JP2006200039A - High carbon steel wire material having excellent wire drawability and manufacturing process thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high carbon steel wire material which is made of high carbon steel as a raw material for wire products such as steel cords, bead wires, PC steel wires and spring steel, allows for these wire products to be manufactured with high productivity and has excellent wire drawability and a manufacturing process thereof. <P>SOLUTION: The high carbon steel wire material having excellent wire drawability is composed of a steel material in which the respective contents of C, Si, Mn, P, S, N, Al and O are specified. The bcc-Fe crystal grains of its metal structure have an average crystal grain diameter (Dave) of 20 μm or less and a maximum crystal grain diameter (Dmax) of 120 μm or less. As a preferred mode of the steel material, the bcc-Fe crystal grains of the above metal structure have an area ratio of crystal grains having a diameter of 80 μm or more of 40% or less, an average sub grain diameter (dave) of 10 μm or less, a maximum sub grain diameter (dmax) of 50 μm or less, and a (Dave/dave) ratio of the average crystal grain diameter (Dave) to the average sub grain diameter (dave) of 4.5 or less. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a high carbon steel used as a material for wire drawing products such as steel cords, bead wires, PC steel wires, spring steel, etc., and these wire drawing products are highly productive under high wire drawing workability. The present invention relates to a carbon steel wire material that can be manufactured and excellent in wire drawability, and a manufacturing method thereof.

  When manufacturing the wire drawing products as described above, the steel wire material is usually drawn for size adjustment and material (physical property) adjustment. Improving processability is extremely useful for improving productivity and the like. Incidentally, when the wire drawing workability is improved, not only can the productivity be improved by increasing the wire drawing speed and the number of wire drawing passes, but also many benefits such as extending the die life can be enjoyed.

  By the way, with regard to wire drawing workability, conventionally, improvement studies have been conducted mainly focusing on the wire breakage resistance during wire drawing. For example, Patent Document 1 discloses a technique for improving disconnection resistance by focusing on the pearlite block size, the amount of proeutectoid cementite, the cementite thickness, the Cr concentration in cementite, and the like, and optimizing them.

  Patent Document 2 reveals that the wire drawing limit is improved by controlling the area ratio of the upper bainite and the size of the intragranular bainite. In addition, Patent Document 3 discloses a technique for improving the wire breakage resistance and the die life by controlling the total oxygen content and the non-viscous inclusion composition in steel. As for the die life, the peelability (descaling property) of the scale existing on the surface of the steel wire is also important, and if the scale peelability is poor and the scale remains on the surface of the steel wire, it will cause die loss during wire drawing. For this reason, Patent Document 4 discloses a technique for improving mechanical descaling properties by controlling holes present in a scale.

However, the conventional technology as described above is a technology that focuses on improving the wire breakage resistance under specific wire drawing conditions. From the viewpoint of wire drawing workability, the wire drawing speed is increased and the number of wire drawing passes is reduced. Little attention has been paid to extending the die life. In addition, as summarized in Chapter 6 of Non-Patent Document 1, it has been clarified that an increase in the drawing speed and an increase in the area reduction rate per pass cause a deterioration in the ductility of the drawn product and a reduction in the die life. ing. However, including the prior art as described above, an effect of improving the wire drawing workability so that an increase in the wire drawing speed and an increase in the area reduction rate can be achieved on a practical scale has not been obtained.
JP 2004-91912 A JP-A-8-295930 JP-A-62-130258 Japanese Patent No. 3544804 "Drawing" edited by the Japan Society for Technology of Plasticity (issued by Corona on October 25, 1990), especially Chapter 6

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to focus on productivity in particular, and it is possible to increase the drawing speed, increase the area reduction rate, and further extend the die life. An object of the present invention is to provide a steel wire rod excellent in wire drawing workability and to provide a method capable of efficiently producing such a steel wire rod.

  The composition of the high carbon steel wire rod excellent in drawability according to the present invention that can solve the above-mentioned problems is expressed by mass% (hereinafter, simply expressed as% in the case of a component composition), and C: 0.6-1. 1%, Si: 0.1 to 2.0%, Mn: 0.1 to 1.0%, P: 0.020% or less, S: 0.020% or less, N: 0.006% or less, Al : 0.03% or less, O: 0.0030% or less is satisfied, the balance is Fe and inevitable impurities, and the average crystal grain size (Dave) is 20 μm or less in the bcc-Fe crystal grains of metal structure, and the largest crystal The gist exists where the particle size (Dmax) is 120 μm or less.

As a more preferable embodiment of the steel wire according to the present invention, in the bcc-Fe crystal grain of the metal structure, the area ratio occupied by the grain size of 80 μm or more is 40% or less, and further, the average subcrystal grain The diameter (dave) is 10 μm or less, the maximum subcrystal grain size (dmax) is 50 μm or less, and the ratio (Dave / dave) of the average crystal grain size (Dave) to the average subcrystal grain size (dave) is Those having a tensile strength of 4.5 or less, and those satisfying the relationship of the following formula (1) when the tensile strength of the steel wire is TS and the C concentration in the steel wire is Wc are mentioned.
TS ≦ 1240 × Wc ^0.52 (1)

  In addition to the above components, the steel wire rod according to the present invention, if necessary, Cr: 1.5% or less (not including 0%), Cu: 1.0% or less (not including 0%), Ni: It contains at least one element selected from 1.0% or less (excluding 0%), or Mg: 5 ppm or less (not including 0 ppm), Ca: 5 ppm or less (not including 0 ppm), REM: It may contain at least one element selected from 1.5 ppm or less (not including 0 ppm).

