JP3327567B2 - Method for manufacturing high strength and high ductility bead wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-ductility bead wire for automobile tires, and more particularly, to a tensile strength of 250.
The present invention relates to a method for producing a high-strength and high-ductility bead wire of kgf / mm ² or more.

[0002]

2. Description of the Related Art As a measure for increasing the strength of a high carbon steel wire such as a bead wire, increasing the C content is the most industrially desirable method because it is inexpensive and has a high effect. However, in the hypereutectoid region, that is, a 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, at the time of wire drawing, grain boundaries along pro-eutectoid cementite tend to occur, and wire drawing with a high area reduction becomes impossible.

Conventionally, as a method of improving the drawability of hypereutectoid steel, a method of suppressing the formation of proeutectoid cementite by heat treatment or addition of an alloy element, or a method of evoking proeutectoid cementite by devising a drawing method. Methods have been developed to prevent ductile degradation.

For example, Japanese Patent Publication No. 56-8893 discloses a method of changing the structure to a pearlite structure in which particulate cementite is dispersed by heat treatment. this is,
The hypereutectoid steel wire is austenitized and subjected to oil quenching to form a martensite structure, and then rapidly heated to a temperature range of 770 to 930 ° C. to precipitate particulate cementite, and immediately after reaching the target heating temperature, 535 to 660. This is a method of performing a patenting process at a temperature of ° C. This method is excellent as a method for raising the drawing limit, but the cementite that has been granulated has a small contribution to strengthening unlike cementite that has developed into a layered form (the strength after patenting is low,
Since the work hardening during wire drawing is small, the effect of increasing the C content cannot be utilized.

The inventors of the present invention have studied a method of suppressing the generation of proeutectoid cementite by utilizing the effect of adding an alloy element.
-76825. All of these are characterized by adding 0.1 to 0.3% of Cr, but even these do not prevent the formation of a small amount of proeutectoid cementite.

JP-A-63-186852 discloses that REM of 5 to 50 ppm and Ca, Mg, B
A method is disclosed in which one or more of a and Sr are added in a total amount of 5 to 50 ppm. All of these elements are elements that simultaneously generate sulfide and oxide. REM, Ca, Mg, Ba, Sr produced by these additions
This is a method in which pearlite transformation is promoted by using fine sulfides and oxides containing nuclei as nuclei to suppress the formation of martensite and proeutectoid cementite. However, this method requires not only the addition of these trace elements but also the control of the contents of S, O, and Al in order to make fine sulfides and oxides appear, and the production management becomes extremely complicated. .

[0007] On the other hand, as an example of improvement from the plastic working surface so that the drawability is not deteriorated even when proeutectoid cementite is present, roller dies or cold rolling is performed before drawing. The method is described in JP-A-63-4016, and the method of drawing by lowering the approach angle of the die to about 10 degrees is described in Japanese Patent Application No. 1-281825. These are all intended to suppress the occurrence of internal cracks caused by proeutectoid cementite by reducing the tensile stress applied to the center of the steel wire during wire drawing. However, this method is effective only when the amount of generated pro-eutectoid cementite is small and thinly present at the grain boundaries, that is, when C is 1% or less or when a small amount of Cr is added, pro-eutectoid cementite is used. Only when the generation of is suppressed. On the other hand, these methods must be equipped with a new roller die and rolling mill separately from the wire drawing machine,
In addition, there are many manufacturing problems such as strict management of dies.

[0008]

As described above, the prior art cannot completely prevent the generation of grain boundary pro-eutectoid cementite in hypereutectoid steel. The purpose of the present invention is
By completely preventing the formation of grain boundary pro-eutectoid cementite in hypereutectoid steel, it is possible to perform wire drawing with high area reduction,
An object of the present invention is to provide a method for producing a high-strength, high-ductility bead wire that fully utilizes the effect of increasing the C content.

[0009]

That is, according to the present invention, C: 0.90 to 1.21 % and Si: 0.15 to 0.15 by weight.
1.5%, Mn: 0.3-1.0%, Cr: 0.1-
1.0%, wire diameter consisting of residual Fe and unavoidable impurities
There After austenite by heating the 2.6mm or more steel wires, with a range defined by (1) the number 2, and, 290
After quenching in a refrigerant maintained at 400 to 650 ° C. at a cooling rate of (° C./sec) as the upper limit, and subsequently completing the isothermal transformation in the refrigerant, a fine pearlite structure containing no proeutectoid cementite is obtained. Degree of processing 86.0-9
Subjected to wire drawing processing at 4.0%, 350 steel wire after drawing
This is a method for producing a high-strength, high-ductility bead wire, characterized by bluing at ~ 450 ° C and further subjecting it to straightening.

[0010]

Y ≦ 0.16 log X + 0.82 (1) where Y is the C content (%) of steel, and X is the cooling rate (° C. /
sec).

[0011]

Hereinafter, the present invention will be described in detail.

The present inventors conducted many experiments to improve the drawability of the hypereutectoid steel, and as shown below, selected the cooling conditions from the austenitizing temperature to produce the proeutectoid cementite. New knowledge that it can be prevented.

