阅读说明：本技术 钢材 (Steel material ) 是由 宫越有祐 高须贺幹 于 2019-03-13 设计创作，主要内容包括：本申请的钢材具有如下化学组成：以质量％计C：0.05～0.55％、Si：0.05～1.00％、Mn：1.51～3.50％、P：0.1000％以下、S：0.3000％以下、Cr：0.05～2.50％、V：0.10～0.75％、Ti：0.005～0.250％、Al：0.003～0.100％、N：0.020％以下以及余量：Fe和杂质,在C含量为0.05％以上且小于0.38％的情况下满足式(1),在C含量为0.38％以上且小于0.55％的情况下满足式(2),钢材中的粗大Al<Sub>2</Sub>O<Sub>3</Sub>系夹杂物的数密度为0.05～1.00个/mm<Sup>2</Sup>。0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50(1)0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65(2)。(The steel material of the present application has the following chemical composition: c in mass%: 0.05 to 0.55%, Si: 0.05 to 1.00%, Mn: 1.51-3.50%, P: 0.1000% or less, S: 0.3000% or less, Cr: 0.05-2.50%, V: 0.10 to 0.75%, Ti: 0.005-0.250%, Al: 0.003-0.100%, N: 0.020% or less and the balance: fe and impurities, and satisfying the formula (1) when the C content is 0.05% or more and less than 0.38%, and the C contentWhen the content is 0.38% or more and less than 0.55%, the formula (2) is satisfied, and coarse Al in the steel material 2 O 3 The number density of the inclusions is 0.05 to 1.00 pieces/mm 2 。0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50(1)0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65(2)。)

1. A steel material having the following chemical composition:

in mass%

C：0.05～0.55％、

Si：0.05～1.00％、

Mn：1.51～3.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Cr：0.05～2.50％、

V：0.10～0.75％、

Ti：0.005～0.250％、

Al：0.003～0.100％、

N: less than 0.020%,

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

Nb：0～0.100％、

Pb：0～0.30％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi: 0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

satisfies formula (1) when the C content is 0.05% or more and less than 0.38%,

satisfying the formula (2) when the C content is 0.38-0.55%,

70.0% or more of Al is contained in mass%₂O₃And inclusions having an AREA of 3 μm or more are defined as coarse Al₂O₃When the foreign matter is included, the mixture is mixed,

the coarse Al in the steel₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm²，

0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50 (1)

0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65 (2)

Here, the symbol of the element in the formula (1) and the formula (2) is substituted into the content in mass% of the corresponding element.

2. The steel product according to claim 1,

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

Cu：0.01～0.60％、

Ni：0.01～0.60％、

Mo: 0.01 to 0.70%, and

nb: 0.005-0.100% of 1 element or more than 2 elements.

3. The steel product according to claim 1 or 2,

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

Pb：0.01～0.30％、

Te：0.0003～0.3000％、

Ca: 0.0003 to 0.0100%, and

bi: 0.0003 to 0.4000% of 1 element or more than 2 elements.

Technical Field

The present invention relates to a steel material, and more particularly, to a steel material used for a hot forged product.

Background

A connecting rod (hereinafter also referred to as "connecting rod") used in an automobile engine or the like is an engine component that connects a piston and a crankshaft, and converts reciprocating motion of the piston into rotational motion of a crank.

Fig. 1 is a front view of a conventional link. As shown in fig. 1, a conventional link 1 includes a large head portion 100, a shaft portion 200, and a small head portion 300. The large head 100 is disposed at one end of the shaft portion 200, and the small head 300 is disposed at the other end of the shaft portion 200. The large head 100 is coupled to the crank pin. Small head 300 is attached to the piston.

The conventional link 1 includes two members (a cover 2 and a lever 3). These parts are typically manufactured by hot forging. One end portions of the cover 2 and the rod 3 correspond to the large head portion 100. The other portions than one end of the shaft 3 correspond to the shaft portion 200 and the small head portion 300. The large head portion 100 and the small head portion 300 are formed by cutting. Therefore, the connecting rod 1 is required to have high machinability.

The connecting rod 1 receives loads from surrounding components when the engine is running. Recently, in order to further reduce fuel consumption, downsizing of the connecting rod 1 and improvement of the in-cylinder pressure in the cylinder have been demanded. Therefore, the connecting rod 1 is required to have excellent yield strength capable of coping with the explosion load transmitted from the piston even if the shaft body portion 200 is narrowed. Further, the connecting rod is required to have excellent fatigue strength because repeated compression load and tensile load are applied.

As described above, the conventional link 1 is manufactured by separately manufacturing the cover 2 and the lever 3. Therefore, a positioning pin processing step is performed for positioning the cover 2 and the rod 3. Further, a cutting process is performed on the joint surface between the cap 2 and the rod 3. Therefore, a fracture rod capable of omitting these steps has begun to be widespread.

In the split link, after the link is integrally molded, a jig is inserted into a hole of the large head 100, and the large head is broken by a load stress, thereby being divided into two parts (corresponding to the cover 2 and the rod 3). Then, the divided two members are joined together when the crankshaft is attached. If the fracture surface of the large head 100 is a brittle fracture surface without deformation, the cap 2 and the fracture surface of the rod 3 can be joined together by bolts. Therefore, in this case, the positioning pin machining step and the cutting machining step are omitted. As a result, the manufacturing cost is reduced.

However, when hot forging is performed using a steel material whose chemical composition is adjusted in order to improve the yield strength and fatigue strength of a hot forged product, such as a split connecting rod, which is required to have high splitting properties, the structure of the steel material after hot forging (hot forged product) may form a structure mainly composed of bainite. When the structure of the steel material after hot forging (hot forged product) is assumed to be a structure mainly composed of bainite, the cracking property is lowered. Specifically, bainite has high toughness, and therefore a ductile fracture surface is easily formed at a fracture surface after cracking. In the case of a ductile section, the large head portion is plastically deformed. Therefore, even if the fracture surfaces are paired, the inner diameter D of the large head 100 in fig. 1 may deviate from the desired value. As a result, the crank connecting portion (large head portion) may partially come into contact with each other, which may cause vibration and noise during traveling of the automobile.

