JP6801782B2 - Steel and parts - Google Patents

Description

The present invention relates to steels with improved hardenability, toughness, surface origin peeling life, and bending fatigue strength, and parts manufactured using such steels.

Since high surface pressure repeatedly acts on mechanical structural parts such as bearings and automobile parts such as constant velocity joints and hub units, excellent rolling fatigue characteristics are required. In recent years, the above parts have been introduced. For example, with the demand for improved fuel efficiency of automobiles and higher output of engines, there is an extremely large demand for weight reduction, miniaturization, and high stress load, and the usage environment of the above parts is harsh. It has become.

In response to the above requirements, with respect to the material of the bearing parts, generally, the amount of non-metal inclusions represented by Al ₂ O ₃ (hereinafter, may be simply referred to as “inclusions”) that cause the rolling parts to peel off. Has been tried to improve the rolling fatigue life by reducing as much as possible.

However, as a result of the recent advances in steelmaking technology, the diameter of oxides has become smaller, and as a result, the size of sulfides has become relatively large, and measures using only oxides as an index may increase the variation in rolling fatigue life. .. Therefore, recently, attempts have been made to improve the rolling fatigue life by controlling the composition and morphology of inclusions.

For example, in Patent Document 1, the component composition is mass%, C: 0.1% or more and less than 0.4%, Si: 0.02 to 1.3%, Mn: 0.2 to 2.0%. , P: 0.05% or less, S: less than 0.010%, Cr: 0.50 to 2.00%, Al: 0.01 to 0.10%, Ca: 0.0003 to 0.0030%, O: 0.0030% or less, N: 0.002 to 0.030%, and the balance: Fe and impurities, 0.7 ≦ Ca / O ≦ 2.0 and Ca / O ≧ 1250S-5.8. A steel for carburized bearings is disclosed.

On the other hand, since the bearing is repeatedly subjected to bending stress, bending fatigue strength is also required. Recently, attempts have been made to suppress the formation of intergranular oxide layers in order to increase the bending fatigue strength of bearings.

For example, in Patent Document 2, the component composition is mass%, C: 0.1 to 0.3%, Si: 0.01 to 0.25%, Mn: 0.2 to 1.5%, S. : 0.003 to 0.05%, Cr: 0.5 to 2.0%, Mo: 0.1 to 0.8%, Al: 0.01 to 0.05%, and N: 0.008 It contains ~ 0.025%, the balance consists of Fe and impurities, Ti in the impurities is 0.005% or less, O (oxygen) is 0.002% or less, and P and Sn are 0.030% or less in total. In addition, the minimum value of A = (1 + 0.681Si) (1 + 3.066Mn + 0.329Mn ² ) (1 + 2.07Cr) (1 + 3.14Mo) in the steel cross section is 13 or more, and the cross section area is 1500 mm ² . A steel material for a carbonized part or a carbonized nitrided part is disclosed, wherein the maximum length of the inclusion group excluding the sulfide is 30 μm or less.

Further, in Patent Document 3, the component composition is mass%, C: 0.15 to 0.30%, Si: 0.02 to 1.0%, Mn: 0.30 to 1.0%, S. : 0.030% or less, Cr: 1.80 to 3.0%, Al: 0.010 to 0.050% and N: 0.0100 to 0.0250%, Si, Mn, Cr and The content of S is the value of fn1 and fn2 represented by the formula (1) Mn / S and the formula (2) Cr / (Si + 2Mn), and 30 ≦ fn1 ≦ 150 and 0.7 ≦ fn2 ≦ 1. It is composed of the balance Fe and impurities, and P, Ti, and O (oxygen) in the impurities are P: 0.020% or less, Ti: less than 0.005%, and O: 0. A skin-baked steel characterized by being 0015% or less is disclosed.

Further, Patent Document 4 describes in terms of mass%, C: 0.05 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.10 to 2.0%, P: 0.050. % Or less, S: 0.008% or less, Cr: 0.4 to 2.0%, Al: 0.010 to 0.050%, N: 0.010 to 0.025% and O: 0.0015% A method for melting a carbonized bearing steel material containing the following and the balance being a chemical composition of Fe and impurities, in the order of step 1: flux blowing treatment, step 2: slag refining treatment, and step 3: molten steel reflux treatment. By performing the pot refining treatment, the average composition of the S-containing compounds constituting the sulfide-based inclusions is CaS: 1.0% or more, MgS: 0 to 20%, and three components of CaS, MgS and MnS. A melting method for controlling sulfide-based impurities so that the total amount of slag is 95% or more is disclosed.

In addition, Patent Document 5 contains a specific amount of C, Si, Mn, P, S, Al, Cr, N and O, the balance of which is Fe and impurities, and maximum oxidation in a longitudinal longitudinal cross section of 100 mm ^2. Measurement of body diameter and maximum sulfide diameter was performed at 30 locations, and prediction of oxides and sulfides in 30000 mm ² calculated using extreme value statistical processing √ AREA _max is 50 μm or less and 60 μm or less, at those 30 locations The average aspect ratio of the measured maximum oxide and maximum sulfide is 5.0 or less, and the contents of the maximum oxide at the 30 locations in the average composition are CaO: 2.0 to 20%, MgO: 0 to 20%, and SiO ₂ : 0. ~ 10%, the balance is Al ₂ O ₃ , and consists of any of the specific 2 to quaternary oxides, and the content of the maximum sulfide at the 30 locations in the average composition is CaS: 100%. A carburized bearing steel material consisting of a specific binary or ternary sulfide with CaS ≥ 1.0%, MgS: 0 to 20%, and the balance of MnS is disclosed.

JP-A-2015-129335 Japanese Patent No. 4243852 Japanese Patent No. 5163242 Japanese Unexamined Patent Publication No. 2014-5520 Japanese Unexamined Patent Publication No. 2013-147689

In the carburized bearing steel disclosed in Patent Document 1, when the intergranular oxide layer is formed thick, the bending fatigue strength may decrease. The steel material for carburized parts or carburized nitrided parts disclosed in Patent Document 2 and the skin-baked steel disclosed in Patent Document 3 may not have an excellent rolling fatigue life in the presence of stretched coarse sulfide. There is sex. Therefore, the techniques disclosed in Patent Documents 1 to 3 may not be able to stably realize all the characteristics of hardenability, toughness, surface origin peeling life, and bending fatigue strength.

Further, the techniques disclosed in Patent Documents 4 and 5 may not be able to stably realize all the characteristics of hardenability, toughness, surface origin peeling life, and bending fatigue strength.

The present invention has been made in view of the above-mentioned current state of the prior art, and an object of the present invention is to obtain steels having improved hardenability, toughness, surface origin peeling life, and bending fatigue strength, and such steels. The purpose is to provide parts manufactured using the product.

