KR102561036B1 - steel - Google Patents

Abstract

Provided is a steel material from which excellent fatigue strength can be obtained even when a carburized steel part is produced by welding before carburizing. Steel materials, in mass%, C: 0.09 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.40 to 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: 0.010~0.050%, Nb: 0.010~0.030%, N: 0.0080% or less, O: 0.0030% or less, B: 0.0003~0.0030%, Ca: 0.0005~0.0050 %, and balance: Fe and impurities, and has a chemical composition satisfying formulas (1) to (3) described in the specification. In a cross section parallel to the axial direction of the steel, Mn sulfides are 70.0/mm ² or less, and oxides are 25.0/mm ² or less.

Description

steel

The present invention relates to steel materials, and more particularly, to steel materials used as raw materials for carburized steel parts.

The steel materials used as the raw material of carburized machine structural parts (carburized steel parts) generally contain Mn, Cr, Mo, Ni, and the like. Steel materials used as raw materials for carburized steel parts are manufactured by casting, forging, rolling, or the like. And, carburized steel parts are manufactured, for example, by the following method. The above-mentioned steel materials are forged. A cutting process is given to the steel materials after forging, and an intermediate member is manufactured. A carburizing treatment is applied to the intermediate member. Through the above manufacturing process, a carburized steel part having a carburized layer, which is a hardened layer of the surface layer part, and a core part, which is a base material that is not affected by the carburizing treatment, is manufactured.

Among the costs of manufacturing carburized steel parts, the cost of cutting is very large. In the cutting process, since a large amount of cutting chips are also generated, the yield is lowered. For this reason, a technique of replacing cutting with forging has been studied. Forging methods can be roughly divided into hot forging, warm forging, and cold forging. Warm forging is performed at a lower temperature than hot forging. Therefore, in warm forging, compared with hot forging, there is little generation|occurrence|production of scale, and dimensional accuracy is improved rather than hot forging. On the other hand, in cold forging, there is no generation of scale, and the dimensional accuracy is equivalent to that of cutting. Therefore, a technique of performing finish machining by cold forging after roughing by hot forging, a technique of performing hard cutting as a finish after performing warm forging, and a technique of forming only by cold forging, etc. this has been reviewed. However, when the cutting process is replaced by warm forging or cold forging, if the deformation resistance of the steel material is large, the surface pressure applied to the mold increases and the life of the mold decreases. Therefore, the cost advantage of replacing cutting with forging becomes small. In addition, in the case of molding into a complex shape, problems such as cracking occur in areas where large processing is applied. Therefore, when cutting is replaced by forging, softening of steel materials or improvement of the limit working rate of steel materials is requested|required. When the steel material used as the raw material of the carburized steel parts is a rolled material, the limit working rate of the steel material can be increased by performing spheroidization 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 processing rate is large, and after carburizing treatment, a hardened layer and core hardness equivalent to conventional steel ( We propose a steel for carburizing with a high degree of hardness. In the steel for carburizing described in Patent Document 1, the chemical components, in mass%, 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: It is limited to 0.0030% or less, the balance is composed of Fe and unavoidable impurities, and the content expressed in mass% of each element in the chemical composition is the following formula 1 as a hardness index, the following formula 2 as a hardenability index, and, It is characterized in that it simultaneously satisfies Equation 3 below as a TiC precipitation index. 0.10 < C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.078 × Al < 0.235 ... (Equation 1), 7.5 < (0.7 × Si + 1) × (5.1 × Mn + 1) × (2.16 × Cr + 1) <44 ... (Equation 2), 0.004 < Ti-N × (48/14) < 0.030 ... (Equation 3). Patent Literature 1 describes that this carburizing steel has the above-mentioned chemical composition, thereby increasing the limit working rate during cold forging, and obtaining a hardened layer and steel part hardness equivalent to those of conventional steel after carburizing treatment.

International Publication No. 2012/108460

By the way, a plurality of carburized steel parts are used for parts for machine structures used in automobiles. For example, carburized steel parts are also used in variable diameter pulleys of continuously variable transmissions (CVT). As described above, large-sized carburized steel parts represented by variable-diameter pulleys are manufactured by cutting after hot forging. Therefore, even for large-sized carburized steel parts, a technique of substituting forging from cutting is being studied. However, when it is going to form a large-sized steel material by cold forging, an excessive load is applied to the cold forging machine. Therefore, when large-sized carburized steel parts are formed by cold forging, a plurality of members are formed by cold forging, and then these plurality of 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 having been prepared has been studied.

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

An object of the present disclosure is to provide steel materials from which excellent fatigue strength is obtained after carburizing even when welding is performed.

Steel according to the present disclosure,

in mass %,

C: 0.09 to 0.16%;

Si: 0.01 to 0.50%;

Mn: 0.40 to 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 to 0.030%;

Ti: 0.010 to less than 0.050%;

Nb: 0.010 to 0.030%;

N: 0.0080% or less;

O: 0.0030% or less;

B: 0.0003 to 0.0030%;

Ca: contains 0.0005 to 0.0050%,

The remainder consists of Fe and impurities, and satisfies formulas (1) to (3),

In a cross section parallel to the axial direction of the steel material, Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in mass% is 70.0 pieces/mm ² or less, and O in mass% The content of oxides of 10.0% or more 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 Equation (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 Equation (3)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in Formulas (1) to (3). When the corresponding element is not contained, "0" is substituted for the element symbol.

Even when the steel materials according to the present disclosure are welded, excellent fatigue strength is obtained in the carburized steel parts after the carburizing treatment.

1 is a cross-sectional view perpendicular to the longitudinal direction of steel materials in this embodiment.
2 is a schematic diagram for explaining a sampling position in microstructure observation of steel materials according to the present embodiment.
Fig. 3 is a schematic diagram for explaining a sampling position of a sample when measuring Mn sulfide and oxide in the present embodiment.

Hereinafter, steel materials according to the present embodiment, which are used as raw materials for carburized steel parts, will be described.

The inventors of the present invention conducted studies in order to obtain excellent characteristics (effective hardened layer depth and improvement in core hardness) of carburized steel parts after carburizing in steel materials used as raw materials for carburized steel parts. As a result, the present inventors obtained the following findings (a) to (f).

(a) Softening of steel materials before cold forging can be aimed at, so that C content is low. However, if the C content is too low, the fatigue strength of the carburized steel parts after carburizing is reduced to a level equivalent to that of conventional steel materials having a C content of about 0.20% (for example, SCR420 specified in JIS G 4052 (2008)). It becomes difficult to do In order to obtain the hardness of the core required as a carburized steel part, an appropriate lower limit of the C content exists.

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

(c) In order to increase the martensite fraction of the microstructure in the core part of the carburized steel part, the content of the alloy element (hardenability improving element) that improves the hardenability of the steel, such as Mn, Cr, Mo, Ni, etc., is quenched as described above. It is effective to contain so as to satisfy Formula (2) of a sex index.

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 above-mentioned hardenability improving element increases, the hardenability improving element strengthens ferrite by solid solution. In this case, the hardness of steel materials increases. Therefore, while suppressing the increase in the hardness of ferrite, while containing B capable of enhancing the hardenability of steel materials, the content of C and the hardenability improving element satisfies the above-mentioned hardness index equation (1).

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 effect of improving the hardenability of B, it is necessary to ensure sufficient solid solution B in the steel materials during carburizing treatment. So, Ti is contained in steel materials. In this case, during the carburizing treatment, most of the N in the steel is fixed as TiN. For this reason, it is possible to suppress that B is bonded to N and precipitated as BN, and a sufficient amount of solute B in the steel material can be secured. In order to obtain this effect effectively, it is preferable to contain Ti so that it may become stoichiometrically excess with respect to N content. In addition, in order to prevent abnormal grain growth of austenite grains during carburizing, TiC is finely dispersed and deposited in the microstructure. In order to ensure a sufficient amount of solid solution B and to finely disperse and precipitate TiC, the contents of Ti and N are made to satisfy the above-described 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, in the case of gas carburizing of the gas type in the denaturing furnace, the effect of improving the hardenability by the B content is low in the carburized layer, which is the surface layer portion of the carburized steel parts. This is because, during the carburizing treatment, nitrogen enters from the surface of the steel component, bonds with solid solution B, precipitates as BN, and the amount of solid solution B is reduced. Therefore, in order to secure hardenability in the carburized layer, which is the surface layer portion of the carburized steel part, it is effective to satisfy Equation (2) of the hardenability index described in (c) above.

The present inventors also studied the fatigue strength (joint fatigue strength) of carburized steel parts manufactured by carburizing after welding in the steel materials of the present embodiment. As a result, for inclusions in the cross section parallel to the longitudinal direction of the steel (ie, the axial direction of the steel), if the following rules are satisfied, the fatigue strength (joint fatigue strength) of carburized steel parts produced by carburizing after welding ) was found to increase.

(I) In terms of mass%, Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% is 70.0 pieces/mm ^{2 or} less.

(II) In terms of mass%, the oxide having an O content of 10.0% or more is 25.0 pieces/mm ² or less.

Hereinafter, this point is demonstrated in detail.

In the steel materials having the chemical composition of the present embodiment, Mn sulfides and oxides exist in the steel materials. Here, in this specification, Mn sulfide and oxide are defined as follows.

Mn sulfide: 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%, in terms of mass%, when the mass% of the inclusion is 100%.

Oxide: Inclusions having an oxygen content of 10.0% or more in terms of mass% when the mass% of the inclusions is 100%.

