# Patent

## Title
Steel wire rod and method of producing same

## Abstract

A steel wire rod which is a material of steel wires includes, as a metallographic structure, by area %, 95% to 100% of a pearlite, wherein an average pearlite block size at a central portion of the steel wire rod is 1 μm to 25 μm, an average pearlite block size at a surface layer portion of the steel wire rod is 1 μm to 20 μm, and, when a minimum lamellar spacing of the pearlite at the central portion of the steel wire rod is S in unit of nm and when a distance from a peripheral surface of the steel wire rod to a center is r in unit of mm, S<12r+65 is satisfied.

## Background
TECHNICAL FIELD
The present invention relates to a steel wire rod with high strength and excellent ductility and which is a material of steel wires such as a prestressed concrete wire, a zinc-coated steel wire, a spring steel wire, and a bridge cable, and a method of producing the same.
This application is a national stage application of International Application No. PCT/JP2012/056377, filed Mar. 13, 2012, which claims priority to Japanese Patent Application No. 2011-056006, filed on Mar. 14, 2011, an amount of which is incorporated herein by reference.
Priority is claimed on Japanese Patent Application No. 2011-056006, filed on Mar. 14, 2011, an amount of which is incorporated herein by reference.
BACKGROUND ART
Commonly, a steel wire is produced by conducting wire-drawing so as to have a predetermined wire diameter and strength by using a steel wire rod which is produced by hot rolling and patenting treatment conducted as necessary. At a stage of a steel wire rod, when the steel wire rod has low strength, work strain should increase in order to be work-hardened to a predetermined strength during wire-drawing. As a result, a steel wire produced by the wire-drawing has poor ductility. In a case where the steel wire has poor ductility, when the steel wire is torsionally deformed, longitudinal cracking which is called as delamination may occur along a wire-drawing direction of the steel wire at an initial stage of deformation. Once the delamination occurs, stress may be concentrated at a site where the delamination occurs, and fracture of the steel wire may be finally promoted. In order to obtain a steel wire with high strength and excellent ductility by suppressing the occurrence of the delamination in the steel wire, the steel wire rod needs to have high strength and excellent ductility at a stage before the wire-drawing.
Generally, it is known that, when grain size is refined, strength is improved. Similarly, reduction of area (RA) that is an index of ductility of the steel wire rod also depends on austenite grain size. When the austenite grain size is refined, the reduction of area is also improved. Therefore, the austenite grain size of the steel wire rod is to be refined by using carbides or nitrides of Nb, B, and the like as pinning particles.
For example, Patent Document 1 suggests a steel wire rod in which at least one selected from a group consisting of, by mass %, 0.01% to 0.1% of Nb, 0.05% to 0.1% of Zr, and 0.02% to 0.5% of Mo is contained in a high carbon steel wire rod.
In addition, Patent Document 2 suggests a steel wire rod in which the austenite grain size is refined by containing NbC in a high-carbon steel wire rod.
However, in the steel wire rod disclosed in Patent Document 1 and Patent Document 2, expensive elements such as Nb are added, and thus the production cost may increase. Furthermore, since Nb forms coarse carbides and nitrides, these may act as fracture origin, and thus ductility of the steel wire rod may decrease.
Patent Document 3 suggests a method of producing a steel wire rod having high strength and large reduction of area by applying a direct patenting treatment (DLP: Direct in-Line Patenting) without using the expensive elements such as Nb.
In fact, the steel wire rod according to the production method disclosed in Patent Document 3 obtains high strength and large reduction of area without adding the expensive elements. However, at the present time, further improvement in strength and ductility is required. In Patent Document 3, as described in examples thereof, in a case of ensuring tensile strength (TS) of 1200 MPa or more, the reduction of area is less than 45%.
In order to improve properties of the prestressed concrete wire, the zinc-coated steel wire, the spring steel wire, the bridge cable, and the like in which the steel wire rod is used as the materials, it is effective to reduce the diameter of the steel wire rod as small as possible. Since reduction during the wire-drawing is controlled to be small by wire-drawing the steel wire rod with small diameter, the wire-drawn steel wire is controlled to excellent ductility. As a result, the occurrence of the delamination in the steel wire is suppressed. Accordingly, the steel wire rod having the small diameter, high strength, and excellent ductility (that is, large reduction of area) has been anticipated. Specifically, in a case where the diameter is 10 mm or less, a steel wire rod having the tensile strength of 1200 MPa or more and the reduction of area of 45% or more has been anticipated.
RELATED ART DOCUMENTS
Patent Documents


    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H04-371549
    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2001-131697
    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2008-007856


SUMMARY OF INVENTION
Technical Problem
In view of the above-mentioned problems, an object of the present invention is to provide a steel wire rod which has higher strength and better ductility than those of the conventional one without adding expensive elements, specifically, tensile strength of 1200 MPa or more and reduction of area of 45% or more, and is to provide a method of producing the same. Particularly, the present invention is to provide the steel wire rod having the tensile strength of 1200 MPa or more and the reduction of area of 45% or more, even when a diameter is 10 mm or less, and is to provide the method of producing the same.
Solution to Problem
An aspect of the present invention employs the following.
(1) A steel wire rod according to an aspect of the invention includes, as a chemical composition, by mass %, 0.70% to 1.00% of C, 0.15% to 0.60% of Si, 0.1% to 1.0% of Mn, 0.001% to 0.005% of N, 0.005% to less than 0.050% of Ni, at least one of 0.005% to 0.10% of Al and 0.005% to 0.10% of Ti, and a balance consisting of iron and unavoidable impurities, and includes, as a metallographic structure, by area %, 95% to 100% of a pearlite, wherein, when a distance from a peripheral surface to a center is r in unit of mm, an average pearlite block size at a central portion which is an area from the center to r×0.99 is 1 μm to 25 μm, wherein an average pearlite block size at a surface layer portion which is an area from the peripheral surface to r×0.01 is 1 μm to 20 μm, and wherein, when a minimum lamellar spacing of the pearlite at the central portion is S in unit of nm, a following Expression 1 is satisfied.

S<12r+65  (Expression 1)

(2) The steel wire rod according to (1) may further includes, as the chemical composition, by mass %, at least one of more than 0% to 0.50% of Cr, more than 0% to 0.50% of Co, more than 0% to 0.50% of V, more than 0% to 0.20% of Cu, more than 0% to 0.10% of Nb, more than 0% to 0.20% of Mo, more than 0% to 0.20% of W, more than 0% to 0.0030% of B, more than 0% to 0.0050% of Rare Earth Metal, more than 0.0005% to 0.0050% of Ca, more than 0.0005% to 0.0050% of Mg, and more than 0.0005% to 0.010% of Zr.
(3) In the steel wire rod according to (1) or (2), when a tensile strength is TS in unit of MPa and a reduction of area is RA in unit of %, both of a following Expression 2 and a following Expression 3 may be satisfied.

RA≧100−0.045×TS  (Expression 2)

RA≧45  (Expression 3)

(4) In the steel wire rod according to any one of (1) to (3), amounts expressed in mass % of each element in the chemical composition may satisfy a following Expression 4.

0.005≦Al+Ti≦0.1  (Expression 4)

(5) A method of producing a steel wire rod according to an aspect of the invention includes: a casting process to obtain a cast piece consisting of the chemical composition according to (1) or (2); a heating process of heating the cast piece to a temperature of 1000° C. to 1100° C.; a hot-rolling process of hot-finish-rolling the cast piece after the heating process by controlling a finishing temperature to be 850° C. to 1000° C. to obtain a hot-rolled steel; a coiling process of coiling the hot-rolled steel within a temperature range of 780° C. to 840° C.; a patenting process of directly immersing the hot-rolled steel after the coiling process in a molten salt, which is held at a temperature of 480° C. to 580° C., within 15 seconds after the coiling process; and a cooling process of cooling the hot-rolled steel after the patenting process to a room temperature to obtain the steel wire rod.
Advantageous Effects of Invention
According to the above aspects of the present invention, it is possible to obtain a steel wire rod having higher strength (tensile strength of 1200 MPa or more) and better ductility (reduction of area of 45% or more) than those of the conventional one without adding expensive elements. As a result, the steel wire after wire-drawing is controlled to excellent ductility, and thus occurrence of delamination in the steel wire is suppressed. Specifically, it is possible to produce the steel wire which has high strength and in which fracture is suppressed.
In addition, by using the above mentioned steel wire rod, it is possible to conduct the wire-drawing of the steel wire rod which has small diameter (10 mm or less), high strength, and excellent ductility. Accordingly, reduction of the wire-drawing is controlled to be small, and thus the wire-drawn steel wire can be controlled to excellent ductility. As a result, it is possible to improve properties of the steel wires such as the prestressed concrete wire, the zinc-coated steel wire, the spring steel wire, the bridge cable, and the like.
Furthermore, according to the above aspects of the present invention, it is possible to produce the steel wire with high strength and excellent ductility under general hot-rolling conditions as described above. It is not necessary to adopt severe hot-rolling conditions such as large rolling reduction and low rolling temperature in order to produce the steel wire rod with high strength and excellent ductility.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a relationship between Ni content of a steel wire rod and reduction of area of the steel wire rod.
FIG. 2 shows a relationship between the reduction of area of the steel wire rod and an average pearlite block size in metallographic structure at a central portion of the steel wire rod.
FIG. 3 shows a relationship between a diameter of the steel wire rod and a minimum lamellar spacing of pearlite in the metallographic structure at the central portion of the steel wire rod.
FIG. 4 shows a relationship between tensile strength of the steel wire rod and the reduction of area of the steel wire rod.



