JP3069256B2 - Nitriding steel with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitriding steel having excellent toughness. More specifically, the present invention relates to a nitriding steel excellent in toughness suitable for steel materials for various sliding parts used in civil engineering construction machines, such as bushes incorporated in a hydraulic breaker of a hydraulic shovel.

[0002]

2. Description of the Related Art Various sliding parts used in civil engineering and construction equipment, such as bushes incorporated in a hydraulic breaker of a hydraulic excavator, are required to have abrasion resistance because they violently slide with pistons and chisels. Is done. On the other hand, civil engineering and construction machines are subject to severe impact during use. Therefore, the above-mentioned various sliding parts are simultaneously required to have excellent impact resistance.

Generally, various sliding parts for civil engineering construction machines are subjected to a nitriding treatment to improve wear resistance. But,
As described above, since excellent impact resistance is also required, SACM645 steel (JIS G 4202 (1
979)) and other steels with inferior impact resistance are not used and are exclusively SCM435 steel and SCM440 steel (both JIS G410
5 (1979)) Furthermore, SNCM420 steel (JIS G 4103 (197
Those obtained by subjecting these steels to nitriding treatment by using 9)) have been used as the above-mentioned various sliding parts.

However, the aforementioned SCM435 steel, SCM
Since both 440 steel and SNCM420 steel are inferior in nitriding properties, there is a problem that it takes a long time for nitriding, resulting in poor productivity and high cost. Furthermore, SC
In the case of M440 steel or SCM435 steel having a high C content, a problem may occur in terms of impact resistance.

Japanese Patent Application Laid-Open No. 49-101216 proposes a rapid nitrided steel containing V. Although this steel is considered to be excellent in wear resistance and fatigue resistance, it is a steel in which impact resistance is not considered at all. Therefore, if this steel is nitrided and used for various sliding parts for civil engineering and construction equipment, it seems that it cannot withstand a severe impact during use.

Therefore, users are expected to develop inexpensive steel having excellent nitriding properties and impact resistance, so that various sliding parts for civil engineering and construction equipment can be produced at low cost and efficiently. I was

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel having excellent impact resistance and nitriding properties suitable for steel materials for various sliding parts for civil engineering construction machines, that is, a steel for nitriding having excellent toughness.
In particular, it is an object of the present invention to provide an inexpensive nitriding steel excellent in toughness suitable for a steel material for bushes or the like incorporated in a hydraulic breaker of a hydraulic shovel.

[0008]

Means for Solving the Problems As a result of repeated studies to achieve the above object, the present inventors have obtained the following findings (a) to (g).

(A) Al, which is well known as an element that significantly improves the nitriding characteristics, is 0.100% by weight.
If it is contained in excess of, the oxide-based inclusions become coagulated and coarse, and the toughness is greatly reduced.

(B) The N content of the steel greatly affects the nitriding characteristics. If the content exceeds 0.0150% by weight, the nitriding characteristics are significantly reduced.

(C) In steels in which the contents of Al and N are 0.100% or less and 0.0150% or less by weight, respectively, the contents of Cr and Mo are set to specific ranges. When controlled, the characteristics of the nitrided layer (“the maximum hardness of the nitrided layer” and “depth of the nitrided layer”) can be arranged by the value of fn1 represented by the following equation. Nitride layer characteristics must be obtained.

Fn1 = (2/3)% Cr + (1/3)% Mo +% Al-25% N where% Cr means weight% of Cr content,% Mo,
The same applies to% Al and% N.

(D) As the characteristics of the nitrided layer, if the maximum hardness of the nitrided layer is 700 or more in Hv and the depth of the nitrided layer up to Hv 500 is 0.3 mm or more, sufficient wear resistance can be obtained. Be done.

(E) So-called "cavitation" occurs in various sliding parts for civil engineering construction machines.

