CN111684094B

CN111684094B - Steel material for carburized parts

Info

Publication number: CN111684094B
Application number: CN201980012493.1A
Authority: CN
Inventors: 根石丰; 天田贵文
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-04-12
Filing date: 2019-03-14
Publication date: 2021-12-03
Anticipated expiration: 2039-03-14
Also published as: KR102402361B1; JPWO2019198415A1; CN111684094A; JP6628014B1; WO2019198415A1; KR20200103821A

Abstract

Provided is a steel material for carburized parts, which can obtain excellent fatigue strength even when parts are manufactured by welding before carburization. The disclosed steel material has a composition consisting of, in mass%: 0.09-0.16%, Si: 0.01 to 0.50%, Mn: 0.40-0.60%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90-2.00%, Mo: 0.10 to 0.40%, Al: 0.005-0.030%, Ti: 0.010% or more and less than 0.050%, Nb: 0.010-0.030%, N: 0.0080% or less, O: 0.0030% or less, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.0050%, and the balance: fe and impurities, and satisfies the chemical compositions of formulae (1) to (3) described in the specification. In a cross section of the steel material parallel to the axial direction, the Mn sulfide content was 70.0 pieces/mm²Oxide of 25.0 pieces/mm²The following.

Description

Steel material for carburized parts

Technical Field

The present invention relates to a steel material, and more particularly, to a steel material used for a member subjected to carburizing treatment.

Background

Steel materials used for machine structural parts generally contain Mn, Cr, Mo, Ni, and the like in combination. Some of the mechanical structural members are subjected to carburizing treatment. The steel material used for the carburized component has the above chemical composition and is produced by casting, forging, rolling, or the like. Hereinafter, in the present specification, a member subjected to a carburizing process is referred to as a "carburized member".

The carburized component is produced, for example, by the following method. An intermediate product is produced by machining such as forging and cutting a steel material. The intermediate product is carburized to produce a carburized part having a carburized layer, which is a hardened layer, in the surface layer portion, and a core portion, which is a base material not affected by the carburization.

Among the costs for manufacturing carburized parts, the cost associated with cutting is very high. The cutting process not only involves expensive cutting tools, but also generates a large amount of chips. Therefore, this is also disadvantageous from the viewpoint of yield. Therefore, an attempt has been made to replace the cutting process with forging. Forging methods are roughly classified into hot forging, warm forging, and cold forging. The warm forging is characterized by less scale generation and improved dimensional accuracy as compared with the hot forging. In addition, cold forging is characterized by no scale generation and dimensional accuracy close to that of cutting. Therefore, studies have been made on rough machining by hot forging, finish machining by cold forging, mild cutting as finish machining after warm forging, molding by cold forging alone, and the like. However, when the cutting work is replaced with warm forging or cold forging, if the deformation resistance of the steel material is large, the surface pressure applied to the die increases, and the die life decreases. Therefore, the cost advantage compared to cutting becomes small. In addition, when a steel material is formed into a complicated shape, there is a problem that cracks occur in a portion subjected to a large processing. Therefore, various techniques have been studied for the purpose of softening a steel material and improving a limit work ratio.

Patent document 1 discloses a steel for carburizing which has a smaller deformation resistance at the time of cold forging and a larger limit working ratio than conventional steels, and which has a hardened layer and core hardness after carburizing which are equivalent to those of conventional steels. The steel for carburizing described in patent document 1 is characterized by containing, in terms of mass%, C: 0.07 to 0.13%, Si: 0.0001 to 0.50 percent, Mn: 0.0001% -0.80%, S: 0.0001 to 0.100 percent, Cr: more than 1.30% and 5.00% or less, B: 0.0005 to 0.0100%, Al: 0.0001 to 1.0 percent, Ti: 0.010% -0.10%, limited to N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, and the balance of Fe and unavoidable impurities, and the content of each element in the chemical composition in mass% satisfies the following formula 1 as an index of hardness, the following formula 2 as an index of hardenability, and the following formula 3 as an index of amount of TiC precipitation. 0.10 < C +0.194 xSi +0.065 xMn +0.012 xCr +0.078 xAl < 0.235 · (formula 1), 7.5 < (0.7 xSi +1) × (5.1 xMn +1) × (2.16 xCr +1) < 44 · (formula 2), 0.004 < Ti-N × (48/14) < 0.030 · (formula 3). The steel for carburizing of patent document 1 has the above chemical composition, and thus the limit working ratio at the time of cold forging is increased, and a hardened layer and a steel part hardness equivalent to those of conventional steels can be obtained after the carburizing treatment.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2012/108460

Disclosure of Invention

Problems to be solved by the invention

On the other hand, a plurality of carburized parts are used for a mechanical structural part used in an automobile. For example, variable diameter pulleys of Continuously Variable Transmissions (CVTs) also utilize carburized components. Large carburized components, such as variable diameter pulleys, are produced by forging and then cutting as described above. However, when a large steel material is formed by cold forging, an excessive load is applied to the cold forging machine. Therefore, when a large carburized part is formed by cold forging, the following method is studied: a large carburized part is manufactured by cold forging a plurality of members, joining the members by welding such as friction joining or laser joining, and carburizing the joined steel members.

In the case of manufacturing a carburized part by welding in this way, the fatigue strength (joint fatigue strength) of the carburized part as a joining material is required.

The purpose of the present disclosure is to provide a steel material for a carburized component, which has a higher limit working ratio during cold forging than conventional steel materials at the stage of the steel material, and which can achieve excellent fatigue strength after carburization even when welded.

Hereinafter, unless otherwise specified, "forging" alone means "cold forging".

Means for solving the problems

The steel material for carburized parts according to the present disclosure comprises

C：0.09～0.16％、

Si：0.01～0.50％、

Mn：0.40～0.60％、

P: less than 0.030%,

S: less than 0.025%,

Cr：0.90～2.00％、

Mo：0.10～0.40％、

Al：0.005～0.030％、

Ti: more than 0.010 percent and less than 0.050 percent,

Nb：0.010～0.030％、

N: less than 0.0080 percent,

O: less than 0.0030%,

B：0.0003～0.0030％、

Ca：0.0005～0.0050％、

Cu：0～0.50％、

Ni: 0 to 0.30%, and

and the balance: fe and impurities, and satisfying the chemical compositions of the formulae (1) to (3),

in a cross section parallel to the axial direction of the steel material, the Mn sulfide content is 70.0 pieces/mm²Below and oxides 25.0 pieces/mm²The following

The Mn sulfide contains 10.0 mass% or more of Mn, 10.0 mass% or more of S, and has a circle-equivalent diameter of 0.5 μm or more,

the oxide contains 10 mass% or more of oxygen and has a circle equivalent 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 XSI +1) × (5.1 XMN +1) × (2.2 XCR +1) × (3.0 XMO +1) × (0.36 XMI +1) < 45.0 formula (2)

0.004 < Ti-Nx (48/14) < 0.030 formula (3)

Wherein the content (mass%) of the corresponding element is substituted into each element symbol of formulae (1) to (3).

ADVANTAGEOUS EFFECTS OF INVENTION

The steel material of the present disclosure has a higher limit working ratio at the cold forging time than the conventional steel material at the stage of the steel material, and can obtain excellent fatigue strength at the stage of the carburized part (carburized part) even when welding is performed.

Drawings

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

Detailed Description

The steel material for a member subjected to carburizing treatment of the present embodiment will be described below.

The present inventors have made detailed studies on a steel material for a carburized component in order to achieve both a reduction in deformation resistance (reduction in hardness) of the steel material before cold forging and an improvement in the limit reduction ratio, and excellent characteristics (for example, effective hardened depth and core hardness) of the carburized component after carburization, and have obtained the following findings (a) to (g).

(a) The lower the C content, the more the steel material before cold forging can be softened. However, if the C content is too low, it becomes difficult to make the properties (e.g., effective hardened depth and core hardness) of the carburized part after carburization at a level equivalent to that of a conventional steel material having a C content of about 0.20% (e.g., JIS-SCR 420). There is a lower limit of the C content in order to obtain the hardness of the core portion necessary for the carburized part.

(b) In order to obtain as large an effective depth of hardened layer and core hardness as possible at the lowest possible C content, the martensite fraction is preferably increased in the metallographic structure of the core of the carburized component.

(c) In order to increase the martensite fraction of the metallographic structure in the core portion of the carburized component, it is necessary to contain alloy elements (hardenability improving elements) such as Si, Mn, Cr, Mo, and Ni for improving the hardenability of the steel so that the content thereof satisfies the below-described formula (2) which is an index of hardenability.

