KR102037089B1 - Steel wire and non-machined parts for non-machined parts - Google Patents

Abstract

In 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%, the balance comprising Fe and impurities, and the metal structure (35 The average aspect ratio of the pearlite block at a depth of 50 μm in the D and L cross sections, and the diameter of the pearlite block at a depth of 50 μm in the AR and C cross sections. When the average block diameter of a pearlite block is GD, AR is 1.4 or more, (AR) / (average aspect ratio of the pearlite block at a depth of 0.25D in the L cross section) is 1.1 or more, and GD is (15 / AR) micrometer or less and (GD) / (steel block for non-machined machine parts which satisfy | fills the average block particle diameter of the pearlite block in the depth 0.25D position in C cross section) less than 1.0.

Description

Steel wire and non-machined parts for non-machined parts

The present disclosure relates to steel wire and non-machined machine parts for non-machined machine parts.

In recent years, in the field of various machines, such as automobiles, construction, etc., the demand for high-strength machine parts is increasing from the viewpoint of light weight or space saving.

However, as the strength of high-strength mechanical parts increases, breakage due to hydrogen embrittlement tends to occur easily, particularly when the tensile strength of high-strength mechanical parts is 1100 MPa or more. .

As a method for improving the hydrogen embrittlement resistance of high-strength mechanical parts, a method is known in which a structure is made of a pearlite structure and the structure is strengthened by fresh processing, and many proposals have been made (for example, Patent Documents 1 to 1). 11).

For example, Patent Literature 11 discloses a high-strength bolt having a tensile strength of 1200 MPa or more, in which a structure is used as a pearlite structure, followed by drawing.

In addition, Patent Document 3 discloses a wire rod of a pearlite structure for high strength bolts having a tensile strength of 1200 MPa or more.

Japanese Patent Laid-Open No. 54-101743 Japanese Patent Laid-Open No. 11-315348 Japanese Patent Publication No. 11-315349 Japanese Patent Laid-Open No. 2000-144306 Japanese Patent Laid-Open No. 2000-337332 Japanese Patent Laid-Open No. 2001-348618 Japanese Patent Laid-Open No. 2002-069579 Japanese Patent Laid-Open No. 2003-193183 Japanese Patent Laid-Open No. 2004-307929 Japanese Patent Laid-Open No. 2005-281860 Japanese Patent Laid-Open No. 2008-261027

As a method of manufacturing high-strength mechanical parts (for example, high-strength bolts) having a tensile strength of 1100 MPa or more, for example, after forming steel wires of alloy steel to which alloy elements such as Cr, Mo, and V are added to a predetermined shape,? There is a method of manufacturing mechanical parts by performing tempering. On the other hand, in order to reduce manufacturing cost, the technique which gives predetermined strength is known by omitting the quenching tempering after shaping | molding, and performing wire drawing to the wire which raised the strength by rapid cooling, precipitation strengthening, etc. Mechanical parts (e.g. bolts) manufactured using this technique are called non-coarse machine parts (e.g. non-coarse bolts).

An unmachined mechanical part having a tensile strength of 1100 MPa or more can be manufactured by cold working a steel wire having a tensile strength of 900 MPa or more.

For example, in a non-machined machine part (for example, a non-coarse bolt) in which a pearlite structure is freshly hardened and reinforced, the pearlite structure captures hydrogen at the interface between cementite and ferrite, thereby preventing the intrusion of hydrogen into the steel material. It is thought that the hydrogen embrittlement characteristic is improved. Even in non-machined machine parts (for example, non-machined bolts) having a tensile strength of 1100 MPa or more, the hydrogen embrittlement resistance is improved to some extent by the technique of freshly processing a pearlite structure. However, it is not easy to sufficiently improve hydrogen embrittlement characteristics only by this technique, and further improvement is desired.

Moreover, in these conventional techniques, when the steel wire strength for obtaining high strength mechanical parts increases by cold working, especially when the tensile strength of the steel wire is 900 MPa or more, when the steel wire is cold worked to obtain high strength mechanical parts, Cold workability may fall. Therefore, it is not easy to improve both hydrogen embrittlement resistance and cold workability.

Due to the above-mentioned circumstances, in the steel wire of 900 Mpa or more of tensile strength for obtaining high strength mechanical parts of 1100 Mpa or more of tensile strength, the cold workability at the time of manufacturing non-manufactured mechanical parts by cold working, and the case where it is set as a non-manufactured mechanical part It may be difficult to make hydrogen embrittlement compatible.

Therefore, the subject of this indication is a steel wire of 900 Mpa or more of tensile strength, being excellent in cold workability at the time of manufacturing a non-machined machine part by cold working, and excellent in the embrittlement resistance of hydrogen in the case of using a non-machined machine part. To provide steel wire for quality machine parts.

Moreover, the subject of this indication is providing the non-machined machine part which can be manufactured using the steel wire excellent in cold workability, and is excellent in tensile strength and hydrogen embrittlement resistance.

Means for solving the above problems include the following aspects.

<1> chemical composition is the mass%,

C: 0.40 to 0.65%,

Si: 0.05 to 0.50%,

Mn: 0.20 to 1.00%,

Al: 0.005 to 0.050%,

P: 0% to 0.030%,

S: 0% to 0.030%,

N: 0% to 0.0050%,

Cr: 0 to 1.00%,

Ti: 0 to 0.050%,

Nb: 0 to 0.050%,

V: 0 to 0.10%,

B: 0 to 0.0050%,

0: 0 to 0.0030% and

Balance: consisting of Fe and impurities,

When the metal structure makes mass% of C [C%], it consists of (35 * [C%] + 65)% or more of pearlite, and the remainder which is at least one of a cornerstone ferrite and bainite in an area ratio,

A cross section parallel to the axial direction of the steel wire and including the central axis is referred to as an L cross section, a cross section perpendicular to the axial direction of the steel wire is referred to as a C cross section, a diameter of the steel wire is referred to as D, and a depth of 50 from the steel wire surface in the L cross section. AR is 1.4 or more when the average aspect ratio of the pearlite block measured by the position of micrometer is AR, and the average block particle diameter of the pearlite block measured by the position of 50 micrometers depth from the steel wire surface in C cross section is GD. (AR) / (average aspect ratio of pearlite block measured at the position of depth 0.25D from the steel wire surface in L cross section) is 1.1 or more, GD is (15 / AR) 탆 or less, and (GD) / (Average block diameter of pearlite block measured at the position of depth 0.25D from the steel wire surface in C cross section) is less than 1.0,

Tensile strength is 900 to 1500 MPa

Steel wire for non-machined parts.

<2> mass%,

Cr: more than 0 and less than 1.00%,

Ti: more than 0 and 0.050% or less,

Nb: more than 0 and 0.050% or less,

V: greater than 0 and 0.10% or less and

B: The steel wire for non-machined machine parts as described in <1> containing 1 type (s) or more than 0 or more and 0.0050% or less types.

<3> The steel wire for non-machined machine parts according to <1> or <2>, in which D is 3 to 30 mm.

