CN107406950B

CN107406950B - High-carbon steel wire rod and steel wire having excellent drawability

Info

Publication number: CN107406950B
Application number: CN201680019334.0A
Authority: CN
Inventors: 石田友信; 增田智一
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2015-03-30
Filing date: 2016-03-29
Publication date: 2020-04-14
Anticipated expiration: 2036-03-29
Also published as: EP3282027B1; US20180087125A1; EP3282027A4; CN107406950A; ES2835325T3; KR20170130527A; CA2980886A1; JP6795319B2; EP3282027A1; CA2980886C; WO2016158901A1; JP2017061740A

Abstract

Provided is a high-carbon steel wire rod having excellent wire drawability. The high-carbon steel wire rod of the present invention contains a predetermined component and contains pearlite and pro-eutectoid cementite, the area ratio of pearlite to the entire structure is 90% or more, the maximum length of the pro-eutectoid cementite is 15 [ mu ] m or less, and the concentration difference between the average value of the Si concentration inside the pro-eutectoid cementite and the maximum value of the Si concentration inside ferrite forming a lamellar structure of pearlite is 0.50 to 3%.

Description

High-carbon steel wire rod and steel wire having excellent drawability

Technical Field

The present invention relates to a high-carbon steel wire rod having excellent drawability, and a steel wire obtained by drawing the high-carbon steel wire rod. More specifically, the present invention relates to a high-carbon steel wire rod produced by hot rolling, which is mainly used as a raw material for a high-strength steel wire used for steel cords, steel cables, wire saw wires, and the like.

Background

Examples of high-strength steel wires used for steel cords and steel cables include piano wires according to JIS G3522 (1991). The piano strings are roughly classified into three types, i.e., type A, type B, and type V, and the high-strength piano string type B includes SWP-B having a string diameter of 0.2mm and a tensile strength of 2840 to 3090 MPa. As a material for a piano wire, pearlite steel such as SWRS82A described in JIS G3502 (2004) is generally used.

A general method for producing a high-strength steel wire is as follows. First, a steel wire rod (also referred to as a rolled wire rod) produced by hot rolling is annularly placed on a cooling conveyor, pearlite transformation is performed, and then the steel wire rod is wound into a coil shape to obtain a coil. Then, wire drawing is performed, and a steel wire having a desired wire diameter and strength is obtained by the work hardening action of pearlite. When it is impossible to process to a desired wire diameter due to the processing limit of the steel wire rod, a heat treatment called patenting is performed during the wire drawing process. For example, in order to obtain an ultra-fine steel wire having a wire diameter of 0.2mm, it is common to repeat the wire drawing process and the patenting process a plurality of times.

Here, in order to increase the strength of the steel wire, it is necessary to increase the amount of C in the steel wire rod as a raw material. However, in the high carbon steel wire containing 0.90% or more of C, there is a problem that proeutectoid cementite precipitates in the structure and the drawability is lowered.

Therefore, various techniques have been proposed for producing a high-carbon steel wire having excellent wire drawability.

For example, patent document 1 describes a wire rod for a high-strength steel wire useful as a material for a galvanized steel wire used for a cable for a bridge or the like, and particularly relates to a wire rod for a high-strength steel wire which is excellent in workability when wire drawing is performed by a so-called green drawing process without performing heat treatment after rolling. In patent document 1, precipitation of pro-eutectoid cementite is suppressed by precipitating fine TiC in the vicinity of grain boundaries, and therefore the lower limit of the Ti content is set to 0.02% or more.

In particular, patent document 2 relates to a thin high-carbon hot-rolled wire rod that can be subjected to wire drawing with a true strain of 2.2 or more in a hot-rolled state. Specifically, patent document 2 describes that when a steel sheet containing Si suppressed to 0.50% or less is hot-rolled, the reduction is increased to make the wire rod diameter as small as 4.5mm or less, thereby making austenite grains (γ grains) fine and performing pearlite transformation, thereby preventing large grain precipitation of proeutectoid ferrite and proeutectoid cementite.

