JP7352069B2 - wire rod and steel wire - Google Patents

Description

The present disclosure relates to wire rods and steel wires.

Wire rods made of steel are high-strength steel used for reinforcing wires in automobile tires, reinforcing wires for aluminum power transmission lines, PC (prestressed concrete) steel wires, rope wires used in bridges, etc. It is widely used as a wire material.
Usually, wire rods are manufactured by hot rolling, and during the wire drawing process to a predetermined wire diameter, intermediate patenting treatment is applied once or twice, and the wire is drawn to a thin steel wire (the final product wire). Processed. For example, for reinforcing materials with a wire diameter of 0.5 mm or less used for reinforcing materials for automobile tires, intermediate patenting is applied to a wire diameter of about 1.5 mm, and then wire drawing is performed to the final wire diameter.

For example, in Patent Document 1, as wire rods that take wire drawability into consideration, C: 0.4-0.65%, Si: 0.1-1.0%, Mn: 0.1-1 0%, Cr: 0.3% or less or B: 100ppm or less, remaining Fe and unavoidable impurities, in which at least one selected from the element group of Ti, Nb, and V is present. 02% or less, the structure is a pearlite structure in which the fraction of pro-eutectoid ferrite is 10% or less, and the rest is 6-10% cementite formed discontinuously. A wire rod for high-strength steel wire with excellent wire drawability is disclosed.
Furthermore, in Patent Document 2, in mass %, C: 0.2 to 0.55%, Si: 0.1 to 1.0%, Mn: 0.1 to 1.1%, Al: 0.01% A wire with a diameter of 4 to 7 mm, characterized by containing the following (including 0%), with the remainder consisting of Fe and unavoidable impurities, with a ferrite area ratio of 20% or more and less than 50%, and 90% or more of the remainder being pearlite. A wire rod for high-strength steel wire is disclosed.

Patent No. 3409055 Patent No. 4267375

When intermediate patenting is applied to a wire rod, the microstructure of the wire rod is destroyed during the intermediate patenting, and the effect of the shaping of the wire rod during and after hot rolling on the final product is reduced. Put it away. For example, even if the wire rod has a fine structure with high ductility, the difference in wire characteristics cannot be maintained after intermediate patenting.

In view of the above problems, an object of the present disclosure is to provide a wire rod with an excellent balance of strength and ductility (drawing area) even after intermediate patenting.
Another object of the present disclosure is to provide a steel wire with an excellent balance of strength and ductility (drawing and twisting properties).

Means for solving the above problems include the following aspects.

<1> C in mass%: 0.40% or more and 0.80% or less,
Si: 0.10% or more and 2.0% or less,
Mn: 0.10% or more and 1.0% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0015% or more and 0.0060% or less, and Ti: 0.005% or more and 0.030% or less,
and has a steel component with the remainder consisting of Fe and impurities, and the ratio of Ti content to N content satisfies 3.3 or more and 6.5 or less,
When the diameter of the wire is D, the number of Ti carbonitrides with a diameter of 10 nm or more and 40 nm or less is 10% or more among Ti carbonitrides with an equivalent circle diameter of 10 nm or more in a region within D/50 from the central axis. , a wire in which the number of Ti carbonitrides with a circular equivalent diameter of 0.5 μm or more in a region within D/9 from the central axis is 40% or less; the number of Ti carbonitrides with a diameter of 3 μm or more is 40% or less;
<2> The steel component is Al: 0.050% or less in mass %,
Cr: 1.0% or less,
Nb: 0.050% or less,
V: 0.15% or less,
Ca: 0.0040% or less,
Mg: 0.0040% or less, and B: 0.0030% or less,
The wire rod according to <1>, which satisfies one or more types selected from the group consisting of:
<3> The average value of the crystal grain size is defined as a crystal grain, which has a pearlite structure and is defined as a region in which the ferrite crystal orientation is surrounded by an angular difference of 15° or more in a region within D/9 from the central axis. , 16 μm or less, the wire according to <1> or <2>.
<4> The wire according to any one of <1> to <3>, which has a pearlite structure and has an area ratio of the pearlite structure in a region within D/9 from the central axis of 90% or more.
<5> The pearlite structure according to any one of <1> to <3>, which has a pearlite structure and has an average lamella spacing of the pearlite structure in a region within D/9 from the central axis of 70 nm or less. wire.

<6> The wire rod according to any one of <1> to <5>, wherein the steel component satisfies Al: 0.005% or more and 0.050% or less in mass %.
<7> The wire rod according to any one of <1> to <6>, wherein the steel component satisfies Cr: 0.05% or more and 1.0% or less in mass %.
<8> Any of <1> to <7>, wherein the steel component satisfies at least one of Nb: 0.003% to 0.050% and V: 0.005% to 0.15% in mass%. The wire rod described in item 1.
<9> Any of <1> to <8>, wherein the steel component satisfies at least one of Ca: 0.0002% or more and 0.0040% or less and Mg: 0.0002% or more and 0.0040% or less. The wire rod described in item 1.
<10> The wire rod according to any one of <1> to <9>, wherein the steel component satisfies B: 0.0001% or more and 0.0030% or less in mass %.
<11> The wire according to any one of <1> to <10>, wherein the wire has a diameter of 1.5 mm or more and 9.0 mm or less.

<12> It has the steel composition described in any one of <1>, <2>, and <6> to <10>, and the ratio of the Ti content to the N content is 3.3. Satisfy the above 6.5 or less,
When a steel wire is heated to 900°C at an average heating rate of 10°C/sec to 30°C/sec and held for 1 minute, the area within d/20 from the central axis, where d is the diameter of the steel wire. The number of Ti carbonitrides with a circle equivalent diameter of 10 nm or more is 10% or more and 40 nm or less is 10% or more, and the circle equivalent diameter is 0.5 μm in a region within d/9 from the central axis. Among the above Ti carbonitrides, the number of Ti carbonitrides of 3 μm or more is 40% or less, and the <110> orientation is parallel to the longitudinal direction of the steel wire in a region within d/9 from the central axis. A steel wire having a degree of accumulation of 2.0 or more.
<13> The steel wire according to <12>, wherein the steel wire has a diameter of 0.5 mm or more and 3.0 mm or less.

According to the present disclosure, a wire rod with an excellent balance of strength and ductility (drawing area) even after intermediate patenting is provided.
Further, according to the present disclosure, a steel wire with an excellent balance of strength and ductility (drawing and twisting characteristics) is provided.

FIG. 2 is a schematic diagram showing a measurement area in a cross section (cross section) perpendicular to the longitudinal direction of a wire according to the present disclosure. FIG. 2 is a schematic diagram showing a measurement area in a cross section (longitudinal cross section) parallel to the longitudinal direction of a wire according to the present disclosure.

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.
In this specification, "%" indicating the content of a component (element) means "mass%".
In this specification, the content of C (carbon) may be referred to as "C content" or "C amount". The contents of other elements may also be expressed in the same manner.
In the numerical ranges described stepwise in this specification, the upper limit or lower limit of one stepwise numerical range may be replaced with the upper limit or lower limit of another stepwise numerical range. , may also be replaced with the values shown in the examples.
In this specification, when only the upper limit is stated as "X% or less" (X is a numerical value) as the content of a component (element), it means that the content of that component (element) is within the range of X% or less. means.

