JP4719320B2 - High strength extra fine steel wire and method for producing the same - Google Patents

Description

The present invention relates to a high-strength steel wire used for a steel cord or saw wire of an automobile tire and a method for manufacturing the same. Specifically, the present invention relates to an ultra fine steel wire having a wire diameter of 0.04 to 0.4 mm and a strength of 4500 MPa class or higher, which is cold-drawn and strengthened by using a die.
This application claims priority on June 22, 2009 based on Japanese Patent Application No. 2009-148051 for which it applied to Japan, and uses the content here.

  In steel cords used for automobile tires, there is an increasing need for higher tensile strength of steel wires due to demands for weight reduction of tires. Similarly, in the saw wire for precisely cutting sapphire crystals, SiC crystals, etc., there is an increasing need for higher tension. Numerous studies have been vigorously developed to answer these needs. As a result, it became clear that it was necessary to ensure sufficient ductility in addition to increasing the tensile strength of the steel wire. There are several ductility indexes. For example, there are the number of twists until breakage in the torsion test and the presence or absence of cracks (delamination) that occur in the longitudinal direction of the steel wire during the torsion test. High strength of steel wire is accompanied by a decrease in ductility, and it is important to suppress this. In addition, it is important that high ductility does not substantially deteriorate due to aging because high strength steel wire has a phenomenon that its properties deteriorate due to room temperature aging (20 ° C. to 40 ° C., several days to several years). is there.

  High-strength steel wires are generally manufactured by drawing a wire having a pearlite structure using a die or the like. This processing reduces the pearlite lamella spacing, and introduces a large amount of dislocations in the ferrite phase, thereby increasing the tensile strength. It has recently been clarified that cementite in a pearlite structure is refined and decomposed when the wire drawing strain becomes very large. However, since the structure is particularly fine, the relationship between the location and state of these carbons and the mechanical properties has not been clarified, and there are many unclear points regarding the cause of ductile deterioration. In an actual high-strength steel wire, the structure and local strain in the steel wire do not always appear to be the same in the surface region and the central region, which also affects the properties of the steel wire. it is conceivable that.

  In order to increase the strength of the ultrafine steel wire, it is necessary to increase the strand strength after the final patenting process or increase the final wire drawing distortion. However, even if the strength of the ultrafine steel wire is increased by increasing the wire strength after the final patenting treatment or the wire drawing strain, if the strength exceeds 4500 MPa, the ductility is remarkably lowered and may be put into practical use. It was extremely difficult.

  On the other hand, for example, Patent Document 1, Patent Document 2, and Patent Document 3 define chemical components such as C, Si, Mn, and Cr, respectively, as conventional knowledge of a high-strength means with little reduction in ductility. A high-strength, high-ductility, high-carbon steel wire rod for ultrafine wires has been proposed. However, as can be seen from the examples disclosed in these publications, the maximum tensile strength of the steel wire is 3500-3600 MPa, and there is a limit to increasing the strength of the ultrafine steel wire.

  Patent Document 4 proposes a high-strength steel and high toughness steel wire rod in which the chemical composition, the non-metallic inclusion structure, and the area fraction of proeutectoid cementite are controlled. Furthermore, Patent Document 5 discloses a method for producing a high-strength steel and a high toughness ultrafine wire steel that controls the chemical composition of the steel and the area reduction rate of the final die. However, even with these techniques, it has been impossible to realize an ultrafine steel wire having a tensile strength of 4500 MPa or more and high ductility.

  In addition, there is another finding that the properties of steel cords are affected by the carbon concentration in the ferrite phase of the pearlite structure, and guidelines for improving the balance between strength and ductility by disclosing these concentrations are disclosed. ing. For example, in patent document 6, it is going to acquire a favorable characteristic by prescribing | regulating the carbon concentration in a steel wire. Patent Document 7 discloses a method of achieving a preferable carbon state and obtaining good characteristics by further devising heat treatment. Furthermore, Patent Document 8 attempts to obtain good characteristics by defining the carbon concentration in the steel wire and the lamellar spacing. However, all of these do not mention the carbon state of the outermost steel wire layer (region from the surface to a depth of 2 μm). This originates from the fact that actual measurement (and control) was not possible with the technology at that time.

  Further, Patent Document 9 defines variation in carbon concentration. Further, Patent Document 10 defines the degree of difference in lamella spacing that affects the variation in carbon concentration. However, these describe the overall variation and do not prescribe the carbon concentration at a specific location. On the other hand, patent document 11 shows the manufacturing method of the steel wire for obtaining a favorable characteristic by prescribing | regulating the C concentration ratio in the ferrite phase in a steel wire surface layer part and a steel wire center part, and a steel wire. ing. However, it is a regulation as a relative value between the center part and the surface layer part, and an absolute value is not prescribed for a clear index. In addition, the actual measurement is made inside 10 μm or more away from the surface, and the C concentration in the region (outermost layer) from the surface to 2 μm is not controlled.