<4> including a cylindrical shaft portion,

The chemical composition is in mass%

C: 0.40 to 0.65%,

Si: 0.05 to 0.50%,

Mn: 0.20 to 1.00%,

Al: 0.005 to 0.050%,

P: 0% to 0.030%,

S: 0% to 0.030%,

N: 0% to 0.0050%,

Cr: 0 to 1.00%,

Ti: 0 to 0.050%,

Nb: 0 to 0.050%,

V: 0 to 0.10%,

B: 0 to 0.0050%,

0: 0 to 0.0030% and

Balance: consisting of Fe and impurities,

When the metal structure makes mass% of C [C%], it consists of (35 * [C%] + 65)% or more of pearlite, and the remainder which is at least one of a cornerstone ferrite and bainite in an area ratio,

A cross section parallel to the axial direction of the cylindrical shaft portion and including a central axis is referred to as an L cross section, a cross section perpendicular to the axial direction of the cylindrical shaft portion is referred to as a C cross section, and a diameter of the cylindrical shaft portion is referred to as D, L The average ratio of the pearlite blocks measured at the position of 50 µm deep from the surface of the cylindrical shaft portion in the cross section is called AR, and the pearlite blocks measured at the position of 50 µm depth from the surface of the cylindrical shaft portion in the C cross section. When the average block particle is GD, AR is 1.4 or more, and (AR) / (average ratio of pearlite blocks measured at a position of 0.25D depth from the surface of the cylindrical shaft portion in the L cross section) is 1.1 or more, GD is (15 / AR) micrometer or less, and (GD) / (average block particle size of the pearlite block measured at the position of depth 0.25D from the surface of the said cylindrical shaft part in C cross section) is 1.0 micrometer And, wherein the tensile strength of the cylindrical shaft portion 1100 to 1500MPa

Non-crude machine parts.

<5> mass%,

Cr: more than 0 and less than 1.00%,

Ti: more than 0 and 0.050% or less,

Nb: more than 0 and 0.050% or less,

V: greater than 0 and 0.10% or less, and

B: The non-machined machine part as described in <4> containing 1 type (s) or 2 or more types exceeding 0 and 0.0050% or less.

<6> The non-machined machine part which is a cold worked product of the steel wire for non-machined machine parts in any one of <1>-<3>, and includes a cylindrical shaft part and the tensile strength of the said cylindrical shaft part is 1100-1500 Mpa.

The non-machined machine part in any one of <4>-<6> which is a <7> non-machined bolt.

According to the present disclosure, a non-crumbing machine which is a steel wire having a tensile strength of 900 MPa or more and is excellent in cold workability when manufacturing non-crunching machine parts by cold working, and also excellent in hydrogen embrittlement resistance when the non-machined machine parts are used. Steel wire for parts is provided.

Moreover, according to this indication, the non-machined machine part which can be manufactured using the steel wire excellent in cold workability, and is excellent in tensile strength and hydrogen embrittlement resistance is provided.

1 is a conceptual diagram illustrating an example of a pearlite block in a steel wire L cross section of the present disclosure.

In this specification, the numerical range displayed using "-" means the range which includes the numerical value described before and after "-" as a lower limit and an upper limit.

In this specification, "%" which shows content of a component (element) means "mass%."

In this specification, content of C (carbon) may be described as "C content." The content of other elements may be similarly described.

In this specification, the term "process" is included in this term as long as the desired purpose of the process is achieved even if it is not only distinguished from the independent process but also from other processes.

[Steel wire for non-machined machine parts]

The steel wire for non-machined mechanical parts of the present disclosure (hereinafter also referred to simply as "steel wire") has a chemical composition of 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%, P: 0 to 0.030%, S: 0 to 0.030%, N: 0 to 0.0050%, Cr: 0 to 1.00%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, V: 0 To 0.10%, B: 0 to 0.0050%, O: 0 to 0.0030% and the balance: Fe and impurities,

When the metal structure makes mass% of C [C%], it consists of (35 * [C%] + 65)% or more of pearlite, and the remainder which is at least one of a cornerstone ferrite and bainite in an area ratio,

A cross section parallel to the axial direction of the steel wire and including the central axis is referred to as an L cross section, a cross section perpendicular to the axial direction of the steel wire is referred to as a C cross section, a diameter of the steel wire is referred to as D, and a depth of 50 from the steel wire surface in the L cross section. AR is 1.4 or more when the average aspect ratio of the pearlite block measured by the position of micrometer is AR, and the average block particle diameter of the pearlite block measured by the position of 50 micrometers depth from the steel wire surface in C cross section is GD. (AR) / (average aspect ratio of pearlite block measured at the position of depth 0.25D from the steel wire surface in L cross section) is 1.1 or more, GD is (15 / AR) 탆 or less, and (GD) / (Average block diameter of pearlite block measured at the position of depth 0.25D from the steel wire surface in C cross section) is less than 1.0,

Tensile strength is 900-1500 Mpa.

Although the steel wire of this indication is a steel wire of 900 Mpa or more of tensile strength, it is excellent in cold workability (henceforth simply "cold workability") at the time of manufacturing a non-machined machine part by cold working.

Further, the steel wire of the present disclosure is excellent in hydrogen embrittlement characteristics (hereinafter, also referred to simply as "hydrogen embrittlement characteristics") in the case of using non-coarse mechanical parts. In other words, by cold working the steel wire of the present disclosure, it is possible to manufacture non-machined machine parts excellent in hydrogen embrittlement resistance.

In the steel wire of the present disclosure, the above-described chemical composition contributes to both cold workability and hydrogen embrittlement resistance. The detail of a chemical composition is mentioned later.

Usually, the steel wire of a chemical composition with a low C content (specifically, C content is 0.65 mass% or less) like a chemical composition mentioned above softens and improves ductility, and favorable cold workability is obtained.

However, with reduction of C content, the biphasic structure of a cornerstone ferrite and a pearlite becomes easy to produce | generate. In particular, in the surface layer of the wire rod, the C content tends to be further lowered by decarburization, and the cornerstone ferrite is easily generated. In addition, in the surface layer of the wire rod, since the cooling rate is large, bainite structure is also easily generated. The biphasic structure of the cornerstone ferrite and pearlite and bainite generally have lower hydrogen embrittlement characteristics than pearlite. When the C content is reduced (specifically, the C content is 0.65% by mass or less), structures such as cornerstone ferrite and bainite tend to be easily formed, and thus hydrogen embrittlement of the surface layer portion of the mechanical part (for example, a bolt) is performed. Characteristics are lowered.

In this regard, the metal structure of the steel wire of the present disclosure is a metal structure mainly composed of pearlite, and more specifically, the metal structure of the steel wire of the present disclosure has an area ratio of pearlite (35 × [C%] + 65). It is a metal structure which is% or more. A pearlite structure may be called the layer which mainly consists of a cementite phase (henceforth simply a "cementite layer"), and the layer which mainly consists of a ferrite phase (henceforth simply a "ferrite layer"). ) Has a laminated structure. It is thought that this laminated structure becomes a resistance (hydrogen embrittlement resistance characteristic) to the growth of a crack. This improves cold workability and hydrogen embrittlement resistance.

In the present disclosure, the reason why the area ratio of pearlite depends on [C%] (that is, C content) is in the range of C content 0.40 to 0.65%, and the lower the C content, the easier the formation of cornerstone ferrite and bainite is. This is because pearlite tends to be hard to be produced.

As for the steel wire of this indication, the average aspect ratio (namely, "AR" in this specification) of the pearlite block measured at the depth of 50 micrometers in L cross section is 1.4 or more, and it is referred to (AR) / (L cross section). The average aspect ratio of the pearlite block measured at the position of depth 0.25D from the steel wire surface in this case is 1.1 or more.