  In the steel wire rod of the present invention, it is also a preferred embodiment that the total decarburization amount (Dm-T) of the surface layer is 100 μm or less and the scale adhesion amount is 0.15 to 0.85 mass%.

Furthermore, the production method of the present invention is positioned as a useful method for producing a high carbon steel wire rod having the above characteristics and excellent wire drawing property,
The first method consists of a steel satisfying the above ingredient composition requirements, cooling a steel wire rod which has been heated to 730-1,050 ° C., to a temperature of four hundred seventy to six hundred and forty ° C. at an average cooling rate of more than 15 ° C. / sec _{(T 1)} Then, it has a gist where it is heated at an average rate of temperature increase of 3 ° C./second or more to a temperature (T ₂ ) of 550 to 720 ° C., which is higher than the temperature (T ₁ ).
In the second manufacturing method, a steel material satisfying the above component composition requirements is heated to 900 to 1260 ° C., hot-rolled at a temperature of 740 ° C. or higher, and finish-rolled at a temperature of 1100 ° C. or lower, and then a temperature of 750 to 950 ° C. Water-cooled to an area and wound on a conveying device, and then cooled at an average cooling rate of 15 ° C./second or more, the steel material temperature minimum value (T ₃ ) was reduced to 500 to 630 within 20 seconds from the end of winding. The temperature is lowered to ℃ and then heated, and within 45 seconds from the end of winding, the steel material maximum value (T ₄ ) is increased to 580 to 720 ℃, which is higher than the above minimum value (T ₃ ). is doing.

  According to the present invention, the contents of C, Si, Mn, P, S, N, Al, and O in steel are specified, and the average crystal grain size and the maximum crystal of the bcc-Fe crystal grains from the metal structure surface. Specify the grain size, preferably further suppress the area ratio of coarse crystal grains, and further specify the average subcrystal grain size and maximum subcrystal grain size of the bcc-Fe crystal grains, and the grain size ratio thereof. Provides a high carbon steel wire that has excellent wire drawing workability, can not only increase productivity by increasing the drawing speed and increasing the area reduction ratio, but also can extend the die life, Furthermore, it is possible to provide a method capable of reliably and efficiently producing a high carbon steel wire having such excellent wire drawing workability.

  Hereinafter, the reason for determining the chemical composition of the steel material in the present invention will be clarified, and subsequently, the reason for determining the crystal grain size of the steel structure will be described in detail.

  First, the reason for determining the chemical composition of the steel material will be described.

C: 0.6% or more, 1.1% or less An element that affects the strength of steel materials, and in order to ensure the strength required for steel cords, bead wires, PC steel wires, etc., which are the subject of the present invention , 0.6% or more of addition is required. If the C content is increased, the strength increases, but if it is too much, the ductility deteriorates, so the upper limit was made 1.1%.

Si: 0.1% or more, 2.0% or less Steel materials that are highly drawn are particularly added for the purpose of deoxidation, and it is necessary to add 0.1% or more. Further, Si contributes to strengthening of the steel material, so the amount is increased as necessary. However, excessive addition increases the solid solution strengthening and promotes decarburization. The upper limit was set at 2.0% from the viewpoint of preventing decarburization. A more preferable Si content is 0.15% or more and 1.8% or less.

Mn: 0.1% or more, 1.0% or less It is necessary to add 0.1% or more for the purpose of deoxidation and fixing S which is a harmful element as MnS and detoxifying it. Mn also has the effect of stabilizing the carbides in the steel, but if it is too much, segregation and supercooled structure occur and the drawability deteriorates, so it must be suppressed to 1.0% or less. A more preferable Mn content is 0.15% or more and 0.9% or less.

P: 0.020% or less (excluding 0%)
P is an element that is particularly harmful to the wire drawing workability. If it is too much, the toughness of the steel material deteriorates, so the upper limit is set to 0.020% in the present invention. Preferably it is 0.015% or less, More preferably, it is 0.010% or less.

S: 0.020% or less (excluding 0%)
Although it is a harmful element, it can be fixed as MnS by adding Mn as described above. However, if the S content increases, the amount and size of MnS increase and the ductility deteriorates. Therefore, in the present invention, the upper limit is set to 0.020%. More preferably, it is 0.015% or less, More preferably, it is 0.010% or less.

N: 0.006% or less (excluding 0%)
Although age hardening contributes to strength increase, in order to deteriorate ductility, the upper limit is set to 0.006% in the present invention. Preferably it is 0.004% or less, More preferably, it is 0.003% or less.

Al: 0.03% or less (excluding 0%)
Al is effective as a deoxidizer, and also contributes to refinement of the metal structure by combining with N to form AlN. However, if the Al content is too large, coarse oxides are formed and the wire drawing is deteriorated. Therefore, in the present invention, the upper limit is set to 0.03%. More preferably, it is 0.01% or less, More preferably, it is 0.005% or less.

O: 0.003% or less As the amount of oxygen in the steel increases, coarse oxides are easily formed and the drawability deteriorates. Therefore, in the present invention, the upper limit is set to 0.003%. Preferably it is 0.002% or less, More preferably, it is 0.0015% or less.

