JPH08311606A - Case hardening steel and carburized parts excellent in impact fatigue characteristic - Google Patents

Abstract

PURPOSE: To produce a case hardening steel and carburized parts having excellent durability to braking caused by impact fatigue. CONSTITUTION: This case hardening steel is the one having a compsn. contg., by weight, 0.05 to 0.40% C, <=0.10% Si, 0.20 to 2.50% Mn, <=0.015% P, <=0.015% S, <=0.20% Cr, <=1.00% Mo, <=0.0030% B, 0.005 to 0.050% Al and 0.005 to 0.030% N, and the balance Fe with inevitable impurities, and in which the value of the following fn1 is regulated to <=1.30. In addition to the same components, one or more kinds among Nb, V and Ti may be incorporated: fu1=3(Si+0.1Mn+0.5Cr)+215P+8S-(0.01/C)-0.3Mo-40B. By using the same steel as the stock, the carburized parts in which, as for the core part after carburizing and quenching, the hardness Hv is regulated to 200 to 400 and the austenitic grain size is regulated to No. >=6 in the JIS size number and excellent in impact fatigue characteristics can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to case-hardening steel and carburized parts made of the same, and more particularly to low-cost case-hardened steel and carburized parts excellent in impact fatigue strength after carburizing and quenching.

[0002]

2. Description of the Related Art In recent years, for the purpose of reducing the weight and increasing the rigidity of a structure, there is an increasing demand for higher strength for mechanical structural parts. In particular, in car gears and various shafts that are carburized for surface hardening, after carburizing and quenching due to the sudden increase in impact force at the time of torque transmission due to high torque of the engine and sudden start of the car. It is required to have high impact resistance and impact fatigue characteristics.

Conventionally, most of the above-mentioned parts have been manufactured by carburizing and quenching JIS standard steels such as SCr420 steel and SCM420 steel. However, conventional carburized and tempered parts have a deep hardened layer, but an intergranular oxide layer with an incompletely hardened layer is formed at the outermost surface of the carburized layer. Fatigue resistance,
In particular, since it is inferior in impact resistance and impact fatigue resistance, breakage occurs during the operation of parts, and thus it is difficult to increase the strength.

To solve the above problem, Japanese Patent Laid-Open No. 6-100
Japanese Patent No. 974 proposes case hardening steel having excellent impact resistance. However, although the case-hardening steel described in this publication is intended to improve impact resistance by reducing the amount of Si to reduce the grain boundary oxide layer, since it contains Cr in a high range, the grain boundary oxide layer In some cases, the effect of suppressing the above is not sufficient, and the desired impact characteristics may not be obtained. In addition, since impact fatigue is not considered at all, sufficient impact fatigue resistance cannot be obtained. In addition, Japanese Unexamined Patent Publication
In JP-A-170944, the amounts of Si, Mn and Cr are limited in order to reduce the grain boundary oxide layer formed during the carburizing process, and the amounts of P, Mo and B are further adjusted to strengthen the grain boundary. A case hardening steel is disclosed. However, even if the case-hardening steel proposed in this publication is used as a raw material, impact resistance and impact fatigue properties have not been significantly improved, and there is a problem in economical efficiency because Mo is added as an essential component. .

[0005]

An object of the present invention is to provide case-hardening steel having excellent impact strength and impact fatigue strength after carburizing and quenching, and carburized parts made of the same, and particularly excellent in damage due to impact fatigue. It is to provide a case hardening steel and a carburized part having durability.

[0006]

Means for Solving the Problems In order to solve the above problems, the present inventor has conducted an investigation and study on the chemical composition of case-hardening steel as a material for carburized parts and the structure of carburized parts. )-(G).

(A) In the grain boundary oxidation, O (oxygen) in the carburizing atmosphere penetrates into the former austenite grain boundary and is bonded to Si, Mn, Cr, etc., which have a stronger affinity for O than Fe, to the grain boundary. It is known to occur by forming an oxide. By the way, if the grain boundary oxide layer is present, when a shocking external stress is repeatedly applied, it leads to stress concentration due to the notch effect, resulting in damage. That is, when a shocking external stress is applied, cracks are generated from the grain boundary oxide layer of the carburized portion, and the starting point of this fracture (grain boundary oxidized layer of the carburized portion) exhibits grain boundary fracture. Therefore, increasing the critical stress for initiation of this initial crack leads to prevention of damage.