  On the other hand, regarding the residual stress of the outermost layer of the steel wire, Patent Literature 12 and Patent Literature 13 define a range of residual stress from the viewpoint of fatigue resistance and longitudinal crack resistance. However, while the residual compressive stress is preferable, the absolute value of the value is small, and the range for obtaining a very good balance between ductility and strength has not yet been defined. Furthermore, there is no example disclosed about the relationship with the carbon state of the outermost layer.

  It is the ductility of the ferrite phase that is responsible for the ductility of the high-strength ultrafine steel wire. If the ductility of the ferrite phase is maintained, ductility is ensured even at high strength. However, when the wire drawing strain increases, cementite is generally decomposed, C atoms diffuse into the ferrite phase, and the carbon concentration in the ferrite phase increases. In Non-Patent Document 1, in a cold-rolled steel sheet, when the carbon concentration in the ferrite phase increases, dynamic strain aging occurs in which the dislocations in the ferrite phase are fixed by carbon during the tensile test, resulting in a significant decrease in ductility. It is stated to cause.

JP 60-204865 A JP 63-24046 A Japanese Patent Publication No. 3-23674 JP-A-6-145895 JP 7-113119 A JP-A-11-199980 JP 2008-208450 A JP 2006-249561 A JP 2001-220649 A Japanese Patent Laid-Open No. 2007-262496 JP 2003-334606 A JP-A-11-199979 JP 2001-279281 A

Journal of the Japan Institute of Metals Vol. 45, No. 9 (1981) 942-947

  By increasing the amount of wire drawing at the time of wire drawing of the steel wire, the tension can be increased even with the prior art, but the problem of reduced ductility is inevitable. The present invention provides a high-strength steel wire, particularly a high-strength ultrafine steel wire, having a high strength of 4500 MPa or more and excellent ductility, against the background of the current situation as described above.

The present invention employs the following means in order to solve the above problems.
(1) The 1st aspect of this invention is the chemistry of C: 0.7-1.2 mass%, Si: 0.05-2.0 mass%, Mn: 0.2-2.0 mass%. A steel wire comprising a component and the balance being Fe and inevitable impurities , the steel wire having a pearlite structure, and an average C concentration in the center of the ferrite phase of the outermost layer of the steel wire being 0.2 mass %, And the residual compressive stress in the longitudinal direction of the outermost steel wire is 600 MPa or more.

(2) In the steel wire described in (1) above, Cr: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0 mass%, V: 0.01 to 0.5 mass%, Nb: 0.001 to 0.1% by mass, Mo: 0.01 to 0.1% by mass, B: 0.0001 to 0.01% by mass may be further contained. .

(3) The steel wire according to the above (1) or (2) may be a high-strength ultrafine steel wire 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 described in (3) above may be a saw wire.

(6) A second aspect of the present invention is a method for producing a steel wire having a tensile strength of 4500 MPa or more, and C: 0.7 to 1.2 mass%, Si: 0.05 to 2.0. A patenting step of producing a pearlite structure by performing a patenting treatment on a steel wire containing a chemical component of mass%, Mn: 0.2 to 2.0 mass%, with the balance being Fe and inevitable impurities; A wire drawing step of drawing the steel wire by controlling an average C concentration of a ferrite phase central portion in the pearlite structure of the outermost layer of the steel wire to 0.2% by mass or less; and a residual compression of 600 MPa or more to the steel wire A method for producing a steel wire comprising: a residual stress applying step for applying stress.
(7) In the steel wire manufacturing method according to (6) above, the steel wire before the patenting treatment is Cr: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0. % By mass, V: 0.01-0.5% by mass, Nb: 0.001-0.1% by mass, Mo: 0.01-0.1% by mass, B: 0.0001-0.01% by mass One or more chemical components may be further contained.
(8) A method for producing a steel wire having a tensile strength of 4500 MPa or more, wherein C: 0.7 to 1.2% by mass, Si: 0.05 to 2.0% by mass, Mn: 0.2 to A patenting step of generating a pearlite structure by applying a patenting process to a steel wire containing 2.0% by mass of a chemical component, the balance being Fe and inevitable impurities; and the pearlite structure of the outermost layer of the steel wire; A wire drawing step of drawing the steel wire by controlling an average C concentration of the ferrite phase center portion to 0.2% by mass or less; a residual stress applying step of applying a residual compressive stress of 600 MPa or more to the steel wire; And after the final patenting process, one manufacturing method from each of the following A group, B group, and C group is employed.
Group A (A1: A skin pass process with a surface reduction rate of 1% to 6% is performed at the final stage. A2: Shot peening is performed after wire drawing.)
Group B (B1: The drawing speed of the final stage is 200 m / min or less. B2: Heat treatment at a temperature of 40 to 400 ° C. is performed for 0.5 second to 5 minutes between drawing processes. B3: Skin pass is performed. (In the final stage and the previous wire drawing step, a die having an approach angle of 8 to 12 ° and a dynamic friction coefficient of 0.1 or less is used.)
Group C (C1: After wire drawing, 60 to 300 ° C. is heated and held for 0.1 minute to 24 hours. C2: During wire drawing excluding 3 steps before the final step, with a reduction in area of 20% or more. Perform wire drawing.)