DESCRIPTION OF EMBODIMENTS
Hereinafter, a preferred embodiment of the present invention will be described in detail. However, the present invention is not limited to the component disclosed in the embodiment, and can employ various modifications as long as the conditions do not depart from the scope of the present invention.
The present inventors have investigated a steel wire rod having higher strength and better ductility than those of the conventional one without adding expensive elements and then found the following results.
First, it is found that the steel wire rod having high strength and excellent ductility can be obtained by adding at least one of Al and Ti which have an effect of suppressing coarsening of austenite grains and by adding a small amount of Ni which has an effect of improving the strength and the ductility only when the addition is the small amount.
The above is derived from the fact that pearlite block size (PBS) is controlled and that lamellar spacing of pearlite is refined in metallographic structure of the steel wire rod. When at least one of Al and Ti is contained, AlN or TiN appropriately precipitates, and thus coarsening of austenite grains is suppressed at a high-temperature region. As a result, the coarsening of the pearlite block size after pearlitic transformation is also suppressed. In addition, when Ni is contained in the small amount, a starting time and a finishing time of the pearlitic transformation during a patenting treatment shift to a longer time side, and thus a pearlitic transformation temperature during production of the steel wire rod substantially decreases. As a result, both of the pearlite block size and the lamellar spacing are refined. By the above effects the steel wire rod obtains high strength and excellent ductility.
In addition, it is found that, as a production method, controlling a time after a coiling process of coiling hot-rolled steel and before a patenting process to be very short is effective.
When the time after the coiling process and before the patenting process is controlled to be very short, the austenite is preferentially transformed to the pearlite in the metallographic structure, and thus the steel wire rod having a small fraction of non-pearlite structure can be obtained. A non-pearlite structure such as upper bainite, pro-eutectoid ferrite, degenerate pearlite, and pro-eutectoid cementite is a factor deteriorating properties of the steel wire rod. When the fraction of the non-pearlite structure is controlled to be a small value and a fraction of the pearlite is controlled to be large, the steel wire rod obtains high strength and excellent ductility.
Hereinafter, limitation range and reasons for the limitation of base elements of the steel wire rod according to the embodiment will be described. In addition, % as described below is mass %.
C: 0.70% to 1.00%
C (carbon) is an element that increases the strength. When an amount of C is less than 0.70%, the strength is insufficient, and it is difficult to obtain uniform pearlite structure because precipitation of the pro-eutectoid ferrite to austenite grain boundaries is promoted. On the other hand, when the amount of C is more than 1.00%, the pro-eutectoid cementite is easily formed at a surface layer portion of the steel wire rod, reduction of area of the steel wire rod at fracture decreases, and thus the fracture of wire at wire-drawing tends to occur. Accordingly, the amount of C is to be 0.70% to 1.00%. The amount of C is preferably 0.70% to 0.95%, and is more preferably 0.70% to 0.90%.
Si: 0.15% to 0.60%
Si (silicon) is an element that increases the strength, and is a deoxidizing element. When an amount of Si is less than 0.15%, the effects may not be obtained. On the other hand, when the amount of Si is more than 0.60%, the ductility of the steel wire rod decreases, the precipitation of the pro-eutectoid ferrite is promoted in hyper-eutectoid steel, and it is difficult to remove surface oxide by mechanical descaling. Accordingly, the amount of Si is to be 0.15% to 0.60%. The amount of Si is preferably 0.15% to 0.35%, and is more preferably 0.15% to 0.32%.
Mn: 0.10% to 1.00%
Mn (manganese) is a deoxidizing element, and is an element that increases the strength. Furthermore, Mn is an element that suppresses hot embrittlement by fixing S in steel as MnS. When an amount of Mn is less than 0.10%, the effects may not be obtained. On the other hand, when the amount of Mn is more than 1.00%, Mn segregates to a central portion of the steel wire rod, martensite or bainite is formed at the segregated portion, and thus the reduction of area and drawability decrease. Accordingly, the amount of Mn is to be 0.10% to 1.00%. The amount of Mn is preferably 0.10% to 0.80%.
N: 0.001% to 0.005%
N (nitrogen) is an element that suppresses the coarsening of austenite grains at a high-temperature region by forming nitrides in steel. When an amount of N is less than 0.001%, the effect may not be obtained. On the other hand, when the amount of N is more than 0.005%, since the amount of nitrides excessively increases and the nitrides act as a fracture origin, the ductility of the steel wire rod may decrease. In addition, solid-soluted N in steel may promote age hardening after the wire-drawing. Accordingly, the amount of N is to be 0.001% to 0.005%. The amount of N is preferably 0.001% to 0.004%.
Ni: 0.005% to less than 0.050%
Ni (nickel) is an element that improves the ductility of steel by solid-soluted in steel. In addition, Ni is an element that suppresses the pearlitic transformation and shifts the starting time and the finishing time of the pearlitic transformation during the patenting treatment to the longer time side. Therefore, in a case where a cooling rate is the same, a temperature further decreases before starting the pearlitic transformation in the patenting treatment in steel which contains Ni as compared with steel which does not contain Ni. The above indicates that the transformation temperature of the pearlitic transformation substantially is to be a lower temperature. As a result, both of the pearlite block size and the lamellar spacing of pearlite are refined. The reduction of area of the steel wire rod is improved with refining the pearlite block size, and the strength of the steel wire rod is improved with refining the lamellar spacing of the pearlite.
When an amount of Ni is less than 0.005%, the effects may not be obtained. On the other hand, when the amount of Ni is 0.050% or more, the pearlitic transformation is excessively suppressed, the austenite remains in the metallographic structure of the steel wire rod during the patenting treatment, and thus a large amount of micro-martensite is formed in the metallographic structure of the steel wire rod after the patenting treatment. As a result, the reduction of area of the steel wire rod decreases. FIG. 1 shows a relationship between Ni content of the steel wire rod and the reduction of area of the steel wire rod. As shown in the figure, when Ni content is 0.005% to less than 0.050%, the effect of improving the reduction of area of the steel wire rod is obtained. The amount of Ni is preferably 0.005% to 0.030%. In addition, approximately 0.0005% of Ni is unavoidably contained under ordinary producing conditions.
Al: 0.005% to 0.10%
Al (aluminum) is a deoxidizing element. In addition, Al is an element that precipitates as AlN by bonding to N. AlN has the effects of suppressing the coarsening of austenite grains at the high-temperature region and of suppressing the age hardening after the wire-drawing by reducing the solid-soluted N in steel. When the coarsening of austenite grains at the high-temperature region is suppressed, the pearlite block size in the metallographic structure of the steel wire rod after the patenting treatment is refined. As a result, the reduction of area of the steel wire rod is improved. When an amount of Al is less than 0.005%, the effects may not be obtained. On the other hand, when the amount of Al is more than 0.10%, a large amount of alumina-based non-metallic inclusions which are hard and undeformable are formed, and thus the ductility of the steel wire rod decreases. Therefore, the amount of Al is to be 0.005% to 0.10%. The amount of Al is preferably 0.005% to 0.050%.
Ti: 0.005% to 0.10%
Similarly to Al, Ti (titanium) is a deoxidizing element. In addition, similarly to Al, Ti is an element that precipitates as TiN by bonding to N. TiN has the effects of suppressing the coarsening of austenite grains at the high-temperature region and of suppressing the age hardening after the wire-drawing by reducing the solid-soluted N in steel. The pearlite block size in the metallographic structure of the steel wire rod after the patenting treatment is refined due to TiN, and as a result, the reduction of area of the steel wire rod is improved. When an amount of Ti is less than 0.005%, the effects may not be obtained. On the other hand, when the amount of Ti is more than 0.1%, coarse carbides are formed in the austenite, and thus the ductility may decrease. Therefore, the amount of Ti is to be 0.005% to 0.10%. The amount of Ti is preferably 0.005% to 0.050%, and is more preferably 0.005% to 0.010%.
As described above, Al and Ti have the same operation and effect. Accordingly, since Al precipitates as AlN by bonding to N in a case where Al is contained, the effects may be obtained even when Ti is not added. Similarly, since Ti precipitates as TiN by bonding to N in a case where Ti is contained, the effects may be obtained even when Al is not added. Therefore, at least one of Al and Ti may be contained. In a case where both of Al and Ti are contained, it is preferable that amounts expressed in mass % of each element satisfy a following Expression A. When a lower limit of the Expression A is less than 0.005, the effects may not be obtained. On the other hand, when an upper limit of the following Expression A is more than 0.10, the alumina-based non-metallic inclusions or Ti-based carbides are excessively formed, and thus the ductility of the steel wire rod decreases. The upper limit of the following Expression A is preferably 0.05% or less.

0.005≦Al+Ti≦0.10  (Expression A)

