JP2003096539A - Case hardening steel, and carburized part using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case hardening steel which has no remarkable increase in material cost and working cost compared with the conventionally used case hardening steel, and has excellent cold forgeability and impact strength, and to provide carburized parts obtained by using the steel. SOLUTION: The case hardening steel has a composition containing 0.1 to 0.3% C, >0.3 to 1.0% or <=0.3% Si, 0.3 to 1.7% Mn, <=0.03% P, <=0.03% S, <=1.0% Mo, <=0.04% Al, and <=0.03% N, and the balance iron with inevitable impurities, and satisfying a fixed relational equation. The carburized parts are obtained by using the above case hardening steel and contain a carburized layer having a fine austenite whose austenite, grain size number according to JIS is not smaller than 7.

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 using the same, and more specifically, it relates to steels used for structural parts such as carburizing and carbonitriding on the surface layer. Parts that require surface hardening (including gas carburizing, solid carburizing, liquid carburizing, salt bath carburizing, plasma carburizing, vacuum carburizing, etc.) to increase the hardness of the surface layer, such as automobile engines, transmissions, differentials The present invention relates to case hardening steel used for engine parts such as piston pins used for motives, parts for gears, shafts and the like, and carburized parts using the same. Note that, although examples of gears will be described below, the scope of application of the present invention is not limited to gears, and the present invention is used for all mechanical structural parts where impact strength characteristics are particularly important.

[0002]

2. Description of the Related Art Conventional case hardening steels include SCr420H, SCM420H, SNCM4 defined by JIS.
20H or the like is used. However, with the recent trend toward higher output and lighter weight of transportation equipment such as automobiles, there is a strong need for improving the impact strength of power transmission parts, and the JIS standard steel is not sufficient. Therefore, in order to meet the above-mentioned needs, Japanese Patent Application Laid-Open No. 9-201644 discloses a method of obtaining a bevel gear that can obtain high impact characteristics by devising forging and heat treatment methods. However, the manufacturing method described in this publication has a problem that the material cost and the processing cost are high. Moreover, the impact strength characteristics were not significantly improved.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and its object is to make a material as compared with the case-hardening steel used conventionally. It is an object of the present invention to provide a case-hardening steel excellent in impact strength and a carburized part using the same, which does not significantly increase costs and processing costs.

[0004]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that the amounts of C, Mn, Mo, P and S, which are the elemental components contained in case-hardening steel. It was found that the above problems can be solved by adjusting the grain size and the carburized and hardened layer to an appropriate balance by adjusting the elements such as B and B in a specific range, and completed the present invention.

That is, the case-hardening steel of the present invention has a C content of 0.1 to 0.1.
0.3%, Si: more than 0.3 to 1.0%, Mn: 0.3 to
1.7%, P: 0.03% or less, S: 0.03% or less,
Mo: 1.0% or less, Al: 0.04% or less and N:
0.03% or less, Fe and unavoidable impurities as the balance, and the following formula (1) [C%] + 5 ([P%] + [S%]) ≦ ([Mn%] + [Mo%] + 1.8. ) / 8 (1) is satisfied.

Further, the carburized component of the present invention is made of the case-hardened steel described above, and the carburized layer has a fine austenite crystal grain size of 7 or more in the austenite grain size number defined in JIS G 0551. .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The case hardening steel of the present invention will be described in detail below. In addition, "%" indicates a mass percentage unless otherwise specified.

The case-hardening steel of the present invention has been found by the present inventors to improve the impact strength of the case-hardening steel by the amounts of C, Mn, Mo, P and S, which are the elemental components contained in the case-hardening steel. This is because it was found that it is effective to balance the addition of B, B, the grain size, and the carburized hardened layer (effective hardened layer). Specifically, it is sufficient to reduce the amounts of P and S, which are impurity elements, without adding many high-cost elements such as Mo.
It is based on the inventors' knowledge that the addition of Mn as a substitute for Mo, the addition of B, and the refinement of crystal grains greatly contribute to the improvement of impact strength.

