JP2019056162A - High-strength steel wire - Google Patents

Abstract

To provide a high-strength steel wire of a hypereutectoid composition, the steel wire having excellent strength and ductility.SOLUTION: A high-strength steel wire comprises, in mass%, C: 0.90-1.10%, Si: more than 0.40% to 0.80% or less, Mn: 0.10-0.70%, and Cr: 0.10-0.40%, with Al: 0.003% or less, P: 0.020% or less, and S: 0.010% or less, and [Si+Cr] of 0.50-0.90% and [Cr+Mn] of 0.40-0.80%, with the balance being Fe and inevitable impurities. The steel wire has a wire diameter of 0.05-0.30 mm. The steel wire has a plurality of exothermic peaks in the temperature range of 100-450°C in a DSC curve of differential scanning calorimetry. The sum of calorific values Hin the temperature range of 100-450°C and calorific values Hin the temperature range of 280-420°C satisfy the following formulae (3): H/H<0.45.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength steel wire, and particularly to a high-strength steel wire manufactured by subjecting a hot-rolled wire rod having a hypereutectoid composition to cold drawing, heat treatment, plating, and the like. The high-strength steel wire of the present invention can be applied to, for example, steel cords, saw wires and the like.

  High-strength steel wires used for steel cords and saw wires have various wire diameters and strengths according to their specifications. These high-strength steel wires are often used in a state where a plurality of strands are finally twisted together (twisted), and even when used as a single wire, they are used in a twisted state. In addition to strength, ductility is required.

  On the other hand, these high-strength steel wires are required to have higher strength in order to reduce manufacturing costs and differentiate products. There is a means to increase the C content to meet the demand for increasing the strength of the steel wire. However, in the hypereutectoid steel with a C content exceeding 0.9%, the proeutectoid cementite is used in the hot rolled wire. Precipitates and the ductility decreases. For this reason, various developments have been made in hypereutectoid steel in order to achieve both strength and ductility after wire drawing.

  Patent Document 1 states that a wire rod for wire drawing having high strength and high ductility can be obtained by controlling the total content of Si and Cr and the lamella spacing.

  Patent document 2 says that the steel wire excellent in intensity | strength and ductility is obtained by controlling the exothermic peak between 60-130 degreeC in a differential scanning calorific value curve.

International Publication No. 2009/084811 JP 2005-126765 A

  In Patent Document 1, the lamella spacing is set to 50 nm or less in the structure of the wire drawing wire after patenting. As a result of the study by the present inventors, a true strain of 3.5 or more is applied to a wire having a lamella spacing of 50 nm or less. It was found that it was difficult to achieve both strength and ductility when the above process was performed to obtain an ultrafine steel wire having a diameter of 0.3 mm or less.

  Moreover, in patent document 2, in the wire drawing process, it was necessary to lower the temperature of the lubricant, to reduce the wire drawing speed, and the like, and there were problems such as an increase in production cost. Furthermore, in Patent Document 2, as described above, the conditions in the wire drawing step are finely limited to suppress the diffusion of N atoms and the fixation to dislocations and improve the ductility, but it is difficult to achieve both strength and ductility. Met.

  This invention is made | formed in order to solve the above problems, and makes it a subject to provide the high strength steel wire excellent in intensity | strength and ductility in the steel wire of a hypereutectoid composition.

  First, the present inventors use a steel wire having a hypereutectoid composition with a carbon concentration of 0.90% to 1.10%, and produce various high-strength steel wires by controlling heat treatment conditions and wire drawing conditions. did. And while evaluating the mechanical properties such as ductility of the obtained high-strength steel wire and analyzing the results of differential scanning calorimetry, the decrease in ductility of the steel wire is due to the decomposition of cementite that occurs during wire drawing, etc. I was able to estimate. Furthermore, as a result of investigating a method for suppressing the decomposition of cementite, it was found that control of the Si amount, Cr amount, and Mn amount and control of the heat treatment structure and the wire drawing method are effective. Furthermore, it has also been found that the decomposition state of cementite can be evaluated by differential scanning calorimetry, and by controlling the exothermic peak between 280 and 420 ° C. in the DSC curve of differential scanning calorimetry of the steel wire, high strength and high ductility steel The knowledge that a line can be obtained was obtained, and the present invention was achieved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.90 to 1.10%, Si: more than 0.40% and 0.80% or less, Mn: 0.10 to 0.70%, Cr: 0.10 to 0.0. Containing 40%, Al: 0.003% or less, P: 0.020% or less, S: 0.010% or less, and satisfying the following formulas (1) and (2) in mass%, the balance Consists of Fe and impurities, has a wire diameter of 0.05 to 0.30 mm in diameter, has a plurality of exothermic peaks in the temperature range of 100 to 450 ° C. of the DSC curve of differential scanning calorimetry, and 100 calorific value _{H 280-420} in the temperature range of the sum _{H 100 to 450} and 280-420 ° C. heating value in the temperature range of to 450 ° C. is, satisfies the following formula (3), a high strength steel wire.
0.50 ≦ Si (%) + Cr (%) ≦ 0.90 (1)
0.40 ≦ Cr (%) + Mn (%) ≦ 0.80 (2)
_H280-420 / _H100-450 <0.45 (3)
The element symbols in the above formulas (1) and (2) mean the content in mass% of the element.