(B) As a result of investigating the member which is damaged by the repeated shocking external stress as described above, the damaged surface shows not a so-called "one-shot failure" by the impact but a failure by the fatigue. It was damaged by "impact fatigue".

(C) In order to suppress damage due to impact fatigue, it is necessary to reduce the grain boundary oxide layer to improve impact resistance, especially impact fatigue strength. For that purpose, Si, Mn and Cr are required. The regulation of the content of is effective.

(D) In order to suppress the development of cracks that have occurred, it is first necessary to strengthen the grain boundaries of the carburized portion. For this purpose, the contents of C, P, S, B and Mo may be regulated.

(E) Due to shocking external stress,
Both the generation of cracks in the carburized portion and the propagation of the generated cracks in the carburized portion can be suppressed when the value of fn1 below is 1.30 or less.

Fn1 = 3 (Si + 0.1Mn + 0.5Cr) +215
P + 8S- (0.01 / C) -0.3 Mo-40 B However, the element symbol in the formula is the content of the element (% by weight)
Represents

(F) The crack generated in the grain boundary oxide layer of the carburized portion propagates and propagates from the carburized portion to the core portion, leading to final damage. Therefore, in order to suppress the development of cracks and prevent damage, secondly, it is necessary to reduce the propagation velocity in the core portion and increase the resistance to propagation. For this purpose, making the core structure fine-grained is an effective means for reducing the fracture surface unit.

(G) If the core has a hardness of HV 200 to 400 and an austenite grain size of JIS No. 6 or more, the propagation speed of crack growth in the core is small and the resistance to propagation is small. growing.

The gist of the present invention based on the above findings is a case-hardened steel of the following (1) and (2) and a carburized component excellent in impact fatigue properties of the following (3).

(1) C: 0.05 to 0.40 in% by weight
%, Si: 0.10% or less, Mn: 0.20 to 2.50
%, P: 0.015% or less, S: 0.015% or less, C
r: 0.20% or less, Mo: 1.00% or less, B: 0.
0030% or less, Al: 0.005 to 0.050%,
N: 0.005-0.030% is contained, the balance consists of Fe and unavoidable impurities, and the value of the above-mentioned fn1 is 1.30 or less.

(2) In addition to the components described in (1) above, 0.005 to 0.030% by weight of Nb, 0.
005 to 0.050% V and 0.005 to 0.050%
1. A case-hardening steel containing at least one of Ti and having a value of fn1 of 1.30 or less.

(3) The material is the steel according to any of (1) and (2) above, and the core after carburizing and quenching is HV
A carburized part having excellent impact fatigue properties with a hardness of 200 to 400 and an austenite grain size of 6 or more in JIS grain size number.

The core portion after carburizing and quenching means a portion which is not carburized.

[0020]

The present invention will be described in detail below along with its effects. "%" Means "% by weight".

(A) Chemical composition C: C is added in order to enhance the hardenability of steel and to secure the core strength, but if its content is less than 0.05%, the effect of addition is poor, while on the other hand, If the content exceeds 40%, not only the toughness of the steel (core portion) is deteriorated, but also the machinability during the component forming process is deteriorated. Therefore, the content of C is set to 0.05 to 0.40%.

Si: Si is an element that is more easily oxidized than Fe so that it is added as a deoxidizer, and forms a grain boundary oxide in the carburized surface layer portion to reduce bendability, impact strength and impact fatigue strength.

However, if the content is 0.10% or less, the grain boundary oxide layer after carburization can be reduced to a negligible level, so this value was made the upper limit. On the other hand, the lower the Si content, the more the grain boundary oxidation can be reduced.

Mn: Mn is an element effective for improving the hardenability of steel and ensuring the core strength. However, if the content is less than 0.20%, sufficient hardenability cannot be secured. On the other hand, if it exceeds 2.50%, the machinability of the steel is greatly reduced, and since it is an element that is more easily oxidized than Fe, although not as much as Si, the grain boundary oxide layer is rapidly increased and bendability and impact strength, Impact fatigue strength deteriorates. Therefore, the Mn content is set to 0.20 to 2.50%.