(d) However, if the content of the hardenability improving element is increased, the hardness of the steel material is increased due to the effect of solid solution strengthening of ferrite by the hardenability improving element. Therefore, B is contained to suppress an increase in the hardness of ferrite and to enhance the hardenability of the steel material, and the contents of C and the hardenability improving element satisfy the hardness index (1) described later.

(e) In order to stably obtain the hardenability improving effect of B, most of N contained in the steel is fixed as TiN at the time of the carburizing treatment. This prevents the solid solution B from precipitating as BN, and ensures the amount of solid solution B in the steel. In order to effectively obtain this effect, Ti is preferably contained in a stoichiometric excess with respect to the N content. In addition, TiC is finely dispersed and precipitated in the metallographic structure in order to prevent abnormal grain growth of austenite grains during carburizing treatment. In this way, in order to finely disperse and precipitate TiC while securing the amount of solid solution B, the contents of Ti and N satisfy the formula (3) derived by the inventors, which is an index of the amount of TiC precipitation described later.

(f) As described above, B effectively improves the hardenability of the core portion of the carburized component. However, in the case of gas carburizing in the converter gas system, the effect of improving hardenability by containing B is low in the carburized layer, which is the surface layer portion of the carburized component. This is because, during the carburizing treatment, nitrogen enters from the surface of the steel material, and is bonded to the solid solution B to precipitate as BN, and the amount of the solid solution B decreases. Therefore, in order to ensure hardenability in a carburized layer, which is a surface layer portion of the carburized component, it is necessary to satisfy the formula (2) which is an index of hardenability described in the above (c).

(g) In order to further soften the steel material for parts subjected to carburization, it is preferable to perform slow cooling under the conditions described below after hot rolling or hot forging in the production of the steel material. This makes it possible to control the metallographic structure of the steel material and soften the steel material. This improves the metallographic structure of the surface layer portion of the steel material, improves the ductility, and can provide a steel material having a high limit work ratio.

The present inventors have further studied the fatigue strength (joint fatigue strength) of the steel material of the present embodiment at the stage of carburized parts manufactured by performing carburization after welding. As a result, it was found that, when inclusions in a cross section of a steel material parallel to the axial direction (i.e., the longitudinal direction of the steel material) satisfy the following requirements, the fatigue strength (joint fatigue strength) at the stage of a carburized component manufactured by carburizing after welding is improved.

(A) Mn sulfide containing 10.0 mass% or more of Mn, 10.0 mass% or more of S, and having a circle equivalent diameter of 0.5 μm or more is 70.0 pieces/mm²The following.

(B) The oxide containing 10 mass% or more of oxygen and having a circle equivalent diameter of 0.5 μm or more is 25.0 pieces/mm²The following.

This point will be described in detail below.

In the steel material having the chemical composition of the present embodiment, Mn sulfide and oxide are present in the steel. Here, in the present specification, Mn sulfide and oxide are defined as follows.

Mn sulfide: an inclusion having a Mn content of 10.0 mass% or more, an S content of 10.0 mass% or more and a circle-equivalent diameter of 0.5 μm or more

Oxide: containing 10 mass% or more of oxygen and having a circle equivalent diameter of 0.5 μm or more

In the present specification, inclusions containing 10.0 mass% or more of S, 10.0 mass% or more of Mn, and 10 mass% or more of oxygen and having a circle equivalent diameter of 0.5 μm or more are not "Mn sulfide" but included in "oxide".

In the present embodiment, when a carburized part is formed by performing a carburizing process after welding, typically friction welding, laser welding, or the like, is performed on a steel material, a HAZ region is present in the carburized part. The HAZ region is sometimes low in strength compared to other regions. Accordingly, in order to secure the strength of the HAZ region, the inclusions are reduced as much as possible. In the present embodiment, as described in the above (a) and (B), the number of Mn sulfides and oxides occupying the most part of the inclusions in the steel is reduced as much as possible. In this case, the strength of the HAZ region can be ensured, and as a result, the fatigue strength of the carburized component can be improved.

The steel material for parts carburized according to the present embodiment completed based on the above findings has the following features.

[1] The steel material for carburized parts of (1) has a composition of

C：0.09～0.16％、

Si：0.01～0.50％、

Mn：0.40～0.60％、

P: less than 0.030%,

S: less than 0.025%,

Cr：0.90～2.00％、

Mo：0.10～0.40％、

Al：0.005～0.030％、

Ti: more than 0.010 percent and less than 0.050 percent,

Nb：0.010～0.030％、

N: less than 0.0080 percent,

O: less than 0.0030%,

B：0.0003～0.0030％、

Ca：0.0005～0.0050％、

Cu：0～0.50％、

Ni: 0 to 0.30%, and

in a cross section parallel to the axial direction of the steel material, the Mn sulfide content is 70.0 pieces/mm²Below and oxides 25.0 pieces/mm²In the following, the following description is given,

0.004 < Ti-Nx (48/14) < 0.030 formula (3)

[2] The steel material of [1], wherein,

the chemical composition comprises a chemical composition selected from the group consisting of

Cu: 0.005% to 0.50%, and

ni: 0.005-0.30% of more than 1 of the group.

[3] The steel of (1) or (2), wherein,

the metallurgical structure contains ferrite and pearlite in an area% of 85-100% in total.

[4] The steel of (1) or (2), wherein,

the metallurgical structure contains ferrite and spherical cementite in an area% of 85-100% in total.

[5] The steel product of (1) to (4), wherein,

the steel material is a bar steel.

The steel material for a member subjected to carburizing treatment of the present embodiment will be described below. The "%" of the chemical composition means mass% unless otherwise specified.

[ chemical composition ]

The chemical composition of the steel material for parts carburized according to the present embodiment contains the following elements.

C：0.09～0.16％

Carbon (C) improves hardenability of steel materials and hardness of a core portion in a carburized component including a carburized layer and the core portion. When the C content is less than 0.09%, the hardness of the core portion of the carburized component is insufficient. On the other hand, when the C content exceeds 0.16%, the cementite fraction and pearlite fraction of the metallographic structure of the steel material increase, the hardness of the steel material before forging significantly increases, and the limit work ratio also decreases. Therefore, the C content is 0.09 to 0.16%. The conventional steel for carburized parts has a C content of about 0.20%. Therefore, the steel material for parts carburized according to the present embodiment has a lower C content than conventional steel materials. The lower limit of the C content is preferably 0.10%, and more preferably 0.11%. The upper limit of the C content is preferably 0.15%, and more preferably 0.14%.

Si：0.01～0.50％

Silicon (Si) improves the temper softening resistance of the carburized part and improves the fatigue strength of the carburized part. If the Si content is less than 0.01%, the above-described 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 limit reduction rate decreases. Therefore, the Si content is 0.01 to 0.50%. When importance is attached to the surface fatigue strength of the carburized component, the lower limit of the Si content is preferably 0.015%, and more preferably 0.02%. When importance is attached to improvement of the limit workability of the carburized part, the upper limit of the Si content is preferably 0.48%, and more preferably 0.46%.

Mn：0.40～0.60％

Manganese (Mn) increases the hardenability of steel materials and increases the strength of the core portion of a carburized component. When the Mn content is less than 0.40%, the 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 limit reduction rate decreases. Therefore, the Mn content is 0.40 to 0.60%. The lower limit of the Mn content is preferably 0.42%, and more preferably 0.44%. The upper limit of the Mn content is preferably 0.58%, and more preferably 0.56%.

P: less than 0.030%

Phosphorus (P) is an impurity inevitably contained. In other words, the P content exceeds 0%. P segregates at austenite grain boundaries to embrittle prior austenite grain boundaries, causing grain boundary cracking. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 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 limit, the productivity is lowered and the production cost is increased. Therefore, in normal operation, the preferable lower limit of the P content is 0.0001%.

S: less than 0.025%

Sulfur (S) is inevitably contained. In other words, the S content exceeds 0%. S combines with Mn to form MnS, thereby improving the machinability of the steel. When the S content exceeds 0%, the effect can be obtained. On the other hand, when the S content exceeds 0.025%, coarse MnS is formed, so that cracks are likely to be generated during forging, and the critical compressibility is lowered. Therefore, the S content is 0.025% or less. The upper limit of the S content is preferably 0.022%, and more preferably 0.020%. In order to improve the machinability more effectively, the lower limit of the S content is preferably 0.0001%, and more preferably 0.003%.

Cr：0.90～2.00％

Chromium (Cr) increases the hardenability of steel materials and increases the strength of the core portion of a carburized component. If the Cr content is less than 0.90%, the 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 limit reduction rate decreases. Therefore, the Cr content is 0.90 to 2.00%. The lower limit of the Cr content is preferably 0.95%, and more preferably 1.00%. The upper limit of the Cr content is preferably 1.95%, and more preferably 1.92%.