[2] Further, in mass%, Ni: 0.50% or less, Co: 1.00% or less, Mo: 0.20% or less, B: 0.0002 to 0.0030%, Cu: 0.15% The high-strength steel wire according to [1] above, containing any one or more of the following.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel wire excellent in intensity | strength and ductility can be provided in the steel wire of a hypereutectoid composition. Moreover, according to this invention, the high strength steel wire excellent in ductility suitable for uses, such as a steel cord and a saw wire, can be produced stably.

It is explanatory drawing which shows the DSC curve obtained by the differential scanning calorimetry of the high strength steel wire which concerns on this embodiment, and the conventional steel wire. It is a schematic diagram for demonstrating the measuring method of the hook strength of the steel wire in a present Example.

  Hereinafter, embodiments of a high-strength steel wire (hereinafter also simply referred to as “steel wire”) and a method for manufacturing the same according to the present invention will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

(DSC curve for differential scanning calorimetry)
The high-strength steel wire according to the present embodiment is, in mass%, C: 0.90 to 1.10%, Si: more than 0.40% and 0.80% or less, Mn: 0.10 to 0.70%, Cr: 0.10 to 0.40%, Al: 0.003% or less, P: 0.020% or less, S: 0.010% or less, and in mass%, the following formula (1) , (2) is satisfied, the balance is made of Fe and impurities, the wire diameter is 0.05 to 0.30 mm in diameter, and a plurality of heat generations in the temperature range of 100 to 450 ° C. in the DSC curve of differential scanning calorimetry The high intensity | _{strength in} which the sum total _H100-450 of the calorific value in the temperature range of 100-450 _degreeC and the calorific value H280-420 in the temperature range of 280-420 degreeC which has a peak satisfy _{| fills following} formula (3). It is a steel wire.
0.50 ≦ Si (%) + Cr (%) ≦ 0.90 (1)
0.40 ≦ Cr (%) + Mn (%) ≦ 0.80 (2)
_H280-420 / _H100-450 <0.45 (3)

  In addition, as an example, this high-strength steel wire is obtained by hot-rolling a steel slab having the above chemical components to form a hot-rolled wire, dry-drawing the hot-rolled wire, and further heat-treating the hot-rolled wire. Thus, the heat treatment wire having a tensile strength is adjusted and having a pearlite structure, brass plating is performed on the heat treatment wire, and wet drawing is performed under the conditions described later using the heat treated wire after plating as a material.

  First, the reasons for defining the DSC curve in the above-described differential scanning calorimetry and the above equation (3) will be described below with reference to the drawings and the like together with the examination results of the present inventors.

First, the present inventors performed differential scanning calorimetry (DSC) on a conventional steel wire, and obtained curves (heat flow (Heat Flow / mW) on the vertical axis and temperature on the horizontal axis). As a result, a plurality of exothermic peaks were confirmed between 100 to 450 ° C. of the DSC curve. Furthermore, it has been found that there is a correlation between the calorific value, particularly the calorific value of 280 to 420 ° C., and the ductility of the steel wire.
As will be described later, the exothermic peak near 280 to 420 ° C. is presumed to be related to reprecipitation of C atoms derived from lamellar cementite fixed to dislocations as cementite. Therefore, it is presumed that the presence / absence of the fixing of C atoms to dislocations and reprecipitation as cementite greatly affect the ductility of the steel wire.
Hereinafter, the correlation between the exothermic peak of the DSC curve and the ductility of the steel wire will be described in more detail.

  FIG. 1 shows a DSC of a high-strength steel wire according to the present embodiment (corresponding to a level “A1” in [Example] described later) and a conventional steel wire (corresponding to a level “B4” in [Example] described later). The curve is shown. In the DSC curve, a case where the DSC curve has an upward peak with respect to a reference line (straight line) shall be “having an exothermic peak”, and a case having a downward peak shall be “having an endothermic peak”. Further, the integrated value of the reference line and each peak is the fluctuating calorific value or endothermic amount at that peak. Here, the “reference line” means a straight line connecting any two points on the curve, and a peak having an absolute value of the maximum height of each peak with respect to the reference line is 3 μW / mg or more is an exothermic peak or endothermic. It is defined as a peak. The reference line in FIG. 1 is a straight line connecting 70 ° C. and 200 ° C., and 250 ° C. and 400 ° C. The reference line in FIG. 1 is merely an example. What is necessary is just to set a reference line by the method normally performed when calculating | requiring calorie | heat amount from a DSC curve.

The DSC curve of the conventional high-strength steel wire has exothermic peaks near 100 to 200 ° C and around 280 to 420 ° C, as shown in FIG. Among these, the exothermic peak near 100 to 200 ° C. is considered to be an exothermic peak indicating the fixation of C atoms to dislocations. This is because lamellar cementite (hereinafter also referred to simply as cementite) that has undergone processing such as wire drawing decomposes due to thermal activity and releases C atoms as the temperature rises during differential scanning calorimetry. It is presumed that it was stuck to the dislocation. On the other hand, it is considered that the exothermic peak near 280 to 420 ° C. was reprecipitated to cementite by further thermal activation of C atoms fixed to dislocations.
Here, C atoms re-deposited into cementite are aged during wire drawing and subsequent storage, in addition to C atoms which cementite decomposes and fixes to dislocations during temperature rise during the differential scanning calorimetry described above. It is presumed that the cementite decomposes at the time, and C atoms fixed to the dislocations are also included. That is, in the DSC curve, the total amount of heat generated _{H 100 to 450} at 100 to 450 ° C., the percentage of heat value _{H from 280 to 420} of _{280~420 ℃ (H 280~420 / H 100~450} ) that is greater Is presumed to indicate that cementite decomposes during wire drawing and after aging, and that the amount of C fixed to dislocations is large.

