KR101260598B1 - High-strength ultrathin steel wire and method of manufacturing the same - Google Patents

Abstract

This invention is a steel wire containing the chemical component of C: 0.7-1.2 mass%, Si: 0.05-2.0 mass%, Mn: 0.2-2.0 mass%, and remainder contains Fe and an unavoidable impurity, The steel wire which has a pearlite structure, the average C density | concentration of the ferrite phase center part of the outermost layer of the said steel wire is 0.2 mass% or less, and the residual compressive stress of the steel wire longitudinal direction of the said outermost layer is 600 Mpa or more.

Description

High-strength ultrafine steel wire and its manufacturing method {HIGH-STRENGTH ULTRATHIN STEEL WIRE AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a high-strength steel wire used in steel cords, saw wires and the like of automobile tires, and a method of manufacturing the same. Specifically, the present invention relates to an ultrafine steel wire with a wire diameter of 0.04 to 0.4 mm and a strength of 4500 MPa or more, which is drawn and hardened by cold using a die.

This application claims priority in June 22, 2009 based on Japanese Patent Application No. 2009-148051 for which it applied to Japan, and uses the content for it here.

In steel cords used in automobile tires, demand for weight reduction of steel wires is increasing from the demand for weight reduction of tires. Similarly, in saw wires for precisely cutting sapphire crystals, SiC crystals, and the like, demand for high tensile strength is increasing. In order to meet these demands, a number of studies have been energetically developed. As a result, it became clear that sufficient ductility needs to be secured in addition to high tensile strength of the steel wire. Although there are some ductility indexes, there exist the number of torsions until breakage by a torsion test, and the presence or absence of generation | occurrence | production of the crack (delamination) which arises in the longitudinal direction of a steel wire during a torsion test, for example. Increasing the strength of the steel wire is accompanied by a decrease in ductility, which is a major problem, and it is important to suppress this. In addition, since high-strength steel wire also exhibits a phenomenon of deterioration in characteristics due to room temperature aging (20 ° C. to 40 ° C., several days to several years), it is also important that good ductility is not substantially lowered by aging.

High-strength steel wire is generally manufactured by carrying out wire drawing using the die | dye etc. for the wire material which has a pearlite structure. By this process, a pearlite lamellar spacing becomes small and a large amount of electric potential is introduce | transduced in a ferrite phase, and tensile strength increases. When this fresh strain becomes very large, it becomes clear recently that the cementite in a pearlite structure will refine | miniaturize and decompose | disassemble. However, in particular, since the structure is fine, the relationship between the presence position and presence state of these carbons and the mechanical properties is not clear, and in particular, many causes are unclear about the cause of ductile deterioration. In actual high-strength steel wire, the structure and local deformation amount in the steel wire are not necessarily the same in the surface area and the center area, and it is considered that this also affects the properties of the steel wire.

In order to increase the strength of the ultrafine steel wire, it is necessary to increase the wire strength after the final parting treatment or to increase the final drawing deformation. By the way, even if the wire strength after the last parting treatment or the wire drawing deformation was increased to increase the strength of the ultrafine steel wire, when the strength exceeded 4500 MPa, the ductility was remarkable, and it was extremely difficult to put it into practical use.

On the other hand, as a conventional knowledge of the high strength increasing means with few ductility decreases, for example, patent document 1, patent document 2, and patent document 3 prescribed | regulated chemical components, such as C, Si, Mn, and Cr, respectively, and it is the high strength and high ductility ultrafine A high carbon steel wire rod for ships has been proposed. However, as can be seen from the examples disclosed in these publications, the tensile strength of the steel wire is at most 3500 to 3600 MPa, and there is a limit in increasing the strength of the ultrafine steel wire.

In addition, Patent Document 4 proposes a high strength steel high toughness steel wire rod having a chemical component, a nonmetallic inclusion structure, and an area fraction of saltpeter cementite. In addition, Patent Document 5 discloses a method for producing a high strength steel high toughness ultrafine steel that controls the chemical composition of steel and the reduction rate in the final die. However, even in these techniques, it was impossible to realize an ultrafine steel wire having a tensile strength of 4500 MPa or more and having high ductility.

In addition, there is another finding that the properties of steel cords are affected by the carbon concentration in the ferrite phase in the pearlite structure, and a guideline for improving the balance between strength and ductility is disclosed by defining these concentrations. For example, Patent Document 6 tries to obtain good characteristics by defining the carbon concentration in the steel wire. In patent document 7, the method of obtaining a favorable characteristic by realizing a preferable carbon state by further studying heat processing is disclosed. Moreover, in patent document 8, it is going to acquire favorable characteristic by defining the carbon concentration and lamellar spacing in steel wire. However, none of these mentions the carbon state of the outermost layer of the steel wire (the area from the surface to a depth of 2 m). This originates in what was not able to be measured (and controlled) by the technique at the time.

