JP6705344B2 - Case-hardening steel excellent in coarse grain prevention characteristics and fatigue characteristics during carburization and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a case-hardening steel excellent in coarse grain prevention characteristics and carburizing characteristics at the time of carburization, and a method for manufacturing the case hardening steel.

Carburized parts such as gears, bearing parts, rolling parts, shafts and constant velocity joint parts are usually manufactured by the following method. For example, medium carbon carbon alloy steel for machine structural use specified in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. is forged, cut into a predetermined shape, and then carburized and quenched.

Cold forging (including rolling) or hot forging is performed in the forging performed when manufacturing a carburized component. Cold forging has good surface texture and dimensional accuracy of the product, lower manufacturing cost than hot forging, and good yield. Therefore, in recent years, there has been a strong tendency to switch from hot forging to cold forging. As a result, the number of carburized parts manufactured by carburizing and quenching after cold forging has significantly increased in recent years.

A major problem of carburized parts manufactured by carburizing and quenching after cold forging is reduction of heat treatment distortion. For example, when the shaft bends due to heat treatment distortion, the function as the shaft is impaired. In gears and constant velocity joint parts, large heat treatment distortion causes noise and vibration.
The largest cause of heat treatment distortion of carburized parts is coarse grains generated during carburization. Conventionally, in order to suppress coarse grains, annealing has been performed after cold forging and before carburizing and quenching. However, in recent years, the tendency to omit annealing has been increasing from the viewpoint of cost reduction. Therefore, there is a strong demand for steel materials that do not generate coarse grains even if annealing is omitted.

On the other hand, among gears, bearing components, and rolling components, bearing components and rolling components to which high surface pressure is applied are subjected to deep carburization. Normally, deep carburization requires a long time of ten to several tens of hours, and therefore shortening the carburizing time is an important issue from the viewpoint of energy saving.
Increasing the carburizing temperature is effective for shortening the carburizing time. The carburizing temperature is about 930° C. in the normal carburizing, but is performed in the temperature range of 990° C. to 1090° C. in the high temperature carburizing. However, when high temperature carburization is performed to shorten the carburizing time, coarse particles are generated, and the fatigue characteristics such as rolling fatigue characteristics required for carburized parts may not be sufficiently obtained. Therefore, there is a demand for case-hardening steel suitable for high-temperature carburization in which coarse grains are not generated even when high-temperature carburization is performed.

In addition, gears, bearing parts, and rolling parts to which high surface pressure is applied are often large parts, and are usually "bar steel-hot forging-heat treatment such as normalization if necessary-cutting-carburizing and quenching-polishing if necessary". It is manufactured through steps. In order to suppress the generation of coarse particles during carburization, the hot forged member after hot forging needs to be an appropriate material capable of suppressing coarse particles during carburization. For that purpose, it is necessary to use, as a material for the steel bar, an appropriate material capable of suppressing coarse grains during carburization.

Patent Document 1 contains Ti: 0.05 to 0.2%, S: 0.001 to 0.15%, limits N: less than 0.0051%, and precipitates AlN after hot rolling. Disclosed is a case-hardening steel which is excellent in coarse grain preventive properties and fatigue properties during carburization in which the amount is limited to 0.01% or less.
Patent Document 2 contains Ti: 0.03 to 0.30%, S: 0.010 to 0.10%, limits N: 0.020% or less, and determines the number density of Ti-based sulfides. A defined case hardening steel is disclosed.

Japanese Patent No. 4448456 JP, 2007-317787, A

However, the conventional case-hardening steel may lack the ability to prevent coarse grains in order to meet future needs for further high-temperature carburization. Further, in gears and shafts, which are major applications of case hardening steel, further improvement in fatigue strength characteristics is desired.
The present invention has been made in view of such circumstances, is excellent in coarse grain prevention characteristics during carburization, can suppress heat treatment distortion due to carburizing and quenching, and excellent fatigue properties after carburizing and quenching An object of the present invention is to provide a hardened steel and a manufacturing method thereof.

[1] Chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.50%,
Mn: 0.30 to 1.80%,
S: 0.003 to 0.020%,
Cr: 0.40 to 2.00%,
Al: 0.005-0.050%,
Ti: 0.06 to 0.20%,
Bi: 0.0001 to 0.0050%
And further,
Sb: 0.0001 to 0.0050%
Sn: 0.0001 to 0.0050% and Pb: 0.0001 to 0.0050%
Containing one or more selected from the group consisting of, the total content of Bi, Sb, Sn and Pb is 0.0050% or less, further P: 0.050% or less,
N: 0.0100% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
The following formula (1) is less than,
The bainite has a structure fraction of 30 area% or less,
A case-hardening steel having excellent coarse grain prevention properties and carburizing properties during carburization, characterized in that the ferrite grain size numbers are 8 to 11 specified in JIS G0552 .
Ti/S≧6.0 Formula (1)
(Ti in the formula (1) is the content of Ti (mass %), and S is the content of S (mass %).)

[2] The chemical composition is mass%,
Mo: 0.02 to 1.50%,
Ni: 0.10 to 3.50%,
V: 0.02-0.50% and B: 0.0002-0.0050%
A case-hardening steel excellent in coarse grain prevention properties during carburization and fatigue properties according to [1], containing one or more selected from the group consisting of:
[3] The chemical composition is mass%,
Nb: A case-hardening steel containing less than 0.04%, which is excellent in the coarse grain preventing property during carburization and fatigue properties according to [1] or [2].

[4] In the longitudinal section in Ma Torikusu, inspection reference area: 100 square mm, the number of check: maximum 30000 Ti-based precipitates by the extreme value statistics measured in terms of square mm: 16 field, the prediction area to perform A case-hardening steel having a diameter of 40 μm or less, which is excellent in coarse grain preventing properties during carburization and fatigue properties according to any one of [1] to [ 3 ].

