JP2011174176A - Case-hardened steel and carburized material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a case-hardened steel not only having good cold forgeability but also having high fatigue strength. <P>SOLUTION: A component composition includes, by mass, 0.10 to 0.35% C; 0.01 to 0.50% Si; 0.40 to 1.50% Mn; no more than 0.02% P; no more than 0.03% S; 0.04 to 0.10% Al; 0.5 to 2.5% Cr; 0.0005 to 0.0050% B; 0.003 to 0.080% Nb; no more than 0.003% Ti; and less than 0.0080% N, with the remainder comprising Fe and unavoidable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a case-hardening steel and a high fatigue strength carburized material that are excellent in cold forgeability and have high fatigue strength by carburization, which are used for machine structural materials used in the fields of construction machinery and automobiles.

  High cold forgeability is required for parts materials such as automobiles manufactured by cold forming steel bars. Therefore, spheroidizing heat treatment is performed to spheroidize carbides to improve cold forgeability. In addition, from the viewpoint of the composition of steel, proposals have been made to reduce Si, which greatly affects deformation resistance. Furthermore, there is a proposal of steel that effectively uses the hardenability of B.

  For example, in Patent Document 1, by reducing the amount of other alloy elements by the effect of improving the hardenability of B, the hardness is reduced from the normalizing step, and the cutting performance is dramatically improved compared to conventional steel. Improved carburized gear steels have been proposed.

  Patent Document 2 proposes a case-hardening steel that secures cold workability by combining the component system that secures hardenability with B by reducing the solid solution strengthening elements Si and Mn and the manufacturing conditions. Has been.

  On the other hand, in recent years, gears used in automobiles and the like have been required to be smaller in size as the weight of the vehicle body has been reduced due to energy saving, and the load on the gears has increased as the output of the engine has increased. is doing. The durability of gears is determined mainly by the root bending fatigue failure and the surface pressure fatigue failure of the tooth surface. Regarding the root bending fatigue strength, it is considered effective to reduce the incompletely hardened layer generated in the surface layer during carburizing and to refine the prior austenite grain size. In addition, regarding the improvement of surface fatigue strength, the relationship with temper softening resistance was pointed out, and the component with increased Si content, the component with Mo added, or the steel with fine carbide dispersed in the carburized surface layer, Each has been proposed.

  For example, Patent Document 3 proposes a steel for carburizing in which fatigue strength and toughness are improved by setting the prior austenite grain size to 7 μm or less. Patent Document 4 proposes finely dispersing carbide in a carburized layer on the surface.

Japanese Patent No. 3551573 Patent No. 3764586 Patent No. 3063399 Patent No. 4056709

However, in Patent Documents 1 and 2 described above, improvements in cold workability and impact properties are recognized, but fatigue properties are comparable to conventional steel.
Further, Patent Documents 3 and 4 have a problem that, when a large amount of carbide generating elements such as Nb, Ti, and V are used and finely precipitated, deformation resistance during processing is remarkably increased.

  The present invention has been developed in view of the above circumstances, and the object thereof is not only excellent in cold forgeability but also case hardening steel having high fatigue resistance after carburizing treatment and carburizing using the same. To provide materials.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge. First, in order to improve the fatigue strength, when a high C concentration carburized layer (hereinafter referred to as a high concentration carburized layer) having a C concentration of 0.85% by mass or more is formed on the case-hardened steel, We have intensively studied ways to finely disperse carbides by suppressing the formation of (mainly cementite).
That is, FIG. 1 shows the relationship among the amounts of Al, B, and Ti on the maximum particle diameter of carbides in the surface layer of the high-concentration carburized layer of case hardening steel. As can be seen from the figure, in order to suppress the formation of coarse carbides and finely disperse the carbides, it is important to control the amounts of Al and B and to suppress the amount of Ti added. Here, FIG. 1 also shows the results of measuring the surface fatigue strength of some steels, but it can also be seen that high surface fatigue strength can be obtained by suppressing the formation of coarse carbides.
In addition, when a carburized layer having a C concentration of 0.70 to 0.84% by mass (hereinafter referred to as a normal carburized layer) was formed on the case-hardened steel, the relationship among the amounts of Al, Ti, and B on the surface fatigue strength was also investigated. The results are also shown in FIG. Even when a normal carburized layer is formed, it can be seen that high surface fatigue strength can be obtained by controlling the amounts of Al and B within a specific range and suppressing Ti to 0.003 mass% or less.