As a result of further investigation, it was found that the ductility value decreased when _H280-420 / _H100-450 increased in the DSC curve. That is, this is considered to indicate that the ductility value decreases when cementite is decomposed during wire drawing and there are excessive C atoms fixed to dislocations.
Generally, dislocations need to move in order for a material to deform. However, since dislocations are difficult to move when C atoms are fixed to dislocations, the material becomes more difficult to deform as the amount of fixing of C atoms to dislocations increases, and the ductility value is considered to decrease as described above.

From these examination results, the present inventors have derived that it is important to suppress the decomposition of cementite and reduce the amount of C atoms fixed to dislocations in order to improve the ductility of the steel wire after drawing. It was.
However, it is very difficult to measure and evaluate whether or not the C atoms are fixed to dislocations in the steel wire structure and the amount thereof by observation using the steel wire structure and other measurement methods.
Accordingly, in this embodiment, the ductility of the steel wire is evaluated using a calorific value H _280-420 of 280-420 ° C. that has a correlation with the dislocation fixing amount of C atoms.
That is, in this embodiment, the ductility of a steel wire is improved by reducing _H280-420 / _H100-450 in the DSC curve.

From the above, the percentage of heat value _{H 280 to 420} of 280 to 420 ° C. for all heating value _{H 100 to 450} between 100 to 450 ° C. in the DSC curve in the present embodiment _(H 280~420 _/ H 100~450) Was less than 0.45. Moreover, although this ratio _H280-420 / _H100-450 may be 0, when it is too low, since the tensile strength of a steel wire will fall, it is good also considering a minimum as 0.05 or more.

Here, the measuring method of DSC of a steel wire is demonstrated below.
A test piece is taken from the high-strength steel wire that has been subjected to wire drawing, the test piece is put into a DSC measurement chamber, and differential scanning calorimetry is performed in a temperature range of 50 ° C to 500 ° C. In the measurement, the reaction may be performed in an Ar atmosphere so that a reaction such as oxidation does not occur, and the rate of temperature rise can be set to 20 ° C./min.
In addition, from each of the obtained DSC curves, a reference line is drawn at each peak to obtain an integrated value of the area surrounded by the peak and the reference line, that is, the height of the peak from the reference line, and this integrated value is obtained as the peak. The amount of heat in

(Component composition)
Next, the steel composition of the high-strength steel wire of this embodiment is demonstrated. Hereinafter, the unit of the content of each element is mass%.

C: 0.90 to 1.10%
C is an essential element for imparting the necessary strength of the steel wire. If it is less than 0.90%, the tensile strength of the steel wire is lowered, so C is contained in an amount of 0.90% or more. Preferably, the amount of C is 0.95% or more, more preferably 1.00%.
On the other hand, when the C content exceeds 1.10%, proeutectoid cementite increases and breakage occurs frequently, and the ductility of the steel wire is reduced. Therefore, the C content is 1.10% or less. Preferably, the amount of C is 1.08% or less.

Si: More than 0.40% and 0.80% or less Si increases the ferrite strength in pearlite, and also has an effect of improving the ductility of the steel wire by suppressing the decomposition of cementite in the wire drawing described later. It is an element. In order to effectively exhibit these actions, it is necessary to contain Si in excess of 0.40%. Preferably, the amount of Si is 0.50% or more.
However, if Si is contained excessively, SiO ₂ -based inclusions that are harmful to wire drawing workability are likely to be generated, so the upper limit was set to 0.80% or less. Preferably, the amount of Si is 0.70% or less.

Mn: 0.10 to 0.70%
Mn is not only useful for deoxidation and desulfurization, but also has an effect of delaying the transformation of pro-eutectoid cementite and grain boundary ferrite from austenite, and is an element useful for obtaining a pearlite-based structure. In order to effectively exhibit such an action, it is necessary to contain 0.10% or more of Mn. Preferably, the amount of Mn is 0.15% or more.
However, even if Mn is contained excessively, the above effect is saturated, and the time to complete transformation at the time of heat treatment becomes long, leading to a decrease in productivity and an increase in equipment cost. Therefore, the upper limit of the amount of Mn is set to 0.70% or less. Preferably, the amount of Mn is 0.50% or less.

Cr: 0.10% to 0.40%
Cr increases the work hardening rate of pearlite, and higher tensile strength can be obtained by wire drawing with a low strain amount. In addition, Cr has an effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In order to effectively exhibit such an action, it is necessary to contain Cr at 0.10% or more. Preferably, the Cr amount is 0.15% or more.
However, if the Cr content exceeds 0.40%, these effects are saturated, and the time to complete transformation at the time of heat treatment becomes long, leading to a decrease in productivity and an increase in equipment costs. Therefore, the upper limit of the Cr amount is set to 0.40% or less. Preferably, the Cr amount is 0.30% or less.