That is, the first case-hardening steel of the present invention is mass%
And 0.1 to 0.3% of carbon (C), more than 0.3 to 1.0
% Silicon (Si), 0.3 to 1.7% manganese (M
n), 0.03% or less phosphorus (P), 0.03% or less sulfur (S), 1.0% or less molybdenum (Mo), 0.
An alloy containing aluminum (Al) in an amount of 04% or less and nitrogen (N) in an amount of 0.03% or less, the balance being iron (Fe) and inevitable impurities, and prepared so as to satisfy the following formula (1). . [C%] + 5 ([P%] + [S%]) ≦ ([Mn%] + [Mo%] + 1.8) / 8 (1)

Here, of the above-mentioned elemental components, C has the function of improving the hardness after carburizing and quenching and improving the strength of the carburized component, and the content thereof is 0.1 to 0. .3%
Is. If the C content is less than 0.1%, the effect of addition is poor. On the other hand, if it exceeds 0.3%, toughness and impact strength are lowered.

Further, Si promotes grain boundary oxidation after carburization,
Since it causes a decrease in strength, it is necessary to set it to 0.3% or less. However, in the heat treatment capable of suppressing the grain boundary oxidation such as vacuum carburization or plasma carburization, the heat treatment need not be limited to 0.3% or less. However, an excessive content significantly impairs machinability and cold forgeability, so the upper limit is made 1.0%.

Further, Mn is an element effective for improving the hardenability of steel. The lower limit is set to 0.3% because it is necessary to appropriately retain austenite after carburization in order to improve toughness. On the other hand, if the content exceeds 1.7% and becomes excessive, the cold forgeability is deteriorated and the intergranular oxidation after carburization is promoted.

Furthermore, P is an element that deteriorates the toughness of the carburized layer. In particular, if the content exceeds 0.03%, the impact strength is significantly reduced. Further, since P is an impurity element, it is preferable to make the content as close to 0% as possible.

Further, S is also an element which deteriorates the toughness of the carburized layer, and like P, if its content exceeds 0.03%, the impact strength remarkably decreases. Further, since S is also an impurity element, it is preferable that the content be as close to 0% as possible.

Further, Mo is an element effective for improving the hardenability of steel and an element effective for improving the toughness of the carburized layer. If the Mo content exceeds 1.0% and is excessively added, these effects are saturated.

Furthermore, Al reacts with N in steel to form A
1N is formed to prevent coarsening of austenite crystal grains during carburization. The content of Al is 0.04%
If it exceeds, the effect of preventing the crystal grain coarsening is saturated. Further, for the same reason, the content of N is 0.
If it exceeds 03%, the effect of preventing the crystal grain coarsening is saturated.

In the case-hardening steel of the present invention, the remaining components other than the above-mentioned elemental components are mostly occupied by Fe, and in addition, inevitable impurities such as Cu and O are contained.

Further, the above equation (1) is used to suppress the generation and propagation of cracks at the crystal grain boundaries which are the starting points of fracture, C, P,
It is a specific formula for optimizing the contents of S, Mn and Mo. That is, by adding an appropriate amount of Mn and Mo, the toughness of the carburized layer after carburization is enhanced, and the impurities P and S
By reducing the toughness, the toughness of the grain boundaries is strengthened and the impact strength is improved.

Next, the second case-hardening steel of the present invention will be described. This case-hardening steel is the above-mentioned first except that it does not contain Mo.
It has almost the same composition and effect as the case-hardening steel. In this case, by adding Mn as a substitute for Mo,
The costly addition of Mo can be avoided. Also,
The elemental component contained is an alloy prepared so as to satisfy the following formula (2) [C%] + 5 ([P%] + [S%]) ≦ ([Mn%] + 1.8) / 8 (2) Is.