[7] The chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.50%,
Mn: 0.30 to 1.80%,
S: 0.003 to 0.020%,
Cr: 0.40 to 2.00%,
Al: 0.005-0.050%,
Ti: 0.06 to 0.20%,
Bi: 0.0001 to 0.0050%
And further,
Sb: 0.0001 to 0.0050%
Sn: 0.0001 to 0.0050% and Pb: 0.0001 to 0.0050%
Containing one or more selected from the group consisting of, the total content of Bi, Sb, Sn and Pb is 0.0050% or less, further P: 0.050% or less,
N: 0.0100% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
Formula steel satisfying (1), a step of hot rolling the wire or bar steel by heating retention time of 10 minutes or more at 1150 ° C. or higher temperatures seen including,
After the hot rolling, the temperature fraction of 800 to 500° C. is gradually cooled at a cooling rate of 1° C./sec or less, and the bainite structure fraction of the steel after hot rolling and cooling is 30 area% or less. West,
The finishing temperature of the hot rolling is set to 840 to 1000° C., and the steel having the ferrite grain size number of 8 to 11 defined in JIS G0552 is obtained. Of manufacturing case hardening steel.
Ti/S≧6.0 Formula (1)
(Ti in the formula (1) is the content of Ti (mass %), and S is the content of S (mass %).)

Since the case-hardening steel of the present invention has a predetermined chemical composition, it is excellent in the property of preventing coarse grains during carburization. Therefore, according to the case-hardening steel of the present invention, heat treatment distortion due to carburizing and quenching can be suppressed, and excellent fatigue characteristics can be obtained after the carburizing and quenching. Further, the carburized parts produced by carburizing and quenching the case-hardening steel of the present invention have little heat treatment distortion and excellent fatigue characteristics.
According to the method for producing a case-hardening steel of the present invention, the case-hardening steel of the present invention is excellent in coarse-grain preventing characteristics during carburization, can suppress heat treatment distortion due to carburizing and quenching, and can obtain excellent fatigue characteristics after carburizing and quenching. Can be manufactured.

The present inventors diligently studied to solve the above problems. As a result, the following findings (1) to (5) were obtained.
(1) By optimizing the relationship between the S content and the Ti content in the case-hardening steel (Ti/S≧6.0), the carburizing of the case-hardening steel reduces the bending fatigue property in the rolling direction. Fine Ti-based carbosulfide is produced instead of MnS which is stretched and coarsened. As a result, excellent fatigue properties are obtained after case hardening of the case-hardened steel.

On the other hand, in the conventional technology, the balance between Ti and S contained in case-hardening steel has not been taken into consideration. For this reason, MnS, which is a starting point of bending fatigue fracture, may crystallize during carburizing of case hardening steel, and sufficient fatigue characteristics may not be obtained after case hardening steel is carburized and quenched.

(2) In order to prevent coarsening of crystal grains during carburization of case hardening steel, Ti-based precipitates mainly composed of TiC and TiCS are finely divided during carburization rather than utilizing AlN and NbN as pinning particles. Precipitation is effective. Further, in the case-hardening steel, in addition to a small amount of Bi, a small amount of one or more kinds selected from the group consisting of Sb, Sn and Pb is contained, so that the growth and coarseness of Ti-based precipitates during carburization Formation is suppressed, and the coarse-grain preventing property is further improved.

In order to stably exert the pinning effect of Ti-based precipitates during carburization of case-hardening steel, Ti-based precipitates are finely precipitated in the steel material after hot rolling and cooling in the manufacturing process of case-hardening steel. I need to keep it. For that purpose, it is necessary to precipitate a Ti-based precipitate at the phase interface during the diffusion transformation from austenite in the cooling process during hot rolling. If bainite is formed in the as-hot-rolled structure, it becomes difficult to precipitate the Ti-based precipitate at the phase interface. Therefore, it is preferable that the structure be substantially free of bainite.

In order to finely precipitate Ti-based precipitates in the steel material after hot rolling and cooling, the conditions of hot rolling may be optimized. That is, the Ti-based precipitate is once solid-dissolved in the matrix by raising the heating temperature in hot rolling. After the hot rolling, the precipitation temperature range of Ti-based precipitates is gradually cooled. As a result, the formation of bainite can be suppressed, and a large amount of Ti-based precipitates can be generated and finely dispersed.

(3) Further, by using in combination with Ti-based precipitates, Nb carbonitrides mainly composed of NbC are finely precipitated at the time of carburizing of case-hardening steel, whereby the coarse grain preventing property is further improved. In order to stably exert the pinning effect of Nb carbonitride during carburizing of case hardening steel, Nb carbonitride is added to the steel material after hot rolling and cooling in the manufacturing process of case hardening steel. It is necessary to deposit finely. To this end, Nb carbonitrides, like Ti-based precipitates, need to undergo phase interface precipitation during the diffusion transformation from austenite in the cooling process during hot rolling. Further, if bainite is generated in the as-hot-rolled structure, it becomes difficult to precipitate the Nb carbonitride at the phase interface. Therefore, it is preferable that the structure be substantially free of bainite.

In order to finely precipitate Nb carbonitride in the steel material after hot rolling and cooling, after heating the heating temperature in hot rolling to a high temperature and once dissolving Nb carbonitride in the matrix, The precipitation temperature range of Nb carbonitride is gradually cooled. As a result, a large amount of Nb carbonitride can be generated and finely dispersed.

(4) If the ferrite crystal grains contained in the steel material after hot rolling and cooling are excessively fine, coarse grains are likely to occur during carburization of case hardening steel. The grain size of the ferrite crystal grains in the steel material after hot rolling and cooling can be optimized by controlling the rolling finishing temperature.

(5) In a carburized component produced by carburizing and quenching case-hardening steel containing Ti, Ti-based precipitates are the starting point of fatigue failure, and thus fatigue characteristics, particularly rolling fatigue characteristics, tend to be insufficient. By lowering the chemical composition of case-hardening steel and increasing the heating temperature in hot rolling to reduce the maximum size of Ti precipitates, it is possible to improve fatigue properties.

The present invention has been made based on the above new findings.
Hereinafter, the case-hardening steel which is excellent in the coarse grain preventive property at the time of carburizing and the fatigue property of the present invention and the manufacturing method thereof will be described in detail.
First, the chemical composition of case hardening steel will be described. In addition, "%" of the content of each element means "mass %".

