JPH08283867A - Production of hyper-eutectoid steel wire rod for wiredrawing - Google Patents

Abstract

PURPOSE: To stably produce a hyper-eutectoid steel wire rod for wiredrawing, having high wiredrawability and ductility, on an industrial scale. CONSTITUTION: At the time of producing a wire rod for wiredrawing a small- diameter high strength steel wire, a hyper-eutectoid steel wire rod, having a composition containing, by weight, >0.9-1.2% C, 0.1-1.5% Si, 0.1-1.5% Mn, and 0-1.0% Cr, is used as a wire rod. In a patenting treatment stage before the final wiredrawing, the wire rod is first heated to 1123-1373K in the austenitic temp. region and then cooled down to a temp. in the region of 1023 to 823K at 'a cooling rate in the range causing not temp. fall down to a temp. below the pearlitic transformation temp. in the isothermal transformation diagram' and subjected to plastic working in this temp. region at 15-80% draft. Then, transformation is allowed to occur at a temp. between 823 and 923K and, in the temp. region satisfying inequality 1233-380X<=T<=1293-380X [where T is transformation temp. (K) and X is C content (%) in the steel], by which the structure of the wire rod is formed into a pearlitic structure free from both pro-eutectoid ferrite and pro-eutectoid cementite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a "hypereutectoid steel wire rod for wire drawing of small diameter high strength steel wire" which is excellent in wire drawing workability and ductility.

[0002]

2. Description of the Related Art Cord wires for tires requiring high strength have generally been formed by twisting a thin high carbon steel filament with a diameter of about 0.2 mm into a strand. In order to meet the recent demand for higher performance, thin-diameter high-strength steel wire with a filament strength of about 3.14 GPa (320 kgf / mm ² ) has been widely used.

This cord wire is generally manufactured in the following steps. That is, first of all, a high carbon steel wire rod with a diameter of about 5.5 mmφ is used as a material, and patenting (LP)
And the wire drawing process are repeated to make a wire with a diameter of about 1.2 mmφ, and the tensile strength is 1.23 GPa in the final patenting process under the conditions of heating to a temperature of about 1173K and immersing in a lead bath at about 873K. (125kgf / mm ² ) Steel wire for wire drawing before and after. Then, this steel wire rod for wire drawing is subjected to pickling and plating, and then drawn up to around 0.2 mmφ, and the tensile strength is:
The final product wire has a strength of around 3.14 GPa (320 kgf / mm ² ).

However, under the above-mentioned processes and conditions that have been conventionally adopted, the wire drawing workability ε to be added is 3.2 {however, ε
= Ln (area of base material / cross-sectional area of wire drawing material)} is the limit, and it was impossible to further increase the wire drawing workability ε to improve product strength due to insufficient ductility.

Therefore, Japanese Patent Application Laid-Open No. 3-240919 proposes a heat treatment method for improving the wire drawing workability of a high carbon steel wire. This method uses 0.7-0.9% (
A steel wire rod containing C (% representing the composition ratio is% by weight) is heated to an austenite region of _three or more points of Ac, and A is cooled at a cooling rate that does not cut the pearlite transformation start temperature of the isothermal transformation curve.
e A technical method of converting a "supercooled austenite state wire" cooled to a temperature of 773K or more below _one point into a steel wire for wire drawing after processing at a workability of 20% or more However, in this case, supercooled austenite is processed, so the pearlite colony is miniaturized to around 5 μm, and therefore tensile strength: 1.13 GPa (115 kgf / mm ² ) class for wire drawing Material is obtained. Finally, this wire drawing material can be drawn up to a wire drawing degree ε: close to 4.9, and therefore the final product wire with a tensile strength of about 4.02 GPa (410 kgf / mm ² ). Can be obtained. However,
The light colony is a range where the cementite plates in pearlite are aligned in a certain direction.

On the other hand, on the other hand, in the case of the above method, proeutectoid ferrite is generated during transformation, which causes decrease in ductility in the final drawing process and insufficient work hardening. It became an impediment factor in achieving higher strength in the wire drawing process to the wire. Therefore, the “tensile strength obtained by the above method is 1.13 GPa (115 kgf
/ mm ² ) class of wire drawing material with good wire drawing workability does not exceed the above-mentioned ε = 4.9, and the wire drawing material can be drawn to the maximum extent. The maximum tensile strength is 4.02 even if the wire drawing degree is close to 4.9.
Only the final product wire with GPa (410 kgf / mm ² ) was obtained. In addition, it has been pointed out that drawing a wire with a high workability causes internal defects, which reduces the ductility of the final product wire and also deteriorates the fatigue strength.