  The steel wire of the present invention has the above chemical components as basic components, and the balance is substantially iron and inevitable impurities, but may contain the following elements as necessary.

Cr: 1.5% or less Although it is an element effective for increasing the strength of steel materials, if added excessively, a supercooled structure is easily formed and the drawability is deteriorated. It must be suppressed to.

Cu: 1.0% or less In addition to having an action of suppressing decarburization of the surface layer part, it also has an action of enhancing corrosion resistance, and can be added as necessary. However, if added excessively, not only does cracking easily occur during hot working, but also the wire drawability is adversely affected by the formation of a supercooled structure, so the upper limit was set to 1.0% in the present invention.

Ni: 1.0% or less Ni is added as necessary because it is effective for suppressing decarburization of the surface layer portion and improving corrosion resistance in the same manner as Cu. However, if added excessively, the drawability deteriorates due to the formation of a supercooled structure, so it must be suppressed to 1.0% or less.

Mg: 5 ppm or less Since Mg has the effect of softening the oxide, it can be added as necessary. However, if added excessively, the properties of the oxide change and the drawability deteriorates, so it is best to keep it at most 5 ppm, preferably 2 ppm or less.

Ca: 5 ppm or less Ca also has an effect of softening an oxide, and may be added as necessary. However, if it is added excessively, the properties of the oxide change and the drawability deteriorates, so it should be suppressed to 5 ppm or less, and more preferably 2 ppm or less.

REM: 1.5 ppm or less REM also has the effect of softening the oxide, and may be added as necessary. However, if excessively added, the properties of the oxide change as in the case of Mg and Ca and the wire drawability deteriorates, so the upper limit was made 1.5 ppm. More preferably, it is 0.5 ppm or less.

  Next, the metal structure will be described.

  In the present invention, on the premise of satisfying the above component composition, the metal structure is “with respect to the bcc-Fe crystal grains, the average crystal grain size (Dave) is 20 μm or less and the maximum crystal grain size (Dmax) is 120 μm or less. ”Is mandatory.

  More preferably, for the bcc-Fe crystal grains, “the area ratio of crystal grains having a grain size of 80 μm or more is 40% or less”, “the average subcrystal grain size (dave) is 10 μm or less and the maximum subcrystal grain size ( dmax) is 50 μm or less ”, or“ the ratio of the average crystal grain size (Dave) to the average subcrystal grain size (dave) (Dave / dave) is 4.5 or less ”.

  Typical wire breaking forms at the time of wire drawing include, for example, “drawing limit of hard steel wire and its controlling factors, plasticity and working” (Takahashi et al.), Vol. 19 (1978), page 726, there are a broken copper line, a vertical crack and a shear crack. According to this, it is said that the disconnection of the coupling occurs when the pearlite block of the wire is rough and the ductility is poor. Therefore, for example, Japanese Patent Application Laid-Open No. 2004-91912 also attempts to improve the disconnection resistance by controlling the pearlite block particle size number to 6-8. However, even this invention has not yet achieved an increase in the drawing speed during the drawing process.

  Therefore, the present inventors have stated that “a disconnection of a cappy is a phenomenon in which a void is generated and grows at a place where crystal rotation does not occur smoothly during wire drawing, leading to fracture. Even if the crystal grain size was made finer, if coarse crystal grains existed, voids were generated in the part, and this caused disconnection ”, and we worked on controlling the size and distribution of the crystal grain size.

  In addition, since the relatively high carbon steel wire targeted by the present invention is often controlled by a pearlite-based structure, the ductility of the wire is often represented by a pearlite block (“eutectoid pearlite steel”). "Takahashi et al., Journal of the Japan Institute of Metals, vol. 42 (1978) p. 708). However, since ordinary steel materials contain other structures such as ferrite and bainite, the present inventors should consider the size and distribution of the entire crystal grain size including structures other than pearlite. I studied repeatedly under my thoughts.

  As a result, as specified in the present invention, if the average crystal grain size (Dave) is refined to 20 μm or less and the maximum crystal grain size (Dmax) is controlled to 120 μm or less, the drawability is greatly improved. I found out. Incidentally, when the average crystal grain size (Dave) exceeds 20 μm, the wire becomes insufficiently ductile. Further, even if the average crystal grain size (Dave) is 20 μm or less, if the maximum crystal grain size (Dmax) exceeds 120 μm, it tends to break during wire drawing. In order to obtain a higher degree of wire drawing, the average crystal grain size (Dave) is preferably 17 μm or less, and the maximum crystal grain size (Dmax) is preferably 100 μm or less.

  From the viewpoint of the metal structure, by specifying the average crystal grain size (Dave) and the maximum crystal grain size (Dmax), the purpose of the present invention can be achieved, but the wire drawing workability is further improved. In addition to these requirements, it is desirable to control to satisfy the following requirements.

  That is, in the same bcc-Fe crystal grains having a metal structure, if the crystal grains having a grain size of 80 μm or more are controlled to an area ratio of 40% or less and the entire crystal grains are uniformly refined, the drawability is further improved. Can do. A more preferable area ratio of crystal grains having a particle diameter of 80 μm or more is 25% or less, and particularly preferably 0%.

