JP6628014B1 - Steel for parts to be carburized - Google Patents

Abstract

Provided is a steel material for a component that is subjected to a carburizing process to obtain excellent fatigue strength even when a component is manufactured by welding before the carburizing process. In the steel material of 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: 0.90-2.00%, Mo: 0.10-0.40%, Al: 0.005-0.030%, Ti: 0.010-0. Less than 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 the balance: Fe and impurities, having a chemical composition satisfying the formulas (1) to (3) described in the specification. In a section parallel to the axial direction of the steel material, Mn sulfide is 70.0 / mm2 or less and oxide is 25.0 / mm2 or less.

Description

  The present invention relates to a steel material, and more particularly, to a steel material used for a part to be carburized.

  A steel material used for a machine structural component generally contains a combination of Mn, Cr, Mo, Ni, and the like. Some parts for machine structure are parts to be carburized. A steel material used for a part to be carburized has the above-mentioned chemical composition, and is manufactured by casting, forging, rolling, or the like. Hereinafter, in this specification, a part to be carburized is referred to as a “carburized part”.

  The carburized part is manufactured, for example, by the following method. For steel products, intermediate products are manufactured by machining such as forging and cutting. Carburizing is performed on the intermediate product to produce a carburized component including a carburized layer that is a hardened layer of a surface layer portion and a core that is a base material that is not affected by the carburizing process.

  Among the costs of manufacturing carburized parts, the cost related to cutting is very large. The cutting process is not only expensive for cutting tools, but also generates a large amount of chips. Therefore, it is disadvantageous from the viewpoint of yield. For this reason, it has been attempted to replace the cutting with forging. Forging methods can be broadly classified into hot forging, warm forging, and cold forging. Warm forging is characterized in that scale is less generated and dimensional accuracy is improved as compared to hot forging. Further, cold forging is characterized in that scale is not generated and dimensional accuracy is close to cutting. Therefore, rough finishing by hot forging and then finishing by cold forging, finishing by warm forging and light cutting as finishing, or molding by cold forging only Etc. have been studied. 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 merit for cutting is reduced. In addition, when a steel material is formed into a complicated shape, there arises a problem that a crack is generated at a portion where a large processing is applied. For this reason, various techniques have been studied for the purpose of softening steel materials and improving the critical working ratio.

  Patent Literature 1 discloses a carburizing steel that has a lower deformation resistance during cold forging than a conventional steel, has a higher critical working rate, and has a hardened layer and a core hardness equivalent to those of the conventional steel after carburizing. Disclose. The carburizing steel described in Patent Literature 1 has a chemical composition in mass% of C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001% to 0.80%, S: 0.0001% to 0.100%, Cr: more than 1.30% to 5.00%, B: 0.0005% to 0.0100%, Al: 0.0001% to 1 0.010% to 0.10%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, the balance being Fe and It consists of unavoidable impurities, and the content represented by mass% of each element in the chemical component is represented by the following formula 1 as a hardness index, the following formula 2 as a hardenability index, and the following formula as a TiC precipitation amount index. It is characterized by satisfying 3. 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). The carburizing steel of Patent Literature 1 has the above-described chemical composition, thereby increasing the critical working ratio at the time of cold forging, and after carburizing, a hardened layer and a steel part hardness equivalent to those of conventional steel are obtained.

WO2012 / 108460

  By the way, a plurality of carburized parts are used as mechanical structural parts used for automobiles. For example, carburized parts are also used in the variable diameter pulley of a continuously variable transmission (CVT). As described above, large-sized carburized parts represented by variable-diameter pulleys are manufactured by cutting after forging. However, if a large steel material is to be formed by cold forging, an excessive load is applied to the cold forging machine. Therefore, when large-sized carburized parts are formed by cold forging, a plurality of members are formed by cold forging, and then these members are joined by welding such as friction welding or laser welding, and the joined steel members are carburized. A method of manufacturing a large-sized carburized part by processing is being studied.

  In the case where a carburized component is manufactured by welding as described above, the fatigue strength (joining fatigue strength) of the carburized component as a joining material is required.

  The purpose of the present disclosure, at the steel material stage, the critical working rate at the time of cold forging is greater than conventional steel materials, and even when welding is performed, excellent fatigue strength is obtained after carburizing treatment. It is to provide a steel material for a part to be carburized.

  Hereinafter, unless otherwise specified, “forging” simply means “cold forging”.

The steel material for the parts to be carburized according to the present disclosure is:
In mass%,
C: 0.09-0.16%,
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 to 0.40%,
Al: 0.005 to 0.030%,
Ti: less than 0.010 to 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 to 0.0050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%, and
The balance: Fe and impurities,
Having a chemical composition that satisfies the formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material,
Mn sulfide containing not less than 10.0% by mass of Mn, not less than 10.0% by mass of S, and having an equivalent circle diameter of 0.5 μm or more is 70.0 / mm ² or less;
25.0 oxides / mm ² or less containing 10% by mass or more of oxygen and having an equivalent circle diameter of 0.5 μm or more.
0.140 <C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.033 × Mo + 0.067 × Ni + 0.097 × Cu + 0.078 × Al <0.235 Formula (1)
13.0 <(0.70 × Si + 1) × (5.1 × Mn + 1) × (2.2 × Cr + 1) × (3.0 × Mo + 1) × (0.36 × Ni + 1) <45.0 Equation (2) )
0.004 <Ti-N * (48/14) <0.030 Formula (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formulas (1) to (3).

  The steel material according to the present disclosure has a higher critical working ratio at the time of cold forging than the conventional steel material at the steel material stage, and is excellent at the stage of the carburized component (carburized component) even when welding is performed. The resulting fatigue strength is obtained.

FIG. 1 is a schematic diagram for explaining a sampling position when measuring Mn sulfide and oxide in the present embodiment.

  Hereinafter, a steel material for a part to be carburized according to the present embodiment will be described.

  The present inventors have proposed a steel material for a part to be subjected to a carburizing treatment, to reduce the deformation resistance (reduced hardness) of the steel material before cold forging, to improve the critical working ratio, and to make the carburized part after the carburizing treatment. In order to achieve both excellent characteristics (for example, effective hardened layer depth and core hardness), detailed studies were performed, and the following findings (a) to (g) were obtained.

  (A) The lower the C content, the softer the steel material can be before cold forging. However, if the C content is too low, the characteristics of the carburized component after carburizing (for example, the effective hardened layer depth and the core hardness) may be reduced by the conventional steel material having a C content of about 0.20% (for example, , JIS-SCR420). In order to obtain the required hardness of the core part as a carburized part, there is a lower limit of the C content.

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

  (C) In order to increase the martensite fraction of the metal structure at the core of the carburized part, alloy elements (hardenability improving elements) such as Si, Mn, Cr, Mo, Ni, etc., which improve the hardenability of steel. It is necessary that the content is contained so as to satisfy the expression (2) of the hardenability index described later.

  (D) However, when the content of the hardenability improving element increases, the hardness of the steel material increases due to the effect of solid solution strengthening of ferrite by the hardenability improving element. Therefore, while containing B which can increase the hardenability of the steel material while suppressing the increase in the hardness of ferrite, the content of C and the hardenability improving element should satisfy the hardness index expression (1) described later. To

  (E) In order to stably obtain the effect of improving the hardenability of B, most of the N contained in the steel is fixed in the form of TiN during the carburizing treatment. Thereby, solid solution B is not precipitated as BN, and the amount of solid solution B in the steel can be secured. In order to effectively obtain this effect, it is preferable to contain Ti in a stoichiometric excess with respect to the N content. Furthermore, in order to prevent abnormal growth of austenite crystal grains during carburizing, TiC is finely dispersed and precipitated in the metal structure. As described above, in order to secure the amount of solid solution B and to finely disperse and precipitate TiC, the contents of Ti and N satisfy the formula (3) of the TiC precipitation amount index, which will be described later, derived by the inventor. To

  (F) As described above, B effectively enhances the hardenability of the core of the carburized part. However, when performing the gas carburizing of the shift furnace gas method, the effect of improving the hardenability due to the B content is low in the carburized layer which is the surface layer of the carburized component. This is because during the carburizing process, nitrogen enters from the surface of the steel material, combines with solid solution B and precipitates as BN, and the amount of solid solution B is reduced. Therefore, in order to secure the hardenability in the carburized layer which is the surface layer of the carburized part, it is necessary to satisfy the equation (2) of the hardenability index described in the above (c).

