JP2003193128A - Method for producing carburized and quenched member and carburized and quenched member - Google Patents

Abstract

(57) [Abstract] (with correction) [PROBLEMS] To provide a carburized and quenched member excellent in fatigue strength and shape accuracy and a method of manufacturing the same. SOLUTION: Fe is a main component and C: 0.12 by weight%.
-0.22%, Si: 0.4-1.5%, Mn: 0.2
5 to 0.45%, Ni: 0.5 to 1.5%, Cr: 1.
3 to 2.3%, B: 0.001 to 0.003%, Ti:
0.02 to 0.06%, Nb: 0.02 to 0.12%,
Al: 0.005 to 0.05%, and 9
When cooled at a constant rate from 00 ° C. to room temperature, bainite is not formed at a cooling rate of at least 0.1 ° C./sec or less, and no ferrite is formed at a cooling rate of at least 12 ° C./sec or more. As described above, the member is made of steel whose component content is adjusted. This is carburized in a reduced-pressure carburizing atmosphere or a carburizing atmosphere mainly composed of an inert gas up to normal pressure to form a carburized layer having a surface carbon concentration of 0.6 to 1.5% by mass on the member. And
Gas quenching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a carburized and hardened member having excellent fatigue strength and shape accuracy, and a carburized and hardened member.

[0002]

2. Description of the Related Art Carburizing and quenching is a heat treatment in which quenching is performed after increasing the carbon concentration in the surface layer of steel. Since the carburized and quenched steel parts have high surface hardness and high internal toughness, they are applied to power transmission mechanism parts that require fatigue strength and toughness. In addition, since the carbon content of the material before carburizing and quenching is relatively small, mechanical removal processing such as cutting and polishing, and plastic processing such as rolling or forging are relatively easy, and it is also excellent in part manufacturability, so mass production is possible. It is often used in parts such as automobiles. Therefore, steel for carburizing and quenching is required on the premise that the manufacturing cost of parts is low as well as the strength.

The surface layer of a member that has been carburized and quenched exhibits the highest hardness as steel. Therefore, if minute defects or weak portions exist near the surface, these act as the origin of fatigue fracture or impact fracture. Will lead to a decrease in strength. Moreover, since the general carburizing treatment is performed at a high temperature in an oxidizing metamorphic gas atmosphere, the surface of the member to be treated is easily oxidized. It is known that such surface oxidation causes deterioration of hardenability of the surface layer portion due to depletion of alloying elements and deteriorates member strength. As a method for improving this, vacuum carburizing treatment and application of high-strength steel containing a non-oxidizing element while reducing elements that promote oxidation have been performed. Further, when abrasion resistance and pitting resistance are required, it is described in Japanese Patent Publication No. 7-116551, etc. that the addition of an element that improves high temperature strength and softening resistance is effective.

[0004]

SUMMARY OF THE INVENTION The strength and cost required for the above-mentioned carburized and hardened material are not always sufficient. In particular, compatibility of fatigue strength and impact strength with wear resistance and pitting resistance in strength, and further, compatibility of these strengths with manufacturing costs such as machinability and shape accuracy is the most important issue in carburizing and quenching members. However, the conventional technology does not sufficiently meet today's market demand for lighter weight and lower cost. The reason will be described below.

Machining accounts for most of the manufacturing cost of carburized and hardened members. The material hardness has the greatest effect on the machinability before carburizing and quenching. For example, the high-strength steels described in Japanese Patent Publication No. 7-116551 and the like described above are generally high-alloy steels and therefore have poor machinability.

For example, fatigue strength and impact strength (hereinafter collectively referred to as "breaking strength") and wear resistance and pitting resistance (hereinafter collectively referred to as "surface fatigue strength"). ), If each is independent, it can be improved by adjusting the chemical composition of the steel material. For example, it is known that reduction of Si, Mn, and Cr and addition of Ni and Mo are effective for breaking strength, and addition of Si, Cr, and Mo is effective for surface fatigue strength. . However, such a high alloying causes an increase in the cost of machining due to an increase in the hardness of the material in addition to an increase in the cost of the steel material, and a deterioration in the carburizing property at the time of carburizing and quenching and an increase in the retained austenite structure.
There is a limit.

On the other hand, among the chemical components that affect the strength of the carburized and hardened member, Si is the most harmful element to the breaking strength but the most beneficial element to the surface fatigue strength. Therefore, in order to achieve both breaking strength and surface fatigue strength with only the chemical composition of steel, as required for gears,
Since it is necessary to add a large amount of a relatively expensive strengthening element other than Si, such as Ni and Mo, and the adverse effect of high alloying becomes large as described above, both strength and manufacturability cannot be achieved at the same time.

Next, when the shape and size tolerances required for the members are strict, if the heat treatment strain generated during carburizing and quenching is large, the required dimensional accuracy cannot be ensured in the state after carburizing and quenching, and grinding finishing is required. May be. Particularly in gears and the like, the precision of shape causes noise and damage, and therefore tooth surface grinding may be performed by a dedicated processing machine. Such a grinding process is generally more expensive than a cutting process, and sometimes accounts for 50% or more of the total manufacturing cost of the carburized and hardened member. Therefore, when carburizing and quenching, it is required that the generated strain be as small as possible,
Conventionally, improvements have been mainly made by quenching technology, but they have not been sufficiently improved.

In most cases, oil or salt is used as a cooling medium in carburizing and quenching, but quenching by gas cooling (hereinafter referred to as "gas quenching").
May also take place. Since gas quenching has a small heat treatment distortion, it is convenient from the viewpoint of securing the accuracy of dimensions and shapes and reducing the processing cost. However, since the gas has a low cooling capacity, the quenching structure may not be sufficiently formed, resulting in incomplete quenching. In such an incompletely quenched state, sufficient strength and mechanical properties cannot be obtained, and as a result, gas cooling can be applied only to small parts.