Cavitation is a type of corrosion phenomenon caused by a strong flow of hydraulic oil (oil for operating a piston) or a change in pressure. In general, when cavitation occurs, the life of various sliding parts is known to be extremely short.However, even if cavitation occurs, if the core hardness of the parts is 250 or more in Hv,
Able to suppress the life of various sliding parts.

(F) As a result of a preliminary test incorporated in an actual machine, in order to ensure that various sliding parts can sufficiently withstand the impact during use and do not break, the toughness of the core of the part is determined by the impact value at room temperature of a JIS No. 3 test piece. Should be 120 J / cm ² or more.

(G) In order to ensure the above-mentioned impact value at the core of the component after nitriding, C, Si, M
The content of n and Al, especially, the content of Al must be strictly controlled.

The present invention based on the above findings provides the following (1) to
The gist is a nitriding steel having the chemical composition shown in (4) and having excellent toughness.

(1) C: 0.15 to 0.35% by weight
%, Si: 0.10 to 1.00%, Mn: 0.30 to
2.00%, Cr: 1.00 to 1.49%, Mo: 0.
30-1.20 %, Al : 0.010-0.100%,
N: 0.0150% or less, the balance being Fe and unavoidable impurities, and the value of fn1 is 0.85
Nitriding steel with excellent toughness of up to 2.00%.

(2) In addition to the components described in the above (1), one or more of 0.005 to 0.050% of Nb and 0.01 to 0.10% of Ti by weight%. And the value of fn1 is 0.85 to 2.00%.

(3) In addition to the components described in the above (1), 0.005 to 0.100% of S, 0.0% by weight.
1 to 0.30% Pb, 0.005 to 0.100% T
e, 0.01-0.30% Bi and 0.0005-
A nitriding steel excellent in toughness containing at least one of 0.0100% of Ca and having a value of fn1 of 0.85 to 2.00%.

(4) In addition to the components described in the above (1), one or more of 0.005 to 0.050% of Nb and 0.01 to 0.10% of Ti by weight%. , And 0.005-0.100% S, 0.01-0.30%
Pb, 0.005 to 0.100% Te, 0.01 to
0.30% Bi and 0.0005-0.0100%
A nitriding steel excellent in toughness, containing at least one of Ca and the value of fn1 is 0.85 to 2.00%. Further, the component according to any one of the above (1) to (4)
2.00% or less by weight of Ni and
At least one of V of 0.40% or less, and 0.01
It can contain up to 00% B.

[0023]

The reason for limiting the chemical composition of steel in the present invention as described above will be described below. “%” Means “% by weight”.

C: C is an element effective for securing desired strength, but is also an element that lowers toughness. In other words, the balance between strength (hardness) and toughness is required to secure core hardness to suppress life reduction due to cavitation and to secure severe core toughness to absorb severe impact and improve breakage resistance. In order to obtain the minimum hardness (core hardness of Hv 250 or more), 0.15% or more is required. On the other hand, if the content exceeds 0.35%, the toughness is reduced, and a predetermined toughness (the toughness of the core is JIS3
(The impact value at room temperature of the No. test specimen is 120 J / cm ² or more) cannot be obtained. Therefore, the content of C is
0.15 to 0.35%.

Si: Si is an element necessary for deoxidizing steel and also for imparting a predetermined strength (hardness). However, if the content is less than 0.10%, the desired effect cannot be obtained, and if it exceeds 1.00%, the toughness is significantly deteriorated and the predetermined toughness cannot be secured. To 1.00%.

Mn: Mn is necessary for deoxidation,
It has the effect of enhancing hardenability and improving hardness and toughness.
However, if the content is less than 0.30%, the desired effect cannot be obtained, and if it exceeds 2.00%, the segregation at the grain boundaries deteriorates and the toughness deteriorates, and the desired toughness cannot be obtained. Was set to 0.30 to 2.00%.

Cr: Cr is an element which is effective for improving the nitriding properties and for improving the hardenability and the hardness and toughness. However, if the content is less than 1.00%, the desired effect cannot be obtained,
Even if the content exceeds 1.49%, the effect tends to be saturated.
Ri it means sacrificing the economic efficiency, the content of 1.0
0 to 1.49 %.

