WO2014175345A1

WO2014175345A1 - Wire rod and method for manufacturing same

Info

Publication number: WO2014175345A1
Application number: PCT/JP2014/061460
Authority: WO
Inventors: 大羽　浩; 新磯; 達誠多田; 敏之真鍋
Original assignee: 新日鐵住金株式会社
Priority date: 2013-04-25
Filing date: 2014-04-23
Publication date: 2014-10-30
Also published as: EP2990499A1; JP6256464B2; KR20150138356A; EP2990499B1; JPWO2014175345A1; EP2990499A4

Abstract

A wire rod that has a definite composition of components with the balance consisting of Fe and impurities, wherein the average particle diameter of pearlite blocks in a cross-section perpendicular to the axial direction of the wire rod is not more than 23 μm, and the number density of the pearlite blocks having the aforesaid average particle diameter of 40 μm or more in the cross-section perpendicular to the axial direction of the wire rod is 0-20 blocks/mm².

Description

Wire rod and manufacturing method thereof

The present invention mainly relates to a wire rod having excellent low-temperature ductility and low-temperature toughness, which is a PC steel stranded wire for reinforcing a PC structure used in an LNG (liquefied natural gas) tank of an energy-related facility.
This application claims priority based on Japanese Patent Application No. 2013-092782 filed in Japan on April 25, 2013 and Japanese Patent Application No. 2013-092775 filed in Japan on April 25, 2013. , The contents of which are incorporated herein.

LNG tanks are classified as above-ground and underground. The present invention relates to a wire for terrestrial LNG and a method for manufacturing the same. As a conventional technique of the above ground type LNG tank, there is a so-called metal double shell type LNG tank provided with a metal inner tank and an outer tank proposed in Patent Document 1 and the like.

In a metal double shell type LNG tank, a gap between the inner tank and the outer tank is usually filled with a cooling agent to suppress the temperature rise in the LNG tank and the accompanying LNG vaporization. However, this structure has a possibility of causing damage such as a large amount of LNG flowing out when defects occur in the inner tank and the outer tank at the same time. For this reason, in recent years, in order to further enhance safety, a PC breakwater equipped with a PC structure (Prestressed Concrete Structure) is arranged outside the LNG tank, and the inside of the PC liquid breakwater is connected to a conventional metal secondary wall. Technology that integrates with the heavy shell LNG tank has appeared.

The PC breakwater in this technology includes concrete that forms a circular bank surrounding the LNG tank, and PC steel strands embedded in the concrete. Prestress is applied to the PC breakwater by tensioning the concrete along the circumferential direction using PC steel strands. When LNG flows out of the LNG tank inside the PC breakwater, tensile stress is applied to the PC breakwater in the circumferential direction due to the fluid pressure of the LNG that flows out, but prestress is applied to the PC breakwater. If given, this tensile stress is relaxed.

Japanese Unexamined Patent Publication No. 2006-234137

However, the PC breakwater is always in a low temperature state due to heat being taken away by the LNG inside the LNG tank. Furthermore, when LNG leaks as described above, the PC steel stranded wire that reinforces the PC breakwater inside the concrete by contacting the LNG with a temperature of −162 ° C. or less with the concrete. The ambient temperature of the can be greatly reduced. If the low-temperature ductility and low-temperature toughness of the PC steel stranded wire are not sufficiently high, the PC steel stranded wire may break and the PC liquid breakwater may be damaged. For this reason, the wire rod which was excellent in low temperature ductility and low temperature toughness compared with the wire rod for the conventional PC steel strand wire came to be calculated | required. In order to meet such market demand, an object of the present invention is to provide a wire rod having a low temperature ductility and a low temperature toughness superior to those of conventional PC steel stranded wires.

In order to enhance the effect of imparting tension to the PC breakwater constituting the PC type LNG tank (LNG tank having a PC breakwater), the present inventors conducted a survey on the actual environment of the PC breakwater usage environment. It was. As a result, it was found that in an actual use environment, the wire may be exposed to an ambient temperature of around −40 ° C. due to heat transfer to the LNG inside the LNG tank.

Therefore, various studies were conducted on methods for obtaining a wire having higher cold ductility and lower temperature toughness than conventional PC steel stranded wires. Specifically, first, in order to examine low temperature ductility, a tensile test piece having a special shape shown in FIG. 3 was prepared from a wire, and a tensile test was performed using the tensile test piece. Furthermore, in order to examine low temperature toughness, a 5 mm sub-size Charpy impact test piece specified in JISZ2202 was prepared from a wire, and a 2 mm U notch test was performed on this test piece. The relationship with Charpy absorbed energy was investigated. From the results of these tests, we found that the average value of the pearlite block particle size of the wire and the number density of coarse pearlite blocks affect the low temperature ductility and fracture surface units at -40 ° C. And it was confirmed that it leads to improvement of low temperature toughness.

And based on the said knowledge, it turned out that it is possible to provide the wire rod which was excellent in low temperature ductility and low temperature toughness than the conventional wire for PC steel strand wire.

That is, the gist of the present invention aimed at solving the above-mentioned problems is as follows.

(1) The wire according to one embodiment of the present invention has a component composition of mass%, C: 0.60 to 1.20%, Si: 0.30 to 1.30%, Mn: 0.30 to 0 90%, P: 0.020% or less, S: 0.020% or less, N: 0.0025 to 0.0060%, Cr: 0 to 1.00%, and V: 0 to 0.800% And further containing one or more of Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, B: 0.0005 to 0.0040%, and the balance The pearlite has an average value of the particle size of the pearlite block in the cross section perpendicular to the wire axis direction and not smaller than 23 μm and the particle size of 40 μm or more in the cross section perpendicular to the wire axis direction. The number density of blocks is 0 to 20 pieces / mm ² .

(2) In the wire according to (1), the component composition is 1% or 2 out of Cr: 0.10 to 1.00% and V: 0.005 to 0.800% in mass%. It may contain seeds.

(3) In the wire according to (1), the component composition is C: 0.70-0.90%, Si: 0.80-1.30%, Mn: 0.60-0 in mass%. .90%, and V: 0 to 0.500%.

(4) In the wire according to (3), the component composition is one or two of Cr: 0.50 to 1.00% and V: 0.300 to 0.500% by mass%. It may contain.