In addition to the above mentioned base elements, the steel wire rod according to the embodiment includes unavoidable impurities. Herein, the unavoidable impurities indicate elements such as P, S, O, Pb, Sn, Cd, and Zn which contaminate unavoidably from auxiliary materials such as scrap and the like and from producing processes. In the elements, P, S, and O may be limited to the following in order to preferably obtain the effect. In addition, % as described below is mass %. Moreover, although a limited range of the unavoidable impurities includes 0%, it is industrially difficult to be stably 0%.
P: 0.020% or less
P (phosphorous) is an impurity and is an element that causes intergranular fracture by segregating to the austenite grain boundaries and by embrittling prior-austenite grain boundaries. When an amount of P is more than 0.02%, the influence may be promoted. Accordingly, it is preferable that the amount of P be limited to 0.02% or less. Since it is preferable that P content is as small as possible, the limited range includes 0%. However, it is not technically easy to control P content to be 0%, and also the production cost of the steel may increase in order to be stably less than 0.001%. Thus, preferable limited range of P content is 0.001% to 0.020%. More preferable limited range of P content is 0.001% to 0.015%. Generally, in ordinary producing conditions, P of approximately 0.020% is contained unavoidably.
S: 0.020% or less
S (sulfur) is an impurity and is an element that forms the sulfides. When an amount of S is more than 0.02%, coarse sulfides are formed, and thus the ductility of the steel wire rod may decrease. Accordingly, it is preferable that the amount of S be limited to 0.020% or less. Since it is preferable that S content is as small as possible, the limited range includes 0%. However, it is not technically easy to control S content to be 0%, and also the production cost of the steel may increase in order to be stably less than 0.001%. Thus, preferable limited range of S content is 0.001% to 0.020%. More preferable limited range of S content is 0.001% to 0.015%. Generally, in ordinary producing conditions, S of approximately 0.020% is contained unavoidably.
O: 0.0030% or less
O (oxygen) is an unavoidably contained impurity and an element that forms oxide-based inclusions. When an amount of O is more than 0.0030%, coarse oxides are formed, and thus the ductility of the steel wire rod may decrease. Accordingly, it is preferable that the amount of O be limited to 0.0030% or less. Since it is preferable that O content is as small as possible, the limited range includes 0%. However, it is not technically easy to control O content to be 0%, and also the production cost of the steel may increase in order to be stably less than 0.00005%. Thus, preferable limited range of O content is 0.00005% to 0.0030%. More preferable limited range of O content is 0.00005% to 0.0025%. Generally, in ordinary producing conditions, O of approximately 0.0035% is contained unavoidably.
In addition to the above mentioned base elements and impurities, the steel wire rod according to the embodiment may further include, as optional elements, at least one of Cr, Co, V, Cu, Nb, Mo, W, B, REM, Ca, Mg, and Zr. Hereinafter, limitation range and reasons for the limitation of the optional elements will be described. In addition, % as described below is mass %.
Cr: more than 0% to 0.50%
Cr (chromium) is an element that refines the lamellar spacing of pearlite and improves the strength of the steel wire rod. In order to obtain the effects, it is preferable that an amount of Cr be more than 0% to 0.5%. The amount of Cr is more preferably 0.0010% to 0.50%. When the amount of Cr is more than 0.50%, the pearlitic transformation may be excessively suppressed, the austenite may remain in the metallographic structure of the steel wire rod during the patenting treatment, and thus supercooled structure such as the martensite and the bainite may be formed in the metallographic structure of the steel wire rod after the patenting treatment. In addition, it may be difficult to remove the surface oxides by the mechanical descaling.
Co: more than 0% to 0.50%
Co (cobalt) is an element that suppresses the precipitation of the pro-eutectoid cementite. In order to obtain the effect, it is preferable that an amount of Co be more than 0% to 0.50%. The amount of Co is more preferably 0.0010% to 0.50%. When the amount of Co is more than 0.50%, the effect may be saturated, and the cost for the addition may be vain.
V: more than 0% to 0.50%
V (vanadium) is an element that suppresses the coarsening of austenite grains at the high-temperature region by forming fine carbonitrides and that increases the strength of the steel wire rod. In order to obtain the effects, it is preferable that an amount of V be more than 0% to 0.50%. The amount of V is more preferably 0.0010% to 0.50%. When the amount of V is more than 0.50%, an amount of the formed carbonitrides may increase, a size of the carbonitrides may also increase, and thus the ductility of the steel wire rod may decrease.
Cu: more than 0% to 0.20%
Cu (copper) is an element that increases corrosion resistance. In order to obtain the effect, it is preferable that an amount of Cu be more than 0% to 0.20%. The amount of Cu is more preferably 0.0001% to 0.20%. When the amount of Cu is more than 0.20%, Cu and may segregate as CuS in the grain boundaries by reacting with S, the ductility of the steel wire rod may decrease, and defects may occur in the steel wire rod.
Nb: more than 0% to 0.10%
Nb (niobium) has an effect of increasing corrosion resistance. In addition, Nb is an element that suppresses the coarsening of austenite grains at the high-temperature region by forming carbides or nitrides. In order to obtain the effects, it is preferable that an amount of Nb be more than 0% to 0.10%. The amount of Nb is more preferably 0.0005% to 0.10%. When the amount of Nb is more than 0.1%, the pearlitic transformation may be suppressed during the patenting treatment.
Mo: more than 0% to 0.20%
Mo (molybdenum) is an element that concentrates at a growth interface of the pearlite and suppresses growth of the pearlite due to so-called solute drag effect. In addition, Mo is an element that suppresses formation of the ferrite and reduces the non-pearlite structure. In order to obtain the effects, it is preferable that an amount of Mo be more than 0% to 0.20%. The amount of Mo is more preferably 0.0010% to 0.20% and further more preferably 0.005% to 0.06%. When the amount of Mo is more than 0.20%, the growth of the pearlite may be suppressed, it may take a long time for the patenting treatment, and a decrease in productivity may occur. In addition, when the amount of Mo is more than 0.20%, coarse Mo2C carbides may precipitate, and thus the drawability may decrease.
W: more than 0% to 0.20%
Similarly to Mo, W (tungsten) is an element that concentrates at the growth interface of the pearlite and suppresses the growth of the pearlite due to the so-called solute drag effect. In addition, W is an element that suppresses the formation of the ferrite and reduces the non-pearlite structure. In order to obtain the effects, it is preferable that an amount of W be more than 0% to 0.20%. The amount of W is more preferably 0.0005% to 0.20% and further more preferably 0.005% to 0.060%. When the amount of W is more than 0.2%, the growth of the pearlite may be suppressed, it may take a long time for the patenting treatment, and the decrease in productivity may occur. In addition, when the amount of W is more than 0.20%, coarse W2C carbides may precipitate, and thus the drawability may decrease.
B: more than 0% to 0.0030%
B (boron) is an element that suppresses the formation of the non-pearlite precipitates such as the ferrite, the degenerate pearlite, and the bainite. In addition, B is an element that forms carbides or nitrides, and suppresses the coarsening of austenite grains at the high-temperature region. In order to obtain the effects, it is preferable that an amount of B be more than 0% to 0.0030%. The amount of B is more preferably 0.0004% to 0.0025%, further more preferably 0.0004% to 0.0015%, and most preferably 0.0006% to 0.0012%. When the amount of B is more than 0.0030%, precipitation of coarse Fe23(CB)6 carbides may be promoted, and the ductility may decrease.
REM: more than 0% to 0.0050%
REM (Rare Earth Metal) is a deoxidizing element. In addition, REM is an element that detoxifies S which is the impurity by forming sulfides. In order to obtain the effects, it is preferable that an amount of REM be more than 0% to 0.0050%. The amount of REM is more preferably 0.0005% to 0.0050%. When the amount of REM is more than 0.0050%, coarse oxides may be formed, the ductility of the steel wire rod may decrease, and the fracture of the wire during the wire-drawing may occur.
Herein, REM indicate a generic name of a total of 17 elements in which scandium of the atomic number 21 and yttrium of the atomic number 39 are added to 15 elements from lanthanum of the atomic number 57 to lutetium of the atomic number 71. In general, misch metal which is a mixture of the elements is supplied and added to the steel.
Ca: more than 0.0005% to 0.0050%
Ca (calcium) is an element that reduces alumina-based hard inclusions. In addition, Ca is an element that precipitates as fine oxides. As a result, the pearlite block size of the steel wire rod is refined, and thus the ductility of the steel wire rod is improved. In order to obtain the effects, it is preferable that an amount of Ca be more than 0.0005% to 0.0050%. The amount of Ca is more preferably 0.0005% to 0.0040%. When the amount of Ca is more than 0.0050%, coarse oxides may be formed, the ductility of the steel wire rod may decrease, and thus the fracture of the wire during the wire-drawing may occur. Generally, in ordinary producing conditions, Ca of approximately 0.0003% is contained unavoidably.
Mg: more than 0.0005% to 0.0050%
Mg (magnesium) is an element that precipitates as fine oxides. As a result, the pearlite block size of the steel wire rod is refined, and thus the ductility of the steel wire rod is improved. In order to obtain the effects, it is preferable that an amount of Mg be more than 0.0005% to 0.0050%. The amount of Mg is more preferably 0.0005% to 0.0040%. When the amount of Mg is more than 0.0050%, coarse oxides may be formed, the ductility of the steel wire rod may decrease, and thus the fracture of the wire during the wire-drawing may occur. Generally, in ordinary producing conditions, Mg of approximately 0.0001% is contained unavoidably.
Zr: more than 0.0005% to 0.010%
Zr (zirconium) is an element that improves a fraction of equiaxial austenite and refines the austenite grains, because Zr is crystallized as ZrO which acts as nuclei of the austenite. As a result, the pearlite block size of the steel wire rod is refined, and thus the ductility of the steel wire rod is improved. In order to obtain the effects, it is preferable that an amount of Zr be more than 0.0005% to 0.010%. The amount of Zr is more preferably 0.0005% to 0.0050%. When the amount of Zr is more than 0.010%, coarse oxides may be formed, and thus the fracture of the wire during the wire-drawing may occur.
Next, the metallographic structure of the steel wire rod according to the embodiment will be described.
The steel wire rod according to the embodiment includes, the metallographic structure, by area %, 95% to 100% of the pearlite. When a distance from a peripheral surface to a center of the steel wire rod is defined as r in a unit of mm, an average pearlite block size at a central portion which is an area from the center of the steel wire rod to r×0.99 is 1 μm to 25 μm. An average pearlite block size at a surface layer portion which is an area from the peripheral surface of the steel wire rod to r×0.01 is 1 μm to 20 μm. When a minimum lamellar spacing of the pearlite at the central portion is defined as S in a unit of nm, a following Expression B is satisfied.

S<12r+65  (Expression B)

Pearlite: 95% to 100%
When 95% to 100% of the pearlite is contained in the metallographic structure, a fraction of the non-pearlite structure such as the upper bainite, the pro-eutectoid ferrite, the degenerate pearlite, and the pro-eutectoid cementite decreases, and thus the strength and the ductility of the steel wire rod is improved. Although it is ideal that the non-pearlite structure is completely suppressed by controlling the pearlite in the metallographic structure to be 100%, in fact it is not necessary that the non-pearlite structure is reduced to zero. In a case where 95% to 100% of pearlite is contained in the metallographic structure, the strength and the ductility of the steel wire rod is sufficiently improved.
The metallographic structure of the steel wire rod may be observed by using a SEM (Scanning Electron Microscope) after subjecting a sample to chemical etching with picric acid. An observed section may be a cross-section (L cross-section) which is parallel to a longitudinal direction of the steel wire rod, metallographic micrographs of at least five visual fields may be taken by the SEM at a magnification of 2000-fold, and an average value of the fraction of the pearlite may be determined by an image analysis.
Average pearlite block size at central portion of steel wire rod: 1 μm to 25 μm
The pearlite block size (PBS) is a factor affecting the ductility of the steel wire rod or the ductility of the steel wire after the wire-drawing. When the austenite grains are refined at the high-temperature region or the pearlitic transformation temperature during the patenting treatment is a low temperature, the PBS is refined. In addition, the ductility of the steel wire rod is improved. FIG. 2 shows a relationship between the reduction of area of the steel wire rod and the average pearlite block size in the metallographic structure of the central portion of the steel wire rod. As shown in the figure, in order to sufficiently increasing and controlling the reduction of area of the steel wire rod to be 45% or more, it is necessary for the average PBS at the central portion of the steel wire rod to be 25 μm or less. The average PBS at the central portion of the steel wire rod is preferably 20 μm or less and more preferably 15 μm or less. In addition, although it is preferable that the PBS at the central portion of the steel wire rod is as fine as possible, the above-described properties of the steel wire rod are satisfied as long as the average PBS is 1 μm or more.
Average pearlite block size at surface layer portion of steel wire rod: 1 μm to 20 μm
The surface layer portion of the steel wire rod is a region at which delamination occurs when the steel wire is torsionally deformed. In order to suppress occurrence of the delamination of the steel wire by sufficiently increasing the drawability of the steel wire rod, the PBS at the surface layer portion of the steel wire rod is refined as compared with that at the central portion of the steel wire rod. Accordingly, it is necessary for the average PBS at the surface layer portion of the steel wire rod to be 20 μm or less. The average PBS at the surface layer portion of the steel wire rod is preferably 15 μm or less and more preferably 10 μm or less. In addition, although it is preferable that the PBS at the surface layer portion of the steel wire rod is as fine as possible, the above-described properties of the steel wire rod are satisfied as long as the average PBS is 1 μm or more.
The pearlite block size of the steel wire rod may be determined by using an EBSD (Electron BackScatter Diffraction Pattern) method. The L cross-section of the steel wire rod which is embedded in resin may be cut and polished, EBSD measurement may be conducted in at least three visual fields which are 150 μm×250 μm at the central portion and the surface layer portion of the steel wire rod, and the average pearlite block size may be determined by the analysis with a method of Johnson-Saltykov in which a region surrounded by boundaries having a misorientation of 9° is regarded as one block.
Minimum Lamellar Spacing s of Pearlite at Central Portion of Steel Wire Rod
The lamellar spacing is a factor affecting the strength of the steel wire rod or the strength of the steel wire after the wire-drawing. When the pearlitic transformation temperature during the patenting treatment is a low temperature, the lamellar spacing is refined. In addition, the strength of the steel wire rod increases. Accordingly, the lamellar spacing can be controlled by adjusting the alloy elements and by changing the pearlitic transformation temperature. In addition, a diameter of the steel wire rod also affects the lamellar spacing. Since the cooling rate of the steel wire rod after hot rolling increases with reducing the diameter of the steel wire rod, the lamellar spacing is refined. FIG. 3 shows a relationship between the diameter of the steel wire rod and the minimum lamellar spacing S of the pearlite in the metallographic structure at the central portion of the steel wire rod. In the figure, results of the steel wire rods, which satisfy the chemical composition and the metallographic structure as mentioned above, are shown as a rhombus, and results of conventional steel wire rods are shown as a quadrangle. In addition, in the figure, S=12r+65 is indicated by a straight line I. As can be seen from the figure, the minimum lamellar spacing S of the steel wire rod, which satisfies the chemical composition and the metallographic structure, is smaller than the minimum lamellar spacing S of the conventional steel wire rod in any diameter by using the straight line I as a border. Specifically, the minimum lamellar spacing S of the steel wire rod according to the embodiment satisfies the above-described Expression B (S<12r+65). As a result, the strength of the steel wire rod further increases as compared with the conventional steel wire rod.
The minimum lamellar spacing S of the pearlite of the steel wire rod may be observed by using the SEM. An observed section may be a cross-section (C cross-section) which is orthogonal to the longitudinal direction of the steel wire rod, the observed section which is embedded in resin may be cut and polished, metallographic micrographs of at least five visual fields at the central portion of the steel wire rod may be taken by the SEM at a magnification of 10000-fold, the minimum lamellar spacing in the visual fields may be measured, and then an average value thereof may be determined.
In addition, in the steel wire rod according to the embodiment, when tensile strength is defined as TS in a unit of MPa and the reduction of area is defined as RA in a unit of %, it is preferable that both of a following Expression C and a following Expression D are satisfied. Generally, it is known that the reduction of area RA is inversely proportional to the tensile strength TS. As described above, a steel wire rod having the reduction of area of 45% or more has been anticipated at present. In addition, in a case of a steel wire rod in which severe tensile strength TS is not required, it is preferable that the reduction of area RA be further larger than 45%. FIG. 4 shows a relationship between the tensile strength of the steel wire rod and the reduction of area of the steel wire rod. In the figure, results of the above-described steel wire rod are shown as a rhombus, and results of the conventional steel wire rod are shown as a quadrangle. In addition, in the figure, RA=100−0.045×TS is indicated by a straight line II, and RA=45 is indicated by a straight line III. As can be seen from the figure, the value of the reduction of area RA of the steel wire rod is larger than that of the conventional steel wire rod by using the straight line II and the straight line III as a border. As mentioned above, it is preferable that the value of the reduction of area RA increases as a function of the value of the tensile strength TS so as to satisfy the following Expression C and the following Expression D. In addition, RA>46 is preferable, RA>48 is more preferable, and RA>50 is most preferable. Although an upper limit of the reduction of area RA is not particularly limited, the wire-drawing can be sufficiently conducted in general when the reduction of area RA is 60%. Accordingly, the upper limit of the reduction of area RA may be 60%.