In this specification, the position of 50 micrometers in depth from a steel wire surface may be called "50 micrometers in depth position" or "surface layer". In other words, "surface layer" in this specification means the position of 50 micrometers in depth from a steel wire surface.

In this specification, when the position of depth 0.25D from a steel wire surface (that is, the depth from a steel wire surface is 0.25 times the diameter of steel wire (that is, D) is called "depth 0.25D position" or "0.25D"), There is.

In this specification, (AR) / (average aspect ratio of the pearlite block measured at the position of depth 0.25D from the steel wire surface in L cross section) is called "the ratio of aspect ratio [surface layer / 0.25D]" of a pearlite block. There is a case.

In the steel wire of this indication, the ratio (surface layer / 0.25D) of aspect ratio is 1.1 or more. That is, in the steel wire L cross section of the present disclosure, the pearlite block in the surface layer of the steel wire (that is, the position of 50 µm in depth) is extended than the pearlite block in the steel wire (that is, the depth of 0.25D position).

Moreover, in the steel wire L cross section of this indication, the average aspect ratio (namely, AR) of the pearlite block in a surface layer is 1.4 or more.

In the steel wire of the present disclosure, by satisfying these conditions, the hydrogen embrittlement resistance characteristic (that is, the hydrogen embrittlement characteristic in the case of making a non-coarse machine part by cold working) is improved. This is because the pearlite block is elongated in the surface layer, so that the direction of the layered structure of the pearlite structure in the surface layer becomes more uniform, and the resistance to hydrogen intrusion from the steel wire surface becomes, and / or the progress of cracking It is because it becomes resistance to. Therefore, in the steel wire of this indication, even if a metal structure contains a cornerstone ferrite and bainite, hydrogen embrittlement resistance improves.

In the steel wire of the present disclosure, the average block particle diameter GD of the pearlite block measured at a depth of 50 µm in the C cross section is (15 / AR) µm or less, and the depth in (GD) / (C cross section) The average block particle diameter of the pearlite block measured at the 0.25D position is less than 1.0.

In this specification, (GD) / (average block particle size of a pearlite block measured at the depth 0.25D position in C cross section) may be called "ratio of block particle diameters [surface layer / 0.25D]" of a pearlite block.

In the steel wire of this indication, the ratio [surface layer / 0.25D] of the block particle diameter of a pearlite block is less than 1.0. That is, in the steel wire C cross section of the present disclosure, the pearlite block in the surface layer of the steel wire (that is, the position of 50 µm in depth) is finer than the pearlite block in the steel wire (that is, the depth of 0.25D position).

Moreover, in the steel wire C cross section of this indication, the average block particle diameter (namely, GD) of the pearlite block in a surface layer is set to (15 / AR) micrometer or less.

In the steel wire of the present disclosure, by satisfying these conditions, the cold workability of the steel wire is improved, and the hydrogen embrittlement resistance property (that is, the hydrogen embrittlement resistance property when the non-coarse machine part is formed by cold working) is improved.

It is considered that the reason why the cold workability of the steel wire is improved by satisfying the above conditions is that the ductility of the steel wire is improved by the fine (that is, (15 / AR) µm or less) of the pearlite block of the surface layer.

The reason why the hydrogen embrittlement resistance is improved by satisfying the above conditions is considered to be related to the fine pearlite block in the surface layer and the tendency of hydrogen to segregate at the grain boundary. That is, the finer pearlite blocks in the surface layer increase the total area of the grain boundaries in the surface layer, and as a result, the hydrogen trapping ability in the surface layer (that is, the ability to prevent hydrogen from intruding into the steel wire) is improved. I think it is.

The steel wire of the present disclosure has a tensile strength of 900 to 1500 MPa.

The steel wire of the present disclosure having a tensile strength of 900 to 1500 MPa (ie, steel wire for non-machined mechanical parts) is suitable for use in the production of non-machined mechanical parts having a tensile strength of 1100 to 1500 MPa by cold working.

Although there is no restriction | limiting in particular as cold working in this indication, Cold forging, rolling, cutting, drawing etc. are mentioned.

Cold processing in the present disclosure may be only one type of processing, or may be a plurality of types of processing (for example, cold forging and rolling).

In addition, the non-machined mechanical parts having the tensile strength of 1100 to 1500 MPa may be manufactured by cold working the steel wire of the present disclosure and then keeping it within a temperature range of 100 to 400 ° C.

The steel wire of the present disclosure is mainly made of pearlite, and in order to satisfy the above-mentioned conditions, the steel wire has a tensile strength of 900 MPa or more, and is excellent in cold workability when obtaining non-machined mechanical parts by cold working.

With respect to the steel wire of the present disclosure, steel wires having a tensile strength of 900 MPa or more and mainly composed of a cornerstone ferrite-pearlite two-phase structure tend to have low cold workability.

<Chemical composition>

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

In addition, the chemical composition of the non-machined machine parts of the present disclosure described later is the same as the steel wire chemical composition of the present disclosure.

Hereinafter, the chemical composition of the steel wire or non-machined machine part of this indication may be called "chemical composition in this indication."

C: 0.40 to 0.65%

C is an element necessary for securing tensile strength.

When the C content is less than 0.40%, it is difficult to obtain the desired tensile strength. Therefore, C content in the chemical composition in this indication is 0.40% or more, Preferably it is 0.45% or more.

On the other hand, when C content is more than 0.65%, cold workability deteriorates. Therefore, C content in the chemical composition in this indication is 0.65% or less, Preferably it is 0.60% or less.

Si: 0.05 to 0.50%

Si is a deoxidation element and an element which raises tensile strength by solid solution strengthening.

When Si content is less than 0.05%, an addition effect is not fully expressed. Therefore, Si content in the chemical composition in this indication is 0.05% or more, Preferably it is 0.15% or more.

On the other hand, when Si content is more than 0.50%, while the addition effect is saturated, the ductility at the time of hot rolling deteriorates, and defects become easy to generate | occur | produce. Therefore, Si content in the chemical composition in this indication is 0.50% or less, Preferably it is 0.30% or less.

Mn: 0.20 to 1.00%

Mn is an element which raises the tensile strength of the steel after pearlite transformation.

When Mn content is less than 0.20%, an addition effect does not fully express. Therefore, Mn content in the chemical composition in this indication is 0.20% or more, Preferably it is 0.40% or more.

On the other hand, when Mn content is more than 1.00%, while the addition effect is saturated, the transformation completion time at the time of constant temperature transformation process of a wire rod becomes long. As transformation completion time becomes long, the area ratio of the pearlite structure of the surface layer part of a wire rod is less than (35 * [C%] + 65) area%, and there exists a possibility that hydrogen embrittlement property and cold workability may deteriorate by this. Moreover, manufacturing cost increases by saturation of an addition effect. Therefore, Mn content in the chemical composition in this indication is 1.00% or less, Preferably it is 0.80% or less.

Al: 0.005 to 0.050%

Al is a deoxidation element and is an element which forms AlN which functions as a pinned particle | grain. AlN makes grains fine, thereby improving cold workability. In addition, Al is an element which has the effect | action which reduces solid solution N, suppresses dynamic strain aging, and raises hydrogen embrittlement resistance characteristic.

When Al content is less than 0.005%, the above-mentioned effect cannot be acquired. Therefore, Al content in the chemical composition in this indication is 0.005% or more, Preferably it is 0.020% or more.