(5) A method for producing a wire according to another aspect of the present invention comprises a method of rough rolling by heating a steel slab having the component composition described in (3) or (4) above to a rough rolling temperature of 950 to 1040 ° C. A step of performing a wire rod finish rolling at a finishing rolling temperature of 750 to 900 ° C., a step of winding at a winding temperature of 730 to 840 ° C., and then a cooling rate of 15 ° C./second or more. And a step of performing blast cooling to room temperature, and the finish rolling temperature and strain rate in the wire finish rolling satisfy the following formula A.
13.7 ≦ log ₁₀ {(dε / dt) × exp (63800 / (1.98 × (T + 273.15))} ≦ 16.5 (Formula A)
Where dε / dt represents the strain rate in the finish rolling of the wire in the unit s ⁻¹ , and T represents the finish rolling temperature in the unit of ° C.

According to the above aspect of the present invention, a PC steel stranded wire suitable for use as a tension member for a PC liquid barrier in a PC-type LNG tank due to a reduction in the particle size of the pearlite block and a limitation on the number density of the coarse pearlite block. It is possible to provide a wire rod that has better ductility and toughness in the vicinity of −40 ° C. than the conventional wire rod.

It is a graph which shows the relationship between the average pearlite block particle size of a wire, and the aperture value of a wire. It is a graph which shows the relationship between Z value in the manufacturing process of a wire, and the pearlite block particle size of a wire. It is a figure which shows the shape of a low-temperature tension test piece. It is a figure which shows the collection position of the 5-mm subsize Charpy impact test piece prescribed | regulated to JISZ2202. It is a figure which shows the pearlite block particle size of the example of an invention manufactured by the Stealmore method. It is a figure which shows the pearlite block particle size of a comparative example. It is a figure which shows the ductility in -40 degreeC of invention example (No6) and a comparative example (No17). It is a figure which shows the pearlite block particle size of the invention example manufactured by DLP method. It is a figure which shows the pearlite block particle size of a comparative example. It is a figure which shows the relationship between a pearlite block particle size (micrometer) and absorbed energy (Charpy impact value) (J). It is a photograph which shows the result of having observed the fracture surface of the impact test piece of the example of this invention by SEM. It is a photograph which shows the result of having observed the fracture surface of the impact test piece of a comparative example by SEM. It is a flowchart which shows an example of the manufacturing method of the wire by a Stealmore method.

The wire for a PC steel stranded wire excellent in low temperature ductility and low temperature toughness according to this embodiment (hereinafter sometimes referred to as “the wire according to this embodiment”) has a component composition of mass%, and C: 0.60 to 1.20%, Si: 0.30 to 1.30%, Mn: 0.30 to 0.90%, P: 0.020% or less, S: 0.020% or less, N: 0.0025 to 0 .0060%, Cr: 0 to 1.00%, and V: 0 to 0.800%, Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, B: Contains one or more of 0.0005 to 0.0040%, the balance is Fe and impurities, and the average value of the particle size of the pearlite block in the cross section perpendicular to the wire axis direction is 23 μm or less. Yes, in the cross section perpendicular to the wire axis direction, the particle size of 40 μm or more The pearlite block has a number density of 0 to 20 pieces / mm ² .

First, the reasons for limiting the component composition of the wire according to this embodiment will be described. Hereinafter,% means mass%.

(C: 0.60 to 1.20%)
C is an element that has the effect of increasing the cementite fraction in the wire and thereby increasing the strength of the wire. By adjusting the patenting conditions, the lamella spacing of the pearlite can be controlled, and the strength of the wire can be increased by processing. However, if the C content is less than 0.60%, even if the above-mentioned patenting conditions are adjusted, it is impossible to obtain a strength that can sufficiently tension the PC liquid bank of the PC type LNG tank.

When the C content of the wire exceeds 1.20%, network-like cementite is generated in the metal structure of the wire. Due to this mesh-like cementite, wire breakage frequently occurs during wire drawing, which may hinder wire production activities.
As described later, as a method for manufacturing a wire according to the present embodiment, either a DLP (Direct in-Line Patenting) method or a Stemmore method can be adopted. However, when a wire is manufactured using the Stemmore method, The C content is preferably 0.70 to 0.90%. When manufacturing a wire using a Stealmore method, the cooling rate of a wire becomes slower than the time of manufacture by a DLP method, and the ductility and toughness of a wire become comparatively low. In order to compensate for this decrease in strength, the lower limit value of the C content is preferably set to 0.70%. Further, if the C content is excessively increased, the wire becomes hypereutectoid steel (steel having a structure in which pearlite and cementite coexist), so that in the process of cooling the wire, the proeutectoid cementite is formed in the prior austenite grain size. To form. This network-like pro-eutectoid cementite significantly reduces the wire drawing workability of the wire. In order to avoid precipitation of network-like pro-eutectoid cementite, the upper limit value of the C content is preferably set to 0.90%. The C content is more preferably 0.80 to 0.90%.

(Si: 0.30 to 1.30%)
Si is an element that acts as a deoxidizing element during refining. This deoxidation effect is sufficiently exhibited when 0.30% or more of Si is contained. Therefore, the lower limit value of the Si content of the wire according to this embodiment is 0.30%. Si also has the effect of improving the strength of the wire, but this strength improvement effect is manifested when 0.80% or more of Si is contained. Therefore, it is good also considering the lower limit of Si content of the wire concerning this embodiment as 0.80%. Further, Si strengthens the solid solution of ferrite, but has the effect of raising the nose of isothermal transformation during heat treatment, so an excessive amount of Si increases the cost of heat treatment. Therefore, considering the capacity of the production facility, the upper limit of the Si content is set to 1.30%.
As a manufacturing method of the wire according to the present embodiment, either the DLP method or the Stemmore method may be adopted. However, when the wire is manufactured using the Stemmore method, the Si content is 0.80 to 1.30%. It is preferable to do. When manufacturing a wire using a Stealmore method, the cooling rate of a wire becomes slower than the time of manufacture by a DLP method, and the ductility and toughness of a wire become comparatively low. In order to compensate for this decrease in strength, the lower limit value of the Si content is preferably set to 0.80%. The Si content is more preferably 0.90 to 1.25%.

(Mn: 0.30-0.90%)
Mn is a solid solution strengthening element and has the effect of improving the ductility and toughness of the wire and the effect of improving the hardenability. In order to ensure the ductility and toughness of the wire, it is necessary to contain 0.30% or more of Mn. In order to further improve the ductility of the wire, the lower limit value of the Mn content may be 0.60%. On the other hand, when the Mn content exceeds 0.90%, a transformation delay occurs in the central portion of the wire during the production of the wire, and micro martensite is generated in the untransformed austenite portion. The micro martensite at the center of the wire causes breakage during wire drawing of the wire. Therefore, the upper limit of the Mn content needs to be 0.90%.
As a method for producing a wire according to the present embodiment, either the DLP method or the Stemmore method can be adopted. However, when producing a wire using the Stemmore method, the Mn content is 0.60 to 0.90%. It is preferable that When manufacturing a wire using a Stealmore method, the cooling rate of a wire becomes slower than the time of manufacture by a DLP method, and the ductility and toughness of a wire become comparatively low. In order to compensate for this decrease in strength, the lower limit value of the Mn content is preferably 0.60%. The Mn content is more preferably 0.70 to 0.90%.