Mo: Mo is an element effective in improving hardenability to increase hardness and toughness and improving nitriding characteristics. However, if the content is less than 0.30%, the desired effect cannot be obtained. On the other hand, if the content exceeds 1.20%, the effect saturates and only the cost rises. The amount was 0.30 to 1.20%.

Ni: Ni may not be added. If added, it has the effect of improving the strength and further greatly improving the toughness. To ensure this effect, Ni is 0.05%
It is preferable to set the content as described above. However, if the content exceeds 2.00%, the above effects are saturated and the cost is increased. Therefore, when the content of Ni is 2.0
0% or less.

V: V may not be added. When added, it has the effect of improving the strength and improving the nitriding properties. To ensure these effects, V should be 0.0
Preferably, the content is 5% or more. However, if the content exceeds 0.40%, the V carbonitrides are agglomerated and coarse, and the toughness is reduced. Therefore, the content of V is 0.40%
It was as follows. In order to further secure toughness, the V content may be less than 0.20%.

B: B may not be added. If added, there is an effect that the hardenability is improved. In order to surely obtain this effect, the content of B is desirably 0.0005% or more.

However, if the content exceeds 0.0100%, the hot workability decreases. Therefore, the upper limit of the B content is set to 0.0100%.

Al: Al is an important element in the present invention, and when it is contained in an appropriate amount, both nitriding characteristics (in other words, wear resistance) and toughness are improved. That is, with regard to the nitriding characteristics, if the amounts thereof are controlled in accordance with the contents of Cr, Mo and N as described later, good nitride layer characteristics are obtained and the wear resistance is improved. Further, by adding a proper amount of C, Si and Mn in combination, toughness is greatly improved, and a severe impact during use is absorbed to improve breakage resistance. However, if the content is less than 0.010%, the desired effect cannot be obtained. If the content exceeds 0.100%, the oxide-based inclusions become coagulated and coarsened, and the toughness is remarkably impaired. Impact strength at room temperature with a JIS No. 3 test piece is not less than 120 J / cm ² ).
0.100%.

N: N does not have to be contained because N has an adverse effect on the characteristics of the nitrided layer, as is clear from the above-mentioned fn1 equation which can arrange the characteristics of the nitrided layer. However, it is impossible to completely remove N at the steelmaking stage,
Conversely, if it is contained, it has an effect of forming a nitride to make crystal grains fine and improve toughness. In order to surely obtain the effect of improving toughness, it is desirable that the content of N be 0.0020% or more. However, if the content exceeds 0.0150%, a large amount of nitride is generated in the steel before the nitriding treatment. As a result, the performance is reduced, the nitriding property is deteriorated, and the wear resistance is reduced.

Therefore, the upper limit of the N content is set to 0.0150%
And

Fn1: The content of each of Al and N is 0.5.
In steels of 100% or less and 0.0150% or less,
When the content of Cr and Mo is controlled within the above range,% X
Is the content (% by weight) of the element X, the characteristics of the nitrided layer are fn1 = (2/3)% Cr + (1/3)% Mo +% Al, as described above.
-25% N, and the value of fn1 is 0.
If it is less than 85%, good nitrided layer properties cannot be obtained.
That is, the desired hardness of the nitrided layer having a maximum hardness of 700 or more in Hv and a depth of 0.3 mm or more up to Hv of 500 cannot be obtained, and the wear resistance is deteriorated. On the other hand, if this value exceeds 2.00%, the effect is saturated and the economy is impaired. Therefore, the value of fn1 is 0.8
5 to 2.00%.

The nitriding steel having excellent toughness of the present invention further comprises one or more of Nb and Ti and / or one or more of S, Pb, Te, Bi and Ca in addition to the above components. The above may be included. The effects and desirable contents of these alloy elements are as follows.