In addition, patent document 9 prescribed | regulated about the dispersion | variation in carbon concentration. Moreover, patent document 10 has prescribed | regulated the grade of the difference of the lamellar spacing which affects the dispersion | variation in carbon concentration. However, these have described the whole deviation and do not prescribe the carbon concentration of a specific location. On the other hand, patent document 11 discloses the manufacturing method of the steel wire and steel wire for obtaining a favorable characteristic by defining the C density | concentration ratio in the ferrite phase in a steel wire surface layer part and a steel wire center part. However, it is a regulation as a relative value of a center part and a surface layer to the last, and the provision of an absolute value for making a clear index is not provided. In addition, actual measurement consists in the inside spaced 10 micrometers or more from the surface, and C density | concentration in the area | region (outermost layer) to 2 micrometers from a surface is not controlled.

On the other hand, about the residual stress of a steel wire outermost layer, in patent document 12 and patent document 13, the range of residual stress is prescribed | regulated from a viewpoint of fatigue property and a crack resistance. However, while the residual compressive stress is preferred, the absolute value of the value is small, and the range of the range for obtaining a very good balance of ductility and strength is not yet established. In addition, there is no disclosed example of the relationship with the carbon state of the outermost layer.

It is the ferrite ductility that is in charge of the ductility of the high strength ultrafine wire, and the ductility is ensured even at high strength if the ductility of the ferrite phase is maintained. However, as the wire drawing strain increases, cementite generally decomposes and C atoms diffuse into the ferrite phase, thereby increasing the carbon concentration in the ferrite phase. Non-Patent Document 1 describes that in a cold rolled steel sheet, when the carbon concentration in the ferrite phase is increased, dynamic strain aging occurs in which the potential in the ferrite phase is fixed by carbon during the tensile test, thereby causing a significant decrease in ductility. .

[Patent Document 1] Japanese Unexamined Patent Publication No. 60-204865 [Patent Document 2] Japanese Unexamined Patent Publication No. 63-24046 [Patent Document 3] Japanese Patent Application Laid-Open No. 3-23674 [Patent Document 4] Japanese Unexamined Patent Publication No. 6-145895 [Patent Document 5] Japanese Patent Application Laid-Open No. 7-113119 [Patent Document 6] Japanese Patent Application Laid-Open No. 11-199980 [Patent Document 7] Japanese Unexamined Patent Publication No. 2008-208450 [Patent Document 8] Japanese Unexamined Patent Publication No. 2006-249561 [Patent Document 9] Japanese Unexamined Patent Publication No. 2001-220649 [Patent Document 10] Japanese Unexamined Patent Publication No. 2007-262496 [Patent Document 11] Japanese Unexamined Patent Publication No. 2003-334606 [Patent Document 12] Japanese Patent Application Laid-Open No. 11-199979 [Patent Document 13] Japanese Unexamined Patent Publication No. 2001-279381

[Non-Patent Document 1] Japanese Society of Metals, Vol. 45, No. 9 (1981) 942 to 947

By increasing the amount of wire drawn at the time of wire drawing of steel wire, the strength of the tension can be increased even by the prior art, but the problem of deterioration in ductility was inevitable. This invention provides the high strength steel wire which is 4500 Mpa or more and excellent in ductility, especially high strength ultrafine wire, based on the above phenomenon.

MEANS TO SOLVE THE PROBLEM This invention employ | adopted the following means in order to solve the said subject.

(1) The 1st aspect of this invention contains the chemical component of C: 0.7-1.2 mass%, Si: 0.05-2.0 mass%, Mn: 0.2-2.0 mass%, and remainder contains Fe and an unavoidable impurity. The steel wire is a steel wire having a pearlite structure, an average C concentration of 0.2% by mass or less in the ferrite phase center of the outermost layer of the steel wire, and a residual compressive stress in the longitudinal direction of the steel wire of the outermost layer of 600 MPa or more.

(2) In the steel wire as described in said (1), Cr: 0.05-1.0 mass%, Ni: 0.05-1.0 mass%, V: 0.01-0.5 mass%, Nb: 0.001-0.1 mass%, Mo: 0.01-0.1 mass %, B: 0.0001-0.01 mass% may contain 1 or more types of chemical components further.

(3) The high strength fine wire may be a steel wire according to the above (1) or (2) having a tensile strength of 4500 MPa or more.