In the present specification, among inclusions, in terms of mass%, inclusions containing 10.0% or more of S, 10.0% or more of Mn, and 10.0% or more of O are not "Mn sulfides", but are included in "oxides".

When steel materials are subjected to welding represented by friction welding, laser welding, etc., and then carburized to produce carburized steel parts, an HAZ region exists in the carburized steel parts. The intensity of the HAZ region may be lower than that of other regions. So, in this embodiment, in order to ensure the intensity|strength of the HAZ area, inclusion is reduced as much as possible. In this embodiment, as described in the above (I) and (II), the number of Mn sulfides and oxides that occupy most of the inclusions in the steel is reduced. In this case, the strength of the HAZ region can be secured, and as a result, the fatigue strength of the carburized steel parts can be increased.

The steel materials of this embodiment completed based on the above knowledge have the following structure.

[One]

As steel,

in mass %,

C: 0.09 to 0.16%;

Si: 0.01 to 0.50%;

Mn: 0.40 to 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 to 0.030%;

Ti: 0.010 to less than 0.050%;

Nb: 0.010 to 0.030%;

N: 0.0080% or less;

O: 0.0030% or less;

B: 0.0003 to 0.0030%;

Ca: contains 0.0005 to 0.0050%,

The remainder consists of Fe and impurities, and satisfies formulas (1) to (3),

In a cross section parallel to the axial direction of the steel material, Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in mass% is 70.0 pieces/mm ² or less, and O in mass% The oxide content is 10.0% or more of 25.0 / 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 (mass %) of the corresponding element is substituted for each element symbol in Formulas (1) to (3). When the corresponding element is not contained, "0" is substituted for the element symbol.

[2]

As the steel described in [1],

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

steel.

[3]

As the steel described in [1],

When the radius in the cross section perpendicular to the axial direction of the steel member is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel member, the surface layer region at least to a depth of 0.1R from the surface is , It is made of ferrite and cementite, and the spheroidization rate of the cementite in the surface layer region is 90.0% or more,

steel.

[4]

As the steel 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 at least one element selected from the group consisting of

steel.

Hereinafter, steel materials according to the present embodiment will be described. "%" related to chemical composition means mass % unless otherwise specified.

[chemical composition]

The steel materials of this embodiment are raw materials for carburized steel parts. After the steel materials of this embodiment are cold forged, they are carburized and become carburized steel parts. The chemical composition of the steel materials of this embodiment contains the following elements.

C: 0.09 to 0.16%

Carbon (C) enhances the hardenability of the steel material and increases the hardness of the core portion in the carburized steel parts including the carburized layer and the core portion. If the C content is less than 0.09%, even if the other element content is within the range of the present embodiment, the hardness of the core portion of the carburized steel part decreases. On the other hand, when the C content exceeds 0.16%, even if the other element content is within the range of the present embodiment, the hardness of the steel materials before cold forging increases remarkably, and the limit working rate decreases. Therefore, the C content is 0.09 to 0.16%. In addition, the C content of steel materials used as materials for conventional carburized steel parts is about 0.20%. Therefore, the C content of the steel materials of this embodiment is lower than that of conventional steel materials. The lower limit of the C content is preferably 0.10%, more preferably 0.11%. The upper limit of the C content is preferably 0.15%, more preferably 0.14%.

Si: 0.01 to 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 is not obtained even if the other element content is within the range of the present embodiment. On the other hand, when the Si content exceeds 0.50%, even if the other element content is within the range of the present embodiment, the hardness of steel materials before cold forging increases and the limit working rate decreases. Therefore, the Si content is 0.01 to 0.50%. When the surface fatigue strength of carburized steel parts is important, the preferable lower limit of the Si content is 0.02%. In the case where importance is placed on improving the limiting processing rate of carburized steel parts, the upper limit of the Si content is preferably 0.48%, more preferably 0.46%.

Mn: 0.40 to 0.60%

Manganese (Mn) enhances 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%, even if the other element content is within the range of the present embodiment, this effect is not sufficiently obtained. On the other hand, when the Mn content exceeds 0.60%, even if the other element content is within the range of the present embodiment, the hardness of the steel materials before forging increases and the limit working rate decreases. Therefore, the Mn content is 0.40 to 0.60%. The lower limit of the Mn content is preferably 0.42%, more preferably 0.44%. The upper limit of the Mn content is preferably 0.58%, more preferably 0.56%.

P: 0.030% or less

Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at austenite grain boundaries, embrittles old austenite grain boundaries, and causes grain boundary cracks. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.026%, more preferably 0.024%. The one where P content is as low as possible is preferable. However, if P content is reduced to the limit, productivity will fall and manufacturing cost will become high. Therefore, in normal operation, the preferable lower limit of the P content is 0.001%.

S: 0.025% or less

Sulfur (S) is inevitably contained. That is, the S content is more than 0%. S combines with Mn to form MnS, and enhances the machinability of steel materials. If the S content is more than 0%, this effect is obtained to some extent. On the other hand, when the S content exceeds 0.025%, even if the other element content is within the range of the present embodiment, coarse MnS is generated, cracks are likely to occur during forging, and the limit working rate of steel materials is lowered. Therefore, the S content is 0.025% or less. The upper limit of the S content is preferably 0.022%, more preferably 0.020%. In the case of increasing the machinability more effectively, 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 increases the strength of the core of carburized steel parts. If the Cr content is less than 0.90%, even if the other element content is within the range of the present embodiment, this effect is not sufficiently obtained. On the other hand, when the Cr content exceeds 2.00%, even if the other element content is within the range of the present embodiment, the hardness of the steel materials before forging increases and the limit working rate decreases. Therefore, the Cr content is 0.90 to 2.00%. The lower limit of the Cr content is preferably 0.95%, more preferably 1.00%, still more preferably 1.10%. The upper limit of the Cr content is preferably 1.95%, more preferably 1.92%.

Mo: 0.10~0.40%

Molybdenum (Mo) enhances 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%, even if the other element content is within the range of the present embodiment, this effect is not sufficiently obtained. On the other hand, when the Mo content exceeds 0.40%, even if the other element content is within the range of the present embodiment, the hardness of the steel materials before forging increases and the limit working rate decreases. Therefore, the Mo content is 0.10 to 0.40%. The lower limit of the Mo content is preferably 0.11%, more preferably 0.12%, still more preferably 0.13%. The upper limit of the Mo content is preferably 0.38%, more preferably 0.36%, still more preferably 0.34%.

Al: 0.005 to 0.030%

Aluminum (Al) deoxidizes steel in the steelmaking process. Al also forms AlN when solid solution N exists in steel. However, in the steel materials according to the present embodiment, N in the steel is fixed as TiN by the addition of Ti. Therefore, solid solution N hardly exists in steel materials. As a result, Al does not form AlN and exists as solid solution Al in the steel material. Al present in a solid solution state enhances the machinability of steel materials. If the Al content is less than 0.005%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, when the Al content exceeds 0.030%, even if the other element content is within the range of the present embodiment, the hardness of the steel materials before forging increases and the limit working rate decreases. Therefore, the Al content is 0.005 to 0.030%. A preferable lower limit of Al is 0.010%, more preferably 0.011%, still more preferably 0.012%. A preferable upper limit of Al is 0.025%, more preferably 0.022%, still more preferably 0.020%.

Ti: 0.010 to less than 0.050%

Titanium (Ti) fixes N in steel materials as TiN and suppresses the formation of BN. For this reason, Ti secures the amount of solid solution B and enhances the hardenability of steel materials. Ti also forms TiC and suppresses coarsening of crystal grains during carburizing. If the Ti content is less than 0.010%, even if the other element content is within the range of the present embodiment, the above effect is not sufficiently obtained. On the other hand, if the Ti content is 0.050% or more, even if the other element content is within the range of the present embodiment, the TiC precipitation amount becomes excessively large. In this case, the limit working rate of steel materials before cold forging falls. Therefore, the Ti content is less than 0.010 to 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 to 0.030%

Nb (niobium) combines with N and C in steel to form Nb carbonitride. Nb carbonitride suppresses coarsening of crystal grains by a pinning effect. If the Nb content is less than 0.010%, even if the other element content is within the range of the present embodiment, the above effect is not sufficiently obtained. On the other hand, when the Nb content exceeds 0.030%, the effect is saturated. Therefore, the Nb content is 0.010 to 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 preferable 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, N content in steel materials is more than 0%. N combines with B to form BN, reducing the amount of dissolved B. In this case, the hardenability of steel materials deteriorates. If the N content exceeds 0.0080%, even if it contains 0.010 to less than 0.050% of Ti, N in the steel cannot be fixed as TiN, making it difficult to secure solid solution B contributing to hardenability. Also, coarse TiN is formed. Coarse TiN becomes a starting point of cracks during forging, and lowers the limit working rate of steel materials before forging. Therefore, the N content is 0.0080% or less. The upper limit of the N content is preferably 0.0078%, more preferably 0.0076%, still more preferably 0.0074%, still more preferably 0.0072%. The one where N content is as low as possible is preferable. However, when the N content is reduced to the limit, productivity decreases and manufacturing cost increases. Therefore, in normal operation, the lower limit of the N content is preferably 0.0001%, more preferably 0.0010%, still more preferably 0.0020%.

