CN108368583B

CN108368583B - Steel wire for non-heat-treated machine part and non-heat-treated machine part

Info

Publication number: CN108368583B
Application number: CN201780004566.3A
Authority: CN
Inventors: 小此木真; 平上大辅; 多田达诚
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-01-20
Filing date: 2017-01-20
Publication date: 2020-05-26
Anticipated expiration: 2037-01-20
Also published as: JP6614245B2; JPWO2017126695A1; KR102037089B1; WO2017126695A1; CN108368583A; KR20180082553A

Abstract

A steel wire for non-heat-treated machine parts, which satisfies the following: contains by mass%: c: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.00% and Al: 0.005 to 0.050%, and the balance of Fe and impurities, wherein the metallic structure contains (35 x [ C% ] + 65)% or more of pearlite, and when the diameter is D, the average aspect ratio of the pearlite block at a position having a depth of 50 μm in an L section is AR, and the average block particle size of the pearlite block at a position having a depth of 50 μm in a C section is GD, the AR is 1.4 or more, (AR)/(average aspect ratio of the pearlite block at a position having a depth of 0.25D in an L section) is 1.1 or more, and the GD is (15/AR) μm or less, (GD)/(average block particle size of the pearlite block at a position having a depth of 0.25D in a C section) is less than 1.0.

Description

Steel wire for non-heat-treated machine part and non-heat-treated machine part

Technical Field

The present application relates to a steel wire for a non-heat-treated machine component and a non-heat-treated machine component.

Background

In recent years, in the fields of various machines such as automobiles, buildings, and the like, demands for high-strength machine parts have been increasing from the viewpoint of weight reduction or space saving.

However, as the strength of the high-strength mechanical component increases, particularly when the tensile strength of the high-strength mechanical component is 1100MPa or more, fracture due to hydrogen embrittlement tends to occur (that is, hydrogen embrittlement resistance tends to decrease).

As a method for improving the hydrogen embrittlement resistance of high-strength mechanical parts, a method of forming a structure into a pearlite structure and reinforcing the structure by wire drawing is known, and a large number of methods have been proposed so far (for example, see patent documents 1 to 11).

For example, patent document 11 discloses a high-strength bolt having a tensile strength of 1200MPa or more, which is obtained by forming a pearlite structure as a structure and then performing wire drawing.

Patent document 3 discloses a pearlite structure wire rod for a high-strength bolt having a tensile strength of 1200MPa or more.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. Sho 54-101743

Patent document 2: japanese laid-open patent publication No. 11-315348

Patent document 3: japanese laid-open patent publication No. 11-315349

Patent document 4: japanese patent laid-open No. 2000-144306

Patent document 5: japanese patent laid-open publication No. 2000-337332

Patent document 6: japanese patent laid-open No. 2001-348618

Patent document 7: japanese laid-open patent publication No. 2002-069579

Patent document 8: japanese patent laid-open publication No. 2003-193183

Patent document 9: japanese patent laid-open publication No. 2004-307929

Patent document 10: japanese patent laid-open publication No. 2005-281860

Patent document 11: japanese patent laid-open No. 2008-261027

Disclosure of Invention

Problems to be solved by the invention

As a method for manufacturing a high-strength mechanical part (for example, a high-strength bolt) having a tensile strength of 1100MPa or more, for example, the following methods are available: steel wires of alloy steel to which alloying elements such as Cr, Mo, and V are added are formed into a predetermined shape, and then quenched and tempered to manufacture machine parts. On the other hand, in order to reduce the manufacturing cost, the following techniques are known: the wire rod having improved strength by rapid cooling, precipitation strengthening, or the like is subjected to wire drawing processing without quenching and tempering after forming, thereby providing a predetermined strength. Machine parts (e.g., bolts) manufactured by this technique are called non-heat treated machine parts (e.g., non-heat treated bolts).

The non-heat-treated machine component having a tensile strength of 1100MPa or more can be produced by cold working a steel wire having a tensile strength of 900MPa or more.

It is believed that: for example, in a non-heat-treated machine component (for example, a non-heat-treated bolt) in which a pearlite structure is wire-drawn and strengthened, hydrogen is trapped in the interface between cementite and ferrite by the pearlite structure, and therefore, the invasion of hydrogen into the inside of a steel material is suppressed, and the hydrogen embrittlement resistance is improved. In a non-heat-treated machine component (for example, a non-heat-treated bolt) having a tensile strength of 1100MPa or more, hydrogen embrittlement resistance is improved to some extent by a technique of wire drawing a pearlite structure. However, it is not easy to sufficiently improve the hydrogen embrittlement resistance by this technique alone, and further improvement is desired.

In addition, in these conventional techniques, as the strength of the steel wire used for obtaining the high-strength machine component by cold working increases, particularly when the tensile strength of the steel wire is 900MPa or more, there is a possibility that the cold workability in cold working the steel wire to obtain the high-strength machine component is lowered. Therefore, it is not easy to improve both the hydrogen embrittlement resistance and the cold workability.

From the above, it is sometimes difficult to achieve both cold workability in the production of a non-heat-treated machine part by cold working and hydrogen embrittlement resistance in the production of a non-heat-treated machine part in a steel wire having a tensile strength of 900MPa or more for obtaining a high-strength machine part having a tensile strength of 1100MPa or more.

Accordingly, an object of the present invention is to provide a steel wire for non heat-treated machine parts, which has a tensile strength of 900MPa or more, is excellent in cold workability when producing non heat-treated machine parts by cold working, and is excellent in hydrogen embrittlement resistance when producing non heat-treated machine parts.

Another object of the present invention is to provide a non-heat-treated machine component which can be produced using a steel wire having excellent cold workability and has excellent tensile strength and hydrogen embrittlement resistance.

Means for solving the problems

Means for solving the above problems include the following means.

<1> a steel wire for non-heat-treated machine parts, which comprises, in terms of mass%, the following chemical composition:

C：0.40～0.65％、

Si：0.05～0.50％、

Mn：0.20～1.00％、

Al：0.005～0.050％、

P：0～0.030％、

S：0～0.030％、

N：0～0.0050％、

Cr：0～1.00％、

Ti：0～0.050％、

Nb：0～0.050％、

V：0～0.10％、

B：0～0.0050％、

o: 0 to 0.0030%, and

the rest is as follows: fe and impurities in the iron-based alloy, and the impurities,

wherein, when the mass% of C is [ C% ], the microstructure is composed of pearlite having an area ratio of (35 × [ C% ] + 65%) or more and the remainder which is at least one of proeutectoid ferrite and bainite,

when a cross section parallel to the axial direction of the wire and including the central axis is an L cross section, a cross section perpendicular to the axial direction of the wire is a C cross section, the diameter of the wire is D, and the average aspect ratio of the pearlite block (Perlite block) measured at a position 50 [ mu ] m deep from the surface of the wire on the L cross section is AR, and the average block particle size of the pearlite block measured at a position 50 [ mu ] m deep from the surface of the wire on the C cross section is GD, AR is 1.4 or more, (AR)/(average aspect ratio of the pearlite block measured at a position 0.25D deep from the surface of the wire on the L cross section) is 1.1 or more, GD is (15/AR) [ mu ] m or less, (GD)/(average block particle size of pearlite measured at a position 0.25D deep from the surface of the wire on the C cross section) is less than 1.0,

the tensile strength is 900-1500 MPa.