Further, patent document 3 relates to a profile for an undersea optical cable using a wire rod for a high-tensile steel wire. Specifically, patent document 3 describes that Si segregation can be prevented from occurring in a wire rod in which Si segregation occurs by using a range of 30nm from a cementite-ferrite interface to a ferrite phase side in a pearlite structure so that the maximum Si segregation degree (maximum Si concentration in a range of 30nm from the cementite-ferrite interface to the ferrite phase side divided by the volume of Si content) of the cementite-ferrite interface satisfies 1.1 or more.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-189855

Patent document 2: japanese patent laid-open publication No. 2001-181789

Patent document 3: japanese patent laid-open publication No. 2003-301240

However, the techniques of patent documents 1 to 3 have the following problems.

First, patent document 1 is directed to a wire rod used for a galvanized steel wire, and is not directed to a steel wire having an ultra-fine wire diameter of about 0.2mm, such as a piano wire. When an ultrafine steel wire is produced using a wire rod having a large Ti content as in patent document 1, breakage during wire drawing due to Ti-based inclusions becomes remarkable. Therefore, the technique of patent document 1 is difficult to be applied to an ultra-fine steel wire for a steel cord or the like.

Further, as in patent document 2, the use of a wire rod having a diameter of 4.5mm or less causes a reduction in productivity, and there is a problem that the wire rod is easily wound during coil production.

Further, the method of providing a difference in Si concentration at the interface between cementite and ferrite in the pearlite structure as in patent document 3 cannot sufficiently reduce pro-eutectoid ferrite which is detrimental to wire drawability. In addition, the reduction of area was only 82.6% in the degree of working performed in patent document 3, even when wire drawing and cold rolling were performed together. The draw reduction rate of drawing required for an extremely fine steel wire such as a steel cord is further large, and therefore the application to the above-mentioned applications is insufficient.

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-carbon steel wire rod and a steel wire which are applicable to an ultra-fine steel wire such as a steel cord and have excellent drawability.

The high-carbon steel wire rod of the present invention, which can solve the above problems, has the following gist: contains, in mass%, C: 0.90-1.3%, Si: 0.4-1.2%, Mn: 0.2-1.5%, P: above 0% and below 0.02%, S: above 0% and below 0.02%, Al: above 0% and below 0.008%, Ti: 0-0.005%, N: 0.001 to 0.008% and the balance being iron and unavoidable impurities, wherein the structure contains pearlite and pro-eutectoid cementite, the area ratio of pearlite to the entire structure is 90% or more, the maximum length of the pro-eutectoid cementite is 15 μm or less, and the concentration difference between the average value of the Si concentration in the pro-eutectoid cementite and the maximum value of the Si concentration in ferrite forming a lamellar structure of pearlite is 0.50 to 3%.

In a preferred embodiment of the present invention, the high-carbon steel wire rod further contains at least one of the following (a) to (d) in mass%.

(a) B: higher than 0% and less than 0.01%

(b) Co: above 0% and below 1.5%

(c) From V: above 0% and below 0.5%, and Cr: more than 0% and less than 0.5%

(d) From Cu: above 0% and below 0.5%, Ni: above 0% and below 0.5%, and Nb: more than 0% and less than 0.5%

In the present invention, the steel wire obtained by drawing the high-carbon steel wire is also included in the scope of the present invention.

According to the present invention, a high-carbon steel wire rod having excellent drawability, which can be applied to an extremely fine steel wire such as a steel cord, can be provided.

Drawings

FIG. 1 is a graph showing the difference in Si concentration at the interface between the proeutectoid cementite phase and the ferrite phase in test No.12 of Table 2 of examples.

Detailed Description

In order to solve the above problems, the present inventors have made extensive studies on the use of a high carbon steel wire rod having a C content of 0.90% or more. As a result, the present inventors have found that precipitation and growth of proeutectoid cementite harmful to drawability can be suppressed by forming an Si concentration difference of 0.50% or more (specifically, an average value of Si concentration in the proeutectoid cementite and a concentration difference of 0.50% or more from a maximum value of Si concentration in the ferrite) at an interface between the proeutectoid cementite and ferrite forming a lamellar structure of pearlite (hereinafter, referred to simply as ferrite), and have completed the present invention.