That is, a columnar sample having a diameter of 3 mm and a height of 10 mm was prepared from a wire rod obtained by hot rolling a vacuum melted steel having the composition shown in Table 1, and this was prepared in an Ar gas at 950-1000.
After austenitizing by induction heating to ° C., continuous cooling was performed at various cooling rates. Polish the sample after cooling
After etching by the method specified in G0551, the state of formation of proeutectoid cementite was examined by an optical microscope.
The formation state of the thin film-like cementite having a small grain boundary was observed using a scanning electron microscope after the polished sample was etched with picral. FIG. 1 shows the relationship between the generation limit of proeutectoid cementite, the C content, and the cooling rate. As described above, the formation of proeutectoid cementite depends on the cooling rate in addition to the C content, and the formation can be prevented by increasing the cooling rate even at the same C content. FIG.
From the above, the conditions under which proeutectoid cementite does not occur are represented by the C content of steel and the cooling rate from the austenite region.
Equation 3 is obtained.

[0014]

[Table 1]

[0015]

Y ≦ 0.16 log X + 0.82 (1) where Y is the C content (%) of the steel, and X is the cooling rate (° C. /
sec).

In the actual patenting of a high-strength steel wire, the pearlite transformation time is insufficient in continuous cooling. Therefore, it is necessary to perform lead patenting or fluidized bed patenting treatment to transform the pearlite into fine pearlite at a constant temperature. At that time, by controlling the temperature of the molten lead or the fluidized bed as the refrigerant and selecting the cooling rate of the steel wire within the range satisfying the expression (1), it is possible to completely prevent the generation of proeutectoid cementite. It is. However, when the refrigerant temperature is lower than 400 ° C., bainite is formed on the surface of the steel wire, and the wire drawing limit is reduced.
On the other hand, when the temperature exceeds 650 ° C., the layer structure of pearlite is broken, and thus both the strength and the limit of wire drawing are reduced. Therefore, the refrigerant temperature is set to 400 to 650 ° C. The temperature in the cooling bath does not need to be uniform. That is,
In order to obtain the cooling rate of equation (1), the temperature of the refrigerant on the side where the glowing steel wire enters is set low, and the temperature of the other parts is maintained at a temperature at which a fine layered pearlite can be obtained according to the steel composition. Should. For such a purpose, it is desirable that the cooling tank has a structure that can perform inclined heating, and it is more preferable to adopt 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 the strength, and is one of the most important elements. 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 drawing, it is advantageous that the C content is high, and in the present invention, the C content is set to 0.90% or more. On the other hand, when the C content exceeds 1.25%, the cooling rate required to prevent the generation of pro-eutectoid cementite exceeds 480 ° C./sec, as shown by the equation (1), so that it is industrially realized. Becomes difficult. Therefore, the upper limit of the C content is 1.25%.

Si is added in an amount of 0.15% or more as a deoxidizing agent. On the other hand, Si forms a solid solution in ferrite as an alloy element and exhibits a remarkable solid solution strengthening action. In addition, Si in ferrite
Is an element indispensable for the production of high-strength steel wires because it has the effect of reducing the strength reduction during hot-dip galvanizing and bluing. However, if it exceeds 1.5%, the ductility of the drawn steel wire is reduced, so the upper limit is 1.5%.

Mn is also added as a deoxidizing agent in an amount of 0.3% or more. In addition, since Mn has a large quenching property improving effect, when the wire diameter is large, it is possible to increase the Mn content to improve the uniformity in the cross section, and to improve the ductility of the steel wire after drawing. It is valid. However, if the content exceeds 1.0%, martensite is generated in the central segregation portion and the wire drawing workability is deteriorated. Therefore, the upper limit is 1.0%.

Cr is used in an amount of 0.1% to reduce the lamella spacing of pearlite and improve the strength and drawability of steel wire.
% Or more. If it is less than 0.1%, the effect is not sufficient. On the other hand, if it exceeds 1.0%, the time required for transformation becomes long and productivity is significantly reduced.

Next, the processing conditions after the heat treatment in the present invention will be described.

The steel wire was a fine pearlite by isothermal transformation, subjected to wire drawing processing of the reduction of area from 86.0 to 94.0%. If it is less than 86.0%, the amount of work hardening is insufficient and the target strength cannot be obtained, so the lower limit is 86.0%. On the other hand, if it exceeds 94.0%, the ductility decreases and the elongation of the steel wire (bead wire) after bluing becomes insufficient.
The upper limit is 4.0%.

The drawn steel wire is subjected to bluing. The purpose of bluing is to impart the necessary elongation to the steel wire, and is usually performed by a method of continuously passing the steel wire through molten lead or a fluidized bed. The bluing temperature is 3
If the temperature is lower than 50 ° C., the required elongation cannot be obtained.
If it exceeds, not only the strength decreases, but also the torsion value decreases. For this reason, the lower limit and upper limit of the bluing temperature are 350 ° C. and 450 ° C., respectively.

