WO2015053311A1 - Wire rod, hypereutectoid bainite steel wire, and method for manufacturing same - Google Patents

Abstract

　The wire rod pertaining to the present invention has a predetermined component composition, the metallographic structure of which includes bainite in an area ratio of 90-100%, and when eight test pieces having a length of 400 mm are manufactured by dividing a length of 3200 mm of the wire rod into eight elements having the same length, the average tensile strength (TS) of the test pieces in units of N/mm² satisfies the relationship "[TS] ≤ 810 × [C] + 475," the difference between the largest and smallest of the tensile strength values of the test pieces is 50 N/mm² or less, and the average reduction of area (RA) of the test pieces in units of percent satisfies the relationship "[RA] ≥ − 0.083 × [TS] + 154."

Description

Wire material, hypereutectoid bainite steel wire, and production method thereof

The present invention relates to a wire for a hypereutectoid bainite steel wire excellent in wire drawing characteristics and delayed fracture resistance, a hypereutectoid bainite steel wire produced from the wire, and a production method thereof.
This application claims priority based on Japanese Patent Application No. 2013-212365 filed in Japan on October 8, 2013, the contents of which are incorporated herein by reference.

Wire is a material for various machine parts such as steel wire. When various machine parts (hereinafter referred to as final products) are manufactured from a wire, usually, the wire is subjected to mechanical processing such as wire drawing and annealing. The tensile strength of the final product is mainly influenced by the component composition of the wire, particularly the C content of the wire. On the other hand, the metal structure of the wire changes due to transformation during annealing. Therefore, when the final product is manufactured by a process including annealing, the metal structure of the wire does not affect the tensile strength of the final product. For the above reasons, when the final product is manufactured by a process including annealing, the component composition of the wire needs to be in accordance with the tensile strength required for the final product.

On the other hand, it is preferable that the tensile strength of the wire is low regardless of the tensile strength of the final product. A wire with high tensile strength has low machinability and wire drawing characteristics. Furthermore, since a wire with high tensile strength is highly sensitive to delayed fracture (fracture due to hydrogen embrittlement), it is easily broken during manufacture, storage, and transportation. In particular, when the C content of the wire is 0.8 mass% or more (that is, when the wire is hypereutectoid steel because the C content of the wire exceeds the eutectoid point), delayed fracture (hydrogen There is a problem that the sensitivity of the wire to embrittlement is increased. As a result, when the manufactured wire is bound in a coil shape for storage and transportation, the wire may be broken due to the stress at the time of binding. The breakage of the wire causes a reduction in the processing efficiency of the wire. Furthermore, when the length of the broken wire is shorter than the length required for the final product, the wire cannot be used as the material of the final product.

In order to prevent breakage due to delayed fracture (hydrogen embrittlement), it is conceivable to relax the binding conditions, for example, to reduce the force for binding the wire. However, by relaxing the bundling condition, the storage property of the coil, the transportability of the coil, the safety when handling the coil, and the like are impaired.

If the tensile strength of the wire is reduced by adjusting the component composition of the wire, for example, by reducing the C content, problems related to delayed fracture and machinability are eliminated. However, as described above, the component composition of the wire needs to be in accordance with the tensile strength required for the final product. Therefore, adjustment of the component composition of the wire cannot be employed as a means for preventing delayed fracture.

The tensile strength of the wire can be reduced by changing the heat treatment conditions during the production of the wire. The metal structure of the conventional hypereutectoid wire (wire whose C content exceeds the eutectoid point) is mainly composed of pearlite. A conventional method for producing a hypereutectoid wire includes a step of rolling a steel material to obtain a wire, and a step of cooling the wire. During the cooling process, the metal structure of the wire becomes pearlite. In this production method, if the rolled wire is first heated to the austenite temperature range and then cooled at a relatively slow cooling rate, the tensile strength of the wire can be reduced. However, when manufacturing conditions with a slow cooling rate are applied to a method for manufacturing a wire having a C content exceeding the eutectoid point, not only pearlite but also proeutectoid cementite is generated during cooling. Proeutectoid cementite deteriorates the workability of the wire. Therefore, slowing the cooling rate of the wire cannot be employed as a means for preventing delayed fracture.

In view of the above circumstances, the present inventors examined adopting adjustment of the metal structure of the wire as a means for reducing the tensile strength. As described above, when the final product is manufactured by a process including annealing, the metal structure of the wire does not affect the tensile strength of the final product. A general wire according to the prior art mainly comprises a pearlite structure, and such a wire is called a pearlite wire. On the other hand, it is known that a wire rod (bainite wire rod) mainly composed of bainite has better wire drawing characteristics than a pearlite wire rod (see, for example, Patent Documents 1 to 7). Moreover, the tensile strength of the hypereutectoid bainite wire in which the C content exceeds the eutectoid point is lower than the tensile strength of the pearlite wire having the same C content as the bainite wire. For example, the present inventors have found that the average tensile strength of a bainite wire having a C content of 1.1% is 200 to 300 MPa lower than the average tensile strength of a pearlite wire having a C content of 1.1%. I found out. By using bainite as the metal structure of the wire, the tensile strength of the wire is reduced regardless of the tensile strength required for the final product after annealing (that is, regardless of the C content required for the steel wire). Improvement of wire drawing characteristics and suppression of delayed fracture can be achieved.

However, the bainite wire has a problem that the tensile strength tends to vary. The state in which the tensile strength of the wire varies is a state in which these measured values vary when the tensile strength is measured at a plurality of locations in one wire. When the tensile strength of the wire varies, the sensitivity to delayed fracture (hydrogen embrittlement) increases at a location where the tensile strength is high, and breakage occurs. Furthermore, when the tensile strength of the wire varies, the workability of the wire varies, so that the machining of the wire becomes difficult. Patent Documents 1 to 7 disclose bainite wire manufacturing methods. However, the present inventors have found that when a bainite wire is produced based on the production methods specifically disclosed in these documents, the tensile strength of the wire varies greatly. The inventors first cut the wire obtained by the above-described manufacturing method into a length of 3200 mm. Next, the present inventors made eight test pieces having a length of 400 mm by dividing the wire into eight equal parts, and performed a tensile test on these test pieces. Among the tensile strengths of these test pieces, the difference between the maximum value and the minimum value (hereinafter referred to as variation width of tensile strength) was more than 100 N / mm ² . On the other hand, as a result of studies by the present inventors, it has been found that a wire having a tensile strength variation width of more than 50 N / mm ² is difficult to be industrially used.

Japanese Unexamined Patent Publication No. 05-117762 Japanese Patent Laid-Open No. 06-017190 Japanese Patent Laid-Open No. 06-017191 Japanese Unexamined Patent Publication No. 06-017192 Japanese Patent Laid-Open No. 06-075032 Japanese Unexamined Patent Publication No. 06-330240 Japanese Unexamined Patent Publication No. 08-003639

As described above, the pearlite wire according to the prior art has a problem that delayed fracture is likely to occur because of its high tensile strength. It has been difficult to reduce the tensile strength by reducing the C content of the pearlite wire in view of the required specifications of the final product obtained from the pearlite wire. On the other hand, reducing the tensile strength by reducing the cooling rate in this method for producing pearlite wire increases the amount of pro-eutectoid cementite, which is not preferable. An increase in the amount of proeutectoid cementite decreases the machinability of the wire. Moreover, the bainite wire by the prior art, especially the hypereutectoid bainite wire in which the C content exceeds the eutectoid point has a problem that the tensile strength tends to vary. Variations in tensile strength increase the frequency of delayed fracture and reduce machinability.

In the present invention, the metal structure of the wire is mainly bainite, thereby reducing the tensile strength and increasing the ductility of the wire whose C content exceeds the eutectoid point. The challenge is to increase it. Furthermore, this invention makes it a subject to suppress the dispersion | variation in the tensile strength of a wire. And this invention aims at providing the wire material which solves these subjects, the hypereutectoid bainite steel wire manufactured using this wire material, and the manufacturing method for manufacturing these stably.