O: 0.0030% or less

Oxygen (O) is an unavoidable impurity. That is, O content in steel materials is more than 0%. O forms an oxide and reduces joinability when welding steel materials before carburizing treatment. In this case, the fatigue strength of the carburized steel parts is lowered. Therefore, the O content is 0.0030% or less. The preferable upper limit of the O content is 0.0029%, more preferably 0.0028%, still more preferably 0.0026%, still more preferably 0.0024%, still more preferably 0.0022%. The one where O content is low is preferable. However, when the O content is reduced to the limit, productivity decreases and manufacturing cost increases. Therefore, in normal operation, the lower limit of the O content is preferably 0.0001%, more preferably 0.0005%, still more preferably 0.0010%.

B: 0.0003~0.0030%

Boron (B) increases the hardenability of steel materials and increases the strength of carburized steel parts. If the B content is less than 0.0003%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, when the B content exceeds 0.0030%, the effect is saturated. Therefore, the B content is 0.0003 to 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%. The upper limit of the B content is preferably 0.0028%, more preferably 0.0026%, still more preferably 0.0024%.

Ca: 0.0005 to 0.0050%

Calcium (Ca) is contained in the oxide to spheroidize the oxide. A spheroidized oxide is difficult to form a cluster. Ca also inhibits the elongation of Mn sulfide. If the Ca content is less than 0.0005%, the above effect is not sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, when the Ca content exceeds 0.0050%, coarse sulfides and coarse oxides are formed, reducing the fatigue strength of the carburized steel parts. Therefore, the Ca content is 0.0005 to 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%. 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 rest of the chemical composition of the steel materials according to the present embodiment consists of Fe and impurities. Here, the term "impurity" refers to a component that is mixed from ore as a raw material, scrap, or the manufacturing environment when steel materials are industrially manufactured, and is not intentionally incorporated into the steel materials.

[For random elements]

The chemical composition of the steel materials of this embodiment may contain 1 type(s) or 2 or more types selected from the group which consists of Cu, Ni, and V instead of a part of Fe. All of these elements increase the strength of carburized steel parts.

Cu: 0.50% or less

Copper (Cu) is an arbitrary element and does not need to be contained. That is, 0% may be sufficient as Cu content. When Cu is contained, that is, when the Cu content exceeds 0%, Cu increases the hardenability of the steel material and increases the strength of the carburized steel parts. Moreover, Cu is an element which does not form an oxide or a nitride in a 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 from them. When even a little Cu is contained, the above effect is obtained to some extent. However, when the Cu content exceeds 0.50%, the ductility of steel materials in a high temperature range of 1000°C or higher decreases even if the other element content is within the range of the present embodiment. In this case, the yield at the time of continuous casting and rolling is reduced. In addition, the hardness of the steel before forging increases and the limit working rate decreases. Therefore, the Cu content is 0.50% or less. That is, the Cu content is 0 to 0.50%. A preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, still more preferably 0.35%.

Ni: 0.30% or less

Nickel (Ni) is an arbitrary element and does not need to be contained. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni increases the hardenability of the steel material and increases the strength of the carburized steel parts. When even a little Ni is contained, the above effect is obtained to some extent. However, when the Ni content exceeds 0.30%, even if the other element content is within the range of the present embodiment, the hardness of the steel before forging increases and the limit working rate decreases. Therefore, the Ni content is 0.30% or less. That is, the Ni content is 0 to 0.30%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%. The upper limit of the Ni content is preferably 0.29%, more preferably 0.28%, still more preferably 0.25%.

V: 0.10% or less

Vanadium (V) is an optional element and does not need to be contained. That is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms carbides and increases the strength of the core portion of the carburized steel part. When even a little V is contained, the above effect is obtained to some extent. However, when the V content exceeds 0.10%, even if the other element content is within the range of the present embodiment, the formability in cold forging of steel materials decreases, and the limit working rate decreases. Therefore, the V content is 0.10% or less. That is, the V content is 0 to 0.10%. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%. The upper limit of the V content is preferably 0.09%, more preferably 0.08%.

[About formula (1) to formula (3)]

The chemical composition of the steel materials of this embodiment also 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 Equation (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 Equation (3)

Here, the content (mass %) of the corresponding element is substituted for the element symbol in Formulas (1) to (3). If the corresponding element is an arbitrary element and is not contained, "0" is substituted for the element symbol. Hereinafter, each expression is demonstrated.

[Formula (1): hardness index]

It is defined as F1 = C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al. F1 is an index of the hardness of carburized steel parts manufactured using steel materials as a material.

In the steel materials of this embodiment, the C content is as low as 0.16% or less. Therefore, in the structure of the steel materials before forging, the ferrite fraction is significantly increased compared to the conventional steel materials having a C content of about 0.20%. In this case, the hardness of the steel material is greatly influenced not only by the C content (perlite fraction) but also by the hardness of ferrite. F1 represents the contribution of each alloying element to the solid solution strengthening of ferrite.

If F1 is 0.235 or more, the hardness of steel materials before cold forging is too high. In this case, the limiting processing rate of steel materials is lowered. On the other hand, when F1 is 0.140 or less, core hardness as a carburized steel part is insufficient. Therefore, F1 is more 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. A preferable upper limit of F1 is less than 0.230, more preferably 0.225, still more preferably 0.220, still more preferably 0.215, still more preferably 0.210. In addition, the F1 value is a value obtained by rounding off the fourth decimal place of the calculated value.

[Equation (2): Hardenability Index]

It is defined as F2 = (0.70 x Si + 1) x (5.1 x Mn + 1) x (2.2 x Cr + 1) x (3.0 x Mo + 1) x (0.36 x Ni + 1). F2 is an index for hardenability of steel.

As described above, B is effective in increasing the hardenability of steel materials and increasing the hardness of the core portion 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 by B content is low in the carburized layer that is the surface layer portion of the carburized steel parts. This is because during the carburizing treatment, N in the atmospheric gas in the furnace penetrates into the surface layer portion of the carburized steel parts, and solid solution B precipitates as BN, and the amount of solid solution B contributing to the hardenability improvement is insufficient. Therefore, in the case of gas carburizing, B can increase the hardness of the core of the carburized steel parts, but it does not contribute to the improvement of the hardness of the carburized layer of the carburized steel parts. Therefore, in order to secure hardenability in the carburized layer, which is the surface layer of the carburized steel part, it is necessary to utilize hardenability improving elements other than B.

F2 is composed of elements other than B that contribute to quenching improvement. When F2 is 13.0 or less, under the same carburizing treatment conditions, compared to the conventional steel material (C content of about 0.20%), equivalent or higher carburized layer depth (Vickers hardness is HV550 or more). Cannot be sufficiently obtained. On the other hand, when F2 is 45.0 or more, the hardness of steel materials before cold forging increases and the limit working rate decreases. Therefore, F2 is more 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 lower limit of F2 is preferably 13.2, more preferably 13.5, still more preferably 14.0, still more preferably 14.5, still more preferably 15.0. In addition, the F2 value is a value obtained by rounding off the second decimal place of the calculated value.

[Equation (3): TiC precipitation index]

It is defined as F3=Ti-N×(48/14). F3 is an index related to the TiC precipitated amount. When Ti is stoichiometrically excess 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" represents the atomic weight of Ti.

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

A steel material whose chemical composition simultaneously satisfies the hardness index F1, the hardenability index F2, and the TiC precipitation index F3 has a limiting work rate during cold forging greater than that of conventional steel by performing spheroidizing heat treatment. And, after the carburizing treatment of this steel material, it is possible to obtain a carburized steel part having a hardened layer and core hardness equivalent to that of conventional steel.

[About inclusions in steel]

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 (ie, the longitudinal direction of the steel material).

(I) Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in terms of mass% is 70.0 pieces/mm ² or less.

(II) Oxide content of 10.0% or more in terms of mass% is 25.0 pieces/mm ² or less.

Here, in this specification, Mn sulfide and oxide are defined as follows.

Mn sulfide: 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%, in terms of mass%, when the mass% of the inclusion is 100%.

Oxide: Inclusions having an O content of 10.0% or more in terms of mass% when the mass% of the inclusions is 100%.

As described above, in the manufacturing process of carburized steel parts, in the case of integrally manufacturing an intermediate member before carburizing 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 obtained carburized steel parts. The joint fatigue strength of the HAZ region may be lower than that of other regions. In order to ensure joint fatigue strength in the HAZ region, inclusions in steel materials are reduced as much as possible. If the Mn sulfide and oxide satisfy the above (I) and (II), joint fatigue strength in the HAZ region can be secured. As a result, it is possible to increase the joint fatigue strength of the carburized steel parts integrated by joining.

[Methods for measuring Mn sulfides and oxides]

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

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

An inclusion in each visual field is specified. For each specific inclusion, using energy dispersive X-ray spectroscopy (EDX), Mn sulfides and oxides are identified. Specifically, using EDX, elemental analysis is performed at at least two measuring points in each inclusion. Then, in each inclusion, the arithmetic average 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 measuring points in one inclusion, the arithmetic mean value of the Mn content, the arithmetic mean value of the S content, and the arithmetic mean value of the O content obtained at the two measuring points are taken as the Mn content in the inclusion. (% by mass), S content (% by mass), and O content (% by mass).

In the result of elemental analysis of the specified inclusions, 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% in terms of mass% are defined as Mn sulfides. Further, in elemental analysis of inclusions, Ti and Ca may also be detected as elements other than Mn and S in some cases. In this case, if the above conditions are satisfied, all are defined as Mn sulfides. Further, in the result of elemental analysis of the identified inclusions, inclusions having an O content of 10.0% or more are defined as oxides. Al, Si, Mg, Ca, Ti, and the like may be detected in inclusions defined as oxides. Even in this case, if the above condition is satisfied, it is identified as an oxide. Incidentally, among inclusions, inclusions containing 10.0% or more of S, 10.0% or more of Mn, and 10.0% or more of O in terms of mass% are identified as oxides.

