JP5272330B2 - Steel for gas carburization, gas carburized parts, and method for manufacturing gas carburized parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for carburizing, provided with alloy composition with which in the case of performing gas-carburizing, internal oxidation is reduced and also, excessive carburization is avoided and C concentration on the steel surface can be controlled to desired C concentration, and to provide a carburized component. <P>SOLUTION: The steel for carburizing and the carburized component in this invention, is composed of 0.10-0.30% C, 0.50-3.00% Si, 0.30-3.00% Mn, &le;0.030% P, &le;0.030% S, 0.01-1.00% Cu, 0.01-3.00% Ni, 0.30-1.50% Cr and the balance Fe with inevitable impurities and satisfies Si[%]+Ni[%]+Cu[%]-Cr[%]&gt;0.3. The carburized component in this invention is obtained by performing the gas-carburization under atmosphere of &ge;0.90% in carbon potential to the pure iron to the steel for carburizing in this invention. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

The present invention relates to a gas carburizing steel, a gas carburized component, and a method of manufacturing a gas carburized component. More specifically, when gas carburizing is performed, grain boundary oxidation is reduced, excessive carburization is avoided, and C concentration on the steel surface is reduced. the desired gas carburized steel and the gas carburized component having an alloy composition can be controlled to C concentration, and a method of manufacturing a gas carburized component capable of controlling the C concentration of the steel surface to a desired C concentration About.

Gas carburization is one of the methods for hardening the steel surface by intruding C into the steel surface. Gas (the main component CO and CO ₂ , H ₂ for adjusting the C concentration) are supplied to the heating furnace containing the steel. For example, and carburizing at a high temperature of 900 ° C. or higher for a long time. The characteristics of carburized parts obtained by gas carburizing depend on the alloy composition and the gas atmosphere during carburizing. Accordingly, various problems have been pointed out. For example, when carburizing JIS SCr420, which is well-known as case hardening steel, Mn, Cr, etc. are oxidized by CO ₂ gas in the atmosphere (grain boundary oxidation), the hardenability is lowered, and the pearlite structure along the grain boundary. It has been pointed out that the strength is lowered and non-patent document 1
Therefore, Patent Documents 1 to 3 disclose a technique for improving the temper softening resistance by reducing the grain boundary oxidation by taking a predetermined alloy composition and adding Si up to 1.5%. Is disclosed.
In Patent Documents 1 to 3, the upper limit of Si is set to 1.5% because, if Si is more than that, the penetration of C is inhibited during carburizing, and it becomes difficult to ensure the C concentration of the surface layer. It is. Therefore, if the C concentration of the surface layer is ensured, for example, in order to further increase the strength of the carburized component, it is desirable to contain more Si.

Electric Steelmaking, Volume 65, P13-P15 (January 1994) JP2003-027142A JP2003-231943 JP 2004-300550 A

However, when carburizing under the general carburizing conditions (carbon potential with respect to pure iron (hereinafter also referred to simply as “CP”) of about 0.8) by increasing the constituent element Si to more than 1.5%, There was a problem that Si was oxidized by the CO ₂ gas and grain boundary oxidation occurred. Therefore, in order to solve this problem, reducing the CO ₂ concentration in the atmosphere, increasing the CO concentration in the atmosphere and making the atmosphere reducible (with a carbon potential of about 1.0 with respect to pure iron), gas carburizing. Can be considered. However, this can reduce grain boundary oxidation, but at the same time, the carbon potential of the atmosphere is increased, resulting in precipitation of carbides and generation of residual γ accompanying the introduction of excess C. There was a problem that I could not. Therefore, there is a demand for an alloy composition that can control the steel surface to a desired C concentration without introducing excessive C even if Si is increased to more than 1.5% and the carbon potential of the atmosphere in gas carburizing is increased. .