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 the residual compressive stress is applied. it can.
Moreover, since it becomes possible to provide the high strength steel wire which has sufficient ductility and tensile strength, the weight reduction of a product is attained.

It is a figure which shows the result of having investigated about the relationship between the average C density | concentration of the ferrite phase center part of the surface of the ultrafine steel wire of 4500 Mpa or more, surface residual stress, and ductility. It is a figure which shows the block cutting-out process in the method of taking out the needle | hook sample of the area | region inside 1 micrometer from the surface of a very fine steel wire. It is a figure which shows the process of fixing the block on a needle base. It is a figure which shows the same block processed with the focused ion beam (FIB) apparatus. It is the figure which observed the block from the upper part. It is the figure which observed the needle sample obtained by carrying out FIB processing to the same block from the top. It is the figure which observed the needle sample from the side. It is a figure which shows C density | concentration measured by the three-dimensional atom probe method (3DAP), and C density | concentration of a ferrite phase center part.

  As a result of various analyzes on the governing factors of ductility of high-strength steel wire, the present inventors have found that the carbon (hereinafter referred to as C) concentration in the ferrite phase in the outermost layer of the steel wire in a strongly processed drawn pearlite structure It has been newly found that the residual stress in the longitudinal direction of the steel wire outermost layer strongly affects the ductility of the steel wire. This is thought to be because, in bending and twisting, the outermost layer of the steel wire is subjected to a stronger stress than the inside and becomes the starting point of fracture. There has been a method for investigating the residual stress in the outermost layer, but there has not been a method for accurately measuring the C concentration in the ferrite phase of the outermost layer of the steel wire within 2 μm from the surface of the steel wire. This time, we developed this method and investigated the relationship with the characteristics. As a result, the C concentration in the ferrite phase of the outermost layer of the steel wire was below the specified value, and at the same time, the residual stress in the longitudinal direction of the steel wire was compressed. It has been found that the balance between the strength and ductility of the ultrafine steel wire is greatly improved by controlling the stress to be a specific value or more.

  On the other hand, the outermost layer of the steel wire is subjected to severer processing with respect to the inside of the steel wire, and undergoes a severe temperature change due to frictional heat generation or the like. Therefore, the structure and state are clearly different from the inside of the steel wire. Accordingly, cementite decomposition further proceeds, and the C concentration in the ferrite phase of the outermost layer is generally higher than the C concentration in the ferrite phase inside the steel wire. Since the outermost layer of the steel wire is the most strongly involved in the properties, it has been found that a steel wire excellent in balance between strength and ductility can be realized almost by controlling the structure of the outermost layer.

  A high-strength steel wire is generally obtained by strengthening a wire having a pearlite structure by applying a high-stretching process using a die or the like. When such a high-strength steel wire is manufactured, a phenomenon in which cementite in the pearlite structure is refined and decomposed and C is dissolved in the ferrite phase due to high wire drawing strain generated during high wire drawing.

  The present inventors have developed a three-dimensional atom probe method (hereinafter referred to as 3DAP) that can measure the local concentration of C in a miniaturized region and a technique for producing a needle sample from the outermost steel wire layer that has been made possible for the first time. In combination, the relationship between the C concentration in the ferrite phase and the strength and ductility of the steel wire at every location in the steel wire was examined in detail. As a result, the ductility may be significantly reduced, particularly when the C concentration in the ferrite phase of the steel wire surface layer is increased, or when the residual stress of the same outermost layer is pulled or weakly compressed in the longitudinal direction of the steel wire. It was located (see FIG. 1).

  That is, in order to ensure sufficient ductility, it was found that the carbon state and residual stress of the outermost steel wire layer must be satisfied at the same time. Such knowledge was discovered for the first time when a new method for examining the C local concentration of the outermost steel wire layer was developed and the carbon state of the outermost steel wire layer could be examined.

  From these findings, in order to realize a strength steel wire with sufficient ductility, the average C concentration in the ferrite phase central portion of the outermost steel wire layer is set to a specific value or less, and the residual in the longitudinal direction of the steel wire on the surface It was concluded that the stress should be a sufficiently large compressive stress.

