JP7168003B2 - steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material, and more particularly to a steel material that is used as a material for carburized steel parts.

Steel materials that are used as materials for carburized mechanical structural parts (carburized steel parts) generally contain Mn, Cr, Mo, Ni, and the like. Steel materials that are used as materials for carburized steel parts are manufactured by casting, forging, rolling, and the like. Carburized steel parts are manufactured, for example, by the following method. Forging the above steel. An intermediate member is manufactured by cutting the forged steel material. A carburizing process is performed on the intermediate member. Through the above-described manufacturing process, a carburized steel component is manufactured that includes a carburized layer that is a hardened layer of the surface layer and a core that is a base material that is not affected by the carburizing treatment.

Machining costs are a significant part of the cost of manufacturing carburized steel parts. Machining also produces a large amount of chips, which reduces yield. For this reason, techniques for replacing cutting with forging are being studied. Forging methods can be broadly classified into hot forging, warm forging, and cold forging. Warm forging is performed in a lower temperature range than hot forging. Therefore, warm forging produces less scale than hot forging and improves dimensional accuracy more than hot forging. On the other hand, cold forging does not generate scale, and the dimensional accuracy is equivalent to cutting. Therefore, there are technologies such as cold forging after rough processing by hot forging, light cutting after warm forging, and molding by cold forging only. has been considered. However, when cutting is replaced with warm forging or cold forging, if the deformation resistance of the steel material is high, the surface pressure applied to the die increases, shortening the life of the die. Therefore, the cost merit by replacing cutting with forging is reduced. In addition, when molding into a complicated shape, problems such as cracks occurring in portions subjected to large processing occur. Therefore, when replacing cutting with forging, it is required to soften the steel material or to improve the limit workability of the steel material. When the steel material that is the raw material for the carburized steel part is as-rolled, the limit workability of the steel material can be increased by performing the spheroidizing heat treatment.

In Patent Document 1, at the stage of carburizing steel before carburizing treatment, the deformation resistance during cold forging is smaller than that of conventional steel, and the limit workability is larger. A carburizing steel with layer and core hardness is proposed. The carburizing steel described in Patent Document 1 has a chemical composition in mass% of C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001% to 0.80%, S: 0.0001% to 0.100%, Cr: more than 1.30% to 5.00%, B: 0.0005% to 0.0100%, Al: 0.0001% to 1 .0%, Ti: 0.010% to 0.10%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, the balance being Fe and Consisting of unavoidable impurities, the content of each element in the chemical composition expressed in mass% is expressed by the following formula 1 as a hardness index, the following formula 2 as a hardenability index, and the following formula as a TiC precipitation amount index. 3, are satisfied at the same time. 0.10<C+0.194×Si+0.065×Mn+0.012×Cr+0.078×Al<0.235 (Formula 1), 7.5<(0.7×Si+1)×(5.1× Mn+1)×(2.16×Cr+1)<44 (Formula 2), 0.004<Ti−N×(48/14)<0.030 (Formula 3). According to Patent Document 1, this carburizing steel has the above-described chemical composition, thereby increasing the limit workability during cold forging and obtaining a hardened layer and steel portion hardness equivalent to those of conventional steel after carburizing. is described in

International Publication No. 2012/108460

By the way, a plurality of carburized steel parts are used for mechanical structural parts used in automobiles. For example, variable diameter pulleys for continuously variable transmissions (CVTs) also utilize carburized steel components. Large carburized steel parts typified by variable diameter pulleys are manufactured by cutting after hot forging, as described above. Therefore, techniques for replacing cutting with forging are being studied even for large-sized carburized steel parts. However, if a large steel material is to be formed by cold forging, an excessive load is applied to the cold forging machine. Therefore, when forming a large carburized steel part by cold forging, a plurality of members are formed by cold forging, and then these members are joined by welding such as friction welding or laser welding, and then joined. A technique for manufacturing large-sized carburized steel parts by carburizing a steel member is being studied.

When manufacturing carburized steel parts by welding in this way, the fatigue strength (joint fatigue strength) of the carburized steel parts, which are joining materials, is required.

An object of the present disclosure is to provide a steel material that exhibits excellent fatigue strength after carburizing treatment even if welding is performed.

The steel according to the present disclosure is
in % by mass,
C: 0.09 to 0.16%,
Si: 0.01 to 0.50%,
Mn: 0.40-0.60%,
P: 0.030% or less,
S: 0.025% or less,
Cr: 0.90 to 2.00%,
Mo: 0.10-0.40%,
Al: 0.005-0.030%,
Ti: less than 0.010 to 0.050%,
Nb: 0.010 to 0.030%,
N: 0.0080% or less,
O: 0.0030% or less,
B: 0.0003 to 0.0030%,
Contains Ca: 0.0005 to 0.0050%,
The remainder consists of Fe and impurities, satisfying formulas (1) to (3),
In the cross section parallel to the axial direction of the steel material, 70% by mass of Mn sulfide having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% .0 pieces/mm ² or less, and the number of oxides having an O content of 10.0% or more in mass% is 25.0 pieces/mm ² or less.
0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 Formula (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 Formula (2 )
0.004<Ti-N×(48/14)<0.030 Formula (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (3). If the corresponding element is not contained, "0" is substituted for the element symbol.

The steel according to the present disclosure provides excellent fatigue strength in carburized steel components after carburizing, even when welded.

FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of the steel material of this embodiment. FIG. 2 is a schematic diagram for explaining the sampling positions in the microstructure observation of the steel material of this embodiment. FIG. 3 is a schematic diagram for explaining sample collection positions when measuring Mn sulfides and oxides in this embodiment.

The steel material according to the present embodiment, which is the material for the carburized steel parts, will be described below.

The inventors of the present invention conducted studies to obtain excellent properties (improved effective case depth and core hardness) of carburized steel parts after carburizing treatment in steel materials that are used as raw materials for carburized steel parts. As a result, the inventors obtained the following findings (a) to (f).

(a) The lower the C content, the softer the steel material before cold forging can be. However, if the C content is too low, the fatigue strength of the carburized steel part after carburizing treatment will be reduced to that of conventional steel materials with a C content of about 0.20% (for example, SCR420 specified in JIS G 4052 (2008) ) will be difficult to reach the same level as In order to obtain the hardness of the core required for carburized steel parts, there is an appropriate lower limit for the C content.

(b) In order to obtain as large an effective case depth and core hardness as possible with a low C content in a carburized steel part, it is desirable to increase the martensite fraction in the microstructure of the core of the carburized steel part. preferable.

(c) In order to increase the martensite fraction of the microstructure at the core of the carburized steel part, the inclusion of alloying elements (hardenability improving elements) that improve the hardenability of steel such as Mn, Cr, Mo and Ni It is effective to contain the amount so as to satisfy the above hardenability index formula (2).
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)

(d) However, when the content of the hardenability improving element increases, the hardenability improving element solid solution strengthens the ferrite. In this case, the hardness of steel material will increase. Therefore, it contains B that can improve the hardenability of the steel material while suppressing the increase in the hardness of ferrite, and the content of C and the hardenability improving element is such that the content of the above-mentioned hardness index formula (1) is satisfied. to
0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)

(e) In order to stably obtain the hardenability-enhancing effect of B, it is necessary to secure a sufficient amount of dissolved B in the steel material during the carburizing treatment. Therefore, the steel is made to contain Ti. In this case, most of the N in the steel is fixed as TiN during the carburizing process. As a result, it is possible to prevent B from bonding with N and precipitating as BN, and to ensure a sufficient amount of dissolved B in the steel material. In order to effectively obtain this effect, it is preferable to contain Ti in a stoichiometric excess with respect to the N content. Furthermore, in order to prevent abnormal grain growth of austenite grains during carburizing, TiC is finely dispersed and precipitated in the microstructure. In order to secure a sufficient amount of B in solid solution and precipitate TiC finely and dispersedly, the contents of Ti and N are made to satisfy the above-mentioned formula (3) of the TiC precipitation amount index.
0.004<Ti-N×(48/14)<0.030 (3)

(f) B effectively enhances the hardenability of the core of the carburized steel part. However, when gas carburizing is performed by the gas carburizing method, the effect of improving the hardenability due to the inclusion of B is low in the carburized layer, which is the surface layer of the carburized steel part. This is because, during the carburizing treatment, nitrogen enters from the surface of the steel part, bonds with solid solution B and precipitates as BN, and the amount of solid solution B decreases. Therefore, in order to ensure the hardenability of the carburized layer, which is the surface layer of the carburized steel part, it is effective to satisfy the hardenability index formula (2) described in (c) above.

The present inventors further examined the fatigue strength (joint fatigue strength) of carburized steel parts manufactured by carburizing the steel material of the present embodiment after welding. As a result, the fatigue strength ( It was found that the joint fatigue strength) increased.
(I) 70.0 pieces/mm ² or less of Mn sulfide having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% in terms of mass% to
(II) The number of oxides having an O content of 10.0% or more is 25.0/mm ² or less in mass %.
This point will be described in detail below.

In the steel material having the chemical composition of this embodiment, Mn sulfides and oxides are present in the steel material. Here, in this specification, Mn sulfides and oxides are defined as follows.
Mn sulfide: Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in mass% when the mass% of inclusions is 100% Certain inclusions Oxide: inclusions having an oxygen content of 10.0% or more in mass% when the mass% of the inclusions is 100%. Inclusions containing 10.0% or more S, 10.0% or more Mn, and 10.0% or more O are included in "oxides" rather than "Mn sulfides" .

When a carburized steel part is manufactured by carrying out carburizing treatment after performing welding represented by friction welding, laser joining, etc. on steel materials, a HAZ region exists in the carburized steel part. The intensity of the HAZ region may be lower than the intensity of other regions. Therefore, in this embodiment, inclusions are reduced as much as possible in order to ensure the strength of the HAZ region. In this embodiment, as described in (I) and (II) above, the number of Mn sulfides and oxides, which account for most of the inclusions in the steel, is reduced. In this case, the strength of the HAZ region can be ensured, and as a result, the fatigue strength of the carburized steel part can be increased.

The steel material of this embodiment completed based on the above findings has the following configuration.

