JPH075992B2 - High-strength steel wire manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a steel cord, a Zn-plated steel stranded wire, a PC steel wire,
The present invention relates to a method for manufacturing high-strength steel wire used for suspension bridge cables and the like.

(Prior Art) As a measure for increasing the strength of a high-strength steel wire, increasing the C content is the most desirable method industrially because it is inexpensive and highly effective. However, in the hypereutectoid region, that is, in the region where C usually exceeds 0.9%, brittle pro-eutectoid cementite is formed in a network form along the prior austenite grain boundaries during patenting. For this reason, during wire drawing, grain boundary cracks along the pro-eutectoid cementite are likely to occur, and wire drawing with a high area reduction ratio becomes impossible.

Conventionally, as a method for improving the wire drawability of hyper-eutectoid steel,
A method for suppressing the formation of pro-eutectoid cementite by heat treatment or addition of alloying elements, or a method for preventing ductility deterioration due to pro-eutectoid cementite by devising a wire drawing method has been developed.

For example, Japanese Examined Patent Publication (Kokoku) No. 56-8893 discloses a method of converting a structure into a pearlite structure in which granular cementite is dispersed by heat treatment. This is because the hyper-eutectoid steel wire is austenitized, oil-quenched to form a martensite structure, and then rapidly heated to a temperature range of 770 to 930 ° C to precipitate granular cementite, and immediately after reaching the target heating temperature.
It is a method of patenting at a temperature of 535 to 660 ° C.

This method is excellent as a method of increasing the wire drawing limit, but granular cementite, unlike layered cementite, has a small contribution to strengthening (strength after patenting is low, Since the work hardening is also small), the effect of increasing the C content cannot be utilized.

The present inventors have so far studied a method of suppressing the generation of pro-eutectoid cementite by utilizing the effect of adding alloy elements,
We have applied for Japanese Patent Application No. 1-281825 and Japanese Patent Application No. 1-76825. All of these are characterized by adding 0.1 to 0.3% of Cr, but they cannot prevent the formation of a small amount of pro-eutectoid cementite.

Further, in JP-A-63-186852, 5 to 50 ppm of REM and one or more kinds of Ca, Mg, Ba and Sr are added in total of 5 to 50 ppm.
A method of adding is disclosed. All of these elements are elements that simultaneously generate sulfides and oxides. REM, Ca, Mg, Ba, Sr generated by these additions promote the pearlite transformation with fine sulfide oxides as a nucleus,
This is a method for suppressing the formation of martensite and pro-eutectoid cementite.

However, in this method, not only the addition of these trace elements, but also the content of S, O, Al must be controlled in order to make fine sulfide oxides appear, and the production control is extremely complicated. Become.

On the other hand, as an example of improving the plastic working surface so that the wire drawing workability does not deteriorate even in the presence of pro-eutectoid cementite, a method of performing roller die processing or cold rolling before drawing is a special method. Japanese Patent Application Laid-Open No. 1-281825 discloses a method of drawing the wire while lowering the approach angle of the die to about 10 degrees.

All of these are intended to suppress the generation of internal cracks due to pro-eutectoid cementite by reducing the tensile stress applied to the central portion of the steel wire during wire drawing. However, this method is effective only when the amount of pro-eutectoid cementite produced is small and is present at grain boundaries thinly, that is, when C is 1% or less or when a small amount of C is present.
Only when Cr addition suppresses the formation of pro-eutectoid cementite.

On the other hand, these methods have many manufacturing problems such as the fact that a roller die and a rolling machine must be newly installed in addition to the wire drawing machine, and the dies must be strictly controlled.

(Problems to be Solved by the Invention) As described above, the conventional techniques cannot completely prevent the generation of grain boundary proeutectoid cementite in hypereutectoid steel.

The object of the present invention is to completely prevent the formation of grain boundary pro-eutectoid cementite in hypereutectoid steel, thereby making it possible to perform wire drawing with a high area reduction rate and making full use of the effect of increasing the C content. It is to provide a manufacturing technology for high strength steel wire.