  (G) In order to further soften the steel material for the part to be carburized, it is preferable to perform slow rolling under the conditions described below after hot rolling or hot forging when manufacturing the steel material. . This makes it possible to control the metal structure of the steel material and to soften the steel material. Thereby, the metal structure of the surface layer portion of the steel material is improved, the ductility is improved, and a steel material having a high critical working rate can be obtained.

The inventor further studied the fatigue strength (joining fatigue strength) at the stage of the carburized component manufactured by carburizing after welding in the steel material of the present embodiment. As a result, regarding the inclusions in the cross section parallel to the axial direction of the steel material (that is, the longitudinal direction of the steel material), if the following rules are satisfied, the fatigue strength at the stage of the carburized part manufactured by welding and carburizing after welding ( (Joint fatigue strength) was found to increase.
(A) Mn sulfide containing 10.0% by mass or more of Mn, 10.0% by mass or more of S, and a circle equivalent diameter of 0.5 μm or more is reduced to 70.0 / mm ² or less.
(B) Oxygen containing 10% by mass or more of oxygen and having an equivalent circle diameter of 0.5 μm or more is reduced to 25.0 / mm ² or less.

  Hereinafter, this point will be described in detail.

In the steel material having the chemical composition of the present embodiment, Mn sulfide and oxide are present in the steel. Here, in this specification, Mn sulfide and oxide are defined as follows.
Mn sulfide: Inclusion of Mn at 10.0 mass% or more, S at 10.0 mass% or more, inclusion having an equivalent circle diameter of 0.5 μm or more Oxide: Oxygen containing at least 10 mass%, circle Inclusions having an equivalent diameter of 0.5 μm or more In this specification, the inclusions contain 10.0% by mass or more of S, 10.0% or more of Mn, and 10% by mass or more of oxygen Inclusions having an equivalent circle diameter of 0.5 μm or more are included not in “Mn sulfide” but in “oxide”.

  In the present embodiment, when a carburizing process is performed to form a carburized component after performing welding represented by friction welding, laser welding, or the like on a steel material, a HAZ region exists in the carburized component. The intensity of the HAZ region may be lower than other regions. Therefore, in order to secure the strength of the HAZ region, inclusions are reduced as much as possible. In this embodiment, as described in (A) and (B) above, the number of Mn sulfides and oxides that occupy most of the inclusions in the steel is reduced as much as possible. In this case, the strength of the HAZ region can be secured, and as a result, the fatigue strength of the carburized component can be increased.

  The gist of the steel material for the part to be carburized according to the present embodiment, which is completed based on the above findings, is as follows.

[1] The steel material for the part to be carburized is
In mass%,
C: 0.09-0.16%,
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 to 0.40%,
Al: 0.005 to 0.030%,
Ti: less than 0.010 to 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 to 0.0050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%, and
The balance: Fe and impurities,
Having a chemical composition that satisfies the formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material,
Mn sulfide containing not less than 10.0% by mass of Mn, not less than 10.0% by mass of S, and having an equivalent circle diameter of 0.5 μm or more is 70.0 / mm ² or less;
25.0 oxides / mm ² or less containing 10% by mass or more of oxygen and having an equivalent circle diameter of 0.5 μm or more.
0.140 <C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.033 × Mo + 0.067 × Ni + 0.097 × Cu + 0.078 × Al <0.235 Formula (1)
13.0 <(0.70 × Si + 1) × (5.1 × Mn + 1) × (2.2 × Cr + 1) × (3.0 × Mo + 1) × (0.36 × Ni + 1) <45.0 Equation (2) )
0.004 <Ti-N * (48/14) <0.030 Formula (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formulas (1) to (3).

The steel material of [2] is the steel material according to [1],
The chemical composition is
Cu: 0.005% to 0.50%, and
Ni: 0.005% to 0.30%,
At least one selected from the group consisting of

[3] The steel material according to [1] or [2],
The metal structure contains 85 to 100% in total of ferrite and pearlite in area%.

[4] The steel material according to [1] or [2],
The metal structure contains 85 to 100% in total of ferrite and spheroidized cementite in area%.

The steel material of [5] is the steel material according to any one of [1] to [4],
The steel material is a steel bar.

  Hereinafter, a steel material for a part to be carburized according to the present embodiment will be described. “%” For a chemical composition means “% by mass” unless otherwise specified.

[Chemical composition]
The chemical composition of the steel material for the part to be carburized according to the present 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 in a carburized component having a carburized layer and a core. If the C content is less than 0.09%, the hardness of the core of the carburized component is insufficient. On the other hand, if the C content exceeds 0.16%, the cementite fraction and the pearlite fraction of the metal structure of the steel material increase, the hardness of the steel material before forging increases remarkably, and the critical working rate decreases. I do. Therefore, the C content is 0.09 to 0.16%. The C content of a conventional steel material for a part to be carburized is about 0.20%. For this reason, in the steel material for a part to be carburized according to the present embodiment, the C content is lower than the conventional steel material. A preferred lower limit of the C content is 0.10%, and more preferably 0.11%. A preferred upper limit of the C content is 0.15%, and more preferably 0.14%.

Si: 0.01 to 0.50%
Silicon (Si) increases the tempering softening resistance of the carburized part and increases the fatigue strength of the carburized part. If the Si content is less than 0.01%, the above effects cannot be obtained. On the other hand, if the Si content exceeds 0.50%, the hardness of the steel material before forging increases, and the critical working rate decreases. Therefore, the Si content is 0.01 to 0.50%. When importance is attached to the surface fatigue strength of a carburized part, a preferable lower limit of the Si content is 0.015%, and more preferably 0.02%. When importance is attached to the improvement of the limit workability of a carburized part, a preferable upper limit of the Si content is 0.48%, more preferably 0.46%.

Mn: 0.40 to 0.60%
Manganese (Mn) enhances the hardenability of the steel material and increases the strength of the core of the carburized part. If the Mn content is less than 0.40%, this effect cannot be obtained. On the other hand, if the Mn content exceeds 0.60%, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, the Mn content is 0.40 to 0.60%. A preferred lower limit of the Mn content is 0.42%, more preferably 0.44%. A preferred upper limit of the Mn content is 0.58%, more preferably 0.56%.

P: 0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is more than 0%. P segregates at austenite grain boundaries, embrittles old austenite grain boundaries, and causes grain boundary cracking. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.026%, and more preferably 0.024%. The P content is preferably as low as possible. However, if the P content is reduced to the utmost, the productivity decreases and the manufacturing cost increases. Therefore, in a normal operation, a preferable lower limit of the P content is 0.0001%.

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 machinability of the steel material. If the S content exceeds 0%, this effect can be obtained. On the other hand, if the S content exceeds 0.025%, coarse MnS is generated, cracks are likely to occur during forging, and the critical compressibility decreases. Therefore, the S content is 0.025% or less. The preferable upper limit of the S content is 0.022%, and more preferably 0.020%. When the machinability is more effectively enhanced, the lower limit of the S content is preferably 0.0001%, and more preferably 0.003%.

Cr: 0.90 to 2.00%
Chromium (Cr) enhances the hardenability of steel and increases the strength of the core of the carburized part. If the Cr content is less than 0.90%, this effect cannot be obtained. On the other hand, if the Cr content exceeds 2.00%, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, the Cr content is 0.90 to 2.00%. A preferred lower limit of the Cr content is 0.95%, more preferably 1.00%. The preferable upper limit of the Cr content is 1.95%, more preferably 1.92%.

Mo: 0.10 to 0.40%
Molybdenum (Mo) enhances the hardenability of steel and increases the strength of the core of the carburized part. If the Mo content is less than 0.10%, this effect cannot be obtained. On the other hand, if the Mo content exceeds 0.40%, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, the Mo content is 0.10 to 0.40%. A preferred lower limit of the Mo content is 0.11%, more preferably 0.12%. The preferred upper limit of the Mo content is 0.38%, and more preferably 0.36%.

Al: 0.005 to 0.030%
Aluminum (Al) is an element that forms AlN when solid solution N exists in steel. However, in the steel material of the present embodiment and the core of the carburized part, since N in the steel is fixed as TiN by the addition of Ti, solute N hardly exists in the steel. For this reason, Al does not form AlN and exists as solid solution Al in the steel. Al present in a solid solution has an effect of improving the machinability of steel. When finishing cutting or the like is performed at the time of manufacturing a carburized part, the Al content is desirably 0.005% or more. However, when the Al content exceeds 0.030%, the hardness of the steel material before forging increases, the deformation resistance increases, and the critical working ratio decreases. Therefore, the Al content is 0.005 to 0.030%. The preferable lower limit of Al is 0.010%. The preferable upper limit of Al is 0.020%.