By the way, it has been known for some time that it is effective to further increase the C content in order to improve the strength of a high carbon steel wire. According to this method, the cost can be reduced and a high effect can be obtained. It was considered to be a desirable strength improvement measure. However, in the case of hyper-eutectoid steel having a C content of more than 0.9%, during the patenting treatment for obtaining a wire drawing material, in the cooling process from the austenite region, prior to the pearlite transformation, the austenite transformation occurs. There was a problem that brittle pro-eutectoid cementite was formed along the grain boundaries of stenite. Therefore, when this material was drawn as it was, intergranular cracks occurred along the pro-eutectoid cementite and frequent wire breakage occurred. As described above, since pro-eutectoid cementite generated in hyper-eutectoid steel is a major cause of deterioration of wire drawing workability, it is practically avoided to use hyper-eutectoid steel for materials such as cord wire. There was

In recent years, however, some proposals have been made to improve the wire drawability of hyper-eutectoid steel. For example,
Japanese Patent Laid-Open No. 5-295448 discloses a C content of 1.1 to 1.3%.
After wire-rolling the hypereutectoid steel containing
Disclosed is a method of obtaining a pearlite structure containing no proeutectoid cementite by winding the film at 23 K and immediately thereafter immersing it in a molten salt bath held at 773 to 923 K to complete the transformation in the molten salt bath. Has been done. However, with this method, a hypereutectoid steel wire that does not contain pro-eutectoid cementite can be obtained, so a great improvement in wire drawability was expected, but in actuality, such sufficient wire drawability and ductility were achieved. As a result, a satisfactory wire rod for drawing small diameter high strength steel wire could not be obtained.

As described above, it has been impossible to industrially stably produce a hyper-eutectoid steel wire rod for wire drawing having sufficiently high wire drawability and ductility by the conventional techniques. Therefore, an object of the present invention was to establish means for stably producing a hypereutectoid steel wire rod for wire drawing having high wire drawing workability and ductility on an industrial scale.

[0010]

Means for Solving the Problems In the process of advancing research to achieve the above-mentioned object, the inventors of the present invention "if considering in view of the technique disclosed in the above-mentioned JP-A-3-240919, The reason why sufficient drawing workability and ductility could not be obtained by the method described in the above-mentioned Japanese Patent Laid-Open No. 5-295448 is due to "insufficient miniaturization of the formed pearlite colony". In order to obtain a fine pearlite colony, various experiments were repeated in which the alloy was processed in the supercooled austenite region and then transformed. As a result, after being processed in the supercooled austenite region, transformation is performed and the pearlite colony is formed.
In order to reduce the size of carbon, the C content was 0.9%.
Even in the case of a hyper-eutectoid steel exceeding the above range, it was revealed that the formation of proeutectoid ferrite or proeutectoid cementite is caused depending on the transformation temperature.

The pro-eutectoid ferrite and pro-eutectoid cementite are harmful in obtaining a steel wire having high strength and excellent wire drawability. Therefore, whether or not there is a "combination condition of C concentration and transformation temperature" in which pro-eutectoid ferrite or pro-eutectoid cementite is not formed even when a process of performing pearlite transformation after processing in the supercooled austenite region is adopted. I examined. And, "If the pearlite transformation temperature is appropriately selected according to the C content (amount) of the material steel, the perusal temperature transformation curve from the austenite region
After processing a "hypereutectoid steel wire with a supercooled austenite structure" that has been cooled in a cooling rate range that does not cut the light transformation start temperature at a specific working degree and then transforming it in a prescribed temperature range, proeutectoid ferrite , Pro-eutectoid cementite is not formed, and a fine patenting structure with a pearlite colony size of 5.0 μm or less is obtained ”.