(C: 0.10 to 0.30%)
C is an element effective in imparting the required strength to steel. If the C content is less than 0.10%, the required tensile strength cannot be secured. If the C content exceeds 0.30%, the steel becomes hard, the cold workability deteriorates, and the toughness of the core portion after carburizing and quenching deteriorates. Therefore, the C content needs to be within the range of 0.10 to 0.30%.

(Si: 0.02 to 1.50%)
Si is an element effective for deoxidizing the steel, and is an element effective for imparting the necessary strength and hardenability to the steel and improving the temper softening resistance of the steel. If the Si content is less than 0.02%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.50%, the hardness of the steel is increased and the cold forgeability deteriorates. For the above reasons, the Si content needs to be within the range of 0.02 to 1.50%.

When the case-hardening steel is to be subjected to cold working, the preferable range of the Si content is 0.02 to 0.30%. In particular, when importance is attached to cold forgeability, it is desirable that the Si content be in the range of 0.02 to 0.15%. Further, Si is an element effective for increasing the grain boundary strength. Further, Si, when case-hardening steel is used as a material for carburized parts such as bearing parts and rolling parts, improves life by suppressing structural changes and material deterioration in the rolling fatigue process of these carburized parts. It is an effective element. When aiming at strengthening by containing Si, the suitable range of Si content is 0.20 to 1.50%. In particular, when case-hardening steel is used as a material for carburized parts having a high level of rolling fatigue strength, it is desirable that the Si content be in the range of 0.40 to 1.50%.

The effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts by containing Si is the amount of retained austenite in the structure after carburizing and quenching (commonly known as residual γ amount). ) Is particularly large when it is 30-40%. In order to control the residual γ amount within this range, it is effective to carry out so-called carburizing and nitrifying treatment. The carburizing and nitriding treatment is a treatment for performing nitriding in the process of diffusion treatment after carburizing. For the carburizing and nitriding treatment, the condition that the surface nitrogen concentration is in the range of 0.20 to 0.60% is suitable. The carbon potential during carburization in this case is preferably in the range of 0.90 to 1.30%.

(Mn: 0.30 to 1.80%)
Mn is an element effective for deoxidizing steel, and also effective for imparting strength and hardenability required for steel. If the Mn content is less than 0.30%, the above effect cannot be sufficiently obtained. If the Mn content exceeds 1.80%, not only the effect is saturated, but also the hardness of steel is increased, and cold forgeability is deteriorated. Therefore, the Mn content needs to be within the range of 0.30% to 1.80%. The preferable range of the Mn content is 0.50 to 1.20%. When importance is attached to the cold forgeability of steel, it is desirable that the Mn content be in the range of 0.50 to 0.75%.

(P: 0.050% or less)
Since P is an element that increases the deformation resistance during cold forging and deteriorates toughness, it deteriorates cold forgeability. Further, P deteriorates the fatigue strength by making the crystal grain boundaries of the parts after quenching and tempering brittle. Therefore, it is desirable to reduce the P content as much as possible, and it is necessary to limit it to 0.050% or less. The preferable range of the P content is 0.015% or less.

(S: 0.003 to 0.020%)
S forms MnS in steel. Since MnS can be the starting point of bending fatigue failure of carburized parts, it is necessary to prevent the generation of MnS. Therefore, the upper limit of the S content is 0.020%, and the range satisfies the following formula (1). When the S content is within the above range, S in the steel exists as Ti-based carbosulfide, so that excellent fatigue properties can be obtained after carburizing and quenching. More preferably, the S content is 0.015% or less. Further, Ti-based carbosulfide exhibits a pinning effect for preventing coarse grains. In order to bring out the effect, the S content needs to be 0.003% or more, and more preferably 0.005% or more.

Ti/S≧6.0 Formula (1)
(Ti in the formula (1) is the content of Ti (mass %), and S is the content of S (mass %).)

(Cr: 0.40 to 2.00%)
Cr is an element effective in imparting strength and hardenability to steel. Further, when the case-hardening steel is used as a material for carburized parts such as bearing parts and rolling parts, Cr increases the amount of residual γ after carburizing and quenching, as well as changes in the structure and material during the rolling fatigue process. It is an element effective in extending the life by suppressing deterioration. If the Cr content is less than 0.40%, the effect is insufficient. If the Cr content exceeds 2.00%, the hardness of steel is increased, and the cold forgeability deteriorates. For the above reasons, the Cr content needs to be within the range of 0.40 to 2.00%. The preferable range of the Cr content is 0.70 to 1.60%.

Note that the effect of suppressing the structural change and material deterioration in the rolling fatigue process of the bearing parts and rolling parts by containing Cr is that the residual γ amount in the structure after carburizing and quenching is 30 to 40%. Sometimes especially big. In order to control the residual γ amount within this range, it is effective to carry out the carburizing/nitriding treatment under the condition that the surface nitrogen concentration is in the range of 0.20 to 0.60%.

(Al: 0.005-0.050%)
Al is contained as a deoxidizing agent. If the Al content is less than 0.005%, the effect is insufficient. On the other hand, when the Al content exceeds 0.050%, AlN remains without being solutionized by the heating in the hot rolling performed at the time of manufacturing the case-hardening steel, and Ti (Ti and Nb in the case of containing Nb). It becomes the precipitation site of the precipitate. As a result, it inhibits fine dispersion of Ti-based precipitates (Ti-based precipitates and Nb carbonitrides when Nb is contained) and promotes coarsening of crystal grains during carburization. For the above reasons, the Al content needs to be within the range of 0.005 to 0.050%. The suitable range of Al content is 0.025 to 0.040%.

(Ti: 0.06 to 0.20%)
Ti is contained in the steel in order to form fine TiC, TiCS, Ti-based carbides such as Ti ₄ C ₂ S ₂ and Ti-based carbosulfide, and thereby to refine the γ grains during carburization. If the Ti content is less than 0.06%, the effect is insufficient. On the other hand, when the Ti content exceeds 0.20%, the effect due to the precipitation of TiC becomes remarkable, the cold workability is significantly deteriorated, and the precipitates mainly composed of TiN become remarkable, and the conversion after carburizing and quenching occurs. Dynamic fatigue characteristics deteriorate. For the above reasons, the Ti content needs to be in the range of 0.06 to 0.20%. The suitable range of Ti content is 0.10 to less than 0.15%.