(Means and Actions for Solving the Problems) The present invention is C: 0.90 to 1.25%, Si: 0.15 to 1.5%, Mn: 0.3 to
1.0%, Cr: 0.1-1.0%, and V: 0.02-
0.30% of 1 type or 2 types, and 1 type or 2 or more types of Al, Ti, Nb, Zr, B and 0.1% or less respectively, and the balance is Fe.
And after heating the steel wire consisting of inevitable impurities to austenite, it is quenched into a refrigerant kept at 400 to 650 ° C at a cooling rate within the range defined by the formula (1), and then isothermized in the refrigerant. A method for producing a high-strength steel wire, characterized in that a fine pearlite structure containing no pro-eutectoid cementite is formed by completing the transformation.

Y ≦ 0.16logX + 0.82 (1) where Y is the C content (%) of steel and X is the cooling rate (° C / s
ec) is shown.

The present invention will be described in detail below.

The present inventors have conducted many experiments to improve the wire drawability of hyper-eutectoid steel, and prevent the formation of pro-eutectoid cementite by selecting the cooling conditions from the austenitizing temperature, as shown below. I got new knowledge that I can do it.

That is, the present inventors produced a cylindrical sample having a diameter of 3 mm and a height of 10 mm from a wire rod obtained by hot-rolling vacuum-melted steel having the composition shown in Table 1, and making a cylindrical sample in Ar gas at 950 to 1000 ° C. After induction heating to form austenite, it was continuously cooled at various cooling rates.

The cooled sample was polished and etched by the method specified in JIS G 0551, and then the generation state of pro-eutectoid cementite was examined by an optical microscope.

In addition, the state of formation of thin film-like cementite with grain boundaries was confirmed by etching the sample after polishing with Picral,
It observed using the scanning electron microscope.

FIG. 1 shows the relationship between the generation limit of proeutectoid cementite, the C content, and the cooling rate.

Thus, the formation of pro-eutectoid cementite depends not only on the C content but also on the cooling rate, and even if the C content is the same, the generation can be prevented by increasing the cooling rate.

From FIG. 1, when the conditions in which pro-eutectoid cementite does not occur are expressed by the C content of steel and the cooling rate from the austenite region,
It becomes like the following formula (1).

Y ≦ 0.16logX + 0.82 (1) where Y is the C content (%) of steel and X is the cooling rate (° C / s
ec) is shown.

In actual patenting of high-strength steel wire, since pearlite transformation time is insufficient in continuous cooling, it is necessary to carry out lead patenting or fluidized bed patenting treatment to transform isothermally into fine pearlite.

At that time, by controlling the temperature of the molten lead or the fluidized bed which is the refrigerant,
If the cooling rate of the steel wire is selected within the range satisfying the expression (1), it is possible to completely prevent the occurrence of pro-eutectoid cementite. However, if the refrigerant temperature is lower than 400 ° C, bainite is generated in the surface layer of the steel wire, and the wire drawing limit is lowered. Also, when the temperature exceeds 650 ° C, the layered structure of pearlite collapses,
Therefore, both strength and wire drawing limit are lowered. Therefore, the refrigerant temperature is 400 to 650 ° C.

The temperature in the cooling tank need not be uniform. That is, in order to obtain the cooling rate of formula (1), the temperature of the refrigerant on the side where the red-hot steel wire enters is set low, and the temperature of the other parts is the temperature at which fine layered pearlite is obtained according to the steel composition. Should be kept at. For this purpose,
The cooling tank preferably has a structure capable of performing gradient heating, and more preferably a cooling tank divided into a plurality of cooling zones.

Next, the reasons for limiting the components of the present invention will be described.

C is an effective and economical element for increasing strength,
It is one of the most important elements of the present invention. As the C content increases, the strength after patenting and the amount of work hardening during wire drawing increase. Therefore, in order to obtain a high strength steel wire by wire drawing, it is advantageous that the C content is high, and in the present invention, it is 0.90% or more. On the other hand, if the C content is
When it exceeds 1.25%, the cooling rate required to prevent the occurrence of pro-eutectoid cementite is 480, as shown in equation (1).
Since it exceeds ℃ / sec, it is difficult to realize industrially. Therefore, the upper limit of the C content is 1.30%.

