KR101676111B1 - Wire having high strength and method for manufacturing thereof - Google Patents

Abstract

A high strength steel wire and a manufacturing method thereof are provided.
The steel wire according to the present invention is characterized by containing 0.05 to 0.15% of carbon (C), 0.1% or less of silicon (Si), 3.0 to 5.0% of manganese (Mn), 0.5 to 2.0% of chromium 0.020% or less of sulfur (P), 0.020% or less of sulfur (S), the balance Fe and unavoidable impurities, the tensile strength is 1150 to 1350 MPa and the ductility is 5%
The method for manufacturing a steel wire according to the present invention comprises the steps of: preparing a steel having the above composition and then reheating the steel; Subjecting the reheated steel material to finish hot rolling and then cooling the steel material to a temperature range of Mf to Mf - 50 占 폚 at a cooling rate of 0.2 占 폚 / s or more; And a step of cold-drawing the cooled steel material at a normal temperature, followed by cold drawing.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel wire,

More particularly, the present invention relates to a steel wire having excellent strength as a steel material for use in industrial machines or automobile parts such as automobiles, which are exposed to various external load environments, and a method of manufacturing the same. will be.

Recently, efforts to reduce the emission of carbon dioxide, which is considered to be the main cause of environmental pollution, have become a global issue. As a part of this, there is also an act of regulating exhaust gas of automobiles, and as a countermeasure, automakers are trying to solve this problem by improving fuel efficiency. However, in order to improve fuel efficiency, the weight and high performance of automobiles are required, and hence the necessity of high strength of automobile materials or parts is increasing.

There is a limitation in securing a high strength in a ferrite or a pearlite structure in a wire rod. Materials having these microstructures are usually characterized by relatively low strength, and additional cold drawing must be performed to increase the strength.

Therefore, in general, to obtain excellent strength, bainite or tempered martensite structure is used. The bainite structure can be obtained by the heat-induced transformation heat treatment using hot-rolled steel, and the tempered martensite structure can be obtained by quenching and tempering. However, since such structures can not be stably obtained only by the ordinary hot rolling and continuous cooling processes, the above-mentioned additional heat treatment process must be performed using the hot-rolled steel material.

If the high strength can be ensured without the above-mentioned additional heat treatment, a number of processes from the material to the part production can be omitted or simplified, thereby improving the productivity and lowering the manufacturing cost.

However, wire rods capable of stably obtaining bainite or martensite structure by using hot rolling and continuous cooling processes have not yet been developed, and there is a demand for development of such wire rods. There are also customer voices that require additional strength enhancement by using cold drafting to achieve higher strength in steels with ferrite or pearlite microstructures.

Accordingly, the present invention has been made to overcome the limitations of the prior art, and it is an object of the present invention to provide a steel wire excellent in strength characteristics which can be manufactured by hot rolling, continuous cooling process and cold drawing without any additional heat treatment process such as constant temperature transformation, quenching and tempering, The present invention has been made in view of the above problems.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention,

(P): 0.05 to 0.15%, silicon (Si): 0.1% or less, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0% Or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities, and having a tensile strength of 1150 to 1350 MPa and a ductility of 5% or more.

In the present invention, the steel wire may be composed of martensite of 95% or more by area and residual austenite (?).

Further, according to the present invention,

(P): 0.05 to 0.15%, silicon (Si): 0.1% or less, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0% Or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities, and reheating the steel material;

Subjecting the reheated steel material to finish hot rolling and then cooling the steel material to a temperature range of Mf to Mf - 50 占 폚 at a cooling rate of 0.2 占 폚 / s or more; And

And a step of cold drawing the cooled steel material at a normal temperature and cold drawing the steel material.

In the present invention, the steel wire may be composed of martensite of 95% or more by area and residual austenite (?).

It is preferable that the cold reduction ratio in the cold drawing process is 10 to 30%.

INDUSTRIAL APPLICABILITY The present invention according to the above-described construction can provide a martensitic high-manganese steel wire having excellent strength characteristics required for industrial machines and automobile materials or parts by using hot rolling, continuous cooling and cold drawing. Therefore, the conventional additional heat treatment process can be omitted, which is very advantageous in reducing the overall manufacturing cost.

Hereinafter, the present invention will be described in detail with reference to various embodiments.

First, the martensitic steel wire of the present invention having excellent strength characteristics will be described.

The steel wire according to the present invention is characterized by containing 0.05 to 0.15% of carbon (C), 0.1% or less of silicon (Si), 3.0 to 5.0% of manganese (Mn), 0.5 to 2.0% of chromium (P): 0.020% or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities.

Hereinafter, the reasons for limiting the composition and the composition range of the steel wire of the present invention will be described in detail.

