JPH10158786A - Case hardening steel excellent in fatigue strength and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine nonmetallic inclusions in a material and to improve fatigue strength and toughness after carburizing or carbonitriding, at the time of manufacture of a case hardening steel, by adding proper amounts of Nd, dispersing and precipitating it in the form of fine NdS during or just after the solidification of S in the steel, and forming the residual S into MnS in the case of an excess of S over the added Nd and precipitating it by using the previourly precipitated NdS as a nucleus. SOLUTION: This case hardening steel has a composition containing, by weight ratio, 0.1-0.3% C, 0.1-2% Mn, <=0.03% P, <0.02% S, 0.005-0.1% Nd, and <=0.0025% O. Although Nd is added to form NdS to be a nucleus of precipitation of MnS and to precipitate MnS into fine and spherical state, NdS cannot be obtained in large quantities when its additive quantity is less than 0.005%, and on the other hand, the MnS-spheroidizing and refining effect is hardly changed even if its additive quantity exceeds 0.1%. Accordingly, its additive quantity is regulated to 0.005-0.1%, however, 0.010-0.006% is preferred.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a case hardened steel having excellent fatigue strength and toughness.

[0002]

2. Description of the Related Art Due to an increase in the output of a power machine and a reduction in weight of parts for improving fuel efficiency, there is a demand for higher strength of carburized steel or carbonitrided steel used for power transmission parts.
Since stress is often applied to these components, fatigue strength is important. In addition, toughness is important because an impact load may also act. For example, in a gear of an automobile, a bending stress repeatedly acts on the tooth root portion during traveling, and an impact load acts on the tooth root portion at the time of sudden start or sudden stop.

[0003] The fatigue strength of carburized steel can be dramatically improved by performing shot peening after carburizing. This is because shot peening introduces a compressive residual stress into the surface layer of the carburized steel, so that the tensile stress acting on the surface by the repeated load is reduced.
Therefore, it is important to obtain a residual stress distribution suitable for the gradient of the applied stress, and various shot peening techniques have been proposed.

However, no matter which method is used, the introduction of compressive residual stress by shot peening is at most about 0.2 to 0.3 mm from the surface, and the relaxation of the load stress by compressive residual stress is deeper than that. No effect. If a defect such as an inclusion is present in such a portion, a fatigue crack propagates from the defect, which often leads to early destruction.

[0005] In order to solve this problem, steels having reduced nonmetallic inclusions have been proposed. However, reduction of nonmetallic inclusions requires out-of-pile refining and degassing at the steelmaking stage, and is not necessarily effective from the viewpoint of manufacturing cost. Further, non-metallic inclusions cannot be completely removed even by making full use of out-of-furnace refining and degassing.

[0006] To improve toughness, lowering the C content of the base material, N
Addition of i and Mo or reduction of P of an impurity element is effective, and various steels have been proposed. In addition, the reduction of non-metallic inclusions is also effective as in the improvement of the fatigue strength. However, as for the reduction of nonmetallic inclusions, similarly to the improvement of the fatigue strength described above, the cost becomes high in the steel making stage, and a reduction that can be rendered harmless cannot be expected.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a non-metallic inclusion is finely precipitated to improve the fatigue strength and toughness after carburizing or carbonitriding. It is an object to provide hardened steel.

[0008]

One of the nonmetallic inclusions present in steel is MnS. Since these inclusions precipitate at a high temperature immediately after solidification of the steel, they generally exist as coarse inclusions in the steel.

The present inventors have conducted intensive studies to develop a case hardening steel capable of precipitating such inclusions finely, and have found that the addition of Nd is effective.

[0010] By the addition of Nd, first, sulfide of Nd precipitates during or immediately after solidification before MnS. Nd
Since the sulfide hardly aggregates, many fine particles are dispersed in the steel. Next, when S is excessive with respect to Nd, the remaining S becomes MnS, but MnS precipitates with NdS finely precipitated as a nucleus. Therefore, MnS is dispersed extremely finely as compared with the case where MnS is precipitated alone.

The present invention has been made based on such findings, and the gist of the invention is that “C: 0.1% to 100% by weight.
0.3%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: less than 0.02%, Nd: 0.005% to 0.1
%, O: 0.0025% or less. "

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for defining the chemical components of the case hardened steel of the present invention will be described in detail below. All “%” are% by weight
Is shown.

C: 0.1% to 0.3% Case hardened steel is used with a high carbon surface layer by carburizing or carbonitriding (hereinafter referred to as "carburizing") to ensure the toughness of the entire member. In order to achieve this, it is fundamental to reduce the C of the base material. If the amount of the base material C is too low, the toughness can be secured, but the strength of the entire member is insufficient, and the fatigue strength is reduced. Therefore, the lower limit of C is set to 0.1%. On the other hand, if the amount of the base material C exceeds 0.3%, not only the toughness is lowered but also the machinability is deteriorated, so the upper limit was made 0.3%. The desirable content of C is 0.15 to 0.23%.