RA≧100−0.045×TS  (Expression C)

RA≧45  (Expression D)

When the steel wire rod satisfies the above-described chemical composition and metallographic structure, the steel wire rod having higher strength and better ductility than those of the conventional one may be obtained. In order to obtain the steel wire rod having the metallographic structure, the steel wire rod may be produced by the following production method.
Next, the method of producing the steel wire rod according to the embodiment will be described.
In a casting process, molten steel which consists of the base elements, the optional elements, and the unavoidable impurities as described above is casted to obtain a cast piece. Although a casting method is not limited particularly, a vacuum casting method, a continuous casting method, and the like may be employed.
In addition, according to the necessity, a soaking, a blooming, and the like may be conducted by using the cast piece after the casting process.
In a heating process, the cast piece after the casting process is heated to a temperature of 1000° C. to 1100° C. The reason why the cast piece is heated to the temperature range of 1000° C. to 1100° C. is to allow the metallographic structure of the cast piece to be the austenite. When the temperature is lower than 1000° C., transformation from the austenite to another structure may occur during the hot rolling that is a subsequent process. When the temperature is higher than 1100° C., austenite grains may grow and coarsen.
In the hot-rolling process, the cast piece after the heating process is hot-finish-rolled so as to control a finishing rolling temperature to be 850° C. to 1000° C. in order to obtain hot-rolled steel. Here, the finish-rolling indicates rolling of a final pass in the hot-rolling process in which plural passes of the hot rolling are conducted. The reason why the finishing rolling temperature is the temperature range of 850° C. to 1000° C. is to control the pearlite block size (PBS). When the finishing rolling temperature is lower than 850° C., transformation from the austenite to another structure may occur during the hot rolling. When the finishing rolling temperature is higher than 1000° C., it is difficult to control a temperature in subsequent processes, and thus the PBS may not be controlled. In addition, it is preferable that rolling reduction in the finish rolling be 10% to less than 60%. When the rolling reduction in the finish rolling is 10% or more, an effect of refining the austenite grains may be appropriately obtained. On the other hand, when the rolling reduction in the finish rolling is 60% or more, load on production facilities may be excessive, and the production cost may increase.
In a coiling process, the hot-rolled steel after the hot-rolling process is coiled within a temperature range of 780° C. to 840° C. The reason why the coiling temperature range is 780° C. to 840° C. is to control the PBS. When the coiling temperature is lower than 780° C., the pearlitic transformation tends to start only at the surface layer portion that is easily cooled. When the coiling temperature is higher than 840° C., unevenness in the PBS may increase due to a difference in the cooling rate between an overlapped portion and a non-overlapped portion during the coiling. The upper limit of the coiling temperature is preferably lower than 800° C. in order to refine the PBS and increase the reduction of area of the steel wire rod.
In a patenting process, within 15 seconds after the coiling process, the hot-rolled steel after the coiling process is directly immersed in a molten salt (DLP) which is held at a temperature of 480° C. to 580° C. The reason why the hot-rolled steel is isothermally maintained at the temperature range of 480° C. to 580° C. within 15 seconds after the coiling process is to preferentially progress the pearlitic transformation. As a result, it is possible to obtain the metallographic structure having the small fraction of the non-pearlite structure. When the temperature of the molten salt is lower than 480° C., the upper bainite which is soft increases, and thus the strength of the steel wire rod is not improved. On the other hand, when the temperature of the molten salt is higher than 580° C., the temperature is high for the pearlitic transformation temperature, the PBS coarsens, and the lamellar spacing also coarsens. In addition, when longer than 15 seconds, the austenite grain size may coarsen, and the fraction of the non-pearlite structure may increase due to the formation of the pro-eutectoid cementite and the like. It is preferable that the immersion is conducted within 10 seconds. Although it is ideal that a lower limit of the number of seconds is 0 seconds, in fact it is preferable that the lower limit is 2 seconds or longer.
In a cooling process, the hot-rolled steel which has been subjected to the patenting treatment and in which the pearlitic transformation has been finished is cooled to room temperature after the patenting process in order to the steel wire rod. The steel wire rod has the above-described metallographic structure.
EXAMPLE
Hereinafter, the effects of an aspect of the present invention will be described in detail with reference to the following examples. However, the condition in the examples is an example condition employed to confirm the operability and the effects of the present invention, so that the present invention is not limited to the example condition. The present invention can employ various types of conditions as long as the conditions do not depart from the scope of the present invention and can achieve the object of the present invention.
Sample Preparation
Examples 1 to 48 and Comparative Examples 49 to 85 with the chemical composition shown in Tables 1 and 2 were casted into cast piece having a shape of 300 mm×500 mm by using a continuous casting machine (casting process). The cast piece was subject to blooming to a shape of a cross-section of 122 mm square. The steel piece (cast piece) was heated to 1000° C. to 1100° C. (heating process). After the heating, finish rolling was conducted so that a finishing rolling temperature was 850° C. to 1000° C., whereby hot-rolled steel having a wire rod diameter (diameter) shown in Tables 3 and 4 was obtained (hot rolling process). The hot-rolled steel was coiled at 780° C. to 840° C. (coiling process). After the coiling, a patenting treatment was conducted (patenting process). Some of the hot-rolled steels were subject to the patenting treatment by immersed in a salt bath held at 480° C. to 580° C. within 15 seconds after the coiling. After the patenting treatment, cooling to room temperature was conducted to obtain steel wire rod (cooling process). In Tables 1 to 4, the underlined value indicates out of the range of the present invention. In Table 1, the blank column indicates that the optional element was not intentionally added.
In addition, wire-drawing was conducted by using the produced steel wire rod. In the wire-drawing, scale of the steel wire rod was removed by pickling, a zinc phosphate film was applied by phosphating, the wire-drawing in which reduction per a pass was 10% to 25% was conducted by using a die having an approach angle of 10°, and whereby a high strength steel wire having a diameter of 1.5 mm to 4.5 mm was obtained. Work strain during the wire-drawing and the wire diameter (diameter) of the steel wire after the wire-drawing are shown in Tables 3 and 4.
Evaluation
Area Fraction of Pearlite
The steel wire rod was embedded in resin and was polished. The steel wire rod was subjected to chemical etching using picric acid and was observed by using a SEM. An observed section was a cross-section (L cross-section) which is parallel to a longitudinal direction of the steel wire rod. In addition, grain boundary ferrite, bainite, pro-eutectoid cementite, and micromartensite were regarded as a non-pearlite structure, and a fraction of the balance was regarded as the area fraction of pearlite. Evaluation of the area fraction of pearlite was conducted by SEM-observing total five areas including, when the diameter of the steel wire rod was defined as D in a unit of mm, total four areas which were obtained by rotating a 1⁄4D region in the L cross-section of the steel wire rod by 90° around the center of the steel wire rod and one area which was the center of the steel of a 1⁄2D region in the L cross-section of the steel wire rod. In the SEM observation, metallographic micrographs with a visual field of vertically 100 μm×horizontally 200 μm were taken at a magnification of 2000-fold, and an average value of the area fraction of pearlite was determined by an image analysis of the metallographic micrographs. A case in which the pearlite was 95% to 100% in a unit of area % was judged to be acceptable.
Average Pearlite Block Size
A pearlite block size (PBS) of the steel wire rod was determined by using an EBSD method. The L cross-section of the steel wire rod was embedded in resin and was polished. When a distance from a peripheral surface to the center of the steel wire rod was r in a unit of mm, a central portion was an area from the center of the steel wire rod to r×0.99, and a surface layer portion was an area from the peripheral surface of the steel wire rod to r×0.01, the central portion and the surface layer portion were evaluated. EBSD measurement was conducted in at least three visual fields which were 150 μm×250 μm at the central portion and the surface layer portion of the steel wire rod, and an average pearlite block size was determined by the analysis with a method of Johnson-Saltykov in which a region surrounded by boundaries having a misorientation of 9° was regarded as one block. A case in which the average pearlite block size at the central portion was 1 μm to 25 μm and a case in which the average pearlite block size at the surface layer portion was 1 μm to 20 μm were judged to be acceptable.
Minimum Lamellar Spacing
A minimum lamellar spacing S at the central portion of the steel wire rod was observed by using the SEM. An observed section was a cross-section (C cross-section) which was orthogonal to the longitudinal direction of the steel wire rod. Metallographic micrographs of at least five visual fields at the central portion of the steel wire rod were taken by the SEM at a magnification of 10000-fold, the minimum lamellar spacing in the visual fields was measured, and then an average value thereof was determined. A case in which the r that is a distance from the peripheral surface to the center of the steel wire rod and the S satisfied S<12r+65 was judged to be acceptable.
Mechanical Properties
Test specimens having a gauge length of 200 mm were prepared so that the longitudinal direction of the steel wire rod and the steel wire was a tensile direction, and tensile tests were conducted under a rate of 10 mm/min. Average values of the tensile strength (TS) and the reduction of area (RA) were determined from results of at least three times of the tests. A case in which the tensile strength (TS) was 1200 MPa or more and a case in which the reduction of area (RA) was 45% were judged to be acceptable.
Occurrence of Delamination
Occurrence of delamination was evaluated by using the steel wire after the wire-drawing. When the diameter of the steel wire was d, the steel wire after the wire-rolling was subjected to a torsion test by using a torsion testing machine under conditions such that a gauge length was 100×d and a rotational speed was 10 rpm. In addition, at least three times of the torsion tests were conducted. A case in which the at least one occurrence of the delamination was confirmed by visual observation was regarded as “occurred”, and a case in which the occurrence of the delamination was not confirmed was regarded as “not occurred”. The delamination “not occurred” was judged to be acceptable.
The production results and the evaluation results are shown in Tables 1 to 4. In Nos. 1 to 48 that were examples, the steel wire rods had excellent strength and ductility. In addition, in the steel wires that were wire-drawn from the steel wire rod, the strength was high strength, and the occurrence of delamination was suppressed.
On the other hand, in Nos. 49 to 85 that were comparative examples, the steel wire rods were out of the range of the present invention. In the steel wires that were wire-drawn from the steel wire rod, the occurrence of delamination was confirmed.
In Comparative Example 49, the amount of Al+Ti was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 50, the amount of Cr was excessive, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 51, the amount of Co was excessive, a large amount of expensive element was contained, and the cost increased. In Comparative Example 52, the amount of V was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 53, the amount of Cu was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 54, the amount of Nb was excessive, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 55, the amount of Mo was excessive, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 56, the amount of W was excessive, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 57, the amount of B was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 58, the amount of REM was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 59, the amount of Ca was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 60, the amount of Mg was excessive, and thus RA of the steel wire rod was insufficient. In Comparative Example 61, the amount of Zr was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 62, the amount of C was insufficient, and thus TS and RA of the steel wire rod were insufficient. In Comparative Example 63, the amount of C was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 64, the amount of Si was insufficient, and thus TS and RA of the steel wire rod were insufficient. In Comparative Example 65, the amount of Si was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 66, the amount of Mn was insufficient, and thus TS and RA of the steel wire rod were insufficient. In Comparative Example 67, the amount of Mn was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 68, the amount of N was insufficient, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient. In Comparative Example 69, the amount of N was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 70, the amount of Ni was insufficient, and thus the average PBS at the central portion of the steel wire rod, the average PBS at the surface layer portion of the steel wire rod, and the minimum lamellar spacing at the central portion of the steel wire rod were insufficient. In Comparative Example 71, the amount of Ni was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 72, the amount of Al was insufficient, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient. In Comparative Example 73, the amount of Al was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 74, the amount of Ti was insufficient, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient. In Comparative Example 75, the amount of Ti was excessive, and thus RA of the steel wire rod was insufficient.
In Comparative Example 76, the heating temperature in the heating process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 77, the heating temperature in the heating process was high, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 78, the reduction of the finish rolling in the hot-rolling process was small, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 79, the finishing rolling temperature in the hot-rolling process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 80, the finishing rolling temperature in the hot-rolling process was high, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 81, the coiling temperature in the coiling process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 82, the coiling temperature in the coiling process was high, and thus the average PBS at the central portion of the steel wire rod and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 83, a time after the coiling process before the patenting process was long, and thus the fraction of the pearlite of the steel wire rod, the average PBS at the central portion of the steel wire rod, and the average PBS at the surface layer portion of the steel wire rod were insufficient.
In Comparative Example 84, the temperature of the molten salt in the patenting process was low, and thus the fraction of the pearlite of the steel wire rod was insufficient. In Comparative Example 85, the temperature of the molten salt in the patenting process was high, and thus the minimum lamellar spacing at the central portion of the steel wire rod was insufficient.