  In addition, the present inventors prepared samples having a tensile strength of 4500 MPa or more by various production methods, the tensile strength and ductility, the average C concentration of the ferrite phase center of the surface pearlite structure, and the surface residual stress. I investigated the relationship with. The average C concentration at the ferrite phase 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 with a tensile tester, and a torsion test, which is one of ductility evaluations, was performed with a torsion tester. As a ductility index, the number of twists to break was measured.

  FIG. 1 shows the average C concentration of the ferrite phase center at a position of 1 μm below the surface of the steel wire, the residual stress in the steel wire longitudinal direction of the outermost layer of the steel wire, and the ductility expressed by the number of twists until breaking by the torsion test. The result of having investigated about the relationship of is shown. Here, samples with 20 or more twists were indicated by white circles (good ductility), and samples with 25 or more twists were indicated by white squares (very good ductility). Samples of less than 20 times are indicated by black triangles (poor ductility). A steel wire having a tensile strength of 4500 MPa or more and good ductility is a large compression having an average C concentration of 0.2 mass% or less at the ferrite phase center portion of the outermost steel wire layer and a residual stress of -600 MPa or less. Only observed when. Furthermore, a steel wire with very good ductility was observed when the average C concentration in the ferrite phase center was 0.1% by mass or less and the residual stress was a strong compressive stress of −600 MPa or less. .

From the above results, in order to realize high strength and sufficient ductility, the average C concentration in the ferrite phase outermost layer of the steel wire is 0.2 mass% or less, more preferably 0.1 mass or less, And it is desirable that the residual stress of the steel wire outermost layer in the longitudinal direction of the steel wire is −600 MPa, more preferably −700 MPa or less. The average C concentration is preferably as low as possible, but the carbon concentration in the center of the ferrite phase of the pearlite structure of the final patenting material is in principle the lowest carbon concentration. Therefore, the lower limit value of the average C concentration at the center of the ferrite phase of the outermost layer may be set to 0.0001% by mass. On the other hand, the maximum value of the residual compressive stress corresponds in principle to the yield stress of the steel wire, but may be substantially −3000 MPa. Applying a compressive stress larger than this leads to a significant cost increase and is not practical.
Here, the steel wire outermost layer refers to a region within a depth of 2 μm from the surface, excluding the plating phase and the surface heterogeneous phase. In addition, the ferrite phase central portion of the pearlite structure of the outermost steel wire layer is a region including a distance of 1/4 of the width of the ferrite phase on both sides from the position of the central surface of the ferrite phase (region of half the width of the ferrite phase) ).

  The steel wire according to one embodiment of the present invention based on the above discovery has a C content of 0.7 to 1.2 mass%, a Si content of 0.05 to 2.0 mass%, and a Mn content of 0.2 to 2.0 mass%. It is a steel wire that is contained by mass and the balance contains Fe and inevitable impurities. This steel wire has a drawn pearlite structure, the average C concentration of the ferrite phase central portion of the outermost layer is 0.2% by mass or less, and the residual in the longitudinal direction of the steel wire of the outermost layer of the steel wire The compressive stress is 600 MPa or more. The reason for limitation will be described in detail below. “%” Shown below means “% by mass” unless otherwise specified.

  C: C has the effect of increasing the tensile strength after the patenting treatment and increasing the wire drawing work hardening rate, and it is possible to increase the tensile strength with less wire drawing strain. If the C content is 0.7% or less, it will be difficult to achieve the intended high strength steel wire in the present invention. On the other hand, if it exceeds 1.2%, the pro-eutectoid cementite will enter the austenite grain boundaries during the patenting process. It precipitates and the wire drawing workability deteriorates, causing breakage during wire drawing. For this reason, the range of C content is prescribed | regulated to 0.7-1.2%.

Si: Si is an effective element for strengthening the ferrite phase in pearlite and for deoxidation of steel. If the Si content is less than 0.05%, the above effect cannot be expected. On the other hand, if it exceeds 2%, hard SiO ₂ inclusions harmful to the wire drawing workability 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 an effective element for improving the hardenability of steel and increasing the tensile strength after patenting. However, if the Mn content is less than 0.2%, the above effect cannot be obtained. On the other hand, if it exceeds 2.0%, the above effect is saturated, and further, the pearlite transformation during the patenting process is completed. The processing time becomes too long and productivity is lowered. For this reason, the range of Mn content is prescribed | regulated to 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 interval between cementite phases of pearlite to increase the tensile strength after the patenting treatment and to improve the wire drawing work hardening rate. However, when the Cr content is less than 0.05%, the effect of the above action is small. On the other hand, when the Cr content exceeds 1.0%, the pearlite transformation finish time during the patenting process becomes long and the productivity is lowered. For this reason, it is preferable to keep Cr content in the range of 0.05 to 1.0%.