The inventors of the present invention can solve the above-mentioned problems by producing a wire based on production conditions capable of generating a bainite structure capable of simultaneously suppressing proeutectoid cementite and reducing the strength of the wire. I found it.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) The wire according to one embodiment of the present invention is unit mass%, C: more than 0.80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0 to 0.02%, S: 0 to 0.02%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 0 50%, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 0.20%, B: 0 to 0.0050%, Al: 0 to 0.10%, and , Ca: 0 to 0.05%, with the balance being composed of Fe and impurities, the metal structure containing 90 to 100 area% bainite, and the same length of 3200 mm length of wire When eight test pieces having a length of 400 mm are manufactured by dividing into length elements, the average value TS of the tensile strength of each test piece is the unit N / mm ² , and Satisfying Equation 1, the difference between the maximum value and the minimum value among the tensile strengths of the test pieces is 50 N / mm ² or less, and the average value RA of the aperture values of the test pieces is expressed in unit%. The following formula 2 is satisfied.
[TS] ≦ 810 × [C] +475 (Formula 1)
[RA] ≧ −0.083 × [TS] +154 (Formula 2)
Here, [C] is the C content of the wire expressed in unit mass%, [TS] is the average value TS of the tensile strength expressed in unit N / mm ² , and [RA] ] Is the average value RA of the aperture values expressed in unit%.

(2) The hyper-eutectoid bainite steel wire according to another aspect of the present invention is obtained by drawing the wire described in (1) above.

(3) A method for manufacturing a wire according to an aspect of the present invention is the method for manufacturing a wire according to (1) above, wherein C: more than 0.80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0 to 0.02%, S: 0 to 0.02%, Cr: 0 to 1.00%, Ni: 0 to 1 0.00%, Cu: 0 to 1.00%, Mo: 0 to 0.50%, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 0.20%, B : A steel material containing 0 to 0.0050%, Al: 0 to 0.10%, and Ca: 0 to 0.05%, with the balance being composed of Fe and impurities, and rolling the steel slab. And immersing the wire at 850 to 1050 ° C. in a first molten salt bath or molten lead bath at 350 to 450 ° C., and then removing the wire from the first molten salt bath or molten lead bath A step of issuing Ri, wherein a time within 5 seconds from the extraction, and the time is t _s seconds before ~ t _s seconds after the start of bainite transformation of the wire, the second of said wires to 530 ~ 600 ° C. A step of immersing in the molten salt bath or molten lead bath, and a step of removing the wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed.
t _s = 0.05 × t _complete (Expression 3)
t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.

(4) In the manufacturing method of the wire described in (3) above, the wire is immersed in the first molten salt bath or the molten lead bath, and the wire is the second molten salt bath or The elapsed time from the time of immersion in the molten lead bath may be 10 to 40 seconds.

(5) In the method for manufacturing a wire according to (3), the reheat of the wire is detected at the time point when the bainite transformation starts in the wire in the first molten salt bath or molten lead bath. You may judge by.

(6) A method for producing a hypereutectoid bainite steel wire according to another aspect of the present invention is the method for producing a hypereutectoid bainite steel wire according to the above (2), wherein C: 0 .80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0.02% or less, S: 0.02% or less, Cr: 0 to 1 0.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 0.50%, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V : 0 to 0.20%, B: 0 to 0.0050%, Al: 0 to 0.10%, and Ca: 0 to 0.05%, with the balance being Fe and impurities. Rolling a steel slab having a wire to obtain a wire; immersing the wire at 850 to 1050 ° C. in a first molten salt bath or molten lead bath at 350 to 450 ° C .; Taking out from a molten salt bath or molten lead bath, the time the a time within 5 seconds from the extraction, and is after t _s seconds before ~ t _s s of the start of bainite transformation of the wire, the Immersing the wire in a second molten salt bath or molten lead bath at 530 to 600 ° C., and removing the wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed. And a step of drawing the wire taken out of the second molten salt bath or molten lead bath.
t _s = 0.05 × t _complete
t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.

(7) In the method for producing a hypereutectoid bainite steel wire described in (6) above, the time during which the wire is immersed in the first molten salt bath or molten lead bath is 10 to 40 seconds. May be.

(8) In the method for producing a hypereutectoid bainite steel wire according to the above (6), the time point at which the bainite transformation starts in the wire in the first molten salt bath or molten lead bath The determination may be made by detecting recuperation.

(9) A method for producing a hypereutectoid bainite steel wire according to another aspect of the present invention is the method for producing a hypereutectoid bainite steel wire according to the above (2), wherein C: 0 .80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0 to 0.02%, S: 0 to 0.02%, Cr: 0 -1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Mo: 0-0.50%, Ti: 0-0.20%, Nb: 0-0.20% V: 0 to 0.20%, B: 0 to 0.0050%, Al: 0 to 0.10%, and Ca: 0 to 0.05%, with the balance being Fe and impurities A step of drawing a wire obtained by rolling a steel slab having a composition to obtain a steel wire, a first molten salt bath or 350 to 450 ° C. of the steel wire of 850 to 1050 ° C. A step of immersing in a molten lead bath and then removing the steel wire from the first molten salt bath or molten lead bath, at a time within 5 seconds from the removal, and of the bainite transformation of the steel wire Immersing the steel wire in a second molten salt bath or molten lead bath at 530-600 ° C. at a time from t _s seconds before start to t _s seconds later; and Removing from the second molten salt bath or molten lead bath after the transformation is completely completed.
t _s = 0.05 × t _complete
t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.

(10) In the method for producing a hypereutectoid bainite steel wire according to (9), when the steel wire is immersed in the first molten salt bath or molten lead bath, the steel wire is The elapsed time from the time of immersion in the second molten salt bath or molten lead bath may be 10 to 40 seconds.

(11) In the method for producing a hypereutectoid bainite steel wire according to (9), the time point at which the bainite transformation has started in the steel wire in the first molten salt bath or molten lead bath is determined as the steel. The determination may be made by detecting the recuperation of the wire.

(12) In the method for producing a hypereutectoid bainite steel wire according to any one of (9) to (11) above, the steel wire taken out from the second molten salt bath or molten lead bath is further added You may provide the process of performing a wire drawing process.

According to the present invention, a wire having a lower tensile strength and higher ductility than a conventional pearlite wire and a smaller variation width of the tensile strength than a conventional bainite wire can be obtained. Occurrence of breakage is suppressed when the wires according to the present invention are bound, or when the wires according to the present invention are bound. Furthermore, the workability of the wire according to the present invention and the workability of the steel wire according to the present invention obtained by drawing the wire are good. Therefore, according to the present invention, a wire for a hypereutectoid bainite steel wire excellent in wire drawing characteristics and delayed fracture resistance, a hypereutectoid bainite steel wire produced using this wire, and stably producing them. A manufacturing method for manufacturing can be provided.

It is a figure explaining the heat processing conditions in the manufacturing method of the wire concerning one embodiment of the present invention. It is a figure which shows an example of the relationship between tensile strength TS (N / mm < ² >) and C content (mass%) in the wire which concerns on one Embodiment of this invention. It is a figure which shows the relationship between the heat processing conditions in the manufacturing method of the wire which concerns on one Embodiment of this invention, and the dispersion | variation in the tensile strength of a wire. It is a flowchart which shows the manufacturing method of the wire which concerns on one Embodiment of this invention, or a steel wire. It is a flowchart which shows the manufacturing method of the steel wire which concerns on another embodiment of this invention. It is a figure which shows the method of calculating | requiring the area ratio of a bainite. It is a schematic diagram of the wire shape at the time of immersing a wire in a molten salt bath or a molten lead bath.