For example, in order to improve the cooling capacity of gas quenching, it is effective to increase the gas pressure in the cooling chamber, but this causes an increase in equipment size and an increase in processing cost. For such a lack of cooling ability, it is effective to enhance the hardenability of steel, but Si, Mn, Cr, Ni, Mo, etc., which are typical elements for enhancing the hardenability of steel, are carburized. It is also an element that increases the hardness of the material before quenching, is harmful to machinability and cold workability, and deteriorates manufacturability of parts. Also,
JIS (Japanese Industrial Standards) defines SNCM616 and SNCM815 as case hardening steels with high hardenability, but these are the most common case hardening steels such as SCr42.
Compared with 0 or SCM420, the hardness of the normalized or annealed state is very high due to the large alloy content, and it is difficult to perform cutting or cold working. That is, it was essentially difficult to achieve both hardenability suitable for gas quenching and manufacturability of parts suitable for mass production.

An object of the present invention is to easily perform carburizing and quenching before carburizing and quenching, and to perform carburizing and quenching with low distortion and high precision by gas cooling regardless of member size, and further, to prevent breaking strength after carburizing and quenching. It is an object of the present invention to provide a method for manufacturing a carburized and hardened member that has excellent surface fatigue strength and that can significantly reduce the manufacturing cost of the entire member, and a carburized and hardened member obtained thereby.

[0012]

In order to solve the above problems, the method for manufacturing a carburized and hardened member according to the present invention comprises Fe as a main component and C, Si, Mn, and
In addition to containing Ni, Cr, B, Ti, Nb and Al, the content range of each sub ingredient is C: 0.12 to 0.2.
2% by mass, Si: 0.4 to 1.5% by mass, Mn: 0.2
5 to 0.45% by mass, Ni: 0.5 to 1.5% by mass, C
r: 1.3 to 2.3 mass%, B: 0.001 to 0.00
3% by mass, Ti: 0.02 to 0.06% by mass, Nb:
0.02-0.12 mass%, Al: 0.005-0.0
When set to 5% by mass and cooled at a constant rate from 900 ° C to room temperature, the cooling rate is at least 0.1 ° C.
/ Sec or less, bainite is not generated, and the member is made of steel whose content of each subcomponent is adjusted so that ferrite is not generated in a cooling rate of at least 12 ° C / sec or more. A carburized layer having a surface carbon concentration of 0.6 to 1.5 mass% is formed by carburizing the member in a carburizing atmosphere composed of a reduced pressure carburizing atmosphere or an inert gas up to normal pressure or nitrogen. It is characterized in that it is formed in the member, and the member is gas-quenched after the carburizing treatment.

According to the method of the present invention, the carburizing and quenching can be easily performed before the carburizing and quenching, and the carburizing and quenching can be performed with low distortion and high precision by the gas cooling regardless of the member size. It excels in breaking strength and surface fatigue strength and can significantly reduce the manufacturing cost of the entire member. The reason will be described in detail below.

The present inventors have found that the breaking strength of the carburized and quenched material is
With the aim of simultaneously improving all of the surface fatigue strength, material hardness and heat treatment strain, the chemical composition and heat treatment of steel materials were studied, including their interactions, and the following was found. First, to improve surface fatigue strength, S
Utilization of i is indispensable, but excessive addition has a very detrimental effect on the breaking strength. On the other hand, in the case of carburizing and quenching,
When the surface of the member to be treated is oxidized, the hardenability of the surface layer portion is deteriorated due to the depletion of the alloy element, and the fracture strength is similarly deteriorated. Therefore, by carburizing the members in a reduced-pressure carburizing atmosphere or a carburizing atmosphere mainly composed of an inert gas, the influence of surface oxidation is significantly reduced, and a margin for lowering the fracture strength caused by the addition of Si is secured more or less. You can

Next, in order to apply gas quenching, which is effective in reducing heat treatment distortion, to members having a wide range of shapes and sizes, it is effective to enhance the hardenability of steel materials. In the present invention, in consideration of adopting a reduced pressure carburizing atmosphere or a carburizing atmosphere mainly containing an inert gas, when cooled at a constant rate from 900 ° C. to room temperature, the cooling rate is at least 0.
Bainite is not formed in the range of 1 ° C / sec or less,
In addition, steel that does not generate ferrite in a region where the cooling rate is at least 12 ° C / sec or more is adopted.

It is known that as the cooling rate increases, the structure of steel generally changes in the following order: F + P → (F + P + B → F + B → B → B + M)
→ M where F is ferrite, P is pearlite, B is bainite, and M is martensite. In addition, depending on the alloy composition, there may not be a condition to be bainite alone. Further, pearlite is a eutectoid structure of ferrite and cementite, and the formation of pearlite also means that ferrite is formed. And F +
The phrase "P" or "F + B" means that there is a ferrite that is independently formed in a form other than pearlite. The range of the cooling rate is the continuous cooling transformation diagram (Co
ntinuous Cooling Transformation diagram (CCT diagram) can be specified by measuring at various cooling rates.

Martensite plays a major role in quench hardening of the carburized layer, as described above. Martensite transformation is
As is well known, the crystal lattice is quasi-shear-deformed in cooperation with each other without causing large diffusion of atoms. Since the martensitic transformation of steel has a large volume expansion, it progresses in a form in which the residual austenite in the surrounding is greatly distorted.
This also contributes to quench hardening of steel. However, the fact that the strain associated with transformation is large means that the hardness improves, but
It also means that the member is liable to be out of size after quenching.

In the case of the present invention, a carburized layer is formed on the surface of the member, and martensite is preferentially generated in the carburized layer to increase the hardness of the surface layer portion and improve the wear resistance. In the present invention, in order to suppress an increase in strain due to cooling, gas cooling having a smaller cooling rate than water or oil is adopted. Therefore, the composition of the steel is adjusted so that the cooling rate is at least 12 ° C /
The setting is made so that ferrite is not generated in the region of seconds or more (hereinafter, this is referred to as the upper limit cooling rate). 12 ° C /
If ferrite is generated even if the cooling rate is increased to more than 2 seconds, martensite is not sufficiently formed in the carburized layer by gas cooling, which leads to insufficient hardness.