The present invention has been made in view of the above circumstances, and a first object thereof is to reduce grain boundary oxidation when performing gas carburizing, avoid excessive carburization, and desired C concentration on the steel surface. An object of the present invention is to provide a gas carburizing steel and a gas carburized component having an alloy composition which can be controlled to a C concentration of 2.
The second object of the present invention is to provide a method for producing a gas carburized component capable of controlling the C concentration on the steel surface to a desired C concentration.

In order to solve the above-mentioned problems, the present inventors have searched for various alloy compositions. As a result, Si, Ni, and Cu indicate the carbon activity of the steel (the carbon content indicating the action as a carburizing steel; the same applies hereinafter). While Cr was increased, the knowledge that Cr decreased the carbon activity was obtained. Therefore, the present inventor added a large amount of Si, Ni, Cu, while adding a small amount of Cr, even if gas carburizing with a higher carbon potential than before, C does not penetrate excessively, It was found that the C concentration on the steel surface can be controlled to a desired C concentration. The present inventor has further obtained the knowledge that the problem of grain boundary oxidation is also solved at a high carbon potential.
Under the conventional gas carburizing conditions, when Si is increased (more than 1.5% in relation to Patent Documents 1 to 3), there is a problem of grain boundary oxidation or excessive carburization. Although it was difficult or out of the question to perform gas carburizing on the treated material of the alloy composition, the present inventor, from the above knowledge, gas carburizing is applied even if the alloy composition is more Si The knowledge that it is possible was obtained.

This invention is made | formed based on these knowledge, The steel for gas carburizing and gas- carburized components which concern on this invention are the mass%, C: 0.10-0.30%, Si: 0.50. 3.00%, Mn: 0.30 to 3.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01 to 1.00 %, Ni: 0.01 to 0 .59% Cr: 0.30 to 1.50% and, Al: includes 0.01 to 0.2% the balance being a gas carburizing steel or gas carburizing component consisting of Fe and unavoidable impurities The gist is to satisfy the following formula (1).
Si [%] + Ni [%] + Cu [%] − Cr [%]> 0.3 (1)
In this case, it may further contain Mo: 0.00 to 2.00 % by mass%, and further, Nb: 0.00 to 0.20%, Ti: 0.00 to 0.5% by mass. It may include at least one of the group consisting of 0.20%, N: 0.00 to 0.05%, and B: 0.00 to 0.01%.

Gas carburizing component and the manufacturing method thereof according to the present invention, with respect to gas carburizing steel according to the present invention, the gas carburized component obtained by performing gas carburizing atmosphere of carbon potential for pure iron is 0.90% or more Or a manufacturing method thereof. In this case, the temperature of the atmosphere is desirably 800 ° C. to 1000 ° C.

The gas carburizing steel according to the present invention has the above composition and satisfies the above formula (1). Therefore, if gas carburizing is performed in an atmosphere of CP ≧ 0.90, grain boundary oxidation is reduced, Even if gas carburizing is performed in that atmosphere, excess carbon does not enter, and the C concentration on the steel surface can be controlled to a desired C concentration. Therefore, carbide and excessive residual γ are not generated during carburizing, and high fatigue strength is obtained.

Since the gas carburized component according to the present invention has the above composition and satisfies the above formula (1), grain boundary oxidation is reduced if gas carburized in an atmosphere of CP ≧ 0.90. In addition, excessive carburization is avoided and the C concentration on the steel surface is controlled to a desired C concentration. Therefore, the gas carburized component according to the present invention does not generate carbide or excessive residual γ, and exhibits high fatigue strength.

The method for producing a gas carburized component according to the present invention is provided with the above composition and gas carburizing in an atmosphere of CP ≧ 0.90 with respect to the carburizing steel satisfying the above formula (1). In the gas carburized component, grain boundary oxidation is reduced, excessive carburization is avoided, and the C concentration on the steel surface is controlled to a desired C concentration. Therefore, the obtained gas carburized parts do not generate carbides or excessive residual γ, and exhibit high fatigue strength.

