JPH11269607A - Wire drawing type high strength steel wire rod and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a wire drawing type high strength steel wire rod free from the generation of vertical cracking in a twisting test and having <=0.4 mm diameter and >=4150 NPa TS and to provide a method for producing it. SOLUTION: This wire drawing type high strength steel wire rod is the one having a compsn. contg., by weight, 0.8 to 1.2%, C, 0.1 to 1.5% Si, 0.2 to 1% Mn, <=0.003% Al, 0 to 1% Cr, 0 to 0.5% Mo, 0 to 0.3% V and 0 to 2% Co and also composed of cementite in which the volume ratio is regulated to 6.0×C (weight %) to 12.0×C (weight %) volume % and the average grain size is regulated to 2 to 10 nm, and the balance ferrite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel wire for a steel cord used for reinforcing automobile tires, conveyor belts, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a steel cord wire generally used for automobile tires and the like (hereinafter referred to as "wire" or "cord wire") is formed by twisting a high carbon steel wire having a diameter of about 0.2 mm into a strand. At present, many steel wires have a tensile strength (TS) of around 3200 MPa. The manufacturing method and characteristics of this conventional code wire are as follows.

[0003] First, a steel wire having a diameter of about 5.5 mm is repeatedly subjected to cold drawing and lead patenting.
After being formed into a steel wire having a diameter of about 2 mm, the steel wire is heated to about 900 ° C. in a final lead patenting step, and then immersed in a lead bath at about 600 ° C. to obtain a steel wire for drawing having a TS of about 1200 MPa. This is pickled and plated, and then drawn to about 0.2 mmφ to obtain a wire having a TS of about 3200 MPa.

[0004] In recent years, there has been an increasing demand for weight reduction of tires for the purpose of improving fuel efficiency of automobiles, and therefore, a higher strength cord wire has been demanded from the industry. In response to this demand, the strength of the cord wire has been increased by a technique such as addition of an alloying element or an increase in the area reduction rate in the final drawing.

[0005] For example, Japanese Patent Application Laid-Open No. 6-279924 discloses a low-alloy steel wire rod for high-strength ultrafine wires having a diameter of 0.40 mm or less and a TS of 4000 MPa or more. However, JP-A-6-279924 does not mention vertical cracking in a twisting test, which is most important in twisting characteristics. Furthermore, although this low alloy steel wire contains B as an alloying element, it is described in the literature (Journal of the Institute of Metals: Kaneko, Nishizawa, Chiba: Volume 30 (1966), p.263: especially
As shown in Fig. 3), B significantly increases the Acm line in the Fe-C quasi-binary phase diagram. Therefore, in the austenitization of heat treatment such as lead patenting, cementite or boride is used. Are likely to remain. The remaining cementite or carbohydrate is usually agglomerated and coarse, and breakage often occurs during cold drawing from these as starting points.

Japanese Patent Application Laid-Open No. Hei 7-113119 discloses that
It discloses a method of obtaining an ultrafine steel wire having a TS of 3800 MPa or more, a drawing of 30% or more, and no longitudinal cracking during a torsion test by reducing the area reduction rate of the final finishing die to 2 to 8%. However, in the example, the TS is the highest, at most 4128 MPa.

The above-mentioned JP-A-6-279924,
By using the method disclosed in Japanese Patent Application Laid-Open No. Hei 7-113119, it is possible to increase the strength of ultrafine steel wire, but it is industrially stable, and the TS after final drawing is 4150 MPa or more.
In addition, vertical cracking cannot be prevented by the torsion test.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to limit the content of alloying elements and the structure in steel after final drawing to prevent longitudinal cracks in a torsion test.
An object of the present invention is to provide a high-strength steel wire having TS4150 MPa or more and a method for manufacturing the same.

[0009]

As a result of repeated tests, the present inventor has found that in order to obtain a high-strength steel wire having a TS of 4150 MPa or more and having no longitudinal crack in a twist test, the following conditions and Have to be met.

