KR20160048991A - Wire rod, hypereutectoid bainite steel wire, and method for manufacturing same - Google Patents

Abstract

The wire material according to the present invention has a predetermined composition and has a metal structure containing 90 to 100% by area of bainite and dividing the wire material having a length of 3200 mm into eight equal length elements, , The average tensile strength TS of each of the test pieces satisfies the relationship of "[TS]? 810 占 [C] +475" in units of N / mm2, The difference between the maximum value and the minimum value of the strengths is 50 N / mm 2 or less, and the average cross sectional shrinkage value RA of each of the test pieces satisfies the relation of "[RA] ≥-0.083 × [TS] +154" in unit%.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod,

The present invention relates to a wire rod for a high purity bainite steel wire having excellent drawability and delayed fracture resistance, a superheated bainite steel wire made from the wire, and a method for producing the same.

The present application claims priority based on Japanese Patent Application No. 2013-211365 filed on October 8, 2013, the contents of which are incorporated herein by reference.

The wire rod is a material for various mechanical parts such as a steel wire. When manufacturing various mechanical parts (hereinafter referred to as final products) from the wire rod, mechanical processing such as drawing processing and annealing are usually performed on the wire rod. The tensile strength of the final product is mainly affected by the composition of the wire, particularly the C content of the wire. On the other hand, the metal structure of the wire changes due to the transformation at the time of annealing. Thus, when the final product is made by a process involving annealing, the metal structure of the wire does not affect the tensile strength of the final product. For the above reason, when the final product is produced by a process including annealing, the composition of the wire needs to be in accordance with the tensile strength required for the final product.

On the other hand, regardless of the tensile strength of the final product, the tensile strength of the wire is preferably low. Wire materials with high tensile strength have low machinability and freshness characteristics. In addition, the wire material having a high tensile strength is susceptible to delayed fracture (fracture due to hydrogen embrittlement), and therefore, is susceptible to breakage during its manufacture, storage and transportation. Particularly, when the C content of the wire rod is 0.8 mass% or more (i.e., the C content of the wire rod is higher than the vacancy point, the wire rod is a quarrying stone) and the problem that the susceptibility of the wire rod to delayed fracture (hydrogen embrittlement) . As a result, when the wire rods after production are bound in a coil shape for storage and transportation, the wire rods may be damaged by the stress at the time of binding. The breakage of the wire causes a decrease in the processing efficiency of the wire. Further, if the length of the cut wire is shorter than the length required for the final product, the wire can not be used as a material for the final product.

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

When the composition of the wire rod is adjusted, for example, by lowering the C content, the tensile strength of the wire rod is lowered, thereby eliminating the problem of delayed fracture and machinability. However, as described above, it is necessary that the composition of the wire material is dependent on the tensile strength required for the final product. Therefore, adjustment of the composition of the wire can not be employed as means for preventing delayed breakdown.

By changing the heat treatment conditions at the time of producing the wire rod, the tensile strength of the wire rod can be lowered. The metal structure of a conventional overlaid wire material (a wire material whose C content exceeds the vacancy point) is mainly composed of pearlite. A conventional method for producing overlaid seam material includes a step of rolling a steel material to obtain a wire material, and a step of cooling the wire material. During the cooling process, the metal structure of the wire becomes pearlite. In this manufacturing method, if the wire after rolling is first heated to the austenite temperature region and then cooled at a relatively slow cooling rate, the tensile strength of the wire can be lowered. However, when the manufacturing conditions in which the C content exceeds the vacancy point and the manufacturing conditions in which the cooling rate is slow are applied to the method of producing a wire rod, many pearlescent cementites as well as pearlite are produced at the time of cooling. Corundum cementite causes poor workability of the wire rod. Therefore, slowing the cooling rate of the wire rod can not be employed as means for preventing delayed fracture.

In view of the above, the inventors of the present invention have studied the adjustment of the metal structure of the wire as a means for lowering the tensile strength. As described above, when the final product is produced by a process involving annealing, the metal structure of the wire does not affect the tensile strength of the final product. Conventional wire rods generally consist of pearlite structure, and these rods are called pearlite rods. On the other hand, it has been known that a wire material (bainite wire material) having bainite as a main structure has superior characteristics to a pearlite wire than a pearlite wire (see, for example, Patent Documents 1 to 7). The tensile strength of the overlaid bainite wire material having a C content exceeding the vacancy point is lower than the tensile strength of the perlite wire material having the same C content as that of the bainite wire material. 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 an average tensile strength of a pearlite wire having a C content of 1.1%. By making the metal structure of the wire material bainite, the tensile strength of the wire material is lowered regardless of the tensile strength required for the final product after annealing (that is, regardless of the C content required for the steel wire) And suppression of delayed fracture can be achieved.

However, the bainite wire rod has a problem that the tensile strength tends to fluctuate. The state in which the tensile strength of the wire rod is fluctuated means a state in which these measured values are varied when tensile strength is measured at a plurality of points in one wire rod. When the tensile strength of the wire rod is varied, susceptibility to delayed fracture (hydrogen embrittlement) increases at a portion with a high tensile strength, and breakage occurs. Further, in the case where the tensile strength of the wire varies, the workability of the wire varies, making it difficult to machine the wire. Patent Literatures 1 to 7 disclose a method for producing a bainite wire. However, the inventors of the present invention have found that when the bainite wire rod is produced based on the production method specifically disclosed in these documents, the tensile strength of the wire rod varies greatly. The inventors of the present invention first cut the wire rod obtained by the above-described manufacturing method to a length of 3200 mm. Next, the present inventors prepared 8 test pieces each having a length of 400 mm by dividing the wire into 8 equal parts, and these test pieces were subjected to a tensile test. The difference between the maximum value and the minimum value of the tensile strengths of these test pieces (hereinafter referred to as the variation width of the tensile strength) was more than 100 N / mm < 2 >. On the other hand, the inventors of the present invention have found that it is difficult to provide a wire rod having a deviation width of tensile strength of more than 50 N / mm < 2 >

Japanese Patent Application Laid-Open No. 05-117762 Japanese Patent Application Laid-Open No. 06-017190 Japanese Patent Application Laid-Open No. 06-017191 Japanese Patent Application Laid-Open No. 06-017192 Japanese Patent Application Laid-Open No. 06-073502 Japanese Patent Application Laid-Open No. 06-330240 Japanese Patent Application Laid-Open No. 08-003639

As described above, the conventional pearlite wire rod has a problem that delayed breakage tends to occur because the tensile strength is high. It is difficult to reduce the tensile strength by decreasing the C content of the pearlite wire material in view of the requirements of the final product obtained from the pearlite wire material. On the other hand, reducing the tensile strength by decreasing the cooling rate in the production method of the pearlite wire material is not preferable because it increases the amount of cobalt cementite. The increase in the amount of cementite cementite lowers the machinability of the wire. In addition, the bainite wire material according to the prior art, particularly the bainite wire material having a C content higher than the vacancy point, has a problem that the tensile strength tends to fluctuate. The deviation of the tensile strength raises the occurrence frequency of the delayed fracture and also lowers the machinability.