TABLE 1





CHEMICAL COMPOSITION (mass %)





























No.
C
Si
Mn
P
S
O
Al
Ti
N
Cr
Mo
Ni





Example
1
0.72
0.33
0.79
0.011
0.013
0.0024
0.023

0.0032


0.023



2
0.71
0.32
0.80
0.010
0.012
0.0022

0.026
0.0033


0.023



3
0.71
0.32
0.79
0.011
0.011
0.0025
0.023
0.025
0.0031


0.023



4
0.73
0.31
0.77
0.011
0.013
0.0024
0.024
0.026
0.0031
0.12

0.022



5
0.72
0.33
0.79
0.011
0.013
0.0024
0.024
0.027
0.0032


0.022



6
0.71
0.33
0.79
0.012
0.012
0.0023
0.022
0.025
0.0033


0.022



7
0.73
0.32
0.78
0.010
0.013
0.0023
0.022
0.025
0.0033


0.022



8
0.73
0.31
0.78
0.010
0.012
0.0023
0.022
0.024
0.0033


0.023



9
0.74
0.32
0.79
0.011
0.013
0.0025
0.021
0.024
0.0031

0.04
0.023



10
0.71
0.33
0.78
0.011
0.013
0.0024
0.025
0.026
0.0032


0.023



11
0.72
0.32
0.80
0.013
0.011
0.0024
0.023
0.025
0.0031


0.024



12
0.72
0.32
0.79
0.011
0.011
0.0025
0.023
0.024
0.0032


0.023



13
0.73
0.31
0.80
0.012
0.013
0.0024
0.024
0.026
0.0031


0.024



14
0.72
0.32
0.77
0.010
0.012
0.0022
0.023
0.025
0.0033


0.022



15
0.73
0.32
0.79
0.011
0.013
0.0024
0.021
0.026
0.0032


0.024



16
0.71
0.15
0.11
0.003
0.011
0.0005
0.006
0.042
0.0049
0.45
0.18
0.005



17
0.72
0.18
0.79
0.011
0.013
0.0024
0.023
0.025
0.0032
0.18
0.057
0.049



18
0.72
0.17
0.99
0.013
0.014
0.0029
0.048
0.048
0.0010
0.0011
0.056
0.034



19
0.72
0.31
0.12
0.014
0.006
0.0023
0.044
0.005
0.0024
0.0012
0.0051
0.010



20
0.72
0.30
0.79
0.006
0.019
0.0028
0.092
0.005
0.0046
0.19
0.0052
0.006



21
0.71
0.32
0.99
0.019
0.014
0.0006
0.032
0.033
0.0033
0.21
0.058
0.009



22
0.72
0.34
0.11
0.014
0.013
0.0024
0.094
0.005
0.0012
0.14
0.023
0.023



23
0.71
0.35
0.78
0.013
0.012
0.0029
0.005
0.092
0.0021
0.13
0.032
0.007



24
0.72
0.34
0.98
0.012
0.009
0.0023
0.046
0.005
0.0025
0.098
0.059
0.006



25
0.72
0.59
0.11
0.009
0.004
0.0022
0.045
0.049
0.0030
0.19
0.055
0.029



26
0.72
0.58
0.79
0.004
0.013
0.0024
0.035
0.013
0.0028
0.47
0.052
0.006



27
0.72
0.59
0.98
0.013
0.003
0.0024
0.036
0.043
0.0039
0.48
0.057
0.030



28
0.81
0.17
0.99
0.013
0.014
0.0029
0.048
0.048
0.0010
0.0011
0.056
0.034



29
0.80
0.31
0.12
0.014
0.006
0.0023
0.044
0.005
0.0024
0.0012
0.0051
0.010



30
0.79
0.30
0.79
0.006
0.019
0.0028
0.092
0.006
0.0046
0.19
0.0052
0.006



31
0.82
0.34
0.11
0.014
0.013
0.0024
0.094
0.005
0.0012
0.14
0.023
0.023



32
0.81
0.35
0.78
0.013
0.012
0.0029
0.005
0.092
0.0021
0.13
0.032
0.007



33
0.84
0.34
0.98
0.012
0.009
0.0023
0.046
0.005
0.0025
0.098
0.059
0.006



34
0.88
0.34
0.11
0.014
0.013
0.0024
0.094
0.005
0.0012
0.0011
0.056
0.034



35
0.89
0.35
0.78
0.013
0.012
0.0029
0.005
0.092
0.0021
0.0012
0.0051
0.010



36
0.88
0.34
0.98
0.012
0.009
0.0023
0.046
0.010
0.0025
0.19
0.0052
0.006



37
0.89
0.17
0.99
0.013
0.014
0.0029
0.048
0.048
0.0010
0.14
0.023
0.023



38
0.89
0.31
0.12
0.014
0.006
0.0023
0.044
0.005
0.0024
0.13
0.032
0.067



39
0.88
0.30
0.79
0.006
0.019
0.0028
0.092
0.006
0.0045
0.098
0.059
0.006



40
0.85
0.31
0.12
0.014
0.006
0.0024
0.094
0.005
0.0012
0.0011
0.0052
0.006



41
0.98
0.30
0.79
0.006
0.019
0.0025
0.005
0.092
0.0021
0.0012
0.0011
0.010



42
0.97
0.34
0.98
0.012
0.009
0.0024
0.094
0.005
0.0012
0.0011
0.056
0.034



43
0.99
0.17
0.99
0.013
0.014
0.0029
0.005
0.092
0.0021
0.0012
0.059
0.006







No.
Cu
V
Co
W
Nb
B
Mg
Ca
REM
Zr
Al + Ti






Example
1










0.023




2










0.026




3










0.048




4










0.050




5


0.1300







0.051




6

0.120








0.047




7
0.12









0.047




8




0.050





0.047




9










0.045




10



0.0700






0.051




11





0.0010




0.048




12








0.0020

0.047




13







0.0020


0.050




14






0.0015



0.046




15









0.0020
0.047




16
0.0011
0.48
0.0900
0.0008
0.056
0.0004
0.0012
0.0010
0.0038
0.0005
0.048




17
0.0012
0.090
0.1500
0.0580
0.055
0.0008
0.0048
0.0039
0.0014
0.0006
0.049




18
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.096




19
0.09
0.0013
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.049




20
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.098




21
0.0013
0.179
0.1800
0.0550
0.057
0.0012
0.0010
0.0038
0.0006
0.0005
0.065




22
0.0014
0.180
0.0600
0.0520
0.056
0.0011
0.0039
0.0014
0.0011
0.0008
0.099




23
0.17
0.060
0.1700
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0028
0.097