Mn: 0.1 to 2% Mn has the effect of increasing the hardenability of steel. In the case of case hardened steel, quenching is performed after carburization, so that hardenability must be ensured. Since 0.1% or more must be contained in order to ensure hardenability, the lower limit is set to 0.1%. When the content exceeds 2%, the cold forgeability deteriorates like Si, so the upper limit is set to 2%. Desirable content of Mn is 0.3-1.
5% or less.

P: not more than 0.03% P has an effect of deteriorating the toughness of the member after carburization.
If it exceeds 0.03%, this effect becomes significant, so the upper limit was made 0.03%. Desirably, it is 0.025% or less.

S: less than 0.02% S forms MnS and degrades the toughness and fatigue strength of the member. In the present invention, the addition of Nd makes MnS, which is usually coarsely formed, spherical and fine, and renders the adverse effects of MnS harmless. However, the S content is 0.02%
Above, although the form of the sulfide is spherical and fine,
As the number increases, toughness and fatigue strength deteriorate. Therefore, the content of S is set to less than 0.02%. Preferably,
0.015% or less.

Nd: 0.005 to 0.1% Nd forms a sulfide serving as a precipitation nucleus of MnS, and MnS
Has the effect of precipitating finely and spherically. If the content is less than 0.005%, a sufficient amount of sulfide cannot be obtained, and the effect of miniaturizing MnS is lost, and a large amount of MnS precipitates, so the lower limit is made 0.005%. .

On the other hand, even if Nd is added in excess of 0.1%, the spherical refining effect of MnS hardly changes. Therefore, the upper limit of Nd is set to 0.1%. Preferably, 0.
010 to 0.06%.

O: 0.0025% or less O becomes oxide-based inclusions and is present in steel, and particularly deteriorates fatigue strength. If the content exceeds 0.0025%, this effect becomes remarkable, and even if MnS is made spherical and fine, the effect is lost, so the upper limit is made 0.0025%. Preferably, 0.
0015 or less.

The chemical components of the case hardening steel of the present invention are, in addition to the above,
The following elements can be contained but are not particularly limited.

Si: 2.0% or less Case-hardened steel is carburized and applied to bearing parts. In bearing components, besides bending fatigue strength and toughness, rolling fatigue life is important. Since Si has an effect of improving the rolling fatigue life, it can be added as necessary.

However, if the content exceeds 2%, the cold forgeability of the steel itself is remarkably deteriorated. Therefore, the upper limit is preferably set to 2%. Desirably, it is 0.2-1%.

Cr: 2% or less Cr has the effect of increasing the hardenability of steel. In the case of case hardened steel, quenching is performed after carburization, so that quenching properties must be ensured. Cr can be added according to the size of the member and the coolant used for quenching. When added in excess of 2%, the cold forgeability deteriorates in the same manner as Si and Mn.
The upper limit is preferably set to 2%. It is desirably 1.5% or less.

Sol. Al: 0.1% or less sol. Al combines with N to form AlN and has the effect of suppressing the austenite grain coarsening during carburization, so sol. Al may be contained as necessary. Even if added over 0.1%, the effect does not change, so the upper limit is preferably set to 0.1%. Desirably, it is 0.02 to 0.04%.

N: 0.03% or less N combines with sol. Al to form AlN and has an action of suppressing austenite coarsening during carburization, so that N can be contained as necessary. If the content exceeds 0.03%, the cold forgeability deteriorates.
%. Desirably, it is 0.01 to 0.02%.

Ni: 3.5% or less, Mo: 1.0% or less, and B: at least one of 0.005% or less Ni, Mo, and B improve the toughness and fatigue strength of the carburized member. Can be added as appropriate according to the application.
If the content exceeds the above amount, Ni and Mo deteriorate the machinability of the member, and B increases the effect of improving toughness and fatigue strength. Therefore, Ni, M
The upper limits of o and B are 3.5%, 1%, and 0.005%, respectively.
It is better to do. Desirable upper limits are 2.5
%, 0.8%, and 0.004%.

Nb: 0.1% or less, V: 0.3% or less,
Ti: at least one of 0.03% Nb, V, and Ti form carbonitrides, refine austenite grains, and improve toughness and fatigue strength. Therefore, Nb, V, and Ti can be added as necessary. Even if it is added in excess of the above amount, the effect of miniaturization is saturated, and conversely, the machinability of the member is deteriorated.
It is good to make it 0.03%. The desired upper limit is
0.06%, 0.2% and 0.02%.

In using the case hardening steel of the present invention, carburizing treatment or carbonitriding treatment is naturally premised. These treatments can be performed by various methods, and can be roughly classified into solid carburization, salt bath carburization, and ion carburization. These methods differ in the mechanism of permeation of C and N into the members, but are still high-temperature treatments, and the effects of the present invention are not lost in any treatment.