(P: 0.020% or less)
(S: 0.020% or less)
P has the effect | action which embrittles steel. Therefore, the upper limit of the P content needs to be 0.020%. In addition, when preventing the low-temperature embrittlement of a wire rod more reliably, it is good also considering the upper limit of P content as 0.010%, 0.005%, or 0.001%.
S is an element that combines with Mn in the wire to form MnS. S is segregated at the center of the steel in the process of refining and solidifying the steel, so MnS accumulates in the center of the steel, but this MnS lowers the low temperature ductility of the steel. When the S content exceeds 0.020%, this decrease in low temperature ductility becomes significant, so the S content is set to 0.020% or less. In addition, when performing prevention of the low temperature embrittlement of a wire more reliably, it is good also considering the upper limit of S content as 0.010%, 0.005%, or 0.001%.
In the wire according to the present embodiment, the P and S contents are preferably as low as possible. Therefore, the lower limit of the contents of P and S is 0%.

(N: 0.0025 to 0.0060%)
N is an element that combines with Al, Ti, and B to form a nitride. Since these nitrides become austenite precipitation nuclei, the austenite grain size can be controlled when the steel is heated by controlling the number of these nitrides. As the amount of nitride increases, the crystal grains become finer. If the N content is less than 0.0025%, nitrides are not sufficiently formed, and the effect of refining the particle size cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.0060%, free N that does not bind to Al, Ti, and B becomes excessive. Due to this excessive free N, age hardening occurs, and the ductility and toughness of the wire are lowered. Therefore, the N content needs to be 0.0025 to 0.0060%. Preferably, the N content is 0.0025 to 0.0040%.

In addition to the above elements, the wire according to this embodiment is one of Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, and B: 0.0005 to 0.0040%. Or 2 or more types are further contained.

(Al: 0.005 to 0.100%)
Al acts as a deoxidizer during steel refining. Further, Al has a function of forming a compound with N in steel and fixing N. By fixing N, age hardening of steel can be prevented. Furthermore, when it contains B simultaneously, the amount of solid solution B can be increased by fixing N.

However, if the Al content is less than 0.005%, the effect of fixing N by Al cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.100%, Al ₂ O ₃ produced by combining with oxygen in the steel forms clusters. This cluster becomes a starting point of cracking during wire drawing. Therefore, the Al content may be 0.005 to 0.100%. Preferably, the Al content is 0.020 to 0.050%.

(Ti: 0.003 to 0.050%)
Ti, like Al, acts as a steel deoxidizer. Further, Ti forms a compound with N in the steel and has an action of fixing N. By fixing N, age hardening of steel can be prevented. Furthermore, when it contains B simultaneously, the amount of solid solution B can be increased by fixing N.

However, if the Ti content is less than 0.003%, the effect of fixing N by Ti cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.050%, TiC generated by combining with carbon in the steel increases. This TiC becomes a starting point of cracking during wire drawing. Therefore, the Ti content may be 0.003 to 0.050%. Preferably, the Ti content is 0.020 to 0.040%.
As described above, Al and Ti have the same effect. Therefore, the amount of Al added can be reduced by adding Ti, and the same effect can be obtained in this case.

(B: 0.0005-0.0040%)
B, when present as a solid solution B in austenite, has the effect of improving the hardenability of the wire. If the B content is less than 0.0005%, the effect of improving hardenability cannot be sufficiently obtained. On the other hand, if the B content exceeds 0.0040%, B forms a compound with Fe and C, and precipitates such as Fe ₂₃ (C, B) ₆ are formed. This precipitate becomes a starting point of cracking during wire drawing. Therefore, the B content may be 0.0005 to 0.0040%. Preferably, the B content is 0.0009 to 0.0030%.

In addition to the above elements, the wire according to the present embodiment may further contain Cr: 0 to 1.00%, V: 0 to 0.800% in mass%.

(Cr: 0 to 1.00%)
In the wire according to this embodiment, the inclusion of Cr is not essential. Therefore, the lower limit of the Cr content is 0%. However, Cr has the effect of increasing the strength of the wire by reducing the lamella spacing of the pearlite. This action increases the strength of the wire during wire drawing. Since this effect is obtained when the Cr content is 0.10% or more, the Cr content is preferably set to 0.10% or more. Moreover, when improving an intensity | strength further, it is preferable to contain 0.50% or more of Cr. When the Cr content exceeds 1.00%, the end time of the pearlite transformation becomes long, a supercooled structure is generated in the process of cooling the wire, and the ductility of the wire is deteriorated. Therefore, the upper limit of the Cr content is preferably 1.00%.
As a method for manufacturing the wire according to the present embodiment, either the DLP method or the Stemmore method can be adopted. However, in the case of manufacturing using the Stemmore method, the Cr content is set to 0.50 to 1.00%. More preferably. In the case of manufacturing using the Stealmore method, the cooling rate of the wire becomes slower than in the case of manufacturing by the DLP method, and the ductility and toughness of the wire become relatively low. In order to compensate for this decrease in strength, the lower limit of the Cr content is more preferably 0.5%. The Cr content is more preferably 0.50 to 0.90%.

(V: 0 to 0.800%)
In the wire according to this embodiment, the inclusion of V is not essential. Therefore, the lower limit of the V content is 0%. However, V is an element that combines with C and precipitates as a carbide in ferrite. By precipitation of this carbide, the ferrite is hardened, and the strength of the wire can be increased. This effect is obtained when V is contained in an amount of 0.005% or more. However, when the V content exceeds 0.800%, coarse carbides precipitate. This coarse carbide becomes a starting point of cracking when the wire is processed. Therefore, the V content is preferably 0.005% or more and 0.800% or less.
As a method for producing the wire according to the present embodiment, either the DLP method or the Stemmore method can be adopted. However, when the wire is produced using the Stemmore method, the V content is 0.300 to 0.500%. More preferably. In the case of manufacturing using the Stealmore method, the cooling rate of the wire becomes slower than in the case of manufacturing by the DLP method, and the ductility and toughness of the wire become relatively low. In order to compensate for this decrease in strength, the lower limit value of the V content is more preferably 0.300%. Moreover, since a microcrack void is formed in the base metal interface part of VC deposit when it receives a processing strain, it is more preferable to make the upper limit of V content into 0.500%. The V content is more preferably 0.300 to 0.400%.