The inclusions to be identified are inclusions 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 it is an inclusion whose circle equivalent diameter is twice or more than the beam diameter of EDX, the precision of elemental analysis increases. In the present embodiment, the beam diameter of EDX used to identify inclusions is set to 0.2 μm. In this case, inclusions having an equivalent circular diameter of less than 0.5 µm cannot increase the accuracy of elemental analysis in EDX. Inclusions having an equivalent circular diameter of less than 0.5 μm also have an extremely small effect on the fatigue strength. Therefore, in the present embodiment, Mn sulfides and oxides having an equivalent circle diameter of 0.5 µm or more are measured. In addition, the upper limit of the equivalent circle diameter of Mn sulfide and oxide is not particularly limited, but is, for example, 100 μm.

Based on the total number of Mn sulfides specified in each visual field and the total area of 20 visual fields, the number of Mn sulfides per unit area (pcs/mm ² ) is determined. Further, the number of oxides per unit area (piece/mm ² ) is determined based on the total number of oxides specified in each visual field and the total area of 20 visual fields.

In the steel material of the present embodiment, the content of each element in the chemical composition is within the above-mentioned range, the hardness index F1 satisfies the formula (1), the hardenability index F2 satisfies the formula (2), and the TiC precipitation amount Index F3 satisfies equation (3). Further, in a cross section parallel to the axial direction of the steel material, Mn content of 10.0% or more, S content of 10.0% or more, and O content of less than 10.0% in mass% is 70.0 pieces/mm ^{2 or} less, and in mass% Oxide content of 10.0% or more is 25.0 pieces/mm ² or less. Therefore, even when welding is performed before carburizing, the carburized steel parts after carburizing have excellent fatigue strength.

[Micro structure of steel materials]

The microstructure of the steel materials of this embodiment is not specifically limited. The steel material of this embodiment may be a rolled material (ie, an As-rolled material) or may be subjected to a spheroidizing treatment.

The radius in the cross section perpendicular to the axial direction (longitudinal direction) of the steel materials of this embodiment is defined as R (mm). The microstructure in the cross section perpendicular to the axial direction of steel materials is either of the following (A) and (B).

(A) In the microstructure, the area ratio of bainite in the surface layer region at least from the surface to a depth of 0.1R is 95.0% or more.

(B) Microstructure WHEREIN: At least the surface layer region from the surface to the depth of 0.1R consists of ferrite and cementite, and the spheroidization rate of cementite in the surface layer region is 90.0% or more.

The microstructure of said (A) is a microstructure in the case where the steel materials of this embodiment are a rolled material. The microstructure of said (B) is a microstructure in case the steel materials of this embodiment are spheroidized.

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

When the steel materials of this embodiment are rolled materials, as described in (A), in a cross section perpendicular to the axial direction of the steel materials, at least the surface layer region is a bainite structure. Here, "a bainite structure" in this specification means that the bainite area ratio is 95.0% or more. That is, in the cross section perpendicular to the axial direction of the steel materials 1 of the present embodiment, the bainite area ratio of at least the surface layer region D is 95.0% or more. Further, "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 materials 1 may be a bainite structure. When the surface layer area|region carries out the spheroidizing process of the steel materials which have a bainite structure, cementite is easy to spheroidize. Therefore, the spheroidization rate of cementite in the surface layer region becomes 90.0% or more.

The microstructure of the steel materials of this embodiment may be (B) instead of (A). As described in case (B) when the steel materials of this embodiment are spheroidized, in the cross section perpendicular to the axial direction of the steel materials of this embodiment, at least the surface layer region is a spheroidized cementite structure. Here, "spheroidized cementite structure" means that a microstructure consists of ferrite and cementite, and the spheroidization rate of cementite in a microstructure is 90.0% or more. In addition, "at least the surface layer region is a spheroidized cementite structure" means that the spheroidized cementite structure may be formed not only to the surface layer region but also to a region deeper 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 deeper than 0.1R. For example, the depth of the spheroidized cementite structure may be 0.2R, 0.3R, or 1.0R. That is, all of the cross sections perpendicular to the axial direction of the steel materials 1 may be spheroidized cementite.

When the microstructure of the steel materials of this embodiment is (A), that is, when steel materials are a rolled material, steel materials are subjected to spheroidization heat treatment before cold forging. For this reason, the microstructure of steel materials becomes (B).

When the microstructure of the steel materials of this embodiment is (B), compared with the structure|tissue in which a microstructure consists of ferrite and pearlite, cold forging property (limit working rate) can be improved.

Microstructure observation in a cross section perpendicular to the longitudinal direction of the steel material is performed by the following method. Referring to FIG. 2, among cross sections perpendicular to the longitudinal direction of the steel material 1, when the radial depth from the surface is d (mm), four places at d = 0.05R position (90 ° pitch in FIG. 2 ) to take samples from In addition, samples are taken from four locations (90° pitch in Fig. 2) at the position of d = 0.1R. The surface of each collected sample is used as the observation surface. After polishing the observation surface of each sample to a mirror surface, it is immersed in a nital etching solution for about 10 seconds, and the structure is exposed by etching. The etched observation surface is observed in three fields of view with a secondary electron image using a scanning electron microscope (SEM). The area of each visual field is 400 μm ² (magnification 5000 times). In each visual field, each phase, such as bainite, ferrite, pearlite, and cementite, can be distinguished as follows. When the observation surface is etched with a nital etchant, a phase having a lamellar structure can be identified as pearlite in SEM observation. A phase in which there is no underlying structure within the particle can be characterized as ferrite. A phase in which a lath-like structure develops from the old γ grain boundary can be identified as bainite. A phase that is granular and has high luminance can be specified as cementite. In addition, the brightness|luminance of the layered cementite in pearlite is about the same as the brightness|luminance of the above-mentioned granular cementite.

Photo images of 3 views at each location (8 locations) are generated. The number of photographic images is 8 places x 3 fields of view = 24. Each phase in each photograph image is specified by the method described above. Image identification by contrast can be realized by a known image processing method.

In the photographic image of each visual field, when the microstructure is a bainite-based structure, it is judged that the microstructure of the steel material is likely to be (A), and the bainite area ratio (%) is obtained by the following method.

Area ratio of bainite (%) = total area of bainite in 24 fields of view / total area of 24 fields of view × 100

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

In the photographic image of each visual field, when the microstructure consists of ferrite and cementite, it is determined that the microstructure of steel materials is likely to be (B), and the spheroidized cementite rate (%) is obtained by the following method.

The major axis (μm) and the minor axis (μm) of each cementite in each visual field (24 visual fields in total) are determined. Among the straight lines connecting arbitrary two points on the interface between cementite and the parent phase (ferrite), the length of the largest straight line is defined as the major axis (μm) of the cementite. Among the straight lines connecting arbitrary two points on the interface between cementite and the parent phase, the length of the straight line perpendicularly intersecting the major axis is defined as the minor axis (μm) of the cementite. Cementite having a calculated major diameter of 0.1 µm or more is used as a measurement target (count target). Next, the aspect ratio (major axis/minor axis) of each cementite made into a measurement object is calculated|required. The aspect ratio can be obtained by 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 rate (%). When the calculated|required spheroidized cementite rate is 90.0 % or more, it recognizes that the surface layer area|region of at least 0.1R is a spheroidized cementite structure.

[Carburized steel parts]

Next, carburized steel parts manufactured using the steel material of the present embodiment as a material will be described.

The carburized steel parts of the present embodiment include a carburized layer formed on the surface layer and a core portion inside the carburized layer. The carburized layer has an effective hardened layer depth of less than 0.4 to 2.0 mm in thickness. Here, the effective hardened layer depth means the depth from the surface at which the Vickers hardness becomes HV550 or more. In this carburized layer, it is preferable that the Vickers hardness at a position at a depth of 50 μm from the surface is 650 to 1000 HV. In the carburized layer, the microstructure at a depth of 0.4 mm from the surface contains 90 to 100% of martensite in area%, and preferably has a Vickers hardness of 600 to 900 HV.

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

When the microstructure in the carburized layer at a depth of 0.4 mm from the surface contains 90 to 100% of martensite, and the Vickers hardness in the carburized layer at a depth of 0.4 mm from the surface is 600 to 900 HV, Fatigue strength and fatigue strength are higher. More preferably, the Vickers hardness at a position at a depth of 0.4 mm from the surface is 620 to 900 HV.

Moreover, it is preferable that the Vickers hardness is 250-500 HV in the deep part at the position of 2.0 mm in depth from the surface. In addition, the chemical composition at a position at a depth of 2.0 mm from the surface becomes the above-mentioned chemical composition. More preferably, the Vickers hardness at a position at a depth of 2.0 mm from the surface is 270 to 450 HV. When the microstructure at a position at a depth of 2.0 mm from the surface contains at least one of martensite and bainite, the above effect is further obtained, so it is preferable.