In general, rolling fatigue is a phenomenon in which a repetitive load is applied to inclusions existing in a steel material, cracks are generated due to stress concentration, and then the cracks gradually develop due to the repetitive load, eventually leading to peeling.

The present inventors have conducted various studies in order to solve the above problems. As a result, the following findings (a) and (b) have been obtained.

(A) By controlling the composition of the sulfide, specifically, for example, by adding Ca to the molten steel and controlling the composition so that (Mn, Ca) S is contained in the sulfide. Coarse sulfide, which is a stress concentration source for rolling fatigue, can be dispersed and reduced in diameter.

(B) By optimizing the amount balance of oxidizing elements, especially Cr, Si, and Mn, the thickness of the intergranular oxide layer and the incompletely hardened layer, which are abnormal carburizing layers, can be reduced. As a result, high bending fatigue strength can be ensured.

The present invention has been made based on the above findings (a) and (b), and the gist thereof is as follows.

[1] Ingredient composition is mass%
C: 0.10 to 0.30%,
Si: 0.01-0.25%,
Mn: 0.25 to 1.50%,
P: 0.001 to 0.015%,
S: 0.001 to 0.010%,
Cr: 0.50 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.005 to 0.100%,
Ca: 0.0002 to 0.0010%,
N: 0.005 to 0.025%,
O: 0.0015% or less,
Cu: 0-0.20%,
Ni: 0 to 0.20%
B: 0 to 0.005%
Nb: 0-0.05%
Ti: 0 to 0.10%
Remaining: Fe and impurities,
Fn1 defined by the following equation (1) is 0.25 to 0.65.
A steel characterized in that Fn2 defined by the following formula (2) is 0.50 to 1.00.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
[Element]: Mass% of element
Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide system containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of inclusions (μm ² )
A2: Total area of sulfide-based inclusions with a circular equivalent diameter of 1.0 μm or more (μm ² ) in the observation area with a total area of 4.0 mm ^2.

[2] The steel according to [1], wherein the component composition contains at least one of Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less in mass%. ..

[3] The steel according to [1] or [2], wherein the component composition contains at least one of Nb: 0.05% or less and Ti: 0.10% or less in mass%.

[4] The steel according to any one of [1] to [3], which is a steel bar.

[5] In a depth region of 500 μm or more from the surface
Ingredient composition is mass%,
C: 0.10 to 0.30%,
Si: 0.01-0.25%,
Mn: 0.25 to 1.50%,
P: 0.001 to 0.015%,
S: 0.001 to 0.010%,
Cr: 0.50 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.005 to 0.100%,
Ca: 0.0002 to 0.0010%,
N: 0.005 to 0.025%,
O: 0.0015%,
Cu: 0-0.20%,
Ni: 0 to 0.20%
B: 0 to 0.005%
Nb: 0-0.05%
Ti: 0 to 0.10%
Remaining: Fe and impurities,
Fn1 defined by the following equation (1) is 0.25 to 0.65.
Fn2 defined by the following equation (2) is 0.50 to 1.00.
A component characterized by excellent surface origin peeling life and bending fatigue strength.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
[Element]: Mass% of element
Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide system containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of inclusions (μm ² )
A2: Total area of sulfide-based inclusions with a circular equivalent diameter of 1.0 μm or more (μm ² ) in the observation area with a total area of 4.0 mm ^2.

[6] The component according to [5], wherein the component composition contains at least one of Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less in mass%. ..

[7] The component according to [5] or [6], wherein the component composition contains at least one of Nb: 0.05% or less and Ti: 0.10% or less in mass%.

[8] The component according to any one of [5] to [7], wherein the absorbed energy vE20 is 43 J / cm ² or more in the central portion.

In the steel according to the present invention, among the sulfide-based inclusions having a predetermined component composition, optimizing the balance of Cr, Si, and Mn, and having a circle-equivalent diameter having a predetermined value, further in the sulfide. The ratio of sulfide-based inclusions in which the ratio of the number of Ca moles is a predetermined value is optimized. Therefore, in the steel according to the present invention, all of hardenability, toughness, surface origin peeling life, and bending fatigue strength can be improved.

It is a figure which shows an example of the brightness distribution of the SEM image in an observation area schematically. It is a figure which shows an example of the SEM image in an observation area schematically. It is a figure which shows the relationship between the temperature and time of a tempering heat treatment.

Hereinafter, the findings of the inventors leading to the present invention, and an embodiment (the present embodiment) relating to the steel, the manufacturing method thereof, and the manufacturing method of the parts according to the present invention will be described in detail. In the following, "%" of the content of each element means "mass%".

<Knowledge of the present inventors>
The present inventors have diligently studied to provide steels having improved hardenability, toughness, surface origin peeling life, and bending fatigue strength, and parts manufactured using such steels. That is, the present inventors investigated and examined the influence of the composition of steel, particularly the influence of Si, Mn, Cr, and Ca on the surface origin peeling life and bending fatigue strength of carburized parts after carburizing treatment. did. As a result, the present inventors have obtained the following findings regarding bending fatigue strength, surface origin peeling life, hardenability and toughness.

(A) Bending fatigue strength In order to ensure high bending fatigue strength in carburized bearing steel, it is necessary to reduce the thickness of the grain boundary oxide layer and the incompletely hardened layer, which are abnormal carburizing layers. Among the sex elements, especially in Si, Mn, and Cr, by optimizing the amount balance, the layer thicknesses of the intergranular oxide layer and the incompletely hardened layer, which are abnormal carburizing layers, can be reduced.

Specifically, when Fn1 defined by the following formula (1) is 0.20 to 0.65, the layer thicknesses of the intergranular oxide layer and the incompletely hardened layer can be reduced.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
[Element]: Mass% of element

Fn1: 0.25 to 0.65
If Fn1 is less than 0.20, the thickness of the abnormal carburized layer becomes thick and it becomes difficult to secure high bending fatigue strength. Therefore, Fn1 is set to 0.20 or more. It is preferably 0.25, more preferably 0.30 or more. On the other hand, when Fn1 exceeds 0.65, the thickness of the abnormal carburized layer becomes thick and it becomes difficult to secure high bending fatigue strength. Therefore, Fn1 is set to 0.65 or less. It is preferably 0.60, more preferably 0.55 or less.

(B) Surface-origin peeling life Since sulfide-based inclusions are usually easily deformed at high temperatures, they are easily deformed and stretched during hot working. The stretched sulfide-based inclusions become a fatigue starting point in the usage environment of the carburized bearing component, and the surface starting point peeling life is shortened. Therefore, in order to extend the surface origin peeling life, it is effective to increase the deformation resistance of the sulfide-based inclusions at high temperature.