Ti: less than 0.010 to 0.050% Titanium (Ti) fixes N in steel as TiN and suppresses the formation of BN. Thereby, Ti secures the amount of solid solution B and enhances the hardenability of the steel material. Ti further forms Ti carbide and suppresses coarsening of crystal grains during carburizing treatment. If the Ti content is less than 0.010%, this effect cannot be obtained. On the other hand, if the Ti content is 0.050% or more, the precipitation amount of Ti carbide becomes excessively large, and the critical working ratio of the steel material before forging decreases. Therefore, the Ti content is less than 0.010 to 0.050%. A preferred lower limit of the Ti content is 0.012%, and more preferably 0.014%. The preferable upper limit of the Ti content is 0.048%, more preferably 0.046%.

Nb: 0.010-0.030%
Nb (niobium) combines with N and C in steel to form Nb carbonitride. Nb carbonitride suppresses coarsening of crystal grains due to a pinning effect. If the Nb content is less than 0.010%, this effect cannot be obtained. On the other hand, if the Nb content exceeds 0.030%, the effect is saturated. Therefore, the Nb content is 0.010 to 0.030%. A preferred lower limit of the Nb content is 0.011%, and more preferably 0.012%. The preferable upper limit of the Nb content is 0.029%, more preferably 0.028%, and further preferably 0.027%.

N: 0.0080% or less Nitrogen (N) is an unavoidable impurity. That is, the N content in the steel material is more than 0%. N combines with B to form BN, and reduces the amount of solid solution B. In this case, the hardenability of the steel material decreases. If the N content exceeds 0.0080%, even if Ti is contained, N in the steel cannot be fixed as TiN, and it is difficult to secure solid solution B that contributes to hardenability. . Further, coarse TiN is formed. Coarse TiN becomes the starting point of cracking during forging, and lowers the critical working ratio of the steel material before forging. Therefore, the N content is 0.0080% or less. The preferable upper limit of the N content is 0.0078%, and more preferably 0.0076%. The lower the N content, the better. However, if the N content is reduced to the utmost, the productivity decreases and the manufacturing cost increases. Therefore, in a normal operation, a preferable lower limit of the N content is 0.0020%.

O: 0.0030% or less Oxygen (O) is an unavoidable impurity. That is, the O content in the steel material is more than 0%. O forms an oxide and reduces the bondability when an intermediate product before carburizing is manufactured by welding. In this case, the fatigue strength decreases. Therefore, the O content is 0.0030% or less. The preferable upper limit of the O content is 0.0029%, and more preferably 0.0028%. A lower O content is preferred. However, if the O content is reduced to the utmost, the productivity decreases and the manufacturing cost increases. Therefore, in a normal operation, a preferable lower limit of the O content is 0.0010%.

B: 0.0003-0.0030%
Boron (B) enhances the hardenability of steel and increases the strength of carburized parts. When the B content is 0.0003% or more, this effect can be obtained. On the other hand, if the B content exceeds 0.0030%, the above-described effect is saturated. Therefore, the B content is 0.0003 to 0.0030%. A preferred lower limit of the B content is 0.0004%, and more preferably 0.0005%. The preferable upper limit of the B content is 0.0028%, and more preferably 0.0026%.

Ca: 0.0005 to 0.0050%
Calcium (Ca) is contained in Mn sulfides and oxides and spheroidizes these inclusions. Furthermore, these inclusions are refined by being contained in Mn sulfide and oxide. If the Ca content is less than 0.0005%, these effects cannot be obtained. On the other hand, if the Ca content exceeds 0.0050%, coarse Mn sulfides and coarse oxides are formed, and the fatigue strength of the carburized part is reduced. Therefore, the Ca content is 0.0005 to 0.0050%. A preferred lower limit of the Ca content is 0.0006%, more preferably 0.0007%. A preferred upper limit of the Ca content is 0.0048%, and more preferably 0.0046%.

Remainder: Fe and impurities The remainder of the chemical composition of the steel material for parts subjected to the carburizing treatment of the present embodiment is composed of Fe and impurities. Here, the impurity is a component that is mixed in from the ore, scrap, or the production environment as a raw material when a steel material is industrially manufactured, and is not intentionally included in the steel. means.

Examples of the impurities include all elements other than the above-described impurities. The impurities may be only one kind or two or more kinds. Other impurities other than the above-described impurities are, for example, Sb, Sn, W, Co, As, Pb, Bi, H, and the like. These elements may have, for example, the following contents as impurities.
Sb: 0.0005% or less, Sn: 0.0005% or less, W: 0.0005% or less, Co: 0.0005% or less, As: 0.0005% or less, Pb: 0.0005% or less, Bi: 0.0005% or less, H: 0.0005% or less.

[About optional elements]
The chemical composition of the steel material for the part to be carburized according to the present embodiment may contain one or more selected from the group consisting of Cu and Ni instead of part of Fe.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and need not be contained. That is, the Cu content may be 0%. When the Cu content exceeds 0%, Cu enhances the hardenability of the steel material and increases the strength of the carburized part. Cu is an element that does not form oxides or nitrides in a gas atmosphere of gas carburization. Therefore, when Cu is contained, it is difficult to form an oxide layer or a nitride layer on the surface of the carburized layer, or an abnormal carburized layer resulting therefrom. However, if the Cu content is too high, the ductility in a high-temperature region of 1000 ° C. or more is reduced, which causes a reduction in the yield during continuous casting and rolling. Further, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, the Cu content is 0 to 0.50%. A preferable lower limit of the Cu content is 0.005%, and more preferably 0.05%. A preferred upper limit of the Cu content is 0.45%, and more preferably 0.40%.

Ni: 0 to 0.30%
Nickel (Ni) is an optional element and need not be contained. That is, the Ni content may be 0%. If the Ni content exceeds 0%, Ni enhances the hardenability of the steel material and increases the strength of the carburized part. However, if the Ni content is too high, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, the Ni content is 0 to 0.30%. A preferred lower limit of the Ni content is 0.005%, and more preferably 0.05%. The preferred upper limit of the Ni content is 0.28%, and more preferably 0.25%.

[About Equations (1) to (3)]
The chemical composition of the steel material for the part to be carburized according to the present embodiment further satisfies the following equations (1) to (3).
0.140 <C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.033 × Mo + 0.067 × Ni + 0.097 × Cu + 0.078 × Al <0.235 Formula (1)
13.0 <(0.70 × Si + 1) × (5.1 × Mn + 1) × (2.2 × Cr + 1) × (3.0 × Mo + 1) × (0.36 × Ni + 1) <45.0 Equation (2) )
0.004 <Ti-N * (48/14) <0.030 Formula (3)
Here, the content (% by mass) of the corresponding element is substituted for the element symbol in the 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 formula will be described.

[Formula (1): hardness index]
F1 = C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.033 × Mo + 0.067 × Ni + 0.097 × Cu + 0.078 × Al F1 is an index of the hardness of the steel material.

  When the C content is low, the structure of the steel material before forging greatly increases the ferrite fraction as compared with the conventional steel material (C content is about 0.20%) for a part to be carburized. In this case, the hardness of the steel material is greatly affected by not only the C content (pearlite fraction) but also the hardness of ferrite. Then, the inventors examined the contribution of each alloy element to the solid solution strengthening amount of ferrite and defined F1.

  If F1 is 0.235 or more, the hardness of the steel material before forging increases, and the critical working rate decreases. On the other hand, if F1 is 0.140 or less, the hardness as a carburized component is insufficient. Thus, F1 is greater than 0.140 and less than 0.235. F1 is preferably as low as possible within a range satisfying a hardenability index (F2) described later. The preferred upper limit of F1 is less than 0.230, more preferably 0.220, and even more preferably 0.210. The F1 value is a value obtained by rounding the calculated value to the fourth decimal place.

[Equation (2): Index of hardenability]
F2 = (0.70 × Si + 1) × (5.1 × Mn + 1) × (2.2 × Cr + 1) × (3.0 × Mo + 1) × (0.36 × Ni + 1) F2 is an index relating to the hardenability of the member.