Nb and Ti: Nb and Ti have the effect of forming carbonitrides to refine crystal grains and improve toughness. Therefore, one or both of Nb and Ti may be added as necessary. However, in the case of Nb, if the content is less than 0.005%, the desired effect cannot be obtained. If the content exceeds 0.050%, the toughness is reduced and the breakage resistance is deteriorated. On the other hand, in the case of Ti,
If the content is less than 0.01%, the desired effect cannot be obtained,
If the content exceeds 0.10%, the crystal grains are rather coarsened and the toughness is reduced, resulting in deterioration of breakage resistance. Therefore, when one or more of these alloy elements are added, N
b: 0.005 to 0.050%, Ti: 0.01 to 0.
The content is preferably 10%.

S, Pb, Te, Bi and Ca: S, P
b, Te, Bi and Ca have an effect of improving machinability. Therefore, one or more of S, Pb, Te, Bi and Ca may be added as necessary.

However, in the case of S, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.100%, the toughness is remarkably reduced. In the case of Pb, the desired effect cannot be obtained if the content is less than 0.01%, and the toughness is reduced if the content exceeds 0.30%. In the case of Te, if the content is less than 0.005%, the desired effect cannot be obtained. If the content exceeds 0.100%, the hot workability deteriorates. On the other hand, in the case of Bi, 0.01%
If the content is less than the desired effect cannot be obtained, and if the content exceeds 0.30%, the toughness is deteriorated. Further, in the case of Ca, if the content is less than 0.0005%, the desired effect cannot be obtained. If the content exceeds 0.0100%, the toughness is reduced. Therefore, when one or more of these alloying elements are added, S: 0.005 to 0.100%, Pb: 0.0
1 to 0.30%, Te: 0.005 to 0.100%, B
i: 0.01 to 0.30% and Ca: 0.0005 to
The content is preferably 0.0100%.

After the steel having the above chemical composition is melted in a usual manner, it is rolled or forged, for example, hot, then is subjected to normalization as required, and then processed into a desired part shape. The refining treatment (quenching and tempering treatment) and the nitriding treatment are performed by an ordinary method. Alternatively, after the above-described hot rolling or forging, normalizing is performed as necessary, then tempering is performed by a normal method, processed into a desired part shape, and then subjected to nitriding by a normal method. You. In addition,
In the tempering treatment, quenching may be performed by heating to a temperature of about 850 to 950 ° C. and then cooling with water or oil. Tempering is performed at a temperature not higher than the Ac ₁ transformation point, and a desired core strength (hardness) after nitriding treatment. And a temperature range in which core toughness can be obtained, for example, 55
What is necessary is just to carry out at the temperature of about 0-600 degreeC. For cooling after tempering, an appropriate method such as accelerated cooling or cooling may be selected. Further, the nitriding treatment may be performed at a temperature of about 520 to 540 ° C.

[0042]

EXAMPLES (Example 1) Steels having the chemical compositions shown in Tables 1 and 2 were melted by a conventional method using a 500 kg test furnace. Steel in Table 1
1, 3, 6, 8, and 13 are steels of the present invention, and steels 16 to 32 in Table 2 are comparative steels in which one of the components is out of the range of the content specified in the present invention. Note that among the comparative steels in Table 2, steels 29, 30, 31, and 32 were SCM4
35 steel, SCM440 steel, SNCM420 steel, SACM
645 steel.

Next, the steel of the present invention and the comparative steel were forged into a 30 mmφ round bar by a usual method, and thereafter, these test steels were heated at 870 ° C. for 1 hour, water-quenched, and then at 600 ° C. Tempering treatment was performed by performing tempering for 6 hours and air cooling. Thereafter, a test piece for nitriding treatment having a diameter of 25 mm and a length of 300 mm was cut out from the treated material, and the test piece was subjected to nitriding treatment at 530 ° C. for 60 hours, followed by air cooling treatment.

