C 38/14

Claims (2)

(57) [Claims] C: 0.35 to 0.65%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002 to 0.020 %, S: 0.020 to 0.10%,
Al: 0.01 to 0.03%, N: 0.001 to 0.0
05%, B: 0.0002% or less, Ca: 0.0003
% As a basic component and an average particle size of 0.5 to 5 μm
A hypoeutectoid graphite precipitated steel having 0.35 to 0.65% of graphite with excellent fatigue strength and cold workability. 2. C: 0.35 to 0.65%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002 to 0.020 %, S: 0.020 to 0.10%,
Al: 0.01 to 0.03%, N: 0.001 to 0.0
05%, B: 0.0002% or less, Ca: 0.0003
% As a basic component, Zr: 0.05 to 0.20%,
V: contains one or more kinds of 0.05 to 0.20%, and has 0.35 to 0.65% of graphite having an average particle size of 0.5 to 5 μm, and is excellent in fatigue strength and cold workability. Hypoeutectoid graphite precipitated steel.