(4) The high strength ultrafine steel wire described in the above (3) may be a steel cord.

(5) The high strength ultrafine steel wire of the said (3) may be a saw wire.

(6) The 2nd aspect of this invention is a manufacturing method of the steel wire which has the tensile strength of 4500 Mpa or more, The chemical component of C: 0.7-1.2 mass%, Si: 0.05-2.0 mass%, Mn: 0.2-2.0 mass% A pitting process for producing a pearlite structure by subjecting the steel wire containing the remaining portion to Fe and an unavoidable impurity to produce a pearlite structure, and an average C concentration of the ferrite phase center in the pearlite structure of the outermost layer of the steel wire. It is a manufacturing method of the steel wire provided with the drawing process which draws the said steel wire by controlling to 0.2 mass% or less, and the residual stress provision process which gives a residual compressive stress of 600 Mpa or more to the said steel wire.

The steel wire according to the present invention can exhibit high strength and ductility because the carbon concentration of the ferrite phase central portion of the outermost layer of the steel wire having a pearlite structure is controlled and residual compressive stress is applied.

In addition, since it becomes possible to provide a high strength steel wire having sufficient ductility and tensile strength, it is possible to reduce the weight of the manufactured product.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the result of having investigated about the relationship of the mean C density | concentration, surface residual stress, and ductility of the ferrite phase center part of the surface of a micro fine wire of 4500 Mpa or more.
It is a figure which shows the block cutting process in the method of taking out the needle sample of the area | region inside 1 micrometer from the surface of an ultrafine steel wire.
2B is a diagram illustrating a process of fixing the block on the needle base.
It is a figure which shows the said block processed by the focused ion beam (FIB) apparatus.
Figure 2d is a view of the block from above.
Fig. 2E is a view of the needle sample obtained by FIB processing on the block.
2F is a view of the needle sample observed from the side.
3 is a diagram showing the C distribution and the C concentration of the ferrite phase center measured by the three-dimensional atom probe method (3DAP).

The present inventors have variously analyzed the dominant factor of the ductility of high strength steel wires, and as a result, the concentration of carbon (hereinafter referred to as C) in the ferrite phase in the outermost layer of steel wires in the steel wire-wrought steel structure, It was newly discovered that the residual stress in the longitudinal direction of the steel wire of the outermost layer strongly influences the ductility of the steel wire. It is considered that this is because, in bending and torsion, the outermost layer of the steel wire is subjected to a stronger stress than the inside and thus becomes a starting point of failure. Although the method of investigating the residual stress of an outermost layer existed previously, the method of measuring the C density | concentration in the ferrite phase of the outermost layer of the steel wire within 2 micrometers from the surface of a steel wire did not exist with high precision. This time, this method was developed and the relationship with the characteristics was investigated. As a result, the C concentration in the ferrite phase of the outermost layer of the steel was below the prescribed value, and at the same time, the residual stress in the longitudinal direction of the steel wire was compressed, and this compressive stress was determined to be a specific value. By controlling so that it may become the above, it discovered that the balance of the intensity | strength and ductility of an ultrafine steel wire improved significantly.

On the other hand, the outermost layer of the steel wire is subjected to more stringent processing on the inside of the steel wire, and undergoes severe temperature change due to frictional heating. Thus, the structure and state are obviously different from those inside the steel wire. Therefore, cementite decomposition progresses more and the C concentration in the ferrite phase of the outermost layer generally shows a higher concentration than the C concentration in the ferrite phase inside the steel wire. Since the outermost layer of the steel wire is most strongly involved in the characteristics, it has been found that steel wire having excellent balance between strength and ductility can be almost realized by control of the structure of the outermost layer.

High-strength steel wire is generally obtained by performing high drawing process and strengthening the wire rod which has a pearlite structure using dice | dies etc. In the production of such a high strength steel wire, the high-strength deformation generated during the high-strength processing causes the cementite in the pearlite structure to be micronized and decomposed to dissolve C in the ferrite phase.

The present inventors combine a three-dimensional Atom probe method (hereinafter referred to as 3DAP) capable of measuring the local concentration of C in the micronized region with a needle sample fabrication technique from the outermost layer of the steel wire first made possible for this time, The relationship between the C concentration in the ferrite phase and the strength and ductility of the steel wire at the place was examined in detail. As a result, in particular, it has been found that the C concentration in the ferrite phase of the steel wire surface layer is increased, or the residual stress of the same outermost layer is stretched in the longitudinal direction of the steel wire, or in the case of weak compression, the ductility is significantly decreased (Fig. 1).