Si is added as a deoxidizer in an amount of 0.15% or more. On the other hand, Si, as an alloying element, forms a solid solution with ferrite and exhibits a remarkable solid solution strengthening effect. Further, Si in ferrite has an effect of reducing the strength reduction during hot-dip galvanizing after wire drawing and bluing, and is an essential element for manufacturing high strength steel wire.
However, if it exceeds 1.5%, the ductility of the steel wire after wire drawing decreases, so 1.5% is made the upper limit.

Mn is also added as a deoxidizing agent in an amount of 0.3% or more. Also, Mn has a large effect of improving hardenability, so if the wire diameter is large, Mn
By increasing the content, it is possible to increase the uniformity in the cross section, which is effective in improving the ductility of the steel wire after drawing.
However, if it exceeds 1.0%, martensite is generated in the central segregated portion, and the wire drawability deteriorates. Therefore, the upper limit is 1.0%.

Cr reduces the lamellar spacing of pearlite and improves the strength and drawability of steel wire, so Cr is added in an amount of 0.1% or more as necessary. If it is less than 0.1%, the effect is not sufficient, while if it exceeds 1.0%, the time required for transformation becomes long and the productivity is remarkably reduced, so 1.0% is made the upper limit.

V, like Mn, improves hardenability, but on the other hand, it forms carbides and strengthens pearlite by precipitation hardening. For this purpose, 0.02% or more is added if necessary. However, the addition of V delays the pearlite transformation and facilitates the formation of martensite and bainite. Furthermore, the precipitation hardening action of V carbide is saturated, so the upper limit is 0.3%.

In addition to the above strengthening elements, if necessary, Al, Ti, Nb, Zr, B
0.1% or less of one or more of the above. All of these elements tend to form nitrides and carbides, and therefore have a strong tendency to make austenite grains fine. The refinement of austenite grains is effective for increasing the drawing value of the steel wire after patenting and improving the drawability.

Therefore, in the case of manufacturing a cable wire for a suspension bridge and the like, preferable results can be obtained by adding these nitride and carbide forming elements. However, even if added over 0.1%, not only the effect is saturated, but also nonmetallic inclusions increase, so 0.1% is made the upper limit.

(Example-1) Hereinafter, the result of manufacturing a thin wire having a tensile strength of 400 kgf / mm ² or more will be described.

A steel wire having a chemical composition of 2.4 mm in diameter shown in Table 2 was wire-drawn after lead patenting to produce a fine wire of 0.4 mm.

Table 3 shows the patenting conditions, the characteristics after patenting, and the characteristics of thin wires after drawing.

When the C content was 0.86% (A-1 steel), the target strength could not be obtained. On the other hand, when the C content was 1.35% (A-4 steel), proeutectoid cementite was generated due to insufficient cooling rate, and wire drawing could not be performed. . Similarly, even if the C content is 1.20% (A-3 steel), as shown in Table 3, when the cooling rate is 20 ° C./sec and the formula (1) is not satisfied, proeutectoid cementite is generated and the elongation is increased. Since the twist value after the wire was reduced, the target thin wire could not be manufactured. The B-added steel (A-3 steel) has a higher drawing value after patenting and a higher twist value after wire drawing than the non-added steel (A-2 steel). The conventional method is the method described in Japanese Patent Application No. 1-281825.

Each of the thin wires produced by the method of the present invention has a strength of 400 kgf / mm ² or more, which is higher than that produced by the conventional method, and is excellent in ductility, particularly in twist value.

(Example-2) Hereinafter, the production results of a high-strength galvanized steel wire supporting a suspension bridge and a cable-stayed bridge will be described.

Table 4 shows the chemical composition of the steel wire.