After final drawing, cementite has an average particle size of 2
It is divided to 10 nm.

Before the final drawing, a volume of 15 × C (by weight)
%) Cementite was about 6.0 × C after final drawing.
Amount (% by weight) to 12.0 x C amount (% by weight)%.

In order to obtain the structure in the steel, it is necessary to control the heat treatment (patenting) and the final drawing.

The present invention has been completed on the basis of the above-mentioned matters and has undergone a production test on site, and has the following draw-strengthened high-strength wire and a method of manufacturing the same.

(1) C: 0.8 to 1.2% by weight,
Si: 0.1 to 1.5%, Mn: 0.2 to 1%, Al:
0.003% or less, Cr: 0 to 1%, Mo: 0 to 0.5
%, V: 0 to 0.3%, and Co: 0 to 2%,
The balance has a chemical composition consisting of Fe and unavoidable impurities, and the structure in the steel has a volume ratio of 6.0 × C (% by weight) to 12.0 ×
A drawn-strengthened high-strength wire rod comprising C (% by weight)%, a cementite having an average particle diameter of 2 to 10 nm and a balance substantially consisting of ferrite.

(2) The steel wire is subjected to the following heat treatment one or more times to perform wire drawing, and before the final wire drawing, the surface of the steel wire is subjected to plating, and the steel wire is cold and is subjected to the following conditions. The method for producing a wire according to the above (1), wherein the final wire drawing is performed.

Heat treatment (a) Austenitizing temperature: 900 to 1100 ° C Holding time: 0 to 120 seconds (b) Transformation temperature: 500 to 650 ° C Holding time: 3 to 30 seconds Final drawing process (a) Degree of work: True strain (ln (S ₀ / S _f ))
4.5 However, ln is the cross-sectional area of the natural logarithm, S ₀ is final drawing ago, S _f represents the cross-sectional area after the final drawing.

(B) Number of dies: 15 to 30 (c) Die angle: 7 to 15 ° (d) Drawing speed at the winding section: 400 to 2000 m / min Here, the final drawing process is the last process. This refers to wire drawing after heat treatment. The “steel wire” before the wire drawing process (2) is performed
Usually, it refers to a steel wire having a diameter of about 5.5 mm manufactured by hot rolling. The above "heat treatment" is generally called "patenting treatment", but in the description of this specification, it is described as "heat treatment" in principle. In the above description, the die angle refers to a taper gradient angle from a portion having a maximum hole diameter to a portion having a minimum hole diameter at the entrance of the die.

[0018]

Next, the reason why the present invention is set will be described. In the following description, “%” of the alloy element means “% by weight”.

(A) Chemical Composition The steel wire to which the present invention is applied is subjected to intermediate drawing after hot rolling, followed by final heat treatment and final drawing to obtain predetermined properties (TS, ductility, etc.). ) Is given. In consideration of imparting properties to the final product and securing industrial productivity, the chemical composition of the steel wire is limited to the following range.

C: 0.8 to 1.2% C is an effective element for increasing the TS of the wire. When C is less than 0.8%, TS after final drawing is 4150MP.
does not reach a. On the other hand, if it exceeds 1.2%, it becomes difficult to prevent the formation of proeutectoid cementite, and disconnection frequently occurs during cold drawing. Therefore, the range of C is set to 0.8 to 1.2%.

Si: 0.1 to 1.5% Si is an effective element for increasing the TS of the wire, and is also required as a deoxidizing agent. Si must be contained in an amount of 0.1% or more as a deoxidizing agent. On the other hand, if it exceeds 1.5%, the torsion characteristics deteriorate, and a vertical crack occurs in the torsion test. Therefore, Si is set to 0.1 to 1.5%.