[1]
is steel,
in % by mass,
C: 0.09 to 0.16%,
Si: 0.01 to 0.50%,
Mn: 0.40-0.60%,
P: 0.030% or less,
S: 0.025% or less,
Cr: 0.90 to 2.00%,
Mo: 0.10-0.40%,
Al: 0.005-0.030%,
Ti: less than 0.010 to 0.050%,
Nb: 0.010 to 0.030%,
N: 0.0080% or less,
O: 0.0030% or less,
B: 0.0003 to 0.0030%,
Contains Ca: 0.0005 to 0.0050%,
The remainder consists of Fe and impurities, satisfying formulas (1) to (3),
In the cross section parallel to the axial direction of the steel material, 70% by mass of Mn sulfide having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% .0 pieces/mm ² or less, and the number of oxides having an O content of 10.0% or more in mass% is 25.0 pieces/mm ² or less,
steel.
0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.004<Ti-N×(48/14)<0.030 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (3). If the corresponding element is not contained, "0" is substituted for the element symbol.

[2]
The steel material according to [1],
When the radius of the cross section perpendicular to the axial direction of the steel material is defined as R (mm), in the microstructure of the cross section perpendicular to the axial direction of the steel material, bainite in the surface layer region at least from the surface to a depth of 0.1R The area ratio of is 95.0% or more,
steel.

[3]
The steel material according to [1],
When the radius of the cross section perpendicular to the axial direction of the steel material is defined as R (mm), in the microstructure of the cross section perpendicular to the axial direction of the steel material, at least the surface layer region from the surface to a depth of 0.1R is ferrite and cementite, and the spheroidization rate of the cementite in the surface layer region is 90.0% or more.
steel.

[4]
The steel material according to any one of [1] to [3],
Instead of part of the Fe, in mass %,
Cu: 0.50% or less,
Ni: 0.30% or less, and
V: 0.10% or less,
containing one or more elements selected from the group consisting of
steel.

The steel material according to this embodiment will be described below. "%" in relation to chemical composition means % by weight unless otherwise specified.

[Chemical composition]
The steel material of this embodiment is a material for carburized steel parts. The steel material of this embodiment is cold forged and then carburized to form a carburized steel part. The chemical composition of the steel material of this embodiment contains the following elements.

C: 0.09-0.16%
Carbon (C) enhances the hardenability of the steel material and enhances the hardness of the core in a carburized steel part having a carburized layer and a core. If the C content is less than 0.09%, the hardness of the core of the carburized steel part is lowered even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.16%, even if the contents of other elements are within the range of the present embodiment, the hardness of the steel material before cold forging significantly increases, and the limit working rate is reduced. descend. Therefore, the C content is 0.09-0.16%. Incidentally, the C content of the steel material used as the raw material for conventional carburized steel parts is about 0.20%. Therefore, the C content of the steel material of this embodiment is lower than that of the conventional steel material. A preferable lower limit of the C content is 0.10%, more preferably 0.11%. A preferable upper limit of the C content is 0.15%, more preferably 0.14%.

Si: 0.01-0.50%
Silicon (Si) increases the temper softening resistance of carburized steel parts and increases the surface fatigue strength of carburized steel parts. If the Si content is less than 0.01%, the above effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the hardness of the steel material before cold forging increases and the critical workability decreases even if the content of other elements is within the range of the present embodiment. . Therefore, the Si content is 0.01-0.50%. When emphasizing the surface fatigue strength of carburized steel parts, the preferred lower limit of the Si content is 0.02%. When emphasizing the improvement of the critical workability of carburized steel parts, the preferred upper limit of the Si content is 0.48%, more preferably 0.46%.

Mn: 0.40-0.60%
Manganese (Mn) increases the hardenability of steel materials and increases the strength of the core of carburized steel parts. If the Mn content is less than 0.40%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.60%, the hardness of the steel material before forging increases and the critical workability decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Mn content is 0.40-0.60%. A preferred lower limit for the Mn content is 0.42%, more preferably 0.44%. A preferable upper limit of the Mn content is 0.58%, more preferably 0.56%.

P: 0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is over 0%. P segregates at the austenite grain boundaries, embrittles the prior austenite grain boundaries, and causes intergranular cracking. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.026%, more preferably 0.024%. The lower the P content is, the better. However, if the P content is reduced to the limit, the productivity will decrease and the manufacturing cost will increase. Therefore, in normal operation, the preferable lower limit of P content is 0.001%.

S: 0.025% or less Sulfur (S) is inevitably contained. That is, the S content is over 0%. S combines with Mn to form MnS and enhances the machinability of the steel material. If the S content exceeds 0%, this effect can be obtained to some extent. On the other hand, if the S content exceeds 0.025%, even if the content of other elements is within the range of the present embodiment, coarse MnS is formed, cracks are likely to occur during forging, and the limit of steel materials Machining rate decreases. Therefore, the S content is 0.025% or less. A preferable upper limit of the S content is 0.022%, more preferably 0.020%. In order to more effectively improve the machinability, the lower limit of the S content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%.

Cr: 0.90-2.00%
Chromium (Cr) enhances the hardenability of steel materials and enhances the strength of the core of carburized steel parts. If the Cr content is less than 0.90%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 2.00%, the hardness of the steel material before forging increases and the critical workability decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Cr content is 0.90-2.00%. A preferable lower limit of the Cr content is 0.95%, more preferably 1.00%, and still more preferably 1.10%. A preferable upper limit of the Cr content is 1.95%, more preferably 1.92%.

Mo: 0.10-0.40%
Molybdenum (Mo) increases the hardenability of steel materials and increases the strength of the core of carburized steel parts. If the Mo content is less than 0.10%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 0.40%, the hardness of the steel material before forging increases and the critical workability decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0.10-0.40%. A preferable lower limit of the Mo content is 0.11%, more preferably 0.12%, and still more preferably 0.13%. A preferable upper limit of the Mo content is 0.38%, more preferably 0.36%, and still more preferably 0.34%.

Al: 0.005-0.030%
Aluminum (Al) deoxidizes steel in the steelmaking process. Al also forms AlN when solute N is present in the steel. However, in the steel material according to this embodiment, N in the steel is fixed as TiN by addition of Ti. Therefore, almost no dissolved N exists in the steel material. As a result, Al does not form AlN, but exists as solid solution Al in the steel material. Al existing in a solid solution state enhances the machinability of the steel material. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.030%, the hardness of the steel material before forging increases and the critical workability decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Al content is 0.005-0.030%. A preferable lower limit of Al is 0.010%, more preferably 0.011%, and still more preferably 0.012%. A preferable upper limit of Al is 0.025%, more preferably 0.022%, and still more preferably 0.020%.

Ti: 0.010 to less than 0.050% Titanium (Ti) fixes N in the steel material as TiN and suppresses the formation of BN. As a result, Ti secures the amount of solid solution B and enhances the hardenability of the steel material. Ti further forms TiC to suppress coarsening of crystal grains during carburizing. If the Ti content is less than 0.010%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ti content is 0.050% or more, the precipitation amount of TiC is excessively increased even if the content of other elements is within the range of the present embodiment. In this case, the critical workability of the steel material before cold forging is lowered. Therefore, the Ti content is between 0.010 and less than 0.050%. The lower limit of the Ti content is preferably 0.012%, more preferably 0.014%, still more preferably 0.016%, still more preferably 0.018%. The upper limit of the Ti content is preferably 0.048%, more preferably 0.046%, still more preferably 0.044%, still more preferably 0.042%, still more preferably 0.040 %.

Nb: 0.010-0.030%
Nb (niobium) combines with N and C in steel to form Nb carbonitrides. Nb carbonitride suppresses coarsening of crystal grains due to the pinning effect. If the Nb content is less than 0.010%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Nb content exceeds 0.030%, the effect is saturated. Therefore, the Nb content is 0.010-0.030%. The lower limit of the Nb content is preferably 0.011%, more preferably 0.012%, still more preferably 0.013%, still more preferably 0.014%. The preferred upper limit of the Nb content is 0.029%, more preferably 0.028%, still more preferably 0.027%, still more preferably 0.026%, still more preferably 0.025 %.

N: 0.0080% or less Nitrogen (N) is an unavoidable impurity. That is, the N content in the steel is over 0%. N combines with B to form BN and reduces the amount of dissolved B. In this case, the hardenability of the steel deteriorates. If the N content exceeds 0.0080%, even if Ti is contained in an amount of less than 0.010 to 0.050%, the N in the steel cannot be fixed as TiN, and a solid that contributes to hardenability. It becomes difficult to secure molten B. Furthermore, coarse TiN is formed. Coarse TiN becomes a starting point of cracks during forging, and lowers the critical workability of the steel material before forging. Therefore, the N content is 0.0080% or less. A preferable upper limit of the N content is 0.0078%, more preferably 0.0076%, still more preferably 0.0074%, still more preferably 0.0072%. N content is preferably as low as possible. However, if the N content is reduced to the limit, the productivity will decrease and the manufacturing cost will increase. Therefore, in normal operation, the preferred lower limit of the N content is 0.0001%, more preferably 0.0010%, still more preferably 0.0020%.

O: 0.0030% or less Oxygen (O) is an unavoidable impurity. That is, the O content in the steel is over 0%. O forms an oxide, which reduces the bondability when welding steel materials before carburizing treatment. In this case, the fatigue strength of the carburized steel part is reduced. Therefore, the O content is 0.0030% or less. The upper limit of the O content is preferably 0.0029%, more preferably 0.0028%, still more preferably 0.0026%, still more preferably 0.0024%, still more preferably 0.0022 %. A lower O content is preferable. However, if the O content is reduced to the limit, the productivity will decrease and the manufacturing cost will increase. Therefore, in normal operations, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

B: 0.0003 to 0.0030%
Boron (B) increases the hardenability of steel and increases the strength of carburized steel parts. If the B content is less than 0.0003%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the B content exceeds 0.0030%, the above effects are saturated. Therefore, the B content is 0.0003-0.0030%. The lower limit of the B content is preferably 0.0004%, more preferably 0.0005%, still more preferably 0.0006%, still more preferably 0.0007%. A preferable upper limit of the B content is 0.0028%, more preferably 0.0026%, and still more preferably 0.0024%.

Ca: 0.0005-0.0050%
Calcium (Ca) is contained in the oxide and spheroidizes the oxide. Spherical oxides are less likely to form clusters. Ca further suppresses the stretching of Mn sulfide. If the Ca content is less than 0.0005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ca content exceeds 0.0050%, it forms coarse sulfides and coarse oxides, which reduces the fatigue strength of carburized steel parts. Therefore, the Ca content is 0.0005-0.0050%. The lower limit of the Ca content is preferably 0.0006%, more preferably 0.0007%, still more preferably 0.0008%, still more preferably 0.0009%, still more preferably 0.0010 % and is. The upper limit of the Ca content is preferably 0.0048%, more preferably 0.0046%, still more preferably 0.0040%, still more preferably 0.0035%.