[0029]

EXAMPLES Thirty types of steel having the chemical components shown in Table 1 were vacuum-melted.

[0030]

[Table 1]

The A series is the case hardened steel of the present invention. A1
The steel is a base steel and is SCr42 of JIS G4104.
This is a steel obtained by adding an Nd amount to steel 0.

A2 to A11 steels are C, Si, Mn, P, C
This is a steel in which the amounts of r, S, solAl, and N are changed.

A12 to A21 steels are Ni, Mo, B, V,
It is a steel to which Nb and Ti are added. Among them, A12, A
18 steel is JIS G4102 SNC815, JIS
G4103 SNCM420 steel with Nd added.

The B series is a comparative steel in which the amounts of S, O and Nd are out of the range specified in the present invention.

The C series is a conventional steel, and C1 to C5 are non-Nd-added steels of A1, A12, A13, A18 and A21 steels, respectively.

A 50 kg ingot of these steels was heated at 1250 ° C., and hot forged to a finish of 1000 ° C. to form round bars having a diameter of 20 mm. Next, these round bars were heated at 925 ° C. × 1 hour, inserted into a furnace at 600 ° C., held for 1 hour, and allowed to cool in the air. Thereafter, from the center of the round bar, a Charpy impact test piece, a JIS Z 2203 No. test piece, a rotary bending fatigue test piece, and a JIS Z 22741 test piece were produced.

The test pieces were carburized and quenched and tempered.

FIG. 1 is a diagram showing carburizing and quenching conditions. The carburizing treatment was performed using a gas carburizing furnace, and the total of the carburizing period and the diffusion period was 10 hours. CP during carburizing and diffusion phases
(Carbon potential) are 1.0 and 0.8, respectively.
%.

Next, the heat-treated test piece was subjected to a shot peening treatment under the following conditions.

Model: impeller type Projection material: SB-6PM (HRC53) Projection speed: 90 m / sec Projection density: 540 kg / m ² Arc height: 0.79 mm Coverage: 300% The portion was masked and projected only on the parallel portion and the R portion (24R) of the test piece.

Using these test pieces, a Charpy impact test and an Ono-type rotary bending fatigue test were performed. All tests were performed at room temperature. Fatigue strength in rotational bending fatigue test is 10
⁷ strength. The target values of the impact value and the fatigue strength were 13 J / cm ² and 1100 MPa, respectively.

Table 2 shows the test results.

[0043]

[Table 2]

A1 to A11 steels are base steel and C, Si,
Mn, P, Cr, S, Nd, solAl, and N are steels whose amounts are changed within the range specified in the present invention, and all steels achieve the target impact value and fatigue strength.

The conventional steels C1 to C5 are respectively A1
Nd-free steel in steel, A12 steel, A13 steel, A18 steel, and A21 steel, but the impact value and the fatigue strength did not reach the targets. This is because Mn in Nd-free steel
This is because S does not become spherical and fine. From this, Nd
It turns out that the content is important.

From the comparison between the A1 steel and the B1 steel, it can be seen that if S exceeds the upper limit, the sulfide itself increases even if Nd is added, so that both the impact value and the fatigue strength deteriorate. Further, from the comparison between the A1 steel and the B2 steel, it can be seen that when O exceeds the upper limit, both the impact value and the fatigue strength deteriorate.

Further, from the comparison between the A1 steel and the B3 steel, it can be seen that both the impact value and the fatigue strength are deteriorated when Nd is below the lower limit. This is because coarse MnS increases. For Ni, Mo and B-added steels, A12 to A14
From the comparison of steel, A18 steel, A19 steel, and the like, it can be seen that both the impact value and the fatigue strength are improved by the addition of these alloy elements.

In the steels to which Nb, V and Ti are added to refine the crystal grains, it can be seen from the comparison between the steels A15 and A17 that both the impact value and the fatigue strength are improved by the addition of these alloying elements. .

Similarly, toughness improving elements (Ni, Mo,
It can be seen that both the impact value and the fatigue strength are excellent in the composite addition of B) and the fine-grained elements (Nb, V, Ti) within the range specified in the present invention.

[0050]

The case hardening steel of the present invention has non-metallic inclusions in a form harmless to fatigue and toughness, and has excellent fatigue strength and toughness after carburizing or carbonitriding. Since there is no need to reduce nonmetallic inclusions by a method such as external refining or degassing, there is an effect that the manufacturing cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a view showing a heat pattern of a gas carburizing process in an embodiment.

Claims (1)

[Claims] C. 0.1% to 0.3% by weight, M
n: 0.1 to 2%, P: 0.03% or less, S: 0.02
%, Nd: 0.005% to 0.1%, O: 0.00
A case hardened steel excellent in fatigue strength and toughness characterized by containing 25% or less.