  Ni: Ni has the effect of making pearlite produced by transformation during the patenting process to have good wire drawing workability. However, if the Ni content is less than 0.05%, the above effect cannot be obtained, and 1.0% Even if the amount exceeds 50%, the effect corresponding to the amount added is small. For this reason, it is preferable to keep Ni content in the range of 0.05 to 1.0%.

  V: V has the effect of increasing the tensile strength during patenting by reducing the interval between the pearlite cementite phases, but this effect is insufficient when the V content is less than 0.01%. If it exceeds 5%, the effect is saturated. For this reason, it is preferable to keep V content in the range of 0.01 to 0.5%.

  Nb: Nb, like V, has the effect of reducing the cementite phase interval and increasing the tensile strength during the patenting treatment, but if the Nb content is less than 0.001%, it is insufficient. If it exceeds%, the effect is saturated. For this reason, it is preferable to keep Nb content in the range of 0.001 to 0.1%.

  Mo: Mo, like V, has the effect of increasing the cementite phase interval and increasing the tensile strength during the patenting treatment. However, if the Mo content is less than 0.01%, it is insufficient. If it exceeds%, the effect is saturated. For this reason, it is preferable to keep Mo content in the range of 0.01 to 0.1%.

  B: B has an effect of fixing N as BN and preventing aging deterioration due to N, and in order to fully exhibit this effect, the B content in the steel material needs to be contained by 0.0001% or more. is there. On the other hand, even if it is added so that the B content in the steel material exceeds 0.01%, the effect is saturated, and B content beyond this is not preferable because it causes the production cost to be increased. For this reason, in the present invention, when B is contained in the steel material, the B content is preferably in the range of 0.0001 to 0.01%.

Other elements are not particularly limited, but as elements contained as impurities, P: 0.015% or less, S: 0.015% or less, and N: 0.007% or less are preferable ranges. Further, if Al exceeds 0.005%, the hardest Al ₂ O ₃ inclusions among the inclusions in the steel are likely to be generated, which may cause disconnection during wire drawing or stranded wire processing. Therefore, 0.005% or less is a preferable range.

  In addition to the above elements, impurities that are inevitably mixed in the manufacturing process may be contained, but it is preferable that impurities are not mixed as much as possible.

  In order to give a sufficient amount of residual compressive stress with an average C concentration of the ferrite phase outermost layer of the steel wire outermost layer being 0.2 mass% or less in a drawn pearlite structure of a strongly processed ultrafine wire In the manufacturing process after the final patenting process, it is most effective to employ one manufacturing method from each of the following A group, B group, and C group. Even if three manufacturing methods are adopted, sufficient effects cannot be obtained if the manufacturing method is biased to one group and all group manufacturing methods are not adopted. In addition, when two types are employed from the same group, the characteristics may be deteriorated. Adopting a manufacturing method from all groups, and even another manufacturing method from some group, will not be very effective. This is because manufacturing methods in the same group basically give similar effects, but if different manufacturing methods are added, the effects may be canceled. Therefore, as described above, it is preferable to adopt the manufacturing method one by one from each group.

(Group A manufacturing method)
A1: The skin pass process is performed once, preferably a plurality of times in the final stage.
Skin pass drawing, which is one of important manufacturing methods, is a method of drawing with a particularly small area reduction than the normal area reduction (10% or more). The area reduction rate is preferably 1% to 6%, and more preferably 2% to 5%. If the area reduction is less than 1%, it becomes difficult to process the entire surface of the steel wire, and if it exceeds 7%, the amount of processing is too large, and preferable compressive residual stress and ferrite phase on the surface. The carbon concentration in can not be obtained. This skin pass wire drawing may be performed independently by a single die method, or may be performed simultaneously with normal wire drawing by a double die method. By applying a skin pass process with a surface reduction rate of 1% to 6% once, preferably multiple times, to apply compressive residual stress to the surface of the steel wire and to make the surface lamella structure more uniform Can do. By applying the appropriate residual compressive stress on the surface and removing the carbon fixed to the dislocations, the amount of local solid solution of carbon is easily reduced, and the cementite decomposition of the outermost layer is suppressed.

A2: Shot peening is performed after wire drawing.
Shot peening is a method of irradiating the entire steel wire with a spherical shot of a specific size for a specific time at a specific pressure, and creating a processed layer or a strained layer only in the surface region of the steel wire. The shot peening is, for example, an air projection type with an air pressure of 4 to 5 × 10 ⁵ Pa, a time of preferably 5 to 10 seconds, and a shot sphere of 10 to 100 μm. It is effective to irradiate a sufficient amount of the entire surface of the steel wire.
By performing shot peening after the wire drawing, compressive residual stress is applied to the surface of the steel wire, and the surface lamella structure is made more uniform. By applying an appropriate residual compressive stress on the surface and removing carbon adhering to dislocations, the amount of local solid solution of carbon is reduced and cementite decomposition of the outermost layer is suppressed.