<2> the steel wire for non-heat-treated machine parts <1> comprising 1 or 2 or more of the following elements in mass%:

cr: more than 0% and not more than 1.00%,

Ti: more than 0% and not more than 0.050%,

Nb: more than 0% and not more than 0.050%,

V: more than 0% and not more than 0.10%, and

b: more than 0% and not more than 0.0050%.

<3> the steel wire for non-heat-treated machine parts <1> or <2>, wherein D is 3 to 30 mm.

<4> a non-heat-treated machine component comprising a cylindrical shaft portion,

the chemical composition thereof comprises by mass percent:

C：0.40～0.65％、

Si：0.05～0.50％、

Mn：0.20～1.00％、

Al：0.005～0.050％、

P：0～0.030％、

S：0～0.030％、

N：0～0.0050％、

Cr：0～1.00％、

Ti：0～0.050％、

Nb：0～0.050％、

V：0～0.10％、

B：0～0.0050％、

o: 0 to 0.0030%, and

when a cross section parallel to the axial direction of the cylindrical shaft portion and including a central axis is set to an L cross section, a cross section perpendicular to the axial direction of the cylindrical shaft portion is set to a C cross section, the diameter of the cylindrical shaft portion is set to D, and the average aspect ratio of the pearlite block measured at a position in the L cross section where the depth from the surface of the cylindrical shaft portion is 50 μm is set to AR, and the average block particle size of the pearlite block measured at a position in the C cross section where the depth from the surface of the cylindrical shaft portion is 50 μm is set to GD, AR is 1.4 or more, (AR)/(average of the pearlite block measured at a position in the L cross section where the depth from the surface of the cylindrical shaft portion is 0.25D) is 1.1 or more, and (15/AR) μm or less, (GD in the C cross section where the depth from the surface of the cylindrical shaft portion is 0.25D), AR is 1.1 or more The average block particle diameter of the pearlite block measured at the position) is less than 1.0,

the tensile strength of the cylindrical shaft part is 1100-1500 MPa.

<5> the non-heat-treated mechanical component according to <4>, which contains 1 or 2 or more of the following elements in mass%:

cr: more than 0% and not more than 1.00%,

Ti: more than 0% and not more than 0.050%,

Nb: more than 0% and not more than 0.050%,

V: more than 0% and not more than 0.10%, and

b: more than 0% and not more than 0.0050%.

<6> A cold-worked product of the steel wire for non-heat-treated machine parts according to any one of <1> to <3>, comprising a cylindrical shaft portion having a tensile strength of 1100 to 1500 MPa.

<7> the non-heat-treated machine component according to any one of <4> to <6>, which is a non-heat-treated bolt.

Effects of the invention

According to the present application, it is possible to provide a steel wire for a non heat-treated machine component which has excellent cold workability in the production of a non heat-treated machine component by cold working, and which has excellent hydrogen embrittlement resistance in the case of producing a non heat-treated machine component, even when the steel wire has a tensile strength of 900MPa or more.

Further, according to the present application, it is possible to provide a non-heat-treated machine component which can be produced using a steel wire having excellent cold workability and which is excellent in tensile strength and hydrogen embrittlement resistance.

Drawings

Fig. 1 is a conceptual diagram showing an example of a pearlite block on an L-section of a steel wire of the present invention.

Detailed Description

In the present specification, the numerical range expressed by the term "to" means a range including the numerical values described before and after the term "to" as the lower limit value and the upper limit value.

In the present specification, "%" indicating the content of a component (element) means "% by mass".

In the present specification, the content of C (carbon) may be referred to as "C content". The contents of other elements are sometimes labeled similarly.

In the present specification, the term "step" is not limited to a separate step, and is also included in the term as long as the desired purpose of the step can be achieved even when the step cannot be clearly distinguished from other steps.

[ Steel wire for non-quenched and tempered mechanical parts ]

The chemical composition of the steel wire for non-heat-treated machine parts (hereinafter also simply referred to as "steel wire") of the present application contains, in mass%: c: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.00%, Al: 0.005-0.050%, P: 0-0.030%, S: 0-0.030%, N: 0-0.0050%, Cr: 0-1.00%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.10%, B: 0-0.0050%, O: 0-0.0030% and the remainder: fe and impurities in the iron-based alloy, and the impurities,

when a cross section parallel to the axial direction of the wire and including the central axis is an L cross section, a cross section perpendicular to the axial direction of the wire is a C cross section, the diameter of the wire is D, the average aspect ratio of the pearlite block measured at a position 50 [ mu ] m deep from the surface of the wire in the L cross section is AR, and the average block particle size of the pearlite block measured at a position 50 [ mu ] m deep from the surface of the wire in the C cross section is GD, AR is 1.4 or more, (AR)/(average aspect ratio of the pearlite block measured at a position 0.25D deep from the surface of the wire in the L cross section) is 1.1 or more, and GD is (15/AR) [ mu ] m or less, (GD)/(average block particle size of pearlite measured at a position 0.25D deep from the surface of the wire in the C cross section) is less than 1.0,

the tensile strength is 900-1500 MPa.

The steel wire of the present application is a steel wire having a tensile strength of 900MPa or more, and is also excellent in cold workability (hereinafter also simply referred to as "cold workability") when producing a non-heat-treated machine part by cold working.

Further, the steel wire of the present application is excellent in hydrogen embrittlement resistance (hereinafter, also simply referred to as "hydrogen embrittlement resistance") when it is formed into a non-heat-treated machine component. In other words, by cold working the steel wire of the present application, a non-heat-treated machine component having excellent hydrogen embrittlement resistance can be produced.

In the steel wire of the present application, the above-described chemical composition contributes to both cold workability and hydrogen embrittlement resistance. Details of the chemical composition will be described later.

In general, a steel wire having a chemical composition with a low C content (specifically, a C content of 0.65 mass% or less) such as the above-described chemical composition is softened and has improved ductility, so that good cold workability is obtained.

However, as the C content decreases, a two-phase structure of pro-eutectoid ferrite and pearlite is easily formed. In particular, in the surface layer of the wire rod, the C content is likely to be further reduced by decarburization, and proeutectoid ferrite is likely to be generated. Further, since the cooling rate is high in the surface layer of the wire rod, the bainite structure is easily formed. The pro-eutectoid ferrite and pearlite two-phase structure and bainite generally have lower hydrogen embrittlement resistance than pearlite. When the C content is reduced (specifically, the C content is set to 0.65 mass% or less), structures such as pro-eutectoid ferrite and bainite are easily formed, and therefore, the surface layer portion of the machine component (for example, a bolt) has low hydrogen embrittlement resistance.

In this regard, the metallic structure of the steel wire of the present invention is a metallic structure mainly containing pearlite, and more specifically, the metallic structure of the steel wire of the present invention is a metallic structure in which the area ratio of pearlite is (35 × [ C% ] + 65)% or more. The pearlite structure has a laminated structure of a layer mainly composed of a cementite phase (hereinafter, also simply referred to as "cementite layer") and a layer mainly composed of a ferrite phase (hereinafter, also simply referred to as "ferrite layer"). It is believed that: this laminated structure becomes resistance to the development of cracks (hydrogen embrittlement resistance). Thereby, cold workability and hydrogen embrittlement resistance are improved.