  As described above, B is effective for enhancing the hardenability of the core of the carburized part. In the case of performing gas carburization of the shift furnace gas system, in the carburized layer which is the surface layer of the carburized part, the effect of improving the hardenability by containing B is low. This is because nitrogen enters the surface layer of the carburized part from the atmosphere during the carburizing treatment, so that solid solution B precipitates as BN, and the amount of solid solution B contributing to the improvement of hardenability is insufficient. Therefore, in order to secure the hardenability in the carburized layer which is the surface layer of the carburized part, it is necessary to use an element other than B that enhances the hardenability of steel.

  F2 is composed of a hardenability improving element. When F2 is 13.0 or less, the hardness of the carburized layer and the effective hardened layer depth are equal to or more than those of the above-mentioned conventional steel material (C content is about 0.20%) under the same carburizing condition. (A depth at which the Vickers hardness becomes HV550 or more) cannot be obtained. On the other hand, if F2 is 45.0 or more, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, F2 is more than 13.0 and less than 45.0. F2 is preferably as large as possible within a range satisfying the hardness index F1. A preferred lower limit of F2 is 13.2, and more preferably 15.0. Note that the F2 value is a value obtained by rounding off the calculated value to the second decimal place.

[Equation (3): TiC precipitation index]
It is defined as F3 = Ti−N × (48/14). F3 is an index relating to the amount of TiC deposited. When Ti is contained in a stoichiometric excess with respect to N, all N is fixed in the form of TiN. That is, F3 means an excessive amount of Ti other than that consumed to form TiN. “14” in F3 is the atomic weight of N, and “48” is the atomic weight of Ti.

  Most of the excess Ti amount defined by F3 is combined with C to form TiC during the carburizing process. This TiC has a pinning effect of preventing coarsening of crystal grains during carburizing. If F3 is 0.004 or less, the amount of TiC deposited becomes insufficient. In this case, it is not possible to suppress coarsening of crystal grains during carburizing. On the other hand, if F3 is 0.030 or more, the precipitation amount of TiC becomes too large, the hardness of the steel material before forging increases, and the critical working ratio decreases. Therefore, F3 is more than 0.004 and less than 0.030. A preferred lower limit of F3 is 0.008. A preferred upper limit of F3 is 0.028. The F3 value is a value obtained by rounding the calculated value to the fourth decimal place.

  When the chemical composition simultaneously satisfies the hardness index F1, the hardenability index F2, and the TiC precipitation index F3, the deformation resistance of the steel material during cold forging is lower than that of the conventional steel, and the critical working ratio is lower. growing. Then, after the carburizing treatment of the steel material, a carburized part having a hardened layer and a core hardness equivalent to those of conventional steel can be obtained.

[Metal structure of steel for parts to be carburized]
The metallographic structure of the steel material for a part to be carburized according to the present embodiment will be described.

[First Metal Structure]
The metal structure of the steel material having the above-mentioned chemical composition preferably contains 85 to 100% in total of ferrite and pearlite in area%.

  If the total area ratio of ferrite and pearlite in the metal structure is 85 to 100%, the hardness of the steel material is further reduced, the deformation resistance is reduced, and the critical working ratio is improved. More preferably, the total area ratio of ferrite and pearlite is 95 to 100%. In the metal structure, the remainder other than ferrite and pearlite includes bainite, martensite, cementite, and the like, which are harder structures than ferrite and pearlite. In order to obtain the above-mentioned effect by ferrite and pearlite, it is preferable that the fraction of the remaining bainite, martensite, cementite, etc. be 15% or less in area%.

  In order to make such a metal structure, the temperature range in which the surface temperature of the hot-worked steel material after the hot-working step in the production of the steel material is 800 ° C to 500 ° C is more than 0 ° C / sec and 1 ° C / sec. It is preferable to perform a slow cooling step of slowly cooling at the following cooling rate. The details of the manufacturing method will be described later.

[Second metallographic structure]
Instead of the above-mentioned metal structure, the steel material made of the above-mentioned chemical components may contain 85 to 100% in total of ferrite and spheroidized cementite in area%. Here, spheroidized cementite is defined as a spheroidized cementite when the area of the cementite is 54% or more with respect to a circle whose diameter is the maximum length of the cementite on the metallographic structure observation surface.

  When the total area ratio of ferrite and spheroidized cementite is 85 to 100%, the hardness of the steel material is further reduced, the deformation resistance is reduced, and the critical working ratio is improved. More preferably, the total area ratio of ferrite and spheroidized cementite is 90 to 100%. The balance of ferrite and spheroidized cementite includes pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, and the like. In order to obtain the above effects of ferrite and spheroidized cementite, the fraction of the remaining pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, etc. should be 15% or less by area%. Is preferred.

  In order to obtain such a metal structure, it is preferable to perform a spheroidizing heat treatment on a hot-worked steel material. In the case of the second metal structure form, the hot-worked steel material may or may not be gradually cooled, but is cooled at a cooling rate at which ferrite is formed without forming bainite and martensite. . Details of the manufacturing method will be described later.

[About inclusions in steel for parts to be carburized]
In the steel material for a part to be carburized according to the present embodiment, Mn sulfide and oxide in the steel satisfy the following conditions in a cross section parallel to the axial direction of the steel material (that is, the longitudinal direction of the steel material).
(A) Mn sulfide containing 10.0% by mass or more of Mn, 10.0% by mass or more of S, and a circle equivalent diameter of 0.5 μm or more is reduced to 70.0 / mm ² or less.
(B) Oxygen containing 10% by mass or more of oxygen and having an equivalent circle diameter of 0.5 μm or more is reduced to 25.0 / mm ² or less.

Here, in this specification, Mn sulfide and oxide are defined as follows.
Mn sulfide: Inclusion of Mn at 10.0 mass% or more, S at 10.0 mass% or more, inclusion having an equivalent circle diameter of 0.5 μm or more Oxide: Oxygen containing at least 10 mass%, circle Inclusions having an equivalent diameter of 0.5 μm or more In this specification, the inclusions contain 10.0% by mass or more of S, 10.0% or more of Mn, and 10% by mass or more of oxygen Inclusions having an equivalent circle diameter of 0.5 μm or more are included not in “Mn sulfide” but in “oxide”.

  As described above, in the manufacturing process of a carburized part, when a plurality of steel members are integrated by joining by welding such as friction welding and laser welding to produce an intermediate member before carburizing, the HAZ is contained in the carburized part. There is an area. The intensity of the HAZ region may be lower than other regions. Inclusions are reduced as much as possible to ensure the strength of the HAZ region. If the Mn sulfide and oxide satisfy the above (A) and (B), the strength of the HAZ region can be secured, and as a result, the fatigue strength of the carburized part integrated by joining can be increased.

[Method for measuring Mn sulfide and oxide]
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 for the part to be carburized. In the case where the steel material is a bar, as shown in FIG. 1, a sample is taken from an R / 2 position (R is the radius of the bar) in the radial direction from the center axis C1 of the bar. The size of the observation surface of the sample is L1 × L2, where L1 is 10 mm and L2 is 5 mm. Further, 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 corresponds to the center position of the observation surface.

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

  Identify inclusions in each field of view. For each specified inclusion, Mn sulfides and oxides are identified using energy dispersive X-ray spectroscopy (EDX). Specifically, in the elemental analysis result of the specified inclusion, when the Mn content is 10.0% by mass or more and the S content is 10.0% by mass or more, the inclusion is regarded as Mn sulfide. Is defined. In addition, in elemental analysis of inclusions, Ti and Ca may be detected as elements other than Mn and S in some cases. In this case, Mn sulfide is defined as long as the above conditions are satisfied. In the elemental analysis result of the identified inclusion, if the O content is 10% by mass or more, the inclusion is defined as an oxide. Al, Si, Mg, Ca, Ti, and the like may be detected as inclusions defined as oxides. Also in this case, if the above conditions are satisfied, it is identified as an oxide. In addition, among the inclusions, those containing 10.0% by mass or more of S, 10.0% by mass or more of Mn, and 10% by mass or more of O are not “Mn sulfides” but “ Oxide ".

  The inclusion to be identified is an inclusion having a circle equivalent 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.

  The inclusion of an inclusion having a circle equivalent diameter equal to or more than twice the beam diameter of EDX increases the accuracy of elemental analysis. In the present embodiment, the beam diameter of EDX used for specifying the inclusion is 0.2 μm. In this case, inclusions having an equivalent circle diameter of less than 0.5 μm cannot improve the accuracy of elemental analysis by EDX. Inclusions having an equivalent circle diameter of less than 0.5 μm have a very small effect on strength. Therefore, in the present embodiment, Mn sulfides and oxides having an equivalent circle diameter of 0.5 μm or more are measured. The upper limit of the equivalent circle diameter of the inclusion is not particularly limited, but is, for example, 100 μm.