(Balance: Fe and impurities)
The balance of the component composition of the wire according to this embodiment is made of Fe and impurities. Impurities are raw materials such as ore or scrap, or components mixed in due to various factors in the manufacturing process when manufacturing steel materials industrially, and have an adverse effect on the characteristics of the wire according to this embodiment. It means what is allowed in the range.

(Average value of pearlite block particle size in cross section perpendicular to wire axis direction: 23 μm or less)
In this embodiment, the pearlite block grain boundary is defined as a boundary between two adjacent pearlites having an orientation difference of 9 degrees or more, and the pearlite block is defined as a region surrounded by the pearlite block grain boundary. , PBS (perlite block particle size) is defined as the equivalent circle diameter of a pearlite block. In the wire according to this embodiment, in addition to defining the component composition as described above, the average PBS (pearlite block particle size) in a cross section perpendicular to the wire axis direction is set to 23 μm or less.

When the average PBS (pearlite block particle size) exceeds 23 μm, the drawing value of the wire decreases. The wire according to this embodiment needs to have an aperture value of 30% or more. In view of past production results, it has been found that a drawing value of 30% or more is necessary in order to prevent disconnection during wire drawing in wire drawing. As a result of studies by the present inventors, it has been found that there is a relationship shown by the graph in FIG. 1 between the average pearlite block particle size and the aperture value. FIG. 1 shows that when the average PBS (pearlite block particle size) is 23 μm or less, an aperture value of 30% or more can be obtained. Furthermore, when the average PBS (pearlite block particle size) exceeds 23 μm, the branching frequency of the crack tip decreases. Since the branching at the crack tip has an effect of suppressing crack propagation, the frequency of branching at the crack tip decreases, so that the fracture surface unit becomes large, and the low temperature ductility and low temperature toughness are lowered. Therefore, the average PBS (perlite block particle size) is 23 μm or less. Preferably, the average pearlite block particle size is 18 μm or less.

Using an EBSD device, it is possible to obtain an average value of the equivalent circle diameters of the pearlite block within an arbitrary viewing angle at an arbitrary position in a cross section perpendicular to the axial direction of the wire. The average PBS in the cross section perpendicular to the axial direction of the wire in the present embodiment is obtained by the following procedure. First, (1) surface layer portion (region having a depth of 30 μm from the surface of the wire), (2) 1 / 4D portion (depth from the surface of the wire to ¼ of the diameter D of the wire) in the cross section perpendicular to the axial direction of the wire Area), (3) center part, (4) 3 / 4D part (area of 3/4 depth of wire diameter D from the surface of the wire. That is, on the opposite side of (2) with respect to the wire center part) Area), and (5) the equivalent circle diameter of the pearlite block within a viewing angle of 300 μm × 180 μm at each of the five locations consisting of the opposite surface layer portion (that is, the region opposite to (1) with respect to the central portion of the wire rod) The average value (primary average value) is measured using an EBSD device. Next, an average value (secondary average value) of each primary average value is calculated. This secondary average value is the average PBS in a cross section perpendicular to the axial direction of the wire in the present embodiment.

(The number density of pearlite blocks having a particle diameter of 40 μm or more in the cross section perpendicular to the wire axis direction is 0 to 20 / mm ² )
In the wire according to the present embodiment, in addition to defining the component composition and the average value of the pearlite block particle size as described above, the pearlite block particle size is 40 μm or more in the cross section perpendicular to the wire axis direction. The number density of pearlite blocks is set to 0 to 20 pieces / mm ² .

Since the pearlite block having a particle size of 40 μm or more serves as a starting point of fracture, even if the number is small, the ductility and toughness of the wire are lowered. In the wire according to the present embodiment, in addition to controlling the average value of the pearlite block particle size as described above, it is also necessary to suppress the generation of coarse pearlite blocks. For this reason, the number density of coarse pearlite blocks is limited. Hereinafter, the “pearlite block having a particle size of 40 μm or more” may be referred to as “coarse pearlite block” or “coarse PB”.
In the cross section perpendicular to the wire axis direction, when the number density of coarse pearlite blocks exceeds 20 / mm ² , the ductility and toughness of the wire do not satisfy the required level. Therefore, it is necessary to limit the number density of coarse pearlite blocks in a cross section perpendicular to the wire axis direction to 20 pieces / mm ² or less. Preferably, the upper limit of the number density of coarse pearlite blocks in a cross section perpendicular to the axial direction of the wire is 18 / mm ² . Since the number of coarse pearlite blocks is preferably small, the lower limit of the number density of coarse pearlite blocks in the cross section perpendicular to the wire axis direction is 0 / mm ² .

Using an EBSD device, the number density of pearlite blocks having a particle size of 40 μm or more within an arbitrary viewing angle can be obtained at an arbitrary position in a cross section perpendicular to the axial direction of the wire. In the present embodiment, the number density of pearlite blocks having a pearlite block particle size of 40 μm or more in a cross section perpendicular to the wire axis direction is obtained by the following procedure. First, (1) surface layer portion (region having a depth of 30 μm from the surface of the wire), (2) 1 / 4D portion (depth from the surface of the wire to 1/4 of the diameter D of the wire) in a cross section perpendicular to the axial direction of the wire Area), (3) center portion, (4) 3 / 4D portion (from the surface of the wire to a region of a depth of 3/4 of the diameter D of the wire, ie, on the opposite side of (2) with respect to the wire center portion) Region), and (5) a particle size of 40 μm or more within a viewing angle of 300 μm × 180 μm at each of five locations consisting of the surface layer portion on the opposite side (that is, the region on the opposite side of (1) with respect to the wire rod central portion). The number density of the pearlite block is measured using an EBSD device. Next, the average value of the number density at each location is calculated. This average value is the number density of pearlite blocks having a particle diameter of 40 μm or more in the cross section perpendicular to the axial direction of the wire in the present embodiment.

Next, a method for manufacturing the wire according to this embodiment will be described.

As a method for manufacturing the wire according to the present embodiment, there is a DLP (Direct in-Line Patenting) method and a Stealmore method.