Although patent document 3 described above also describes Si segregation, the above-mentioned patent document 3 differs from the present invention in that the Si concentration at the interface between pro-eutectoid cementite and ferrite is controlled not for cementite in the pearlite structure but for the interface between pro-eutectoid cementite and ferrite in the point that the Si concentration difference at the interface between cementite (lamellar cementite forming a lamellar structure of pearlite) and ferrite in the pearlite structure is controlled. The cementite in the pearlite structure is substantially different from pro-eutectoid cementite and also has a different precipitation start temperature, and pro-eutectoid cementite is about 750 ℃ and higher than the precipitation start temperature of pearlite precipitated at about 590 to 650 ℃. Therefore, the technique of patent document 3 is considered to be unable to sufficiently reduce the pro-eutectoid cementite that is detrimental to wire drawability. Patent document 3 describes that, in order to efficiently segregate Si at the above-described interface, it is effective to set the speed of air-cooling after wire rod rolling to 1 to 10 ℃/sec, and in the examples, air-cooling of about 7 ℃/sec is performed in all cases. However, in No.6 of Table 2 described later, which is rolled under the above cooling conditions, the difference in Si concentration specified in the present invention is not obtained, the maximum length of the pro-eutectoid cementite is also increased, and the wire drawing property is deteriorated.

The steel wire rod of the present invention will be described in detail below.

First, the steel of the steel wire rod of the present invention has the following components. The units of each component are mass% unless otherwise specified.

C：0.90～1.3％

C is effective for increasing the strength, and the strength of the cold-worked steel wire rod is improved with an increase in the C content. In order to achieve the desired strength of 4000MPa or more, the lower limit of the C content is set to 0.90% or more, preferably 0.93% or more, and more preferably 0.95% or more. However, if the C content is too large, the pro-eutectoid cementite harmful to drawability cannot be sufficiently reduced, and drawability is degraded. Therefore, the upper limit of the C content is 1.3% or less, preferably 1.25% or less.

Si：0.4～1.2％

Si is an effective deoxidizing agent, and has an effect of reducing oxide inclusions in steel, and in addition, has an effect of increasing the strength of a steel wire rod. Further, as described later, there is also an effect of suppressing the growth of proeutectoid cementite. In order to effectively exhibit these effects, the lower limit of the Si content is set to 0.4% or more, preferably 0.45% or more, more preferably higher than 0.50%, and still more preferably 0.55% or more. However, if Si is excessively added, embrittlement during wire drawing is promoted, and the twisting characteristics of the drawn wire material are degraded. Therefore, the upper limit of the Si content is 1.2% or less, preferably 1.15% or less.

Mn：0.2～1.5％

Mn greatly improves the hardenability of steel, and therefore, has the effect of lowering the transformation temperature during blast cooling and improving the strength of the pearlite structure. In order to effectively exhibit these effects, the lower limit of the Mn content is set to 0.2% or more, preferably 0.3% or more. However, Mn is an element that is easily segregated in the center of the wire, and if it is excessively added, hardenability of the Mn segregation portion is excessively increased, and there is a possibility that a super-cooled structure such as martensite may be generated. Therefore, the upper limit of the Mn content is 1.5% or less, preferably 1.0% or less, and more preferably 0.95% or less.

P: above 0% and below 0.02%

P is contained as an impurity, but segregates at the prior austenite grain boundary to embrittle the grain boundary, which causes cracking of the steel sheet and also degrades the fatigue characteristics of the steel wire after drawing. Therefore, in order to prevent these disadvantages, the upper limit of the P content is set to 0.02% or less, preferably 0.018% or less. Further, it is difficult to industrially produce P at a lower limit of 0%.