However, the excessively good hardenability is rather disadvantageous in the present invention. That is, if martensite is excessively generated even in the inner layer portion that is not affected by carburization, the amount of martensite generated in the entire member becomes large, which leads to a decrease in dimensional accuracy. Therefore, it is important to select a composition so that martensite is sufficiently generated in the carburized layer during gas quenching, but excessive martensite is not formed in the inner layer portion. Specifically, bainite is prevented from being formed in a range where the cooling rate is at least 0.1 ° C./second or less (hereinafter referred to as the lower limit cooling rate). 0.1
If bainite is formed even at a cooling rate of ° C / sec or less, the inner layer portion, which is not affected by the carburized layer, is deeply quenched and the strain increases, so that the object of the present invention cannot be achieved.

On the other hand, another object of the present invention is to facilitate the processing before carburizing and quenching, and for that purpose, the annealing rate, that is, the cooling rate from austenite is set to cool or air cool. Corresponding range (hereinafter,
In the annealing cooling rate range), a sufficiently low material hardness must be obtained to improve workability. Therefore, assuming this, when the cooling rate is set to less than 0.1 ° C./sec, the composition of the steel in the non-carburized state is selected so that bainite is not formed. Therefore, as described above, if bainite is not formed at a cooling rate of less than 0.1 ° C./sec, the formation of bainite is sufficiently suppressed in the actual annealing cooling rate range, resulting in workability with a large amount of ferrite and pearlite. A rich organization can be obtained.

As a method for experimentally determining the critical cooling rate for the formation of ferrite or bainite in the inner layer portion, after normalizing the steel, it is processed into a test piece having the shape and dimensions shown in FIG. A bottomed hole is formed in this test piece from one end to the central portion in the longitudinal direction, and a thermocouple is fixed to the bottom of the hole by welding. In this state, the test piece is placed in a predetermined constant flow rate of cooling gas, and further heated to 900 ° C. which is an austenitizing temperature by high frequency induction heating. If the temperature is kept substantially constant at 900 ° C., the power of the high-frequency induction heating is controlled and decreased while monitoring the temperature at the center position of the test piece with a thermocouple, and cooling is performed at a constant cooling rate. During cooling, one end of the test piece is fixed, and the position of the support rod attached to the other end position is monitored using a differential transformer to measure the temperature change of the test piece size in real time. Read the temperature of ferrite or bainite formation from the inflection point. After cooling, the test piece is cut at a position corresponding to the thermocouple along a plane orthogonal to the axis, and the presence or absence of ferrite or bainite is visually confirmed by observation with an optical microscope. This measurement is performed by changing the cooling rate variously, and the limiting cooling rate is determined by confirming the presence or absence of ferrite and bainite. In addition, in this measurement method, the formation of ferrite and bainite is confirmed by both the inflection point of the specimen size change and the optical microscope structure, but in the former, it is constant to determine the inflection point by well-known differential analysis. If there is an error and no inflection point corresponding to the transformation is found within the range of the error, it is determined that ferrite or bainite is not formed. On the other hand, when observing with an optical microscope, nine visual fields of 0.1 mm square are randomly extracted on the cross section, and no ferrite or bainite is generated unless ferrite or bainite is visually observed in each visual field. To determine. In either case, even if ferrite or bainite is generated in an area ratio of about 1 to 2%, it is determined that ferrite or bainite is not substantially generated due to the problem of measurement limit.

As described above, in order to obtain the hardenability suitable for gas quenching from the viewpoint of both heat treatment strain and strength, it is effective to increase the amount of alloying elements, but the adverse effects of high alloying are minimized. Need to fasten. On the other hand, to obtain low material hardness,
In the range where the cooling rate from austenite is equivalent to freezing or air cooling, it is necessary to suppress the formation of bainite and make the structure rich in ferrite / pearlite.
It has been generally considered difficult to achieve both high hardness after quenching and low hardness of the material, but in the present invention, it is possible to achieve both within a preferable range by the combination of specific alloy elements. The details will be described below.

First, boron (B) is the most effective element as a chemical component that produces the above effects. When B exists as a solid solution in austenite, it suppresses the formation of ferrite and bainite and enhances hardenability in cooling at a cooling rate of 2 ° C / sec or more, but at a cooling rate of 1 ° C / sec or less, The same structure and hardness as when B is not added are obtained.

On the other hand, the hardenability and normalizing hardness can be adjusted by S
It is made by adding i, Mn, Cr, Ni, Mo and the like. Here, in the case of general case-hardening steel and the like, the behavior of the effects of these contents has been sufficiently analyzed from the past, and prediction to some extent was possible. However, in the present invention, the addition of B is essential to obtain a low material hardness,
Solid solution B is always present in the steel material. Regarding the steel having such a solid solution B, the influence of the above alloying elements, that is, the interaction with B has not been sufficiently clarified so far, and naturally no proposal has been made regarding their proper composition range. .

As a result of a detailed study on this point, the inventors of the present invention have made a particularly large contribution to the improvement of the hardenability when Ni and Si coexist with the solid solution B, but have little influence on the material hardness. I found that. On the contrary, it was also found that Mn and Mo, when coexisting with solid solution B, have a particularly large effect of increasing the material hardness. The steel material composition adopted in the present invention achieves both high hardenability and low material hardness by adjusting the alloying element content based on the addition or increase of B, Ni and Si and the reduction of Mn and Mo. It is possible and unique.

The reasons for limiting the composition of the steel used in the present invention will be described below. (1) C: 0.12 to 0.22 mass% C is an element that improves the strength of the non-carburized portion of the carburized and quenched material. However, if the content is less than 0.12% by mass, the effect is small, and if it exceeds 0.22% by mass, the material hardness increases. Therefore, the C content is 0.12 to 0.22% by mass.

(2) Si: 0.40 to 1.50 mass% Si is an element effective for improving surface fatigue strength, and
It is an element that is positively added in the present invention because it is an element that particularly improves the hardenability when coexisting with solid solution B and has a relatively small effect on the material hardness. If the content is less than 0.40% by mass, the effect of improving surface fatigue strength and hardenability is small, and if it exceeds 1.50% by mass, the increase in the A3 transformation point makes it difficult to form uniform austenite before quenching. Therefore, the Si content is 0.40 to 1.50% by mass.
And

(3) Mn: 0.25 to 0.45 mass% Mn is an element that improves hardenability, but 0.25 mass%
If it is less than 0.45%, the effect is small, and if it exceeds 0.45% by mass, the effect of increasing the material hardness when coexisting with the solid solution B is particularly large, and the manufacturability of parts is significantly deteriorated. Therefore, the Mn content is 0.25 to 0.45 mass%.