24
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.0006
0.0048
0.0039
0.0038
0.051




25
0.06
0.180
0.3400
0.0570
0.058
0.0011
0.0028
0.0005
0.0004
0.0014
0.094




26
0.17
0.34
0.2300
0.0560
0.023
0.0610
0.0038
0.0006
0.0008
0.0012
0.048




27
0.18
0.23
0.4800
0.0051
0.032
0.0012
0.0014
0.0011
0.0028
0.0048
0.049




28
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.096




29
0.09
0.0013
0.0014
0.0320
0.057
0.0024
0.0006
0.0008
0.0048
0.0010
0.049




30
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.098




31
0.0014
0.180
0.0600
0.0520
0.056
0.0011
0.0039
0.0014
0.0011
0.0006
0.099




32
0.17
0.050
0.1700
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0028
0.097




33
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0039
0.0098
0.051




34
0.16
0.150
0.0013
0.0230
0.052
0.0026
0.0006
0.0006
0.0012
0.0011
0.099




35
0.09
0.0013
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.097




36
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.056




37
0.0005
0.180
0.0600
0.0520
0.056
0.0011
0.0038
0.0014
0.0011
0.0006
0.098




38
0.17
0.080
0.1700
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0028
0.049




39
0.18
0.176
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0039
0.0038
0.098




40
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.099




41
0.09
0.0013
0.0014
0.0320
0.057
0.0015
0.0008
0.0008
0.0048
0.0010
0.097




42
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.099




43
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0039
0.0038
0.097
















TABLE 2





CHEMICAL COMPOSITION (mass %)





























No.
C
Si
Mn
P
S
O
Al
Ti
N
Cr
Mo
Ni





Example
44
0.98
0.34
0.11
0.014
0.013
0.0023
0.044
0.005
0.0024
0.13
0.032
0.007



45
0.97
0.35
0.78
0.013
0.012
0.0028
0.092
0.006
0.0046
0.098
0.023
0.023



46
0.89
0.31
0.98
0.025
0.013
0.0024
0.094
0.006
0.0012
0.0011
0.056
0.034



47
0.88
0.30
0.11
0.013
0.024
0.0029
0.005
0.013
0.0021
0.0012
0.0051
0.010



48
0.96
0.34
0.78
0.014
0.019
0.0034
0.046
0.043
0.0025
0.19
0.0052
0.006


Comparative
49
0.89
0.17
0.78
0.014
0.019
0.0023
0.054
0.051
0.0025
0.098
0.059
0.006


Example
50
0.98
0.35
0.98
0.006
0.014
0.0023
0.094
0.005
0.0010
0.51
0.023
0.023



51
0.87
0.34
0.99
0.014
0.013
0.0028
0.005
0.005
0.0021
0.13
0.032
0.007



52
0.88
0.34
0.12
0.013
0.012
0.0006
0.046
0.006
0.0025
0.098
0.059
0.006



53
0.89
0.35
0.79
0.012
0.009
0.0024
0.046
0.005
0.0012
0.0011
0.056
0.034



54
0.89
0.34
0.78
0.014
0.004
0.0029
0.044
0.005
0.0021
0.0012
0.0051
0.010



55
0.88
0.17
0.98
0.013
0.013
0.0023
0.006
0.048
0.0046
0.19
0.23
0.006



56
0.96
0.31
0.11
0.012
0.003
0.0022
0.046
0.006
0.0012
0.14
0.032
0.007



57
0.98
0.30
0.78
0.013
0.014
0.0024
0.094
0.005
0.0021
0.13
0.0051
0.010



58
0.97
0.31
0.98
0.014
0.006
0.0024
0.005
0.005
0.0025
0.10
0.056
0.034



59
0.82
0.30
0.99
0.014
0.019
0.0029
0.046
0.048
0.0012
0.0011
0.032
0.007



60
0.81
0.35
0.12
0.006
0.013
0.0024
0.048
0.043
0.0021
0.0012
0.059
0.006



61
0.84
0.34
0.79
0.014
0.004
0.0024
0.005
0.048
0.0025
0.19
0.0052
0.006



62
0.68
0.31
0.12
0.014
0.006
0.0023
0.044
0.005
0.0024
0.0012
0.0051
0.010



63
1.02
0.30
0.79
0.006
0.019
0.0028
0.092
0.006
0.0046
0.19
0.0052
0.006



64
0.80
0.12
0.12
0.014
0.006
0.0023
0.044
0.005
0.0024
0.0012
0.0051
0.010



65
0.79
0.64
0.79
0.006
0.019
0.0028
0.092
0.006
0.0046
0.19
0.0062
0.006



66
0.72
0.17
0.08
0.013
0.019
0.0023
0.045
0.013
0.0024
0.13
0.032
0.007



67
0.81
0.31
1.08
0.014
0.014
0.0026
0.035
0.043
0.0046
0.098
0.059
0.006



68
0.80
0.30
0.98
0.006
0.013
0.0006
0.006
0.048
0.0009
0.0011
0.0052
0.006



69
0.79
0.32
0.11
0.014
0.012
0.0024
0.048
0.005
0.0053
0.0012
0.0051
0.010



70
0.82
0.34
0.79
0.013
0.009
0.0029
0.044
0.006
0.0025
0.098
0.059
0.0047



71
0.81
0.35
0.98
0.012
0.004
0.0023
0.092
0.005
0.0030
0.19
0.055
0.051



72
0.84
0.34
0.99
0.014
0.013
0.0022
0.0046
0.005
0.0024
0.0012
0.0051
0.010



73
0.88
0.59
0.12
0.013
0.003
0.0024
0.11
0.048
0.0046
0.19
0.0052
0.006



74
0.89
0.58
0.79
0.012
0.014
0.0024
0.005
0.0046
0.0012
0.14
0.023
0.023



75
0.88
0.59
0.11
0.013
0.006
0.0029
0.048
0.11
0.0021
0.13
0.032
0.007



76
0.72
0.17
0.99
0.013
0.014
0.0029
0.048
0.05
0.001
0.0011
0.058
0.034



77
0.72
0.31
0.12
0.014
0.006
0.0023
0.044
0.01
0.0024
0.0012
0.0051
0.010



78
0.81
0.17
0.99
0.013
0.014
0.0029
0.048
0.05
0.001
0.0011
0.056
0.034



79
0.84
0.34
0.98
0.012
0.009
0.0023
0.046
0.01
0.0025
0.098
0.059
0.006



80
0.88
0.34
0.11
0.014
0.013
0.0024
0.094
0.01
0.0012
0.0011
0.056
0.034



81
0.88
0.34
0.98
0.012
0.009
0.0023
0.046
0.01
0.0025
0.19
0.0052
0.006



82
0.89
0.17
0.99
0.013
0.014
0.0029
0.048
0.05
0.001
0.14
0.023
0.023



83
0.96
0.31
0.12
0.014
0.006
0.0024
0.094
0.01
0.0012
0.0011
0.0052
0.006



84
0.98
0.30
0.79
0.006
0.019
0.0029
0.005
0.09
0.0021
0.0012
0.0051
0.010



85
0.97
0.34
0.98
0.012
0.009
0.0024
0.094
0.01
0.0012
0.0011
0.056
0.034







No.
Cu
V
Co
W
Nb
B
Mg
Ca
REM
Zr
Al + Ti






Example
44
0.17
0.060
0.1700
0.570
0.0005
0.0005
0.0006
0.0012
0.0010
0.0028
0.049




45
0.0014
0.180
0.0600
0.0520
0.056
0.0011
0.0039
0.0014
0.0011
0.0008
0.098




46
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.100




47
0.09
0.0013
0.0014
0.0320
0.059
0.0015
0.0006
0.0008
0.0048
0.0010
0.018




48
0.15
0.0014
0.1700
0.0580
0.080
0.0014
0.0011
0.0028
0.0005
0.0039
0.089



Comparative
49
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.008
0.0048
0.0039
0.0038
0.105



Example
50
0.0014
0.180
0.0600
0.0520
0.056
0.0011
0.0039
0.0014
0.0011
0.0008
0.099




51
0.18
0.170
0.53
0.520
0.056
0.0011
0.0038
0.0014
0.0011
0.0008
0.010




52
0.09
0.53
0.0013
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0029
0.052




53
0.22
0.170
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0039
0.0038
0.053




54
0.17
0.060
0.1700
0.0570
0.11
0.0014
0.0011
0.0028
0.005
0.0039
0.049




55
0.15
0.0014
0.1700
0.0582
0.090
0.0015
0.0006
0.0008
0.0048
0.0010
0.054




56
0.17
0.060
0.1700
0.23
0.056
0.0028
0.0005
0.0006
0.0012
0.0011
0.052




57
0.09
0.0013
0.0014
0.0320
0.057
0.0034
0.0039
0.0014
0.001
0.0039
0.089




58
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0055
0.0008
0.010




59
0.17
0.060
0.1700
0.0571
0.0051
0.006
0.006
0.0054
0.0012
0.0008
0.094




60
0.1800
0.170
0.1822
0.1800
0.0052
0.0006
0.0053
0.0028
0.0005
0.0028
0.091




61
0.1500
0.001
0.1700
0.0582
0.09
0.0014
0.0006
0.0008
0.0048
0.0110
0.053




62
0.0900
0.001
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.049




63
0.1500
0.0014
0.1700
0.0590
0.09
0.0014
0.0011
0.0028
0.0005
0.0039
0.098




64
0.09
0.0013
0.0614
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.049




65
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0038
0.098




66
0.1700
0.060
0.1700
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0028
0.058




67
0.1800
0.170
0.1800
0.1800
0.0052
0.0006
0.0006
0.0046
0.0039
0.0038
0.078




68
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.054




69
0.09
0.0013
0.0014
0.0320
0.057
0.0015
0.0008
0.0008
0.0048
0.0010
0.053




70
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.060




71
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0038
0.0038
0.097




72
0.09
0.0013
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.010




73
0.15
0.0014
0.1700
0.0590
0.090
0.0014
0.0011
0.0028
0.0005
0.0039
0.158




74
0.0014
0.180
0.0600
0.520
0.056
0.0011
0.0039
0.0014
0.0014
0.0008
0.010




75
0.17
0.060
0.1700
0.0570
0.0051
0.0005
0.0006
0.0012
0.0010
0.0028
0.158




76
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.096




77
0.09
0.001
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.049




78
0.16
0.150
0.0013
0.230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.096




79
0.18
0.170
0.1800
0.1800
0.0052
0.0006
0.0008
0.0048
0.0039
0.0038
0.051




80
0.16
0.150
0.0013
0.020
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.099




81
0.15
0.001
0.1700
0.0580
0.09
0.0014
0.0011
0.0028
0.0005
0.0039
0.051




82
0.0014
0.180
0.0600
0.0520
0.056
0.0011
0.0039
0.0014
0.0011
0.0008
0.096




83
0.15
0.001
0.1700
0.0590
0.09
0.0014
0.0011
0.0028
0.0005
0.0039
0.099




84
0.09
0.001
0.0014
0.0320
0.057
0.0015
0.0006
0.0008
0.0048
0.0010
0.097




85
0.16
0.150
0.0013
0.0230
0.052
0.0028
0.0005
0.0006
0.0012
0.0011
0.099
















TABLE 3





PRODUCTION CONDITIONS






















(1)
3
(7)
(9)


















(2)
(4)
(5)
(6)
(8)


(12)



No.
(° C.)
(%)
(° C.)
(um)
(° C.)
(10)
(11)
(° C.)