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 including a position at a depth of 0.4 mm from the surface of the carburized steel part is taken on the surface. The surface of the sample is etched with a picral liquid. Of the surface after etching, arbitrary 3 fields of view are observed as secondary electron images using SEM. The area of each visual field is 400 μm ² (magnification 5000 times). In SEM observation, martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite can be discriminated as follows. Specifically, a phase having a lamellar structure can be specified as pearlite. A phase in which there is no underlying structure within the particle can be characterized as ferrite. A phase containing a lath-like structure can be specified as martensite and bainite. Further, tempered martensite and tempered bainite contain a lath-like structure and also contain carbides in the lath. A phase that is granular and has high luminance can be specified as cementite. The luminance of layered cementite in pearlite is about the same as that of the above-mentioned granular cementite. In addition, martensite and bainite both contain lath-like structures as described above, and in the present specification, martensite and bainite are not distinguished in the microstructure of carburized steel parts.

The total area of martensite in 3 visual fields of the sample at a depth of 0.4 mm was obtained. The ratio of the calculated total area of martensite to the total area of the three visual fields is defined as the area ratio (%) of martensite at a depth of 0.5 μm. In the calculation of the area ratio of martensite, ferrite, pearlite, martensite and bainite, tempered martensite, tempered bainite, spheroidized cementite, and cementite are considered. Calculation of the above area ratio does not include precipitates other than cementite such as BN, TiC, TiN, and AlN, inclusions, retained austenite, and the like.

The Vickers hardness of carburized steel parts is measured by the following method. The cross section perpendicular to the arbitrary surface of the carburized steel part is taken as 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 determined by a Vickers hardness test based on JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force is 0.49 N. The Vickers hardness HV at 10 locations at a depth of 50 μm is measured, and the average value is taken as the Vickers hardness HV at a location at a depth of 50 μm. Moreover, the Vickers hardness HV of 10 locations at a depth of 0.4 mm is measured, and the average value is taken as the Vickers hardness HV at a location at a depth of 0.4 mm. If the Vickers hardness at a depth of 0.4 mm is 550 HV or more, it is determined that the depth of the carburized layer is at least 0.4 mm or more. Further, 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 is 0.49 N. The Vickers hardness HV at 10 positions at a depth of 2.0 mm is measured, and the average value is taken as the Vickers hardness HV at a position at a depth of 2.0 mm. The measurement surface of the Vickers hardness is not particularly limited, but may be a cut surface orthogonal to the axial direction (longitudinal direction) of the carburized steel parts.

[Method of manufacturing steel and carburized steel parts]

The steel materials according to the present embodiment and the manufacturing method of the carburized steel parts will be described.

[Method of manufacturing steel]

First, an example of the manufacturing method of the steel materials of this embodiment is demonstrated. An example of the manufacturing method of steel materials includes a steelmaking process, a casting process, a hot working process, and a cooling process. Hereinafter, each process is demonstrated.

[Steel making process]

A steelmaking process includes a refining process and a casting process.

[Refining process]

In the refining step, first, molten iron produced by a known method is refined in a converter (primary refining). Secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, alloying elements are added to molten steel to produce molten steel that satisfies the above chemical composition.

Specifically, deoxidation treatment is performed by adding Al to molten steel tapped from a converter. After the deoxidation treatment, a sawing treatment is performed. After the sawn treatment, a secondary refinement is performed. The secondary refining is performed, for example, complex refining. For example, at first, a refining process using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed. Further, RH (Ruhrstahl-Hausen) vacuum degassing is performed. After that, final adjustments are made to the other alloy components except for Si and Ca.

After performing secondary refining and adjusting the components of molten steel other than Si and Ca, the following treatments (heating and holding step and final component adjustment step) are performed on the molten steel.

[Heating and holding process]

The molten steel in the ladle after secondary refining (final component adjustment) is heated at a temperature of 1500 to 1600°C with a holding time ts twice or more than 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, V _g : Gas flow rate (Nm ³ /min), M _l : Mass of molten steel in the ladle (ton), T _l : Temperature of molten steel (K), h ₀ : Depth of gas injection (m), P ₀ : Surface of molten steel 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 aggregate and coalesce. For this reason, the oxide cannot be removed by floating, and the number of oxides increases. Also, when the holding time ts is less than twice the uniform mixing time τ, Mg or the like mixed in from the slag is combined with S in the molten steel to form MgS or the like, and MgS is dispersed in the molten steel. This dispersed MgS serves as a precipitation site for MnS. As a result, the number of Mn sulfides increases.

When the holding time ts is twice or more than the uniform mixing time τ, the number of oxides in the steel can be suppressed. In addition, since once-formed MgS becomes MgO by re-oxidation, the number of MnS precipitation sites decreases, and as a result, the number of Mn sulfides in steel can also be suppressed. As a result, after the final component adjustment step, which is the next step, the Mn sulfide content is 70.0/mm ² or less, and the oxide content is 25.0/mm ² or less.

[Final component adjustment process]

Si and Ca are added to the molten steel after the heating and holding step to manufacture molten steel that satisfies the chemical composition and formulas (1) to (3) described above. Si and Ca may be added to molten steel as independent raw materials, respectively. A Si-Ca alloy may be added to molten steel as a raw material.

When Si and Ca are added to sufficiently uniformly heated molten steel in the heating and holding step, oxides are reformed from Al ₂ O ₃ to complex inclusions including SiO ₂ and CaO, and Mn sulfides are also reformed into sulfides containing Ca. do. Therefore, assuming that the holding time ts is twice or more than the uniform mixing time τ, the Mn sulfide content is 70.0 particles/mm ² or less, and the oxide content is 25.0 particles/mm ² or less.

If Si is added before adding Al to molten steel, deoxidation is not sufficiently performed, and as a result, the oxide content exceeds 25.0 pieces/mm ² . By adding Si and Ca to the molten steel after adding Al, the Mn sulfide becomes 70.0/mm ² or less, and the oxide 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 of addition of Si and Ca is not particularly limited. Si and Ca may be added simultaneously. Either Si or Ca may be added first.

[Casting process]

A raw material (cast piece or ingot) is manufactured using the molten steel manufactured by the refining process. Specifically, a cast steel is manufactured by a continuous casting method using molten steel. Alternatively, it is good also as an ingot by the ingot method using molten steel. A hot working process, which is the next step, is performed using this cast steel or ingot. Hereinafter, the cast steel or ingot is referred to as "material".

[Hot working process]

In the hot working step, hot working is performed on the raw material (bloom or ingot) prepared in the casting step to manufacture steel materials. The shape of the steel is not particularly limited, but 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 steel bar, it can be manufactured in the same hot working process.

Hot working includes a rough rolling process and a finish rolling process. In the rough rolling process, a raw material is hot-worked to manufacture a billet. A rough rolling process uses a bundling mill, for example. Bulking is performed on the raw material with a bulking mill to manufacture a billet. In the case where a continuous rolling mill is provided downstream of the lump rolling mill, a billet having a smaller size may be produced by additionally performing hot rolling on the billet after the lump rolling using the continuous rolling mill. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. Through the above process, the raw material is manufactured into a billet in the rough rolling process. The heating temperature in the rough rolling step is not particularly limited, but is, for example, 1100 to 1300°C.

In the finish rolling step, the billet is heated using a heating furnace or a hot-dipping furnace. With respect to the billet after heating, hot rolling is performed using a continuous rolling mill to manufacture steel materials (steel bars). The heating temperature in the finish rolling step is not particularly limited, but is, for example, 1000 to 1250°C.

[Cooling process]

In a cooling process, steel materials immediately after completion of a hot working process are cooled. Specifically, the temperature range in which the surface temperature of steel materials becomes 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 in the surface layer region from at least to a depth of 0.1R from the surface in the microstructure in the cross section perpendicular to the axial direction is 95.0% or more. That is, the steel materials (rolled material) whose microstructure is (A) are manufactured by the above manufacturing process.

[Spheroidization heat treatment process]

With respect to the steel materials after the said cooling process, a spheroidization heat treatment process may be further performed, and it is good also considering the steel materials of this embodiment as steel materials whose microstructure is (B). That is, in this case, spheroidizing heat treatment is performed to manufacture steel materials having a microstructure (B).

The spheroidizing heat treatment may be performed by a known method. The spheroidization heat treatment is performed, for example, by the following method. The steel material after the cooling process is heated to a temperature just below or just above the A _c1 point (the temperature at which austenite starts to form during heating) (for example, within the A _c1 point + 50 ° C) and maintained for a predetermined time After that, cool it down. Alternatively, the steel material after the cooling step is heated to a temperature just above the A _c1 point, and cooled to a temperature just below the A _r1 point (temperature at which austenite completes transformation into ferrite, ferrite, or cementite during cooling) The process may be repeated several times. Alternatively, the steel materials after the cooling step may be quenched once, and then tempered for 3 to 100 hours in a temperature range of 600 to 700°C. In addition, the known annealing or spheroidizing heat treatment method as described above may be applied to the method of the spheroidization heat treatment, and is not particularly limited.

In the steel material manufactured by performing the spheroidization heat treatment, in the microstructure in the cross section perpendicular to the axial direction, at least the surface layer region from the surface to a depth of 0.1R consists of ferrite and cementite, and the spheroidization rate of the cementite is 90.0 more than % That is, the steel materials whose microstructure is (B) are manufactured by the above manufacturing process.

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

[Method of manufacturing 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 material will be described. This manufacturing method includes a cold forging step of performing cold forging on the above-mentioned steel materials to manufacture a plurality of intermediate members, a welding step of welding the plurality of intermediate members manufactured as needed to form a single product, and According to this, it includes a cutting process for cutting the intermediate member, a carburizing process for carburizing the intermediate member, and a tempering process for tempering the intermediate steel after the carburizing process. In addition, in this specification, carburizing treatment also includes carburizing nitriding treatment.