The present inventors have considered controlling the characteristics of the final product that has undergone intermediate patenting during hot rolling and in the fabrication of the wire rod after hot rolling.
Heat treatment changes the structure of ferrite, cementite, etc., but factors with high thermal stability such as inclusions and precipitates should be controlled. Therefore, the present inventors thought of controlling fine carbonitrides that can be used as pinning particles, and as a result of extensive research, they maintained the fine structure even after intermediate patenting by controlling Ti carbonitrides. Ti carbonitrides are particularly easy to precipitate in the center, and the number density (number %) of fine Ti carbonitrides and coarse Ti carbonitrides can be controlled in a specific region of the center. As a result, the inventors have discovered that it is possible to provide wire rods and steel wires with an excellent balance of strength and ductility even after lead patenting (LP), and have completed the wire rods and steel wires according to the present disclosure.

Hereinafter, wire rods and steel wires according to embodiments of the present disclosure will be specifically described.
In addition, in this specification, "wire rod" includes not only steel material after hot rolling but also steel material subjected to LP as intermediate patenting after primary wire drawing. Furthermore, the term "steel wire" refers to a steel material that has been subjected to a wire drawing process to a final wire diameter (final wire drawing process).

[wire]
The wire according to this embodiment is
C in mass%: 0.40% or more and 0.80% or less,
Si: 0.10% or more and 2.0% or less,
Mn: 0.10% or more and 1.0% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0015% or more and 0.0060% or less, and Ti: 0.005% or more and 0.030% or less,
, the balance being Fe and impurities, and the ratio of Ti content to N content satisfies 3.3 or more and 6.5 or less,
When the diameter of the wire is D, the number of Ti carbonitrides with a diameter of 10 nm or more and 40 nm or less is 10% or more among Ti carbonitrides with an equivalent circle diameter of 10 nm or more in a region within D/50 from the central axis. , the number of Ti carbonitrides with an equivalent circle diameter of 3 μm or more is 40% or less among the Ti carbonitrides with an equivalent circle diameter of 0.5 μm or more in a region within D/9 from the central axis.
In the wire rod according to this embodiment, instead of Fe,
Al: 0.050% or less,
Cr: 1.0% or less,
Nb: 0.050% or less,
V: 0.15% or less,
Ca: 0.0040% or less,
Mg: 0.0040% or less, and B: 0.0030% or less,
It may contain one or more selected from the group consisting of:

<Steel composition of wire rod>
・C: 0.40-0.80%
C is an element that strengthens steel. In order to obtain this effect, 0.40% or more of C is contained.
On the other hand, when the C content exceeds 0.80%, the cementite fraction increases and the drawing of the wire decreases. Therefore, a suitable C content is 0.40-0.80%.
Furthermore, from the viewpoint of suppressing crack formation, the C content is preferably 0.45% or more, and more preferably 0.50% or more.
On the other hand, from the viewpoint of improving the drawing area of the wire, the C content is preferably less than 0.67% or 0.65% or less.

・Si:0.10~2.0%
Si is an element that strengthens steel. In order to obtain this effect, 0.10% or more of Si is contained. However, when Si is contained in an amount exceeding 2.0%, the drawing area of the wire decreases. Therefore, a suitable Si content is 0.10% to 2.0%.
When it is desired to further increase the strength of the wire, Si is preferably contained in an amount of 0.15% or more, more preferably 0.20% or more, and even more preferably 0.30% or more.
On the other hand, from the viewpoint of improving the drawing of the wire, the Si content is preferably less than 1.90%, more preferably 1.85% or less, and even more preferably 1.80% or less.

・Mn: 0.10-1.0%
Mn is an element that has the effect of increasing the strength of steel and also has the effect of fixing S in steel as MnS to prevent hot embrittlement of the steel wire. However, if the Mn content is less than 0.10%, the above effects are not sufficient. Therefore, the lower limit of the Mn content is set to 0.10% or more.
On the other hand, Mn is an element that tends to segregate. If Mn is contained in an amount exceeding 1.0%, Mn will be concentrated particularly in the center, martensite or bainite will be generated in the center, and the drawing of the wire will be reduced. Therefore, a suitable Mn content is 0.10 to 1.0%.
Furthermore, in order to ensure the strength of the steel wire after wire drawing and prevent hot embrittlement at a higher level, the Mn content is preferably 0.35% or more, and 0.40% or more. It is more preferable to do so. The Mn content may be 0.50% or more, or 0.55% or more.
On the other hand, the formation of coarse MnS also causes a decrease in the drawing area of the wire. From the viewpoint of improving the drawing area of the wire, the Mn content is preferably 0.90% or less, and even more preferably 0.80% or less. The Mn content may be 0.75% or less, or 0.70% or less.

- P: 0.030% or less P is an element that segregates at the grain boundaries of the wire rod and deteriorates the torsional properties of the steel. When the P content of the wire rod exceeds 0.030%, the torsional properties deteriorate significantly. Therefore, the P content of the wire is limited to 0.030% or less. The upper limit of the P content is preferably 0.025% or less. The lower the P content, the more preferable it is, but from the viewpoint of reducing manufacturing costs (dephosphorization costs), the P content may be more than 0%, may be 0.0005% or more, and may be 0.0010%. % or more.

-S: 0.030% or less S is an element that forms MnS and reduces the drawing area of the wire. When the S content of the wire rod exceeds 0.030%, the reduction in the drawing area of the wire rod becomes significant. From this, the S content of the wire is limited to 0.030% or less. A preferable upper limit of the S content is 0.015% or less. The lower the S content, the more preferable it is, but from the viewpoint of reducing manufacturing costs (desulfurization costs), the S content may be more than 0%, may be 0.002% or more, and may be 0.005%. It may be more than that.

・N: 0.0015-0.0060%
N is an element that improves thermal stability when Ti carbonitride is formed. In order to obtain the effect of high thermal stability, the N content should be 0.0015% or more.
On the other hand, N is an element that increases the strength of the wire by fixing to dislocations during cold wire drawing, but at the same time reduces the torsional properties. When the N content of the wire rod exceeds 0.0060%, the torsional properties deteriorate significantly. Therefore, the N content of the wire is limited to 0.0060% or less. Therefore, an appropriate N content is 0.0015 to 0.0060%.
From the viewpoint of improving the thermal stability of Ti carbonitride, the N content is preferably 0.0020% or more, more preferably 0.0025% or more.
On the other hand, from the viewpoint of suppressing deterioration of the torsional properties of steel, the preferable upper limit of the N content is 0.0050% or less, or 0.0040% or less.

・Ti: 0.005-0.030%
Ti combines with N and/or C to form carbonitrides, and their pinning effect refines austenite grains during hot rolling, thereby improving the torsional properties of steel. In order to obtain this effect, it is preferable to contain Ti in an amount of 0.005% or more.
On the other hand, if the Ti content exceeds 0.030%, not only the effect will be saturated, but also the steel manufacturability will be affected, such as cracking of the steel slab during the process of blooming the steel ingot or slab into steel slabs. Adversely affect. Therefore, the Ti content is set to 0.030% or less. Therefore, a suitable Ti content is 0.005 to 0.030%.
From the viewpoint of improving the torsional properties of steel, the Ti content is preferably 0.007% or more, and more preferably 0.010% or more.
On the other hand, from the viewpoint of suppressing cracking of the steel slab during the blooming process, the Ti content is more preferably 0.025% or less.