Hereinafter, an embodiment of the present invention will be described in detail. In the following description, “%” means “mass%” unless otherwise specified.
(Ingredient composition and relationship and reasons for limitation)
The carburizing steel and carburized component according to an embodiment of the present invention include the following elements (1) to (8 ) and (10) as basic constituent elements.
(1) C: 0.10 to 0.30%.
C is an element necessary for obtaining strength required as a machine part. The reason why the lower limit of C is set to 0.10% is that if C is too small, ferrite is generated in the core and the strength is lowered. On the other hand, the upper limit of C is set to 0.30% because workability, particularly machinability, is deteriorated when C is too much.

(2) S i: 0.50 to 3.00%.
Si, together with Cu and Ni described later, is an element that increases the carbon activity, and is an element that is added to increase strength and increase temper softening resistance. The reason why the lower limit of Si is set to 0.50% is that if there is too little Si, the hardenability is lowered and the strength is lowered. On the other hand, the upper limit of Si is set to 3.00% because workability, particularly machinability, is deteriorated when there is too much Si.

(3) Mn: 0.30 to 3.00%.
Mn is an element added as a deoxidizer during the melting of steel, but does not affect gas carburization. The reason why the lower limit of Mn is set to 0.30% is that if the amount of Mn is too small, ferrite is generated in the core and the strength is lowered. On the other hand, the upper limit of Mn is set to 3.00% because if Mn is too much, workability, particularly machinability, deteriorates.

(4) P : 0.030% or less.
P is an element which is an impurity and inevitably contained. The upper limit of P is set to 0.030% because it is brittle when P is too much.

(5) S : 0.030% or less.
S is an element which is an impurity and is inevitably included. The reason why the upper limit of S is set to 0.030% is that when S is too much, embrittlement occurs.

(6) Cu: 0.01-1.00%.
Cu is an element that increases the carbon activity together with the above-described Si and Ni described later, and is an element that is included to increase the strength. The reason why the lower limit of Cu is set to 0.01% is that if there is too little Cu, the hardenability is lowered and the strength is lowered. On the other hand, the reason why the upper limit of Cu is set to 1.00% is that if there is too much Cu, workability, particularly hot forgeability deteriorates.

(7) Ni: 0.01 to 3.00%.
Ni, together with Si and Cu, is an element that increases the carbon activity, and is an element that is included to increase the strength. The reason why the lower limit of Ni is set to 0.01% is that if Ni is too small, the hardenability is lowered and the strength is lowered. On the other hand, the upper limit of Ni is set to 3.00% because if Ni is too much, workability, particularly hot forgeability, deteriorates.

(8) Cr: 0.30 to 1.50%.
Since Cr is an element that decreases the carbon activity, it cannot be contained in a large amount. Further, if the amount of Cr is too much, workability, particularly machinability, is deteriorated, so the upper limit was made 1.50%. On the other hand, if the Cr content is too small, the hardenability decreases and the strength decreases, so the lower limit was made 0.30%.

The carburizing steel and carburized component according to an embodiment of the present invention may include the following elements (9 ) and ( 11) as optional constituent elements.
(9) Mo: 0.00-2.00%.
Mo can be added in order to improve hardenability and increase resistance to temper softening, but if it is too much, workability, particularly machinability, is deteriorated, so the upper limit was made 2.00%.

(10) Al: 0.01 to 0.20%.
Al can be added in order to refine crystal grains and improve the strength, but if it is too much, alumina is generated in the steel and the strength is lowered, so the upper limit was made 0.20%.

(11) From Nb: 0.00 to 0.20%, Ti: 0.00 to 0.20%, N: 0.00 to 0.05%, and B: 0.00 to 0.01% At least one of the group consisting of.
Nb and Ti can be added to suppress the growth of crystal grains caused by gas carburization and to maintain a sized structure. On the other hand, if Nb or Ti is too much, workability is adversely affected.
N can be added to prevent coarsening of crystal grains, but its effect is saturated at about 0.05%, so 0.05% was made the upper limit.
B can be added to improve hardenability, but if it is too much, workability is adversely affected, so 0.01% was made the upper limit.