Balance: Fe and Impurities The balance of the chemical composition of the steel according to the present embodiment consists of Fe and impurities. Here, the term "impurities" refers to components that are mixed in from raw materials such as ores, scraps, or the manufacturing environment when steel materials are manufactured industrially, and that are not intentionally included in steel materials. means.

[Regarding arbitrary elements]
The chemical composition of the steel material of the present embodiment may contain one or more selected from the group consisting of Cu, Ni, and V in place of part of Fe. Both of these elements increase the strength of carburized steel parts.

Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content exceeds 0%, Cu enhances the hardenability of the steel material and enhances the strength of the carburized steel part. Cu is an element that does not form oxides or nitrides in the gas atmosphere of gas carburizing. Therefore, when Cu is contained, it becomes difficult to form an oxide layer or a nitride layer on the surface of the carburized layer, or an abnormal carburized layer resulting therefrom. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.50%, the ductility of the steel material in a high temperature range of 1000° C. or higher decreases even if the content of other elements is within the range of the present embodiment. In this case, the yield during continuous casting and rolling decreases. Furthermore, the hardness of the steel material before forging increases, and the limit workability decreases. Therefore, the Cu content is 0.50% or less. That is, the Cu content is 0-0.50%. The lower limit of the Cu content is preferably over 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%. A preferable upper limit of the Cu content is 0.45%, more preferably 0.40%, more preferably 0.35%.

Ni: 0.30% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni enhances the hardenability of the steel material and enhances the strength of the carburized steel part. If Ni is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.30%, the hardness of the steel material before forging increases and the limit workability decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0.30% or less. That is, the Ni content is 0-0.30%. A preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and still more preferably 0.05%. A preferable upper limit of the Ni content is 0.29%, more preferably 0.28%, and still more preferably 0.25%.

V: 0.10% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is included, ie when the V content is greater than 0%, V forms carbides to increase the strength of the core of the carburized steel component. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.10%, the cold forgeability of the steel deteriorates and the critical workability decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the V content is 0.10% or less. That is, the V content is 0-0.10%. A preferable lower limit of the V content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. A preferable upper limit of the V content is 0.09%, more preferably 0.08%.

[Regarding formulas (1) to (3)]
The chemical composition of the steel material of this embodiment further satisfies the following formulas (1) to (3).
0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 Formula (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 Formula (2 )
0.004<Ti-N×(48/14)<0.030 Formula (3)
Here, the contents (% by mass) of the corresponding elements are substituted for the element symbols in formulas (1) to (3). If the corresponding element is an optional element and is not contained, "0" is substituted for the element symbol. Each formula will be described below.

[Formula (1): hardness index]
Define F1=C+0.194*Si+0.065*Mn+0.012*Cr+0.033*Mo+0.067*Ni+0.097*Cu+0.078*Al. F1 is an index of hardness of a carburized steel part manufactured using steel as a raw material.

The steel material of the present embodiment has a low C content of 0.16% or less. Therefore, the structure of the steel material before forging has a significantly increased ferrite fraction than the conventional steel material having a C content of about 0.20%. In this case, the hardness of the steel material is greatly affected not only by the C content (pearlite fraction) but also by the hardness of ferrite. F1 indicates the contribution of each alloying element to solid-solution strengthening of ferrite.

If F1 is 0.235 or more, the hardness of the steel material before cold forging is too high. In this case, the critical workability of the steel material is lowered. On the other hand, if F1 is 0.140 or less, the hardness of the core portion as a carburized steel part is insufficient. Therefore, F1 is greater than 0.140 and less than 0.235. F1 is preferably as low as possible within a range that satisfies the hardenability index (F2) described later. The upper limit of F1 is preferably less than 0.230, more preferably 0.225, still more preferably 0.220, still more preferably 0.215, still more preferably 0.210. Note that the F1 value is a value obtained by rounding off the calculated value to the fourth decimal place.

[Formula (2): hardenability index]
Define F2=(0.70*Si+1)*(5.1*Mn+1)*(2.2*Cr+1)*(3.0*Mo+1)*(0.36*Ni+1). F2 is an index relating to the hardenability of steel materials.

As described above, B is effective in increasing the hardenability of steel materials and increasing the hardness of the core of carburized steel parts. On the other hand, when gas carburizing (transformation furnace gas method) is performed as the carburizing treatment, the hardenability improvement effect due to the addition of B is low in the carburized layer, which is the surface layer of the carburized steel part. This is because N in the atmosphere gas in the furnace penetrates into the surface layer of the carburized steel part during the carburizing process, and solid solution B precipitates as BN, and the amount of solid solution B that contributes to the improvement of hardenability is insufficient. . Therefore, when gas carburizing treatment is performed, B can increase the hardness of the core of the carburized steel part, but does not contribute to the improvement of the hardness of the carburized layer of the carburized steel part. Therefore, in order to secure the hardenability of the carburized layer, which is the surface layer of the carburized steel part, it is necessary to use a hardenability improving element other than B.

F2 is composed of elements other than B that contribute to the improvement of hardening. When F2 is 13.0 or less, the carburized layer depth (Vickers hardness is HV 550 depth) cannot be sufficiently obtained. On the other hand, if F2 is 45.0 or more, the hardness of the steel material before cold forging increases, and the limit workability decreases. Therefore, F2 is greater than 13.0 and less than 45.0. F2 is preferably as large as possible within a range that satisfies the hardness index F1. The preferred lower limit of F2 is 13.2, more preferably 13.5, still more preferably 14.0, still more preferably 14.5, still more preferably 15.0. Note that the F2 value is a value obtained by rounding off the calculated value to the second decimal place.

[Formula (3): TiC precipitation amount index]
Define F3=Ti−N×(48/14). F3 is an index relating to the TiC precipitation amount. When Ti is stoichiometrically excessive with respect to N, all N is fixed as TiN. That is, F3 means an excess amount of Ti other than the amount of Ti consumed to form TiN. "14" in F3 is the atomic weight of N, and "48" is the atomic weight of Ti.

Most of the excess Ti defined by F3 is combined with C to form TiC during carburizing. This TiC has a pinning effect that prevents coarsening of crystal grains during carburizing. If the content of each element in the chemical composition of the steel satisfies the numerical range of the above-described embodiment and F3 is 0.004 or less, the amount of TiC precipitated is insufficient. In this case, coarsening of crystal grains during carburizing cannot be suppressed. As a result, the toughness of the carburized steel part is lowered, and the amount of deformation of the steel material after carburizing treatment is increased. On the other hand, if the content of each element in the chemical composition of the steel satisfies the numerical range of the above-described embodiment and F3 is 0.030 or more, the amount of TiC precipitated is too large, and The hardness of the steel material increases, and the critical workability decreases. Therefore, F3 is greater than 0.004 and less than 0.030. A preferred lower limit for F3 is 0.006, more preferably 0.008. A preferred upper limit for F3 is 0.028, more preferably 0.025. Note that the F3 value is a value obtained by rounding off the calculated value to the fourth decimal place.

A steel material whose chemical composition simultaneously satisfies the hardness index F1, the hardenability index F2, and the TiC precipitation amount index F3 is subjected to spheroidizing heat treatment, so that the limit workability during cold forging is higher than that of conventional steel. growing. After the carburizing treatment of this steel material, a carburized steel part having a hardened layer and core hardness equivalent to those of conventional steel can be obtained.

[Regarding inclusions in steel]
Further, in the steel material of the present embodiment, Mn sulfides and oxides in the steel satisfy the following conditions in a cross section parallel to the axial direction of the steel material (that is, the longitudinal direction of the steel material).
(I) Mn sulfide having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% by mass% is 70.0 pieces/mm ² or less be.
(II) The number of oxides having an oxygen content of 10.0% or more in mass % is 25.0/mm ² or less.

Here, in this specification, Mn sulfides and oxides are defined as follows.
Mn sulfide: Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in mass% when the mass% of inclusions is 100% Certain inclusions Oxide: inclusions with an O content of 10.0% or more in mass% when the mass% of inclusions is taken as 100%

As described above, in the manufacturing process of carburized steel parts, when an intermediate member before carburization treatment is manufactured by joining a plurality of steel members by welding such as friction welding or laser welding, the intermediate member is carburized. A HAZ region exists in the resulting carburized steel part. The HAZ region may have lower joint fatigue strength than other regions. Inclusions in the steel material are reduced as much as possible in order to secure the joint fatigue strength in the HAZ region. If the Mn sulfides and oxides satisfy (I) and (II) above, the joint fatigue strength in the HAZ region can be ensured. As a result, the joint fatigue strength of the carburized steel parts integrated by joining can be increased.

[Method for measuring Mn sulfides and oxides]
The number of Mn sulfides and the number of oxides in steel can be measured by the following methods. A sample is taken from the steel material. Specifically, as shown in FIG. 3, a sample is taken from a position R/2 (where R is the radius of the steel bar) in the radial direction from the center axis C1 of the steel material 1 . The size of the observation surface of the sample is L1×L2, where L1 is 10 mm and L2 is 5 mm. Furthermore, the sample thickness L3 in the direction perpendicular to the viewing plane is set to 5 mm. The normal N of the viewing plane is perpendicular to the central axis C1 (that is, the viewing plane is parallel to the axial direction of the steel material), and the R/2 position is approximately the central position of the viewing plane.

The observed surface of the collected sample is mirror-polished, and 20 fields of view (evaluation area per field of view of 100 μm×100 μm) are randomly observed at a magnification of 1000 using a scanning electron microscope (SEM).

Identify inclusions in each field of view. For each identified inclusion, energy dispersive X-ray spectroscopy (EDX) is used to identify Mn sulfides and oxides. Specifically, using EDX, each inclusion is subjected to elemental analysis at at least two measuring points. Then, in each inclusion, the arithmetic mean value of the element content obtained at each measurement point is defined as the content (mass %) of each element in the inclusion. For example, when elemental analysis is performed at two measurement points in one inclusion, the arithmetic average value of the Mn content, the arithmetic average value of the S content, and the arithmetic average value of the O content obtained at the two measurement points is defined as the Mn content (mass%), S content (mass%), and O content (mass%) in the inclusion.

In the elemental analysis results of the specified inclusions, Mn sulfurization of inclusions having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% by mass% defined as things. Note that Ti and Ca may also be detected as elements other than Mn and S in the elemental analysis of inclusions. Also in this case, if the above conditions are satisfied, all are defined as Mn sulfides. Also, inclusions having an O content of 10.0% or more in the results of elemental analysis of the identified inclusions are defined as oxides. Al, Si, Mg, Ca, Ti and the like may be detected in inclusions defined as oxides. In this case as well, if the above conditions are met, it is identified as an oxide. Among the inclusions, inclusions containing 10.0% or more of S, 10.0% or more of Mn, and 10.0% or more of O are identified as oxides.