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

[Metal structure of carburized parts]
Next, the metallographic structure of the carburized part according to the present embodiment will be described.

  The carburized component according to the present embodiment includes a carburized layer and a core portion inside the carburized layer. The carburized layer has an effective hardened layer depth greater than 0.4 mm and less than 2.0 mm. Here, the carburized layer means a depth from the surface where the Vickers hardness is HV550 or more. In this carburized layer, the metal structure at a position 50 μm deep from the surface preferably contains 90 to 100% martensite by area%, and has a Vickers hardness of HV650 to HV1000. In addition, in this carburized layer, it is preferable that the metal structure at a position of 0.4 mm in depth from the surface contains 90 to 100% of martensite by area% and the Vickers hardness is HV550 to HV900. .

  When the metal structure in the carburized layer at a depth of 50 μm from the surface contains 90 to 100% martensite and the Vickers hardness is HV650 to HV1000, the wear resistance and the fatigue strength are further increased. More preferably, the metal structure in the carburized layer at a depth of 50 μm from the surface contains 95 to 100% of martensite, and the Vickers hardness is HV700 to HV1000.

  When the metal structure in the carburized layer at a depth of 0.4 mm from the surface contains 90 to 100% of martensite and the Vickers hardness is HV550 to HV900, the surface fatigue strength and the fatigue strength are further increased. Increase. More preferably, the metal structure in the carburized layer at a depth of 0.4 mm from the surface contains 92 to 100% of martensite, and the Vickers hardness is HV560 to HV900.

  Further, in the above-mentioned core portion, it is preferable that Vickers hardness at a position at a depth of 2.0 mm from the surface is HV250 to HV500. In addition, in the core, the chemical component at this position needs to be composed of the chemical component described above. More preferably, the Vickers hardness is HV270 to HV450. It is preferable that the metal structure at the core at a depth of 2.0 mm from the surface contains at least one of martensite and bainite, since the above-mentioned effect can be further obtained.

  The above-mentioned observation of the metal structure can be performed with nital corrosion or picral corrosion and observed with an optical microscope. At this time, it is preferable that the specimen subjected to the spheroidizing heat treatment is subjected to picral corrosion.

  The fraction of ferrite, pearlite, bainite, martensite, tempered martensite, tempered bainite, cementite, and the like can be determined by image analysis. In addition, the spheroidized cementite, the number of cementite, and the maximum length of cementite can be determined by mirror-polishing the observation surface, performing EBSD mapping without performing corrosion treatment, identifying cementite, and performing image analysis. The observation surface is not particularly limited, but a cut surface orthogonal to the longitudinal direction may be used as the observation surface.

  In calculating the area fraction of the metal structure, ferrite, pearlite, martensite, bainite, tempered martensite, tempered bainite, spheroidized cementite, and cementite are considered. The calculation of the area fraction does not include nitrides and carbides such as BN, TiC, TiN, and AlN, other fine precipitates, inclusions, and retained austenite.

  In the measurement of the Vickers hardness described above, it is preferable to perform a total of ten measurements on one sample using a micro Vickers measuring machine with a load of 0.49 N to calculate an arithmetic average value. The measurement surface is not particularly limited, but may be a cut surface orthogonal to the axial direction (longitudinal direction).

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

[Method of manufacturing steel for parts to be carburized]
First, an example of the method for manufacturing a steel material according to the present embodiment will be described. An example of a method for producing a steel material for a part to be carburized includes a steelmaking step, a hot working step, and a slow cooling step. Hereinafter, each step will be described.

[Steel making process]
The steel making process includes a refining process, a heating and holding process, a final component adjusting process, and a casting process.

[Refining process]
In the refining process, first, refining in a converter (primary refining) is performed on hot metal manufactured by a known method. Secondary refining is performed on the molten steel discharged from the converter. In the secondary refining, an alloying element is added to molten steel to produce molten steel satisfying the above chemical composition.

  Specifically, Al is added to molten steel discharged from the converter to perform deoxidation. After the deoxidizing treatment, a residue removing treatment is performed. After the slag removal process, secondary refining is performed. In the secondary refining, for example, compound refining is performed. For example, first, a refining process using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed. Further, RH (Ruhrstahl-Hausen) vacuum degassing is performed. Then, final adjustment of other alloy components except Si and Ca is performed.

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

[Heating and holding step]
The molten steel in the ladle after the secondary refining (final component adjustment) is heated at a temperature of 1500 to 1600 ° C. for a holding time ts which is at least twice the uniform mixing time τ (s) calculated by the following equation. .
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 : molten steel temperature (K), h ₀ : gas blowing depth (m), P ₀ : Surface pressure of molten steel (Pa), ε: stirring power value (W / ton), τ: uniform mixing time (s).

  If the holding time ts is less than 2.0 times the uniform mixing time τ, the oxides present in the molten steel in the ladle cannot sufficiently coagulate and coalesce. Therefore, the oxide cannot be removed by floating, and the number of oxides increases. When the holding time ts is less than 2.0 times the uniform mixing time τ, Mg and the like mixed from the slag combine with S in the molten steel to form MgS and the like, and the MgS is dispersed in the molten steel. . This dispersed MgS becomes a precipitation site of MnS. As a result, the number of Mn sulfides increases.

If the holding time ts is at least 2.0 times the uniform mixing time τ, the number of oxides in the steel can be reduced. Further, once formed MgS becomes MgO by reoxidation, the number of MnS precipitation sites is reduced, and as a result, the number of Mn sulfides in the steel can be suppressed. As a result, after the final component adjustment step in the next step, the amount of Mn sulfide becomes 70.0 / mm ² or less and the amount of oxide becomes 25.0 / mm ² or less.

[Final component adjustment step]
Si and Ca are added to the molten steel after the heating and holding step to produce molten steel that satisfies the above chemical composition and equations (1) to (3). Si and Ca may be added to the molten steel as independent raw materials, respectively. The Si-Ca alloy may be added to the molten steel as a raw material.

If Si and Ca are added to the molten steel heated sufficiently uniformly in the heating and holding step, the oxide is changed from Al ₂ O ₃ to a composite inclusion containing SiO ₂ and CaO, and the Mn sulfide is also reduced to Ca. To a sulfide containing Therefore, assuming that the holding time ts is at least twice the uniform mixing time τ, the Mn sulfide is 70.0 / mm ² or less and the oxide is 25.0 / mm ² or less. become.

If Si is added before adding Al to molten steel, deoxidation is not sufficiently performed, and as a result, the amount of oxide exceeds 25.0 particles / mm ² . By adding Si and Ca to the molten steel after the addition of 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. Any of Si and Ca may be added first.

[Casting process]
A raw material (slab or ingot) is manufactured using the molten steel manufactured in the refining process. Specifically, a slab is manufactured by continuous casting using molten steel. Alternatively, an ingot may be formed by ingot making method using molten steel. If necessary, slab rolling or the like may be performed on the cast slab or ingot after the casting step. By using the slab or ingot and selecting the following manufacturing method, the steel material of the present embodiment having the above-described metal structure can be manufactured.

  In order to obtain a steel material having a metal structure containing 85 to 100% in total of ferrite and pearlite in area%, it is preferable to perform the following manufacturing method.

[Hot working process]
As a hot working step, the slab after the casting step is subjected to hot rolling, hot forging, or the like to obtain a hot worked steel material. Hot rolling is, for example, bulk rolling, finish rolling using a continuous rolling mill having a plurality of rolling stands arranged in a row, and the like. Bulk rolling may be performed as needed. The plastic working conditions such as the working temperature, working rate, and strain rate in this hot working step are not particularly limited, and suitable conditions may be appropriately selected. The heating temperature in the hot working step is a known heating temperature, for example, 1100 to 1300 ° C.

[Slow cooling step]
Immediately after this hot working step, the temperature range in which the surface temperature of the hot-worked steel material is 800 ° C. to 500 ° C. is increased to 0 ° C. Slow cooling is performed at a cooling rate of 1.00 ° C./sec to obtain the steel material of the present embodiment.