Hereinafter, embodiments of the present invention will be described.

The hypereutectoid bainite steel wire rod (hereinafter, also referred to as “wire rod according to this embodiment”) having excellent wire drawing characteristics and delayed fracture resistance according to the present embodiment will be described.

The wire according to the present embodiment is unit mass%, C: more than 0.80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0 to 0 0.02%, S: 0 to 0.02%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 0.50%, Ti : 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 0.20%, B: 0 to 0.0050%, Al: 0 to 0.10%, and Ca: 0 to A test piece having a length of 8 mm is obtained by dividing a wire having a length of 3200 mm into 8 elements having the same length by containing 0.05% and the balance being Fe and impurities. When the average tensile strength TS of each test piece is a unit N / mm ² , the following formula 1 is satisfied, and the maximum value and the minimum value among the tensile strengths of the test pieces are Difference is at 50 N / mm ² or less, the average aperture RA of each of the test piece, in units%, and satisfies the following expression 2.

[TS] ≦ 810 × [C] +475 (Formula 1)
[RA] ≧ −0.083 × [TS] +154 (Formula 2)
Here, [C] is the C content of the wire expressed in unit mass%, and [TS] is the average tensile strength TS expressed in unit N / mm ² .

First, the component composition of the wire according to this embodiment will be described. Hereinafter, the unit “%” means “% by mass”.

C: Over 0.80 to 1.20%
C is an element that enhances the hardenability and tensile strength of the wire. By improving the hardenability of the wire, the main structure of the wire is bainite. When the C content exceeds 0.80%, the required hardenability and tensile strength can be obtained. On the other hand, when the C content is more than 1.20%, pro-eutectoid cementite is generated, and disconnection is likely to occur during wire drawing of the wire. Therefore, in order to suppress the formation of proeutectoid cementite, the upper limit value of the C content is set to 1.20%. In order to further facilitate bainite generation, the lower limit value of the C content may be 0.85%, 0.90%, or 0.95%. In addition, when the tensile strength is too high, the sensitivity of the wire to delayed fracture increases, so the lower limit of the C content may be 1.15%, 1.10%, or 1.05%.

Si: 0.10 to 1.50%
Si is an element that increases the tensile strength of the wire. Si is an element that functions as a deoxidizer. When the Si content is less than 0.10%, the above effect cannot be obtained, so the lower limit of the Si content is set to 0.10%. However, in hypereutectoid steel, Si promotes precipitation of proeutectoid ferrite. Proeutectoid ferrite may cause breakage during wire drawing. Further, Si may reduce the limit working degree in wire drawing in hypereutectoid steel. Therefore, the upper limit of Si content is set to 1.50%. In order to further enhance the above-described effects due to Si, the lower limit value of the Si content may be set to 0.15%, 0.20%, or 0.25%. In order to further facilitate the wire drawing, the upper limit value of the Si content may be 1.45%, 1.40%, or 1.35%.

Mn: 0 to 1.00%
The wire according to this embodiment does not need to contain Mn. Therefore, the lower limit of the Mn content of the wire according to this embodiment is 0%. However, Mn has the effect of increasing the strength of the wire by increasing the hardenability of the wire. Further, Mn is an element that acts as a deoxidizing agent, similarly to Si. Therefore, Mn may be included in the wire as necessary. When Mn content exceeds 1.00%, hardenability improves in the segregation part of Mn, and time until completion | finish of transformation becomes long. That is, in this case, the hardenability is not uniform in the wire, and martensite is generated at a place with high hardenability, and this martensite causes disconnection during wire drawing. Therefore, the upper limit of the Mn content needs to be 1.00%. In order to further improve the wire drawing characteristics, the upper limit value of the Mn content may be 0.90% or 0.80%. The lower limit value of the Mn content is 0%, but in order to obtain the above-described effect, the lower limit value of the Mn content is preferably 0.20%, more preferably 0.40%.

P: 0 to 0.02%
S: 0 to 0.02%
P and S are impurity elements. When P and S are present in a large amount in the wire, the ductility of the wire is lowered. Therefore, the upper limit values of P and S are both 0.02%. Preferably, the upper limits of the P content and the S content are both 0.01%, and more preferably both 0.005%. The smaller the P content and the S content, the better. Therefore, the lower limit of the P content and the S content is 0%. However, reducing the content of these elements to 0.001% or less causes an increase in the manufacturing cost of the wire. Therefore, in practical steel, the lower limit of the P content and the S content is usually 0.001%.

In the wire according to the present embodiment, in addition to the above elements, Cr, Ni, Cu, Mo, Ti, Nb, V, B, Al, and Ca are appropriately selected within a range that does not hinder the characteristics of the wire according to the present embodiment. You may contain. However, since the content of these elements is not essential, the lower limit of the content of these elements is 0%.

Cr: 0 to 1.00%
Cr is an element that improves the hardenability of the wire and thereby promotes bainite transformation. When the Cr content exceeds 1.00%, the time required from the start of transformation to the end of transformation becomes longer, which is not preferable because the heat treatment time until bainite transformation is completed becomes longer. Further, like Mn, Cr exceeding 1.00% may cause martensite to be generated in the wire. Therefore, the upper limit of the Cr content is set to 1.00%. The Cr content is preferably 0.50% or less, and more preferably 0.30% or less. The lower limit of the Cr content is 0%, but in order to obtain the above effect, 0.01% or more, more preferably 0.05% or more of Cr may be contained.

Ni: 0 to 1.00%
Ni, like Cr, is an element that improves the hardenability of the wire and thereby promotes bainite transformation. When the Ni content exceeds 1.00%, the ductility of the ferrite phase decreases. Therefore, the upper limit of the Ni content is set to 1.00%. The Ni content is preferably 0.70% or less, and more preferably 0.50% or less. The lower limit of the Ni content is 0%, but in order to obtain the above-mentioned effect, 0.05% or more, more preferably 0.10% or more of Ni may be contained.

Cu: 0 to 1.00%
Cu is an element that improves the corrosion fatigue characteristics of the wire. When Cu content exceeds 1.00%, the ductility of the ferrite in bainite falls. Therefore, the upper limit of the Cu content is set to 1.00%. The Cu content is preferably 0.70% or less, more preferably 0.50% or less. The lower limit value of the Cu content is 0%, but in order to obtain the above-mentioned effect, 0.05% or more, more preferably 0.10% or more of Cu may be contained.

Mo: 0 to 0.50%
Mo is an element that improves the hardenability of the wire. When the Mo content exceeds 0.50%, the hardenability of the wire material is excessively improved, which may cause micromartensite to precipitate in the Mo segregation part. Micro martensite may reduce the ductility of the wire. Therefore, the upper limit of the Mo content is set to 0.50%. Mo content becomes like this. Preferably it is 0.30% or less, More preferably, it is 0.10% or less. The lower limit of the Mo content is 0%, but in order to obtain the above-described effect, 0.01% or more, more preferably 0.03% or more of Mo may be contained.

Ti: 0 to 0.20%
Nb: 0 to 0.20%
V: 0 to 0.20%
Ti, Nb, and V refine the γ grain size of the heated wire. In this case, since the structure formed when the wire is cooled is refined, the toughness of the wire is improved. On the other hand, when the content of Ti, Nb, and V exceeds 0.20%, the characteristics of the wire according to this embodiment are adversely affected. Accordingly, the upper limit values of the Ti, Nb, and V contents are set to 0.20%. The contents of Ti, Nb, and V are preferably all 0.15% or less, more preferably 0.10% or less. The lower limit values for the contents of Ti, Nb, and V are all 0%, but in order to obtain the above-mentioned effects, the lower limit values for the contents of Ti, Nb, and V are preferably 0.01%. %, More preferably 0.02%.