- Al: 0.050% or less Al is an arbitrary element. That is, the Al content may be 0% or more than 0%.
Al is an element that has a deoxidizing effect, and may be added to reduce the amount of oxygen in the wire. Furthermore, Al is an element that forms nitrides in the wire and makes the austenite grain size finer, thereby reducing the pearlite block grain size (PBS). In order to obtain these effects, the Al content is preferably 0.005% or more.
On the other hand, if the Al content exceeds 0.050%, the electrical resistivity of the wire may become excessively high. The reason for this is that when the Al content exceeds 0.050%, coarse oxide-based inclusions are significantly likely to be formed, and the torsional properties are significantly deteriorated. Therefore, the upper limit of the Al content is 0.050%. A preferable upper limit of the Al content is 0.040% or less, a more preferable upper limit is 0.035% or less, and an even more preferable upper limit is 0.030% or less.

- Cr: 1.0% or less Cr is an optional element, and the Cr content may be 0% or more than 0%. Like Mn, Cr is an element that improves the hardenability of steel and increases its strength. In order to reliably obtain this effect, it is preferable to contain 0.05% or more of Cr.
On the other hand, when the Cr content exceeds 1.0%, the torsional properties deteriorate. Therefore, the Cr content is 1.0% or less.
In addition, when increasing the hardenability of steel, it is preferable to contain Cr in an amount of 0.10% or more, and it is more preferable to contain Cr in an amount of 0.30% or more. The upper limit of the Cr content is preferably 0.90% or less, and even more preferably 0.80% or less.

- Nb: 0.050% or less Nb is an optional element, and the Nb content may be 0% or more than 0%. Nb combines with N and/or C to form nitrides, carbides, or carbonitrides, and their pinning effect refines austenite grains during hot rolling, which has the effect of improving the torsional properties of steel. . In order to reliably obtain such effects, it is preferable to contain Nb in an amount of 0.003% or more. From the viewpoint of improving torsion properties, the Nb content is more preferably 0.004% or more, and even more preferably 0.005% or more.
On the other hand, if the Nb content exceeds 0.050%, not only will the effect be saturated, but also the steel manufacturability will be affected, such as cracks occurring in the steel slab during the process of blooming the steel ingot or slab into steel slabs. Since it has an adverse effect, the Nb content is set to 0.050% or less. More preferably, the Nb content is 0.030% or less.

-V: 0.15% or less V is an optional element, and the V content may be 0% or more than 0%. V combines with N and/or C to form nitrides, carbides, or carbonitrides, and their pinning effect has the effect of refining austenite grains during hot rolling and improving the torsional properties of steel. . In order to reliably obtain this effect, it is preferable to contain 0.005% or more of V. From the viewpoint of improving torsional properties, the V content is preferably 0.02% or more, and more preferably 0.03% or more.
On the other hand, if the V content exceeds 0.15%, not only will the effect be saturated, but also the steel manufacturability will be affected, such as cracking of the steel billet during the process of blooming the steel ingot or slab. Since V has an adverse effect, the V content is set to 0.15% or less. The V content is preferably 0.10% or less, and even more preferably 0.07% or less.

From the viewpoint of improving the torsional properties of steel and suppressing cracking of steel slabs, the steel components in the wire rod of this embodiment are Nb: 0.003% or more and 0.050% or less and V: 0.005% in mass%. It is preferable that at least one of the above and 0.15% is satisfied.

-Ca: 0.0040% or less Ca is an optional element, and the Ca content may be 0% or more than 0%. Ca forms a solid solution in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, wire breakage during wire drawing caused by MnS can be suppressed. In order to reliably obtain the effect of Ca, it is preferable to contain Ca in an amount of 0.0002% or more. If a higher effect is desired, 0.0005% or more of Ca may be contained.
However, when the Ca content exceeds 0.0040%, the effect is saturated. Furthermore, when the Ca content exceeds 0.0040%, the oxides produced by reacting with oxygen in the steel become coarse, which actually causes a decrease in the drawing area of the wire. Therefore, when Ca is included, the appropriate Ca content is 0.0040% or less. The Ca content is preferably 0.0030% or less, more preferably 0.0025% or less.

- Mg: 0.0040% or less Mg is an optional element, and the Mg content may be 0% or more than 0%. Mg is a deoxidizing element and generates oxides, but it also generates sulfides, so it is an element that has a mutual relationship with MnS, and has the effect of finely dispersing MnS. This effect can suppress wire breakage during wire drawing due to MnS. In order to reliably obtain the effect of Mg, it is preferable to contain Mg in an amount of 0.0002% or more. If a higher effect is desired, 0.0005% or more of Mg may be contained.
However, when the Mg content exceeds 0.0040%, the effect is saturated, and a large amount of MgS is generated, which results in a decrease in the drawing capacity of the wire. Therefore, when Mg is included, the appropriate Mg content is 0.0040% or less. The Mg content is preferably 0.0035% or less, more preferably 0.0030% or less.

From the viewpoint of suppressing wire breakage during wire drawing and suppressing a reduction in the area of area of the wire rod, the steel components in the wire rod of this embodiment include Ca: 0.0002% or more and 0.0040% or less, and Ca: 0.0002% or more and 0.0040% or less Mg: It is preferable that at least one of 0.0002% and 0.0040% is satisfied.

- B: 0.0030% or less B is an optional element, and the B content may be 0% or more than 0%. When B is contained in a small amount, it has the effect of reducing the ferrite structure of steel. If this effect is to be achieved reliably, it is preferable to contain 0.0001% or more of B. When it is desired to increase the area ratio of the pearlite structure, the B content is preferably 0.0004% or more, and even more preferably 0.0007% or more.
On the other hand, even if B is contained in an amount exceeding 0.0030%, not only the effect is saturated, but also coarse nitrides are formed, resulting in a decrease in torsional properties. Therefore, when B is included, the B content is set to 0.0030% or less. Note that the B content for improving torsional properties is preferably 0.0025% or less, and even more preferably 0.0020% or less.

・Ratio of Ti content to N content If the ratio of Ti content to N content (hereinafter sometimes abbreviated as [Ti/N]) is less than 3.3, solid solution due to Ti The fixation of N becomes insufficient, and the torsional properties of the steel wire after wire drawing deteriorate. On the other hand, when [Ti/N] exceeds 6.5, fine Ti carbonitrides tend to become coarse and an effective pinning effect cannot be obtained. Therefore, the wire rod of this embodiment contains Ti and N so that [Ti/N] of the steel component satisfies 3.3 or more and 6.5 or less.
The lower limit of [Ti/N] is preferably 3.8 or more, more preferably 4.0 or more.
The upper limit of [Ti/N] is preferably 6.0 or less, more preferably 5.5 or less.