(Group B manufacturing method)
B1: The final stage drawing speed is 200 m / min or less, preferably 50 m / min or less.
By performing the low-speed wire drawing, it is possible to reduce the heat generation amount due to friction and plastic deformation, thereby suppressing the decomposition of cementite in the pearlite structure and reducing the amount of carbon diffused in the ferrite phase.

B2: Heat treatment at a temperature of 40 to 400 ° C. is performed for 0.5 second to 5 minutes, more preferably at a temperature of 100 to 300 ° C. for 1 second to 3 minutes between wire drawing passes.
The wire temperature due to wire drawing increases instantaneously and decreases immediately. Separately, by applying heat treatment at an appropriate temperature between the wire drawing passes, supersaturated carbon dissolved in the ferrite phase by decomposition of cementite during wire drawing is obtained by heat treatment between passes. It is possible to discharge from the ferrite phase to lower the C concentration in the ferrite phase, and to eliminate unnecessary point defects (atomic vacancies, etc.) and dislocations. As a result, ductility is restored and processing with a high strain amount, that is, the ferrite phase interval can be made finer. However, it is effective to perform this process between specific passes rather than between all wire drawing passes.

B3: In the final stage including the skin pass and the previous drawing step, a die having an approach angle of 8 to 12 ° and a dynamic friction coefficient of 0.1, preferably 0.05 or less is used.
By using a die having a small approach angle and a small dynamic friction coefficient, frictional heat generation during wire drawing is suppressed, and an increase in the C concentration in the ferrite phase due to cementite decomposition due to temperature rise in the outermost layer is suppressed. It is effective to use this in a process close to the final stage.

(Group C manufacturing method)
C1: After wire drawing, heating and holding at 60 to 300 ° C. is performed for 0.1 minute to 24 hours, more preferably at 180 to 260 ° C. for 20 seconds to 15 minutes.
Due to aging during or after wire drawing, cementite decomposes and supersaturated carbon dissolved in the ferrite phase is discharged, reducing the carbon concentration in the ferrite phase. However, when this temperature is too high, spherical cementite or transition carbide is formed, and when it is too low, the effect is small. It is necessary to set a temperature suitable for the steel material type and wire drawing conditions.

C2: A process with a large area reduction rate of 20% or more is inserted once, preferably a plurality of times during wire drawing excluding the three stages before the final stage.
By introducing a process of a large area reduction rate of 20% or more once, preferably a plurality of times, it is possible to uniformly apply strain to the inside without biasing the wire drawing strain to the surface. It is effective to perform this before the third stage before the final stage.

  The C concentration in the ferrite phase in the steel wire can be accurately measured by a three-dimensional atom probe method (3DAP). However, conventionally, the C concentration in the ferrite phase in the drawn pearlite structure of the outermost layer of the steel wire could not be measured. Using a focused ion beam (FIB) device, we have developed a technology to produce a needle sample by cutting a small piece from the surface of a steel wire and processing it with FIB, so that the carbon concentration in the outermost layer can be measured with high accuracy. I can do it now.

  Care must be taken because the solute C concentration may show different values depending on the position in the ferrite phase. When cementite decomposes and C diffuses into the ferrite phase, generally, the C concentration at the ferrite phase / cementite phase interface position is high, and the value is the smallest at the ferrite phase center position. In the present embodiment, the average C concentration of a region (a region that is a half of the width of the ferrite phase) including a distance of 1/4 of the width of the ferrite phase on both sides from the position of the center plane of the ferrite phase is defined.

  By analyzing with 3DAP, it is possible to measure the C concentration in the ferrite phase including the ferrite phase / cementite phase interface, so by selecting and cutting out a box of a specific size in the region to be examined from the measurement data, By calculating the ratio of C atoms to all atoms in the box, the C concentration in the ferrite phase can be obtained in atomic%. This can be converted to mass% by multiplying by 12/56. Such measurement was performed for a plurality of ferrite phase center portions to obtain an average, and this was taken as the average C concentration of the ferrite phase center portion.

  As an example, FIGS. 2A to 2F show a method for preparing a needle sample for measuring the C concentration of the ferrite phase center within 1 μm from the surface of the steel wire, and FIG. 3 uses the prepared needle sample. The C distribution measured by 3DAP and the C concentration at the center of the ferrite phase are shown.