(4) Ni: 0.50 to 1.50 mass% Ni is an element that improves the strength and hardenability of carburized steel, and when coexisting with solid solution B, its effect is particularly great. However, if the content is less than 0.50% by mass, the effect is not remarkable,
If it exceeds 1.50% by mass, a bainite structure is likely to be formed and the machinability is significantly deteriorated. Therefore, Ni
Content of 0.50 to 1.50% by mass.

(5) Cr: 1.30 to 2.30% by mass Cr is an element that improves carburizing properties, and particularly Si and Ni.
It is an element having an effect of preventing deterioration of carburizing property due to. However, if the content is less than 1.30% by mass, the effect is insufficient, and if it exceeds 2.30% by mass, a bainite structure is likely to be formed in the material, and machinability is significantly deteriorated. Therefore, the content of Cr is 1.30.
To 2.30% by mass.

(6) B: 0.0010 to 0.0030% by mass B is an element that significantly improves the hardenability, but 0.001
If it is less than 0% by mass, a stable effect cannot be obtained, and if it is less than 0.
Even if it is added in an amount of more than 0030% by mass, the effect is saturated and it is not economical. Therefore, the content of B is 0.0010
The amount is 0.0030% by mass.

(7) Ti: 0.02 to 0.06 mass% Ti is an element which prevents the B from becoming a nitride by forming a nitride and stabilizes the hardenability improving effect of B. is there. However, if its content is less than 0.02% by mass, the effect is small, and if it is added in excess of 0.06% by mass, the effect is saturated and it is not economical. Therefore, the Ti content is 0.02 to 0.06 mass%.

(8) Nb: 0.02 to 0.12 mass% Nb is an element effective in suppressing the growth of crystal grains and improving the strength. If the content is less than 0.02% by mass, the effect is small, and if it exceeds 0.12% by mass, coarse carbonitrides are formed during solidification and the grain growth suppressing effect is reduced. Therefore, the Nb content is 0.02 to 0.12 mass%.

(9) Al: 0.005 to 0.050 mass% Al promotes deoxidation in the melting process and is effective in reducing the amount of oxide inclusions, but if less than 0.005 mass%, deoxidation occurs. There is no effect, and the effect is saturated even if added in an amount of more than 0.050% by mass, which is not economical. Therefore, the content of Al is set to 0.005 to 0.050 mass%.

Mo, which is often added to general carburizing steel, is an element which has a particularly large effect of improving hardenability when coexisting with B. However, in the state after hot working, or because it significantly increases the material hardness in the normalized state, in the present invention aiming to achieve both manufacturability and strength, it is not an element to be positively added, and preferably, It is preferable to limit the Mo content of the adopted steel to 0.05 mass% or less.

Further, in the present invention, the surface carbon concentration of the carburized layer is set to 0.6 to 1.5 mass%. Although the surface carbon concentration of the carburized layer affects the surface hardness of the carburized and hardened material,
If it is less than 100% by mass, the surface hardness will be insufficient, and if it exceeds 1.5% by mass, the precipitation amount of carbides will be large, and the hardenability of the matrix will be significantly reduced, leading to insufficient surface hardness.

In the carburizing atmosphere, nitrogen gas is inexpensive and can be preferably used in the present invention, but an inert gas such as argon can be used instead of nitrogen gas.

The requirements that can be further added in the method for manufacturing a carburized and quenched member of the present invention will be described below. The present inventors employ steel containing up to 1.5 mass% of Si, but from the viewpoint of compensating for the decrease in fracture strength due to the addition of silicon, as described above, carburizing treatment in a constant reduced pressure atmosphere Preferably, specifically, the carburizing treatment is
It is desirable to perform in a reduced pressure carburizing atmosphere in which the atmospheric pressure is adjusted to 30 hPa or less. However, if the pressure is unnecessarily reduced, the time required for carburizing becomes long, the manufacturing cost increases, and the carburizing unevenness easily occurs.
Further, Si in the steel is an element which has a strong effect of suppressing the precipitation of carbides and a tendency to inhibit carburization due to the formation of an oxide film. From this point of view, it is desirable that the reduced pressure carburizing atmosphere has a pressure of 1 hPa or more (especially, since Si is a carburizing inhibiting element, if its content is close to the upper limit of 1.5% by mass, the carburizing property is It is indispensable to carry out the carburizing treatment at a pressure higher than the above pressure so as not to lower it). By adjusting the atmospheric pressure during carburizing to such a range, it is possible to obtain a carburized and hardened member that is less likely to cause surface oxidation and carburizing concentration unevenness and insufficient carburizing depth during carburizing, and that has excellent surface fatigue strength. can do.

Next, in order to secure the surface hardness of the member, it is important that the quenched structure is sufficiently formed. To sufficiently generate martensite, which is the main constituent of the quenched structure,
During quenching cooling, the temperature near the Ms point must be passed at a constant cooling rate (critical cooling rate) or higher. Therefore, in order to sufficiently increase the hardness of the surface of the member after quenching, it is necessary to increase the pressure of the gas cooling medium to a certain level or more and ensure the cooling ability for sufficiently forming the quenched structure.
On the other hand, if the cooling rate becomes too large (ie,
If the cooling capacity of the gas cooling medium becomes too high), quenching strain becomes large, and good dimensional and shape accuracy cannot be obtained. Therefore, there is an appropriate range for the gas pressure that determines the cooling capacity. However, even in the same cooling atmosphere, when the materials and dimensions of the members are different, the cooling rate distribution inside the members also changes, and the same quenched state is not always obtained.