<Metal structure of wire>
・Number density of fine Ti carbonitrides in the area within D/50 from the central axis When the diameter of the wire is D, the wire of this embodiment has a circular equivalent diameter in the area within D/50 from the central axis. Among Ti carbonitrides with a diameter of 10 nm or more, Ti carbonitrides with a diameter of 10 nm or more and 40 nm or less (herein, "Ti carbonitrides with an equivalent circle diameter of 10 nm or more and 40 nm or less" are referred to as "fine Ti carbonitrides") ) is 10% or more.
Fine Ti carbonitrides having an equivalent circle diameter of 10 nm or more and 40 nm or less act as pinning particles and promote refinement of prior austenite. If the number percent of fine Ti carbonitrides in the area within D/50 from the central axis is 10% or more, the wire structure is sufficiently refined, and the wire drawing and steel wire drawing and twisting characteristics are improved. becomes good. The number percent of fine Ti carbonitrides in the region within D/50 from the central axis is preferably 11% or more, more preferably 12% or more, and the larger the number, the better, and may be 100%.

The number percent of fine Ti carbonitrides in a region within D/50 from the central axis is determined by measuring as follows.
After cutting a cross section perpendicular to the longitudinal direction of the wire (i.e., a cross section of the wire), as shown in FIG. From an area within D/50 from the central axis C (area within a radius D/50 from the central axis C), perpendicular to the cutting plane (i.e. vertical section) using a FIB (FOCUSED ION BEAM) device, A sample of 100 μm ² (in the direction) is taken, and its longitudinal section is observed using a transmission electron microscope (TEM) at a magnification of 40,000 times. The observation position is any position within the sample, and observation is performed at 10 locations. The area per field of view is 3.15×10 ⁻¹ μm ² (vertical 0.45 μm, horizontal 0.70 μm).
Next, elemental analysis is performed on the precipitates in each photograph by obtaining a characteristic X-ray spectrum using an energy dispersive X-ray analyzer (EDS). At this time, if Ti and at least one of C or N is detected, it is determined that it is Ti carbonitride. Further, if O or S is detected at the same time, there is a possibility that it is an oxide or sulfide, but since these also have a pinning effect if they are small in size, they are considered to be Ti carbonitrides.
However, if carbonitrides, oxides, or sulfides other than Ti and Ti carbonitrides are precipitated in contact with each other (compounded), it is difficult to act as a pinning particle. Carbonitrides will be counted.
Next, the number of particles determined to be Ti carbonitride is counted, and the diameter is evaluated using the size as a circular equivalent diameter. The ratio of the total number of Ti carbonitrides with a diameter of 10 nm or more and 40 nm or less (fine Ti carbonitrides) to the total number of Ti carbonitrides with an equivalent circle diameter of 10 nm or more in the images taken at 10 locations is calculated, and the ratio is calculated to determine the fine Ti carbonitrides. Calculate the number density (number %) of objects. Note that particles with an equivalent circular diameter of less than 10 nm may not be visible by TEM, and particles with an equivalent circle diameter of less than 10 nm are not counted.

・Number density of coarse Ti carbonitrides in a region within D/9 from the central axis In addition, the wire of this embodiment has an equivalent circle diameter of 0.5 μm or more in a region within D/9 from the central axis of the wire. Among the Ti carbonitrides, the number of Ti carbonitrides with a diameter of 3 μm or more (in this specification, “Ti carbonitrides with an equivalent circle diameter of 3 μm or more” may be referred to as “coarse Ti carbonitrides”) is It is 40% or less.
Coarse Ti carbonitrides with an equivalent circle diameter of 3 μm or more accelerate the formation of cracks during a torsion test on a steel wire after wire drawing. If the number percent of coarse Ti carbonitrides in the region within D/9 from the central axis exceeds 40%, the twisting characteristics of the steel wire after wire drawing will be significantly deteriorated, so it is set to 40% or less. The number percent of coarse Ti carbonitrides in the region within D/9 from the central axis is preferably 38% or less, more preferably 36% or less, the smaller the number, the better, and it may be 0%.

The number percent of coarse Ti carbonitrides in the region within D/9 from the central axis is determined by measuring as follows.
After mirror-polishing the cross section parallel to the longitudinal direction of the wire and including the central axis (i.e., the longitudinal section of the wire), as shown in FIG. Field emission scanning is performed for each of five arbitrary positions in the area within D/9 from the center axis C (area within the radius D/9 from the center axis C) with the center axis C at the position D/2 in the radial direction. Observe at 1000x magnification using an electron microscope (FE-SEM) and take a photograph.
Next, elemental analysis is performed on the precipitates in each photograph by obtaining a characteristic X-ray spectrum using an energy dispersive X-ray analyzer (EDS). At this time, if Ti and at least one of C or N is detected, it is determined that it is Ti carbonitride. Further, if O or S is detected at the same time, there is a possibility that it is an oxide or sulfide, but if these are large in size, they can reduce the twisting properties, so they are considered to be Ti carbonitrides.
Next, the number of particles determined to be Ti carbonitride is counted, and the diameter is evaluated using the size as a circular equivalent diameter. The ratio of the total number of Ti carbonitrides with a diameter of 3 nm or more (coarse Ti carbonitrides) to the total number of Ti carbonitrides with an equivalent circle diameter of 0.5 μm or more in the photographs taken at five locations was calculated, and the number density (number %) was calculated. ) to measure. Note that particles with an equivalent circle diameter of 0.5 μm or less may not be visible by SEM, and particles with a circle equivalent diameter of 0.5 μm or less are not counted.

- Average value of the area where the ferrite crystal orientation is surrounded by an angle difference of 15° or more in the area within D/9 from the central axis The wire of this embodiment has the ferrite crystal orientation in the area within D/9 from the central axis It is preferable that the average value of the area surrounded by an angular difference of 15 degrees or more is 16 μm or less. If the average value of this area is 16 μm or less, it means that the pearlite block grains (PBS) are refined, which acts as propagation resistance during crack formation and improves ductility.

The average value of the crystal grain size in which the ferrite crystal orientation is surrounded by an angle difference of 15 degrees or more is determined by measuring as follows.
After mirror-polishing the cross-section parallel to the longitudinal direction of the wire and including the central axis (i.e., the longitudinal cross-section of the wire), polishing with colloidal silica produces electric field radiation at any position within D/9 from the central axis. Each of the four fields of view was observed at a magnification of 400 times using a type scanning electron microscope (FE-SEM), and measurement by electron beam backscatter diffraction method (EBSD measurement) was performed. The area per field of view is 0.0324 mm ² (vertical 0.18 mm, horizontal 0.18 mm), and the step during measurement is 0.3 μm.
Next, the grain boundary is defined as 15°, a weighted average of the crystal grain sizes is calculated, and the average value of the four measured fields of view is taken as the crystal grain size of the region surrounded by an angular difference of 15° or more. For example, the crystal grain size can be obtained by using OIM analysis (EBSD analysis software, OIM: Orientation Imaging Microscopy, manufactured by TSL Solutions Co., Ltd.). When using OIM analysis, pixels with a CI value of 0.1 or less and clusters of 9 or fewer pixels are considered to be noise and excluded because the reliability of the data is low.

- Area ratio of pearlite structure in the area within D/9 from the central axis The wire of this embodiment preferably has a pearlite structure, and the area ratio of the pearlite structure in the area within D/9 from the central axis is 90% or more It is preferable that If the area ratio of the pearlite structure in the region within D/9 from the central axis is 90% or more, the wire rod can have a particularly excellent balance between high strength and ductility. From this viewpoint, the area ratio of the pearlite structure in the region within D/9 from the central axis is more preferably 92% or more, and even more preferably 95% or more.
Note that the upper limit of the area ratio of the pearlite structure in the region within D/9 from the central axis is not particularly limited, and may be 100%, but may be 99% or less from the viewpoint of variation. Note that metal structures other than the pearlite structure (non-pearlite structure) in the wire of this embodiment include ferrite, bainite, and martensite.