Further, the case-hardening steel may contain Cr in an amount of 1.6% or less. Cr is an element effective for improving the hardenability of steel. However, excessive addition may cause embrittlement of crystal grain boundaries.
6% or less is preferable. The lower limit of the Cr content is determined according to the required hardenability, and is not particularly limited.

Next, the third case-hardening steel of the present invention will be described. The case-hardening steel is 0.1% to 0.3% C by mass%,
Si of 0.3% or less, Mn of 0.3 to 1.7%, 0.0
P less than 3%, S less than 0.03%, M less than 1.0%
O, 0.04% or less Al and 0.03% or less N, and the balance being Fe and inevitable impurities. Further, the contained elemental component is an alloy prepared so as to satisfy the above formula (1). That is, the case-hardening steel has substantially the same composition as the first case-hardening steel described above except that the Si content is 0.3% or less. Further, Si is an element effective for improving the hardenability of steel, but if added in a large amount, it promotes grain boundary oxidation after carburization and causes a decrease in strength, so it is limited to 0.3% or less.

Next, the fourth case-hardening steel of the present invention will be described. The present case-hardening steel has the same composition range as the above-mentioned second case-hardening steel except that the Si content is 0.3% or less and Cr is not contained. Further, the contained elemental component is an alloy prepared so as to satisfy the above formula (2).

Further, the above-mentioned third case-hardened steel or fourth case-hardened steel can contain Cr in an amount of 1.6% or less. Cr is effective because it improves the hardenability of steel. However, excessive addition may cause embrittlement of crystal grain boundaries, so the upper limit is preferably set to 1.6%. Note that C
The lower limit of the r content is determined according to the required hardenability and is not particularly limited.

Furthermore, in the above-mentioned third case-hardened steel or fourth case-hardened steel, the elemental composition is 80 (Si%) + 24 [Mn%] + 33 [Mo%] + 13≤40 ... It is preferable to satisfy (4). As a result, the hardness of the material before cold forging can be reduced, the deformation resistance can be reduced and the deformability can be improved, and further the press load at the time of cold forging can be reduced. That is, the cold forgeability can be improved by preparing so as to satisfy the above formula (4).

The above-mentioned first to fourth case-hardening steels include
0.001 to 0.005% of boron (B) and 0.01
~ 0.10% niobium (Nb) and / or titanium (T
i) and can be included. B is an element effective for improving the hardenability of steel. Further, it is an element effective in strengthening the grain boundaries of the carburized layer by segregating at the crystal grain boundaries of the carburized layer. To obtain this effect, addition of 0.001% or more is preferable. However, if added in excess of 0.005%, not only the effect of improving hardenability saturates, but
This is not preferable because cold workability is reduced. Nb above
Either or both of Ti and Ti can be contained, and when both elements are contained, the content of each element is preferably 0.01 to 0.10%. Nb or Ti
Has a function of reacting with C and N in steel to form carbonitrides and preventing coarsening of austenite crystal grains during carburization. However, if it is less than 0.01%, it is difficult to obtain the sufficient effect of preventing the crystal grain coarsening. If it exceeds 0.10%, the effect may be saturated.

Further, in the above-mentioned first to fourth case-hardening steels, the elemental components are represented by the following formula (3) [C%] + 5.2 ([P%] + [S%]) ≦ ([Mn%] + [Mo%] + 3.8) / 22 + 96 [B%] + [Austenite grain size number defined by JIS G0551] / 111 (3) It is preferable to satisfy. Thus, the effect of the addition of B can eliminate impurities at the crystal grain boundaries and strengthen the grain boundaries. Furthermore, it is effective because the crystal grain size becomes finer and the grain boundaries are less likely to break.