  In order to produce a needle sample in a region 1 μm from the surface of the steel wire, for example, as shown in FIG. 2A, a rod-like block including the steel wire surface on one side is cut out from the steel wire surface region by FIB. This block is fixed on the needle base as shown in FIG. 2B by using vapor deposition (deposition) of tungsten or the like, for example. As shown in FIG. 2C, this block is processed by FIB so that the tip end portion is thin. FIG. 2D is a diagram of the processed block observed from above, and it can be seen that the tip has a rod shape including the steel wire surface. Then, the tip part was processed into a needle shape by irradiating a ring-shaped beam from the upper part. FIG. 2F is a view of the needle sample thus made, observed from the side. As shown in FIG. 2E, the needle tip position was prepared so as to correspond to the inside of 1 μm from the steel wire surface. By using such a needle sample preparation technique, a needle sample of the outermost steel wire layer can be prepared.

  Further, in FIG. 3, the dark portion indicates that the C concentration is high and the light portion indicates that the C concentration is low. Accordingly, a dark band-like region indicates a cementite phase that has undergone wire drawing, and a lightly colored region between them indicates a ferrite phase that has undergone wire drawing. It is shown that C is also dissolved in the ferrite phase.

  As shown in the figure, by cutting out a 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, the carbon concentration in the ferrite phase center can be estimated. In this example, the C concentration is 0.18% by mass. The ferrite phase center is located in the middle of the two cementite phases, and includes a region that includes up to a quarter of the width of the ferrite phase on both sides from the center surface of the ferrite phase (region that is half the width of the ferrite phase). ).

  The width of the ferrite phase is not necessarily constant depending on the amount of processing and the location of the sample, and there is a region of 10 nm or less in a narrow portion. If the cementite region is included in the box position, it becomes higher than the actual C concentration in the ferrite phase. Therefore, the box position to be analyzed was the ferrite phase center, and the box width was half the width of the ferrite phase. Further, the average C concentration is estimated by averaging the measured values of the C concentration at the central portion of 5 or more, preferably 10 or more different ferrite phases.

  The residual stress of the steel wire outermost layer can be accurately measured by, for example, the X-ray diffraction method. In particular, it can be measured accurately by the Debye fitting method using a micro-region X-ray diffractometer capable of measuring a local region. This method is a method of fitting the reflection of the crystal grain of the steel wire as a Debye ring and examining the magnitude direction of the residual stress from the Debye distortion. The depth region including the surface is determined from the penetration depth of X-rays. For example, when the X-ray source is Cr, an integrated value with a depth of several μm on the surface can be obtained. Another method for examining the residual stress on the surface of the steel wire is a melting method (Hane method) as needed. This is a method of examining the residual stress in the longitudinal direction of the steel wire by measuring the difference in length of the steel wire before and after melting the outermost layer to be examined. Both of these methods can accurately determine the residual stress of a high-strength steel wire with a developed texture.

  Hereinafter, the feasibility and effects of the present invention will be described more specifically with reference to examples.

  After making the test material having the chemical composition shown in Table 1 into a predetermined wire diameter by hot rolling, performing a patenting treatment and a wire drawing using a lead bath, so that the tensile strength becomes 4500 MPa or more, A high-strength ultrafine wire steel made of a drawn pearlite structure having a brass plating with a wire diameter of 0.04 to 0.40 mm was manufactured. Brass plating was performed after pickling after the final patenting treatment.

  Table 2 shows the true strain, the manufacturing method, the wire diameter, the average C concentration of the ferrite phase in the outermost layer of the steel wire, the residual stress in the outermost layer of the steel wire, the tensile strength, and the torsion test. Shows the number of twists to break. In Table 2, the manufacturing method is represented by symbols indicating the contents described above. In the torsion test, the test piece was fixed at a gripping distance 100 times the diameter of both ends of the test piece, and the number of torsion until breaking was examined. When the tensile strength was 4500 MPa or more and the number of twists was 20 times or more, the ductility was evaluated as good, and when the tensile strength was 25 times or more, the ductility was evaluated as very good. The C concentration in the ferrite phase of the steel wire outermost layer was measured at the surface of 1 μm by 3DAP using the method described above, and the residual stress in the longitudinal direction of the steel wire outermost layer was measured by the Debyling fitting method described above. . When the residual stress is negative, it represents compressive stress, and when it is positive, it represents tensile stress.

  In Table 2, test no. 1-6 are examples of the present invention, and others are comparative examples. As seen in the table, all of the inventive examples have a tensile strength of 4500 MPa or more, an average C concentration of the ferrite phase central portion of the outermost layer of 0.2% by mass or less, and a residual stress of −600 MPa or less ( The residual compressive stress is 600 MPa or more. As a result, an ultrafine steel wire having a sufficient ductility with a high number of twists can be realized. In particular, test no. In Nos. 1 and 2, the number of twists was very good at 25 times or more.