B: 0 to 0.0050%
B improves the hardenability of the wire. If the B content exceeds 0.0050%, the hardenability of the wire becomes too high, so that martensite is formed in the wire, which may reduce the ductility of the wire. Therefore, the upper limit of the B content is set to 0.0050%. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less. The lower limit of the B content is 0%, but in order to obtain the above-described effect, 0.0005% or more, more preferably 0.0010% or more of B may be contained.

Al: 0 to 0.10%
Al is an element that functions as a deoxidizer. When the Al content exceeds 0.10%, hard alumina inclusions are generated, and the inclusions reduce the ductility and wire drawing of the wire. Therefore, the upper limit of the Al content is 0.10%. The Al content is preferably 0.07% or less, and more preferably 0.05% or less. The lower limit of the Al content is 0%, but in order to obtain the above-described effect, Al may preferably be contained by 0.01% or more, more preferably 0.02% or more.

Ca: 0 to 0.05%
Ca improves the delayed fracture resistance of the wire by controlling the form of MnS, which is an inclusion in the wire. However, when the Ca content exceeds 0.05%, Ca generates coarse inclusions, thereby reducing the delayed fracture resistance of the wire. Therefore, the upper limit value of the Ca content is set to 0.05%. The Ca content is preferably 0.04% or less, more preferably 0.03% or less. The lower limit of the Ca content is 0%, but in order to obtain the above effect, 0.001% or more, and more preferably 0.005% or more of Ca may be contained.

The remainder of the component composition of the wire according to this embodiment is made of Fe and impurities. Impurities are components that are mixed due to various factors in the production process, such as ore or scrap, when industrially producing steel materials, and do not adversely affect the wire according to the present embodiment. Means what is allowed.

Next, the metal structure of the wire rod according to this embodiment will be described.

Bainite: 90-100% area
The metal structure of the wire according to this embodiment contains 90 to 100 area% bainite. The wire drawing characteristics of a wire rod containing bainite having a metal structure of 90 to 100 area% (bainite wire rod) are superior to those of a wire rod having a metal structure mainly composed of pearlite (pearlite wire rod). Moreover, since cementite contained in bainite is finer than cementite contained in pearlite, when comparing bainite wire and pearlite wire with the same composition, the tensile strength of bainite wire is higher than the tensile strength of pearlite wire. Low. When the tensile strength of a wire is low, the ductility, wire drawing characteristics, and workability of a wire and a steel wire obtained by drawing the wire are high. In order to further improve these characteristics, the lower limit of the bainite content may be 95 area% or 98 area%. In addition to bainite, for example, micromartensite (MA), proeutectoid cementite, and the like may be included in the metal structure of the wire. These contents are permissible as long as the bainite content is 90 area% or more.

The content of bainite can be obtained by observing a cross section of the wire perpendicular to the wire drawing direction. An example of a method for measuring the content of bainite is as follows. First, metal structure images are obtained at a plurality of locations on the wire cross section perpendicular to the wire drawing direction. Next, the average value of the area ratio of bainite in each metal structure image is obtained. The imaging region for obtaining a metallographic image is not particularly limited. For example, as shown in FIG. 6, the center portion 11 of the wire cross section 1 perpendicular to the wire drawing direction, the surface layer portion 12, and the intermediate portion 13, which is a region having a depth of ¼ of the wire diameter, are mutually possible. It is preferable to include four imaging regions 2 that are separated as much as possible. A means for obtaining a metallographic image is not particularly limited. For example, it is preferable to take a metal structure image at an imaging magnification of 1000 using an SEM (scanning electron microscope). The means for discriminating bainite in the metal structure image is not particularly limited. In view of the component composition, it can be considered that the wire according to the present embodiment does not contain a structure other than pearlite, martensite (including micromartensite), proeutectoid cementite, and bainite. In the metal structure image of the wire according to the present invention, a structure other than pearlite, martensite, and proeutectoid cementite may be regarded as bainite.

Next, the mechanical characteristics of the wire according to this embodiment will be described.

Average tensile strength TS of wire: 810 × [C] +475 N / mm ² or less The mechanical properties of the wire according to this embodiment are obtained by dividing a wire having a length of 3200 mm into eight elements having the same length. Evaluation is made by measuring the characteristics of eight test pieces 400 mm long. The average value of the tensile strength of the eight test pieces described above is defined as the average tensile strength TS of the wire. The average tensile strength TS of the wire according to this embodiment satisfies the following formula 1.
[TS] ≦ 810 × [C] +475 (1)
Here, [C] is the C content of the wire expressed in unit mass%, and [TS] is the average tensile strength TS expressed in unit N / mm ² .

The main factors that increase the tensile strength of the wire are the C content of the wire and the heat treatment conditions during wire manufacture. The increase in tensile strength due to the C content of the wire does not vary the tensile strength of the wire. This is because the increase in tensile strength that occurs with an increase in C content occurs uniformly throughout the wire. On the other hand, an increase in tensile strength due to heat treatment conditions during wire manufacture may cause the wire tensile strength to vary. In particular, when the diameter of the wire is small, the heat capacity per unit length of the wire is small and the temperature distribution in the length direction of the wire is large, making it difficult to perform heat treatment uniformly throughout the wire, Variations are likely to occur. The greater the effect of heat treatment on tensile strength, the greater the variation in tensile strength. When the tensile strength of the wire varies, the workability of the wire and the steel wire varies, so that the machining of the wire and the steel wire becomes difficult. Further, in this case, the sensitivity to delayed fracture (hydrogen embrittlement) is increased at a portion where the tensile strength of the wire is high, and breakage occurs.

In view of the above matters, the average tensile strength of the wire according to this embodiment needs to be lower than the upper limit value defined only by the C content. The present inventors limited the upper limit value of the average tensile strength TS by the above formula 1.

The coefficients “810” and “475” in the above formula 1 were experimentally determined by the present inventors for a wire having a C content exceeding 0.80%, that is, a wire having a C content exceeding the eutectoid point. It is a coefficient. When the average tensile strength TS of the wire exceeds the upper limit defined by Equation 1 (that is, when the average tensile strength is too high for the C content), the effect of heat treatment on the tensile strength is inappropriate. The present inventors have found that the variation in the tensile strength of the wire is increased, which increases the stability of machining and easily breaks. In this case, it is considered that the heat treatment conditions at the time of manufacturing the wire are not appropriate, and therefore the tensile strength of the wire is increased unevenly.

In FIG. 2, an example of the relationship between average tensile strength TS (N / mm < ² >) and C content (mass%) is shown. From the figure, it can be seen that the average tensile strength TS of the wire according to the present embodiment is in the region of “[TS] ≦ 810 × [C] +475”.

The lower limit value of the tensile strength of the wire is not particularly specified. However, a certain amount of tensile strength is usually required for industrially used wires. Even when the average tensile strength of the wire is too low with respect to the C content, it is difficult to industrially use the wire. Therefore, you may prescribe | regulate the average tensile strength of the wire which concerns on this embodiment by the following formula | equation 1 ', Formula 1'', or Formula 1'''.
810 × [C] + 425 ≦ [TS] ≦ 810 × [C] +475 (Formula 1 ′)
810 × [C] + 435 ≦ [TS] ≦ 810 × [C] +475 (Formula 1 ″)
810 × [C] + 445 ≦ [TS] ≦ 810 × [C] +475 (Formula 1 ″ ′)

Average drawing value RA of wire rod: −0.083 × TS + 154 or more Evaluation of the mechanical properties of the wire rod according to the present embodiment is obtained by dividing a wire rod having a length of 3200 mm into eight elements having the same length. This is done by measuring the characteristics of eight 400 mm long test pieces. The average value of the above-mentioned eight test pieces is defined as the average drawing value RA of the wire. The average drawing value RA of the wire according to the present embodiment satisfies the following formula 2.
[RA] ≧ −0.083 × [TS] +154 (Formula 2)
Here, [TS] is the average tensile strength TS expressed in the unit N / mm ² .