The experiment whose results are shown in FIG. 1 is based on 0.2 mass% C-0.1 mass% Si-0.6 mass% Mn-1.5 mass% Cr-0.02 mass% Nb steel. And B added steel materials (the balance is Fe and inevitable impurities), and after these steel materials are treated under the following conditions, the maximum particle size (μm) and surface fatigue strength (MPa) of carbides Is evaluated.
That is, for high-concentration carburization, a 25mmφ round bar is processed from steel material, carbon concentration is 2%, high-concentration carburization at 950 ℃ for 5 hours, once cooled to 600 ℃, then again at 850 ℃ for 30 minutes It was kept, oil cooled at 60 ° C., and then tempered at 170 ° C. for 2 hours. After cutting the sample subjected to this treatment, the cut surface was corroded with Piracool liquid, the region from the surface to a depth of 30 μm was observed over a scanning electron microscope over 6000 μm ^2, and the maximum particle size of the carbide was determined by image analysis. . In addition, a roller pitching test piece was collected from the round bar, and a roller pitching test was performed on the samples subjected to the above-described high-concentration carburization to tempering treatment at a sliding rate of 40% and an oil temperature of 80 ° C. And the strength was evaluated 10 ⁷ times (the limit strength at which pitting occurs on the surface of the test piece).
For normal carburizing, a 25mmφ round bar is machined from a steel material, and a roller pitching test piece is taken from this round bar, and then carburized under conditions of 930 ° C, 7 hours, and carbon potential of 1.1% by mass. Then, it was oil-cooled at 60 ° C. and tempered at 170 ° C. for 2 hours. Thus to the sample thus obtained is subjected to roller pitting test under the conditions of the slip ratio of 40% and oil temperature 80 ° C., was evaluated (pitching limiting intensity generated in the test piece surface) 10 ⁷ times the strength.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.40-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
B: 0.0005-0.0050 mass%,
Nb: 0.003 to 0.080 mass%,
Case-hardened steel containing Ti: 0.003 mass% or less and N: less than 0.0080 mass%, with the balance being composed of Fe and inevitable impurities.

(2) The component composition further includes:
Cu: 1.0 mass% or less,
Ni: 0.50 mass% or less,
The case-hardening steel according to (1), which contains one or more selected from Mo: 0.50% by mass or less and V: 0.5% by mass or less.

(3) The component composition further includes:
Ca: 0.0005 to 0.0050 mass% and
Mg: 0.0002 to 0.0020 mass%
The case-hardened steel according to (1) or (2), which contains one or two of the above.
(4) A carburized material obtained by carburizing the case-hardened steel according to any one of (1) to (3), wherein the carbon content in the surface layer region from the surface to 0.4 mm is 0.70 mass. Carburizing material that is more than%.

(5) A carburized material obtained by carburizing the case-hardened steel according to any one of (1) to (3), wherein the carbon content in the surface layer region from the surface to 0.4 mm is 0.85 mass. Carburizing material having a maximum carbide diameter of 10 μm or less and an average particle diameter of 4 μm or less.

  According to the present invention, it is possible to provide a case-hardened steel that is not only excellent in cold forgeability but also excellent in fatigue resistance after carburizing treatment, which is very useful industrially.

It is a graph which shows the influence of the amount of Al, B, and Ti which acts on the precipitation state of carbide.

Hereinafter, the case-hardened steel of the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described in detail for each component.
C: 0.10 to 0.35 mass%
In order to increase the hardness of the central part by quenching after the carburizing heat treatment, 0.10% by mass or more of C is required, but when the content increases by 0.35% by mass, the toughness of the core part decreases, so the C amount is 0.10%. It was limited to the range of ˜0.35 mass%. Preferably, it is 0.3 mass% or less.

Si: 0.01 to 0.50 mass%
Si is necessary as a deoxidizing agent, and at least 0.01% by mass or more must be added. However, Si is an element that preferentially oxidizes in the carburized surface layer and promotes grain boundary oxidation. In addition, the upper limit is set to 0.50% by mass in order to enhance the solid solution strengthening of ferrite and increase the deformation resistance to deteriorate the cold forgeability. Preferably it is 0.03-0.35 mass%.