  If the cooling rate at 800 ° C. to 500 ° C., which is the temperature at which austenite transforms into ferrite and pearlite, exceeds 1.00 ° C./sec, the structural fraction of bainite and martensite increases. As a result, the hardness of the steel material increases, the deformation resistance increases, and the critical working rate decreases. Therefore, it is preferable to limit the cooling rate in the above temperature range to more than 0 to 1.00 ° C./sec. More preferably, it is more than 0 to 0.70 ° C./sec. In order to reduce the cooling rate of the hot-worked steel material after the hot-working step as a slow cooling step, a heat-retaining cover, a heat-retaining cover with a heat source, or a holding furnace is installed after the rolling line or hot forging line. do it.

  In order to obtain a steel material having a metal structure containing 85 to 100% in total of ferrite and spheroidized cementite in area%, the following production method is preferably performed.

[Sphering process]
A spheroidizing heat treatment step may be further performed on the hot-worked steel material subjected to the above-described slow cooling. In this case, the steel material of the present embodiment can be manufactured by performing spheroidizing heat treatment.

As the spheroidizing heat treatment, for example, the following heat treatment may be performed. The hot-worked steel material subjected to the slow cooling is heated to a temperature immediately below the Ac ₁ point (a temperature at which austenite starts to be generated at the time of heating) or immediately above the Ac, and then slowly cooled. The annealed hot-worked steel material is heated to a temperature just above the Ac ₁ point and cooled to a temperature just below the Ar ₁ point (the temperature at which austenite completes transformation to ferrite or ferrite or cementite during cooling). Repeat the process several times. Alternatively, the quenched hot-worked steel material is once quenched, and then tempered in a temperature range of 600 to 700 ° C. for 3 to 100 hours. The method of the spheroidizing heat treatment may be any of the above-described conventionally known annealing or spheroidizing heat treatment methods, and is not particularly limited.

  Even if the steel material subjected to the spheroidizing heat treatment step has high hardness, the critical working ratio can be increased.

  Through the above manufacturing steps, the steel material of the present embodiment can be manufactured. The steel material of the present embodiment is, for example, a steel bar.

[Method of manufacturing carburized parts]
Next, an example of the method for manufacturing the carburized component according to the present embodiment will be described. The present manufacturing method performs a cold working on the steel material of the present embodiment described above, a cold working step of manufacturing a plurality of intermediate members, and welding the manufactured plurality of intermediate members to form an integrated product. A welding process, a carburizing process of performing a carburizing process or a carbonitriding process on the intermediate member after welding, and a finishing heat treatment process of performing a quenching process, or a quenching / tempering process on the intermediate member after the carburizing process. including.

[Cold working process]
As a cold working step, the steel material manufactured by the above-described manufacturing method is subjected to cold plastic working to give a shape to manufacture a plurality of intermediate members. Plastic working conditions such as a working ratio and a strain rate in the cold working step are not particularly limited, and suitable conditions may be appropriately selected. The cold working is, for example, cold forging. The plurality of intermediate members are welded and integrated in a subsequent welding process.

[Welding process]
In the welding process, the plurality of intermediate members are welded by friction joining or laser joining to form an integrated product. The welding method is not particularly limited. The joining surface of the intermediate member may be formed flat by machining. In the above-mentioned steel material, Mn sulfide is not more than 70.0 pieces / mm ² and oxide is not more than 25.0 pieces / mm ² . Therefore, it has excellent jointability and excellent joint fatigue strength of carburized parts.

[Carburizing process]
Carburizing or carbonitriding is performed as a carburizing step on the intermediate members integrally joined by the welding step. In order to obtain a carburized part having the above-described metal structure and hardness, the conditions of the carburizing treatment or the carbonitriding treatment were performed at a temperature of 830 to 1100 ° C., a carbon potential of 0.5 to 1.2%, and a carburizing time of one hour. It is preferable to make the above.

[Finish heat treatment process]
After the carburizing step, a quenching treatment or a quenching / tempering treatment is performed as a finishing heat treatment step to obtain a carburized part. In order to obtain a carburized part having the above-described metal structure and hardness, it is preferable that the temperature of the quenching medium be room temperature to 250 ° C. as a condition of the quenching treatment or the quenching / tempering treatment. If necessary, a sub-zero treatment may be performed after quenching.

[Other steps]
If necessary, an annealing step may be further performed on the steel material before the cold working step. By performing the annealing treatment in the annealing step, the hardness of the steel material is reduced, the deformation resistance is reduced, and the critical working ratio is improved. Annealing conditions are not particularly limited, and well-known annealing conditions may be appropriately selected.

  If necessary, a cutting step may be further performed on the steel material before the carburizing step after the cold working step. In this case, a cutting process is performed in the cutting process to give a shape to the steel material. By performing the cutting process, it is possible to impart a precise shape to the steel material, which is difficult only by cold plastic working.

  If necessary, a shot peening step may be further performed on the carburized component after the above finishing heat treatment step. By performing the shot peening process in the shot peening step, compressive residual stress is introduced into the surface layer of the carburized component. Since the compressive residual stress suppresses the generation and propagation of fatigue cracks, the tooth root and tooth surface fatigue strength of the carburized component can be further improved. The shot peening treatment is desirably performed using shot grains having a diameter of 0.7 mm or less and an arc height of 0.4 mm or more.

  The effects of one embodiment of the present invention will be described more specifically with reference to examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one condition example. The present invention can employ various conditions without departing from the gist of the present invention and as long as the object of the present invention is achieved.

  A molten steel having a chemical composition shown in Table 1 was prepared. At this time, refining was performed under the conditions shown in Table 2. The smelted molten steel was cast by continuous casting to obtain a slab.

  "Steel making conditions (1)" in Table 2 indicates a ratio (= ts / τ) of the holding time ts at 1500 to 1600 ° C. after the secondary refining to the uniform mixing time τ. “Steel making conditions (2)” in Table 2 indicates the order of addition of Al, Si, and Ca. "1" in the column "Steel making conditions (2)" means that Si and Ca were added after Al was added and deoxidized. "2" means that Al and Ca were added after Si was added.

  After the produced slab was heated, it was subjected to slab rolling to obtain a steel material having a cross section perpendicular to the longitudinal direction of 162 mm × 162 mm. Using this steel material, hot rolling was performed by a continuous rolling mill to obtain a round bar-shaped hot-worked steel material having a circular cut surface orthogonal to the longitudinal direction and a diameter of the cut surface of 35 mm. As a cooling step, the hot-worked steel material was gradually cooled using a heat insulating cover or a heat insulating cover with a heat source installed after the rolling line as necessary. The average cooling rate (° C./sec) at 800 ° C. to 500 ° C. during the slow cooling was as shown in Table 2. For each test number, a plurality of hot-worked steel materials after slow cooling were prepared.

  In each test number, spheroidizing heat treatment was performed as a spheroidizing annealing process (SA process: Spheroidizing Annealing) on some of the prepared hot-worked steel materials. In the spheroidizing treatment, the above hot-worked steel material was heated to 740 ° C. Thereafter, the steel material was gradually cooled at a cooling rate of 8 ° C./hr until the temperature of the steel material reached 650 ° C. Air cooling was performed until the steel material temperature reached 650 ° C. to room temperature.

  A steel material was manufactured by the above manufacturing method.

[Evaluation test]
The following properties were evaluated for the manufactured steel materials.

[Production of Metallographic Observation Test Specimen and Test Specimen for Limiting Compressibility]
A metal structure observation test piece was sampled from a bar-shaped steel material at a position at a depth of 1/4 of the diameter of the cut surface from the peripheral surface. Further, a test piece (6 mmφ × 9 mm, notch shape: 30 degrees, depth 0.8 mm, radius of curvature of the tip part 0.15 mm) was sampled so that the longitudinal direction of the steel material was the compression direction. . The test piece for observing the metallographic structure was collected from a steel material after the slow cooling step but not subjected to the SA step, and further from a steel material after the SA step. Similarly, a test piece for measuring the critical compression ratio was sampled from a steel material after the slow cooling process and not subjected to the SA process (as-slowly cooled material), and further sampled from the steel material after the SA process (SA material). . Hereinafter, in each of the test pieces, a sample taken from a steel material after the slow cooling step and not subjected to the SA process (as slowly cooled material) is referred to as a “slowly cooled test piece”, and a steel material after the SA process (SA material). The sample collected from the sample is referred to as “post-SA test piece”.