The inclusions to be identified are those having an equivalent circle diameter of 0.5 μm or more. Here, the equivalent circle diameter means the diameter of a circle when the area of each inclusion is converted into a circle having the same area.

If the inclusion has an equivalent circle diameter of at least twice the diameter of the EDX beam, the accuracy of the elemental analysis is enhanced. In this embodiment, the EDX beam diameter used to identify inclusions is 0.2 μm. In this case, inclusions with an equivalent circle diameter of less than 0.5 μm cannot improve the accuracy of elemental analysis by EDX. Inclusions with an equivalent circle diameter of less than 0.5 μm further have a very small effect on fatigue strength. Therefore, in the present embodiment, Mn sulfides and oxides having an equivalent circle diameter of 0.5 μm or more are to be measured. Although the upper limit of the equivalent circle diameter of Mn sulfide and oxide is not particularly limited, it is, for example, 100 μm.

Based on the total number of Mn sulfides identified in each field of view and the total area of the 20 fields of view, the number of Mn sulfides per unit area (number/mm ² ) is determined. Also, based on the total number of oxides specified in each field of view and the total area of the 20 fields of view, the number of oxides per unit area (pieces/mm ² ) is obtained.

In the steel material of the present embodiment, the content of each element in the chemical composition is within the above range, the hardness index F1 satisfies the formula (1), the hardenability index F2 satisfies the formula (2), TiC precipitation amount index F3 satisfies equation (3). Furthermore, in a cross section parallel to the axial direction of the steel material, Mn sulfide having a Mn content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% by mass% 70.0 pieces/mm ² or less, and oxides having an O content of 10.0% or more in mass% are 25.0 pieces/mm ² or less. Therefore, even if welding is performed before carburizing, the carburized steel part after carburizing has excellent fatigue strength.

[Microstructure of steel]
The microstructure of the steel material of this embodiment is not particularly limited. The steel material of this embodiment may be an as-rolled material (that is, an As-rolled material) or may be spheroidized.

The radius of the cross section perpendicular to the axial direction (longitudinal direction) of the steel material of this embodiment is defined as R (mm). The microstructure in the cross section perpendicular to the axial direction of the steel material is either (A) or (B) below.
(A) In the microstructure, the area ratio of bainite is 95.0% or more in at least the surface region from the surface to the depth of 0.1R.
(B) In the microstructure, the surface layer region at least from the surface to the depth of 0.1R is composed of ferrite and cementite, and the cementite spheroidization rate in the surface layer region is 90.0% or more.
The microstructure of (A) above is a microstructure when the steel material of the present embodiment is an as-rolled material. The microstructure of (B) above is the microstructure when the steel material of the present embodiment is spheroidized.

FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction (axial direction) of the steel material of this embodiment. In FIG. 1, the radius of the steel material 1 is defined as R (mm). At this time, the area from the surface of the steel material 1 to the 0.1R depth D is defined as a "surface layer area". That is, the depth D (mm) means 10% of the radius R.

When the steel material of the present embodiment is an as-rolled material, as described in (A), at least the surface layer region has a bainite structure in a cross section perpendicular to the axial direction of the steel material. Here, in the present specification, "having a bainite structure" means having a bainite area ratio of 95.0% or more. That is, in the cross section perpendicular to the axial direction of the steel material 1 of this embodiment, at least the surface layer region D has a bainite area ratio of 95.0% or more. Moreover, "at least the surface layer region has a bainite structure" means that the bainite region may be formed not only in the surface layer region but also in a region deeper than the surface layer region. Specifically, in FIG. 1, the depth of the bainite structure from the surface is at least 0.1R, and the depth of the bainite structure may be deeper than 0.1R. For example, the depth of the bainite structure may be 0.2R, 0.3R, or 1.0R. That is, the entire cross section perpendicular to the axial direction of the steel material 1 may be a bainite structure. When a steel material having a bainite structure in the surface layer region is spheroidized, cementite tends to be spheroidized. Therefore, the cementite spheroidization rate in the surface layer region is 90.0% or more.

The microstructure of the steel material of this embodiment may be (B) instead of (A). When the steel material of the present embodiment is spheroidized, as described in (B), at least the surface region of the steel material of the present embodiment in a cross section perpendicular to the axial direction has a spheroidized cementite structure. Here, the "spheroidized cementite structure" means that the microstructure is composed of ferrite and cementite, and the cementite spheroidization rate in the microstructure is 90.0% or more. Further, "at least the surface layer region has a spheroidized cementite structure" means that the spheroidized cementite structure may be formed not only in the surface layer region but also in a deeper region than the surface layer region. Specifically, in FIG. 1, the depth of the spheroidized cementite structure from the surface is at least 0.1R, and the depth of the spheroidized cementite structure may be greater than 0.1R. For example, the depth of the spheroidized cementite structure may be 0.2R, 0.3R, or 1.0R. That is, the entire cross section perpendicular to the axial direction of the steel material 1 may be spheroidized cementite.

When the steel material of the present embodiment has a microstructure of (A), that is, when the steel material is an as-rolled material, the steel material is subjected to a spheroidizing heat treatment before cold forging. As a result, the microstructure of the steel becomes (B).

When the microstructure of the steel material of the present embodiment is (B), the cold forgeability (limit workability) can be improved as compared with a structure in which the microstructure consists of ferrite and pearlite.

Observation of the microstructure in the section perpendicular to the longitudinal direction of the steel material is performed by the following method. Referring to FIG. 2, in the cross section perpendicular to the longitudinal direction of the steel material 1, when the depth in the radial direction from the surface is d (mm), d=0.05R at four locations (90 in FIG. 2). ° Pitch). Furthermore, samples are taken from four locations (90° pitch in FIG. 2) at the d=0.1R position. The surface of each sample taken is used as the observation surface. After polishing the observation surface of each sample to a mirror surface, the sample is immersed in a nital corrosive solution for about 10 seconds to expose the structure by etching. The etched observation surface is observed with a secondary electron image from three views using a scanning electron microscope (SEM). The area of each field of view is assumed to be 400 μm ² (magnification of 5000 times). In each field, each phase such as bainite, ferrite, pearlite, and cementite can be distinguished as follows. When the observed surface is corroded with a nital etchant, the phase having a lamellar structure can be identified as pearlite in SEM observation. A phase with no substructure in grains can be identified as ferrite. Bainite can be identified as a phase in which a lath-like structure develops from prior γ grain boundaries. The granular, bright phase can be identified as cementite. The brightness of layered cementite in pearlite is approximately the same as the brightness of granular cementite described above.

Photographic images of 3 fields of view are generated at each location (8 locations). The number of photographic images is 8 points×3 fields of view=24. Each phase within each photographic image is identified in the manner described above. Identification of phases by contrast can be realized by well-known image processing methods.

If the microstructure in the photographic image of each field of view is mainly composed of bainite, it is determined that the microstructure of the steel material may be (A), and the bainite area ratio (%) is obtained by the following method.
Bainite area ratio (%) = total area of bainite in 24 fields of view/total area of 24 fields of view x 100

If the obtained bainite area ratio is 95.0% or more, at least the surface layer region of 0.1R is recognized as having a bainite structure (that is, the microstructure is (A)). In addition, the calculation of the above area ratio does not include precipitates other than cementite such as BN, TiC, TiN, and AlN, inclusions, and retained austenite.

In the photographic image of each field of view, if the microstructure consists of ferrite and cementite, it is judged that the microstructure of the steel material may be (B), and the spheroidized cementite rate (%) is determined by the following method.

The major axis (μm) and the minor axis (μm) of each cementite are determined in each visual field (24 visual fields in total). Among the straight lines connecting any two points on the interface between the cementite and the mother phase (ferrite), the maximum straight line length is defined as the major axis (μm) of the cementite. Of the straight lines connecting any two points on the interface between the cementite and the matrix phase, the length of the straight line perpendicular to the major axis is defined as the minor axis (μm) of the cementite. The obtained cementite having a major axis of 0.1 μm or more is to be measured (counted). Next, the aspect ratio (major axis/minor axis) of each cementite to be measured is obtained. The aspect ratio can be obtained by well-known image processing. Cementite having an aspect ratio of 3.0 or less is defined as "spheroidized cementite". The ratio (%) of the total number of spheroidized cementite in 24 visual fields to the total number of cementite in 24 visual fields is defined as the spheroidized cementite ratio (%). When the calculated spheroidized cementite ratio is 90.0% or more, at least the surface region of 0.1R is identified as a spheroidized cementite structure.

[Carburized steel parts]
Next, a carburized steel part manufactured using the steel material of this embodiment as a raw material will be described.

The carburized steel part of the present embodiment includes a carburized layer formed on the surface layer and a core portion inside the carburized layer. The carburized layer has an effective case depth of less than 0.4-2.0 mm in thickness. Here, the effective hardened layer depth means the depth from the surface at which the Vickers hardness is HV550 or higher. The carburized layer preferably has a Vickers hardness of 650 to 1000 HV at a depth of 50 μm from the surface. Further, in the carburized layer, the microstructure at a depth of 0.4 mm from the surface preferably contains 90 to 100% martensite in terms of area % and has a Vickers hardness of 600 to 900 HV.

When the carburized layer at a depth of 50 μm from the surface has a Vickers hardness of 650 to 1000 HV, wear resistance and fatigue strength are further enhanced. More preferably, the Vickers hardness at a depth of 50 μm from the surface is 700-1000 HV.

The microstructure in the carburized layer at a depth of 0.4 mm from the surface contains 90 to 100% martensite, and the carburized layer at a depth of 0.4 mm from the surface has a Vickers hardness of 600 to 900 HV. In the case of , the surface fatigue strength and fatigue strength are further increased. More preferably, the Vickers hardness at a depth of 0.4 mm from the surface is 620-900 HV.

Further, it is preferable that the Vickers hardness of the core portion at a depth of 2.0 mm from the surface is 250 to 500 HV. Furthermore, the chemical composition at a depth of 2.0 mm from the surface is the chemical composition described above. More preferably, the Vickers hardness at a depth of 2.0 mm from the surface is 270-450 HV. It is preferable that the microstructure at a depth of 2.0 mm from the surface contains at least one of martensite and bainite because the above effect can be further obtained.