Moreover, in the wire according to the present embodiment, the lower limit value of the average drawing value RA is limited by the lower limit value calculated from the average tensile strength TS.

The coefficients “−0.083” and “154” in the above equation 2 are obtained by investigating the average tensile strength and the average drawing value of various wires whose C content is in the hypereutectoid region. Is a coefficient obtained experimentally. The aperture value of the wire obtained by the manufacturing method according to this embodiment, which will be described later, had an average aperture value of at least “−0.083 × [TS] +154”. This average aperture value exceeds the average aperture value of the conventional pearlite wire. On the other hand, the average drawing value of the wire rod having no bainite having a metal structure of 90 to 100% was lower than the above lower limit value. Moreover, although the metal structure is mainly composed of bainite, this bainite is obtained by heating the supercooled austenite before the start of the bainite transformation. It was low.

Variation width of the tensile strength of the wire: The difference between the maximum value and the minimum value among the tensile strengths of the eight test pieces is 50 N / mm ² or less. The evaluation of the mechanical properties of the wire according to this embodiment is 3200 mm in length. This is done by measuring the properties of eight 400 mm long specimens obtained by dividing the wire into eight elements having the same length. In the wire according to the present embodiment, the difference between the maximum value and the minimum value among the tensile strengths of the test pieces described above is defined as the variation width of the tensile strength of the wire. The variation width of the tensile strength of the wire according to this embodiment is 50 N / mm ² or less.

When the tensile strength of the wire is large, the workability of the wire and the steel wire obtained by drawing the wire is reduced. When the variation width of the tensile strength of the wire is more than 50 N / mm ² , it becomes difficult to process the wire and a steel wire obtained by drawing the wire under a certain condition. Further, in this case, the sensitivity to delayed fracture (hydrogen embrittlement) is increased at a portion where the tensile strength of the wire is high, and breakage occurs. Wires and to further facilitate the processing of the steel wire, and in order to further suppress the occurrence of breakage of the wire, the variation range of the tensile strength of the wire is 45N / mm ² or less, 40N / mm ² or less, 35N / mm ² or less, Alternatively, it may be 30 N / mm ² or less.

The diameter of the wire according to this embodiment is not particularly specified. However, in order to further suppress variations in the tensile strength of the wire, the diameter of the wire may be set to 3.5 to 16.0 mm. As described above, when the diameter of the wire is less than 3.5 mm, the heat capacity per unit length of the wire is small and the temperature distribution in the length direction of the wire is large, so the heat treatment is performed uniformly over the entire wire. This makes it difficult to produce variations in tensile strength. On the other hand, when the diameter of the wire is more than 16.0 mm, it becomes difficult to uniformly cool the center portion and the surface layer portion of the wire, and it becomes difficult to make the metal structure of the center portion of the wire a predetermined one. There is a fear.

Next, a method for manufacturing a wire and a steel wire according to the present embodiment (hereinafter sometimes referred to as “manufacturing method according to the present embodiment”) will be described.

As shown in FIG. 4, the method for manufacturing a wire according to the present embodiment includes (a) a step of obtaining a wire by rolling a steel piece having the above-described composition of the wire according to the present embodiment, and (b). Immersing a wire at 850 to 1050 ° C. in a first molten salt bath or molten lead bath at 350 to 450 ° C., and then removing the wire from the first molten salt bath or molten lead bath; from a time within 5 seconds, and the time is t _s seconds before the start of bainite transformation of the wire ~ t _s seconds after, the second molten salt bath or molten lead bath the wire 530 ~ 600 ° C. Dipping in, and (d) removing the wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed. t _s is obtained by the following Equation 3.
t _s = 0.05 × t _complete (Expression 3)
t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath. In the hypereutectoid bainite steel wire manufacturing method according to this embodiment, as shown in FIG. 4, in addition to the above (a) to (d), the wire is immersed in the first molten salt bath or molten lead bath. Before performing, the wire rod is drawn to provide a steel wire. In addition to the above (a) to (d), the method for producing a hypereutectoid bainite steel wire according to another embodiment of the present invention includes (e) a second molten salt as shown in FIG. A step of drawing the wire taken out of the bath or molten lead bath. 4 and 5, “molten salt bath or molten lead bath” is simply referred to as “bath”. Since, starting substantially bainite transformation "wire to being dipped into the wire rods second molten salt bath or molten lead bath at a time is t _s seconds before ~ t _s seconds after the start of bainite transformation of the wire At the same time, the wire may be dipped in the second molten salt bath or molten lead bath.

In FIG. 1, the heat processing of the manufacturing method which concerns on this embodiment is shown. In the figure, the arrow with the symbol (b) indicates that a wire at 850 to 1050 ° C. is immersed in the first molten salt bath or molten lead bath at a temperature T ₁ in the range of 350 to 450 ° C. Taking out, that is, the above-mentioned (b) is shown. (B), the wire rod is retention at a temperature T _1, taken out and then transferred to a second molten salt bath or molten lead bath. T ₁ in the figure indicates the time for immersing the wire in the first molten salt bath or molten lead bath, and the wire is transferred from the first molten salt bath or molten lead bath to the second molten salt bath or molten lead bath. (I.e., the time from when the wire is immersed in the first molten salt bath or molten lead bath to the time when the wire is immersed in the second molten salt bath or molten lead bath) . In the figure, the arrow with the symbol (c) indicates that the wire is melted in the second molten salt bath or melt at a temperature in the range of 530 to 600 ° C. (T ₁ + ΔT) almost simultaneously with the start of the bainite transformation. Immersion in a lead bath, that is, the above (c) is shown. In the figure, the arrow with the symbol (d) indicates that the wire is held in the second molten salt bath or molten lead bath until the bainite transformation is completely completed, that is, the above (d). Indicates.

Temperature of the wire before being immersed in the first molten salt bath or molten lead bath: 850 to 1050 ° C.
In the manufacturing method of the wire according to the present embodiment, first, a steel piece having the component composition of the wire according to the present embodiment is rolled to obtain a wire. Next, this wire is immersed in the first molten salt bath or molten lead bath. The wire may be cooled once between rolling and dipping and then reheated, or cooling and reheating may not be performed between rolling and dipping. Moreover, you may wire-draw a wire between rolling and immersion. In either case, the temperature of the wire immersed in the first molten salt bath or molten lead bath is 850 to 1050 ° C. Usually, since the temperature of the wire immediately after rolling is 1050 ° C. or less, the wire after rolling or the steel wire after drawing is directly immersed in the first molten salt bath or molten lead bath (that is, cooling and reheating are performed). In the case of immersion), the upper limit of the temperature of the immersed wire or steel wire is substantially 1050 ° C. In addition, even if the wire rod after rolling or the steel wire after drawing is once cooled and then reheated and then immersed in the first molten salt bath or molten lead bath, the first molten salt bath or molten It is good also considering the upper limit of the temperature of the wire rod or steel wire immersed in a lead bath as 1050 degreeC. This is because there is no advantage of heating the wire or steel wire to 1050 ° C. or higher. When the temperature of the wire or steel wire immersed in the first molten salt bath or molten lead bath is less than 850 ° C., the wire or steel wire is not sufficiently quenched, so the first molten salt bath or molten lead The lower limit of the temperature of the wire rod or steel wire immersed in the bath is 850 ° C. In addition, when wire drawing is performed on a wire between rolling and dipping, the description of “wire” in the description of the subsequent steps is appropriately read as “steel wire”.