In order to improve the yield strength and fatigue strength of a hot forged product requiring high cracking properties, hot forging is performed using a steel material having an adjusted chemical composition, and as a result, if the structure of the steel material after hot forging (hot forged product) is assumed to be a structure mainly composed of bainite, the machinability of the steel material is further lowered, and the cutting resistance at the time of drilling the bolt hole is increased. If the cutting resistance during drilling increases, the tool life decreases or the load on the driving parts in the cutting machine increases. Therefore, in the case of improving the yield strength and fatigue strength of a hot forged product, it is required to further improve the machinability (suppress the cutting resistance) of the steel material in the production process of the hot forged product.

Japanese patent laid-open Nos. 2004-277817 (patent document 1), 2011-195862 (patent document 2), International publication No. 2009/107282 (patent document 3), 2006-336071 (patent document 4), and 2016-027204 (patent document 5) propose steels having high cracking properties.

The high-strength non-heat-treated steel disclosed in patent document 1 has the following composition: c in weight percent: 0.2 to 0.6%, Si: 0.1-2%, Mn: 0.1-1.5%, S: 0.03-0.2%, P: 0.02 to 0.15%, Cu: 0.03-1%, Ni: 0.03-1%, Cr: 0.05-1%, V: 0.02 to 0.4%, Ti: 0.01 to 0.8%, s-Al: 0.005-0.045%, N: 0.008 to 0.035%, and the balance of unavoidable impurities and Fe, and having a ferrite pearlite structure. The maximum diameter of TiN inclusions in the steel is 5 μm or more, and the amount thereof is 5/mm in number density²The above. Patent document 1 describes that the non-heat-treated steel has high strength and good machinability, and is excellent in fracture separation performance and capable of forming good irregularities on a fracture surface.

The non-heat-treated steel for hot forging disclosed in patent document 2 contains, in mass%, C: 0.35 to 0.55%, Si: 0.15-0.40%, Mn: 0.50-1.00%, P: 0.100% or less, S: 0.040 to 0.100%, Cr: 1.00% or less, V: 0.20 to 0.50%, Ca: 0.0005 to 0.0100%, N: less than 0.0150 percent, the balance being Fe and inevitable impurities, 2Mn +5Mo + Cr being less than or equal to 3.1, C + Si/5+ Mn/10+10P +5V being greater than or equal to 1.8, Ceq being C + Si/7+ Mn/5+ Cr/9+ V being 0.90-1.10, hardness being more than or equal to HV330, yield ratio being more than or equal to 0.73, and structure being ferrite/pearlite structure with bainite being less than or equal to 10 percent. Patent document 2 describes that the hot forging non heat-treated steel can provide a hot forging non heat-treated steel member capable of ensuring high strength and excellent machinability and fracture separability.

The non-heat-treated steel for hot forging disclosed in patent document 3 contains, in mass%, C: more than 0.35% and 0.60% or less, Si: 0.50 to 2.50%, Mn: 0.20-2.00%, P: 0.010-0.150%, S: 0.040-0.150%, V: 0.10 to 0.50%, Zr: 0.0005 to 0.0050%, Ca: 0.0005 to 0.0050%, N: 0.0020 to 0.0200%, Al is limited to less than 0.010%, and the balance is substantially Fe and unavoidable impurities. Patent document 3 describes that the non-heat-treated steel for hot forging is excellent in fracture splittability and machinability.

The steel for a connecting rod disclosed in patent document 4 contains, in mass%, C: 0.1 to 0.5%, Si: 0.1-2%, Mn: 0.5-2%, P: 0.15% or less (excluding 0%), S: 0.06-0.2%, N: 0.02% or less (excluding 0%), Ca: 0.0001 to 0.005% and Al: 0.001 to 0.02% and the balance of Fe and inevitable impurities. The steel for a connecting rod controls the composition of oxide inclusions existing in the steel to be within a predetermined range. Specifically, Al is added to oxide inclusions₂O₃In the case of the host, SiO₂When the main component is a metal, fracture splittability is insufficient. Therefore, in this document, Al is not added to oxide inclusions₂O₃、SiO₂And CaO. Patent document 4 describes that this can improve fracture splittability (see [0009 ] of patent document 4)]Segment).

The age-hardening bainite non-heat-treated steel disclosed in patent document 5 contains, in mass%, C: 0.10 to 0.40%, Si: 0.01 to 2.00%, Mn: 0.10-3.00%, P: 0.001-0.150%, S: 0.001 to 0.200%, Cu: 0.001 to 2.00%, Ni: 0.40% or less, Cr: 0.10 to 3.00%, further comprising Mo: 0.02-2.00%, V: 0.02 to 2.00%, Ti: 0.001 to 0.250%, Nb: 0.01 to 0.10% of any 1 element or 2 or more elements, and the balance Fe and inevitable impurities, and the content of specific chemical components in mass% satisfies 3 x [ C ] +10 x [ Mn ] +2 x [ Cu ] +2 x [ Ni ] +12 x [ Cr ] +9 x [ Mo ] +2 x [ V ] ≧ 20, 32 x [ C ] +3 x [ Si ] +3 x [ Mn ] +2 x [ Ni ] +3 x [ Cr ] +11 x [ Mo ] +32 x [ V ] +65 x [ Ti ] +36 x [ Nb ] ≧ 24, 321 x [ C ] -31 x [ Mo ] +213 x [ V ] +545 x [ Nb ] + 100, 321 x [ C ] -31 x [ Mo ] +545 x [ V ] +280 ≧ 100. Patent document 5 describes that plastic deformation during fracture splitting can be satisfactorily suppressed even in a part produced by fracture splitting.

Disclosure of Invention

Problems to be solved by the invention

However, patent documents 1, 3 and 4 presuppose that the microstructure of the hot forged product is mainly composed of ferrite and pearlite. Therefore, when bainite is formed in the hot forged product, a ductile fracture surface may be formed in the fracture surface, and the inner diameter of the large head may be deformed, thereby degrading the cracking property.

