JP2022055308A - Carburized steel and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carburized steel which can be formed by cold forging and in which the formation of cementite in a surface layer portion at the time of vacuum carburizing is suppressed.
SOLUTION: C: 0.10 to 0.35% by mass, Si: 0.50% by mass or less, Mn: 0.30 to 1.50% by mass, Cr: 1.10 to 2.00% by mass, P: 0.02% by mass or less, S: 0.03% by mass or less, Al. : 0.01-0.05% by mass and N: 0.030% by mass or less, the balance has a component composition of Fe and unavoidable impurities, the cementite content at a depth of 0.05 mm from the surface is 5% or less, from the surface. The hardness at a depth of 0.05 mm is HV600 or more, the austenite particle size at a depth of 0.05 mm from the surface is No. 5 or more, and the hardness at a depth of 0.10 mm from the surface is HV650 or more.
[Selection diagram] None

Description

The present invention relates to a carburized steel having a carburized layer on the surface layer, particularly a carburized steel in which the formation of cementite in the surface layer is suppressed during carburizing, and a method for producing the same, which is used as a material for machine structural use in the construction machinery and automobile fields. be. In the field of construction machinery, for example, gears of traveling speed reducers (gears such as planetary gears and sun gears), gears of large speed reducers, valve plates of hydraulic pumps, ball screws, etc. Examples include nuts, curve plates and pins of cyclone reducers, and blocks of linear motion bearings. Similarly, in the automobile field, various bearings, engine piston pins, cam shafts and timing gears, and transmission gears ( Missing gears, ring gears, sun gears, planetar gears, etc.), as well as drive train differential bevel gears, tripports, inners, balls, etc. In addition to the construction machinery and automobile fields, there are bearings and reduction gears for wind power generators in the electrical equipment field.

Since cold forging is capable of near-net shape forming, it has the advantage that the amount of cutting after forging can be reduced as compared with hot forging, and the decrease in yield can be suppressed. Further, the carburizing, quenching and tempering treatment is a heat treatment for improving the fatigue characteristics of steel parts, and is applied to various parts including gears for automobiles. Among them, vacuum carburizing is a method in which a carburizing gas (acetylene gas or ethylene gas) is introduced into a batch furnace under reduced pressure, and the carburizing gas serves as a carbon supply source to permeate and diffuse carbon from the surface of the component to the inside. be. This vacuum carburizing remains a problem that carburization becomes excessive at the edge portion of the component and causes the formation of cementite, but it is advantageous in that it can suppress intergranular oxidation of the surface layer.

For example, Patent Document 1 proposes a vacuum carburizing steel capable of suppressing excessive carburizing (cementite formation) at the edge portion of a component by increasing the Si addition amount and lowering the Cr addition amount of the steel. Further, in Patent Document 2, a hydrocarbon gas pulse is introduced once in the carburizing period, and after the carbon concentration on the surface of the steel material exceeds the carbon concentration corresponding to the Acm line, the carbon concentration on the surface of the steel material becomes the Acm line. A method for producing a high-concentration carburized steel has been proposed, which can suppress the formation of coarse cementite by introducing the next pulse after the carbon concentration becomes less than the corresponding carbon concentration. Further, in Patent Document 3, by increasing the amount of N added and the amount of Nb added to steel and performing precipitation heat treatment after hot forging, it is possible to suppress the coarsening of crystal grains even if the treatment temperature of carburizing under reduced pressure is raised. A method for manufacturing forged parts that can shorten the carburizing time due to high temperature has been proposed.

Japanese Unexamined Patent Publication No. 2019-7063 Japanese Patent No. 6497208 Japanese Patent No. 6148995

In recent years, price competition for parts has become more and more fierce, and there is a strong need for parts manufacturing in cold forging, which is cheaper than hot forging. In addition, from the viewpoint of improving the performance of parts, there is an increasing demand for suppressing excessive carburizing (cementite formation) during vacuum carburizing.
However, in the technique described in Patent Document 1, since the amount of Si added to the steel is increased in order to suppress excessive carburizing, the cold forging property is lowered and it is difficult to apply the cold forging. .. Further, in the technique described in Patent Document 2, there is a problem that the heat treatment time is increased because a time is required to wait for the diffusion of C after the introduction of the hydrocarbon gas pulse in the carburizing period. Further, the technique described in Patent Document 3 can shorten the carburizing time by high-temperature carburizing of hot forged parts, but even if the technique is applied to cold forged parts, coarsening of crystal grains during carburizing is suppressed. The problem was that I couldn't.