In addition, when the microstructure of the steel materials of this embodiment is (A), that is, when the steel materials of this embodiment are a rolled material, after performing the above-mentioned spheroidizing heat treatment process with respect to steel materials, a cold forging process is implemented. Before performing the spheroidization heat treatment step for the steel materials having a microstructure of (A), a cold drawing step such as a wire drawing step is performed as needed.

[Cold forging process]

In a cold forging process, cold forging is given to the steel materials manufactured by the manufacturing method mentioned above, a shape is provided, and several intermediate members are manufactured. Cold forging conditions, such as a working rate and a deformation rate, in this cold forging process are not specifically limited. What is necessary is just to select suitable conditions suitably for cold forging conditions. The plurality of intermediate members are welded and integrated in the next step, the welding step.

[welding process]

The welding process is an arbitrary process and does not need to be performed. In the case of carrying out, in a welding process, a plurality of intermediate members described above are welded and integrated by friction welding or laser welding. The welding method is not particularly limited. After welding, you may form the joint surface of an intermediate member flat by machining. In the steel of this 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 is excellent in joinability, and even when an intermediate member is welded to form a carburized steel part, the joint fatigue strength of the carburized steel part is excellent.

[Cutting process]

The cutting process is an arbitrary process and does not need to be performed. In the case of carrying out, in the cutting process, the intermediate member after the cold forging process and before the carburizing process described later is cut and shaped. By performing the cutting process, it is possible to give the carburized steel parts a precise shape that is difficult only in the cold forging process.

[Carburizing process]

In the carburizing process, carburizing is performed on an intermediate member (when the welding process is performed, an intermediate member joined integrally). In the carburizing process, a well-known carburizing process is performed. The carburizing process includes a carburizing process, a diffusion process, and a quenching process.

Carburizing treatment conditions in the carburizing process and the diffusion process may be appropriately adjusted. The carburizing temperature in the carburizing process and the diffusion process is, for example, 830 to 1100 ° C. The carbon potential in the carburizing process and the diffusion process is, for example, 0.5 to 1.2%. The holding time in the carburizing process is, for example, 60 minutes or more, and the holding time in the diffusion process is 30 minutes or more. The carbon potential in the diffusion step is preferably lower than the carbon potential in the carburizing step. However, the conditions in the carburizing process and the diffusion process are not limited to the above-mentioned conditions.

After the diffusion process, a well-known quenching process is performed. In the quenching process, the intermediate member after the diffusion process is maintained at a quenching temperature equal to or higher than the _Ar3 transformation point. The holding time at the quenching temperature is not particularly limited, but is, for example, 30 to 60 minutes. Preferably, the quenching temperature is lower than the carburizing temperature. It is preferable to set the temperature of the quenching medium to room temperature to 250°C. The quenching medium is, for example, water or oil. Further, if necessary, sub-zero treatment may be performed after quenching.

[Tempering process]

A well-known tempering process is implemented with respect to the intermediate member after a carburizing process process. Tempering temperature is 100-250 degreeC, for example. The holding time at the tempering temperature is, for example, 60 to 150 minutes.

[Other processes]

If necessary, further grinding or shot peening may be performed on the carburized steel parts after the tempering step. By performing the grinding process, precise shapes can be imparted to the carburized steel parts. In addition, by performing the shot peening process, compressive residual stress is introduced into the surface layer portion of the carburized steel part. The compressive residual stress inhibits the initiation and propagation of fatigue cracks. Therefore, the fatigue strength of carburized steel parts is increased. For example, when the carburized steel part is a toothed wheel, the fatigue strength of the tooth root and tooth surface of the carburized steel part can be improved. The shot peening process may be performed by a known method. It is preferable to carry out the shot peening process using shot grains having a diameter of, for example, 0.7 mm or less, under conditions of an arc height of 0.4 mm or more.

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

Example

The effect of one aspect of the steel materials of the present disclosure will be described more specifically by examples. The conditions in the examples are examples of conditions employed to confirm the feasibility and effect of the steel materials of the present disclosure. The steel materials of this indication are not limited to this one conditional example. Various conditions can be employed for the steel materials of the present disclosure as long as the purpose 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 cast steel. A blank part in Table 1 means that the content of the corresponding element was less than the detection limit. That is, the blank part means that the minimum number of digits of the corresponding element content was less than the detection limit. For example, in the case of Cu content in Table 1, the minimum number of digits is the second decimal place. Therefore, the Cu content of Test No. 1 means that it was not detected in the number of digits to the second decimal place (significant digits were 0% in the content to the second decimal place).

"Steelmaking conditions (1)" in Table 2 shows the ratio (=ts/τ) of the holding time ts at 1500 to 1600°C after the secondary refining to the uniform mixing time τ. "Steelmaking conditions (2)" in Table 2 shows the order of addition of Al, Si, and Ca. In the column of "Steelmaking conditions (2)", "1" means that Si and Ca were added after Al was added and deoxidized. "2" means that after adding Si, Al and Ca were added. In addition, about test numbers 22 and 24, the steelmaking process was performed targeting the chemical composition of steel number B1. About Test Nos. 23 and 25, the steelmaking process was performed targeting the chemical composition of steel No. C1.

After heating the manufactured cast steel at 1100 to 1300 ° C., a rough rolling step was performed to obtain a billet having a cross section perpendicular to the longitudinal direction of 162 mm × 162 mm. The finish rolling process was performed using this billet. In the finish rolling step, hot rolling was performed using a continuous rolling mill using a billet heated to 1000 to 1250 ° C. to produce a steel bar having a circular cut surface orthogonal to the longitudinal direction and a diameter of 30 mm. A cooling process was performed on the steel bar immediately after the finish rolling process. The average cooling rate (°C/sec) at 800 to 500°C in the cooling step was as shown in Table 2. For each test number, a plurality of steel bars (hereinafter referred to as "rolled stock") after the cooling step were prepared.

In each test number, a spherodizing heat treatment process (SA process: Spherodizing Annealing) was performed on some of the prepared steel bars. In the spheroidization heat treatment, the above steel bar was heated at 740°C. Thereafter, the steel bar 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 normal temperature, and a steel bar subjected to the spheroidization heat treatment process was manufactured (hereinafter referred to as “SA material”). Steel materials (rolled material, SA material) of each test number were manufactured by the above manufacturing method.

[Evaluation test]

About the steel materials of each test number, the following test was implemented.

[Microstructure observation test]

A microstructure observation test was conducted by the following method. Specifically, among the cross sections perpendicular to the longitudinal direction of the steel of each test number, when the radial depth from the surface is d (mm), d = 0.05R 4 locations (90 ° pitch in Fig. 2) samples were taken from In addition, samples were taken from four locations (90° pitch in Fig. 2) at the position of d = 0.1R. The surface S of each collected sample was used as the observation surface. After the observation surface of each sample was polished to a mirror surface, it was immersed in a nital corrosion solution for about 10 seconds, and the structure was exposed by etching. The etched observation surface was observed for three views on the secondary electron image using SEM. The visual field area was 400 µm ² (magnification 5000 times). In each visual field, bainite and other phases (ferrite, pearlite, cementite, etc.) could be distinguished as described above.

For the as-rolled material, the bainite area ratio (%) was determined by the following method.

Area ratio of bainite (%) = total area of bainite in 24 fields of view / total area of 24 fields of view × 100

When the obtained bainite area ratio was 95.0% or more, it was recognized that at least the surface layer region at a depth of 0.1R from the surface was a bainite structure ("Y" in "microstructure of rolled material" in Table 2). On the other hand, when the obtained bainite area ratio was less than 95.0%, it was recognized that the surface layer region at a depth of 0.1R from the surface was not a bainite structure ("N" in "microstructure of rolled material" in Table 2).

About SA material, the spheroidized cementite rate (%) was calculated|required by the following method. First, the major axis (μm) and the minor axis (μm) of each cementite in each visual field (24 visual fields) were determined. Among the straight lines connecting arbitrary two points on the interface between cementite and the parent phase (ferrite), the length of the largest straight line was defined as the major axis (μm) of the cementite. Among the straight lines connecting arbitrary two points on the interface between cementite and the parent phase, the length of the straight line perpendicularly intersecting the major axis was defined as the minor axis (μm) of the cementite. Cementite having a determined major axis of 0.1 µm or more was used as a measurement target (count target). Next, the aspect ratio (major axis/minor axis) of each cementite as a measurement object was determined. Cementite having an aspect ratio of 3.0 or less was defined as "spheroidized cementite". The ratio (%) of the total number of objects of spheroidized cementite in 24 visual fields to the total number of objects of cementite in 24 visual fields was defined as the spheroidized cementite rate (%). When the calculated spheroidized cementite rate was 90.0% or more, it was recognized that the surface layer region of at least 0.1R was a spheroidized cementite structure ("Y" in "microstructure of SA material" in Table 2). On the other hand, when the area ratio of the obtained spheroidized cementite was less than 90.0%, it was recognized that the surface layer region at a depth of 0.1R from the surface was not a spheroidized cementite structure ("N" in "microstructure of SA material" in Table 2). Further, in the microstructures of the 24 fields of view of the SA materials of each test number, all were structures composed of ferrite and cementite. That is, in the SA material of each test number, all the microstructures in the surface layer region were structures composed of ferrite and cementite.