The microstructure at a depth of 0.4 mm from the surface of the carburized steel part is obtained by the following method. A sample is taken from the surface of the carburized steel part to a depth of 0.4 mm. The surface of the sample is etched with a picral liquid. Any 3 fields of view of the surface after etching are observed with a secondary electron image using an SEM. The area of each field of view is assumed to be 400 μm ² (magnification of 5000 times). In SEM observation, martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite can be distinguished as follows. Specifically, a phase having a lamellar structure can be identified as pearlite. A phase with no substructure in grains can be identified as ferrite. Phases containing a lath-like structure can be identified as martensite and bainite. Tempered martensite and tempered bainite contain a lath-like structure and further contain carbides in the laths. The granular, bright phase can be identified as cementite. The brightness of layered cementite in perlite is comparable to the brightness of granular cementite described above. As described above, both martensite and bainite contain a lath-like structure, and in this specification, martensite and bainite are not distinguished in terms of the microstructure of carburized steel parts.

Obtain the total area of martensite in three fields of view of the 0.4 mm depth sample. The ratio of the obtained total area of martensite to the total area of the three fields of view is defined as the area ratio (%) of martensite at a depth of 0.5 μm. In calculating the area fraction of martensite, ferrite, pearlite, martensite and bainite, tempered martensite, tempered bainite, spheroidized cementite, and cementite are considered. Precipitates other than cementite such as BN, TiC, TiN, and AlN, inclusions, retained austenite, and the like are not included in the calculation of the above area ratio.

Vickers hardness of carburized steel parts is measured by the following method. The cross-section perpendicular to any surface of the carburized steel part shall be the measurement plane. On the measurement surface, the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface are tested using a micro Vickers hardness tester in accordance with JIS Z 2244 (2009). Calculated by The test force shall be 0.49N. The Vickers hardness HV is measured at 10 positions at a depth of 50 μm, and the average value is defined as the Vickers hardness HV at the position at a depth of 50 μm. Also, the Vickers hardness HV at 10 positions at a depth of 0.4 mm is measured, and the average value is taken as the Vickers hardness HV at the position at a depth of 0.4 mm. If the Vickers hardness at the 0.4 mm depth position is 550 HV or more, it is determined that the carburized layer depth is at least 0.4 mm or more. Also, on the measurement surface, the Vickers hardness at a depth of 2.0 mm from the surface is determined by a Vickers hardness test based on JIS Z 2244 (2009) using a micro Vickers hardness tester. The load during the test shall be 0.49N. The Vickers hardness HV is measured at 10 positions at a depth of 2.0 mm, and the average value thereof is taken as the Vickers hardness HV at the position at a depth of 2.0 mm. Although the Vickers hardness measurement plane is not particularly limited, it may be a cut plane perpendicular to the axial direction (longitudinal direction) of the carburized steel part.

[Manufacturing method for steel materials and carburized steel parts]
A method for manufacturing a steel material and a carburized steel part according to the present embodiment will be described.

[Manufacturing method of steel]
First, an example of the steel manufacturing method of the present embodiment will be described. An example of a steel manufacturing method includes a steelmaking process, a casting process, a hot working process, and a cooling process. Each step will be described below.

[Steelmaking process]
The steelmaking process includes a refining process and a casting process.

[Refining process]
In the refining process, first, molten iron manufactured by a well-known method is subjected to refining (primary refining) in a converter. Secondary refining is performed on the molten steel tapped from the converter. In secondary refining, alloying elements are added to molten steel to produce molten steel that satisfies the above chemical composition.

Specifically, Al is added to the molten steel tapped from the converter for deoxidation treatment. After the deacidification treatment, the slag removal treatment is carried out. After the slag removal treatment, secondary refining is carried out. Secondary refining, for example, implements compound refining. For example, first, a refining treatment using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed. Furthermore, an RH (Ruhrstahl-Hausen) vacuum degassing process is performed. After that, the final adjustment of the alloy components other than Si and Ca is performed.

After carrying out secondary refining and adjusting components of the molten steel other than Si and Ca, the molten steel is subjected to the following treatments (heating and holding step and final component adjustment step).

[Heating and holding step]
Heat the molten steel in the ladle after secondary refining (final composition adjustment) at a temperature of 1500 to 1600 ° C. with a holding time ts that is at least twice the uniform mixing time τ (s) calculated by the following formula. .
τ=800×ε− ^0.4
ε=((6.18×V _g ×T _l )/M _l ) ln(1+(h ₀ /(1.46×10 ⁻⁵ ×P ₀ )))
Here, Vg: gas flow rate (Nm ³ /min), M _l : mass of molten steel in ladle (ton), T _l : molten steel temperature (K), h ₀ : gas injection depth (m), P ₀ : molten steel Surface pressure (Pa), ε: stirring power value (W/ton), τ: uniform mixing time (s).

If the holding time ts is less than twice the uniform mixing time τ, the oxides present in the molten steel in the ladle cannot sufficiently agglomerate and coalesce. Therefore, the floating removal of the oxide cannot be performed, and the number of oxides increases. When the holding time ts is less than twice the uniform mixing time τ, Mg and the like mixed in the slag combine with S in the molten steel to form MgS and the like, and the MgS is dispersed in the molten steel. This dispersed MgS serves as precipitation sites for MnS. As a result, the number of Mn sulfides increases.

If the holding time ts is at least twice the uniform mixing time τ, the number of oxides in the steel can be suppressed. Furthermore, since MgS once formed becomes MgO by reoxidation, the precipitation sites of MnS are reduced, and as a result, the number of Mn sulfides in the steel can also be suppressed. As a result, the number of Mn sulfides becomes 70.0 pieces/mm ² or less and the number of oxides becomes 25.0 pieces/mm ² or less after the final component adjustment step in the next step.

[Final component adjustment step]
Si and Ca are added to the molten steel after the heating and holding process to produce molten steel that satisfies the above chemical composition and formulas (1) to (3). Si and Ca may be added to the molten steel as individual raw materials. Si—Ca alloy may be used as a raw material and added to molten steel.

If Si and Ca are added to molten steel that has been sufficiently uniformly heated in the heating and holding process, oxides are modified from Al ₂ O ₃ to composite inclusions containing SiO ₂ and CaO, and Mn sulfides are also Ca is modified to a sulfide containing Therefore, assuming that the holding time ts is at least twice the uniform mixing time τ, the number of Mn sulfides is 70.0/mm ² or less, and the number of oxides is 25.0/mm ² or less. become.

If Si is added to molten steel before Al is added, deoxidation will not be sufficiently performed, and as a result, the number of oxides will exceed 25.0/mm ² . By adding Si and Ca to molten steel after addition of Al, the number of Mn sulfides becomes 70.0/mm ² or less and the number of oxides becomes 25.0/mm ² or less. Therefore, in this embodiment, Al is added to molten steel, and then Si and Ca are added. The order in which Si and Ca are added is not particularly limited. Si and Ca may be added simultaneously. Either Si or Ca may be added first.

[Casting process]
A raw material (a slab or an ingot) is manufactured using the molten steel manufactured by the above refining process. Specifically, a slab is produced by a continuous casting method using molten steel. Alternatively, an ingot may be made by an ingot casting method using molten steel. Using this slab or ingot, the next hot working step is carried out. Hereinafter, the slab or ingot will be referred to as "raw material".

[Hot working process]
In the hot working process, the material (bloom or ingot) prepared in the casting process is subjected to hot working to manufacture steel materials. Although the shape of the steel material is not particularly limited, it is, for example, a steel bar or a wire rod. In the following description, as an example, a case where the steel material is a steel bar will be described. However, even if the steel material has a shape other than the steel bar, it can be manufactured by the same hot working process.

Hot working includes a rough rolling process and a finish rolling process. In the rough rolling process, the raw material is hot worked to produce a billet. The rough rolling process uses, for example, a blooming mill. The material is bloomed by a blooming mill to produce a billet. When a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous rolling mill to produce a smaller billet. may In a continuous rolling mill, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are alternately arranged in a row. Through the above steps, the raw material is manufactured into a billet in the rough rolling step. Although the heating temperature in the rough rolling step is not particularly limited, it is, for example, 1100 to 1300.degree.

In the finish rolling step, the billet is heated using a heating furnace or a soaking furnace. The billet after heating is subjected to hot rolling using a continuous rolling mill to produce a steel material (steel bar). Although the heating temperature at the finishing rolling temperature is not particularly limited, it is, for example, 1000 to 1250°C.

[Cooling process]
In the cooling process, the steel material is cooled immediately after the completion of the hot working process. Specifically, the temperature range in which the surface temperature of the steel material is 800° C. to 500° C. is cooled at a cooling rate of more than 1.0° C./second to 30.0° C./second.

In the steel material manufactured by the above manufacturing process, the area ratio of bainite is 95.0% or more in the surface layer region at least from the surface to the depth of 0.1R in the microstructure in the cross section perpendicular to the axial direction. That is, a steel material (as-rolled material) having a microstructure of (A) is manufactured by the above manufacturing process.

[Spheroidizing heat treatment step]
The steel material after the cooling process may be further subjected to a spheroidizing heat treatment process, so that the steel material of the present embodiment has a microstructure of (B). That is, in this case, the steel material having the microstructure (B) is manufactured by carrying out the spheroidizing heat treatment.

A well-known method may be used for the spheroidizing heat treatment. The spheroidizing heat treatment is performed, for example, by the following method. The steel material after the cooling step is heated to a temperature just below or above the _Ac1 point (the temperature at which austenite starts to form when heated) (for example, within _Ac1 point + 50 ° C.) and held for a predetermined time, and then gradually Cool. Alternatively, the steel material after the cooling step is heated to a temperature just above the A _c point, and cooled to a temperature just below the A _r point (at the time of cooling, austenite completes transformation to ferrite, ferrite, or cementite). It may be repeated several times. Alternatively, the steel material after the cooling process may be quenched once and then tempered at a temperature of 600 to 700° C. for 3 to 100 hours. The method of the spheroidizing heat treatment is not particularly limited, and any known annealing or spheroidizing heat treatment method as described above may be applied.

In the steel material manufactured by performing the spheroidizing heat treatment, in the microstructure in the cross section perpendicular to the axial direction, the surface layer region at least from the surface to the depth of 0.1R is composed of ferrite and cementite, and the spherical shape of the cementite conversion rate becomes 90.0% or more. That is, the steel material having the microstructure (B) is manufactured by the above manufacturing process.

The steel material of this embodiment can be manufactured by the manufacturing process described above.

[Manufacturing method of carburized steel parts]
Next, an example of a method for manufacturing a carburized steel part using the steel material of the present embodiment as a raw material will be described. This manufacturing method includes a cold forging step of cold forging the steel material described above to manufacture a plurality of intermediate members, and if necessary, welding the manufactured intermediate members to form an integrated product. and, if necessary, a cutting process for cutting the intermediate member, a carburizing process for carburizing the intermediate member, and an intermediate steel material after the carburizing process and a tempering step of performing tempering. In this specification, carburizing treatment also includes carbonitriding treatment.