Mo：0.10～0.40％

Molybdenum (Mo) increases the hardenability of steel materials and increases the strength of the core portion of a carburized component. When the Mo content is less than 0.10%, the 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 limit reduction rate decreases. Therefore, the Mo content is 0.10 to 0.40%. The lower limit of the Mo content is preferably 0.11%, and more preferably 0.12%. The upper limit of the Mo content is preferably 0.38%, and more preferably 0.36%.

Al：0.005～0.030％

Aluminum (Al) is an element that forms AlN in the presence of solid-solution N in steel. However, in the core portion of the steel material and the carburized component according to this embodiment, since N in the steel is fixed as TiN by the addition of Ti, there is almost no solid solution N in the steel. Therefore, Al does not form AlN, and exists as solid-solution Al in the steel. Al present in a solid solution state has an effect of improving the machinability of steel. When finish machining or the like is performed during the production of a carburized component, the Al content is preferably 0.005% or more. However, if 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: more than 0.010 percent and less than 0.050 percent

Titanium (Ti) fixes N in the steel material in the form of TiN, and suppresses the formation of BN. Thus, Ti secures a solid solution B amount and improves the hardenability of the steel material. Ti further forms Ti carbide, and coarsening of crystal grains at the time of carburizing treatment is suppressed. When the Ti content is less than 0.010%, the effect cannot be obtained. On the other hand, when the Ti content is 0.050% or more, the precipitation amount of Ti carbide becomes too large, and the limit reduction ratio of the steel material before forging is lowered. Therefore, the Ti content is 0.010% or more and less than 0.050%. The lower limit of the Ti content is preferably 0.012%, and more preferably 0.014%. The upper limit of the Ti content is preferably 0.048%, and more preferably 0.046%.

Nb：0.010～0.030％

Nb (niobium) combines with N and C in the steel to form Nb carbonitride. Nb carbonitride suppresses coarsening of crystal grains by the pinning effect. If the Nb content is less than 0.010%, the 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%. The lower limit of the Nb content is preferably 0.011%, and more preferably 0.012%. The preferable upper limit of the Nb content is 0.029%, more preferably 0.028%, and still more preferably 0.027%.

N: 0.0080% or less

Nitrogen (N) is an impurity inevitably contained. In other words, the N content in the steel exceeds 0%. N and B combine to form BN, thereby reducing the amount of B dissolved in the solution. In this case, the hardenability of the steel material is reduced. 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 contributing to hardenability. Further, coarse TiN is formed. The coarse TiN becomes a starting point of cracks during forging, and lowers the limit machining rate of the steel before forging. Therefore, the N content is 0.0080% or less. The upper limit of the N content is preferably 0.0078%, and more preferably 0.0076%. Preferably, the N content is low. However, if the N content is reduced to the limit, the productivity is lowered and the production cost is increased. Therefore, in normal operation, the preferable lower limit of the N content is 0.0020%.

O: less than 0.0030%

Oxygen (O) is an impurity inevitably contained. In other words, the O content in the steel material exceeds 0%. O forms an oxide, and when an intermediate product before carburizing is manufactured by welding, the bondability is reduced. At this time, the fatigue strength is reduced. 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%. Preferably, the O content is low. However, if the O content is reduced to the limit, the productivity is lowered and the production cost is increased. Therefore, in normal operation, the preferable lower limit of the O content is 0.0010%.

B：0.0003～0.0030％

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

Ca：0.0005～0.0050％

Calcium (Ca) is contained in Mn sulfide and oxide to spheroidize these inclusions. Further, these inclusions are refined by being contained in Mn sulfide and oxide. When the Ca content is less than 0.0005%, these effects cannot be obtained. On the other hand, when the Ca content exceeds 0.0050%, coarse Mn sulfides and coarse oxides are formed, and the fatigue strength of the carburized component is reduced. Therefore, the Ca content is 0.0005 to 0.0050%. The lower limit of the Ca content is preferably 0.0006%, and more preferably 0.0007%. The upper limit of the Ca content is preferably 0.0048%, and more preferably 0.0046%.

And the balance: fe and impurities

The remainder of the chemical composition of the steel material for parts carburized according to the present embodiment is composed of Fe and impurities. Here, the impurities mean: in the industrial production of steel materials, substances are mixed from ores, waste materials, production environments, and the like as raw materials, and are components which are not intended to be contained in steel.

As the impurities, all elements other than the above-mentioned impurities can be cited. The number of impurities may be only 1, or 2 or more. Examples of the impurities other than the above impurities include Sb, Sn, W, Co, As, Pb, Bi, and H. These elements may be contained in the following amounts as impurities, for example.

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: less than 0.0005%.

[ with respect to any element ]

The chemical composition of the steel material for parts to be carburized according to the present embodiment may contain 1 or more selected from the group consisting of Cu and Ni instead of part of Fe.

Cu：0～0.50％

Copper (Cu) is an arbitrary element, and may not be contained. In other words, the Cu content may be 0%. When the Cu content exceeds 0%, Cu improves the hardenability of the steel material, and improves the strength of the carburized component. Cu is an element that does not form an oxide or nitride in a gas atmosphere of gas carburization. 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 a carburized abnormal layer derived therefrom. However, if the Cu content is too high, the ductility in a high temperature region of 1000 ℃ or higher is lowered, which causes a reduction in the yield in continuous casting and rolling. Further, the hardness of the steel material before forging increases, and the limit reduction rate decreases. Therefore, the Cu content is 0 to 0.50%. The lower limit of the Cu content is preferably 0.005%, and more preferably 0.05%. The upper limit of the Cu content is preferably 0.45%, and more preferably 0.40%.

Ni：0～0.30％

Nickel (Ni) is an arbitrary element, and may not be contained. In other words, the Ni content may be 0%. When the Ni content exceeds 0%, Ni increases hardenability of the steel material, and increases the strength of the carburized component. However, if the Ni content is too high, the hardness of the steel material before forging increases, and the limit reduction ratio decreases. Therefore, the Ni content is 0 to 0.30%. The lower limit of the Ni content is preferably 0.005%, and more preferably 0.05%. The upper limit of the Ni content is preferably 0.28%, and more preferably 0.25%.

[ concerning formulae (1) to (3) ]

The chemical composition of the steel material for parts to be carburized according to the present embodiment further satisfies the following formulae (1) to (3).

0.004 < Ti-Nx (48/14) < 0.030 formula (3)

Wherein the content (mass%) of the corresponding element is substituted into the symbol of the element in the formulae (1) to (3). In the case where the corresponding element is an arbitrary element and is not contained, "0" is substituted at the symbol of the element.

Hereinafter, each formula will be described.

[ formula (1): hardness index

Definition F1 ═ C +0.194 × Si +0.065 × Mn +0.012 × Cr +0.033 × Mo +0.067 × Ni +0.097 × Cu +0.078 × Al. F1 is an index of hardness of steel.

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

When F1 is 0.235 or more, the hardness of the steel material before forging increases, and the limit reduction ratio decreases. On the other hand, if F1 is 0.140 or less, the hardness of the carburized part is insufficient. Therefore, F1 exceeds 0.140 and falls short of 0.235. F1 is preferably as low as possible within a range satisfying the hardenability index (F2) described later. A preferable upper limit of F1 is less than 0.230, more preferably 0.220, and still more preferably 0.210. The F1 value is obtained by rounding the 4 th digit after the decimal point of the calculated value.

[ formula (2): index of hardenability)

F2 is defined as (0.70 × Si +1) × (5.1 × Mn +1) × (2.2 × Cr +1) × (3.0 × Mo +1) × (0.36 × Ni + 1). F2 is an index for the hardenability of the member.

As described above, B is effective for improving the hardenability of the core portion of the carburized component. In the case of gas carburizing in the converter gas system, the effect of improving hardenability by the inclusion of B is low in the carburized layer, which is the surface layer portion of the carburized component. This is because nitrogen enters from the atmosphere into the surface layer portion of the carburized component during the carburizing treatment, solid solution B precipitates as BN, and the amount of solid solution B contributing to improvement of hardenability is insufficient. Therefore, in order to ensure hardenability in a carburized layer, which is a surface layer portion of the carburized component, it is necessary to use an element other than B that improves hardenability of steel.