Mn: 0.2-1% Mn is an effective element for increasing the TS of the wire, and is necessary for preventing hot brittleness due to S. In order to exhibit these effects, it is necessary to contain Mn at 0.2% or more. On the other hand, if it exceeds 1%, Mn tends to segregate in the center of the steel wire, so that martensite and bainite are formed in the center of the wire after hot rolling. As a result, the workability in the primary drawing decreases. Therefore, the range of Mn is 0.2 to 1
%.

Al: 0.003% or less Al forms oxide inclusions mainly composed of Al ₂ O ₃ in steel and causes wire breakage at the time of final drawing. The above adverse effects can be prevented by setting the content of Al to 0.003% or less.
Therefore, the range of the Al content is set to 0.003% or less.

The wire-strengthened high-strength wire according to the present invention satisfies the above-mentioned component ranges, and reaches the target performance if the balance is Fe and inevitable impurities. Cr, Mo, V, C
If one or more of the components o are contained in the following range, the performance can be further enhanced.

Cr: 0 to 1% Cr may not be contained. However, when Cr is further contained, the lamella spacing of pearlite is reduced, and the TS can be increased. However, if it exceeds 1%, the time required for transformation from austenite to pearlite exceeds 30 seconds,
Productivity decreases. Therefore, Cr is set to 0 to 1%.

Mo: 0 to 0.5% Mo may not be contained. However, Mo has a T
Since it is effective for increasing S, it is added when higher strength is required. When Mo is contained, the effect is not clear if it is less than 0.1%, so that the content is desirably 0.1% or more. On the other hand, if the content exceeds 0.5%, the time required for transformation from austenite to pearlite exceeds 30 seconds and the productivity is reduced.

V: 0 to 0.3%, V may not be contained. However, V is effective for increasing the TS of the wire, and is added to further increase the strength. When V is included, the effect is less likely to be exhibited if the content is less than 0.05%, so that the content is preferably 0.05% or more. On the other hand, if it exceeds 0.3%, coarse VC is generated during steelmaking and remains after hot rolling, and breakage frequently occurs during cold drawing. I do.

Co: 0 to 2% Co may not be contained. However, Co is effective for suppressing the formation of proeutectoid cementite, and is added particularly when the cooling rate cannot be increased in the patenting treatment. However, if the content is less than 0.2%, a clear effect cannot be obtained. Therefore, in order to exhibit the effect, it is desirable to include 0.2% or more. On the other hand, if it exceeds 2%, the time required for transformation from austenite to pearlite is longer than 30 seconds, and productivity is reduced.

There is no need to impose any particular restrictions on the composition of the other chemical components of the wire to which the present invention is directed. It is only necessary that the component range be such that the characteristics required in the final product can be imparted and industrial productivity can be ensured.

Specifically, for example, the alloy contains the alloy in the above range, and contains Cu: 0 to 1%, N
i: 0 to 2%, Ti: 0 to 0.2%, Nb: 0 to 0.2
%, N: 0 to 0.03%, B: 0 to 0.005%, P
b: 0 to 0.3%, rare earth element: 0 to 0.1%, Ca:
Any steel may be used as long as it contains 0 to 0.01%, Mg: 0 to 0.01%, and contains P: 0.05% or less and S: 0.05% or less as impurities.

When the purpose is to improve the properties of steel materials or final products, the range of elements other than those described above is further limited to Cu: 0.05 to 1% and Ni: 0.3.
~ 2%, Ti: 0.03-0.1%, Nb: 0.02-
0.1%, N: 0.001 to 0.03%, B: 0.00
03-0.005%, Pb: 0.02-0.3%, rare earth element: 0.002-0.1%, Ca: 0.0005-
It is preferable to set 0.01% and Mg: 0.0005 to 0.01%. Further, it is preferable that P as an impurity is 0.05% or less and S: 0.05% or less.

(B) Structure in Steel after Final Drawing Next, the reason for limiting the structure in steel after final drawing will be described.

In the present invention, a steel material satisfying the above requirements for the chemical components is hot-rolled, subjected to primary drawing, then subjected to final heat treatment and final drawing, and given predetermined characteristics (TS, ductility, etc.). You.