[Limited Compression Ratio Measurement Test]

Compression test pieces were produced from the steel materials of each test number having a diameter of 30 mm so that the longitudinal direction of the steel materials became the compression direction. The diameter of the compression test piece was 29.5 mm, and the length was 44 mm. The central axis of the compression test piece substantially coincided with the central axis of the steel material. Notches were formed in the circumferential direction of the central position in the longitudinal direction of the compression test piece. The notch angle was 30°, the notch depth was 0.8 mm, and the radius of curvature of the notch tip was 0.15 mm. Compression test pieces were collected from the rolled material and the SA material, respectively. Hereinafter, in each test piece, a sample taken from a rolled material is referred to as a "rolled test piece", and a sample taken from an SA material is referred to as an "SA test piece".

The compression test piece described above (rolled test piece, SA test piece) was subjected to a limit compression test by the following method. For the limit compression test, a 500 ton hydraulic press was used. For each test piece, cold compression was performed at a speed of 10 mm/min using a restraining die. Compression was stopped when a micro crack of 0.5 mm or more occurred in the vicinity of the notch, and the compression rate (%) at that time was calculated. This measurement was performed 10 times in total to determine the compression ratio (%) at which the cumulative breakage probability was 50%, and the compression ratio was defined as the limit compression ratio (%). The limiting compressibility (%) of each test number is shown in Table 2.

As for the rolled material, as described above, cold drawing such as wire drawing may be performed before the spheroidization heat treatment step. In this case, the rolled material needs to have workability that does not cause breakage due to internal cracks (chevron cracks) in cold drawing. Therefore, for the test piece in the rolled state, it was judged that the limit working rate was excellent when the limit compression ratio was 50% or more. In addition, for test numbers in which the limit compressibility of the test piece in the rolled state was less than 50%, subsequent evaluation tests of carburized steel parts were not conducted.

Regarding the SA test piece, since the limit compressibility of conventional steel used as a material for carburized steel parts is approximately 65%, it is judged that the limit working rate is excellent when it is 75% or more, which can be regarded as a value clearly higher than this value. did. In addition, the evaluation test of the carburized steel part was not performed about the test number whose limit compressibility was less than 75%.

[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 position R/2 in the radial direction from the central axis C1 of the rolled material and the SA material. The size of the observation surface of the sample was L1×L2, where L1 was 10 mm and L2 was 5 mm. In addition, the sample thickness L3 in the direction perpendicular to the observation plane was set to 5 mm. The normal line N of the observation surface was perpendicular to the central axis C1, and the R/2 position corresponded to the central position of the observation surface.

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

Inclusions in each visual field were identified. For each specific inclusion, Mn sulfides and oxides were identified using energy dispersive X-ray spectroscopy (EDX). Specifically, using EDX, elemental analysis was performed at at least two measurement points for each inclusion. Then, in each inclusion, the arithmetic average value of the element content obtained at each measurement point was defined as the content (mass%) of each element in the inclusion. In the case of elemental analysis at two measuring points in one inclusion, the arithmetic mean value of the Mn content, the arithmetic mean value of the S content, and the arithmetic mean value of the O content obtained at the two measuring points were taken as the Mn content (% by mass) in the inclusion. , S content (mass %), and O content (mass %). In the result of elemental analysis of the specified inclusion, when the Mn content was 10.0% or more, the S content was 10.0% or more, and the O content was less than 10.0%, the inclusion was recognized as Mn sulfide. Further, in the result of elemental analysis of the specified inclusion, when the O content was 10.0% or more, the inclusion was recognized as an oxide. Inclusions made into a specific target were inclusions having an equivalent circle diameter of 0.5 µm or more. In addition, the beam diameter of EDX used for identification of inclusions was set to 0.2 µm.

In each of the 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 specified in each visual field and the total area of 20 visual fields, the number of Mn sulfides per unit area (pcs/mm ² ) was determined. Further, the number of oxides per unit area (piece/mm ² ) was determined based on the total number of oxides specified in each visual field and the total area of 20 visual fields.

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

[Manufacture of carburized steel parts]

Spheroidization annealing was performed on the rolled material of each test number. Specifically, the rolled material was heated to 740°C. Thereafter, it was slowly cooled at a cooling rate of 8°C/hr until the temperature of the rolled material reached 650°C. The steel material temperature was air-cooled from 650 ° C. to normal temperature to manufacture a rolled state material subjected to spheroidization annealing.

Test pieces were taken from the rolled material subjected to spheroidizing annealing. The test piece was a round bar, had a diameter of 29.5 mm, and a length of 44 mm. The longitudinal direction of the test piece was the same as that of the rolled material.

Cold forging was simulated to this test piece, and upsetting compression of 50% of a compression ratio was performed cold. Upsetting compression was performed at room temperature, and restraining dies were used. The strain rate at the time of upsetting compression was set to 1/sec. The test piece after upsetting compression was subjected to gas carburization by a gas method in a denaturing furnace. In this gas carburizing, the carbon potential was set to 0.8%, holding was performed at 950°C for 5 hours, and then holding was performed at 850°C for 0.5 hours. After gas carburizing, oil quenching at 130°C was performed as a finish heat treatment step. After quenching, tempering was performed at 150°C for 90 minutes. Through the above process, a test piece simulating a carburized steel part was produced from the rolled material.

Also for the SA material of each test number, a test piece simulating a carburized steel part was produced in the same way as for the above-described rolled material. Specifically, test pieces were sampled from the SA material of each test number. The test piece was a round bar, had a diameter of 29.5 mm, and a length of 44 mm. The longitudinal direction of the test piece was the same as that of the SA material. Cold forging was simulated to this test piece, and upsetting compression of 50% of a compression ratio was performed cold. Upsetting compression was performed at room temperature, and restraining dies were used. The strain rate at the time of upsetting compression was set to 1/sec. The test piece after upsetting compression was subjected to gas carburization by a gas method in a denaturation furnace. In this gas carburizing, the carbon potential was set to 0.8%, holding was performed at 950°C for 5 hours, and then holding was performed at 850°C for 0.5 hours. After gas carburizing, oil quenching at 130°C was performed as a finish heat treatment step. After quenching, tempering was performed at 150°C for 90 minutes. Through the above process, a test piece simulating a carburized steel part was produced from the SA material.

[Evaluation test of carburized steel parts]

The following tests were conducted on the carburized layer and the core of the simulated carburized steel parts produced above (rolled material, SA material).

[Vickers hardness test of carburized layer]

In the cross section perpendicular to the longitudinal direction of the test piece (rolled material, SA material) simulating the carburized steel parts of each test number, 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 , Using a micro Vickers hardness tester, it was determined by a Vickers hardness test based on JIS Z 2244 (2009). The test force was 0.49 N. The Vickers hardness HV at 10 locations at a depth of 50 μm was measured, and the average value was taken as the Vickers hardness HV at a location at a depth of 50 μm. In addition, 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 a position at a depth of 0.4 mm.

If the hardness at a position at a depth of 0.4 mm from the surface was 600 HV or more, it was determined that the carburized layer existed from the surface to at least 0.4 mm. In addition, when the Vickers hardness at a position at a depth of 50 μm from the surface was 650 to 1000 HV, it was determined that the hardness of the carburized layer of the carburized steel part was 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 (rolled material, SA material) simulating a carburized steel part was determined by the following method. A sample containing a position at a depth of 0.4 mm from the surface of the test piece was collected on the surface. The surface of the sample was etched with picral liquid. Among the surfaces after etching, three arbitrary fields of view were observed as secondary electron images using SEM. The area of each visual field was 400 µm ² (magnification: 5000 times). Martensite and bainite (including tempered martensite and tempered bainite), ferrite, pearlite, and cementite could be discriminated from contrast. The total area of martensite in 3 visual fields of the sample at a depth of 0.4 mm was determined. The ratio of the calculated total area of martensite to the total area of the three visual fields was defined as the area ratio (%) of martensite at a depth of 0.5 μm.

[Vickers hardness test of core and chemical composition specification]

The Vickers hardness and chemical composition of the core part of the test piece simulating a carburized steel part were measured by the following method. In the cut surface perpendicular to the longitudinal direction of the carburized steel part, the Vickers hardness at a depth of 2.0 mm from the surface was obtained by a Vickers hardness test based on JIS Z 2244 (2009) using a micro Vickers hardness tester. The test force was 0.49 N. Measurements were made 10 times 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. The obtained Vickers hardness is shown in Table 3. When the Vickers hardness at a depth of 0.2 mm was 250 to 500 HV, the core hardness was sufficient and judged as pass. 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 judged to have an effective hardened layer of 0.4 to less than 2.0 mm. That is, in this case, a position at a depth of 2.0 mm from the surface was recognized as a deep part.

[Presence or absence of granulation of carburized steel parts]

For the core part of the test piece (rolled material and SA material) simulating the carburized steel part, prior austenite crystal grains were observed at a depth of 2 mm from the surface. Specifically, the cut plane perpendicular to the longitudinal direction of the carburized steel parts was used as the observation plane. After the observation surface was mirror-polished, it was etched with a saturated picric acid aqueous solution. A field of view (300 μm × 300 μm) covering a position at a depth of 2 mm from the surface of the etched observation surface was observed with an optical microscope (400 times) to identify prior austenite crystal grains. For the specified prior austenite crystal grains, the crystal grain size of each prior austenite crystal grain was determined as an equivalent circular diameter (μm) in accordance with JIS G 0551 (2013). Among the prior austenite crystal grains, if at least one crystal grain with an equivalent circle diameter exceeding the equivalent circle diameter (88.4 μm) corresponding to No. 4 of the crystal grain size number of the JIS stipulation was present, it was judged as “coarse grain generation present”. The determination results are shown in Table 3.