Carbon (C): 0.05 to 0.15%

Carbon is an indispensable element for ensuring strength and is either dissolved in steel or in the form of carbides or cementites. The easiest way to increase the strength is to increase the carbon content to form carbide or cementite, but because of the reduced ductility, it is necessary to limit the addition of carbon to a certain extent. In the present invention, it is preferable to limit the content of carbon (C) in the range of 0.05-0.15%. If the carbon content is less than 0.05%, it is difficult to obtain the target strength, and if the carbon content exceeds 0.15%, the ductility may be rapidly decreased.

Silicon (Si): not more than 0.1%

It is known that silicon is added to ferrite when added and is very effective in increasing the strength through solid solution strengthening of steel. However, since the strength is greatly increased by the addition of silicone but the ductility is rapidly reduced, the addition of silicon is very limited in the case of cold forging parts requiring sufficient ductility. In the present invention, the content of silicon is preferably limited to 0.1% or less.

Manganese (Mn): 3.0 to 5.0%

Manganese increases the strength of the steel and improves the hardenability, facilitating the formation of low temperature structures such as bainite or martensite at a wide range of cooling rates. However, when the manganese content is less than 3.0%, the hardenability is not sufficient, and it is difficult to stably secure the low-temperature structure by the continuous cooling process after the hot rolling. When the manganese content is more than 5.0%, segregation of Mn during coagulation tends to be facilitated. In consideration of this, it is preferable to limit the manganese content to 3.0 to 5.0% in the present invention.

Chromium (Cr): 0.5 to 2.0%

Chromium increases the strength and hardenability of steel similar to manganese. However, when the content of chromium is less than 0.5%, the effect of improving the strength and hardenability is not significant. When the content of chromium exceeds 2.0%, the ductility is deteriorated although it is effective for improving the strength and hardenability. In view of this, in the present invention, the content of chromium is preferably limited to a range of 0.5 to 2.0%.

Phosphorus (P): not more than 0.020%

Phosphorus is segregated at the grain boundaries and is the main cause of decreasing toughness and reducing delayed fracture resistance, so the upper limit is limited to 0.020%.

Sulfur (S): not more than 0.020%

Sulfur is segregated at crystal grain boundaries to lower toughness and form a low melting point emulsion to inhibit hot rolling, so that the upper limit is preferably limited to 0.020%.

In addition, the martensitic high-manganese steel wire of the present invention may have a substantially martensitic structure, and may be composed of martensite of 95% by area or more and residual austenite (?).

Next, a method of manufacturing a steel wire having high strength characteristics of the present invention will be described.

The method for manufacturing a steel wire according to the present invention comprises the steps of: preparing a steel having the above composition and reheating the steel; Subjecting the reheated steel material to finish hot rolling and then cooling the steel material to a temperature range of Mf to Mf - 50 占 폚 at a cooling rate of 0.2 占 폚 / s or more; And a step of cold-drawing the cooled steel material at a normal temperature, followed by cold drawing.

First, in the present invention, a steel material having the above-mentioned composition components is prepared and reheated. The reheating temperature range that can be employed in the present invention may be in the range of 1000 to 1100 占 폚.

In the present invention, the reheated steel material is hot-rolled, and the finish hot-rolling temperature may be controlled within a range of 850 to 950 ° C.

The finished hot rolled steel is subjected to cooling treatment, and it is preferable to terminate the cooling in a temperature range of Mf to Mf - 50 ° C. If the cooling end temperature exceeds Mf, it is difficult to obtain a sufficient amount of martensite structure. If it is lower than Mf-50 ° C, the steel material is sufficiently cooled to facilitate handling. However, since the productivity is lowered, the cooling end temperature is preferably from Mf to Mf - It is preferable to control it in the range.

Further, in the present invention, it is preferable that the section from the finish hot rolling to the cooling end temperature is cooled at a cooling rate of 0.2 DEG C / s or more and then air-cooled. By cooling at a cooling rate of 0.2 DEG C / s or higher, a steel structure composed of martensite having an area fraction of 95% or more can be obtained.

Meanwhile, the air-cooled steel material is subjected to cold working through a cold drawing die to produce a final steel wire. In the present invention, the cold reduction ratio is preferably in the range of 10 to 30%. If the cold reduction ratio is less than 10%, it is difficult to obtain the strength to be achieved in the present invention. If the cold reduction ratio is more than 30%, the required strength range is exceeded and the ductility is greatly deteriorated.

The steel wire of the present invention manufactured through the above-described manufacturing process may have a tensile strength of 1150 to 1350 MPa and a ductility of 5% or more.

Hereinafter, the present invention will be described more specifically by way of examples.

(Example)

Molten steel having the composition components shown in Table 1 were each cast into an ingot, and homogenized at 1200 ° C for 12 hours. The homogenized steel material was hot-rolled to a thickness of 25 mm and then air-cooled.

Then, each of the steel materials prepared as described above was subjected to solution treatment at 900 ° C, followed by cooling at a cooling rate shown in Table 2, followed by air cooling. The cold-rolled steel was subjected to cold drawing at a cold reduction ratio as shown in Table 2, and tensile strength and ductility were measured and are also shown in Table 2.