That is, in order to ensure sufficient ductility, it turned out that the carbon state and residual stress of an outermost layer of steel wire need to be satisfied at the same time. Such a discovery was discovered for the first time this time by newly developing a method for investigating the C local concentration of the outermost layer of the steel wire and enabling the investigation of the carbon state of the outermost layer of the steel wire.

From these findings, in order to realize a strength steel wire with sufficient ductility, the average C concentration of the ferrite phase center of the outermost layer of the steel wire is set to a specific value or less, and the residual stress in the longitudinal direction of the steel wire length is sufficient to compress the compressive stress of sufficient magnitude. We came to the conclusion that it is necessary to do so.

In addition, the inventors produced samples having a tensile strength of 4500 MPa or more by various manufacturing methods, and investigated the relationship between the tensile strength and ductility, and the average C concentration of the ferrite phase central part of the surface pearlite structure and the residual stress on the surface. The average C concentration of the ferrite phase core of the outermost layer of the steel wire was measured by 3DAP, and the residual stress was examined by X-ray diffraction. Tensile strength measurement was performed by the tensile tester, and the torsion test which is one of ductility evaluation was performed by the torsion tester. As the ductility index, the number of twists to break was measured.

Fig. 1 shows the relationship between the average C concentration of the ferrite phase center at the position of 1 μm below the steel wire surface, the residual stress in the longitudinal direction of the steel wire of the steel wire outermost layer, and the ductility indicated by the number of twists until breakage by the torsion test. The result of the investigation is shown. Here, the sample with 20 or more times of twists was shown with the white circle (good ductility), and the 25 times or more sample was shown with the white square (good ductility). In addition, the sample of less than 20 times was shown with the black triangle (duty). A steel wire having a tensile strength of 4500 MPa or more and good ductility was observed only in the case where the average C concentration of the ferrite phase center of the outermost layer of the steel wire was 0.2% by mass or less and the residual stress was -600 MPa or less. In addition, the steel wire which becomes very ductile was observed only when it became strong compressive stress whose average C concentration of a ferrite phase center is 0.1 mass% or less and residual stress is -600 Mpa or less.

From the above results, in order to realize high strength and sufficient ductility, the average C concentration of the ferrite phase center portion of the steel wire outermost layer is 0.2% by mass or less, more preferably 0.1% by mass or less, and the length of the steel wire in the steel wire outermost layer The residual stress is preferably -600 MPa, more preferably -700 MPa or less. The lower the average C concentration is, the more preferable it is, but the carbon concentration at the center of the ferrite phase of the pearlite structure of the final parting material is, in principle, the lowest carbon concentration. Therefore, you may set the lower limit of the average C density | concentration of the ferrite phase center part of outermost layer to 0.0001 mass%. On the other hand, the maximum value of the residual compressive stress corresponds to the yield stress of the steel wire in principle, but may be substantially -3000 MPa. Applying a compressive stress greater than this leads to a significant cost increase and is not practical.

Here, a steel wire outermost layer shows the area | region within 2 micrometers from the surface except the plating phase and the heterogeneous phase of the surface. In addition, the ferrite phase center part of the pearlite structure of the outermost layer of a steel wire means the area | region (half area | region of the width of a ferrite phase) containing the distance from the position of the center surface of a ferrite phase to 1/4 of the width of a ferrite phase to both sides. do.

The steel wire which concerns on one Embodiment of this invention based on the above-mentioned discovery contains 0.7-1.2 mass% of C, 0.05-2.0 mass% of Si, 0.2-2.0 mass% of Mn, and remainder is Fe and an unavoidable impurity. It is a steel wire having. This steel wire has the pearlite structure processed by wire, the average C concentration of the ferrite phase center part of an outermost layer is 0.2 mass% or less, and the residual compressive stress of the steel wire longitudinal direction of the outermost layer of the said steel wire is 600 Mpa or more, It is characterized by the above-mentioned. Below, the reason for limitation is explained in full detail. In addition, "%" shown below means "mass%" unless there is particular notice.

C: C has the effect of increasing the tensile strength after the patting treatment and the drawing work hardening rate, and it is possible to increase the tensile strength with less drawing work deformation. When the C content is 0.7% or less, it is difficult to realize the high strength steel wire which is the object of the present invention. On the other hand, when the C content exceeds 1.2%, the cornerstone cementite precipitates at the austenite grain boundary during the parting treatment and the wire workability is deteriorated, which leads to the drawing. It may cause disconnection during processing. For this reason, the range of C content is prescribed | regulated as 0.7 to 1.2%.