The thus obtained 25 mmφ after nitriding treatment
A hardness test piece having a diameter of 25 mm and a JIS No. 3 Charpy impact test piece were cut out from the position of the test piece in the length direction shown in FIG. 1, and the core hardness and the core toughness at room temperature and the characteristics of the nitrided layer were investigated. The results are shown in Tables 3 and 4.

Steels 1, 3, 6, 8 and 1 of the steel of the present invention
No. 3 shows very good properties of the nitrided layer (the maximum hardness of the nitrided layer is 700 or more in Hv, the depth of the nitrided layer up to Hv500 is 0.3 mm or more), and the core hardness at normal temperature (Hv
It is evident that it has a core toughness (120 J / cm ² or more) of 250 or more (see Table 3).

On the other hand, steels 16 to 16 which are comparative steels in which one of the components is out of the range of the content specified in the present invention.
In No. 32, the above-mentioned values are not satisfied in any of the properties of the nitrided layer, the core hardness at room temperature, and the core toughness (see Table 4).

[0047]

[0048]

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

According to the present invention, nitriding steel excellent in toughness suitable for various sliding part steel materials used in civil engineering and construction equipment, such as bushes incorporated in the hydraulic breaker of a hydraulic shovel, can be manufactured at low cost. It can be obtained, and the industrial effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a view showing a cut-out position in a length direction of a test piece used in an example.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masaki Sakamoto 1 Konomi-cho, Kokurakita-ku, Kitakyushu-shi, Fukuoka Prefecture Sumitomo Metal Industries, Ltd. Inside the Kokura Works (56) References JP-A-58-171558 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (6)

(57) [Claims] (1) C: 0.15 to 0.35% by weight, S
i: 0.10 to 1.00%, Mn: 0.30 to 2.00
%, Cr: 1.00 to 1.49%, Mo: 0.30
1.20 %, Al : 0.010 to 0.100%, N:
0.0150% or less, the balance being Fe and unavoidable impurities, and the following fn1 value of 0.85 to 0.85%
Nitriding steel with excellent toughness of 2.00%. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] where [% X] is the weight% of the element X. 2. The composition of claim 1 further comprising 0.005 to 0.050% by weight of Nb and 0.01% by weight.
Containing at least one of Ti of about 0.10%, and
A nitriding steel excellent in toughness having the following fn1 value of 0.85 to 2.00%. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] where [% X] is the weight% of the element X. 3. The composition according to claim 1, further comprising 0.005 to 0.100% of S, 0.01 to 0.1% by weight.
30% Pb, 0.005 to 0.100% Te, 0.
01-0.30% Bi and 0.0005-0.01
Contains at least one of Ca of at least 100% and the following f
An excellent toughness nitriding steel having a value of n1 of 0.85 to 2.00%. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] where [% X] is the weight% of the element X. 4. The composition of claim 1, further comprising 0.005 to 0.050% by weight of Nb and 0.01% by weight.
At least one of 0.1% to 0.10% Ti;
5 to 0.100% S, 0.01 to 0.30% Pb,
0.005 to 0.100% Te, 0.01 to 0.30
% Bi and 0.0005 to 0.0100% Ca and at least one of the following fn1 values:
A nitriding steel having an excellent toughness of 85 to 2.00%. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] where [% X] is the weight% of the element X. (5) In addition to the component according to any one of claims 1 to 4,
Furthermore, it contains 0.0100% or less of B by weight%.
And the value of fn1 below is 0.85 to 2.00%
Nitriding steel with excellent toughness. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] Here, [% X] is the weight% of the element X. 6. The composition according to claim 1 , wherein
In addition, in weight percent, less than 2.00% Ni and 0.1%.
Contains at least one of V of 40% or less, and
Excellent toughness with fn1 value of 0.85 to 2.00%
Steel for nitriding. fn1 = (2/3) [% Cr] + (1/3) [% Mo] + [% Al] -25 [% N] where [% X] is the weight% of the element X.