  In addition, when further studies were made with the aim of further improving the drawability, so-called subcrystal grains, which are crystal units having a boundary with a small misorientation with the adjacent crystal, also affect the crystal rotation. It has been found that the drawability can be further improved if the grain size (dave) is 10 μm or less and the maximum subcrystal grain size (dmax) is 50 μm or less. That is, it is considered that when the number of coarse subcrystal grains is reduced and uniform refinement is performed, the stress concentration portion is reduced and the generation of voids is suppressed. In order to exert such effects, the average subcrystal grain size (dave) is more preferably 7 μm or less, and the maximum subcrystal grain size (dmax) is 40 μm or less.

  Furthermore, with regard to the average crystal grain size (Dave) and the average subcrystal grain size (dave), it is confirmed that the wire drawability is further improved if the ratio (Dave / dave) of both is kept within the above range. It was. This is probably because crystal rotation during wire drawing is smooth over the entire steel material and stress concentration is less likely to occur. A preferable (Dave / dave) ratio is 4.5 or less, more preferably 4.0 or less, for effectively exhibiting such an action.

Further, in the present invention, in order to further improve the drawability, the relationship of “TS ≦ 1240 × Wc ^0.52 ” (TS represents the tensile strength of the steel wire and Wc represents the C concentration in the steel wire) is satisfied. It is also effective to control this.

  Increasing the wire drawing speed and increasing the area reduction rate not only facilitates the generation of voids, but also increases the temperature of the steel wire and the die, leading to wire breakage (longitudinal cracking / shear cracking) and a reduction in die life. When the drawing speed and the area reduction rate are the same, the temperature rise has a great influence on the strength of the wire, and the lower the tensile strength, the lower the temperature rise. Further, the tensile strength of the steel wire as a whole is greatly affected by variations in strength within the wire, the tensile strength is almost determined by the C content in the steel wire, and the tensile strength (TS) and the steel. It was confirmed that if the relationship of the C content (Wc) of the wire is adjusted so as to satisfy the relationship of the above formula, disconnection due to a temperature rise during wire drawing is significantly suppressed and the die life is also improved.

  In addition, in the present invention, in order to further improve the wire drawing workability, the influence of the surface layer decarburization amount and the scale adhesion amount of the steel wire on the wire drawing property was also examined. It was confirmed that “(Dm-t) is 100 μm or less and the surface layer scale deposition amount is 0.15 to 0.85 mass%” also shows excellent wire drawing workability.

  By the way, even when the wire drawing property is improved by the component design and the structure control of the steel wire material, the wire drawing property is affected by the scale property of the surface of the steel wire material. Steel wire rods are chemically and mechanically descaled prior to wire drawing. However, if wire drawing is performed while the scale cannot be completely removed in the process, die loss is caused. The influence of the scale adhesion amount on the scale peelability is large, and the larger the scale adhesion amount, the better the peelability. However, if the amount is too large, rusting may occur after peeling. In addition, when decarburization occurs on the surface of the steel wire rod, even if the amount of scale attached is sufficient, the scale bites into the decarburized part, and the scale peeling becomes difficult. Therefore, in the present invention, as a result of pursuing the requirements for reducing the scale-derived wire drawing obstruction factor as much as possible, the surface total decarburization amount (Dm-T) is controlled to 100 μm or less as described above, and the surface scale is further reduced. It has been confirmed that if the adhesion amount is controlled within the range of 0.15 to 0.85 mass%, the reduction in wire drawing due to scale can be suppressed as much as possible.

  Next, the manufacturing method of the high carbon steel wire rod provided with the said characteristic is demonstrated.

First, the first method is to cool a steel wire heated to 730 to 1050 ° C. made of steel that satisfies the above-described requirements for the component composition at an average cooling rate of 15 ° C./second or more, and a temperature of 470 to 640 ° C. _{(T 1)} after cooling to a method of heating at that temperature _{(T 1)} is higher than five hundred and fifty to seven hundred and twenty ° C. of the temperature _{(T 2)} to 3 ° C. / sec or more average heating rate.

In the second method, a steel material that satisfies the above-described requirements for the component composition is heated to 900 to 1260 ° C., hot-rolled at a temperature of 740 ° C. or higher, and finish-rolled at a temperature of 1100 ° C. or lower, and then 750 to 750 ° C. Water cooling to a temperature range of 950 ° C. and winding on a transfer device, followed by cooling at an average cooling rate of 15 ° C./second or more, the steel material temperature minimum value (T ₃ ) within 20 seconds from the end of winding The temperature is lowered to 500 to 630 ° C. and then heated, and within 45 seconds from the end of winding, the maximum value (T ₄ ) of the steel material is increased to 580 to 720 ° C., which is higher than the minimum value (T ₃ ). It is.

  That is, in order to obtain a steel wire rod having the above characteristics, it is necessary to first heat to 730 ° C. or higher in order to make the carbide once solid solution and to make the structure before transformation uniform. At this time, the higher the heating temperature, the better the scale peelability. However, if the heating temperature exceeds 1050 ° C., the crystal grains before transformation become coarse, and it becomes difficult to control the structure by transformation in the subsequent cooling step. The temperature should be kept below 1050 ° C. The more preferable heating temperature at this time is 750 degreeC or more and 1000 degrees C or less.

