JP2579640B2 - Manufacturing method of high fatigue strength case hardened product - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a case hardened product having a high fatigue strength, and more particularly, in producing a case hardened product such as a gear of an automobile, a construction machine, and various shafts. The present invention relates to a method for producing a case-hardened product which can improve the material properties, particularly the fatigue strength.

[Prior art] Among mechanical parts such as gears of automobiles and construction machines, various shafts, and the like, particularly those requiring high fatigue strength, a surface hardening treatment is performed after processing into a desired product shape. ,
For this reason, these products are usually called case-hardened products. As the surface hardening method, the method by carburizing and quenching is the most common.

What is carburizing and quenching is, for example, as shown in “Heat treatment of steel 5th revised edition” published on Mar. 1, 1969 by Maruzen Co., Ltd. In this method, only the surface layer is made of high-carbon martensite by tempering if necessary, and the fatigue strength is improved.

Such surface hardening method using only carburizing and quenching
Published by Japan Heat Treatment Technology Association in June 1959, "Heat Treatment Volume 24, Volume 3
No. pp. 128-136, hardness and residual stress are reduced due to the formation of an abnormal surface layer along the austenite grain boundaries in the carburized surface layer, and sufficient fatigue strength cannot always be imparted. Has traditionally been a problem. In contrast, JP-A-61-253346 discloses that Si: 0.10% or less, M:
n: reduced to 0.5% or less to suppress generation of grain boundary oxides, P:
A material used by performing carburizing and quenching with the grain boundary strength improved to 0.010% or less is shown.

However, even if such a material is used and the case hardening method using only carburizing and quenching, the fatigue limit evaluated by the Ono-type rotary bending fatigue test using a smooth specimen is 80 kgf /
The upper limit is mm ² , which is not yet sufficient to guarantee the fatigue strength of case-hardened products.

[Problems to be Solved by the Invention] An object of the present invention is a fatigue limit evaluated by an Ono-type rotating bending fatigue test using a smooth test piece, and the upper limit of the conventional method.
It is an object of the present invention to provide a method for producing a case-hardened product that can impart an excellent fatigue strength of at least 1.2 times or more and 96 kgf / mm ² or more.

[Means and Actions for Solving the Problems] The present inventors have conducted various studies in order to achieve high fatigue strength of case-hardened products, and as a result, have clarified the following points.
(1) Fatigue cracks in case-hardened products grow due to grain boundary fracture of old austenite. Therefore, the point of increasing the fatigue strength of the case-hardened product is to increase the grain boundary strength of the former austenite. Further, applying a compressive residual stress to the surface is also effective for increasing the fatigue strength. (2) As a method for increasing the grain boundary strength of the prior austenite, it is effective to refine the prior austenite crystal grains and to reduce the P. (3) In order to refine the prior austenite crystal grains, it is effective to carry out induction hardening after carburizing under specific conditions, containing Al and N in specific component ranges. This also imparts a compressive residual stress to the surface. (4) When Nb and Ti are contained, the effect of refining prior austenite crystal grains becomes even more remarkable. From the above, the steel material in a specific component range is carburized and quenched at a specific carbon potential, and then induction hardened under specific conditions, and evaluated by the Ono-type rotating bending fatigue test using a smooth specimen. 96 with reduced fatigue limit
It is possible to realize a fatigue strength of kfg / mm ² or more,
The present invention has been made based on a novel finding that it is possible to impart an excellent fatigue strength that can sufficiently satisfy the required characteristics of a case-hardened product according to such a manufacturing method.

That is, the present invention has been made based on the above findings, and the gist of the present invention is as follows: C: 0.15 to 0.35%, Al: 0.01 to 0.15%, N: 0.005 to 0.025%, Mn : 0.30 to 1.2%, Cr: 0.30 to 1.20%, S: 0.01 to 0.20%, or one or two of Nb: 0.020 to 0.120%, Ti: 0.005 to 0.10% P: 0.01% or less, Si: 0.50% or less, or one or more of Mo: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less, V: 1.0% or less A steel material containing at least one species and the balance of Fe and unavoidable impurities is processed into a required product shape, the carbon potential Cp is in the range of 0.4 to 0.9 wt%, and the difference between the carbon potential and the carbon concentration of the material is 0.2%. After carburizing and quenching with a carbon potential of at least wt%, austenite 0.3 to 1.5 times the total hardened layer depth when carburizing part or all of the product. The present invention is directed to a method for producing a high fatigue strength case hardened article, characterized by performing induction hardening for heat treatment.