That is, the inventors of the present invention prepared a plurality of cylindrical test pieces having a diameter of 5 mm and a height of 7 mm from a hot-rolled wire rod of vacuum-melted steel having the composition shown in Table 1, and then vacuum-formed them. After induction heating to a temperature of 1273K to form austenite, it is cooled to 923K at a cooling rate within a range not exceeding the pearlite transformation temperature in the isothermal transformation curve, and at this temperature, plastic working with a working rate of 40% is performed. After applying (rolling),
948K, 923K, 898K, 873K,
Isothermally maintained at 848K and 823K for transformation.

[0013]

[Table 1]

Then, the sample after cooling is polished and pickled.
After the etching, the microstructure was observed using a scanning electron microscope and the formation of pro-eutectoid ferrite and pro-eutectoid cementite was observed. The results of this observation are shown in FIG.

From FIG. 1, it is possible to clearly understand the relationship between the formation state of pro-eutectoid ferrite and pro-eutectoid cementite, the C content, and the transformation temperature. That is, the hatched region in FIG. 1 is a region where pro-eutectoid ferrite and pro-eutectoid cementite are not formed, and pro-eutectoid ferrite is produced on the left side of this region and pro-eutectoid cementite is produced on the right side. When the hatched region is mathematically expressed by the relationship between the C content and the transformation temperature, the following formula (1) is obtained. 1233-380X ≤ T ≤ 1293-380X (1) [however, T: transformation temperature (K), X: C content (%) of steel] In this region, the chemical composition of the present invention described later. Was applicable to any of the hyper-eutectoid steels.

The present invention has been made on the basis of the above findings and the like. "When manufacturing a wire rod for drawing thin and high strength steel wire, the weight ratio of C: more than 0.9 to 1.2%, Si :
0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 0 to 1.0%
In the patenting treatment step before the final wire drawing for obtaining the wire rod for wire drawing from the above, a hyper-eutectoid steel wire rod containing is used as a raw material, and is first heated to an austenite region temperature of 1123 to 1373K, and then " After cooling to a temperature range of 1023 to 823K at a cooling rate within a range that does not cut the pearlite transformation temperature in the isothermal transformation curve, and performing plastic working at a working rate of 15 to 80% in this temperature range, 823 to
In the temperature range of 923K, and in the temperature range satisfying the formula 1233-380X ≤ T ≤ 1293-380X [where T: transformation temperature (K), X: C content (%) of steel] is transformed, By making the wire structure a pearlite structure that does not contain both proeutectoid ferrite and proeutectoid cementite, it has become possible to stably manufacture hypereutectoid steel wire for wire drawing with high wire drawability and ductility. Has a great feature.

The "plastic working" performed in the supercooled austenite region may be carried out by "rolling using a rolling mill", "pulling using a warm die", or any other suitable method.

Next, the reason why the chemical composition of the material and the processing conditions are limited as described above in the present invention will be explained together with its action.

[Action]

(A) Chemical composition of material a) C The transformation temperature is 823 to 923K, and the range of C content satisfying the condition of the above formula (1) is 0.82 to 1.2%. However, when the C content is 0.9% or less, the strength after wire drawing is 4.12.
It does not reach GPa (420kgf / mm ² ). Therefore, the C content is 0.9
Determined as super ~ 1.2%.

B) Si Si is a component effective for increasing the strength of the steel wire rod, and is also a component necessary as a deoxidizing agent. However, its content is 0.1
If it is less than%, a sufficient deoxidizing effect cannot be obtained, while
If Si is contained in excess of%, the wire drawing workability decreases. Therefore, the Si content is set to 0.1 to 1.5%.

C) Mn Mn is also a component effective for increasing the strength of the steel wire rod, and is also a component necessary as a deoxidizing agent. However, its content is 0.1
If it is less than%, a sufficient deoxidizing effect cannot be obtained, while
If Mn is contained in excess of%, the ductility of pearlite decreases. Therefore, the Mn content is set to 0.1 to 1.5%.

D) Cr It is possible to obtain the target strength and reduction value without adding Cr to the steel material, but if Cr is added, the lamella spacing of the pearlite is narrowed and the strength of the steel wire is further stabilized. It is possible to improve. However, if the Cr content exceeds 1.0%, the wire drawability decreases, so the Cr content is 0 to
1.0%.

(B) Manufacturing Conditions The hypereutectoid steel wire rod having the above chemical composition, which is used as a raw material for the method of the present invention, can be manufactured by converter melting, continuous casting and hot rolling as usual. Yes (usually diameter: 5.5
mm and the wire).