When the case-hardened steel of the present invention or a forged member obtained by forging the case-hardened steel is carburized and quenched, solid solution Ti reacts with carbon and nitrogen that enter during carburization to form fine TiC in the carburized layer. And TiN (hereinafter sometimes referred to as “Ti(C,N)”) are deposited in large amounts. These Ti(C, N) contribute to the improvement of rolling contact fatigue life in carburized parts such as bearing parts and rolling parts obtained after carburizing and quenching. Therefore, in the case of manufacturing bearing parts and rolling parts aiming at a particularly high level of rolling fatigue life, the carbon potential during carburization should be set higher in the range of 0.9 to 1.3%, or That is, it is effective to promote the precipitation of Ti(C,N) by performing so-called carburizing and nitriding treatment. The carburizing and nitriding treatment is a treatment for carrying out the nitriding in the process of the diffusion treatment after carburizing as described above, and it is suitable that the nitrogen concentration on the surface is in the range of 0.2 to 0.6%.

(Bi: 0.0001 to 0.0050%)
Bi is an important element in the present invention. When a trace amount of Bi is contained in steel, sulfides are finely dispersed as the solidification structure of steel is refined. Furthermore, by containing a trace amount of Bi in the steel, it is possible to suppress the growth and coarsening of precipitates such as Ti-based precipitates that suppress the coarsening of crystal grains during carburization. To obtain the above effects, the Bi content needs to be 0.0001% or more. However, if the Bi content exceeds 0.0050%, the effect of finely dispersing the dendrite structure is saturated, and the hot workability of the steel deteriorates, making it difficult to carry out hot rolling during the production of case-hardening steel. .. For these reasons, the Bi content needs to be within the range of 0.0001 to 0.0050%. The suitable range of Bi content is 0.0010 to 0.0040%.

(One or more selected from the group consisting of Sb: 0.0001 to 0.0050%, Sn: 0.0001 to 0.0050% and Pb: 0.0001 to 0.0050%)
The present invention is characterized by containing one or more kinds selected from the group consisting of Sb, Sn and Pb in addition to a trace amount of Bi. By containing a trace amount of these elements, the sulfide represented by MnS is finely dispersed as the solidification structure of steel becomes finer. In order to obtain the effect of finely dispersing sulfides, in addition to the Bi content of 0.0001% or more, 0.001% of one or two or more kinds selected from the group consisting of Sb, Sn and Pb is added. % Or more must be contained. However, if the total content of Bi, Sb, Sn, and Pb exceeds 0.0050%, the effect of finely dispersing the dendrite structure is saturated, and the hot workability of steel deteriorates, making hot rolling difficult. Become. From these, in the present invention, the total content of Bi, Sb, Sn and Pb needs to be 0.0050% or less. The suitable range of the total content of Bi, Sb, Sn and Pb is 0.0002 to 0.0050%. In order to further improve machinability, the total content of Bi, Sb, Sn and Pb is more preferably 0.0010% or more. Further, the upper limit of the total content of Bi, Sb, Sn and Pb may be 0.0048%.

(N: 0.0100% or less)
N, when combined with Ti in the steel, produces coarse TiN that makes little contribution to grain control. TiN serves as a precipitation site for TiC, TiCS-based Ti-based precipitates, NbC-based NbC and NbN (hereinafter sometimes referred to as “Nb(C,N)”), and Ti-based precipitates and Nb carbons. It inhibits the fine precipitation of nitrides and promotes the formation of coarse grains. The above adverse effect is particularly remarkable when the N content exceeds 0.0100%. For the above reasons, the N content needs to be 0.0100% or less. The N content is preferably limited to less than 0.0051%.

(O: 0.0025% or less)
In high-Ti steel such as the case-hardening steel of the present invention, O in the steel forms Ti-based oxide inclusions. If a large amount of Ti-based oxide inclusions is present in the steel, it becomes a precipitation site for TiC, and TiC coarsely precipitates during hot rolling performed during the production of case-hardening steel, which can suppress the coarsening of crystal grains during carburization. Disappear. Therefore, it is desirable to reduce the O content as much as possible. For the above reasons, it is necessary to limit the O content to 0.0025% or less. The preferred range of the O content is 0.0020% or less. In carburized parts such as bearing parts and rolling parts, oxide inclusions are the starting point of rolling fatigue failure. Therefore, the lower the O content of case-hardening steel, the longer the rolling life of carburized parts. .. Therefore, when case-hardening steel is used as a material for carburized parts such as bearing parts and rolling parts, it is desirable to limit the O content to 0.0012% or less.

The chemical composition of the case-hardening steel of the present invention may further contain one or more selected from the group consisting of Mo, Ni, V and B.
(Mo: 0.02 to 1.50%)
Mo has the effect of imparting strength and hardenability to steel, and further increases the amount of residual γ after carburization in bearing parts and rolling parts, while suppressing structural changes and material deterioration in the rolling fatigue process. It is an element effective in extending the life. In order to obtain the effect, the Mo content needs to be 0.02% or more. However, if the Mo content exceeds 1.50%, the hardness is increased, and the machinability and cold forgeability deteriorate. For the above reasons, the Mo content needs to be within the range of 1.50% or less. The preferable range of the Mo content is 0.05 to 0.50%.

It should be noted that the effect of suppressing the structural change and material deterioration in the rolling fatigue process of the bearing parts and rolling parts by containing Mo is similar to the above effect by containing Cr after carburizing and quenching. It is particularly large when the amount of residual γ in the tissue is 30 to 40%.

(Ni: 0.10 to 3.50%)
Ni has the effect of imparting strength and hardenability to steel. In order to obtain the effect, the Ni content needs to be 0.10% or more. However, if the Ni content exceeds 3.50%, the hardness is increased, and the machinability and cold forgeability deteriorate. For the above reasons, the Ni content needs to be within the range of 3.50% or less. The suitable range of Ni content is 0.20 to 2.00%.

