KR20170130527A - A high carbon steel wire rod excellent in freshness, - Google Patents

Abstract

A high carbon steel wire rod excellent in freshness is provided. The high carbon steel wire rod according to the present invention comprises a pearlite and a corner stone cementite and includes a pearlite area ratio of 90% or more, a maximum length of the cornerstone cementite is 15 탆 or less, And the concentration difference between the maximum value of the Si concentration in the ferrite forming the lamellar structure of pearlite is 0.50 to 3%.

Description

A high carbon steel wire rod excellent in freshness,

The present invention relates to a high carbon steel wire rod excellent in freshness and a steel wire obtained by drawing the high carbon steel wire rod. More particularly, the present invention relates to a high strength carbon steel wire rod produced by hot rolling, and more particularly to a steel wire rod which is a raw material for a high strength steel wire used mainly for steel cord, wire rope, saw wire and the like.

As a high strength steel wire used for steel cord or wire rope, for example, a piano wire described in JIS G 3522 (1991) is known. The piano wire is broadly divided into three types of A, B, and V, and SWP-B species having a high strength of piano B class and having a wire diameter of 0.2 mm and a tensile strength of 2840 to 3090 MPa. As the material of the piano wire, pearlite steels such as SWRS82A described in JIS G 3502 (2004) are generally used.

A general manufacturing method of 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 placed on a cooling conveyor in a ring shape, pearlitic transformation is carried out, and then coiled in a coil form to obtain a wire rod coil. Thereafter, drawing is carried out, and a steel wire having a desired wire diameter and strength is obtained by using the work hardening action of pearlite. When the steel wire rod can not be machined to a desired wire diameter by the working limit of the steel wire rod, a heat treatment called patterning is performed between the drawing processes. For example, in order to obtain a micro-fine wire having a wire diameter of 0.2 mm, it is common to perform drawing and patterning processes several times.

Here, in order to increase the strength of the steel wire, it is necessary to increase the amount of C of the steel wire material to be the material. However, in a high-carbon steel wire containing 0.90% or more of C, there has been a problem in that cornerstone cementite is precipitated in the structure and the freshness is lowered.

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

For example, Patent Document 1 relates to a wire rod for high-strength steel wire useful as a material of a galvanized steel wire used for a rope for bridges and the like, and particularly relates to a wire rod for high strength steel wire which is not subjected to heat treatment after rolling, A wire rod for a high strength steel wire with good workability is described. In Patent Document 1, precipitation of fine-grain cementite is suppressed by precipitating fine TiC in the vicinity of the grain boundary, so that the lower limit of the Ti content is set to 0.02% or more.

Further, Patent Document 2 relates to a narrow-diameter, high-carbon hot-rolled wire rod which can be subjected to drawing processing at a true strain of 2.2 or more, in particular, as hot rolling. Specifically, in Patent Document 2, a steel strip in which Si is suppressed to 0.50% or less is made thinner to a wire rod diameter of 4.5 mm or less by increasing the amount of reduction during hot rolling, thereby finely austenitizing the , It is disclosed that grain boundary precipitation of pro-eutectoid ferrite and cobalt cementite can be prevented.

Also, Patent Document 3 relates to a discrete wire for a submarine optical fiber cable using a wire for a high-tension steel wire. Specifically, Patent Document 3 discloses that the maximum Si saturation at the cementite / ferrite interface (the maximum Si concentration in the range of 30 nm from the cementite and ferrite interface to the ferrite surface in the range of 30 nm from the ferrite interface to the cementite in the pearlite structure) Quot ;, Si content in the bulk, Si content in the bulk) > = 1.1, it is possible to prevent the single wire ratio during the mold release process.

Japanese Patent Application Laid-Open No. 2014-189855 Japanese Patent Laid-Open No. 2001-181789 Japanese Unexamined Patent Application Publication No. 2003-301240

However, the techniques of the above-described Patent Documents 1 to 3 each have the following problems.