Si + Cr: 0.50-0.90%
Furthermore, by containing the above Si and Cr in combination, decomposition of cementite during wire drawing, which will be described later, can be suppressed, and the strength and ductility of the steel wire can be improved. If the total amount of Si and Cr is less than 0.50%, decomposition of cementite proceeds and ductility is lowered, and the tensile strength may be insufficient. On the other hand, if the total amount of Si and Cr exceeds 0.90%, the tensile strength of the steel wire increases excessively and ductility decreases. Therefore, the total amount of Si and Cr is set to satisfy the following formula (1). In addition, the preferable minimum of the total amount of Si and Cr is 0.55% or more, and a preferable upper limit is 0.80% or less.
0.50 ≦ Si (%) + Cr (%) ≦ 0.90 (1)

Cr + Mn: 0.40% to 0.80%
By containing Cr and Mn in a composite manner, a pearlite structure can be stably obtained, and an effect of increasing the tensile strength of the steel wire can be obtained. If the total amount of Mn and Cr is less than 0.40%, these effects cannot be obtained sufficiently. On the other hand, if the total amount of Mn and Cr exceeds 0.80%, it takes a long time to complete the transformation during the heat treatment. , Leading to reduced productivity and increased equipment costs. Therefore, the total amount of Mn and Cr is made to satisfy the following formula (2). In addition, the minimum with the preferable total amount of Mn and Cr is 0.45% or more, and a preferable upper limit is 0.60% or less.
0.40 ≦ Cr (%) + Mn (%) ≦ 0.80 (2)

Al: 0.003% or less Al reacts with O to generate a hard oxide such as Al ₂ O ₃ , which causes a reduction in wire drawing workability and ductility of the steel wire. Therefore, the upper limit is limited to 0.003% or less. Al is very useful as a deoxidizing element, and considering the refining technique and production cost, the lower limit of the Al content is preferably 0.001% or more.

P: 0.020% or less P is an impurity. If the P content exceeds 0.020%, there is a risk of segregating at the grain boundaries and impairing the wire drawing workability. Therefore, the P content is limited to 0.020% or less. Preferably, the P content is limited to 0.015% or less. Moreover, since it is desirable that the P content is small, the lower limit of the P content may be 0%. However, it is not technically easy to make the P content 0%, and even if it is stably made less than 0.001%, the steelmaking cost becomes high. Therefore, the lower limit of the P content may be 0.001% or more.

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

  The basic component composition of the high-strength steel wire according to the present embodiment is as described above, but it is preferable to selectively contain one or more of the following elements in addition to the above components. The elements shown below may or may not be contained, and the lower limit of the content of these elements is 0%.

Ni: 0.50% or less Ni has an effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In addition, it is an element that increases the toughness of steel wires. In order to obtain these effects, Ni is desirably contained in an amount of 0.10% or more. On the other hand, when Ni is contained excessively, it takes a long time to complete transformation at the time of heat treatment, leading to a decrease in productivity and an increase in equipment cost. Therefore, the upper limit is made 0.50% or less. Preferably, the Ni content is 0.30% or less.

Co: 1.00% or less Co is an element effective for suppressing precipitation of pro-eutectoid ferrite during rolling or heat treatment. Moreover, it is an element effective in improving the ductility of a steel wire. In order to exhibit such an action effectively, it is preferable to contain 0.10% or more. On the other hand, even if Co is contained excessively, the effect is saturated and economically useless, so the upper limit was made 1.00% or less. Preferably, the Co content is 0.90% or less.

Mo: 0.20% or less Mo has an effect of delaying the transformation of proeutectoid cementite and grain boundary ferrite from austenite, and is a useful element for obtaining a pearlite-based structure. In order to acquire this effect, it is desirable to contain Mo 0.05% or more. However, if the Mo content exceeds 0.20%, it takes a long time to complete transformation at the time of heat treatment, leading to a decrease in productivity and an increase in equipment costs. Therefore, the upper limit was made 0.20% or less. Preferably, the Mo amount is 0.15% or less.

B: 0.0002 to 0.0030%
B is an element that concentrates at the grain boundary and is effective in suppressing pro-eutectoid ferrite. In order to acquire this effect, it is necessary to contain B 0.0002% or more. On the other hand, if B is contained excessively, carbides such as Fe ₂₃ (CB) ₆ are formed in austenite and wire drawing workability is lowered, so the upper limit was made 0.0030% or less. Preferably, the amount of B is 0.0005 to 0.0020%.

Cu: 0.15% or less Cu is a useful element that contributes to increasing the strength of a steel wire by precipitation hardening or the like. On the other hand, when Cu is contained excessively, grain boundary embrittlement is caused, which becomes a cause of flaws and a factor of lowering ductility. Therefore, the upper limit is made 0.15% or less. Preferably, the amount of Cu is 0.12% or less.

The high-strength steel wire of this embodiment contains the above components, and the balance is substantially formed of Fe and impurities. It should be noted that other elements can be contained in minute amounts within a range that does not impair the effects of the present invention.
Here, an impurity refers to what is mixed from ore as a raw material, scrap, a manufacturing environment, etc. when manufacturing steel materials industrially.