(V: 0.02-0.50%)
V has an effect of giving strength and hardenability to steel. In order to obtain the effect, the V content needs to be 0.02% or more. However, if the V content exceeds 0.50%, the hardness is increased, and the machinability and cold forgeability deteriorate. For the above reasons, the V content needs to be within the range of 0.50% or less. The preferable range of the V content is 0.05 to 0.20%.

(B: 0.0002 to 0.0050%)
B is an element effective in imparting strength and hardenability to steel. In addition, B has an effect of increasing the growth rate of ferrite by forming boron iron carbide in the cooling process after rolling in bar steel/wire rod rolling and promoting softening as it is rolled. Furthermore, B also has the effect of improving the grain boundary strength of the carburized material and improving the fatigue strength and impact strength of the carburized part. In order to obtain those effects, the B content needs to be 0.0002% or more. However, when the B content exceeds 0.0050%, the above effects are saturated, and there is a concern that adverse effects such as impact strength deterioration may occur. Therefore, the B content needs to be within the range of 0.0050% or less. The preferable range of the B content is 0.0005 to 0.0030%.

The chemical composition of the case-hardening steel of the present invention may further contain Nb.
(Nb: less than 0.04%)
Nb is an element effective in suppressing coarsening of crystal grains by forming Nb (C, N) in association with C and N in steel during carburization. By containing Nb, the effect of preventing coarse particles due to Ti-based precipitates becomes more effective. This is because Nb is solid-dissolved in the Ti-based precipitate and the coarsening of the Ti-based precipitate is suppressed. The above-mentioned effect of containing Nb increases with an increase in Nb content, but even in a trace amount such as less than 0.03%, less than 0.02%, or even less than 0.01%, The coarse-grain preventing property is remarkably improved as compared with the case where Nb is not contained.

However, the inclusion of Nb causes deterioration of machinability, cold forgeability, and carburizing characteristics. In particular, when the content of Nb is 0.04% or more, the hardness of the raw material becomes hard and the machinability and cold forgeability deteriorate, and the Nb carbon is heated by the heating when the rolling material is hot-rolled. It becomes difficult for the nitride to form a solid solution. For the above reasons, the Nb content needs to be less than 0.04%. When the machinability such as machinability and cold forgeability is emphasized, the preferable range of the Nb content is less than 0.03%. Moreover, when carburizing property is emphasized in addition to workability, the preferable range of the Nb content is less than 0.02%. Furthermore, when carburizing property is particularly important, the preferable range of the Nb content is less than 0.01%.

Further, in order to achieve both the coarse-grain preventing property and the workability, it is recommended that the Nb content be adjusted according to the Ti content. Specifically, the preferable range of the total content (Ti+Nb) of the Nb content and the Ti content is 0.07 to less than 0.17%. In particular, when the case-hardening steel is one that is carburized at high temperature or one that is cold forged, the desirable range of the total content of the Nb content and the Ti content is more than 0.091% to 0.17%. Is less than.

(Bainite structure fraction: 30 area% or less)
The case hardening steel of the present invention preferably has a bainite structure fraction of 30 area% or less. When the bainite structure is mixed in the case-hardening steel, it causes coarse grains during carburization. Further, it is desirable that the bainite structure in case-hardening steel is small from the viewpoint of improving cold workability. The adverse effect of the bainite structure in case-hardening steel becomes particularly remarkable when the structure fraction of bainite exceeds 30 area %. For the above reasons, it is preferable to limit the structural fraction of bainite to 30 area% or less. When the case hardening steel is one which is carburized at a high temperature and the carburizing conditions are strict with respect to the prevention of coarse grains during carburizing, the preferable range of the structural fraction of bainite is 20 area% or less. When the case hardening steel is cold forged, and when the carburizing conditions are more strict with respect to the prevention of coarse grains during carburizing, the preferable range of the structural fraction of bainite is 10 area% or less.

(Ferrite grain size: No. 8-11)
The case hardened steel of the present invention preferably has a ferrite grain size number of 8 to 11 defined by JIS G0552. If the ferrite grains of the case-hardening steel are excessively fine, the austenite grains become excessively fine during carburization. If the austenite grains become too fine, coarse grains are likely to be generated. In particular, when the ferrite grain size exceeds No. 11 specified in JIS G0552, the tendency becomes remarkable. Further, if the austenite grain size becomes excessively finer than No. 11 specified in JIS G 0551, as in the case of the steel material disclosed in Japanese Patent Laid-Open No. 2003-34843, there is a problem such as insufficient strength due to deterioration of hardenability. On the other hand, if the ferrite crystal grain size number is less than No. 8 specified in JIS G0552, the ferrite grain size is coarse, so the ductility deteriorates and the cold forgeability deteriorates. For the above reasons, it is preferable to set the ferrite grain size number within the range of 8 to 11 defined in JIS G0552.

(Maximum diameter of Ti-based precipitate: 40 μm or less)
The case-hardening steel of the present invention is a Ti system based on extreme value statistics measured under conditions of an inspection reference area: 100 square mm, the number of inspections: 16 fields of view, and an area for prediction: 30,000 square mm in a longitudinal section in a matrix. The maximum diameter of the precipitate is preferably 40 μm or less.
As one of the required characteristics of the carburized component targeted by the present invention, improvement of contact fatigue strength such as rolling fatigue characteristics and surface fatigue strength can be mentioned. The presence of coarse Ti-based precipitates in case-hardened steel causes contact fatigue fracture in carburized parts produced by carburizing and quenching the Ti-based precipitates, resulting in deterioration of fatigue properties.

According to the extreme value statistics, when the maximum diameter of the Ti-based precipitates exceeds 40 μm when measured under the conditions of the inspection reference area: 100 square mm, the number of inspections 16 fields of view, and the area for prediction: 30,000 square mm, contact fatigue The adverse effect of Ti-based precipitates on the characteristics becomes remarkable. For the above reasons, the maximum diameter of the Ti-based precipitate measured by the extreme value statistics measured under the above conditions is preferably 40 μm or less, and more preferably 30 μm or less.