 Hereinafter, the present invention will be described in detail.

First, the reason for limiting the component content range as described above for the steel subject to the present invention will be described.

First, C is an effective element for increasing the strength of the core of a case-hardened product, but if it is less than 0.15%, the strength is insufficient.
If the content exceeds 0.35 wt%, the toughness is deteriorated, and a compressive residual stress useful for the fatigue strength of the case hardened product is hardly generated. Therefore, the content is set to 0.15 to 0.35%.

Next, Al and N form the compound AlN in the steel and have the function of refining austenite crystal grains.
If it is less than the lower limits of Al: 0.01% and N: 0.005%, the effect is insufficient. On the other hand, if the content exceeds Al: 0.15 wt% and N: 0.025 wt%, the effect is saturated and the toughness is rather deteriorated. Therefore, these contents are set to Al: 0.01 to 0.15 wt% and N: 0.005 to 0.025 wt%. .

Mn and Cr are effective elements for improving hardenability.
If it is less than 0.30%, the effect is insufficient, while both are 1.
If the content exceeds 20%, the formation of a grain boundary oxide layer along the austenite grain boundaries in the carburized surface layer becomes remarkable and deteriorates the fatigue strength. Therefore, the content of these elements is Mn: 0.30 to 1.20%, Cr:
0.30 to 1.20%.

S is an element effective for improving machinability, but its effect is
If it is less than 0.01%, it is not sufficient. On the other hand, if it exceeds 0.20%, sulfide-based inclusions are formed and fatigue strength is deteriorated. Therefore, the content is made 0.01 to 0.20%.

Further, P causes grain boundary skew in the steel, and causes deterioration of fatigue strength through reduction in grain boundary strength. In particular, when P exceeds 0.01 wt%, fatigue strength deterioration becomes remarkable, so 0.01% was made the upper limit.

Si is an element that has a strong tendency to form a grain boundary oxide layer along the austenite grain boundaries in the carburized surface layer, and when the content exceeds 0.50%, the tendency is particularly remarkable, so the content is set to 0.50% or less.

The above is the basic component system. In the present invention, one or both of Nb and Ti can be contained in order to further refine the austenite crystal grains.

First, Nb forms the compound Nb (C, N) in steel and has the function of refining austenite crystal grains. However, if it is less than 0.020%, its effect is insufficient, while if it exceeds 0.120%, its effect is Since it saturates and rather deteriorates toughness, its content is made 0.020 to 0.120%.

In addition, Ti forms the compound Ti (C, N) in steel and has a function of refining austenite crystal grains. However, if the content is less than 0.005%, the effect is insufficient, and if it exceeds 0.10%, the effect is reduced. Since it saturates and rather deteriorates toughness, its content is made 0.005 to 0.10%.

Further, in the steel subject to the present invention, one or more of Mo: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less, and V: 1.0% or less can be contained as necessary. All of these elements are effective in improving the hardenability, but the addition of a large amount is not preferable in terms of economy, so the upper limit is specified as described above.

Next, in the present invention, such a material is processed into a required product shape, the carbon potential Cp is in the range of 0.4 to 0.9 wt%, and the difference between the carbon potential and the carbon concentration of the material is 0.2 watts.
Carburizing and quenching at a carbon potential of at least t% and then induction hardening to austenite 0.3 to 1.5 times the total hardened layer depth during carburizing of part or all of the product. The reason will be described.