Si: Si is an effective element for strengthening the ferrite phase in pearlite and also for deoxidation of steel. If the Si content is less than 0.05%, the above-described effects cannot be expected. On the other hand, if the Si content is more than 2%, hard SiO ₂ inclusions that are detrimental to drawability are likely to be generated. For this reason, the range of Si content is prescribed | regulated to 0.05 to 2.0%.

Mn: Mn is not only necessary for deoxidation and desulfurization but is also an effective element for improving the hardenability of steel to increase tensile strength after parting treatment. However, when the Mn content is less than 0.2%, the above effects are not obtained. On the other hand, when the Mn content exceeds 2.0%, the above effects are saturated, and the processing time until the completion of pearlite transformation in the parting process becomes too long. Productivity is lowered. For this reason, the range of Mn content is prescribed | regulated as 0.2 to 2.0%.

The steel wire which concerns on one Embodiment of this invention mentioned above may further contain 1 or more types of Cr, Ni, V, Nb, Mo, B for the following reasons.

Cr: Cr refines the spacing of the cementite phase of pearlite to increase the tensile strength after the patting treatment and improves the drawing work hardening rate. However, when Cr content is less than 0.05%, the said effect is little, On the other hand, when it exceeds 1.0%, the pearlite transformation end time in a parting process will become long, and productivity will fall. For this reason, it is preferable to make Cr content into 0.05 to 1.0% of range.

Ni: Ni has the effect of making the pearlite produced by transformation during the patting treatment to have good wire workability. However, when the Ni content is less than 0.05%, the above-described effect is not obtained. Is less. For this reason, it is preferable to make Ni content into 0.05 to 1.0% of range.

V: V has the effect of making the gap between the cementite phase of pearlite finer to increase the tensile strength at the time of parting treatment. However, this effect is insufficient when the V content is less than 0.01%, while the effect is saturated when the content exceeds 0.5%. For this reason, it is preferable to make V content into 0.01 to 0.5% of range.

Nb: Like V, Nb has an effect of making the gap between the cementite phase finer and increasing the tensile strength during the parting process. However, when the Nb content is less than 0.001%, the effect is saturated. For this reason, it is preferable to make Nb content into 0.001 to 0.1% of range.

Mo: Mo, like V, has the effect of making the gap between the cementite phase finer and increasing the tensile strength at the time of the tenting treatment, but when the Mo content is less than 0.01%, the effect is saturated when it exceeds 0.1%. For this reason, it is preferable to make Mo content into 0.01 to 0.1% of range.

B: B has the effect of fixing N as BN and preventing aging deterioration by N, and in order to fully exhibit this effect, it is necessary to contain B content in steel materials 0.0001% or more. On the other hand, even if it adds so that B content in steel materials may exceed 0.01%, since the effect is saturated and further B content causes a raise in manufacturing cost, it is unpreferable. For this reason, in this invention, when B is contained in steel materials, it is preferable to make content of B into 0.0001 to 0.01% of range.

Although other elements are not specifically limited, P: 0.015% or less, S: 0.015% or less, N: 0.007% or less are preferable ranges as an element contained as an impurity. In addition, Al is exceeding 0.005%, and most likely to be turned hard Al ₂ O ₃ inclusions are produced in the inclusions in the steel, because it may cause breakage at the time of processing fresh or processed along the line, is not more than 0.005%, the preferred range.

In addition to the above elements, impurities which are inevitably mixed in the manufacturing process or the like may be contained, but it is preferable to prevent the impurities from mixing as much as possible.

In order to give the average C concentration of the ferrite phase center part of the outermost layer of the steel wire in the freshly prepared fine pearlite structure of the steel wire to be 0.2% by mass or less, and to provide a sufficient residual compressive stress, the manufacturing process after the final parting treatment In the following, it is most effective to employ one production method each from the following A group, B group, and C group. For example, even if three manufacturing methods are adopted, when one of the manufacturing methods is shifted to one group and all manufacturing methods are not adopted, sufficient effects are not obtained. In addition, when two types are employ | adopted from the same group, a characteristic may rather fall. Even if a manufacturing method from all groups is adopted and another manufacturing method is adopted from a group, no effect is obtained. This is because the preparations in the same group basically give a similar effect, whereas if another preparation is added, the effects may be canceled out. Therefore, as described above, it is preferable to employ the production method one by one from each group.

(A group manufacturing method)

A1: The skin pass process is put into the last stage once, preferably several times.

