JP6922726B2 - Hot rolled wire - Google Patents

Description

The present invention relates to hot-rolled wire rods, and in particular, for high-strength steel wires (for example, steel cords, bead wires, wire ropes, PC steel wires, saw wires, etc.) manufactured through a cold wire drawing process after hot rolling. It relates to a hot-rolled wire rod as a material.

High-strength steel wires used for steel cords such as bead wires and wire ropes have various wire diameters and strengths according to their respective specifications. These high-strength steel wires are rarely used as single wires, and are finally used in a state where a plurality of high-strength steel wires are twisted (twisted). Therefore, high-strength steel wire is required to have ductility (twisting characteristics) sufficient to withstand twisting.

In addition, these high-strength steel wire products are usually made of high-carbon steel wire with a C content of about 0.7 to 0.9%, and undergo various processes such as wire drawing, heat treatment, plating, and twisting. Manufactured. In particular, steel cords and the like are drawn and reduced in diameter, but if the amount of processing at this time is large, ductility including twisting characteristics is lowered, and disconnection is likely to occur. In order to suppress this disconnection, heat treatment before wire drawing and heat treatment during dry wire drawing (so-called "intermediate heat treatment") are performed, but it is desirable to omit this heat treatment from the viewpoint of cost and productivity. ing.

On the other hand, these high-strength steel wires are required to have higher strength in order to reduce manufacturing costs and differentiate products. In response to such demands for higher strength of steel wire, there are means such as increasing the amount of C, but in hypereutectile steel with an amount of C exceeding 0.9%, the hot-rolled wire is pre-eposited cementite. Is deposited, and it is said that the ductility is reduced. Therefore, various developments have been made on hypereutectoid steel in order to achieve both strength and ductility after wire drawing.

Patent Document 1 states that a wire drawing wire having high strength and high ductility can be obtained by controlling the total content of Si and Cr and the lamella spacing. Further, as a method for manufacturing this wire drawing wire, it is said that the heated wire rod is cooled at 10 to 20 ° C./s.

Patent Document 2 states that by controlling the shape of lamella cementite according to lead patenting conditions, a steel wire having excellent strength and ductility can be obtained after wire drawing.

International Publication No. 2009/084811 International Publication No. 2016/024635

However, in the examples described in Patent Document 1, the level at which the ductility was evaluated is the wire drawing material obtained by subjecting the hot rolled wire material to lead patenting and then wire drawing, and the wire drawing material using the hot rolled wire material. Has not been evaluated. Further, the control of the amount of Si and the amount of Cr and the control of the lamella interval as described in Patent Document 1 are insufficient to secure the strength and ductility of the wire drawing material.
Further, Patent Document 2 is premised on lead patenting and does not cover hot-rolled wire rods.
Further, Patent Document 2 evaluates a wire drawing material produced by wet wire drawing. On the other hand, in a wire drawing material manufactured by dry wire drawing, which has a high processing heat generation, it is possible to further improve the wire drawing workability and the ductility of the wire drawing material by controlling the composition and structure of the hot rolled wire material which is the material. It is said that it can be expected, but the details have not been clarified yet, and further examination is desired.

The present invention has been made to solve the above problems, and can have excellent strength and ductility in a wire drawing material or a brewing material (steel wire) that has undergone bluing treatment after wire drawing. An object of the present invention is to provide a hot-rolled wire rod having a hypereutectility composition.

The present inventors have produced hot-rolled wire rods in which the metallographic structure and tensile strength are controlled under various hot-rolling conditions using hypereutectoid steel having a carbon concentration of 0.90% to 1.10%. Using these hot-rolled wire rods, wire drawing and bluing treatment were performed, and the effects of the structure and tensile strength of the hot-rolled wire rod on the mechanical properties of the wire drawn and bluing material were examined in detail. As a result, by controlling the C amount, Si amount, Cr amount, Mn amount, shape of lamella cementite, area ratio of proeutectile cementite and bainite of the hot-rolled wire rod, and setting the tensile strength in an appropriate range, such heat can be obtained. Obtaining high strength and high ductility characteristics in wire drawing materials that have been subjected to wire drawing with a machining strain of 2.0 or more and brewing materials (steel wires) that have been brewed from the wire drawing material. The present invention was reached with the finding that

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] In terms of mass%, C: 0.99 to 1.10%, Si: more than 0.40% and 0.80% or less, Mn: 0.10 to 0.70%, Cr: 0.10 to 0. It contains 40%, is limited to Al: 0.003% or less, P: 0.020% or less, S: 0.010% or less, and satisfies the following formulas (1) and (2) in mass%, and the balance. Is composed of Fe and impurities. A region consisting of any one or more of martensite and having a lamella cementite length of 1.5 μm or less in the central portion has an area ratio of 20% or less, and in the central portion, proeutectoid cementite A hot-rolled wire rod having an area ratio of 0.50% or less and a total area ratio of grain boundary ferrite, bainite and martensite of 8.0% or less.
0.50 ≤ Si (%) + Cr (%) ≤ 0.90 ... (1)
0.40 ≤ Cr (%) + Mn (%) ≤ 0.80 ... (2)
The element symbol in the above formulas (1) and (2) means the content of the element in mass%.

[2] Further, in mass%, Ni: 0.50% or less, Co: 1.00% or less, Mo: 0.20% or less, B: 0.0030% or less, Cu: 0.15% or less. The hot-rolled wire rod according to the above [1], which contains one kind or two or more kinds selected from the group.