When Al content is more than 0.050%, while the above-mentioned effect is saturated, defect will become easy to occur at the time of hot rolling. Therefore, Al content in the chemical composition in this indication is 0.050% or less, Preferably it is 0.040% or less.

P: 0% to 0.030%

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

When P content is more than 0.030%, deterioration of hydrogen embrittlement characteristic and deterioration of cold workability become remarkable. Therefore, P content in the chemical composition in this indication is 0.030% or less, Preferably it is 0.015% or less.

Since the steel wire of this indication does not need to contain P, the lower limit of P content is 0%. However, from a viewpoint of reduction of manufacturing cost (dephosphorization cost), P content may be more than 0%, 0.002% or more, and 0.005% or more may be sufficient.

S: 0% to 0.030%

S, like P, is segregated at grain boundaries to deteriorate hydrogen embrittlement characteristics and to deteriorate cold workability.

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

Since the steel wire of this indication does not need to contain S, the lower limit of S content is 0%. However, from a viewpoint of reduction of manufacturing cost (desulfurization cost), S content may be more than 0%, 0.002% or more, and 0.005% or more may be sufficient.

N: 0% to 0.0050%

N is an element which may deteriorate cold workability by dynamic strain aging and also deteriorate hydrogen embrittlement characteristics. In order to avoid such a bad influence, in the chemical composition in this indication, N content is made into 0.0050% or less. N content becomes like this. Preferably it is 0.0040% or less.

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

Cr: 0 to 1.00%

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

Cr is an element which raises the tensile strength of the steel after pearlite transformation. From the viewpoint of obtaining such an effect, the Cr content is preferably more than 0%, more preferably 0.01% or more, still more preferably 0.03% or more, still more preferably 0.05% or more, particularly preferably 0.10% or more.

On the other hand, when Cr content is more than 1.00%, martensite becomes easy to generate | occur | produce, thereby degrading cold workability. Therefore, Cr content in the chemical composition in this indication is 1.00% or less, Preferably it is 0.70% or less, More preferably, it is 0.50% or less.

Ti: 0 to 0.050%

Ti is an arbitrary element. That is, the minimum of Ti content in the chemical composition in this indication is 0%.

Ti is a deoxidation element, and it is an element which has the effect | action which forms TiN, reduces solid-solution N, suppresses dynamic strain aging, and raises hydrogen embrittlement resistance characteristic. From a viewpoint of obtaining such an effect, Ti content becomes like this. Preferably it is more than 0%, More preferably, it is 0.005% or more, More preferably, it is 0.015% or more.

On the other hand, when Ti content is more than 0.050%, while the above-mentioned effect is saturated, defects will arise easily at the time of hot rolling. Therefore, Ti content in the chemical composition in this indication is 0.050% or less, Preferably it is 0.035% or less.

Nb: 0 to 0.050%

Nb is an arbitrary element. That is, the minimum of Nb content in the chemical composition in this indication is 0%.

Nb is an element which has the effect | action which forms NbN, reduces solid-solution N, suppresses dynamic strain aging, and raises hydrogen embrittlement resistance characteristic. From a viewpoint of obtaining such an effect, Nb content becomes like this. Preferably it is more than 0%, More preferably, it is 0.005% or more, More preferably, it is 0.015% or more.

On the other hand, when Nb content is more than 0.05%, while the above-mentioned effect is saturated, defect will become easy to occur at the time of hot rolling. Therefore, Nb content in the chemical composition in this indication is 0.050% or less, Preferably it is 0.035% or less.

V: 0 to 0.10%

V is an arbitrary element. That is, the minimum of V content in the chemical composition in this indication is 0%.

V is an element which has a function which forms VN, reduces solid-solution N, suppresses dynamic strain aging, and improves hydrogen embrittlement resistance. From a viewpoint of obtaining such an effect, V content becomes like this. Preferably it is more than 0%, More preferably, it is 0.02% or more.

On the other hand, when V content is more than 0.10%, while the above-mentioned effect is saturated, defect will become easy to occur at the time of hot rolling. Therefore, V content in the chemical composition in this indication is 0.10% or less, Preferably it is 0.05% or less.

B: 0% to 0.0050%

B is an arbitrary element. That is, the lower limit of B content in the chemical composition in this indication is 0%.

B has the effect of suppressing grain boundary ferrite and grain boundary bainite, improving cold workability and hydrogen embrittlement resistance, and increasing tensile strength after pearlite transformation. From a viewpoint of obtaining such an effect, B content becomes like this. Preferably it is more than 0%, More preferably, it is 0.0003% or more.

On the other hand, when B content exceeds 0.0050%, the above-mentioned effect will be saturated. Therefore, B content in the chemical composition in this indication is 0.0050% or less.

The chemical composition in the present disclosure is, in terms of mass, from the viewpoint of obtaining the respective effects of any of the above-described arbitrary elements, in terms of mass%: Cr: more than 0 and 1.00% or less, Ti: more than 0 and 0.050% or less and Nb: more than 0 and 0.050% or less. , V: more than 0 and 0.10% or less, and B: more than 0 and 0.0050% or less, or may contain one or two or more kinds.

O: 0% to 0.0030%

O exists as oxides, such as Al and Ti, in a steel wire.

When the O content is more than 0.0030%, coarse oxide is formed in the steel, and fatigue breakdown is likely to occur. Therefore, O content in the chemical composition in this indication is 0.0030% or less, Preferably it is 0.0020% or less.

Since the steel wire of this indication does not need to contain O, the lower limit of O content is 0%. However, from a viewpoint of reduction of manufacturing cost (deoxidation cost), O content may be more than 0%, 0.0002% or more, and 0.0005% or more may be sufficient.

Remainder: Fe and impurities

In the chemical composition of the present disclosure, the balance except for the above-described elements is Fe and impurities.

Here, an impurity is a component contained in a raw material, or a component mixed in the manufacturing process, and refers to the component which was not intentionally contained in steel.

As an impurity, all elements other than the above-mentioned element are mentioned. 1 type may be sufficient as an element as an impurity, and 2 or more types may be sufficient as it.

Metal organization

Next, the metal structure of the steel wire of this indication is demonstrated.

(Area ratio of pearlite)

The metal structure of the steel wire of the present disclosure is at least one of (35 x [C%] + 65)% or more of pearlite, cornerstone ferrite and bainite when the mass% of C is [C%]. It consists of the balance.

This improves cold workability and hydrogen embrittlement resistance.

If the area ratio of pearlite in the metal structure of the steel wire is less than (35 × [C%] + 65)%, the strength (tensile strength, hardness, etc.) of the steel wire becomes nonuniform, and thus cold working on the non-machined machine part Cracking tends to occur at the time (that is, cold workability falls).

Further, when the area ratio of pearlite in the metal structure of the steel wire is less than (35 × [C%] + 65)%, the pearlite area ratio of the metal structure also in the non-machined mechanical parts obtained by cold working the steel wire ( It becomes less than 35 * [C%] + 65)%. As a result, the hydrogen embrittlement resistance of the non-machined machine parts is deteriorated.

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

From the viewpoint of production aptitude, the area ratio of pearlite is preferably 99% or less, more preferably 97% or less, and even more preferably 95% or less.

In the metal structure of the steel wire of this indication, although the specific preferable range of the area ratio of pearlite changes with [C%], 80-99% is preferable, 83-97% is more preferable, 85-95% Is particularly preferred.

