JP2907700B2 - Hypoeutectoid graphite precipitated steel with excellent quenching performance and fatigue strength - 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, and more particularly, a hypoeutectoid graphite-precipitated steel having improved hardenability and fatigue strength by finely dispersing graphite. It is related to.

[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.
After processing in the state of ferrite + graphite structure, which has low strength, it is quenched and tempered in order to secure the necessary strength as a part. The excellent cold forgeability of this steel has already been disclosed in Japanese Patent Publication No. 54-30366. Regarding that the machinability is remarkably improved by the lubricating action of graphite, Japanese Patent Publication No. 53-15450, Japanese Patent Publication No. 53-15451, Japanese Patent Publication No. 53-46774,
JP-B-54-5367, JP-B-54-11773,
It is disclosed in JP-A-2-111842.

On the other hand, Cr, M having a large hardenability multiple
Since o and V have a large carbide forming ability and significantly inhibit graphitization, their use is limited. Mn with large hardenability multiple
In addition, the content is suppressed as small as possible to delay the graphitization. Therefore, the graphite-precipitated steel is excellent in workability such as cold forgeability and machinability, but has a problem in hardenability.

It has been reported that the fatigue limit of hypoeutectoid graphite-precipitated steel decreases as the average graphite grain size increases, in the Journal of the Japan Institute of Metals, Vol. 48, No. 10 (1984) 965-971. 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. Hypoeutectoid graphite-precipitated steel has hardenability despite excellent workability,
Due to poor fatigue strength, it has not actually been used industrially.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hypoeutectoid graphite-precipitated steel having excellent hardenability and an average graphite grain size, that is, a pore having a small size, and excellent fatigue characteristics. It is.

[0006]

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%.
As indications: (1) C: 0.35 to 0.75%, Si: 0.30 to
0.5%, Mn: 0.3-1.0%, P: 0.002-
0.020%, S: 0.015 to 0.10%, Al:
0.01 to 0.03%, Ti: 0.010 to 0.030
%, B: 0.001 to 0.005%, N: 0.005%
Hereinafter, Ca: 0.0003% or less as a basic component, and 0.35 to 0.7% of graphite having an average particle size of 0.5 to 5 μm.
Hypoeutectoid graphite-precipitated steel with 5% quenching performance and excellent fatigue strength. (2) C: 0.35 to 0.75%, 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 to 0.03%, Ti: 0.010 to 0.030
%, B: 0.001 to 0.005%, N: 0.005%
Hereinafter, Ca: 0.0003% or less as a basic component;
r: 0.05 to 0.30%, Mo: 0.05 to 0.30
% Or more, and an average particle size of 0.5 to 5 μm
A hypoeutectoid graphite-precipitated steel having excellent quenching performance and fatigue strength having 0.35 to 0.75% of graphite. It is in.

[0007]

The present inventors have made various studies and found that the graphite of the present hypoeutectoid graphite-precipitated steel precipitates at the ferrite crystal grain boundaries, and that BN, which is hexagonal like graphite at the grain boundaries, is present.
Found that when BN is present, BN becomes a nucleus generation site for graphite, and graphite easily precipitates and grows, and the average particle size increases to 5 to 10 μm. In order to reduce the average particle size of graphite,
It was clarified that there should be no BN that promotes graphitization. However, B is an element which has the highest hardenability multiple among various elements and is desired to be used to enhance hardenability. Therefore, Ti is added to fix N as TiN,
The present inventors have found that by reducing the amount of B precipitated as BN at the grain boundary as much as possible and utilizing it as B useful for quenching properties, it is possible to satisfy both properties of quenching properties and fatigue properties.

The reasons for defining the scope of the steel of the present invention as described above will be described below. Regarding item (1), the lower limit of C is set to 0.35% in order to secure the strength after quenching and to secure the amount of graphite necessary for obtaining sufficient machinability. . The upper limit is set to 0.65% in order to prevent cracking during cooling during quenching. Si is a powerful element that promotes graphitization because of its low bonding force with carbon atoms in steel.
It is an essential element to be one. In order to precipitate graphite by quenching + annealing treatment, it is necessary to add Si, and its lower limit must be 0.30%. Although the graphitization rate becomes 0.50% or more, the amount of Si dissolved in the ferrite phase increases and the size of the casta becomes large, so that the effect of reducing the casta by the graphitization is canceled out. .50%. Mn
It is necessary to add an amount necessary to fix and disperse sulfur in steel as MnS and an amount necessary to secure solid hardenability by dissolving 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%.