In patent document 2, bainite formation in the hot forged product is allowed to some extent. However, when the area ratio of bainite in the structure is increased, a ductile fracture surface may be formed at the fracture surface, and the cracking property may be lowered.

Patent document 5 assumes a hot forged product having a microstructure mainly composed of bainite. And that even a microstructure mainly composed of bainite can suppress toughness. However, by a method different from the non-heat-treated steel disclosed in patent document 5, high machinability, high yield strength, and high fatigue strength are possible, and even if the microstructure after hot forging forms a structure mainly composed of bainite, excellent cracking properties can be obtained.

An object of the present invention is to provide a steel material which, when a steel material is hot forged to produce a hot forged product, has high machinability, high yield strength and high fatigue strength after hot forging, and which, even if the microstructure of the steel material after hot forging forms a structure mainly composed of bainite, can attain excellent cracking properties after hot forging.

Means for solving the problems

The steel material of the present application has the following chemical composition:

in mass%

C：0.05～0.55％、

Si：0.05～1.00％、

Mn：1.51～3.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Cr：0.05～2.50％、

V：0.10～0.75％、

Ti：0.005～0.250％、

Al：0.003～0.100％、

N: less than 0.020%,

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

Nb：0～0.100％、

Pb：0～0.30％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi: 0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

satisfies formula (1) when the C content is 0.05% or more and less than 0.38%,

satisfying the formula (2) when the C content is 0.38-0.55%,

70.0% or more of Al is contained in mass%₂O₃And inclusions having an AREA of 3 μm or more are defined as coarse Al₂O₃When the foreign matter is included, the mixture is mixed,

the coarse Al in the steel₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm²。

0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50 (1)

0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65 (2)

Here, the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (1) and the formula (2).

ADVANTAGEOUS EFFECTS OF INVENTION

The steel material of the present invention has high machinability, high yield strength, and high fatigue strength after hot forging when the steel material is hot forged to produce a hot forged product, and even if the microstructure of the steel material after hot forging forms a structure mainly composed of bainite, excellent cracking properties can be obtained after hot forging.

Drawings

Fig. 1 is a front view of a conventional tie rod.

FIG. 2A is a plan view of a test piece used in the cleavage evaluation test in the examples.

FIG. 2B is a cross-sectional view of the test piece shown in FIG. 2A.

Fig. 2C is a plan view of the test piece shown in fig. 2A after the test piece is fractured and separated.

Fig. 2D is a plan view of the test piece in the state where the test piece of fig. 2C is fastened with a bolt.

Detailed Description

The steel material of the present embodiment will be described in detail below.

The present inventors have conducted investigations and studies on machinability, hot workability, and cracking properties in a process for producing a hot forged product using a steel material, and yield strength and fatigue strength of a hot forged product produced using a steel material. As a result, the present inventors have obtained the following findings.

(A) For strength and machinability

Strength and machinability are mutually contradictory mechanical properties. These mechanical properties can be achieved at the same time if the chemical composition of the steel material can be appropriately adjusted.

In a composition having a composition comprising, in mass%, C: 0.05 to 0.55%, Si: 0.05 to 1.00%, Mn: 1.51-3.50%, P: 0.1000% or less, S: 0.3000% or less, Cr: 0.05-2.50%, V: 0.10 to 0.75%, Ti: 0.005-0.250%, Al: 0.003-0.100%, N: 0.020% or less, Cu: 0-0.60%, Ni: 0-0.60%, Mo: 0-0.70%, Nb: 0-0.100%, Pb: 0-0.30%, Te: 0-0.3000%, Ca: 0 to 0.0100%, Bi: 0 to 0.4000%, and the balance being Fe and impurities, is defined as fn1 ═ C +0.11Mn +0.08Cr +0.75V +0.20 Mo. fn1 is an index of strength and machinability of a steel material, and is positively correlated with yield strength.

When the C content is 0.05% or more and less than 0.38%, if fn1 is higher than 1.50, the strength of the steel material after hot forging becomes too high, and the machinability of the steel material after hot forging is lowered. If fn1 is less than 0.38, the strength of the steel material becomes too low, and the steel material after hot forging (hot forged product) cannot obtain sufficient yield strength. When the C content is 0.05% or more and less than 0.38%, the steel material having the above chemical composition can have excellent yield strength and machinability if fn1 is 0.38 to 1.50.

In addition, if fn1 is higher than 1.65 when the C content is 0.38 to 0.55%, the strength of the steel material after hot forging becomes too high, and the machinability of the steel material after hot forging is lowered. If fn1 is less than 0.73, the strength of the steel material becomes too low, and the steel material after hot forging (hot forged product) cannot obtain sufficient yield strength. When the C content is 0.38 to 0.55%, the steel material having the above chemical composition can have excellent yield strength and machinability if fn1 is 0.73 to 1.65.

(B) Against lysis

As described above, in order to improve the yield strength and fatigue strength of a hot forged product which requires high cracking properties, hot forging is performed on a steel material which is a blank, and as a result, if the structure of the steel material after hot forging (hot forged product) is assumed to be a structure mainly composed of bainite, the cracking properties are reduced. This is because: bainite has high toughness, and ductile fracture surfaces are easily formed on fracture surfaces after cracking. Therefore, in order to improve the cracking property, it is preferable that the area ratio of bainite in the microstructure is low.

However, bainite increases the fatigue strength and yield strength of the steel. Therefore, if a technique for making the structure of a hot forged product into a bainite body and further improving the cracking property can be obtained, the yield strength and the fatigue strength of the hot forged product can be improved and the cracking property can be improved.

Accordingly, the present inventors have found that sufficient cracking can be obtained even when a bainite-based structure is formed as a structure of a hot forged product after hot forgingFurther, investigation and study were conducted on the steel material having a high toughness. As a result, they found that: among various oxide-based inclusions, with SiO₂The inclusions mainly containing CaO are made of Al in comparison with the inclusions mainly containing CaO₂O₃Al as a main component₂O₃The inclusion further affects the cracking properties of the hot forged product having the structure of the bainite matrix. This point will be described in detail below.