When the wire rod according to the present embodiment is manufactured by the Stemmore method, as shown in FIG. 10, (1) mass%, C: 0.70 to 0.90%, Si: 0.80 to 1.30% , Mn: 0.60 to 0.90%, P: 0.020% or less, S: 0.020% or less, N: 0.0025 to 0.0060%, Cr: 0 to 1.00%, and V : 0 to 0.500%, Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, B: one of 0.0005 to 0.0040% or A steel slab containing two or more components, the balance of which is composed of Fe and impurities, is heated to a rough rolling temperature of 950 to 1040 ° C. and then subjected to rough rolling, and (2) finish rolling at 750 to 900 ° C. (3) Winding is performed at a winding temperature of 730 to 840 ° C., and (4) Performing blast cooling to room temperature at 15 ° C. / sec or more cooling rate. At this time, the finish rolling temperature and the strain rate must satisfy the relationship of the following formula 1.
13.7 ≦ log ₁₀ {(dε / dt) × exp (63800 / (1.98 × (T + 273.15)))} ≦ 16.5 (Formula 1)
Here, dε / dt represents the strain rate in the wire finish rolling in the unit s ⁻¹ , and T represents the finish rolling temperature in the unit ° C.
In addition, when manufacturing by the Stealmore method, the steel slab further contains one or two of Cr: 0.50 to 1.00% and V: 0.300 to 0.500% by mass%. May be.

(Component composition of steel slabs for rough rolling: within the above specified range)
When manufacturing the wire which concerns on this embodiment by the Stealmore method, the component composition of the steel slab used for rough rolling needs to be in the above-mentioned specified range. This specified range is narrower than the specified range described above as the component composition of the wire according to the present embodiment. The Stemmore method is a manufacturing method that performs blast cooling after winding, and the cooling rate by blast cooling is slower than the cooling rate of direct heat treatment with molten salt in the DLP method described later. When the cooling rate is low, the ductility and toughness of the finally obtained wire are relatively low. Therefore, when the wire according to the present embodiment is manufactured by the Stemmore method, C, Mn, and Si, which are alloy elements for improving ductility and toughness, are compared to the case of performing the direct heat treatment with the molten salt in the DLP method. It is necessary to contain a relatively large amount. In addition, when manufacturing the wire according to this embodiment by the Stealmore method, when Cr and V are included to improve the properties of the wire, Cr and V are also subjected to direct heat treatment with molten salt in the DLP method. It is preferable to contain relatively more than the case.

(Heating temperature of steel slab before being subjected to rough rolling: 950 to 1040 ° C)
In the wire manufacturing method according to the present embodiment by the Stealmore method, the heating temperature (rough rolling temperature) of the steel slab before being subjected to rough rolling is set to 950 to 1040 ° C. When the heating temperature of the steel slab before rough rolling is less than 950 ° C., the roll reaction force at the time of wire rod rolling increases rapidly, and equipment troubles such as roll breakage may occur. Therefore, the heating temperature of the steel slab before rough rolling is set to 950 ° C. or higher. On the other hand, when the heating temperature of the steel slab before rough rolling exceeds 1040 ° C., solutionization of aluminum nitride (AlN) precipitated in the steel slab proceeds excessively. As described above, AlN serves as austenite precipitation nuclei and contributes to refinement of the austenite crystal grain size. By refining the particle size of the austenite of the wire rod in the production stage, the particle size of the pearlite of the finally obtained wire rod can be reduced. However, when the solution of AlN proceeds excessively, the austenite crystal grain size increases. In this case, the PBS of the wire becomes coarse when the wire is manufactured later. In this case, in the cross section perpendicular to the wire axis direction, the average value of the particle size of the pearlite block is more than 23 μm, and / or the number density of the pearlite block having the particle size of 40 μm or more is more than 20 / mm ^2. . In order to avoid such a phenomenon, the heating temperature of the steel slab before rough rolling is set to 1040 ° C. or less.

(Temperature of wire finish rolling: 750-900 ° C)
In the wire manufacturing method according to the present embodiment by the Stealmore method, the wire finish rolling is performed in a temperature range of 750 to 900 ° C. When the temperature of the wire finish rolling (finish rolling temperature) is less than 750 ° C., an increase in the roll reaction force during rolling may cause equipment trouble such as roll breakage. ℃ or more. On the other hand, if the temperature of the wire finish rolling exceeds 900 ° C., the austenite crystal grain size becomes coarse. In this case, in the cross section perpendicular to the wire axis direction, the average value of the particle size of the pearlite block is more than 23 μm, and / or the number density of the pearlite block having the particle size of 40 μm or more is more than 20 / mm ^2. . The ductility of the wire deteriorates due to the coarsening of the pearlite block. In order to avoid such a phenomenon, the wire finishing rolling temperature is set to 900 ° C. or less.

(Relationship between finish rolling temperature and strain rate in wire finish rolling: 13.7 ≦ log ₁₀ Z ≦ 16.5)
In the manufacturing method of the wire rod according to the present embodiment by the Stealmore method, it is further necessary to define the relationship between the finish rolling temperature and the strain rate in the wire finish rolling. Specifically, the strain rate dε / dt of the wire during the wire finish rolling, the activation energy Q of the plastic deformation of the wire, and the finish rolling temperature T during the wire finish rolling are expressed by the following equation 2 -The common logarithm (log ₁₀ Z) of the Z value (Zener-Holomon parameter) obtained by substituting into the Hollomon equation needs to be 13.7 to 16.5.
Z = (dε / dt) × exp (Q / RT) (Formula 2)
“Dε / dt” indicates the strain rate in the unit s ⁻¹ . “R” is a gas constant, and its value is 1.98 cal / mol · deg. “T” indicates the finish rolling temperature in K. When the unit of the finish rolling temperature is ° C., “T” in Equation 2 needs to be “(T + 273.15)”. “Q” indicates the activation energy of plastic deformation of the wire. The activation energy of plastic deformation of the wire according to the present embodiment having the chemical composition described above is considered to be 63800 cal / mol.
In the manufacturing method according to this embodiment, the strain rate dε / dt of the wire during the finish rolling of the wire can be obtained by the following equation.
dε / dt = ε × 2 × π × (N / 60) × L _d (Formula 3)
ε = ln (h2 / h1) (Formula 4)
L _d = (r / Δh) ^1/2 (Formula 5)
Δh = h1-h2 (Formula 6)
“Ε” is a strain amount in the finish rolling of the wire rod, and is a dimensionless number. “H1” indicates the diameter of the wire before the wire finish rolling in unit mm, and “h2” indicates the diameter of the wire after the wire finish rolling in unit mm. “N” indicates the number of rotations of the roll for performing the wire finish rolling in the unit of rpm. “L _d ” indicates the projected contact length in the unit mm in the wire finishing rolling. The projected contact length is the length along the rolling direction of the region where the roll and the material to be rolled (wire) are in contact during rolling. r represents the roll radius in the unit mm in the wire finishing rolling. Δh represents the amount of reduction in unit mm in the wire finish rolling. As can be seen from Equations 3 to 6, the strain rate is a rolling condition related to both the rolling rate and the rolling reduction.
As a result of investigations by the present inventors, it has been found that there is a correlation shown in FIG. 2 between the Z value at the time of wire finish rolling and the average pearlite block particle size of the finally obtained wire. . As can be seen from FIG. 2, in order to make the average pearlite block particle size in the cross section perpendicular to the wire axis direction 23 μm or less, log ₁₀ Z needs to be 13.7 or more. Further, in order to suppress the number density of coarse pearlite blocks having a particle diameter of 40 μm or more to 0 to 20 / mm ² , log ₁₀ Z needs to be 13.7 or more in the same manner. On the other hand, it is difficult to set log ₁₀ Z to more than 16.5 in consideration of equipment capacity. This is because, as can be seen from the Zener-Holomon equation, to increase the Z value, it is necessary to lower the rolling temperature and increase the strain rate. Therefore, in the method for manufacturing the wire rod according to the present embodiment using the Stemmore method, the upper limit value of log ₁₀ Z is set to 16.5. log ₁₀ Z is preferably 14.0 to 16.0. When log ₁₀ Z is less than the above range, the average value of the particle size of the pearlite block exceeds 23 μm and / or the number density of the pearlite block having the particle size of 40 μm or more in the cross section perpendicular to the wire axis direction. Is over 20 pieces / mm ² .