The area ratio of the pearlite structure in the region within D/9 from the central axis is determined by measuring as follows.
After mirror-polishing the cross section parallel to the longitudinal direction of the wire and including the central axis (i.e., the longitudinal cross-section of the wire), it was corroded with Picral and examined using a field emission scanning electron microscope (FE-SEM) at a magnification of 2000. Observe and photograph five arbitrary positions in an area within D/9 from the central axis with the center axis at a position D/2 from the outer circumferential surface of the wire at double magnification. The area per field of view is 2.7×10 ⁻³ mm ² (vertical 0.045 mm, horizontal 0.060 mm).
Next, a transparent sheet (for example, an OHP (Over Head Projector) sheet) is placed over each of the obtained photographs. In this state, color is applied to the "non-pearlite structure" on each transparent sheet.
Next, the area ratio of the "colored area" in each transparent sheet is determined using image analysis software (image-J ver. 1.51), and the average value is calculated as the average value of the area ratio of the non-pearlite structure. The area ratio of the pearlite structure is calculated by subtracting the obtained area ratio of the non-pearlite structure from 100%, and the average value of the five fields of view is taken as the area ratio of the pearlite structure.

- Average value of lamella spacing in the area within D/9 from the central axis The wire of this embodiment preferably has a pearlite structure, and the average value of the lamella spacing of the pearlite structure in the area within D/9 from the central axis is The thickness is preferably 70 nm or less. If the average value of the lamella spacing of the pearlite structure in the region within D/9 from the central axis is 70 nm or less, a high-strength wire can be more reliably obtained. From this viewpoint, the average value of the lamella spacing of the pearlite structure in the region within D/9 from the central axis is more preferably 67 nm or less, and even more preferably 65 nm or less.
Note that the lower limit of the average value of the lamella spacing of the pearlite structure in the region within D/9 from the central axis is not particularly limited, but may be 40 nm or more from the viewpoint of reducing ductility due to increased strength.

The lamella spacing of the pearlite structure in the region within D/9 from the central axis is determined by measuring as follows.
After mirror-polishing the cross section parallel to the longitudinal direction of the wire and including the central axis (i.e. the longitudinal cross-section of the wire), it was corroded with Picral and examined using a field emission scanning electron microscope (FE-SEM) at a magnification of 10,000. As shown in FIG. 2, with the central axis C set at a position D/2 from the outer circumferential surface of the wire 10, observe and photograph five arbitrary positions in the area within D/9 from the central axis C. . Specifically, using photographs of each field of view, in the range where pearlite lamellae are aligned in the same direction within the field of view, five lamella intervals can be measured, and the lamella interval is the smallest, and the second lamella interval is measured. For small locations, draw a straight line perpendicular to each lamella, find the length of 5 lamella intervals, and divide it by 5 to find the pearlite lamella spacing at each location (2 locations for each field of view). . The average value of the lamella spacings at the 10 locations thus determined can be used as the "average lamella spacing" of the sample.

<Wire diameter>
The diameter (D) of the wire of this embodiment is not particularly limited, but from the viewpoint of forming a uniform structure within the cross section, it is preferably 1.5 mm or more and 9.0 mm or less, and 2.0 mm or more and 8.0 mm or less. It is more preferable that

[Steel wire]
Next, the steel wire of this embodiment will be explained.
The steel wire of this embodiment has C: 0.40% or more and 0.80% or less in mass %,
Si: 0.10% or more and 2.0% or less,
Mn: 0.10% or more and 1.0% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0015% to 0.0060%, Ti: 0.005% to 0.030%, the balance being Fe and impurities, and the content of N The ratio of Ti content to 3.3 or more and 6.5 or less,
When a steel wire is heated to 900°C at an average heating rate of 10°C/sec to 30°C/sec and held for 1 minute, the area within d/20 from the central axis, where d is the diameter of the steel wire. The number of Ti carbonitrides with a circle equivalent diameter of 10 nm or more is 10% or more and 40 nm or less is 10% or more, and the circle equivalent diameter is 0.5 μm in a region within d/9 from the central axis. Among the above Ti carbonitrides, the number of Ti carbonitrides of 3 μm or more is 40% or less, and the <110> orientation is parallel to the longitudinal direction of the steel wire in a region within d/9 from the central axis. The degree of integration is 2.0 or more.
The steel composition of the steel wire of this embodiment is replaced with Fe,
Al: 0.050% or less,
Cr: 1.0% or less,
Nb: 0.050% or less,
V: 0.15% or less,
Ca: 0.0040% or less,
Mg: 0.0040% or less, and B: 0.0030% or less,
It may contain one or more selected from the group consisting of:

<Steel composition of steel wire>
The steel wire of this embodiment is obtained by drawing the wire rod of this embodiment described above, and the steel composition is the same as that of the wire rod, so a description thereof will be omitted here.

<Metal structure of steel wire>
The size and number of Ti carbonitrides do not change significantly before and after wire drawing, but when the cross section of a steel wire obtained by wire drawing is observed with an SEM or TEM, The cementite structure is so fine that it is difficult to observe the ferrite matrix, and in TEM it is difficult to identify Ti carbonitrides because the dislocation structure and cementite structure are intricately intertwined. Therefore, the steel wire of this embodiment was quenched by heating the steel wire to 900°C at an average heating rate of 10°C/sec to 30°C/sec and holding it for 1 minute to observe Ti carbonitrides. For simplicity, the respective number percentages of fine Ti carbonitrides and coarse Ti carbonitrides are specified.

・Number density of fine Ti carbonitrides in the area within d/20 from the central axis The steel wire of this embodiment is heated to 900°C at an average heating rate of 10°C/sec to 30°C/sec. When held (quenched) for 1 minute, when the diameter of the steel wire is d, Ti carbonitrides with a circular equivalent diameter of 10 nm or more in the area within d/20 from the central axis are 10 nm or more and 40 nm or less The number of Ti carbonitrides is 10% or more.
Fine Ti carbonitrides having an equivalent circle diameter of 10 nm or more and 40 nm or less act as pinning particles and promote refinement of prior austenite. If the number percent of fine Ti carbonitrides in the region within d/20 from the central axis of the steel wire is 10% or more, the structure of the steel wire is sufficiently refined, and the drawing and twisting characteristics of the steel wire are improved. Become good. The number percent of fine Ti carbonitrides in the region within d/20 from the central axis is preferably 11% or more, more preferably 12% or more, and the larger the number, the better, and may be 100%.

The method for determining the number % of fine Ti carbonitrides in the steel wire of this embodiment is to heat the steel wire to 900°C at an average heating rate of 10°C/sec to 30°C/sec and hold it for 1 minute. After quenching, the method is the same as the method for determining the number % of fine Ti carbonitrides in the wire described above, except that the area within d/20 from the central axis is the measurement target, and will be explained here. is omitted.