That is, if the deformation resistance of the sulfide-based inclusions at a high temperature is increased, the sulfide-based inclusions are less likely to stretch during hot working and maintain a spherical shape, so that the sulfide-based inclusions are less likely to become a fatigue starting point.

The deformation resistance of the sulfide containing Ca is larger than that of the sulfide containing Ca. Therefore, if Ca is dissolved in the sulfide-based inclusions, that is, if Mn of MnS is replaced with Ca, the deformation resistance at high temperature increases as a result. Let (Mn, Ca) S be a sulfide in which Mn of MnS is replaced with Ca. Specifically, the total number of moles in each sulfide is increased by performing secondary refining in a state where the oxygen concentration is lowered as much as possible so that the sulfide inclusions are mainly (Mn, Ca) S. It can contain 1.0 mol% or more of Ca.

Since the sulfide-based inclusions in which Ca is dissolved in this way can maintain a spherical shape even after hot working, the aspect ratio (major axis / minor axis of the sulfide-based inclusions) is small. Specifically, the sulfide-based inclusions containing Ca in an amount of 1.0 mol% or more based on the total number of moles in each sulfide are less than 1.0 mol% based on the total number of moles of Ca in each sulfide. The aspect ratio after hot working is smaller than that of sulfide-based inclusions that do not contain it, and 90% of them have an aspect ratio of 3 or less. As a result of the experiment, it has been found that the upper limit of the total number of moles of Ca in each sulfide is 50 mol%.

Based on the above findings, the present inventors consider that if the sulfide-based inclusions in the carburized bearing steel have Fn2 defined by the following formula (2) of 0.50 to 1.00, the sulfide-based inclusions. It was found that the deformation resistance during hot working is increased and the surface origin peeling life of carburized bearing parts is extended.

Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide-based interposition containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of objects (μm ² )
A2: The equivalent circle diameter in the observation area with a total area of 4.0 mm ² is 1.0 μm or more.
Total area of sulfide-based inclusions (μm ² )

Fn2 (= A1 / A2): 0.50 to 1.00
Fn2 is an index related to the aspect ratio of sulfide-based inclusions in steel for carburized bearings after hot working. When Fn2 is 0.50 or less, the sulfide-based inclusions are stretched during hot working, and the aspect ratio of the sulfide-based inclusions after hot working becomes large.

When the aspect ratio of the sulfide-based inclusions after hot working becomes large, the sulfide-based inclusions become the starting point of fatigue under the usage environment of the carburized bearing parts after the carburizing treatment, and the surface starting point peeling life is shortened. Is 0.50 or more. It is preferably 0.55 or more, more preferably 0.60 or more. The upper limit of Fn2 is 1.00 by definition.
(C) Hardenability and toughness Conventionally, it has been difficult to maintain hardenability or toughness of steel for carburized bearings while improving bending fatigue strength or surface origin peeling life. When the bending fatigue strength or the surface origin peeling life is improved, there is a problem that the hardenability or toughness is lowered.
The present inventors have found that the steel according to the present embodiment satisfying the predetermined component composition, the formulas (1) and (2), improves the bending fatigue strength or the surface origin peeling life in the part after the carburizing treatment. It was found that it is also excellent in hardenability and toughness.
Excellent hardenability means that the hardness of HRC is 22 or more at 500 μm or less from the surface of the part after quenching.
Excellent toughness means that the absorbed energy vE20 is 43 J / cm2 or more in the central portion.

<Steel>
[Ingredient composition]
(Required element)
C: 0.10 to 0.30%
C is an element that enhances the hardenability of steel and enhances the strength and toughness of the core of the steel material after quenching. Further, C is an element having an action of extending the surface origin peeling life of the carburized bearing component after the carburizing treatment.

If C is less than 0.10%, the addition effect cannot be sufficiently obtained, so C is set to 0.10% or more. It is preferably 0.13% or more, more preferably 0.15% or more. On the other hand, if C exceeds 0.30%, the toughness decreases, so C is set to 0.30% or less. It is preferably 0.29% or less, more preferably 0.28% or less, still more preferably 0.25% or less.

Si: 0.01-0.25%
Si is an element that not only functions as an antacid but also contributes to the improvement of hardenability. Further, Si is an element that increases temper softening resistance and suppresses softening of steel at high temperatures. However, Si is an oxidizing element, and when the amount is increased, it is selectively oxidized by a small amount of H ₂ O and / or CO ₂ in the carburized gas, and the grain boundary oxide layer and the incompletely hardened layer which are abnormal carburizing layers The layer thickness becomes thicker and the bending fatigue strength decreases.

If Si is less than 0.01%, the addition effect cannot be sufficiently obtained, so Si is set to 0.01% or more. It is preferably 0.03% or more, more preferably 0.06% or more. On the other hand, when Si exceeds 0.25%, the thickness of the intergranular oxide layer and the incompletely hardened layer, which are abnormal carburizing layers, becomes thick and the bending fatigue strength decreases, so that Si is 0.25% or less. To do. It is preferably 0.20% or less, more preferably 0.15% or less.

Mn: 0.25 to 1.50%
Mn is an element that not only functions as an antacid but also contributes to the improvement of hardenability. However, Mn is an oxidizing element like Si, and when the amount increases, it is selectively oxidized by a trace amount of H ₂ O and / or CO ₂ in the carburized gas, and the grain boundary oxide layer which is an abnormal carburizing layer and The incompletely hardened layer becomes thicker and the bending fatigue strength decreases.

If Mn is less than 0.20%, the addition effect cannot be sufficiently obtained, so Mn is set to 0.20% or more. It is preferably 0.30% or more, more preferably 0.40% or more. On the other hand, when Mn exceeds 1.50%, the hardness increases, the machinability decreases significantly, the thickness of the abnormal carburized layer increases, and the bending fatigue strength decreases significantly. Therefore, Mn is 1. .50% or less. It is preferably 1.48% or less, more preferably 1.30% or less, still more preferably 1.10% or less.

P: 0.001 to 0.015%
P is an impurity element, which segregates at grain boundaries and inhibits the toughness of steel and the surface origin peeling life of carburized bearing parts.

If P exceeds 0.015%, the toughness of the steel and the surface origin peeling life of the carburized bearing component are significantly reduced, so P is set to 0.015% or less. It is preferably 0.013% or less, more preferably 0.010% or less. It is preferable that P is as small as possible, but if it is reduced to less than 0.001%, the manufacturing cost increases, so P is set to 0.001% or more. It is preferably 0.003% or more.

S: 0.001 to 0.010%
S is an impurity element, which forms sulfide, inhibits the toughness and cold forging property of steel, and inhibits the surface origin peeling life of carburized bearing parts.