In the present application, the reason why the area ratio of pearlite depends on [ C% ] (i.e., C content) is because: in the range of the C content of 0.40 to 0.65%, as the C content is lower, pro-eutectoid ferrite and bainite tend to be more easily generated and pearlite tends to be less easily generated.

The steel wire of the present application has an average aspect ratio of the pearlite block measured at a position having a depth of 50 μm in L-section (i.e., "AR" in the present specification) of 1.4 or more, and has an average Aspect Ratio of (AR)/(the average aspect ratio of the pearlite block measured at a position having a depth of 0.25D from the surface of the steel wire in L-section) of 1.1 or more.

In the present specification, a position at a depth of 50 μm from the surface of the steel wire may be referred to as a "position at a depth of 50 μm" or a "surface layer". In other words, the "surface layer" in the present specification means a position at a depth of 50 μm from the surface of the steel wire.

In the present specification, a position having a depth of 0.25D from the surface of the steel wire (i.e., a position having a depth of 0.25 times the diameter (i.e., D)) from the surface of the steel wire may be referred to as a "position having a depth of 0.25D" or "0.25D".

In the present specification, (AR)/(average aspect ratio of the pearlite block measured at a position having a depth of 0.25D from the surface of the steel wire in the L-section) may be referred to as "aspect ratio of the pearlite block [ surface layer/0.25D ].

The aspect ratio [ surface layer/0.25D ] of the steel wire of the present application is 1.1 or more. That is, in the L-section of the steel wire of the present application, the pearlite blocks in the surface layer of the steel wire (i.e., at a position having a depth of 50 μm) are more elongated than those in the inside of the steel wire (i.e., at a position having a depth of 0.25D).

In the L-section of the steel wire of the present application, the average aspect ratio (i.e., AR) of the pearlite blocks in the surface layer is 1.4 or more.

The steel wire of the present application satisfies the above conditions, and thereby improves the hydrogen embrittlement resistance (i.e., hydrogen embrittlement resistance in the case of a non-heat-treated machine component produced by cold working). The reason is considered to be that: by extending the pearlite block in the surface layer, the direction of the lamellar structure of the pearlite structure in the surface layer becomes more uniform, and resistance to hydrogen intrusion from the surface of the steel wire and/or resistance to the development of cracks becomes. Therefore, in the steel wire of the present application, even if the microstructure contains pro-eutectoid ferrite and bainite, the hydrogen embrittlement resistance is improved.

The steel wire of the present application has an average block particle diameter (GD) of a pearlite block measured at a position having a depth of 50 μm in a C-section of not more than (15/AR) μm, and (GD)/(average block particle diameter of a pearlite block measured at a position having a depth of 0.25D in a C-section) of less than 1.0.

In the present specification, (GD)/(average block particle size of a pearlite block measured at a position having a depth of 0.25D in a C section) may be referred to as a "ratio of block particle sizes [ surface layer/0.25D ] of the pearlite block.

The steel wire of the present invention has a ratio [ surface layer/0.25D ] of the block particle size of the pearlite block of less than 1.0. That is, in the C-section of the steel wire of the present application, the pearlite blocks in the surface layer of the steel wire (i.e., at the position of a depth of 50 μm) are finer than those in the inside of the steel wire (i.e., at the position of a depth of 0.25D).

In the C-section of the steel wire of the present application, the average block particle size (i.e., GD) of the pearlite block in the surface layer is not more than (15/AR) μm.

The steel wire of the present application satisfies the above conditions, and thereby improves cold workability of the steel wire and improves hydrogen embrittlement resistance (that is, hydrogen embrittlement resistance in the case of forming a non-heat-treated machine component by cold working).

The reason why the cold workability of the steel wire is improved by satisfying the above conditions is considered to be that: the ductility of the steel wire is improved by making the pearlite block in the surface layer fine (i.e., (15/AR) μm or less).

The reason why the hydrogen embrittlement resistance is improved by satisfying the above conditions is considered to be related to the fact that the pearlite block in the surface layer is fine and that hydrogen tends to segregate in grain boundaries. Namely, it is considered that: since the pearlite blocks in the surface layer are fine, the total area of the grain boundaries in the surface layer increases, and as a result, the hydrogen trapping ability of the surface layer (i.e., the ability to prevent hydrogen from penetrating into the inside of the steel wire) improves.

The tensile strength of the steel wire is 900-1500 MPa.

The steel wire (i.e., steel wire for non-heat-treated machine parts) of the present application having a tensile strength of 900 to 1500MPa is suitable for use in manufacturing non-heat-treated machine parts having a tensile strength of 1100 to 1500MPa by cold working.

The cold working in the present application is not particularly limited, and cold forging, roll forming, cutting, drawing, and the like can be mentioned.

The cold working in the present application may be only one type of working or may be a plurality of types of working (for example, cold forging and roll forming).

The non-heat-treated machine part having a tensile strength of 1100 to 1500MPa may be produced by cold working the steel wire of the present invention and then maintaining the temperature within a range of 100 to 400 ℃.

The steel wire of the present application is mainly composed of pearlite and satisfies the above-described conditions, and therefore, the steel wire is excellent in cold workability when a non-heat-treated machine component is obtained by cold working in addition to a steel wire having a tensile strength of 900MPa or more.

The steel wire of the present application has a tensile strength of 900MPa or more and tends to have low cold workability mainly based on the pro-eutectoid ferrite-pearlite two-phase structure.

< chemical composition >

Next, the chemical composition of the steel wire of the present application will be described.

The chemical composition of the non-heat-treated mechanical parts of the present application described later is also the same as the chemical composition of the steel wire of the present application.

Hereinafter, the chemical composition of the steel wire or the non-heat-treated machine component of the present application may be referred to as "chemical composition of the present application".

·C：0.40～0.65％

C is an element necessary for securing tensile strength.

In the case where the C content is less than 0.40%, it is difficult to obtain a desired tensile strength. Therefore, the content of C in the chemical composition in the present application is 0.40% or more, preferably 0.45% or more.

On the other hand, if the C content exceeds 0.65%, cold workability may deteriorate. Therefore, the content of C in the chemical composition in the present application is 0.65% or less, preferably 0.60% or less.

·Si：0.05～0.50％

Si is a deoxidizing element and also an element that improves the tensile strength by solid solution strengthening.

When the Si content is less than 0.05%, the effect of addition is not sufficiently exhibited. Therefore, the chemical composition in the present application has an Si content of 0.05% or more, preferably 0.15% or more.

On the other hand, if the Si content exceeds 0.50%, the addition effect is saturated, and ductility during hot rolling deteriorates, and defects are likely to occur. Therefore, the Si content in the chemical composition in the present application is 0.50% or less, preferably 0.30% or less.

·Mn：0.20～1.00％

Mn is an element for improving the tensile strength of the pearlite transformed steel.

When the Mn content is less than 0.20%, the effect of addition is not sufficiently exhibited. Therefore, the Mn content in the chemical composition in the present application is 0.20% or more, preferably 0.40% or more.

On the other hand, when the Mn content exceeds 1.00%, the addition effect is saturated, and the time for completing the transformation at the time of the isothermal transformation treatment of the wire rod becomes long. The longer the transformation completion time, the less the area ratio of the pearlite structure (35 × [ C% ] +65) in the surface layer portion of the wire rod, and the hydrogen embrittlement resistance and cold workability may deteriorate. Further, saturation of the addition effect leads to an increase in manufacturing cost. Therefore, the Mn content in the chemical composition in the present application is 1.00% or less, preferably 0.80% or less.