(Tensile strength)
The steel wire according to the present embodiment aims to increase the strength of the ultrafine steel wire. Therefore, the steel wire according to the present embodiment is a steel wire made by wet wire drawing with a true strain amount ε of 3.5 or more, and the lower limit of the tensile strength TS of the steel wire is 1000 times the true strain amount ε. It is desirable to be super (see the following formula (4)). The upper limit of the tensile strength TS of the steel wire is not particularly limited, but if it is excessively high, the ductility may be lowered. Therefore, the upper limit of the tensile strength TS is preferably less than 1070 times the true strain amount ε.
TS (MPa)> 1000 × ε (4)
(True strain amount ε: −2 × Ln (D / D0), D: steel wire diameter (mm), D0: heat treated wire diameter (mm))

In addition, the tensile strength TS of the steel wire subjected to the wet wire drawing greatly depends on the tensile strength TS _b of the heat-treated wire that is a material before wire drawing. Here, the tensile strength TS _b annealing wire, depending on the component, there is a suitable range. Tensile strength TS _b annealing wire is on the left side following the following formula (5), decreases the tensile strength TS of the steel wire after drawing. On the other hand, there is a possibility that the tensile strength TS _b annealing wire rod tensile strength TS of the steel wire becomes excessively high in the above right side of formula (5), ductility value decreases. Therefore, the tensile strength TS _b annealing wire as a material of the steel wire according to the present embodiment preferably satisfies the following formula (5).

On the other hand, even an appropriate level of tensile strength TS _b annealing wire rod according to the components, if excessively high, decrease the ductility of the steel wire, if excessively low Conversely, the steel wire tensile strength TS is descend. Therefore, in addition to the following formula (5), tensile strength TS _b of the heat treatment wire is more than 1500MPa to the lower limit, it is more preferable to control the upper limit below 1700 MPa.

600 × C amount (%) + 100 × Si amount (%) + 250 × Cr amount (%) + 860 <TS _b (MPa) <600 × C amount (%) + 100 × Si amount (%) + 250 × Cr amount (%) +900 Formula (5)

(Measurement method of tensile strength)
As a method for measuring the tensile strength TS of a steel wire, three or more samples are continuously collected from the steel wire except for the unsteady portion, and each sample is subjected to a tensile test. Their average value can be used as the tensile strength TS of the steel wire.
The measurement method of the tensile strength TS _b annealing wire from heat treatment wire, three or more samples except unsteady portion was collected, subjected to tensile each test. The average thereof can be tensile strength TS _b annealing wire.

(Metal structure)
Next, the high-strength steel wire according to the present embodiment and the metal structure of the heat-treated wire used as the material will be described.
In order to achieve both high strength and high ductility, the metallographic structure of the heat-treated wire that is the material of the steel wire of this embodiment has pearlite as the main structure, and the remaining structure is any one of proeutectoid cementite, grain boundary ferrite, and bainite. It consists of two or more species.
Proeutectoid cementite, intergranular ferrite, and bainite may be a propagation path of fracture, and if these area ratios increase in the heat-treated wire, they will cause a decrease in the tensile strength and ductility of the steel wire. Furthermore, if the area ratio of bainite or proeutectoid ferrite in the heat-treated wire increases, the strain at the time of wire drawing concentrates unevenly, and the decomposition of cementite locally proceeds. As a result, the calorific value H _280-420 may increase and the ductility may decrease. Therefore, it is preferable that the area ratio of the pearlite of the heat-treated wire used as the material before wire drawing is 95% or more.

The measurement of the pearlite area ratio of the heat-treated wire can be performed as follows. A heat-treated wire is cut and filled with a resin so that a cross section perpendicular to the length direction can be observed, and then polished to a mirror surface by polishing paper or alumina abrasive grains. This is corroded with a 3% nital solution or picral and observed using a scanning electron microscope (SEM). Subsequently, using a photographic apparatus attached to the SEM, a plurality of fields of view were photographed so that an arbitrary spot was 2000 times or more and the total observation field of view was 0.08 mm ² or more, and image analysis software such as particle solution software was used. The area ratio of pearlite is measured.

  The metal structure of the steel wire of this embodiment is presumed to have a metal structure substantially equivalent to the heat-treated wire. That is, it is presumed to have a structure including pearlite with an area ratio of 95% or more and any one or more of pro-eutectoid cementite, grain boundary ferrite and bainite as the remaining structure. However, the metal structure of the steel wire can be confirmed to be a structure mainly composed of pearlite by microscopic observation of the cross section, but its area ratio is difficult to measure accurately because the cross section of the steel wire is an extremely small area. It is.

(Production method)
Next, the manufacturing method of the high strength steel wire which concerns on this embodiment is demonstrated.
When manufacturing the high-strength steel wire according to the present embodiment, the steel composition, each process, and each process so as to satisfy the above-described conditions such as the definition of the calorific value and the area ratio of the metal structure in the DSC curve. What is necessary is just to set the conditions in.
That is, manufacturing conditions can be set according to the wire diameter of the steel wire after wire drawing and the required strength and ductility.
The manufacturing method described below is an example, and is not limited by the following procedures and methods, and any method can be adopted as long as the method can realize the configuration of the high-strength steel wire of the present invention. It is.

  First, normal manufacturing conditions can be adopted as the manufacturing conditions of the material according to the present embodiment, that is, the material subjected to hot rolling. For example, a cast piece is produced by continuous casting of the steel of the above components, and the cast piece is produced by split rolling to produce a steel piece of a size suitable for wire rod rolling (generally called a billet called a billet). Used for hot rolling.