Disclosure of the Invention Problems to be Solved by the Invention The present invention is intended to improve the low-tensile strength and high ductility of a wire material having a C content exceeding the vacancy point by mainly making the metal structure of the wire material bainite, thereby improving the wire drawing characteristics and delayed fracture resistance. Further, the present invention aims to suppress the variation of the tensile strength of the wire rod. It is another object of the present invention to provide a wire rod which solves these problems, a superabsorbent bainite steel wire produced by using the wire rod, and a manufacturing method for stably producing the wire rod.

The inventors of the present invention have found that the aforementioned problems can be solved by producing a wire material on the basis of the production conditions capable of producing bainite structure capable of achieving both suppression of cornerstone cementite and low strength of wire material.

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

(1) A wire according to one aspect of the present invention comprises, in unit mass%, C: 0.80 to 1.20%, Si: 0.10 to 1.50%, Mn: 0 to 1.00%, P: 0 to 0.02% 0 to 1.00% of Cr, 0 to 1.00% of Cr, 0 to 1.00% of Ni, 0 to 1.00% of Cu, 0 to 0.50% of Mo, 0 to 0.20% of Ti, 0 to 0.20% of Nb, 0.20%, B: 0 to 0.0050%, Al: 0 to 0.10% and Ca: 0 to 0.05%, the balance being Fe and impurities, and the metal structure is 90 to 100% Wherein the average value TS of the tensile strengths of the respective test pieces is expressed in units of N / mm < 2 > when the test pieces of eight lengths 400 mm are produced by dividing the wire material having a length of 3200 mm into eight equal length elements, And a difference between a maximum value and a minimum value of the respective tensile strengths of the respective test pieces is 50 N / mm 2 or less, and the average value RA of the cross-sectional shrinkage percentage values of the respective test pieces satisfies the following formula (2).

Where [C] is the C content of the wire rod expressed in unit mass%, [TS] is the average value TS of the tensile strength in N / mm 2, [RA] Is the above average value RA of the values.

(2) The overlaid bainite steel wire according to another embodiment of the present invention is obtained by drawing the wire described in (1) above.

(3) A method for producing a wire according to one embodiment of the present invention is a method for producing a wire as described in (1) above, wherein C is in a range of 0.80 to 1.20%, 0.10 to 1.50% 0 to 1.0%, 0 to 1.0% of P, 0 to 0.02% of P, 0 to 0.02% of S, 0 to 1.00% of Cr, 0 to 1.00% of Ni, 0 to 1.00% of Cu, 0 to 0.50% 0 to 0.20% of Al, 0 to 0.05% of Ca, 0 to 0.20% of N, 0 to 0.20% of N, 0 to 0.20% of V, 0 to 0.20% A step of dipping the wire rod at 850 to 1050 캜 in a first molten salt bath or a melt-spinning bath at 350 to 450 캜, and then the wire rod is taken out from the first molten bath or melt- the time of within 5 seconds from the step of the extraction of, and the second molten salt bath or melt of 530~600 ℃ the wire at the time of t seconds before ~t _{s s} seconds after the start of bainite transformation of the wire, Dipping The Jung, the wire, after which the bainite transformation is completely shut down and a step of taking out from said second molten salt bath or melt yeonyok.

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath.

(4) The method for producing a wire according to (3) above, wherein the time when the wire rod is immersed in the first molten bath or melt bath is compared with a time point when the wire rod is immersed in the second molten bath or melt bath The time may be 10 to 40 seconds.

(5) In the method of manufacturing a wire according to (3), the point of time at which the bainite transformation is started in the wire in the first molten salt bath or melt bath may be determined by detecting the double heat of the wire.

(6) A method for producing a superalloy bainite steel wire according to another embodiment of the present invention is a method for producing a superalloy bainite steel wire according to the above (2), wherein C is from 0.80 to 1.20% 0.10 to 1.50%, Mn: 0 to 1.00%, P: not more than 0.02%, S: not more than 0.02%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00% 0 to 0.50%, Ti to 0 to 0.20%, Nb to 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% Fe and impurities to obtain a wire rod; dipping the wire rod at 850 to 1050 占 폚 in a first molten bath or melt bath at 350 to 450 占 폚; or a step of taking out from the molten yeonyok, a time within 5 seconds from the take-out, and the 530~600 ℃ of the wire at the time of t seconds before ~t _{s s} seconds after the start of bainite transformation of the wire, 2 A step of removing the wire rod from the second molten salt bath or the melting and burning bath after the completion of the bainite transformation is completely finished, and a step of immersing the wire rod in the second molten salt bath or the melt- And a step of performing a drawing process on the surface.

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath.

(7) In the above-mentioned method for producing overlaid bainite steel wire according to (6), the time for which the wire rod is immersed in the first molten bath or melt bath may be 10 to 40 seconds.

(8) In the above-mentioned method for producing overlaid bainite steel wire according to (6), the time point at which the bainite transformation is started in the wire material in the first molten salt bath or the melting and burning bath is detected by detecting the double- You can.

(9) A method for producing a superalloy bainite steel wire according to another embodiment of the present invention is a method for producing a superalloy bainite steel wire according to the above (2), wherein C is in a range of 0.80 to 1.20% 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% 0 to 0.50% of Ti, 0 to 0.20% of Nb, 0 to 0.20% of Nb, 0 to 0.20% of V, 0 to 0,0050% of B, 0 to 0.10% of Al and 0 to 0.05% of Ca, A step of drawing a wire rod obtained by rolling a steel strip having a component composition composed of Fe and an impurity in a remainder to obtain a steel wire; and a step of immersing the steel wire at 850 to 1050 캜 in a first molten bath or a melt- followed by a step of extracting the liner from said first molten salt bath or melt yeonyok, a time within 5 seconds from the take-out, and t _s seconds before the start of bainite transformation of the steel wire ~t _s seconds , The step of immersing the steel wire in a second molten salt bath or a melting and burning bath at 530 to 600 占 폚 and a step of taking out the steel wire from the second molten bath or melt bath after the bainite transformation is completely finished Respectively.

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath.

(10) In the method for producing a superalloy bainite steel wire according to (9), when the steel wire is immersed in the first molten bath or melt bath, and when the steel wire is immersed in the second molten bath or melt bath The elapsed time between the points may be 10 to 40 seconds.

(11) In the above-mentioned method for producing a bainitic superalloy steel wire, the time point at which the bainite transformation is started in the steel wire in the first molten salt bath or the melting and burning bath is detected by detecting the double- You can.

(12) In the method for producing an overlaid bainite steel wire according to any one of (9) to (11), a step of drawing the steel wire taken out from the second molten salt bath or melt- You can.

According to the present invention, a wire rod having a lower tensile strength and a higher ductility than a conventional pearlite wire and having a smaller variation width of tensile strength than a conventional bainite wire can be obtained. When the wire according to the present invention is bound, or when the wire according to the present invention is engaged, the occurrence of breakage is suppressed. 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, it is possible to provide a wire rod for a brittle bainite steel wire having excellent drawability and delayed fracture resistance, a bainite bainite wire produced by using the wire rod, and a manufacturing method for stably producing the same. .