The wire finish rolling speed (finish rolling speed) is not particularly limited as long as log ₁₀ Z is in the range of 13.7 to 16.5. For example, when a wire rod having a wire diameter of 12 mm is produced, the wire rod finish rolling speed is preferably 15.5 to 25.2 m / sec. When the speed of the wire finish rolling is less than 15.5 m / sec, the strain rate decreases. In this case, the PBS may not be made sufficiently small. Accordingly, the wire finishing rolling speed is preferably 15.5 m / sec or more. On the other hand, when the wire finish rolling speed exceeds 25.2 m / sec, heat generation during processing increases, and the austenite crystal grain size may become coarse. Also in this case, the PBS cannot be miniaturized. As can be seen from the above formulas 3 to 6, when manufacturing a wire having a wire diameter other than 12 mm, the wire finish rolling speed is set so that the log ₁₀ Z is in the range of 13.7 to 16.5. It is necessary to change appropriately from the range of.

(Winding temperature: 730-840 ° C)
(Shock cooling rate after winding: 15 ℃ / second or more)
In the method for manufacturing the wire according to the present embodiment by the Stealmore method, after the wire finish rolling, the wire is wound at 730 to 840 ° C., and then blast cooling is performed at a cooling rate of 15 ° C./second or more. If the coiling temperature is less than 730 ° C, the amount of scale generation is reduced and the mechanical descaling property is deteriorated, so the coiling temperature is set to 730 ° C or higher. When the coiling temperature exceeds 840 ° C., the austenite particle size increases. In this case, the pearlite block particle diameter of the finally obtained wire cannot be adjusted to 23 μm or less, and / or the number density of pearlite blocks having a particle diameter of 40 μm or more is more than 20 / mm ² . Accordingly, the winding temperature is set to 840 ° C. or lower. The winding temperature is preferably 750 to 820 ° C.

The wound wire is cooled to room temperature by blast cooling. The room temperature basically indicates a temperature range of 5 to 35 ° C. as defined in JIS Z 8703. At that time, if the cooling rate is less than 15 ° C./second (ie, slow cooling), the austenite particle size becomes large, the average value of the particle size of the pearlite block in the cross section perpendicular to the wire axis direction becomes more than 23 μm, and / Or the number density of the pearlite blocks having the particle diameter of 40 μm or more is more than 20 / mm ² . Thereby, the ductility of the wire finally obtained deteriorates. Therefore, the cooling rate during blast cooling is set to 15 ° C./second or more. The cooling rate at the time of blast cooling is preferably 25 ° C./second or more. Although it is not necessary to define the upper limit value of the cooling rate at the time of blast cooling, the upper limit value is about 50 ° C./second in consideration of the facility capacity.

When manufacturing the wire according to the present embodiment by the DLP method, (1) component composition is mass%, C: 0.60 to 1.20%, Si: 0.30 to 1.30%, Mn: 0 .30 to 0.90%, P: 0.020% or less, S: 0.020% or less, N: 0.0025 to 0.0060%, Cr: 0 to 1.00%, and V: 0 to 0 800%, Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, B: 0.0005 to 0.0040%, or one or more of them The steel slab containing the composition containing Fe and impurities in the balance is heated to 950 to 1040 ° C., and then wire rolling is performed. (2) Winding is performed in a temperature range of 750 to 800 ° C. (3) Immediately after completion of winding, heat treatment is directly performed with molten salt at 500 to 600 ° C.
When the wire according to the present embodiment is manufactured by the DLP method, the steel slab as the raw material is in mass%, Cr: 0.10 to 1.00%, and V: 0.005 to 0.800%. One or two of them may be further contained.

(Component composition of steel slab used for wire rod rolling: within the above-mentioned specified range)
When manufacturing the wire which concerns on this embodiment by DLP method, the component composition of the steel slab used for wire rolling needs to be in the above-mentioned specified range. This specified range is the same as the specified range described above as the component composition of the wire according to the present embodiment. The production method by the DLP method has an advantage that a wire having excellent ductility and toughness can be obtained from a steel piece having relatively few alloy elements for improving ductility and toughness. However, since a direct heat treatment with a molten salt is essential in the production method by the DLP method, more equipment is required for carrying out this production method than in the production method by the Stemmore method.

(Heating temperature for wire rolling: 950-1040 ° C)
When manufacturing the wire according to the present embodiment by the DLP method, the heating temperature of the steel slab before wire rod rolling is set to 950 to 1040 ° C. When the heating temperature is less than 950 ° C., the roll reaction force during wire rod rolling increases remarkably, and equipment troubles such as roll breakage may occur. Therefore, the heating temperature before wire rod rolling is set to 950 ° C. or higher. . On the other hand, when the heating temperature before the wire rod rolling exceeds 1040 ° C., the solution of aluminum nitride (AlN) precipitated in the steel slab proceeds, the austenite crystal grain size increases, and finally obtained. The pearlite block particle size (PBS) of the wire becomes coarse. In this case, the average value of the particle sizes of the pearlite blocks in the cross section perpendicular to the wire axis direction is more than 23 μm, and / or the number density of the pearlite blocks having a particle size of 40 μm or more is more than 20 / mm ² . In order to avoid such a phenomenon, the heating temperature before wire rod rolling is set to 1040 ° C. or lower. The heating temperature before wire rolling is preferably 980 to 1030 ° C. In addition, the finishing temperature in wire rod rolling is not particularly limited, and a reasonable temperature can be appropriately selected.