In the cooling step after the heating, the crystal grain size to be controlled by the present invention is determined. In order to make the crystal grain size as uniform and fine as possible, the cooling rate after heating should be as high as possible. In the present invention, the average cooling rate is set to 15 ° C./second or more. In addition, the lower the ultimate temperature (T ₁ ) at the time of cooling, the finer the crystal grains, but when cooled to a temperature below 470 ° C., it becomes easy to generate a supercooled structure that impairs the drawability, so the lower limit is set to 470 ° C. It was. If the reached temperature exceeds 640 ° C., the average particle size becomes coarse, so it must be cooled to at least 640 ° C. or less. A more preferable temperature reached during the cooling is 480 ° C. or more and 630 ° C. or less.

In the present invention, it is essential to raise the temperature to 550 to 720 ° C. (T ₂ ), which is higher than the ultimate temperature (T ₁ ), following the above cooling step for crystal grain refinement. The temperature (T ₂ ) at the time of the temperature rise has a significant influence on the strength of the steel material, and the strength decreases as the temperature (T ₂ ) increases, which is advantageous for wire drawing workability. If the temperature is lower than 550 ° C., the strength is not sufficiently lowered. On the other hand, if the temperature exceeds 720 ° C. and the temperature is excessively high, transformation may be insufficient and the strength may be increased. A more preferable reached temperature at the time of the temperature rise is 580 ° C. or more and 715 ° C. or less.

That is, after cooling once to a temperature (T ₁ ) of 470 ° C. or more and 640 ° C. or less (more preferably 480 ° C. or more and 630 ° C. or less), T ₁ <T ₂ and 550 ° C. or more and 720 ° C. or less (more By reheating to a temperature (T ₂ ) of preferably 580 ° C. or more and 715 ° C. or less, more preferably 710 ° C. or less, a steel material with uniform and fine crystal grains and low strength can be obtained.

However, at this time, if the average rate of temperature increase from the temperature (T ₁ ) to the temperature (T ₂ ) is too slow, the reduction in strength to the level intended by the present invention cannot be achieved. It is an indispensable requirement to set the temperature to 3 ° C./second or more. That is, in order to obtain a steel wire excellent in drawability by the first method, a wire heated to 730 ° C. or higher and 1050 ° C. or lower (more preferably 750 ° C. or higher and 1000 ° C. or lower) is cooled at an average cooling rate of 15 ° C. / After cooling to a temperature (T ₁ ) of 470 ° C. or more and 640 ° C. or less (more preferably 480 ° C. or more and 630 ° C. or less) in a second or more, 550 ° C. or more and 720 ° C. which are higher than the temperature (T ₁ ) It is important to raise the temperature (T ₂ ) below (more preferably 580 ° C. or more, 715 ° C. or less, more preferably 710 ° C. or less) at a rate of 3 ° C./second or more.

  On the other hand, when the steel wire to which the present invention is applied is a hot-rolled wire, the second method is applied and the following control is performed.

  First, after heating to 900-1260 degreeC with a heating furnace, it hot-rolls at the temperature of 740 degreeC or more, and finish rolling temperature is controlled to 1100 degrees C or less. This is because if the heating temperature is less than 900 ° C., the heating is insufficient, and if it exceeds 1260 ° C., the surface decarburization region becomes wide. A more preferable heating temperature is 900 ° C. or more and 1250 ° C. or less. Further, when the rolling temperature is lowered, surface layer decarburization is promoted, resulting in deterioration of scale peelability. Therefore, the lower limit temperature of hot rolling is set to 740 ° C. A more preferable lower limit temperature is 780 ° C. Further, when the finish rolling temperature exceeds 1100 ° C., it becomes difficult to control the transformation structure by cooling and reheating performed in the next step, so the upper limit of the finish rolling temperature is set to 1100 ° C.

  After finish rolling, it is water-cooled to 750 to 950 ° C., wound up and placed on a conveyor such as a conveyor. The temperature management performed after water cooling is for the subsequent transformation control and scale control, but when the temperature reached during cooling becomes lower than 750 ° C, a supercooled structure is formed on the surface layer, while the high temperature region exceeding 950 ° C. Then, the deformability of the scale is lost, and it peels off during transportation and causes rust.

After winding, the steel sheet is cooled at an average cooling rate of 15 ° C./second or more, and the minimum steel material temperature is set to a temperature of 500 to 630 ° C. (T ₃₎ within 20 seconds after winding on the conveying device. and controls to), followed by the temperature _{(T 3),} the maximum value of the steel material temperature until after loading 45 seconds five hundred eighty to seven hundred twenty ° C. is higher than the temperature _{(T 3)} temperature _(T 4) It is an important requirement to obtain a metal structure excellent in drawability.

That is, the crystal grains can be made uniform and fine by controlling cooling at a rate of 15 ° C./second or more so that the minimum temperature (T ₃ ) within 20 seconds after winding and placing becomes 500 to 630 ° C. it can. If the cooling rate is less than 15 ° C./second, the metal structure cannot be sufficiently refined uniformly due to insufficient cooling rate, and some coarse grains are generated. The faster the cooling rate at this time, the more effective is the refinement of the metal structure. However, when blast cooling is performed after hot rolling, the variation in the cooling rate in the steel wire tends to increase. Therefore, the cooling rate should be 120 ° C./second or less, more preferably 100 ° C./second or less. Further, even when cooled to below 480 ° C. in this cooling step, a supercooled structure is generated on the surface layer, and conversely, when it exceeds 630 ° C., coarse particles are easily generated. Even when the temperature is not cooled to a suitable temperature range within 20 seconds after the winding and mounting, the metal structure becomes coarse.