In addition, when the microstructure of the steel material of the present embodiment is (A), that is, when the steel material of the present embodiment is an as-rolled material, the steel material is subjected to the above-described spheroidizing heat treatment step, and then cold-rolled. Carry out the forging process. Before performing the spheroidizing heat treatment process on the steel material with the microstructure of (A), a cold drawing process such as a wire drawing process is performed as necessary.

[Cold forging process]
In the cold forging step, the steel material manufactured by the manufacturing method described above is subjected to cold forging to give a shape, thereby manufacturing a plurality of intermediate members. Cold forging conditions such as working ratio and strain rate in this cold forging process are not particularly limited. Suitable cold forging conditions may be appropriately selected. The plurality of intermediate members are welded and integrated in the next welding process.

[Welding process]
The welding step is an optional step and need not be performed. When implemented, the welding process welds and integrates the plurality of intermediate members by friction bonding or laser bonding. A welding method is not particularly limited. After welding, the joint surface of the intermediate member may be formed flat by machining. In the steel material of the present embodiment, Mn sulfides are 70.0 pieces/mm ² or less and oxides are 25.0 pieces/mm ² or less. Therefore, the steel material of the present embodiment has excellent bondability, and even when the intermediate member is welded to form a carburized steel part, the carburized steel part has excellent joint fatigue strength.

[Cutting process]
The cutting process is an optional process and may not be performed. When implemented, in the cutting process, the intermediate member after the cold forging process and before the carburizing treatment process described later is subjected to cutting to give a shape. By performing cutting, it is possible to impart a precise shape to the carburized steel part, which is difficult to do with the cold forging process alone.

[Carburizing process]
In the carburizing process, the intermediate member (the integrally joined intermediate member when the welding process is performed) is carburized. In the carburizing process, well-known carburizing is performed. The carburizing process includes a carburizing process, a diffusion process, and a hardening process.

Carburizing conditions in the carburizing process and the diffusion process may be adjusted as appropriate. The carburizing temperature in the carburizing process and the diffusion process is, for example, 830-1100.degree. The carbon potential in the carburizing and diffusion steps is, for example, 0.5-1.2%. The holding time in the carburizing step is, for example, 60 minutes or longer, and the holding time in the diffusion step is 30 minutes or longer. The carbon potential in the diffusion process is preferably lower than the carbon potential in the carburizing process. However, the conditions in the carburizing step and the diffusion step are not limited to the above conditions.

After the diffusion step, a known hardening step is performed. In the quenching step, the intermediate member after the diffusion step is held at a quenching temperature equal to or higher than the _Ar3 transformation point. Although the holding time at the quenching temperature is not particularly limited, it is, for example, 30 to 60 minutes. Preferably, the quenching temperature is lower than the carburizing temperature. The temperature of the quenching medium is preferably room temperature to 250°C. The quenching medium is for example water or oil. Moreover, you may implement a sub-zero process after hardening as needed.

[Tempering process]
A well-known tempering process is performed on the intermediate member after the carburizing process. The tempering temperature is, for example, 100-250°C. The holding time at the tempering temperature is, for example, 60-150 minutes.

[Other processes]
If necessary, the carburized steel part after the tempering process may be further subjected to grinding or shot peening. A precision shape can be imparted to the carburized steel component by performing a grinding operation. In addition, compressive residual stress is introduced into the surface layer of the carburized steel part by performing the shot peening treatment. Compressive residual stress suppresses initiation and propagation of fatigue cracks. Therefore, it increases the fatigue strength of the carburized steel part. For example, if the carburized steel part is a gear, the fatigue strength of the tooth root and tooth surface of the carburized steel part can be improved. Shot peening treatment may be performed by a well-known method. The shot peening treatment is preferably carried out, for example, using shot grains having a diameter of 0.7 mm or less and an arc height of 0.4 mm or more.

As described above, the steel material of the present embodiment can be applied as a material for carburized steel parts integrated by welding a plurality of intermediate members. Naturally, the steel material of this embodiment can also be applied as a material for carburized steel parts without welding.

The effects of one aspect of the steel material of the present disclosure will be described more specifically by way of examples. The conditions in the examples are an example of conditions adopted to confirm the feasibility and effect of the steel materials of the present disclosure. The steel material of the present disclosure is not limited to this one condition example. Various conditions can be adopted for the steel material of the present disclosure as long as the object of the present disclosure is achieved without departing from the gist of the present disclosure.

Molten steel having the chemical composition shown in Table 1 was prepared. At this time, refining was performed under the conditions shown in Table 2. The molten steel after refining was cast by continuous casting to obtain a slab. A blank portion in Table 1 means that the content of the corresponding element was below the detection limit. That is, blanks mean that the corresponding elemental content was below the detection limit at the lowest order of magnitude. For example, for the Cu content in Table 1, the least significant digit is two decimal places. Therefore, it means that the Cu content of Test No. 1 was not detected to two decimal places (the content to two decimal places was 0%).

"Steelmaking conditions (1)" in Table 2 shows the ratio (=ts/τ) of the holding time ts at 1500 to 1600°C after secondary refining to the uniform mixing time τ. "Steelmaking conditions (2)" in Table 2 indicates the order of addition of Al, Si and Ca. "1" in the "steelmaking conditions (2)" column means that Si and Ca were added after deoxidizing by adding Al. "2" means that Al and Ca were added after Si was added. For test numbers 22 and 24, the steelmaking process was carried out with the chemical composition of steel number B1 as the target. For test numbers 23 and 25, the steelmaking process was carried out with the chemical composition of steel number C1 as the target.

After heating the produced slab at 1100 to 1300° C., it was subjected to a rough rolling process to obtain a billet having a cross section of 162 mm×162 mm perpendicular to the longitudinal direction. Using this billet, the finish rolling process was carried out. In the finish rolling step, a billet heated to 1000 to 1250° C. is hot-rolled by a continuous rolling mill to produce a steel bar having a circular cross section perpendicular to the longitudinal direction and a diameter of 30 mm. manufactured. A cooling process was performed on the steel bar immediately after the finish rolling process. Table 2 shows the average cooling rate (°C/sec) at 800 to 500°C in the cooling step. A plurality of steel bars after the cooling process (hereinafter referred to as "as-rolled material") were prepared for each test number.

In each test number, a spheroidizing heat treatment process (SA process: Spherodizing Annealing) was performed on some of the prepared steel bars. In the spheroidizing heat treatment, the steel bars were heated to 740°C. After that, it was slowly cooled at a cooling rate of 8°C/hr until the temperature of the steel bar reached 650°C. The steel material temperature was air-cooled from 650° C. to room temperature, and a steel bar was manufactured by performing a spheroidizing heat treatment process (hereinafter referred to as “SA material”). Steel materials (as-rolled material, SA material) of each test number were manufactured by the above manufacturing method.

[Evaluation test]
The following tests were performed on the steel materials of each test number.

[Microstructure Observation Test]
A microstructure observation test was performed by the following method. Specifically, in the cross section perpendicular to the longitudinal direction of the steel material of each test number, when the depth in the radial direction from the surface is d (mm), 4 points at d = 0.05R position (in FIG. 2, 90° pitch). Furthermore, samples were taken from four locations (90° pitch in FIG. 2) at the d=0.1R position. The surface S of each sample taken was used as an observation surface. After polishing the observation surface of each sample to a mirror surface, the sample was immersed in a nital corrosive solution for about 10 seconds to expose the structure by etching. The etched observation surface was observed in 3 fields of view with a secondary electron image using an SEM. The visual field area was 400 μm ² (magnification: 5000 times). Bainite and other phases (ferrite, pearlite, cementite, etc.) could be distinguished in each field of view, as described above.

For the as-rolled material, the bainite area ratio (%) was obtained by the following method.
Bainite area ratio (%) = total area of bainite in 24 fields of view/total area of 24 fields of view x 100

If the obtained bainite area ratio is 95.0% or more, at least the surface layer region at a depth of 0.1R from the surface was recognized as a bainite structure ("Y ”). On the other hand, when the obtained bainite area ratio is less than 95.0%, the surface layer region at a depth of 0.1R from the surface was recognized as not having a bainite structure ("Microstructure of as-rolled material" in Table 2, " N").

For the SA material, the spheroidized cementite rate (%) was obtained by the following method. First, the major axis (μm) and the minor axis (μm) of each cementite were obtained in each visual field (24 visual fields). Among the straight lines connecting any two points on the interface between the cementite and the mother phase (ferrite), the maximum straight line length was defined as the major axis (μm) of the cementite. Of the straight lines connecting any two points on the interface between the cementite and the parent phase, the length of the straight line perpendicular to the major axis was defined as the minor axis (μm) of the cementite. Cementite having a length of 0.1 μm or more was measured (counted). Next, the aspect ratio (major axis/minor axis) of each cementite to be measured was obtained. Cementite having an aspect ratio of 3.0 or less was defined as "spheroidized cementite". The ratio (%) of the total number of spheroidized cementite in 24 visual fields to the total number of cementite in 24 visual fields was defined as the spheroidized cementite ratio (%). When the obtained spheroidized cementite ratio was 90.0% or more, the surface layer region of at least 0.1R was recognized as a spheroidized cementite structure ("Y" in "SA material microstructure" in Table 2). ). On the other hand, when the obtained area ratio of spheroidized cementite was less than 90.0%, the surface layer region at a depth of 0.1R from the surface was recognized as not having a spheroidized cementite structure (see "SA material microstructure “N” in “Organization”). In addition, all of the microstructures of 24 fields of view of the SA material of each test number consisted of ferrite and cementite. In other words, in the SA material of each test number, the microstructures of the surface layer regions were all composed of ferrite and cementite.

[Limited compressibility measurement test]
A compression test piece was prepared from a steel material having a diameter of 30 mm and having each test number so that the longitudinal direction of the steel material was the compression direction. The compression specimen had a diameter of 29.5 mm and a length of 44 mm. The central axis of the compression test piece almost coincided with the central axis of the steel material. A notch was formed in the circumferential direction at the central position in the longitudinal direction of the compression test piece. The notch angle was 30° C., the notch depth was 0.8 mm, and the radius of curvature of the notch tip was 0.15 mm. Compression test pieces were taken from the as-rolled material and the SA material. Hereinafter, in each test piece, the one taken from the as-rolled material will be referred to as the "as-rolled test piece", and the one taken from the SA material will be referred to as the "SA test piece".