First, Patent Document 1 does not target a steel wire used for a galvanized steel wire, and a steel wire having a fine wire diameter of about 0.2 mm, such as a piano wire. When an ultra fine wire is manufactured using a wire having a large amount of Ti as in Patent Document 1, disconnection at the time of drawing processing becomes remarkable due to the Ti inclusion. Therefore, it is difficult to apply the technique of Patent Document 1 to a micro-fine wire provided on a steel cord or the like.

In addition, the use of a wire having a diameter of 4.5 mm or less as in Patent Document 2 causes a decrease in productivity and causes a problem that the wire is easily entangled at the time of manufacturing the coil.

Also, as in Patent Document 3, it is impossible to sufficiently reduce the pro-eutectoid ferrite which is detrimental to the drawability, as a method of making the Si concentration difference between the cementite and the ferrite interface in the pearlite structure. In addition, the processing degree in Patent Document 3 is a reduction ratio of 82.6% when the drawing processing and the cold rolling are combined. The reduction ratio of the drawing process required for a micro-fine wire such as a steel cord is larger, and therefore, it is insufficient for application to the above applications.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high carbon steel wire rod and a steel wire which are applicable to an ultra fine wire such as a steel cord.

The high carbon steel wire rod according to the present invention capable of solving the above problems is characterized in that it comprises 0.90 to 1.3% of C, 0.4 to 1.2% of Si, 0.2 to 1.5% of Mn, more than 0% , Ti: 0 to 0.005%, N: 0.001 to 0.008%, the balance being iron and inevitable impurities, the structure including pearlite and basic stone cementite , And the difference in density between the average value of the Si concentration in the cornerstone cementite and the maximum value of the Si concentration in the ferrite forming the lamellar structure of pearlite is 90% or more, the maximum percentage of the cornerstone cementite is 15 m or less, 0.50 to 3%.

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

(a) B: more than 0% and not more than 0.01%

(b) Co: more than 0% and not more than 1.5%

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

(d) at least one selected from the group consisting of Cu: more than 0% to 0.5%, Ni: more than 0% to 0.5%, and Nb: more than 0% to 0.5%

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

According to the present invention, it is possible to provide a high carbon steel wire rod excellent in freshness, which can be applied to an ultra fine wire such as a steel cord.

Fig. 12 shows the difference in Si concentration at the interface between the cornerstone cementite phase and the ferrite phase.

In order to solve the above problems, the inventors of the present invention have extensively studied using a high carbon steel wire having a C content of 0.90% or more. As a result, a Si concentration difference of at least 0.50% is formed at the interface between ferrite cementite and ferrite forming a lamellar structure of pearlite (hereinafter, simply referred to as ferrite) (specifically, Si concentration difference And the concentration difference between the average value and the maximum value of the Si concentration in the ferrite is not less than 0.50%), the present inventors have completed the present invention by discovering that the precipitation and growth of the elemental cementite can be inhibited.

On the other hand, Patent Document 3 mentioned above also describes Si segregation, but Patent Document 3 discloses that the difference in Si concentration at the interface between cementite (lamellar cementite forming a lamellar structure of pearlite) and ferrite in the pearlite structure is controlled , The structure to be subjected is different from the present invention which controls the Si concentration at the interface between the cornerstone cementite and the ferrite, not the cementite in the pearlite structure. The cementite in the pearlite structure is essentially different from the basic stone cementite, and the precipitation start temperature is higher than that of the pearlite precipitated at about 590 to 650 占 폚. Therefore, in the technique of Patent Document 3, it is considered that the base stone cementitious which is detrimental to the drawability can not be sufficiently reduced. In Patent Document 3, it is described that it is effective to set the speed of the air-blast cooling after the wire rolling to 1 to 10 ° C / second in order to efficiently segregate Si on the interface. In the embodiment, . However, in Table 2, which is rolled under the above-mentioned cooling conditions, 6, the Si concentration difference defined in the present invention was not obtained, and the maximum length of the crude stone cementite was also prolonged, thereby deteriorating the drawability.

Hereinafter, the steel wire rod of the present invention will be described in detail.

First, the steel components of the steel wire according to the present invention are as follows. The unit of each component is mass%, unless otherwise specified.