[3] The first portion is a region having a length of 20 m arbitrarily selected between a position 500 m away from one end of the wire and a position 600 m away from the one end, and a position 500 m away from the other end of the wire and the above. When a region having a length of 20 m arbitrarily selected from the other end at a distance of 600 m is used as the second portion, the average tensile strength TSave, which is the average of the tensile strengths of the first portion and the second portion, respectively. Satisfies the following formula (3), and the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin is 100 MPa or less in each section obtained by dividing the first portion and the second portion into five equal parts in the length direction. The hot-rolled wire rod according to the above [1] or [2], wherein the difference in average tensile strength between adjacent sections is 20 MPa or less.
1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 200 <T Save (MPa) <1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) +300 … (3)
The element symbol in the above formula (3) means the content of the element in mass%.

[4] The above [1] to [3], wherein the lamella spacing L observed in the central portion satisfies the following formula (4), and the standard deviation of the lamella spacing L is 30 nm or less. The hot-rolled wire rod according to any one item.
-65 x (Si (%) + Cr (%)) + 130 <L (nm) ... (4)
The element symbol in the above formula (4) means the content of the element in mass%.

By using the hot-rolled wire rod of the present invention as a material, excellent strength and ductility can be obtained in a wire drawing material or a brewing material that has undergone brewing treatment after wire drawing.
Further, by using the hot-rolled wire rod of the present invention as a material, it is suitable for applications such as PC steel wire and steel cord, and has excellent wire drawing workability and excellent ductility after wire drawing and bluing. Stable production of high-strength steel wire is possible.

Hereinafter, embodiments of the hot-rolled wire rod of the present invention (hereinafter, may be simply referred to as “wire rod”) will be described. It should be noted that this embodiment is described in detail in order to better understand the gist of the invention, and therefore, the present invention is not limited unless otherwise specified.

<Ingredient composition>
First, the steel composition of the hot-rolled wire rod of the present embodiment will be described. Hereinafter, the unit of the content of each element is mass%.

C: 0.99-1.10%
C is an essential element for imparting the required strength of the steel material. If it is less than 0.90%, the tensile strength of the wire drawing material and the bluing material is lowered, so that C is contained in an amount of 0.90% or more. Preferably, the amount of C is 0.95% or more, more preferably 1.00% or more.
On the other hand, when the amount of C exceeds 1.10%, the pro-eutectile cementite increases and disconnection occurs frequently, and the strength of the hot-rolled wire becomes excessively high, resulting in a decrease in wire drawing workability and a wire drawing material. Causes a decrease in ductility. Therefore, the amount of C is set to 1.10% or less. Preferably, the amount of C is 1.08% or less.

Si: More than 0.40% and 0.80% or less Si has the effect of increasing the ferrite strength in pearlite, improving the ductility of the wire drawing material, and suppressing the decrease in tensile strength during bluing treatment. In order to effectively exert these effects, it is necessary that Si is contained in an amount of more than 0.40%. Preferably, the amount of Si is 0.50% or more.
However, if Si is excessively contained, SiO ₂ inclusions that are harmful to the wire drawing workability are likely to be generated, the ferrite strength is excessively increased, and the ductility of the wire drawing material is lowered. Therefore, the upper limit of the amount of Si is set to 0.80% or less. Preferably, the amount of Si is 0.70% or less.

Mn: 0.10 to 0.70%
Mn is not only useful for deoxidation and desulfurization, but also has the effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is an element useful for obtaining a pearlite-based structure. In order to effectively exert such an action, it is necessary to contain Mn of 0.10% or more. Preferably, the amount of Mn is 0.15% or more.
However, even if Mn is excessively contained, the above effect is saturated, supercooled structures such as bainite and martensite are likely to occur in the cooling process after hot rolling, and it takes a long time to complete the transformation. This leads to a decrease in productivity and an increase in equipment costs. Therefore, the upper limit of the amount of Mn is set to 0.70% or less. Preferably, the amount of Mn is 0.50% or less.

Cr: 0.10 to 0.40%
Cr increases the work hardening rate of pearlite, and higher tensile strength can be obtained by wire drawing with a low strain amount. In addition, Cr has the effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In order to effectively exert such an action, it is necessary to contain Cr in an amount of 0.10% or more. Preferably, the amount of Cr is 0.15% or more.
However, when the amount of Cr exceeds 0.40%, these effects are saturated, hardenability becomes high, and supercooled structures such as bainite and martensite are likely to occur in the cooling process after hot rolling, and transformation is completed. It takes a long time to complete, leading to a decrease in productivity and an increase in equipment cost. Therefore, the upper limit of the amount of Cr is set to 0.40% or less. Preferably, the amount of Cr is 0.30% or less.

Si + Cr: 0.50 to 0.90%
Further, by adding the above Si and Cr in a composite manner, the strength and ductility of the wire drawing material and the bluing material can be improved. When the total amount of Si and Cr is less than 0.50%, the tensile strength of the wire drawing material and the bluing material is insufficient, while when the total amount of Si and Cr exceeds 0.90%, the ductility of the wire drawing material and the bluing material is high. descend. Therefore, make sure that the total amount of Si and Cr satisfies the following equation (1). The preferable lower limit of the total amount of Si and Cr is 0.55% or more, and the upper limit is 0.80% or less.
0.50 ≤ Si (%) + Cr (%) ≤ 0.90 ... (1)