·Al：0.005～0.050％

Al is a deoxidizing element and is an element that forms AlN functioning as a needle-punched particle. AlN causes grain refinement, thereby improving cold workability. Further, Al is an element having an action of reducing solid-solution N to suppress dynamic strain aging and an action of improving hydrogen embrittlement resistance.

If the Al content is less than 0.005%, the above-described effects cannot be obtained. Therefore, the Al content in the chemical composition in the present application is 0.005% or more, preferably 0.020% or more.

When the Al content exceeds 0.050%, the above effects are saturated, and defects are likely to occur during hot rolling. Therefore, the Al content in the chemical composition in the present application is 0.050% or less, and preferably 0.040% or less.

·P：0～0.030％

P is an element that segregates in grain boundaries to deteriorate hydrogen embrittlement resistance and cold workability.

When the P content exceeds 0.030%, deterioration of hydrogen embrittlement resistance and deterioration of cold workability become remarkable. Therefore, the P content in the chemical composition in the present application is 0.030% or less, preferably 0.015% or less.

Since the steel wire of the present application does not need to contain P, the lower limit of the P content is 0%. However, from the viewpoint of reducing the production cost (dephosphorization cost), the P content may be more than 0%, or 0.002% or more, or 0.005% or more.

·S：0～0.030％

S is an element that segregates in grain boundaries to deteriorate hydrogen embrittlement resistance and cold workability, similarly to P.

When the S content exceeds 0.030%, deterioration of hydrogen embrittlement resistance and deterioration of cold workability become remarkable. Therefore, the S content is 0.030% or less, preferably 0.015% or less, and more preferably 0.010% or less.

Since the steel wire of the present application does not need to contain S, the lower limit of the S content is 0%. However, from the viewpoint of reducing the production cost (desulfurization cost), the S content may be more than 0%, or 0.002% or more, or 0.005% or more.

·N：0～0.0050％

N is an element that may deteriorate cold workability and hydrogen embrittlement resistance due to dynamic strain aging. In order to avoid such adverse effects, the chemical composition in the present application is set to have an N content of 0.0050% or less. The N content is preferably 0.0040% or less.

The lower limit of the N content is 0%. However, from the viewpoint of reducing the production cost (denitrification cost), the N content may be more than 0%, or 0.0010% or more, or 0.0020% or more, or 0.0030% or more.

·Cr：0～1.00％

Cr is an optional element. That is, the lower limit of the Cr content in the chemical composition in the present application is 0%.

Cr is an element that increases the tensile strength of the steel after pearlite transformation. From the viewpoint of obtaining this effect, the Cr content is preferably more than 0%, more preferably 0.01% or more, further preferably 0.03% or more, further preferably 0.05% or more, and particularly preferably 0.10% or more.

On the other hand, if the Cr content exceeds 1.00%, martensite is likely to be generated, thereby deteriorating cold workability. Therefore, the Cr content in the chemical composition in the present application is 1.00% or less, preferably 0.70% or less, and more preferably 0.50% or less.

·Ti：0～0.050％

Ti is an optional element. That is, the lower limit of the Ti content in the chemical composition in the present application is 0%.

Ti is a deoxidizing element, and is an element that forms TiN, and has an effect of reducing solid-solution N to suppress dynamic strain aging and an effect of improving hydrogen embrittlement resistance. From the viewpoint of obtaining these effects, the Ti content is preferably more than 0%, more preferably 0.005% or more, and further preferably 0.015% or more.

On the other hand, when the Ti content exceeds 0.050%, the above-described effects are saturated, and defects are likely to be generated during hot rolling. Therefore, the Ti content in the chemical composition in the present application is 0.050% or less, preferably 0.035% or less.

·Nb：0～0.050％

Nb is an optional element. That is, the lower limit of the Nb content in the chemical composition in the present application is 0%.

Nb is an element that forms NbN and has an effect of reducing solid-solution N to suppress dynamic strain aging and an effect of improving hydrogen embrittlement resistance. From the viewpoint of obtaining these effects, the Nb content is preferably more than 0%, more preferably 0.005% or more, and further preferably 0.015% or more.

On the other hand, when the Nb content exceeds 0.05%, the above-described effects are saturated, and defects are likely to be generated during hot rolling. Therefore, the Nb content in the chemical composition in the present application is 0.050% or less, preferably 0.035% or less.

·V：0～0.10％

V is an optional element. That is, the lower limit of the V content in the chemical composition in the present application is 0%.

V is an element which forms VN and has an effect of reducing solid-solution N to suppress dynamic strain aging and an effect of improving hydrogen embrittlement resistance. From the viewpoint of obtaining these effects, the V content is preferably more than 0%, more preferably 0.02% or more.

On the other hand, when the V content exceeds 0.10%, the above-described effects are saturated, and defects are likely to be generated during hot rolling. Therefore, the V content in the chemical composition in the present application is 0.10% or less, preferably 0.05% or less.

·B：0～0.0050％

B is an optional element. That is, the lower limit of the B content in the chemical composition in the present application is 0%.

B suppresses grain boundary ferrite and grain boundary bainite, and has the effect of improving cold workability and hydrogen embrittlement resistance, and the effect of improving the tensile strength after pearlite transformation. From the viewpoint of obtaining these effects, the B content is preferably more than 0%, and more preferably 0.0003% or more.

On the other hand, if the B content exceeds 0.0050%, the above-described effects are saturated. Therefore, the content of B in the chemical composition in the present application is 0.0050% or less.

From the viewpoint of obtaining the respective effects of the above-described optional elements, the chemical composition in the present application may further contain 1 or 2 or more of the following elements in mass%: cr: more than 0% and 1.00% or less, Ti: more than 0% and 0.050% or less, Nb: more than 0% and 0.050% or less, V: more than 0% and 0.10% or less and B: more than 0% and not more than 0.0050%.

·O：0～0.0030％

O is present in the steel wire in the form of oxides of Al, Ti, etc.

When the O content exceeds 0.0030%, coarse oxides are generated in the steel, and fatigue fracture is likely to occur. Therefore, the O content in the chemical composition in the present application is 0.0030% or less, preferably 0.0020% or less.

Since the steel wire of the present application does not need to contain O, the lower limit of the O content is 0%. However, from the viewpoint of reducing the production cost (deoxidation cost), the O content may be more than 0%, or may be 0.0002% or more, or may be 0.0005% or more.

The remainder: fe and impurities

In the chemical composition in the present application, the balance other than the above-described elements is Fe and impurities.

Here, the impurities refer to components contained in the raw material or components mixed in the manufacturing process, and are not components intentionally contained in the steel.

Examples of the impurities include all elements other than the above-mentioned elements. The impurity element may be 1 kind or 2 or more kinds.

< Metal Structure >

Next, the metal structure of the steel wire of the present application will be described.

(pearlite area ratio)

When the mass% of C is set to [ C% ], the microstructure of the steel wire of the present application is composed of pearlite having an area ratio of (35 × [ C% ] + 65%) or more and the remainder which is at least one of proeutectoid ferrite and bainite.

Thereby, cold workability and hydrogen embrittlement resistance are improved.