In particular, when satisfying the above formulas (1) and (3), C and P for suppressing the generation and propagation of cracks at the crystal grain boundaries which are the starting points of fracture, which are necessary for improving the impact strength. ,
Optimization of the amounts of S, Mn, and Mo, addition of B, and grain refinement of the crystal grain can be achieved. That is, in order to strengthen the toughness of the carburized layer after carburization, Mn and Mo are added in appropriate amounts,
It is preferable to reduce P and S which are impurities in order to strengthen the toughness of the crystal grain boundaries, and the limitation based on the formula (1) is based on this idea. Further, it is preferable to eliminate impurities at the crystal grain boundaries by the effect of adding B and strengthen the grain boundaries.
Further, it is preferable to make the crystal grain size fine so as not to break at the grain boundaries. Based on this idea, the limitation by the equation (3) is given.

Furthermore, in the above-mentioned first to fourth case-hardening steels, 0.3% or less of lead (Pb), 0.15% or less of bismuth (Bi) or 0.1% or less of calcium (Ca) is used. ),
And any combination thereof. These elements are effective for improving machinability, but if Pb exceeds 0.3%, Bi is 0.15%, and Ca exceeds 0.1%, machinability is increased. Not only the improvement effect is saturated, but the toughness may decrease.

Next, the carburized component of the present invention will be described in detail. Such a carburized component is made of the above case-hardened steel, and the carburized layer has a fine austenite crystal grain size of 7 or more in the austenite crystal grain size number defined in JIS G 0551. As described above, making the crystal grains fine during carburization is effective in increasing the resistance to crack propagation against impact input. It should be noted that excellent impact strength characteristics cannot be obtained when the grain size of the austenite grain size is less than 7, which is not refined.

As described above, the case-hardening steel of the present invention can be used for gears, shafts, etc. used in transmissions and differentials of automobiles.
It is used for parts that need to increase the hardness of the surface layer.

[0031]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. Further, although gears are taken up in the present embodiment and comparative examples, the scope of application of the present invention is not limited to gears, and it is applied to all mechanical structural parts where impact strength characteristics are particularly important.

(Examples and Comparative Examples) Steels having the chemical compositions shown in Table 1 were vacuum-melted at 150 kg by a conventional method.
Steels A to I, K to M and R are steels embodying the present invention, and steel N
~ Q is a comparative steel. The comparative steel N is the conventional JI
Equivalent to SCr420H of S standard case-hardening steel.

[0033]

[Table 1]

Next, these steels were processed by a conventional method.
After rolling and normalizing, the gear test piece (module 1.5) shown in FIG. 1 was processed. Furthermore, for each test piece,
Carburizing, quenching, and tempering were performed in the heat pattern shown in (1) to finish. An impact test was carried out with a falling weight type impact tester using each gear test piece thus produced. Regarding the austenite grain size, JIS
It was measured by the determination method based on the intersection line segment described in G0551. In the impact test, as shown in FIG. 3, after the gear test piece is meshed with the mating gear, the torque arm connected to the gear test piece is repeatedly subjected to the impact load, and the number of impact load times until breakage at each impact torque is performed. It is a test for. Figure 4
Based on the test results shown in, the impact load torque (hereinafter "100
It may be abbreviated as “round impact strength”) and the number of times of breakage, and the impact load torque after 100 breaks was found. The test results are shown in Table 2. The broken line arrow in FIG. 4 shows a method of obtaining the impact strength 100 times from the relational expression of the load impact torque and the number of times of breakage.

[0035]

[Table 2]

The practical steels A to I, K to M and R satisfy one of the above formulas (1) and (2) and the formula (3) by optimizing the balance of the impurity element and the additive element, and are compared. The impact strength was higher than that of steel. On the other hand, Comparative Steel N did not satisfy the above formulas (2) and (3) and had a low impact strength. Further, Comparative Steel O had a large amount of Cr, did not satisfy the above formulas (2) and (3), and had a low impact strength. Further, the comparative steel P had a grain size of less than 7, did not satisfy the above formulas (1) and (3), and had a low impact strength. Furthermore, Comparative Steel Q had a grain size below No. 7 and did not satisfy the above formulas (2) and (3), so the impact strength was low.