Skin pass drawing which is one of the important manufacturing methods is a method of drawing with the reduction rate especially small than the reduction rate (10% or more) of normal drawing. As this reduction ratio, 1% or more and 6% or less are preferable, and 2% or more and 5% or less are more preferable. When the reduction ratio is less than 1%, it is difficult to apply the entire surface layer of the steel wire, and when the reduction rate exceeds 7%, the processing amount is too large to obtain the residual compressive stress of the desired surface and the carbon concentration in the ferrite phase. It becomes impossible. This skin path drawing may be performed independently by a single dice system, or may be performed simultaneously with the normal drawing by a double dice system. By inserting the skin pass step having a reduction rate of 1% to 6% into the final stage once, preferably a plurality of times, it is possible to apply compressive residual stress to the steel wire surface and to make the surface lamellar structure more uniform. The application of the appropriate residual compressive stress on this surface and the effect of removing the carbon stuck to the potential make it easy to reduce the local high capacity of carbon and suppress the cementite decomposition of the outermost layer.

A2: Shot peening is carried out after drawing.

Shot peening is a method in which a spherical shot of a specific size and a specific size is irradiated to the entire steel wire at a specific pressure, thereby forming a processed layer or a strained layer only on the surface area of the steel wire. The shot peening is preferably, for example, by air projection, with an air pressure of 4 to 5 x 10 ⁵ Pa, a time of 5 to 10 seconds, and a short spherical shape of 10 to 100 m. It is effective to irradiate a sufficient amount on the entire surface of the steel wire.

By short peening after drawing, the compressed residual stress is applied to the steel wire surface, and the lamellar structure of the surface is aligned to be more uniform. By application of appropriate residual compressive stress on this surface and the effect of removing the carbon stuck to the potential, the local high capacity of carbon is reduced and the cementite decomposition of the outermost layer is suppressed.

(B group manufacturing method)

B1: The drawing speed of the last stage is performed by low speed drawing of 200 m / min or less, preferably 50 m / min or less.

By performing low-speed drawing, the amount of heat generated by friction or plastic deformation can be reduced, thereby suppressing decomposition of cementite in the pearlite structure and reducing the amount of carbon diffused in the ferrite phase.

B2: The heat processing of the temperature of 40-400 degreeC is performed between the wire | drawing process paths at 0.5 second-5 minutes, More preferably, it is the temperature of 100-300 degreeC for 1 second-3 minutes.

The wire temperature by the drawing process rises instantaneously and drops immediately. Apart from this, heat treatment at an appropriate temperature is carried out between the drawing passes, so that supersaturated carbon dissolved in cementite during the drawing process and dissolved in the ferrite phase is discharged from the ferrite phase by heating treatment between the passes. The concentration of C in the mixture can be lowered, and unnecessary point defects (atomic vacancy, etc.) and dislocations can be eliminated. As a result, the ductility is recovered to enable processing of a high strain amount, that is, miniaturization of the ferrite phase gap. However, this process is not carried out in the whole between the drawing process passes, but it is effective to carry out between specific passes.

B3: In the final step including the skin pass and the drawing process before the one, a die having an approach angle of 8 to 12 ° and a dynamic friction coefficient of 0.1, preferably 0.05 or less.

By using a die with a small approach angle and a small coefficient of dynamic friction, frictional heat generation during drawing is suppressed, and an increase in the C concentration in the ferrite phase due to cementite decomposition due to the temperature rise of the outermost layer is suppressed. It is effective to use this in a process close to the final stage.

(C group manufacturing method)

C1: After drawing, the heating and holding at 60 to 300 ° C. is carried out at 0.1 to 24 hours, more preferably at 180 to 260 ° C. for 20 seconds to 15 minutes.

By aging during or after drawing, cementite is decomposed to release supersaturated carbon dissolved in the ferrite phase, thereby lowering the carbon concentration in the ferrite phase. However, when this temperature is too high, spherical cementite and transition carbide are formed, and when too low, the effect is small. It is necessary to set it to suitable temperature according to steel type and drawing condition.

C2: The process of 20% or more of large reduction | restoration rate is put into 1 time, and preferably multiple times during the wire | drawing process except three steps before the last stage.

By putting the process of large reduction ratio of 20% or more once, preferably a plurality of times, strain can be uniformly introduced to the inside without biasing the wire strain on the surface. It is effective to perform this before 3 steps before a last step.

The C concentration in the ferrite phase in the steel wire can be accurately measured by the three-dimensional atom probe method (3DAP). However, conventionally, the C concentration in the ferrite phase in the fresh pearlite structure of the outermost layer of the steel wire cannot be measured. By using a focused ion beam (FIB) device, small pieces are cut out from the surface of the wire and processed by FIB to develop a technique for preparing a needle sample, and the carbon concentration of the outermost layer can be measured with high accuracy. .