The present invention has been developed in view of the above circumstances, and proposes a carburized steel and a method for producing the same, which can be formed by cold forging and suppress the formation of cementite in the surface layer portion during vacuum carburizing. The purpose is to do.

In order to achieve the above objectives, the inventors have diligently studied the influence of alloying elements on cold forging and the formation of cementite on the surface layer during vacuum carburizing. As a result, they have excellent cold forging and are excellent in cold forging. We have completed a carburized steel in which the formation of cementite in the surface layer is suppressed and a method for producing the same. That is, the gist of the present invention is as follows.

1. 1. C: 0.10 to 0.35% by mass,
Si: 0.50% by mass or less,
Mn: 0.30 to 1.50% by mass,
Cr: 1.10-2.00 mass%,
P: 0.02% by mass or less,
S: 0.05% by mass or less,
It contains Al: 0.01-0.05% by mass and N: 0.030% by mass or less, and the balance has a component composition of Fe and unavoidable impurities.
The cementite fraction at a depth of 0.05 mm from the surface is 5% or less, the hardness at a depth of 0.05 mm from the surface is HV600 or more, the austenite particle size at a depth of 0.05 mm from the surface is a particle size of 5 or more, and Carburized steel with a hardness of HV650 or higher at a depth of 0.10 mm from the surface.

2. 2. The composition of the steel further
Mo: 1% by mass or less,
Cu: 1% by mass or less,
The carburized steel according to 1 above, which contains one or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less.

3. 3. The composition of the steel further
Nb: 0.1% by mass or less,
The carburized steel according to 1 or 2 above, which contains one or more selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less.

4. The composition of the components further
Sn: 0.1% by mass or less and
Sb: The carburized steel according to any one of 1 to 3 above, which contains 1 or 2 selected from 0.1% by mass or less.

5. The composition of the components further
Se: 0.3% by mass or less,
Ca: 0.1% by mass or less,
Pb: 0.3% by mass or less and
Bi: The carburized steel according to any one of 1 to 4 above, which contains one or more selected from 0.3% by mass or less.

6. The carburized steel according to any one of 1 to 5 above, which has a component shape.

7. A steel material having the component composition according to any one of 1 to 5 is subjected to vacuum carburizing treatment in which the carburizing period temperature Tc (° C) and the diffusion period temperature Td (° C) satisfy the following equations (1) and (2). How to make carburized steel.
Tc + 22 × ([Cr] -0.8) ≤ Td ≤ 1100 ・ ・ ・ (1)
Tc ≤ 1000 ・ ・ ・ (2)
Here, [Cr]: Cr concentration in steel (mass%)

According to the present invention, it is possible to provide a carburized steel having excellent cold forging property and suppressing the formation of cementite in the surface layer portion at the time of vacuum carburizing, which is very useful industrially.

Hereinafter, the present invention will be specifically described. That is, the carburized steel of the present invention has a predetermined composition and surface structure, and will be described in detail in order from the composition.
C: 0.10 to 0.35% by mass
C requires 0.10% by mass or more in order to increase the hardness of the central portion after carburizing. On the other hand, if the content exceeds 0.35% by mass, the hardness of the rolled material increases and the life of the forging die is shortened due to the increase in load during cold working, so the C amount is limited to the range of 0.10 to 0.35% by mass. bottom. It is preferably in the range of 0.15 to 0.30% by mass.