Further, 150 kg of the steel having the chemical composition shown in Table 3 was vacuum-melted by a usual method. Invention Steels 1 to 3 are preferred embodiments of the present invention that satisfy the expressions (1), (3), and (4), and Steels 4 to 9 are one of the expressions (1), (3), and (4). It is a comparative steel that does not satisfy the above. The comparative steel 8 corresponds to the conventional JIS standard case hardening steel SCM418H. Next, these steels were rolled into a bar material by a usual method, cut, spheroidized, shot blasted, lubricated, and cold toothed forged. Then, the final shape of the gear shown in FIG. 5 was processed by cutting such as turning, carburizing and tempering was performed, and finish grinding was performed. In addition, as a manufacturing method of the gear described in the present embodiment, it is possible to manufacture by a method in which a predetermined blank shape is formed by cold forging, followed by turning and gear cutting. Inventive steels 1 to 3 can be sufficiently formed by cold forging even if spheroidizing annealing, which is a softening heat treatment before cold forging, is omitted. An impact test was carried out using a drop weight type impact tester using the gear thus produced. The impact test is almost the same as in Examples AQ. The relational expression between the impact load torque and the number of breakages obtained by this experimental method was obtained, and the impact load torque after 100 breaks was obtained.
Further, regarding the cold forgeability, at the time of the cold tooth die forging,
The load was measured by a load meter installed in a press machine for the actual part.
The lower the press load, the better the cold forgeability.

[0038]

[Table 3]

The results of the cold forging and the impact test results are shown in Table 4.
Shown in. The results are comparative steel 8 (JIS standard case hardening steel S
(CM418H) is shown as a ratio when the cold forging load and the impact strength at 100 times are 100 respectively.

[0040]

[Table 4]

From Table 4, the invention steels 1 to 3 are represented by the formula (1),
Since (3) and (4) are satisfied, both cold forgeability and impact strength are excellent. On the other hand, Comparative Steels 4, 5 and 9 satisfy the formulas (1) and (3) and are excellent in impact strength, but they do not satisfy the formula (4) and are inferior in cold forgeability. Since Comparative Steel 6 does not satisfy the formulas (3) and (4), both impact strength and cold forgeability are poor. Comparative steels 7 and 8 satisfy the formulas (1) and (4), but do not satisfy the formula (3), and therefore have poor impact strength.

[0042]

As described above, according to the present invention,
C, Mn, Mo, P, which are the elemental components contained in case-hardening steel
By adjusting the amount of B, S, and elements such as B within a specific range, an appropriate balance of grain size and carburizing layer can be obtained, so material cost and processing are better than those of conventional case hardening steels. It is possible to provide a case-hardening steel excellent in impact strength and a carburized component using the same, without significantly increasing the cost.

[Brief description of drawings]

FIG. 1 is a side view showing a gear test piece.

FIG. 2 is a diagram showing a heat pattern for carburizing, quenching, and tempering.

FIG. 3 is a configuration diagram showing an impact test.

FIG. 4 is a graph showing the relationship between impact torque and the number of damages.

FIG. 5 is a diagram showing a gear that has been prototyped by cold forging and cutting.

FIG. 6 is a graph showing the relationship between the hardness of the spheroidized annealed material and the load value during cold forging (ratio when the comparative steel 5 is 100).

FIG. 7 is a graph showing a relationship between [left side value]-[right side value] of equation (2) and impact strength (ratio when comparative steel 5 is 100).