Mn: 0.40 to 1.50 mass%
Mn is an element effective for improving the hardenability, and requires addition of at least 0.40% by mass. However, Mn tends to cause grain boundary oxidation, and excessive addition increases residual austenite, leading to a decrease in surface hardness, so the upper limit was made 1.50% by mass. Preferably it is the range of 0.60-1.40 mass%.

P: 0.02% by mass or less P is segregated at the grain boundary and lowers the toughness. Therefore, the lower the content, the better, but 0.02% by mass is acceptable. Preferably, it is 0.018 mass% or less.

S: 0.03 mass% or less S is an element that exists as sulfide inclusions and is effective in improving machinability. However, excessive addition causes a decrease in fatigue strength, so the upper limit was made 0.03% by mass.

Al: 0.04-0.10 mass%
Al is an important element for obtaining the hardenability effect of B by fixing N in steel as AlN. In order to obtain this effect, it is necessary to add at least 0.04% by mass. However, if the content exceeds 0.10 mass%, in order to facilitate the generation of harmful A1 ₂ 0 ₃ inclusions against fatigue strength, Al content is limited to the range of 0.04 to 0.10 wt%.

Cr: 0.5-2.5% by mass
Cr is an element that contributes not only to hardenability but also to improving resistance to temper softening, and also useful for promoting the spheroidization of carbides. On the other hand, if it exceeds 2.5% by mass, the formation of retained austenite in the carburized part is promoted, and the fatigue strength may be adversely affected. Therefore, the Cr content is limited to the range of 0.5 to 2.5 mass%. Preferably it is the range of 0.6-2.0 mass%.

B: 0.0005 to 0.0050 mass%
B is the most important element in the present invention. B segregates at the austenite grain boundaries during the quenching heat treatment, thereby improving the hardenability and contributing to an increase in the hardness of the material. By this effect, other strengthening elements can be reduced, and as a result, an improvement in cold forgeability due to a decrease in deformation resistance can be obtained. In order to exhibit this effect, it is necessary to add at least 0.0005% by mass or more. On the other hand, excessive addition causes a decrease in toughness, forgeability, etc., so the upper limit was made 0.0050% by mass. A preferable upper limit of the B content is 0.0030% by mass.

Nb: 0.003 to 0.080 mass%
Nb forms NbC in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment by the pinning effect. In order to obtain this effect, at least 0.003% by mass or more must be added. On the other hand, if added over 0.080% by mass, there is a risk of reducing the coarsening suppression ability and deterioration of fatigue strength due to coarse precipitation of NbC, so 0.080% by mass or less. Preferably, it is 0.010-0.060 mass%.

Ti: 0.003 mass% or less
Ti is a component that preferably avoids mixing into steel as much as possible. Ti bonds with N and easily forms coarse TiN. In this way, the upper limit is set to 0.003 mass% in order to cause coarsening of the carbide on the carburized surface layer and decrease in fatigue strength.

N: Less than 0.008% by mass N is a component that preferably avoids mixing into steel as much as possible. Therefore, N is made less than 0.008% by mass in order to secure the hardenability of B and to suppress the formation of TiN.

In the present invention, in order to improve hardenability, the above components are further selected from Cu: 1.0 mass% or less, Ni: 0.50 mass% or less, Mo: 0.5 mass% or less, and V: 0.5 mass% or less. 1 type, or 2 or more types can be contained.
Cu is an element effective for improving hardenability, and is preferably added in an amount of 0.1% by mass or more. However, addition of a large amount leads to deterioration of the surface properties of steel materials and an increase in alloy costs, so the upper limit is 1.0% by mass. It was.

  Ni, Mo, and V are effective elements for improving hardenability and toughness, and are preferably 0.1% by mass or more, 0.05% by mass or more, and 0.02% by mass or more, respectively. It was set as mass%.