Si: 0.50% by mass or less
Si has an action as a deoxidizing agent, but is an alloying element that is not always necessary if Al is added. Since Si has the effect of strengthening the steel by solid solution to increase the hardness of the rolled material and shortening the die life during cold forging, the upper limit is limited to 0.50% by mass. It is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less. It goes without saying that Si may be 0% by mass, but since a large refining cost is required to reduce it to less than 0.001% by mass, a content of 0.001% by mass or more is allowed.

Mn: 0.30 to 1.50 mass%
Mn has the effect of improving hardenability and increasing hardness after carburizing. In order to obtain this effect, an addition of at least 0.30% by mass is required. However, excessive addition of Mn increases the hardness of the rolled material and shortens the life of the die during cold forging, so the upper limit was set to 1.50% by mass. It is preferably 0.35 to 0.65% by mass.

Cr: 1.10-2.00 mass%
Cr has the effect of improving hardenability and increasing hardness after carburizing. To obtain this effect, an addition of at least 1.10% by weight is required. Cr also has the effect of increasing the hardness of the rolled material like Si and Mn, but the effect is smaller than that of Si and Mn. That is, in order to obtain sufficient hardenability and low rolled material hardness, a design that reduces Si and Mn and increases Cr is effective. However, excessive Cr addition increases the stability of cementite in steel, increases the cementite fraction after vacuum carburizing, and causes a decrease in fatigue characteristics, so the upper limit is specified as 2.00% by mass. It is preferably in the range of 1.20 to 1.80% by mass.

P: 0.02% by mass or less P segregates at the grain boundaries and lowers the toughness. Therefore, the lower the mixing, the more desirable, but up to 0.02% by mass is allowed. Further, the lower limit is not particularly limited, but excessively low P will increase the refining time and the refining cost, so it is preferable to set it to 0.001% by mass or more.

S: 0.05% by mass or less S is an element that exists as a sulfide-based inclusion and is effective for improving machinability, and for that purpose, it is preferable to add 0.001% by mass or more. However, since excessive addition causes a decrease in cold workability, the upper limit is set to 0.05% by mass. Further, the lower limit is not particularly limited, but excessively low S increases the refining cost, so it is preferable to set it to 0.001% by mass or more. It is preferably 0.005 to 0.03% by mass, and more preferably 0.005 to 0.02% by mass.

Al: 0.01-0.05 mass%
Al is an element that forms an oxide and is effective for deoxidation, and has an effect of suppressing the formation of coarse oxide-based inclusions, but if the content is less than 0.01% by mass, its addition effect is poor. .. However, excessive addition causes an increase in inclusions, increases the starting point of fatigue fracture, and causes low fatigue strength. Therefore, the upper limit is set to 0.05% by mass. It is preferably 0.015 to 0.035% by mass.

N: 0.030% by mass or less Since excessive addition of N causes cracks on the surface of steel pieces after casting, the upper limit is 0.030% by mass. The lower limit is not particularly limited, but excessively low N increases the refining cost, so it is preferable to set it to 0.0010% by mass or more. It is preferably 0.0030 to 0.0180% by mass.

The basic components of the present invention have been described above, but in the present invention, each component shown below can be further added as appropriate, if necessary.
Mo: 1% by mass or less,
Cu: 1% by mass or less,
Ni: 1% by mass or less and B: 1 or more selected from 0.01% by mass or less

Mo: 1% by mass or less
Mo has the effect of improving hardenability and increasing hardness after carburizing. However, if the content exceeds 1% by mass, the hardenability becomes excessive, the hardness after rolling increases, and there is a concern that the workability and machinability deteriorate. Therefore, it is preferable to limit the Mo content to the range of 1% by mass or less. In addition, in order to exhibit the effect of improving the strength of the steel material by Mo, it is preferable that Mo is contained in an amount of 0.005% by mass or more. More preferably, it is in the range of 0.03 to 0.50% by mass. More preferably, it is 0.05 to 0.25% by mass.

Cu: 1% by mass or less
Cu has the effect of improving hardenability and increasing hardness after carburizing. In order to obtain this effect, Cu is preferably contained in an amount of 0.01% by mass or more. On the other hand, if the Cu content exceeds 1% by mass, the surface surface of the rolled material becomes rough, and there is a concern that it may remain as a flaw. Therefore, it is preferable to limit the amount of Cu to the range of 1% by mass or less. More preferably, it is in the range of 0.015 to 0.5% by mass. More preferably, it is 0.03 to 0.3% by mass.