When the area percentage of pearlite in the metal structure of the steel wire is less than (35 × [ C% ] + 65)%, the strength (tensile strength, hardness, and the like) of the steel wire becomes uneven, and therefore, cracks are likely to occur (that is, cold workability is decreased) when cold working is performed on a non-heat treated machine component.

In addition, when the area percentage of pearlite in the metallic structure of the steel wire is less than (35 × [ C% ] + 65%), the area percentage of pearlite in the metallic structure is also less than (35 × [ C% ] + 65%) in a non-heat treated machine component obtained by cold working the steel wire. As a result, the hydrogen embrittlement resistance of the non-heat-treated machine part deteriorates.

From the viewpoint of further improving cold workability and hydrogen embrittlement resistance, the area fraction of pearlite is preferably (35 × [ C% ] + 70)% or more, and more preferably (35 × [ C% ] + 75)% or more.

From the viewpoint of manufacturing suitability, the area fraction of pearlite is preferably 99% or less, more preferably 97% or less, and still more preferably 95% or less.

In the microstructure of the steel wire of the present application, a specific preferable range of the area ratio of pearlite is preferably 80 to 99%, more preferably 83 to 97%, and particularly preferably 85 to 95%, although it depends on [ C% ].

The remainder of the microstructure of the steel wire of the present application is at least one of proeutectoid ferrite and bainite.

When the remainder contains martensite, cold workability and hydrogen embrittlement resistance in the case of producing a non-heat-treated mechanical part are degraded.

In the present specification, the pearlite area ratio (%) is a value obtained by the following procedure.

First, the C-section of the steel wire was etched with bitter alcohol to develop a metal structure.

Then, 4 observation positions were selected at intervals of 90 ° in the circumferential direction from positions having a depth of 50 μm (i.e., circumferential positions) on the etched C section, and SEM photographs were taken at 1000-fold magnification for each observation position using FE-SEM (Field Emission-scanning electron Microscope).

Similarly, 4 observation positions were selected at 90 ° intervals in the circumferential direction from positions (i.e., circumferential positions) having a depth of 0.25D in the C-section after etching, and SEM photographs having a magnification of 1000 times were taken at each observation position using FE-SEM.

In the 8 SEM photographs obtained, structures other than pearlite (pro-eutectoid ferrite, bainite, and the like) were visually marked, and the area ratio (%) of the structures other than pearlite with respect to the entire metal structure was determined by image analysis. The area percentage (%) of pearlite can be obtained by subtracting the obtained area percentage (%) of the structure other than pearlite from 100%.

(AR)

The steel wire of the present invention has an AR (i.e., an average aspect ratio of the pearlite block measured at a position having a depth of 50 μm in an L-section) of 1.4 or more. Whereby the hydrogen embrittlement characteristics can be improved. The reason for this is considered as follows. As described above, the pearlite structure has a laminated structure of the cementite layer and the ferrite layer, and the direction of the lamellar structure of the pearlite structure of the elongated pearlite blocks (i.e., pearlite blocks having an AR of 1.4 or more) in the surface layer becomes more uniform. It is believed that: the homogenized layered structure becomes a resistance to the intrusion of hydrogen from the surface of the steel wire and/or a resistance to the development of cracks.

When the AR of the steel wire is less than 1.4, the AR of the non-heat-treated machine part obtained by cold working the steel wire also becomes less than 1.4. In this case, since it is difficult to obtain the above-described effects (the effect of becoming resistance to hydrogen intrusion and/or the effect of becoming resistance to the development of cracks), the hydrogen embrittlement resistance of the non-heat-treated mechanical part is not improved.

From the viewpoint of further improving the hydrogen embrittlement resistance, AR is preferably 1.5 or more, and more preferably 1.6 or more.

From the viewpoint of the manufacturing suitability of the steel wire, AR is preferably 2.5 or less, and more preferably 2.0 or less.

In the present specification, a pearlite block means: from a crystal orientation diagram of ferrite obtained by an EBSD (electron back scattering diffraction) method, pearlite units having uniform ferrite orientation with a misorientation of 15 degrees or less are observed. That is, the boundaries where the above-mentioned misorientation is 15 ° or more are the block grain boundaries of the pearlite block.

In the present specification, AR means a value measured by the following procedure.

First, 4 observation positions were selected at intervals of 2.0mm from a straight line indicating a position having a depth of 50 μm in an L-section of the steel wire, and a crystal orientation pattern of ferrite in a region having a depth direction of 50 μm and an axial direction of 250 μm centered around each observation position was obtained using an EBSD apparatus.

In all the 4 crystal orientation diagrams obtained, 10 pearlite blocks were selected in order from the pearlite block having the largest equivalent circle diameter in the group of pearlite blocks crossed by the straight line indicating the position having a depth of 50 μm.

Then, the aspect ratio of each of the selected 10 pearl essence blocks was determined, and the average of the aspect ratios (i.e., 10 values) of the 10 pearl essence blocks was set to AR (i.e., the average aspect ratio of the pearl essence blocks measured at a position having a depth of 50 μm in the L-cross section).

In the present specification, the aspect ratio of a pearlite block is a value obtained by dividing the long diameter of the pearlite block by the short diameter (i.e., long diameter/short diameter). Here, the long diameter of the pearlite block means the maximum length of the pearlite block, and the short diameter of the pearlite block means the maximum length in the direction perpendicular to the long diameter direction.

Fig. 1 is a conceptual diagram showing an example of a pearlite block on an L-section of a steel wire according to an example of the present application.

In fig. 1, not only grain boundaries of pearlite blocks but also a Major diameter (Major axis) and a Minor diameter (Minor axis) of the pearlite blocks are shown.

The shape of the pearlite block may be a polygonal shape as shown in fig. 1, an elliptical shape, or a shape other than a polygonal shape and an elliptical shape (for example, an indefinite shape).

In short, the shape of the pearlite block is not particularly limited as long as the AR is 1.4 or more.

(ratio of aspect ratio [ skin/0.25D ]

The aspect ratio of the steel wire of the present application [ surface layer/0.25D ] (i.e., (AR)/(average aspect ratio of pearlite block measured at a position having a depth of 0.25D in an L-section)) is 1.1 or more.

When the aspect ratio [ surface layer/0.25D ] of the steel wire of the present application is 1.1 or more, the hydrogen embrittlement resistance is improved as described above. The reason is considered to be that: the direction of the lamellar structure of the pearlite structure in the pearlite block with the surface layer extended is more uniformed, and the lamellar structure becomes resistance to hydrogen intrusion from the surface of the steel wire and/or resistance to the development of cracks.

Further, since the ratio of the aspect ratio [ surface layer/0.25D ] of the steel wire of the present application is 1.1 or more, strain is concentrated on the surface layer of the steel wire, and therefore, the hydrogen embrittlement resistance can be effectively improved.

If the ratio of the aspect ratio [ surface layer/0.25D ] is less than 1.1, it is necessary to increase not only the strain in the surface layer of the steel wire but also the strain in the inside of the steel wire, and therefore there is a possibility that the hydrogen embrittlement resistance cannot be effectively improved and the productivity of the steel wire may be deteriorated.

From the viewpoint of improving the hydrogen embrittlement resistance, the ratio of the aspect ratio [ surface layer/0.25D ] is preferably 1.2 or more.