The method for measuring and predicting the maximum diameter of precipitates by extreme value statistics is the method described in "Effects of Metal Fatigue Minute Defects and Inclusions", pages 233 to 239, published by Yokendo on March 8, 1993. In addition, what is used in the present invention is a two-dimensional inspection method in which the maximum precipitate observed within a certain area (area for prediction: 30,000 mm 2) is estimated by a two-dimensional inspection. The detailed measurement procedure will be described in the Example section.

Next, the method for manufacturing case-hardening steel of the present invention will be described in detail.
First, a converter, an electric furnace, and the like are used to melt steel, adjust the composition, and form a slab of the above chemical composition by casting, and if necessary, through a slab rolling process, wire or A rolling material to be hot-rolled into steel bars. In the present embodiment, the size of the slab, the cooling rate at the time of solidification, and the slab rolling condition are not particularly limited, and any condition may be satisfied as long as the requirements of the present invention are satisfied.
Next, the rolling material having the above chemical composition, by the method shown below, hot rolled into a wire rod or steel bar, the case hardening steel of the present embodiment is a steel material obtained after hot rolling and cooling. obtain.

(Heating temperature, holding time)
In the present embodiment, a rolling material having the above chemical composition is heated at a temperature of 1150° C. or more for a holding time of 10 minutes or more and hot-rolled into a wire rod or a steel bar. When the heating temperature in hot rolling is 1150° C. or higher and the holding time is 10 minutes or longer, Ti-based precipitates can be sufficiently dissolved in the matrix, and the property of preventing coarse grains during carburization is excellent.

On the other hand, when the heating temperature in hot rolling is less than 1150° C. and/or the holding time is less than 10 minutes, when Ti-based precipitates and AlN (Nb are contained, Cannot sufficiently dissolve Ti-based precipitates, Nb precipitates, and AlN) in the matrix. As a result, Ti-based precipitates (Ti-containing precipitates and Nb-containing precipitates when Nb is contained) cannot be finely precipitated in the steel material after hot rolling and cooling, and thus hot rolling After cooling by cooling, the steel material contains coarse Ti-based precipitates and AlN (in the case of containing Nb, coarse Ti-based precipitates, Nb precipitates, and AlN). Therefore, the steel material after hot rolling and cooling cannot suppress the generation of coarse particles during carburization. Therefore, when hot rolling, it is necessary to heat at a temperature of 1150° C. or more for a holding time of 10 minutes or more. A preferred range of heating conditions in hot rolling is a temperature of 1180° C. or higher and a holding time of 10 minutes or longer.

(Finishing temperature)
In the present embodiment, the finishing temperature for hot rolling is preferably 840°C to 1000°C. By setting the finishing temperature of hot rolling within the above range, steel having a ferrite grain size number of 8 to 11 specified in JIS G0552 can be obtained. If the finishing temperature is lower than 840°C, the ferrite grain size becomes excessively fine, and coarse grains are likely to occur during carburization. On the other hand, when the finishing temperature exceeds 1000° C., the grain size of ferrite becomes coarse, the hardness of the steel material after hot rolling and cooling becomes hard, and the cold forgeability deteriorates. For the above reasons, the finishing temperature for hot rolling is preferably 840°C to 1000°C, more preferably 920°C to 1000°C. The finishing temperature of the hot rolling is such that the case-hardening steel is cold forged, and after the cold forging and before the carburizing and quenching, when annealing is not performed, it is in the range of 840°C to 920°C. Is more preferable.

(Cooling rate)
In the present embodiment, it is preferable to gradually cool the temperature range of 800° C. to 500° C. after the hot rolling at a cooling rate of 1° C./second or less. By cooling under the above cooling conditions after hot rolling, it is possible to secure a sufficient passage time in the precipitation temperature range of Ti-based precipitates, promote the dispersion of fine Ti-based precipitates, and increase the structural fraction of bainite. Can be suppressed. As a result, the structural fraction of bainite is 30 area% or less, and a steel having further excellent coarse grain preventing characteristics during carburization can be obtained. When the cooling rate in the above temperature range exceeds 1° C./sec, the bainite structure fraction increases. Further, if the cooling rate in the above temperature range is high, the hardness of the steel material after hot rolling and cooling increases, and the cold forgeability deteriorates. For this reason, it is desirable that the cooling rate within the above temperature range be as low as possible. A suitable range of the cooling rate in the above temperature range is 0.7° C./second or less.

As a method of reducing the cooling rate, for example, a method of installing a heat retaining cover or a heat retaining cover with a heat source behind the hot rolling line and gradually cooling the steel material after hot rolling with the heat retaining cover can be mentioned.

In the present embodiment, the steel material ((wire rod or bar steel): case-hardened steel) obtained after hot rolling and cooling may be subjected to spheroidizing annealing as necessary.

When a case-hardened steel of the present embodiment is used to manufacture a carburized component, it may be hot forged and then carburized and quenched, or cold forged and then carburized and quenched.

When a case-hardened steel is hot forged and then carburized and quenched to produce a carburized part, for example, "case hardening steel (wire rod or bar steel)-hot forging-heat treatment such as normalizing (normalizing) as necessary- A method of manufacturing through the steps of "cutting-carburizing and quenching-polishing if necessary" can be mentioned.
Specifically, for example, hot forging can be performed at a heating temperature of 1150° C. or higher.
The conditions for carburizing and quenching are not particularly limited. For example, high temperature carburization can be performed with the carburizing temperature in the temperature range of 950°C to 1090°C. Further, in order to improve the rolling fatigue life of the carburized component, the carbon potential during carburizing may be set higher in the range of 0.9 to 1.3%. Further, carburizing and nitrifying treatment may be performed in which nitrification is performed in the diffusion process after carburizing. In the carburizing and nitriding treatment, in order to improve the rolling fatigue life of the carburized component, the condition that the surface nitrogen concentration is in the range of 0.2 to 0.6% is appropriate.