  On the other hand, test no. 7 to 20 are comparative examples, and the tensile strength was 4500 MPa or more, but the number of twists was insufficient.

  No. 7 to 9 are comparative examples in which the components of the steel wire are outside the scope of the present invention. No. In No. 7, since the amount of C in the steel wire was too small, and the amount of wire drawing strain was increased, the C concentration at the ferrite phase central portion exceeded the specified value, and the ductility decreased. No. 8 is the amount of Si in the steel wire. 9 is a comparative example in which the amount of C is higher than the range of the present invention. In these comparative examples, the residual stress and the C concentration at the ferrite phase center were within the specified range, but the ductility was lowered.

  No. Nos. 10 to 13 are comparative examples in which the steel wire components and residual stress are within the scope of the present invention, but the C concentration in the ferrite phase central portion of the outermost layer is not less than a specified value. In these comparative examples, the ductility decreased. No. Nos. 14 to 16 are comparative examples in which the component of the steel wire and the C concentration of the ferrite phase central portion are within the range of the present invention, but the residual stress is out of the range. In these comparative examples, the ductility decreased. No. 17 to 20 are comparative examples in which the C concentration and the residual stress at the ferrite phase center of the outermost layer are both out of the range. In these comparative examples, the ductility decreased.

  According to the present invention, it is possible to provide a high-strength steel wire having sufficient ductility, so that the contribution to the industry is very large.

Claims (8)

C: 0.7 to 1.2% by mass,
Si: 0.05 to 2.0% by mass,
Mn: 0.2 to 2.0% by mass,
A steel wire comprising the chemical components of the balance, the balance being Fe and inevitable impurities,
The steel wire has a pearlite structure,
The average C concentration of the ferrite phase central portion of the outermost layer of the steel wire is 0.2% by mass or less,
The steel wire, wherein the outermost steel wire has a residual compressive stress in the longitudinal direction of 600 MPa or more. Cr: 0.05 to 1.0% by mass,
Ni: 0.05 to 1.0% by mass,
V: 0.01 to 0.5 mass%,
Nb: 0.001 to 0.1% by mass,
Mo: 0.01 to 0.1% by mass,
B: 0.0001 to 0.01% by mass,
The steel wire according to claim 1, further comprising one or more chemical components.   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. A method for producing a steel wire having a tensile strength of 4500 MPa or more,
C: 0.7 to 1.2% by mass, Si: 0.05 to 2.0% by mass, Mn: 0.2 to 2.0% by mass, the balance being Fe and inevitable impurities and patenting step of generating a pearlite structure subjected to patenting treatment to become more steel wires;
A wire drawing step of drawing the steel wire by controlling an average C concentration of a ferrite phase central portion in the pearlite structure of the outermost layer of the steel wire to 0.2% by mass or less;
A residual stress applying step of applying a residual compressive stress of 600 MPa or more to the steel wire;
A method for producing a steel wire, comprising:   The steel wire before performing the patenting process is
Cr: 0.05 to 1.0% by mass,
Ni: 0.05 to 1.0% by mass,
V: 0.01 to 0.5 mass%,
Nb: 0.001 to 0.1% by mass,
Mo: 0.01 to 0.1% by mass,
B: 0.0001 to 0.01% by mass,
The method for producing a steel wire according to claim 6, further comprising one or more chemical components. A method for producing a steel wire having a tensile strength of 4500 MPa or more,
C: 0.7 to 1.2% by mass, Si: 0.05 to 2.0% by mass, Mn: 0.2 to 2.0% by mass, the balance being Fe and inevitable impurities A patenting step of generating a pearlite structure by performing a patenting treatment on the steel wire comprising;
  A wire drawing step of drawing the steel wire by controlling an average C concentration of a ferrite phase central portion in the pearlite structure of the outermost layer of the steel wire to 0.2% by mass or less;
  A residual stress applying step of applying a residual compressive stress of 600 MPa or more to the steel wire;
With
  A method of manufacturing a steel wire, characterized in that after the final patenting process, one manufacturing method is adopted from each of the following A group, B group, and C group.
  Group A (A1: A skin pass process with a surface reduction rate of 1% to 6% is performed at the final stage. A2: Shot peening is performed after wire drawing.)
  Group B (B1: The drawing speed of the final stage is 200 m / min or less. B2: Heat treatment at a temperature of 40 to 400 ° C. is performed for 0.5 second to 5 minutes between drawing processes. B3: Skin pass is performed. (In the final stage and the previous wire drawing process, a die having an approach angle of 8 to 12 ° and a dynamic friction coefficient of 0.1 or less is used.)
  Group C (C1: After wire drawing, 60-300 ° C. is heated for 0.1 minutes to 24 hours. C2: During wire drawing excluding 3 steps before the final step, with a reduction in area of 20% or more. Perform wire drawing.)