After the cooling, subsequently, from the temperature (T ₃ ), a temperature of 580 to 720 ° C. (T ₃ ) in which the maximum value of the steel material is higher than the temperature (T ₃ ) by 45 seconds from the winding and mounting. _By controlling to ₄ ), the strength of the hot rolled material can be significantly reduced. In order to more effectively reduce the strength at this time, it is preferable to set the time from the winding and mounting to the temperature range within 42 seconds, more preferably within 40 seconds. Here, when the temperature T ₄ is lower than the temperature T ₃ or when the temperature T ₄ is less than 580 ° C., the strength is insufficient, and when the temperature T ₄ exceeds 720 ° C., the ductility is lowered with the strength.

As described above, in order to obtain a hot-rolled wire rod excellent in drawability, the second method is adopted, and after heating to 900 to 1260 ° C. (more preferably 900 to 1250 ° C.) in a heating furnace, the rolling temperature After hot rolling at 740 ° C. or higher (more preferably 780 ° C. or higher), the finish rolling temperature is controlled to 1100 ° C. or lower, subsequently cooled to 750 to 950 ° C. and wound on a conveying device, then 15 The steel material is cooled at a rate of at least ° C./second, and the minimum steel material temperature within 20 seconds after winding is controlled to a temperature (T ₃ ) of 500 to 630 ° C., and then the temperature (T ₃ ) is taken up. By controlling the maximum value of the steel material temperature from 45 to 45 seconds to a temperature (T ₄ ) of 580 to 720 ° C. with T ₃ <T ₄ , a high carbon steel wire material excellent in wire drawing workability is efficiently obtained. be able to. The preferable range of the temperature (T ₄ ) is 580 to 715 ° C, and more preferably 580 to 710 ° C.

  Hereinafter, the configuration and operational effects of the present invention will be described in more detail with reference to experimental examples.However, the present invention is not limited by the following experimental examples, and is appropriate within a range that can be adapted to the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

Experimental example 1
A hot rolled steel wire rod having a chemical composition shown in Table 1 and having a diameter of 5.5 mm was produced. Note that REM in Table 1 is the total amount of La, Ce, Pr, and Nd. Each hot-rolled steel wire obtained was heated in an atmospheric furnace under the conditions shown in FIG. 1 and Tables 2 and 3, and continuously charged into a lead furnace and heat-treated to obtain various steel wires. In this experimental example, an example of heat treatment using an atmospheric furnace and a lead furnace was shown, but the present invention is not limited to the use of these facilities, and other heating furnaces and holding furnaces may be used. Of course it is possible.

  Each steel wire obtained was evaluated for structural characteristics, scale characteristics and tensile characteristics. Among the structural features, regarding crystal units of crystal grains and sub-crystal grains, since it is important to evaluate the variation of each crystal unit in the present invention, SEM / EBSP (Electron Back Scatter diffraction Pattern) is used as the evaluation method. ) Method was adopted. The trade name “JSM-5410” manufactured by JEOL was used as the SEM, and the “OIM (Orientation Imaging Microscopy) system” manufactured by TSL was used for the EBSP method.

  Samples were collected from each steel wire by wet cutting, and then wet polishing, buff polishing, and chemical polishing were employed for sample preparation for EBSP measurement, and samples with reduced distortion and irregularities in the polishing process were made as much as possible. At this time, it grind | polished so that an observation surface might become the longitudinal cross-section of a steel wire.

The obtained sample was used, and the measurement was performed using the center portion of the wire diameter of the steel wire as the EBSP measurement position. The measurement step was 0.5 μm or less, and the measurement area of each steel wire was 60000 μm ² or more. The crystal orientation was analyzed after the measurement. In order to increase the reliability of the analysis, the analysis was performed using the measurement result having an average CI (Confidence Index) value of 0.3 or more.

  As a result of the analysis of the bcc-Fe crystal orientation, the crystal unit intended by the present invention is defined as a region surrounded by a boundary line having an orientation angle difference of 10 ° or more, and a boundary line having an orientation angle difference of 2 ° or more. Each sub-grain is the area surrounded by, and each analysis result (boundary map: an example is shown in Fig. 2) is obtained, and each boundary map is processed by image analysis software "Image-Pro" Units were calculated and evaluated.

  First, the area of each region (crystal unit) surrounded by the boundary line in the boundary map is obtained by “Image-Pro”. Next, the circle diameter calculated by approximating each crystal unit to the equivalent circle diameter based on the area was adopted as the individual crystal grain size. Based on the calculation results, statistical processing is performed as shown in FIG. 3 as an example to obtain an average crystal grain size (Dave), an average subcrystal grain size (dave), a maximum crystal grain size (Dmax), and a maximum subcrystal grain size. The diameter (dmax), the area ratio of crystal grains of 80 μm or more, and the ratio of the average crystal grain size to the average subcrystal grain size (Dave / dave) were determined.