The remainder in the metal structure of the steel wire of this indication is at least one of a cornerstone ferrite and bainite.

When the remainder contains martensite, the cold workability and the hydrogen embrittlement resistance in the case of using non-machined machine parts are deteriorated.

In this specification, the area ratio (%) of pearlite points out the value calculated | required by the following procedures.

First, the C cross section of the steel wire is etched using a picral to reveal a metal structure.

Subsequently, four observation positions are selected at a depth of 50 μm (ie, a circumferential position) in the C cross section after etching at intervals of 90 ° in the circumferential direction, and FE-SEM (for each observation position) is selected. Using a Field Emission-Scanning Electron Microscope, SEM images with a magnification of 1000x are taken.

Similarly, from the depth 0.25D position (that is, the circumferential position) in C cross section after etching, four observation positions are selected by 90 degree space in the circumferential direction, and, for each observation position, FE-SEM Using, a SEM photograph at 1000 times magnification is taken.

In the eight obtained SEM photographs, structures other than pearlite (stone-ferrite, bainite, etc.) are visually marked, and the area ratio (%) of the structures other than pearlite with respect to the whole metal structure is calculated | required by image analysis. By subtracting the area ratio (%) of structures other than the obtained pearlite from 100%, the area ratio (%) of pearlite is obtained.

(AR)

As for the steel wire of this indication, AR (namely, the average aspect ratio of the pearlite block measured at the depth of 50 micrometers in L cross section) is 1.4 or more. This improves the hydrogen embrittlement characteristic. This reason is considered as follows. As described above, the pearlite structure has a laminated structure of a cementite layer and a ferrite layer, and the stretched pearlite block (that is, the pearlite block having an AR of 1.4 or more) in the surface layer becomes more uniform in the direction of the layered structure of the pearlite structure. . This homogeneous layered structure is considered to be because the resistance to hydrogen ingress from the steel wire surface and / or the resistance to the progress of cracking.

When the AR of the steel wire is less than 1.4, the AR of the non-machined machine part obtained by cold working the steel wire is also less than 1.4. In this case, since the above effects (the effect of resisting hydrogen intrusion and / or the effect of resisting crack propagation) are hardly obtained, the hydrogen embrittlement resistance of non-machined machine parts is not improved.

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

It is preferable that it is 2.5 or less, and, as for AR, from a viewpoint of the manufacture ability of steel wire, it is more preferable that it is 2.0 or less.

In the present specification, the pearlite block means a tissue unit of pearlite in which the orientation of the ferrite is arranged within an orientation difference of 15 ° from the crystal orientation map of the ferrite obtained by the electron back scattering diffraction (EBSD) method. That is, the boundary where the said orientation difference becomes 15 degrees or more is a block boundary of a pearlite block.

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

First, four observation positions are selected at 2.0 mm intervals from a straight line indicating a depth of 50 µm position in the L cross section of the steel wire, and each of the observation positions having a depth direction of 50 µm and an axial direction of 250 µm. The crystal orientation map of the ferrite in the area is obtained using an EBSD device, respectively.

In all the obtained four crystal orientation maps, ten pearlite blocks are selected in order from the largest equivalent diameter of a circle from the group of the pearlite blocks which the straight line which shows a depth of 50 micrometers traverses.

Next, each aspect ratio of the selected ten pearlite blocks is obtained, and the average value of the aspect ratios (that is, ten values) in the ten pearlite blocks is calculated at AR (i.e., at a depth of 50 µm in the L cross section). Average aspect ratio of the measured pearlite block).

In this specification, the aspect ratio of a pearlite block means a value obtained by dividing the long diameter of the pearlite block by the short diameter (ie, the long diameter / short diameter). Here, the long diameter of a pearlite block means the maximum length of a pearlite block, and the short diameter of a pearlite block means the maximum length of the direction orthogonal to a long radial direction.

1 is a conceptual diagram illustrating an example of a pearlite block in an L cross section of a steel wire according to an example of the present disclosure.

In FIG. 1, not only the grain boundary of a pearlite block but also the major axis and minor axis of this pearlite block are shown.

The shape of a pearlite block may be polygonal shape as shown in FIG. 1, may be oval shape, and shape other than a polygonal shape and an ellipse shape (for example, indefinite shape) may be sufficient as it.

In short, the pearlite block may have an AR of 1.4 or more, and the shape thereof is not particularly limited.

(Ratio of aspect ratio [surface layer / 0.25D])

In the steel wire of the present disclosure, the aspect ratio (surface layer / 0.25D) (that is, (AR) / (average aspect ratio of pearlite blocks measured at a depth of 0.25D in the L cross section)) is 1.1 or more.

In the steel wire of the present disclosure, the hydrogen embrittlement resistance is improved as described above because the aspect ratio (surface layer / 0.25D) is 1.1 or more. This is because the direction of the layered structure of the pearlite structure in the pearlite block extended in the surface layer becomes more uniform, and this layered structure becomes a resistance to hydrogen intrusion from the steel wire surface, and / or the progress of cracking. It is because it becomes resistance to.

In addition, since the deformation | concentration concentrates on the surface layer of steel wire by the aspect ratio [surface layer /0.25D] of 1.1 or more, the steel wire of this indication can improve hydrogen embrittlement resistance efficiently.

If the aspect ratio ratio (surface layer / 0.25D) is less than 1.1, not only the surface layer of the steel wire but also the strain inside the steel wire must be increased, so that the hydrogen embrittlement resistance characteristics cannot be efficiently improved and the productivity of the steel wire is increased. It may fall.

It is preferable that ratio (surface layer /0.25D) of aspect ratio is 1.2 or more from a viewpoint of improving hydrogen embrittlement characteristic.

The aspect ratio [surface layer / 0.25D] of the aspect ratio is preferably 2.0 or less, more preferably 1.8 or less, and particularly preferably 1.6 or less, from the viewpoint of the production aptitude of the steel wire.

In this specification, the average aspect ratio of the pearlite block measured at the depth 0.25D position in L cross section changes a observation position from the depth of 50 micrometers in the L cross section to the depth 0.25D position in L cross section. Other than that is measured by the same method as the measuring method of AR mentioned above.

(GD)

As for the steel wire of this indication, GD (namely, the average block particle diameter of the pearlite block measured at the depth of 50 micrometers in C cross section) is (15 / AR) micrometers or less. As the pearlite block is fine (that is, GD is (15 / AR) µm or less), as described above, cold workability and hydrogen embrittlement resistance are improved.

This reason is considered as follows. When the pearlite block in the surface layer of the steel wire is coarsened (that is, when the average block diameter of the pearlite block exceeds (15 / AR) µm), the ductility of the steel wire is lowered, whereby cold workability of the steel wire is reduced. Degrades. Moreover, the block particle diameter of the surface perlite block of the mechanical component obtained by cold working this steel wire is coarsened. Hydrogen tends to segregate at the pearlite block boundary. When the surface pearlite block of the steel wire is coarse, the total area of the block boundary of the pearlite block is reduced, so that the hydrogen trapping ability of the surface layer (that is, the ability to prevent hydrogen from penetrating into the wire rod) is lowered. Thereby, when the pearlite block of a surface layer becomes coarse, it is thought that hydrogen embrittlement characteristic falls.

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.

It is preferable that it is 7.0 micrometers or more, and, as for GD, from a viewpoint of manufacture ability of a steel wire, it is more preferable that it is 8.0 micrometers or more.