Since the case-hardened steel of the present embodiment is excellent in the coarse grain preventing property during carburization, it is possible to suppress the heat treatment distortion due to carburizing and quenching, and to obtain a carburized component having excellent fatigue properties after carburizing and quenching. Therefore, for example, the carburizing time can be shortened by performing high-temperature carburizing after forging the case-hardening steel of the present embodiment. Further, it is possible to switch to cold forging even for a carburized part that could not be conventionally switched from hot forging to cold forging due to deterioration of dimensional accuracy due to heat treatment distortion. Further, it is possible to omit the annealing conventionally performed after cold forging for suppressing heat treatment distortion.
As described above, the industrial effect of the present invention is extremely remarkable.

Hereinafter, the present invention will be specifically described with reference to examples.
A converter molten steel having the composition shown in Table 1 was continuously cast into a slab, and a slab rolling process was performed as necessary to obtain a 162 mm square rolled material.
Subsequently, heating is performed at a heating temperature shown in Table 2 for a holding time of 10 minutes or more, hot rolling is performed at a finishing temperature of hot rolling shown in Table 2, and a temperature range of 800°C to 500°C is shown after hot rolling. It cooled at the cooling rate shown in 2 and manufactured the steel bar with a diameter of 24-30 mm.

The microstructure of each steel bar (case hardening steel) after hot rolling and cooling was observed, and the bainite structure fraction was measured by the following method.
With respect to each steel bar (case hardening steel), the ferrite crystal grain size was measured according to JIS G0552, and the grain size number was examined.
In addition, the maximum diameter of Ti-based precipitates was examined by extreme value statistics for each steel bar (case hardening steel) by the method described below.
Further, the Vickers hardness of each steel bar (case hardening steel) was measured by the following method as an index of cold workability.
The results are shown in Table 2.

"Bainite structure fraction"
A sample was obtained by cutting (crossing) each steel bar (case hardening steel) in a direction perpendicular to the axial direction. After embedding the obtained sample in resin, the cut surface (observation surface) was polished. Nital corrosion was performed on the observation surface after polishing to observe the microstructure, and the bainite structure in the microstructure was specified. Further, on the observation surface, the area ratio of the bainite structure was obtained and used as the structure fraction (area%) of the bainite.

"Maximum diameter of Ti precipitates"
The maximum diameter of the Ti-based precipitate was predicted by the extreme value statistical method by the following method. Whether or not the deposits were Ti-based was determined from the difference in contrast under an optical microscope. The validity of the discrimination method based on the difference in contrast was confirmed in advance with a scanning electron microscope equipped with an energy dispersive X-ray spectroscopic analyzer.
A test piece was sampled from each steel bar (case hardening steel), and an area having an inspection reference area of 100 square mm (area of 10 mm×10 mm) was prepared in advance for 16 fields of view in the longitudinal section of the steel bar. Then, the maximum precipitate of Ti-based precipitates in each inspection reference area of 100 square mm was detected, and this was photographed with an optical microscope at a magnification of 1000 times. The test was repeated 16 times with respect to 16 fields of view having an inspection reference area of 100 square mm (that is, 16 fields of inspection). The diameter of the maximum precipitate in each inspection reference area was measured from the obtained photograph. When the precipitate has an elliptical shape, the geometric mean of the major axis and the minor axis was calculated and used as the diameter of the precipitate. The 16 data of the diameters of the obtained maximum precipitates were plotted on the extreme value probability sheet by the method described in "Kenfudo, "Fatigue of Minor Defects and Effects of Inclusions", pages 233 to 239. The precipitation distribution line (maximum precipitation diameter and linear function of the extreme value standardized variable) is obtained, and the maximum precipitation distribution line is extrapolated to obtain the maximum precipitation diameter at an area of 30,000 square mm for prediction. I predicted.

"Vickers hardness (HV)"
Each rolled steel bar (case hardening steel) was cut (crossed) in a direction perpendicular to the axial direction to obtain a sample. After embedding the obtained sample in resin, the cut surface (observation surface) was polished. According to "Vickers hardness test-test method" in JIS Z 2244 (2009), the total Vickers hardness of a portion having a depth of 1/4 of the diameter from the surface with respect to the observation surface after polishing is applied in accordance with "Vickers hardness test-test method". It measured 5 times and made the average value into Vickers hardness.

For each steel bar (case-hardened steel), spheroidizing annealing was performed, then an upsetting test piece was produced, and upsetting with a rolling reduction of 50% was performed, and then a carburization simulation was performed under the following conditions.
In the carburization simulation, the heating temperature was set to three kinds of 1000° C., 1050° C. and 1100° C., and at any heating temperature, after heating for 5 hours, water cooling was performed. After the carburization simulation, the cut surface of each test piece was polished and then corroded, and the former austenite grain size was observed to determine the coarse grain generation temperature (grain coarsening temperature). The former austenite grain size was measured according to JIS G 0551, observed at 400 times for about 10 fields of view, and it was judged that coarse grains having a grain size number 5 or less were generated.
It was determined that the crystal grain coarsening property was good when the coarse grain generation temperature exceeded 1100°C, and was poor when the coarse grain generation temperature was 1100°C or lower. Table 2 shows the coarse particle generation temperature.

Next, each steel bar (case hardening steel) was subjected to cold forging at a rolling reduction of 50% to form a cylindrical rolling fatigue test piece having a diameter of 12.2 mm and an Ono-type rotary bending test piece having a diameter of 9 mm in the parallel portion. (R1.14 with notch) was prepared and carburized at 1050° C. for 5 hours under the condition of carbon potential of 0.8%. The temperature of the quenching oil was 130° C. and the tempering was 180° C. for 2 hours.

For each of the obtained carburized and quenched materials, the γ (austenite) grain size of the carburized layer was investigated by the method described below.
The parallel part of the Ono-type rotary bending after carburizing, quenching and tempering was cut (crossed) in a direction perpendicular to the axial direction to obtain a sample. After embedding the obtained sample in resin, the cut surface (observation surface) was polished. Corrosion to expose austenite grains was performed on the observation surface after polishing, and the austenite grain size was measured in a visual field centered on a position 200 μm deep from the surface according to JIS G0551.