In rolling the wire, the steel slab is heated to 950 to 1150 ° C., the finish rolling start temperature is controlled to 800 ° C. or more and 950 ° C. or less, and hot rolled to a diameter of 3.6 to 5.5 mm.
After hot rolling, the wire is wound into a ring shape at 800 ° C. or higher and 940 ° C. or lower. Then, the cooling rate to 700 ° C. is cooled at 10 ° C./s or more, and then cooled to 580 ° C. at a cooling rate of 5 ° C./s to 15 ° C./s to produce a hot rolled wire rod.

The hot-rolled wire thus obtained is subjected to scale peeling and film treatment by a usual method. After that, dry drawing and cold drawing are performed. The dry wire drawing is performed with a surface reduction rate of 20% or less in one pass, and when the true strain exceeds 2.5, an intermediate heat treatment is performed. This intermediate heat treatment is performed by a general patenting process.
Although the wire diameter of the wire after dry wire drawing is not particularly limited, it may be in the range of 0.4 to 2.5 mm.

Next, a final heat treatment is performed after drawing to the above wire diameter by dry drawing. The final heat treatment, the tensile strength TS _b annealing wire after the heat treatment is adjusted to a range satisfying the above expression (5). Thereby, the tensile strength TS of the steel wire of this embodiment can be adjusted to the range which satisfy | fills said Formula (4).

Final heat treatment conditions for adjusting the tensile strength TS _b annealing wire in a range satisfying the above expression (5), for example, argon, etc., in an atmosphere which does not oxidize, and heated to a range of 985 ℃ ~1020 ℃, Austenitic hot rolled wire rod. Specifically, the temperature is maintained in the range of 985 ° C. to 1020 ° C. for 5 seconds to 10 seconds. Then, it is immersed in a lead bath having a bath temperature of 590 ° C. to 605 ° C. within 2 seconds and held for 6 seconds to 20 seconds. Immediately cool to room temperature with air or water.

By performing such a final heat treatment, it is possible to obtain a tissue or tensile strength TS _b annealing wire as described above. However, as described above, these manufactures are merely examples, and the optimum heat treatment conditions vary depending on the composition of the steel, the wire diameter for heat treatment, the processing process up to the heat treatment, etc., and therefore the range satisfying the above formula (5) May be changed as appropriate.

Next, the heat-treated wire thus manufactured is subjected to brass plating, and then drawn to a diameter of 0.05 to 0.30 mm in a wet drawing process. In this embodiment, it is important to perform the wire drawing step after the final heat treatment by wet wire drawing.
When the wire drawing step after plating is performed by dry wire drawing, the decomposition of cementite proceeds due to large processing heat generation. Accordingly, the final heat treatment is performed by wet drawing. In the wet wire drawing process, a liquid such as a lubricant is attached to the wire, but the applied heat removes the processing heat generated during wire drawing, and the contact pressure between the wire and the die is reduced. Decomposition can be suppressed. As a result, _H280-420 / _H100-450 can be reduced, and the ductility of the steel wire after wire drawing can be improved.
An example of manufacturing conditions for wet wire drawing is shown below, but this is only an example, and depending on the wire diameter of the steel wire, the type and capacity of the manufacturing equipment such as wire drawing machine and die, etc. What is necessary is just to set each condition in the range which can be adapted to the meaning.

  First, wet wire drawing should be continuous wet wire drawing with dies arranged in multiple stages, with a 1-pass area reduction of 10 to 17% and a final wire drawing speed of 50 to 700 m / min. Can do. If the area reduction during wire drawing is too large, the contact pressure with the die applied to the surface of the steel wire may increase, and decomposition of cementite may proceed. On the other hand, if the wire drawing speed is excessively low, the pull-in of the lubricant is reduced, and the heat generated by the work due to friction is increased. As a result, decomposition of cementite may proceed. Similarly, when the drawing speed is excessively high, the processing heat generated by friction increases, and as a result, decomposition of cementite may proceed.

  Further, within the range of 5 passes from the final pass to the upstream side, wire drawing is performed while stably controlling the reverse tension, thereby suppressing an increase in contact pressure with the die on the steel wire surface layer and frictional heat. be able to. However, if the reverse tension is too large, the steel wire may be broken. Therefore, it is preferable that the magnitude of the reverse tension to be applied is 5 to 40% of the tensile strength of the steel wire in each pass.

  The steel wire of this embodiment obtained by wire drawing by wet wire drawing satisfies the above formula (4) and has excellent strength.

  As described above, the steel wire can be brought within the scope of the present invention by having the component composition of the present embodiment and adjusting the manufacturing conditions as described above. Moreover, by producing a steel wire as described above, a high-strength ultrafine steel wire excellent in strength and ductility of the present invention can be obtained. However, the above production is merely an example, and the optimum production conditions vary depending on the composition of the steel, the wire diameter of the final ultrafine steel wire, etc., and the production method is not specified and may be changed as appropriate.

  Hereinafter, examples of the steel wire according to the present invention will be given and the present invention will be described more specifically. However, the present invention is not limited to the following examples from the beginning, and may be adapted to the purpose described above and below. It is also possible to carry out by appropriately changing the range, and these are all included in the technical scope of the present invention.