A limit compression test was performed on the above-described compression test pieces (as-rolled test piece, SA test piece) by the following method. A 500-ton hydraulic press was used for the limit compression test. Cold compression was performed on each specimen using a constraining die at a speed of 10 mm/min. Compression was stopped when microcracks of 0.5 mm or more were generated in the vicinity of the notch, and the compression rate (%) at that time was calculated. This measurement was performed a total of 10 times, and the compression rate (%) at which the cumulative failure probability was 50% was obtained, and the compression rate was defined as the critical compression rate (%). Table 2 shows the limit compression rate (%) for each test number.

As described above, the as-rolled material may be subjected to cold drawing such as wire drawing before the spheroidizing heat treatment step. In this case, the as-rolled material must have workability that does not cause breakage due to internal cracks (chevron cracks) during cold drawing. Therefore, for the as-rolled test piece, it was judged that the critical working rate was excellent when the critical compression rate was 50% or more. For the test numbers in which the as-rolled test piece had a critical compressibility of less than 50%, the carburized steel parts were not subjected to subsequent evaluation tests.

Regarding the SA test piece, the critical compressibility of the conventional steel used as the raw material for the carburized steel parts is about 65%. judged to be superior to For test numbers with a critical compressibility of less than 75%, the carburized steel parts were not evaluated.

[Mn sulfide number and oxide number measurement test]
Samples were taken from each of the as-rolled material and the SA material of each test number described above. Specifically, as shown in FIG. 3, samples were taken from the R/2 position in the radial direction from the center axis C1 of the as-rolled material and the SA material. The observation surface of the sample had a size of L1×L2, where L1 was 10 mm and L2 was 5 mm. Furthermore, the sample thickness L3, which is the direction perpendicular to the observation surface, was set to 5 mm. The normal N of the viewing plane was perpendicular to the central axis C1, and the R/2 position corresponded to the central position of the viewing plane.

The observation surface of the collected sample was mirror-polished, and 20 fields of view (evaluation area per field of view of 100 μm × 100 µm) were randomly observed at a magnification of 1000 using a scanning electron microscope (SEM) (as-rolled material 20 fields of view, 20 fields of view for SA material).

Inclusions in each field were identified. For each identified inclusion, energy dispersive X-ray spectroscopy (EDX) was used to identify Mn sulfides and oxides. Specifically, using EDX, each inclusion was subjected to elemental analysis at at least two measurement points. Then, in each inclusion, the arithmetic mean value of the element content obtained at each measurement point was defined as the content (mass %) of each element in the inclusion. When elemental analysis is performed on one inclusion at two measurement points, the arithmetic average value of the Mn content, the arithmetic average value of the S content, and the arithmetic average value of the O content obtained at the two measurement points are It was defined as Mn content (mass%), S content (mass%), and O content (mass%) in the inclusion. In the elemental analysis results of the identified inclusions, if the Mn content is 10.0% or more, the S content is 10.0% or more, and the O content is less than 10.0%, the inclusion The material was identified as Mn sulfide. Moreover, when the O content was 10.0% or more in the elemental analysis result of the specified inclusion, the inclusion was identified as an oxide. Inclusions to be specified were inclusions having an equivalent circle diameter of 0.5 μm or more. The EDX beam diameter used to identify inclusions was set to 0.2 μm.

In each of the as-rolled material and the SA material, Mn sulfides and oxides having an equivalent circle diameter of 0.5 μm or more were measured. Based on the total number of Mn sulfides identified in each field of view and the total area of the 20 fields of view, the number of Mn sulfides per unit area (number/mm ² ) was determined. Also, the number of oxides per unit area (pieces/mm ² ) was obtained based on the total number of oxides identified in each field of view and the total area of the 20 fields of view.

Table 2 shows the number of Mn sulfides (pieces/mm ² ) in the as-rolled material and the number of oxides (pieces/mm ² ) in the as-rolled material. In each test number, the number of Mn sulfides in the SA material was the same as the number of Mn sulfides in the as-rolled material, and the number of oxides in the SA material was the same as that in the as-rolled material.

[Manufacture of carburized steel parts]
The as-rolled material of each test number was subjected to spheroidizing annealing. Specifically, the as-rolled material was heated to 740°C. Thereafter, the as-rolled material was slowly cooled at a cooling rate of 8°C/hr until the temperature reached 650°C. The steel material temperature was air-cooled from 650° C. to room temperature, and a spheroidized and annealed as-rolled material was manufactured.

A test piece was taken from an as-rolled material that had been spheroidized and annealed. The specimen was a round bar with a diameter of 29.5 mm and a length of 44 mm. The longitudinal direction of the test piece was the same as the longitudinal direction of the as-rolled material.

This test piece was subjected to cold upset compression at a compression rate of 50% to simulate cold forging. Upsetting compression was performed at ambient temperature and used a constrained die. The strain rate during upset compression was set to 1/sec. After upsetting compression, gas carburizing was performed on the test piece by a metamorphic furnace gas method. In this gas carburizing, the carbon potential was set to 0.8%, and the temperature was maintained at 950° C. for 5 hours, and then at 850° C. for 0.5 hours. After gas carburizing, oil quenching to 130° C. was performed as a finishing heat treatment step. After quenching, tempering was performed at 150° C. for 90 minutes. Through the above steps, a test piece simulating a carburized steel part was produced from the as-rolled material.

A test piece simulating a carburized steel part was also prepared for the SA material of each test number in the same manner as for the as-rolled material described above. Specifically, a test piece was taken from the SA material of each test number. The specimen was a round bar with a diameter of 29.5 mm and a length of 44 mm. The longitudinal direction of the test piece was the same as the longitudinal direction of the SA material. This test piece was subjected to cold upset compression at a compression rate of 50% to simulate cold forging. Upsetting compression was performed at ambient temperature and used a constrained die. The strain rate during upset compression was set to 1/sec. After upsetting compression, gas carburizing was performed on the test piece by a metamorphic furnace gas method. In this gas carburizing, the carbon potential was set to 0.8%, and the temperature was maintained at 950° C. for 5 hours, and then at 850° C. for 0.5 hours. After gas carburizing, oil quenching to 130° C. was performed as a finishing heat treatment step. After quenching, tempering was performed at 150° C. for 90 minutes. Through the above steps, a test piece simulating a carburized steel part was produced from the SA material.

[Evaluation test of carburized steel parts]
The following tests were carried out on the carburized layer and the core of the carburized steel parts simulative test pieces (as-rolled material, SA material) manufactured as described above.

[Vickers hardness test of carburized layer]
The Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface on the cut surface perpendicular to the longitudinal direction of the test piece (as-rolled material, SA material) simulating the carburized steel parts of each test number. was obtained by a Vickers hardness test based on JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force was 0.49N. The Vickers hardness HV was measured at 10 positions at a depth of 50 μm, and the average value was defined as the Vickers hardness HV at the position at a depth of 50 μm. Also, the Vickers hardness HV at 10 positions at a depth of 0.4 mm was measured, and the average value was taken as the Vickers hardness HV at the position at a depth of 0.4 mm.

If the hardness at a depth of 0.4 mm from the surface was 600 HV or more, it was determined that the carburized layer was present at least 0.4 mm from the surface. Also, when the Vickers hardness at a depth of 50 μm from the surface is 650 to 1000 HV, it was determined that the hardness of the carburized layer of the carburized steel part is sufficient. Table 3 shows the measurement results.

[Microstructure Observation Test of Carburized Layer]
The microstructure at a depth of 0.4 mm from the surface of the test piece (as-rolled material, SA material) simulating a carburized steel part was obtained by the following method. A sample was taken from the surface of the test piece with a depth of 0.4 mm on the surface. The surface of the sample was etched with a picral liquid. Three arbitrary fields of view of the surface after etching were observed as secondary electron images using an SEM. The area of each field of view was 400 μm ² (magnification of 5000 times). Martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite were distinguishable from the contrast. The total area of martensite in 3 fields of view of the 0.4 mm depth position sample was determined. The ratio of the obtained total area of martensite to the total area of the three fields of view was defined as the area ratio (%) of martensite at a depth of 0.5 μm.

[Vickers Hardness Test of Core Part and Specification of Chemical Composition]
The Vickers hardness and chemical composition of the core of the test piece simulating a carburized steel part were measured by the following methods. Vickers hardness at a depth of 2.0 mm from the surface of the carburized steel part perpendicular to the longitudinal direction is measured using a micro Vickers hardness tester in accordance with JIS Z 2244 (2009). rice field. The test force was 0.49N. Ten measurements were performed at a depth of 2.0 mm, and the average value was taken as the Vickers hardness (HV) at a depth of 2.0 mm from the surface. Table 3 shows the Vickers hardness obtained. When the Vickers hardness at the 0.2 mm depth position was 250 to 500 HV, the hardness of the core was sufficient and it was judged as acceptable. In addition, when the Vickers hardness at the 0.4 mm depth position is 600 HV or more and the Vickers hardness at the 2.0 mm position is less than 600 HV, the carburized layer is effective for less than 0.4 to 2.0 mm It was judged to have a hardened layer. That is, in this case, the 2.0 mm depth position from the surface was recognized as the core portion.

[Presence or absence of coarse grains in carburized steel parts]
Prior austenite grains were observed at a depth of 2 mm from the surface of the cores of the test pieces (as-rolled material and SA material) simulating the carburized steel parts. Specifically, a cut surface perpendicular to the longitudinal direction of the carburized steel part was used as the observation surface. After the observation surface was mirror-polished, it was etched with a picric acid saturated aqueous solution. A field of view (300 μm×300 μm) including a position 2 mm deep from the surface of the etched observation surface was observed with an optical microscope (400×) to identify prior austenite grains. For the identified prior austenite crystal grains, the crystal grain size of each prior-stenite crystal grain was determined in terms of equivalent circle diameter (μm) in accordance with JIS G 0551 (2013). Among the prior austenite crystal grains, if there is even one crystal grain with a circle equivalent diameter exceeding the equivalent circle diameter (88.4 μm) corresponding to No. 4 of the JIS specified crystal grain size number, "coarse grain generation Yes.” Table 3 shows the determination results.

[Fatigue strength evaluation test of carburized steel parts after joining]
The as-rolled material and SA material of each test number were machined to produce a round bar with a diameter of 20 mm and a length of 150 mm. Using this round bar (as-rolled material, SA material), a basic fatigue test piece and a joint fatigue test piece were produced. Before preparing the following basic fatigue test pieces and joint fatigue test pieces, the as-rolled material was subjected to spheroidizing annealing under the same conditions as described above, and then machined to a diameter of 20 mm and a length of 150 mm. A round bar was produced.