Al is added as a deoxidizer in the deoxidation treatment in the refining step, and is bonded to oxygen in molten steel to form Al₂O₃. Usually, Al₂O₃Aggregation, combination and floating up occur in the molten steel and are removed. On the other hand, a part of Al₂O₃Will remain in the steel to form Al₂O₃Is an inclusion. Here, in the present specification, Al₂O₃The term "inclusions" means Al in the inclusions₂O₃The proportion of (b) is 70.0% or more of inclusions in mass%. Al remaining in steel₂O₃The inclusions do not remain as solid solution in the steel material and hot forged product produced by hot rolling.

Al in steel₂O₃The toughness of the inclusions is extremely low as compared with that of the base metal (steel material matrix). Thus, upon cracking, Al₂O₃Brittle fracture of the inclusions occurs. Al having brittle fracture₂O₃The inclusions further become the starting points of fracture in Al₂O₃The interface between the inclusions and the matrix generates sharp initial cracks. The tip of the initial crack is strongly plastically constrained, and thus the steel material is likely to be brittle. Cracks formed by increased brittleness of initial cracks and adjacent Al₂O₃Cracks generated by the inclusions are combined with each other, thereby obtaining a brittle fracture surface.

According to the above mechanism, even if a steel material (hot forged product) having a microstructure mainly composed of bainite having high toughness is formed by hot forging, the reason for Al is as long as₂O₃The initial cracks are generated by inclusion, and the brittle cracks are easily increased. Whereby the fracture surface forms a brittle fracture surfaceDuctile fracture is suppressed. As a result, excellent cracking properties were obtained.

On the other hand, as a deoxidizer other than Al, Si, Ca, and the like are widely used. Si and Ca form SiO in molten steel₂And CaO. SiO 2₂The fatigue strength and hot workability of the steel material are easily reduced. In addition, CaO and Al₂O₃Has higher toughness than Al, so₂O₃It is more difficult to improve the cracking property of the steel.

As described above, in order to maintain hot workability of the steel material and improve the cracking property, SiO is not used among oxide inclusions in the steel₂And CaO to utilize Al₂O₃Inclusions are suitable. Based on the above considerations, the present inventors have further focused on Al₂O₃The appropriate number density of the inclusions was investigated and investigated. As a result, they found that: if Al is 3 μm or more in √ AREA₂O₃Inclusions (hereinafter, also referred to as "coarse Al₂O₃Inclusions) of 0.05 to 1.00 pieces/mm²As a result of the improvement of yield strength and fatigue strength of the hot forged product while maintaining hot workability of the steel material, excellent cracking performance can be obtained even when the structure of the steel material after hot forging (hot forged product) is assumed to be a structure mainly composed of bainite.

The gist of the steel material of the present application completed based on the above findings is as follows.

[1] The steel material has the following chemical composition:

in mass%

C：0.05～0.55％、

Si：0.05～1.00％、

Mn：1.51～3.50％、

P: less than 0.1000 percent,

S: less than 0.3000 percent,

Cr：0.05～2.50％、

V：0.10～0.75％、

Ti：0.005～0.250％、

Al：0.003～0.100％、

N: less than 0.020%,

Cu：0～0.60％、

Ni：0～0.60％、

Mo：0～0.70％、

Nb：0～0.100％、

Pb：0～0.30％、

Te：0～0.3000％、

Ca：0～0.0100％、

Bi: 0 to 0.4000%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

satisfies formula (1) when the C content is 0.05% or more and less than 0.38%,

satisfying the formula (2) when the C content is 0.38-0.55%,

70.0% or more of Al is contained in mass%₂O₃And inclusions having an AREA of 3 μm or more are defined as coarse Al₂O₃When the foreign matter is included, the mixture is mixed,

the coarse Al in the steel₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm²。

0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50 (1)

0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65 (2)

Here, the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (1) and the formula (2).

Here, in the present specification, "Al₂O₃The term "inclusions" means Al in the inclusions₂O₃The proportion of (b) is 70.0% or more of inclusions in mass%.

[2] The steel product according to [1],

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

Cu：0.01～0.60％、

Ni：0.01～0.60％、

Mo: 0.01 to 0.70%, and

nb: 0.005-0.100% of 1 element or more than 2 elements.

[3] The steel according to [1] or [2],

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

Pb：0.01～0.30％、

Te：0.0003～0.3000％、

Ca: 0.0003 to 0.0100%, and

bi: 0.0003 to 0.4000% of 1 element or more than 2 elements.

The steel material according to the present invention will be described in detail below. The "%" relating to an element means mass% unless otherwise specified.

[ chemical composition ]

The chemical composition of the steel material of the present invention contains the following elements.

C：0.05～0.55％

Carbon (C) increases the yield strength and fatigue strength of the steel material after hot forging, on the premise that formula (1) or formula (2) is satisfied. If the C content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content is too high, the machinability of the steel material after hot forging is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.05 to 0.55%. The lower limit of the C content is preferably 0.06%, more preferably 0.07%, and still more preferably 0.10%. The upper limit of the C content is preferably 0.54%, more preferably 0.53%, and still more preferably 0.52%.

Si：0.05～1.00％

Silicon (Si) is dissolved in the steel material to improve the fatigue strength of the steel material after hot forging. If the Si content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content is too high, the above effects are saturated. If the Si content is too high, the hot workability of the steel material is lowered and the manufacturing cost of the steel material is increased even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.06%, more preferably 0.07%, and still more preferably 0.08%. The upper limit of the Si content is preferably 0.99%, more preferably 0.95%, and still more preferably 0.90%.