P: P is an undesirable impurity element that segregates at the austenite grain boundaries during carburization and significantly reduces the strength of the austenite grain boundaries in the carburized layer. Therefore, in order to reduce the grain boundary embrittlement due to the grain boundary segregation of P and prevent the deterioration of desired characteristics (bendability, impact resistance, impact fatigue resistance), 0.015
% Was set as the upper limit.

S: S is an undesired impurity element which remains in the crystal grain boundaries and remarkably lowers the grain boundary strength, resulting in deterioration of bendability, impact resistance and impact fatigue resistance. Therefore, in order to reduce the decrease in the grain boundary strength due to the remaining S crystal grain boundaries and prevent the deterioration of desired characteristics (bendability, impact resistance, impact fatigue resistance), 0.015% is made the upper limit.

Cr: Cr may not be added. If added, the hardenability of steel is improved, core strength can be secured, and toughness is improved. In order to reliably obtain this effect, the content of Cr is preferably 0.05% or more. However, Cr has a much larger affinity with O than Fe, and forms grain boundary oxides in the carburized surface layer portion to deteriorate bendability and impact fatigue properties. In particular, the content is 0.
If it exceeds 20%, the various strengths of the carburized hardened layer are significantly reduced. Therefore, the upper limit of the Cr content is set to 0.20%.

Mo: Mo may not be added. Addition has the effects of ensuring good hardenability of steel and improving toughness. In order to reliably obtain this effect, Mo is set to 0.
It is desirable that the content be at least 05%. But 1.0
Even if the content exceeds 0%, the above effect is saturated and the cost is disadvantageous. Therefore, the upper limit of the Mo content is set to 1.00%.

B: B may not be added. If added, it has the effect of improving the hardenability of steel and segregating to the austenite grain boundaries of the carburized layer to strengthen the grain boundaries of the carburized layer. In order to surely obtain this effect, the content of B is preferably 0.0003% or more. However, if the content exceeds 0.0030%, not only the effect of improving the hardenability is saturated, but also the workability in hot or cold is deteriorated.
The upper limit of the B content was 0.0030%.

Al: Al reacts with N in steel to form AlN, and has an action of preventing coarsening of austenite grains during carburizing and heating. However, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.050%, not only the effect is saturated, but also cold workability and machinability deteriorate. Therefore, the Al content is set to 0.
It was set to 005 to 0.050%.

N: N reacts with Al to produce AlN,
It has the effect of refining the austenite grains during carburizing and heating to improve the toughness of the steel. However, its content is 0.005
%, The amount of AlN is insufficient and the desired effect cannot be obtained.
If it exceeds 0.030%, not only the effect is saturated but also the cold workability is deteriorated. Therefore, the content of N is set to 0.005 to 0.030%.

Fn1: As described above, suppression of both occurrence of cracks in the carburized portion and propagation of the generated cracks in the carburized portion due to shocking external stress can be said to be an impact improvement index fn1. Can be achieved when the value of is 1.30 or less. Therefore, it is necessary to set the value of fn1 to 1.30 or less.

The case-hardening steel of the present invention may further contain one or more of Nb, V and Ti in addition to the above components. The effects and desirable contents of these alloying elements are as follows.

Nb, V and Ti: Nb, V and Ti
Has the effect of reacting with C and N in the steel to form carbonitrides and refining the austenite grains during carburizing and heating to improve the toughness of the steel. Therefore, Nb, V and Ti may be added if necessary. However, if the content is less than 0.005%, it is difficult to obtain the above effects, while Nb is less than 0.
Even if the content exceeds 030%, or the content of V and Ti exceeds 0.050% respectively, the effect is saturated,
In addition, the toughness will deteriorate. Therefore, when adding one or more of these elements, Nb: 0.005
0.030%, V: 0.005 to 0.050%, Ti:
The content is preferably 0.005 to 0.050%.