At this time, it was found that the structure in the steel of the steel wire after the final drawing was closely related to the TS and the torsion characteristics together with the alloy components.

FIG. 1 is a graph in which test values of Examples described later are plotted with the vertical axis representing the cementite particle diameter after final drawing and the horizontal axis representing (volume% of cementite) / {C (weight%)}. is there. The chemical composition of the specimen in each plot was selected within the scope of the present invention. In FIG. 1, those having a TS of 4150 MPa after the final wire drawing and having no longitudinal crack in the torsion test are plotted with ○ marks. On the other hand, those that do not satisfy one or both of “TS after final drawing is 4150 MPa or more” and “No longitudinal crack occurs in twisting test” are indicated by ×. From FIG. 1, the cementite particle size is 2 to 10 nm, and part of the cementite is decomposed (volume% of cementite) / ΔC (weight)
%) ６ is 6 to 12, it can be seen that the TS after the final drawing is 4150 MPa or more, and that no vertical cracks occur in the twist test.

When the average diameter of the cementite is less than 2 nm or more than 10 nm, the drawing is reduced in the tensile test and the vertical crack is generated in the twist test.

The ratio of the volume% of cementite to the C content is 6
If it is less than 1, the drawing is reduced in the tensile test, and a vertical crack is generated in the twist test. On the other hand, when the above ratio exceeds 12, TS becomes insufficient or vertical cracks are liable to occur in a twist test.

The reason why the volume% of cementite is reduced by the drawing treatment has not been elucidated yet. Further, it has been actively discussed that the structural change of cementite and ferrite is caused by wire drawing.
There is no covenant document that the decline will occur. The present inventor takes a position to explain the phenomenon that the volume fraction of cementite decreases from the following thermodynamics. As a result of the cementite being divided by wire drawing, the diameter is nanometer (nm)
When the order is reached, the area of the interface between cementite and steel increases, and the interface energy increases. As a result, cementite becomes unstable and a part of the cementite decomposes. The present inventor, for the first time, will explain with this concept, but does not intend to be bound by this concept, if the target structure in steel can be obtained under certain manufacturing conditions, adopt that condition, The production conditions of the present invention were set.

The method for determining the average particle size of cementite and the content of (cementite volume%) / C will be described in detail in Examples.

(C) Manufacturing Method The wire drawing process is usually performed by drawing through a die hole.

(C-1) Heat Treatment (Patenting) The steel wire obtained by ordinary hot rolling may be subjected to wire drawing as it is, and may be subjected to heat treatment after work hardening, or the steel after hot rolling. The wire may be immediately heat-treated and subjected to wire drawing. The heat treatment may be performed only once or a plurality of times during the drawing process. In this heat treatment, the austenitizing temperature was 9
The reason why the temperature is set to 00 ° C. or higher is to obtain a uniform structure by dissolving all cementite. On the other hand, when the austenitizing temperature is 1100 ° C. or higher, the austenite grain size becomes coarse, the ductility of the steel wire after the heat treatment is reduced, and the wire is easily broken during drawing. The lower limit of the austenitization holding time includes substantially zero, since cementite may be in solid solution. On the other hand, if the holding time exceeds 120 seconds, productivity is impaired, so the time is set to 0 to 120 seconds. The holding time is
After the temperature transition of the steel wire is determined in advance by temperature measurement using a thermocouple or calculation simulation, it is actually managed based on the furnace time.

The transformation treatment is performed to obtain fine pearlite or fine pearlite and a small amount of upper bainite.
Therefore, in the case of the C content of the present invention, the isothermal holding temperature for causing transformation is in a temperature range of 500 to 650 ° C. If the temperature is lower than 500 ° C., the upper bainite becomes the main phase, and the wire tends to break during drawing. On the other hand, when the temperature exceeds 650 ° C., coarse pearlite or the like is generated, a target TS cannot be secured, and disconnection is likely to occur during wire drawing. The holding time at the transformation temperature is 3 to 30 seconds. If the time is less than 3 seconds, all of the austenite has not been transformed into fine pearlite or the like, so that martensite or the like is generated by subsequent cooling, which causes disconnection. On the other hand, if the time exceeds 30 seconds, the productivity is adversely affected.