・Number density of coarse Ti carbonitrides in the area within d/9 from the central axis The steel wire of this embodiment is heated to 900°C at an average heating rate of 10°C/sec to 30°C/sec. When held for 1 minute, the number of Ti carbonitrides with an equivalent circle diameter of 3 μm or more is 40% or less of the Ti carbonitrides with an equivalent circle diameter of 0.5 μm or more in a region within d/9 from the central axis.
Coarse Ti carbonitrides with an equivalent circle diameter of 3 μm or more accelerate the formation of cracks during the torsion test. If the number percent of coarse Ti carbonitrides in the region within d/9 from the central axis of the steel wire exceeds 40%, the twisting characteristics of the steel wire will be significantly degraded, so it is set to 40% or less. The number percent of coarse Ti carbonitrides in the region within d/9 from the central axis is preferably 38% or less, more preferably 36% or less, the smaller the number, the better, and it may be 0%.

The method of determining the number % of coarse Ti carbonitrides in the area within d/9 from the central axis of the steel wire is to calculate the number % of coarse Ti carbonitrides in the area within D/9 from the central axis of the wire as described above. The method is the same as the method for finding , so the explanation here will be omitted.

・Agglomeration degree of <110> direction parallel to the longitudinal direction of the steel wire in the area within d/9 from the central axis In addition, the steel wire of this embodiment The degree of integration in the <110> direction parallel to the longitudinal direction of the line (hereinafter sometimes simply referred to as "the degree of integration in <110>") is 2.0 or more. Here, the degree of integration of <110> is how many times the frequency of crystal grains with orientation <110> is present compared to a structure with a completely random orientation distribution (in this case, the degree of integration is 1). This is an indicator that shows whether
In the steel wire of this embodiment, the degree of integration of <110> in the region within d/9 from the central axis is 2.0 or more, so that the structure variation in the cross section can be reduced and the twisting characteristics can be improved. I can do it. From this point of view, the steel wire of the present embodiment preferably has a <110> integration degree of 2.1 or more, more preferably 2.2 or more in a region within d/9 from the central axis.
Note that the upper limit of the integration degree of <110> in the area within d/9 from the central axis is not particularly limited, but from the viewpoint of suppressing wire breakage during wire drawing, it is preferably 4.0 or less, and 3.8 It is more preferable that it is below.

The degree of accumulation of <110> directions parallel to the longitudinal direction of the steel wire in a region within d/9 from the central axis is determined by measuring as follows.
A cross section perpendicular to the longitudinal direction of the steel wire (i.e., a cross section of the steel wire) was polished to a mirror surface, then polished with colloidal silica, and the central axis was examined using a field emission scanning electron microscope (FE-SEM) at a magnification of 400x. Three fields of view are each observed at any position within a radius of d/9 from , and EBSD measurement (measurement by electron beam backscatter diffraction method) is performed. The area per field of view is 0.0324 mm ² (vertical 0.18 mm, horizontal 0.18 mm), and the step during measurement is 0.1 μm.
Next, the degree of accumulation of <110> as seen from the longitudinal direction of the steel wire is calculated. For example, the degree of integration can be calculated using the OIM analysis described above. When using OIM analysis, pixels with a CI value of 0.1 or less and clusters of 9 or fewer pixels are considered to be noise and are excluded.

<Diameter of steel wire>
The diameter (d) of the steel wire of this embodiment is not particularly limited, but in order to obtain stable twisting characteristics, it is preferably 0.5 mm or more and 3.0 mm or less, and 0.7 mm or more and 2.5 mm or less. It is more preferable that

[Wire manufacturing method]
Although the method for manufacturing the wire rod of this embodiment is not particularly limited, a preferred method for manufacturing the wire rod of this embodiment will be described below as an example.

<Heating slab>
A slab having the above-mentioned steel components is heated to 1200° C. or higher. As a result, coarse Ti carbonitrides having an equivalent circle diameter of 3 μm or more (formed during solidification) can be dissolved, and the proportion can be reduced. The solid solution Ti increases by dissolving the Ti carbonitride, and the Ti carbonitride is finely dispersed during blooming. The maximum heating temperature of the slab is preferably 1350°C or less. This is because if the temperature of the slab is raised to over 1350°C, decarburization becomes severe.

<Slab heating time>
By heating at 1200°C or more for 30 minutes or more, coarse Ti carbonitrides with an equivalent circle diameter of 3 μm or more can be dissolved, and the proportion can be reduced. The solid solution Ti increases by dissolving the Ti carbonitride, and the Ti carbonitride is finely dispersed during blooming. The heating time at 1200° C. or higher is preferably 300 minutes or less. This is because heating for more than 300 minutes does not show a significant effect.

<Wire heating temperature>
The heating temperature of the wire is 1000°C or higher. This is because heating at less than 1000° C. causes a large reaction force during wire rolling. The maximum temperature T (°C) when heating the wire can be a value calculated by the following formula (I), where the Ti content in the wire is [Ti] and the C content is [C]. preferable.
T=-9575/(log([Ti]×[C])-4.4)-360...(I)
When the wire rod is heated to a temperature exceeding T (℃) calculated by the above formula (I), the fine Ti carbonitrides precipitated during blooming will be dissolved again, and the dissolved Ti carbonitrides will be dissolved during the wire rod rolling. precipitates in the nucleus, which reduces the pinning effect. In order to utilize the pinning effect, it is effective to leave the fine Ti carbonitrides precipitated during blooming.

<Wire heating time>
By setting the heating time at 1000° C. or higher to 35 minutes or longer, the reaction force during wire rod rolling can be sufficiently reduced. On the other hand, if the heating time at 1000° C. or more and T° C. or less is less than 80 minutes, fine Ti carbonitrides can be sufficiently left.

By heating the slab and the wire rod under the above conditions, it is possible to obtain a wire rod in which Ti carbonitride has a fine grain size.
By subjecting the above-mentioned wire rod to wire drawing, a steel wire with excellent twisting properties can be obtained.
Moreover, even if the wire rod obtained by performing the above-mentioned wire rod rolling is re-LPed, the refinement of the structure can be reproduced.

<LP heating temperature>
The heating temperature in LP is preferably 900°C or higher. This is because if the temperature is lower than 900°C, a two-phase structure of ferrite/austenite is formed, and a uniform structure may not be obtained. The maximum temperature T, which is the upper limit of the heating temperature in the LP, is preferably a value calculated by the above formula (I). At a temperature exceeding T (° C.) calculated by the above formula (I), Ti carbonitride is dissolved and no pinning effect is obtained. The LP heating temperature is more preferably 1000°C or less.

<LP heating time>
Since complete austenitization can be achieved by performing LP heating for 30 seconds or more, the heating time is preferably 30 seconds or more. On the other hand, since LP heating for more than 300 seconds may cause austenite grains to become coarse, the heating time is preferably 300 seconds or less.

<Lead bath temperature>
During LP, the lead bath temperature is preferably 500°C or higher. This is because if the temperature is lower than 500°C, the temperature becomes a bainite transformation region, and the transformation may not be completed. The maximum temperature of the lead bath is preferably 630°C or less. This is because if the temperature exceeds 630°C, not only the strength of the wire will decrease, but also the transformation may not be completed.

<Lead bath immersion time>
The immersion time in the lead bath is preferably 10 seconds or more. This is because pearlite transformation may not be completed in less than 10 seconds. On the other hand, the upper limit of the lead bath immersion time is preferably 60 seconds. This is because tensile strength may decrease if immersed for more than 60 seconds.