In this case, the quenching quenching degree H is used instead of the gas pressure.
Is convenient to use. That is, the thermal conductivity of the steel constituting the member is λ (unit: kcal / mh ° C.), and the surface heat transfer coefficient of the member in the gas cooling atmosphere is α (unit: kc).
al / mh ² ° C.), the quenching quenching degree H is H≡0.5.
It is defined by x (α / λ). Of these, the thermal conductivity λ is
It is a physical property value that is uniquely determined by the material of steel, and the heat transfer coefficient α depends on the specific heat, thermal conductivity, weight, shape and size of the member, as well as the type of gas cooling medium (specific heat), pressure, flow velocity, etc. Once determined, the cooling atmosphere and the shape, material and dimensions of the members are parameters that are uniquely determined by the well-known main method of heat transfer analysis. Then, in the present invention which employs the steel having the above composition, the gas cooling atmosphere when performing gas quenching is adjusted according to the material and shape / dimension of the member, such as gas species, pressure and flow velocity, It is desirable to set the quenching quenching degree H to be 0.01 to 0.08. In a cooling atmosphere in which H is less than 0.01, a quenched structure is not sufficiently formed and the hardness of the member is insufficient. Conversely, H is 0.
If it exceeds 08, quenching strain becomes large, and good dimensional and shape accuracy cannot be obtained.

As the cooling gas used for the gas quenching, it is desirable to use an inert gas (eg, argon gas) or nitrogen gas from the viewpoint of suppressing member oxidation. Nitrogen gas, in particular, has a relatively large specific heat and an excellent cooling capacity (1.03 J · K ⁻¹ · g ⁻¹ , and argon has an oxygen content of 0.
52 J · K ⁻¹ · g ⁻¹ ), and the availability, cost, and handleability in mass-production operation are suitable for use in the present invention.

Further, as the steel constituting the member, N≡106 × C (mass%) + 10.8 × Si (mass%)
+19.9 x Mn (mass%) + 16.7 x Ni (mass%) + 8.55 x Cr (mass%) + 45.5 x Mo (mass%) + 28 so that the component parameter N is 95 or less. It is preferable to use carburizing steel whose composition has been adjusted. N is 9
If it exceeds 5, the hardness of the rolled steel and the hardness of the normalized steel will remarkably increase, and the machinability and cold workability cannot be obtained. Therefore, when importance is attached to manufacturability, it is necessary to control the composition of the steel so that the composition parameter N is 95 or less.

Further, the Vickers hardness measured on the surface of the carburized layer is preferably 700 Hv or more by the gas quenching after the carburizing treatment. The surface hardness after carburizing and quenching affects member strength (particularly fatigue strength), and if the surface hardness is less than 700 Hv, sufficient member strength may not be ensured. Therefore, when importance is attached to fatigue strength, the surface hardness should be 70
It is desirable to set it to 0 Hv or more. There is no limitation on the upper limit of the Vickers hardness measured on the surface of the carburized layer. For example, if it is up to about 900 Hv, carburization can be performed without problems while suppressing the precipitation of carbides. If the surface hardness exceeds 900 Hv due to excessive generation of carbides such as cementite, the strength may be insufficient and the toughness may be deteriorated.

On the other hand, the Vickers hardness of the non-carburized portion inside the carburized layer is preferably 250 Hv or more. When the Vickers hardness of the non-carburized portion inside the carburized layer is less than 250 Hv, fatigue fracture from the internal origin is likely to occur and the fatigue strength decreases. Therefore, when the fatigue strength is particularly important, it is preferable that the Vickers hardness of the non-carburized portion is 250 Hv or more, whereby a component having both strength and toughness can be obtained. In the present invention, the Vickers hardness refers to that measured with a test load of 2.94 N by a test method defined in JIS: Z2244 (1998).

In order to secure the hardness of the non-carburized part after quenching as described above, it is necessary that bainite is formed in a sufficient amount, and preferably the structure of the non-carburized part is mainly composed of bainite. Good thing. In the present specification, “mainly composed of bainite” means that the area ratio of bainite in the sectional structure is 50% or more. Unlike martensite, bainite undergoes transformation while the iron atoms forming the lattice partially diffuse. Therefore, the occurrence of strain associated with transformation is smaller than that of martensite, and since the hardness is higher than that of pearlite that is generated when the cooling rate is further reduced, the strength of the inner non-carburized portion is appropriately increased. be able to. In order to configure the inner layer portion mainly with bainite, the cooling rate at which the structure mainly with bainite is obtained is 0.5 ° C./second to 10 ° C. / It is desirable to select the composition so that it exists in the range of seconds (more preferably, when the cooling rate is 3 ° C./second, a structure mainly composed of bainite is obtained).

Next, the hardness of the normalized steel material is
From the viewpoint of improving the workability described above, the Rockwell B scale hardness is preferably 95 H _RB or less. In addition,
In the present invention, the Rockwell B scale is JIS: Z.
2245 (1998) means the value measured by the test method.

The amount of retained austenite on the surface of the carburized layer is 2
It is preferably 5% or less. When the amount of retained austenite exceeds 25%, the hardness decreases, and it is intended to be applied to gear members (eg gears for power and automobiles (gear for transmissions), etc.) that are particularly susceptible to heavy loads or impacts. In this case, there is a problem that initial damage such as deformation of the gear and waviness of the tooth surface (lip ring) is likely to occur.
The amount of retained austenite on the surface of the carburized layer is more preferably 20% or less.

On the other hand, the lower limit of the area ratio of retained austenite is not particularly limited, and the amount can be reduced by forcibly converting the retained austenite into martensite by, for example, shot peening. In this case, the final retained austenite amount can be reduced to, for example, an area ratio of up to 1% (which is actually a value below the measurement limit). Since the martensitic transformation of steel has a large volume expansion, the austenite remaining after the influence of back stress may not be completely reduced to zero.

The amount of retained austenite is the amount of a body-centered cubic (ferrite) or body-centered tetragonal (martensite) phase when an X-ray diffraction profile is measured by the diffractometer method on the surface of the carburized and quenched layer. If the integrated area of the main diffraction peak is If ({200} and {21}
1}; peak splitting occurs in the tetragonal system, so all peak areas belonging to the group are summed) and the integrated area of the diffraction peaks of the face-centered cubic system, that is, the austenite phase is Ia ({200}, {220} , {311}} is represented by {Ia / (Ia + If)} × 100 (%).