When S exceeds 0.010%, the toughness and cold forging property of the steel are remarkably lowered, and the surface origin peeling life of the carburized bearing component is remarkably lowered. Therefore, S is set to 0.010% or less. It is preferably 0.008% or less, more preferably 0.005% or less. It is preferable that S is small, but if it is reduced to less than 0.001%, the manufacturing cost increases, so S is set to 0.001% or more. It is preferably 0.002% or more, more preferably 0.003 or more, still more preferably 0.005% or more.

Cr: 0.50 to 2.00%
Cr is an element that enhances hardenability, enhances temper softening resistance, and suppresses softening of steel at high temperatures. However, Cr is an oxidizing element like Si and Mn, and when the amount is increased, it is selectively oxidized by a trace amount of H ₂ O and / or CO ₂ in the carburized gas, and grain boundary oxidation which is an abnormal carburized layer The layer thickness of the layer and the incompletely hardened layer becomes thicker, and the bending fatigue strength decreases.

If Cr is less than 0.50%, the addition effect cannot be sufficiently obtained, so Cr is set to 0.50% or more. It is preferably 0.70% or more, more preferably 0.90% or more. On the other hand, when Cr exceeds 2.00%, the hardness increases, the machinability decreases remarkably, the thickness of the abnormal carburized layer becomes thick, and the bending fatigue strength decreases remarkably. It shall be 00% or less. It is preferably 1.98% or less, more preferably 1.80% or less, still more preferably 1.60% or less.

Mo: 0.10 to 0.50%
Mo is an element that enhances hardenability, improves surface hardness, hardened layer depth, and core hardness after carburizing and quenching, and contributes to ensuring the strength of carburized parts. Further, since Mo is a non-oxidizing element, it is an element that acts to toughen the steel surface and increase bending fatigue strength without increasing the thickness of the intergranular oxide layer during carburizing.

If Mo is less than 0.10%, the addition effect cannot be sufficiently obtained, so Mo is set to 0.10% or more. It is preferably 0.20% or more, more preferably 0.30% or more. On the other hand, when Mo exceeds 0.50%, the hardness increases and the machinability decreases remarkably. Further, the surface origin peeling life of the carburized bearing component is shortened. In addition, since the manufacturing cost also increases, Mo is set to 0.50% or less. It is preferably 0.48% or less, more preferably 0.45% or less.

Al: 0.005 to 0.100%
Al is an element that acts to deoxidize steel. If Al is less than 0.005%, the addition effect cannot be sufficiently obtained, so Al is set to 0.005% or more. It is preferably 0.010% or more, more preferably 0.015% or more. On the other hand, if Al exceeds 0.100%, coarse oxides are generated and the surface origin peeling life of the carburized bearing component is shortened. Therefore, Al is set to 0.100% or less. It is preferably 0.070% or less, more preferably 0.050% or less.

Ca: 0.0002 to 0.0010%
Ca is an element that dissolves in sulfide-based inclusions and acts to spheroidize sulfide-based inclusions. In addition, Ca increases the deformation resistance of sulfide-based inclusions at high temperatures, suppresses the elongation of sulfide-based inclusions during hot working, maintains the spherical shape, and prolongs the surface origin peeling life of carburized bearing parts. It is an element that makes up.

If Ca is less than 0.0002%, the addition effect cannot be sufficiently obtained, so Ca is set to 0.0002% or more. It is preferably 0.0003% or more, more preferably 0.0004% or more. On the other hand, if Ca exceeds 0.0010%, coarse oxides are generated and the surface origin peeling life of the carburized bearing component is shortened. Therefore, Ca is set to 0.0010% or less. It is preferably 0.0009% or less, more preferably 0.0008% or less.

N: 0.005 to 0.025%
N is an element that combines with Al, Nb, and / or Ti to form AlN, NbN, and / or TiN, which are effective for refining crystal grains, and contributes to the improvement of bending fatigue strength.

If N is less than 0.005%, the addition effect cannot be sufficiently obtained, so N is set to 0.005% or more. It is preferably 0.010% or more, more preferably 0.012% or more. On the other hand, if N exceeds 0.025%, coarse nitrides are generated and the toughness and bending fatigue strength are lowered, so N is set to 0.025% or less. It is preferably 0.022% or less, more preferably 0.020% or less.

O (oxygen): 0.0015% or less O (oxygen) is an element that forms an oxide and inhibits the strength, as well as the bending fatigue strength and surface origin peeling life of carburized bearing parts.

If O (oxygen) exceeds 0.0015%, the strength, bending fatigue strength of carburized bearing parts, and surface origin peeling life decrease, so O (oxygen) should be 0.0015% or less. It is preferably 0.0013% or less, more preferably 0.0010% or less. It is preferable that the amount of O (oxygen) is small, but if the amount of O (oxygen) is reduced to 0.0001% or less, the manufacturing cost increases significantly. Therefore, 0.0001% is a practical lower limit for practical steel.

(Selected element)
In the present embodiment, in addition to the above elements, the composition of the steel is, in terms of mass%, (a) Cu: 0.20% or less, Ni: 0.20% or less, and in order to improve the characteristics of the steel. , B: at least one of the element groups of 0.005% or less, and (b) Nb: 0.05% or less, and Ti: at least one of the element groups of 0.10%. It may be.

(a) Group element Cu: 0.20% or less Cu is an element that has the effect of enhancing hardenability. If Cu exceeds 0.20%, the hot workability is lowered and the steel cost is increased. Therefore, Cu is preferably 0.20% or less. More preferably, it is 0.16% or less. Cu is preferably 0.05% or more from the viewpoint of surely obtaining the effect of adding Cu. More preferably, it is 0.10% or more.

Ni: 0.20% or less Ni is an element that contributes to the improvement of toughness as well as the improvement of hardenability. Further, Ni is a non-oxidizing element, which acts to toughen the steel surface without increasing the layer thickness of the intergranular oxide layer during carburizing.

If Ni exceeds 0.20%, the addition effect is saturated and the steel cost increases. Therefore, Ni is preferably 0.20% or less. More preferably, it is 0.16% or less. Ni is preferably 0.05% or more in order to surely obtain the effect of adding Ni. More preferably, it is 0.10% or more.

B: 0.005% or less B is an element that not only enhances hardenability but also suppresses segregation of P and S at austenite grain boundaries during quenching. If B exceeds 0.005%, BN is generated and the toughness of the steel is lowered. Therefore, B is preferably 0.005% or less. More preferably, it is 0.003% or less. B is preferably 0.0003% or more in order to surely obtain the effect of adding B. More preferably, it is 0.0005% or more.

(b) Group element Nb: 0.05% or less Nb combines with C and / or N to form fine carbides, nitrides, and / or carbonitrides to refine the crystal grains. , An element that contributes to the improvement of bending fatigue strength.