Temperature of the first molten salt bath or molten lead bath: 350 to 450 ° C.
In the manufacturing method of the wire according to the present embodiment, the wire at 850 to 1050 ° C. is rapidly cooled by being immersed in the first molten salt bath or molten lead bath ((b) in FIG. 1). The temperature T ₁ of the first molten salt bath or molten lead bath is 350 to 450 ° C. By this rapid cooling, the metal structure of the wire becomes a supercooled austenite. When the wire is kept isothermal in this state, the bainite transformation of the austenite in the supercooled state starts.

When the temperature T1 of the _first molten salt bath or the molten lead bath is higher than 450 ° C., the cooling rate of the wire decreases, so that the metal structure of the wire is transformed into bainite before becoming a supercooled austenite. In this case, the tensile strength of the wire is lowered, but pro-eutectoid cementite is precipitated in the wire. Proeutectoid cementite deteriorates the wire drawing properties of the wire. Therefore, in order to rapidly cool the wire, the temperature T1 of the _first molten salt bath or molten lead bath needs to be 450 ° C. or lower. On the other hand, if the temperature T ₁ of the first molten salt bath or molten lead bath is below 350 ° C., there is a possibility that the first molten salt bath or molten lead bath solidifies. The time for immersing the wire in the first molten salt bath or molten lead bath is appropriately adjusted so that the step of immersing the wire in the second molten salt bath or molten lead bath can be performed as prescribed. Need to be done.

When the wire is immersed in the second molten salt bath or molten lead bath: within 5 seconds from the removal of the wire from the first molten salt bath or molten lead bath, and the start of the bainite transformation of the wire Time point before t _s seconds to time point after t _s seconds In the wire manufacturing method according to this embodiment, a time point within 5 seconds after the wire is taken out from the first molten salt bath or molten lead bath at temperature T _1. a is, and the time is t _s seconds before ~ t _s seconds after the start of bainite transformation of the wire, immersing the wire in a second molten salt bath or molten lead bath at a temperature T _2.

The inventors of the present invention have determined the time from when the wire is immersed in the first molten salt bath or the molten lead bath to the time when the wire is immersed in the second molten salt bath or the molten lead bath (that is, the first molten salt bath or the molten lead bath). The total time of the time during which the wire is immersed in the molten salt bath or molten lead bath and the time for transferring the wire from the first molten salt bath or molten lead bath to the second molten salt bath or molten lead bath ) Wires were produced under various production conditions in which t ₁ and the temperature T _{1 of} the first molten salt bath or molten lead bath were changed, and the variation widths of the tensile strengths of these wires were measured. Using the data thus obtained, the relationship between the temperature T ₁ , the time t ₁ and the variation width of the tensile strength was investigated. As a result, the result shown in FIG. 3 was obtained.

The curve to which the symbol “S” is attached in FIG. 3 is a curve (hereinafter referred to as S curve) indicating the temperature and time at which the bainite transformation starts. This curve changes according to the component composition of the wire. Data points that are described in FIG. 3 represents the temperature T ₁ and the time t ₁ at the time of manufacture wire according to the data points. The wire according to the data point on the left side of the curve is a wire immersed in the second molten salt bath or the molten lead bath before the start of the bainite transformation, and the wire according to the data point on the right side of the curve is the bainite transformation. Is a wire rod immersed in a second molten salt bath or a molten lead bath after the start of the above. In FIG. 3, the dotted line described regarding each data point shows the thermal history of the wire which concerns on each data point. The variation width of the tensile strength of the wire having the data point type “BAD” is more than 50 N / mm ² , and the variation width of the tensile strength of the wire having the data point type “GOOD” is more than 40 N / mm ^{2 to} 50 N. / Mm ² or less, and the variation width of the tensile strength of the wire material whose data point type is “VERY GOOD” is 40 N / mm ² or less.

As shown in FIG. 3, in the wire according to the data point close to the curve (that is, the wire immersed in the second molten salt bath or molten lead bath almost simultaneously with the start of the bainite transformation), The variation width of the tensile strength was small.

Time t _1, as the start of t _s seconds before ~ t _s seconds after a which point the wire of bainite transformation of the wire is immersed in a second molten salt bath or in molten lead bath is appropriately set. t _s is a value obtained by Equation 3 shown below.
t _s = 0.05 × t _complete (Expression 3)
t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.

Time from immersing the wire in a first molten salt bath or molten lead bath until bainite transformation of the wire is started, and t _s, and S curve corresponding to the composition of the wire rod, a first molten salt It depends on the temperature of the bath or molten lead bath. Accordingly, the time t ₁ is determined by simulation and / or preliminary experiments based on the component composition of the wire and the temperature of the first molten salt bath or molten lead bath. Further, as will be described later, by detecting the recuperation of the wire, the time from when the wire is immersed in the first molten salt bath or molten lead bath until the start of bainite transformation of the wire is obtained. Therefore, a preliminary investigation for determining the time t ₁ may be performed by the above-mentioned means before manufacturing the wire.

It is not clear why the variation in tensile strength of the wire is suppressed by immersing the wire in the second molten salt bath or molten lead bath substantially simultaneously with the start of the bainite transformation of the wire. However, the reason explained below is estimated. If the bainite transformation of the wire occurs during immersion in the first molten salt bath or molten lead bath or during transfer to the second molten salt bath or molten lead bath, recuperation (transformation exotherm) causes The wire temperature rises during immersion in the first molten salt bath or molten lead bath or during transfer to the second molten salt bath or molten lead bath. In this case, the wire temperature may increase unevenly. This is because when a wire is heat-treated in a molten salt bath or a molten lead bath, the wire is immersed in a molten salt bath or a molten lead bath in a state having a coil shape as shown in FIG. 7, for example, and then taken out. Because. When the wire during heat treatment has a coil shape, the temperature rise due to recuperation is greater in the portion where the wires overlap each other than in the other portions. This is because the cooling effect of the first molten salt bath or the molten lead bath is relatively difficult to reach the portion where the wires overlap. Thus, by the time t ₁ of the above becomes longer, if the delayed start of the heating of the wire, the non-uniform rise of the wire temperature, variation in the tensile strength of the wire occurs. In addition, it is indispensable to make a wire shape into a coil shape at the time of heat processing, in order to improve the manufacture efficiency of a wire. Unless there is a special reason, a wire is not immersed in a molten salt bath or a molten lead bath in a state where the wires do not overlap each other. On the other hand, the above time t ₁ is short, if the wire to t _s s ultra before the start of bainite transformation is immersed in a second molten salt bath or molten lead bath, since the start of the transformation is accelerated, transformation starting temperature Becomes higher. In this case, the strength of the wire increases and the ductility of the wire decreases.

In view of the above-mentioned reason, it is most preferable that the immersion of the wire in the second molten salt bath or the molten lead bath is performed completely simultaneously with the start of the bainite transformation of the wire. However, the inventors of the present invention, in the wire material in which the bainite transformation proceeds quickly and the temperature rise due to recuperation is relatively large, if the time between the immersion of the wire material and the start of the transformation of the wire material is 5 seconds or less, For wire rods that can sufficiently suppress the variation in tensile strength, and that the progress of bainite transformation is slow and the temperature rise due to recuperation is relatively low, the time between the immersion of the wire rod and the start of transformation of the wire rod is over 5 seconds. It was found from experimental facts that even if it exists, variation can be suppressed. Based on this finding, in the manufacturing method of the wire according to the present embodiment, the time between the start of the transformation of immersion and wire of the wire, the value t _s which is determined in accordance with the rate of progression of bainite transformation Stipulated. Note that, in the wire according to the present _embodiment, since _{t complete} will not be less than 100 seconds, the upper limit of _{t s} may be 5 seconds.

In many cases, the elapsed time t ₁ between the time when the wire is immersed in the first molten salt bath or the molten lead bath and the time when the wire is immersed in the second molten salt bath or the molten lead bath is 10 ~ 40 seconds is preferred. In view of the chemical composition of the wire according to the present embodiment, the time when t ₁ to a or 40 seconds greater than 10 seconds, to properly carry out the immersion of the wire to the subsequent second molten salt bath or molten lead bath in Is difficult.