Mn：1.51～3.50％

Manganese (Mn) deoxidizes a steel material at a molten steel stage in a manufacturing process. Mn further increases the yield strength and fatigue strength of the steel material after hot forging on the premise that the formula (1) or (2) is satisfied. If the Mn content is too low, these effects cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content is too high, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content is 1.51 to 3.50%. The lower limit of the Mn content is preferably 1.52%, more preferably 1.53%, and still more preferably 1.55%. The upper limit of the Mn content is preferably 3.49%, more preferably 3.48%, and still more preferably 3.45%.

P: less than 0.1000%

Phosphorus (P) is an impurity that is inevitably contained. In other words, the P content exceeds 0%. If the P content exceeds 0.1000%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the P content is 0.1000% or less, more specifically, the P content exceeds 0% and is 0.1000% or less. The upper limit of the P content is preferably 0.0800%, more preferably 0.0500%. The P content is preferably as low as possible. However, if the P content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the P content is 0.0001%, and more preferably 0.0005%.

S: less than 0.3000%

Sulfur (S) is an impurity inevitably contained. In other words, the S content exceeds 0%. If the S content exceeds 0.3000%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the S content is 0.3000% or less, more specifically, the S content exceeds 0% and is 0.3000% or less. The preferable upper limit of the S content is 0.2000%, more preferably 0.1500%. The S content is preferably as low as possible. However, if the S content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the S content is 0.0001%, and more preferably 0.0005%.

Cr：0.05～2.50％

Chromium (Cr) increases the yield strength and fatigue strength of the steel material after hot forging on the premise that the formula (1) or (2) is satisfied. If the Cr content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content is too high, the hardness of the steel material after hot forging becomes too hard even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging is lowered. If the Cr content is too high, the production cost also increases. Therefore, the Cr content is 0.05 to 2.50%. The lower limit of the Cr content is preferably 0.10%, more preferably 0.12%, and still more preferably 0.15%. The upper limit of the Cr content is preferably 2.00%, more preferably 1.80%, and still more preferably 1.60%.

V：0.10～0.75％

Vanadium (V) precipitates as carbides in the ferrite during cooling after hot forging, and increases the yield strength and fatigue strength of the steel after hot forging on the premise that formula (1) or formula (2) is satisfied. If the content of V is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content is too high, the production cost of the steel material becomes high even if the content of other elements is within the range of the present embodiment. If the V content is too high, the machinability of the steel material is also lowered. Therefore, the V content is 0.10 to 0.75%. The lower limit of the V content is preferably 0.11%, more preferably 0.12%, and still more preferably 0.15%. The upper limit of the V content is preferably 0.70%, more preferably 0.68%, and still more preferably 0.66%.

Ti：0.005～0.250％

Titanium (Ti) precipitates as carbide together with V during cooling and heating after hot forging, and improves the fatigue strength of the steel after hot forging. Ti also forms Ti sulfides and Ti carbosulfides during solidification of molten steel based on continuous casting, improving machinability of the steel. If the Ti content is too low, the effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ti content is too high, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Ti content is 0.005-0.250%. The lower limit of the Ti content is preferably 0.010%, more preferably 0.020%. The preferable upper limit of the Ti content is 0.240%, more preferably 0.220%.

Al：0.003～0.100％

Aluminum (Al) deoxidizes steel at the molten steel stage in the manufacturing process. Al bonds with oxygen to form coarse Al₂O₃Is an inclusion. Coarse Al₂O₃The inclusions remain in the steel material, and the cracking property of the steel material after hot forging is improved. If the Al content is too low, these effects cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the content of Al is too high, coarse Al is excessively generated even if the content of other elements is within the range of the present embodiment₂O₃Inclusions decrease the hot workability of the steel material and the fatigue strength of the steel material after hot forging. If the Al content is too high, the production cost also becomes high. Therefore, the Al content is 0.003 to 0.100%. The lower limit of the Al content is preferably 0.004%, more preferably 0.005%, still more preferably 0.006%, and still more preferably 0.011%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%, and still more preferably 0.050%. In the steel material according to the embodiment of the present invention, the Al content means all Al content.

N: 0.020% or less

Nitrogen (N) is inevitably contained. In other words, the N content exceeds 0%. N bonds with Al to form AlN, which blocks Al₂O₃Is performed. As a result, the cracking property of the steel material after hot forging is reduced. Therefore, the N content is 0.020% or less, more specifically, the N content exceeds 0% and is 0.020% or less. The preferable upper limit of the N content is 0.015%, and more preferably 0.010%. The N content is preferably as low as possible. However, if the N content is reduced to the limit by the refining step, the productivity is lowered and the production cost is increased. Therefore, in consideration of usual operations, the preferable lower limit of the N content is 0.0001%, and more preferably 0.0005%.

The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the impurities are not substances which are actively contained in the steel, but substances which are mixed from ores, scraps, manufacturing environments, and the like which are raw materials in the industrial production of the steel material.

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

B: 0.01% or less, Sb: 0.30% or less, Sn: 0.30% or less, W: 0.30% or less, Co: 0.30% or less, As: 0.30% or less and H: less than 0.005%.

[ against optional elements ]

The steel material according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Cu, Ni, Mo, and Nb instead of a part of Fe. These elements all improve the strength of the steel.

Cu：0～0.60％

Copper (Cu) is an optional element and may or may not be contained. In other words, the Cu content may be 0%. If contained, Cu is dissolved in the steel material, and increases the fatigue strength of the steel material after hot forging. If Cu is contained only slightly, the above-described effects can be obtained to some extent. However, if the Cu content is too high, the manufacturing cost of the steel material becomes high and the machinability of the steel material after hot forging is also deteriorated even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0 to 0.60%. The lower limit of the Cu content for more effectively improving the above effect is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cu content is preferably 0.59%, more preferably 0.55%, and still more preferably 0.50%.