[Explanation of symbols]

1 Gear test piece 2 Mating gear 3 fixed end 4 torque arm 5 Impact load application position

Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 8/46 C23C 8/46 8/66 8/66 (72) Inventor Go Nakamura Daido-cho, Minami-ku, Nagoya-shi, Aichi 30-chome, Daido Steel Co., Ltd., Technical Development Laboratory (72) Inventor, Masatoshi Honda 2--30, Daido-cho, Minami-ku, Nagoya-shi, Aichi Prefecture Daido Steel Co., Ltd., Technical Development Laboratory (72) Inventor, Toyohashi, Nagoya, Aichi Prefecture 2-30, Daido-cho, Minami-ku, Daido Steel Co., Ltd. Technical Research Institute (72) Inventor Takao Hayashi Yokohama, Kanagawa 2 Takaracho, Kanagawa-ku Nissan Motor Co., Ltd. (72) Hideki Usuki Yokohama, Kanagawa 2 Takaracho, Kanagawa-ku Nissan Motor Co., Ltd. (72) Inventor Naomi Miura 2 Takara-cho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (72) Yoichi Murakami 2 Takara-cho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. In the company

Claims (10)

[Claims] 1. 0.1% to 0.3% by mass of carbon,
More than 0.3-1.0% silicon, 0.3-1.7% manganese, 0.03% or less phosphorus, 0.03% or less sulfur,
It contains 1.0% or less of molybdenum, 0.04% or less of aluminum and 0.03% or less of nitrogen, and the balance is composed of iron and unavoidable impurities. The following formula (1) [C%] + 5 ([P% ] + [S%]) ≦ ([Mn%] + [Mo%] + 1.8) / 8 (1) The case hardening steel characterized by the following. 2. Carbon in an amount of 0.1 to 0.3% by mass,
More than 0.3-1.0% silicon, 0.3-1.7% manganese, 0.03% or less phosphorus, 0.03% or less sulfur,
It contains 0.04% or less of aluminum, 0.03% or less of nitrogen and 0 to 1.6% of chromium, and the balance is composed of iron and unavoidable impurities, and has the following formula (2) [C%] + 5 ([ P%] + [S%]) ≦ ([Mn%] + 1.8) / 8 (2) The case hardening steel characterized by the following. 3. Mass% of 0.1-0.3% carbon,
0.3% or less of silicon, 0.3 to 1.7% of manganese,
0.03% or less phosphorus, 0.03% or less sulfur, 1.0
% Or less of molybdenum, 0.04% or less of aluminum and 0.03% or less of nitrogen, and the balance of iron and unavoidable impurities. The following formula (1) [C%] + 5 ([P%] + [[ S%]) ≦ ([Mn%] + [Mo%] + 1.8) / 8 (1) The case hardening steel characterized by the following :. 4. 0.1% to 0.3% by mass of carbon,
0.3% or less of silicon, 0.3 to 1.7% of manganese,
0.03% or less phosphorus, 0.03% or less sulfur, 0.0
It contains 4% or less of aluminum and 0.03% or less of nitrogen, and the balance is composed of iron and unavoidable impurities. The following formula (2) [C%] + 5 ([P%] + [S%]) ≦ ([ Mn%] + 1.8) / 8 ... (2) The case hardening steel characterized by the above-mentioned. 5. The case-hardening steel according to claim 3, further containing more than 0 to 1.6% of chromium. 6. The method according to claim 1, further comprising 0.001 to 0.005% of boron and 0.01 to 0.10% of niobium and / or titanium. The case-hardening steel according to one item. 7. The elemental component has the following formula (3) [C%] + 5.2 ([P%] + [S%]) ≦ ([Mn%] + [Mo%] + 3.8) / 22 + 96 [ B%] + [Austenite grain size number defined by JIS G0551] / 111 (3), The case-hardening steel according to any one of claims 1 to 6, wherein 8. The elemental component satisfies the following expression (4): 80 [Si%] + 24 [Mn%] + 33 [Mo%] + 13 ≦ 40 (4). The case-hardening steel according to one item. 9. Further comprising at least one element selected from the group consisting of 0.3% or less of lead, 0.15% or less of bismuth and 0.1% or less of calcium. The case-hardening steel according to any one of claims 1 to 8. 10. A carburized part formed by using the case-hardening steel according to claim 1, wherein the carburized layer has a grain size of austenite grain size number 7 defined by JIS G 0551. Carburized parts characterized by a finer austenite grain size than No.