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a view for explaining heat treatment conditions in a method of manufacturing a wire according to an embodiment of the present invention; FIG.
2 is a diagram showing an example of a relationship between a tensile strength TS (N / mm 2) and a C content (mass%) in a wire according to an embodiment of the present invention.
Fig. 3 is a diagram showing the relationship between the heat treatment conditions in the method of manufacturing the wire according to the embodiment of the present invention and the deviation of the tensile strength of the wire. Fig.
4 is a flowchart showing a method of manufacturing a wire or a wire according to an embodiment of the present invention.
5 is a flowchart showing a method of manufacturing a steel wire according to another embodiment of the present invention.
6 is a view showing a method for obtaining the area ratio of bainite.
Fig. 7 is a schematic view of a wire rod shape when the wire rod is immersed in a molten salt bath or melt bath.

Hereinafter, embodiments of the present invention will be described.

(Hereinafter sometimes referred to as " wire material according to the present embodiment ") which is excellent in the fresh characteristic and the delayed fracture resistance characteristic according to the present embodiment.

The wire material according to the present embodiment contains, as a unit mass%, 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% 0 to 1.00%, 0 to 1.00% of Ni, 0 to 1.00% of Cu, 0 to 0.50% of Mo, 0 to 0.20% of Ti, 0 to 0.20% of Nb, By dividing the wire having a composition of 0 to 0.0050%, Al: 0 to 0.10% and Ca: 0 to 0.05%, the balance being Fe and impurities, and having a length of 3200 mm, Wherein the average tensile strength TS of each of the test pieces satisfies the following formula 1 at a unit of N / mm 2, and the difference between the maximum value and the minimum value of the respective tensile strengths of the test pieces is 50 N / Mm 2, and the average cross-sectional shrinkage value RA of each of the test pieces satisfies the following formula (2) in unit of%.

Where [C] is the C content of the wire rod expressed in unit mass% and [TS] is the average tensile strength TS expressed in units N / mm 2.

First, the composition of the wire material according to the present embodiment will be described. Hereinafter, the unit "%" means "% by mass".

C: greater than 0.80 to 1.20%

C is an element that increases the hardenability and tensile strength of the wire. By increasing the quenching of the wire, the main structure of the wire becomes bainite. When the C content exceeds 0.80%, required hardenability and tensile strength are obtained. On the other hand, when the C content is more than 1.20%, cornerstone cementite is generated, and breakage is likely to occur during wire drawing. Therefore, in order to suppress the generation of cornerstone cementite, the upper limit of the C content is set to 1.20%. In order to further facilitate the production of bainite, the lower limit of the C content may be set to 0.85%, 0.90%, or 0.95%. When the tensile strength is too high, the susceptibility of the wire to delayed fracture becomes high, so the lower limit of the C content may be set to 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 rod. Further, Si is an element which functions as a deoxidizer. When the Si content is less than 0.10%, the above-mentioned effect can not be obtained, so the lower limit of the Si content is set to 0.10%. However, in the overfilled quartz, Si promotes the precipitation of pro-eutectoid ferrite. Pro-eutectoid ferrite may cause disconnection at the time of wire drawing. In addition, there is a possibility that Si limits the degree of finishing in the drawing process in the overcrystallized steel. Therefore, the upper limit value of the Si content is set to 1.50%. The lower limit of the Si content may be set to 0.15%, 0.20%, or 0.25% in order to further increase the above-mentioned effect of Si. In order to further facilitate the drawing process, the upper limit of the Si content may be set to 1.45%, 1.40%, or 1.35%.

Mn: 0 to 1.00%

The wire material according to the present embodiment need not contain Mn. Therefore, the lower limit value of the Mn content of the wire material according to the present embodiment is 0%. However, Mn has the effect of increasing the strength of the wire rod by increasing the quenching of the wire rod. Mn, like Si, is an element that acts as a deoxidizer. Therefore, Mn may be contained in the wire if necessary. When the Mn content exceeds 1.00%, the quenching is improved in the segregated portion of Mn, and the time until the transformation is completed is prolonged. That is, in this case, the quenching is not uniform in the wire rod, and martensite is generated at a portion having high quenching property, and this martensite causes disconnection at the time of drawing processing. Therefore, it is necessary to set the upper limit value of the Mn content to 1.00%. Further, in order to further improve the fresh characteristics, the upper limit value of the Mn content may be set to 0.90% or 0.80%. The lower limit value of the Mn content is 0%, but in order to obtain the above-mentioned 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 deteriorates. Therefore, the upper limit values of P and S are all 0.02%. Preferably, the upper limit value of the P content and the S content are all 0.01%, more preferably 0.005%. The smaller the P content and the S content, the better, so that 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 leads to an increase in production cost of the wire rod. Therefore, in practical steels, the lower limit of the P content and the S content is usually 0.001%.

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

Cr: 0 to 1.00%

Cr improves the quenching of the wire rod, thereby promoting bainite transformation. When the Cr content exceeds 1.00%, the time required from the start of transformation to the end of transformation is prolonged, thereby undesirably increasing the heat treatment time until completion of bainite transformation. In addition, as with Mn, Cr exceeding 1.00% may cause martensite to be produced in the wire rod. Therefore, the upper limit value 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%. However, in order to obtain the above-mentioned effect, Cr may be contained in an amount of 0.01% or more, more preferably 0.05% or more.

Ni: 0 to 1.00%

Like Ni, Ni is an element that improves the quenching of a wire rod and thereby promotes bainite transformation. When the Ni content exceeds 1.00%, the ductility of the ferrite phase is lowered. Therefore, the upper limit value 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 value of the Ni content is 0%. However, in order to obtain the above-mentioned effect, it is preferable that the Ni content is 0.05% or more, more preferably 0.10% or more.

Cu: 0 to 1.00%

Cu is an element which improves the corrosion fatigue characteristic of the wire rod. When the Cu content exceeds 1.00%, the ductility of the ferrite in the bainite decreases. Therefore, the upper limit of the Cu content is set to 1.00%. The Cu content is preferably 0.70% or less, and more preferably 0.50% or less. The lower limit of the Cu content is 0%. However, in order to obtain the above-mentioned effect, Cu may be contained preferably at 0.05% or more, and more preferably at 0.10% or more.

Mo: 0 to 0.50%

Mo is an element that improves the quenching of the wire rod. When the Mo content exceeds 0.50%, the quenching of the wire rod is excessively improved, and there is a possibility that micro martensite is precipitated in the Mo segregation portion. Micro-martensite may deteriorate the ductility of the wire rod. Therefore, the upper limit value of the Mo content is made 0.50%. The Mo content is preferably 0.30% or less, and more preferably 0.10% or less. The lower limit value of the Mo content is 0%. However, in order to obtain the above-mentioned effect, it is preferable that the Mo content is 0.01% or more, more preferably 0.03% or more.

Ti: 0 to 0.20%

Nb: 0 to 0.20%

V: 0 to 0.20%

Ti, Nb and V reduce the? Grain size of the heated wire. In this case, since the structure formed when the wire rod is cooled becomes finer, the toughness of the wire rod is improved. On the other hand, when the content of Ti, Nb and V exceeds 0.20%, the characteristics of the wire according to the present embodiment are adversely affected. Therefore, any upper limit value of the contents of Ti, Nb and V is 0.20%. The content of Ti, Nb and V is preferably 0.15% or less, more preferably 0.10% or less. The lower limit values of the contents of Ti, Nb and V are all 0%. To obtain the above-mentioned effect, the lower limit values of the contents of Ti, Nb and V may preferably be 0.01%, more preferably 0.02%.