Ni：0～0.60％

Nickel (Ni) is an optional element and may or may not be contained. In other words, the Ni content may be 0%. If contained, Ni is dissolved in the steel material, and increases the fatigue strength of the steel material after hot forging. If Ni is contained only slightly, the above-described effects can be obtained to some extent. However, if the Ni content is too high, the manufacturing cost becomes high. If the Ni content is too high, the toughness of the steel material after hot forging becomes too high even if the content of other elements is within the range of the present embodiment. As a result, when the steel material after hot forging (hot forged product) is fractured, a ductile fracture surface is formed on the fracture surface after fracture separation, and the cracking property is lowered. Therefore, the Ni content is 0 to 0.60%. The lower limit of the Ni content for more effectively improving the above effect is preferably 0.01%, more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Ni content is preferably 0.59%, more preferably 0.58%, and still more preferably 0.55%.

Mo：0～0.70％

Molybdenum (Mo) is an optional element and may or may not be present. In other words, the Mo content may be 0%. When contained, Mo forms carbides in the steel material. Therefore, the yield strength and fatigue strength of the steel material after hot forging are improved on the premise that the formula (1) or the formula (2) is satisfied. If Mo is contained slightly, the above-described effects can be obtained to some extent. However, if the Mo content is too high, the hardness of the steel material becomes too high even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging is lowered. If the Mo content is too high, the production cost also becomes high. Therefore, the Mo content is 0 to 0.70%. The lower limit of the Mo content for more effectively improving the above effect is preferably 0.01%, more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.69%, more preferably 0.68%, and still more preferably 0.65%.

Nb：0～0.100％

Niobium (Nb) is an optional element and may be absent. In other words, the Nb content may be 0%. When contained, Nb forms carbides in the steel material, and improves the fatigue strength of the steel material after hot forging. If Nb is contained slightly, the above-described effects can be obtained to some extent. However, if the Nb content is too high, the hardness of the steel material becomes too high even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material after hot forging is lowered. If the Nb content is too high, the crystal grains are further refined, and the toughness of the steel after hot forging becomes too high. As a result, when the steel material after hot forging (hot forged product) is fractured, a ductile fracture surface is formed on the fracture surface after fracture separation, and the fracture properties of the steel material are lowered. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content for more effectively improving the above effect is preferably 0.005%, more preferably 0.010%, and still more preferably 0.015%. The upper limit of the Nb content is preferably 0.095%, more preferably 0.090%, and still more preferably 0.085%.

The steel material according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Pb, Te, Ca, and Bi, instead of a part of Fe. These elements all improve the machinability of the steel.

Pb：0～0.30％

Lead (Pb) is an optional element and may or may not be present. In other words, the Pb content may be 0%. When contained, Pb improves the machinability of the steel material after hot forging. The above-described effect can be obtained to some extent by containing only a slight amount of Pb. However, if the Pb content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Pb content is 0 to 0.30%. The lower limit of the Pb content for more effectively improving the above effect is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Pb content is preferably 0.29%, more preferably 0.25%, and still more preferably 0.20%.

Te：0～0.3000％

Tellurium (Te) is an optional element and may or may not be present. In other words, the Te content may be 0%. When Te is contained, the machinability of the steel material after hot forging is improved. If Te is contained slightly, the above-described effect can be obtained to some extent. However, if the Te content is too high, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Te content is 0 to 0.3000%. The lower limit of the Te content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Te content is preferably 0.2900%, more preferably 0.2500%, and still more preferably 0.2000%.

Ca：0～0.0100％

Calcium (Ca) is an optional element and may or may not be present. In other words, the Ca content may be 0%. When included, Ca improves the machinability of the steel material after hot forging. If Ca is contained slightly, the above-described effect can be obtained to some extent. However, if the Ca content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ca content is 0 to 0.0100%. The lower limit of the Ca content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0080%, and still more preferably 0.0050%.

Bi：0～0.4000％

Bismuth (Bi) is an optional element and may or may not be contained. In other words, the Bi content may be 0%. When contained, Bi improves the machinability of the steel material after hot forging. The above-mentioned effects can be obtained to some extent by slightly containing Bi. However, if the Bi content is too high, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Bi content is 0 to 0.4000%. The lower limit of the Bi content for more effectively enhancing the above effect is preferably 0.0003%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Bi content is preferably 0.3900%, more preferably 0.3000%, and still more preferably 0.2000%.

[ against formula (1) ]

The chemical composition of the steel material according to the embodiment of the present invention further satisfies formula (1) when the C content is 0.05% or more and less than 0.38%, and satisfies formula (2) when the C content is 0.38 to 0.55%.

0.38≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.50 (1)

0.73≤C+0.11Mn+0.08Cr+0.75V+0.20Mo≤1.65 (2)

Here, the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (1) and the formula (2).

fn1(═ C +0.11Mn +0.08Cr +0.75V +0.20Mo) is an indicator of strength, and is positively correlated with yield strength.

[ the range of fn1 for a C content of 0.05% or more and less than 0.38% ]

When the C content is 0.05% or more and less than 0.38%, if fn1 is less than 0.38, the strength of the steel material becomes too low, and a sufficient yield strength cannot be obtained in the steel material after hot forging (hot forged product). On the other hand, if fn1 is higher than 1.50, the strength of the steel material after hot forging becomes too high, and the machinability of the steel material after hot forging is lowered. Therefore, when the C content is 0.05% or more and less than 0.38%, fn1 is 0.38 to 1.50. The lower limit of fn1 is preferably 0.39, more preferably 0.40, and still more preferably 0.41. The upper limit of fn1 is preferably 1.49, more preferably 1.48, and still more preferably 1.47.

[ the range of fn1 when the C content is 0.38-0.55% ]

When the C content is 0.38 to 0.55%, if fn1 is less than 0.73, the strength of the steel material becomes too low, and the steel material after hot forging (hot forged product) cannot obtain sufficient yield strength. On the other hand, if fn1 is higher than 1.65, the strength of the steel material after hot forging becomes too high, and the machinability of the steel material after hot forging is lowered. Therefore, when the C content is 0.38 to 0.55%, fn1 is 0.73 to 1.65. The lower limit of fn1 is preferably 0.74, more preferably 0.75, and further preferably 0.76. The upper limit of fn1 is preferably 1.64, more preferably 1.63, and still more preferably 1.62.