S: above 0% and below 0.02%

S is contained as an impurity in the same manner as P, but segregates at the prior austenite grain boundary to embrittle the grain boundary, and causes cracking of the steel sheet and also lowers the fatigue characteristics of the steel wire after drawing. Therefore, in order to prevent these disadvantages, the upper limit of the S content is set to 0.02% or less, preferably 0.018% or less. Further, it is difficult to industrially produce S at a lower limit of 0%.

Al: higher than 0% and below 0.008%

Al is contained as an impurity to form Al₂O₃Such Al-based inclusions increase the breakage rate during wire drawing. Therefore, in order to ensure sufficient drawability, the upper limit of the Al content is set to 0.008% or less, preferably 0.006% or less. Further, it is difficult to industrially produce Al at a lower limit of 0%.

Ti：0～0.005％

Ti is contained as an impurity, but Ti-based inclusions such as TiN are generated to increase the wire breakage rate during wire drawing. Therefore, in order to ensure sufficient drawability, the upper limit of the Ti content is set to 0.005% or less, preferably 0.003% or less.

N：0.001～0.008％

N is dissolved in steel to cause strain aging during wire drawing, thereby lowering the toughness of the steel wire. In order to prevent such adverse effects, the upper limit of the N content is set to 0.008% or less, preferably 0.007% or less. The smaller the amount of N, the better, but the lower limit thereof is 0.001% or more, preferably 0.0015% or more in terms of industrial production.

The steel wire rod of the present invention contains the above components, the balance: iron and inevitable impurities.

The steel wire rod of the present invention may contain the following selective elements in order to further improve properties such as strength, toughness, and ductility.

B: higher than 0% and less than 0.01%

B is concentrated at austenite grain boundaries, and has an effect of preventing the formation of grain boundary ferrite and improving drawability. In addition, the compound forms a nitride such as BN by combining with N, and thus, there is an effect of suppressing a decrease in toughness due to the solid-solution N and improving the twisting characteristics. In order to effectively exhibit the drawability and twisting characteristics of the steel wire rod by the addition of B, the lower limit of the B content is preferably 0.0005% or more. However, if added excessively, compounds (B-dependent) with Fe precipitate and cause cracks during hot rolling, and therefore the upper limit of the B content is preferably 0.01% or less, and more preferably 0.008% or less.

Co: above 0% and below 1.5%

Co promotes pearlite transformation and has the effect of reducing proeutectoid cementite. In particular, the wire drawability can be improved by adding Co in addition to Si. In order to effectively exhibit such an effect, the lower limit of the Co content is preferably 0.05% or more, and more preferably 0.1%. However, since Co is a very expensive element and its effect is saturated even when added in excess, and it is economically wasteful, the upper limit of the Co content is preferably 1.5% or less, more preferably 1.3% or less, and still more preferably 1% or less.

From V: above 0% and below 0.5%, and Cr: more than 0% and less than 0.5%

V and Cr are elements contributing to the improvement of the strength of the steel wire rod. These elements may be added alone or in combination.

Specifically, V produces fine carbonitrides, and has an effect of increasing strength, and also has an effect of improving the twist characteristics by reducing the amount of dissolved N. In order to effectively exert such an effect, the lower limit of the V content is preferably 0.05% or more, and more preferably 0.1% or more. However, since V is an expensive element and its effect is saturated even when added in excess, and it is economically wasteful, the upper limit of the V content is preferably 0.5% or less, and more preferably 0.4% or less.

Cr has an effect of refining the lamellar spacing of pearlite and improving the strength of the steel wire rod. In order to effectively exert such an effect, the lower limit of the Cr content is preferably 0.05% or more, and more preferably 0.1% or more. However, since the effect is saturated even when the Cr content is excessively added and is economically wasteful, the upper limit of the Cr content is preferably 0.5% or less, and more preferably 0.4% or less.

From Cu: above 0% and below 0.5%, Ni: above 0% and below 0.5%, and Nb: more than 0% and less than 0.5%

These elements contribute to the improvement of the steel wire in terms of manufacturability, corrosion resistance, and the like. These elements may be added alone or in combination.