Example
1
1026
21
901
9.0
795
7
DLP
530



2
1023
20
901
9.0
790
6
DLP
535



3
1022
21
900
9.0
785
8
DLP
530



4
1028
22
900
9.0
790
7
DLP
530



5
1028
22
902
9.0
790
9
DLP
525



6
1025
21
908
9.0
795
8
DLP
530



7
1026
22
900
9.0
765
7
DLP
530



8
1026
21
901
9.0
795
7
DLP
530



9
1028
20
903
9.0
785
8
DLP
525



10
1025
21
901
9.0
790
7
DLP
530



11
1022
23
902
9.0
785
8
DLP
535



12
1026
22
800
9.0
780
6
DLP
530



13
1025
22
801
9.0
785
6
DLP
530



14
1026
21
803
9.0
785
7
DLP
535



15
1023
20
801
9.0
795
7
DLP
530



16
1001
36
851
5.5
795
5
DLP
540



17
1002
32
875
10.0
840
15
DLP
530



18
1024
27
888
12.5
780
5
DLP
550



19
1025
15
900
5.5
795
6
DLP
580



20
1026
21
901
9.0
795
7
DLP
480



21
1048
25
924
12.0
795
8
DLP
550



22
1050
52
925
10.0
790
4
DLP
550



23
1051
29
926
12.0
840
14
DLP
560



24
1074
28
948
12.5
820
18
DLP
545



25
1075
48
950
12.0
795
6
DLP
580



26
1076
31
851
12.0
786
5
DLP
550



27
1086
23
975
12.5
825
12
DLP
535



28
1088
56
896
11.0
780
5
DLP
540



29
1002
33
852
9.5
795
5
DLP
540



30
1003
33
876
10.0
840
15
DLP
530



31
1025
25
900
12.5
780
8
DLP
550



32
1026
16
901
535
795
8
DLP
580



33
1027
22
902
9.0
795
7
DLP
450



34
1050
26
925
12.0
795
8
DLP
550



35
1051
53
926
10.0
790
4
DLP
560



36
1052
30
927
12.0
840
14
DLP
580



37
1075
29
950
12.5
820
13
DLP
545



38
1076
49
951
12.0
795
8
DLP
580



39
1077
32
952
12.0
780
5
DLP
550



40
1099
24
976
12.5
825
12
DLP
535



41
1099
59
999
11.0
790
5
DLP
540



42
1002
37
852
5.5
795
5
DLP
540



43
1003
33
876
10.0
840
15
DLP
530


















(13)

















(14)
(22)
(27)


















(16)
(19)

(24)
(28)
(31)






















(15)
(17)
(18)
(20)

(23)
(25)

(29)


(33)



No.
(%)
(μm)
(μm)
(nm)
(21)
(MPa)
(%)
(26)
(mm)
(30)
(32)
(MPa)





Example
1
95.1
22.5
19.7
113
119
1225
45
44.9
3.0
2.20
(34)
2225



2
95.2
22.8
19.8
113
119
1228
45
44.8
3.0
2.20
(34)
2227



3
95.2
20.5
18.6
113
119
1238
46
43.5
3.0
2.20
(34)
2282



4
85.2
20.4
18.7
100
119
1322
46
40.9
3.0
2.20
(34)
2035



5
95.1
20.0
18.9
113
119
1251
48
43.7
3.0
2.20
(34)
2283



6
85.1
18.3
17.4
113
119
1204
47
61.3
3.0
2.20
(34)
2316



7
95.0
20.6
18.5
113
119
1253
46
43.6
3.0
2.20
(34)
2264



8
95.5
14.9
14.5
113
119
1231
48
62.4
3.0
2.20
(34)
2298



9
97.1
19.9
18.7
113
119
1301
48
61.5
3.0
2.20
(34)
2313



10
97.2
19.7
18.8
113
119
1305
48
61.4
3.0
2.20
(34)
2315



11
97.0
18.3
17.4
113
119
1314
48
60.9
3.0
2.20
(34)
2322



12
96.1
20.3
19.0
113
119
1262
48
63.7
3.0
2.20
(34)
2282



13
95.2
19.9
18.7
113
119
1275
48
62.6
3.0
2.20
(34)
2285



14
95.0
18.7
18.8
113
119
1276
48
42.5
3.0
2.20
(34)
2287



15
95.1
19.8
18.7
113
119
1275
48
62.8
3.0
2.20
(34)
2284



16
95.1
9.8
9.3
93
88
1380
50
37.9
1.5
2.60
(34)
2323



17
95.2
15.4
14.1
119
123
1222
47
45.0
2.8
2.55
(34)
2219



18
95.8
4.2
4.0
133
140
1304
55
41.3
3.0
2.65
(34)
2337



19
97.7
15.3
14.5
93
88
1643
49
25.8
1.5
2.60
(34)
2484



20
97.7
8.7
8.3
113
119
1252
51
43.8
3.0
2.20
(34)
2159



21
97.7
10.3
9.8
130
137
1286
52
42.1
3.8
2.30
(34)
2179



22
99.1
7.2
6.8
119
125
1445
54
35.5
3.0
2.41
(34)
2285



23
88.5
19.9
18.9
139
137
1315
45
40.7
4.0
2.20
(34)
2252



24
98.1
17.6
16.7
133
140
1385
47
38.6
4.5
2.04
(34)
2265



25
88.5
18.3
17.4
139
137
1395
48
37.2
3.8
2.30
(34)
2107



26
97.7
3.5
3.3
130
157
1380
56
379
3.8
2.50
(34)
2218



27
96.5
17.0
16.2
133
140
1365
47
38.0
4.5
2.00
(34)
2108



28
96.3
12.4
11.8
124
101
1427
47
35.8
4.0
2.02
(34)
2115



29
95.1
9.8
9.3
93
98
1380
50
37.0
1.5
2.80
(34)
2023



30
95.2
21.1
20.0
119
125
1222
47
45.0
2.3
2.55
(34)
2216



31
85.8
4.2
4.0
133
140
1304
55
41.3
3.0
2.85
(34)
2337



32
97.7
15.3
14.5
93
95
1649
49
25.8
1.5
2.60
(34)
2484



33
87.7
8.7
8.3
113
119
1252
51
43.6
3.0
2.20
(34)
2159



34
97.7
10.8
9.8
130
107
1288
52
42.1
3.8
2.30
(34)
2179



35
99.1
7.2
6.5
119
125
1445
54
35.9
3.0
2.41
(34)
2285



36
98.5
19.9
18.9
130
137
1318
48
60.7
4.0
2.20
(34)
2252



37
98.1
17.8
16.7
133
140
1365
47
38.8
4.5
2.04
(34)
2205



38
98.8
18.3
17.4
130
137
1395
48
37.2
3.8
2.30
(34)
2107



39
97.7
3.8
3.2
130
137
1380
56
39.9
3.8
2.30
(34)
2218



40
96.5
17.0
18.2
133
140
1365
47
38.6
4.5
2.04
(34)
2103



41
96.3
12.4
11.8
124
131
1427
47
35.8
4.0
2.02
(34)
2115



42
95.1
9.8
9.3
93
98
1380
50
37.9
1.5
2.60
(34)
2323



43
95.2
21.1
20.0
119
123
1222
47
45.0
2.8
2.55
(34)
2219





(1) HEATING PROCESS


(2) HEATING TEMPERATURE


(3) HOT-ROLL PROCESS


(4) ROLLING REDUCTION IN FINISH ROLLING


(5) FINISHING ROLLING TEMPERATURE


(6) DIAMETER 2 r AFTER FINISH ROLLING


(7) COILING PROCESS


(8) COILING TEMPERATURE


(9) PATENTING PROCESS


(10) TIME AFTER COILING (sec.)


(11) PATENTING METHOD


(12) TEMPERATURE OF MOLTEN SALT


(13) EVALUATION RESULTS OF STEEL WIRE ROD


(14) METALLOGRAPHIC STRUCTURE


(15) FRACTION OF PEARLITE


(16) AVERAGE PEARLITE BLOCK SIZE


(17) CENTRAL PORTION


(18) SURFACE LAYER PORTION


(19) LAMELLAR SPACING


(20) MINIMUM LAMELLAR SPACING AT CENTRAL PORTION


(21) VALUE OF (12 r + 65)


(22) MECHANICAL PROPERTIES


(23) TENSILE STRENGTH TS


(24) REDUCTION OF AREA


(25) REDUCTION OF AREA RA


(26) VALUE OF (100-0.045 × TS)


(27) STEEL WIRE AFTER WIRE-DRAWING


(28) PRODUCTION CONDITIONS


(29) DIAMETER AFTER WIRE-DRAWING


(30) STRAIN DURING WIRE-DRAWING


(31) EVALUTION RESULTS


(32) OCCURRENCE OF DELAMINATION


(33) TENSILE STRENGTH TS


(34) NOT OCCURED













TABLE 4





PRODUCTION CONDITIONS






















(1)
(3)
(7)
(9)


















(2)
(4)
(5)
(6)
(8)


(12)



No.
(° C.)
(%)
(° C.)
(mm)
(° C.)
(10)
(11)
(° C.)