The fatigue strength of a case hardened product is determined mainly by the prior austenite grain size and the surface compressive residual stress. The smaller the former austenite grain size and the larger the surface compressive residual stress, the higher the fatigue strength. Fatigue limit was evaluated by the test fatigue Ono-type rotating bending with smooth specimens, in order to impart 96kgf / mm ² or more excellent fatigue strength is 1.2 times the upper limit of the conventional method, the old austenite crystal Granularity
JIS particle size number of 10 or more miniaturization, at the same time -30k
It is necessary to impart a surface compressive residual stress of gf / mm ² or more. In order to realize this, it is impossible with the conventional method using only carburizing and quenching, but according to the method of the present invention, this is possible for the first time. That is, with respect to the steel material of the above-described composition range of the present invention, after carburizing and quenching with a carbon potential in the range of 0.4 to 0.9 wt%, followed by induction quenching, the old austenite crystal grain size is 10 or more in JIS grain size number. It can be refined. Also, the difference between the carbon potential and the carbon concentration of the material is 0.2 wt%.
As described above, when performing the induction hardening further, by austenitizing 0.3 to 1.5 times the total hardened layer depth at the time of carburizing, a sufficient surface compressive residual stress of −30 kgf / mm ² or more is given to the case hardened product. It becomes possible. If the difference between the surface carbon concentration and the core carbon concentration is less than 0.2 wt%, the difference between the surface layer and the core in the amount of transformation expansion due to martensitic transformation becomes small, and the austenitization depth is reduced by the If the depth of the hardened layer is less than 0.3 times, the absolute amount of transformation expansion accompanying martensitic transformation is small, while if it exceeds 1.5 times, the contribution of thermal stress to the surface layer residual stress is greater than the transformation stress. On the other hand, it is impossible to apply a compressive residual stress of −30 kgf / mm ² or more to a case-hardened product. The carbon potential Cp during carburization was set in the range of 0.4 to 0.9 wt% because the hardness of martensite was determined by the carbon concentration.
If Cp is less than 0.4 wt%, it is difficult to secure surface hardness, and 0.9
If the content is more than wt%, pro-eutectoid cementite will precipitate and deteriorate the fatigue strength.

In the present invention, carburizing and quenching conditions other than those specified above (for example, carburizing temperature, carburizing time, etc.) and induction hardening conditions (for example, frequency, heating time, cooling method, etc.)
Any condition can be selected as long as it is within a range satisfying the requirements of the present invention. In the present invention, after induction hardening, tempering can be performed if necessary.

Hereinafter, the effects of the present invention will be more specifically described with reference to examples.

[Example] A smooth test piece having a parallel portion of 8 mm in diameter was prepared from a steel material having the composition shown in Table 1 and heat-treated under the conditions shown in Table 2.
The fatigue properties were evaluated by the Ono-type rotating bending fatigue test. Table 3 shows the results.

It can be seen that according to the method of the present invention, a case-hardened product having an excellent fatigue limit of 96 kgf / mm ² or more can be produced.

On the other hand, Comparative Examples 1, 23, 24, and 25 are requirements of the present invention. Induction hardening after carburizing was not performed,
Comparative Examples 4, 6, and 8 are cases in which the content of C, Al, or N was below the range required for the present invention, respectively, and Comparative Examples 5, 7, 9, 10, 12, and 14 were , Al, N, P, Nb, the content of Ti is more than the range required for the present invention, respectively, Comparative Examples 17 and 18, the austenitizing depth when performing induction hardening is the present invention. Comparative example 20 is the case where the difference between the carbon potential and the carbon concentration of the material was lower than the range required for the present invention. Is the case where the carbon potential exceeds the range required by the present invention, and in each case, the fatigue limit of less than 96 kgf / mm ² was obtained.