Specifically, Cu thickens on the surface of the steel wire rod to improve the scale removability, and has an effect of improving the Mechanical Descaling (MD) property. In order to effectively exhibit such an effect, the lower limit of the Cu content is preferably 0.05% or more. However, if added excessively, bubbles are generated on the surface of the steel wire rod, and therefore the upper limit of the Cu content is preferably 0.5% or less, and more preferably 0.4% or less.

Ni has an effect of improving the corrosion resistance of the steel wire rod. In order to effectively exhibit such an effect, the lower limit of the Ni content is preferably 0.05% or more. However, since the effect of excessive addition is saturated and is economically wasteful, the upper limit of the Ni content is preferably 0.5% or less, more preferably 0.4% or less.

Nb has the effect of refining crystal grains and improving ductility of the wire rod. In order to effectively exhibit such an effect, the lower limit of the Nb content is preferably 0.05% or more. However, since the effect of excessive addition is saturated and is economically wasteful, the upper limit of the Nb content is preferably 0.5% or less, and more preferably 0.4% or less.

Next, the structure of the steel wire rod of the present invention will be described. As described above, the steel wire rod of the present invention contains pearlite and pro-eutectoid cementite, the area ratio of pearlite to the entire structure is 90% or more, the maximum length of the pro-eutectoid cementite is 15 μm or less, and the concentration difference between the average Si concentration in the pro-eutectoid cementite and the maximum Si concentration in the ferrite (hereinafter, sometimes simply referred to as the Si concentration difference) is 0.50 to 3%.

Area ratio of pearlite to the entire structure: over 90 percent

As described above, the steel wire rod of the present invention contains pearlite and pro-eutectoid cementite. Since a low-temperature transformation structure (also referred to as a supercooled structure) such as bainite or martensite hinders drawability, the area ratio of the pearlite structure is 90% or more, preferably 95% or more, in order to ensure sufficient drawability. The upper limit thereof may be appropriately controlled depending on the relationship with the proeutectoid cementite, but is preferably approximately 99 area% or less.

The steel wire rod of the present invention may contain a residual structure, which is inevitably included in the production, in addition to pearlite and pro-eutectoid cementite. Examples of such a residual structure include a non-pearlite structure such as bainite and pro-eutectoid ferrite. In order to effectively exhibit the effect of the present invention, the non-pearlite structure (including proeutectoid cementite) is preferably controlled to about 10 area% or less with respect to the total of all structures.

Maximum length of proeutectoid cementite: less than 15 μm

The proeutectoid cementite precipitated in a plate shape is a structure detrimental to wire drawability, and interferes with orientation of pearlite colonies of the steel wire rod, and becomes a starting point of cracks to increase wire breakage. However, the above disadvantage is small in the proeutectoid cementite having a short maximum length. The mechanism of such proeutectoid cementite is described in detail in patent document 1. In order to ensure sufficient drawability, the upper limit of the maximum length of the proeutectoid cementite is set to 15 μm or less, preferably 13 μm or less, and more preferably 10 μm or less. The lower limit of the maximum length of the proeutectoid cementite is not particularly limited, and may be, for example, about 0.1 μm.

Concentration difference between the average value of Si concentration inside proeutectoid cementite and the maximum value of Si concentration inside ferrite (Si concentration difference): 0.50 to 3 percent

Si is an element that is difficult to be dissolved in a cementite, and is discharged from a cementite phase to the outside during the precipitation of proeutectoid cementite, and a concentration difference of Si is formed at the interface (interface between proeutectoid cementite and ferrite phase). From the experimental results of the present inventors, it was found that the larger the difference in Si concentration, the more the growth of the proeutectoid cementite phase is suppressed, and the maximum length of the proeutectoid cementite is reduced. Since the Si concentration distribution formed at this time continues pearlite transformation even after the passage, when the structure of the produced steel wire rod is observed, it can be confirmed as the Si concentration difference at the interface between the pro-eutectoid cementite phase and the ferrite phase in the periphery thereof.