Further, the area ratio of the troostite structure on the surface of the carburized layer is preferably 10% or less. Troostite is an incompletely quenched structure formed in the carburized layer after carburizing and quenching, and has a small hardness. Therefore, when the area ratio of the carburized layer surface exceeds 10%, the surface fatigue strength is significantly deteriorated. Therefore, when importance is attached to surface fatigue strength, it is desirable to regulate the area ratio of the above-mentioned troostite to 10% or less.

Further, it is desirable that the depth of grain boundary oxidation from the surface of the member is within 3 μm. Oxygen invading the member (steel) from the atmosphere during carburization is often dominated by grain boundary diffusion and forms an oxide phase at the grain boundary. When the grain boundary oxide phase is formed, the grain boundary strength decreases, which may lead to a decrease in wear resistance due to insufficient strength of the carburized layer or grain removal. In addition, as a result of part of the surrounding steel additive elements (auxiliary components) being taken into the grain boundary oxide phase when the grain boundary oxide phase is generated,
A depletion region of the additional element is formed around the grain boundary oxide phase, which may cause insufficient hardenability of the carburized and hardened layer and eventually insufficient hardness and strength. Therefore, by adjusting the composition of the steel material, the atmosphere during carburization (particularly oxygen partial pressure), the carburizing temperature and the time, the grain boundary oxidation depth can be suppressed to 3 μm or less.

Since the grain boundary oxidation phase can be easily visually discriminated from the non-oxidized region by polishing the member cross section, the grain boundary oxidation depth can be measured from the cross section optical micrograph.

Next, the surface compressive residual stress of the member is 300 to
It is desirable that the pressure is 800 MPa. By allowing the compressive residual stress to remain on the surface of the member, when a crack is formed in the surface layer, the expansion / propagation of the crack is suppressed. Therefore, the strength of the member, especially the dynamic strength (surface fatigue strength,
Bending fatigue strength, torsional fatigue strength, etc.) can be significantly improved. As described above, when the carburized layer is subjected to quenching treatment to generate martensite, a compressive stress field is generated due to volume expansion accompanying transformation, which is convenient for imparting the above-mentioned surface compressive residual stress. However, when the amount of martensite produced is small, that is, when the amount of retained austenite is large, a sufficient compressive residual stress field cannot be formed. Therefore, it can be said that reducing the retained austenite (specifically, 25% or less) is advantageous from the viewpoint of enhancing the compressive residual stress effect.
When the martensite amount is small, absorption of volume expansion during martensitic transformation is stress-relaxed because it proceeds by plastically deforming the surrounding austenite and does not contribute so much to the increase in surface compressive residual stress. However, when the amount of martensite increases and the retained austenite decreases as described above, the density of dislocations introduced by plastic deformation increases,
The surface compressive residual stress increases rapidly because the slip deformation is constrained. There is also a method of increasing the compressive residual stress by performing surface layer processing such as shot peening after quenching. In the latter case, if the retained austenite is converted to martensite by the shot peening treatment, it is more advantageous in improving the compressive residual stress. Compressive residual stress is 3
If it is less than 00 MPa, the effect of improving the strength by suppressing crack propagation cannot be sufficiently obtained, and if the compressive residual stress exceeds 800 MPa, the amount of martensite is excessively increased or the strain imparted by the post-processing becomes excessive. Either must be increased, but the former must increase the cooling rate during quenching beyond the limit, and the latter must increase the processing strain excessively. In any case, in view of the subject of the present invention of ensuring the dimensional accuracy of the members, the result of the end fall of the book is brought about.

The residual stress is measured by measuring the X-ray diffraction profile on the surface of the carburized and quenched layer by the diffractometer method, and measuring the body-centered cubic (ferrite) or body-centered tetragonal (martensite) phase. Of (21
1) It can be performed based on the relationship between the half-value width of the peak and the peak center position. An apparatus for performing stress analysis by this method is well known as Fastress (registered trademark) stress analyzer (for example, "New Edition X-ray Diffraction Principles" (written by Karity, Agne (1979), pp. 431-433), The description is omitted.

The carburized and quenched member of the present invention can be realized by the above-described manufacturing method of the present invention. The steel constituting the member non-carburized part has: Fe as a main component and C and S as sub-components.
i, Mn, Ni, Cr, B, Ti, Nb and Al are contained, and the content range of each sub ingredient is C: 0.1.
2 to 0.22% by mass, Si: 0.4 to 1.5% by mass, M
n: 0.25 to 0.45 mass%, Ni: 0.5 to 1.5
% By mass, Cr: 1.3 to 2.3% by mass, B: 0.001
-0.003 mass%, Ti: 0.02-0.06 mass%, Nb: 0.02-0.12 mass%, Al: 0.00
5 to 0.05 mass% is set; N≡106 × C (mass%) + 10.8 × Si (mass%)
+ 19.9 x Mn (mass%) + 16.7 x Ni (mass%) + 8.55 x Cr (mass%) + 45.5 x Mo (mass%) + 28, the component parameter N is 95 or less; , When cooled from 900 ℃ to room temperature at a constant rate,
Bainite does not form at a cooling rate of at least 0.1 ° C./sec. And ferrite does not form at a cooling rate of at least 12 ° C./sec. And a carburized layer is formed on the surface of the member, and the Vickers hardness measured on the surface of the carburized layer is 700 Hv or more,
In addition, the Vickers hardness of the non-carburized part inside the carburized layer is 250
It is characterized by being Hv or more.

The carburized and quenched member of the present invention can be easily worked before carburizing and quenching by adopting the steel having the above composition, and a sufficiently quenched structure can be formed by gas cooling regardless of the member size. Therefore, a member with low distortion and high accuracy can be realized. In addition, the breaking strength and surface fatigue strength after carburizing and quenching are excellent, and the manufacturing cost of the entire member can be significantly reduced.
Since the critical meaning of each numerical value has already been explained, it is omitted. The composition of the carburized layer is equivalent to the composition of the non-carburized portion with the carbon content increased by carburization.