(12) Si [%] + Ni [%] + Cu [%]-Cr [%]> 0.3 (1)
As described above, since Si, Cu, and Ni increase the carbon activity, and Cr decreases the carbon activity, the amount of Cr is subtracted from the total amount of Si, Cu, and Ni. The value was used as a standard for judging the carbon activity. The value exceeding 0.3 is an empirically obtained value in the examples described later. If the relationship of the above formula (1) is satisfied, even if the carbon potential is increased in gas carburizing, excess carbon does not enter, the C concentration on the steel surface can be controlled to a desired C concentration, and the carbon potential If the height is increased, grain boundary oxidation can be reduced.

(Production method)
In the carburizing steel according to an embodiment of the present invention, the raw materials are dissolved so as to have the above composition, ingot-formed, processed into a predetermined shape by forging, and further subjected to necessary heat treatment (normalizing). Is obtained.
The carburized component according to one embodiment of the present invention is manufactured as follows. That is, first, for the carburizing steel according to one embodiment of the present invention, gas carburizing is performed for several hours in an atmosphere having a carbon potential of 0.90% or more with respect to pure iron and a temperature of 800 ° C to 1000 ° C. Next, quenching is performed (gas carburizing quenching). Here, the gas carburization may be performed by carburization at a temperature of about 930 ° C. and diffusion at about 850 ° C. Next, oil tempering is performed at 150 to 200 ° C. for several hours. Thereby, the carburized component which concerns on one Embodiment of this invention is obtained.

(Function)
Since the carburizing steel according to an embodiment of the present invention has the above composition and satisfies the above formula (1), on the other hand, a method for manufacturing a carburized component according to an embodiment of the present invention, That is, if the method of gas carburizing is performed in an atmosphere of CP ≧ 0.90, grain boundary oxidation is reduced because CP is high in the first place. Further, since the carbon activity derived from the component composition is high, excessive carbon does not enter even when gas carburizing is performed in an atmosphere of CP ≧ 0.90, and the C concentration on the steel surface is 0.70 to 0.88%. The desired C concentration is controlled. In addition, carbide and excessive residual γ are not generated during gas carburization, and the resulting carburized parts have high fatigue strength.

Examples of the present invention and comparative examples will be described below.
(Examples 1-21 and Comparative Examples 1-6)
(Production of carburizing steel)
About Examples 1-21 and Comparative Examples 1-6, the raw material was thrown into an electric furnace so that it might become the component composition shown in Table 1, and it melted and casted and produced the steel ingot. Table 1 also shows the value of the above formula (1) in addition to the component composition.

(Production of carburized parts)
Next, for each of the examples and comparative examples, the steel ingot was processed into a bar (φ100 mm) by rolling, and then hot forged to form a set of gear shapes, that is, the shape of the driving gear (module 2. 5, number of teeth 26, tooth width 20mm, pressure angle 20 °, helix angle 25 °) and driven gear shape (module 2.5, number of teeth 30, tooth width 20mm, pressure angle 20 °, helix angle 25 °) ) And processed.
Next, the processed steel ingots on the drive side and the driven side are put in a furnace and heated to 930 ° C., and gas (CO 2) is obtained so as to have the carbon potential shown in Table 2 (carbon potential with respect to pure iron). , CO ₂ , H ₂ ), carburizing treatment (carburizing) at 930 ° C. for 120 minutes, then lowering the furnace temperature to 850 ° C. by furnace cooling, and carburizing treatment (diffusion) at 850 ° C. for 30 minutes. went. Next, oil quenching was performed. Then, oil tempering was performed at 180 ° C. for 1 hour (see FIG. 1). As a result, for each of the examples and comparative examples, a set of gears with increased C concentration on the steel surface, that is, a driving gear and a driven gear (carburized parts) were obtained.
The gas introduction for obtaining the carbon potential shown in Table 2 was performed based on these relationships at 930 ° C. among the relationship between CO ₂ % -CO% and the carbon potential with respect to pure iron shown in FIG. When performing at other temperature, it may carry out based on those relationships at other temperatures. In the same figure, examples of other temperatures include 816 ° C, 843 ° C, 871 ° C, and 900 ° C.