Each of the steels B-1 to B-4 is intended to manufacture a steel wire having a diameter of 7 mm and a tensile strength of 200 kgf / mm ² or more, and each steel of C-1 to C-4 has a diameter of 5 mm and a tensile strength. The purpose is to manufacture steel wire with strength of 220 kgf / mm ² or more. In addition, the conventional methods are all Japanese Patent Application No. 1-76
This is the method described in No. 825.

As shown in Table 5, steel wires having diameters of 13 mm and 11 mm were subjected to lead patenting, and then hot-dip galvanized after wire drawing to a target wire diameter. The strength increased with the increase of Si content, but the ductility was insufficient at 1.61% (B-3 steel), and the twist value of the plated steel wire decreased. On the other hand, even when Mn is 1.08% (C-3 steel), due to the martensite formed in the central segregation part,
The twist value of the galvanized steel wire was significantly reduced. Further, in Table 5, B-1 steel, which was patented at a lead bath temperature of 375 ° C., had a short transformation time and produced martensite, so that the twist value of the plated steel wire was significantly reduced.

As described above, when the hot-dip galvanized steel wire is produced by the method of the present invention, the high-strength steel wire which cannot be obtained by the conventional method, that is, the strength is about 20 kgf / mm ² higher, and the twisting property is further excellent. You can get a good steel wire.

(Example-3) Hereinafter, the production results of a high-strength galvanized steel twisted wire (steel wire for ACSR) will be described.

Table 6 shows the chemical composition of the steel wire.

Each steel from D-1 to D-5 has a diameter of 2.8 mm, and E-1 to E-5
Each steel up to 2.0mm in diameter, each tensile strength 240kgf / mm
^The purpose is to manufacture high strength steel wire of ² or more. 10 mm and 7 mm steel wires were subjected to fluidized bed patenting, drawn to a target wire diameter, and then hot-dip galvanized.

The results are shown in Table 7.

When the Cr content is 0.06% (D-1 steel), the effect is small,
The target strength cannot be obtained. On the other hand, at 1.11% (D-4 steel),
Martensite was generated in the patenting structure due to insufficient transformation time, so wire breakage occurred frequently and wire drawing was impossible. The conventional method (D-5 steel) is the method disclosed in Japanese Patent Laid-Open No. 63-4016, in which the roller die is drawn and then the ordinary drawing is performed. The conventional method (E-5 steel) is the method disclosed in JP-A-63-186852.

As shown in Table 7, the strength that can be achieved by the conventional method is 220 kgf.
Although it is / mm ² grade, according to the method of the present invention, it is possible to manufacture 240 kgf / mm ² grade, and further, since the formation of pro-eutectoid cementite is completely suppressed, the strength is increased. Despite this, the twisting characteristic is improved on the contrary.

(Effects of the Invention) As described above, according to the method of the present invention, a steel cord having higher strength and excellent twisting characteristics than the conventional one, AC
It is possible to manufacture high-strength steel wires such as SR steel wires, suspension bridge cables, and PC steel wires.

[Brief description of drawings]

FIG. 1 is a table showing the relationship between the generation limit of pro-eutectoid cementite, the C content, and the cooling rate.

Claims (2)

[Claims] 1. A weight ratio of C: 0.90 to 1.25%, Si: 0.15 to 1.5%, Mn: 0.3 to 1.0%, the balance being austenitized by heating a steel wire consisting of Fe and unavoidable impurities. To obtain a fine pearlite structure that does not contain pro-eutectoid cementite by quenching in a refrigerant maintained at 400 to 650 ° C at a cooling rate within the range defined by formula (1) and then completing isothermal transformation in the refrigerant. A method for manufacturing a high-strength steel wire. Y ≦ 0.16logX + 0.82 (1) where Y is the C content (%) of steel and X is the cooling rate (° C / s
ec) is shown. 2. A weight ratio of Cr: 0.1 to 1.0%, V: 0.02 to 0.30%, one or two kinds, and Al, Ti, Nb, Zr, B, one or more kinds, respectively.
The method for producing a high-strength steel wire according to claim 1, containing 0.1% or less.