Mn + Cr: 0.40 to 0.80%
By adding the above Mn and Cr in combination, the effect of suppressing the formation of proeutectoid cementite and grain boundary ferrite during hot rolling is improved, and the tensile strength of hot rolled wire and wire drawing is increased. can get. If the total amount of Mn and Cr is less than 0.40%, these effects cannot be sufficiently obtained, while if the total amount of Mn and Cr exceeds 0.80%, the hardenability becomes excessively high and during hot rolling. Supercooled structures such as bainite and martensite are likely to occur, and it takes a long time to complete the transformation, leading to a decrease in productivity and an increase in equipment cost. Therefore, make sure that the total amount of Mn and Cr satisfies the following formula (2). The preferable lower limit of the total amount of Mn and Cr is 0.45% or more, and the upper limit is 0.60% or less.
0.40 ≤ Cr (%) + Mn (%) ≤ 0.80 ... (2)

Al: 0.003% or less Al reacts with O to _{generate hard oxides such as Al 2} O ₃ , which causes a decrease in wire drawing workability and ductility of the wire drawing material. Therefore, the upper limit is limited to 0.003% or less.

P: 0.020% or less P is an impurity. If the P content exceeds 0.020%, segregation may occur at the grain boundaries and the wire drawing workability may be impaired. Therefore, the P content is limited to 0.020% or less. Preferably, the P content is limited to 0.015% or less. Further, since the smaller the P content is, the more desirable it is, the lower limit of the P content may be 0%. However, it is technically not easy to set the P content to 0%, and even if the P content is stably set to less than 0.001%, the steelmaking cost is high. Therefore, the lower limit of the P content may be 0.001% or more.

S: 0.010% or less S is an impurity. If the S content exceeds 0.010%, coarse MnS may be formed and the wire drawing workability may be impaired. Therefore, the S content is limited to 0.010% or less. Preferably, the S content is limited to 0.008% or less. Further, since the smaller the S content is, the more desirable it is, the lower limit of the S content may be 0%. However, it is technically not easy to reduce the S content to 0%, and even if the S content is stably set to less than 0.001%, the steelmaking cost is high. Therefore, the lower limit of the S content may be 0.001% or more.

The basic composition of the hot-rolled wire rod according to the present embodiment is as described above. The elements Ni, Co, Mo, B, and Cu shown below may not be contained. If you want to obtain the effects described below, Ni: 0.5% or less, Co: 1.00% or less, Mo: 0.20% or less, B: 0.0030% or less, Cu: 0.15% or less It may contain one or more selected from the group consisting of.

Ni: 0.50% or less Ni has the effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In addition, it is an element that enhances the toughness of the wire drawing material. In order to obtain these effects, it is desirable that Ni is contained in an amount of 0.10% or more. On the other hand, if Ni is excessively contained, the hardenability becomes excessive, supercooled structures such as bainite and martensite are generated in the cooling process after hot rolling, and the wire drawing workability is lowered. Therefore, the upper limit is 0.50. % Or less. Preferably, the amount of Ni is 0.30% or less.

Co: 1.00% or less Co is an element effective for suppressing the precipitation of proeutectoid ferrite in rolled wire. Moreover, it is an effective element for improving the ductility of the wire drawing material. In order to effectively exert such an action, it is preferably contained in an amount of 0.10% or more. On the other hand, even if Co is excessively contained, the effect is saturated and economically wasteful, so the upper limit is set to 1.00% or less. Preferably, the amount of Co is 0.90% or less.

Mo: 0.20% or less Mo has the effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In order to obtain this effect, it is desirable that Mo is contained in an amount of 0.05% or more. However, if the amount of Mo exceeds 0.20%, the hardenability becomes excessive and supercooled structures such as bainite and martensite are likely to occur in the cooling process after hot rolling, so the upper limit is 0.20% or less. And said. Preferably, the amount of Mo is 0.15% or less.

B: 0.0030% or less B is an element that is concentrated at the grain boundaries and is effective in suppressing proeutectoid ferrite. In order to obtain this effect, B is preferably contained in an amount of 0.0002% or more. On the other hand, if B is excessively contained _{, carbides such as Fe 23} (CB) ₆ are formed in austenite and the wire drawing workability is lowered. Therefore, the upper limit is set to 0.0030% or less. Preferably, the amount of B is 0.0005 to 0.0020%.

Cu: 0.15% or less Cu is an element that can be expected to have the effect of increasing the strength of rolled wire and wire drawn by precipitation hardening. In order to exert this effect, it is preferably contained at least 0.05% or more. On the other hand, excessive inclusion of Cu causes intergranular embrittlement and causes defects. Therefore, the upper limit is set to 0.15% or less. Preferably, the amount of Cu is 0.12% or less.

The hot-rolled wire rod of the present embodiment contains the above components, and the balance is substantially formed of Fe and impurities. In addition, other elements can be contained in a trace amount within a range that does not impair the action and effect of the present invention.
Here, the impurity refers to an impurity mixed in from ore, scrap, or a manufacturing environment as a raw material when a steel material is industrially manufactured.

<Metal structure>
Next, the metal structure of the hot-rolled wire rod according to the present embodiment will be described.
As described above, the metal structure of the hot-rolled wire according to the present embodiment has pearlite and the residual structure at the center within R / 3 from the center of the cross section of the wire when the radius of the hot-rolled wire is R. It consists of. Organizations other than the rest of the organization are pearlite. The residual structure is composed of any one or more of proeutectoid cementite, grain boundary ferrite, bainite and martensite.