[Manufacturing method of steel wire]
Although the method for manufacturing the steel wire of this embodiment is not particularly limited, an example of a preferable manufacturing method is a method of manufacturing the steel wire of this embodiment by manufacturing a wire rod by the above method and then drawing it. .
In addition, even if wire drawing is performed using the wire produced by the above method, if the amount of strain is insufficient, the degree of integration of <110> aligned in the longitudinal direction of the steel wire will be less than 2.0. There is a possibility that the wire may break during wire drawing when the amount of strain is too large. Therefore, when the diameter of the wire before wire drawing is D and the diameter of the steel wire is d, the true strain ε expressed as ε = 2 × ln (D/d) is 1.4 or more and 4.0 or less. It is preferable to manufacture the steel wire by drawing the wire so that the following is achieved.

The use of the wire rod and steel wire of this embodiment is not particularly limited, but for example, it can be used as a reinforcing material for automobile tires, a reinforcing wire for aluminum power transmission lines, a PC (prestressed concrete) steel wire, a bridge, etc. It can be widely used as a material for high-strength steel wire used in rope wires.

Hereinafter, examples of the wire rod and steel wire according to the present disclosure will be specifically explained using Examples, but the wire rod and steel wire according to the present disclosure are not limited to the following Examples.

Steels A and B having the chemical compositions (steel components) shown in Table 1 below and steels 1 to 32 having the chemical compositions shown in Table 2 were melted, respectively, and wire rods were produced by the method described below.
Note that the notation "-" in Tables 1 and 2 indicates that the content of the element is at an impurity level, and it can be determined that it is not substantially contained. The remainder of the chemical composition (mass %) of the steel shown in Tables 1 and 2 is iron (Fe) and impurities. Further, the underlined values in Table 2 are values that do not satisfy the steel composition of the wire rod and steel wire according to the present disclosure.

Steels having the chemical compositions shown in Tables 1 and 2, respectively, were melted and wire rods were produced by the following method.
First, steels A and B having the chemical compositions shown in Table 1 were respectively melted, heated to approximately 1200°C to 1300°C, and subjected to blooming rolling to obtain a 122mm square billet (steel billet), which was then wire rolled. Ta. Heating during blooming and wire rod rolling and adjustment cooling after finish rolling were performed under the conditions shown in wire rod numbers (R1) to (R30) shown in Table 3, respectively. That is, after heating the steel piece to about 1000 to 1120 °C, it is subjected to wire rolling, coiled at 810 to 920 °C, and cooled for about 30 seconds at an average cooling rate of 4 °C / sec to 33 °C / sec. , hot rolled to a diameter of 5.5 to 8.5 mm.

Regarding wire rod numbers (R21) to (R30), wire rods were obtained by rolling the wire rods under conditions different from the above manufacturing conditions.

[Metal structure]
The metal structure of the obtained wire was measured by the method described above.
In Table 3, PBS (pearlite block grain size) is the grain size (circle equivalent diameter) of the area where the ferrite crystal orientation is surrounded by an angle difference of 15° or more, and the pearlite block is the grain size of the area where the ferrite crystal orientation is surrounded by an angle difference of 15° or more. The crystal grains are approximately the same.
Note that ferrite was observed as the non-pearlite structure.

[Tensile strength and area reduction rate (aperture value) of wire rod]
The wire rod was cut into a length of 340 mm, and both ends of 70 mm each were fixed with wedge chucks, and a tensile test was performed. The tensile strength was calculated by dividing the obtained maximum load by the cross-sectional area.
Then, measure the wire diameter at the thinnest point after the tensile test of the wire, divide the change in cross-sectional area before and after the tensile test by the cross-sectional area before the tensile test, and multiply by 100% to calculate the aperture value. did.
Since it is preferable that the tensile strength is 900 MPa or more, a product having a tensile strength of 900 MPa or more was evaluated as an acceptable product.
Since the aperture value is preferably 59% or more, an aperture value of 59% or more was evaluated as an acceptable product.

Table 3 shows the manufacturing conditions, steel composition [Ti/N], wire structure, and wire properties of wire rod numbers (R1) to (R30).
Note that the value T (°C) calculated by the above-mentioned formula (I) as a preferable upper limit of the wire heating temperature is 1129°C for steel type A and 1067°C for steel type B.
The underlined values in Table 3 are inappropriate values under the manufacturing conditions of the wire according to the present disclosure. Furthermore, the underlined values for the wire structure in Table 3 are values that do not satisfy the structure of the wire according to the present disclosure. The underlined values for the characteristics of the wire are the values evaluated as rejected. The same applies to the underlined values in Tables 4 to 7.
In addition, in Tables 3 to 7, "number % of fine Ti carbonitrides" means the number % of Ti carbonitrides with a size of 10 nm or more and 40 nm or less among Ti carbonitrides with an equivalent circle diameter of 10 nm or more, and "coarse Ti carbonitrides""Number % of Ti carbonitrides" means the number % of Ti carbonitrides with an equivalent circle diameter of 3 μm or more among Ti carbonitrides with an equivalent circle diameter of 0.5 μm or more.

As shown in Table 3, wire rod numbers (R21) to (R23) and (R27) have a high proportion of coarse Ti carbonitrides due to the low heating temperature or short heating time during blooming rolling. The amount of fine Ti carbonitrides precipitated decreased. Therefore, the aperture value of the wire rod was less than 59%.
For wire rod numbers (R24) to (R26) and (R28) to (R30), the temperature during wire rod rolling was high or the heating time was long, so fine Ti carbonitrides precipitated during blooming were dissolved. The aperture value of the wire rod has become lower.

After rolling, the wire rod was treated with zinc phosphate film and drawn to a diameter of 1.8 mm.
Next, the wire rods numbered (R1) and (R21) were heated to 950°C to 1150°C and immersed in a lead bath at 550°C to undergo pearlite transformation.
The metal structure (wire structure) and properties of the obtained wire were measured by the method described above. Table 4 shows the LP conditions, steel composition [Ti/N], wire structure, and properties.

As shown in Table 4, LP numbers (L1) to (L4) that were subjected to LP treatment under appropriate heating conditions were able to have good apertures even after LP.
On the other hand, in the case of LP number (L5), fine Ti carbonitrides were dissolved and the aperture value decreased.
LP numbers (L6) to (L10) had few fine Ti carbonitrides in the wire stage, and the aperture value was low even after LP.

The wire rods (R1) to (R30) after rolling were treated with zinc phosphate film and drawn to a wire diameter d shown in Table 5 to produce steel wires.
The metal structure (wire structure) and properties of the obtained steel wire were measured by the method described above. Table 5 shows the wire rod diameter D before wire drawing, the steel wire diameter d after wire drawing, the steel composition [Ti/N], the steel wire structure, and the characteristics of the steel wire.

[Tensile strength and area reduction rate (aperture value) of steel wire]
A steel wire was cut into a length of 340 mm, and both ends of 70 mm were fixed with wedge chucks and a tensile test was conducted. The tensile strength was calculated by dividing the obtained maximum load by the cross-sectional area.
Then, measure the wire diameter at the thinnest point of the steel wire after the tensile test, divide the change in cross-sectional area before and after the tensile test by the cross-sectional area before the tensile test, and multiply by 100% to calculate the aperture value. Calculated.
Since it is preferable that the tensile strength is 1500 MPa or more, a product with a tensile strength of 1500 MPa or more was evaluated as an acceptable product.
Since the aperture value is preferably 59% or more, an aperture value of 59% or more was evaluated as an acceptable product.