In addition to the above-mentioned rules, the wire must be immersed in the second molten salt bath or molten lead bath within 5 seconds after being taken out from the first molten salt bath or molten lead bath. If the time between wire rod removal and immersion, that is, the wire rod transfer time exceeds 5 seconds, the temperature of the wire rod may fluctuate during the wire rod transfer. At the same time, it is extremely difficult to immerse the wire in the second molten salt bath or molten lead bath.

You may determine the time of the bainite transformation start in the wire in a 1st molten salt bath or a molten lead bath by detecting the recuperation (transformation heat_generation | fever) of a wire. The recuperation in the present embodiment is a phenomenon in which the temperature of the wire increases due to the start of bainite transformation in the wire. The recuperation can be detected, for example, by comparing the temperature of the wire taken out after being immersed in the first molten salt bath or molten lead bath and the temperature of the first molten salt bath or molten lead bath. When the temperature of the wire is higher than the temperature of the first molten salt bath or molten lead bath, it is determined that reheating has occurred in the wire. In each of the wires in which the immersion time in the first molten salt bath or the molten lead bath is variously changed, the shortest immersion time t _min that can cause reheating of the wire is determined by examining the presence or absence of recuperation. Can be sought. The time when t _min has elapsed since the wire is immersed in the first molten salt bath or molten lead bath can be regarded as the time when the bainite transformation has started in the wire. As described above, it is more preferable to use the recuperation to obtain in advance the time point at which the bainite transformation starts in the wire, and to manufacture the wire based on that.

If the time of immersion in the first molten salt bath or molten lead bath is less than 5 seconds, the wire is restored even if the temperature of the wire is higher than that of the first molten salt bath or molten lead bath. It cannot be determined whether or not heat is generated. This is because the temperature of the wire may increase due to insufficient dipping time rather than recuperation.

Temperature of second molten salt bath or molten lead bath: 530 to 600 ° C
Second molten salt bath or when taking out the wire from the molten lead bath: bainite transformation completely finished start of the bainite transformation point wire after substantially the same time, a second molten salt bath or molten lead is temperature T ₂ Immerse the wire in the bath. Temperature _{T 2} is 530 ~ 600 ° C.. As a result, the wire can be rapidly heated to a temperature of 530 to 600 ° C. ((c) in FIG. 1) and maintained at that temperature until the bainite transformation is completely completed. When the wire is rapidly heated to a temperature of 530 to 600 ° C. almost simultaneously with the start of the bainite transformation of the wire, the space between the cementite in the bainite becomes wide. As a result, the strength of the wire is reduced as compared with the case where rapid heating is not performed. When the temperature of the second molten salt bath or molten lead bath is less than 530 ° C. or more than 600 ° C., it takes a long time to complete the bainite transformation. Therefore, the temperature of the second molten salt bath or molten lead bath is set to 530 to 600 ° C. in order to reliably complete the bainite transformation in a short time. The heating rate at the time of heating a wire to said temperature range is not specifically limited. However, in order to shorten the time until completion of the bainite transformation, it is preferable that the heating rate is high, specifically 10 to 50 ° C./second. Such a heating rate can be obtained by immersing the wire in a molten salt bath or a molten lead bath at a temperature of 530 to 600 ° C. When the wire is taken out from the second molten salt bath or molten lead bath before the bainite transformation is completed, MA is generated in the wire, and this MA may lower the workability of the wire.

When the wire material is held as it is after the wire material is immersed in the first molten salt bath or molten lead bath, the dense bainite structure grows. A wire rod with a dense bainite structure has higher strength than a wire rod that is rapidly heated substantially simultaneously with the start of the bainite transformation. Therefore, in the wire according to the present embodiment, by rapidly heating the wire, the interval between the precipitated cementite is widened and the strength is lowered.

The hyper-eutectoid bainite steel wire (hereinafter, also referred to as “steel wire according to this embodiment”) having excellent delayed fracture resistance according to the present embodiment is the wire rod according to the present embodiment having excellent wire drawing properties. It is drawn. The drawing process may be a normal drawing process, and the area reduction rate is not particularly limited. Since the steel wire which concerns on this embodiment is excellent in the delayed fracture resistance, the use of a steel wire expands significantly.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. 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 1
The hypereutectoid steel slab having the component composition shown in Table 1 was rolled into a wire having the wire diameter shown in Table 2, and the bainite transformation was completed under the temperature conditions shown in Table 2. Average tensile strength of the wire after completion bainite transformation (N / mm ^2), the average aperture (%) and tensile strength of the variation width (N / mm ²⁾ was measured. The average tensile strength of the wire is an average value of the tensile strength of each of eight 400 mm-long test pieces obtained by dividing a wire having a length of 3200 mm into eight elements having the same length. The average drawing value of the wire is an average value of the drawing values of each of eight 400 mm-long test pieces obtained by dividing a wire having a length of 3200 mm into eight elements having the same length. The variation width of the tensile strength of the wire is the maximum value among the tensile strengths of each of the eight 400 mm-long test pieces obtained by dividing the wire having a length of 3200 mm into eight elements having the same length. It is the difference from the minimum value. The measurement results are also shown in Table 2. The heating rate when the wire was immersed in the second molten salt bath or molten lead bath was 10 to 50 ° C./second.

In Table 2, T ₀ is the temperature of the wire immersed in the first molten salt bath or molten lead bath, T ₁ is the temperature of the first molten salt bath or molten lead bath, and t ₁ is the first molten salt bath. Or the time from immersing the wire in the molten lead bath to immersing the wire in the second molten salt bath or molten lead bath, ΔT is that the wire is immersed in the second molten salt bath or molten lead bath , T ₂ is the temperature of the second molten salt bath or molten lead bath, the TS upper limit is the C content and the upper limit of the average tensile strength calculated from Equation 1, and the TS average is the average tensile strength (N / mm ² ), TS maximum is the maximum value of tensile strength (N / mm ² ), TS minimum is the minimum value of tensile strength (N / mm ² ), and TS variation width is the difference between TS maximum and TS minimum (N / mm) ^2), RA lower the upper limit value and the upper limit value of the average aperture value calculated from equation 2 of average tensile strength, RA mean average aperture (%), RA largest maximum aperture value (%), RA minimum minimum aperture value (%), RA variation width is the difference between the RA maximum and RA Min (%). No. The immersion time t of the wire in the first molten salt bath or molten lead bath when manufacturing the inventive examples 1 to 7 is the same as that of the second molten salt bath or molten lead bath. It is appropriately selected so as to be almost simultaneously with the start of the transformation. No. In Comparative Example 8, the wire was not immersed in the second molten salt bath or molten lead bath. No. In Comparative Examples 9 and 10, the wire was immersed in the second molten salt bath or molten lead bath after a long time had elapsed after the start of the bainite transformation. Inventive Example No. 1-7, Comparative Example No. 9 and Comparative Example No. In 10, the wire was immersed in the second molten salt bath or molten lead bath within 5 seconds after being removed from the first molten salt bath or molten lead bath.

From Table 2, no. In the inventive examples 1 to 7, the TS average and RA average satisfy the formulas 1 and 2, and the TS variation width is 50 N / mm ₂ or less. As a result, no. In the inventive examples 1 to 7, it can be seen that the delayed fracture resistance is improved, and no breakage occurs when the wires are bound and in the bound state.

As described above, according to the present invention, the wire rod has a lower strength and a higher ductility than pearlite steel, and the breakage during the binding work of the wire rod or in the bound state is suppressed, and the wire drawing characteristics and It is possible to provide a wire having excellent delayed fracture resistance, a hypereutectoid bainite steel wire produced using the wire, and a production method for producing them stably. Therefore, the present invention has high applicability in the steel industry.