[ coarse Al₂O₃Number density of inclusions]

In the steel material according to the embodiment of the present invention, √ AREA is Al 3 μm or more₂O₃Inclusions (i.e., coarse Al)₂O₃Inclusions) of 0.05 to 1.00 pieces/mm². As mentioned above, Al₂O₃The term "inclusions" means that 70.0% or more of Al is contained by mass%₂O₃The inclusion of (2).

Coarse Al if₂O₃The number density of the inclusions is less than 0.05 pieces/mm²The steel material after hot forging cannot be sufficiently cracked. On the other hand, if coarse Al is contained₂O₃The number density of the inclusions exceeds 1.00/mm²The hot forged steel material can be excellent in cracking property, but hot forgedThe fatigue strength and hot workability of the steel material are reduced. Coarse Al if₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm²Even if bainite is formed in the structure of the steel material by hot forging, the hot workability of the steel material and the fatigue strength of the steel material after hot forging can be maintained, and the steel material after hot forging can be excellent in cracking properties.

Coarse Al for further improving the cracking properties of steel after hot forging₂O₃The preferable lower limit of the number density of the inclusions is 0.07 pieces/mm²More preferably 0.10 pieces/mm²More preferably 0.11 pieces/mm²More preferably 0.12 pieces/mm². Coarse Al for further improving hot workability of steel material and fatigue strength of steel material after hot forging₂O₃The preferable upper limit of the number density of the inclusions is 0.80 pieces/mm²More preferably 0.60 pieces/mm²。

Coarse Al₂O₃The number density of the inclusions can be measured by the following method. When the steel material is a steel bar, a sample is taken from the R/2 portion in the cross section perpendicular to the axial direction (rolling direction) of the steel bar. The R/2 part is: in a cross section perpendicular to the axial direction of the bar steel, the center of a line segment (radius R) connecting the center and the surface is located at the center. Out of the surfaces of the samples, 30 samples each having a length of 4mm × a width of 2.5mm were sampled from the surface corresponding to a cross section (vertical cross section) including the axial direction of the steel bar. The observation surfaces of 30 samples were observed by an optical microscope of 200 magnifications without etching, and a photographic image was generated. The sum of the inspected areas is 300mm²。

The inclusions in the observation surface (4 mm. times.2.5 mm) of each sample were subjected to spot analysis using an Electron Probe Microanalyzer (EPMA). The mass% of each element including Al and O in each inclusion was determined from the analysis results. Calculating Al in the inclusions from the analyzed contents of the respective elements₂O₃In mass%. Specifically, arbitrary 3 points of the inclusions were determined, and the Al content (% by mass) was measured using an electron beam having a beam diameter of 1 μm for each point. Based on the obtained Al content, Al and Al are used₂O₃Mass ratio of (3) to calculate Al₂O₃Content (mass%). Obtaining the calculated Al₂O₃Average value of the contents, defined as Al₂O₃In mass%.

Al in the inclusions₂O₃The inclusions in an amount of 70% by mass or more are determined as Al₂O₃Is an inclusion. Calculating each of the determined Al by using an image analyzer₂O₃V-AREA which is an inclusion. Specifically, each of the determined Al is obtained₂O₃The length L (. mu.m) and width W (. mu.m) of the inclusion. Each Al is added₂O₃The inclusions are assumed to be rectangular and have an area (═ L × W (μm)²) Is obtained in the form of (c). The square root of the calculated area was determined and defined as each Al₂O₃V AREA (μm) of inclusion series.

Each Al was obtained₂O₃After v AREA of inclusions, coarse Al of 3 μm or more is determined₂O₃Is an inclusion. Finding the determined coarse Al₂O₃The number of inclusions was divided by the total area of the specimens (300 mm)²) The value thus obtained is defined as coarse Al₂O₃Number density of inclusions/mm²)。

[ production method ]

An example of the method for producing the steel material will be described. The manufacturing method of this example includes a refining step, a casting step, and a hot working step.

[ refining step ]

Molten steel satisfying the chemical composition and formula (1) (C content of 0.05% or more and less than 0.38%) or formula (2) (C content of 0.38 to 0.55%) is produced by a known method. Specifically, decarburization, dephosphorization and desiliconization in the converter are performed by a known method. After tapping, an aluminum deoxidizer is added to the ladle to perform deoxidation treatment. Note that, in order to prevent SiO₂And mixing CaO, wherein a ladle is an aluminum deoxidation special ladle. The aluminum deoxidizer is metallic Al or an Al alloy having an Al content of 80% by mass or more.

After the deoxidation treatment, vacuum degassing treatment was performed. Here, the Al content in the molten steel is adjusted by confirming the molten steel components in the production process and adding the aluminum deoxidizer (metallic Al or Al alloy having an Al content of 80% by mass or more) to the vacuum degassing treatment. The aluminum deoxidizer added in the vacuum degassing treatment accounts for 50-70% by mass of the aluminum deoxidizer to be added.

Note that, in order to suppress SiO₂The addition of Si is performed after the steel is sufficiently deoxidized by an aluminum deoxidizer. The addition of Si is performed, for example, after 10 minutes or more has elapsed since the addition of the additional aluminum deoxidizer. Furthermore, in order to make Al₂O₃The aggregation is carried out in an appropriate range, and a preferable holding time for the molten steel temperature to be 1600 ℃ or more from the time when the deoxidizer is added after tapping to the time when casting is started is 15 to 60 minutes. The lower limit of the time for which the temperature of molten steel is 1600 ℃ or higher is preferably 30 minutes, and more preferably 40 minutes. By the refining step, Al satisfying the chemical composition and satisfying the formula (1) or (2) and √ AREA at 3 μm or more can be obtained₂O₃Inclusions (i.e., coarse Al)₂O₃Inclusions) of 0.05 to 1.00 pieces/mm²The molten steel of (1).

[ casting Process ]

Using the molten steel, a cast slab (slab or bloom) or a steel ingot (ingot) is produced by a known method. Examples of the casting method include a continuous casting method and an ingot casting method.