(Winding temperature: 750-800 ° C)
When manufacturing the wire according to the present embodiment by the DLP method, the winding temperature after wire rolling is set to 750 to 800 ° C. When the coiling temperature is less than 750 ° C., the variation in the tensile strength in the longitudinal direction of the wire increases after the constant temperature transformation in the subsequent constant temperature transformation treatment step. Accordingly, the winding temperature is set to 750 ° C. or higher. When the coiling temperature exceeds 800 ° C., the austenite particle size increases. In this case, the pearlite block particle diameter of the finally obtained wire cannot be adjusted to 23 μm or less, and the number density of pearlite blocks having a particle diameter of 40 μm or more cannot be adjusted to 20 pieces / mm ² or less. The winding temperature is set to 800 ° C. or lower.

(Constant temperature transformation treatment method: direct heat treatment)
(Constant temperature transformation treatment temperature: 500-600 ° C)
When the wire according to this embodiment is manufactured by the DLP method, the wire is wound up and immediately immersed in a molten salt at 500 to 600 ° C. to perform a constant temperature transformation treatment. When the isothermal transformation temperature is less than 500 ° C., a lot of non-pearlite structure is generated in the surface layer portion of the wire. In this case, unevenness of processing strain occurs at the interface between the pearlite structure generated inside the wire and the non-pearlite structure of the surface part of the wire, and this non-uniformity causes breakage at the wire drawing stage. There is. Therefore, the temperature of the isothermal transformation treatment is set to 500 ° C. or higher. If the isothermal transformation temperature exceeds 600 ° C., operational problems such as increased thermal deformation of the equipment occur. Therefore, the isothermal transformation temperature is set to 600 ° C. or less. Moreover, this isothermal transformation process needs to be performed by direct heat treatment (on-line heat treatment). If direct heat treatment is not performed (that is, isothermal transformation is performed by off-line heat treatment), γ grains grow by the reheating step included in off-line heat treatment. This phenomenon prevents the particle diameter of the PBS of the wire rod from being controlled to 23 μm or less.

Next, examples of the present invention will be described. The conditions at the time of creating the examples are condition examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these condition examples. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Hereinafter, examples of the manufacturing method using the Stealmore method will be described.

First, molten steel having the composition shown in Table 1-1 was continuously cast into a 300 mm × 500 mm slab, and this slab was then rolled into 122 mm square steel slabs by split rolling. Thereafter, the steel slab was heated at the heating temperature shown in Table 1-2, and the steel slab was rolled under the conditions shown in Table 1-2 to obtain a 12 mmφ wire rod. The radius of the roll at the time of wire-finishing rolling was 75.5 mm. No. S1 to S16 are invention examples that satisfy the conditions of the present invention. S17 to S41 are comparative examples that do not satisfy the conditions of the present invention.

The tensile test piece shown in FIG. 3 was manufactured from the rolled wire. Tensile strength and ductility of the wire at −40 ° C. were measured by conducting a tensile test on the tensile test piece in a low temperature atmosphere of −40 ° C. while adjusting the temperature with dry ice and alcohol.
Furthermore, a Charpy impact test piece defined in JISZ2202 was collected from the wire by the sampling method shown in FIG. 4 to produce a 2 mm U notch Charpy impact test piece of 5 mm subsize. By performing a Charpy impact test at −40 ° C. on these Charpy impact test pieces, the impact value of the wire at a temperature of −40 ° C., which is close to the actual use environment temperature of the PC liquid barrier in the LNG tank, was obtained.

The average PBS (pearlite block particle size) of the wire was obtained by the following procedure. First, (1) surface layer portion (region having a depth of 30 μm from the surface of the wire), (2) 1 / 4D portion (depth from the surface of the wire to ¼ of the diameter D of the wire) in the cross section perpendicular to the axial direction of the wire Area), (3) center portion, (4) 3 / 4D portion (region having a depth of 3/4 of the diameter D of the wire from the surface of the wire, ie, the region opposite to (2) with respect to the wire center portion) ), And (5) the equivalent circle diameter of the pearlite block within the viewing angle of 300 μm × 180 μm at each of the five locations consisting of the surface layer portion on the opposite side (that is, the region opposite to (1) with respect to the central portion of the wire rod) The average value (primary average value) was measured using an EBSD device. Next, the average value (secondary average value) of each primary average value was calculated. This secondary average value was defined as the average PBS in the cross section perpendicular to the axial direction of the wire. In the measurement with the EBSD apparatus, the boundary between two adjacent pearlites having an orientation difference of 9 degrees or more was determined to be a pearlite block grain boundary.

The number density of coarse pearlite blocks in the wire was determined by the following procedure. First, (1) surface layer portion (region having a depth of 30 μm from the surface of the wire), (2) 1 / 4D portion (depth from the surface of the wire to ¼ of the diameter D of the wire) in the cross section perpendicular to the axial direction of the wire Area), (3) center part, (4) 3 / 4D part (area of 3/4 depth of wire diameter D from the wire surface. Region), and (5) a particle size of 40 μm or more within a viewing angle of 300 μm × 180 μm at each of five locations consisting of the surface layer portion on the opposite side (that is, the region on the opposite side of (1) with respect to the wire center). The number density of pearlite blocks was measured using an EBSD apparatus. Next, the average value of the number density at each location was calculated. This average value was defined as the number density of pearlite blocks having a particle diameter of 40 μm or more in a cross section perpendicular to the axial direction of the wire.

The heating temperature, finishing temperature, and winding temperature in the invention examples are within an appropriate temperature range. As a result, the pearlite block of the inventive example was made fine, and the average PBS and coarse PB number density of the inventive example were controlled to appropriate levels. On the other hand, the number density of average PBS and coarse PB in the comparative examples in which the heating temperature, finishing temperature, and winding temperature are higher than the appropriate temperature range were outside the specified range of the present invention. The inventive examples exhibited better properties than the comparative examples with respect to low temperature strength, low temperature toughness, and room temperature toughness. In Comparative Example S36, the finish rolling temperature was below the appropriate temperature range, so the mill load increased and rolling could not be performed.