(Dental fatigue test)
The tooth root fatigue test is a test for investigating deterioration of fatigue strength due to deformation at the tooth root when a large load is continuously applied. In the tooth root fatigue test, the set of gears described above, that is, the driving side gear (module 2.5, number of teeth 26, tooth width 20 mm, pressure angle 20 °, helix angle 25 °) and driven side gear (module 2). .5 number of teeth 30, face width 20 mm, pressure angle 20 °, helix angle 25 °) and engaging a determined 10 ⁷ times, the applied stress that does not cause pitting destruction be rotated as dedendum bending fatigue strength It was. In the tooth root fatigue test, the rotation speed was 3500 rpm, and the oil temperature was 80 ° C. The results are shown in Table 2.

(Dental surface carbon concentration)
The tooth surface carbon concentration was determined by taking a sample from the produced gear and analyzing the cross section in the plate thickness direction with EPMA. The results are shown in Table 2.

(Grain boundary oxidation depth and presence or absence of carbides)
The grain boundary oxidation depth is obtained by taking a sample from the prepared gear, performing Ni plating on the surface (to make the micrograph easier to see), corroding the cross section in the plate thickness direction with nital, and then the blackened portion of the plate It was determined by measuring the length in the thickness direction. The results are shown in Table 2.
3 (a) and 3 (b) are photomicrographs after corroding with nital in the cross-section in the plate thickness direction of Example 9 and a general case-hardened steel, SCr420 (not included in the comparative example). Shown in comparison. In Example 9, grain boundary oxidation was not observed, but in SCr420, grain boundary oxidation (beard-like black part) was remarkably recognized.
Further, FIGS. 4 (a) and 4 (b) show a comparison of micrographs of Example 9 and a general case-hardened steel SCM420 (Comparative Example 1) after being corroded with nital in the cross section in the plate thickness direction. . In Example 9, the formation of carbide was not recognized, but the formation of carbide was recognized in SCM420 (Comparative Example 1).

(Evaluation)
Comparative Examples 1 and 2 are directed to SCM420, which is a general case-hardened steel, and are different in CP. In Comparative Example 1, CP is set to 0.8 as usual. In this case, the root surface carbon concentration is a desired value, but the grain boundary oxidation depth is deep, and the root bending fatigue strength is also high. It became low. Therefore, as in Comparative Example 2, when CP is set to 1.09, which is higher as in Examples 1 to 21 of the present invention, even if the grain boundary oxidation depth can be reduced, C enters the surface too much. The tooth root surface carbon concentration became too high, and the tooth root bending fatigue strength was lowered. From these facts, it was confirmed that the general case-hardened steel does not have the desired characteristics only by adjusting the CP, and it is necessary to devise the component composition to obtain the desired characteristics.
Therefore, as in Examples 1 to 21, when a predetermined component composition is satisfied and the above formula (1) is satisfied, excessive carbon does not enter even if gas carburizing is performed in an atmosphere of CP ≧ 0.90. The tooth surface carbon concentration could be a desired C concentration of about 0.70 to 0.88%. Moreover, since Examples 1-21 performed gas carburizing in the atmosphere of CP> = 0.90, it resulted in the grain boundary oxidation depth being reduced (refer Fig.3 (a)). Further, in Examples 1 to 21, since excessive carbon does not enter as described above, carbide and excessive residual γ are not generated during gas carburization (the residual γ in Examples 1 to 21 is 25 vol% or less, In Comparative Examples 2 to 6, the residual γ was more than 40 vol%), and the obtained gear exhibited a significantly higher root bending fatigue strength of 980 MPa or higher than that of Comparative Examples 1 to 6.