Ni: 1% by mass or less
Ni is an element useful for improving toughness. In order to obtain these effects, Ni is preferably contained in an amount of 0.01% by mass or more. On the other hand, even if it is contained in an amount of more than 1% by mass, the above effect is saturated. Therefore, the Ni content is preferably limited to the range of 1% by mass or less. More preferably, it is in the range of 0.015 to 0.5% by mass. More preferably, it is 0.03 to 0.3% by mass.

B: 0.01% by mass or less B is effective in improving hardenability by segregating at grain boundaries and suppressing diffusion-type transformation, and in addition, strengthens grain boundaries and suppresses the generation and growth of fatigue cracks. It also has the effect of improving fatigue strength. In order to obtain this effect by B, it is preferable to contain B in an amount of 0.0003% by mass or more. On the other hand, if it exceeds 0.01%, the toughness decreases, so it is preferable to limit the amount of B to the range of 0.01% by mass or less. More preferably, it is in the range of 0.0005 to 0.005% by mass. More preferably, it is 0.0007 to 0.002% by mass.

In addition to the above components, if necessary, each component shown below can be added as appropriate.
Nb: 0.1% by mass or less,
One or more selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less
Nb: 0.1% by mass or less
Nb binds to carbon and nitrogen to form fine precipitates, thereby suppressing grain growth during carburizing and having a grain refinement effect. However, even if it is added in an amount of more than 0.1% by mass, the effect is only saturated, so that the Nb content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.08% by mass. More preferably, it is 0.01 to 0.06% by mass.

Ti: 0.1% by mass or less
Ti binds to carbon and nitrogen to form fine precipitates, thereby suppressing grain growth during carburizing and having a grain refinement effect. However, even if it is added in an amount of more than 0.1% by mass, the effect is only saturated, so that the Ti content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.08% by mass. More preferably, it is 0.01 to 0.06% by mass.

V: 0.1% by mass or less V has the effect of suppressing crystal grain growth during carburizing and refining the crystal grains by the action of binding to carbon and nitrogen to form fine precipitates. However, even if it is added in an amount of more than 0.1% by mass, the effect is only saturated, so that the V content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.08% by mass. More preferably, it is 0.01 to 0.06% by mass.

In addition to the above components, if necessary, each component shown below can be added as appropriate.
Sn: 0.1% by mass or less and
Sb: 1 or 2 selected from 0.1% by mass or less

Sn: 0.1% by mass or less
Sn is an effective element for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable that Sn is contained in an amount of 0.003% by mass or more. On the other hand, since excessive addition deteriorates processability, the Sn content is preferably 0.1% by mass or less. It is more preferably 0.0010 to 0.050% by mass, and even more preferably 0.0015 to 0.035% by mass.

Sb: 0.1% by mass or less
Sb is an element effective for suppressing decarburization of the steel surface and preventing a decrease in surface hardness. In order to exhibit this effect, it is preferable to contain Sb in an amount of 0.0003% by mass or more. On the other hand, since excessive addition deteriorates processability, the Sb content is preferably 0.1% by mass or less. It is more preferably 0.001 to 0.05% by mass, and even more preferably 0.0015 to 0.035% by mass.

In addition to the above components, if necessary, each component shown below can be added as appropriate.
Se: 0.3% by mass or less,
Ca: 0.1% by mass or less,
Pb: 0.3% by mass or less and
Bi: One or more selected from 0.3% by mass or less

Se: 0.3% by mass or less
Se binds to Mn and Cu and disperses as a precipitate in the steel to improve machinability. In order to obtain this effect, it is preferable to add Se in an amount of at least 0.001% by mass or more. On the other hand, even if it is added in excess of 0.3% by mass, the effect is saturated. Therefore, the Se content is preferably 0.3% by mass or less. More preferably, it is 0.005 to 0.1% by mass. More preferably, it is 0.008 to 0.09% by mass.