Further, in the present invention, in order to control the form of the sulfide and improve the machinability and cold forgeability, the above components are further added to one of Ca: 0.0005 to 0.0050 mass% and Mg: 0.0002 to 0.0020 mass%. Or it can contain 2 types.
That is, in order to obtain the above effects by Ca and Mg, it is necessary to add at least 0.0005 mass% and 0.0002 mass%, respectively. On the other hand, when excessively added, coarse inclusions are formed and the fatigue strength is adversely affected. Therefore, the upper limits of Ca and Mg were set to 0.0050 mass% and 0.0020 mass%, respectively.
The balance other than the elements described above is Fe and inevitable impurities.

The case-hardened steel having the component composition described above is subjected to a cold carving process and then carburized. The carburizing treatment can be performed under the conditions generally used for case-hardened steel (hereinafter referred to as normal carburizing). That is, by holding at a carbon potential of 0.8 to 1.1 mass% and 900 ° C. or more for 3 to 7 hours, a carburized layer having a C concentration of 0.7 mass% or more at least up to a surface layer of 0.4 mm is formed on the surface layer. After the carburized layer is formed, quenching and tempering that is usually applied to case-hardened steel is performed. That is, by quenching with oil at 60 to 140 ° C. and making the structure of the surface layer (carburized layer) into a martensite structure containing 10 to 40% of retained austenite, and then tempering at 160 to 200 ° C. for 1 to 2 hours, A carburized material excellent in rotational bending fatigue strength and surface fatigue strength can be obtained. The temperature for forming the carburized layer is preferably 900 ° C. or higher so that the carburized layer formation is not prolonged, and is preferably 950 ° C. or lower from the viewpoint of the durability of the carburizing furnace. Moreover, it is preferable that the temperature of the oil at the time of quenching is 60 ° C. or more from the viewpoint of suppressing deformation of the material at the time of quenching, and a necessary steel structure (a martensite structure including 10 to 40% of retained austenite) is obtained. From the viewpoint of ensuring hardness, the temperature is preferably 140 ° C. or lower. The C concentration in the carburized layer in normal carburizing is less than 0.85% by mass.
Further, the case-hardened steel material of the present invention increases the C concentration of the carburized layer to 0.85% by mass or more with respect to the normal carburizing described above, and further increases the hardness of the carburized layer by precipitating carbides, thereby increasing the surface fatigue strength. It is particularly suitable for high-concentration carburizing for the purpose of improving the quality. In the case of high-concentration carburizing, the amount of coarse carbide increases in conventionally used case-hardened steel, and further improvement in surface fatigue strength cannot be expected. However, in the case-hardened steel of the present invention, even if the C concentration of the carburized layer is increased to 0.85% by mass or more, precipitation of coarse carbides can be suppressed and surface fatigue strength is improved. That is, after the carburization treatment, the carbon content is 0.85 mass% or more in the surface layer region up to 0.4 mm below the surface, and the maximum diameter of the carbide formed here is 10 μm or less and the average particle diameter is 4 μm or less. it can. Within this range, the surface fatigue strength is particularly improved. Conversely, if this range is exceeded, further improvement in surface fatigue strength cannot be expected.

If the amount of carbon in the surface layer area is less than 0.85% by mass, a sufficient amount of carbide cannot be obtained, and the surface fatigue strength cannot be further improved. In addition, when the maximum diameter of carbide exceeds 10 μm, the fatigue life is reduced due to the coarse carbide becoming the starting point of fatigue cracks. Similarly, when the average particle diameter exceeds 4 μm, the fatigue life is reduced.

  In addition, in order to obtain the carbide according to said prescription | regulation, it is preferable to perform carburizing heat processing on the following conditions. That is, the carbon potential is kept at 1.2 to 2.5 mass%, held at 930 to 1050 ° C for about 1 to 5 hours, carburized, once cooled to 550 to 650 ° C, and then again at 830 to 880 ° C for 30 to 60 minutes. After holding, it is preferable to quench with oil at 60 to 140 ° C. and then temper, and the tempering temperature is preferably within the range of 170 to 200 ° C. By performing the above treatment, the steel structure of the carburized layer formed on the surface layer contains 10-40% of retained austenite in which carbides are finely dispersed to a maximum diameter of 10 μm or less and an average particle diameter of 4 μm or less as described above. Become a martensite organization.

Next, examples of the present invention will be described.
Steel with the composition shown in Table 1 (the balance is Fe and inevitable impurities) was melted and heated to 1150 ° C or higher, then used as an intermediate material with a 170 mm x 170 mm square cross section, and further heated to Ac ₃ + 100 ° C or higher. Thereafter, it was formed into a round bar having a diameter of 60 mm by hot rolling. The obtained bar steel was evaluated for cold forgeability.