In the method of the present invention, the raw steel wire is first heated to an austenite temperature range of 1123 to 1373K. Here, the heating temperature is set to 1123 K or higher in order to completely dissolve the carbide in the austenite. On the other hand, if the heating temperature exceeds 1373 K, the austenite grains become coarse and a fine pearlite colony structure cannot be obtained.

The material steel wire rod heated to the austenite region temperature is subsequently cooled at 1023 to 8 at "cooling rate within a range not exceeding the pearlite transformation start temperature in the isothermal transformation curve".
It is cooled to a temperature of 23K. This cooling is for not starting the pearlite transformation until the end of processing, and the cooling rate at that time is not particularly limited as long as it is "the cooling rate within the range not exceeding the pearlite transformation start temperature in the constant temperature transformation curve". Not something. The cooling rate at which pearlite transformation is not started is AISI standard-C1080 steel (C:
0.80%, Si: 0.20%, Mn: 0.45%)
A temperature of 180 ° C./sec or more is sufficient.

By the way, since the alloying elements Si, Mn and Cr shift the pearlite transformation start temperature in the isothermal transformation curve to the longer side, the cooling rate is slowed by increasing the addition amount of these alloying elements. It is possible. Therefore, for example, C: 0.93%, Si: 0.24%, Mn: 0.80%, C
For steel with a chemical composition of r: 0.30%, the cooling rate is 30 ° C.
Perlite transformation does not occur for more than 1 second.

The material steel wire rod cooled to the working temperature of 1023 to 823K in this manner has the following working degree:
15-80% plastic working is performed. It can be said that “rolling using a rolling mill” is preferable for this plastic working,
"Pulling using a warm die" and "other processing methods"
Of course, there is no problem in applying.

The processing temperature at this time is 1023 to 8
The reason why the working degree is set to 23K and the working degree is set to 15 to 80% is as follows. That is, the present inventors have found that C: 0.93
%, Si: 0.24%, Mn: 0.80%, Cr: 0.30% vacuum melted steel hot-rolled wire with a chemical composition 5 mm in diameter and 7 mm in height
A plurality of cylindrical test pieces of No. 1 were prepared, and then these were induction-heated in vacuum to a temperature of 1273K to be austenitized, and then at a cooling rate within a range not exceeding the pearlite transformation temperature in the isothermal transformation curve. Cooling to various processing temperatures,
After performing plastic working (rolling) with various working degrees at this temperature,
It was kept isothermal at 873K and transformed. Then, the cooled sample was polished and etched with a picular, and then the structure was observed with a scanning electron microscope to obtain a pearlite colony.
I checked the size. Here, the pearlite colony size is a factor that greatly affects the wire drawing workability and ductility,
Unless the pearlite colony size is reduced to 5 μm or less, sufficient wire drawability and ductility cannot be obtained. The "effects of processing temperature and processing degree on the pearlite colony size" thus grasped are summarized in Fig. 2.

As is clear from the results shown in FIG. 2, the fine pearlite colony size (5.0 μm or less) can be achieved at a processing temperature of 1023 K or less and a processing degree of 15% or more. I understand.

On the other hand, if the working temperature is lower than 823K, the wire drawing workability is deteriorated, and if the working ratio exceeds 80%, the pearlite layered structure is not sufficiently developed and the wire drawing work is also performed. It was also confirmed that the sex was reduced. It has been confirmed that the above conditions apply to any of the hypereutectoid steels having the chemical composition according to the present invention. Therefore, the working temperature in the plastic working is 1023 to 823K, and the working degree is 15
Each was limited to ~ 80%.

Next, after the plastic working is completed under the above-mentioned conditions, the wire is within the temperature range of 823 to 923K and has the formula 1233-380X ≤ T ≤ 1293-380X [where T: transformation temperature (K)]. , X: C content (%) of steel] is transformed in a temperature range satisfying the condition.
Here, if the transformation temperature is less than 823 K (550 ° C.), bainite is generated and wire drawability deteriorates. Further, if the transformation temperature exceeds 923K, the lamellar spacing of pearlite becomes wide and the wire drawability also deteriorates. Further, proeutectoid ferrite or proeutectoid cementite is produced in a temperature range that does not satisfy the conditions shown by the above formula, which deteriorates the wire drawing workability.