  Among the structural characteristics, the total decarburization amount was determined by the measurement method described in JIS G0558. The sample was cut and collected from the steel wire, embedded in the resin so that the cross section of the wire became the observation surface, wet-polished, buffed, and exposed to an optical microscope by revealing the metal structure with 5% nital, The decarburization amount of the steel wire surface layer was measured. In the decarburization evaluation, two or more pieces were measured for each steel wire, and the average value was obtained.

  The scale characteristics were evaluated by the amount of scale attached to the steel wire surface layer. Specifically, a sample having a length of 200 mm was cut and collected from each steel wire, and the amount of scale adhesion was calculated from the weight difference between the samples before and after pickling with hydrochloric acid. For the scale evaluation, 10 or more steel wires were measured and the average value was used.

  For the evaluation of the tensile properties, a sample having a length of 400 mm was cut from each steel wire, and a tensile test was performed with a universal testing machine at a crosshead speed of 10 mm / min and a gauge length of 150 mm. More than 40 wires were measured for each steel wire, and the average values were taken as the tensile strength (TS: MPa) and the drawing value (RA:%).

  Next, evaluation of wire drawing property will be described. Each steel wire was subjected to descaling and lubricating film treatment as a pre-drawing process. Hydrochloric acid was used for descaling, and the scale was removed by pickling. After removing the scale, a phosphate film was applied to the surface of each steel wire as a lubricating film treatment, and then subjected to wire drawing. Thereafter, dry drawing was performed with a continuous wire drawing machine to a final wire diameter of 0.9 mm.

  In this experimental example, the purpose is to improve the productivity at the time of wire drawing, and the wire drawing conditions are the condition (1) in which the final wire drawing speed is 600 m / min and the number of dies is 14 and the number of dies is 14 pieces. The test was carried out under three conditions: a condition (2) in which the final drawing speed was increased to 800 m / min, and a condition (3) in which the number of dies was reduced to 12 at a final drawing speed of 800 m / min.

  As the conditions (1) to (3) change, the productivity of wire drawing improves, but the wire drawing conditions become severe, and the steel wire used for wire drawing requires high wire drawing. Under these three conditions, 50 tons of wire was drawn for each steel wire, and the presence or absence of wire breakage during wire drawing and the die life were evaluated. The die life is evaluated as (x) when the die breaks during wire drawing. Die breakage does not occur during wire drawing of 50 tons, but the die is worn and requires die change after wire drawing. In the case, the evaluation was (Δ), and the case where there was no need to change the die due to die breakage or wear after 50 tons wire drawing was evaluated as (（) evaluation. What was shown by (-) was not able to evaluate die life because it was disconnected.

  The results are shown in Table 4 and FIG.

  From Tables 1 to 4, it can be analyzed as follows.

First, the average crystal grain size (Dave) and the maximum crystal grain size (Dmax) are improved by controlling (Dave) to 20 μm or less and (Dmax) to 120 μm or less as summarized in FIG. However, even if the drawing speed is increased, high-speed drawing is possible without breaking. Furthermore, as additional requirements, (Dave) is 17 μm or less, (Dmax) is 100 μm or less, the structure is uniformly refined, the average subcrystal grain size (dave) is 10 μm or less, and the maximum subcrystal grain size (dmax) is 50 μm or less. If the (Dave / dave) ratio is controlled to 4.5 or less and TS is reduced to 1240 x Wc ^0.52 or less, drawing is possible without disconnection even if the number of dies is reduced in addition to high-speed drawing. Thus, the wire drawing workability can be further improved.

Although the requirements of the average crystal grain size (Dave) and the maximum crystal grain size (Dmax) are satisfied, No. in Tables 2 to 4 which do not satisfy the above additional requirements. Although 2, 14, 18, 24, 29, 30, 40, and 41 can be drawn at high speed, fracture occurs when the number of dies decreases. Moreover, from the viewpoint of the die life, Nos. No. 3 does not cause disconnection during wire drawing even if the wire drawing conditions are strict, but the die life is adversely affected to the extent that die replacement is required after wire drawing. In addition, No. in Tables 2 to 4 where steel does not satisfy “TS ≦ 1240 × Wc ^0.52 ” due to insufficient softening. 29, 30, and 40 also have poor die life.

  In addition, the effect of the component composition on the drawability is shown in Tables 3 and 4. It appears in 43-48. That is, No. in Tables 3 and 4. Steel types A16 and A17 used in Nos. 43 and 44 have a high P and S content, and thus disconnection occurs even though the metal structure is appropriately controlled. In Tables 3 and 4, No. Steel type A18 used in No. 45 has a significant decarburization because of its excessive Si content, has poor scale peelability, and is too high in strength, resulting in die breakage and disconnection during wire drawing.

  No. in Tables 3 and 4 Steel type A19 used in No. 46 has a too high Mn content, resulting in a supercooled structure and high strength. Steel No. 47, No. 47, is too ductile because of its excessive N content, and is susceptible to strain aging embrittlement during wire drawing. No. Similarly, 48 steel type A21 has a C content exceeding the specified value, so that it has poor ductility and is susceptible to strain aging embrittlement during wire drawing.

  As described above, if the steel component is out of the specified range of the present invention, satisfactory wire drawing cannot be obtained even if the structural features of the present invention are satisfied.