If Nb exceeds 0.05%, the hot ductility is remarkably lowered, and defects are likely to occur on the steel surface during hot rolling or hot forging, and the toughness of the steel is lowered, so that Nb is 0. It is preferably 0.05% or less. More preferably, it is 0.02% or less. The Nb is preferably 0.005% or more from the viewpoint of surely obtaining the effect on the addition of Nb. More preferably, it is 0.008% or more.

Ti: 0.10% or less Ti is an element that forms fine carbides and the like to refine crystal grains and contribute to the improvement of steel strength. If Ti exceeds 0.10%, the toughness and bending fatigue strength of the steel decrease, so Ti is preferably 0.10% or less. More preferably, it is 0.08% or less. Ti is preferably 0.005% or more in order to surely obtain the effect of adding Ti. More preferably, it is 0.010% or more.

(Remaining)
Regarding the composition of the steel according to the present embodiment, the balance is Fe and impurities. Here, the impurity is an element that is inevitably mixed from the steel raw material (ore, scrap, etc.) and / or in the steelmaking process, and is an element that is allowed within a range that does not impair the characteristics of the steel according to the present embodiment. Specific examples thereof include Sb, Sn, W, Co, As, Mg, Pb, Bi, and H. In addition, Sb, Sn, W, Co, As, Mg, Pb, Bi, and H are 0.010%, 0.10%, 0.50%, and 0, respectively, in realizing the effects of the present application. It is acceptable to include up to 50%, 0.005%, 0.005%, 0.10%, 0.10%, and 0.0010%.

Next, the composition of the steel according to the present embodiment is defined by the following formula (1), and the sulfide-based inclusions of the steel according to the present embodiment are defined by the following formula (2). Fn2 will be described in detail.
In this specification, the sulfide-based inclusions are considered to be MnS, (Mn, Ca) S, CaS, and FeS. The abundance of FeS is very small. Consider FeS in the calculation.

Fn1: 0.25 to 0.65
In the composition of the steel according to the present embodiment, Fn1 defined by the following formula (1) is set to 0.25 to 0.65.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
The mass% of the element is introduced in the parentheses in the formula (1).

Fn1 is an index related to the layer thickness of the abnormal carburized layer. When Fn1 is less than 0.20 (the amount of Si is excessively large), the intergranular oxide layer and the like become thick. Further, when Fn1 exceeds 0.65 (the amount of Cr is excessively large), Cr is selectively oxidized by a trace amount of H ₂ O and / or CO ₂ in the carburized gas. Therefore, in each of these cases, the layer thickness of the abnormal carburized layer increases and the bending fatigue strength decreases, so that Fn1 is 0.20 or more and Fn1 is 0.65 or less. Fn1 is preferably 0.25 or more, more preferably 0.3 or more. Fn1 is preferably 0.60 or less, more preferably 0.55 or less.

Fn2: 0.50 to 1.00
For the sulfide-based inclusions of the steel of the present invention, Fn2 defined by the following formula (2) is set to 0.50 to 1.00.
Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide system containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of inclusions (μm ² )
A2: The equivalent circle diameter in the observation area with a total area of 4.0 mm ² is 1.0 μm or more.
Total area of sulfide-based inclusions (μm ² )

Fn2 (= A1 / A2) is an index related to the aspect ratio of sulfide-based inclusions after hot working. When Fn2 is less than 0.50, the proportion of sulfide-based inclusions having a large aspect ratio increases.

Sulfide-based inclusions with a large aspect ratio become fatigue starting points in the usage environment of carburized bearing parts after carburizing treatment, and hinder the peeling life from the surface origin. Therefore, the proportion of sulfide-based inclusions having a large aspect ratio is reduced. Therefore, Fn2 is 0.5 or more. Fn2 is preferably 0.55 or more, more preferably 0.60 or more. Fn2 is 1.00 or less by its definition.

Fn2 is obtained by the following method. The region 1 / 10d-7 / 16d from the surface of the cross section parallel to the rolling direction including the diameter of the rod-shaped or linear steel is set as the observation target region. Here, the diameter of the steel is shown as d.

The observation target area, which has a cross section parallel to the rolling direction, is mirror-polished with diamond to prepare the surface to be inspected. Sulfide-based inclusions on the surface to be inspected are identified by SEM (scanning electron microscope). Specifically, 100 arbitrary observation areas in the test surface are selected at a magnification of 500 times. That is, the observation region refers to an region in which an arbitrary region of the test surface whose observation target region is mirror-polished is observed at a magnification of 500 times. The total area of the observation area shall be at least 4.0 mm ² . The total area of the observation area may be more than 4.0 mm ² . It is sufficient that the test surface is created so that the total area of the observation area satisfies at least 4.0 mm ^2, and the size of the test surface itself is not particularly specified.

In each observation region, sulfide-based inclusions are identified based on the contrast of the reflected electron image observed by SEM. In the backscattered electron image, the observation area is displayed as a grayscale image. The contrasts of the Fe base material, sulfide-based inclusions, and oxide-based inclusions in the backscattered electron image are different.

The numerical range of the brightness (multiple gradations) indicating the sulfide-based inclusions is determined in advance by SEM and EDS (energy dispersive X-ray microanalyzer). Hereinafter, the numerical range determined in advance as the brightness indicating the sulfide-based inclusions is referred to as a reference range. In the observation area, the area where the brightness is within the reference range is determined. Hereinafter, the region whose brightness is within the reference range is referred to as a sulfide region.

FIG. 1 schematically shows an example of the brightness distribution of the SEM image in the observation area. In FIG. 1, the vertical axis is the area ratio (%) in the observation area, and the horizontal axis is the brightness. In FIG. 1, region R1 shows a region of oxide-based inclusions, region R2 shows a region of sulfide-based inclusions, and region R3 shows a region of Fe base material.

B1 to B2 in FIG. 1 are set as the reference range of brightness, and the region of the reference range B1 to B2 is selected from the observation area. FIG. 2 schematically shows an example of an SEM image in the observation area. In FIG. 2, the sulfide regions X1 to X4 are regions having the brightness of the reference ranges B1 to B2, and the regions correspond to regions of sulfide-based inclusions.

In FIG. 2, the regions Z1 to Z3 in the inclusions Y1 to Y3 correspond to the regions of the oxide-based inclusions. That is, inclusions Y1 to Y3 are composite inclusions composed of sulfide-based inclusions and oxide-based inclusions.