C: 0.90 to 1.3%

C is effective for increasing the strength, and the strength of the steel wire after cold working is improved with an increase in the C content. In order to achieve the desired strength of 4000 MPa 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 excessively large, it is not possible to sufficiently reduce the elemental cementitious detrimental to the drafting workability and the drawability is lowered. Therefore, the upper limit of the C content is set to 1.3% or less, preferably 1.25% or less.

Si: 0.4 to 1.2%

Si is an effective deoxidizing material and has the effect of reducing the oxide inclusions in the steel and also has the effect of increasing the strength of the steel wire rod. It also has an effect of suppressing the growth of Crushed stone cementite as described later. 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 0.50% or more, and still more preferably 0.55% or more. However, if Si is added excessively, embrittlement at the time of drawing is promoted, and the twisting property of the drawn material is lowered. Therefore, the upper limit of the Si content is 1.2% or less, preferably 1.15% or less.

Mn: 0.2 to 1.5%

Mn significantly increases the hardenability of the steel. Therefore, Mn has an effect of lowering the transformation temperature at the time of air blast cooling and increasing the strength of the pearlite structure. In order to effectively exhibit these effects, the lower limit of the Mn content is set to not less than 0.2%, preferably not less than 0.3%. However, Mn is an element which is easily segregated at the center of the wire rod, and if it is added in excess, the quenching property of the Mn segregation portion is excessively increased, and there is a fear that a supercooled structure such as martensite is produced. Therefore, the upper limit of the Mn content is set to 1.5% or less, preferably 1.0% or less, more preferably 0.95% or less.

P: more than 0% and not more than 0.02%

P is contained as an impurity but segregates in the old austenite grain boundaries to embrittle the grain boundaries to cause cracking of the steel billet and to lower the fatigue characteristics of the steel wire after the drawing. Therefore, in order to prevent these harmful effects, the upper limit of the P content is set to 0.02% or less, preferably 0.018% or less. On the other hand, setting the lower limit of P to 0% is difficult for industrial production.

S: more than 0% and less than 0.02%

S is contained as an impurity in the same manner as P above, but is segregated at the old austenite grain boundaries to embrittle the grain boundaries to cause cracking of the steel sheet, and to lower the fatigue characteristics of the steel wire after the sintering. Therefore, in order to prevent these harmful effects, the upper limit of the S content is made 0.02% or less, preferably 0.018% or less. On the other hand, setting the lower limit of S to 0% is difficult for industrial production.

Al: more than 0% and not more than 0.008%

Al is contained as an impurity, and Al-based inclusions such as Al ₂ O ₃ are generated, thereby increasing the burning rate during 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. On the other hand, setting the lower limit of Al to 0% is difficult for industrial production.

Ti: 0 to 0.005%

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

N: 0.001 to 0.008%

N is dissolved in the steel to cause strain aging at the time of drawing, thereby lowering the toughness of the steel wire. In order to prevent such harm, the upper limit of the N content is 0.008% or less, preferably 0.007% or less. The amount of N is preferably small, but the lower limit of the industrial production is 0.001% or more, preferably 0.0015% or more.

The steel wire material of the present invention includes the above components, and the balance is iron and inevitable impurities.

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

B: more than 0% and not more than 0.01%

B is concentrated in the austenite grain boundaries, thereby preventing generation of intergranular ferrite and improving the freshness. In addition, there is also an effect of compounding N with N to form nitride such as BN, thereby suppressing toughness deterioration due to solid solution N, thereby improving the solar cycling characteristics. It is preferable that the lower limit of the B content is 0.0005% or more in order to effectively exhibit the freshness and the thinning characteristics of the steel wire material by the addition of B. However, when it is added in excess, the compound (B-constituent) with Fe precipitates and causes cracking during hot rolling. Therefore, the upper limit of the B content is preferably 0.01% or less, more preferably 0.008% or less.

Co: more than 0% and not more than 1.5%

Co has an effect of promoting pearlite transformation and reducing cornerstone cementite. Particularly, by adding Co in addition to Si, the drawing workability is promoted. 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 it is added in excess, it is economically wasteful. Therefore, the upper limit of the Co content is preferably 1.5% or less, more preferably 1.3% %.