(C-2) Final Drawing Processing The final drawing processing is performed by drawing through a die. The number of dice in the final wire drawing process is 15 to 30.
If the number of dies is less than 15, the degree of processing per one die is excessively large and disconnection or the like is likely to occur when trying to satisfy the condition of the degree of processing described below. On the other hand, when the number of dies exceeds 30, the processing amount in each die becomes small, and cementite is not effectively crushed and cannot be divided.

The die angle of each die is 7 to 15 °. If the die angle is less than 7 °, the processing degree described below cannot be secured. On the other hand, when the die angle exceeds 15 °,
The degree of processing per die becomes excessive, and the risk of disconnection increases.

The degree of working applied to the steel wire by drawing all the dies is true strain (ln (S ₀ /
S _f )) is set to 3.0 to 4.5. If the true strain is less than 3.0, the workability is insufficient, and the TS becomes less than 4150 MPa even if the average particle size of cementite and the volume% of cementite fall within the defined range of the present invention. On the other hand, 4.5
If it exceeds, the structure in the steel falls within the definition range of the present invention, but the possibility of disconnection increases.

The final drawing process is performed cold. Cold means that the heat treatment is not performed from the outside as described above, and the temperature naturally rises as a result of the wire drawing. Although lubricating oil, emulsion, and the like are used in wire drawing, the lubrication is not limited to liquid lubrication but may be solid lubrication such as graphite. The temperature of the lubricant (the temperature at the start of use) is not particularly limited, but is preferably around the temperature shown in Table 2 in Examples described later from the viewpoint of lubrication characteristics.

The steel wire before the final wire drawing must be plated. If plating is not applied, lubrication will be insufficient and seizure etc. will occur and disconnection will easily occur,
Even if there is no break, the surface is easily scratched. The type of plating may be anything, such as brass plating or copper plating.

The drawing speed at the winding portion is 400 to 2
000 m / min. When it is less than 400 m / min, the productivity is insufficient from an industrial point of view. On the other hand, 2000m /
Exceeding the minutes increases the risk of breakage of the steel wire.

[0049]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

Table 1 shows the chemical composition of the test steel.

[0051]

[Table 1]

The steels A to V shown in Table 1 were prepared by the usual method.
It was melted using a 50 kg vacuum furnace. Of these steels,
Steels BE, G, I, K, M, N, P, Q, S and T are examples of the present invention, while steels A, F, H, J, L, O,
R, U and V are comparative examples.

Next, these steel ingots were hot-rolled into steel wires having a wire diameter of 5.5 mm by a usual method, wound up in a coil shape, and cooled to room temperature. After the 5.5 mmφ steel wire was cold drawn to 1.4 mmφ, heat treatment was performed.

FIG. 2 shows an outline of an apparatus for performing this heat treatment. The steel wire is heated to 1000 ° C by the heating furnace shown in FIG.
After holding for 2 seconds, it was immersed in a lead bath at 570 ° C. for 30 seconds to perform heat treatment. Further, after performing brass plating by a usual method, the wire was finally drawn to 0.2 mmφ under the conditions shown in Table 2, and a tensile test and a twist test were performed. In the torsion test, a portion having a length of 20 mm, which is 100 times the diameter after the final drawing, was twisted at 15 revolutions / minute until the test material broke.
By observing the shape of the test piece after breaking, it was determined whether or not there was a vertical crack.

[0055]

[Table 2]

The average particle size of cementite in the structure of the steel after the final drawing was determined by observing the cementite by a dark-field image using a transmission electron microscope and calculating the (minor axis + major axis) of 100 cementite grains. / 2.