[0057]

EXAMPLES The results of experiments conducted to confirm the effects of the present invention will be described below. First, steel having the chemical composition shown in Table 1 was melted in an arc furnace and then hot-rolled to a diameter of 150.
A round bar having a diameter of 32 mm and a diameter of 32 mm was held at 925 ° C. for 1 hour and then air-cooled to normalize. Steel types A1, A2, and A3 are steel types corresponding to the steel composition described in the claims of the present invention, and steel types B and C are steel types corresponding to JIS case-hardening steels SCM420 and SNCM815, respectively (component parameters The calculated value of N is also shown).

[0058]

[Table 1]

Rockwell hardness was measured at the center of the cross section of the normalized material having a diameter of 32 mm for all the steel types. Then, the lower limit critical cooling rate βLC of bainite formation and the upper limit critical cooling rate βUC of ferrite formation of each steel material were measured by the method already described by separately preparing the test piece of FIG. A round bar test piece having a diameter of 25 mm and a length of 50 mm and a rotating bending fatigue test piece having the shape shown in FIG. 1 were processed. Also, diameter 32 mm and diameter 1
A pitting test roller and its counterpart roller shown in FIG. 2 were processed from a normalizing material of 50 mm. On the other hand,
The normalized material having a diameter of 150 mm was cut along a cross section orthogonal to the axis, and further, forged and cut to form a gear having a shape shown in FIG.

Among these members, a round bar test piece having a diameter of 25 mm was subjected to low pressure carburization and gas quenching under the conditions shown in Table 2 for all steel types A1, A2, A3, B and C (invention example A, respectively). , Comparative Example B and Comparative Example C). The gears, the rotary bending fatigue test piece, the pitting test roller and the mating roller are steel types A1, A2,
A3 and steel type C were subjected to low pressure carburization and gas quenching under the conditions shown in Table 2 (Invention Examples A1, A2, A3, Comparative Example C),
Steel type B was gas-carburized and oil-quenched under the conditions shown in Table 3.

[0061]

[Table 2]

[0062]

[Table 3]

Further, the axial cross-section of the test piece was mirror-polished, and the XMA (X-ray Micro Analysis) analysis of the surface layer portion was carried out so that the carbon concentration in the surface layer portion after carburization was 50 μm from the surface.
It was examined at the position of m (the results are shown in Table 1). On the other hand, with respect to a round bar test piece having a diameter of 25 mm, the hardness distribution in the cross section was examined by a Vickers hardness meter. The surface hardness of the carburized and quenched material was measured at a position 0.02 mm from the surface. Further, the area ratio of troostite was measured at the same position as this by image-analyzing a scanning electron micrograph. Further, the amount of retained austenite and the compressive residual stress in the surface layer portion were measured by the method described above. The results are shown in Table 4.

[0064]

[Table 4]

In Table 4, invention examples A1 to A3 are 95H
Normalizing hardness of _RB or less is shown, and hardness of the central portion of the carburized and hardened material is 350 Hv or more. The surface (carburized) structure is martensite, and the central part (non-carburized part)
All of the structures were bainite or bainite + martensite (however, bainite was 50% or more), and no remarkable incompletely quenched structure was present. On the other hand, Comparative Example B has a low normalizing hardness, but the surface layer hardness and the center hardness of the carburized and quenched material are lower than those of the invention examples. In Comparative Example C, the carburized and quenched material has surface hardness and center hardness almost equal to those of Invention Example A, but the normalizing hardness is extremely high.

In the rotary bending fatigue test, an Ono rotary bending fatigue tester was used to determine the fatigue strength based on the number of repetitions of 10 million times. In the pitting test, the test roller and the mating roller are rotated and contacted in lubricating oil while controlling the load and sliding speed, and the pitting damage that occurs on the surface of the test roller is detected by vibration, and the number of repetitions is 10 million times. The surface pressure strength based on the above was determined. The relative slip velocity was 800 mm / s. Table 4 shows the results of these strength tests (however, each strength is represented by a relative value based on Comparative Example B). As can be seen from these results, in the invention examples, the properties in which the rotating bending fatigue strength and the pitting resistance strength greatly exceed those of the comparative examples are obtained.

Next, for the gears, the accuracy after carburizing and quenching was evaluated by the following method. First, regarding gear accuracy, set the gear in a dedicated precision gear accuracy measuring machine,
The error amount in the pressure angle direction of the gear and the error amount in the torsion angle direction were measured on each of the left and right tooth surfaces. The height of the tooth space was measured over the entire circumference, and the value obtained by subtracting the minimum value from the maximum value was calculated as the tooth space runout. On the other hand, the dimensional accuracy of the gear was measured as follows. That is, balls are placed in two tooth spaces of a gear that face each other, and the outer peripheral dimension of the balls is set to a special O.D. B. It was measured with a D (Over Ball Diaphragm) measuring device. O. B. As shown in FIG. 3, the measurement positions of D were two directions (X, Y) at right angles in the circumferential direction, and three positions (A, B, C) in the tooth width direction were upper, middle, and lower. In the table, "O.B.D. ellipse" means O.D. B.
It is the absolute value of the difference of D. "OBD taper" is the upper O.D. in the tooth width direction. B. D and lower O. B. It is the difference of D.
The above results are shown in Table 5.

[0068]

[Table 5]

That is, it can be seen that the gear configured as the carburizing and quenching member according to the present invention has extremely small heat treatment distortion, and therefore has very good characteristics over the entire gear precision and dimensional precision.

[Brief description of drawings]

FIG. 1 is a view showing a rotating bending fatigue test piece.

FIG. 2 is a view showing a pitting test roller and a counterpart roller.

FIG. 3 is a diagram showing a gear for heat treatment distortion evaluation.