Next, the circle-equivalent diameters of the specified sulfide regions X1 to X4 are calculated. The circle-equivalent diameter is the diameter of a circle when the area of the sulfide region is converted into a circle having the same area. When calculating the equivalent circle diameter of the sulfide regions X1 to X4, the area of oxide-based inclusions (regions Z1 to Z3 in FIG. 2) existing in each sulfide region is excluded. In 100 observation regions (total area 4.0 mm ² ), the total area (μm ² ) of the sulfide region where the calculated equivalent circle diameter is 1.0 μm or more is defined as A2.

Next, the total area A1 of the sulfide-based inclusions containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide and having a circle equivalent diameter of 1.0 μm or more is obtained by the following method. Ask. In the above 100 observation regions (total area 4.0 mm ² ), sulfide regions having a circle equivalent diameter of 1.0 μm or more are quantitatively analyzed by EDS. Among the sulfide regions analyzed quantitatively, regions of sulfide-based inclusions containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide are specified.

When quantitatively analyzing Ca in sulfide-based inclusions by EDS, a semi-quantitative analysis method is used. In the observation region, not only a single sulfide-based inclusion is present, but also a composite inclusion containing a sulfide-based inclusion and an oxide-based inclusion is also present as described above.

It is assumed that the sulfide region identified by the SEM image is a sulfide-based inclusion of a composite inclusion. In this case, even if electrons are incident from the EDS device aiming at the sulfide-based inclusions, the incident electrons hit not only the sulfide-based inclusions but also the oxide-based inclusions adjacent to the sulfide-based inclusions. In some cases.

In such a case, the analysis result includes not only the analysis value of the sulfide-based inclusions but also the analysis value of the oxide-based inclusions. The oxide-based inclusions may be Ca oxides. To avoid this problem, a semi-quantitative measurement method is adopted. The semi-quantitative measurement method is as follows. The content shown below is mol%.

The S content and the Mn content in the sulfide-based inclusions measured by EDS quantitative analysis are compared. In the EDS quantitative analysis, the measurement was performed in the region where the entire inclusions enter for each inclusion, and the measurement was performed at a voltage of 5 kV and a beam diameter of 20 nm at a pitch of 100 nm.
(I) When the S content is Mn content or less Since S has a stronger binding force with Mn than Ca, S in the analyzed sulfide region is formed as MnS and does not contain Ca. .. That is, (Ca, Mn) S does not exist, and the area of the analyzed sulfide region is not included in A1 of equation (2).
The difference value Mn (hereinafter [Mn] *) obtained by subtracting the S content from the Mn content is calculated to be contained in the oxide-based inclusions.
[Mn] * = Mn content-S content ... Formula (A)

(Ii) When the S content exceeds the Mn content When the Ca content is greater than the [S] * amount in the following formula (B), the Ca corresponding to [S] * is designated as (Ca, Mn) S. It is calculated that it is contained in the sulfide region. The amount of [Ca] * in the following formula (C) forms an oxide as CaO. Therefore, [Ca] * is excluded from the number of moles of the analyzed sulfide region.
When the Ca content is less than the [S] * amount of the following formula (B), the [S] * amount of S is combined with Fe to form FeS. In this case, the Ca content is contained in the sulfide region as (Ca, Mn) S.
[S] * = S content-Mn content ... (B) formula [Ca] * = Ca content- [S] * ... (C) formula

By the above semi-quantitative measurement method, the Ca content in the sulfide region having a circle equivalent diameter of 1.0 μm or more is specified. Then, the total area (μm ² ) of the sulfide region containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide and having a circle equivalent diameter of 1.0 μm or more was obtained and obtained. The total area is defined as A1. When calculating A1, the area of oxide-based inclusions (regions Z1 to Z3 in FIG. 2) existing in the sulfide region is excluded from the calculation.

Fn2 is obtained using the total area A1 and the total area A2 calculated by the above method.
The steel of the present invention is a steel for carburized bearings. Usually, steel bars and wires are used as steel for carburized bearings. Generally distributed steel bars have a diameter of 16 mm to 200 mm and wire rods have a diameter of 4 mm to 20 mm. The steel according to the embodiment of the present invention may be defined as a steel bar having a diameter of 16 mm to 200 mm or a wire rod having a diameter of 4 mm to 20 mm.

<Steel manufacturing method>
Next, an example of a manufacturing method for manufacturing the steel of the present invention will be described.

Molten steel having the above composition and satisfying the above formula (1) is continuously cast into slabs. Ca is wire-added to the molten steel after Al addition and before insertion into the tundish. By adding Ca after adding Al, it becomes difficult to generate coarse Ca oxide, and by adding a wire to the molten steel before insertion into the tundish, the amount of coarse (Mn, Ca) S crystallized in the molten steel is reduced. Since it can be reduced and the presence of supersaturated solid-dissolved Ca makes it easier to crystallize fine (Mn, Ca) S during solidification, it becomes possible to satisfy the above formula (2). The fine CaO and CaS may be formed prior to the fine (Mn, Ca) S. The molten steel may be made into an ingot (steel ingot) by an ingot forming method.

Steel pieces are manufactured by hot working slabs or ingots. For example, by block rolling, the slab or ingot is made into a steel piece. Hot-worked steel pieces to produce steel for carburized bearings such as steel bars or wires. The hot working may be hot rolling or hot forging (hot forging or the like). The manufactured steel material for carburized bearings may be subjected to a normalizing treatment or a spheroidizing annealing treatment, if necessary. Steel for carburized bearings is manufactured by the above steps.

<Manufacturing method of parts>
An example of a method for manufacturing a part (for example, a carburized bearing) using the steel according to the present embodiment is as follows. That is, first, the steel according to the present embodiment is processed into a predetermined shape to produce an intermediate product. The processing method is, for example, machining typified by cutting.

Next, the intermediate product is carburized. The carburizing treatment may be carried out under well-known conditions. The quenching conditions and tempering conditions in the carburizing treatment are appropriately adjusted by a well-known method, and the surface hardness, surface C concentration, etc. of the parts are appropriately adjusted.

By the above steps, (carburized bearing) parts can be manufactured. The parts manufactured by the well-known carburizing treatment using the steel according to the present embodiment are excellent in hardenability, toughness, surface origin peeling life and bending fatigue strength.

The thickness of the carburized layer of the parts obtained by the method for manufacturing the parts according to the present embodiment is 0.5 to 2.0 mm from the surface. When the thickness of the carburized layer is 0.5 mm or more, the surface origin peeling life can be improved. On the other hand, in order to make it 2.0 mm or more, the carburizing time becomes long and the cost becomes high. The thickness of the carburized layer is preferably 0.5 to 2.0 mm.