V: not less than 0% and not more than 0.5%, and Cr: not less than 0% and not more 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 singly or in combination.

Specifically, V produces fine carbonitride and has an effect of enhancing the strength. In addition, it can exhibit an improvement in the characteristics of thinning by reduction of solid solution N. In order to effectively exhibit such effects, the lower limit of the V content is preferably 0.05% or more, and more preferably 0.1% or more. However, V is an expensive element, and even if added in excess, the effect is saturated, which is economically wasteful. Therefore, the upper limit of the V content is preferably 0.5% or less, more preferably 0.4% or less.

Cr also has an effect of increasing the strength of the steel wire rod by making the lamella interval of pearlite small. In order to effectively exhibit such effects, the lower limit of the Cr content is preferably 0.05% or more, and more preferably 0.1% or more. However, even if added in excess, the effect is saturated, which is economically wasteful. Therefore, the upper limit of the Cr content is preferably 0.5% or less, more preferably 0.4% or less.

At least one selected from the group consisting of Cu: more than 0% to 0.5%, Ni: more than 0% to 0.5%, and Nb: more than 0% to 0.5%

These are all elements contributing to improvement of the composition and corrosion resistance of the steel wire. These elements may be added singly or in combination.

Specifically, Cu is concentrated on the surface of the steel wire to increase the peelability of the scale, thereby improving the mechanical descaling (MD) property. In order to effectively exhibit such an effect, it is preferable to set the lower limit of the Cu content to 0.05% or more. However, when added in excess, blisters are formed on the surface of the steel wire material, so the upper limit of the Cu content is preferably 0.5% or less, more preferably 0.4% or less.

Ni has an effect of enhancing the corrosion resistance of steel wire rods. In order to effectively exhibit such an effect, it is preferable that the lower limit of the Ni content is 0.05% or more. However, even if added in excess, the effect is saturated, which is economically wasteful. Therefore, the upper limit of the Ni content is preferably 0.5% or less, more preferably 0.4% or less.

Nb has the effect of increasing the ductility of the wire by refining the crystal grains. In order to effectively exhibit such an effect, it is preferable that the lower limit of the Nb content is 0.05% or more. However, even if added in excess, the effect is saturated, which is economically wasteful. Therefore, the upper limit of the Nb content is preferably 0.5% or less, more preferably 0.4% or less.

Next, the structure of the steel wire rod according to the present invention will be described. As described above, the steel wire material of the present invention includes pearlite and crushed stone cementite, and has an area ratio of pearlite of 90% or more with respect to the whole texture, a maximum length of the crushed stone cementite of 15 탆 or less, (Hereinafter sometimes simply referred to as a Si concentration difference) of 0.5% to 3%.

Area percentage of pearlite to whole structure: 90% or more

As described above, the steel wire rod of the present invention includes pearlite and quartz cementite. A low-temperature transformation structure (sometimes called supercooled structure) such as bainite or martensite hinders the freshness. Therefore, in order to ensure sufficient freshness, the area ratio of the pearlite structure should be 90% or more, preferably 95% Or more. On the other hand, its upper limit can be suitably controlled in relation to cobalt cementite, but it is preferably about 99% or less by area.

The steel wire rod of the present invention may also include a residual structure which is inevitably included in manufacturing, in addition to pearlite and cobblestone cementite. Examples of such residual structure include non-pearlite structures such as bainite and pro-eutectoid ferrite. In order to effectively exhibit the action of the present invention, it is preferable to control the total amount of non-pearlite structure (including crude stone cementite) for the whole texture to about 10 percent by area or less.

Maximum length of corner stone cementite: 15μm or less

Crushed stone cementite precipitated in the form of a plate is a structure which is detrimental to the drafting workability and interferes with the orientation of the pearlite colony of the steel wire material, and becomes a starting point of the crack, thereby increasing the breakage. However, the crude cementite having the shortest maximum length is less likely to cause the aforementioned problems. The mechanism by such a corner stone cementite is as described in the above-mentioned Patent Document 1. In order to ensure sufficient freshness, the upper limit of the maximum length of cornerstone cementite is set to 15 탆 or less, preferably 13 탆 or less, and more preferably 10 탆 or less. On the other hand, the lower limit of the maximum length of cornerstone cementite is not particularly limited, and may be, for example, about 0.1 탆.