[Fatigue strength evaluation test of carburized steel parts after joining]

A round bar having a diameter of 20 mm and a length of 150 mm was produced by machining the rolled material and the SA material of each test number. Using this round bar (rolled material, SA material), a basic fatigue test piece and a bonded fatigue test piece were produced. In addition, before producing the following basic fatigue test piece and joint fatigue test piece, the rolled material is subjected to spheroidization annealing under the same conditions as described above, and then a round bar having a diameter of 20 mm and a length of 150 mm is produced by machining. did.

The basic fatigue test piece was produced by the following method. An Ono type rotary bending fatigue test piece having an evaluation part diameter of 8 mm and a parallel part length of 15 mm was prepared from the center of the cross section 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.

Joint fatigue test pieces were produced by the following method. Round bars with a diameter of 20 mm and a length of 150 mm of the same test piece number were butted together to create a joined round bar under the following friction welding conditions.

Friction welding conditions:

Frictional pressure: 100MPa;

Friction time: 5 seconds;

Upsetting pressure (pressing force from both ends of the round bar to the junction): 200 MPa,

Upsetting time (pressing time to joint): 5 seconds;

RPM: 2000 rpm,

Loss length: 5-12mm.

An Ono type rotational 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 central portion of the cross section of the joined round bar, and it was used as a pressure welding fatigue test piece. In the pressure welding fatigue test piece, the central part in the longitudinal direction of the parallel part was used as the bonding 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 was performed on the basic fatigue test piece and the bonded fatigue test piece to obtain carburized steel parts (test pieces using rolled materials after spheroidizing heat treatment, test pieces using SA materials). In the carburizing and quenching treatment, gas carburizing was performed by a gas method in a denaturation furnace. Specifically, the carbon potential was set to 0.8% and maintained at 950°C for 5 hours. After that, it was held at 850°C for 0.5 hour at the same carbon potential. Then, it was immersed in 130 degreeC oil and oil quenching was performed. After oil quenching, tempering was performed at 150°C for 90 minutes. By the above method, Ono-type rotary bending fatigue test pieces (basic fatigue test pieces, joined fatigue test pieces) simulating carburized steel parts were produced.

The Ohno type rotational bending fatigue test was performed on the fabricated basic fatigue test pieces and joint fatigue test pieces. Specifically, an Ono-type rotational bending fatigue test conforming to JIS Z 2274 (1978) was conducted using each of the above-mentioned Ono rotational bending fatigue test pieces (basic fatigue test piece, bonded fatigue test piece) at room temperature in an air atmosphere. . The rotational speed was 3000 rpm, the stress ratio R was -1, and the maximum stress that did not break after a cycle of 1×10 ⁷ cycles of stress loading was defined as fatigue strength (MPa).

The ratio (%) of the fatigue strength (MPa) of the bonded 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 equation.

Fatigue strength ratio (%) = Fatigue strength of bonded fatigue test piece/Fatigue strength of basic fatigue test piece × 100

The obtained fatigue strength ratios are shown in Table 3. When the fatigue strength ratio was 85% or more, it was judged that excellent fatigue strength was obtained even after bonding.

[Test result]

The test results are shown in Table 2 and Table 3. Referring to Tables 2 and 3, the chemical compositions of Test Nos. 1 to 11 and 28 were appropriate and satisfied the formulas (1) to (3). Also, the steelmaking conditions were appropriate. Moreover, the cooling rate in the cooling process was also appropriate. Therefore, the number of MnS in the 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. In addition, in the 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 column of "Microstructure of the rolled material" in Table 2), In this case, the microstructure of the surface layer region from at least the surface to a depth of 0.1R is made of ferrite and cementite, and the spheroidization rate of cementite in the surface layer region is 90.0% or more ("Y" in the column "Microstructure of SA material" in Table 2 ) was As a result, the limit compression ratio in the rolled material was 50% or more, and the limit compression ratio in the SA material was 75% or more, showing excellent limit compression ratio.

In addition, referring to Table 3, in a test piece simulating a carburized steel part manufactured using a rolled material, the Vickers hardness at a depth of 50 μm is 650 to 1000 HV, and the martensite area ratio at a depth of 0.4 mm is 90.0% or more, and the Vickers hardness at a depth of 0.4 mm was 600 to 900 HV or more. In addition, 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 less than 0.4 to 2.0 mm. Further, prior austenite grain boundaries were not coarsened in the core portion. In addition, the fatigue strength ratio based on the bonded fatigue test piece and the basic fatigue test piece were both as high as 85% or more, showing excellent fatigue strength even when joined or after welding.

On the other hand, in test number 12, the C content was too high. Therefore, the limit compressibility of the rolled material was less than 50%. Also, the limit compression rate of the SA material was less than 75%, and a sufficient limit compression rate was not obtained.

In Test No. 13, the C content was too low. Therefore, sufficient hardness was not obtained in the core part of the test piece simulating a carburized steel part.

In Test No. 14, the oxygen content was too high. For that reason, the number of oxides was too large. As a result, the fatigue strength ratio based on the bonded fatigue test specimens and basic fatigue test specimens of the rolled material and SA material was as low as less than 85%, and the fatigue strength after welding was low.

In Test No. 15, the N content was too high. Therefore, solid solution B could not be secured, and core hardness was too low.

In Test Nos. 16 and 29, F1 exceeded the upper limit of Formula (1). Therefore, the limit processing rate of the rolled material and the SA material was low.

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

In Test Nos. 18 and 30, F2 exceeded the upper limit of Formula (2). Therefore, the limit processing rate of the rolled material and the SA material was low.

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

In test number 20, F3 exceeded the upper limit of formula (3). For this reason, the limit working rate of steel materials (rolled material and SA material) was low.

In test number 21, F3 was less than the lower limit of formula (3). Therefore, in the core part of the carburized part of the rolled material and the deep part of the carburized steel part of the SA material, some of the prior austenite grains were granulated.

In Test Nos. 22 and 23, for the molten steel in the ladle after secondary refining, the holding time ts at a temperature of 1500 to 1600°C was less than 2.0 times the uniform mixing time τ. Therefore, in the 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, in the joint fatigue test piece simulating the carburized parts of the rolled material and the carburized steel parts of the SA material, the fatigue strength ratio was as low as less than 85%.

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

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

In the above, the embodiment of the present invention has been described. However, the embodiments described above are only examples for carrying out the present invention. Therefore, this invention is not limited to the above-mentioned embodiment, Within the range which does not deviate from the meaning, the above-mentioned embodiment can be changed suitably and can be implemented.

Claims (4)

As steel,
in mass %,
C: 0.09 to 0.16%;
Si: 0.01 to 0.50%;
Mn: 0.40 to 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 to 0.030%;
Ti: 0.010 to less than 0.050%;
Nb: 0.010 to 0.030%;
N: 0.0080% or less;
O: 0.0030% or less;
B: 0.0003 to 0.0030%;
Ca: 0.0005 to 0.0050%;
Cu: 0~0.50%,
Ni: 0 to 0.30%;
V: 0 to 0.10%, and
The remainder consists of Fe and impurities, and satisfies formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material, Mn content is 10.0% or more, S content is 10.0% or more, O content is less than 10.0%, and the equivalent circle diameter is 0.5 μm or more, in mass%, 70.0 pieces/mm ² or less, the O content in mass% is 10.0% or more, and the oxides having an equivalent circle diameter of 0.5 μm or more are 25.0 pieces/mm ² or less,
When the radius in the cross section perpendicular to the axial direction of the steel member is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel member, at least in the surface layer region to a depth of 0.1R from the surface. The area ratio of bainite is 95.0% or more,
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 (mass %) of the corresponding element is substituted for each element symbol in Formulas (1) to (3). When the corresponding element is not contained, "0" is substituted for the element symbol. As steel,
in mass %,
C: 0.09 to 0.16%;
Si: 0.01 to 0.50%;
Mn: 0.40 to 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 to 0.030%;
Ti: 0.010 to less than 0.050%;
Nb: 0.010 to 0.030%;
N: 0.0080% or less;
O: 0.0030% or less;
B: 0.0003 to 0.0030%;
Ca: 0.0005 to 0.0050%;
Cu: 0~0.50%,
Ni: 0 to 0.30%;
V: 0 to 0.10%, and
The remainder consists of Fe and impurities, and satisfies formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material, Mn content is 10.0% or more, S content is 10.0% or more, O content is less than 10.0%, and the equivalent circle diameter is 0.5 μm or more, in mass%, 70.0 pieces/mm ² or less, the O content in mass% is 10.0% or more, and the oxides having an equivalent circle diameter of 0.5 μm or more are 25.0 pieces/mm ² or less,
When the radius in the cross section perpendicular to the axial direction of the steel member is defined as R (mm), in the microstructure in the cross section perpendicular to the axial direction of the steel member, the surface layer region at least to a depth of 0.1R from the surface is , It is made of ferrite and cementite, and the spheroidization rate of the cementite in the surface layer region is 90.0% or more,
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 (mass %) of the corresponding element is substituted for each element symbol in Formulas (1) to (3). When the corresponding element is not contained, "0" is substituted for the element symbol. According to claim 1 or claim 2,
in mass %,
Cu: 0.01~0.50%,
Ni: 0.01 to 0.30%, and
V: 0.01 to 0.10%;
Containing at least one element selected from the group consisting of
steel. delete