A billet of case-hardening steel having the above chemical composition is
For example, it is hot rolled into a round bar or forged, then subjected to normalization and machining as required to be processed into required carburized parts, and then carburized and quenched by a usual method. In addition,
If tempering is performed at a low temperature, the toughness can be improved without a large decrease in surface hardness and core hardness, so tempering may be performed after carburizing and quenching as necessary. When tempering, a normal method may be used, but the temperature is preferably 150 to 200 ° C. in order to secure hardness.

(B) Hardness of core of carburized part after carburizing and quenching Carburizing and quenching is carried out to harden the surface of the steel part and to secure necessary abrasion resistance as a product. It is necessary that the core hardness of HV is 200 to 400. When the hardness is in the above range, the strength of the core portion can be secured, and further, the propagation speed of crack growth in the core portion becomes small and the resistance to propagation becomes large, so that good impact fatigue characteristics can be obtained. If the core hardness is less than HV 200, the desired depth of the hardened layer cannot be obtained after carburizing and quenching, and if it exceeds HV 400, the propagation speed of crack growth is increased and the impact fatigue strength is significantly reduced.

(C) O of the core of carburized parts after carburizing and quenching
Stenite grain size In the austenite grains of the core part of the carburized parts when carburized and quenched, cracks generated in the grain boundary oxide layer of the carburized part due to repeated shocking external stress propagate and propagate from the carburized part to the core part. Then, in the process leading to the final damage, the propagation velocity and the resistance to the propagation at the core are affected. When the core structure has an austenite grain size of 6 or more in JIS grain size number, a small fracture surface unit can be obtained, so crack propagation in the core portion is suppressed, which leads to prevention of damage and good impact. Fatigue characteristics can be achieved. It is preferable to make the grain size number as large as possible, in other words, make the crystal grains as small as possible, and the upper limit of the grain size number is not particularly specified.

[0038]

【Example】

(Example 1) Steels having the chemical compositions shown in Tables 1 to 4 were melted by a usual method using a 3 ton test furnace. Tables 1 and 2
Steels 1 to 15 in the present invention are steels of the present invention, Steels 1 to 2 in Tables
Nos. 6 to 36 are comparative steels in which any of the components is out of the content range specified in the present invention. Steels 35 and 36 in the comparative steel are conventional JIS standard case-hardening steels SCr420 steel and SCM420 steel.

Then, these steels of the present invention and comparative steels were made into steel pieces of 160 mm square by an ordinary method, and then 1
After heating to 100 ℃, finishing temperature is 950 ℃ and diameter is 30
It was hot forged into a round bar of mm.

Then, from these test round bars, as shown in FIG.
A test piece (for a three-point bending test, a Charpy impact test and an impact fatigue test) having a 0 mm square × 55 mm length and a semicircular notch of 3 mmR was cut out, and this test piece was cut at 925 ° C. × 5.
hr (however, the carbon potential is 0.9%)-> oil quenching and carburizing quenching. Note that some of them were also tempered at 180 ° C. × 2 hrs → air cooling.

Next, a room temperature Charpy impact test and a room temperature three-point bending test with a span of 45 mm and a load speed of 0.05 mm / s were performed as one of the evaluations of the root bending characteristics of gears representing carburized parts. Furthermore, in order to investigate the impact fatigue characteristics, a test was conducted by dropping the weight from a position 500 mm above the test piece under the condition of 0.5 Hz by the method shown in FIG. The impact fatigue characteristics were evaluated by the impact torque at 10 ³ times of the above test. Further, using a Charpy impact test piece, the grain boundary oxide layer of the carburized portion and the hardness and austenite grain size of the center portion of the test piece, that is, the core portion were measured by microscopic observation.

The test results are shown in Tables 5-8. The results of the 3-point bending test were evaluated by the stress at the maximum load, and this was expressed as 3-point bending strength.

From Tables 5 to 8, steels 1 to 15 which are steels of the present invention
Is a very small grain boundary oxide layer of 1 μm or less in the carburized part,
Further, since the hardness of the core and the austenite grain size both satisfy the requirements of the present invention, the three-point bending strength, the Charpy absorbed energy and the impact fatigue torque are all excellent,
It is clear that it has great resistance not only to so-called “one-shot fracture” due to external impact stress but also to “impact fatigue” due to repeated impactive external stress.