(Si concentration difference) between the average value of the Si concentration inside the cornerstone cementite and the maximum value of the Si concentration inside the ferrite: 0.50 to 3%

Si is an element hardly solidified in cementite, and when the cobbstone cementite is precipitated, it is discharged from the cementite phase to the external austenite phase, and a difference in concentration of Si is formed at the interface (interfacial cementite and ferrite phase interface). According to the experimental results of the present inventors, it has been found that the larger the Si concentration difference is, the more the growth of the cornerstone cementite phase can be suppressed and the maximum length of the cornerstone cementite can be reduced. Since the Si concentration distribution formed at this time can be inherited even after the subsequent pearlite transformation, it can be confirmed as a difference in Si concentration between the cornerstone cementite phase and the surrounding ferrite phase at the texture of the produced steel wire.

For reference, the test No. 2 of Table 2 of the later-mentioned Examples. Fig. 1 shows a graph showing the difference in Si concentration in Fig. In Fig. 1, the average value of the Si concentration on the cornerstone cementite in the center and the maximum value of the Si concentration on each ferrite phase around the cornerstone cementite phase are measured, and these differences are defined as the Si concentration difference. The method of measuring the Si concentration will be described in detail in the column of the later-described embodiment.

In the present invention, the Si concentration difference calculated as described above is set to 0.50% or more. As a result, the maximum length of cornerstone cementite can be made 15 탆 or less. The Si concentration difference is preferably 0.6% or more. However, even if the Si concentration difference is excessively formed, the above effect is saturated, so that the upper limit thereof is 3% or less, preferably 2.8% or less.

On the other hand, in the present invention, the difference in the Si concentration occurs at the interface between ferrite cementite phase and ferrite in the pearlite structure, and the Si concentration difference is generated at the interface between cementite phase and pearlite crystal phase (lamellar cementite forming a lamellar structure of pearlite) Do not.

Next, a preferable method of manufacturing the steel wire rod of the present invention will be described.

The high carbon steel wire rod according to the present invention is generally made by heating a steel strip adjusted to a predetermined chemical composition to austenitize the steel wire rod and subjecting the steel wire rod to a predetermined wire diameter by hot rolling.

After hot rolling, place on a cooling conveyor in the form of a ring and cool. At this time, the setting temperature is preferably 880 to 980 캜. If the setting temperature is excessively high or too low, the scale property may change, which may adversely affect the mechanical descaling (MD) processing before drawing. The preferable setting temperature is 900 DEG C or more and 960 DEG C or less. On the other hand, in order to solve the above problem, other descaling processes such as pickling may be used. However, in consideration of productivity and the like, it is recommended to control the wafer temperature to the above range.

Subsequently, cooling is started at a temperature of 800 캜 or higher. The cooling condition here is extremely important for controlling the desired Si concentration difference to a predetermined range. On the other hand, the range of the cooling stop temperature and the holding temperature described below needs to be such that the whole of the coil placed in the ring shape is within this range.

Specifically, it is cooled to a cooling stop temperature of 480 to 620 캜 at an average cooling rate of 12 to 60 캜 / s. If the average cooling rate at this time is low, the Si concentration difference formed at the cornerstone cementite interface is lost due to diffusion of Si atoms, and a desired Si concentration difference can not be obtained. On the other hand, if the average cooling rate is increased, supercooled structure is generated, and the pearlite area ratio becomes less than 90%. A more preferable average cooling rate is not less than 15 ° C / s and not more than 55 ° C / s.

Further, when the cooling start temperature is low, the precipitation of the cornerstone cementite is initiated during the cooling, so that the difference in the Si concentration becomes small corresponding to the case where the above-mentioned average cooling rate is low. When the cooling stop temperature is low, supercooled structure such as bainite is generated and the pearlite area ratio is lowered. On the other hand, if the cooling stop temperature is high, the Si atoms diffuse and the Si concentration difference becomes small. The more preferable cooling stop temperature is 500 ° C or more and 600 ° C or less.