Table 1 wire diameter of the component composition and the final heat treatment wire, tissue (pearlite area ratio) and tensile strength TS _b, shows the results of evaluating the tensile properties and ductility values of the wet-drawing conditions and the steel wires in Table 2.

In Tables 1 to 4, A1 to 23 are examples of the present invention, and B1 to 24 are at least one of the component composition or production conditions outside the proper range, and H _{280 to 420} in the mechanical characteristics and DSC curve of the steel wire. / H _100-450 is outside the proper range of the present invention.
In Tables 1 to 4, numerical values deviating from the appropriate range of the present invention are underlined.

In both the inventive examples and comparative examples, the heat-treated wire was produced by the following method.
Steels having the compositions shown in Tables 1 and 2 were melted by vacuum melting or a converter, and then hot-rolled wire having a diameter of 3.6 to 5.5 mm was produced by hot forging and hot rolling. At that time, the structure of the hot-rolled wire rod was controlled so that a supercooled structure such as martensite was not generated, and a pearlite structure in which pro-eutectoid cementite was suppressed as much as possible. Specifically, the billet was heated to 1000 to 1200 ° C. in a heating furnace, and then hot-rolled, and then wound into a ring at 750 to 950 ° C. After winding, it was cooled at a cooling rate of up to 700 ° C. at 20 ° C./s, and then at a cooling rate of up to 580 ° C. at 8 ° C./s.

The hot-rolled wire thus obtained is subjected to scale peeling and film treatment by a normal method, and then cold-drawn by a dry method to produce wires having the wire diameters shown in Tables 1 and 2, and finally Subjected to heat treatment.
The dry wire drawing was performed with a surface reduction rate of 20% or less in one pass, and when the true strain exceeded 2.5, an intermediate heat treatment was performed by a general patenting process.
The final heat treatment is performed in an argon atmosphere within a range of 985 ° C. to 1020 ° C., held in that temperature range for 5 to 10 seconds to austenite, and then the bath temperature is 590 ° C. to 605 ° C. within 2 seconds. It was transformed into a pearlite structure by being immersed in a lead bath and held for 6 to 20 seconds.
Then, it cooled immediately to normal temperature with air | atmosphere or water, and manufactured the heat processing wire. Incidentally, B15, B21 of the comparative example results produced under conditions out of the above production method, the tensile strength TS _b or pearlite area ratio is an example of a heat treatment wire out of the preferable range of the present invention.

Pearlite area ratio and tensile strength TS _b annealing wire was measured by the above method.
In the examples of the present invention and the comparative examples, the metal structures of the heat-treated wire were all composite structures of pearlite, pro-eutectoid cementite, grain boundary ferrite, and bainite. In addition, the tensile test of the final heat-treated wire was performed with an N number of 3, a sample length of 400 mm, a crosshead speed of 10 mm / min, and a gap between jigs of 200 mm.

  The final heat-treated wire thus obtained is subjected to brass plating by a usual method, and under the conditions shown in Table 2, wet wire drawing is performed with a wire diameter of 0.06 to 0.28 mm and a true strain ε of 3.5 or more. It was. In Table 2, “area reduction rate” and “drawing speed” indicate the area reduction rate of one pass (each die) and the drawing speed of the final pass, respectively. Further, “reverse tension” is given in the range from the final pass to the upstream five passes, and the numerical value indicates a ratio to the tensile strength (MPa) of the steel wire in each pass.

  After wet wire drawing, as an evaluation of the obtained steel wire, measurement, analysis, tensile test and ductility evaluation of exothermic peak in a temperature range of 50 ° C. to 500 ° C. in the DSC curve were performed.

In differential scanning calorimetry, an exothermic peak is measured in a temperature range from 50 ° C. to 500 ° C. in an Ar atmosphere, and a calorific value H _{280 to 280} to 420 ° C. with respect to a total calorific value H ₁₀₀ to 450 at ₁₀₀ to 450 ° C. A ratio of ₄₂₀ H _280-420 / H _100-450 was measured. In addition, the temperature increase rate from 50 degreeC to 500 degreeC was 20 degreeC / min.

  In the tensile test, the N number was 3, the sample length was 200 mm, the crosshead speed was 10 mm / min, and the gap between the jigs was 200 mm, and the average tensile strength TS was evaluated. In addition, in this invention, what satisfy | filled Formula (4) was evaluated as the outstanding tensile strength.

The ductility value was measured in the same manner as the method for measuring the hook strength retention rate described in JP 2011-52269 A.
First, 20 or more samples are collected continuously from the place where the samples were collected in the tensile test described above. Thereafter, as shown in FIG. 2, the two steel wires 1 are looped and hooked with each other, and in this state, each steel wire 1 is fixed between the chucks 2 of the tensile tester until it breaks. The load (hook strength) when it was pulled and broken was measured. Then, by comparing the hook strength with the tensile strength TS of the steel wire, the hook strength retention ratio = ((hook strength) / tensile strength TS × 100) was obtained, and the value was defined as the ductility value. In the present invention, a ductility value of 60% or more was evaluated as an excellent ductility value.