For reference, a graph showing the difference in Si concentration of test No.12 of table 2 of the later-described examples is shown in fig. 1. In fig. 1, the average Si concentration of the pro-eutectoid carburized phase at the center is measured, and the difference from the maximum Si concentration of each ferrite phase around the pro-eutectoid carburized phase is defined as the Si concentration difference. The method of measuring the Si concentration is described in detail in the section of examples described later.

In the present invention, the difference in Si concentration calculated as described above is 0.50% or more. This makes it possible to set the maximum length of the proeutectoid cementite to 15 μm or less. The difference in Si concentration is preferably 0.6% or more. However, since the above-mentioned effect is saturated by excessively forming the Si concentration difference, the upper limit thereof is set to 3% or less, preferably 2.8% or less.

In the present invention, the above-described Si concentration difference occurs at the interface between the pro-eutectoid cementite phase and ferrite in the pearlite structure, and no Si concentration difference occurs at the interface between the pro-eutectoid cementite phase and the cementite (lamellar cementite forming a lamellar structure of pearlite) phase in the pearlite structure.

Next, a preferred method for producing the steel wire rod of the present invention will be described.

The high carbon steel wire rod of the present invention is generally produced by heating a steel sheet adjusted to a predetermined chemical composition to austenitize the steel sheet, and hot rolling the steel sheet to obtain a steel wire rod having a predetermined wire diameter.

After hot rolling, the steel sheet is placed in a loop on a cooling conveyor and cooled. The mounting temperature is preferably 880 to 980 ℃. If the placing temperature is too high or too low, the properties of the scale change, and the Mechanical Descaling (MD) treatment before drawing is adversely affected. The mounting temperature is preferably 900 ℃ to 960 ℃. In order to solve the above-mentioned problems, other iron oxide removal treatment such as pickling may be used, but in consideration of productivity and the like, it is recommended to control the placing temperature within the above-mentioned range.

Next, cooling was started at a temperature of 800 ℃ or higher. The cooling conditions are extremely important for controlling the desired Si concentration difference within a predetermined range. The ranges of the cooling stop temperature and the holding temperature described below are required to be within the ranges of the entire coil placed in the loop.

Specifically, the cooling is carried out at an average cooling rate of 12 to 60 ℃/s to a cooling stop temperature of 480 to 620 ℃. If the average cooling rate is slow, the Si concentration difference formed at the proeutectoid cementite interface is lost by the diffusion of Si atoms, and a desired Si concentration difference cannot be obtained. On the other hand, if the average cooling rate is high, a supercooled structure is formed, and the pearlite area ratio is less than 90%. More preferably, the average cooling rate is 15 ℃/s or more and 55 ℃/s or less.

Further, when the cooling start temperature is low, precipitation of pro-eutectoid cementite starts during cooling, and therefore, the difference in Si concentration is small as compared with the case where the average cooling rate is slow. When the cooling stop temperature is low, a supercooled structure such as bainite is formed, and the pearlite area ratio is reduced. On the other hand, when the cooling stop temperature is high, Si atoms diffuse and the Si concentration difference becomes small. The cooling stop temperature is more preferably 500 ℃ to 600 ℃.

After the cooling is stopped, the temperature is raised to a holding temperature of 590 to 650 ℃ to cause pearlite transformation. If the holding temperature is too high, Si atoms diffuse, and the difference in Si concentration decreases. On the other hand, if the holding temperature is too low, a supercooled structure occurs, and the pearlite area ratio decreases. The holding temperature is more preferably 600 ℃ to 640 ℃.

After the steel wire rod of the present invention is obtained as described above, it is wound into a coil shape to obtain a wire coil. Then, wire drawing is performed to obtain a steel wire having a desired wire diameter and strength.

Further, it is preferable to perform patenting treatment after the wire drawing process. After patenting, drawing is performed to obtain an ultra-fine steel wire having a wire diameter of about 0.2 mm. The conditions for patenting treatment are not particularly limited, and for example, conditions such as a heating temperature of 950 ℃ and a patenting temperature of 600 ℃ can be adopted. The patenting treatment may be performed not only 1 time but also a plurality of times (for example, 2 to 3 times).