The component thus obtained has an absorbed energy vE20 of 43 J / cm ² or more in the central portion thereof and has excellent toughness.
Since the shape of the part differs depending on the type of the part, it is difficult to uniformly define the central portion from the shape of the part. Therefore, the central portion is defined for the material after the carburizing treatment before the shape processing of the parts. The central portion means a range of 2 / 5T to 3 / 5T from the surface of the material after carburizing before shaping the part in a cross section parallel to the rolling direction. Here, T means the thickness of the material. When the parts are analyzed, it is possible to certify the central part.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

<Example 1>
[Making steel bars]
Molten steel having each component composition shown in Table 1 was produced in a 300 kg vacuum melting furnace and cast into an ingot. The ingot was heated at 1150 ° C. for 30 minutes and then hot forged so that the finishing temperature was 950 ° C. or higher to prepare a steel bar having a diameter of 60 mm.

A part of a steel bar having a diameter of 60 mm was cut, and the cut steel bar was hot forged to produce a steel bar having a diameter of 30 mm. These steel bars were held at 1250 ° C. for 12 hours and then allowed to cool to room temperature, further heated and held at 925 ° C. for 1 hour, and then allowed to cool to room temperature.

[Various evaluations using steel bars]
Using the steel bars after normalizing (diameter 60 mm and diameter 30 mm), inclusion evaluation test, hardenability evaluation test, toughness evaluation test, surface origin peeling life evaluation test, and rotary bending fatigue strength evaluation test, as shown below. Was done.

(Contains evaluation test)
The inclusion evaluation test was carried out by the following method. From a steel bar having a diameter of 30 mm, a position of 3.00 to 13.12 mm was observed from the surface of the surface parallel to the rolling direction of the steel bar. The observation surface parallel to the rolling direction was mirror-polished with diamond. The sulfide-based inclusions on the observation surface after mirror polishing were identified by the above method, and Fn2 (= A1 / A2) at each test number was determined. The results related to Fn2 are shown in Table 2 together with the calculation results of Fn1.

(Hardenability evaluation test)
The hardenability evaluation test was carried out by the following method. A Jominy test piece with a flange having a diameter of 25 mm and a length of 100 mm was produced from a steel bar having a diameter of 30 mm by machining. A jominy test conforming to JIS G 0561 (2011) was performed on the test pieces of each test number. The quenching temperature was 950 ° C., and the steel bars 1-32 were treated over 6 hours.

After the test, the hardness J ₁₁ at a position 11 mm from the water-cooled end was measured, and the hardenability was evaluated by the measured hardness J ₁₁ . The hardness test was measured under the condition of 150 kgf using a diamond cone indenter with a tip radius of 0.2 mm and a tip angle of 120 degrees. When the hardness J ₁₁ was 22 or more in Rockwell hardness HRC, it was judged that the hardenability was high (“pass” in Table 2). When the hardness J ₁₁ was less than 22 in Rockwell hardness HRC, it was judged that the hardenability was low (“Failed” in Table 2). The results are also shown in Table 2.

(Toughness evaluation test)
The toughness evaluation test was carried out by the following method. A steel bar having a diameter of 30 mm was subjected to a heat treatment for tempering the heat pattern shown in FIG. Specifically, a steel bar having a diameter of 30 mm was held at 900 ° C. for 4 hours, and then oil quenching was carried out (“OQ” in FIG. 3). The oil-quenched steel bars were further subjected to a tempering treatment of holding at 180 ° C. for 2 hours, and then air-cooled (“AC” in FIG. 3).

A Charpy test piece having a V notch was prepared from the steel bar subjected to the temper heat treatment so that the center of the surface on the V notch side in the width direction was at the position of 1 / 8D'. The Charpy impact test according to JIS Z 2242 (2009) was carried out at room temperature for the Charpy test pieces of each test number. Here, D'indicates the diameter of the steel bar that has undergone the heat treatment.

The absorbed energy obtained in the test was divided by the original cross-sectional area of the notch (the cross-sectional area of the notch of the test piece before the test) to obtain the impact value vE ₂₀ (J / cm ² ). When the impact value vE ₂₀ was 43 J / cm ² or more, it was judged that the toughness was high (“pass” in Table 2). When the impact value vE ₂₀ was less than 43 J / cm ² , it was judged that the toughness was low (“Fail” in Table 2). The results are also shown in Table 2.

(Surface origin peeling life evaluation test)
The surface origin peeling life evaluation test was carried out by the following method. A disk-shaped rough test piece having a diameter of 60 mm and a thickness of 5.5 mm was prepared from a steel bar having a diameter of 60 mm. The thickness of the rough test piece (5.5 mm) corresponds to the longitudinal direction of the steel bar.

For the rough test piece of each test number, after carburizing treatment at 950 ° C. in a gas atmosphere having a carbon equivalent of 0.8 mass% for 6 hours (carburizing condition A) or at 950 ° C. in a gas atmosphere having a carbon equivalent of 0.8 mass%. Carburizing treatment for 3 hours (carburizing condition B), quenching in oil at 60 ° C., immediately tempering at 150 ° C. for 1.5 hours and then allowing to cool, produced a test piece simulating a carburized bearing part. Next, the surface of the produced test piece was slid with free abrasive grains (abrasive), and a rolling fatigue test piece was obtained by performing a wrapping process in which the rolling contact surface was finely cut and polished. ..

A rolling fatigue test was carried out using a thrust type rolling fatigue tester. The maximum contact surface pressure at the time of the test was 5.0 GPa, and the repetition rate was 1800 cpm (cycle per minute). Gas atomizing powder was mixed as a foreign substance in the lubricating oil used at the time of the test. The gas atomized powder was made into a fine powder by gas atomizing using a high-speed steel having a Vickers hardness of 750 Hv, and classified into a particle size of 100 to 180 μm. The amount of gas atomizing powder mixed was 0.02% with respect to the lubricating oil. For the Vickers hardness, an arbitrary 5-point average value was used with a measured load of 10 kgf. As the steel ball used in the test, the tempered material of SUJ2 specified in JIS G 4805 (2008) was used.

The rolling fatigue test results were plotted on a Weibull probability paper, and the L10 life showing a 10% breakage probability was defined as the "surface origin peeling life". Harsh use environments of contamination in (this study), L10 life if 7.0 × 10 ⁵ or more, it is determined that excellent surface origin flaking life (Table 2 "pass"). If L10 life is less than 7.0 × 10 ^5, surface-originated flaking life is determined that a short (in Table 2, "fail"). The results are also shown in Table 2.

(Rotary bending fatigue strength evaluation test)
The rotary bending fatigue strength evaluation test was carried out by the following method. Ono-type rotary bending fatigue test pieces having parallel portions having diameters and lengths of 8 mm and 25 mm and shoulder radii of 12 mm were prepared from steel bars having a diameter of 30 mm. The longitudinal direction of the Ono-type rotary bending fatigue test piece corresponds to the longitudinal direction of the steel bar.