B: 0 to 0.0050%

B improves the quenching of the wire rod. When the B content exceeds 0.0050%, the quenching of the wire becomes too high, and martensite is formed in the wire, thereby deteriorating the ductility of the wire. Therefore, the upper limit value of the B content is 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%. However, in order to obtain the above-mentioned effect, the B content may preferably be 0.0005% or more, more preferably 0.0010% or more.

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 produced, and the inclusions deteriorate the ductility and the drawability of the wire rod. Therefore, the upper limit value of the Al content is set to 0.10%. The Al content is preferably 0.07% or less, and more preferably 0.05% or less. The lower limit value of the Al content is 0%. However, in order to obtain the above-mentioned effect, the Al content may preferably be 0.01% or more, more preferably 0.02% or more.

Ca: 0 to 0.05%

Ca improves the delayed fracture characteristics of the wire by controlling the shape of MnS which is an inclusion in the wire. However, when the Ca content exceeds 0.05%, Ca generates coarse inclusions, whereby the delayed fracture characteristics of the wire are deteriorated. Therefore, the upper limit value of the Ca content is set to 0.05%. The Ca content is preferably 0.04% or less, and more preferably 0.03% or less. The lower limit of the Ca content is 0%. However, in order to obtain the above-mentioned effect, Ca may preferably be contained in an amount of 0.001% or more, more preferably 0.005% or more.

The balance of the composition of the wire according to the present embodiment is composed of Fe and impurities. The term impurity refers to a raw material such as ore or scrap when manufacturing a steel material industrially, or a component incorporated by various factors in the manufacturing process, and is not limited to the range of not adversely affecting the wire material according to the present embodiment .

Next, the metal structure of the wire material according to the present embodiment will be described.

Bainite: 90 to 100 area%

The metal structure of the wire according to the present embodiment contains bainite of 90 to 100% by area. The drawing characteristic of the wire material (bainite wire material) containing 90 to 100% by area of bainite in the metal structure is superior to the wire material (perlite wire material) in which the metallic structure is mainly composed of pearlite. Since the cementite contained in bainite is finer than cementite contained in pearlite, when the bainite wire and the pearlite wire having the same composition are compared with each other, the tensile strength of the bainite wire is lower than that of the pearlite wire. When the tensile strength of the wire rod is low, the wire rod and the steel wire obtained by drawing the wire rod have high ductility, drawability and workability. In order to further improve these properties, the lower limit value of the bainite content may be 95% by area or 98% by area. In addition to bainite, for example, micro martensite (MA), cobalt cementite, and the like may be included in the metal structure of the wire. The content thereof is permissible as long as the content of bainite is 90% by area or more.

The content of bainite is determined by observing a section of the wire rod perpendicular to the drawing direction. An example of a method for measuring the content of bainite is as follows. First, a metal texture image is obtained at a plurality of places of the wire material section perpendicular to the drawing direction. Next, an average value of the area ratio of bainite on each metal structure is obtained. The photographing area for obtaining the metallic texture image is not particularly limited. 6, each of the center portion 11, the surface layer portion 12 and the intermediate portion 13, which is a region having a depth of 1/4 of the wire diameter, of the wire member section 1 perpendicular to the drawing direction , And preferably includes four shooting regions 2 as far as possible from each other. Means for obtaining a metallic texture phase is not particularly limited. For example, it is preferable to take a metal tissue image at a magnification of 1000 times using an SEM (Scanning Electron Microscope). The means for discriminating bainite in the metallic texture is not particularly limited. It can be considered that the wire material according to the present embodiment does not include a structure other than pearlite, martensite (including micro martensite), cornerstone cementite, and bainite in view of the composition of the wire. Therefore, A structure other than pearlite, martensite and cornerstone cementite may be regarded as bainite.

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

Average tensile strength of wire TS: 810 x [C] + 475 N / mm2 or less

The mechanical properties of the wire according to the present embodiment were evaluated by measuring the characteristics of test pieces having eight lengths of 400 mm obtained by dividing the wire having a length of 3200 mm into eight elements having the same length. The average value of the tensile strengths of the eight specimens described above is defined as the average tensile strength TS of the wire rods. The average tensile strength TS of the wire according to the present embodiment satisfies the following formula (1).

Where [C] is the C content of the wire rod expressed in unit mass% and [TS] is the average tensile strength TS in N / mm2.

The main factors that increase the tensile strength of the wire are the C content of the wire and the heat treatment conditions at the time of producing the wire. The increase in tensile strength due to the C content of the wire does not change the tensile strength of the wire. This is because an increase in the tensile strength caused by the increase in the C content occurs uniformly throughout the wire rod. On the other hand, the increase in the tensile strength due to the heat treatment conditions at the time of producing the wire rod may change the tensile strength of the wire rod. Particularly, when the diameter of the wire rod is small, since the heat capacity per unit length of the wire rod is small and the temperature distribution in the longitudinal direction of the wire rod becomes large, it becomes difficult to uniformly perform the heat treatment on the entire wire rod and the tensile strength is easily varied. The greater the effect of heat treatment on the tensile strength, the greater the variation in tensile strength. When the tensile strength of the wire rod is varied, the workability of the wire rod and the steel wire is varied, which makes it difficult to machine the wire rod and the steel wire. In this case, susceptibility to delayed fracture (hydrogen embrittlement) is increased at a portion where the tensile strength of the wire rod is high, and breakage occurs.

In view of the above, the average tensile strength of the wire according to the present 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-mentioned formula (1).

The coefficients " 810 " and " 475 " in the formula (1) are coefficients obtained experimentally by the present inventors in a wire rod having a C content exceeding 0.80%, that is, a wire having a C content exceeding the vacancy point. Since the influence of the heat treatment on the tensile strength is increased to an inappropriate level when the upper limit value defined by the formula 1 is higher than the average tensile strength TS of the wire rod (that is, when the average tensile strength is excessively high with respect to the C content) The inventors of the present invention have found that the stability of the machining process is deteriorated and the breakage is likely to occur. In this case, the heat treatment conditions at the time of producing the wire rod are not appropriate, and therefore, it is considered that the tensile strength of the wire rod is unevenly raised.

Fig. 2 shows an example of the relationship between the average tensile strength TS (N / mm2) and the C content (mass%). It can be seen from the drawing that the average tensile strength TS of the wire according to the present embodiment is within the range of "[TS]? 810 占 [C] +475".

The lower limit value of the tensile strength of the wire rod is not specifically defined. However, a wire material used industrially is usually required to have a certain tensile strength. Even when the average tensile strength of the wire rod is too low relative to the C content, it is difficult to industrially use the wire rod. Therefore, the average tensile strength of the wire according to the present embodiment may be defined by the following formula 1 ', formula 1' ', or formula 1' ''.