On the other hand, among comparative steels having high Si, Mn and Cr contents, steels 18, 20, 27 and 30 and JI
In the steels 35 and 36 of S standard steel, a large intergranular oxide layer is formed, and the three-point bending strength, the Charpy absorbed energy and the impact fatigue torque are all inferior.

Comparative steels 16 and 19 having a low C and Mn content do not form an intergranular oxide layer, but have a low core hardness and thus are inferior in impact fatigue torque.

In the steel 17 having a high C content and the steel 29 having a high C and V content, the impact fatigue torque is extremely low and the Charpy absorbed energy is also low because the core hardness exceeds 400.

Steel 21 with high P and S contents and weak grain boundaries
22 and 22, steels 23 to 26 and 28 whose austenite grain size is out of the regulation of the present invention, and steels 31 to 34 whose fn1 value exceeds 1.3 also have small Charpy absorbed energy and impact fatigue torque. .

Example 2 Steels 1 and 11 (inventive steels) shown in Tables 1 and 2 above were prepared in the same manner as in Example 1 and shown in FIG.
The test piece of is cut out, and this test piece is cut at 950 ° C. for 5 hours.
(However, carbon potential is 0.9%) → Oil quenching, 1000 ° C. × 5 hr (however, carbon potential is 0.9%) → Oil quenching, After carburizing and quenching,
Tempering was performed by 180 ° C. × 2 hr → air cooling. Using the test piece thus obtained, the room temperature Charpy impact test, the room temperature three-point bending test, the impact fatigue test, the measurement of the grain boundary oxide layer of the carburized part by the microscopic observation, and the hardness and the oh of the core part were carried out in the same manner as in Example 1. Stenite grain size was measured.

The test results are shown in Table 9. It is clear from Table 9 that even when the steel of the present invention is used, the Charpy absorbed energy and impact fatigue torque are significantly deteriorated when the austenite grain size deviates from the values specified in the present invention.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

[Table 4]

[0054]

[Table 5]

[0055]

[Table 6]

[0056]

[Table 7]

[0057]

[Table 8]

[0058]

[Table 9]

[0059]

As described above, since the carburized component according to the present invention has excellent durability against damage due to impact fatigue, it is used as a gear or shaft for automobiles which is subjected to repeated impactive external stress. be able to. Since this carburized component can be relatively easily obtained by using the low-cost case-hardening steel of the present invention as a raw material, it has a great industrial effect.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of test pieces used in a room temperature Charpy impact test, a room temperature three-point bending test, and an impact fatigue test.

FIG. 2 is an explanatory diagram of an impact fatigue test method.

[Explanation of symbols]

 1: test piece, 2: weight, 3: saucer,

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazuhiko Nishida Sumitomo Metal Industry Co., Ltd. Kokura Steel Works, No. 1 Konomi-cho, Kokurakita-ku, Kitakyushu City, Fukuoka Prefecture

Claims (3)

[Claims] 1. C: 0.05 to 0.40% by weight, S
i: 0.10% or less, Mn: 0.20 to 2.50%,
P: 0.015% or less, S: 0.015% or less, Cr:
0.20% or less, Mo: 1.00% or less, B: 0.00
30% or less, Al: 0.005 to 0.050%, N:
0.005 to 0.030%, the balance consists of Fe and unavoidable impurities, and the value of fn1 below is 1.3.
Case hardening steel characterized by being 0 or less. fn1 = 3 (Si + 0.1Mn + 0.5Cr) +2 15P + 8S- (0.01
/ C) -0.3 Mo-40 B 2. In addition to the components of claim 1, 0.005 to 0.030% by weight of Nb, 0.005 to 0.005.
It contains one or more of 0.050% V and 0.005-0.050% Ti, and has the following fn1 value of 1.
Case hardening steel characterized by being 30 or less. fn1 = 3 (Si + 0.1Mn + 0.5Cr) +2 15P + 8S- (0.01
/ C) -0.3 Mo-40 B 3. The steel as claimed in claim 1 or 2, wherein the core after carburizing and quenching is HV 200-40.
Carburized parts excellent in impact fatigue characteristics with hardness 0 and JIS austenite grain size of 6 or more.