From the viewpoint of the suitability for steel wire production, the ratio of aspect ratios [ skin layer/0.25D ] is preferably 2.0 or less, more preferably 1.8 or less, and particularly preferably 1.6 or less.

In the present specification, the average aspect ratio of the pearlite block measured at a position having a depth of 0.25D in the L-section is measured by the same method as the above-described AR measurement method except that the observation position is changed from a position having a depth of 50 μm in the L-section to a position having a depth of 0.25D in the L-section.

(GD)

The steel wire of the present application has GD (i.e., the average block particle size of a pearlite block measured at a position having a depth of 50 μm in a C-section) of not more than (15/AR) μm. When the pearlite block is fine (that is, GD is (15/AR) μm or less), cold workability and hydrogen embrittlement resistance are improved as described above.

The reason for this is considered as follows. When the pearlite block in the surface layer of the steel wire is coarsened (that is, when the average block particle size of the pearlite block exceeds (15/AR) μm), the ductility of the steel wire is lowered, and the cold workability of the steel wire is lowered. Further, the block particle size of the pearlite block in the surface layer of the machine part obtained by cold working the steel wire is coarsened. Hydrogen tends to segregate in pearlite block grain boundaries. When the pearlite block in the surface layer of the steel wire is coarsened, the total area of the block grain boundaries of the pearlite block is reduced, and therefore the hydrogen trapping ability of the surface layer (i.e., the ability to prevent hydrogen from penetrating into the wire rod) is reduced. Thus, it is believed that: when the pearlite block in the surface layer is coarsened, the hydrogen embrittlement resistance is lowered.

From the viewpoint of further improving cold workability and hydrogen embrittlement resistance, GD is preferably 11.0 μm or less, more preferably 10.0 μm or less.

From the viewpoint of the manufacturing suitability of the steel wire, GD is preferably 7.0 μm or more, and more preferably 8.0 μm or more.

In the present specification, GD means a value measured according to the following procedure.

First, 8 observation positions were selected at 45 ° intervals along the circumferential direction on the circumference indicating the position of 50 μm depth on the C-section of the steel wire, and a crystal orientation pattern of ferrite in a region of 50 μm × 50 μm centered around each observation position was obtained using an EBSD apparatus.

All of the obtained 8 crystal orientation patterns were measured for the equivalent circle diameter of all of the pearlite blocks. The average value of the obtained measurement values was set to GD (i.e., the average block particle diameter of the pearlite block measured at a position having a depth of 50 μm on the C section).

(ratio of particle diameters [ skin layer/0.25D ]

The ratio of the grain size of the steel wire of the present application [ surface layer/0.25D ] (i.e., (GD)/(average block grain size of pearlite block measured at a position having a depth of 0.25D in a C section)) is less than 1.0.

The steel wire of the present invention has improved cold workability and hydrogen embrittlement resistance when the ratio [ GD/0.25D ] of the grain size is less than 1.0.

From the viewpoint of further improving cold workability and hydrogen embrittlement resistance, the ratio [ GD/0.25D ] of the particle diameter is preferably 0.98 or less, more preferably 0.96 or less, and particularly preferably 0.94 or less.

From the viewpoint of the manufacturing suitability of the steel wire, the ratio [ GD/0.25D ] of the particle diameter is preferably 0.80 or more, more preferably 0.85 or more, and particularly preferably 0.90 or more.

In the present specification, the average block particle size of the pearlite block measured at a position having a depth of 0.25D in the C section is measured by the same method as the above-mentioned GD measurement method except that the observation position is changed from a position having a depth of 50 μm in the C section to a position having a depth of 0.25D in the C section.

The Tensile Strength (TS) of the steel wire is 900-1500 MPa.

The TS of the steel wire of the present application is 900MPa or more, and therefore, a non-heat-treated machine component having a TS of 1100MPa or more can be easily manufactured by cold working the steel wire.

In addition, in the conventional steel wire, if the TS of the steel wire is 900MPa or more, the cold workability tends to be lowered.

However, the steel wire of the present application has a TS of 900MPa or more and is excellent in cold workability by having the above-described chemical composition and metal structure.

Further, the TS of the steel wire of the present application is 1500MPa or less, so that the steel wire is excellent in the manufacturing suitability and cold workability.

In the present specification, the Tensile Strength (TS) of the steel wire and the Tensile Strength (TS) of the non-heat-treated machine component are both values measured by the test method described in JIS Z2201 (2011) using a 9A test piece according to JIS Z2201 (2011).

From the viewpoint of further improving the manufacturing suitability and cold workability of the steel wire, the TS of the steel wire of the present application is preferably 900 to 1300MPa, and more preferably 900 to 1200 MPa.

In the steel wire of the present application, D (i.e., the diameter of the steel wire) is preferably 3 to 30mm, more preferably 5 to 25mm, and particularly preferably 5 to 20 mm.

From the viewpoint of cold workability, the steel wire of the present application preferably has a limiting compressibility of 75% or more. The method of measuring the ultimate compressibility is shown in examples described later.

As an example of a method for producing the steel wire of the present application, the following production method a can be cited.

The preparation method A comprises the following steps:

a step of obtaining a wire rod by heating a billet having a chemical composition of the present application to 1000 to 1150 ℃ and setting a finish rolling temperature to 800 to 950 ℃ to perform hot rolling;

a step of performing a constant temperature phase change treatment by immersing the wire rod having a temperature of 800 to 950 ℃ in a molten salt bath at 400 to 550 ℃ for 50 seconds or longer;

a step of cooling the wire rod subjected to the constant temperature phase change treatment with water to a temperature of 300 ℃ or lower; and

and a step of obtaining a steel wire by drawing the water-cooled wire rod so that the total reduction of area is 15 to 25%.

The chemical composition of the steel wire (target) obtained by the production method a can be considered to be the same as the chemical composition of the billet (raw material) in the production method a. The reason is that: the hot rolling, the constant temperature transformation treatment, the water cooling and the wire drawing do not affect the chemical composition of the steel.

The steel wire of the present application in which the area ratio of pearlite and the remaining part satisfy the above conditions can be easily produced by the production method a including the step of the constant temperature transformation treatment and the step of the water cooling.

For example, in the step of the constant temperature transformation treatment, the wire rod is immersed in a molten salt bath for 50 seconds or more, so that the area fraction of pearlite and the remaining part easily satisfy the above conditions.

The upper limit of the impregnation time is not particularly limited. From the viewpoint of productivity of the steel wire, the dipping time is preferably 100 seconds or less, and more preferably 80 seconds or less.

In addition, a steel material having a tensile strength of 900MPa or more can be easily produced by setting the total reduction of area to 15% or more in the step of obtaining the steel material (i.e., the step including wire drawing; hereinafter also referred to as "wire drawing step").

Further, by setting the total reduction of area to 25% or less in the drawing step, it is easy to manufacture a steel material having an AR of 1.4 or more and an aspect ratio [ surface layer/0.25D ] of 1.1 or more (i.e., a steel material in which the pearlite blocks in the surface layer of the steel material are extended as compared with the pearlite blocks in the interior of the steel material).

The drawing step may include only one drawing step, or may include a plurality of drawing steps.

That is, the total reduction of area of 15 to 25% in the drawing process can be realized by one-time drawing or by multiple times of drawing.