Average cross-sectional shrinkage factor of wire: RA: -0.083 × TS + 154 or more

The evaluation of the mechanical properties of the wire according to the present embodiment is performed by measuring the characteristics of test specimens having eight lengths of 400 mm obtained by dividing a wire having a length of 3200 mm into eight elements having the same length. The average values of the cross-sectional shrinkage values of the above-mentioned eight test pieces are defined as the average cross-sectional shrinkage value RA of the wire rods. The average cross-sectional shrinkage rate RA of the wire according to the present embodiment satisfies the following formula (2).

Here, [TS] is the average tensile strength TS expressed in units of N / mm < 2 >.

Further, in the wire according to the present embodiment, the lower limit value of the average cross-sectional shrinkage value RA is limited by the lower limit value calculated from the average tensile strength TS.

The coefficients " -0.083 " and " 154 " in the formula (2) are empirically determined by the present inventors by examining the average tensile strength and average cross-sectional shrinkage percentage values of various wire materials having a C content in the region of overlaid seams. The cross-sectional shrinkage percentage value of the wire obtained by the manufacturing method according to the present embodiment described later had an average cross-sectional shrinkage percentage value of at least "-0.083 × [TS] +154" or more. This average cross-sectional shrinkage percentage value is higher than the average cross-sectional shrinkage percentage value of the conventional pearlite wire. On the other hand, the average cross-sectional shrinkage value of the wire rod having no metal structure of 90 to 100% of bainite was lower than the above-mentioned lower limit value. Further, although the metal structure mainly consists of bainite, the average cross-sectional shrinkage value of the wire rod obtained by heating the bainite before the initiation of the bainite transformation in the supercooled state was also lower than the above-mentioned lower limit value.

Width of Deviation of Tensile Strength of Wire Rod: The difference between the maximum value and the minimum value of each tensile strength of 8 specimens is 50 N / mm 2 or less

The evaluation of the mechanical properties of the wire according to the present embodiment is performed by measuring the characteristics of test specimens having eight lengths of 400 mm obtained by dividing a wire having a length of 3200 mm 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 of the respective tensile strengths of the above-mentioned test pieces is defined as the variation width of the tensile strength of the wire. The variation width of the tensile strength of the wire rod according to the present embodiment is 50 N / mm 2 or less.

When the tensile strength of the wire rod is large, the workability of the wire rod obtained by drawing the wire rod and the wire rod becomes small. When the variation width of the tensile strength of the wire rod exceeds 50 N / mm 2, it is difficult to process the wire rod and the steel wire obtained by drawing the wire rod under a predetermined condition. In this case, susceptibility to delayed fracture (hydrogen embrittlement) is increased at a portion where the tensile strength of the wire rod is high, and breakage occurs. The width of the tensile strength of the wire rod is 45 N / mm 2 or less, 40 N / mm 2 or less, 35 N / mm 2 or less, or 30 N / mm 2 or less in order to further facilitate processing of the wire rod and the steel wire, It may be.

The diameter of the wire according to the present embodiment is not specifically defined. However, in order to further suppress the deviation of the tensile strength of the wire rod, the diameter of the wire rod may be 3.5 to 16.0 mm. As described above, when the diameter of the wire rod is less than 3.5 mm, since the heat capacity per unit length of the wire rod is small and the temperature distribution in the longitudinal direction of the wire rod becomes large, it becomes difficult to uniformly perform the heat treatment throughout the wire rod, . On the other hand, when the diameter of the wire rod exceeds 16.0 mm, it is difficult to uniformly cool the center portion and the surface layer portion of the wire rod, and it is difficult to make the metal structure of the center portion of the wire rod to be predetermined.

Next, a method for manufacturing a wire rod and a steel wire according to the present embodiment (hereinafter, also referred to as a " manufacturing method according to the present embodiment ") will be described.

As shown in Fig. 4, the method of manufacturing the wire according to the present embodiment includes the steps of: (a) rolling a piece of steel having the component composition of the wire according to the above-described embodiment to obtain a wire; and (b) (C) a step of immersing the wire rod in a first molten salt bath or a melting and burning bath at a temperature of 350 to 450 DEG C and then taking out the wire rod from the first molten salt bath or the melting and burning bath; of the process and the bay, (d) of the wire, the bainite transformation of immersing a _s t _s seconds before ~t second wire, the time after the start of the transformation of the night during the second molten salt bath or melt yeonyok of 530~600 ℃ And then taking it out of the second molten salt bath or the melting and burning bath. t _s is obtained by the following equation (3).

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath. 4, the method of manufacturing the overlaid bainite steel wire according to the present embodiment is characterized in that, in addition to the above (a) to (d), before the wire rod is immersed in the first molten bath or melt bath, So as to obtain a steel wire. 5, in addition to the above-mentioned (a) to (d), (e) a method for producing a bainite steel wire according to another embodiment of the present invention, And a step of performing drawing processing on the wire rope taken out from the wire rope. In Figs. 4 and 5, " molten salt bath or melt bath " is simply referred to as " bath ". Then, the first wire at approximately the same time that the wire is immersed in the second molten salt bath or in the molten yeonyok bay time t _s of seconds before ~t _s seconds after the start of the transformation of the pre-existing and nitro bay start of night transformation of "wire 2 In a molten salt bath or a melt-spinning bath ".

Fig. 1 shows a heat treatment of the manufacturing method according to the present embodiment. In the drawing, the symbol indicated by (b) indicates that the wire rod at 850 to 1050 ° C is immersed in the first molten salt bath or the melt bath at a temperature T ₁ in the range of 350 to 450 ° C and then taken out , And (b) described above. (b), the wire rod is corrected at the temperature T ₁ , taken out, and then transferred to the second molten bath or melt bath. In the figure, t ₁ represents the sum of the time for immersing the wire into the first molten salt bath or the melt bath and the time for transferring the wire from the first molten bath or the melt bath to the second molten bath or melt bath (that is, 1 time from the time when the wire rod is immersed in the molten bath or melt bath to the time when the wire rod is immersed in the second molten bath or melt bath). The symbol indicated by (c) in the drawing indicates that the wire is immersed in the second molten salt bath or the melt bath at a temperature (T ₁ +? T) in the range of 530 to 600 ° C at about the same time as the start of bainite transformation That is, (c) described above. In the drawing, the symbol (d) indicates that the wire material is corrected in the second molten salt bath or melt-spinning bath until the bainite transformation is completely terminated, that is, (d) described above.

Temperature of the wire before immersing in the first molten salt bath or melt bath: 850 - 1050 캜

In the wire rod manufacturing method according to the present embodiment, first, the steel strip having the composition of the wire rod according to the present embodiment is rolled to obtain a wire rod. Subsequently, the wire rod is immersed in a first molten salt bath or a melt-laden bath. The wire rod may be cooled between the rolling and the dipping, then reheated, and cooling and reheating may not be performed between the rolling and the dipping. Further, drawing processing may be performed between the rolling and the dipping. In either case, the temperature of the wire rod immersed in the first molten salt bath or melt bath is 850 to 1050 캜. Since the temperature of the wire immediately after the rolling is 1050 占 폚 or less, when the wire after rolling or the steel wire after the drawing is directly immersed in the first molten salt bath or the melt-burning bath (i.e., immersed without cooling and reheating) Or the upper limit of the temperature of the steel wire is substantially 1050 占 폚. The upper limit of the temperature of the wire or steel wire to be immersed in the first molten salt bath or the melt-burning bath is 1050 or more, even if the wire after the rolling or the steel wire after the drawing is once cooled and then reheated and then immersed in the first molten bath or melt- Deg.] C. This is because there is no advantage of heating the wire or steel wire to 1050 DEG C or higher. If the temperature of the wire or steel wire immersed in the first molten salt bath or the melt bath is less than 850 DEG C, quenching of the wire or steel wire will not be performed sufficiently, so that the lower limit of the temperature of the wire or steel wire immersed in the first molten bath or melt- Lt; / RTI > Further, in the case of performing drawing processing on the wire rod between rolling and immersion, the description "wire rod" in the following description of the process can be appropriately read as "steel wire".