[Torsional properties of steel wire after wire drawing]
In the torsion test, a steel wire with a length 100 times the wire diameter was twisted at 15 rpm until it broke, and the number of twists was measured. The torsion test was conducted on 10 steel wires, and when the average number of twists of the 10 steel wires was 38 times or more, the torsion properties were evaluated as being good.

Regarding the steel wire after wire drawing, the wire rod numbers (R1) to (R19), which control fine Ti carbonitrides and coarse Ti carbonitrides, can achieve both a high reduction of area and a high number of twists. ing.
On the other hand, in the wire rod number (R20), the number of twists decreased because the wire drawing strain was small and the <110> texture was insufficient.
Wire rod numbers (R21) to (R23) and (R27) had a high proportion of coarse Ti carbonitrides remaining and few fine Ti carbonitrides, so the aperture value and number of twists were low.
Wire rod numbers (R24) to (R26) and (R28) to (R30) had fewer fine Ti carbonitrides, and the aperture value and number of twists were low.

After melting steel grades 1 to 32 with chemical compositions shown in Table 2, wire rods (r1) to (r32) were manufactured using the same manufacturing method as wire rod number (R1) in Table 3.
The metal structure (wire structure) and properties of the obtained wire were measured by the method described above. Table 6 shows the wire rod diameter D, steel composition [Ti/N], wire rod structure, and wire rod characteristics.

Furthermore, the wire rod after rolling was treated with zinc phosphate coating, and then wire-drawn to a diameter of 1.8 mm.
The metal structure (steel wire structure) and properties of the obtained steel wire were measured by the method described above. Table 7 shows the steel wire diameter d, steel composition [Ti/N], steel wire structure, and properties of the steel wire.

Regarding wire rod numbers (r1) to (r22) and steel wire numbers (w1) to (w22), both the composition and manufacturing method were appropriate, and the wire rods and steel wires had both high strength and ductility.
The wire rod number (r23) and steel wire number (w23) had a low C content and low tensile strength.
The wire rod number (r24) and the steel wire number (w24) had a high C content, so the strength was high, and the drawing of the wire rod and the drawing of the steel wire were low.
The wire rod number (r25) and the steel wire number (w25) had a low Mn content, solute S remained, and the wire rod drawing and steel wire drawing were low.
Wire rod number (r26) and steel wire number (w26) had a high Mn content, and Mn was segregated in the center, resulting in low drawing of the wire and low drawing of the steel wire.
Wire rod number (r27) and steel wire number (w27) had a low Si content and low tensile strength.
The wire rod number (r28) and the steel wire number (w28) had a high Si content and the transformation was not completed at the wire rod stage, so the drawing of the wire rod and the drawing of the steel wire were low.
Wire rod number (r29) and steel wire number (w29) had a high Ti content, a high proportion of coarse Ti carbonitrides remained, and the number of wire rod drawings, steel wire drawings, and twists was low.
Wire rod numbers (r30), (r31) and steel wire numbers (w30), (w31) had low Ti/N and solid solution N remained, so the number of wire rod drawings, steel wire drawings, and twists was low. Ta.
Wire rod number (r32) and steel wire number (w32) have high Ti/N, many coarse Ti carbonitrides, and few fine Ti carbonitrides, so the wire rod drawing, steel wire drawing, and number of twists are difficult. It was low.

10 Wire rod C Central axis D Wire rod diameter

Claims (13)

C in mass%: 0.40% or more and 0.80% or less,
Si: 0.10% or more and 2.0% or less,
Mn: 0.10% or more and 1.00 % or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0015% or more and 0.0060% or less, and Ti: 0.005% or more and 0.030% or less,
and has a steel component with the remainder consisting of Fe and impurities, and the ratio of Ti content to N content satisfies 3.3 or more and 6.5 or less,
When the diameter of the wire is D, the number of Ti carbonitrides with a diameter of 10 nm or more and 40 nm or less is 10% or more among Ti carbonitrides with an equivalent circle diameter of 10 nm or more in a region within D/50 from the central axis. , a wire in which the number of Ti carbonitrides with a circular equivalent diameter of 0.5 μm or more in a region within D/9 from the central axis is 40% or less; the number of Ti carbonitrides with a diameter of 3 μm or more is 40% or less; The steel component is Al: 0.050% or less in mass %,
Cr: 1.0% or less,
Nb: 0.050% or less,
V: 0.15% or less,
Ca: 0.0040% or less,
Mg: 0.0040% or less, and B: 0.0030% or less,
The wire rod according to claim 1, which satisfies one or more types selected from the group consisting of: Having a pearlite structure, in a region within D/9 from the central axis, the average value of the crystal grain size is 16 μm or less when defining the region where the ferrite crystal orientation is surrounded by an angular difference of 15° or more as a crystal grain. The wire according to claim 1 or 2. The wire according to any one of claims 1 to 3, which has a pearlite structure, and the area ratio of the pearlite structure in a region within D/9 from the central axis is 90% or more. The wire according to any one of claims 1 to 4, which has a pearlite structure and has an average lamella spacing of the pearlite structure in a region within D/9 from the central axis of 70 nm or less. The wire rod according to any one of claims 1 to 5, wherein the steel component satisfies Al: 0.005% or more and 0.050% or less in mass %. The wire rod according to any one of claims 1 to 6, wherein the steel component satisfies Cr: 0.05% or more and 1.0% or less in mass %. Any one of claims 1 to 7, wherein the steel component satisfies at least one of Nb: 0.003% to 0.050% and V: 0.005% to 0.15% in mass %. Wire rod described in . Any one of claims 1 to 8, wherein the steel component satisfies at least one of Ca: 0.0002% or more and 0.0040% or less and Mg: 0.0002% or more and 0.0040% or less. Wire rod described in . The wire rod according to any one of claims 1 to 9, wherein the steel component satisfies B: 0.0001% or more and 0.0030% or less in mass %. The wire rod according to any one of claims 1 to 10, wherein the wire rod has a diameter of 1.5 mm or more and 9.0 mm or less. 6. It has the steel composition according to claim 1, claim 2, and any one of claims 6 to 10, and has a ratio of Ti content to N content of 3.3 or more. 5 or less,
When a steel wire is heated to 900°C at an average heating rate of 10°C/sec to 30°C/sec and held for 1 minute, the area within d/20 from the central axis, where d is the diameter of the steel wire. The number of Ti carbonitrides with a circle equivalent diameter of 10 nm or more is 10% or more and 40 nm or less is 10% or more, and the circle equivalent diameter is 0.5 μm in a region within d/9 from the central axis. Among the above Ti carbonitrides, the number of Ti carbonitrides of 3 μm or more is 40% or less, and the <110> orientation is parallel to the longitudinal direction of the steel wire in a region within d/9 from the central axis. A steel wire having a degree of accumulation of 2.0 or more. The steel wire according to claim 12, wherein the diameter of the steel wire is 0.5 mm or more and 3.0 mm or less.