A basic fatigue test piece was prepared by the following method. An Ono-type rotary bending fatigue test piece having a diameter of 8 mm and a parallel portion length of 15 mm was prepared from the cross-sectional central portion of a round bar with a diameter of 20 mm and a length of 150 mm. This test piece was used as a basic fatigue test piece. The longitudinal direction of the basic fatigue test piece was the same as the longitudinal direction of the round bar.

A joint fatigue test piece was produced by the following method. A round bar having a diameter of 20 mm and a length of 150 mm having the same test piece number was butted against each other to prepare a joined round bar under the following friction welding conditions.

Friction welding conditions:
Friction pressure: 100 MPa,
Friction time: 5 seconds,
Upset pressure (applied pressure from both ends of the round bar to the joint): 200 MPa,
Upset time (pressurization time to joint): 5 seconds,
Rotation speed: 2000 rpm,
Approach allowance: 5 to 12 mm.

An Ono-type rotary bending fatigue test piece having an evaluation portion diameter of 8 mm and a parallel portion length of 15.0 mm was prepared from the cross-sectional central portion of the joined round bar, and used as a pressure contact fatigue test piece. In the pressure contact fatigue test piece, the longitudinal central portion of the parallel portion was used as the joint surface. The longitudinal direction of the joint fatigue test piece was the same as the longitudinal direction of the round bar.

The following carburizing and quenching treatment is performed on the basic fatigue test piece and the joint fatigue test piece, and the carburized steel part (test piece using as-rolled material after spheroidizing heat treatment, test piece using SA material) and did. In the carburizing and quenching treatment, gas carburizing was carried out using a gas conversion furnace gas system. Specifically, the carbon potential was set to 0.8% and the temperature was maintained at 950° C. for 5 hours. After that, it was held at 850° C. for 0.5 hours at the same carbon potential. After that, it was immersed in oil at 130° C. to perform oil quenching. After oil quenching, tempering was performed at 150° C. for 90 minutes. By the above method, Ono-type rotary bending fatigue test specimens (basic fatigue test specimens, joint fatigue test specimens) simulating carburized steel parts were produced.

An Ono-type rotary bending fatigue test was performed on the prepared basic fatigue test piece and joint fatigue test piece. Specifically, using the above-mentioned Ono-type rotating bending fatigue test pieces (basic fatigue test pieces, bonding fatigue test pieces), Ono-type rotation in accordance with JIS Z 2274 (1978) is performed at room temperature in an air atmosphere. A bending fatigue test was performed. Fatigue strength (MPa) was defined as the maximum stress at which no fracture occurred after 1×10 ⁷ cycles of repeated stress loading with a rotation speed of 3000 rpm and a stress ratio R of −1.

The ratio (%) of the fatigue strength (MPa) of the joint fatigue test piece to the fatigue strength (MPa) of the basic fatigue test piece was defined as the fatigue strength ratio. That is, the fatigue strength ratio was defined by the following formula.
Fatigue strength ratio (%) = fatigue strength of joint fatigue test piece/fatigue strength of basic fatigue test piece x 100

Table 3 shows the obtained fatigue strength ratios. If the fatigue strength ratio was 85% or more, it was judged that excellent fatigue strength was obtained even after joining.

[Test results]
Test results are shown in Tables 2 and 3. With reference to Tables 2 and 3, the chemical compositions of Test Nos. 1-11 and 28 were appropriate and satisfied Formulas (1)-(3). Furthermore, the steelmaking conditions were also appropriate. Moreover, the cooling rate in the cooling process was also appropriate. Therefore, the number of MnS in the as-rolled material and the SA material was 70.0/mm ² or less, and the number of oxides was 25.0/mm ² or less. Furthermore, in the as-rolled material, the bainite area ratio in the surface layer region at least from the surface to the depth of 0.1R is 95.0% or more ("Y" in the "microstructure of as-rolled material" column in Table 2). , In the SA material, the microstructure of the surface layer region at least from the surface to the depth of 0.1R consists of ferrite and cementite, and the cementite spheroidization rate in the surface layer region is 90.0% or more ("SA material in Table 2 "Y" in the "microstructure" column. As a result, the limit compressibility of the as-rolled material was 50% or more, and the limit compressibility of the SA material was 75% or more, showing excellent limit compressibility.

Furthermore, referring to Table 3, in test pieces simulating carburized steel parts produced using as-rolled materials, the Vickers hardness at a depth of 50 μm was 650 to 1000 HV, and at a depth of 0.4 mm The martensite area ratio was 90.0% or more, and the Vickers hardness at a depth of 0.4 mm was 600 to 900 HV or more. Furthermore, the Vickers hardness at a depth of 2.0 mm from the surface was 250 to 500 HV, and the effective hardened layer depth of the carburized layer was 0.4 to less than 2.0 mm. Furthermore, the prior austenite grain boundaries were not coarsened in the core. Furthermore, the fatigue strength ratios based on the joint fatigue test piece and the basic fatigue test piece were both as high as 85% or more, and excellent fatigue strength was exhibited both when joined and after welding.

On the other hand, in test number 12, the C content was too high. Therefore, the critical compressibility of the as-rolled material was less than 50%. Furthermore, the critical compressibility of the SA material was less than 75%, and a sufficient critical compressibility was not obtained.

In test number 13, the C content was too low. Therefore, sufficient hardness was not obtained in the core portion of the test piece simulating the carburized steel part.

In test number 14 the oxygen content was too high. Therefore, the number of oxides was too large. As a result, the fatigue strength ratio based on the joint fatigue test piece and the basic fatigue test piece of the as-rolled material and the SA material was as low as less than 85%, and the fatigue strength after welding was low.

In test number 15, the N content was too high. Therefore, solid solution B could not be ensured, and the hardness of the core was too low.

In test numbers 16 and 29, F1 exceeded the upper limit of formula (1). As a result, the as-rolled material and the SA material had low marginal working rates.

In test number 17, F1 was less than the lower limit of formula (1). Therefore, the core hardness of the carburized parts of the as-rolled material and the core hardness of the carburized steel parts of the SA material were too low.

In test numbers 18 and 30, F2 exceeded the upper limit of formula (2). As a result, the as-rolled material and the SA material had low marginal working rates.

In test number 19, F2 was less than the lower limit of formula (2). Therefore, the hardness at a depth of 0.4 mm was too low in the carburized parts of the as-rolled material and the carburized steel parts of the SA material.

In test number 20, F3 exceeded the upper limit of formula (3). Therefore, the critical working rate of steel materials (as-rolled materials and SA materials) was low.

In test number 21, F3 was less than the lower limit of formula (3). As a result, part of the prior austenite grains became coarse grains in the core of the carburized part of the as-rolled material and the core of the carburized steel part of the SA material.

In test numbers 22 and 23, the holding time ts at a temperature of 1500 to 1600° C. for the molten steel in the ladle after secondary refining was less than 2.0 times the uniform mixing time τ. Therefore, in the as-rolled material and the SA material, the number of MnS exceeded 70.0/mm ² and the number of oxides exceeded 25.0/mm ² . As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test pieces simulating the carburized part of the as-rolled material and the carburized steel part of the SA material.

In test numbers 24 and 25, Si was added before adding Al in the refining process. Therefore, in the as-rolled material and the SA material, the number of oxides exceeded 25.0/mm ² . As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test pieces simulating the carburized part of the as-rolled material and the carburized steel part of the SA material.

In test numbers 26 and 27, the average cooling rate between 800 and 500 in the slow cooling process after hot rolling was too fast. Therefore, the microstructure of the as-rolled steel material became a microstructure mainly composed of martensite. As a result, the critical compressibility of the as-rolled material was less than 50%. However, in the microstructure of the SA material, the microstructure of the surface layer region at least from the surface to the depth of 0.1R consisted of ferrite and cementite, and the cementite spheroidization rate in the surface layer region was 90.0% or more. Therefore, the limit compressibility of the SA material was 75% or more. The Vickers hardness of the carburized layer of the SA material carburized steel part was appropriate, and the martensite fraction at a depth of 0.4 mm was 90.0% or more. Furthermore, the core hardness and chemical composition were also appropriate, and the prior austenite grain size was not coarsened. Furthermore, in the joint fatigue test piece, the fatigue strength ratio was as high as 85% or more, indicating excellent fatigue strength even when joined.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (4)

is steel,
in % by mass,
C: 0.09 to 0.16%,
Si: 0.01 to 0.50%,
Mn: 0.40-0.60%,
P: 0.030% or less,
S: 0.025% or less,
Cr: 0.90 to 2.00%,
Mo: 0.10-0.40%,
Al: 0.005-0.030%,
Ti: less than 0.010 to 0.050%,
Nb: 0.010 to 0.030%,
N: 0.0080% or less,
O: 0.0030% or less,
B: 0.0003 to 0.0030%,
Contains Ca: 0.0005 to 0.0050%,
The remainder consists of Fe and impurities, satisfying formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material, the Mn content is 10.0% or more, the S content is 10.0% or more, and the O content is less than 10.0% in mass%, and the equivalent circle diameter The number of Mn sulfides of 0.5 μm or more is 70.0/mm ² or less, the O content is 10.0% or more in mass%, and the oxide having an equivalent circle diameter of 0.5 μm or more is 25. .0 pieces / mm ² or less,
steel.
0.140<C+0.194×Si+0.065×Mn+0.012×Cr+0.033×Mo+0.067×Ni+0.097×Cu+0.078×Al<0.235 (1)
13.0<(0.70×Si+1)×(5.1×Mn+1)×(2.2×Cr+1)×(3.0×Mo+1)×(0.36×Ni+1)<45.0 (2)
0.004<Ti-N×(48/14)<0.030 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (3). If the corresponding element is not contained, "0" is substituted for the element symbol. The steel material according to claim 1,
When the cross section perpendicular to the axial direction of the steel material is circular and the radius of the cross section is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel material, at least from the surface to the depth of 0.1R The area ratio of bainite in the surface layer region is 95.0% or more,
steel. The steel material according to claim 1,
When the cross section perpendicular to the axial direction of the steel material is circular and the radius of the cross section is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel material, at least from the surface to the depth of 0.1R The surface layer region of is composed of ferrite and cementite, and the cementite spheroidization rate in the surface layer region is 90.0% or more.
steel. The steel material according to any one of claims 1 to 3,
Instead of part of the Fe, in mass %,
Cu: 0.50% or less,
Ni: 0.30% or less, and
V: 0.10% or less,
containing one or more elements selected from the group consisting of
steel.

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