Temperature of the first molten salt bath or melting and burning bath: 350 to 450 캜

In the wire rod manufacturing method according to the present embodiment, the wire rod at 850 to 1050 占 폚 is quenched by immersing the wire rod in the first molten salt bath or melt-spinning bath (Fig. 1 (b)). The temperature T ₁ of the first molten salt bath or the melting and burning bath is 350 to 450 캜. By this quenching, the metal structure of the wire becomes a supercooled austenite. When the wire rod is kept isothermal in this state, bainite transformation of a supercooled austenite is started.

If the temperature T ₁ of the first molten salt bath or the melting and burning bath is higher than 450 ° C, the cooling rate of the wire rod is lowered, so that the metal structure of the wire rod is transformed into bainite before becoming a supercooled austenite. In this case, the tensile strength of the wire rod is lowered, but the crushed stone cementite is precipitated in the wire rod. Corundum cementite deteriorates the wire 's fresh characteristics. Therefore, in order to quench the wire rod, it is necessary to set the temperature T ₁ of the first molten salt bath or the melting and burning bath at 450 캜 or lower. On the other hand, the first molten salt bath or the temperature T ₁ of the melting yeonyok there is a fear that the solidification is less than 350 ℃, first molten salt bath or melt yeonyok. The time for immersing the wire rod in the first molten salt bath or the melt-spinning bath needs to be appropriately adjusted so that the step of immersing the wire rod into the second molten salt bath or the melt-spinning bath can be carried out as prescribed.

At the time when the wire rod is immersed in the second molten salt bath or the melting and burning bath: within 5 seconds from the take-out of the wire rod from the first molten salt bath or the melting and burning bath, and from t _s to t _s Seconds later

In the wire rod manufacturing method according to the present embodiment, the time is within 5 seconds after the wire rods are taken out from the first molten bath or the hot melt bath at temperature T ₁ , and from t _s to t _s , the wire rod is immersed in a second molten salt bath having a temperature T ₂ or a melt-spinning bath.

The inventors of the present invention have found that the time from when the wire rod is immersed in the first molten salt bath or the melt-spinning bath until the time when the wire rod is immersed in the second molten bath or the melt-laden bath (i.e., the wire rod is immersed in the first molten bath or melt bath) And the time for transferring the wire from the first molten salt bath or the melt bath to the second molten salt bath or the melting and burning bath) t ₁ and the temperature T ₁ of the first molten salt bath or melt bath, , And the width of variation in the tensile strength of the wire rods was 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 examined. As a result, the results shown in Fig. 3 were obtained.

In Fig. 3, the curve to which the symbol " S " is applied is a curve indicating the temperature and time at which the bainite transformation starts (hereinafter referred to as the S curve). This curve changes according to the composition of the wire rod. The data points shown in Fig. 3 indicate the temperatures T ₁ and t ₁ at the time of manufacturing the wire material with respect to the data points. The wire material on the data point on the left side of the curve is a wire material immersed in the second molten salt bath or melt bath before the initiation of the bainite transformation and the wire material on the data point on the right- 2 It is a wire rod which is immersed in a molten salt bath or a melting and burning bath. In Fig. 3, the dotted line described for each data point represents the thermal history of the wire material with respect to each data point. The variation width of the tensile strength of the wire material of which the kind of the data point is "BAD" is more than 50 N / mm 2, the width of the tensile strength of the wire material of which the data point type is "GOOD" is not less than 40 N / The variation width of the tensile strength of the wire material in which the data point type is "VERY GOOD" is 40 N / mm 2 or less.

As shown in Fig. 3, in the wire material (i.e., the wire material immersed in the second molten salt bath or the melting and burning bath at about the same time as the start of the bainite transformation) relative to the data point near the curve, Was small.

Time t ₁ is such that the wire is immersed in the second molten salt bath or in the molten yeonyok bay time t _s of seconds before ~t _s seconds after the start of the transformation of the pre-existing night, is set appropriately. t _s is a value obtained by the following expression (3).

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath.

The time ts until the bainite transformation of the wire rod is started after the wire rod is immersed in the first molten salt bath or the melt and burning bath and the time t _s are determined by the S curve corresponding to the composition of the wire rod and the S curve corresponding to the composition of the wire rod It depends on the temperature. Therefore, the time t ₁ is obtained by a simulation and / or a preliminary experiment based on the composition of the wire rod material and the temperature of the first molten salt bath or the melting and burning bath. Further, as will be described later, the time from when the wire rod is immersed in the first molten salt bath or the melting and burning bath to when the bainite transformation of the wire rod is started is obtained by detecting the double bond of the wire rod. Therefore, before the wire rod is manufactured, a preliminary investigation for determining the time t ₁ may be performed by the above-described means.

It is not clear why the deviation of the tensile strength of the wire rod is suppressed by immersing the wire rod in the second molten salt bath or melt-burning bath at about the same time as the start of bainite transformation of the wire rod. However, the reason explained below is presumed. When the bainite transformation of the wire material occurs during the immersion in the first molten salt bath or in the melting and burning bath, or during the transfer to the second molten salt bath or the melting and burning bath, the first molten bath or melt bath The wire rod temperature rises during the immersion in the second molten salt bath or the melting and burning bath. In this case, the rise of the wire rod temperature may occur unevenly. This is because, when the wire material is subjected to the heat treatment in the molten salt bath or the melt-spinning bath, the wire material is immersed in the molten salt bath or the melt-spinning bath and then taken out in the state of a coil shape as shown in Fig. 7, for example. In the case where the wire rod in the heat treatment has a coil shape, the temperature rise due to the double heat is larger in the portion where the wire rods overlap each other than in the other portions. This is because the cooling effect by the first molten salt bath or the melting and burning is relatively difficult to reach the portion where the wire rods overlap each other. Therefore, when the start of heating of the wire is delayed due to the elongation of the time t ₁ described above, a variation in the tensile strength of the wire occurs due to an uneven increase in the wire temperature. Further, it is indispensable to increase the manufacturing efficiency of the wire material by making the wire material into a coil shape at the time of heat treatment. The wire material is not immersed in the molten salt bath or the melt-spinning bath in a state in which the wire rods do not overlap each other, unless there is a special reason. On the other hand, when the above-described one hours t ₁ is the shorter, the excess wire than before bay t _s early start of the night transformation immersed in a second molten salt bath or melt yeonyok, since the transformation of the start faster, the higher the transformation starting temperature. In this case, the strength of the wire rod is increased and the ductility of the wire rod is lowered.