The steel wire of the present invention thus obtained has a high tensile strength of substantially 4000MPa or more. According to the present invention, since a steel wire having a wire diameter of about 0.1 to 0.4mm can be obtained, the steel wire is suitably used for, for example, a steel cord, a wire rope, a wire saw wire, and the like.

The present application is based on the benefit of claiming priority from Japanese patent application No. 2015-. The specifications of Japanese patent application No. 2015-070095, applied on 30/3/2015, and Japanese patent application No. 2015-188843, applied on 25/9/2015, are incorporated by reference in their entirety in this application.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples, and can be modified and practiced within the scope that can meet the purpose described above and below, and all of these are included in the technical scope of the present invention.

Steel grades A to Z (cross-sectional shape: 155 mm. times.155 mm) described in Table 1 were hot-rolled by heating to 1000 ℃ and worked to a predetermined wire diameter

Subsequently, the steel sheet was placed on a cooling conveyor in an endless manner, pearlite transformation was performed by controlled cooling by air-blast cooling, and the steel sheet was wound into a coil shape to obtain a rolled material coil. The cooling conditions after rolling and the wire diameter after rolling are shown in table 2.

The rolled material coil thus obtained was used to measure the following items.

Measurement of pearlite (P) area fraction

After the unstable part at the end of the rolled material coil was cut off, the end of the non-defective product was extracted, and a test piece having a length of 5cm was extracted. The cross section of the test piece thus obtained, which was perpendicular to the longitudinal direction of the wire rod, was photographed with a Scanning Electron Microscope (SEM), and the areas of the pearlite structure and the non-pearlite structure were determined by a point counting method using the structure photograph thereofAnd (4) rate. The dot counting method is a method of dividing a tissue photograph into meshes, counting tissues existing at the mesh points, and easily obtaining an area ratio of the tissues. Specifically, 3 SEM photographs were prepared in which the center portion of the cross section was photographed 4000 times, and the area fraction of pearlite was obtained by dividing each of the SEM photographs into 100 grid points, and the average value of the area fractions was calculated. The evaluation area of 1 SEM photograph was 868 μm². The pearlite area ratio and the details of the structure of each test piece are shown in table 2. In table 2, the non-pearlite structure (pro-eutectoid cementite structure, bainite structure) detected by the above point counting method is also described. In the table, P represents a pearlite structure, B represents a bainite structure, and θ represents proeutectoid cementite.

Evaluation of maximum Length of proeutectoid cementite (. theta.)

The maximum length was determined by measuring the length of each of the observed proeutectoid cementites using the SEM photograph obtained as described above. When the proeutectoid cementite precipitates in the form of a plate, and a plurality of branches of the plate-like cementite occur, the total length of each branch is used.

Measurement of Si concentration difference

Using the SEM photograph obtained as described above, the tensile analysis was performed on the Si concentration of the observed pro-eutectoid cementite by means of an EDX (Energy dispersive X-ray spectrometry) using a Cs-STEM (spherical aberration corrected Scanning Electron Microscope) to determine the Si concentration difference between the inside of the pro-eutectoid cementite and the ferrite phase located around the pro-eutectoid cementite. Specifically, the average value of the Si concentration in the pro-eutectoid carburized body phase and the maximum value of the Si concentration in the ferrite phase are measured, and the difference is defined as the Si concentration difference. The step size of the linear analysis was 2nm and the evaluation length was 200 nm.

Evaluation of mechanical Properties of rolled Material coil

The unstable part at the end of the rolled material coil was cut off, 1 turn was taken from the coil end of a non-defective product, and the coil end was equally divided in the longitudinal direction by 8 minutes, and a tensile test was performed in accordance with JIS Z2201 to measure the tensile strength TS. An average of 8 rolled coils was obtained, and the TS of the rolled material coil was calculated.