As shown in Tables 1 and 3, A1 to 23 of the test examples are all examples of the present invention, and all high-strength steel wires are subjected to wire drawing with a true strain of ε3.5 or more, and then DSC. In the curve, an exothermic peak in the temperature range of 280 to 450 ° C. can be suppressed (H _{280 to 420} / H ₁₀₀ to ₄₅₀ can be reduced), and properties excellent in strength and ductility were obtained.

  On the other hand, since the test examples B1-24 did not satisfy any of the requirements of the present invention, the strength and ductility of the steel wire were inferior.

B1 to B12 are examples in which the component range of the present invention is outside and the tensile strength and ductility value of the steel wire are reduced.
B1 and B2 are examples in which the ductility value of the steel wire decreased due to a decrease in the pearlite area ratio and an increase in the tensile strength due to an increase in the proeutectoid cementite in the heat treated wire because C was added excessively.
Since B3-5 and B10 had too little Si amount, Cr amount and Si + Cr amount, decomposition of cementite proceeds in the wet wire drawing process, resulting in an increase in _H280-420 / _H100-450 , This is an example in which the tensile strength and ductility value of the steel wire are reduced.
B6 is an example in which the amount of Si was excessively added, so that SiO-based inclusions were precipitated and disconnection occurred.
B7 is an example in which the ductility value of the steel wire is reduced because the Cr amount and the Si + Cr amount are excessive, and B8 is the excessive Si + Cr amount.
Moreover, since the amount of Cr + Mn in B9 is excessive, the transformation completion time becomes longer, the transformation is not completed within a predetermined time (during immersion in the lead bath), and a lot of bainite is precipitated, and the pearlite area ratio results There was lowered, reduces the tensile strength TS _b annealing wire, an example in which ductility is lowered in the steel wire.
B11 Because Cr + Mn amount is small, the decrease in the decrease and tensile strength TS _b pearlite area ratio in the heat treatment wire with increasing pro-eutectoid cementite, an example in which the tensile strength TS and ductility values drops of steel wire.
B12 is an example in which since Cu was added excessively, coarse surface defects were generated in the heat-treated wire, and the ductility value of the steel wire was lowered.

B13-19, B22, and 23 show a DSC curve exothermic peak ( _H280-420 / _H100-450 ) due to the progress of decomposition of lamellar cementite during wet drawing, where the pearlite area ratio of the heat-treated wire is low. This is an example in which the ductility of the steel wire is lowered outside the scope of the invention.
Since B13 has a large area reduction ratio during wet drawing, B14, 16, 19, and 22 have low reverse tension or do not stably control reverse tension. This is an example in which the heat generation amount due to contact pressure or friction with the steel increases, decomposition of cementite proceeds, _H280-420 / _H100-450 is outside the scope of the present invention, and the ductility of the steel wire is reduced. .
As for B17, since the drawing speed at the time of wet drawing was excessively high, the drawing speed of B18, 23 was excessively low, so that the pulling-in of the lubricant was reduced at the time of wet drawing and the processing heat generation due to friction was increased. This is an example in which decomposition of cementite proceeds, _H280-420 / _H100-450 falls outside the scope of the present invention, and the ductility of the steel wire is lowered.
B20 is an example in which disconnection occurred because the area reduction rate during wet drawing was small, and B24 was large in reverse tension.
B15 is a result of pearlite area ratio bainite or ferrite ends up deposited drops in the heat treatment wire, distortion at the time of wet-drawing is unevenly concentrated, would proceeds decomposition of locally cementite, H _{280 to 420} / H _100-450 is outside the scope of the present invention, and the ductility of the steel wire is reduced.
B21 has an excessively large tensile strength of the heat-treated wire, so that the deformation resistance during wet drawing increases, the processing heat generation increases, the decomposition of lamellar cementite during wet drawing progresses, and the ductility of the steel wire decreases. This is an example.

  According to the present invention, it is possible to stably produce a high-strength steel wire excellent in ductility suitable for applications such as a steel cord and saw wire, and the industrial contribution is extremely remarkable.

1: Steel wire 2: Chuck of testing machine

Claims (2)

% By mass
C: 0.90 to 1.10%,
Si: more than 0.40% and 0.80% or less,
Mn: 0.10 to 0.70%,
Cr: 0.10 to 0.40%
Containing
Al: 0.003% or less,
P: 0.020% or less,
S: Limiting to 0.010% or less and satisfying the following formulas (1) and (2) in mass%, the balance is made of Fe and impurities,
The wire diameter is 0.05-0.30 mm in diameter,
In the temperature range of 100 to 450 ° C. of the DSC curve of differential scanning calorimetry, it has a plurality of exothermic peaks, and the total amount of heat generation H in the temperature range of ₁₀₀ to 450 ° C. H ₁₀₀ to 450 and 280 to 420 ° C. A high-strength steel wire having a calorific value H ₂₈₀ to ₄₂₀ in the range satisfying the following formula (3).
0.50 ≦ Si (%) + Cr (%) ≦ 0.90 (1)
0.40 ≦ Cr (%) + Mn (%) ≦ 0.80 (2)
_H280-420 / _H100-450 <0.45 (3)
The element symbols in the above formulas (1) and (2) mean the content in mass% of the element.   Furthermore, in mass%, Ni: 0.50% or less, Co: 1.00% or less, Mo: 0.20% or less, B: 0.0002 to 0.0030%, Cu: 0.15% or less The high-strength steel wire according to claim 1, comprising one or more of them.

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