In view of the above-mentioned reason, it is most preferable that the dipping of the wire into the second molten salt bath or melt-spinning bath is performed completely simultaneously with the start of the bainite transformation of the wire rod. However, the inventors of the present invention found that if the time between the immersion of the wire and the start of the transformation of the wire is 5 seconds or less, the deviation of the tensile strength of the wire can be sufficiently It is possible to suppress deviation even when the time between the immersion of the wire rod and the start of the transformation of the wire rod exceeds 5 seconds in the case of a wire rod in which the progress of bainite transformation is slow and the temperature rise due to double heat is relatively low , From experimental facts. Based on this finding, in the wire rod manufacturing method according to the present embodiment, the time between the immersion of the wire rod and the start of the transformation of the wire rod is defined by the value t _s determined according to the traveling speed of the bainite transformation. In the wire according to the present embodiment, since t _complete does not become less than 100 seconds, the upper limit value of t _s may be set to 5 seconds.

In most cases, the elapsed time t ₁ between the point of time when the wire rod is immersed in the first molten salt bath or the melt-spinning bath and the point of time when the wire rod is immersed in the second molten bath or the melt bath is preferably 10 to 40 seconds. In view of the composition of the wire material according to the present embodiment, when the time t ₁ is less than 10 seconds or more than 40 seconds, it is difficult to adequately immerse the wire material in the subsequent second molten salt bath or melt bath.

The wire rod needs to be immersed in the second molten salt bath or the melt burning bath within 5 seconds after being taken out of the first molten salt bath or the melt burning bath, in addition to the above-mentioned provision. If the time between the extraction of the wire rod and the immersion, that is, the feeding time of the wire rod is more than 5 seconds, the temperature of the wire rod may fluctuate during the feeding of the wire rod. In the second molten salt bath or the melt-spinning bath is extremely difficult.

The time point at which the bainite transformation is started in the wire material in the first molten salt bath or the melting and burning bath may be determined by detecting the double heat (transformation heat generation) of the wire rods. The term " double heat " in this embodiment means a phenomenon in which the temperature of the wire rod rises due to the initiation of bainite transformation in the wire rod. The double heat can be detected, for example, by comparing the temperature of the wire drawn after being immersed in the first molten salt bath or the melting and burning bath and the temperature of the first molten salt bath or melting and burning bath. When the temperature of the wire rod is higher than the temperature of the first molten salt bath or the melting and burning bath, it is judged that double wire is generated in the wire rod. The shortest immersion time t _min that can generate double heat in the wire rod can be obtained by examining the presence or absence of double heat in each of the wire rods in which the immersion time in the first molten salt bath or the melt bath is varied. The time point at which the wire rod has passed t _min after immersing the wire rod in the first molten bath or melt bath can be regarded as the point in time at which the bainite transformation in the wire rod is started. In this manner, it is more preferable to obtain the timing at which the bainite transformation starts in the wire rods in advance using the double heat, and to produce the wire rods based on the timing.

Further, when the time immersed in the first molten salt bath or the melt bath is less than 5 seconds, it can not be judged whether or not a double heat is generated in the wire rod even if the temperature of the wire rod is higher than the first molten salt bath or the molten soft bath. This is because the temperature of the wire rod may increase due to an insufficient immersion time, not a double heat.

Temperature of the second molten salt bath or melting and burning bath: 530 to 600 ° C

When the wire rod is taken out from the second molten salt bath or melt-spinning bath: the time point after the bainite transformation is completely finished

Substantially simultaneously with the initiation of bainite transformation of the wire rod, the wire rod is immersed in the second molten salt bath at temperature T ₂ or in the melt-spinning bath. The temperature T ₂ is 530 to 600 ° C. Thereby, the wire rod is rapidly heated to a temperature of 530 to 600 占 폚 (FIG. 1 (c)) and can be corrected at the above temperature until the bainite transformation is completely completed. Simultaneously with the start of the bainite transformation of the wire rod, if the wire rod is rapidly heated to a temperature of 530 to 600 ° C, the spacing of the cementite in the bainite increases. As a result, the strength of the wire rod is lowered as compared with the case where the rapid heating is not performed. If the temperature of the second molten salt bath or melt bath is below 530 占 폚 or exceeds 600 占 폚, it takes a long time until the end of bainite transformation. Therefore, the temperature of the second molten salt bath or the melt bath is set to 530 to 600 占 폚 in order to surely complete the bainite transformation in a short time. The heating rate at the time of heating the wire rod to the above-mentioned temperature range is not particularly limited. However, in order to shorten the time until completion of the bainite transformation, the heating rate is preferably as high as possible, specifically, 10 to 50 占 폚 / second. This heating rate is obtained by immersing the wire into a molten salt bath or melt bath at a temperature of 530 to 600 占 폚. When the wire rod is taken out from the second molten salt bath or melt-laden bath before the bainite transformation is completed, MA is generated in the wire rod, and this MA may lower the workability of the wire rod.

After the initiation of the bainite transformation during the immersion of the wire material in the first molten salt bath or melt-spinning bath, the wire material is directly corrected, and a dense bainite structure grows. The wire material in which the dense bainite structure is grown has a higher strength than the wire material which is rapidly heated at the same time as the start of bainite transformation. Therefore, in the wire material according to the present embodiment, the wire material is rapidly heated to widen the interval of the deposited cementite, thereby lowering the strength.

(Hereinafter sometimes referred to as " steel wire according to the present embodiment ") having excellent resistance against delayed fracture according to the present embodiment is obtained by drawing a wire material according to the present embodiment having excellent freshness characteristics. The drawing process may be a normal drawing process, and the reduction ratio is not particularly limited. Since the steel wire according to the present embodiment has excellent resistance to delayed fracture, the use of the steel wire is greatly expanded.

Example

Next, the embodiment of the present invention will be described, but the conditions in the embodiment are examples of conditions employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to this one conditional example . The present invention can adopt various conditions as long as the objects of the present invention are achieved without departing from the gist of the present invention.

(Example 1)

The overlaid steel strips having the composition shown in Table 1 were rolled with a wire having a wire diameter as shown in Table 2, and bainite transformation was completed under the temperature conditions shown in Table 2. (N / mm < 2 >), an average cross-sectional shrinkage percentage value (%) and a variation width (N / mm < 2 >) of the tensile strength after the bainite transformation was completed. The average tensile strength of the wire is an average value of the tensile strength of each test piece of 8 lengths of 400 mm obtained by dividing the wire having a length of 3200 mm into eight elements having the same length. The average value of the sectional shrinkage factor of the wire rod is an average value of the sectional shrinkage rate of each test piece of 8 lengths of 400 mm obtained by dividing the wire rod having a length of 3200 mm into 8 elements having the same length. The deviation width of the tensile strength of the wire rod is the difference between the maximum value and the minimum value among the tensile strengths of the test specimens of 8 lengths of 400 mm obtained by dividing the wire rod having a length of 3200 mm into 8 elements having the same length. The measurement results are shown in Table 2 together. The heating rate when the wire rod was immersed in the second molten salt bath or the melt-burning bath was 10 to 50 占 폚 / second.