F2 is composed of a hardenability improving element. When F2 is 13.0 or less, the hardness and effective hardened layer depth (depth at which vickers hardness is HV550 or more) of the carburized layer are not equal to or more than those of the conventional steel material (C content is about 0.20%) under the same carburizing conditions. On the other hand, if F2 is 45.0 or more, the hardness of the steel material before forging increases, and the limit reduction ratio decreases. Therefore, F2 exceeded 13.0 and was less than 45.0. F2 is preferably as large as possible within the range satisfying the hardness index F1. The lower limit of F2 is preferably 13.2, and more preferably 15.0. The F2 value is obtained by rounding off the 2 nd digit after the decimal point of the calculated value.

[ formula (3): TiC precipitation index

Definition F3 ═ Ti-N × (48/14). F3 is an index for the amount of TiC precipitated. When Ti is contained in a stoichiometric excess to N, N is fixed as TiN. In other words, F3 means an excess Ti amount other than the amount consumed for forming TiN. "14" in F3 represents the atomic weight of N, and "48" represents the atomic weight of Ti.

The most part of the excess Ti defined by F3 is bonded to C during the carburizing treatment to become TiC. This TiC has a pinning effect of preventing coarsening of crystal grains at the time of carburizing treatment. If F3 is 0.004 or less, the amount of TiC precipitated is insufficient. In this case, the coarsening of crystal grains during the carburizing treatment cannot be suppressed. On the other hand, if F3 is 0.030 or more, the amount of TiC precipitated becomes too large, the hardness of the steel material before forging increases, and the rate of limit machining decreases. Therefore, F3 exceeded 0.004 and was 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 obtained by rounding the 4 th digit after the decimal point of the calculated value.

By satisfying the hardness index F1, the hardenability index F2, and the TiC precipitation amount index F3 at the same time in the chemical composition, the steel material has a smaller deformation resistance at the time of cold forging and a larger limit working ratio than the conventional steel. Further, after the carburizing treatment of the steel material, a carburized component having a hardened layer and a core hardness equivalent to those of conventional steels can be obtained.

[ metallographic Structure of Steel Material for Member carburized ]

The metallurgical structure of the steel material for a member subjected to carburizing treatment according to the present embodiment will be described.

[ first metallographic Structure morphology ]

The metallurgical structure of the steel material formed by the chemical composition preferably contains ferrite and pearlite in an area% in a total amount of 85 to 100%.

When the total area ratio of ferrite and pearlite in the metallographic structure is 85 to 100%, the hardness of the steel material is further reduced, the deformation resistance is reduced, and the limit workability is improved. Further preferably, the total area ratio of ferrite and pearlite is 95 to 100%. In the metallographic structure, the remainder excluding ferrite and pearlite includes bainite, martensite, cementite, and the like which are structures harder than ferrite and pearlite. In order to obtain the above-described effects due to ferrite and pearlite, the fraction of bainite, martensite, cementite, and the like as the balance is preferably 15% or less in area%.

In order to form such a metallographic structure, the following slow cooling step is preferably performed: the surface temperature of the hot-worked steel material after the hot-working step in the production of the steel material is in the temperature range of 800 to 500 ℃, and slow cooling is performed at a cooling rate of more than 0 ℃/sec but 1 ℃/sec or less. The manufacturing method will be described in detail later.

[ second metallographic Structure morphology ]

Instead of the above-described metallographic structure, the steel material formed of the above-described chemical components may contain ferrite and spherical cementite in an area% in a total of 85 to 100%. Here, the spherical cementite means that the following is taken as the spherical cementite: in the metallographic structure observation surface, the area of the cementite is 54% or more with respect to a circle having the maximum length of the cementite as the diameter.

When the total area ratio of ferrite and spherical cementite is 85 to 100%, the hardness of the steel is reduced, the deformation resistance is reduced, and the limit working ratio is improved. Further preferably, the total area ratio of ferrite and spherical cementite is 90 to 100%. The balance of ferrite and spherical cementite includes pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, and the like. In order to obtain the above-described effects of ferrite and spherical cementite, the fraction of pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, and the like as the balance is preferably 15% or less in area%.

In order to form such a metallographic structure, it is preferable to perform spheroidizing heat treatment on the hot-worked steel material. When the second microstructure form is formed, the hot worked steel may be gradually cooled or may not be gradually cooled, but is cooled at a cooling rate at which ferrite is generated and bainite and martensite are not generated. The manufacturing method will be described in detail later.

[ inclusions in a steel material for a member to be carburized ]

In the steel material for a carburized component of the present embodiment, Mn sulfide and oxide in the steel further satisfy the following conditions in a cross section parallel to the axial direction (i.e., the longitudinal direction of the steel material) of the steel material.

In the present specification, Mn sulfide and oxide are defined as follows.

In the present specification, the inclusions containing 10.0 mass% or more of S, 10.0 mass% or more of Mn, and 10 mass% or more of oxygen and having a circle equivalent diameter of 0.5 μm or more are not "Mn sulfide" but included in "oxide".

As described above, in the case where a plurality of steel members are joined by welding such as friction joining or laser joining to be integrated in the process of manufacturing a carburized part to manufacture an intermediate member before carburization, a HAZ region is present in the carburized part. The HAZ region is sometimes low in strength compared to other regions. To ensure the strength of the HAZ region, inclusions are reduced as much as possible. When the Mn sulfide and the oxide satisfy the above (a) and (B), the strength of the HAZ region can be ensured, and as a result, the fatigue strength of the carburized component integrated by joining can be improved.

[ method for measuring Mn sulfide and oxide ]

The number of Mn sulfides and the number of oxides in the steel can be measured by the following methods. A sample was taken from a steel material for a carburized part. In the case where the steel material is a steel bar, as shown in FIG. 1, a sample is taken from a position (R is the radius of the steel bar) at R/2 from the center axis C1 of the steel bar in the radial direction. The size of the observation surface of the sample was L1 XL 2, and L1 was set to 10mm and L2 was set to 5 mm. Further, the sample thickness L3 in the direction perpendicular to the observation plane was set to 5 mm. The normal N of the observation plane is perpendicular to the center axis C1 (in other words, the observation plane is parallel to the axial direction of the steel material), and the R/2 position corresponds to the center position of the observation plane.

The observation surface of the collected sample was mirror-polished, and 20 fields of view (evaluation area per 1 field of view 100. mu. m.times.100 μm) were observed at random at 1000-fold magnification using a Scanning Electron Microscope (SEM).

Inclusions in each field of view were determined. For each inclusion identified, energy dispersive X-ray spectroscopy (EDX) was used to identify Mn sulfides and oxides. Specifically, in the elemental analysis result of the identified inclusion, when the Mn content is 10.0 mass% or more and the S content is 10.0 mass% or more, the inclusion is defined as Mn sulfide. In the elemental analysis of inclusions, Ti and Ca may be detected as elements other than Mn and S. In this case, all Mn sulfides are defined as long as the above conditions are satisfied. In addition, in the elemental analysis result of the identified inclusion, when the O content is 10 mass% or more, the inclusion is defined as an oxide. In the inclusions defined as oxides, Al, Si, Mg, Ca, Ti, etc. may be detected. In this case, the oxide is also recognized as long as the above condition is satisfied. Among the inclusions, inclusions containing 10.0 mass% or more of S, 10.0 mass% or more of Mn, and 10 mass% or more of O are identified as "oxides" rather than "Mn sulfides".

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

When the number of inclusions is 2 times or more the beam diameter of EDX, the accuracy of elemental analysis is improved. In the present embodiment, the beam diameter of EDX used for specifying inclusions is set to 0.2 μm. At this time, the inclusions having a circle-equivalent diameter of less than 0.5 μm cannot improve the accuracy of elemental analysis by EDX. Further, inclusions having a circle-equivalent diameter of less than 0.5 μm have a very small influence on the strength. Therefore, in the present embodiment, Mn sulfides and oxides having a circle-equivalent diameter of 0.5 μm or more are measured. The upper limit of the circle-equivalent diameter of the inclusions is not particularly limited, and is, for example, 100 μm.

The number of Mn sulfides per unit area (one/mm) was determined based on the total number of Mn sulfides identified in each field and the total area of 20 fields²). Further, the number of oxides per unit area (number/mm) was determined based on the total number of oxides identified in each field and the total area of 20 fields²)。

[ metallographic Structure of carburized part ]

Next, the metallic structure of the carburized component of the present embodiment will be described.

The carburized component of the present embodiment includes a carburized layer and a core portion inside the carburized layer. The carburized layer has an effective depth of hardened layer of more than 0.4mm and less than 2.0mm in thickness. Here, the carburized layer is a depth from the surface with a vickers hardness of HV550 or more. In the carburized layer, the metallographic structure at a position of a depth of 50 μm from the surface preferably contains 90 to 100% by area of martensite, and the Vickers hardness is HV650 to HV 1000. In the carburized layer, the metallographic structure at a position 0.4mm deep from the surface preferably contains 90 to 100% by area of martensite, and the vickers hardness is HV550 to HV 900.