The Ono type rotary bending fatigue test piece of each test number is carburized, that is, after carburizing for 6 hours at 950 ° C. in a gas atmosphere having a carbon equivalent of 0.8 mass% (carburizing condition A) or a carbon equivalent of 0.8 mass%. Carburizing treatment at 950 ° C for 3 hours in a gas atmosphere (carburizing condition B), quenching in oil at 60 ° C, immediately tempering at 150 ° C for 1.5 hours and then allowing to cool, a test simulating carburized bearing parts Pieces were made.

Ono and rotary bending fatigue test number in the test are present each 7, by conventional methods, were tested at room temperature in air, "rotating bending the highest stress among that did not break up repeated several 1.0 × 10 ⁷ "Fatigue strength". When the rotational bending fatigue strength was 800 MPa or more, it was judged that the bending fatigue strength was excellent (“Pass” in Table 2). If the rotational bending fatigue strength was less than 800 MPa, it was judged that the bending fatigue strength was inferior (“Fail” in Table 2). The results are also shown in Table 2.

Then, all of the above test results (hardenability evaluation test, toughness evaluation test, surface origin peeling life evaluation test, and rotary bending fatigue strength evaluation test) are passed as a comprehensive evaluation of steel bars 1 to 17 as "pass". On the other hand, the steel bars 18 to 32 in which at least one of these test results failed was evaluated as "failed" as a comprehensive evaluation. The results are also shown in Table 2.

As is clear from Tables 1 and 2, the steel bars 1 to 17 having the components specified in the present application, having Fn1 of 0.25 to 0.65 and Fn2 of 0.50 to 1.00. It can be seen that excellent results are obtained in all of the hardenability evaluation test, the toughness evaluation test, the surface origin peeling life evaluation test, and the rotary bending fatigue strength evaluation test.

On the other hand, hardenability evaluation of steel bars 18 to 31 which do not satisfy at least one of the components specified in the present application and Fn1 (0.20 to 0.65) and Fn2 (0.50 to 1.00) specified in the present application is evaluated. It can be seen that excellent results have not been obtained in any of the test, the toughness evaluation test, the surface origin peeling life evaluation test, and the rotational bending fatigue strength evaluation test. The results of each comparative example are described below in detail.

The steel bar 18 has a low C concentration and a low hardenability (J ₁₁ ), so that the bending fatigue strength is low.

The toughness of the steel bar 19 is low due to the high C concentration.

As for the steel bar 20, the bending fatigue strength is low because the Si concentration is high.

The steel bar 21 has a low Mn concentration and a low hardenability (J ₁₁ ), so that the bending fatigue strength is low.

As for the steel bar 22, the bending fatigue strength is low because the Mn concentration is high.

Since the Cr concentration of the steel bar 23 is low, both the surface origin peeling life and the bending fatigue strength are low.

As for the steel bar 24, the bending fatigue strength is low because the Cr concentration is high.

Since the Mo concentration of the steel bar 25 is low, both the surface origin peeling life and the bending fatigue strength are low.

Since the Mo concentration of the steel bar 26 is high, the surface origin peeling life is short.

The toughness of the steel bar 27 is low due to the high Nb concentration.

As for the steel bars 28 and 29, the Ca concentration is low and Fn2 is low, so that the surface origin peeling life is short.

The bending fatigue strength of the steel bar 30 is low because Fn1 is low.

As for the steel bar 31, the bending fatigue strength is low because Fn1 is high.

The steel bar 32 had the components specified in the present application, Fn1 was 0.25 to 0.65, and Fn2 was 0.50 to 1.00, but the carburizing was insufficient, so that the surface origin peeling life and It can be seen that the bending fatigue strength is not obtained.

Claims (8)

Ingredient composition is mass%,
C: 0.10 to 0.30%,
Si: 0.01-0.25%,
Mn: 0.25 to 1.50%,
P: 0.001 to 0.015%,
S: 0.001 to 0.010%,
Cr: 0.50 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.005 to 0.100%,
Ca: 0.0002 to 0.0010%,
N: 0.005 to 0.025%,
O: 0.0015% or less,
Cu: 0-0.20%,
Ni: 0 to 0.20%
B: 0 to 0.005%
Nb: 0-0.05%
Ti: 0 to 0.10%
Remaining: Fe and impurities,
Fn1 defined by the following equation (1) is 0.25 to 0.65.
A steel characterized in that Fn2 defined by the following formula (2) is 0.50 to 1.00.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
[Element]: Mass% of element
Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide system containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of inclusions (μm ² )
A2: Total area of sulfide-based inclusions with a circular equivalent diameter of 1.0 μm or more (μm ² ) in the observation area with a total area of 4.0 mm ^2. The steel according to claim 1, wherein the component composition contains at least one of Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less in mass%. The steel according to claim 1 or 2, wherein the component composition contains at least one of Nb: 0.05% or less and Ti: 0.10% or less in mass%. The steel according to any one of claims 1 to 3, which is a steel bar. In a depth region of 500 μm or more from the surface
Ingredient composition is mass%,
C: 0.10 to 0.30%,
Si: 0.01-0.25%,
Mn: 0.25 to 1.50%,
P: 0.001 to 0.015%,
S: 0.001 to 0.010%,
Cr: 0.50 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.005 to 0.100%,
Ca: 0.0002 to 0.0010%,
N: 0.005 to 0.025%,
O: 0.0015%,
Cu: 0-0.20%,
Ni: 0 to 0.20%
B: 0 to 0.005%
Nb: 0-0.05%
Ti: 0 to 0.10%
Remaining: Fe and impurities,
Fn1 defined by the following equation (1) is 0.25 to 0.65.
Fn2 defined by the following equation (2) is 0.50 to 1.00.
A component characterized by excellent surface origin peeling life and bending fatigue strength.
Fn1 = 4.2 x [Cr] / (7.0 x [Si] + 16.0 x [Mn]) ... (1)
[Element]: Mass% of element
Fn2 = A1 / A2 ・ ・ ・ (2)
A1: A sulfide system containing 1.0 mol% or more of Ca with respect to the total number of moles in each sulfide in the observation region having a total area of 4.0 mm ² and having a circle equivalent diameter of 1.0 μm or more. Total area of inclusions (μm ² )
A2: Total area of sulfide-based inclusions with a circular equivalent diameter of 1.0 μm or more (μm ² ) in the observation area with a total area of 4.0 mm ^2. The component according to claim 5, wherein the component composition contains at least one of Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less in mass%. The component according to claim 5 or 6, wherein the component composition contains at least one of Nb: 0.05% or less and Ti: 0.10% or less in mass%. The component according to any one of claims 5 to 7, wherein the absorbed energy vE20 is 43 J / cm ² or more in the central portion.

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