[Metal structure observation test]
A metallographic observation test was performed using the above-described metallographic observation specimens (test specimens with slow cooling, specimens after SA). Specifically, the test piece and the test piece after SA were mirror-polished while being gradually cooled. The surface (observation surface) of the mirror-polished test piece was etched for 5 to 10 seconds with a nital etching solution (2% nitric acid alcohol) while cooling slowly. Further, the surface (observation surface) of the mirror-polished post-SA test specimen was etched for 10 to 20 seconds with a picral etchant (5% picric acid alcohol). In addition, picric acid is 2,4,6-trinitrophenol.

  The observation surface after etching was observed with an optical microscope to generate a photographic image. Using a photographic image (100 μm × 100 μm per visual field, observing 20 visual fields), the total fraction of ferrite and pearlite and the total fraction of ferrite and spheroidized cementite were determined by image analysis. Table 2 shows the total area ratio (%) of ferrite and pearlite obtained from the test piece while being cooled slowly, and the total area ratio (%) of ferrite and spheroidized cementite obtained from the test piece after SA. In the metal structure, the rest of the structure other than the above was pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, or the like.

[Limit compression test]
A limit compression test was performed on the above-described test piece for measuring the limit compression ratio (a test piece while being cooled slowly, a test piece after SA) by the following method. Each test piece was subjected to cold compression at a speed of 10 mm / min using a restraining die. The compression was stopped when a minute crack of 0.5 mm or more occurred near the notch, and the compression ratio (%) at that time was calculated. This measurement was performed 10 times in total, the compression ratio (%) at which the cumulative probability of failure was 50% was obtained, and the compression ratio was defined as the limit compression ratio (%). Table 2 shows the limit compression ratio (%) of each test number. Since the critical compressibility of the conventional steel material for the part to be carburized is approximately 65%, the case where the critical compressibility is 68% or more, which can be regarded as a value clearly higher than this value, is judged to be excellent in the critical working ratio. . In addition, the evaluation test of the carburized parts was not performed for the test numbers having a critical compression ratio of less than 68%.

[Mn sulfide number and oxide number measurement test]
In each of the test numbers described above, samples were taken from each of the steel material after the slow cooling process and without the SA process (as-cooled material) and the steel material after the SA process (SA material). Specifically, as shown in FIG. 1, a sample was taken from the R / 2 position in the radial direction from the central axis C1 of the as-cooled 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. Further, the sample thickness L3 in a direction perpendicular to the observation surface was set to 5 mm. The normal N of the observation surface was perpendicular to the center axis C1, and the R / 2 position corresponded to the center position of the observation surface.

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

  Inclusions in each field were identified. For each of the identified inclusions, Mn sulfides and oxides were identified using energy dispersive X-ray spectroscopy (EDX). Specifically, in the elemental analysis result of the specified inclusion, when the Mn content is 10.0% or more and the S content is 10.0% or more, the inclusion is recognized as Mn sulfide. did. In addition, in the elemental analysis result of the specified inclusion, when the O content was 10% or more, the inclusion was recognized as an oxide. Inclusions to be specified were inclusions having an equivalent circle diameter of 0.5 μm or more. The beam diameter of EDX used for specifying inclusions was 0.2 μm.

In each of the as-cooled material and the SA material, Mn sulfides and oxides having an equivalent circle diameter of 0.5 μm or more were measured. The number (number / mm ² ) of Mn sulfides per unit area was determined based on the total number of Mn sulfides specified in each visual field and the total area of the 20 visual fields. In addition, the number of oxides per unit area (pieces / 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 as-cooled material (pieces / mm ² ) and the number of oxides in the as-cooled material (pieces / 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 as-cooled material, and the number of oxides in the SA material was the same as the number of oxides in the as-cooled material. Was.

[Manufacture of carburized parts]
A test piece for carburizing (20 mmφ × 30 mm) was taken from the position of a depth of 1/4 of the diameter of the cut surface from the peripheral surface of the as-cooled steel material of each test number so that the longitudinal direction was the compression direction. Collected. The test piece for carburizing was subjected to upsetting at a compression ratio of 50% in a cold working step as a cold working step. The conditions for the upset compression were room temperature, use of a restricted die, and a strain rate of 1 / sec. The test piece for carburizing after the upset compression was subjected to gas carburizing by a shift furnace gas method as a carburizing step. This gas carburization was performed at 950 ° C. for 5 hours, with a carbon potential of 0.8%, and subsequently at 850 ° C. for 0.5 hour. After the carburizing step, as a finishing heat treatment step, oil quenching was performed to 130 ° C., and tempering was performed at 150 ° C. for 90 minutes to obtain a carburized part (using the material while being gradually cooled).

  For the SA material of each test number, carburized parts (using SA material) were manufactured under the same manufacturing conditions as the above-described slowly cooled material.

[Evaluation test of carburized parts]
The properties of the carburized layer and the core of the carburized parts (the carburized parts using the as-cooled material, the carburized parts using the SA material) manufactured above were evaluated. Table 3 shows the evaluation results.

[Vickers hardness test of carburized layer]
In the cut surface perpendicular to the longitudinal direction of the carburized parts of each test number (the carburized parts using the as-cooled material, the carburized parts using the SA material), the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness from the surface were measured. The Vickers hardness at a depth of 4 μm was determined by a Vickers hardness test based on JIS Z 2244 (2009) using a micro Vickers hardness meter. The load during the test was 0.49N. Vickers hardness HV was measured at 10 positions at a depth of 50 μm, and the arithmetic mean value was defined as Vickers hardness HV at a position of 50 μm depth. The Vickers hardness HV at 10 places at a depth of 0.4 μm was measured, and the arithmetic average value was defined as the Vickers hardness HV at a 0.4 μm depth. When the hardness at the position 50 μm deep from the surface is HV650 to HV1000, and when the hardness at the position 0.4 mm deep from the surface is HV550 to HV900, the hardness is determined to be sufficient and pass. did. Table 3 shows the measurement results.

[Metal structure observation test of carburized layer]
With respect to the carburized layer of the carburized part (the carburized part using the as-cooled material, the carburized part using the SA material), the metallographic structure at a depth of 0.4 mm from the surface was evaluated. The metal structure was subjected to nital corrosion and observed with an optical microscope. The magnification of the optical microscope was 200 times, and the observation field was 500 μm × 500 μm.

  The fraction (area ratio (%)) of martensite in the observation visual field was calculated by image analysis. In the metal structure, the remainder other than the above was ferrite, pearlite, bainite, tempered martensite, tempered bainite, spheroidized cementite, cementite, or the like.

[Chemical composition of core and Vickers hardness]
The Vickers hardness and the chemical composition of the core of the carburized parts (the carburized parts using the as-cooled material and the carburized parts using the SA material) were measured by the following methods. The Vickers hardness at a position 2 mm deep from the surface of the cut surface perpendicular to the longitudinal direction of the carburized component was determined by a Vickers hardness test based on JIS Z 2244 (2009) using a Vickers hardness meter. The load during the test was 49N. The measurement was performed ten times at a depth of 2 mm, and the arithmetic mean value was defined as Vickers hardness (HV) at a depth of 2 mm from the surface. Table 2 shows the obtained Vickers hardness. When the Vickers hardness was HV250 to HV500, the hardness was sufficient and judged to be acceptable. Table 3 shows the measurement results.

  The chemical composition at a depth of 2 mm from the surface was quantitatively analyzed using EPMA (Electron Probe MicroAnalyzer) for elements having an atomic number of 5 or more. Then, a case where the composition was almost the same as the chemical component in the slab as the starting material was determined to be equivalent. Table 3 shows the determination results.

[Presence of coarse particles in carburized parts]
With respect to the core of the carburized part (the carburized part using the as-cooled material, the carburized part using the SA material), the old austenite crystal grains were observed at a position 2 mm deep from the surface. Specifically, the cut surface perpendicular to the longitudinal direction of the carburized component was used as the observation surface. After mirror-polishing the observation surface, etching was performed with a saturated aqueous solution of picric acid. A field of view (300 μm × 300 μm) including a position at a depth of 2 mm from the surface of the etched observation surface was observed with an optical microscope (× 400) to identify old austenite crystal grains. With respect to the specified prior austenite crystal grains, the crystal grain size number of each prior austenite grain was determined in accordance with JIS G 0551 (2013). No. in the crystal grain size number. When at least one crystal grain was present, it was determined that "coarse grains occurred". Table 3 shows the determination results.

[Fatigue strength evaluation test of carburized parts after joining]
A round bar having a diameter of 35 mm and a length of 150 mm was prepared by machining a round bar having a diameter of 35 mm, which was a steel material (as-annealed material, SA material) of each test number. Using this round bar, a basic fatigue test specimen and a joint fatigue test specimen were produced.