Ca: 0.1% by mass or less
Ca binds to S and disperses as sulfide in steel to improve machinability. In order to obtain this effect, it is preferable to add Ca in an amount of at least 0.0005% by mass or more. On the other hand, even if it is added in an amount of more than 0.1% by mass, the effect is saturated. Therefore, the Ca content is preferably 0.1% by mass or less. More preferably, it is 0.0010 to 0.0500% by mass. More preferably, it is 0.0015 to 0.0300% by mass.

Pb: 0.3% by mass or less
Bi: 0.3% by mass or less
Pb and Bi have the effect of refining chips during cutting, and the addition of these elements is effective when improving the chip disposability. In order to obtain this effect, it is preferable to add Pb and Bi in an amount of 0.01% by mass or more. However, even if these elements are added excessively, the effect of improving the chip treatment property is saturated. Therefore, in order to suppress the increase in alloy cost, the upper limit of the amount of Pb and Bi is set to 0.3% by mass. More preferable amounts of Pb and Bi are 0.01 to 0.2% by mass, and more preferably 0.01 to 0.1% by mass.
The rest other than the elements described above are Fe and unavoidable impurities.

Further, the carburized steel of the present invention has a surface layer that satisfies the following conditions.
Cementite fraction at a depth of 0.05 mm from the surface is 5% or less If cementite is present on the surface layer on the outermost surface side after carburizing, it promotes fatigue fracture and causes early fracture of carburized steel (parts). Therefore, in order to suppress premature fracture of carburized steel (parts), the cementite fraction (area ratio) at a depth of 0.05 mm from the surface is set to 5% or less. It is preferably 3% or less, and more preferably 1.5% or less. Of course, it may be 0%.
The rest of cementite on the surface layer is composed of martensite, bainite, retained austenite, and pearlite. Preferably, retained austenite is 35% or less and pearlite is 15% or less.

When the hardness is HV600 or more at a depth of 0.05 mm from the surface, the fatigue strength is improved by the effect of increasing the hardness of the outermost layer. That is, if the hardness of the outermost layer is insufficient, the effect of improving the fatigue characteristics cannot be obtained. In order to improve fatigue strength, the hardness at a depth of 0.05 mm from the surface must be HV600 or higher. More preferably, it is HV650 or higher.

Hardness at a depth of 0.10 mm from the surface is HV650 or more In addition to the hardness at a depth of 0.05 mm described above, hardness at a depth of 0.10 mm also affects fatigue strength. That is, in order to improve the fatigue strength, the hardness at a depth of 0.10 mm from the surface needs to be HV650 or more.

The austenite particle size at a depth of 0.05 mm from the surface is 5 or more. The old austenite particle size after carburizing affects the growth of fatigue cracks. When the particle size becomes coarse, the fatigue characteristics deteriorate, but in order to suppress this, fine particles having a particle size of 5 or more are appropriate. That is, if the particle size is less than 5, the fatigue characteristics are significantly deteriorated. More preferably, the particle size is 6 or more.

Next, a method for manufacturing carburized steel will be described. That is, after cutting a steel bar or a wire having the above-mentioned composition into small pieces, cold forging is performed to form a desired part shape. After that, the cold forged material in the shape of the part is carburized to obtain carburized steel. Hot forging may be added prior to such cold forging. Further, a part of the part may be machined at any stage prior to the carburizing treatment.

[Vacuum carburizing treatment]
It is particularly important to perform the above carburizing treatment by vacuum carburizing under the following conditions in order to obtain the above-mentioned surface layer structure. That is, the carburizing period temperature Tc (° C) and the diffusion period temperature Td (° C) in vacuum carburizing satisfy the following equations (1) and (2). The carburizing period temperature Tc is the temperature during which the carburized gas is introduced into the carburizing furnace, and the diffusion period temperature Td is the temperature during which carbon, which has a high concentration in the surface layer of steel, is infiltrated into the inside. Is. Tc and Td are determined in consideration of the required carburizing depth, but the following equations (1) and (2) must be satisfied in order to suppress the formation of excess cementite and the decrease in the particle size of the former austenite. be.
Tc + 22 × ([Cr] -0.8) ≤ Td ≤ 1100 ・ ・ ・ (1)
Tc ≤ 1000 ・ ・ ・ (2)
Here, [Cr]: Cr concentration in steel (mass%)