The volume percentage of cementite was measured by the following method. As a result of the decomposition of cementite, the amount of carbon decomposed was measured by a differential scanning calorimeter, utilizing the fact that the carbon discharged as a cementite reprecipitated when the temperature was raised and exhibited an exothermic reaction. The volume% of the cementite after the final drawing was determined by subtracting the amount of decomposed cementite from 15 × C (% by weight) which is the volume% before the final drawing.

Table 3 shows the results of these tests.

[0059]

[Table 3]

According to the table, in the case of the comparative example which is out of the defined range of the present invention, TS after drawing is less than 4150 MPa, longitudinal cracking occurs in a twist test, or both. Is applicable. In Test Nos. 27 and 37, since the average particle size of cementite was less than 2 nm, and in Test No. 41, since cementite exceeded 10 nm, vertical cracks occurred in the twist test.

Test Nos. 25, 35 and 36 have (Cementite volume%) / {C content (% by weight)} of less than 6, and Test Nos. 30 and 32 have (Cementite volume%) / ΔC content (weight%). %)｝ Exceeds 12, vertical cracks occurred in the twisting test.

Test No. 1 has a C content (hereinafter, “%” means “% by weight”) of less than 0.8%.
Since (cementite volume%) / (C content) exceeds 12, TS after final drawing was less than 4150 MPa.

Test No. 6 had a C content exceeding 1.2%, test No. 18 had a V content exceeding 0.3%, and test No. 22 had an Al content of 0.003%. , The wire drawing workability was poor, and the wire was broken during the final wire drawing.

Test No. 10 has a Mn content of more than 1%, test No. 12 has a Cr content of more than 1%, and test No. 15 has a Mo content of more than 0.5%. Test No. 21 has a Co content of more than 2%,
Martensite was formed and thus broke during the final drawing.

In Test No. 8, since the Si content exceeded 1.5%, the torsion characteristics were poor, and vertical cracks occurred in the torsion test.

On the other hand, in the example of the present invention, T
It is clear that S is 4150 MPa or more and that no vertical crack occurs in the twist test.

[0067]

According to the present invention, it is possible to stably obtain a drawn-strengthened high-strength steel wire which has a higher TS after final drawing and does not cause longitudinal cracks in a twist test, as compared with the prior art. It is.

[Brief description of the drawings]

FIG. 1 shows the average particle diameter of cementite having a TS of 4150 MPa or more and no longitudinal cracking in a twist test, and the range of (volume% of cementite) / {C (weight%)}.

FIG. 2 is a schematic diagram of an apparatus used for heat treatment in a test of an example.

Claims (2)

[Claims] (1) C: 0.8-1.2% by weight, Si:
0.1-1.5%, Mn: 0.2-1%, Al: 0.0
03% or less, Cr: 0 to 1%, Mo: 0 to 0.5%,
V: 0 to 0.3%, and Co: 0 to 2%, with the balance having a chemical composition of Fe and unavoidable impurities,
The structure in steel has a volume ratio of 6.0 × C (% by weight) to 12.0 × C
(% By weight)%, a wire-strengthened high-strength steel wire comprising cementite having an average particle diameter of 2 to 10 nm and the balance substantially of ferrite. 2. A wire rod is subjected to a drawing process including one or more of the following heat treatments, and a plating process is performed on the surface of the steel wire before the final drawing process. The method for producing a wire according to claim 1, wherein the treatment is performed. Heat treatment (a) Austenitizing Temperature: 900 to 1100 ° C Holding time: 0 to 120 seconds (b) Transformation temperature: 500 to 650 ° C Holding time: 3 to 30 seconds Final wire drawing (a) Degree of work: True strain ( ln (S ₀ / S _f ))
4.5 However, ln is the cross-sectional area of the natural logarithm, S ₀ is final drawing ago, S _f represents the cross-sectional area after the final drawing. (b) Number of dies: 15 to 30 (c) Die angle: 7 to 15 ° (d) Drawing speed at the winding section: 400 to 2000 m / min