Evaluation of drawing characteristics

Using the rolled material coil, cold drawing was performed with the drawing strain shown in table 2 to obtain a predetermined wire diameter, and the tensile strength TS after drawing was obtained. The amounts of the strands were 200kg, respectively. The occurrence of a wire breakage during the wire drawing process is described as "wire breakage".

These results are also shown in Table 2.

[ Table 1A ]

[ Table 1B ]

[ Table 2A ]

[ Table 2B ]

From these results, the following examination can be made.

Test Nos. 1 to 3, 11 to 21, and 24 to 32 are examples satisfying the requirements of the present invention, and no wire breakage occurred, and good wire drawability was confirmed. In particular, test Nos. 3, 11 to 14, 16 to 18, 20 and 21 using the steel grades C to G, I to K, M, N of Table 1 containing B were all able to draw without breaking up the wire until the drawing strain was high. Among them, the steel types D, E of Table 1, which used the steel types D, E containing Co in addition to B, were able to be drawn in the higher drawing strain region (2.13 or more).

In contrast, the following example has the following problems.

In all of the tests nos. 4 to 10, the steel type C in table 1 satisfying the requirements of the present invention was used, but the steel type C was produced without satisfying any of the conditions recommended in the present invention, and therefore, the wire was broken during wire drawing.

Specifically, in test No.4, since the cooling start temperature was low, the difference in Si concentration was reduced, the maximum length of the proeutectoid cementite was increased, and the wire was broken during wire drawing.

In test No.5, since the average cooling rate from the cooling start temperature to the cooling stop temperature was high, the pearlite area ratio decreased, and the wire broke during wire drawing.

In test No.6, since the average cooling rate from the cooling start temperature to the cooling stop temperature was small, the difference in Si concentration was reduced, the maximum length of the proeutectoid cementite was long, and the wire was broken during wire drawing.

In test No.7, the cooling stop temperature was low, and therefore the pearlite area ratio was low, and wire breakage occurred during wire drawing.

In test No.8, since the cooling stop temperature was high, the difference in Si concentration was reduced, the maximum length of the proeutectoid cementite was long, and the wire breakage occurred during wire drawing.

In test No.9, the pearlite area ratio decreased because the holding temperature was low, and the wire breakage occurred during wire drawing.

In test No.10, since the holding temperature was high, the difference in Si concentration was reduced, the maximum length of the proeutectoid cementite was long, and the wire breakage occurred during wire drawing.

Next, in test No.22, since steel type O of table 1 having a large C content was used, the maximum length of the proeutectoid cementite was long, and the wire was broken by wire drawing.

In test No.23, since steel type P shown in Table 1 having a small Si content was used, the difference in Si concentration was small, the maximum length of the proeutectoid cementite was long, and the wire breakage occurred during wire drawing.

Claims

1. A steel wire rod having excellent drawability and suitable for use in an ultrafine steel wire, characterized by containing, in mass%, C: 0.90-1.3%, Si: 0.55-1.2%, Mn: 0.2-1.5%, P: above 0% and below 0.02%, S: above 0% and below 0.02%, Al: above 0% and below 0.008%, Ti: 0-0.005%, N: 0.001 to 0.008% and the balance of iron and inevitable impurities,

the structure contains pearlite and pro-eutectoid cementite,

the pearlite area ratio to the entire structure is 90% or more,

the maximum length of the proeutectoid cementite is 15 μm or less,

the concentration difference between the average value of the Si concentration in the proeutectoid cementite and the maximum value of the Si concentration in the ferrite forming the pearlite lamellar structure is 0.50-3%.

2. The steel wire rod according to claim 1, further comprising one or more members selected from the group consisting of (a) to (d) below in mass%:

(a) b: above 0% and below 0.01%;

(b) co: above 0% and below 1.5%;

(c) from V: above 0% and below 0.5%, and Cr: at least one selected from the group consisting of more than 0% and 0.5% or less;

(d) from Cu: above 0% and below 0.5%, Ni: above 0% and below 0.5%, and Nb: more than 0% and less than 0.5%.

3. A steel wire obtained by drawing the steel wire rod according to claim 1 or 2.