The steel after vacuum carburizing has a hardness distribution that follows the carbon concentration distribution from the surface layer to the inside. In particular, the surface layer has the highest carbon concentration and high hardness. This high surface hardness contributes to the improvement of fatigue characteristics, but sufficient fatigue characteristics cannot be obtained only with high surface hardness. For example, in gas carburizing, intergranular oxidation of the surface may be the starting point of fatigue fracture and sufficient fatigue characteristics may not be exhibited. In that respect, vacuum carburizing suppresses intergranular oxidation, which is advantageous for fatigue characteristics.
However, in vacuum carburizing, cementite is likely to be generated due to excessive carburizing, which may be the starting point of fatigue fracture. This cementite formation is promoted as the amount of Cr added increases and the amount of Si added decreases. On the other hand, increasing the Cr and lowering the Si of steel is an effective means for improving the cold forging property, so it is desired to increase the Cr and lower the Si. Therefore, after diligent studies on raising the temperature of the diffusion period from the temperature of the carburizing period, which is effective as a means of suppressing the formation of cementite in vacuum carburizing, the amount of increase should be set appropriately according to the amount of Cr added. As a result, we have found that it is possible to achieve both high Cr of steel and suppression of cementite production.

That is, if the temperature of the carburizing period is set to the same high temperature as the diffusion period, there is no particular problem from the viewpoint of suppressing the formation of cementite, but the temperature is maintained at a high temperature for the total time of the carburizing period and the diffusion period. Therefore, the crystal grains are likely to grow, which is disadvantageous in obtaining an austenite particle size having a particle size number of 5 or more. Therefore, it was decided to set an upper limit on the temperature in the carburizing period, that is, set the carburizing period temperature to 1000 ° C. or lower to suppress the growth of austenite grains in the carburizing period. Furthermore, it was decided to set a lower limit on the temperature of the diffusion period so that excess cementite generated during the carburizing period disappears during the diffusion period. Here, as a result of studying that the lower limit of the diffusion period temperature will change depending on the carburizing period temperature that affects the formation of cementite and the Cr content in the steel, the carburizing period temperature Tc And the diffusion period temperature Td
Tc + 22 × ([Cr] -0.8) ≤ Td
It was found that the production of excess cementite can be suppressed if the above conditions are satisfied.
In addition, if the maximum temperature reached by the steel during the diffusion period is too high, grain growth of austenite grains will occur during the diffusion period, so the upper limit of the diffusion period temperature Td was set to 1100 ° C.
For the above reasons, the temperature in the carburizing period and the temperature in the diffusion period need to satisfy the above equations (1) and (2). If the diffusion period is too long, there is a concern that austenite grains may grow, so the diffusion period is preferably 9 hours or less.

Further, the temperature and holding time of the carburizing period and the temperature and holding time of the diffusion period can be appropriately set according to the desired carburizing depth. However, there is a concern that excessively high temperature and lengthening of the carburizing period and prolonging of the diffusion period lead to the growth of crystal grains and increase the austenite particle size after carburizing. From this point, the temperature during the carburizing period should be 1000 ° C or lower. The holding time is preferably 9 hours or less at 880 to 1000 ° C.

Further, the conditions other than the above are not particularly limited in the vacuum carburizing, but it is preferable to perform the vacuum carburizing according to the following. That is, the carburized gas should be acetylene or ethylene, the pressure inside the furnace should be reduced to 10 kPa or less, and the atmosphere inside the furnace should be made uniform by appropriate stirring.