In this specification, GD means the value measured by the following procedures.

First, in the circumference which shows the depth of 50 micrometers position in C cross section of a steel wire, 8 observation positions are selected by 45 degree space in the circumference direction, and the area | region of 50 micrometers x 50 micrometers centering on each observation position Crystal orientation maps of ferrites in the interior are obtained using an EBSD device, respectively.

The circle equivalent diameters of all the pearlite blocks contained in all eight crystal orientation maps obtained are measured, respectively. The average value of the obtained measured value is called GD (that is, the average block particle diameter of the pearlite block measured at the depth of 50 micrometers in C cross section).

(Ratio of particle size [surface layer / 0.25D])

In the steel wire of the present disclosure, the ratio (surface layer / 0.25D) (that is, (GD) / (average block particle size of the pearlite block measured at a depth of 0.25D in the C cross section)) of the particle diameter is less than 1.0.

In the steel wire of the present disclosure, cold workability and hydrogen embrittlement resistance are improved when the ratio [GD / 0.25D] of the particle 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.

It is preferable that it is 0.80 or more, as for the ratio [GD / 0.25D] of a particle diameter, it is more preferable that it is 0.85 or more, and it is especially preferable that it is 0.90 or more.

In this specification, the average block particle diameter of the pearlite block measured at the depth 0.25D position in C cross section changes an observation position from the depth of 50 micrometers in the C cross section to the depth 0.25D position in C cross section. Other than that is measured by the method similar to the measuring method of GD mentioned above.

Tensile Strength (TS) of the steel wire of the present disclosure is 900 to 1500 MPa.

Since the TS of the steel wire of this indication is 900 Mpa or more, by cold-processing this steel wire, it is easy to manufacture the non-machined machine parts whose TS is 1100 Mpa or more.

Moreover, in the conventional steel wire, there exists a tendency for cold workability to fall that TS of steel wire is 900 Mpa or more.

However, in the steel wire of this indication, by having the chemical composition and metal structure mentioned above, it is a steel wire whose TS is 900 Mpa or more, and is excellent in cold workability.

Moreover, when TS of the steel wire of this indication is 1500 Mpa or less, it is excellent in the manufacturing aptitude and cold workability of a steel wire.

In the present specification, both the tensile strength (TS) of the steel wire and the tensile strength (TS) of the non-machined machine parts are based on the test method described in JIS Z2201 (2011), using a 9A test piece of JIS Z2201 (2011). Means the measured value.

The TS of the steel wire of the present disclosure is preferably 900 to 1300 MPa, more preferably 900 to 1200 MPa, from the viewpoint of further improving the production suitability and cold workability of the steel wire.

In the steel wire of this indication, 3-30 mm is preferable, as for D (namely, diameter of a steel wire), 5-25 mm is more preferable, 5-20 mm is especially preferable.

The steel wire of the present disclosure preferably has a limit compressibility of 75% or more from the viewpoint of cold workability. The measuring method of a limit compression rate is as showing in the Example mentioned later.

The following manufacturing method A is mentioned as an example of the method of manufacturing the steel wire of this indication.

The manufacturing method A is a process of obtaining a wire rod by heating a steel piece which has a chemical composition in this indication to 1000-1150 degreeC, and hot-rolling by making finish rolling temperature 800-950 degreeC,

A step of constant temperature transformation by dipping the wire rod having a temperature of 800 to 950 ° C. in a molten salt bath at 400 to 550 ° C. for at least 50 seconds,

The process of water-cooling the wire rod processed by constant temperature transformation to the temperature below 300 degreeC,

The process of obtaining a steel wire by giving a wire drawing which a total reduction of reduction is 15 to 25% with respect to water-cooled wire rod.

It includes.

The chemical composition of the steel wire (purpose object) obtained by the manufacturing method A can be considered to be the same as the chemical composition of the steel piece (raw material) in the manufacturing method A. The reason is that the hot rolling, the constant temperature transformation treatment, the water cooling and the drawing processing all do not affect the chemical composition of the steel.

The manufacturing method A is easy to manufacture the steel wire of this indication by which the area constant and remainder of a pearlite satisfy | fill the conditions mentioned above by including the process of the constant temperature transformation process and the said water-cooling process.

For example, in the process of the constant temperature transformation treatment, the immersion time for immersing the wire rod in the molten salt bath is 50 seconds or more, whereby the area ratio of the pearlite and the balance remain to satisfy the above-described conditions, respectively.

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

Moreover, in the process of obtaining the said steel material (namely, the process including drawing process; hereafter also called a "drawing process"), when the total reduction rate is 15% or more, it is easy to manufacture steel materials with a tensile strength of 900 MPa or more.

In the drawing process, when the total reduction ratio is 25% or less, the AR is 1.4 or more, and the aspect ratio (surface layer / 0.25D) is 1.1 or more (ie, steel materials in comparison with the pearlite block inside the steel). Steel material in which the pearlite blocks of the surface layer are stretched).

The wire drawing process may be a process including only one drawing process or may be a step including a drawing process a plurality of times.

That is, 15-25% of the total reduction | restoration rate in a wire drawing process may be achieved by one draw process, and may be achieved by multiple draw process.

In the case where the drawing process includes drawing only once, a die having an approach half-angle exceeding 10 ° is used as a die to be used for drawing. Thereby, it is easy to manufacture steel materials whose aspect ratio (surface layer /0.25D) is 1.1 or more.

In addition, when a wire drawing process includes wire drawing in multiple times, it carries out several times of drawing process on the conditions that the reduction rate in a last pass will be 10% or less. Thereby, it is easy to manufacture steel materials whose aspect ratio (surface layer /0.25D) is 1.1 or more.

It is preferable that it is 5 to 10%, It is more preferable that it is 5 to 9%, It is especially preferable that it is 5 to 8%, as for the reduction ratio in the last pass | pass in the case where a wire drawing process includes drawing process multiple times. .

The steel wire of the present disclosure is particularly suitable as a steel wire for producing an unmachined mechanical part including a cylindrical shaft portion having a tensile strength of 1100 to 1500 MPa.

That is, by cold working the steel wire of the present disclosure (and preferably maintaining the temperature at 100 to 400 ° C. after the cold working), an unmachined mechanical part including a cylindrical shaft portion having a tensile strength of 1100 to 1500 MPa is obtained. Easy to manufacture

Here, the chemical composition of the non-machined machine parts obtained by cold working the steel wire of the present disclosure (and preferably after cold working (keeping at 100 to 400 ° C) is the same as the steel wire chemical composition of the present disclosure. This is because cold working and heat treatment do not affect the chemical composition of the steel.

In addition, the metal structure of the non-machined mechanical part obtained by cold working the steel wire of this indication (and performing a heat processing of 100-400 degreeC after cold processing as needed) is the metal structure of the steel wire of this indication. Can be considered equal. The reason is that the amount of cold working for obtaining the non-machined machine part having the cylindrical shaft portion is minute.

[Non-machined machine parts]

EMBODIMENT OF THE INVENTION Hereinafter, 1st Embodiment and 2nd Embodiment of the non-coarse machine part (henceforth simply a "machine part") of this indication are demonstrated.