Since the solid solution C concentration may show a different value depending on the difference in position in the ferrite phase, care should be taken. When cementite decomposes and C diffuses in the ferrite phase, in general, the C concentration at the interface position of the ferrite phase / cementite phase is high, and the value is smallest at the center position of the ferrite phase. In this embodiment, the average C density | concentration of the area | region (half area | region of the width | variety of a ferrite phase) containing the distance from the position of the center plane of a ferrite phase to 1/4 of the width | variety of a ferrite phase is prescribed | regulated.

By analyzing with 3DAP, the C concentration in the ferrite phase including the interface of the ferrite phase / cementite phase can be measured. Therefore, by selecting and cutting out a box of a specific size to the area to be examined from the measurement data, the C atoms and the whole of the box are cut out. By calculating the ratio of atoms, the C concentration in the ferrite phase can be obtained in atomic%. By multiplying this by 12/56, it can convert into mass%. These measurements were performed for a plurality of ferrite phase centers to obtain an average, which was taken as the average C concentration of the ferrite phase centers.

As an example, FIGS. 2A to 2F show a method of preparing a needle sample for measuring the C concentration of a ferrite-like central part within 1 μm from the wire surface, and FIG. 3 shows a C distribution measured by 3DAP using a manufactured needle sample. And C concentrations in the center of the ferrite phase.

In order to produce the needle sample of the area | region inside 1 micrometer from a steel wire surface, for example, as shown in FIG. 2A, the rod-shaped block which includes a steel wire surface on one side is cut out by FIB from a steel wire surface area | region. This block is fixed on the needle base as shown in Fig. 2B, for example by deposition of tungsten or the like. This block is processed by FIB so that the tip part becomes thin as shown in FIG. 2C. Fig. 2D is a view of the block after processing from the top, and it can be seen that the tip portion has a rod shape including the steel wire surface. Then, the tip was processed into a needle shape by irradiating a ring-shaped beam from the top. Fig. 2F is a view of the needle sample made in this way from the side. As shown in Fig. 2E, the needle tip position was produced to correspond to 1 mu m inside of the steel wire surface. By using such a needle sample preparation technique, a needle sample of the steel wire outermost layer can be produced.

In addition, in FIG. 3, the dark part shows high C density and the light part shows low C concentration. Therefore, the dark band region shows a cementite phase that has been drawn, and the light region between them shows a ferrite phase that has been drawn. Among the ferrite phases, C is shown to be employed.

As shown in the figure, the carbon concentration at the center of the ferrite phase can be estimated by cutting out the box from the center position of the ferrite phase and dividing the number of C atoms contained in the box by the total number of atoms in the box. In this example, C concentration is 0.18 mass%. The center of the ferrite phase is located in the middle of two cementite phases and corresponds to a region (half the width of the ferrite phase) from the position of the center surface of the ferrite phase to a distance of 1/4 of the width of the ferrite phase on both sides.

The width of the ferrite phase is not necessarily constant depending on the amount of processing and the location of the sample, and a region of 10 nm or less also exists in a narrow portion. If the cementite region is included in the box position, the concentration becomes higher than the actual C concentration in the ferrite phase. Therefore, the box position to analyze was made into the ferrite phase center part, and the box width was made into half the width of the ferrite phase. In addition, as an expectation of average C density | concentration, 5 or more are made into the average of the measured value of C density | concentration of 10 or more other ferrite phase center part.

The residual stress of the outermost layer of the steel wire can be measured with high accuracy by, for example, X-ray diffraction. In particular, it can be measured accurately by the debye ring fitting method using a microregion X-ray diffraction apparatus capable of measuring the local region. This method is a method of fitting the reflection of the grain of steel wire as a divider ring, and examining the magnitude direction of residual stress from the deformation of the divider ring. From the depth of penetration of the X-rays a depth region comprising the surface is determined. For example, when the X-ray source is Cr, an integrated value of a depth of several micrometers on the surface is obtained. In addition, another method of examining the residual stress on the surface of the steel wire is the occasional dissolution method (Heyn method). This is a method of investigating the residual stress of the steel wire longitudinal direction by measuring the difference of the length of the steel wire before and after melt | dissolving the outermost layer to irradiate. In all of these methods, the residual stress of the high strength steel wire in which the aggregate structure was developed can be calculated | required with high precision.

(Example)

Hereinafter, the feasibility and the effect of this invention are demonstrated further more concretely by an Example.

After the test-providing material having the chemical composition shown in Table 1 was hot rolled to a predetermined wire diameter, the wire diameter was 0.04 to 0.40 so that the tensile strength was 4500 MPa or more by carrying out parting treatment and wire drawing using a bath. A high-strength ultrafine steel made of a fresh pearlite structure having a brass plating of mm was started. Brass plating was performed after pickling after the final parting process.