After cooling is stopped, the temperature is raised to a holding temperature of 590 to 650 캜 to cause pearlite transformation. If the holding temperature is too high, the Si atoms are diffused and the Si concentration difference becomes small. On the other hand, if the holding temperature is too low, supercooled structure occurs and the pearlite area ratio decreases. A more preferable holding temperature is 600 占 폚 or more and 640 占 폚 or less.

After obtaining the steel wire rod according to the present invention as described above, it is coiled in a coil shape to obtain a wire rod coil. Then, drawing is performed to obtain a steel wire having a desired wire diameter and strength.

On the other hand, it is preferable to perform the patterning process after the drawing process. By applying additional drawing processing after the patterning processing, a micro-fine wire having a wire diameter of about 0.2 mm can be obtained. The conditions for the patterning treatment are not particularly limited, and for example, conditions such as a heating temperature of 950 deg. C, a patterning temperature of 600 deg. The patterning process may be performed not only once, but also a plurality of times (for example, two to three times).

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

This application claims the benefit of priority based on Japanese Patent Application No. 2015-070095 filed on March 30, 2015, and Japanese Patent Application No. 2015-188843 filed on September 25, 2015. The entire specification of Japanese Patent Application No. 2015-070095 filed on March 30, 2015 and Japanese Patent Application No. 2015-188843 filed on September 25, 2015 are incorporated herein by reference in their entirety.

Example

The present invention will now be described in more detail with reference to the following examples. However, it should be understood that the present invention is not limited to the following examples, And are included in the technical scope of the present invention.

The steel types A to Z (sectional shape: 155 mm x 155 mm) shown in Table 1 were heated to a temperature of 1000 占 폚, hot-rolled, and processed to a predetermined diameter (? 5.5 mm). Subsequently, the laminate was placed on a cooling conveyor in a ring shape, pearlite transformation was carried out by controlled cooling by air cooling, and then coiled in a coiled state to obtain a rolled material coil. Table 2 shows cooling conditions after rolling and wire diameter after rolling.

Using the rolled material coil thus obtained, the following items were measured.

Measurement of area ratio of pearlite (P)

The abnormal portion of the terminal of the rolled material coil was cut off and the terminal of the good article was taken and a test piece having a length of 5 cm was taken. The cross-sectional area of the thus-obtained test piece perpendicular to the longitudinal direction of the wire rod was taken by a scanning electron microscope (SEM), and the area ratio between the pearlite structure and the non-pearlite structure was determined by the point cloud method . The grading method is a method of dividing a tissue photograph into meshes and counting the tissues existing at the lattice points, thereby easily obtaining the tissue area ratio. Specifically, three SEM photographs were taken at 4000 times the central portion of the cross section, and each pearlite area ratio was divided by 100 grid points, and the average value was calculated. The evaluation area of one SEM photograph is 868 mu m < ^{2 &} gt ;. Table 2 shows the pearlite area ratio and the texture of each test piece. Table 2 also shows the non-pearlite structure (cornerstone cementite structure, bainite structure) detected by the above-mentioned point-of-view method. In the table, P is a pearlite structure, B is a bainite structure, and? Is a corner stone cementite.

Evaluation of maximum length of cornerstone cementite (θ)

Using the SEM photograph thus obtained, the lengths of the observed quartzite cementites were measured and the maximum length was obtained. On the other hand, the crushed stone cementite precipitates in the form of a plate, but when a plurality of the cementites in the form of a plate are branched, the sum of the lengths of the branches is adopted.

Measurement of Si concentration difference

Using the SEM photograph thus obtained, the observed SiC cementite was analyzed by Cs-STEM (spherical aberration corrected scanning transmission electron microscope) to determine the Si concentration by EDX (energy dispersive X-ray analysis : Energy dispersive X-ray spectrometry) to determine the Si concentration difference between the inside of the cornerstone cementite phase and the ferrite phase around it. Specifically, the average value of the Si concentration on the cornerstone cementite and the maximum value of the Si concentration on the ferrite phase were respectively measured, and the difference was defined as the Si concentration difference. The line width of the line analysis was 2 nm and the evaluation length was 200 nm.