When the drawing step includes only one drawing, it is preferable to use a drawing die having a half cone angle of over 10 ° as the drawing die used for the drawing. Thus, a steel material having an aspect ratio [ surface layer/0.25D ] of 1.1 or more can be easily produced.

When the drawing step includes a plurality of times of drawing, it is preferable to perform the plurality of times of drawing under the condition that the reduction of area in the final pass is 10% or less. Thus, a steel material having an aspect ratio [ surface layer/0.25D ] of 1.1 or more can be easily produced.

When the drawing step includes a plurality of times of drawing, the reduction of area in the final pass is more preferably 5 to 10%, still more preferably 5 to 9%, and particularly preferably 5 to 8%.

The steel wire is particularly suitable for manufacturing a non-heat-treated machine component having a cylindrical shaft portion with a tensile strength of 1100-1500 MPa.

That is, the steel wire of the present invention is cold worked (and preferably kept at 100 to 400 ℃ after cold working), thereby facilitating the production of a non-heat-treated machine component including a cylindrical shaft portion having a tensile strength of 1100 to 1500 MPa.

Here, the chemical composition of the non-heat-treated machine component obtained by cold working the steel wire of the present application (and preferably, maintaining the temperature at 100 to 400 ℃ after cold working) can be considered to be the same as the chemical composition of the steel wire of the present application. The reason is that: cold working and heat treatment do not affect the chemical composition of the steel.

The microstructure of a non-heat-treated machine part obtained by cold working (and, if necessary, heat treatment at 100 to 400 ℃ after cold working) the steel wire of the present application can be considered to be the same as the microstructure of the steel wire of the present application. The reason is that: the amount of cold working for obtaining a non-heat-treated machine component having a cylindrical shaft portion is very small.

[ non-quenched and tempered mechanical parts ]

Embodiments 1 and 2 of the non-heat-treated machine component (hereinafter also simply referred to as "machine component") according to the present application will be described below.

The mechanical component of embodiment 1 of the present application includes a cylindrical shaft portion,

the chemical composition is the chemical composition in the present application described above,

when the mass% of C is [ C% ], the microstructure is composed of pearlite having an area ratio of (35 × [ C% ] + 65%) or more and the remainder which is at least one of proeutectoid ferrite and bainite,

when a cross section parallel to the axial direction of the cylindrical shaft portion and including the central axis is set to an L cross section, a cross section perpendicular to the axial direction of the cylindrical shaft portion is set to a C cross section, the diameter of the cylindrical shaft portion is set to D, and the average aspect ratio of the pearlite block measured at a position at a distance of 50 [ mu ] m from the surface of the cylindrical shaft portion in the L cross section is set to AR, and the average block particle diameter of the pearlite block measured at a position at a distance of 50 [ mu ] m from the surface of the cylindrical shaft portion in the C cross section is set to GD, AR is 1.4 or more, (AR)/(average aspect ratio of the pearlite block measured at a position at a depth of 0.25D from the surface of the cylindrical shaft portion in the L cross section) is 1.1 or more, GD is (15/AR) [ mu ] m or less, (GD)/(average aspect ratio of the pearlite block measured at a position at a depth of 0.25D from the surface of the cylindrical shaft portion in the C cross section is set to GD Block particle size) of less than 1.0,

the Tensile Strength (TS) of the cylindrical shaft portion is 1100-1500 MPa.

The chemical composition and the metallic structure of the columnar shaft portion (i.e., the ratio of the pearlite area ratio, AR, the aspect ratio [ skin/0.25D ], the ratio of GD and the average block particle diameter [ skin/0.25D ], the same as below) in the machine component of embodiment 1 are the same as those in the steel wire of the present application, respectively.

Therefore, the mechanical component of embodiment 1 is excellent in hydrogen embrittlement resistance.

The machine component according to embodiment 1 can be manufactured using a steel wire having excellent cold workability (for example, the steel wire of the present application).

The chemical composition and the preferred form of the metal structure of the cylindrical shaft portion in the machine component according to embodiment 1 are the same as those in the steel wire of the present application.

The machine component according to embodiment 2 of the present application is a cold-worked product of the steel wire of the present application (i.e., a machine component obtained by cold-working the steel wire of the present application), and the tensile strength of the cylindrical shaft portion is 1100 to 1500 MPa.

Therefore, the mechanical component of embodiment 2 is excellent in hydrogen embrittlement resistance.

The chemical composition and the preferred form of the metal structure of the cylindrical shaft portion in the machine component according to embodiment 2 are the same as those in the steel wire of the present application.

In the machine component of the present application, embodiment 1 and embodiment 2 may have overlapping portions.

That is, not only the machine component according to any one of embodiment 1 and embodiment 2 is included in the scope of the machine component of the present application, but also the machine component according to both embodiment 1 and embodiment 2 is naturally included in the scope of the machine component of the present application.

The machine component of the present application is not particularly limited as long as it is a non-heat-treated machine component including a cylindrical shaft portion, but among them, a non-heat-treated bolt is particularly preferable.

As an example of a method for manufacturing a machine component of the present application, the following manufacturing method X may be mentioned.

The manufacturing method X includes a step of obtaining a machine component by cold working the steel wire of the present application.

The production method X preferably includes a step of maintaining the machine component obtained by cold working at a temperature in the range of 100 to 400 ℃ (hereinafter also referred to as "maintaining step").

By including the holding step, a machine component having a tensile strength of 1100 to 1500MPa can be more easily manufactured.

The holding temperature in the holding step is 100 to 400 ℃, preferably 200 to 400 ℃, and more preferably 300 to 400 ℃.

The holding time in the holding step (i.e., the time for holding the mechanical component within the above temperature range) is preferably 10 to 120 minutes, and more preferably 10 to 60 minutes.

The steel wire for non heat-treated machine parts and the non heat-treated machine parts of the present application described above can be used in various machines such as automobiles and buildings.

Examples

Examples of the present application are shown below, but the present application is not limited to the following examples.

[ Condition (condition)1 ~ 28 ]

< production of Steel wire >

Steel wires having diameters (D) shown in table 3 were produced using billets (billets) having chemical compositions shown in table 1.

In the chemical compositions of the respective steel types in table 1, the balance other than the elements shown in table 1 is Fe and impurities.

The slabs were subjected to hot rolling, constant temperature transformation treatment, water cooling and wire drawing under the conditions shown in Table 2 at levels 1 to 6, 8 to 9, 11 to 13, 15 to 24 and 27 to 28 in this order to obtain steel wires having diameters (D) shown in Table 3.

At levels 14, 25 and 26, steel wires having diameters (D) shown in table 3 were obtained by subjecting a slab to hot rolling under the conditions shown in table 2, then to air cooling, reheating at a heating temperature of 950 ℃, patenting and natural cooling under a patenting temperature of 580 ℃, and then to wire drawing under the conditions shown in table 2.

At levels 7 and 10, steel wires having diameters (D) shown in table 3 were obtained by subjecting the slabs to hot rolling under the conditions shown in table 2, air cooling, and wire drawing under the conditions shown in table 2.