In Table 2, until the true deformation of the ultrafine steel wire, the manufacturing method, the wire diameter, the average C concentration of the ferrite phase center of the steel wire outermost layer, the residual stress of the steel wire outermost layer, the tensile strength and the fracture in the torsion test Indicates the number of twists. In Table 2, the manufacturing method was shown by the symbol which shows the content mentioned above. The torsion test was carried out at a holding interval of 100 times the diameter of the both ends of the test piece, and the number of twists until fracture was examined. Tensile strength was 4500 Mpa or more and the number of twists was 20 or more, and ductility was good, and 25 or more times were evaluated as ductility very good. The C concentration in the ferrite phase of the steel outermost layer was measured by 3DAP using the method described above, and the residual stress in the longitudinal direction of the steel wire of the outermost steel layer was measured by the above-described dividing fitting method. In the case where the residual stress is negative, the compressive stress is shown; in the positive case, the tensile stress is shown.

In Table 2, Test Nos. 1 to 6 are examples of the present invention, and others are comparative examples. As shown in Table 2, all of the examples of the present invention had a tensile strength of 4500 MPa or more, an average C concentration of 0.2 mass% or less, and a residual stress of -600 MPa or less (residual compressive stress of 600 MPa) of the ferrite phase center of the outermost layer. Above). As a result, an ultrafine steel wire having sufficient ductility with a high number of twists can be realized. In particular, Test Nos. 1 to 2 had very good twist times of 25 or more.

On the other hand, although the test numbers 7-20 are comparative examples and the tensile strength is 4500 Mpa or more, the number of twists was inadequate.

Nos. 7 to 9 are comparative examples in which the components of the steel wire are outside the scope of the present invention. Since the C amount of steel wire was too small, and the wire strain amount was increased too much, C density | concentration of the ferrite phase center became more than a prescribed value, and ductility fell. In addition, No. 8 is a comparative example in which the Si amount of steel wire and No. 9 are C amounts higher than the range of this invention. In these comparative examples, although the residual stress and the C concentration of the ferrite phase center were within the prescribed range, the ductility was lowered.

In addition, although the components and residual stress of steel wire are in the scope of the present invention, numbers 10-13 are comparative examples whose C concentration of the ferrite phase center part of outermost layer is more than a prescribed value. In these comparative examples, ductility fell. Nos. 14 to 16 are comparative examples in which the components of the steel wire and the C concentration of the ferrite phase center are within the scope of the present invention, but the residual stress is outside the range. In these comparative examples, ductility fell. Nos. 17 to 20 are comparative examples in which both the C concentration and the residual stress in the ferrite phase central part of the outermost layer were out of range. In these comparative examples, ductility fell.

Since the present invention enables the provision of a high strength steel wire having sufficient ductility, the contribution to the industry is very large.

Claims (6)

C: 0.7-1.2 mass%,
Si: 0.05-2.0 mass%,
Mn: 0.2-2.0 mass%
Is a steel wire containing a chemical component of and the remainder containing Fe and an unavoidable impurity,
The steel wire has a pearlite structure,
The average C concentration of the ferrite phase center part of the outermost layer of the said steel wire is more than 0 mass% and 0.2 mass% or less,
Residual compressive stress of the steel wire longitudinal direction of the said outermost layer is 600 Mpa or more, The steel wire characterized by the above-mentioned. Cr: 0.05-1.0 mass%,
Ni: 0.05-1.0 mass%,
V: 0.01-0.5 mass%,
Nb: 0.001-0.1 mass%,
Mo: 0.01-0.1 mass%,
B: 0.0001-0.01 mass%
The steel wire, further containing at least one chemical component of. The steel wire according to claim 1 or 2, wherein the steel wire is a high strength ultrafine steel wire having a tensile strength of 4500 MPa or more. The steel wire according to claim 3, wherein the high strength ultrafine steel wire is a steel cord. The steel wire according to claim 3, wherein the high strength ultrafine steel wire is a saw wire. It is a manufacturing method of steel wire which has tensile strength of 4500 Mpa or more,
A pearlite structure was prepared by subjecting a steel wire containing C: 0.7 to 1.2% by mass, Si: 0.05 to 2.0% by mass, and Mn: 0.2 to 2.0% by mass, with the remaining portion containing Fe and unavoidable impurities. Parting process to generate,
A drawing process of drawing the steel wire by controlling the average C concentration of the ferrite phase central part in the pearlite structure of the outermost layer of the steel wire to more than 0% by mass and 0.2% by mass or less;
And a residual stress imparting step of imparting a residual compressive stress of 600 MPa or more to the steel wire.

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