Furthermore, in order to achieve both tensile strength and ductility (twisting characteristics) of the wire drawing material after wire drawing or the brewing material that has been subjected to bluing treatment after wire drawing, the shape of cementite at the center of the wire is controlled. This is very important. Specifically, when the radius of the hot-rolled wire is R, the length of cementite (lamella cementite) in pearlite is 1.5 μm or less in the center within R / 3 from the center of the cross section of the wire. (Hereinafter, also referred to as "micro region") is reduced. If the area ratio of the minute region exceeds 20%, it is difficult to achieve both strength and twisting characteristics in the wire drawing material and the bluing material. Therefore, the area ratio of the minute region in the central portion is set to 20% or less. In order to achieve more stable strength and twisting characteristics, the area ratio of the area where the length of lamella cementite is 1.5 μm or less is preferably 15% or less, and more preferably 10% or less.

In addition, proeutectoid cementite, grain boundary ferrite, bainite, and martensite may serve as propagation paths for fracture, and if the area ratio increases, the tensile strength and ductility of the wire drawing material will decrease. Therefore, when the radius of the wire is R, the area ratio of proeutectoid cementite is 0.50% or less at the center within R / 3 from the center of the cross section of the wire, and the grain boundary ferrite, bainite and martensite The total area ratio shall be 8.0% or less. Preferably, the area ratio of proeutectoid cementite is 0.25% or less, and the total area ratio of grain boundary ferrite, bainite and martensite is 5.0% or less. More preferably, the area ratio of proeutectoid cementite is 0.10% or less, and the total area ratio of grain boundary ferrite, bainite and martensite is 4.5% or less. The area ratio of pearlite is 100% minus the area ratio of the remaining structure.
The area ratio of proeutectoid cementite, grain boundary ferrite, bainite, and martensite may be 0% (that is, the area ratio of the pearlite structure is 100%), but this is the first component system of the present invention as described above. It is difficult to completely suppress the precipitation of cementite, grain boundary ferrite, bainite, and martensite, and achieving an area ratio of 0% requires extremely excellent cooling capacity, resulting in an increase in equipment cost. Leads to.

Further, the structure other than the central part is not particularly limited because it does not particularly affect the characteristics aimed at by the present invention, but pearlite is the main structure and the balance is proeutectoid cementite, grain boundary ferrite, bainite and martensite. It is preferable that it is composed of any one or more of the above. Further, the area ratio of the tissue other than the central part may be the same as or different from the area ratio of the tissue in the central part.

<Tensile strength>
When the hot-rolled wire rod is placed on the cooling conveyor immediately after hot-rolling, it is in a ring state in which a part of the hot-rolled wire is overlapped while being continuously displaced, and is cooled in that state. That is, since a plurality of ring-shaped wire rods have some overlaps and sparse and dense differences between the rings, the ring-shaped hot-rolled wire rod during cooling can be used in each ring and between adjacent rings. A temperature distribution occurs, resulting in variations in tensile strength.
Since the work hardening ability of hot-rolled wire depends on the tensile strength, if the variation in tensile strength of hot-rolled wire increases, the variation in tensile strength of the drawn wire becomes even larger, and during wire drawing in actual machine manufacturing, This leads to disconnection during twisting and variations in the characteristics of the final product.

Therefore, in the present embodiment, it is desirable to specify the tensile strength of the hot-rolled wire rod as follows.
A region having a length of 20 m arbitrarily selected between a position 500 m away from one end of the hot-rolled wire and a position 600 m away from the one end is set as the first part, and a position 500 m away from the other end of the wire and the above. When a region having a length of 20 m arbitrarily selected from the other end at a distance of 600 m is used as the second portion, the average tensile strength TSave in the first portion and the second portion satisfies the following formula (3). Further, in each section obtained by dividing the first part and the second part into five equal parts in the length direction, the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin is 100 MPa or less, and the average tensile strength of the adjacent sections is 100 MPa or less. The difference in strength is 20 MPa or less.
1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 200 <T Save <1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 300 ... (3)

The average tensile strength TSave (MPa) in the first portion and the second portion of the hot-rolled wire rod falls within the range specified by the above formula (3) according to the content (mass%) of C, Si, and Cr. Is preferable. If the TSave of the hot-rolled wire rod is less than the lower limit value shown in the equation (3), the tensile strength of the wire drawn wire rod or the bluing material may decrease. On the other hand, when the TSave of the hot-rolled wire exceeds the upper limit value shown in the formula (3), the work hardening rate at the time of wire drawing increases, the tensile strength of the wire drawing material and the brewing material increases excessively, and the ductility decreases. There is a risk of More preferably, the constant term on the left side of the equation (3) is +210 MPa, the constant term on the right side is +290 MPa, and even more preferably, the constant term on the left side of the equation (3) is +220 MPa and the constant term on the right side is +280 MPa. The preferred T Save range is shown by the formula (3'). The element symbol in the formulas (3) and (3') means the content of the element in mass%.
1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 210 <T Save <1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 290 ... (3')

Further, in each section in which the first part and the second part of the hot-rolled wire rod are divided into five equal parts in the length direction, the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin is 100 MPa or less, and adjacent sections. It is desirable that the difference in average tensile strength between the two is 20 MPa or less.
By controlling the components within the present invention, the hardenability, and the cooling rate of 650 ° C. or lower, which will be described later, the variation in the tensile strength of the hot-rolled wire rod is reduced, and as a result, the elongation It is possible to suppress variations in the tensile strength of the wire rod. In addition, it also leads to reduction of wire breakage during wire drawing and twisting in actual machine manufacturing and variation in characteristics of final products. As a result, the quality and characteristics of the wire drawing material and the brewing material can be stabilized even if the wire drawing process is performed while the hot rolling material is used.

<Lamellar interval>
Further, in the present embodiment, it is desirable to control the standard deviations of the lamella spacing L and the lamella spacing L in the center of the wire rod from the viewpoint of achieving both ductility and strength. Specifically, when the radius of the wire rod is R, the lamella spacing L at the center within R / 3 from the center of the cross section of the wire rod satisfies the following formula (4), and the standard deviation of the lamella spacing L is 30 nm or less. Is preferable.
When the lamella spacing L is small, the strength of the wire drawing material and the bluing material is increased, but the ductility is decreased. Further, when the variation of the lamella spacing L becomes large, the ductility of the wire drawing material and the bluing material decreases. Therefore, it is preferable that the lamella spacing L satisfies the following formula (4) and the standard deviation of the lamella spacing L is 30 nm or less. On the other hand, if the lamella spacing L is excessively large, the tensile strength of the hot-rolled wire rod and the work hardening rate during wire drawing are lowered, so that the tensile strength of the wire drawing material and the brewing material is lowered. The upper limit of is preferably 150 nm or less.
-65 x (Si (%) + Cr (%)) + 130 <L (nm) ... (4)

The wire diameter of the hot-rolled wire material affects the cooling rate after winding, and as a result, affects the metal structure, tensile strength, and the like. If the diameter of the wire rod exceeds 6.0 mm, the cooling rate at the center of the wire rod tends to be slow, and proeutectoid cementite is likely to be generated. On the other hand, if the diameter of the wire rod is less than 3.0 mm, it is difficult to manufacture, the production efficiency is lowered, and the cost of the hot-rolled wire rod is increased. Therefore, the wire diameter of the hot-rolled wire rod of the present embodiment is set to 3.0 to 6.0 mm.

<Measurement method of area ratio of metal structure>
The area ratio of proeutectoid cementite, grain boundary ferrite, and bainite and the area ratio of the region where the length of lamella cementite is 1.5 μm or less can be measured as follows.
The hot-rolled wire is cut, embedded in resin so that the cross section perpendicular to the longitudinal direction can be observed, and then polished with abrasive paper or alumina abrasive grains to obtain a mirror-finished sample. This is corroded with a 3% nital solution or picral, observed using a scanning electron microscope (SEM), and the central part of the sample (with the radius of the wire as R, the distance of R / 3 or less from the center of the wire) is magnified. Take multiple visual fields so that the total observation field area is 0.08 mm ^{2 or more at 2000 times or more.} In each photographed field, mark the object for which the area ratio is to be measured (the area ratio of proeutectoid cementite, grain boundary ferrite, bainite, and the region where the length of lamella cementite is 1.5 μm or less), and image-analyze the marked portion. Measure the area ratio of the tissue. Then, the average of the area ratio of each field of view was taken as the area ratio of each structure of the hot-rolled wire rod. For image analysis, an ordinary image analysis device such as "LUZEX" (manufactured by Nireco Corporation) can be used.
In the present invention, the "length of lamella cementite" is the length in the major axis direction, and the "region where the length of lamella cementite is 1.5 μm or less" is the lamella cementite having a maximum length of 1.5 μm or less. The area of.

<Measurement method of lamella interval L and its standard deviation>
The lamella interval L and its standard deviation can be measured as follows.
The hot-rolled wire is cut, embedded in resin so that the cross section perpendicular to the longitudinal direction can be observed, and then polished with abrasive paper or alumina abrasive grains to make a mirror finish, and five pieces are prepared. These are corroded with a 3% nital solution or picral, and using a field emission scanning electron microscope (FE-SEM), the center of each sample (with the radius of the wire as R, R / 3 or less from the center of the wire). Area) is observed at a magnification of 5000 times or more.
Next, in the observation field of view, a portion where the directions of the lamella cementite are aligned and the lamella cementite is determined not to be tilted with respect to the observation surface is photographed at a magnification of 10000 times or more. If a plurality of lamella cementites satisfy the conditions in the same observation field, the part having the smallest lamella interval is photographed. In the same way, images for 5 fields of view are taken so that the fields of view do not overlap in each sample.
In each photograph, a straight line having a length of 10 intervals of the lamella is drawn perpendicular to the longitudinal direction of the lamella cementite, and the length of the straight line is measured. By dividing the length of the straight line by 10, the lamella spacing at that location can be measured.
The lamella spacing was measured by the same method in all 25 locations of the 5 samples prepared and 5 fields of each sample, and the average was taken as the lamella spacing L of the hot-rolled wire rod, and the lamella spacing of all 25 locations was further measured. The standard deviation is defined as the standard deviation of the hot-rolled wire rod.

<Measurement method of tensile strength of hot-rolled wire>
First, the unsteady part is removed from the hot-rolled wire rod (wire rod coil) wound into a coil, and the length is 20 m from the position 500 m away from one end of the wire rod and the position 600 m away from this one end. Collect the wire (first part). A plurality of eight or more samples are collected at equal intervals from each section obtained by further dividing the collected 20 m wire (first part) into five in the length direction, and subjected to a tensile test. Let the average of them be the tensile strength of the first part. Further, a wire rod (second part) having a length of 20 m was collected from a position 500 m away from the other end of the wire rod and a position 600 m away from the other end of the wire rod, and thereafter, the tensile strength of the second portion was similarly collected. Ask for. The average of the tensile strengths of the obtained first portion and the second portion is defined as the average tensile strength TSave of the hot-rolled wire rod.

Further, in each of the first part and the second part described above, the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin in each section is measured in each section, and the largest value among them is the variation in tensile strength within the section. evaluate.
Further, from the result of the above tensile test, the average tensile strength in each of the above-mentioned sections obtained by dividing each of the first portion and the second portion into five is measured, and the difference in the average tensile strength of the adjacent sections is measured in the length direction of the wire rod. It is assumed that the tensile strength varies.
The total length of the hot-rolled wire rod (wire rod coil) according to the present embodiment is preferably 1200 m or more.

Next, a method for manufacturing the hot-rolled wire rod according to the present embodiment will be described.
The method for producing a hot-rolled wire rod described below is an example, and is not limited to the following procedure and method. Any method can be used as long as the configuration of the hot-rolled wire rod of the present invention can be realized. It is also possible to adopt it.

When the hot-rolled wire rod according to the present embodiment is manufactured, the steel composition, each process, and the conditions in each process so as to satisfy the above-mentioned conditions such as the area ratio of the metal structure and the shape of cementite in pearlite. Should be set.
Further, the manufacturing conditions can be set according to the wire diameter of the wire drawing material after the wire drawing process and the required strength and ductility.

First, as the material to be subjected to hot rolling, ordinary manufacturing conditions can be adopted. For example, steel with the above components is cast, and the cast pieces are lump-rolled to produce steel pieces of a size suitable for wire rod rolling (generally called billets, steel pieces before wire rod rolling) and used for hot rolling. ..

When rolling the wire rod, the steel pieces are heated to 950 to 1150 ° C., and the finish rolling start temperature is controlled to 800 ° C. or higher and 950 ° C. or lower. The rolling temperature of the wire is measured by a radiation thermometer and means the surface temperature of the steel.

The wire rod after finish rolling rises above the finish rolling start temperature due to processing heat generation, but the take-up temperature is controlled to 800 ° C. or higher and 940 ° C. or lower. When the winding temperature is less than 800 ° C., the austenite particle size becomes finer, pro-eutectoid cementite and intergranular ferrite are likely to be precipitated, and the mechanical scale peeling property is also lowered. On the other hand, above 940 ° C., the austenite particle size becomes excessively large, and the wire drawing workability deteriorates. The winding temperature is more preferably 830 ° C. or higher and 920 ° C. or lower, and further preferably 850 ° C. or higher and 900 ° C. or lower.

The hot-rolled wire is transformed from austenite to pearlite during cooling after winding. Therefore, the cooling rate after winding is an important factor that controls the structure, strength, and ductility.
Specifically, after winding, the cooling rate 1 up to 650 ° C. is set to 10.0 ° C./s or more and 40.0 ° C./s or less, and the cooling rate 2 from 650 ° C. to 590 ° C. is set to 5.0 ° C./s. The cooling rate 3 from 590 ° C. to 400 ° C. is cooled at 10.0 ° C./or or higher with s or more and 10.0 ° C./s or lower.

If the cooling rate 1 is less than 10.0 ° C./s, it is difficult to suppress the pro-eutectoid cementite, so the temperature should be 10.0 ° C./s or higher. It is preferably 15.0 ° C./s or higher. On the other hand, if the cooling rate 1 is set to more than 40.0 ° C./s, there is a high possibility that the mechanical scale peeling property will decrease and the cooling equipment cost will increase. Therefore, the cooling rate will be 40.0 ° C./s or less. do. Preferably, it is 30.0 ° C./s or less.

Further, at a cooling rate of 2, if the temperature exceeds 10.0 ° C./s, the transformation temperature decreases, the lamella interval becomes excessively small, the tensile strength becomes excessively high, the variation in tensile strength becomes large, and the lamella cementite length becomes large. Regions with a size of 1.5 μm or less increase. Therefore, the cooling rate 2 is set to 10.0 ° C./s or less. It is preferably 9.5 ° C./s or less, more preferably 9.0 ° C./s or less, and even more preferably 8.0 ° C./s or less. On the other hand, if the cooling rate 2 is less than 5.0 ° C./s, the tensile strength becomes too low, and there is a high possibility that the tensile strength of the wire drawing material or the bluing material becomes insufficient. In addition, the standard deviation of the lamella spacing L becomes large, and the ductility of the wire drawing material and the bluing material may decrease. Therefore, the cooling rate 2 is 5.0 ° C./s or higher, preferably 6.0 ° C./s or higher.

Further, when the cooling rate 3 is less than 10.0 ° C./s, cementite is divided and the region where the lamella cementite length is 1.5 μm or less increases. Therefore, the cooling rate 3 is set to 10.0 ° C./s or higher. Preferably, it is 11.0 ° C./s or higher.

The temperature of the rolled wire was measured with a radiation thermometer. Further, in general, in rolling of a wire rod, after rolling, the rolled wire rod is wound into a ring shape and cooled, and there are a dense portion where the rolled wire rod has a large overlap and a sparse portion where the overlap is small. In the present invention, the temperature of the rolled wire after winding is measured at the place where the rings overlap (dense part).

By having the component composition of the present invention and adjusting the production conditions as described above, the structure and tensile strength of the hot-rolled wire can be within the range of the present invention.

Hereinafter, the present invention will be described in more detail with reference to examples of the hot-rolled wire rod according to the present invention. It is also possible to make appropriate changes to the extent possible, and all of them are included in the technical scope of the present invention.

Tables 1A and B show the chemical composition, Tables 2A and B show the hot rolling conditions, Tables 3A and B show the texture evaluation of the hot rolled wire, and Tables 4A and B show the tensile properties and the tension of the wire drawing and bluing materials. The results of evaluating the characteristics and twisting characteristics are shown. The cooling rates 1 to 3 in Tables 2A and B are as follows. Further, in Tables 1 to 4, the numerical values outside the range of the present invention and the suitable range are underlined.
Cooling speed 1: Cooling speed up to 650 ° C after winding Cooling speed 2: Cooling speed from 650 ° C to 590 ° C Cooling speed 3: Cooling speed from 590 ° C to 400 ° C

A1 to 24 are examples of the present invention, and B1 to 17 are those in which either the component or the hot rolling condition is out of the appropriate range, and the structure and mechanical properties of the hot rolled wire are out of the appropriate range of the present invention. be.

In both the examples of the present invention and the comparative examples, the billet was heated to 1000 to 1200 ° C. in a heating furnace, and then hot rolling was performed by controlling the temperature before finish rolling and the temperature of the steel material increased due to processing heat generation in finish rolling.
After that, it is wound and cooled in a ring shape, and the winding temperature at this time, the cooling rate up to 650 ° C after winding (cooling rate 1 in Table 1), and the cooling rate from 650 ° C to 590 ° C (cooling rate 2). The cooling rate from 590 ° C. to 400 ° C. (cooling rate 3) was set as the value shown in Table 1.

The pro-eutectoid cementite area ratio at the center of the obtained hot-rolled wire and the total area ratio of grain boundary ferrite, bainite, and martensite were evaluated according to the above-mentioned method. In both the examples of the present invention and the comparative examples, the structure at the center of the wire is mainly pearlite, and the balance is one or more composite structures of proeutectoid cementite, grain boundary ferrite, bainite, and martensite. there were.

Further, the measurement of the lamella interval L and the standard deviation of the lamella interval L also followed the above-mentioned method. At the time of measurement, 5 samples were taken from each hot-rolled wire rod, and 5 samples were photographed from each sample in a total of 25 fields of view at a magnification of 1000 to 20000 times, and the standard deviation of the lamella interval and the lamella interval was measured.

Moreover, the tensile test of the obtained hot-rolled wire rod was carried out as follows.
First, among the obtained hot-rolled wire rods, a hot-rolled wire rod (first part) having a length of 20 m is collected from a position (front portion) of 550 m from the rolling tip to the tail end side by 10 m in the front-rear direction. Similarly, a hot-rolled wire rod (second part) having a length of 20 m is collected from a position (tail portion) of 550 m from the rolled tail end to the tip side by 10 m in the front-rear direction.
Next, each of the collected 20 m hot-rolled wire rods (first part and second part) was divided into five in the length direction (1 section; 4 m), and eight samples were taken from each section at equal intervals. A total of 80 pieces of each 20 m hot-rolled wire rod (first part and second part) were sampled and subjected to a tensile test. The average of the tensile strengths of the 80 samples was taken as the tensile strength TSave of the hot-rolled wire rod.
In each of the first part and the second part, the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin in each section was measured, and the largest value among them was defined as "variation in tensile strength within the section".
In addition, the average tensile strength in each of the above-mentioned sections obtained by dividing each of the first part and the second part into five (1 section; 4 m) was measured, the average tensile strengths of the adjacent sections were compared, and the difference was obtained. .. The largest difference among them was defined as the "difference in average tensile strength between adjacent sections".
A tensile test was conducted with the sample length set to 400 mm, the crosshead speed set to 10 mm / min, and the distance between jigs set to 200 mm.

In general, hot-rolled wire rods are often in a ring state (wire rod coil) in which parts of the hot-rolled wire rods overlap while being continuously displaced when cooled after hot-rolling. The length of one ring in this case varies depending on the wire diameter of the wire rod, the manufacturing apparatus, etc., but in general, it is often formed into a ring shape with a number of meters per ring, and in such a case, this embodiment. Each "section" obtained by dividing the first part and the second part (1 section; 4 m) in the above corresponds to this one ring. That is, when the length of one ring is, for example, about 4 mm, a hot-rolled wire rod having a large "variation in tensile strength within a section" is evaluated as having a large "variation in tensile strength within one ring". It is also possible. Similarly, a product having a large "difference in average tensile strength between adjacent sections" can be evaluated as having a large "difference (variation) between adjacent rings".

Using the hot-rolled wire obtained as described above, a wire drawing process (dry wire drawing process) was performed without performing a patenting treatment to obtain a wire drawing material.
In the dry wire drawing process, a true strain of 2.0 or more was processed using a hot-rolled wire rod 10 ring. The dry wire drawing process was performed as a wire drawing pretreatment, the scale was removed by pickling, and then a lime film treatment was performed, and the wire drawing was performed with a surface reduction rate of 17 to 23% per pass.

A tensile test and a twist test were carried out using each of the obtained wire drawing material and a brewing material that had been subjected to a bluing treatment in which the wire drawing material was held at 450 ° C. for 10 seconds in a salt bath.

The tensile test was performed with 3 samples, a sample length of 200 mm, a crosshead speed of 10 mm / min, and a distance between jigs of 100 mm, and the tensile strength was evaluated by averaging them.
In the present invention, the tensile strength of the wire drawing material was set to 2300 MPa or more, and the tensile strength of the bluing material was set to 2100 MPa or more.

In the twisting test, when the diameter of the sample was d (mm), a load of 1% of the tensile strength of each sample was applied at a distance of 100 × d (mm) between jigs, and twisting was applied until the sample was broken. This test was performed three times each, and the average number of total twisted rotations until breaking (hereinafter referred to as the twist value) and whether or not delamination occurred were evaluated.
In the present invention, the twist value of the wire drawing material is 31 rotations or more, and that of the bluing material is 26 rotations or more.

In Table 2, A1 to 24 of the test examples are all examples of the present invention, and all the hot-rolled wire rods are drawn with a true strain of 2.0 or more without performing a patenting treatment. , Both the wire drawing material and the brewing material were obtained with excellent strength and ductility.

On the other hand, since the test examples of B1 to 17 did not satisfy any of the requirements of the present invention, the strength and ductility of the wire drawing material and the bluing material were inferior.

B1 to B12 are examples in which the component range of the present invention is out of range and the tensile strength and twisting characteristics of the wire drawing material or the bluing material are lowered.
B1 is an example in which the twisting characteristics of the wire drawing material and the brewing material are deteriorated as a result of the increase of the proeutectoid cementite and the reduction of the lamella spacing due to the excessive addition of C.
B2 is an example in which the amount of Si is reduced, and B3 and B4 are examples in which the amount of Si and the amount of Si + Cr are reduced, and the tensile strength and twisting characteristics of the wire drawing material and the bluing material are reduced.
B5 and B8 are examples in which the twisting characteristics of the wire drawing material and the bluing material are deteriorated because Si and Si + Cr are excessively added.
Further, since at least one of Cr amount, Mn amount, and Cr + Mn amount is excessive in B6, B7, and B9, martensite is deposited on the hot-rolled wire rod, and the wire is broken.
B10 is an example in which the area ratio of grain boundary ferrite and bainite increases because the amount of Cr decreases, and as a result, the tensile strength and twisting characteristics of the wire drawing material and the bluing material decrease.
B11 is an example in which the amount of Cr + Mn is small and the tensile strength of the hot-rolled wire rod is lowered, so that the tensile strength and ductility of the wire drawing material and the bluing material are lowered.
B12 is an example in which coarse surface flaws are generated in the hot-rolled wire rod due to excessive addition of Cu, and the twisting characteristics of the wire drawing material and the bluing material are deteriorated.
Regarding B13 to B17, although the components are within the range of the present invention, the structure is out of the range, so that the tensile strength and the twisting property of the wire drawing material and the bluing material are deteriorated.

The hot-rolled wire rod for high-strength steel wire of the present invention can have excellent strength and ductility in a wire drawing material or a bluing material (steel wire) that has undergone bluing treatment after wire drawing. Therefore, by using the hot-rolled wire rod of the present invention as a material, high-strength steel wire with excellent ductility after wire drawing and after bluing, which is suitable for applications such as PC steel wire and steel cord, can be stably produced. And the industrial contribution is extremely significant.

Claims (4)

By mass%
C: 0.99-1.10%,
Si: More than 0.40% and less than 0.80%,
Mn: 0.10 to 0.70%,
Cr: 0.10 to 0.40%
Contains,
Al: 0.003% or less,
P: 0.020% or less,
S: Limited to 0.010% or less, and satisfies the following formulas (1) and (2) in mass%, and the balance is composed of Fe and impurities.
When the radius of the wire is R, the metal structure in the center within R / 3 from the center of the cross section of the wire is pearlite, and the remaining structure is one of proeutectoid cementite, grain boundary ferrite, bainite, and martensite. Or it consists of two or more types
Moreover, in the central portion, a region having a lamella cementite length of 1.5 μm or less has an area ratio of 20% or less.
A hot-rolled wire rod characterized in that the area ratio of proeutectoid cementite is 0.50% or less and the total area ratio of grain boundary ferrite, bainite and martensite is 8.0% or less in the central portion.
0.50 ≤ Si (%) + Cr (%) ≤ 0.90 ... (1)
0.40 ≤ Cr (%) + Mn (%) ≤ 0.80 ... (2)
The element symbol in the above formulas (1) and (2) means the content of the element in mass%. Further, selected from the group consisting of Ni: 0.50% or less, Co: 1.00% or less, Mo: 0.20% or less, B: 0.0030% or less, Cu: 0.15% or less in terms of mass%. The hot-rolled wire rod according to claim 1, wherein the hot-rolled wire rod contains one kind or two or more kinds. The first portion is a region having a length of 20 m arbitrarily selected between a position 500 m away from one end of the wire and a position 600 m away from the one end.
When a region having a length of 20 m arbitrarily selected between a position 500 m away from the other end of the wire and a position 600 m away from the other end is used as the second portion.
The average tensile strength TSave, which is the average of the tensile strengths of the first portion and the second portion, satisfies the following formula (3).
Further, in each section obtained by dividing the first part and the second part into five equal parts in the length direction, the difference between the maximum tensile strength TSmax and the minimum tensile strength TSmin is 100 MPa or less, and the average of the adjacent sections. The hot-rolled wire rod according to claim 1 or 2, wherein the difference in tensile strength is 20 MPa or less.
1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) + 200 <T Save (MPa) <1000 x C amount (%) + 100 x Si amount (%) + 125 x Cr amount (%) +300 ... (3)
The element symbol in the above formula (3) means the content of the element in mass%. The invention according to any one of claims 1 to 3, wherein the lamella spacing L observed in the central portion satisfies the following formula (4), and the standard deviation of the lamella spacing L is 30 nm or less. Hot rolled wire rod.
-65 x (Si (%) + Cr (%)) + 130 <L (nm) ... (4)
The element symbol in the above formula (4) means the content of the element in mass%.