KR101676113B1 - Wire rod having high strength, and method for manufacturing thereof - Google Patents

Abstract

A high strength wire and a manufacturing method thereof are provided.
The wire material of the present invention preferably contains 0.05 to 0.15% of carbon (C), 0.5 to 2.0% of silicon (Si), 3.0 to 5.0% of manganese (Mn), 0.5 to 2.0% of chromium (Cr) 0.020% or less phosphorus (P), 0.020% or less sulfur (S), the balance Fe and unavoidable impurities,
A method for manufacturing a wire rod 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 air-cooling the cooled steel material to a normal temperature.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength wire rod and a method of manufacturing the same,

More particularly, the present invention relates to a wire material 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 regarded as the main cause of environmental pollution, have become global issues. As a part of this, there is also an act of regulating automobile exhaust gas. As a countermeasure, automakers are trying to solve this problem by improving fuel efficiency. In order to improve the fuel efficiency, it is required to reduce the weight of the automobile and to improve the performance thereof, and accordingly, 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.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and it is an object of the present invention to provide a wire rod which can have excellent strength and impact toughness only by a hot rolling process and a continuous cooling process without additional heat treatment such as constant temperature transformation, quenching and tempering, The purpose is to provide.

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,

(C): 0.05 to 0.15%, silicon (Si): 0.5 to 2.0%, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0%, phosphorus (P): 0.020 Or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities.

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

In the present invention, the wire rod may have a tensile strength of 1200 to 1400 MPa and a ductility of 15% or more.

Further, according to the present invention,

(C): 0.05 to 0.15%, silicon (Si): 0.5 to 2.0%, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0%, phosphorus (P): 0.020 % Or less, sulfur (S): 0.020% or less, the balance Fe and inevitable 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 air-cooling the cooled steel material at a normal temperature.

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

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

The present invention relates to a martensitic high-manganese steel wire having excellent strength characteristics only by a hot rolling and a continuous cooling process without an additional heat treatment step such as constant temperature transformation, quenching and tempering, and a manufacturing method thereof.

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

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

The wire material of the present invention preferably contains 0.05 to 0.15% of carbon (C), 0.5 to 2.0% of silicon (Si), 3.0 to 5.0% of manganese (Mn), 0.5 to 2.0% of chromium (Cr) 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), the remainder Fe and unavoidable impurities.

The reasons for limiting the composition and the composition range of the wire of the present invention will be described in detail below.

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): 0.5 to 2.0% or less

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. If the content of silicon is less than 0.5%, the effect of strengthening solubility of silicon can not be sufficiently obtained. Therefore, the increase in strength is small, and if the silicon content exceeds 2.0%, the increase in strength becomes too large and the ductility deteriorates excessively .

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 the solidification 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%.

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

Next, a method of manufacturing a wire rod excellent in strength characteristics of the present invention will be described.

A method of manufacturing a wire rod 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 air-cooling the cooled steel material to a normal temperature.

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 less 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 to cool the section from the finish hot rolling to the cooling end temperature at a cooling rate of 0.2 DEG C / s or more. Cooling at a cooling rate of 0.2 DEG C / s or more, and then air cooling to obtain a steel wire structure composed of martensite having an area fraction of 95% or more.

Also, the high manganese steel wire of the present invention manufactured by the above-described manufacturing process may have a tensile strength of 1200 to 1400 MPa and a ductility of 15% 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 캜 and then cooled at a cooling rate shown in Table 2. The mechanical properties were measured for each of the cooled steels, and the results are also shown in Table 2.

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

Pseudo-No.
                            Composition Component (% by weight) C Si Mn Cr P S One 0.08 0.9 4.3 1.0 0.016 0.007 2 0.05 1.8 3.5 1.3 0.012 0.017 3 0.11 1.2 3.8 0.7 0.010 0.015 4 0.09 1.5 4.1 1.1 0.011 0.010 5 0.07 2.0 3.6 0.9 0.017 0.015 6 0.13 0.6 4.6 0.6 0.012 0.012 7 0.06 0.7 4.7 0.5 0.018 0.008 8 0.10 1.7 3.3 1.9 0.009 0.010 9 0.11 1.5 4.8 3.2 0.015 0.012 10 0.23 1.1 3.9 0.7 0.016 0.015 11 0.08 1.6 5.5 1.0 0.013 0.016 12 0.05 1.3 2.5 0.1 0.016 0.014 13 0.07 3.1 4.5 0.5 0.011 0.013 14 0.12 0.8 3.6 1.2 0.014 0.017 15 0.06 0.1 3.8 1.1 0.010 0.008

division Pseudo-No. Cooling rate (° C / s) Martensite fraction (%) Tensile Strength (MPa) Elongation (%)


Honor



One One 97 1250 17 2 10 99 1340 16 3 15 99 1380 15 4 2 98 1322 16 5 3 98 1375 15 6 7 98 1276 17 7 5 98 1234 17 8 0.5 97 1348 16

Comparative Example


9 5 99 1489 10 10 2 98 1562 7 11 One 99 1440 11 12 3 30 1046 23 13 7 99 1523 9 14 0.05 10 1135 21 15 0.5 95 1192 19

As shown in Tables 1 and 2, in the case of Inventive Examples 1-8 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 is satisfied, martensitic structure is obtained and tensile strength of 1200-1400 MPa Strength and ductility of more than 15%

On the other hand, Comparative Examples 9 and 11, in which the chromium and manganese components are different from the upper limit of the present invention, show a large increase in strength and a decrease in ductility.

Also, in Comparative Example 10, when the content of carbon was added in excess of the component range of the present invention, the strength was greatly increased and the ductility decreased sharply due to an increase in the effect of strengthening the solid solution of martensite base of carbon.

In Comparative Example 12, manganese and chromium were added in a smaller amount than the component range of the present invention. However, since the manganese and chromium were added to the composition range of the present invention, they satisfied the cooling rate but had relatively low hardenability. It is showing.

In addition, in Comparative Example 13, it was confirmed that the silicon contained in the composition exceeded the range of the present invention, the tensile strength was greatly increased, and the ductility was greatly reduced.

In Comparative Example 14, the bainite structure was formed instead of martensite when the steel composition component of the present invention was satisfied but the cooling rate was too slow, so that the strength was decreased and the ductility was increased.

Comparative Example 15 shows that when the amount of silicon is less than the range of the present invention, the tensile strength is decreased and the ductility is increased because the cooling rate is satisfied but the effect of strengthening the solid solution by silicon is hardly obtained.

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)

(C): 0.05 to 0.13%, silicon (Si): 0.6 to 2.0%, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 1.9%, phosphorus (P): 0.020 Or less, sulfur (S): 0.020% or less, the balance Fe and unavoidable impurities,
A wire having a microstructure composed of martensite of 95% or more by area and residual austenite (?) Having an excellent strength property.
delete The wire according to claim 1, wherein the wire has a tensile strength of 1200 to 1400 MPa and a ductility of 15% or more.
(C): 0.05 to 0.13%, silicon (Si): 0.6 to 2.0%, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 1.9%, phosphorus (P): 0.020 % Or less, sulfur (S): 0.020% or less, the balance Fe and inevitable 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 cooling the cooled steel material at room temperature to produce a wire material having a microstructure composed of martensite and residual austenite (?) Of 95% or more by area.
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