The wear resistance and fatigue strength are further improved when the metallurgical structure of the carburized layer at a depth of 50 μm from the surface contains 90 to 100% of martensite and the Vickers hardness is HV650 to HV 1000. More preferably, the carburized layer at a depth of 50 μm from the surface has a metallographic structure containing 95 to 100% of martensite and has a Vickers hardness of HV700 to HV 1000.

The metallurgical structure of the carburized layer at a depth of 0.4mm from the surface contains 90 to 100% martensite, and the surface fatigue strength and the fatigue strength are further improved when the Vickers hardness is HV550 to HV 900. More preferably, the carburized layer at a depth of 0.4mm from the surface has a metallographic structure containing 92 to 100% of martensite and has a Vickers hardness of HV560 to HV 900.

In the core, the Vickers hardness at a position of a depth of 2.0mm from the surface is preferably HV250 to HV 500. Further, in the core, the chemical component at the position needs to be composed of the above chemical component. More preferably, the Vickers hardness is HV270 to HV 450. The metallographic structure in the core at a position of a depth of 2.0mm from the surface preferably contains at least 1 of martensite and bainite because the above-described effects can be further obtained.

The metallographic structure can be observed by an optical microscope by performing nitrol etching or picric alcohol etching. In this case, the sample subjected to the spheroidizing heat treatment is preferably subjected to bitter alcohol etching.

The fractions of ferrite, pearlite, bainite, martensite, tempered bainite, cementite, and the like can be determined by image analysis. The spherical cementite, the number of cementite, and the maximum length of cementite can be determined as follows: the observation surface was mirror-polished, and then EBSD mapping was performed without etching treatment to identify cementite, and the cementite was obtained by image analysis. The observation surface is not particularly limited, and a cross section perpendicular to the longitudinal direction may be used as the observation surface.

In the calculation of the area fraction of the metallographic structure, ferrite, pearlite, martensite, bainite, tempered martensite, tempered bainite, spherical cementite, and cementite are considered. The above-mentioned area fraction is calculated without including nitrides such as BN, TiC, TiN, AlN, etc., carbides, other fine precipitates, inclusions, retained austenite, etc.

The vickers hardness is preferably measured by a microscopic vickers hardness measuring machine with a load of 0.49N for a total of 10 times for one sample, and an arithmetic average value is calculated. The measurement surface is not particularly limited, and a cross section perpendicular to the axial direction (longitudinal direction) may be used as the measurement surface.

[ Steel for carburized Member and method for producing carburized Member ]

The steel material for a carburized component and the method for manufacturing a carburized component according to the present embodiment will be described.

[ method for producing Steel Material for Components to be carburized ]

First, an example of the method for producing a steel material according to the present embodiment will be described. An example of a method for producing a steel material for parts subjected to carburizing treatment includes a steel-making step, a hot working step, and a slow cooling step. Hereinafter, each step will be explained.

[ Steel-making Process ]

The steel-making process includes a refining process, a heating-holding process, a final composition adjusting process, and a casting process.

[ refining step ]

In the refining step, first, molten iron produced by a known method is refined in a converter (primary refining). The molten steel tapped from the converter is subjected to secondary refining. In the secondary refining, alloy elements are added to molten steel to produce molten steel satisfying the above chemical composition.

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

After the secondary refining is performed to adjust the components of the molten steel other than Si and Ca, the molten steel is subjected to the following treatments (heat holding step and final component adjustment step).

[ Heat-holding step ]

The molten steel in the ladle after the secondary refining (final composition adjustment) is heated at a temperature of 1500 to 1600 ℃ for a holding time ts that is 2 times or more the uniform mixing time τ(s) calculated from the following expression.

τ＝800×ε^-0.4

ε＝((6.18×V_g×T_l)/M_l)ln(1+(h₀/(1.46×10^-5×P₀)))

Here, V_g: gas flow (Nm)³/min)、M_l: molten steel quality (ton), T in ladle_l: temperature (K), h) of molten steel₀: depth (m) of gas injection and P₀: molten steel surface pressure (Pa), ε: stirring power value (W/ton), τ: homogeneous mixing time(s).

When the retention time ts is less than 2.0 times the homogeneous mixing time τ, oxides present in the molten steel in the ladle are not sufficiently aggregated and integrated. Therefore, the oxide is not removed by floating up, and the number of oxides increases. When the holding time ts is less than 2.0 times the homogeneous mixing time τ, Mg and the like mixed in from the slag are bonded to S in the molten steel to form MgS and the like, and MgS is dispersed in the molten steel. The dispersed MgS becomes a precipitation site of MnS. As a result, the number of Mn sulfides increases.

When the retention time ts is 2.0 times or more the uniform mixing time τ, the number of oxides in the steel can be suppressed. Further, the MgS formed once becomes MgO by reoxidation, so that the precipitation sites of MnS decrease, and as a result, the number of Mn sulfides in the steel can be suppressed. As a result, after the final composition adjustment step in the subsequent step, the Mn sulfide content was 70.0 pieces/mm²Below, and the oxide number is 25.0 pieces/mm²The following.

[ Final component adjustment step ]

Si and Ca are added to the molten steel after the heat-holding step, thereby producing molten steel satisfying the chemical composition and the formulas (1) to (3). Si and Ca may be added to molten steel as separate raw materials, respectively. The Si-Ca alloy may be added to molten steel as a raw material.

When Si and Ca are added to the molten steel which is sufficiently uniformly heated in the heat-holding step, the oxides are Al₂O₃Is modified to contain SiO₂And/or CaO, and further, Mn sulfide is also modified into Ca-containing sulfide. Therefore, assuming that the retention time ts is 2 times or more the homogeneous mixing time τ, the Mn sulfide content is 70.0 pieces/mm²Below, and the oxide number is 25.0 pieces/mm²The following.

If Si is added before Al is added to molten steel, deoxidation does not proceed sufficiently, and as a result, the amount of oxides exceeds 25.0 oxides/mm². By adding Si and Ca to the molten steel after Al addition, Mn sulfide becomes 70.0 pieces/mm²Below, and the oxide number is 25.0 pieces/mm²The following. Therefore, in the present embodiment, Al is added to the 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 one of Si and Ca may be added first.

[ casting Process ]

Using the molten steel produced in the refining step, a billet (cast slab or ingot) is produced. Specifically, a cast slab is produced by a continuous casting method using molten steel. Alternatively, an ingot may be produced by an ingot casting method using molten steel. Further, the cast slab or ingot after the casting process may be subjected to blooming or the like as necessary. By using the cast slab or ingot and selecting the following production method, the steel material of the present embodiment having the above-described metallographic structure can be produced.

In order to produce the steel material having a metallurgical structure containing 85 to 100% by area of ferrite and pearlite in total, the following production method is preferably performed.

[ Hot working Process ]

As the hot working step, hot rolling, hot forging, or the like is performed on the cast slab after the casting step to obtain a hot worked steel material. The hot rolling is, for example, a rough rolling, a finish rolling using a continuous rolling mill having a plurality of rolling mills arranged in a row, or the like. And (4) carrying out primary rolling according to the requirement. The plastic working conditions such as working temperature, working ratio, strain rate and the like in the hot working step are not particularly limited, and appropriate conditions may be appropriately selected. The heating temperature in the hot working step is a known heating temperature, and is, for example, 1100 to 1300 ℃.

[ Slow Cooling Process ]

Immediately after the hot working step, the hot-worked steel material which has not been cooled is subjected to slow cooling at a cooling rate of more than 0 and 1.00 ℃/sec or less in a temperature range in which the surface temperature of the hot-worked steel material is 800 to 500 ℃ as a slow cooling step, thereby obtaining the steel material of the present embodiment.

When the cooling rate at 800 to 500 ℃ which is the temperature at which austenite is transformed into ferrite and pearlite exceeds 1.00 ℃/sec, the microstructure fraction of bainite and martensite becomes large. As a result, the hardness of the steel material increases, the deformation resistance increases, and the limit working ratio decreases. Therefore, the cooling rate in the above temperature range is preferably controlled to be more than 0 and 1.00 ℃/sec or less. More preferably, the temperature is more than 0 and 0.70 ℃/sec or less. In the slow cooling step, a jacket with a heat source, a holding furnace, or the like may be provided after the rolling line and/or the hot forging line in order to reduce the cooling rate of the hot worked steel material after the hot working step.

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

[ spheroidizing treatment Process ]

The hot worked steel material subjected to the slow cooling may be further subjected to a spheroidizing heat treatment step. At this time, the steel material of the present embodiment can be manufactured by performing the spheroidizing heat treatment.