Experimental example 2
In order to improve the drawability as it is in hot rolling, examination was performed using the steel types shown in Table 5 below. All the steel types shown in Table 5 satisfy the component composition requirements defined in the present invention. REM in Table 5 is the total content of La, Ce, Pr and Nd.

  The steel types listed in Table 5 were hot-rolled under the conditions shown in Table 6 and FIG. In the case of a hot-rolled material, it is necessary to control in all steps from the heating furnace to rolling and cooling, and the management items are more complicated than in the case of Experimental Example 1 (FIG. 1) as shown in FIG. The obtained hot-rolled material was evaluated for structural characteristics, scale characteristics, tensile characteristics, and wire drawing properties in the same manner as in Experimental Example 1.

  The results are summarized in Tables 6 to 8 and FIG. 6, and even in hot rolling, by appropriately controlling each management item in a series of processes from heating to winding cooling, the structural characteristics, scale The properties and tensile properties can be controlled within the specified range of the present invention, and it can be confirmed from the results of wire drawing evaluation that excellent wire drawing can be obtained even in hot rolling.

  As described above, in the carbon steel wire satisfying the requirements of the predetermined component composition, the average crystal grain size (Dave) is set to 20 μm or less, the maximum crystal grain size (Dmax) is set to 120 μm or less, and the size variation of the metal structure unit is A high carbon steel wire having excellent drawability can be obtained by reducing the size and making it uniform and uniform.

6 is a schematic diagram of a manufacturing pattern employed in Experimental Example 1. FIG. It is a figure which shows one example of the boundary map of the steel wire obtained by this invention. It is a graph which shows the example of evaluation of the crystal unit of the steel wire obtained by experiment. 4 is a graph collectively showing the influence of the average crystal grain size and the maximum crystal grain size obtained in Experimental Example 1 on performance. It is the schematic of the manufacturing pattern employ | adopted in Experimental example 2. FIG. It is a graph which shows collectively the influence which the average crystal grain size obtained in Experimental Example 2 and the maximum crystal grain size have on performance.

Claims (10)

  In mass%, C: 0.6 to 1.1%, Si: 0.1 to 2.0%, Mn: 0.1 to 1.0%, P: 0.020% or less, S: 0.020 %, N: 0.006% or less, Al: 0.03% or less, O: 0.0030% or less, the balance being Fe and inevitable impurities, and the average crystal in the bcc-Fe crystal grains of metal structure A high carbon steel wire rod excellent in wire drawing, characterized by having a grain size (Dave) of 20 μm or less and a maximum crystal grain size (Dmax) of 120 μm or less.   2. The high carbon steel wire rod according to claim 1, wherein in the bcc-Fe crystal grains of the metal structure, an area ratio occupied by particles having a particle size of 80 μm or more is 40% or less.   The high carbon steel wire rod according to claim 1 or 2, wherein the bcc-Fe crystal grains of the metal structure have an average subcrystal grain size (dave) of 10 µm or less and a maximum subcrystal grain size (dmax) of 50 µm or less.   The ratio (Dave / dave) of the average crystal grain size (Dave) to the average subcrystal grain size (dave) in the metal structure bcc-Fe crystal grains is 4.5 or less. The high carbon steel wire described. The high carbon steel wire according to any one of claims 1 to 4, wherein when the tensile strength of the steel wire is TS and the C concentration in the steel wire is Wc, they satisfy the relationship of the following formula (1). .
TS ≦ 1240 × Wc ^0.52 (1)   Still other elements of steel are Cr: 1.5% or less (not including 0%), Cu: 1.0% or less (not including 0%), Ni: 1.0% or less (including 0%) The high carbon steel wire according to any one of claims 1 to 5, which contains at least one element selected from:   Steel is at least one element selected from Mg: 5 ppm or less (not including 0 ppm), Ca: 5 ppm or less (not including 0 ppm), REM: 1.5 ppm or less (not including 0 ppm) as other elements. The high carbon steel wire according to any one of claims 1 to 6, which contains an element. The high carbon steel wire according to any one of claims 1 to 7, wherein a total decarburization amount (D _mT ) of the surface layer is 100 µm or less, and a scale adhesion amount is 0.15 to 0.85 mass%. A steel wire made of steel having a component composition defined in any one of claims 1, 6, and 7 and heated to 730 to 1050 ° C. at a temperature of 470 to 640 ° C. at an average cooling rate of 15 ° C./second or more. _{(T 1)} after cooling to, wherein the heating at said temperature _{(T 1)} is higher than five hundred and fifty to seven hundred and twenty ° C. of the temperature _{(T 2)} to 3 ° C. / sec or more average heating rate, A method for producing high carbon steel wire rods with excellent drawability. After heating the steel material having the component composition defined in any one of claims 1, 6 and 7 to 900 to 1260 ° C, hot rolling at a temperature of 740 ° C or higher and finish rolling at a temperature of 1100 ° C or lower, Water cooling to a temperature range of 750 to 950 ° C. and winding on a transfer device, followed by cooling at an average cooling rate of 15 ° C./second or more, the steel material temperature minimum value (T ₃ ) The temperature is lowered to 500 to 630 ° C. and then heated, and the maximum value (T ₄ ) of the steel material temperature is increased to 580 to 720 ° C. which is higher than the above minimum value (T ₃ ) within 45 seconds from the end of winding. A method for producing a high carbon steel wire rod excellent in drawability, characterized by increasing.

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