The carburized steel subjected to vacuum carburizing under the above-mentioned composition and carburizing conditions, that is, the conditions satisfying the conditions (1) and (2), suppresses intergranular oxidation of the surface which is the starting point of fatigue fracture. Moreover, the cementite content of the surface layer can be suppressed to a low level, and the old austenite grain size is also fine, so that sufficient fatigue characteristics can be ensured.
The grain boundary oxidation depth is preferably 5 μm or less. It is more preferably 3 μm or less, and further preferably 1.5 μm or less.

Hereinafter, the constitution and the action and effect of the present invention will be described more specifically according to Examples. However, the present invention is not limited by the following examples, and can be appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Will be.

The steel having the composition shown in Table 1 was melted and hot-rolled to form a round bar having a diameter of 50 mm. The Vickers hardness of the obtained rolled material was measured with a load of 1 kgf. If the hardness of the rolled material is HV180 or less, it can be used as a steel material for cold forging. Also, from a round bar, a cylinder of φ20 × 100 mm is collected so that 1/4 of the diameter of the round bar is the central axis, and after vacuum carburizing, Vickers hardness measurement with a load of 0.3 kgf is performed 0.05 mm below the surface. And at the position of 0.10 mm. The treatment temperatures in the carburizing period and the diffusion period in vacuum carburizing are as shown in Table 2. Other conditions for vacuum carburizing are as follows. Vacuum carburization was carried out under a reduced pressure of 10 kPa or less, acetylene was used as the carburizing gas, and the carburizing period was maintained for 2 hours and the diffusion period was maintained for 9 hours.

The cross section perpendicular to the height direction of the cylinder after vacuum carburizing (corresponding to the longitudinal direction of the round bar) was mirror-polished, and then the grain boundary oxidation depth from the surface was examined using a scanning electron microscope. In addition, after etching with a 1.5% nital solution, the cementite fraction was determined at a position 0.05 mm below the surface using a scanning electron microscope. Then, mirror polishing was performed, and the crystal grain size number of the old austenite grains at a position 0.05 mm below the surface was evaluated according to JIS G0551.

As shown in Table 2, according to the present invention, a carburized steel that is formed by cold forging and can suppress the formation of surface cementite during vacuum carburizing can be obtained.

Claims (7)

C: 0.10 to 0.35% by mass,
Si: 0.50% by mass or less,
Mn: 0.30 to 1.50% by mass,
Cr: 1.10-2.00 mass%,
P: 0.02% by mass or less,
S: 0.05% by mass or less,
It contains Al: 0.01-0.05% by mass and N: 0.030% by mass or less, and the balance has a component composition of Fe and unavoidable impurities.
The cementite fraction at a depth of 0.05 mm from the surface is 5% or less, the hardness at a depth of 0.05 mm from the surface is HV600 or more, the austenite particle size at a depth of 0.05 mm from the surface is a particle size of 5 or more, and Carburized steel with a hardness of HV650 or higher at a depth of 0.10 mm from the surface. The composition of the steel further
Mo: 1% by mass or less,
Cu: 1% by mass or less,
The carburized steel according to claim 1, which contains one or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less. The composition of the steel further
Nb: 0.1% by mass or less,
The carburized steel according to claim 1 or 2, which contains one or more selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less. The composition of the components further
Sn: 0.1% by mass or less and
Sb: The carburized steel according to any one of claims 1 to 3, which contains one or two selected from 0.1% by mass or less. The composition of the components further
Se: 0.3% by mass or less,
Ca: 0.1% by mass or less,
Pb: 0.3% by mass or less and
Bi: The carburized steel according to any one of claims 1 to 4, which contains one kind or two or more kinds selected from 0.3% by mass or less. The carburized steel according to any one of claims 1 to 5, which has a part shape. Vacuum carburizing treatment of a steel material having the component composition according to any one of claims 1 to 5 in which the carburizing period temperature Tc (° C) and the diffusion period temperature Td (° C) satisfy the following equations (1) and (2). A method of manufacturing carburized steel.
Tc + 22 × ([Cr] -0.8) ≤ Td ≤ 1100 ・ ・ ・ (1)
Tc ≤ 1000 ・ ・ ・ (2)
Here, [Cr]: Cr concentration in steel (mass%)