FIG. 4 is a diagram showing a shape of a sample for measuring a continuous cooling transformation diagram.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yutaka Koubayashi             2-30 Daido-cho, Minami-ku, Nagoya-shi, Aichi             Daido Steel Co., Ltd. Technology Development Laboratory (72) Inventor Takao Taniguchi             10 Akane, Takane, Fujii-cho, Anjo City, Aichi Prefecture             N AW Co., Ltd. (72) Inventor Kazumasa Tsukamoto             10 Akane, Takane, Fujii-cho, Anjo City, Aichi Prefecture             N AW Co., Ltd. (72) Inventor Takuji Obayashi             10 Akane, Takane, Fujii-cho, Anjo City, Aichi Prefecture             N AW Co., Ltd. F-term (reference) 4K028 AA01 AB01 AC03 AC07

Claims (17)

[Claims] 1. Fe as a main component, and C and S as auxiliary components
i, Mn, Ni, Cr, B, Ti, Nb and Al are contained, and the content range of each sub ingredient is C: 0.1.
2 to 0.22% by mass, Si: 0.4 to 1.5% by mass, M
n: 0.25 to 0.45 mass%, Ni: 0.5 to 1.5
% By mass, Cr: 1.3 to 2.3% by mass, B: 0.001
-0.003 mass%, Ti: 0.02-0.06 mass%, Nb: 0.02-0.12 mass%, Al: 0.00
It is set to 5 to 0.05% by mass, and when cooled at a constant rate from 900 ° C. to room temperature, bainite is not formed in the cooling rate of at least 0.1 ° C./sec or less, and cooling is performed. The member is made of steel in which the content of each of the above subcomponents in the non-carburized state is adjusted so that ferrite will not be generated at a speed of at least 12 ° C / sec or more, and a reduced pressure carburized atmosphere or normal pressure By carburizing the member in a carburizing atmosphere mainly composed of inert gas or nitrogen, a carburized layer having a surface carbon concentration of 0.6 to 1.5 mass% is formed on the member, and the carburizing treatment is performed. A method for manufacturing a carburized and hardened member, characterized in that the member is gas-quenched later. 2. The carburizing treatment is performed at an atmospheric pressure of 1 to 30.
The method is performed in a reduced pressure carburizing atmosphere adjusted to hPa.
A method for manufacturing the carburized and hardened member described. 3. The gas cooling atmosphere when performing the gas quenching has a thermal conductivity of λ (unit:
kcal / mh ° C), the surface heat transfer coefficient of the member in the gas cooling atmosphere is α (unit: kcal / mh ² ° C)
As the carburizing and quenching member according to claim 1 or 2, an atmosphere having a quenching quenching degree H defined by H≡0.5 × (α / λ) of 0.01 to 0.08 is used. Production method. 4. The method for manufacturing a carburized and quenched member according to claim 1, wherein the gas cooling atmosphere is an inert gas atmosphere or a nitrogen gas atmosphere. 5. The steel constituting the member includes: N≡106 × C (mass%) + 10.8 × Si (mass%)
+19.9 x Mn (mass%) + 16.7 x Ni (mass%) + 8.55 x Cr (mass%) + 45.5 x Mo (mass%) + 28 so that the component parameter N is 95 or less. The method for manufacturing a carburized and hardened member according to claim 1, wherein the composition-adjusted carburizing steel is used. 6. A Vickers hardness of 700 Hv measured on the surface of the carburized layer by gas quenching after the carburizing treatment.
The method for producing a carburized and hardened member according to any one of claims 1 to 5, wherein the non-carburized portion inside the carburized layer has a Vickers hardness of 250 Hv or more. 7. The method for manufacturing a carburized and hardened member according to claim 6, wherein the area ratio of troostite on the surface of the carburized layer is 10% or less. 8. The method for producing a carburized and hardened member according to claim 6, wherein the amount of retained austenite on the surface of the carburized layer is 25% or less. 9. The method for manufacturing a carburized and quenched member according to claim 6, wherein the structure of the non-carburized portion is made of bainite. 10. The depth of grain boundary oxidation from the surface of the member within 3 μm.
Item 4. A method for manufacturing a carburized and quenched member according to Item. 11. The compressive residual stress on the surface of the member is 300 to 8
The method for manufacturing a carburized and quenched member according to any one of claims 1 to 10, wherein the pressure is 00 MPa. 12. The steel constituting the non-carburized portion of the member comprises: Fe as a main component, and C, Si, Mn, Ni, C as a minor component.
While containing r, B, Ti, Nb, and Al, the content range of each subcomponent is C: 0.12 to 0.22 mass%, Si: 0.4 to 1.5 mass%, Mn: 0. .25 ~
0.45% by mass, Ni: 0.5 to 1.5% by mass, Cr:
1.3-2.3 mass%, B: 0.001-0.003 mass%, Ti: 0.02-0.06 mass%, Nb: 0.0
2 to 0.12% by mass, Al: 0.005 to 0.05% by mass; N≡106 × C (mass%) + 10.8 × Si (mass%)
+ 19.9 x Mn (mass%) + 16.7 x Ni (mass%) + 8.55 x Cr (mass%) + 45.5 x Mo (mass%) + 28, the component parameter N is 95 or less; , When cooled from 900 ℃ to room temperature at a constant rate,
In order that bainite may not be formed in a cooling rate of at least 0.1 ° C./sec or less and ferrite may not be formed in a cooling rate of at least 12 ° C./sec or more; Adjusted steel, a carburized layer is formed on the surface of the member, the Vickers hardness measured on the surface of the carburized layer is 700 Hv or more, and the Vickers hardness of the non-carburized portion inside the carburized layer is 250 Hv. A carburized and quenched member characterized by the above. 13. The carburized and hardened member according to claim 12, wherein the area ratio of the troostite structure on the surface of the carburized layer is 10% or less. 14. The carburized and hardened member according to claim 12, wherein the amount of retained austenite on the surface of the carburized layer is 25% or less. 15. The carburized and quenched member according to claim 12, wherein the structure of the non-carburized portion is mainly made of bainite. 16. The carburized and quenched member according to claim 12, wherein a depth at which grain boundary oxidation is generated from the surface of the member is within 3 μm. 17. A member surface has a compressive residual stress of 300 to 8
The carburized and quenched member according to any one of claims 12 to 16, which has a pressure of 00 MPa.