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 0.015%. The upper limit is set to 0.10% from the viewpoint of cold workability.

[0010] Al deoxidizes molten steel to increase the yield of Ti. The lower limit of Al required for this purpose is 0.01%, and the effect of deoxidation is saturated at 0.03%.
03%. Ti must have a lower limit of 0.010% in order to secure an amount necessary to fix N as TiN. This can suppress generation of BN that combines with B and contributes to the growth of graphite. This is an essential condition for improving the fatigue characteristics. The amount of Ti necessary for suppressing the formation of BN and improving the hardenability is 0.03.
%, The effect does not increase even if this value is exceeded, so the upper limit was set to 0.03%. B is added in order to ensure hardenability.
001%, and the saturation point of the effect on hardenability is 0.005%, so the lower limit is 0.001%.
%, And the upper limit is 0.005%. N reacts with B to precipitate BN at the crystal grain boundary and becomes a nucleation site for graphite precipitation, thereby facilitating graphite precipitation. However, since the average particle size of graphite needs to be increased, it is necessary to reduce as much as possible. The upper limit must be 0.005%. 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 component (2) of the present invention will be described. C, Si, Mn, P, S, Al,
The average particle diameters of Ti, N, B, Ca and graphite, and the weight of graphite are exactly the same as those in item (1). Cr and Mo are undesirable elements from the viewpoint of workability because they suppress graphitization. The upper limit is 0.3% in each case. In order to improve the hardenability, 0.05% or more is required, so the lower limit was made 0.05%.

Here, means for producing the steel of the present invention will be described. Immediately after the steelmaking and rolling processes, the steel of the present invention is produced by immediately spraying water uniformly on the surface of the steel material by a water cooling device installed on the line immediately after the end of rolling, rapidly cooling, and then annealing in a heating furnace. did.

[0014]

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. The method of manufacturing the test steel will be described. The 39 mmφ round bar immediately after finish rolling was water-cooled from 920 ° C., and then annealed at 680 ° C. for 15 hours to precipitate graphite. 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. Table 2 shows the hardenability and the fatigue limit. The round bar on which graphite was deposited was quenched at 825 ° C. × 60 min. The quenching performance was determined by Katasa in this state. As a fatigue test piece, a round bar on which graphite was deposited was heat-treated under the following conditions. Quenching: 825 ° C. × 60 min-oil cooling, tempering: 600 ° C. × 60 min-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.

[0015]

[Table 1]

[0016]

[Table 2]

The hardenability of the steel of the present invention is determined based on the size of the cast after hardening. It can be seen that the catasa of the steel of the present invention is HV650 or more and is larger than that of the comparative steel and has excellent hardenability. Further, the fatigue strength of the hypoeutectoid graphite precipitated steel of the present invention steel having a small average graphite particle size of 5 μm or less is remarkably superior to the comparative steel having a large average particle size of 6 to 9 μm.

[0018]

As apparent from the above examples, according to the present invention, it is possible to provide a hypoeutectoid graphite-precipitated steel having extremely excellent hardenability and fatigue properties, and the industrial effect is extremely high. Some are remarkable.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/14 C22C 38/32

Claims (2)

(57) [Claims] 1. C: 0.35 to 0.75%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002-0.020%, S: 0.015-0.10%,
Al: 0.01-0.03%, Ti: 0.010-0.
030%, B: 0.001 to 0.005%, N: 0.0
0.05% or less, Ca: 0.0003% or less as a basic component, and 0.35 to 5% of graphite having an average particle size of 0.5 to 5 μm.
A hypoeutectoid graphite-precipitated steel having excellent quenching performance and fatigue strength having 0.75%. 2. C: 0.35 to 0.75%, Si: 0.
30 to 0.50%, Mn: 0.3 to 1.0%, P: 0.
002-0.020%, S: 0.015-0.10%,
Al: 0.01-0.03%, Ti: 0.010-0.
030%, B: 0.001 to 0.005%, N: 0.0
0.05% or less, Ca: 0.0003% or less as basic components, Cr: 0.050 to 0.30%, Mo: 0.05 to
0.30% or more, and average particle size: 0.5
A hypoeutectoid graphite-precipitated steel having excellent quenching performance and fatigue strength having 0.35 to 0.75% of graphite having a size of 5 to 5 μm.