< measurement of Steel wire >

The steel wires of each level were each processed by the above-described method: the area ratio of pearlite was measured, the remaining portions were confirmed, AR (i.e., the average aspect ratio of the pearlite block at a position having a depth of 50 μm in the L-section) was measured, the aspect ratio [ surface layer/0.25D ] (i.e., (AR)/(average aspect ratio of the pearlite block measured at a position having a depth of 0.25D in the L-section)) was measured, GD (i.e., the average block particle size of the pearlite block at a position having a depth of 50 μm in the C-section) was measured, the particle size ratio [ surface layer/0.25D ] (i.e., (GD)/(average block particle size of the pearlite block measured at a position having a depth of 0.25D in the C-section)) was measured, and the Tensile Strength (TS) was measured.

The measurement results are shown in table 3.

< Cold workability of Steel wire (measurement of Limit compressibility) >

The cold workability of the steel wire of each level was evaluated by measuring the following ultimate reduction ratio.

First, a steel wire was machined to produce a sample having a diameter D (i.e., the diameter of the steel wire) and a length of 1.5 × D.

Both end faces of the obtained sample were constrained using a pair of molds. As the pair of dies, dies each having concentric grooves on a contact surface with the sample end surface are used. In this state, the sample is compressed in the longitudinal direction. The compression ratio of the sample during the compression was variously changed and a test was performed to obtain the maximum compression ratio at which no crack of the sample occurred.

The maximum compressibility at which no cracks of the specimen were generated was set as the limit compressibility (%).

As a result, the cold workability was judged to be good when the ultimate reduction ratio was 70% or more (G), and poor when the ultimate reduction ratio was less than 70% (NG).

The above results are shown in table 3.

< production of machine component >

The steel wire of each level is cold worked (cold forging) to form a bolt with a flange. The processed steel wire was heated to 350 ℃ and held at that temperature for 30 minutes, thereby obtaining a non heat-treated bolt as a machine part.

< measurement of Tensile Strength (TS) of machine component >

The TS of the shaft portion of the obtained machine component (non-heat-treated bolt) was measured by the above-described measurement method.

The results are shown in Table 3.

< evaluation of Hydrogen embrittlement resistance of mechanical parts >

The hydrogen embrittlement resistance of the obtained machine part (non-heat-treated bolt) was measured by the following method.

First, the mechanical parts were charged with hydrogen by electrolysis to contain 0.5ppm of diffused hydrogen.

Then, in order to prevent hydrogen from being released into the air from the mechanical parts in the test, the sample was subjected to Cd plating.

Then, a load of 90% of the maximum tensile load of the machine member is applied to the machine member in the atmosphere, and the machine member is held in this state for 100 hours or more.

As a result, the case where the fracture did not occur at the time of 100 hours was judged as good hydrogen embrittlement resistance (G), and the case where the fracture occurred at the time of 100 hours was judged as poor hydrogen embrittlement resistance (NG).

The above results are shown in table 3.

TABLE 1

TABLE 2

TABLE 3

Description of Table 3

In the remaining part structure column, F and B refer to proeutectoid ferrite and bainite, respectively.

As shown in table 3, the steel wire of each level of the examples having the chemical composition in the present application, the pearlite area ratio of (35 × [ C% ] + 65%) or more, the remaining portion structure of at least one of pro-eutectoid ferrite (F) and bainite (B), AR of 1.4 or more, the ratio of aspect ratio [ surface layer/0.25D ] of 1.1 or more, GD of (15/AR) μm or less, the ratio of particle size [ GD/0.25D ] of less than 1.0, and TS of 900 to 1500MPa was a steel wire having a TS of 900MPa or more, and also excellent cold workability, and also excellent hydrogen embrittlement resistance when made into a machine part.

Further, machine parts having a TS of 1100MPa or more can be manufactured by cold working the steel wires of the respective levels of examples.

The steel wires of levels 7 and 10 (both comparative examples) having a pearlite area ratio lower than (35 × [ C% ] + 65)% are inferior to those of the examples in hydrogen embrittlement resistance characteristics in the case of machine parts.

Further, the steel wires of levels 3, 5, 12 and 27 (all comparative examples) having an AR of less than 1.4 have poor hydrogen embrittlement resistance characteristics when they are formed into mechanical parts.

Further, the steel wires of levels 9, 21, 22 and 27 (all comparative examples) having an aspect ratio [ skin/0.25D ] of less than 1.1 were inferior in hydrogen embrittlement resistance characteristics in finished mechanical parts.

Further, the steel wires of levels 14 and 25 (both comparative examples) in which GD exceeded (15/AR) μm had poor cold workability.

Further, the steel wires of levels 14 and 26 (both comparative examples) in which the grain size ratio [ GD/0.25D ] was 1.0 or more were inferior in cold workability.

Further, the steel wires of levels 23 and 24 (both comparative examples) having TS lower than 900MPa could not produce machine parts having TS of 1100MPa or more.

Steel wires with poor cold workability (limit compressibility of less than 70%) have a high frequency of occurrence of processing cracks when manufacturing machine parts. Further, mechanical parts manufactured using steel wires with poor cold workability (limit compressibility lower than 70%) have poor dimensional accuracy.

The disclosure of Japanese patent application 2016-008708 is incorporated in its entirety by reference into the present specification.

All documents, patent applications, and technical standards described in the present specification are incorporated by reference into the present specification to the same extent as if each document, patent application, and technical standard was specifically and individually described to be "incorporated by reference".

Claims

1. A steel wire for non-heat-treated machine parts, which has a chemical composition comprising, in mass%:

C：0.40～0.65％、

Si：0.05～0.50％、

Mn：0.20～1.00％、

Al：0.005～0.050％、

P：0～0.030％、

S：0～0.030％、

N：0～0.0050％、

Cr：0～1.00％、

Ti：0～0.050％、

Nb：0～0.050％、

V：0～0.10％、

B：0～0.0050％、

o: 0 to 0.0030%, and

the tensile strength is 900-1500 MPa.

2. The steel wire for non-heat-treated machine parts according to claim 1, which contains 1 or 2 or more of the following elements in mass%:

cr: more than 0% and not more than 1.00%,

Ti: more than 0% and not more than 0.050%,

Nb: more than 0% and not more than 0.050%,

V: more than 0% and not more than 0.10%, and

b: more than 0% and not more than 0.0050%.

3. The steel wire for non-heat-treated machine parts according to claim 1 or 2, wherein D is 3 to 30 mm.

4. A non-thermal mechanical component comprising a cylindrical shaft portion,

the chemical composition thereof comprises by mass percent:

C：0.40～0.65％、

Si：0.05～0.50％、

Mn：0.20～1.00％、

Al：0.005～0.050％、

P：0～0.030％、

S：0～0.030％、

N：0～0.0050％、

Cr：0～1.00％、

Ti：0～0.050％、

Nb：0～0.050％、

V：0～0.10％、

B：0～0.0050％、

o: 0 to 0.0030%, and

the tensile strength of the cylindrical shaft part is 1100-1500 MPa.

5. The non-heat-treated mechanical part according to claim 4, comprising 1 or 2 or more of the following elements in mass%:

cr: more than 0% and not more than 1.00%,

Ti: more than 0% and not more than 0.050%,

Nb: more than 0% and not more than 0.050%,

V: more than 0% and not more than 0.10%, and

b: more than 0% and not more than 0.0050%.

6. A cold-worked product of the steel wire for non-heat-treated machine parts according to any one of claims 1 to 3, comprising a columnar shaft portion having a tensile strength of 1100 to 1500 MPa.

7. The non heat treated machine part according to any one of claims 4 to 6, which is a non heat treated bolt.