The spheroidizing heat treatment may be, for example, the following heat treatment. Heating the hot worked steel material subjected to the slow cooling to a temperature slightly lower than or slightly higher than Ac₁After the temperature of the point (temperature at which austenite starts to be generated during heating), the steel sheet is slowly cooled. The following processes were repeated several times: heating the hot-worked steel subjected to the slow cooling to a temperature slightly higher than Ac₁Cooling to a temperature slightly below Ar₁Point (temperature at which austenite completes transformation to ferrite, or cementite upon cooling). Alternatively, the hot worked steel subjected to the slow cooling is subjected to primary quenching and then to tempering at a temperature of 600 to 700 ℃ for 3 to 100 hours. The method of the spheroidization heat treatment is not particularly limited, and any conventionally known annealing or spheroidization heat treatment method may be used as long as it is described above.

The steel material subjected to the spheroidization heat treatment step can have an improved limit working ratio even when the hardness is high.

The steel material of the present embodiment can be produced by the above-described production steps. The steel material of the present embodiment is, for example, a bar steel.

[ method for producing carburized component ]

Next, an example of the method for manufacturing a carburized component according to this embodiment will be described. The manufacturing method comprises the following steps: a cold working step of cold working the steel material of the present embodiment to produce a plurality of intermediate members; a welding step of welding the plurality of intermediate members to form an integrated product; a carburizing step of performing carburizing treatment or carbonitriding treatment on the welded intermediate member; and a finish heat treatment step of performing a quenching treatment or a quenching/tempering treatment on the intermediate member after the carburizing step.

[ Cold working Process ]

The steel material produced by the above-described production method is subjected to cold plastic working as a cold working step to impart a shape thereto, thereby producing a plurality of intermediate members. The plastic working conditions such as the reduction ratio and strain rate in the cold working step are not particularly limited, and appropriate conditions may be appropriately selected. The cold working is, for example, cold forging. The plurality of intermediate members are welded and integrated by a welding step which is a subsequent step.

[ welding Process ]

In the welding step, the plurality of intermediate members are welded by friction welding or laser welding 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 steel, the Mn sulfide content is 70.0 pieces/mm²Below, and the oxide is 25.0 pieces/mm²The following. Therefore, the joining property is excellent, and the joining fatigue strength of the carburized component is excellent.

[ carburizing step ]

An intermediate member integrally joined by a welding process is subjected to a carburizing process or a carbonitriding process as a carburizing process. In order to obtain a carburized part having the above-described metallographic structure and hardness, it is preferable that the conditions of the carburizing treatment or the carbonitriding treatment are: the temperature is 830-1100 ℃, the carbon potential is 0.5-1.2%, and the carburizing time is more than 1 hour.

[ Fine Heat treatment Process ]

After the carburizing step, as a finish heat treatment step, quenching treatment or quenching/tempering treatment is performed to obtain a carburized component. In order to obtain a carburized part having the above-described metallographic structure and hardness, the temperature of the quenching medium is preferably set to room temperature to 250 ℃ as the condition for the quenching treatment or the quenching/tempering treatment. Further, if necessary, the treatment of sub-zero may be performed after quenching.

[ other Processes ]

If necessary, the steel material before the cold working step may be further subjected to an annealing step. By annealing in the annealing step, the hardness of the steel material is reduced, the deformation resistance is reduced, and the limit processing rate is improved. The annealing conditions are not particularly limited, and known annealing conditions may be appropriately selected.

If necessary, after the cold working step, the steel material before the carburizing step may be further subjected to a cutting step. In this case, the cutting process is performed to give a shape to the steel material. By performing the cutting process, a precise shape which is difficult to realize by cold plastic working alone can be imparted to the steel material.

If necessary, the carburized component after the finish heat treatment step may be subjected to a shot blasting step. By performing shot blasting in a shot blasting step, compressive residual stress is introduced into the surface layer portion of the carburized component. The compressive residual stress suppresses the generation and development of fatigue cracks, and therefore, the fatigue strength of the tooth root and the tooth surface of the carburized component can be further improved. The shot blasting is preferably performed under a condition that the arc height is 0.4mm or more by using shot blasting having a diameter of 0.7mm or less.

Examples

The effect of one embodiment of the present invention will be described in further detail with reference to examples. The conditions in the examples are one example of conditions employed for confirming the feasibility and the effect of the present invention. The present invention is not limited to this one condition example. Various conditions may be adopted as long as the object of the present invention can be achieved without departing from the gist of the present invention.

Molten steels having chemical compositions shown in table 1 were prepared. At this time, refining was performed under the conditions shown in table 2. And casting the refined molten steel through continuous casting to obtain a casting blank.

[ Table 1]

[ Table 2]

In table 2, "steel making condition (1)" represents a ratio of the holding time ts to the uniform mixing time τ at 1500 to 1600 ℃ after the secondary refining (ts/τ). "steelmaking conditions (2)" in Table 2 indicate the order of addition of Al, Si and Ca. In the column of "steel making conditions (2)", "1" means: after Al is added for deoxidation, Si and Ca are added. "2" means: after addition of Si, Al and Ca were added.

The produced cast slab was heated and then subjected to preliminary rolling to produce a steel material having a cross section perpendicular to the longitudinal direction of 162mm × 162 mm. Using this steel material, hot rolling was carried out by a continuous rolling mill to obtain a round bar-shaped hot worked steel material having a circular cross section perpendicular to the longitudinal direction and a diameter of the cross section of 35 mm. As the slow cooling step, the hot worked steel material is slowly cooled using a jacket installed after the pass line or a jacket with a heat source as necessary. The average cooling rate (. degree. C./sec) at the time of slow cooling at 800 ℃ to 500 ℃ is shown in Table 2. A plurality of pieces of hot-worked steel materials after slow cooling were prepared for each test number.

In each test number, some of the prepared hot-worked steel materials were subjected to Spheroidizing heat treatment as a Spheroidizing heat treatment step (SA step: Spheroidizing Annealing). In the spheroidization treatment, the hot worked steel material is heated to 740 ℃. Then, slow cooling was carried out at a cooling rate of 8 ℃/hr until the steel temperature became 650 ℃. And air cooling the steel at 650-normal temperature.

The steel material is produced by the above production method.

[ evaluation test ]

The produced steel material was evaluated for the following properties.

[ preparation of metallographic Structure Observation specimen and Critical compressibility measurement specimen ]

The metallographic structure observation test piece was sampled from a position on the rod-shaped steel material at a depth of 1/4 mm from the circumferential surface of the cross section. Further, a critical compressibility measurement test piece (6 mm. phi. times.9 mm, notch shape: 30 degrees, depth: 0.8mm, curvature radius of the tip portion: 0.15mm) was sampled so that the longitudinal direction of the steel material became the compression direction. The test piece for metallographic structure observation was collected from the steel material after the annealing step and without the SA step, and further the test piece for metallographic structure observation was collected from the steel material after the SA step. Similarly, the critical compressibility measurement test piece was collected from the steel material (material immediately after slow cooling) after the slow cooling step and not subjected to the SA step, and further, the critical compressibility measurement test piece was collected from the steel material (SA material) after the SA step. Hereinafter, among the test pieces, the test piece collected from the steel material (material immediately after slow cooling) after the slow cooling step and not subjected to the SA step is referred to as "test piece immediately after slow cooling", and the test piece collected from the steel material (material SA) after the SA step is referred to as "test piece after SA".

[ metallographic structure observation test ]

The metallographic structure observation test was performed using the metallographic structure observation test piece (test piece immediately after slow cooling, test piece after SA). Specifically, the test piece immediately after slow cooling and the test piece after SA were mirror polished. The surface (observation surface) of the test piece immediately after the slow cooling after mirror polishing was etched with a nital etching solution (2% nital) for 5 to 10 seconds. The surface (observation surface) of the mirror-polished SA post-test piece was etched with a bitter alcohol etchant (5% picric acid alcohol) for 10 to 20 seconds. The picric acid is 2,4, 6-trinitrophenol.

The observation surface after etching was observed with an optical microscope to generate a photographic image. The total fraction of ferrite and pearlite and the total fraction of ferrite and spherical cementite were determined by image analysis using photographic images (100 μm × 100 μm per 1 visual field, 20 visual fields observed). Table 2 shows the total area ratio (%) of ferrite and pearlite obtained from the test piece immediately after slow cooling and the total area ratio (%) of ferrite and spherical cementite obtained from the test piece after SA. In the metallographic structure, the balance of the structure other than the above is pearlite, martensite, bainite, tempered martensite, tempered bainite, cementite, or the like.