In Table 2, the room temperature tensile test was performed at a crosshead speed of 0.9 mm / min until the yield point and then at a rate of 6 mm / min.

Psalter
No.
Composition Component (% by weight) C Si Mn Cr P S One 0.07 0.05 4.1 1.0 0.017 0.020 2 0.09 0.04 3.9 1.4 0.014 0.017 3 0.10 0.02 4.9 0.6 0.011 0.015 4 0.06 0.06 4.7 0.7 0.016 0.013 5 0.08 0.05 3.3 0.9 0.013 0.015 6 0.12 0.03 3.5 1.6 0.015 0.014 7 0.07 0.09 3.7 1.8 0.019 0.013 8 0.11 0.04 4.5 0.8 0.015 0.016 9 0.09 0.05 3.1 1.3 0.010 0.018 10 0.07 0.05 3.4 2.5 0.014 0.013 11 0.18 0.04 4.2 0.5 0.011 0.015 12 0.11 0.02 5.3 0.8 0.018 0.014 13 0.06 0.03 2.6 0.3 0.016 0.017 14 0.10 0.52 3.8 1.8 0.012 0.011 15 0.08 0.07 3.6 1.4 0.015 0.012 16 0.14 0.07 4.3 1.2 0.011 0.012 17 0.13 0.08 3.2 1.9 0.018 0.019

division Pseudo-No. Cooling rate
(° C / s) Martensite
Fraction (%) Cold reduction ratio
(%) The tensile strength
(MPa) Elongation
(%)



Honor One 6 98 26 1203 8 2 10 99 18 1195 9 3 2 98 13 1186 9 4 20 99 21 1214 8 5 One 97 15 1153 11 6 5 98 30 1327 6 7 15 99 10 1220 8 8 8 98 23 1212 8 9 12 99 27 1266 7



Comparative Example 10 3 98 29 1365 4 11 5 98 25 1382 3 12 2 98 24 1363 4 13 0.5 15 15 927 16 14 8 98 26 1371 3 15 0.05 10 13 1076 13 16 3 98 37 1393 3 17 0.5 97 3 1140 12

As shown in Tables 1 and 2, in the case of Inventive Examples 1-9 in which the steel composition component is within the range of the present invention and the cooling rate of 0.2 DEG C / s or more and the cold reduction ratio of 10-30% is satisfied, The tensile strength of 1150-1350 MPa and ductility of 5% or more can be seen.

On the other hand, Comparative Examples 10 and 12, in which the chromium and manganese components are each different from the upper limit of the present invention, show a large increase in strength but a decrease in ductility.

Comparative Example 11 shows a case where the content of carbon exceeds the range of the present invention, showing that the strength is greatly increased due to an increase in the effect of strengthening the solid solution of martensite base of carbon, but the ductility is drastically decreased.

In Comparative Example 13, manganese and chromium were added in a smaller amount than the component range of the present invention. However, since the manganese and chromium were less than the martensite because of the relatively low hardening ability, the strength decreased and the ductility increased. It is showing.

In addition, in Comparative Example 14, when the content of silicon exceeds the range of the present invention, the tensile strength is greatly increased, and the ductility is greatly reduced.

In Comparative Example 15, when the steel composition of the present invention is satisfied but the cooling rate is too slow, a bainite structure is formed instead of martensite to decrease the strength and increase ductility.

Comparative Example 16 shows that the steel composition component satisfies the range of the present invention, but the cold reduction ratio exceeds the range of the present invention, and the work hardening becomes severe, the strength increases, and the ductility decreases.

In Comparative Example 17, the steel composition component satisfies the range of the present invention, but the cold reduction ratio is not within the range of the present invention, and sufficient work hardening effect can not be obtained, so that the strength range of the present invention is not satisfied .

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (5)

(Si): 0.1% or less (0% is not included), manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 1.8%, carbon (P): not more than 0.020%, sulfur (S): not more than 0.020%, the balance Fe and unavoidable impurities,
A steel wire having a microstructure composed of martensite of 95% by area or more and residual austenite (?) Having a residual tensile strength of 1150 to 1350 MPa and a ductility of 5% or more.
delete (Si): 0.1% or less (0% is not included), manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 1.8%, carbon (P): 0.020% or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities, and then reheating the steel material;
Subjecting the reheated steel material to finish hot rolling and then cooling the steel material to a temperature range of Mf to Mf - 50 占 폚 at a cooling rate of 0.2 占 폚 / s or more; And
And a step of cold drawing the cooled steel material at a cold temperature of 10 to 30% after cold-cooling the cooled steel material at a normal temperature.
The method for manufacturing a steel wire according to claim 3, wherein the steel wire is composed of martensite of 95% or more by area and residual austenite (?).
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