Evaluation of mechanical properties of rolled coil

The unsteady portion of the terminal of the rolled material coil was cut off and one ring was taken from the coil terminal of the good product and divided into eight in the longitudinal direction and subjected to tensile test on the basis of JIS Z 2201 to measure the tensile strength TS. And the TS of the rolled material coil was calculated.

Evaluation of freshness characteristics

Using the rolled material coil, cold drawing was carried out with the drawing deformation described in Table 2 to a predetermined wire diameter, and the tensile strength TS after drawing was obtained. The freshness is 200 kg each. On the other hand, it is described as "disconnection" that the disconnection occurred during the drawing process.

These results are shown in Table 2.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

From these results, consideration can be made as follows.

Test No. 1 to 3, 11 to 21 and 24 to 32 are examples satisfying the requirements of the present invention, and good drawability was confirmed without causing disconnection. Particularly, in Test No. 1 using the steel types C to G, I to K, M and N in Table 1 containing B, 3, 11 to 14, 16 to 18, 20, and 21 could be fresh without breaking up to a high fresh deformation. Of these, the test No. 1 using the steel types D and E of Table 1 containing Co in addition to Co. 11, 12 could be fresh to the higher fresh strain range (above 2.13).

On the other hand, the following examples have the following problems.

Test No. 4 to 10 were all made of the steel type C of Table 1 which satisfied the requirements of the present invention but were manufactured without satisfying any of the conditions recommended in the present invention,

Specifically, 4 had a lower cooling start temperature, the Si concentration difference was lowered, and the maximum length of the cobalt cementite became longer and broken at the time of freshness.

Test No. 5, since the average cooling rate from the cooling start temperature to the cooling stop temperature was large, the pearlite area ratio was lowered and broken at the time of freshness.

Test No. Since the average cooling rate from the cooling start temperature to the cooling stop temperature was small, the Si concentration difference was lowered, and the maximum length of the cornerstone cementite was lengthened and broken at the time of freshness.

Test No. 7 had a low cooling stop temperature, the pearlite area ratio decreased and was broken at the time of freshness.

Test No. 8 had a high cooling stop temperature, the Si concentration difference was lowered, and the maximum length of the cobblestone cementite became longer and broken at the time of freshness.

Test No. 9 had a low holding temperature, and thus the pearlite area ratio was lowered and broken at the time of freshness.

Test No. 10 had a high holding temperature, the Si concentration difference was lowered, and the maximum length of the cobalt cementite was lengthened and broken at the time of freshness.

Next, 22 used the steel grade O of Table 1 with a large amount of C, the maximum length of the cornerstone cementite became longer and broken during the drawing.

Test No. 23 used the steel grade P in Table 1 having a small amount of Si, the Si concentration difference was small and the maximum length of the corner stone cementite was long and broken at the time of freshness.

Claims (3)

P: not less than 0% and not more than 0.02%, S: not less than 0% and not more than 0.02%, Al: more than 0% and not more than 0.008% , Ti: 0 to 0.005%, N: 0.001 to 0.008%, the balance being iron and inevitable impurities,
The tissue includes pearlite and corner stone cementite,
The area ratio of pearlite to the entire structure is 90% or more,
The maximum length of the corner stone cementite is 15 占 퐉 or less,
Wherein the difference in concentration between the average value of the Si concentration inside the cornerstone cementite and the maximum value of the Si concentration inside the ferrite forming the lamellar structure of the pearlite is 0.50 to 3%. The method according to claim 1,
A steel wire rod further comprising at least one member selected from the following (a) to (d) in mass%.
(a) B: more than 0% and not more than 0.01%
(b) Co: more than 0% and not more than 1.5%
(c) at least one selected from the group consisting of V: more than 0% to 0.5% or less and Cr: more than 0% to 0.5%
(d) at least one selected from the group consisting of Cu: more than 0% to 0.5%, Ni: more than 0% to 0.5%, and Nb: more than 0% to 0.5% A steel wire obtained by drawing a steel wire according to any one of claims 1 to 3.

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