[ Critical compressibility measurement test ]

The critical compression test was performed on the critical compression ratio measurement test pieces (test piece immediately after slow cooling and test piece after SA) by the following method. For each test piece, cold compression was performed at a rate of 10 mm/min using a confining die. When a micro crack of 0.5mm or more occurred in the vicinity of the notch, the compression was stopped, and the compressibility (%) at that time was calculated. The measurement was performed 10 times in total to obtain a compressibility (%) at which the cumulative breakage probability became 50%, and this compressibility was defined as a critical compressibility (%). The critical compressibility (%) of each test number is shown in table 2. The critical reduction ratio of a conventional steel material for a carburized component is about 65%, and therefore, a case where the steel material reaches 68% or more, which is considered to be a value significantly higher than this value, is judged to be excellent in the limit reduction ratio. In the test numbers with the critical compressibility of less than 68%, the evaluation test of the carburized parts was not performed.

[ determination of Mn sulfide number and oxide number test ]

In each of the above test numbers, samples were taken from the steel material (material immediately after slow cooling) after the slow cooling step and not subjected to the SA step and the steel material (SA material) after the SA step. Specifically, as shown in FIG. 1, the sample was taken from a position of the material immediately after the slow cooling and the SA material at a distance of R/2 in the radial direction from the center axis C1. The size of the observation surface of the sample was L1 XL 2, and L1 was set to 10mm and L2 was set to 5 mm. Further, the sample thickness L3 in the direction perpendicular to the observation plane was set to 5 mm. The normal N of the observation plane is perpendicular to the central axis C1, and the R/2 position corresponds to the central position of the observation plane.

The observation surface of the collected sample was mirror-polished, and 20 fields (evaluation area per 1 field 100 μm × 100 μm) were observed at random at 1000 × magnification using a Scanning Electron Microscope (SEM) (20 fields in the material immediately after slow cooling, 20 fields in the SA material).

Inclusions in each field of view were determined. For each inclusion identified, energy dispersive X-ray spectroscopy (EDX) was used to identify Mn sulfides and oxides. Specifically, when the Mn content is 10.0% or more and the S content is 10.0% or more in the elemental analysis results of the inclusions thus identified, the inclusions are regarded as Mn sulfides. In addition, when the O content was 10% or more in the elemental analysis results of the inclusions thus identified, the inclusions were identified as oxides. The inclusions to be identified are those having a circle-equivalent diameter of 0.5 μm or more. The beam diameter of EDX used for the determination of inclusions was set to 0.2. mu.m.

Mn sulfide and oxide having a circle equivalent diameter of 0.5 μm or more were measured for the material immediately after annealing and the SA material, respectively. The number of Mn sulfides per unit area (one/mm) was determined based on the total number of Mn sulfides identified in each field and the total area of 20 fields²). Further, the number of oxides per unit area (number/mm) was determined based on the total number of oxides identified in each field and the total area of 20 fields²)。

Table 2 shows the number of Mn sulfides in the material immediately after the slow cooling (pieces/mm)²) The number of oxides in the material immediately after the annealing (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 material immediately after slow cooling, and the number of oxides in the SA material was the same as the number of oxides in the material immediately after slow cooling.

[ production of carburized component ]

Test pieces (20 mm. phi. times.30 mm) for carburization were collected from the positions of the test numbers immediately after the slow cooling where the distance from the peripheral surface was the depth of the diameter 1/4 of the cross section, with the longitudinal direction being the compression direction. The carburized test piece was subjected to cold working by upsetting with a compressibility of 50% under cold conditions. The conditions for upsetting compression were room temperature, using a constraint die, and strain rate 1/sec. As the carburizing step, gas carburizing by the converter gas system was performed on the carburized test piece after the upsetting compression. The gas carburization was performed so that the carbon potential was 0.8%, and the carbon was held at 950 ℃ for 5 hours and then at 850 ℃ for 0.5 hours. After the carburizing step, as a finish heat treatment step, oil quenching was performed up to 130 ℃, and then tempering was performed at 150 ℃ for 90 minutes to obtain a carburized part (a material immediately after slow cooling was used).

For the SA material of each test number, a carburized part was also produced under the same production conditions as for the material immediately after slow cooling (SA material was used).

[ evaluation test of carburized parts ]

The properties of the carburized layer and the core of the carburized parts produced as described above (carburized parts using a material immediately after slow cooling, carburized parts using an SA material) were evaluated. The evaluation results are shown in Table 3.

[ Table 3]

[ Vickers hardness test of carburized layer ]

In a cross section perpendicular to the longitudinal direction of each test number carburized part (carburized part using a material immediately after slow cooling, carburized part using an SA material) was measured for vickers hardness at a depth position of 50 μm from the surface and for vickers hardness at a depth position of 0.4 μm from the surface by a vickers hardness test according to JIS Z2244 (2009) using a micro vickers hardness tester. The load during the test was set to 0.49N. The Vickers hardness HV at the 50 μm depth position 10 was measured, and the arithmetic average thereof was taken as the Vickers hardness HV at the 50 μm depth position. The Vickers hardness HV at the 0.4 μm depth position 10 was measured, and the arithmetic average thereof was taken as the Vickers hardness HV at the 0.4 μm depth position. The hardness was judged to be sufficient and acceptable when the hardness at a position 50 μm deep from the surface was HV650 to HV1000 and when the hardness at a position 0.4mm deep from the surface was HV550 to HV 900. The measurement results are shown in table 3.

[ metallographic structure observation test of carburized layer ]

The carburized layer of the carburized part (carburized part using a material immediately after slow cooling, carburized part using an SA material) was evaluated for the metallographic structure at a position 0.4mm deep from the surface. The metallographic structure was subjected to etching with a nital solution and observed with an optical microscope. The magnification of the optical microscope was 200 times, and the observation field was 500. mu. m.times.500. mu.m.

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

[ chemical composition and Vickers hardness of core ]

The vickers hardness and chemical composition of the core portion of the carburized part (carburized part using a material immediately after slow cooling, carburized part using an SA material) were measured by the following methods. In a cross section perpendicular to the longitudinal direction of the carburized part, vickers hardness was determined at a depth position of 2mm from the surface by a vickers hardness test in accordance with JIS Z2244 (2009) using a vickers hardness tester. The load at the time of the test was 49N. The measurement was performed 10 times at a depth position of 2mm, and the arithmetic average thereof was taken as the Vickers Hardness (HV) at a depth position of 2mm from the surface. The vickers hardness obtained is shown in table 2. The hardness was judged to be sufficient and acceptable when the Vickers hardness was HV250 to HV 500. The measurement results are shown in table 3.

Further, with respect to the chemical composition at a depth position of 2mm from the surface, quantitative analysis was performed with respect to the elements of atomic number No. 5 or more using EPMA (Electron beam microanalyzer). In addition, it is judged that the same composition as the chemical composition in the cast slab as the starting material is substantially the same. The results of the determination are shown in table 3.

[ Presence or absence of coarse grains in carburized component ]

For the core portion of the carburized part (carburized part using a material immediately after slow cooling, carburized part using an SA material) described above, the prior austenite crystal grains were observed at a position 2mm deep from the surface. Specifically, a cross section perpendicular to the longitudinal direction of the carburized component is taken as an observation plane. The surface of the observation mirror is ground and then etched with a picric acid saturated aqueous solution. The field of view (300. mu. m.times.300. mu.m) including a depth position of 2mm from the surface of the etched observation surface was observed with an optical microscope (400 times), and the prior austenite crystal grains were determined. The grain size number of each prior austenite grain was determined in accordance with JIS G0551 (2013) for the determined prior austenite grain. When there is even one crystal grain having a grain size of No.4 or less, it is judged that "coarse crystal grains are generated". The results of the determination are shown in table 3.

[ fatigue Strength evaluation test of carburized parts after joining ]

A round bar of 35mm in diameter, which was a steel material (material immediately after annealing, SA material) of each test No. was machined to prepare a round bar of 20mm in diameter and 150mm in length. Using the round bar, a basic fatigue test piece and a joint fatigue test piece were produced.

The basic fatigue test piece was produced by the following method. A small field type rotary bending fatigue test piece having an evaluation portion diameter of 8mm and a parallel portion length of 15.0mm was prepared from the center of the cross section of a round bar having a diameter of 20mm and a length of 150 mm. The test piece was set as a basic fatigue test piece.

The joint fatigue test piece was produced by the following method. Round bars of the same test piece number and 20mm in diameter and 150mm in length were butted against each other, and joined round bars were produced under the following friction welding conditions.