The rolling contact fatigue characteristics of each carburized and quenched material were evaluated using a point contact type rolling contact fatigue tester (Hertz maximum contact stress 5884 MPa). As a measure of the fatigue life of rolling contact fatigue property, L10 life defined as “the stress repetition number until fatigue failure at a cumulative failure probability of 10% obtained by plotting test results on Weibull probability paper” was used. The rolling fatigue life of the comparative steel No. The relative value of the L10 life of each material when the L10 life of 18 was set to 1 is shown.

The bending fatigue strength of each carburized and hardened material was evaluated using an Ono-type rotary bending fatigue tester. Regarding the rotational bending fatigue strength, those that were durable 10,000,000 times at a stress of 550 MPa were evaluated as “◯”, and those that were fractured were evaluated as “x”.
The results are summarized in Table 2.

As shown in Table 2, the grain coarsening temperature of the steel of the present invention is over 1100°C, the γ grain size of the 1050°C carburized material is also fine grain, and the rolling fatigue life and the result of the rotating bending fatigue test are also good. Met.

On the other hand, the comparative steel No. Since No. 14 did not contain Bi, Sb, Sn and Pb, the crystal grain coarsening temperature was lower than that of the steel of the present invention.
In addition, No. In No. 15, since the total content of Bi, Sb, and Sn exceeds the upper limit specified in the present invention, there are initial cracks that are presumed to have occurred during hot rolling, and rolling fatigue life and rotation The result of the bending fatigue test was inferior to that of the steel of the present invention.

Comparative steel No. No. 16 had a large S content and did not satisfy the formula (1), so that fatigue fracture starting from MnS occurred, and the results of rolling fatigue life and rotary bending fatigue test were inferior to those of the steel of the present invention. .. In addition, No. In No. 16, Ti-based carbonitride precipitates, which are effective in preventing coarsening due to the formation of a large amount of Ti-based sulfides, were not sufficiently obtained, and the crystal grain coarsening temperature was lower than that of the steels of the present invention.

Comparative steel No. Since Nos. 17 and 18 did not satisfy the formula (1), fatigue fracture occurred with MnS as the starting point, and the rolling fatigue life and the results of the rotating bending fatigue test were inferior to those of the steels of the present invention.

Comparative steel No. Since No. 19 had a large N content, the rolling fatigue life was inferior to the steel of the present invention due to the formation of coarse TiN. Furthermore, No. In No. 19, since coarse TiN was generated, the precipitates of fine Ti-based carbonitrides effective for preventing coarse grains were reduced, so that the coarse grain generation temperature was inferior to the steel of the present invention.
Comparative steel No. In Nos. 20 to 29, the total content of Bi, Sb, Sn, and Pb exceeded the upper limit specified in the present invention, so that an initial crack presumed to have occurred during hot rolling was present and rolling occurred. The fatigue life and the results of the rotary bending fatigue test were inferior to those of the steel of the present invention.

Claims (5)

The chemical composition is% by mass,
C: 0.10 to 0.30%,
Si: 0.02 to 1.50%,
Mn: 0.30 to 1.80%,
S: 0.003 to 0.020%,
Cr: 0.40 to 2.00%,
Al: 0.005-0.050%,
Ti: 0.06 to 0.20%,
Bi: 0.0001 to 0.0050%
And further,
Sb: 0.0001 to 0.0050%
Sn: 0.0001 to 0.0050% and Pb: 0.0001 to 0.0050%
Containing one or more selected from the group consisting of, the total content of Bi, Sb, Sn and Pb is 0.0050% or less, further P: 0.050% or less,
N: 0.0100% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
The following formula (1) is less than,
The bainite has a structure fraction of 30 area% or less,
A case-hardening steel having excellent coarse grain prevention properties and carburizing properties during carburization, characterized in that the ferrite grain size numbers are 8 to 11 specified in JIS G0552 .
Ti/S≧6.0 Formula (1)
(Ti in Formula (1) is the content of Ti (mass %), and S is the content of S (mass %). The chemical composition is mass%,
Mo: 0.02 to 1.50%,
Ni: 0.10 to 3.50%,
V: 0.02-0.50% and B: 0.0002-0.0050%
The case-hardening steel excellent in coarse grain preventive properties during carburization and fatigue properties according to claim 1, containing one or more selected from the group consisting of: The chemical composition is% by mass,
Nb: A case-hardening steel containing less than 0.04%, which is excellent in coarse grain preventive properties during carburization and fatigue properties according to claim 1 or 2. In the longitudinal section in the matrix, the maximum diameter of the Ti-based precipitate is 40 μm or less according to the extreme value statistics measured under the conditions of the inspection reference area: 100 square mm, the number of inspections: 16 fields of view, and the area for prediction: 30,000 square mm. The case-hardening steel according to any one of claims 1 to 3 , which is excellent in the coarse grain preventing property during carburization and the fatigue property. The chemical composition is% by mass,
C: 0.10 to 0.30%,
Si: 0.02 to 1.50%,
Mn: 0.30 to 1.80%,
S: 0.003 to 0.020%,
Cr: 0.40 to 2.00%,
Al: 0.005-0.050%,
Ti: 0.06 to 0.20%,
Bi: 0.0001 to 0.0050%
And further,
Sb: 0.0001 to 0.0050%
Sn: 0.0001 to 0.0050% and Pb: 0.0001 to 0.0050%
Containing one or more selected from the group consisting of, the total content of Bi, Sb, Sn and Pb is 0.0050% or less, further P: 0.050% or less,
N: 0.0100% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
Formula steel satisfying (1), a step of hot rolling the wire or bar steel by heating retention time of 10 minutes or more at 1150 ° C. or higher temperatures seen including,
After the hot rolling, the temperature fraction of 800 to 500° C. is gradually cooled at a cooling rate of 1° C./sec or less, and the bainite structure fraction of the steel after hot rolling and cooling is 30 area% or less. West,
The finishing temperature of the hot rolling is set to 840 to 1000° C., and the steel having the ferrite grain size number of 8 to 11 defined in JIS G0552 is obtained. Method for manufacturing case hardening steel.
Ti/S≧6.0 Formula (1)
(Ti in the formula (1) is the content of Ti (mass %), and S is the content of S (mass %).)