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

EXAMPLES First, steels A to R each having the chemical composition shown in Table 2 were melted in a 150 kg vacuum melting furnace and hot-rolled into a raw steel wire rod having a diameter of 5.5 mmφ.

[0032]

[Table 2]

Next, these raw steel wire rods were processed by cold drawing to a diameter of 2.3 mmφ and processed and heat-treated under the conditions shown in Tables 3 and 4 to draw steel wire rods (base metal). ) Got. Then, the structure of each of the base materials was inspected and the mechanical properties thereof were investigated. At the same time, these were drawn to the wire drawing limit, and the limit workability and the mechanical properties of the wire drawn materials were investigated. The results of these investigations are also shown in Tables 3 and 4.

[0034]

[Table 3]

[0035]

[Table 4]

From the results shown in Tables 3 and 4, the following can be confirmed. That is, test numbers 1, 2,
Nos. 4, 5, 7, 8 and 24 are regions where the transformation temperature produces pro-eutectoid ferrite or pro-eutectoid cementite as shown in Fig. 1, so the obtained base metal has a low limit workability, and the wire drawing material is sufficient. I can't get enough strength. Further, in Test No. 21, the transformation temperature exceeds 923 K, and thus the obtained base material has a low limit working degree.

On the other hand, the test number 9 has a heating temperature of 1123.
Since it was less than K, the carbides were not completely solid-dissolved, so that the base material had a low limit workability, and the wire-drawn material could not obtain sufficient strength. On the other hand, in Test No. 12, the heating temperature exceeded 1373 K, so that the size of the obtained pearlite block of the base material was not 5 μm or less, and the limit workability was low.

In the test No. 14, since the working temperature exceeds 1023K, the size of the pearlite colony of the obtained base material does not become 5 μm or less, and the limit working degree is also low.
Test No. 15 has a low limit working degree of the base material obtained because the working temperature is less than 823K.

In Test No. 16, since the workability is less than 15%, the pearlite colony of the obtained base material does not have a size of 5 μm or less and the limit workability is low. Also,
Test No. 20 has a low limit workability of the base material obtained because the workability exceeds 80%.

Test number 28 has a Si content of 1.5%, test number 31 has a Mn content of 1.5%, and test number 33 has a Cr content of more than 1.0%. The limit workability is low, and therefore the strength of the drawn wire is low.

On the other hand, all of the base materials (hyper-eutectoid steel wire for wire drawing) manufactured according to the specified conditions of the present invention have a high limit workability, and the wire drawing material has a tensile strength of 4.12 GPa (420 kgf). / mm
It can be seen that excellent properties are obtained such as having a strength of ² ) or more and an aperture value of 40% or more.

[0042]

[Summary of Effects] As described above, according to the present invention, it becomes possible to stably manufacture a hyper-eutectoid steel wire rod for wire drawing having high strength and excellent wire drawing workability. It has a useful effect.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the C content of a raw steel wire and the pearlite transformation temperature and the formation of proeutectoid ferrite and proeutectoid cementite.

FIG. 2 is a graph showing the influence of the working temperature and the working amount in the plastic working before the pearlite transformation on the pearlite colony size.

Claims (1)

[Claims] 1. When manufacturing a wire rod for drawing thin high-strength steel wire, C: more than 0.9 to 1.2% and Si: 0.1 by weight.
In the patenting process before final drawing for obtaining a wire for hyperdrawing from a hypereutectoid steel wire containing ~ 1.5%, Mn: 0.1-1.5%, Cr: 0-1.0% as a raw material. First, after heating to an austenite region temperature of 1123 to 1373 K, "10" at "cooling rate within a range not exceeding the pearlite transformation temperature in the isothermal transformation curve"
After cooling to a temperature range of 23 to 823 K and performing plastic working with a working ratio of 15 to 80% in this temperature range, 823 to 92
The wire drawing is characterized in that the structure of the wire is a pearlite structure not containing both proeutectoid ferrite and proeutectoid cementite by transforming within a temperature range of 3 K and satisfying the following formula. For manufacturing hyper-eutectoid steel wire rods for automobiles. 1233-380X ≤ T ≤ 1293-380X [where, T: transformation temperature (K), X: C content (%) of steel]