  The basic fatigue test piece was produced by the following method. An Ono-type rotary bending fatigue test piece having a diameter of 8 mm and a parallel portion length of 15.0 mm was prepared from the center of the cross section of a round bar having a diameter of 20 mm and a length of 150 mm from the center. This test piece was used as a basic fatigue test piece.

  The joint fatigue test piece was produced by the following method. Round rods having the same test piece number and having a diameter of 20 mm and a length of 150 mm were butted together to form a jointed round bar under the following friction welding conditions.

Friction welding conditions:
Friction pressure: 100 MPa, friction time: 5 sec,
Upset pressure (pressing force from both ends of the round bar to the joint): 200 MPa
Upset time (time to pressurize the joint): 5 sec,
Rotation speed: 2000 rpm,
Offset: 5-12mm

  An Ono-type rotating bending fatigue test piece having a diameter of 8 mm at the evaluation portion and a length of the parallel portion of 15.0 mm was prepared from the center of the cross section of the joined round bar, and was used as a pressure contact fatigue test piece. In the press-contact fatigue test specimen, the central part in the longitudinal direction of the parallel part was used as the joining surface.

  The following carburizing and quenching treatment was performed on the basic fatigue test piece and the joint fatigue test piece to obtain a carburized part (a carburized part using a material as slowly cooled and a carburized part using an SA material). In the carburizing and quenching treatment, gas carburizing was performed by a shift furnace gas method. Specifically, the carbon potential was set to 0.8%, and held at 950 ° C. for 5 hours. Then, it was kept at 850 ° C. for 0.5 hour at the same carbon potential. Then, it was immersed in oil at 130 ° C. to perform oil quenching. After oil quenching, tempering was performed at 150 ° C. for 90 minutes. By the above method, Ono-type rotating bending fatigue test pieces (basic fatigue test pieces and joining fatigue test pieces) simulating carburized parts were produced.

An Ono-type rotating bending fatigue test was performed on the prepared basic fatigue test piece and joint fatigue test piece. Specifically, the above-mentioned Ono-type rotating bending fatigue test pieces (basic fatigue test pieces and joining fatigue test pieces) were used at room temperature and in an air atmosphere in accordance with JIS Z 2274 (1978). A bending fatigue test was performed. The number of rotations was 3000 rpm, the stress ratio R was -1, and the maximum stress that did not break after 1 × 10 ⁷ cycles of the stress load was defined as the fatigue strength (MPa).

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

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

[Test results]
The test results are shown in Tables 2 and 3. Referring to Table 2 and Table 3, the chemical compositions of Test No. 1 to Test No. 11 were appropriate, and satisfied Expressions (1) to (3). Furthermore, the steelmaking conditions were also appropriate. Also, the cooling rate in the slow cooling step was appropriate. Furthermore, the spheroidizing treatment was also appropriate. Therefore, the number of Mn sulfides in the as-cooled material and the SA material was 70.0 / mm ² or less, and the number of oxides was 25.0 / mm ² or less. Furthermore, in the as-cooled material, the total area ratio of ferrite and pearlite was 85 to 100%, and in the SA material, the total area ratio of ferrite and spheroidized cementite was 85 to 100%. As a result, the critical compression ratio was 68% or more for both the as-cooled material and the SA material, showing excellent critical compressibility.

  Furthermore, the Vickers hardness of the carburized layer of the carburized part of the as-cooled material and the carburized part of the SA material was both appropriate, and the fraction of martensite at a depth of 0.4 was 90% or more. Further, the core hardness and the chemical composition were appropriate, and the prior austenite particle size was not coarsened. Furthermore, in the joint fatigue test pieces, the fatigue strength ratio was as high as 85% or more in each case, and excellent fatigue strength was exhibited even in the case of joining.

  On the other hand, in Test No. 12, the C content was too high. Therefore, in the as-cooled material, the total area ratio of ferrite and pearlite was less than 85%. Further, the critical compressibility of both the as-cooled material and the SA material was less than 68%, and a sufficient critical compressibility was not obtained.

  In Test No. 13, the C content was too low. For this reason, sufficient hardness was not obtained in the core portion of the carburized component made of the as-cooled material and the core portion of the carburized component made of the SA material.

  In test number 14, the oxygen content was too high. Therefore, the number of oxides in both the as-cooled material and the SA material was too large. As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test pieces simulating the carburized parts of the as-cooled material and the carburized parts of the SA material.

  In test number 15, the N content was too high. Therefore, solid solution B could not be secured, and the core hardness was too low. Furthermore, since coarse TiN was generated, the critical working rate of the steel material (as-cooled material and SA material) was low.

  In Test No. 16, F1 was less than the lower limit of Expression (1). For this reason, the core hardness of the carburized part of the as-cooled material and the core hardness of the carburized part of the SA material were too low.

  In Test No. 17, F1 exceeded the upper limit of Expression (1). Therefore, in the as-cooled material, the total area ratio of ferrite and pearlite was less than 85%. Therefore, the critical working rates of the as-cooled steel and the SA steel were low.

  In Test No. 18, F2 was less than the lower limit of Expression (2). Therefore, the hardness at the depth of 0.4 mm was too low in the carburized parts of the as-cooled material and the carburized parts of the SA material.

  In Test No. 19, F2 exceeded the upper limit of Expression (2). Therefore, the critical working rate of the steel material (as-cooled material and SA material) before forging was too low.

  In Test No. 20, F3 was less than the lower limit of Expression (3). Therefore, in the core part of the carburized part made of the as-cooled material and the core part of the carburized part made of the SA material, part of the old austenite grains became coarse.

  In Test No. 21, F3 exceeded the upper limit of Expression (3). Therefore, the critical working rate of the steel material (as-cooled material and SA material) was low.

In Test Nos. 22 and 23, for the molten steel in the ladle after the 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 as-cooled material and the SA material, the number of MnS exceeded 70.0 / mm ² and the number of oxides exceeded 25.0 / mm ² . As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test pieces simulating the carburized parts of the as-cooled material and the carburized parts of the SA material.

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

  In Test Nos. 26 and 27, the average cooling rate at 800 ° C. to 500 ° C. in the slow cooling step after hot rolling was too high. Therefore, in the structure of the as-cooled steel material, the total area ratio of ferrite and pearlite was less than 85%, and the critical compression ratio was less than 68%. On the other hand, in the structure of the steel material of the SA material, the total area ratio of ferrite and spherical cementite was 85% or more, and the critical compressibility of the SA material exceeded 68%. Then, the Vickers hardness of the carburized layer of the carburized part of the SA material was appropriate, and the fraction of martensite at a depth of 0.4 mm was 90% or more. Further, the core hardness and the chemical composition were appropriate, and the prior austenite particle size was not coarsened. Further, in the joint fatigue test specimen, the fatigue strength ratio was as high as 85% or more, and even when joined, excellent fatigue strength was exhibited.

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

Claims (3)

A steel material for a part to be carburized,
In mass%,
C: 0.09-0.16%,
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 to 0.40%,
Al: 0.005 to 0.030%,
Ti: less than 0.010 to 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 to 0.0050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.30%, and
The balance: Fe and impurities,
Having a chemical composition that satisfies the formulas (1) to (3),
In a cross section parallel to the axial direction of the steel material,
Mn sulfide containing not less than 10.0% by mass of Mn, not less than 10.0% by mass of S, and having an equivalent circle diameter of 0.5 μm or more is 70.0 / mm ² or less;
Oxygen containing more than 10 wt%, oxide of equivalent circle diameter of more than 0.5μm is Ri der 25.0 pieces / mm ² or less,
The metal structure contains, in area%, ferrite and pearlite in a total of 85 to 100%, or
The metal structure contains ferrite and spheroidized cementite in an area% of 85 to 100% in total.
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 Formula (1)
13.0 <(0.70 × Si + 1) × (5.1 × Mn + 1) × (2.2 × Cr + 1) × (3.0 × Mo + 1) × (0.36 × Ni + 1) <45.0 Equation (2) )
0.004 <Ti-N * (48/14) <0.030 Formula (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formulas (1) to (3). The steel material according to claim 1,
The chemical composition is
Cu: 0.005% to 0.50%, and
Ni: 0.005% to 0.30%,
Containing one or more selected from the group consisting of
Steel. It is a steel material according to claim 1 or claim 2 ,
The steel material is a steel bar,
Steel.

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