Friction welding conditions:

friction pressure: 100MPa, friction time: 5 seconds,

Butt pressure (pressure applied to the joint from both ends of the round bar): 200MPa, and,

Docking time (pressing time for joint): 5 seconds,

Rotating speed: 2000rpm, 2000rpm,

Total retention: 5-12 mm

A small field type rotary bending fatigue test piece having an evaluation portion diameter of 8mm and a parallel portion length of 15.0mm was prepared from the center portion of the cross section of the joined round bar as a pressure welding fatigue test piece. In the pressure welding fatigue test piece, the longitudinal center portion of the parallel portion was used as a joint surface.

The basic fatigue test piece and the joint fatigue test piece were subjected to carburizing and quenching treatment as follows to prepare carburized parts (carburized parts using a material immediately after slow cooling, carburized parts using an SA material). In the carburizing and quenching treatment, gas carburizing by a converter gas system is performed. Specifically, the carbon potential was set to 0.8%, and the reaction solution was held at 950 ℃ for 5 hours. Then, it was kept at 850 ℃ for 0.5 hour under the same carbon potential. Then, the steel sheet was immersed in oil at 130 ℃ to carry out oil quenching. After oil quenching, tempering was performed at 150 ℃ for 90 minutes. In the above manner, small field type rotational bending fatigue test pieces (basic fatigue test pieces, joint fatigue test pieces) simulating carburized parts were produced.

The prepared basic fatigue test piece and the prepared joint fatigue test piece were subjected to a small field type rotational bending fatigue test. Specifically, the small field type rotary bending fatigue test was carried out at room temperature in the atmosphere according to JIS Z2274 (1978) using the small field type rotary bending fatigue test pieces (basic fatigue test pieces, joint fatigue test pieces) described above. The rotation speed was set to 3000rpm, the stress ratio R was set to-1, and the number of repetitions of stress loading was set to 1X 10⁷The maximum stress without fracture after one cycle was taken as the fatigue strength (MPa).

The fatigue strength ratio is defined as the ratio (%) of the fatigue strength (MPa) of the joint fatigue test piece to the fatigue strength (MPa) of the basic fatigue test piece. In other words, the fatigue strength ratio is defined by the following formula.

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

The obtained fatigue strength ratio is shown in table 3. When the fatigue strength ratio is 85% or more, it is judged that excellent fatigue strength is obtained also after joining.

[ test results ]

The test results are shown in tables 2 and 3. Referring to tables 2 and 3, the chemical compositions of test nos. 1 to 11 satisfy formulas (1) to (3) as appropriate. Further, the steel-making conditions are also appropriate. Further, the cooling rate in the slow cooling stepThe degree is also appropriate. Further, the spheroidizing treatment is also suitable. Therefore, the number of Mn sulfides in both the material immediately after the annealing and the SA material was 70.0 Mn sulfides/mm²The number of oxides is 25.0 oxides/mm²The following. Further, the total area ratio of ferrite and pearlite in the material immediately after slow cooling is 85 to 100%, and the total area ratio of ferrite and spherical cementite in the SA material is 85 to 100%. As a result, the material immediately after the slow cooling and the SA material both had a critical compressibility of 68% or more, and showed excellent critical compressibility.

Furthermore, the carburized layers of the carburized part of the material immediately after slow cooling and the carburized part of the SA material both had appropriate vickers hardnesses, and the martensite fraction at the 0.4 depth position was 90% or more. Further, the core hardness and chemical composition are also appropriate, and the prior austenite grain diameter is not coarsened. Furthermore, the fatigue strength ratios of the joining fatigue test pieces were all as high as 85% or more, and excellent fatigue strength was exhibited even when the joining was performed.

On the other hand, test No. 12 had too high a C content. Therefore, the total area ratio of ferrite and pearlite in the material immediately after slow cooling is less than 85%. Further, the critical compressibility of both the material immediately after slow cooling 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. Therefore, sufficient hardness is not obtained in the core portion of the carburized part of the material immediately after slow cooling and the core portion of the carburized part of the SA material.

In test No. 14, the oxygen content was too high. Therefore, the number of oxides of the material immediately after the slow cooling and the SA material is too large. As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test piece simulating the carburized part of the material immediately after slow cooling and the carburized part of the SA material.

In test No. 15, the N content was too high. Therefore, solid solution B cannot be secured, and the core hardness is too low. Further, coarse TiN is produced, and therefore the limit working ratio of steel materials (materials immediately after slow cooling and SA materials) is low.

In test No. 16, F1 was less than the lower limit of formula (1). Therefore, the core hardness of the carburized part of the material immediately after slow cooling and the core hardness of the carburized part of the SA material are too low.

In test No. 17, F1 exceeded the upper limit of formula (1). Therefore, the total area ratio of ferrite and pearlite in the material immediately after slow cooling is less than 85%. Therefore, the limit reduction ratio of the steel material of the material immediately after slow cooling and the SA material is low.

In test No. 18, F2 was less than the lower limit of formula (2). Therefore, the hardness of the carburized part of the material immediately after slow cooling and the carburized part of the SA material at the 0.4mm depth position is too low.

In test No. 19, F2 exceeded the upper limit of formula (2). Therefore, the limit reduction ratio of the steel material before forging (material immediately after slow cooling and SA material) is too low.

In test No. 20, F3 was less than the lower limit of formula (3). Therefore, in the core portion of the carburized part of the material immediately after slow cooling and the core portion of the carburized part of the SA material, a part of the prior austenite grains become coarse grains.

In test No. 21, F3 exceeded the upper limit of formula (3). Therefore, the limit processing rate of the steel material (material immediately after slow cooling and SA material) is low.

In test numbers 22 and 23, the retention time ts at 1500 to 1600 ℃ for the molten steel in the ladle after the secondary refining was less than 2.0 times the uniform mixing time τ. Therefore, the number of MnS exceeds 70.0 pieces/mm in the material immediately after the slow cooling and the SA material²The number of oxides exceeds 25.0/mm². As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test piece simulating the carburized part of the material immediately after slow cooling and the carburized part of the SA material.

In test nos. 24 and 25, Si was added before Al was added in the refining step. Therefore, in the material immediately after the slow cooling and the SA material, the number of oxides exceeds 25.0 oxides/mm². As a result, the fatigue strength ratio was as low as less than 85% in the joint fatigue test piece simulating the carburized part of the material immediately after slow cooling and the carburized part of the SA material.

In test nos. 26 and 27, the average cooling rate in the slow cooling step after hot rolling was too high at 800 to 500 ℃. Therefore, in the structure of the steel material as a material immediately after slow cooling, the total area ratio of ferrite and pearlite is less than 85%, and the critical compressibility is less than 68%. On the other hand, in the structure of the steel material for the SA material, the total area ratio of ferrite and spherical cementite is 85% or more, and the critical compressibility of the SA material exceeds 68%. The carburized layer of the SA material carburized component has an appropriate vickers hardness, and the martensite fraction at the 0.4mm depth position is also 90% or more. Further, the core hardness and chemical composition are also appropriate, and the prior austenite grain diameter is not coarsened. Furthermore, the joint fatigue test piece showed a fatigue strength ratio as high as 85% or more, and even when the joint was joined, it showed excellent fatigue strength.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriately changing the above embodiments within a range not departing from the gist thereof.

Claims

1. A steel material for a carburized component,

which has a composition in mass%

C：0.09～0.16％、

Si：0.01～0.50％、

Mn：0.40～0.60％、

P: less than 0.030%,

S: less than 0.025%,

Cr：0.90～2.00％、

Mo：0.10～0.40％、

Al：0.005～0.030％、

Ti: more than 0.010 percent and less than 0.050 percent,

Nb：0.010～0.030％、

N: less than 0.0080 percent,

O: less than 0.0030%,

B：0.0003～0.0030％、

Ca：0.0005～0.0050％、

Cu：0～0.50％、

Ni: 0 to 0.30%, and

in a cross section of the steel material parallel to the axial direction, Mn sulfide is 70.0 pieces/mm²Below and oxides 25.0 pieces/mm²In the following, the following description is given,

the oxide contains 10 mass% or more of oxygen, has a circle equivalent diameter of 0.5 μm or more,

0.004 < Ti-Nx (48/14) < 0.030 formula (3)

Wherein the content in mass% of the corresponding element is substituted at each element symbol of formulae (1) to (3).

2. The steel product according to claim 1,

Cu: 0.005% to 0.50%, and

ni: 0.005-0.30% of more than 1 of the group.

3. The steel product according to claim 1,

4. The steel product according to claim 2,

5. The steel product according to claim 1,

6. The steel product according to claim 2,

7. The steel product as claimed in any one of claims 1 to 6 wherein the steel product is a steel bar.