Although the elongation of the steel wire is recovered by bluing, a straightening process is performed after bluing to obtain a higher elongation. The straightening is performed using a roller type straightening machine or a rotary top straightening machine. By performing the straightening process, the distribution of the movable dislocation density in the steel wire longitudinal direction becomes more uniform,
Uniform elongation increases. Since the straightening does not affect the local elongation, the total elongation is increased by the increase in the uniform elongation.

[0026]

EXAMPLES Hereinafter, 250 kgf / mm ² according to the method of the present invention will be described.
The result of manufacturing a high-strength, high-ductility bead wire having the above tensile strength will be described. The target ductility of the bead wire is elongation of 8% or more and twist value of 35 times or more.

The diameters of the chemical components shown in Table 2 are 2.60-4.
After lead patenting a 20 mm steel wire, a thin wire having a diameter of 1.00 mm was manufactured by wire drawing. Next, these three
Blueing was performed for 15 s in a lead bath maintained at 40 to 460 ° C., followed by straightening using a 20-stage roller straightener, and finally bronzing by displacement plating to produce a high-strength bead wire. Tables 2 and 3 show the chemical composition of the steel, the patenting conditions, the structure after the patenting, the degree of wire drawing, the bluing temperature, and the mechanical properties of the bead wire.

[0028]

[Table 2]

[0029]

[Table 3]

The groups A, B, C and D mainly show the influence of the contents of C, Si, Mn and Cr, respectively.

When A-1 has a C content of 0.86%, the target strength cannot be obtained. On the other hand, when A-8 has a C content of 1.35%, proeutectoid cementite is formed due to insufficient cooling rate, and drawing cannot be performed. Was. Similarly, in the case of A-5 having a C content of 1.20%, if the cooling rate is not 20 ° C./sec and does not satisfy the expression (1), proeutectoid cementite is formed, and the elongation and twist value of the product decrease. As a result, the target bead wire could not be manufactured.

Although the strength increased with an increase in the Si content, the ductility was insufficient with 1.63% of B-5, and the elongation and twist of the plated steel wire were reduced.

In the case of C-4 with Mn of 1.06%, the elongation and twist value of the plated steel wire were remarkably reduced due to martensite formed in the center segregation part.

With D-1 having a Cr content of 0.05%, the effect was small and the target strength could not be obtained. On the other hand, 1.
In D-8 of 20%, disconnection frequently occurred because martensite was generated in the patenting structure due to insufficient transformation time.
The elongation and torsion values also deteriorated remarkably.

E-1 of the conventional method 1 is disclosed in JP-A-63-4016.
This is a method disclosed in Japanese Patent Application Laid-Open Publication No. H10-260, in which normal drawing is performed after drawing a roller die. F-1 of the conventional method 2 is a method disclosed in JP-A-63-186852, and has a REM of 0.0012 in addition to the chemical components shown in Table 2.
%, 0.0008% of Ca. In each case, not only did not reach the target strength, but also elongation and aperture value were low due to the incorporation of pro-eutectoid cementite.

B-2, when the lead bath temperature is less than 400 ° C.
In addition, D-3 is a case exceeding 650 ° C., and martensite and proeutectoid cementite were generated, respectively, so that the ductility of the bead wire was reduced.

A-3 indicates a case where the degree of drawing is less than 86.0%, and D-4 indicates a case where the degree of drawing exceeds 94.0%. The former is strength, and the latter is elongation and reduction. Fell below the target.

A-6 indicates a case where the bluing temperature is lower than 350 ° C., and D-6 indicates a case where the bluing temperature is higher than 450 ° C. The former has insufficient elongation, and the latter has elongation and torsion target values. Below the value.

Each of the bead wires manufactured by the method of the present invention has a strength of 250 kgf / mm ² or more, has higher strength and is excellent in ductility (elongation and torsion value) as compared with those manufactured by the conventional method. Was.

[0040]

As described above, according to the method of the present invention, it is possible to manufacture a bead wire having higher strength than the conventional one and excellent in elongation and twisting characteristics.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the generation limit of proeutectoid cementite, the C content, and the cooling rate.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-4016 (JP, A) JP-A-2-294426 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/06 C21D 9/52

Claims (1)

(57) [Claims] 1. A weight ratio of C: 0.90 to 1.21 %, Si: 0.15 to 1.5%, Mn: 0.3 to 1.0%, Cr: 0.1 to 1.0. %, The wire diameter consisting of residual Fe and unavoidable impurities is 2.6 mm
After heating the above steel wire to austenite,
(1) Within the range defined by equation (1) and 290 (° C./s
ec) is quenched in a cooling medium maintained at 400 to 650 ° C. at a cooling rate with the upper limit set, followed by isothermal transformation in the cooling medium to obtain a fine pearlite structure containing no proeutectoid cementite, and then a workability of 86 0.0 to 94.0%
In subjected to a wire drawing process, the steel wire after drawing 350 to 450
A method for producing a high-strength, high-ductility bead wire, comprising bluing at a temperature of ° C. and further performing a straightening process. Y ≦ 0.16 log X + 0.82 (1) where Y is the C content (%) of steel, and X is the cooling rate (° C. /
sec). "