The first embodiment mechanical component of the present disclosure includes a cylindrical shaft portion,

The chemical composition is the chemical composition in the present disclosure described above,

When a metal structure makes mass% of C [C%], it consists of a pearlite (35 * [C%] + 65)% or more by area ratio, and remainder which is at least one of a cornerstone ferrite and a pearlite,

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

The tensile strength TS of the cylindrical shaft portion is 1100 to 1500 MPa.

Chemical composition in the mechanical component of the first embodiment, and the metal structure of the cylindrical shaft portion (that is, the ratio of the pearlite area ratio, the AR, the aspect ratio [surface layer / 0.25D], the ratio of the GD and the average block particle diameter [surface layer) /0.25D], which is the same as the chemical composition and metal structure in the steel wire of the present disclosure, respectively.

Therefore, the mechanical component of 1st Embodiment is excellent in hydrogen embrittlement resistance characteristic.

The mechanical part of 1st Embodiment can be manufactured with the steel wire excellent in cold workability (for example, the steel wire of this indication).

Preferable forms of the chemical composition and the metal structure of the cylindrical shaft portion in the mechanical part of the first embodiment are the same as those of the chemical composition and the metal structure in the steel wire of the present disclosure, respectively.

The mechanical component of the second embodiment of the present disclosure is the steel wire cold worked product of the present disclosure (that is, the mechanical component obtained by cold working the steel wire of the present disclosure), and the tensile strength of the cylindrical shaft portion is 1100 to 1500 MPa.

Therefore, the mechanical component of 2nd Embodiment is excellent in hydrogen embrittlement resistance characteristic.

In the mechanical part of 2nd Embodiment, the preferable form of the chemical composition and the metal structure of a cylindrical shaft part is the same as that of the chemical composition and metal structure in the steel wire of this indication, respectively.

In the mechanical parts of the present disclosure, the first embodiment and the second embodiment may have overlapping portions.

That is, not only the mechanical parts corresponding to any of the first embodiment and the second embodiment, but also the mechanical parts corresponding to both the first embodiment and the second embodiment are naturally included in the scope of the mechanical parts of the present disclosure.

The mechanical component of the present disclosure is not particularly limited as long as it is an unstructured mechanical component including a cylindrical shaft portion. Among them, an unstructured bolt is particularly preferable.

The following manufacturing method X is mentioned as an example of the method of manufacturing the mechanical component of this indication.

The manufacturing method X includes the process of obtaining a mechanical component by cold working the steel wire of this indication.

It is preferable that manufacturing method X includes the process (henceforth a "maintenance process") which keeps the mechanical component obtained by cold working in the temperature range of 100-400 degreeC.

By including the holding step, it is easier to manufacture mechanical parts having a tensile strength of 1100 to 1500 MPa.

Although the holding temperature in a holding process is 100-400 degreeC, it is preferable that it is 200-400 degreeC, and it is more preferable that it is 300-400 degreeC.

The holding time in the holding step (ie, the time for keeping the mechanical part within the above temperature range) is preferably 10 to 120 minutes, more preferably 10 to 60 minutes.

The steel wire for non-machined machine parts and non-machined machine parts of the present disclosure described above can be used for various machines such as automobiles, construction, and the like.

Example

Hereinafter, although the Example of this indication is shown, this indication is not limited to the following Example.

(Conditions 1 to 28)

<Manufacture of Steel Wire>

The steel wire of diameter D shown in Table 3 was manufactured using the steel plate (billet) of the chemical composition shown in Table 1.

In the chemical composition of each steel species in Table 1, the balance other than the elements shown in Table 1 is Fe and impurities.

At levels 1 to 6, 8 to 9, 11 to 13, 15 to 24, and 27 to 28, diameters (by performing hot rolling, constant temperature transformation treatment, water cooling, and drawing) under the conditions shown in Table 2 were sequentially performed on the steel slab. D) The steel wire as shown in this Table 3 was obtained.

At levels 14, 25, and 26, the steel slab is subjected to hot rolling under the conditions shown in Table 2, followed by air cooling, reheating at a heating temperature of 950 ° C, and lead patenting and cooling of conditions at a condition of a bath temperature of 580 ° C. Was carried out sequentially, and then, the wire drawing of the conditions shown in Table 2 was performed, and the steel wire as shown in Table 3 of diameter (D) was obtained.

At levels 7 and 10, the diameter D is shown in Table 3 by performing hot rolling on the steel slab under the conditions shown in Table 2, followed by air cooling, and then drawing the steel under the conditions shown in Table 2. Got the same liner.

<Measurement in steel wire>

For each level of steel wire, by the method described above,

Measurement of the area ratio of pearlite,

Confirmation of balance,

Measurement of AR (that is, the average aspect ratio of the pearlite block at a depth of 50 µm in the L cross section),

Measurement of the ratio [surface layer / 0.25D] (that is, (AR) / (average aspect ratio of the pearlite block measured at the depth 0.25D position in the L cross section)) of the aspect ratio,

Measurement of GD (ie, average block particle diameter of pearlite blocks at a depth of 50 μm in the C cross section),

Measurement of the ratio of the particle diameter [surface layer / 0.25D] (that is, (GD) / (average block particle size of the pearlite block measured at a depth of 0.25D in the C cross section)), and

Measurement of Tensile Strength (TS)

Were performed respectively.

Table 3 shows the results of each measurement.

<Cold workability of steel wire (measurement of limit compressibility)>

For each level of steel wire, cold workability was evaluated by measuring the following limit compression ratio.

First, by machining the steel wire, a sample having a diameter of D (ie, the diameter of the steel wire) and having a length of 1.5 × D was produced.

Both cross sections of the obtained sample were restrained using a pair of molds. As a pair of metal mold | die, the metal mold | die which has concentric circular groove in the contact surface of the cross section of a sample was used, respectively. In this state, the sample was compressed in the longitudinal direction. By carrying out the test which variously changed the compression rate of the sample in this compression, the maximum compression rate which a crack of a sample does not generate | occur | produce was calculated | required.

The maximum compression ratio at which a crack of a sample does not generate | occur | produce was made into the limit compression ratio (%).

As a result, it was judged that the cold workability was good (G) when the limit compressibility was 70% or more, and the cold workability was judged poor (NG) when the limit compressibility was less than 70%.

The above result is shown in Table 3.

<Manufacture of mechanical parts>

The steel wire of each level was cold worked (cold forging), and it processed into the shape of the bolt with a flange. The processed steel wire was heated to 350 ° C. and held at this temperature for 30 minutes to obtain crude bolts as mechanical parts.

Measurement of Tensile Strength (TS) of Mechanical Components

The TS of the shaft part of the obtained mechanical component (non-rigid bolt) was measured by the above-mentioned measuring method.

The results are shown in Table 3.

<Evaluation of Hydrogen Embrittlement Characteristics of Mechanical Parts>

About the obtained mechanical part (non-crumb bolt), hydrogen embrittlement resistance was measured with the following method.

First, 0.5 ppm of diffusible hydrogen was contained in a mechanical component by charging an electric field hydrogen in a mechanical component.

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

Subsequently, in the atmosphere, a load of 90% of the maximum tensile load of the mechanical component was loaded on the mechanical component, and maintained at 100 h or more in this state.

As a result, it was judged that the hydrogen embrittlement characteristic was good (G) when no fracture occurred at 100 h elapsed, and that the hydrogen hydrogen embrittlement characteristic was poor (NG) when the fracture occurred at 100 h elapsed. .

The above result is shown in Table 3.