In Table 2, T ₀ is the temperature of the wire rod immersed in the first molten salt bath or melt-spinning bath, T ₁ is the temperature of the first molten bath or melting and burning bath, t ₁ is the number of times the wire rod is immersed in the first molten bath or melting bath ΔT is the temperature elevated by immersing the wire into the second molten salt bath or melt bath, T ₂ is the temperature of the second molten bath or melt bath, TS upper limit (N / mm 2), TS maximum is the maximum value of the tensile strength (N / mm 2), TS minimum is the minimum value of the tensile strength (N / mm 2), TS is the maximum value of the tensile strength The upper limit value of the average tensile strength and the upper limit value of the average sectional shrinkage value calculated from the formula 2 and the RA average are the average sectional shrinkage value (%) and the TS deviation width (N / mm 2) ), RA maximum is the maximum value (%) of the section shrinkage value, RA minimum A Min (%), RA RA is a maximum deviation width with RA minimum difference (%) of reduction of area value. The immersion time t of the wire rods in the first molten salt bath or the melt-spinning bath at the time of manufacturing the inventions of Nos. 1 to 7 is determined by the fact that the immersion of the wire rods in the second molten bath or the melt- Are selected appropriately so as to be approximately simultaneous. In Comparative Example No. 8, the wire rod was not immersed in the second molten bath or the melt bath. In the comparative examples Nos. 9 and 10, the wire rod was immersed in the second molten salt bath or the melt-spinning bath after elapse of a long time after the initiation of bainite transformation. In the inventive examples 1 to 7, comparative example No. 9 and comparative example No. 10, the wire rod was immersed in the second molten bath or melt bath after being taken out from the first molten bath or melt- .

From Table 2, in the inventions of Nos. 1 to 7, TS averages and RA averages satisfy the equations 1 and 2, and the TS deviation width was 50 N / mm 2 or less. As a result, in the inventions of Nos. 1 to 7, the delayed fracture resistance was improved, and it was found that no breakage occurred in the wire bonding and in the binding state.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to provide a wire rod which is lower in strength and higher in ductility than pearlitic steel and which is capable of suppressing cutting during wire bonding or in a bundled state, , Overlaid bainite steel wire produced by using the wire material, and a production method for stably producing them. Therefore, the present invention is highly available in the steel industry.

1: wire section
11: center
12:
13: Middle part
2: shooting area

Claims (12)

As the unit mass%
C: greater 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%
≪ / RTI >
The balance being Fe and an impurity,
Wherein the metal structure comprises 90 to 100% by area of bainite,
The average value TS of the tensile strengths of the respective test pieces was measured in units of N / mm < 2 > in the case where test pieces having 8 lengths of 400 mm were produced by dividing the wire material having a length of 3200 mm into 8 equal length elements, Satisfaction,
The difference between the maximum value and the minimum value of each tensile strength of each of the test pieces is 50 N / mm 2 or less,
Wherein the average value RA of the cross-sectional shrinkage percentage values of the respective test pieces satisfies the following formula (2) in unit of%.

Where [C] is the C content of the wire rod expressed in unit mass%, [TS] is the average value TS of the tensile strength in N / mm 2, [RA] Is the above average value RA of the values. A superalloy bainite steel wire characterized by being obtained by drawing the wire according to claim 1. The method for producing a wire according to claim 1,
As the unit mass%
C: greater 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%
And the remainder being Fe and impurities to obtain a wire rod,
A step of immersing the wire rod at 850 to 1050 占 폚 in a first molten bath or a hot melt bath at 350 to 450 占 폚 and then taking out the wire rod from the first molten bath or melt-
The time of within 5 seconds from the take-out, and in a bay of time t _s after the start of the second before ~t _s second night transformation of the wire, immersing the wire in a second molten salt bath or melt yeonyok of 530~600 ℃ The process,
And removing the wire rod from the second molten salt bath or melt bath after the bainite transformation is completely completed.

t _complete indicates the time from the start of the bainite transformation of the wire to the end of the wire when the wire is immersed in the first molten salt bath or the melt bath. The method of claim 3,
Wherein an elapsed time between a point of time when the wire rod is immersed in the first molten salt bath or the melting and burning bath and a point of time when the wire rod is immersed in the second molten bath or melt bath is 10 to 40 seconds . The method of claim 3,
Wherein the determination is made by detecting the double refinement of the wire rod at the time point at which the bainite transformation is started in the wire rod in the first molten salt bath or the melting and burning bath. The method for producing the overlaid bainite steel wire according to claim 2,
As the unit mass%
C: greater 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%
And the remainder being Fe and impurities to obtain a wire rod,
A step of dipping the wire rod at 850 to 1050 占 폚 in a first molten bath or a hot melt bath at 350 to 450 占 폚 and then taking out the wire rod from the first molten bath or melt-
The time of within 5 seconds from the take-out, and in a bay of time t _s after the start of the second before ~t _s second night transformation of the wire, immersing the wire in a second molten salt bath or melt yeonyok of 530~600 ℃ The process,
Removing the wire rod from the second molten bath or melt bath after the bainite transformation is completely finished,
And a step of subjecting the wire rod taken out from the second molten salt bath or melt bath to a wire drawing process.

t _complete indicates the time from the initiation of the bainite transformation of the wire rod to the end of the wire rod in the unit of second when the wire rod is continuously immersed in the first molten salt bath or melt bath. The method according to claim 6,
Wherein the time during which the wire rod is immersed in the first molten salt bath or the melt-spinning bath is 10 to 40 seconds. The method according to claim 6,
Wherein the determination is made by detecting the double refinement of the wire rod at the time point at which the bainite transformation is started in the wire rod in the first molten salt bath or the melting and burning bath. The method for producing the overlaid bainite steel wire according to claim 2,
As the unit mass%
C: greater 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%
And the remainder being Fe and impurities, to obtain a steel wire,
Immersing the steel wire at 850 to 1050 占 폚 in a first molten salt bath or a melting and burning bath at 350 to 450 占 폚 and then taking out the steel wire from the first molten bath or melt-
The time of within 5 seconds from the take-out, also at the time of t seconds before ~t _{s s} seconds after the start of bainite transformation of the steel wire, immersing the steel wire in the second molten salt bath or melt yeonyok of 530~600 ℃ The process,
And a step of taking out the steel wire from the second molten bath or melt bath after the bainite transformation is completely finished.

t _complete indicates the time from the initiation of the bainite transformation of the steel wire to the end of the bainite transformation in the unit seconds when the steel wire is continuously immersed in the first molten salt bath or melt bath. 10. The method of claim 9,
Characterized in that the elapsed time between the point at which the steel wire is immersed in the first molten salt bath or the melting and burning bath and the point at which the steel wire is immersed in the second molten bath or melt bath is 10 to 40 seconds, A method of manufacturing a steel wire. 10. The method of claim 9,
Wherein the determination is made by detecting the double refinement of the steel wire at the time point at which the bainite transformation is started in the steel wire in the first molten salt bath or the melting and burning bath. 12. The method according to any one of claims 9 to 11,
Further comprising a step of subjecting the steel wire taken out from the second molten salt bath or melt bath to a drawing process.

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