5A and 5B show an SEM photograph of the pearlite block of the invention example and an SEM photograph of the pearlite block of the comparative example. It was possible to discriminate from these SEM photographs that the pearlite block particle size of the inventive example was smaller than the pearlite block particle size of the comparative example.

FIG. 6 shows the results of a tensile test at −40 ° C. for the inventive example (No. S6) and the comparative example (No. S17). In a low temperature environment of −40 ° C., the ductility of the inventive example was higher than that of the comparative example, and it was determined that it was good. Also from Table 2, it can be determined that the ductility of the inventive example tends to be higher than the ductility of the comparative example. It is estimated that the difference in ductility was caused by the difference in the pearlite block particle size shown in FIGS. 5A and 5B.

Next, examples of the manufacturing method using the DLP method will be described below.

First, molten steel having the composition shown in Table 3 was continuously cast to form a 300 mm × 500 mm slab, and then rolled into a 122 mm square steel slab. Thereafter, the steel slab is heated at the heating temperature shown in Table 3, and further, under the conditions shown in Table 3, the steel slab is subjected to rolling, winding, and heat treatment using a molten salt to obtain a 12 mmφ wire. It was. No. D1 to D16 and No. D30 to D36 were produced by heat treatment (direct heat treatment) immersed in the molten salt without being reheated after winding. No. In D17 to D29, winding was performed under the conditions shown in Table 3, and thereafter, heat treatment (offline heat treatment) for reheating to 950 ° C. and performing lead patenting treatment was performed.

The tensile strength at −40 ° C., the ductility at −40 ° C., and the impact value at −40 ° C. of the wire were determined. The test method for obtaining these values is described in No. 1 above. S1-No. This is the same as the method of each test performed for S41. Moreover, the Charpy impact test at room temperature was performed on the wire, and the impact value of the wire at room temperature was obtained. The method for performing the Charpy impact test at room temperature was the same as the method for performing the Charpy impact test at −40 described above, except for the test temperature. The average density of PBS and the number density of coarse pearlite blocks of the above-mentioned wire rods are No. S1-No. It was measured by the measurement method applied to S16.

No. in Table 3 D1 to D16 are invention examples that satisfy the conditions of the present invention. On the other hand, no. D17 to D38 are comparative examples that do not satisfy the conditions of the present invention. The pearlite block particle size and coarse PB number density of the inventive example were controlled at appropriate levels, while the pearlite block particle size and coarse PB number density of the comparative example were outside the specified range of the present invention. The inventive examples exhibited better properties than the comparative examples with respect to low temperature strength, low temperature toughness, and room temperature toughness.

7A and 7B show an SEM photograph of the pearlite block of the invention example and an SEM photograph of the pearlite block of the comparative example. It was possible to discriminate from the SEM photograph that the pearlite block particle sizes (PBS) of the invention example and the comparative example were clearly different.

FIG. 8 shows the relationship between the pearlite block particle size (μm) and the impact value based on the impact values shown in Tables 4 and 4. From FIG. 8, it was determined that the impact value (PBS: 15 to 23 μm) of the example of the present invention was higher than the impact value of the comparative example (PBS: 30 to 45 μm) both at room temperature and at −40 ° C.

FIG. 9A and FIG. 9B show the results of observing the fracture surface of the Charpy impact test piece of the invention example and the comparative example with an SEM. FIG. 9A shows a fracture surface unit of the invention example, and FIG. 9B shows a fracture surface unit of the comparative example. The fracture surface unit of the invention example was finer than the fracture surface unit of the comparative example. This indicates that the inventive example is superior in terms of toughness. Also in this point, the effect of PBS miniaturization could be confirmed.

From the above, the toughness of the inventive example is higher than the toughness of the comparative example both at room temperature and in the environment of −40 ° C. that is exposed when the wire is actually used as the reinforcing PC of the LNG tank. Was able to be determined.

According to the present invention, the ductility in the vicinity of −40 ° C. of the PC steel stranded wire used for the tension material of the PC type LNG PC breakwater is better than that of the conventional material due to the reduction of the pearlite block particle size. It became possible to provide wire rods. Therefore, the present invention contributes to improving the reliability of PC steel stranded wire, which is a member of LNG tank-related equipment, which has been increasingly demanded in recent years, in a low temperature use environment, and can be used industrially. It is highly probable.

1 Wire material 2 2 mm U-notch Charpy impact test piece 3 2 mm U-notch

Claims

Ingredient composition is mass%,
C: 0.60 to 1.20%,
Si: 0.30 to 1.30%
Mn: 0.30 to 0.90%,
P: 0.020% or less,
S: 0.020% or less,
N: 0.0025 to 0.0060%,
Cr: 0 to 1.00%, and V: 0 to 0.800%,
Furthermore,
Al: 0.005 to 0.100%,
Ti: 0.003 to 0.050%,
B: 0.0005 to 0.0040%
Containing one or more of them,
The balance consists of Fe and impurities,
The average value of the particle size of the pearlite block in the cross section perpendicular to the wire axis direction is 23 μm or less,
A wire rod, wherein the number density of the pearlite blocks having the particle diameter of 40 μm or more in the cross section perpendicular to the wire rod axis direction is 0 to 20 / mm 2 .
The component composition is mass%,
Cr: 0.10 to 1.00%, and V: 0.005 to 0.800%
The wire according to claim 1, wherein one or two of them are contained.
The component composition is, by mass%, C: 0.70 to 0.90%,
Si: 0.80 to 1.30%,
Mn: 0.60 to 0.90%, and V: 0 to 0.500%
The wire according to claim 1, comprising:
The component composition is, in mass%, Cr: 0.50 to 1.00%, and V: 0.300 to 0.500%
The wire according to claim 3, wherein one or two of them are contained.
Heating the steel slab having the component composition according to claim 3 or 4 to a rough rolling temperature of 950 to 1040 ° C., and performing rough rolling;
A step of performing wire finish rolling at a finish rolling temperature of 750 to 900 ° C .;
Next, a step of winding at a winding temperature of 730 to 840 ° C.,
Thereafter, blast cooling to room temperature at a cooling rate of 15 ° C./second or more,
With
The finish rolling temperature and the strain rate in the finish finish rolling satisfy the following formula A.
13.7 ≦ log 10 {(dε / dt) × exp (63800 / (1.98 × (T + 273.15))} ≦ 16.5 (Formula A)
Where dε / dt represents the strain rate in the finish rolling of the wire in the unit s −1 , and T represents the finish rolling temperature in the unit of ° C.