Example
44
1025
28
900
12.5
780
3
DLP
550



45
1026
16
901
5.5
795
6
DLP
580



46
1050
26
825
12.0
795
8
DLP
550



47
1051
53
926
10.0
790
4
DLP
580



48
1052
30
927
12.0
840
14
DLP
580



49
1027
22
902
9.0
705
7
DLP
480


Example
50
1075
29
950
12.5
820
13
DLP
545


Comparative
51
1076
49
951
12.0
795
6
DLP
580



52
1077
32
952
12.0
780
5
DLP
550



53
1089
24
976
12.5
825
12
DLP
535



54
1099
59
999
11.0
790
5
DLP
540



55
1003
37
878
5.5
840
15
DLP
530



56
1025
33
900
10.0
780
3
DLP
550



57
1026
28
901
12.5
795
6
DLP
580



58
1027
16
902
5.5
795
7
DLP
480



59
1050
22
925
9.0
795
8
DLP
550



60
1051
26
928
12.0
790
4
DLP
580



61
1052
53
927
10.0
840
14
DLP
560



62
1001
36
851
5.5
795
5
DLP
540



63
1002
32
875
10.0
840
15
DLP
530



64
1024
27
899
12.5
780
3
DLP
550



65
1025
15
900
5.5
795
6
DLP
580



66
1026
21
901
9.0
795
7
DLP
480



67
1049
25
924
12.0
795
8
DLP
550



68
1050
52
925
100
790
4
DLP
560



69
1051
29
926
12.0
840
14
DLP
560



70
1074
28
949
12.5
820
13
DLP
545



71
1075
48
950
12.0
795
6
DLP
580



72
1076
31
951
12.0
780
5
DLP
550



73
1058
23
975
12.5
825
12
DLP
535



74
1099
58
999
11.0
790
5
DLP
540



75
1002
37
852
5.5
795
5
DLP
540



76
995
22
904
10.0
795
7
(36)
—



77
1104
26
927
12.5
755
—
(37)
575



78
1027
8
928
5.5
790
5
(36)
—



79
1029
29
945
12.0
820
5
(36)
—



80
1052
49
1003
10.0
795
10
(36)
—



81
1053
32
954
12.0
778
6
(36)
—



82
1054
24
978
12.5
845
—
(37)
579



83
1077
59
995
12.0
780
17
(36)
—



84
1025
15
900
5.5
795
6
DLP
470



85
1026
21
901
9.0
795
7
DLP
600


















(13)

















(14)
(22)
(27)


















(16)
(19)

(24)
(28)
(31)






















(15)
(17)
(18)
(20)

(23)
(25)

(29)


(33)



No.
(%)
(μm)
(μm)
(nm)
(21)
(MPa)
(%)
(26)
(mm)
(30)
(32)
(MPa)





Example
44
95.8
4.2
4.0
133
140
1304
55
41.3
3.0
2.85
(34)
2337



45
97.7
16.8
14.6
93
98
1849
49
25.6
1.5
2.80
(34)
2484



46
87.7
10.3
9.8
130
137
1286
45
42.1
3.8
2.30
(34)
2179



47
90.1
7.2
5.8
112
125
1445
46
35.0
3.0
2.48
(34)
2285



48
88.5
19.9
18.9
130
137
1318
46
40.7
4.0
2.20
(34)
2252



49
87.7
8.7
8.3
113
119
1252
44
43.6
3.0
2.20
(35)
2159


Example
50
84.2
17.4
16.7
133
140
1182
44
45.8
4.5
2.04
(35)
2205


Comparative
51
98.8
18.3
17.4
130
137
1095
46
37.2
3.8
2.30
(34)
2107



52
97.7
3.5
3.3
130
137
1980
49
37.9
3.8
2.30
(35)
2218



53
98.5
17.9
16.2
133
140
1385
41
38.6
4.5
2.04
(35)
2103



54
94.8
12.4
11.8
124
138
1197
44
46.1
4.0
2.02
(35)
2115



55
94.2
9.8
8.3
83
98
1192
44
46.4
1.5
2.60
(35)
2323



56
94.0
21.1
20.0
119
125
1185
49
46.7
2.8
2.55
(35)
2219



57
95.8
4.2
4.0
93
140
1304
43
41.3
3.0
2.85
(35)
2337



58
97.7
15.3
14.5
93
98
1548
42
25.6
1.5
2.80
(35)
2484



59
97.7
8.7
8.3
113
119
1252
44
43.6
3.0
2.20
(35)
2159



60
87.7
10.3
9.8
119
137
1255
42
42.3
3.8
2.30
(35)
2179



61
89.1
7.2
5.8
119
125
1445
42
35.0
3.0
2.42
(35)
2285



62
85.1
9.8
9.3
93
98
1100
44
50.5
1.5
2.80
(35)
2323



63
95.2
21.1
20.0
119
125
1222
39
45.0
2.8
2.55
(35)
2219



64
95.8
4.2
4.0
133
140
1115
44
49.8
3.0
2.85
(35)
2337



65
97.7
15.3
14.5
93
88
1600
26
28.3
1.5
2.80
(35)
2484



66
97.7
8.7
8.3
113
119
1175
44
47.1
3.0
2.20
(35)
2159



67
97.7
10.3
9.8
130
137
1280
39
42.1
3.8
2.30
(35)
2179



68
89.1
26.5
25.5
119
125
1445
37
35.0
3.0
2.41
(35)
2288



69
98.5
19.9
18.9
130
137
1318
38
40.7
4.0
2.20
(35)
2252



70
98.1
25.9
25.6
166
140
1165
37
47.6
4.5
2.04
(35)
2205



71
98.8
18.3
17.4
130
137
1395
42
37.2
3.8
2.30
(35)
2187



72
87.7
26.5
25.3
130
137
1380
37
37.5
3.8
2.30
(23)
2218



73
86.5
17.0
16.2
133
140
1305
39
36.0
4.5
2.04
(35)
2103



74
96.3
28.4
25.8
124
131
1427
36
35.8
4.0
2.02
(35)
2115



75
95.1
9.8
9.3
93
98
1380
39
37.9
1.5
2.60
(35)
2323



76
83.1
22.1
19.9
119
125
1113
38
49.9
2.8
2.55
(35)
2316



77
95.5
29.3
28.6
133
140
1304
38
41.3
3.0
2.85
(35)
2212



78
96.3
28.6
25.7
93
98
1649
32
25.8
1.5
2.60
(35)
2330



79
92.5
6.3
6.0
130
137
1186
39
46.6
3.8
2.36
(35)
2153



80
95.6
26.9
25.6
119
125
1445
30
35.0
3.0
2.41
(35)
2172



81
93.2
13.5
12.8
130
137
1119
37
49.7
4.0
2.26
(35)
2278



82
87.7
26.2
24.9
133
140
1365
38
38.6
4.5
2.04
(35)
2245



83
92.1
25.9
25.6
130
137
1195
36
46.2
3.8
2.36
(25)
2198



84
92.7
15.3
14.5
93
98
1178
30
47.0
1.5
2.00
(35)
2216



85
97.7
8.7
8.3
179
119
1128
38
49.2
3.0
2.20
(35)
1981





(1) HEATING PROCESS


(2) HEATING TEMPERATURE


(3) HOT-ROLLING PROCESS


(4) ROLLING REDUCTION IN FINISH ROOLING


(5) FINISHING ROLLING TEMPERATURE


(6) DIAMETER 2 R AFTER FINISH ROLLING


(7) COILING PROCESS


(8) COILING TEMPERATURE


(9) PATENTING PROCESS


(10) TIME AFTER COILING (sec.)


(11) PATENTING METHOD


(12) TEMPERATURE OF MOLTEN SALT


(36) STELMOR


(37) REHEAT LP


(13) ELALUATION RESULTS OF STEEL WIRE ROD


(14) METALLOGRAPHIC STRUCTURE


(15) FRACTION OF PEARLLITE


(16) AVERAGE PEARLITE BLOCK SIZE


(17) CENTRAL PORTION


(18) SURFACE LAYER PORTION


(19) LAMELLAR SPACING


(20) MINIMUM LAMELLAR SPACING AT CENTRAL PORTION


(21) VALUE OF (12 r + 65)


(22) MECHANICAL PROPERTIES


(23) TENSILE STRENGTH TS


(24) REDUCTION OF AREA


(25) REDUCTION OF AREA RA


(26) VALUE OF (100-0.045 × TS)


(27) STEEL WIRE AFTER WIRE-DRAWING


(28) PRODUCTION CONDITIONS


(29) DIAMETER AFTER WIRE-DRAWING


(30) STRAIN DURING WIRE-DRAWING


(31) EVALUATION RESULTS


(32) OCCURRENCE OF SELAMINATION


(33) TENSILE STRENGTH TS


(34) NOT OCCURES


(35) OCCURRED






INDUSTRIAL APPLICABILITY
According to the aspects of the present invention, it is possible to obtain a steel wire rod having higher strength and better ductility than those of the conventional one without adding expensive elements. As a result, it is possible to produce a steel wire in which the occurrence of delamination is suppressed and in which strength is high. Accordingly, the present invention has significant industrial applicability.

## Claims

1. A steel wire rod having a diameter of 5.5 mm to 12.5 mm after finish rolling consisting of, as a chemical composition, by mass %:
0.70% to 1.00% of C;
0.15% to 0.35% of Si;
0.1% to 1.0% of Mn;
0.001% to 0.005% of N;
0.005% to less than 0.050% of Ni;
at least one of 0.005% to 0.10% of Al or 0.005% to 0.10% of Ti;
at least one of
more than 0% to 0.50% of Cr,
more than 0% to 0.50% of Co,
more than 0% to 0.50% of V,
more than 0% to 0.20% of Cu,
more than 0% to 0.10% of Nb,
more than 0% to 0.20% of Mo,
more than 0% to 0.0020% of W,
more than 0% to 0.0050% of Rare Earth Metal,
more than 0.0005% to 0.0050% of Ca,
more than 0.0005% to 0.0050% of Mg, or
more than 0.0005% to 0.010% of Zr; and
a balance consisting of iron and unavoidable impurities, and
as a metallographic structure, by area %, 95% to 100% of a pearlite,
wherein, when a distance from a peripheral surface to a center is r in unit of mm, an average pearlite block size at a central portion which is an area from the center to r×0.99 is 1 μm to 25 μm,
an average pearlite block size at a surface layer portion which is an area from the peripheral surface to r×0.01 is 1 μm to 20 μm,
when a minimum lamellar spacing of the pearlite at the central portion is S in unit of nm, a following Expression 1 is satisfied,
wherein the steel wire rod has a tensile strength of 1200 MPa or more, and
the tensile strength, TS in unit of MPa, and a reduction of area, RA in unit of %, satisfy a following Expression 2 and a following Expression 3, and
amounts expressed in mass % of each element in the chemical composition satisfy a following Expression 4,


S<12r+65  (Expression 1)



RA≧100−0.045×TS  (Expression 2)



RA≧45  (Expression 3)



0.005≦Al+Ti≦0.1  (Expression 4).

2. A method of producing a steel wire rod, the method comprising:
a casting process to obtain a cast piece consisting of the chemical composition according to claim 1;
a heating process of heating the cast piece to a temperature of 1000° C. to 1100° C.;
a hot-rolling process of hot-finish-rolling the cast piece after the heating process by controlling a finishing temperature to be 850° C. to 1000° C. to obtain a hot-rolled steel;
a coiling process of coiling the hot-rolled steel within a temperature range of 780° C. to 840° C.;
a patenting process of directly immersing the hot-rolled steel after the coiling process in a molten salt, which is held at a temperature of 480° C. to 580° C., within 15 seconds after the coiling process; and
a cooling process of cooling the hot-rolled steel after the patenting process to a room temperature to obtain the steel wire rod.