Here, the cold workability was evaluated by two items, the limit upsetting rate and the deformation resistance.
In other words, a specimen with a diameter of 10 mm and a height of 15 mm was taken from a depth position (1 / 4D position) of 1/4 of the diameter from the surface of the steel bar, and installed at 60% using a 300-ton press. The compression load was measured, and the deformation resistance measurement method was obtained by end face constrained compression proposed by the Japan Society for Technology of Plasticity.
Further, the limit upsetting rate was defined as the upsetting rate when the end portion was cracked by compressing by a method of measuring deformation resistance. If the deformation resistance value is 899 MPa or less and the critical crack rate is 74% or more, it can be said that the cold forgeability is good.

Next, the fatigue characteristics were evaluated by two items of rotational bending fatigue and surface fatigue.
That is, rotating bending test pieces and roller pitching test pieces are collected from 1 / 4D positions of the above steel bars, and two types of heat treatment are performed on these test pieces: normal carburizing and high-concentration carburizing to generate a large amount of carbides. It was. Normally, carburization was performed at 930 ° C for 7 hours under conditions of carbon potential of 1.1% by mass, and then oil-cooled at 60 ° C and tempered at 170 ° C for 2 hours. On the other hand, high-concentration carburization is held at 950 ° C for 5 hours under the condition of 2% by mass of carbon potential, once cooled to 600 ° C, then again held at 850 ° C for 30 minutes, oil-cooled at 60 ° C and 170 ° C A tempering treatment for 2 hours was performed.

Here, in the measurement of the carbide after carburizing, after etching with a picral solution, a region from the surface to a depth of 30 μm was observed with a scanning electron microscope over 6000 μm ² and the maximum diameter and average diameter of the carbide were obtained by image analysis. That is, the maximum value of the equivalent circle diameter was defined as the maximum diameter, and the average value of the equivalent circle diameter was defined as the average diameter. Carbides were also observed in other depth regions from the surface to 0.4 mm, and it was confirmed that the maximum diameter and the average diameter were the largest from the surface to a depth of 30 μm. Here, in the observation of carbides, those having an equivalent circle diameter of 0.5 μm or more can be identified as carbides.
The carbon concentration was measured by EPMA line analysis from the surface to a depth of 0.4 mm.
A rotating bending test and a roller pitching test were performed on each test piece after the carburization. First, the rotating bending fatigue test was carried out at a rotational speed of 3500 rpm and evaluated with a fatigue strength of 10 ⁷ times. Further, the roller pitting test, the slip ratio of 40% was evaluated at 10 ⁷ times strength under conditions of oil temperature 80 ° C. (limit strength pitching occurs in the test piece surface).
The obtained evaluation results are shown in Table 2.

  As shown in Table 2, it can be seen that all the inventive examples according to the present invention are excellent in cold workability and fatigue resistance.

Claims (5)

C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.40-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
B: 0.0005-0.0050 mass%,
Nb: 0.003 to 0.080 mass%,
Case-hardened steel containing Ti: 0.003 mass% or less and N: less than 0.0080 mass%, with the balance being composed of Fe and inevitable impurities. The component composition further includes:
Cu: 1.0 mass% or less,
Ni: 0.50 mass% or less,
The case hardening steel of Claim 1 containing 1 type, or 2 or more types chosen from Mo: 0.50 mass% or less and V: 0.5 mass% or less. The component composition further includes:
Ca: 0.0005 to 0.0050 mass% and
Mg: 0.0002 to 0.0020 mass%
The case hardening steel of Claim 1 or 2 containing 1 type or 2 types of these.  A carburized material obtained by carburizing the case-hardened steel according to any one of claims 1 to 3, wherein the carbon content in a surface layer region from the surface to 0.4 mm is 0.70% by mass or more. . A carburized material obtained by carburizing the case-hardened steel according to any one of claims 1 to 3, wherein a carbon amount in a surface layer region from the surface to 0.4 mm is 0.85 mass% or more, A carburized material having a maximum carbide diameter of 10 μm or less and an average particle diameter of 4 μm or less in the surface layer region.