[Effects of the Invention] As described above, according to the method of the present invention, it is possible to impart excellent fatigue strength that can sufficiently satisfy the required characteristics against breakage of a case-hardened product, and the industrial effect is extremely significant. There is something.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-185718 (JP, A) JP-A-59-182952 (JP, A) JP-A-59-185759 (JP, A) JP-A-62 63653 (JP, A) JP-A-57-94516 (JP, A) JP-A-62-70512 (JP, A)

Claims (4)

(57) [Claims] (1) As a weight ratio, C: 0.15 to 0.35%, Al: 0.01 to
0.15%, N: 0.005-0.025%, Mn: 0.30-1.2%, Cr: 0.30
~ 1.20%, S: 0.01 ~ 0.20%, P: 0.01% or less, S
i: limited to 0.50% or less, steel material consisting of the balance Fe and unavoidable impurities is processed into the required product shape, carbon potential Cp is in the range of 0.4 to 0.9 wt%, and carbon potential and carbon concentration of the material are Carburizing and quenching with a carbon potential with a difference of 0.2wt% or more, then induction hardening to austenite 0.3 to 1.5 times the total hardened layer depth when carburizing part or all of the product. A method for producing a high-fatigue-strength case hardening product, characterized in that the former austenite crystal grain size is 10 or more in JIS grain size number and the residual stress on the surface is -30 kgf / mm ² or less. 2. The weight ratio of C: 0.15 to 0.35%, Al: 0.01 to 2.
0.15%, N: 0.005-0.025%, Mn: 0.30-1.2%, Cr: 0.30
~ 1.20%, S: 0.01 ~ 0.20%, Nb: 0.020 ~
0.120%, contains one or two of Ti: 0.005 to 0.10%, P: 0.01% or less, Si: 0.50% or less, balance Fe
And steel material consisting of unavoidable impurities is processed into the required product shape, and carbon potential Cp is in the range of 0.4 to 0.9 wt%,
And the difference between carbon potential and carbon concentration of material is 0.2wt%
After carburizing and quenching with the above carbon potential, part or all of the product is subjected to the total hardened layer depth during carburizing.
Performed induction hardening to austenite 0.3 to 1.5 times, and the austenite crystal grain size of the surface layer was 1 in JIS grain size number.
A method for producing a high-fatigue-strength skin-hardened product, characterized in that the surface residual stress is 0 kg or more and the residual stress on the surface is −30 kgf / mm ² or less. 3. The weight ratio of C: 0.15 to 0.35%, Al: 0.01 to 3.
0.15%, N: 0.005-0.025%, Mn: 0.30-1.2%, Cr: 0.30
~ 1.20%, S: 0.01 ~ 0.20%, P: 0.01% or less, S
i: 0.50% or less, Mo: 1.0% or less, Ni: 4.0%
Hereinafter, a steel material containing one or more of Cu: 2.0% or less and V: 1.0% or less, the balance being Fe and unavoidable impurities,
Processed to required product shape, carbon potential Cp 0.4 ~
After carburizing and quenching at a carbon potential in the range of 0.9 wt% and the difference between the carbon potential and the carbon concentration of the material is 0.2 wt% or more, the depth of the entire hardened layer when carburizing part or all of the product Induction quenching to austenite 0.3 to 1.5 times of that of the former austenite crystal grain size of the surface layer is 10 or more in JIS grain size number, the surface residual stress is -30 kgf
/ mm ² or less. 4. The weight ratio of C: 0.15 to 0.35%, Al: 0.01 to
0.15%, N: 0.005-0.025%, Mn: 0.30-1.2%, Cr: 0.30
~ 1.20%, S: 0.01 ~ 0.20%, Nb: 0.020 ~
0.120%, contains one or two of Ti: 0.005 to 0.10%, P: 0.01% or less, Si: 0.50% or less, M
o: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less, V: 1.0% or less, containing one or two or more types, with the balance of Fe and unavoidable impurities into the required product shape After processing and carburizing and quenching with a carbon potential Cp in the range of 0.4 to 0.9 wt% and a difference between the carbon potential and the carbon concentration of the material of 0.2 wt% or more, part or all of the product Induction hardening to austenite 0.3 to 1.5 times the total hardened layer depth during carburizing, the old austenite crystal grain size of the surface layer is 10 or more in JIS grain number, and the residual stress on the surface is -30 kgf / mm ² or less A method for producing a high-fatigue-strength case-hardened product, characterized in that: