WO2018079781A1

WO2018079781A1 - Wire rod and manufacturing method therefor

Info

Publication number: WO2018079781A1
Application number: PCT/JP2017/039166
Authority: WO
Inventors: 昌坂本; 児玉　順一; 圭佑齋藤
Original assignee: 新日鐵住金株式会社
Priority date: 2016-10-28
Filing date: 2017-10-30
Publication date: 2018-05-03
Also published as: EP3533898B1; EP3533898A4; CN109963960A; CN109963960B; JPWO2018079781A1; EP3533898A1; KR20190073456A; JP6733741B2; KR102247234B1

Abstract

A wire rod according to this embodiment has a predetermined chemical composition, and, in a structure observed in a center portion of a lateral cross-section of the wire rod, the area proportion of perlite is equal to or greater than 90.0%, the area proportion of pro-eutectoid cementite is equal to or less than 1.00%, the average thickness of the pro-eutectoid cementite is equal to or less than 0.25 μm, the total length of the pro-eutectoid cementite per unit area is less than 40.0 mm/mm², the tensile strength satisfies expression (1), and the diameter is 3.0-5.5 mm. Expression (1): 1000 × C content (%) + 300 × Cr content (%) + 70 ≤ TS ≤ 1000 × C content (%) + 300 × Cr content (%) + 160

Description

Wire rod and manufacturing method thereof

The present invention relates to a wire and a method for manufacturing the same.
The present application claims priority based on Japanese Patent Application No. 2016-212590 filed in Japan on October 28, 2016, the contents of which are incorporated herein by reference.

High-strength steel wires such as steel cords and sawing wires are usually manufactured by drawing a high carbon steel wire having a C content of about 0.7 to 0.9%. Since high carbon steel has high strength, disconnection is likely to occur during wire drawing. When the processing strain increases in the wire drawing process, the wire drawing material becomes higher in strength and lower in ductility, so that disconnection is particularly likely to occur. Disconnection during wire drawing significantly reduces productivity. Therefore, a high carbon steel wire rod that is difficult to be disconnected at the time of wire drawing (that is, a high carbon steel wire rod having good wire drawing workability) is demanded.

On the other hand, steel wire is required to have high strength. For example, steel cords are required to have high strength in order to reduce the weight of tires and improve the fuel efficiency of automobiles. Sewing wires are required to have high strength and a small diameter in order to prevent disconnection when cutting silicon wafers and reduce cutting allowances. In order to meet the strength requirements for these steel wires, high carbon steel, particularly hypereutectoid steel containing an amount of C more than eutectoid steel, is used as a steel material.

In hypereutectoid steel, in general, proeutectoid cementite precipitates on a hot-rolled wire, so that the wire drawing workability of the wire is significantly reduced. Therefore, suppression of the precipitation amount of pro-eutectoid cementite in the hot rolled wire rod of hypereutectoid steel has been desired. Here, in the present specification, the “hot rolled wire” means a wire that is not hot-rolled after hot rolling and is not subjected to heat treatment.

Patent Document 1 discloses that the drawing processability of a hot-rolled wire is improved by defining the pearlite lamella spacing of the hot-rolled wire. However, Patent Document 1 does not examine the effect of pro-eutectoid cementite on wire drawing workability. Moreover, in patent document 1, the cooling rate from winding to predetermined temperature shall be 20 degrees C / s or more, and it has the process of heating after that, and a manufacturing process is complicated. Furthermore, there is a problem that the cooling capacity after winding is heavy and the manufacturing cost is high.

Patent Document 2 aims to improve the drawing workability of the hot rolled wire by limiting the tensile strength, fracture drawing, nodule diameter, and the like of the hot rolled wire. However, Patent Document 2 does not examine the effect of proeutectoid cementite on the wire drawing workability, as in Patent Document 1. When a wire with a high C content achieves the fracture drawing and the nodule diameter, which are limited in Patent Document 2, a large amount of proeutectoid cementite precipitates, which may reduce wire drawing workability.

Patent document 3 refines the austenite grains of the wire after hot rolling, and sets the area fraction and aspect ratio of the pro-eutectoid cementite after cooling within a predetermined range, thereby improving the wire drawing workability of the wire. It is improving. The wire disclosed in Patent Document 3 is expected to reduce the manufacturing cost by further reducing the tensile strength, thereby improving the wire drawing workability and reducing the load during wire drawing.

Japanese Patent No. 5179331 Japanese Patent No. 4088220 Japanese Unexamined Patent Publication No. 2001-181789

The present invention has been made to solve the above problems. That is, the present invention provides a wire having excellent wire drawing workability and a method for producing the same, which contains C in an amount equal to or greater than eutectoid steel, and is obtained without performing a heat treatment to be heated again after hot rolling. Objective.

The present inventors have used a steel material having a C content of 0.90 to 1.15% and a high-carbon steel hot-rolled wire material (hereinafter referred to as “wire material”) in which the metal structure and tensile strength are controlled under various rolling conditions. May be described). The present inventors evaluated the wire drawing workability of these wires, and examined in detail the influence of the wire structure and tensile strength on the wire drawing workability. As a result, the present inventors control the tensile strength within a predetermined range according to the C content and Cr content, suppress the area fraction and thickness of pro-eutectoid cementite, and further, per unit area It was found that the wire drawing processability of the wire is improved by controlling the total length of proeutectoid cementite. In the present specification, “drawing workability” refers to the property of drawing without disconnection. In this specification, the wire drawing workability of the wire is evaluated based on the true strain when disconnection occurs during the wire drawing.

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

(1) The wire according to one embodiment of the present invention is, in mass%, C: 0.90 to 1.15%, Si: 0.10 to 0.50%, Mn: 0.10 to 0.80%, Cr: 0.10 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 1.00%, Mo: 0 to 0.20% and B: 0 to 0.0030%, P: 0.020% or less and S: 0.010% or less, the balance is Fe and impurities, and when the radius of the wire is R, (1/5) R from the center of the cross section of the wire The area fraction of pearlite is 90.0% or more and the area fraction of pro-eutectoid cementite is 1.00% or less in the structure observed in the central part of In the central part, the proeutectoid cementer per unit area The total length of the bets is less than 40.0 mm / mm ^2, a tensile strength satisfies the equation (1), having a diameter of 3.0 ~ 5.5 mm.
1000 × C amount (%) + 300 × Cr amount (%) + 70 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 160 Expression (1)
The total length of pro-eutectoid cementite per unit area (mm / mm ² ) is the total length of pro-eutectoid cementite observed per unit area. The TS in the formula (1) indicates the tensile strength of the wire when the unit is MPa. The “C amount (%)” in the formula (1) indicates the C content mass% in the wire, and the “Cr amount (%)” indicates the Cr content mass% in the wire.
(2) In the wire described in the above (1), by mass%, Ni: 0.10 to 0.50%, Co: 0.10 to 1.00%, Mo: 0.05 to 0.20% and B: Any one or more of 0.0002 to 0.0030% may be contained.
(3) In the wire described in (1) or (2) above, the area fraction of the pro-eutectoid cementite may be more than 0% to 1.00%.
(4) In the wire according to any one of the above (1) to (3), the structure observed in the central portion includes one or more of pro-eutectoid cementite, grain boundary ferrite, and bainite. You may go out.
(5) In the method for producing a wire according to another aspect of the present invention, a steel piece having the component described in (1) above is hot rolled to a diameter of 3.0 to 5.5 mm, and then 940 to 800 ° C. After winding, cool to 650 ° C at an average cooling rate of 6.0 to 15.0 ° C / s, and cool at 650 to 600 ° C at an average cooling rate of 1.0 to 3.0 ° C / s. Then, it is cooled at 600 to 300 ° C. at an average cooling rate of 10.0 ° C./s or more.

According to the above aspect, it is possible to provide a wire rod having excellent wire drawing workability and a method for producing the same, which is obtained without performing heat treatment again after hot rolling, containing C in an amount equal to or greater than eutectoid steel. Can do. Moreover, according to the said aspect, the wire drawing workability of the wire of a hypereutectoid steel composition can be improved, without requiring extra equipment cost. In addition, according to the above-described aspect, the cost increase factor accompanying the increase in strength of the steel cord, the sawing wire, etc. (increased disconnection rate during wire drawing, implementation of intermediate patenting, increased die wear, and wire drawing) Time load etc.) can be suppressed. Therefore, the wire according to the above aspect is useful as a material for high-strength steel wires such as steel cords used as reinforcing materials for tires and hoses, and sawing wires used for cutting silicon wafers and the like.

It is the schematic which shows the precipitation state of pro-eutectoid cementite in a prior austenite grain boundary. It is a figure explaining the measuring method of the thickness and length of pro-eutectoid cementite. It is a figure explaining the measuring method of the thickness and length of pro-eutectoid cementite. It is a figure explaining the measuring method of the thickness and length of pro-eutectoid cementite.

Hereinafter, the wire according to the present embodiment will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the present invention, the present invention is not limited unless otherwise specified.
First, the steel composition of the wire according to this embodiment will be described. Hereinafter, unless otherwise specified, “%” regarding the steel composition indicates “% by mass”.

C: 0.90 to 1.15%
C is an essential element for securing the strength of the steel wire. If the C content is less than 0.90%, the strength of the steel wire is reduced. Therefore, the lower limit of the C content is set to 0.90%. The lower limit of the preferable C content is 0.96% or 1.00%. On the other hand, if the C content exceeds 1.15%, a large amount of pro-eutectoid cementite precipitates in the wire, and disconnection is likely to occur. On the other hand, if the C content exceeds 1.15%, the wire and steel wire strength becomes excessively high, so that the wire workability of the wire and steel wire is lowered. Therefore, the upper limit of the C content is 1.15%. The upper limit of the preferable C content is 1.10% or 1.08%.

Si: 0.10 to 0.50%
Si has an action of increasing the strength of ferrite in pearlite. In order to effectively exhibit the above action, the lower limit of the Si content is 0.10%. The lower limit of the preferred Si content is 0.15% or 0.20%. However, if Si is excessively contained, SiO ₂ inclusions that are harmful to the wire drawing workability of the wire may be generated. Therefore, the upper limit of Si content is 0.50%. The upper limit of the preferable Si content is 0.40% or 0.35%.

Mn: 0.10 to 0.80%
Mn has an action of delaying transformation from austenite to pro-eutectoid cementite and pro-eutectoid ferrite, and is an element useful for obtaining a pearlite-based structure. In order to effectively exhibit the above action, the lower limit of the Mn content is 0.10%. The lower limit of the preferable Mn content is 0.20% or 0.30%. However, even if Mn is contained excessively, the above action is saturated. Furthermore, Mn has the effect | action which improves the hardenability of steel. Therefore, when the wire contains excessive Mn, an overcooled structure such as bainite and martensite is generated in the wire during the cooling process after hot rolling, or the wire strength is excessively increased and the wire drawing workability is increased. to degrade. Therefore, the upper limit of the Mn content is 0.80%. The upper limit of the preferable Mn content is 0.70%, 0.60%, or 0.50%.

Cr: 0.10 to 0.50%
Cr has the effect of increasing the work hardening rate of steel pearlite. When the work hardening rate of pearlite increases, a higher tensile strength can be obtained by a low strain drawing process. Cr is an element useful for obtaining a pearlite-based structure because it has the effect of delaying the transformation from austenite to pro-eutectoid cementite and pro-eutectoid ferrite. In order to effectively exhibit the above action, the lower limit of the Cr content is 0.10%. The lower limit of the preferred Cr content is 0.15% or 0.20%. However, if the Cr content exceeds 0.50%, the hardenability of the wire becomes high, and a supercooled structure such as bainite and martensite is generated in the cooling process after hot rolling, or the wire has an excessively high strength. And wire drawing processability is reduced. Therefore, the upper limit of Cr content is 0.50%. The upper limit of preferable Cr content is 0.40% or 0.35%.

Both Mn and Cr are elements that have the effect of improving the hardenability of the steel and delaying the transformation to proeutectoid cementite. In order to suppress the occurrence of non-pearlite structures (such as pro-eutectoid cementite, bainite and martensite) in the wire, it is preferable to control the total content of Mn and Cr. The lower limit of the total content of Mn and Cr is preferably 0.40% or 0.45%. The upper limit of the total content of Mn and Cr is preferably 0.60% or 0.55%.

In addition to the basic elements described above, the wire according to this embodiment may further contain one or more of Ni, Co, Mo, and B shown below. When these elements are not contained, the content of these elements is 0%.

Ni: 0 to 0.50%
Ni has a function of delaying transformation from austenite to pro-eutectoid cementite and pro-eutectoid ferrite, and is a useful element for obtaining a pearlite-based structure. Ni is an element that also has an effect of increasing the toughness of the wire drawing material. In order to obtain the above action, the lower limit of the Ni content is preferably 0.10%. A more preferable lower limit of the Ni content is 0.15% or 0.20%. On the other hand, when Ni is contained excessively, the hardenability becomes excessive, and the wire wire drawing workability deteriorates due to the occurrence of supercooled structures such as bainite and martensite in the wire during the cooling process after hot rolling. There is a case. For this reason, the upper limit of the Ni content is preferably 0.50%. A more preferable upper limit of the Ni content is 0.30% or 0.25%.

Co: 0 to 1.00%
Co has a function of suppressing precipitation of pro-eutectoid ferrite in the rolled material. Co has an effect of improving the ductility of the wire drawing material. In order to effectively exhibit the above action, the lower limit of the Co content is preferably 0.10%. A more preferable lower limit of the Co content is 0.20%, 0.30%, or 0.40%. On the other hand, even if Co is excessively contained, the above-mentioned action is saturated, and the cost increases. Therefore, the upper limit of the Co content is preferably 1.00%. A more preferable upper limit of the Co content is 0.90% or 0.80%.

Mo: 0 to 0.20%
Mo has an action of delaying transformation from austenite to pro-eutectoid cementite and pro-eutectoid ferrite, and is an element useful for obtaining a pearlite-based structure. In order to acquire the said effect | action, it is preferable to make the minimum of Mo content into 0.05%. A more preferable lower limit of the Mo content is 0.08%. However, if the Mo content exceeds 0.20%, the hardenability becomes excessive, and a supercooled structure such as bainite and martensite is generated in the cooling process after hot rolling, or the wire drawing workability of the wire is lowered. There is a case. Therefore, it is preferable that the upper limit of the Mo content is 0.20%. A more preferable upper limit of the Mo content is 0.15% or 0.11%.

B: 0 to 0.0030%
B has an effect of concentrating on the grain boundary and suppressing precipitation of pro-eutectoid ferrite. In order to obtain the above action, the lower limit of the B content is preferably 0.0002%. A more preferable lower limit of the B content is 0.0005%, 0.0007%, 0.0008%, or 0.0009%. On the other hand, when B is contained excessively, B may form carbides such as Fe ₂₃ (CB) _{6 in} austenite, which may reduce the wire drawing workability of the wire. Therefore, it is preferable that the upper limit of the B content be 0.0030%. A more preferable upper limit of the B content is 0.0020%.

The wire according to the present embodiment contains one or more of Ni, Co, Mo, and B as necessary, and the balance is substantially Fe and impurities. The wire according to the present embodiment may include P and S as impurities mixed during the manufacture of molten steel.

P: 0.020% or less P is an element that decreases the wire drawing workability of the wire by segregating at the grain boundaries. Therefore, it is preferable to reduce the P content as much as possible. In order to ensure the wire drawing workability of the wire, the upper limit of the P content is 0.020%. A preferable upper limit of the P content is 0.014% or 0.010%. P may be mixed as an impurity during the production of molten steel, but its lower limit is not particularly limited, and the lower limit is 0%. If the P content is excessively reduced, the melting cost may increase, so the lower limit of the P content may be 0.003% or 0.005%.

S: 0.010% or less S is an element that reduces the wire drawing workability of the wire by forming precipitates with Mn and the like. Therefore, it is preferable to reduce the S content as much as possible. In order to ensure the wire drawing workability of the wire, the upper limit of the S content is 0.010%. The upper limit of the preferable S content is 0.008%, 0.007%, or 0.005%. S may be mixed as an impurity during the production of molten steel, but its lower limit is not particularly limited, and the lower limit is 0%. If the S content is excessively reduced, the melting cost may increase, so the lower limit of the S content may be 0.001% or 0.003%.

The wire according to the present embodiment has pearlite as a main structure, and the remaining structure is composed of one or more of proeutectoid cementite, grain boundary ferrite, and bainite. The remaining structures, proeutectoid cementite, intergranular ferrite, and bainite, may be the propagation path of fracture, and the wire area drawability of the wire may be reduced by increasing the area fraction of these remaining structures. is there. Therefore, the wire according to the present embodiment has a pearlite area fraction of 90 in the structure observed in the central portion within (1/5) R from the center of the cross section of the wire, where R is the radius of the wire. 0.0% or more, and the area fraction of pro-eutectoid cementite is 1.00% or less. The area fraction of pearlite is preferably 93.0% or more, 95.0% or more, or 97.0% or more. A preferred area fraction of pro-eutectoid cementite is 0.50% or less, or 0.20% or less.

In the structure observed in the central portion within (1/5) R from the center of the cross section of the wire rod, the area fraction of pearlite may be 100%, but with the chemical composition of the wire rod according to the present embodiment, It is difficult to completely suppress the precipitation of proeutectoid cementite, grain boundary ferrite and bainite. In the structure observed in the central portion within (1/5) R from the center of the cross section of the wire rod, if an attempt is made to make the area fraction of pearlite 100%, a very good cooling capacity is required and the equipment cost is reduced. If it increases, the drawing processability may decrease due to an increase in the tensile strength of the wire, and the cost may increase in the secondary processing due to an increase in the load during the drawing process. Therefore, in the structure observed at the central portion within (1/5) R from the center of the cross section of the wire, the area fraction of pearlite may be less than 100%.

Proeutectoid cementite does not deteriorate the wire drawing workability of the wire if the precipitation amount is small. On the other hand, in the structure observed in the central portion within (1/5) R from the center of the cross section of the wire rod, in order to reduce the area fraction of proeutectoid cementite to 0%, an excellent cooling capacity is required. Cost may increase. Therefore, the area fraction of pro-eutectoid cementite may be more than 0% in the structure observed in the central portion within (1/5) R from the center of the cross section of the wire.

In the structure observed in the central portion within (1/5) R from the center of the cross section of the wire rod, it is preferable to reduce the area fraction of grain boundary ferrite and bainite as much as possible. The total area fraction of the grain boundary ferrite and bainite is preferably 5.0% or less, or 4.5% or less. Setting the total area fraction of the grain boundary ferrite and bainite to 0% may cause an increase in manufacturing cost, so the total area fraction of the grain boundary ferrite and bainite may be more than 0%.

The proeutectoid cementite in the wire becomes a cause of wire breakage during wire drawing. However, if the precipitation amount of pro-eutectoid cementite is small, it is possible to suppress a decrease in wire drawing workability, particularly by appropriately adjusting the relationship with the prior austenite grain boundaries. Specifically, by reducing the thickness of pro-eutectoid cementite and shortening the total length of pro-eutectoid cementite per unit area, it is possible to suppress a reduction in wire drawing workability of the wire.

The thickness and total length of proeutectoid cementite will be described with reference to FIGS. FIG. 1 is a schematic diagram showing the precipitation state of pro-eutectoid cementite at the prior austenite grain boundaries. FIG. 2 is a view for explaining a method for measuring the thickness and length of the pro-eutectoid cementite 10a of FIG. FIGS. 3 and 4 are diagrams for explaining a method of measuring the thickness and length of the

pro-eutectoid cementite

10b and 10c in FIG. 1, respectively.

Proeutectoid cementite precipitates in a shape along the former austenite grain boundary. Specifically, as shown in FIG. 1, the pro-eutectoid cementite 10a to 10d precipitates along the prior austenite grain boundary 20. In each proeutectoid cementite, the length is defined in the direction along the prior austenite grain boundary, and the thickness is defined in the direction perpendicular to the prior austenite grain boundary. About the thickness of pro-eutectoid cementite, the thickness is measured at three places at intervals equal to the length along the former austenite grain boundary, and the average of these measured values is defined as the thickness of the pro-eutectoid cementite. To do. In the measurement of the thickness of proeutectoid cementite, if it is determined that the measurement location is different from usual, such as a branch point or an end, the location is not included in the average. That is, in FIG. 2, the length of pro-eutectoid cementite 10a is L1, and the thickness of pro-eutectoid cementite 10a is the average of T1, T2, and T3. As for the pro-eutectoid cementite 10b in FIG. 1, for the pro-eutectoid cementite having branches, the total length of each branch is defined as the length of the pro-eutectoid cementite. That is, in FIG. 3, the length of the pro-eutectoid cementite 10b is the sum of OA, OB and OC. Further, the thickness of pro-eutectoid cementite was measured at three locations at intervals equal to the length of the former austenite grain boundary in each branch as described above, and the average of these measured values was measured for the pro-eutectoid cementite. Defined as thickness. That is, in FIG. 3, the thickness of proeutectoid cementite 10b is the average of TA1, TA2, TA3, TB1, TB2, TB3, TC1, TC2, and TC3. As for the pro-eutectoid cementite having a shape bent along the prior austenite grain boundary, as in the pro-eutectoid cementite 10c in FIG. 1, the length is measured along the prior austenite grain boundary. That is, in FIG. 4, the length of pro-eutectoid cementite 10c is the sum of O'D and O'E. In addition, the thickness is divided at the bent part, and each part is measured at three points at intervals of four equal lengths in the direction along the former austenite grain boundary as described above, and the average of the measured values is analyzed for the first analysis. It is defined as the thickness of cementite. That is, in FIG. 4, the thickness of proeutectoid cementite 10c is the average of TD1, TD2, TD3, TE1, TE2, and TE3. The total length of pro-eutectoid cementite in FIG. 1 is the total length of pro-eutectoid cementite 10a to 10d.

The wire according to this embodiment has an average thickness of pro-eutectoid cementite of 0.25 μm or less in a structure observed in the central portion within (1/5) R from the center of the cross section of the wire, and per unit area The total length of pro-eutectoid cementite is less than 40.0 mm / mm ² . The average thickness of preferable pro-eutectoid cementite is 0.20 μm or less. The total length of pro-eutectoid cementite per unit area is 30.0 mm / mm ² or less, 20.0 mm / mm ² or less, or 10.0 mm / mm ² or less. When the average thickness of pro-eutectoid cementite exceeds 0.25 μm, or the total length of pro-eutectoid cementite per unit area is 40.0 mm / mm ² or more, defects during wire drawing of the wire become large, It may be a factor.

The wire according to the present embodiment, in the structure observed in the central portion within (1/5) R from the center of the cross section of the wire, by reducing the degree of occupancy in the prior austenite grain boundaries of proeutectoid cementite, You may further reduce the wire drawing workability of a wire. The degree of occupation of pro-eutectoid cementite in the prior austenite grain boundaries is evaluated by the product of the total length of pro-eutectoid cementite per unit area and the prior austenite grain size, as shown on the left side of the following formula (A). It is preferable that the left side of following formula (A) is less than 1.2. More preferably, the left side of the following formula (A) is less than 1.0.

Total length of pro-eutectoid cementite per unit area (mm / mm ² ) × old austenite particle size (mm) <1.2 Formula (A)

The tensile strength (MPa) of the wire according to this embodiment is defined by the following formula (1) according to the C content (mass%) and the Cr content (mass%). If the tensile strength of the wire falls below the lower limit (left side) shown in the following formula (1), it causes coarsening of pro-eutectoid cementite, increase in area fraction of pro-eutectoid cementite, or increase in thickness of lamellar cementite. The wire drawing workability of the wire may be reduced. On the other hand, if the tensile strength of the wire exceeds the upper limit (right side) shown in the following formula (1), the work hardening rate during wire drawing increases, the tensile strength of the wire increases, and ductility decreases. The wire drawing workability of the wire drawing material may be reduced. Moreover, when the tensile strength of a wire exceeds the upper limit (right side) shown in the following formula (1), the manufacturing cost may increase due to an increase in the load on the die and the wire drawing machine.

The constant term on the right side of the preferred formula (1) is +150 (MPa). In other words, the tensile strength of the wire preferably satisfies the following formula (2). A more preferable constant term on the left side of the formula (1) is +80 (MPa), and a more preferable constant term on the right side is +150 (MPa). In other words, it is more preferable that the tensile strength of the wire satisfies the following formula (3). A more preferable constant term on the left side of the formula (1) is +90 (MPa), and a more preferable constant term on the right side is +140 (MPa). In other words, it is more preferable that the tensile strength of the wire satisfies the following formula (4). In the following formulas (1) to (4), TS represents the tensile strength of the wire, “C amount (%)” represents the mass content of C in the wire, and “Cr amount (%)”. Indicates the mass content of Cr in the wire.

1000 × C amount (%) + 300 × Cr amount (%) + 70 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 160 Expression (1)
1000 × C amount (%) + 300 × Cr amount (%) + 70 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 150 (2)
1000 × C amount (%) + 300 × Cr amount (%) + 80 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 150 (3)
1000 × C amount (%) + 300 × Cr amount (%) + 90 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 140 Expression (4)

The wire diameter of the wire affects the cooling rate after winding, and as a result, the metal structure and tensile strength of the wire. When the diameter of the wire exceeds 5.5 mm, a large amount of proeutectoid cementite may be generated in the wire due to a slow cooling rate at the center of the wire. On the other hand, if the diameter of the wire is less than 3.0 mm, it may be difficult to manufacture the wire, and the production efficiency may decrease, which may increase the cost of the wire. Therefore, the wire diameter of the wire according to this embodiment is set to 3.0 to 5.5 mm.

The area fraction of pearlite and pro-eutectoid cementite is measured by the following method.
First, the wire is cut, and the wire is filled with a resin so that a cross section perpendicular to the longitudinal direction of the wire can be observed. The resin-filled wire is polished with polishing paper and alumina abrasive grains, and further mirror-finished to prepare a sample. After observing the observation surface of the sample (that is, the cross section of the wire) with a nital solution or picral solution, the observation surface of the sample is observed using a scanning electron microscope (SEM).

The nital solution is a mixture of oxalic acid and ethyl alcohol. Corrosion of the observation surface of the sample includes a method of immersing the observation surface in a nital solution having a concentration of 5% or less and a temperature of about 15 to 30 ° C. for several seconds to 1 min, and a nital solution having the above-described concentration and temperature. This is done by wiping the observation surface with absorbent cotton soaked in water. The picral solution is a mixed solution of picric acid and ethyl alcohol. Corrosion of the observation surface of the sample is performed by immersing the observation surface in a picral solution having a concentration of about 5% and a temperature of about 40 to 60 ° C. for 30 seconds to 2 minutes. After corrosion, immediately rinse the observation surface of the sample thoroughly with water, and then quickly dry it with cold or warm air.

Subsequently, using the photographic apparatus attached to the SEM, the central area of the sample (the area within (1/5) R from the center of the wire, where R is the radius of the wire) is a magnification of 2000 times or more, and the total viewing field area A plurality of fields of view are photographed so that becomes 0.08 mm ² or more. Using these SEM photographs and image analysis software such as particle analysis software, the area fraction of pearlite and proeutectoid cementite at the center of the wire is obtained.

The average thickness and length of proeutectoid cementite are measured using the SEM photograph. The average thickness of pro-eutectoid cementite is obtained by calculating the average value of the thicknesses of all pro-eutectoid cementite in the SEM photograph. The thickness of pro-eutectoid cementite can be obtained by measuring the thickness in the direction perpendicular to the prior austenite grain boundaries. In the case of the cementite 10a in FIG. 2, the thicknesses T1, T2, and T3 are measured, and the average of these is taken as the thickness of the proeutectoid cementite. Moreover, the length (mm) of pro-eutectoid cementite draws the line which imagines the prior austenite grain boundary based on the shape of pro-eutectoid cementite in the said SEM photograph, and measures length along the line. If the cementite does not have a particularly bent shape like the cementite 10a in FIG. 2, a straight line imagining the prior austenite grain boundary is drawn along the major axis direction, and the length L1 is measured along the straight line. . If it is a pro-eutectoid cementite with a specific curved part like the cementite 10c in Fig. 4, a line imagining the prior austenite grain boundary is drawn according to the shape, and the pro-eutectoid cementite length is measured along that line. To do. If it is a pro-eutectoid cementite with a branch like the cementite 10b of FIG. 3, the length for every branch is totaled. The total length (mm / mm ² ) of pro-eutectoid cementite per unit area is a value obtained by dividing the total length of pro-eutectoid cementite in the measured visual field by the visual field area. That is, the total length of pro-eutectoid cementite per unit area (mm / mm ² ) is the total length of pro-eutectoid cementite observed per unit area. In measurement, if necessary, an area containing pro-eutectoid cementite may be photographed at a higher magnification to measure the average thickness and length of pro-eutectoid cementite.

The prior austenite grain size is measured by using a wire rod that has been quenched by water cooling several rings from the final end of the coil after hot rolling and immediately after winding. The hardened wire is cut, and the wire is filled with resin so that the cross section can be observed. The resin-filled wire is polished with abrasive paper and alumina, and further mirror-finished to obtain a sample. The prior austenite grain boundaries are exposed by corroding the observation surface of the sample (that is, the cross section of the wire) with an alkali picric acid solution. Corrosion of the observation surface of the sample is performed by immersing the observation surface of the sample for about 10 to 20 minutes in an alkali picrate solution at a temperature of 75 to 90 ° C. After corrosion, immediately wash the observation surface of the sample thoroughly with water, and then quickly dry it with cold or warm air. In addition, the picric acid alkali solution used for corrosion of an observation surface is a mixed solution of the ratio of picric acid 2, sodium hydroxide 5, and water 100 by weight ratio.

After corroding the observation surface, using an optical microscope, the central portion of the observation surface of the sample (the radius of the wire is R and the region within (1/5) R from the center of the wire) is a total observation field of view at a magnification of 400 times or more. Multiple fields of view are taken so that the area is 0.15 mm ² or more. The prior austenite particle size is measured using the photographed photograph and the cutting method described in JIS G 0551: 2013. In the cutting method, 10 or more straight lines having a length of 400 μm are drawn so as not to overlap each other at 100 μm intervals, and evaluation is performed based on the number of captured crystal grains captured by a total of 4 mm or more.

The tensile strength of the wire is measured by the following method. Three or more samples are collected from the front part, middle part, and tail part of the wire coil except for the unsteady part. A tensile test is performed according to JIS Z 2241: 2011 using the collected samples. By calculating the average value of the tensile strength of all the samples, the tensile strength of the wire is obtained.

Next, a method for manufacturing the wire according to this embodiment will be described. In addition, the manufacturing method demonstrated below is an example, and is not limited by the following procedures and methods, What kind of method can be employ | adopted if it is a method which can implement | achieve the structure of the wire which concerns on this embodiment.

The material used for hot rolling can be obtained under normal manufacturing conditions. For example, after casting steel having the above-described components and performing a soaking process (heat treatment for reducing segregation generated in casting) that holds the slab at about 1100 to 1200 ° C. for 10 to 20 hours, By applying, a steel slab having a size suitable for hot rolling (a steel slab before hot rolling generally called a billet) is obtained.

Next, hot rolling is performed under the following conditions. First, the steel slab is heated to 900 to 1200 ° C., and the start temperature of finish rolling is controlled to 750 to 950 ° C. The temperature of the wire during hot rolling indicates the surface temperature of the wire. What is necessary is just to measure the temperature of the wire at the time of hot rolling using a radiation thermometer.

The temperature of the wire rod after finish rolling rises higher than the finish rolling start temperature due to processing heat generation. In this embodiment, the winding temperature is controlled to 800 to 940 ° C. When the coiling temperature is less than 800 ° C., the austenite grain size of the wire becomes finer, so that pro-eutectoid cementite, intergranular ferrite and bainite are likely to precipitate, and the mechanical scale peelability of the wire may decrease. is there. On the other hand, when the coiling temperature exceeds 940 ° C., the austenite grain size of the wire becomes excessively large, and the wire drawing workability of the wire may be reduced. A preferable winding temperature is 830 to 920 ° C. A more preferable winding temperature is 850 to 900 ° C.

It is preferable that the grain size of the prior austenite of the wire is 15 to 60 μm by controlling the start temperature and the winding temperature of the finish rolling as described above. A more preferable prior austenite particle size is 20 to 45 μm.

Austenite in the wire transforms into pearlite during cooling after winding. Therefore, the cooling rate after winding is an important factor for controlling the structure and tensile strength of the wire. In this embodiment, the cooling after winding is divided into three temperature ranges, and the average cooling rate in each temperature range is controlled.

After winding, if the average cooling rate to 650 ° C. is less than 6.0 ° C./s, it may be difficult to suppress the precipitation of proeutectoid cementite. On the other hand, when the average cooling rate up to 650 ° C. after winding is over 15.0 ° C./s, transformation from austenite to bainite, deterioration of wire drawing process due to high strength, and mechanical scale peeling property of the wire. May occur. In addition, if the average cooling rate up to 650 ° C. exceeds 15.0 ° C./s after winding, equipment costs may increase due to the need for large-scale cooling equipment. Therefore, after winding, the average cooling rate to 650 ° C. is 6.0 to 15.0 ° C./s. After winding, the preferred average cooling rate up to 650 ° C. is 7.0 to 10.0 ° C./s.

In the temperature range of 650 to 600 ° C., the average cooling rate is controlled to 1.0 to 3.0 ° C./s in order to transform austenite in the wire into pearlite. If the average cooling rate at 650 to 600 ° C. is less than 1.0 ° C./s, the tensile strength of the wire may decrease or the thickness of the proeutectoid cementite may increase, which may reduce the wire drawing workability of the wire. is there. On the other hand, when the average cooling rate at 650 to 600 ° C. exceeds 3.0 ° C./s, the transformation from austenite to pearlite is not completed by 600 ° C., and the tensile strength of the wire is increased. May decrease, and the life of the wire drawing die may decrease. A preferable average cooling rate at 650 to 600 ° C. is 1.5 to 2.8 ° C./s.

In the temperature range of 600 ° C. or lower, the average cooling rate is set to 10.0 ° C./s or higher and the cooling is performed to 300 ° C. or lower. This is because the tensile strength of the wire may decrease if the wire is held near the transformation temperature even after austenite is transformed into pearlite. A preferable average cooling rate at 600 to 300 ° C. is 15.0 ° C./s or more. If the average cooling rate at 600 to 300 ° C. is to exceed 50 ° C./s, an excellent cooling facility is required, which increases the equipment cost. Therefore, the upper limit of the average cooling rate at 600 to 300 ° C. may be 50 ° C./s or less.

¡The temperature of the wire during cooling should be measured with a radiation thermometer. Generally, cooling after hot rolling of a wire rod is performed after winding it in a coil shape. The wire wound up in a coil shape includes a dense portion where the overlapping of the wires is large and a sparse portion where the overlapping of the wires is small. In the method for manufacturing a wire according to the present embodiment, the temperature of the wire after winding is measured at a portion (dense portion) where the wires are overlapped with each other in the wire wound in a coil shape.

By having the above-described component composition and adjusting the manufacturing conditions as described above, the structure and tensile strength of the wire can be made within the scope of the present invention.

Hereinafter, the present invention will be described more specifically with reference to examples of the wire according to the present invention. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the spirit of the present invention. It is included in the range.

Table 1 shows the chemical composition and hot rolling conditions of steel, and Table 2 shows the results of evaluating the structure of the wire, and the results of evaluating tensile properties and wire drawing workability. The cooling rates 1 to 3 in Table 1 are as follows. The average cooling rate was controlled by adjusting the amount of blast. In Tables 1 and 2, numbers outside the scope of the present invention are underlined.

Cooling rate 1: Average cooling rate from 650 ° C after winding up Cooling rate 2: Average cooling rate from 650 ° C to 600 ° C Cooling rate 3: Average cooling rate from 600 ° C to 300 ° C

No. in Table 1. A1 to A22 are examples of the present invention. In Table 1, No. B1 to B13 are comparative examples in which any one or more of the components and hot rolling conditions are out of the proper range.

In both the inventive example and the comparative example, the billet was heated to 1000 to 1200 ° C. in a heating furnace, and then the finish rolling start temperature was set to 750 to 950 ° C. The wire temperature increased by heat generated during finish rolling was controlled, and the coil was wound into a coil at the winding temperature shown in Table 1. Cooling after winding is performed by means of an average cooling rate from 650 ° C. after cooling (cooling rate 1 in Table 1), an average cooling rate from 650 ° C. to 600 ° C. (cooling rate 2 from Table 1), and 600 ° C. to 300 The average cooling rate up to ° C. (cooling rate 3 in Table 1) was performed under the conditions shown in Table 1. By the above method, the wire which has a wire diameter shown in Table 1 was obtained.

The area fraction of pearlite in the wire and the area fraction of pro-eutectoid cementite were measured by the following methods.

First, the wire was cut and filled with resin so that a cross section perpendicular to the longitudinal direction could be observed. The resin-filled wire was polished with polishing paper and alumina abrasive grains, and further mirror finished to prepare a sample. After observing the observation surface of the sample (that is, the cross section of the wire) with a nital solution or picral solution, the observation surface of the sample was observed using a scanning electron microscope (SEM). The used nital solution was a mixed solution of oxalic acid and ethyl alcohol. Corrosion of the observation surface of the sample is performed by immersing the observation surface in a nital solution for several seconds to 1 min with a concentration of 5% or less and a temperature of about 15 to 30 ° C., and the above-described concentration and temperature of the nital solution. The observation surface was wiped with a dipped absorbent cotton. The picral solution used was a mixed solution of picric acid and ethyl alcohol. Corrosion of the observation surface of the sample was performed by immersing the observation surface in a picral solution having a concentration of about 5% and a temperature of about 40 to 60 ° C. for 30 seconds to 2 minutes. After the corrosion, the observation surface of the sample was immediately washed thoroughly with water and quickly dried with cold air or hot air.

Subsequently, using a photographic apparatus attached to the SEM, the central portion of the sample (the radius of the wire is R, and the region within (1/5) R from the center of the wire) is 2000 times magnification or more, and the total observation visual field area is A plurality of fields of view were photographed so as to be 0.08 mm ² or more. Using these SEM photographs and image analysis software such as particle analysis software, the area fraction of pearlite and proeutectoid cementite at the center of the wire was obtained. Note that Luzex (registered trademark, manufactured by Nireco Corporation) was used as the image analysis software.

In both the inventive examples and the comparative examples, the metal structure observed in the central portion was one or more composite structures of pearlite, proeutectoid cementite, grain boundary ferrite, and bainite.

The average thickness and length of proeutectoid cementite were measured using the SEM photograph. The average thickness of pro-eutectoid cementite was obtained by measuring the thickness of all pro-eutectoid cementite in the SEM photograph and calculating the average value. The thickness of pro-eutectoid cementite was obtained by measuring the thickness in the direction perpendicular to the prior austenite grain boundaries. In the case of cementite having a shape like the cementite 10a in FIG. 2, the thicknesses T1, T2, and T3 were measured, and the average of these was the thickness of the pro-eutectoid cementite. The length of pro-eutectoid cementite was measured by drawing a line imagining a prior austenite grain boundary based on the shape of pro-eutectoid cementite in the SEM photograph, and measuring the length of pro-eutectoid cementite along the line. If the cementite does not have a particularly bent shape like the cementite 10a in FIG. 2, a straight line imagining the prior austenite grain boundary is drawn along the major axis direction, and the length L1 is measured along the straight line. . If it is a pro-eutectoid cementite with a specific curved part like the cementite 10c in Fig. 4, a line imagining the prior austenite grain boundary is drawn according to the shape, and the pro-eutectoid cementite length is measured along that line. did. In the case of a pro-eutectoid cementite having branches like the cementite 10b in FIG. 3, the length of each branch was totaled. The total length of pro-eutectoid cementite per unit area was obtained by dividing the total length of pro-eutectoid cementite in the measured visual field by the visual field area. That is, the total length (mm / mm ² ) of pro-eutectoid cementite per unit area was the total length of pro-eutectoid cementite observed per unit area. In measurement, if necessary, an area containing pro-eutectoid cementite was photographed at a magnification of 3000 to 5000 times, and the average thickness and length of pro-eutectoid cementite were measured.

The prior austenite grain size was measured by using a wire rod that was water-cooled after several rings from the final end of the coil after hot rolling and immediately after winding. The quenched wire was cut and filled with a resin so that the cross section could be observed, and then polished with alumina to obtain a sample. Thereafter, the polished sample was corroded with an alkali picrate solution to reveal prior austenite grain boundaries. Corrosion of the observation surface of the sample was performed by immersing the observation surface of the sample for about 10 to 20 minutes in an alkali picrate solution at a temperature of 75 to 90 ° C. After corrosion, the observation surface of the sample was immediately washed thoroughly with water, and then quickly dried with cold air or hot air. The alkaline picric acid solution used for corrosion of the observation surface was a mixed solution of picric acid 2, sodium hydroxide 5 and water 100 in weight ratio.

The observation surface of the sample was corroded by immersing the observation surface of the sample in an alkaline picric acid solution at a temperature of 75 to 90 ° C. for about 10 to 20 minutes. After the corrosion, the observation surface of the sample was immediately washed thoroughly with water and quickly dried with cold air or hot air. Thereafter, using an optical microscope, the central portion of the observation surface of the sample (the radius of the wire is R, and the region within (1/5) R from the center of the wire) is 400 × magnification and the total observation field area is 0.18 mm. ^A plurality of fields of view were taken so as to be ² . Using these SEM photographs and the cutting method described in JIS G0551: 2013, the prior austenite particle size was measured. In the cutting method, 15 or more straight lines having a length of 400 μm were drawn at intervals of 100 μm, and evaluation was performed based on the number of captured crystal grains captured by a total of 6 mm straight lines.

The tensile strength was measured by the following method. Of the wire, front part (location on the tail end side of the 50 ring from the tip), middle part (within 100 rings from the middle of the tip and tail end in the coil), and tail part (location on the tip end side of the 50 ring from the tail end) From each ring, 3 rings were collected, and 8 samples were collected from each ring at equal intervals, for a total of 72 samples. Using these samples, a tensile test was performed according to JIS Z 2241: 2011. The tensile strength of the wire was obtained by calculating the average value of the tensile strength obtained from these 72 samples. The tensile test was performed with the sample length of 400 mm, the crosshead speed of 10 mm / min, and the distance between jigs of 200 mm.

The wire drawing workability of the wire was evaluated by the following method. Ten rings were collected from the wire, pickled and scaled, and then subjected to lime film treatment. Thereafter, wire drawing (dry wire drawing) was performed without performing a patenting treatment. The area reduction per pass during wire drawing was 17-23%. When the wire strain was performed and the true strain at the time of disconnection was 2.9 or more, it was determined to be acceptable because of excellent wireworkability. On the other hand, when the wire drawing was performed and the true strain at the time of disconnection was less than 2.9, it was determined to be rejected because the wire drawing workability was poor. The true strain was obtained by calculating −2 × ln (the wire diameter of the drawn wire / the wire diameter of the wire). “Ln” is a natural logarithm.

No. A1 to A22 are all examples of the present invention, and showed excellent wire drawing workability that enables wire drawing with a true strain of 2.9 or more without performing a patenting treatment.

On the other hand, No. Since B1 to B13 did not satisfy any of the requirements of the present invention, the true strain at the time of disconnection was less than 2.9, and the wire drawing workability was inferior to the examples of the present invention.

No. Since B1 has a high C content, the area fraction of pro-eutectoid cementite of the wire increases, the average thickness of pro-eutectoid cementite increases, and the total length of pro-eutectoid cementite per unit area increases. The wire drawing workability was reduced.
No. B2 has a high Si content. Since B3 had a high Mn content, the tensile strength of the wire increased and the wire drawing workability decreased.
No. Since B4 had a high Cr content, the area fraction of pearlite decreased, the tensile strength increased, and the wire drawing processability of the wire decreased.

No. B5, and No. Since B11 had a large average cooling rate (cooling rate 2) of 650 to 600 ° C., the tensile strength increased and the wire drawing workability of the wire decreased.
No. In B6, since the average cooling rate (cooling rate 1) from 650 ° C. after winding was small, the average thickness of pro-eutectoid cementite increased, and the wire drawing workability of the wire decreased.
No. In B7, since the average cooling rate (cooling rate 2) at 650 to 600 ° C. was small, the tensile strength was lowered and the wire drawing workability of the wire was lowered.

No. In B8, since the average cooling rate (cooling rate 1) up to 650 ° C. after winding was large, the wire was excessively cooled, the tensile strength increased, and the wire drawing workability decreased.
No. B9 has a low coiling temperature and a small average cooling rate (cooling rate 1) from 650 ° C. after winding, so that the prior austenite grain size is refined and a large amount of proeutectoid cementite is precipitated. The total length of pro-eutectoid cementite per area increased, and the wire drawing processability of the wire decreased.
No. B10 had a small average cooling rate (cooling rate 3) of 600 to 300 ° C., so that the tensile strength of the wire was lowered and the wire drawing workability was lowered.

No. Since the coiling temperature of B12 was high, the prior austenite grain size was increased, the total length of proeutectoid cementite per unit area was increased, and the wire drawing workability of the wire was lowered.
No. In B13, since the average cooling rate (cooling rate 1) from 650 ° C. after winding was small, the area fraction of pro-eutectoid cementite increased, the average thickness of pro-eutectoid cementite increased, and further per unit area The total length of proeutectoid cementite became longer, and the wire drawing processability of the wire decreased.

Claims

% By mass
C: 0.90 to 1.15%,
Si: 0.10 to 0.50%,
Mn: 0.10 to 0.80%,
Cr: 0.10 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 1.00%,
Mo: 0 to 0.20% and B: 0 to 0.0030%
Containing
P: 0.020% or less and S: 0.010% or less, the balance consists of Fe and impurities,
When the radius of the wire is R, the area fraction of pearlite is 90.0% or more in the structure observed in the central portion within (1/5) R from the center of the cross section of the wire, The area fraction of cementite is 1.00% or less,
In the central part, the average thickness of the pro-eutectoid cementite is 0.25 μm or less,
In the central portion, the total length of the pro-eutectoid cementite per unit area is less than 40.0 mm / mm 2 ,
The tensile strength satisfies the formula (1),
A wire having a diameter of 3.0 to 5.5 mm.
1000 × C amount (%) + 300 × Cr amount (%) + 70 ≦ TS ≦ 1000 × C amount (%) + 300 × Cr amount (%) + 160 Expression (1)
The total length of pro-eutectoid cementite per unit area (mm / mm 2 ) is the total length of pro-eutectoid cementite observed per unit area. The TS in the formula (1) indicates the tensile strength of the wire when the unit is MPa. The “C amount (%)” in the formula (1) indicates the C content mass% in the wire, and the “Cr amount (%)” indicates the Cr content mass% in the wire.
% By mass
Ni: 0.10 to 0.50%,
Co: 0.10 to 1.00%,
Mo: 0.05-0.20% and B: 0.0002-0.0030%
The wire according to claim 1, which contains any one or more of the above.
The wire according to claim 1 or 2, wherein an area fraction of the pro-eutectoid cementite is more than 0% to 1.00%.
The wire according to any one of claims 1 to 3, wherein the structure observed in the central portion contains one or more of pro-eutectoid cementite, grain boundary ferrite, and bainite.
The steel slab having the composition according to claim 1 is hot rolled to a diameter of 3.0 to 5.5 mm, wound at 940 to 800 ° C., and wound up to 650 ° C. to 6.0 to 15.0. Cooling at an average cooling rate of ° C / s, cooling from 650 to 600 ° C at an average cooling rate of 1.0 to 3.0 ° C / s, and cooling from 600 to 300 ° C at an average cooling rate of 10.0 ° C / s or more The method for producing a wire material for producing the wire material according to any one of claims 1 to 4, wherein the wire material is cooled by heating.