Description of Table 3

In the remainder of the structure column, F and B mean a cornerstone ferrite and bainite, respectively.

As shown in Table 3, it has the chemical composition in this indication, a pearlite area ratio is (35 * [C%] + 65)% or more, and a remainder structure is at least of cornerstone ferrite (F) and bainite (B). One side, AR is 1.4 or more, aspect ratio ratio [surface layer / 0.25D] is 1.1 or more, GD is (15 / AR) micrometer or less, particle diameter ratio [GD / 0.25D] is less than 1.0, TS The steel wire of each level of Example which is 900-1500 Mpa is the steel wire of TS 900 Mpa or more, and was excellent in cold workability, and also the hydrogen embrittlement resistance characteristic in the case of making into a mechanical component.

Moreover, by cold working the steel wire of each level of an Example, it was possible to manufacture the mechanical component whose TS is 1100 Mpa or more.

As for the steel wires of the level 7 and 10 (all comparative examples) whose pearlite area ratio is less than (35x [C%] + 65)% with respect to an Example, the hydrogen embrittlement characteristic at the time of using it as a mechanical component fell.

Moreover, the hydrogen embrittlement resistance of the steel wire of the level 3, 5, 12, and 27 (all comparative examples) whose AR is less than 1.4 fell as a mechanical component.

In addition, the hydrogen embrittlement resistance of the steel wires of the levels 9, 21, 22, and 27 (all comparative examples) whose ratio [surface layer / 0.25D] of aspect ratio is less than 1.1 fell.

In addition, the steel wires of the level 14 and 25 (all comparative examples) whose GD is more than (15 / AR) micrometers fell the cold workability of the steel wire.

Moreover, the cold wire workability of the steel wire of the steel wire of the level 14 and 26 (all comparative examples) whose ratio [GD / 0.25D] of particle diameter is 1.0 or more fell.

Moreover, in the steel wires of the level 23 and 24 (all comparative examples) whose TS is less than 900 Mpa, the machine part whose TS was 1100 Mpa or more was not manufactured.

In steel wires with poor cold workability (limit compressibility is less than 70%), the frequency of work cracking was high when manufacturing mechanical parts. Moreover, the dimensional precision of the mechanical parts manufactured using the steel wire (limit compression rate less than 70%) whose cold workability is bad was reduced.

As for the indication of the Japan patent application 2016-008708, the whole is taken in into this specification by reference.

All documents, patent applications, and technical specifications described herein are incorporated by reference herein to the same extent as if the individual documents, patent applications, and technical specifications were specifically and individually described.

Claims (10)

The chemical composition is in mass%
C: 0.40 to 0.65%,
Si: 0.05 to 0.50%,
Mn: 0.20 to 1.00%,
Al: 0.005 to 0.050%,
P: 0% to 0.030%,
S: 0% to 0.030%,
N: 0% to 0.0050%,
Cr: 0 to 1.00%,
Ti: 0 to 0.050%,
Nb: 0 to 0.050%,
V: 0 to 0.10%,
B: 0 to 0.0050%,
0: 0 to 0.0030% and
Balance: consisting of Fe and impurities,
When the metal structure makes mass% of C [C%], it consists of (35 * [C%] + 65)% or more of pearlite, and the remainder which is at least one of a cornerstone ferrite and bainite in an area ratio,
A cross section parallel to the axial direction of the steel wire and including the central axis is referred to as an L cross section, a cross section perpendicular to the axial direction of the steel wire is referred to as a C cross section, a diameter of the steel wire is referred to as D, and a depth of 50 from the steel wire surface in the L cross section. AR is 1.4 or more when the average aspect ratio of the pearlite block measured by the position of micrometer is AR, and the average block particle diameter of the pearlite block measured by the position of 50 micrometers depth from the steel wire surface in C cross section is GD. (AR) / (average aspect ratio of pearlite block measured at the position of depth 0.25D from the steel wire surface in L cross section) is 1.1 or more, GD is (15 / AR) 탆 or less, and (GD) / (Average block size of pearlite block measured at the position of depth 0.25D from the steel wire surface in C cross section) is less than 1.0,
Tensile strength is 900 to 1500 MPa
Steel wire for non-machined parts. The method according to claim 1, wherein in mass%,
Cr: more than 0 and less than 1.00%,
Ti: more than 0 and 0.050% or less,
Nb: more than 0 and 0.050% or less,
V: greater than 0 and 0.10% or less and
B: Steel wire for non-machined mechanical parts, containing one or two or more types of more than 0 and 0.0050% or less. The steel wire for non-machined machine parts according to claim 1 or 2, wherein D is 3 to 30 mm. Including a cylindrical shaft,
The chemical composition is in mass%
C: 0.40 to 0.65%,
Si: 0.05 to 0.50%,
Mn: 0.20 to 1.00%,
Al: 0.005 to 0.050%,
P: 0% to 0.030%,
S: 0% to 0.030%,
N: 0% to 0.0050%,
Cr: 0 to 1.00%,
Ti: 0 to 0.050%,
Nb: 0 to 0.050%,
V: 0 to 0.10%,
B: 0 to 0.0050%,
0: 0 to 0.0030% and
Balance: consisting of Fe and impurities,
When a metal structure makes mass% of C [C%], it consists of remainder which is at least one of (35 * [C%] + 65)% or more of pearlite, a cornerstone ferrite, and bainite in an area ratio,
A cross section parallel to the axial direction of the cylindrical shaft portion and including a central axis is referred to as an L cross section, a cross section perpendicular to the axial direction of the cylindrical shaft portion is referred to as a C cross section, and a diameter of the cylindrical shaft portion is referred to as D, L The average ratio of the pearlite blocks measured at the position of 50 µm deep from the surface of the cylindrical shaft portion in the cross section is called AR, and the pearlite blocks measured at the position of 50 µm depth from the surface of the cylindrical shaft portion in the C cross section. When the average block diameter is GD, AR is 1.4 or more, and (AR) / (average aspect ratio of the pearlite block measured at a position of 0.25D depth from the surface of the cylindrical shaft portion in the L cross section) is 1.1 or more. , GD is (15 / AR) µm or less, and (GD) / (average block size of pearlite block measured at a position of 0.25D depth from the surface of the cylindrical shaft portion in the C cross section) Less than 1.0,
An unmachined mechanical part having a tensile strength of the cylindrical shaft portion of 1100 to 1500 MPa. The method according to claim 4, wherein in mass%,
Cr: more than 0 and less than 1.00%,
Ti: more than 0 and 0.050% or less,
Nb: more than 0 and 0.050% or less,
V: greater than 0 and 0.10% or less, and
B: A non-machined machine part containing 1 type or 2 or more types which are more than 0 and 0.0050% or less. The non-machined machine part which is a cold-worked product of the steel wire for non-machined machine parts of Claim 1 or 2, and includes a cylindrical shaft part and the tensile strength of the cylindrical shaft part is 1100-1500 Mpa. The non-machined machine part which is a cold-worked product of the steel wire for non-machined machine parts of Claim 3, Comprising: A cylindrical shaft part and the tensile strength of the said cylindrical shaft part are 1100-1500 Mpa. 6. The non-machined machine part according to claim 4, wherein the machine-tool is an non-machined bolt. The non-machined machine part of claim 6, wherein the machined part is an unassembled bolt. 8. The non-machined machine part of claim 7, wherein the machine-assembly is a non-machined bolt.

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