DESCRIPTION OF SYMBOLS 1 Wire rod cross section 11 Center part 12 Surface layer part 13 Middle part 2 Imaging | photography region

Claims (12)

1. In unit mass%
   C: more than 0.80 to 1.20%,
   Si: 0.10 to 1.50%,
   Mn: 0 to 1.00%
   P: 0 to 0.02%,
   S: 0 to 0.02%,
   Cr: 0 to 1.00%,
   Ni: 0 to 1.00%,
   Cu: 0 to 1.00%,
   Mo: 0 to 0.50%,
   Ti: 0 to 0.20%,
   Nb: 0 to 0.20%,
   V: 0 to 0.20%,
   B: 0 to 0.0050%,
   Al: 0 to 0.10%, and
   Ca: 0 to 0.05%
   Containing
   The balance has a component composition consisting of Fe and impurities,
   The metal structure comprises 90-100 area% bainite;
   When a test piece having a length of 3 mm is divided into eight elements having the same length to produce eight test pieces having a length of 400 mm, the average value TS of the tensile strength of each test piece is expressed in the unit N. / Mm ² , the following formula 1 is satisfied,
   The difference between the maximum value and the minimum value among the tensile strengths of the test pieces is 50 N / mm ² or less,
   An average value RA of the aperture value of each test piece satisfies the following formula 2 in unit%.
   [TS] ≦ 810 × [C] +475 (Formula 1)
   [RA] ≧ −0.083 × [TS] +154 (Formula 2)
   Here, [C] is the C content of the wire expressed in unit mass%, [TS] is the average value TS of the tensile strength expressed in unit N / mm ² , and [RA] ] Is the average value RA of the aperture values expressed in unit%.
2. A hypereutectoid bainite steel wire obtained by drawing the wire according to claim 1.
3. It is a manufacturing method of the wire according to claim 1,
   In unit mass%
   C: more than 0.80 to 1.20%,
   Si: 0.10 to 1.50%,
   Mn: 0 to 1.00%
   P: 0 to 0.02%,
   S: 0 to 0.02%,
   Cr: 0 to 1.00%,
   Ni: 0 to 1.00%,
   Cu: 0 to 1.00%,
   Mo: 0 to 0.50%,
   Ti: 0 to 0.20%,
   Nb: 0 to 0.20%,
   V: 0 to 0.20%,
   B: 0 to 0.0050%,
   Al: 0 to 0.10%, and
   Ca: 0 to 0.05%
   A step of rolling a steel slab having a component composition consisting of Fe and impurities, and obtaining a wire rod,
   Immersing the wire at 850 to 1050 ° C. in a first molten salt bath or molten lead bath at 350 to 450 ° C., and then removing the wire from the first molten salt bath or molten lead bath;
   A time within 5 seconds from the extraction, and the time is after t _s seconds before the start of bainite transformation of the wire rod ~ t _s s, a second molten salt bath of the wire 530 ~ 600 ° C. or Soaking in a molten lead bath;
   And a step of taking out the wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed.
   t _s = 0.05 × t _complete (Expression 3)
   t _complete indicates the time from the start to the end of the bainite transformation of the wire in a unit of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.
4. Elapsed time between the time when the wire is immersed in the first molten salt bath or molten lead bath and the time when the wire is immersed in the second molten salt bath or molten lead bath. The method for producing a wire according to claim 3, wherein is 10 to 40 seconds.
5. The wire rod according to claim 3, wherein the time point at which the bainite transformation starts in the wire rod in the first molten salt bath or molten lead bath is determined by detecting recuperation of the wire rod. Manufacturing method.
6. A method for producing a hypereutectoid bainite steel wire according to claim 2,
   In unit mass%
   C: more than 0.80 to 1.20%,
   Si: 0.10 to 1.50%,
   Mn: 0 to 1.00%
   P: 0 to 0.02%,
   S: 0 to 0.02%,
   Cr: 0 to 1.00%,
   Ni: 0 to 1.00%,
   Cu: 0 to 1.00%,
   Mo: 0 to 0.50%,
   Ti: 0 to 0.20%,
   Nb: 0 to 0.20%,
   V: 0 to 0.20%,
   B: 0 to 0.0050%,
   Al: 0 to 0.10%, and
   Ca: 0 to 0.05%
   A step of rolling a steel slab having a component composition consisting of Fe and impurities, and obtaining a wire rod,
   Immersing a wire at 850-1050 ° C. in a first molten salt bath or molten lead bath at 350-450 ° C., and then removing the wire from the first molten salt bath or molten lead bath;
   A time within 5 seconds from the extraction, and the time is after t _s seconds before the start of bainite transformation of the wire rod ~ t _s s, a second molten salt bath of the wire 530 ~ 600 ° C. or Soaking in a molten lead bath;
   Removing the wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed;
   Applying wire drawing to the wire taken out from the second molten salt bath or molten lead bath;
   A method for producing a hypereutectoid bainite steel wire, comprising:
   t _s = 0.05 × t _complete
   t _complete indicates the time from the start to end of the bainite transformation of the wire in units of seconds when the wire is continuously immersed in the first molten salt bath or molten lead bath.
7. The method for producing a hypereutectoid bainite steel wire according to claim 6, wherein the time during which the wire is immersed in the first molten salt bath or molten lead bath is 10 to 40 seconds.
8. 7. The process according to claim 6, wherein the time point at which the bainite transformation has started in the wire in the first molten salt bath or molten lead bath is determined by detecting reheating of the wire. A method for producing a eutectoid bainite steel wire.
9. A method for producing a hypereutectoid bainite steel wire according to claim 2,
   In unit mass%
   C: more than 0.80 to 1.20%,
   Si: 0.10 to 1.50%,
   Mn: 0 to 1.00%
   P: 0 to 0.02%,
   S: 0 to 0.02%,
   Cr: 0 to 1.00%,
   Ni: 0 to 1.00%,
   Cu: 0 to 1.00%,
   Mo: 0 to 0.50%,
   Ti: 0 to 0.20%,
   Nb: 0 to 0.20%,
   V: 0 to 0.20%,
   B: 0 to 0.0050%,
   Al: 0 to 0.10%, and
   Ca: 0 to 0.05%
   A step of drawing a wire obtained by rolling a steel slab having a component composition consisting of Fe and impurities in the balance to obtain a steel wire,
   Immersing the steel wire at 850 to 1050 ° C. in a first molten salt bath or molten lead bath at 350 to 450 ° C., and then removing the steel wire from the first molten salt bath or molten lead bath; ,
   A time within 5 seconds from the extraction, and the time is t _s seconds before ~ t _s seconds after the start of bainite transformation of the steel wire, a second molten salt of the steel wire 530 ~ 600 ° C. Soaking in a bath or molten lead bath;
   And a step of taking out the steel wire from the second molten salt bath or molten lead bath after the bainite transformation is completely completed, and a method for producing a hypereutectoid bainite steel wire.
   t _s = 0.05 × t _complete
   t _complete indicates the time from the start to the end of the bainite transformation of the steel wire in units of seconds when the steel wire is continuously immersed in the first molten salt bath or molten lead bath.
10. Between the time when the steel wire is immersed in the first molten salt bath or the molten lead bath and the time when the steel wire is immersed in the second molten salt bath or the molten lead bath. The method for producing a hypereutectoid bainite steel wire according to claim 9, wherein the elapsed time is 10 to 40 seconds.
11. The time point at which the bainite transformation starts in the steel wire in the first molten salt bath or molten lead bath is determined by detecting recuperation of the steel wire. Method for producing hypereutectoid bainite steel wire.
12. The hypereutectoid bainite according to any one of claims 9 to 11, further comprising a step of drawing the steel wire taken out from the second molten salt bath or molten lead bath. Manufacturing method of steel wire.

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