In addition to Comparative Example 2, Comparative Examples 3 to 5 have poor component balance and do not satisfy the above formula (1). Therefore, when gas carburizing is performed with the same CP as in Examples 1 to 21, the grain boundary oxidation depth is reduced. Even if it could be reduced, C entered too much and the tooth surface carbon concentration became too high. Therefore, the root bending fatigue strength was also low. This is thought to be because the residual γ remained because the tooth surface carbon concentration was high and did not transform into martensite.
In Comparative Example 6, since Si was small, even if the above formula (1) was satisfied, if gas carburization was performed with the same CP as in Example 1, C entered excessively and the tooth surface carbon concentration became too high. It was. Therefore, the root bending fatigue strength was also low. This is thought to be because the residual γ remained because the tooth surface carbon concentration was high and did not transform into martensite.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

The gas carburizing steel according to the present invention is suitable as a steel material for machine structural parts such as shafts and gears, and the gas carburizing component according to the present invention is suitable as a part for various machines. Industrial use value is extremely high for parts manufacturers.
The method for producing a gas carburized component according to the present invention includes a specific element (for example, Si) in an amount that has been conventionally considered difficult by gas carburizing if attention is paid to a specific element by devising a component balance. Since it can be applied to various processing materials, there is a wide range of options for manufacturing methods, and the industrial utility value is extremely high for steel material manufacturers and machine structural component manufacturers.

It is the figure which showed typically the gas carburizing quenching process of Examples 1-21 and Comparative Examples 1-6. It is a graph showing the relationship between the carbon potential for _CO 2% -CO% of pure iron in 800 ℃ ~930 ℃. It is a microscope picture after corroding the plate | board thickness direction cross section of Example 9 and the gas carburizing quenching and tempering of SCr420 with nital. It is a microscope picture after corroding the plate | board thickness direction cross section of Example 9 and SCM420 (comparative example 1) after gas carburizing quenching and tempering with nital.

Claims (10)

In mass%, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, P: 0.030% or less, S: 0.030 %: Cu: 0.01 to 1.00% , Ni: 0.01 to 0.59% , Cr: 0.30 to 1.50% , and Al: 0.01 to 0.2% the balance a gas carburized steel consisting of Fe and unavoidable impurities, the following (1) steel for gas carburizing to satisfy the equation.
Si [%] + Ni [%] + Cu [%] − Cr [%]> 0.3 (1) The gas carburizing steel according to claim 1, further comprising Mo: 0.00 to 2.00 % by mass. Furthermore, by mass%, Nb: 0.00 to 0.20%, Ti: 0.00 to 0.20%, N: 0.00 to 0.05%, and B: 0.00 to 0.01. The steel for gas carburization according to claim 2, comprising at least one of the group consisting of%. In mass%, C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, P: 0.030% or less, S: 0.030 %: Cu: 0.01 to 1.00% , Ni: 0.01 to 0.59% , Cr: 0.30 to 1.50% , and Al: 0.01 to 0.2% the balance a gas carburizing component consisting of Fe and unavoidable impurities, gas carburizing component and satisfies the following formula (1).
Si [%] + Ni [%] + Cu [%] − Cr [%]> 0.3 (1) The gas carburized component according to claim 4, further comprising, by mass%, Mo: 0.00 to 2.00 % . Furthermore, by mass%, Nb: 0.00 to 0.20%, Ti: 0.00 to 0.20%, N: 0.00 to 0.05%, and B: 0.00 to 0.01. The gas carburized component according to claim 5, comprising at least one of the group consisting of%. A gas carburized part obtained by gas carburizing the steel for gas carburizing according to any one of claims 1 to 3 in an atmosphere having a carbon potential of 0.90% or more with respect to pure iron. The gas carburized part according to claim 7, wherein the temperature of the atmosphere according to claim 7 is 800C to 1000C. A method for producing a gas carburized component, comprising performing gas carburizing on the gas carburizing steel according to any one of claims 1 to 3 in an atmosphere having a carbon potential of 0.90% or more with respect to pure iron. The method for producing a gas carburized component according to claim 9, wherein the temperature of the atmosphere according to claim 9 is 800C to 1000C.

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