[ Hot working Process ]

In the hot working step, the cast slab or steel ingot produced in the casting step is hot worked to produce a steel material. The steel material is, for example, bar steel. The hot working step is performed by a known method. The hot working process includes, for example, a rough rolling process and a finish rolling process. The rough rolling step is, for example, rough rolling using a blooming mill. The finish rolling step is, for example, rolling using a continuous rolling mill. In a continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The heating temperature in the rough rolling step is, for example, 1000 to 1300 ℃. Addition in the finish rolling procedureThe thermal temperature is, for example, 1000 to 1300 ℃. In a heating temperature range of 1000 to 1300 ℃, Al₂O₃The form of the inclusions is not particularly changed. The hot working step may be performed by hot forging instead of hot rolling. In the above description, the hot working step includes two steps, i.e., the rough rolling step and the finish rolling step, but only the finish rolling step may be performed without the rough rolling step.

The steel material is produced by the above-described production process. In the above-described manufacturing method, the steel bar is manufactured as a steel material, but the steel material according to the embodiment of the present invention may be a wire rod. The cross section perpendicular to the axial direction of the steel material is not particularly limited. The cross-sectional shape perpendicular to the axial direction of the steel material is, for example, rectangular, circular, elliptical, or polygonal.

The method for producing the steel material according to the present embodiment is not limited to the above-described production method. The above-described manufacturing method is one of the preferable manufacturing methods, but the steel material according to the embodiment of the present invention can be manufactured by other manufacturing methods. Provided that √ AREA in the steel is 3 μm or more Al₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm²The above-mentioned production method is not particularly limited.

[ method for producing Hot forged products ]

A method for manufacturing a split connecting rod will be described as an example of a method for manufacturing a hot forged product using the steel material.

First, the steel material is heated in a high-frequency induction heating furnace. In this case, the preferable heating temperature is 1000 to 1300 ℃, and the preferable heating time is 10 to 15 minutes. Since the heating temperature in the high-frequency induction heating furnace is low, Al in the steel material₂O₃The form of the inclusions is not particularly changed. The heated steel material is hot forged to produce an intermediate product (a split connecting rod having a substantially shape). The degree of working at the time of hot forging is not particularly limited. The degree of working at the time of hot forging is preferably 0.22 or more. Here, the degree of working is the maximum value of the logarithmic strain generated in the portion other than the burr in the forging step. The intermediate product after hot forging is subjected to machining, and rough cutting is performed on the intermediate product.Thereafter, the large head 100 of the intermediate product is subjected to fracture division (cleavage). The intermediate product after fracture splitting is subjected to finish cutting to produce a final hot forged product (split connecting rod). The hot forged product is produced by the above steps.

In the above method of manufacturing a hot forged product, the hot forged product is formed into a split connecting rod, but the hot forged product is not limited to a split connecting rod. The hot forged product may be other parts for machine structural use.

[ microstructure of Hot forged product ]

The microstructure of the produced hot forged product is not particularly limited. However, when a steel material having the above chemical composition is hot forged to produce a hot forged product in order to improve yield strength and fatigue strength, the structure of the steel material after hot forging (hot forged product) may form a structure mainly composed of bainite. Here, "microstructure of bainite body" means: in the microstructure of the hot forged product, the area ratio of bainite is 80% or more. In this specification, the bainite also includes martensite.

When the area ratio of bainite is not 100%, the balance of the matrix structure is ferrite or ferrite and pearlite. The lower limit of the bainite area ratio in the microstructure is preferably 85%, more preferably 90%, further preferably 95% or more, and most preferably 100%. An example of the area ratio of bainite is 95 to 100%.

As a result of producing a hot forged product by hot forging a steel material having the above chemical composition in order to improve yield strength and fatigue strength, it is assumed that the structure of the hot forged product is mainly bainite. Further, a case is assumed where the hot forged product is a split connecting rod. At this time, when the large head 100 is broken and divided into 2 pieces (the cap 2 and the rod 3), the broken portion is plastically deformed, the broken surface is likely to have a ductile cross section, and the cracking property is likely to be lowered. However, the steel material of the present embodiment contains 70.0% by mass or more of Al₂O₃Al of (2)₂O₃Coarse Al of √ AREA at 3 μm or more among the inclusions₂O₃The number density of the inclusions is 0.05 to 1.00 pieces/mm². Therefore, the steel material of the present embodiment is also hot forgedEven when the microstructure of the product is mainly bainite, the fracture surface of the hot forged product easily forms a brittle fracture surface, and excellent cracking properties can be maintained.

As described above, in the steel material according to the present embodiment, even when the bainite area ratio in the steel material structure after hot forging is 80% or more (in other words, when the structure is mainly composed of bainite), excellent cracking property can be obtained. Therefore, when the area fraction of bainite in the structure is less than 80% (in other words, the structure is not mainly composed of bainite) as a result of hot forging the steel material according to the present embodiment, the hot forged product is certainly excellent in cracking properties.

The area fraction of bainite in the microstructure of the steel material after hot forging (hot forged product) can be measured by the following method. In the steel material after hot forging, 10 samples were taken from the portion (inner region) from which the region (surface layer region) up to the depth position of 1mm or more from the surface was removed. Any surface of each sample collected was used as an observation surface. The observation surface was polished and then etched with 3% nitroethanol (nitroethanol etching solution). The etched observation surface was observed with an optical microscope at 200 magnifications, and a photographic image with arbitrary 5 visual fields was generated. The area of each field was 475. mu. m.times.475. mu.m.

In each field, the ferrite, pearlite, and bainite phases have different contrast ratios. Thus, each phase is determined based on the contrast. As described above, martensite is not distinguished from bainite in the present specification. Therefore, in the present specification, the region excluding ferrite and pearlite in each field is defined as "bainite". The area (. mu.m) of bainite in each visual field was determined in the determined phase²). The bainite area ratio (%) is defined as the ratio of the area of bainite in all the fields of view to the total area of all the fields of view (5 fields × 10).